POWER QUALITY IMPROVEMENT FOR GRID

CONNECTED WINDENERGY SYSTEM USING

FACTS -MATLAB (SIMULINK)

Project Report

Submitted on partial fulfillment of the requirements for the award of the degree of

BACHELOR OF TECHNOLOGYIn

ELECTRICAL & ELECTRONICS ENGINEERING By

Batch ID:-08EEE-17

Y. Karuna Reddy (08L31A0293) L. Naga Chaitanya

(07L31A0250)

P.V.S. Naveen (08L31A0268) T. Anitha (08L31A0283)

Under the esteemed guidance ofMs. M.ARUNA KUMARI Assistant Professor, EEE

Department of Electrical & Electronics Engineering

Vignan’s Institute of Information Technology(Approved by AICTE and affiliated to JNT University, KAKINADA)

NAAC & NBA Accredited & ISO9001:2008, ISO14001:2004, OHSAS 18001:2007 Certified Institution, Besides VSEZ, Duvvada, Visakhapatnam-530046

2011-2012

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CHAPTER – 1

INTRODUCTION

1.0 Introduction

With increase in the demand for Electricity due to increase in population and

industrialization, the Generation of power was really a challenge now a day. If we want

to increase the power generated in the conventional way i.e., by means of non-

renewable energy sources like coal, diesel, natural gases and similar fossil fuels, the

pollution increases which degrades the Environment and human life style.

Disadvantage of using Non-Renewable energy sources are:

Non-renewable sources will expire some day and we have to us our endangered

resources to create more non-renewable sources of energy.

The speed at which such resources are being utilized can have serious

environmental changes.

Non-renewable sources release toxic gases in the air when burnt which are the

major cause for global warming.

Since these sources are going to expire soon, prices of these sources are soaring

day by day.

Thus there is a great need for electric power which has to be produced in a clean

way that is through the Renewable energy sources like solar, wind, tidal,

geothermal, biomass energy sources. These resources are very cheap and are

abundant in nature. We can completely depend on these sources if we got the

technology to do so.

Compared to the non-renewable energy sources these have the advantages of the

following:

The sun, wind, geothermal, ocean energy are available in the abundant quantity

and free to use.

The non-renewable sources of energy that we are using are limited and are

bound to expire one day.

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Renewable sources have low carbon emissions, therefore they are considered as

green and environment friendly.

Renewable helps in stimulating the economy and creating job opportunities.

The money that is used to build these plants can provide jobs to thousands to

lakhs of people.

You don't have to rely on any third country for the supply of renewable sources

as in case of non-renewable sources.

Renewable sources can cost less than consuming the local electrical supply. In

the long run, the prices of electricity are expected to soar since they are based

on the prices of crude oil, so renewable sources can cut your electricity bills.

Various tax incentives in the form of tax waivers, credit deductions are

available for individuals and businesses who want to go green.

But even though they have their advantages, they are not preferred due to

economical criteria of investing huge funds. Also the problems that we face when we

integrate these energy sources to the grid are quite many like power quality

maintenance.

In this paper we consider Wind power that can be utilised for generation of electrical

power using Wind farms with FACTS device P-STATCOM to compensate the

disturbances that occur due to the fluctuating nature of the wind. This nature of wind

also effects the current and voltage in the grid to which wind turbine is connected.

1.1 OBJECTIVE OF THE PROJECT

The causes of power quality problems are generally complex and difficult to

detect when we integrate a wind turbine to the grid. Technically speaking, the ideal AC

line supply by the utility system should be a pure sine wave of fundamental frequency

(50/60Hz). We can therefore conclude that the lack of quality power can cause loss of

production, damage of equipment or appliances or can even be detrimental to human

health. It is therefore imperative that a high standard of power quality is maintained.

This project demonstrates that the power electronic based power conditioning using

custom power devices like P-STATCOM can be effectively utilized to improve the

quality of power supplied to the customers.

The aim of the project is to implement Wind turbine connected to a Grid

consisting of Distribution generation and P-STATCOM with Back Up energy storage

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system (BESS) in the MATLAB, simulink using Simpower systems tool box and to

verify the results through various case studies applying Non-linear loads and study

them in detail.

1.2 OVERVIEW OF THE PROJECT

The Renewable energy sources, which have been expected to be a promising

alternative energy source, can bring new challenges when it is connected to the power

grid. However, the generated power from renewable energy source is always

fluctuating due to environmental condition. In the same way Wind power injection into

an electric grid affects the power quality due to the fluctuation nature of the wind and

the comparatively new types of its generators.

On the basis of measurements and norms followed according to the guidelines

specified in IEC-61400 (International Electro-technical Commission) standard, the

performance of the wind turbine and thereby power quality are determined. The power

arising out of the wind turbine when it connected to grid system concerning the power

quality measurements are-the active power, reactive power, voltage sag, voltage swell,

flicker, harmonics, and electrical behaviour of switching operation and these are

measured according to national/international guidelines. The paper clearly shows the

existence of power quality problem due to installation of wind turbine with the grid.

In this proposed scheme a FACTS device {STATIC COMPENSATOR

(STATCOM)} is connected at a point of common coupling with a battery energy

storage system (BESS) to reduce the power quality problems. The battery energy

storage system is integrated to support the real power source under fluctuating wind

power. The FACTS Device (STATCOM) control scheme for the grid connected wind

energy generation system to improve the power quality is simulated using

MATLAB/SIMULINK in power system block set. The intended result of the proposed

scheme relives the main supply source from the reactive power demand of the load and

the induction generator. From the obtained results, we have consolidated the feasibility

and practicability of the approach for the applications considered.

The STATCOM is a compensating device which is used to control the flow of

active and reactive power required to the Induction Generator of the wind turbine. It is

a custom power device which is gaining a fast publicity during these days due to its

exceptional features like it provides fast response, suitable for dynamic load response

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or voltage regulation and automation needs, Both leading and lagging VARS can be

provided, to correct voltage surges or sags caused by reactive power demands pulse

STATCOM can be applied on wide range of distribution and transmission voltage,

overload capability of this provides reserve energy for transients from the BESS.

The pulse STATCOM is controlled using the PI controller. The complete

background of the compensating devices and power electronic application in

compensating devices is discussed and also the compensation using the STATCOM

modeling is also discussed.

Theoretical analyses of the Different types of control strategies use for the

control of STATCOM are discussed and the necessary block diagrams and the

transformations required are discussed.

Conclusions are drawn basing on the simulated results obtained and also the

future scope of the project is also included.

CHAPTER – 2

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FLEXIBLE AC TRANSMISSION SYSTEMS AND

THEORETICAL ANALYSIS

2.1.0 POWER QUALITY AND RELIABILITY:

Power quality and reliability cost the industry large amounts due to mainly sags and

short-term interruptions. Distorted and unwanted voltage wave forms, too. And the

main concern for the consumers of electricity was the reliability of supply. Here we

define the reliability as the continuity of supply. As shown in fig.2.1, the problem of

distribution lines is divided into two major categories. First group is power quality,

second is power reliability. First group consists of harmonic distortions, impulses and

swells. Second group consists of voltage sags and outages. Voltage sags is much more

serious and can cause a large amount of damage. If exceeds a few cycle, motors, robots,

servo drives and machine tools cannot maintain control of process.

Fig.2.1.1 power quality and reliability

Both the reliability and quality of supply are equally important. For example, a

consumer that is connected to the same bus that supplies a large motor load may have

to face a severe dip in his supply voltage every time the motor load is switched on. In

some extreme cases even we have to bear the black outs which is not acceptable to the

consumers. There are also sensitive loads such as hospitals (life support, operation

theatre, and patient database system), processing plants, air traffic control, financial

institutions and numerous other data processing and service providers that require clean

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and uninterrupted power. In processing plants, a batch of product can be ruined by

voltage dip of very short duration. Such customers are very wary of such dips since

each dip can cost them a substantial amount of money. Even short dips are sufficient to

cause contactors on motor drives to drop out. Stoppage in a portion of process can

destroy the conditions for quality control of product and require restarting of

production. Thus in this scenario in which consumers increasingly demand the quality

power, the term power quality (PQ) attains increased significance.

Transmission lines are exposed to the forces of nature. Furthermore, each

transmission line has its load ability limit that is often determined by either stability

constraints or by thermal limits or by the dielectric limits. Even though the power

quality problem is distribution side problem, transmission lines are often having an

impact on the quality of the power supplied. It is however to be noted that while most

problems associated with the transmission systems arise due to the forces of nature or

due to the interconnection of power systems, individual customers are responsible for

more substantial fraction of the problems of power distribution systems.

2.1.1 Types of Power Quality Problem

Some of the power quality disturbance wave forms are shown in fig 2.1.2

2.1.2 Transients These are sub cycle disturbances with a very fast voltage change.

They typically have frequencies often to hundreds of kilohertz and sometimes

megahertz. The voltage excursions range from hundreds to thousands of volts.

Transients are also called spikes, impulses and surges. Two categories of transients are

described, impulsive transient and oscillatory transient. Examples of transients include

lightning, electro-static discharge; load switching, line/ cable switching, capacitor bank

or transformer energizing and Ferro-resonance.

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Fig.2.1.2. Some PQ disturbances

2.1.3 Long- Duration Voltage Variations

Long-duration variations encompass root-mean-square (rms) deviations at

power frequencies for longer than 1 min. A voltage variation is considered to be long-

duration when the limits are exceeded for greater than 1 min. These variations are

categorized below:

Over voltage: An over voltage is an increase in the rms voltage greater than 110

percent at power frequency for duration longer than 1 min. Examples include load

switching, incorrect tap settings on transformers, etc.

Under voltage: An under voltage is a decrease in the rms ac voltage to less than 90

percent at power frequency for duration longer than 1 min. Examples include load

switching, capacitor bank switching off, overloaded circuits, etc.

Sustained interruptions: These come about when the supply voltage stays at zero

longer than 1 min. They are often permanent and require human intervention to repair

the system restoration. Examples include system faults, protection maltrip, operator

intervention, etc.

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2.1.4 Short- Duration Voltage Variations

Short-duration variations encompass the voltage dips and short interruptions.

Each type of variations can be designated as instantaneous, momentary, or temporary,

depending on its duration these variations can be categorized as:

Interruptions: This occurs when the supply voltage or load current decreases to less

than 0.1 pu for a time not exceeding 1 min. The voltage magnitude is always less than

10 percent of nominal. Examples include system faults, equipment failures, control

malfunctions, etc.

Sags (dips): Sag is a decrease to between 0.1 and 0.9 Pu in rms voltage or current at

power frequency for durations from 0.5 cycle to 1 min. Examples include system faults,

energization of heavy loads, starting of large motors, etc.

