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CHAPTER 2

LITERATURE REVIEW

Any research work foundation depends on literature survey. Based on the studies

carried out by several researchers and their contribution to research field motivates

for further scope of research. In this chapter review of several research papers by

various authors and technical reports has been discussed. Distributed Generation

(DG)and their grid integration issues and later on solutions presented by several

authors are presented. Also studies on the hybrid combination of PV/ Wind modelling

and simulation by several authors using various tools have been discussed.

2.1 Distributed Generation

The earliest electric power systems were DG systems intended to cater the

requirements of local areas. Different definitions of DG have been proposed. Some

have linked this to size of the plant, suggesting that DG should be from a few KW to

sizes less than 10 or 50 MW. A review of alternative definitions of DG [(Ackerman et

al., 2001), (IEEE), (CIRED, 1999), (CIGRE)], suggested that DG can be defined as

the installation and operation of electric power generation units connected directly to

the distribution network or connected to the network on the customer site of the

meter. Comparison of renewable and non renewable DG options in context to their

current status, evaluation of their future potential and cost of generation in Indian

power sector are available [Banergee, 2006].

DG has a key role to play in power sector. The Energy Information Administration

(EIA) provides statistics and data analysis about the growing demand of electricity

and the role of DG. It offers great potential to offset traditional utility investments in

generation and transmission facilities. A report from EIA suggested that DG‟s

contribution to electricity is predicted to increase drastically. Many researchers have

investigated the importance of the DG and its applications in enhancing the electrical

system.

Daly et al., [2001] discussed different applications of DG and its cost analysis in

their study. They presented different DG technologies and the potential benefits

12

of DG that included reliability, deferral of power delivery investments, and

environmental benefits. Joos et al., [2000], discussed the potential of DG to provide

ancillary services. Their study demonstrated the potential of different types of

DG along with proper power electronic interface to provide ancillary services to the

main grid. The high performance development of power sector by DG is highlighted

by Matthew [2009] and with the existing technologies, the developers and urban

planners have a key ingredient to create sustainable cities with DGs.

Banergee, [2006] and Gerwen, [2006], summarised the benefits of DGs by comparing

different technologies of renewable sources. The viability of DG integration depends

heavily on energy prices and stable policies to encourage serious investments by

market parties [Gerwen, 2006]. Malcolm [2003] suggested with incentives,

recognition and rewards the possibility of encouragement to the distributed network

operators.

Benysek [2009], presented the fundamental problems of the electrical power systems

and the efficiency improvement services to grid by DG. He also predicted that, DGs

are very suitable for end users connected to the weak grid. Abimbola [2003], assessed

that over 2 billion people lack access to electricity in developing countries and

suggests that DG can quickly increase the quality of those individuals.

2.2 Impact of Distributed Generation

Interconnecting a DG to the distribution feeder can have significant effects on the

system such as power flow, voltage regulation, reliability etc. A DG installation

changes traditional characteristics of the distribution system. Most of the distribution

systems are designed such that the power flows in one direction. The installation of a

DG introduces another source in the system. When the DG power is more than the

downstream load, it sends power upstream reversing the direction of power flow and

at some point between the DG and substation; the real power flow is zero due to

back flow of power from DG.

Few papers presented guidelines for DG interconnection. Baghzouz [2006], defined

rules for studying the impacts of interconnecting DG to a distribution feeder. The

rules are defined for power flow reversal, optimal DG placement for reduction of

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losses and the impacts of DG on over-current protection. Willis [2000], discussed

zero point analysis and rules for modelling of DG interaction with the system.

The 1547 series of IEEE standards for interconnecting distributed resources to the

power system is a set of standards consisting of 6 parts [ieee.org]. These standards

provide criteria and requirement for interconnecting distributed resources to the

power system. The IEEE 1547.1, [2005] defines the requirement for interconnecting

equipment that connects the DG to the electric power system. The IEEE 1547.2,

[2005] , provides the technical details and application to understand the IEEE

standard. The IEEE 1547.3, [2005], guided and addressed engineering concerns of

design, operation and integration of DG island systems. The IEEE 1547.6, [2005],

focuses on standard criteria, test and requirements for interconnection distribution

secondary network of area electric power system with local electric power system

having distributed resource generation.