Swells: A swell is an increase to between 1.1 and 1.8 Pu in rms voltage or current at

power frequency for durations from 0.5 cycle to 1 min. Swells are not as common as

sags. Sometimes the term momentary over voltage is used as a synonym for the term

swell. Examples include system faults, switching off heavy loads, energizing a large

capacitor bank, etc.

2.1.5 Voltage and Current Imbalance

Unbalance, or three-phase unbalance, is the phenomenon in a three-phase

system, in which the rms values of the voltages or the phase angles between

consecutive phases are not equal. Examples include unbalanced load, large single-phase

load, blown fuse in one phase of a three-phase capacitor bank, etc.

2.1.6 Voltage Fluctuation

The fast variation in voltage magnitude is called “voltage fluctuation”, or “light

flicker”. Sometimes the term “voltage flicker” is also used. This voltage magnitude

ranges from 0.9 to 1.1 pu of nominal. One example is an arc furnace.

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2.1.7 Power Frequency Variations

Power frequency variations are defined as deviation of the power system

fundamental frequency from its specified nominal value (e.g. 50 or 60Hz). This

frequency is directly related to the rotational speed of the generators supplying the

system. There are slight variations in frequency as the dynamic balance between load

and generation changes. The size of the frequency shift and its duration depends on the

load characteristics and the response of the generation control system to load changes.

Examples include faults on transmission system, disconnection of large load,

disconnection of large generator, etc.

2.1.8 Waveform Distortion

Waveform distortion is defined as a steady-state deviation from an ideal sine

wave of power frequency principally characterized by the spectral content of the

deviation.

Three types of waveform distortion are listed below:

Harmonics: These are steady-state sinusoidal voltages or currents having frequencies

that are integer multiples of the fundamental frequency. Harmonic distortion originates

in the nonlinear characteristics of devices and loads on the power system. Examples

include computers; fax machines, UPS systems, variable frequency drives (VFDs), etc.

Inter harmonics: These are voltages and currents having frequency components which

are not integer multiples of the fundamental frequency. Examples include static

frequency converters, cyclo-converters, induction motors and arcing devices.

Noise: This is unwanted electrical signals with broadband spectral content lower than

200 kHz superimposed on system voltage or current in phase conductors, or found on

neutral conductors or signal lines. Examples include power electronics applications,

control circuits, solid-state rectifiers, switching power supplies, etc.

2.1.9 Causes of Power Quality Variations

The main causes of poor power quality come from the customers themselves

(internal), generated from one customer that may impact other customers (neighbours),

and also from the utility. Neighbours here include those in separate buildings near the

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customer and separate businesses under the same roof such as a small business park.

The types and causes of power quality variations are as follows:

Table 2.1.1 Internal Causes of Power Quality Variations

Types Causes

Transient

Small lightning strikes at low voltage levels (e.g.500V) can

disrupt or damage electronic equipment. Reactive loads turning

on and off generate spikes. Poor connections in the wiring

system lead to arcing-caused transients. Switching of power

electronics devices.

Long-duration

voltage variations

Over- and under-voltages are caused by load variations on the

system. Overloaded circuits results in under voltages. Sustained

interruptions are caused by lightning strikes.

Short-duration

voltage variations

Sags and swells occurs whenever there is a sudden change in

the load current or voltage. Sags result when a load turns on

suddenly (e.g. starting of large motors). Sags do not directly

cause damage but initiate problems indirectly. Swells caused

by the sudden turning off of loads can easily damage user

equipment.

Waveform

distortions

Current distortion affects the power system and distribution

equipment. Overheating and failure in transformer and high

neutral currents are some direct problems. Current harmonics

may excite resonant frequencies in the system, which can cause

extremely high harmonic voltages to damage equipment.

Nonlinear loads (e.g. Variable frequency drives, induction

motors, and power electronics components) cause voltage

distortions, which can cause motor to overheat and vibrate

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excessively, resulting in damage to the shaft of motors.

Components in computers may also be damaged. Electrical

noise indirectly causes damage and loss of product Process

control equipment and telecommunications are sensitive to

such noise.

Wiring& grounding Inappropriate or poor wiring and grounding can affect the

operation and reliability of sensitive loads and local area

networks.

Table 2.1.2 Neighbouring Causes of Power Quality Variations

Types Causes and effects

Transient

Transients are generated from the switching of loads. In situations

where multiple, separate businesses share wiring or other parts of the

power system, arcing-based transients are possible. Reactive loads,

regardless of light or heavy motors, generate spikes.

Long /

Short

duration

voltage

variations

Changing currents interact with the system impedance. Loads in the

neighbour’s facility must be large and changing enough to affect the

voltage feeding the customer’s facility or office. If shared wiring is

present, then even simple devices may cause similar concerns.

Overloading may be the cause as well.

Waveform

distortion

If a customer’s neighbours draw large amount of distorted current,

this current will subsequently distort the utility supply voltage, which

is then fed back to the customer. Hence, loads within the customer’s

business are subjected to potential problems.

Table 2.1.3 Utility Causes of Power Quality Variations

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Types Causes

Transient

The most common causes of transients come from lightning surges.

Other causes include capacitor bank energization, transformer

energization, system faults

Long-duration

voltage

variations

These voltage variations are the result of load switching (e.g.

switching on/off a large load, or on/off a capacitor bank). Incorrect

tap settings on transformers can also cause system over voltages.

Overloaded circuits can result in under voltages as well.

Short-duration

voltage

variations

These variations are caused by fault conditions, energization of large

loads that require high starting currents, or intermittent loose

connections in power wiring. Delayed reclosing of protective devices

may cause momentary or temporary interruptions.

Voltage and

current

imbalance

Primary source of voltage unbalance is unbalanced load (thus current

unbalance). This is due to an uneven spread of single-phase, low

voltage customers over the three phases, but more commonly due to a

large single-phase load. Three-phase unbalance can also result

because of capacitor bank anomalies, such as a blown fuse in one

phase of a three-phase bank.

Power

frequency

variation

The frequency of the supply voltage is not constant. This frequency

variation is due to unbalance between load and generation. Short

circuits also contribute to this variation.

Waveform

distortion

The amount of harmonic distortion originating from the power

system is normally small. The increasing use of power electronics for

control of power flow and voltage (flexible ac transmission systems

or FACTS) carries the risk of increasing the amount of harmonic

distortion originating in the power system.

Harmonic current distortion requires over-rating of series components

like transformers and cables

Inter harmonics can excite unexpected resonance between

transformer inductances and capacitor banks. More dangerous are

sub-harmonic currents, which can lead to saturation of transformers

and damage to synchronous generators and turbines.

2.2.0. Power electronic applications in power transmission system

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The rapid development of power electronics technology provides exciting

opportunities to develop new power system equipment for better utilization of existing

systems. Since 1990, a number of devices under the term FACTS (flexible AC

transmission systems) technology have been proposed and implemented. FACTS

devices can be effectively used for power flow control, load sharing among parallel

corridors, voltage regulation, and enhancement of transient stability and mitigation of

system oscillations. By giving additional flexibility, FACTS controllers can enable a

line to carry power close to its thermal rating. Mechanical switching has to be

supplemented by rapid response power electronics. It may be noted that FACTS is

enabling technology, and not a one-on-one substitute for mechanical switches.

FACTS employ high speed Thyristor for switching in or out transmission line

components such as capacitors, reactors or phase shifting transformers for desirable

performance of systems. The FACTS technology is not a single high power controller,

but rather a collection of controllers, which can be applied individually or in

coordination with others to control one or more of system parameters. it started with the

high voltage DC current (HVDC) transmission, static VAR compensator (SVC)

systems were employed later for the reactive power compensation of power

transmission lines . Subsequently, devices like thyristor controlled series compensator

(TCSC), static compensator (STATCOM), static synchronous series compensator

(SSSC), unified power flow controller (UPFC) were proposed and installed under the

generic name of flexible AC transmission systems (FACTS) controllers.

2.2.1 PRINCIPLE AND OPERATION OF CONVERTERS:

The switching converter forms the heart of the FACTS controllers.

Controllable reactive power can be generated by the DC to AC switching converters

which are switched in synchronism with the line voltage with which the reactive power

is exchanged. A switching power converter consists of an array of solid state switches

which connect the input terminals to the output terminals. It has no internal storage and

so the instantaneous input and output power are equal. Further the input and output

terminations are complementary, that is, if the input is terminated by a voltage source

(charged capacitor or battery), output is a current source (which means a voltage source

having an inductive impedance) and vice versa. Thus, the converter can be voltage

sourced (shunted by a capacitor or battery) or current sourced (shunted by an inductor).

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Fig:-2.2.1. Operation of converter

Single line diagram of basic voltage sourced converter scheme for reactive power

generation is shown in fig.2.2.1 for reactive power flow bus voltage V and converter

terminal voltage V0 are in phase.

Then on per phase basis

I= V- V0 / X

The reactive power exchange is

Q = VI = V (V- V0 ) / X

The switching circuit is capable of adjusting V0 , the output voltage of the converter.

For V0 < V, I lags V and Q drawn from the bus is inductive, while for V0 >V, I leads V

and Q drawn from the bus is leading. Reactive power drawn can be easily and smoothly

varied by adjusting V0 by changing the on time of the solid state solid state switches. It

is to be noted that the transformer leakage reactance is quite small, which means that a

small difference in of voltage (V- V0) causes the required I and Q flow. Thus the

converter acts as the static synchronous condenser or VAR generator. As the converter

draws only reactive power, the real power drawn from the capacitor is zero. Also at DC

(zero frequency) the capacitor doesn’t change and the capacitor establishes only a

voltage level for the converter.

2.3 FACTS CONTROLLERS:

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The development of FACTS controllers has followed two different approaches. The

first approach employs reactive impedances or a tap changing transformer with

thyristor switches as the controlled elements, the second approach employs self

commutated static converters as voltage sources. In general these are three categories.

in series with the power system (series compensation)

in shunt with the power system (shunt compensation)

both in series and in shunt with the power system

2.3.1. Series compensation

In series compensation, FACTS is connected in series with the power system. It

works as a controllable voltage source. In series compensation generally inductors are

connected in series with the transmission line that is because in case of long

transmission line due to series inductance when a large current flows through it causes

a large voltage drop. Now to compensate that large voltage drop due to inductance,

series capacitances are connected. All series controllers inject voltage in series with the

line. If the voltage is in phase quadrature with the line, series controller only supplies or

consumes variable reactive power. Any other phase relationship will involve real power

also.

Tasks of dynamic series compensation:

Reduction of load dependent voltage drops

Reduction of system transfer impedance

Reduction of transmission angle

Increase of system stability

Load flow control for specified power paths

Damping of active power oscillations

Static synchronous series compensation (SSSC):

Series compensation can also be built up by the use of STATCOM converter

technology. Similar valve configurations are used. Above figure shows the connection

principle of an SSSC. A series voltage formed by the DC storage capacitor and the

converter configuration will be introduced to the system in quadrature to the line

current. Capacitive as well inductive compensation is possible. Such SSSC

configurations are also used in the Unified Power Flow Controller (UPFC, described

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later) as series part of the whole device. Two or more of the SSSC can be installed in a

system in parallel lines or at major substations with several lines leaving to different

areas. Such arrangement allows power flow control under severe system conditions.

Fig.2.2.2.Static synchronous series compensator (SSSC)

Thyristor-Controlled Series Capacitor (TCSC)

The two basic schemes of thyristor-controlled series capacitors, using thyristor-

switched capacitors and a fixed capacitor in parallel with a thyristor-controlled Reactor,

are shown schematically in Fig- 2.2.3 a and b. In the thyristor-switched capacitor

scheme of Figure 2.2.3 a, the degree of series compensation is controlled by Increasing

or decreasing the number of capacitor banks in series. To accomplish this, each

capacitor bank is inserted or bypassed by a thyristor valve (switch). To minimize

switching transients and utilize “natural” commutation, the operation of the thyristor

valves is coordinated with voltage and current zero crossings. In the fixed-capacitor,

thyristor-controlled reactor scheme of Figure 2.2.3 b, the degree of series compensation

in the capacitive operating region (the admittance of the TCR is kept below that of the

parallel connected capacitor) is increased (or decreased) by increasing (or decreasing)

the thyristor conduction period, and thereby the current in the TCR. Minimum series

compensation is reached when the TCR is off. The TCR may be designed to have the

capability to limit the voltage across the capacitor during faults and other system

contingencies of similar effect. The two schemes may be combined by connecting a

number of TCRs plus a fixed capacitor in series in order to achieve greater control

range and flexibility.

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(a)

(b)

Fig.2.2.3. (a) TCSC with thyristor switched capacitance

(b) TCSC with fixed capacitor

2.3.2. Shunt compensation:

This may be variable impedance, variable source or combination of these. All shunt

controllers inject current into the system at the point of connection. Combined series-

series controllers can be combination of separate series controllers which are controlled

in a coordinated manner. Combined series and shunt controllers either controlled in

coordinated manner as in fig. or a unified power flow controller with series and shunt

elements as in fig. for a unified controller there can be real power exchange between

the series and shunt controllers via dc power link.

Tasks of dynamic shunt compensation:

Steady state and dynamic voltage control

Reactive power control of dynamic loads

Damping of active power oscillations

Improvement of system stability

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Examples of shunt compensation:

STATIC VAR COMPENSATOR (SVC):

SHUNT-connected static var compensators (SVCs) are used extensively to control the

AC voltage in transmission networks. Power electronic equipment, such as the thyristor

controlled reactor (TCR) and the thyristor switched capacitor (TSC) have gained a

significant market, primarily because of well-proven robustness to supply dynamic

reactive power with fast response time and with low maintenance. With the advent of

high power gate turn-off thyristors and transistor devices (GTO, IGBT, …) a new

generation of power electronic equipment, STATCOM, shows great promise for

application in power systems .Installation of a large number of SVCs and experience

gained from recent STATCOM projects throughout the world motivates us to clarify

certain aspects of these devices.

Fig .2 .3 .1 S ta t i c var compensa tor

Fig.2.3.1 shows a schematic diagram of a static var compensator. The compensator

normally includes a thyristor controlled reactor (TCR), thyristor-switched capacitors

(TSCs) and harmonic filters. It might also include mechanically switched shunt

capacitors (MSCs), and then the term static var system is used. The harmonic filters

(for the TCR-produced harmonics) are capacitive at fundamental frequency. The TCR

is typically larger than the TSC blocks so that continuous control is realized. Other

possibilities are fixed capacitors (FCs), and thyristor switched reactors (TSRs). Usually

a dedicated transformer is used, with the compensator equipment at medium voltage.

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The transmission side voltage is controlled, and the Mvar ratings are referred to the

transmission side.

TYPICAL CONFIGURATIONS IN SVC:

The SVC typically consists of a TCR (Thyristor Controlled Reactor), a TSC

(Thyristor Switched Capacitor) and fixed capacitors (FC) in a harmonic filter

arrangement as shown in Figure 2.3.2. The TCR consists of reactors and thyristor

valves. The TCR continuously controls reactive power by varying the current

amplitude flowing through the reactors. The TSC consists of capacitors, reactors and

thyristor valves. The TSC switches on and off the capacitors. The AC filters provide

fixed reactive power and absorb the harmonic current generated by the TCR. The

TCR+FC is the most basic configuration of the SVC. The TCR+TSC+FC, the more

advanced configuration, can be tuned to minimize the losses at the most frequent

operation point.

Fig .2 .3 .2 . Typ ica l conf igura t ion o f SVC

TCR (Thyristor Controlled Reactor)

The amplitude of the TCR current can be changed continuously by varying the

thyristor firing angle (Figure 2.3.3). The firing angle can be varied from 90 degrees to

180 degrees. The TCR firing angle can be fully changed within one cycle of the

fundamental frequency, thus providing smooth and fast control of reactive power

supply to the system

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Fig .2 .3 .3 . TCR current and f i r ing ang l e

TSC (Thyristor Switched Capacitor):

The TSC is used to switch on and off the capacitor bank. The TSC does not generate

any harmonic current components. The capacitor switching operation is completed

within one cycle of the fundamental frequency. The TSC provides a faster and more

reliable solution to capacitor switching than conventional mechanical switching

devices. The TSC can operate in coordination with the TCR so that the sum of the

reactive power from the TSC and the TCR becomes linear. Applications with only

TSC's are also available, providing stepwise control of capacitive reactive power.

Fig .2 .3 .4 TCS current and f i r ing ang l e

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Unified power flow conditioner (UPFC):

The Unified Power Flow Controller (UPFC) is the first member of an emerging

family of advanced Flexible AC Transmission System (FACTS) controllers that use

multiple Synchronous voltage sources (SVS) operated conjunctively to optimize the

use of electric power transmission networks. Each such SVS is typically an electronic

voltage-sourced inverter that can be shunt-connected (STATCOM) or series-connected

(SSSC) to the power network. A STATCOM or an SSSC can operate on its own, using

the inherent ability to generate or absorb reactive power at its ac terminals. These

devices are, however, unable to negotiate real power from the network unless they are

equipped with an additional source or sink of real power at their dc terminals. This

leads to the concept of joining multiple STATCOMs and/or SSSCs together at their dc

terminals. The joined units are thus free to negotiate real power at their ac terminals,

subject only to the constraint that the total average power at the dc bus must be zero.

Fig.2.3.5. Unified power flow controller

Dynamic voltage restorer (DVR):

The DVR mitigates voltage sags by injecting a compensating voltage into the power

system in synchronous real time. The DVR is a high-speed switching power electronic

converter that consists of an energy storage system that feeds three independent single-

phase pulse width modulated (PWM) inverters. As shown in Fig.2.4.1, the energy

storage system for the DVR is a dc capacitor bank, which is interfaced to the PWM

inverters by using a boost converter (dc to dc). The boost converter regulates the

voltage across the dc link capacitor that serves as a common voltage source for the

PWM inverters. The three voltage source single-phase PWM inverters (dc to ac)

synthesize the appropriate voltage waveform as determined by the DVR’s digital

22

control system. This compensating voltage waveform is injected into the power system

through three single-phase series injection transformers. The DVR control system

compares the input voltage to an adaptive reference signal and injects voltage so that

the output voltage remains within specifications (e.g.,1.0 per unit).

Fig.2.3.6 dynamic voltage regulator (DVR)

Under normal operating conditions (no sag), the DVR injects only a small voltage to

compensate for the series reactance of the injection transformers and device losses.

During sag, the DVR control system calculates and synthesizes the voltage required to

maintain the output voltage and injects this voltage in synchronous real time.

2.4.1. STATCOM

In 1999 the first SVC with Voltage Source Converter called STATCOM (static

compensator) went into operation. The STATCOM has a characteristic similar to the

synchronous condenser, but as an electronic device it has no inertia and is superior to

the synchronous condenser in several ways, such as better dynamics, a lower

investment cost and lower operating and maintenance costs.

A STATCOM is build with Thyristors with turn-off capability like GTO or

today IGCT or with more and more IGBTs. The static line between the current

limitations has a certain steepness determining the control characteristic for the voltage.

The advantage of a STATCOM is that the reactive power provision is

independent from the actual voltage on the connection point. This can be seen in the

23

diagram for the maximum currents being independent of the voltage in comparison to

the SVC. This means, that even during most severe contingencies, the STATCOM

keeps its full capability.

In the distributed energy sector the usage of Voltage Source Converters for grid

interconnection is common practice today. The next step in STATCOM development is

the combination with energy storages on the DC-side. The performance for power

quality and balanced network operation can be improved much more with the

combination of active and reactive power.

2.4.1 STATCOM structure and voltage / current characteristic

STATCOMs are based on Voltage Sourced Converter (VSC) topology and

utilize either Gate-Turn-off Thyristors (GTO) or Isolated Gate Bipolar Transistors

(IGBT) devices. The STATCOM is a very fast acting, electronic equivalent of a

synchronous condenser.

If the STATCOM voltage, Vs, (which is proportional to the dc bus voltage Vc)

is larger than bus voltage, Es, then leading or capacitive VARS are produced. If Vs is

smaller then Es then lagging or inductive VARS are produced.

24

Fig 2.4.1.1 6 Pulses STATCOM

The three phases STATCOM makes use of the fact that on a three phase,

fundamental frequency, steady state basis, and the instantaneous power entering a

purely reactive device must be zero. The reactive power in each phase is supplied by

circulating the instantaneous real power between the phases. This is achieved by firing

the GTO/diode switches in a manner that maintains the phase difference between the ac

bus voltage ES and the STATCOM generated voltage VS. Ideally it is possible to

construct a device based on circulating instantaneous power which has no energy

storage device (ie no dc capacitor).

A practical STATCOM requires some amount of energy storage to

accommodate harmonic power and ac system unbalances, when the instantaneous real

power is non-zero. The maximum energy storage required for the STATCOM is much

less than for a TCR/TSC type of SVC compensator of comparable rating.

2.4.1.2. STATCOM Equivalent Circuit

Several different control techniques can be used for the firing control of the

STATCOM. Fundamental switching of the GTO/diode once per cycle can be used.

This approach will minimize switching losses, but will generally utilize more complex

transformer topologies. As an alternative, Pulse Width Modulated (PWM) techniques,

which turn on and off the GTO or IGBT switch more than once per cycle, can be used.