The DG installation can impact the overall voltage profile of the system. Inclusion of

DG can improve feeder voltage of distribution networks in areas where voltage

dip or blackouts are of concern for utilities. The voltage issues related to the

installation of DG on current electrical system have been discussed in many papers.

Several debates on voltage impact on the grid when wind DG integrates have been

carried out as it is most necessary for grid code maintenance.

Borges and Falcao [2003], have discussed the impact of DG on electric losses,

voltage profile and reliability. The purpose is to find optimal DG allocation and

sizing for minimal losses and proper voltage and reliability support. The impacts

of DG on power system have been analyzed by Barker and Mello [2000]. The

research work analyzes the impact of DG on voltage regulation and losses, as well as

the voltage flicker and harmonics that can be caused by the DG. The work also

addresses DG impacts on short circuit levels and the islanding operation of DG.

However the impact of DG can alter the voltage regulator settings and can cause the

voltage to deviate above or below the permissible range. A technique to regulate

voltage using DG was proposed by Le et al., [2006]. Their study presented a

simulation that uses a voltage control method for optimal power injection from DG.

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The impacts of DG on voltage regulation by Load Tap Changing (LTC) transformer

were studied by Dai and Baghzouz [2004]. They found that the DG can cause under-

voltages and over-voltages if proper LTC transformer controls are not applied.

Several researchers have worked on the control models for the efficient operation of

DG. Kashem and Ledwich [2004], studied on operation and control for DG

installation in their work. They contributed a DG control model to improve the

network voltage efficiently. Further they addressed network issues when multiple

DGs are included in the network and presented analytical methods and solutions to

develop design criteria for DG installation [Kashem and Ledwich, 2005].

Dai and Baghzouz [2003], presented a simple analytical method to estimate voltage

profile for radial distribution system when placing DG units with specific active

and reactive power generation.

Kim and Kim [2001], also discussed voltage regulation coordination methods of DG

in a distribution system. The approach makes use of controlling DGs reactive power

based on its real power to satisfy system voltage requirements. Installation of DG

also impacts the losses and power factor of the distribution system. Few research

papers discusses about the reduction of losses by inserting power from DG into the

system.

Borges and Falcao [2003] discussed the role of DG in loss reduction based on a

power summation method. Barker and Mello [2000], briefly presented on the impact

of DG on losses of the feeder but analysis was not carried out.

2.3 Wind DG and its Grid Integration Issues

Among renewable energy sources, wind power is the most attractive for mass

production. This is supported by exponential growth into the installed wind generation

capacity worldwide. The rapid increase in wind power installations over the past ten

years have benefited tremendously from the technology advancements that have

brought about significant reduction in the investment costs associated with wind

generation. A wind power project typically has a fast payback period, a relatively small

installation period and a low operation/maintenance cost, making it very attractive for

15

potential investors. This inspired the investors and lead to the growth of wind

generation.

The wind power programme in India was initiated towards the end of the Sixth Plan,

in 1983-84. Indian wind energy outlook report of 2009 outlines Indian renewable

installed capacity of 13.2 GW [IWEO, 2009] and the installed capacity has increased

from 41.3 MW in 1992 to reach 13065.78 MW by December 2010 [IWEO, 2010].

The gross potential is 48,561 MW and a total of about 14,158.00 MW of commercial

projects have been established until March 31, 2011. In terms of wind power installed

capacity, India is ranked fifth in the World. The reports discusses not only about

status of wind energy in India but also policy environment for wind energy, costs and

benefits, demand projections, energy efficiency and growth rates.

IWEO [2011], reported on the importance of the grid stability which is a key

consideration for interconnection of any new system to the existing grid. Stresses upon

the variable nature of wind power necessitate the development of interconnection

standards to enable the grid to sustain the variability without affecting the power

quality adversely.

The report of OECD [2005] discussed that „Grid integration concerns have come to the

fore in recent years as wind power penetration levels have increased in a number of

countries as an issue that may impede the widespread deployment of wind power

systems‟ and mention that two of the strongest challenges to wind power‟s future

prospects are the problems of intermittency and grid reliability. The report also

highlights the most of the international collaborative work specifically categorises the

current focus into four main areas-

Wind power prediction tools to improve forecasting for electricity

production

Modelling and grid simulation studies and practices to ensure grid system

optimization.