This approach allows for simpler transformer topologies at the expense of higher

switching losses.

The 6 Pulse STATCOM using fundamental switching will of course produce

the 6 N1 harmonics. There are a Variety of methods to decrease the harmonics. These

methods include the basic 12 pulse configuration with parallel star / delta transformer

connections, a complete elimination of 5th and 7th harmonic current using series

25

connection of star/star and star/delta transformers and a quasi 12 pulse method with a

single star-star transformer, and two secondary windings, using control of firing angle

to produce a 300 phase shift between the two 6 pulse bridges.

This method can be extended to produce a 24 pulse and a 48 pulse STATCOM,

thus eliminating harmonics even further. Another possible approach for harmonic

cancellation is a multi-level configuration which allows for more than one switching

element per level and therefore more than one switching in each bridge arm. The ac

voltage derived has a staircase effect, dependent on the number of levels. This staircase

voltage can be controlled to eliminate harmonics.

2.4.2. REAL AND REACTIVE POWER CONTROL:-

Basic operating principle of a SATCOM is similar to that of synchronous machine.

The synchronous machine will provide lagging current when under excited and leading

current when over excited.

STATCOM can generate and absorb reactive power similar to that of

synchronous machine and it can also exchange real power if provided with an external

device DC source.

1) Exchange of reactive power:- If the output voltage of the voltage source converter

is greater than the system voltage then the SATCOM will act as capacitor and generate

reactive power(i.e.. provide lagging current to the system)

2) Exchange of real power: - As the switching devices are not loss less there is a

need for the DC capacitor to provide the required real power to the switches. For long

duration of real power requirement even after the primary supply failed back up energy

storage system (BESS) is used. Hence there is a need for real power exchange with an

AC system to make the capacitor voltage constant in case of direct voltage control.

There is also a real power exchange with the AC system if STATCOM is provided with

an external DC source to regulate the voltage in case of very low voltage in the

distribution system or in case of faults.

And if the VSC output voltage leads the system voltage then the real power from the

capacitor or the DC source will be supplied to the AC system to regulate the system

voltage to the =1p.u or to make the capacitor voltage constant.

26

Hence the exchange of real power and reactive power of the voltage source

converter with AC system is the major required phenomenon for the regulation in the

transmission as well as in the distribution system.

2.4.3 BASIC OPERATING PRINCIPLES OF STATCOM:

The STATCOM is connected to the power system at a PCC (point of common

coupling), through a step-up coupling transformer, where the voltage-quality problem

is a concern. The PCC is also known as the terminal for which the terminal voltage is

UT. All required voltages and currents are measured and are fed into the controller to be

compared with the commands. The controller then performs feedback control and

outputs a set of switching signals (firing angle) to drive the main semiconductor

switches of the power converter accordingly to either increase the voltage or to

decrease it accordingly. A STATCOM is a controlled reactive-power source. It

provides voltage support by generating or absorbing reactive power at the point of

common coupling without the need of large external reactors or capacitor banks. Using

the controller, the VSC and the coupling transformer, the STATCOM operation is

illustrated in Figure below.

Fig 2.5 . STATCOM operation in a power system

The charged capacitor Cdc provides a DC voltage, Udc to the converter, which

produces a set of controllable three-phase output voltages, U in synchronism with the

27

AC system. The synchronism of the three-phase output voltage with the transmission

line voltage has to be performed by an external controller. The amount of desired

voltage across STATCOM, which is the voltage reference, Uref, is set manually to the

controller. The voltage control is thereby to match UT with Uref which has been

elaborated. This matching of voltages is done by Varying the amplitude of the output

voltage U, which is done by the firing angle set by the controller. The controller thus

sets UT equivalent to the Uref. The reactive power exchange between the converter and

the AC system can also be controlled. This reactive power exchange is the reactive

current injected by the STATCOM, which is the current from the capacitor produced

by absorbing real power from the AC system.

Where, Iq is the reactive current injected by the STATCOM

UT is the STATCOM terminal voltage

Ueq is the equivalent Thevinen’s voltage seen by the STATCOM

Xeq is the equivalent Thevinen’s reactance of the power system seen by the STATCOM

If the amplitude of the output voltage U is increased above that of the AC

system voltage, UT, a leading current is produced, i.e. the STATCOM is seen as a

conductor by the AC system and reactive power is generated. Decreasing the amplitude

of the output voltage below that of the AC system, a lagging current results and the

STATCOM is seen as an inductor. In this case reactive power is absorbed. If the

amplitudes are equal no power exchange takes place.

A practical converter is not lossless. In the case of the DC capacitor, the energy

stored in this capacitor would be consumed by the internal losses of the converter. By

making the output voltages of the converter lag the AC system voltages by a small

angle, δ, the converter absorbs a small amount of active power from the AC system to

balance the losses in the converter. The diagram in Figure below illustrates the phasor

diagrams of the voltage at the terminal, the converter output current and voltage in all

four quadrants of the PQ plane.

28

Fig 2.6 Phasor diagrams for STATCOM applications

The mechanism of phase angle adjustment, angle δ, can also be used to control the

reactive power generation or absorption by increasing or decreasing the capacitor

voltage Udc, with reference with the output voltage U.

Instead of a capacitor a battery can also be used as DC energy. In this case the

converter can control both reactive and active power exchange with the AC system.

The capability of controlling active as well as reactive power exchange is a significant

feature which can be used effectively in applications requiring power oscillation

damping, to level peak power demand, and to provide uninterrupted power for critical

load.

2.4.4 CHARACTERISTICS OF STATCOM:

The derivation of the formula for the transmitted active power employs

considerable calculations. Using the Variables defined in Figure below and applying

Kirchoffs laws the following equations can be written;

Fig 2.7 .Two machine system with STATCOM

29

By equaling right-hand terms of the above formulas, a formula for the current I1 is

obtained as

Where UR is the STATCOM terminal voltage if the STATCOM is out of operation, i.e.

when Iq = 0. The fact that Iq is shifted by 90◦ with regard to UR can be used to express Iq

as

Applying the sine law to the diagram in Figure below the following two equations

result

from which the formula for sin α is derived as

The formula for the transmitted active power can be given as

30

To dispose of the term UR the cosine law is applied to the diagram in Figure above

Therefore,

Fig 2.8 Transmitted power versus transmission angle characteristic of a

STATCOM

With these concepts of STATCOM, it is thus important to utilize these

principles in accommodating shunt compensation to any system. Since this thesis only

reflects on the voltage control and power increase, the requirements of the STATCOM

would be further elaborated.

2.4.5 FUNCTIONAL REQUIREMENTS OF STATCOM:

The main functional requirements of the STATCOM in this thesis are to

provide shunt compensation, operating in capacitive mode only, in terms of the

following;

• Voltage stability control in a power system, as to compensate the loss voltage along

transmission. This compensation of voltage has to be in synchronism with the AC

system regardless of disturbances or change of load.

• Transient stability during disturbances in a system or a change of load.

• Direct voltage support to maintain sufficient line voltage for facilitating increased

reactive power flow under heavy loads and for preventing voltage instability

• Reactive power injection by STATCOM into the system

The design phase and implementation phase (as presented in the next chapter) would

refer to the theoretical background of STATCOM in providing the requirements

CHAPTER – 3

31

DISTRIBUTION GENERATION

3.1.1 .DISTRIBUTED GENERATION

The centralized and regulated electric utilities have always been the major

source of electric power production and supply. However, the increase in demand for

electric power has led to the development of distributed generation (DG) which can

complement the central power by providing additional capacity to the users. These

are smal l generating units which can be located at the consumer end or anywhere

within the distribution system. DG can be beneficial to the consumers as well

as the utility. Consumers are interested in DG due to the various benefits associated

with it: cost saving during peak demand charges, higher power quality and increased

energy efficiency. The utilities can also benefit as it generally eliminates the

cost needed for laying new transmission/distribution lines.

Distributed generation employs alternate resources such as micro-turbines,

solar photovoltaic systems, fuel cells and wind energy systems. This thesis lays

emphasis on the fuel cell technology and its integration with the utility grid.

3.1.2 DISTRIBUTED GENERATION SYSTEMS BACKGROUND

Today, new advances in power generation technologies and new

environmental regulations encourage a significant increase of distributed

generation resources around the world. Distributed generation systems (DGS)

have mainly been used as a standby power source for critical businesses. For

example, most hospitals and office buildings had stand-by diesel generators as an

emergency power source for use only during outages. However, the diesel

generators were not inherently cost-effective, and produce noise and exhaust that would

be objectionable on anything except for an emergency basis. On the other hand,

environmental-friendly distributed generation systems such as fuel cells, micro

turbines, biomass, wind turbines, hydro turbines or photovoltaic arrays can be a

solution to meet both the increasing demand of electric power and

environmental regulations due to green house gas emission.

As illustrated in these figures, the currently competitive DGS units will be

constructed on a conventional distribution network, instead of large central power

32

plants because the DGS can offer improved service reliability, better economics

and a reduced dependence on the local utility

Figure 3.1 A large central power plant and distributed generation systems

Recently, the use of distributed generation systems under the 500 kW level is

rapidly increasing due to technology improvements in small generators,

power electronics, and energy storage devices. Efficient clean fossil-fuels technologies

such as micro-turbines, fuel cells, and environmental-friendly renewable energy

technologies such as biomass, solar/photovoltaic arrays, small wind turbines and

hydro turbines, are growingly used for new distributed generation systems. These

DGS are applied to a standalone , a grid-interconnected, a standby, peak shavings ,

a cogeneration etc. and have a lot of benefits such as environmental-friendly and

modular electric generation, increased reliability/stability, high power quality, load

management, fuel flexibility, uninterruptible service, cost savings, on-site generation,

expandability, etc.

3.2 Benefits

33

In the last decade, the concept of many small scale energy sources dispersed

over the grid gain a considerable interest. Most of all, technological innovations and a

changing economic and regulatory environment were that main triggers for this

interest. International Energy Agency IEA lists five major factors that contribute to

this evolution, such as developments in distributed generation technologies, constraints

on the construction of new transmission lines, increased customer demand for highly

reliable electricity, the electricity market liberalization and concerns about climate

change. Especially the last two points seem to offer the most significant benefits, as it is

unlikely that distributed generation would be capable of avoiding the development of

new transmission lines. At minimum, the grid has to be available as backup supply. In

the liberalized market environment, the distributed generation offers a number of

benefits to the market participants. As a rule, customers look for the electricity services

best suited for them. Different customers attach different weights to features of

electrical energy supply, and distributed generation technologies can help electricity

suppliers to supply the type of electricity service they prefer. One of the most

interesting features is the flexibility of DG that could allow market participants to

respond to changing market conditions, i.e. due to their small sizes and the short

construction lead times compared to most types of larger central power plants.

3.3. Flexibility in price response

Important aspects of the abovementioned flexibility of distributed generation

technologies are operation, size and expandability. Flexible reaction to electrical energy

price evolutions can be one of the examples, allowing a DG to serve as a hedge against

these price fluctuations. Apparently, using distributed generation for continuous use or

for peak shaving is the major driver for the US demand for distributed generation. In

Europe, market demand for distributed generation is driven by heat applications, the

introduction of renewable and by potential efficiency improvements.

1) Large generation units connected on the customer’s side are however difficult to

classify in view of this definition.

2) The value of their flexibility is probably understated when economic assessments of

distributed generation are made. Recent work based on option value theory suggests

that flexible power plants.

34

3.4. Flexibility in reliability needs

Reliability considerations of the second major driver of US demand for distributed

generation is quality of supply or reliability considerations. Reliability problems refer

to sustained interruptions in electrical energy supply (outages). The liberalization of

energy markets makes customers more aware of the value of reliable electricity supply.

In many European countries, the reliability level has been very high, mainly because of

high engineering standards. High reliability level implies high investment and

maintenance costs for the network and generation infrastructure. Due to the incentives

for cost effectiveness that come from the introduction of competition in generation and

from the re-regulation of the network companies, it might be that reliability levels will

decrease. However, for some industries, such as chemical, petroleum, refining, paper,

metal, telecommunication, a reliable power supply is very important. Such companies

may find the reliability of the grid supplied electricity too low and thus be willing to

invest in distributed generation units in order to increase their overall reliability of

supply. The IEA recognizes the provision of reliable power as the most important

future market niche for distributed generation. Fuel cells and backup systems combined

with an UPS (Uninterruptible Power Supply) are identified as the technologies that

could provide protection against power interruptions, though it has to be noted that the

fuel cell technology is currently not easily commercially available.

3.5. Flexibility in power quality needs

Apart from large voltage drops to near zero (reliability problems), one can also have

smaller voltage deviations. The latter deviations are aspects of power quality. Power

quality refers to the degree to which power characteristics align with the ideal

sinusoidal voltage and current waveform, with current and voltage in balance. Thus,

strictly speaking, power quality encompasses reliability. Insufficient power quality can

be caused by failures and switching operations in the network (voltage dips and

transients) and by network disturbances from loads (flickers, harmonics and phase

imbalance).

35

3.6. Environmental friendliness

Environmental policies or concerns are probably the major driving force for the

demand for distributed generation in Europe. Environmental regulations force players

in the electricity market to look for cleaner energy- and cost-efficient solutions. Many

of the distributed generation technologies are recognized environmentally friendly.

Combined Heat and Power (CHP) technology, allowing for portfolio optimization of

companies needing both heat and electrical energy, is one of the examples. Compared

to separate fossil-fired generation of heat and electricity, CHP generation may result in

a primary energy conservation, varying from 10% to 30%, depending on the size (and

efficiency) of the cogeneration units.

Furthermore, as renewable energy sources are by nature small-scale and dispersed

over the grid. Installing distributed generation allows the exploitation of cheap fuel

opportunities. For example, DG units could burn landfill gasses in the proximity of

landfills, or other locally available biomass resources. Most government policies that

aim to promote the use of renewable will also result in an increased impact of

distributed generation.

3.7. Impacts on power quality

The installation and connection of distributed generation units can positively affect

the power quality. However, a converse effect could also be noted. DG units are likely

to affect the system frequency. As they are often not equipped with a load-frequency

control, they will free ride on the efforts of the transmission grid operator or the

regulatory body to maintain system frequency. Therefore, connecting a large number of

DG units to the grid should be carefully evaluated and planned.

Moreover, the impact on the local voltage level of distributed generation connected

to the distribution grid can be significant. Especially raising voltage levels in radial for

Some times the power injections need to be corrected for the transmission losses,

meaning that an ARP should inject some 3¸4% more than it withdraws.

3.8. Connection issues

36

It can be taken as given that the electric power flows from the higher voltage grid to

the lower voltage grid. Increased share of distributed generation units may lead to

inducing power flows from the low voltage into the medium-voltage grid. This bi-

directional power flows asks for different protection schemes at both voltage levels.

Moreover, the added flexibility of DG asks for extra efforts on the grid operation side.

As some customers might want to switch to the “island” mode during an outage, they

should also meet the requirements for such operation mode. Next to guarantying no

power supplied to the grid, they must be able to provide the auxiliary services needed.

Moreover, once the distribution grid is back into operation, the DG unit must be able to

be re-synchronized.

CHAPTER – 4

37

WIND POWER GENERATION

4.1. INTRODUCTION:

Wind power is the conversion of wind energy into useful form, such as electricity,

using wind turbines. In windmills, wind energy is directly used to crush grain or to

pump water. Wind energy is plentiful, renewable, widely distributed, clean, and

reduces greenhouse gas emissions when it displaces fossil-fuel-derived electricity. The

intermittency of wind seldom creates insurmountable problems when using wind power

to supply a low proportion of total demand, but it presents extra costs when wind is to

be used for a large fraction of demand.

4.1.1Wind Turbine Types:

Modern wind turbines fall into two basic groups; the horizontal-axis variety, like the

traditional farm windmills used for pumping water, and the vertical-axis design, like

the eggbeater-style Darrieus model, named after its French inventor. Most large modern

wind turbines are horizontal-axis turbines.

Turbine Components

Horizontal turbine components include:

1) Blade or rotor, which converts the energy in the wind to rotational shaft

energy;

2) A drive train, usually including a gearbox and a generator;

3) A tower that supports the rotor and drive train;

4) And other equipment, including controls, electrical cables, ground support

equipment, and interconnection equipment. Wind turbines are often grouped together

into a single wind power plant, also known as a wind farm, and generate bulk electrical

power. Electricity from these turbines is fed into a utility grid and distributed to

customers, just as with conventional power plants.

38

FIGURE.4.1 Wind turbine

4.1.2. Wind Turbine Systems:

Wind turbines can operate with either fixed speed (actually within a speed range

about 1 %) or variable speed. For fixed-speed wind turbines, the generator (induction

generator) is directly connected to the grid. Since the speed is almost fixed to the grid

frequency, and most certainly not controllable, it is not possible to store the turbulence

of the wind in form of rotational energy. Therefore, for a fixed-speed system the

turbulence of the wind will result in power variations, and thus affect the power quality

of the grid. For a variable-speed wind turbine the generator is controlled by power

electronic equipment, which makes it possible to control the rotor speed. In this way

the power fluctuations caused by wind variations can be more or less absorbed by

changing the rotor speed and thus power variations originating from the wind

conversion and the drive train can be reduced. Hence, the power quality impact caused

by the wind turbine can be improved compared to a fixed-speed turbine.

The rotational speed of a wind turbine is fairly low and must therefore be

adjusted to the electrical frequency. This can be done in two ways: with a gearbox or

with the number of pole pairs of the generator. The number of pole pairs sets the

mechanical speed of the generator with respect to the electrical frequency and the

gearbox adjusts the rotor speed of the turbine to the mechanical speed of the generator.

39

FIGURE 1.14

FIGURE.4.2

In this section the following wind turbine systems will be presented:

1. Fixed-speed wind turbine with an induction generator.

2. Variable-speed wind turbine equipped with a cage-bar induction generator or

synchronous generator.

3. Variable-speed wind turbine equipped with multiple-pole synchronous generator or

multiple-pole permanent-magnet synchronous generator.

4. Variable-speed wind turbine equipped with a doubly-fed induction generator.

4.2.0. Fixed-Speed Wind Turbine

For the fixed-speed wind turbine the induction generator is directly connected to the

electrical grid according to Fig.4.3 The rotor speed of the fixed-speed wind turbine is in

principle

40

FIGURE 4.3

determined by a gearbox and the pole-pair number of the generator. The fixed-speed

wind turbine system has often two fixed speeds. This is accomplished by using two

generators with different ratings and pole pairs, or it can be a generator with two

windings having different ratings and pole pairs. This leads to increased aerodynamic

capture as well as reduced magnetizing losses at low wind speeds. This system (one or

two-speed) was the “conventional” concept used by many Danish manufacturers in the

1980s and 1990s.

4.2.1. Variable-Speed Wind Turbine

The system presented in Fig. 4.4. consists of a wind turbine equipped with a converter

connected to the stator of the generator. The generator could either be a cage-bar

induction generator or a synchronous generator. The gearbox is designed so that

maximum rotor speed corresponds to rated speed of the generator. Synchronous

generators or permanent-magnet synchronous generators can be designed with multiple

poles which imply that there is no need for a gearbox, see Fig. 4.4. Since this “full-

power” converter/generator system is commonly used for other applications, one

advantage with this system is its well-developed and robust control.

4.2.2 Variable-Speed Wind Turbine with Doubly-Fed Induction Generator

The system, see Fig. 4.5, consists of a wind turbine with doubly-fed induction

generator. This means that the stator is directly connected to the grid while the rotor

winding is connected via slip rings to a converter. This system has recently become

very popular as generators for variable-speed wind turbines. This is mainly due to the

fact that the power electronic converter only has to handle a fraction (20–30%) of the

41

FIGURE 4.4

total power .Therefore, the losses in the power electronic converter can be reduced,

compared to a system where the converter has to handle the total power. In addition,

the cost of the converter becomes lower. There exists a variant of the DFIG method that

uses controllable external rotor resistances

(compare to slip power recovery). Some of the drawbacks of this method are that

energy is unnecessary dissipated in the external rotor resistances and that it is not

possible to control the reactive power.

Of all the above mentioned types we are going to use the first type because of its

simplicity that is fixed speed wind turbine method with induction generator. Due to the

advantages that we considered in real time application and also in simulink designing

we adopted the induction generators.

4.3. INDUCTON MACHINE:

Induction machines are often described as the ‘workhorse of industry’. This reflects

the reality of the qualities of these machines. They are cheap to manufacture, rugged

and reliable and find their way in most possible applications. Variable speed drives

require inexpensive power electronics and computer hardware, and allowed induction

machines to become more versatile. In particular, vector or field oriented control allows

induction motors to replace DC motors in many applications

4.3.0. DESCRIPTION

42

FIGURE 4.5

The stator of an induction machine is a typical three phase one. The rotor can be one of

two major types. Either of the following :-

a) It is wound in a fashion similar to that of the stator with the terminals led to slip rings

on the shaft, as shown in figure , or

b) It is made with shorted Fig Wound rotor

slip rings and connections bars..The picture

of the rotor bars is not easy to obtain, since

the bars are formed by casting aluminium in

the openings of the rotor laminations. In this

case the iron laminations were chemically

removed.