Investigations and planning of designs to reinforce and extend the grid

Analysis and development of grid access rules, technical code requirements

and international standards.

16

From these four areas the second one has been taken for this research study and

presented and discussed in chapter 5 and 6.

CREDP discussed on the degree of the success of grid integration and the level of wind

penetration variation are due to, largely on voltage levels, power quality issues such as

harmonic distortion, voltage transients, sags, voltage flicker and response to abnormal

conditions. Xie et al., [2011], surveys a major technical challenge for power system

operations in support of large scale wind energy integration and discusses the impact

of wind, on unit commitment, economic dispatch, automatic generation control and

frequency stabilization in their study.

Researchers Martinez et al., [2004] have analysed on the suitability of wind turbines

with Doubly Fed Induction Generators (DFIG) for new grid operator norms that

require ride through operations and shown that the use of power error vector control

and active & reactive power reference reduction during voltage dips may be a good

solution for low voltage ride through.

Khorrami [2010], presented on challenges in cyber- controlled smart grid and he

developed a technology for integration by controlling renewable energy source, control

of energy consumption and load management. He also suggested to empower energy

user for a sustainable living and to develop DG system where energy user is also an

energy producer and usage of FACTS for controlling smart switches.

Smith et al., [2007], Parsons et al., [2006], Demeo et al., [2005], summarises and

updates on many of the salient points on the current state of knowledge regarding

utility wind integration issues. All provided the impact of wind generation on system

dynamic performance by comparing with and without wind integration and suggests

that actual operational experience will contribute significantly to understand the wind

impact on the system.

Matias et al., [2010] study findings provided supportive evidence for the need to

rethink, upgrade and redesign the current electricity markets structures to

accommodate deep penetration of wind power and stressed on to harness the benefits

associated with wind energy.

17

Sorensen et al., [2000] reported on critical power quality issues, in connection with

joint Danish and Indian project and obtained power quality data such as reactive

power, voltage imbalance, current imbalance, frequency range harmonics, inter

harmonic distortion and voltage fluctuations. They suggested for power factor

improvement by capacitor compensation, synchronous condenser, and thyristor

controlled reactors and after the survey they concluded that usage of automatic tap

changers for steady state voltage control and dedicated feeders to the wind farms for

better quality.

Several researchers opinion is that in order to maintain the grid stable even after wind

integrates there is a need for grid code maintenance. Rajesh, [C-WET] reported that

there is a need for Indian wind grid code as wind energy constitutes 12% of the

installed capacity in the power scenario in India. With high penetration, overall power

system gets affected and hence grid code envisages establishing a standard operating

practice.

The grid codes for wind in general deal with active power control, frequency, voltage

and reactive power issues, fault ride through capability, protection and power quality

issues like flicker, harmonics. According to the report variation of frequency can lie

between 47.5-51.5 Hertz for stable operation.

Douglas and Orme [2006] traced out that hooking up wind farms to the grid is not

always easy but network operators are rising to the challenge. Brian et al., [2006]

discussed about higher wind penetration impacts the power system operation due to

variability of wind. Goggin [2009] overviews the recent wind growth, factors driving

the growth and also transmission and grid integration issues. He also mentioned that

wind is easier to integrate on more flexible power systems and integrating wind would

be cost issue but not a reliability issue. The New Indian Electricity Grid Code declares

that the operational frequency band has been tightened from 50.3-49.2 Hertz to 50.2-

49.5 Hertz. Also all users of inter-state grid including distribution utilities will now be

also directly responsible for grid discipline and load management, in addition to load

despatch centres [Pradeep, 2010].

All these research papers present the importance of grid code maintenance when

renewable DG integrates with the grid and there is an emergency in taking care of

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reliable and secure of operation of power system. There is also a call for upgrading the

transmission system infrastructure in order to reach power to every corner of the

world.

For all the above necessary statements FACTS Controllers are the solution as they are

more reliable and best option. Improved utilization of the existing power system is

provided through the application of advanced control technologies.

2.4 FACTS Controllers

Power electronics based equipment or FACTS provide proven technical solutions to

address the new operating challenges. FACTS technology allows the improved

transmission system operation with minimal infrastructure investment, environmental

impact, and implementation time compared to the construction of new transmission

lines. Constructing new transmission lines becomes extremely difficult, expensive and

time consuming. FACTS technology provides advanced solutions for the existing

transmission lines which would be cost effective alternative for new transmission line

construction.

Major problem for maintenance of grid codes are with respect to voltage variations.

These problems are traditionally solved by reactive power compensation by capacitor

banks or with on load tap changers. But FACTS controllers bring a wide opening for

all the problems.

Paserba [2007] discussed the issues and benefits of applying FACTS controllers to AC

power systems. He also presented all the associated problems of power system for

stable operation and suggested which type of FACTS controller to be used to solve

those problems. Tyll and Frank [2009] presented different types of reactive power

compensation by FACTS controllers and addressed real implementation of several

controllers at different regions.

Breuer et al., [2004] elaborates the need for HVDC and FACTS for transmission

system and presented how long distance AC transmission issues in several countries

like Brazil, UK, etc., have been solved by FACTS Controllers. Adamczyk et al, [2010]

discussed how transmission system operators were forced to impose grid codes for grid

stability and continues with FACTS beneficiary to power system with respect to

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voltage stability, frequency stability, power oscillations, fault ride through, and power

quality.

Wei et al., [2006] investigated the application of FACTS devices to enhance the

dynamic and transient performance of power systems which includes large wind farm.

Simulation study on three phase short circuit test at PCC on the test system using

PSCAD/EMDTC has carried out in their research work. The study was limited to only

three phase fault at PCC. SSSC and STATCOM combination were used to damp

oscillations in their simulation studies. Results showed that the FACTS devices

provide an effective means in dynamic voltage control of the wind farm, dynamic

power flow control, improving power oscillation damping and transient stability.

Jones [2007] addressed how FACTS technology helped in improving at least 10%-

15% of transmission capability of lines and mentions ABB taken initiative of installing

over 600 FACTS applications in 50 countries.

Panda and Padhy, [2007] investigated the dynamic behavior of WTG during an

external three phase fault with STATCOM and without STATCOM under various

wind speed changes. Simulation results show that STATCOM prevents large

deviations of bus voltage magnitude induced by reactive power drawn from

distribution network by WTGs.

Zobaa et al., [2006] discussed the disadvantage of wind energy converting systems

dependency on the grid for supplying its own reactive power. The grid has to supply

not only the load and lines but also generator. They suggested that installing

centralized VAr compensator at the point of common coupling regulates the fluctuated

VAr demand, mitigates voltage flicker and compensates line losses.

The techniques of correcting the supply voltage sag, swell, and interruption in a

distributed system were suggested [(Ren et al, 2008), (Kumar and Nagaraju, 2007)].

Ren et al, [2008], proposed an algorithm and simulation procedure to deal with the

installation of the DFACTS in the distribution network, allowing better and higher

penetration of DGs. Stephan and Stefan [2007], discussed the technical and

economical benefits of wind energy converters with FACTS capabilities for power

systems and grid integration of wind power. Their investigation is mainly on the wind

turbine generators recent developments.

20

Salman and Teo [2005] investigated the dynamic behaviour of multiple wind farms

that are integrated to the same network. The authors concluded that the behaviour of

wind farm under fault condition connected to the network, influence the stability of

other wind farms connected to the same network. To solve this problem critical

clearing time of multiple wind farms can be slightly improved using VAr injection

technique. They achieved by installing Static Synchronous Compensators (SSC) at

selected location.

Rajiv et al., [2008] studied on mitigation of sub synchronous resonance (SSR) in series

compensated wind farm using two thyristor based FACTS devices-Static VAr

Compensator [SVC] and Thyristor Controlled Series Capacitor [TCSC]. They found

that SVC and TCSC are effective in damping sun synchronous resonance [SSR]

oscillations when the system is subjected to severe fault.

Very interesting point considered by the authors Wessels and Fuchs [2009] about the

voltage swell during grid faults that is caused by switching off large loads or switching

on capacitor banks. They suggested the applicability of Dynamic Voltage Restorer

[DVR] and the Static Synchronous Compensator [SSC] to mitigate the three phase

swell. Cartwright et al, [2004] presented the solution for integrating large off shore

wind farms into transmission networks using both FACTS and a hybrid HVDC

system.

Keane et al, [2011] proposed a passive solution to reduce the impact on transmission

system voltages and to overcome the distribution voltage rise barrier. They developed

a methodology which optimizes the power factor and tap changer settings of the

distribution network section such that distribution voltages are obeyed at all times.