4.3.1. CONCEPT OF OPERATION

As these rotor windings or bars rotate within the magnetic field created by the stator

magnetizing currents, voltages are induced in them. If the rotor were to stand still, then

the induced voltages would be very similar to those induced in the stator windings. In

the case of squirrel cage rotor, the voltage induced in the bars will be slightly out of

phase with the voltage in the next one, since the flux linkages will change in it after a

short delay.

If the rotor is moving at synchronous speed, together with the field, no voltage will be

induced in the bars or the windings.

Generally when the synchronous speed is ωs = 2πfs, and the rotor speed ω0, the

frequency of the induced voltages will be fr, where 2πfr = ωs - ω0. Maxwell’s equation

becomes here:

έ = v x Bg---------------------------------------- (1)

where v is the relative velocity of the rotor with respect to the field:

43

FIGURE 4.6

FIGURE 4.7

v = ωs - ω0----------------------------------------------------------- (2)

Since a voltage is induced in the bars, and these are short circuited, currents will flow

in them. The current density J (θ) will be:

J (θ) = (1/ρ) έ

J (θ) = (1/ρ). ωs - ω0 Bg(θ)

J (θ)= (1/ρ). (ωs - ω0) Bg sin (θ)

We define as slip s the ratio: s= (ωs - ω0)/ ωs

At starting the speed is zero, hence s = 1, and at synchronous speed, ωs = ω0, hence s =

0. Above

Synchronous speed s < 0, and when the rotor rotates in a direction opposite of the

magnetic field 1< s

Three-phase motors: Operation principles

• The interaction between the rotor current and the stator field produces a force that

drives the motor: Force = B I L sin Φ. The induced voltage magnitude is dependent

upon the speed difference between the rotating stator field and the rotor.

• The speed difference is maximum during starting when the motor draws large current.

The frequency of the rotor current is 50 Hz when the rotor is stationary.

• As the motor starts to rotate the speed difference are reduced, which results in

– reduction on the frequency of the induced voltage in the rotor.

– reduced magnitude of rotor current and induced voltage.

Force generation:

• Rotating field induces current in the bar .

• The current and field interaction generates the driving force.

• Force = BIL

The force drives the motor

44

FIGURE 4.8

FIGURE 4.9

• L is the length of the rotor

• If the rotor speed is equal to the angular speed of the stator field, the induced voltage,

current and torque become zero. Therefore the motor speed must be less than the

synchronous speed.

• Motor operation requires speed difference between the stator generated rotating field

and the actual rotor speed. The speed difference is called slip (s) and defined as:

s = (ns - nr) / ns where ns =120 f / p

• The frequency of the rotor current is: fr = s f

• The slip in normal operation is between 1 and 5 %

4.3.2. Development of equivalent circuit:

• The induction motor/generator consists of a two magnetically connected systems,

Stator and rotor.

• This is similar to a transformer that also has two magnetically connected systems:

primary and secondary windings.

• The stator is supplied by a balanced three-phase voltage that drives a three-phase

current through the winding. This current induces a voltage in the rotor.

• The applied voltage (V1) across phase A is equal to the sum of the

– induced voltage (E1).

– Voltage drop across the stator resistance (I1 R1).

– Voltage drop across the stator leakage reactance (I1 j X1).

• The stator voltage equation is:

V1 = E1+ I1 ( R1+ j X1)

• The E1 induced voltage generates a voltage E2 in the rotor through the magnetic

coupling.

– If the rotor is at stand still, the induced voltage E2 is proportional to E1 times the turn

ratio. T = Nstat / Nrot = N1 /N2. The value is:

E2 = E1 (N2 /N1 ) = E1 / T

If the rotor is rotating, the voltage induced in the rotor is multiplied by the slip s,

because the induced voltage is proportional to the speed difference between the stator

field and rotor.

E2 = s E1 / T

• The rotor induced voltage is equal to the sum of the voltage drop across the rotor

resistance (I2 R2), and the leakage inductance (I2 X2).

45

• The voltage drop across the secondary leakage inductance L2 is:

I2 j wr L2 = I2 j (2 π fr) L2 = I2 j (2 π f ) s L2 = I2 j s (w L2) = I2 j s X2

• The rotor voltage equation is:

E2 = I2 (R2 + j s X2 )

• The equations derived for the induction motors are:

V1 = E1+ I1 ( R1+ j X1) E2 = s E1 / T

E2 = I2 (R2 + j s X2 ) I2 = I1 (N1/ N2) = I1 T

• Combining the equations we have:

E1 = E2 T / s = T I2 (R2 + j s X2 ) /s = I1 T2 (R2 /s + j X2 )

= I1 [(R2 T2 /s) + j (T2 X2 )] = I1 (R*2 /s) + j X*2 )

where: R*2 = R2 T2 and X*2 = T2 X2 are rotor resistance & reactance referred to stator.

• The derivation results in the following equations:

V1 = E1+ I1 ( R1+ j X1) E1 = I1 (R2* / s + j X2* )

• We substitute the second equation into the first one to obtain the following equation

for the induction motor:

V1 = I1 (R2* / s + j X2* ) + I1 ( R1+ j X1) = I1 [( R1 + R2* / s) + j ( X1+ X2*)]

• The final equation is:

V1 = I1 [( R1 + R2* / s) + j ( X1+ X2*)]

• The induction motor equation is:

V1 = I1 [(R1 + R2* / s) + j ( X1+ X2*)]

• This equations suggests that the induction motor equivalent circuit contains two

resistances and reactance’s connected in series.

• The magnetizing current can be represented by a resistance Rc and a reactance Xm

connected in parallel.

– The resistance represents the hysteresis and eddy current losses.

– The reactance represents the magnetizing current that generates the air-gap

magnetizing flux.

The induction motor/generator equivalent circuit is:

46

4.4. INDUCTION GENERATOR OPERATION

Figure 5 shows the speed torque characteristics of an induction motor operating from a

constant frequency power source. Most readers are familiar with this characteristic of

the Induction motor operation. The operation of the induction motor occurs in a stable

manner in the region of the speed torque curve indicated in Figure 5. The torque output

as well as the power delivered by the motor varies as the motor speed changes. At

synchronous speed no power is delivered at all. The difference between the

synchronous speed and the operating speed is called the slip. The output torque and

power vary linearly with the slip. If the induction motor is driven to a speed higher than

the synchronous speed, the speed torque curve reverses as shown in Figure 6. In the

stable region of this curve, electric power is generated utilizing the mechanical input

power from the prime mover. Once again the generated power is a function of the slip,

and varies with the slip itself

47

FIGURE 4.10

Figure 4.11: Induction Motor Torque v/s Speed

Figure 4.12: Induction Generator Torque v/s Speed

48

In the generator mode, if the slip is controlled in accordance with the load

requirements, the induction generator will deliver the necessary power. It must be

remembered that the synchronous speed is a function of the electrical frequency applied

to the generator terminals. On the other hand, the operating shaft speed is determined

by the prime mover. Therefore to generate power, the electrical frequency must be

adjusted as the changes in the load and the prime mover speed occur. In addition to the

requirement stated above, the excitation current must be provided to the generator

stator windings for induction into the rotor. The magnitude of the excitation current

will determine the voltage at the bus. Thus the excitation current must be regulated at

specific levels to obtain a constant bus voltage. The controller for the induction

generator has the dual function as follows:

i) Adjust the electrical frequency to produce the slip corresponding to the load

requirement.

ii) Adjust the magnitude of the excitation current to provide the desirable bus voltage.

Figure 7 depicts the region of generator mode operation for a typical induction

generator. A number of torque speed characteristic curves in the stable region of

operation are shown to explain the operation. As an example, consider the situation

when the prime mover is at the nominal or 100% speed. The electrical frequency must

be adjusted to cater for load changes from 0 to 100% of the load. If a vertical line is

drawn along the speed of 100%, it can be observed that the electrical frequency must be

changed from 100% at no load to about 95% at full load if the prime mover speed is

held at 100%.

Figure 4.13: Induction Generator Torque v/s Speed in Operating Range

49

4.5. BENEFITS OF INDUCTION GENERATOR TECHNOLOGY

Induction generator has several benefits to offer for the micro, mini power systems

under consideration. These benefits relate to the generator design as follows:

i) Cost of Materials: Use of electromagnets rather than permanent magnets means lower

cost of materials for the induction generator. Rare earth permanent magnets are

substantially more expensive than the electrical steel used in electromagnets. They also

must be contained using additional supporting rings.

ii) Cost of Labour: PM’s require special machining operations and must be retained on

the rotor structure by installation of the containment structure. Handling of permanent

magnets that are pre-charged is generally difficult in production shops. These

requirements increase the cost of labour for the PM generator.

iii) Generator Power Quality: The PM generator produces raw ac power with

unregulated voltage. Depending upon the changes in load and speed, the voltage

variation can be wide. This is all the more true for generators exceeding about 75 kW

power rating. The induction generator produces ac voltage that is reasonably sinusoidal

as shown in the example from an actual test in Figure 9. This voltage can be rectified

easily to produce a constant dc voltage. Additionally, the ac voltage can be stepped up

or down using a transformer to provide multiple levels of voltages if required.

Figure 4.14: Induction Generator AC Output Voltage Waveform

iv) Fault Conditions: When an internal failure occurs in a PM generator, the failed

winding will continue to draw energy until the generator is stopped. For high-speed

generators, this may mean a long enough duration during which further damage to

electrical and mechanical components would occur. It could also mean a safety hazard

for the individuals working in the vicinity. The induction generator on the other hand is

50

safely shut down by de-excitation within a few milliseconds, preventing the hazardous

situations.

4.6. MATHEMATICS OF WIND POWER

The amount of mechanical power captured from wind by a wind turbine can be

formulated as:

Pm=(1/2)ACpv3 -------------------(1)

= Air density (Kg/m3)

A = Swept area (m2)

CP = Power coefficient of the wind turbine

V = Wind speed (m/s)

Therefore, if the air density, swept area and wind speed are constant the output power

of the turbine will be a function of power coefficient of the turbine. In addition, the

wind turbine is normally characterized by its CP-λ curve; where the tip speed ratio, λ, is

given by:

=(R)/ -----------------(2)

In (2), , R and v are the turbine rotor speed in “rad/s”, radius of the turbine blade in

“m”, and wind speed in “m/s” respectively. Figure above shows a typical “CP- λ” curve

for a wind turbine. It shows that CP has its maximum value at λopt, which results in

optimum efficiency; therefore, maximum power is captured from wind by the turbine.