Wilch et al., [2007] addressed the reactive power generation of offshore wind parks

using DFIG connected to the main grid with long cables along with reactive power

compensating devices. Both studies had not used FACTS controllers for compensation.

Salehi et al., [2006] found that wind turbine generators tend to drain large amount of

VAr‟s from the grid, potentially causing low voltage and may be voltage stability

problems for the utility. Using SVC and STATCOM they could expand voltage

stability margin even during transient and dynamic load variations. Han et al., [2006

and 2008], studied the impact of STATCOM on large wind farm integrating weak

21

power system. They presented a STATCOM control strategy for voltage fluctuation

suppression.

Roohollah et al, [2008] used TurbSim, AeroDyn, FAST, and SIMULINK to model the

aerodynamic, mechanical and electrical aspects of a wind power system including

STATCOM. They studied the performance of STATCOM controller for different wind

disturbances to the wind energy converting systems. They also showed that wind speed

and wind direction changes have different effects on the generated power and voltage.

Sharad and Mohan [2010] and Arulampalam et al., [2006] discussed on

implementation of STATCOM in wind farm for improvement of the system. Yuvaraj

and Deepa [2011] and Meera and Ratna [2012] also demonstrated the power quality

problem and its solution by STATCOM. Bhanu et al., [2003] discussed the additional

features of UPFC and mentions that today FACTS devices are individually controlled

but according to a new EPRI report, inventive strategies incorporating system wide

control logic could further increase power transfer capability, stability, and reliability

of transmission systems. Francisco et al., [2012] presented three strategies for reactive

power control in wind farms with STATCOM.

Oskoui et al., [2010] presents the implementation of Holly STATCOM at Austin.

Since its inception, STATCOM for overall system performance was appreciated.

Grunbaum et al.,[2004] showed that when the network is weak local dynamic reactive

power support may prevent voltage collapse and make it possible for the wind farms to

recover and remain in service after a short circuit event. They also discussed a dynamic

voltage control scheme based on a combination of SVC and STATCOM technology.

Verma et al., [2009] presented a very interesting concept of using solar farm as

STATCOM during night time when there is no active power at solar farm. The solar

farm inverter is used to regulate the distribution voltage at PCC. The proposed

strategy of PV solar farm control will facilitate integration of more wind plants in the

system without needing additional of more wind plants in the system and voltage

regulating devices. Trond [2005] and Suwannarat [2005] in their reports on wind

farms in weak grids compensated with STATCOM. Both as their part of curriculum

have taken up the study and developed a suitable simulation models for different faults

and disturbances and he reports that through simulation study it would give a rough

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picture of expected dynamic behaviour of the system and found that as long as fixed

speed machine is complemented with sufficient reactive power control, such as

STATCOM, the transient response would improve.

A report from ABB [2012], on renewable energy, presented wind integration with the

grid and the challenges to maintain grid code at various countries like, Great Britain,

Australia, Poland, Norway, Korea etc. with the help of PCS100, a fully dynamic

STATCOM. ABB also reported that the outstanding performance for dynamic

operation of the wind farm is possible by STATCOM.

Madhavan [2010] reported on the usage of AMSCs D-VAR STATCOM to integrate

wind energy to the grid. The voltage provided to the grid is always steady even when

the wind slows. This was possible with D-VAR STATCOM implementation in wind

farms which enabled wind farms to look more like conventional power plants.

It is known that the SVCs with an auxiliary injection of a suitable signal can

considerably improve the dynamic stability performance of a power system. In the

literature, SVCs have been applied successfully to improve the transient stability of a

synchronous machine [Byerly, 1982]. Hammad [1986] presented a fundamental

analysis of the application of SVC for enhancing the power systems stability.

Then, the low frequency oscillation damping enhancement via SVC has been

analyzed by various investigators [(Padiyar and Varma, 1991), (Zhou, 1993),

(Oliveira, 1994), (Wang and Swift, 1996)].

SVC enhances the system damping of local as well as inter area oscillation modes. It

was observed that SVC controls can significantly influence nonlinear system

behaviour especially under high-stress operating conditions and increased SVC gains.