The output power of a wind turbine versus rotor speed while speed of wind is changed

from v1 to v3 (v3>v2>v1). They show that if the speed of wind is v1, then the maximum

power could be captured when the rotor speed is 1; in other words, the operating point

of the system is point A, which corresponds to the maximum output power. If wind

speed changes from v1 to v2 while the rotor speed is fixed at 1, the operating point of

system is point B, which does not correspond to maximum power tracking. The rotor

speed should be increased from, 1 to 2, which results in the maximum power at

operating point C.

Fig 4.15: Power Coefficient vs. Tip-Speed Ratio.

51

Fig 4.16: power with different TSR

Based on (2), the optimum speed of rotor can be estimated as follows:

ωopt=v❑opt

R

v=R ωopt

❑opt --------(3)

Unfortunately, measuring the wind speed in the rotor of turbine is very difficult; thus,

to avoid using wind speed, (1) needs to be revised. By substituting the wind speed

equivalent from (3) into (1), the output power of the turbine is given as:

Pm=12

ρACp( R ωopt

❑opt)

3

Finally, the target torque can be written as:

T target=kopt ωopt2 Where, k opt=

12

ρAC pmax( Ropt )

3

CHAPTER – 5

BACK UP ENERGY STORAGE SYSTEM AND NON-

LINEAR LOAD

5.1 Energy Storage:

Electricity is more versatile in use than other types of power, because it is a

highly ordered form of energy that can be converted efficiently into other forms. For

example, it can be converted into mechanical form with efficiency near 100% or into

heat with 100% efficiency. Heat energy, on the other hand, cannot be converted into

electricity with such high efficiency, because it is a disordered form of energy in atoms.

For this reason, the overall thermal-to-electrical conversion efficiency of a typical fossil

thermal power plant is less than 50%.

Disadvantage of electricity is that it cannot be easily stored on a large scale.

Almost all electric energy used today is consumed as it is generated. This poses no

52

hardship in conventional power plants, in which fuel consumption is continuously

varied with the load requirement. Wind and photovoltaic’s (PVs), both being

intermittent sources of power, cannot meet the load demand at all times, 24 h a day,

365 d a year.

The present and future energy storage technologies that may be considered for stand-

alone wind or PV power systems fall into the following broad categories:

• Electrochemical battery

• Flywheel

• Compressed air

• Superconducting coil

5.2 BATTERY:

The battery stores energy in an electrochemical form and is the most widely

used device for energy storage in a variety of applications. There are two basic types of

electrochemical batteries:

The primary battery, which converts chemical energy into electric energy. The

electrochemical reaction in a primary battery is non-reversible, and the battery is

discarded after a full discharge. For this reason, it finds applications where a high

energy density for one-time use is required.

The secondary battery, which is also known as the rechargeable battery. The

electrochemical reaction in the secondary battery is reversible. After a discharge, it can

be recharged by injecting a direct current from an external source. This type of battery

converts chemical energy into electric energy The internal construction of a typical

electrochemical cell is shown in Figure. It has positive and negative electrode plates

with insulating separators and a chemical electrolyte in between. The two groups of

electrode plates are connected to two external terminals mounted on the casing. The

cell stores electrochemical energy at a low electrical potential, typically a few volts.

The cell capacity, denoted by C, is measured in ampere-hours (Ah), meaning it can

deliver C A for one hour or C/n A for n hours.

The battery is made of numerous electrochemical cells connected in a series–

parallel combination to obtain the desired battery voltage and current. The higher the

battery voltage, the higher the number of cells required in series. The battery rating is

stated in terms of the average voltage during discharge and the ampere-hour capacity it

53

can deliver before the voltage drops below the specified limit. The product of the

voltage and ampere-hour forms the watt-hour (Wh) energy rating the battery can

deliver to a load from the fully charged condition. The battery charge and discharge

rates are stated in units of its capacity in Ah. For example, charging a 100-Ah battery at

C/10 rate means charging at 100/10 = 10 A. Discharging that battery at C/2 rate means

drawing 100/2 = 50 A, at which rate the battery will be fully discharged in 2 h. The

state of charge (SOC) of the battery at any time is defined as the following:

SOC¿ Ah Capacity remaining∈the batteryRapid Ahcapacity

5.3 TYPES OF BATTERY:

There are at least six major rechargeable electro-chemistries available today.

They are as follows:

• Lead-acid (Pb-acid)

• Nickel-cadmium (NiCd)

• Nickel-metal hydride (NiMH)

• Lithium-ion (Li-ion)

• Lithium-polymer (Li-poly)

• Zinc-air

5.3.1 LEAD-ACID

This is the most common type of rechargeable battery used today because of its

maturity and high performance-over-cost ratio, even though it has the least energy

density by weight and volume. In a Pb-acid battery under discharge, water and lead

sulphate are formed, the water dilutes the sulphuric acid electrolyte, and the specific

gravity of the electrolyte decreases with the decreasing SOC. Recharging reverses the

reaction, in which the lead and lead dioxide are formed at the negative and positive

plates, respectively, restoring the battery into its originally charged state. The Pb-acid

battery comes in various versions. The shallow-cycle version is used in automobiles, in

which a short burst of energy is drawn from the battery to start the engine. The deep-

cycle version, on the other hand, is suitable for repeated full charge and discharge

cycles. Most energy storage applications require deep cycle batteries. The Pb-acid

battery is also available in a sealed “gel-cell” version with additives, which turns the

54

electrolyte into non-spillable gel. The gel-cell battery, therefore, can be mounted

sideways or upside down. The high cost, however, limits its use in military avionics.

5.3.2 NICKEL-CADMIUM

The NiCd is a matured electrochemistry, in which the positive electrode is made

of cadmium and the negative electrode of nickel hydroxide. The two electrodes are

separated by Nylon TM separators and placed in potassium hydroxide electrolyte in a

stainless steel casing. With a sealed cell and half the weight of the conventional Pb-

acid, the NiCd battery has been used to power most rechargeable consumer

applications. It has a longer deep-cycle life and is more temperature tolerant than the

Pb-acid battery. However, this electrochemistry has a memory effect (explained later),

which degrades the capacity if not used for a long time. Moreover, cadmium has

recently come under environmental regulatory scrutiny. For these reasons, NiCd is

being replaced by NiMH and Li-ion batteries in laptop computers and other similar

high-priced consumer electronics.

5.3.3 NICKEL-METAL HYDRIDE

NiMH is an extension of the NiCd technology and offers an improvement in

energy density over that in NiCd. The major construction difference is that the anode is

made of a metal hydride. This eliminates the environmental concerns of cadmium.

Another performance improvement is that it has a negligible memory effect. NiMH,

however, is less capable of delivering high peak power, has a high self-discharge rate,

and is susceptible to damage due to overcharging. Compared to NiCd, NiMH is

expensive at present, although the price is expected to drop significantly in the future.

This expectation is based on current development programs targeted for large-scale

application of this technology in electric vehicles.

5.4 EQUIVALENT ELECTRICAL CIRCUIT:

For steady-state electrical performance calculations, the battery is represented

by an equivalent electrical circuit shown in the figure. In its simplest form, the battery

works as a constant voltage source with a small internal resistance. The open-circuit (or

electrochemical) voltage Ei of the battery decreases linearly with the Ah discharged ,

and the internal resistance Ri increases . That is, the battery open-circuit voltage is

55

lower, and the internal resistance is higher in a partially discharged state as compared to

the E0 and R0 values in a fully charged state.

Fig 5.1 Equivalent electrical circuit of battery showing internal voltage and

resistance.

5.5.0 NON LINEAR LOADS:

Applies to those ac loads where the current is not proportional to the voltage.

Foremost among loads meeting their definition is gas discharge lighting having

saturated ballast coils and thyritor (SCR) controlled loads. The nature of non-linear

loads is to generate harmonics in the current waveform. This distortion of the current

waveform leads to distortion of the voltage waveform. Under these conditions, the

voltage waveform is no longer proportional to the current.

Non Linear Loads are: COMPUTER, LASER PRINTERS, SMPS,

RECTIFIER, PLC, ELECTRONIC BALLAST, REFRIGERATOR, TV ETC.

5.5.1 LINEAR LOAD:

AC electrical loads where the voltage and current waveforms are sinusoidal. The

current at any time is proportional to voltage. Linear Loads are: POWER FACTOR

IMPROVEMENT CAPACITORS, INDESCENT LAMPS, HEATERS ETC.

5.5.2 DIFFERENCE BETWEEN LINEAR AND NON LINEAR LOADS

56

Fig: 5.2 Difference between linear and non linear loads

If the system is integrated with non linear loads then the system shall be earthed

through conducting material suitable to carry the fault current. The minimum cross

section of the earth conductor shall be calculated based on maximum current, which

can flow at the time of short circuit/earth fault. It’s an effort to linearise the waveforms,

though in some cases the wave forms will be distorted in nature.

5.5.3 DIFFERENCE IN CURRENT WAVEFORMS

57

Fig : 5.3 Current waveforms of linear and non linear loads

CHAPTER – 6

58

IMPLEMENTATION IN MATLAB (SIMULINK)

6.1 Introduction to MATLAB

The name MATLAB stands for Matrix Laboratory. MATLAB is a software package

for high performance numerical computation and visualization. It provides an

interactive environment with hundreds of built-in functions for technical computation,

graphics and animations.

The combination of analysis capabilities, flexibility, reliability and powerful

graphics makes MATLAB the premier software package for electrical engineers. best

of all, MATLAB provides easy extensibility with its own high level programming

language.

MATLAB provides an interactive environment with hundreds of reliable and

accurate built-in mathematical functions, these built-in functions provide excellent

tools for linear algebra computations, data analysis, signal processing, optimization,

numerical solution of ODEs, quadrature and man other types of scientific

computations. They provide solutions to a broad range of mathematical problems

including matrix algebra and complex arithmetic. There are also numerous an external

interface to run programs written in FORTAN or C language from MATLAB.

TYPICAL USES OF MATLAB

1. Math and computation.

2. Algorithm development.

3. Modelling, simulation and prototyping.

4. Data analysis, exploration and visualization.

5. Scientific and engineering graphics.

6. Application development including graphical user interface.

Since the basic data element in MATLAB is an array which does not require

dimensioning, this allows us to solve many technical computing problems in a fraction

of time it would take to write a program in a scalar non-interactive language such as C

or Fortran.

THE MATLAB system has five main parts:

1. MATLAB language.

2. MATLAB Working Environment.

3. Handle Graphics.

59

4. MATLAB Mathematical Function Library.

5. MATLAB Application Program Interface (API).

6.2 INTRODUCTION TO SIMPOWER SYSTEMS:

SimPowerSystems and other products of the Physical Modeling product family

work together with Simulink to model electrical, mechanical, and control systems.

SimPowerSystems operates in the Simulink environment. Power systems are

combinations of electrical circuits and electromechanical devices like motors and

generators. Engineers working in this discipline are constantly improving the

performance of the systems. Requirements for drastically increased efficiency have

forced power system designers to use power electronic devices and sophisticated

control system concepts that tax traditional analysis tools and techniques. Further

complicating the analyst's role is the fact that the system is often so nonlinear that the

only way to understand it is through simulation. Land-based power generation from

hydroelectric, steam, or other devices is not the only use of power systems. A common

attribute of these systems is their use of power electronics and control systems to

achieve their performance objectives.

SimPowerSystems is a modern design tool that allows scientists and engineers to

rapidly and easily build models that simulate power systems. SimPowerSystems uses

the Simulink environment, allowing you to build a model using simple click and drag

procedures. Not only can you draw the circuit topology rapidly, but your analysis of the

circuit can include its

FIGURE 6.0

60

interactions with mechanical, thermal, control, and other disciplines. This is possible

because all the electrical parts of the simulation interact with the extensive Simulink

modeling library. SimPowerSystems and SimMechanics share a special Physical

Modeling block and connection line interface.

6.3 SIMULINK BLOCKS USED AND THEIR FUNCTIONS

Synchronous Machine:

Model the dynamics of three-phase round-rotor or salient-pole

synchronous machine. The synchronous Machine block operates in

generator or motor modes. The operating mode is dictated by the

sign of the mechanical power (positive for generator mode, negative

for motor mode). The electrical part of the machine is represented by a sixth-order

state-space model and the mechanical part is the same as in the Simplified Synchronous

Machine block.

Three-Phase Transformer (Two Windings): Implements three-phase transformer

with configurable winding connections. he Three-Phase Transformer (Two

Windings) block implements a three-phase transformer using three single-

phase transformers. You can simulate the saturable core or not simply by

setting the appropriate check box in the parameter menu of the block

Asynchronous Machine: Model the dynamics of three-phase asynchronous machine,

also known

as induction machine. The Asynchronous Machine block operates

in either generator or motor mode. The mode of operation is

dictated by the sign of the mechanical torque: If Tm is positive,

the machine acts as a motor. If Tm is negative, the machine acts as

a generator.

Universal Bridge: Implements universal power converter with selectable topologies

and power electronic devices. The Universal Bridge block implements a

universal three-phase power converter that consists of up to six power

61

switches connected in a bridge configuration. The type of power switch and converter

configuration is selectable from the dialog box.

Wind Turbine: Implements model of variable pitch wind turbine. The model is based

on the steady-state power characteristics of the turbine. The stiffness of

the drive train is infinite and the friction factor and the inertia of the

turbine must be combined with those of the generator coupled to the

turbine.

6.4 MODELLING THE SUB SYSTEMS:

6.4.1 DISTRIBUTION GENERATOR

FIGURE 6.1 Hydraulic generation as a distribution generation

Here from the fig 6.8 we can see the synchronous alternator that is used in the hydraulic

power plant. HTG is the hydraulic turbine governor used to give the power input to the

Alternator. The poles are excited by the excitation system. Here in the simulation we

are virtually regenerating the 3 phase power output by giving the power output as input

to the hydraulic turbine governor. Similarly the voltage at the direct and quadrature axis

are fed back to generate the required dc voltage by the excitation system. The

generation is at13.8 KV at power of 200MVA. A step up transformer is used to step up

the voltage to 230 KV. Here the shunt loads are just to avoid the error being induced

due to connecting the machines alternator and transformer in series. The output ports A,

62

B, C are given to the grid through a step down transformer, as the grid voltage is at

415V.

6.4.2 WIND POWER GENERATION

FIGURE 6.2 wind power generation

Wind generation using wind turbine, pitch control, Induction Generator. Here we are

using the induction generator as generating machine due to its advantages over other

machines for its simplicity and economical factors. The pitch angle controller makes

the angle of the turbine blade to adjust in such a way that the speed of rotation at every

velocity of the wind is maintained constant. And the parallel capacitive bank is to

supply the reactive power to the IM running as the generator. Here we considered the

per unit values in the closed loop that can be seen from the fig 6.9. The rms values of

the current and voltage generated is taken and the power is being calculated at every

sampling time interval and the wave form is being traced in the scope. A timer is used

in fig for assigning the wind velocity at 3 different states which will be linearise after

some loop operations.

63

6.4.3 STATCOM (VSI) WITH BESS AND CONTROLLER

FIGURE 6.3 Voltage source inverter with battery and controller.

Here the VSI used comprises of the 6 pulse converter in which the components are the

IGBT’s with anti parallel diodes. It consists of a capacitor and back up energy storage

system for back up under long duration real power outage. Here we are using the PI

controller and PWM (pulse width modulation technique) to generate the gate pulses to

the IGBT’s.

FIGURE 6.4 CONTROLLER WITH PWM

64

The DC link voltage Vdc is sensed and is given to the controller. And also the grid

voltage is sensed Vabc and is given using ‘goto’ and ‘from’ blocks from signal routing.

From fig 6.11 the error from the Vabc and the 1.0 pu value is given to the PI controller

the transfer function generates the control voltage. Similarly from the Vdc voltage the

phase angle is adjusted accordingly. Here the control method adopted is phase shift

control and Regulation of ac bus and dc link voltage.

6.4.4 NON LINEAR LOAD

FIGURE 6.5 NON LINEAR LOAD (RECTIFIER)

The nature of non-linear loads is to generate harmonics in the current waveform. This

distortion of the current waveform leads to distortion of the voltage waveform. Under

these conditions, the voltage waveform is no longer proportional to the current. Non

Linear Loads are: COMPUTER, LASER PRINTERS, SMPS, RECTIFIER, PLC,

ELECTRONIC BALLAST, REFRIGERATOR, and TV ETC.

6.5 PARAMETERS

Grid voltage - 415 V.

Operating frequency - 60 HZ.

Induction generator - 3.35KVA, 415V, 60 Hz, P=4,

Speed=1440rpm, Rr=0.01Ω,Rs=0.015Ω, Ls=Lr=0.06H.

Inverter - DC Link Voltage=800V, DC Link Capacitance=100μF,

Switching Frequency=2 kHz.

Non linear load – 25 KW.

65

FIGURE 6.6 over all circuit diagram in simulink with sub systems

6.6 Simulation Result

FIGURE 6.7.1 grid voltage without statcom

66

FIGURE 6.7.2 Voltage and current from DG

FIGURE 6.7.3 Injected current into the grid

67

FIGURE 6.7.4 compensated wind output

FIGURE 6.7.5 compensated grid voltage

Here the voltage is in P.U. That is as below

415 V/ 13.8 KV = 0.03007 p.u

68

CHAPTER – 7

CONCLUSION AND FUTURE SCOPE

7.1 CONCLUSION

In this paper we present the FACTS device (STATCOM) -based control scheme

for power quality improvement in grid connected wind generating system and with

nonlinear load. The power quality issues and its consequences on the consumer and

electric utility are presented. The operation of the control system developed for the

STATCOM in MATLAB/SIMULINK for maintaining the power quality is to be

simulated. It has a capability to cancel out the harmonic parts of the load current. It

maintains the source voltage and current in-phase and support the reactive power

demand for the wind generator and load at PCC in the grid system, thus it gives an

opportunity to enhance the utilization factor of transmission line.

Thus the integrated wind generation and FACTS device with BESS have shown

the outstanding performance in maintaining the voltage profile as per requirement.

Thus the proposed scheme in the grid connected system fulfils the power quality

requirements and maintains the grid voltage free from distortion and harmonics.

7.2 FUTURE SCOPE:

STATCOM can be replaced with UPQC for better power control.

Replacing the Induction Generator with Doubly fed Induction generator is

preferred for better results.

Now a day the statcom control scheme is based on various methods mentioned

in chapter 2 basing on the requirements.

In future the off-shore wind turbines will be well implemented due to its

advantages of producing high power.

69

REFERENCES:

[1] Yuvaraj and Pratheep Raj, Anna University of Technology “Power Quality

Improvement for Grid Connected Wind Energy System using FACTS device”, ,” IEEE

Trans. on E. Conv., vol. 23, no. 1, pp. 163–169, 2008.

[2] R. Billinton and Y. Gao, “Multistate wind Energy conversion system models for

adequacy assessment of generating systems incorporating wind energy,” IEEE Trans.

on E. Conv., vol. 23, no. 1, pp. 163–169, 2008.

[3] J. Manel Carrasco, “Power electronic system for grid integration of renewable

energy source: A survey,” IEEE Trans. Ind. Electron., vol. 53, no. 4, pp. 1002–1014,

2006.

[4] M. Tsili and S. Papathanassiou, “A review of grid code technology requirements

for wind

turbine,” Proc. IET Renew.power gen., vol. 3, pp. 308–332, 2009.

[5] J. J. Gutierrez, J. Ruiz, L. Leturiondo, and A. Lazkano, “Flicker measurement

system for

wind turbine certification,” IEEE Trans. Instrum. Meas., vol. 58, no. 2, pp. 375–382,

Feb. 2009.

[6] Indian Wind Grid Code Draft report on, Jul. 2009, pp. 15–18, C-NET.

[7] C. Han, A. Q. Huang, M. Baran, S. Bhattacharya, and W. Litzenberger,

“STATCOM impact study on the integration of a large wind farm into a weak loop

power system,” IEEE Trans. Energy Conv., vol. 23, no. 1, pp. 226–232, Mar. 2008.

[8] F. Zhou, G. Joos, and C. Abhey, “Voltage stability in weak connection wind farm,”

in IEEE PES Gen. Meeting, 2005, vol. 2, pp. 1483–1488.

70

APPENDIX

Sinusoidal Pulse Width Modulation (SPWM) generator:

For each arm of the VSC shown in fig pulses are generated by PWM

generator. It compares a triangular carrier waveform to a reference modulating signal as

shown in fig .The modulating signals can be generated by the PWM generator itself.

Three reference signals are needed to generate the pulses for a three-phase, single or

double bridge. The reference signals used here are three-phase sinusoidal signals. These

are generated by controller circuit. The output of PWM generator is given by:

When Va0> VT T+ on; T- off; Va0 = ½Vd, and

when Va0 < VT T- on; T+ off; Va0 = -½Vd

Fig

Sinusoidal Pulse Width Modulation (SPWM) technique

The DC link voltage Vdc is sensed and is given to the controller. And also the grid

voltage is sensed Vabc and is given using ‘goto’ and ‘from’ blocks from signal routing.

From fig the error from the Vabc and the 1.0 pu value is given to the PI controller the

transfer function generates the control voltage. Similarly from the Vdc voltage the

phase angle is adjusted accordingly. Here the control method adopted is phase shift

control and Regulation of ac bus and dc link voltage.

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