Rodriguez et al., [2004] showed the results of stable operation of the wind energy

conversion systems with the aid of SVC. The reference list of SVCs installed all over

world is given in www.siemens.com. The Siemens states that over 30,000 MVAr

capacities of SVCs have been installed.

Various authors discussed the performance comparison of SVC and STATCOM. Ali

et al., [2010] showed the results with SVC and STATCOM and concluded

STATCOM is much more capable of stabilizing the network due to its inherent

factors. Noroozian et al., [2003] examined the overall performance of SVCs and

23

STATCOMs. The impact of SVCs and STATCOMs are studied separately on a

system and simulation results showed that the STATCOM solution allows faster

voltage recovery compared to SVC.

Tariq et al., [2010] in their study reported that STATCOMs superior functional

characteristics subsides SVC. Arthit and Mithulanathan [2004] compared Shunt

Capacitor, SVC and STATCOM operations with respect to static voltage stability

margin enhancement and found that from cost point of view shunt capacitor is

cheapest but STATCOM is better option when performance wise considered.

Mehrdad et al., [2009] also found in their research work that STATCOM is a better

option than SVC.

Based on the literatures, for this study, STATCOM was chosen to solve the grid

integration issues. Most of the investigators analyzed the system behavior

considering three phase fault for some milliseconds and for fixed STATCOM rating.

A complete study on different type of faults at PCC and at wind turbine generator

(WTG) are essential to achieve overall power quality of the system.

Since the study is not only to see the better performance of the existing wind farm

using STATCOM, but also to suggest hybrid combination of PV energy with the wind

energy with in and around the space availability of Wind Farm. So literature survey

on PV Modelling and simulation of PV arrays were carried out.

2.5 Photo Voltaic Energy

Villava et al., [2009] analysed the development of a method for the mathematical

modelling of PV arrays. They have proposed an effective and straight forward method

to fit the mathematical I-V curve to the three, remarkable points without the need to

guess or to estimate any other parameters except the diode constant. They have also

provided all the necessary information to easily develop a single- diode photovoltaic

array model for analysing and simulating a photovoltaic array.

Altas and Sharaf [2007] introduced a simulation model for photovoltaic arrays to be

used in Matlab-SIMULINK GUI Environment. The model is simulated connecting a

three phase inverter showing that, the generated DC voltage can be converted to AC

and interfaced to AC loads as well as AC utility grid system. Lloyd et al., [2000]

24

developed a reliable and repeatable methodology for the assessment of Maximum

Power Point Tracking (MPPT) performance. They had modelled a simulator using

Ispice and SIMULINK.

Ropp and Gonzalez [2009] addressed a Matlab SIMULINK model of a single phase

grid connected PV inverter that have been developed and experimentally tested its

performance.

Patel and Agarwal [2008], presented Matlab based modelling and simulation scheme

suitable for studying the I-V and P-V characteristics of a PV array under a non

uniform insolation due to partial shading. The situation is of particular interest in case

of large PV installations such as those used in distributed power generation schemes.

Huan et al., [2008] presented the implementation of a generalized photovoltaic model

using Matlab/SIMULINK software package, which can be representative of PV cell,

module and array for easy use on simulation platform. The proposed model takes

sunlight irradiance and cell temperature as input parameters and outputs the I-V and

P-V characteristics under various conditions.

Hamrouni and Cherif [2007] found an approach of modelling and control of a grid

connected photovoltaic system. Here a MPPT controller is used to extract the optimal

photovoltaic power; a current and a DC link voltage regulator are used to transfer the

photovoltaic power and to synchronize the output inverter with the grid. Hernanz et

al., [2010], developed a model based on other studies and that model has been

validated with experimental data of a commercial PV module, Mitsubishi PV-

TD1185MF5.

Ciobotaru et al., [2006] compared the models developed by MATLAB/SIMULINK

and PLECS toolbox. Both simulation models were tested and the results were

compared. Jayachandran et al., [2011] presented a simple model wherein no complex

computational techniques were used. The developed model was validated in the lab

using 80WPV module with Autosys SMT-INV 20-11 module tester and Quicksun

700A Sun simulator.

Kishore et al., [2010] presented the determination of resistances of PV cell with

adjustments of characteristics that correspond to maximum power point. The

25

influential environmental factors like; dust, solar radiation intensity, shadow,

temperature and wind velocity are considered. The study by Villalva et al., [2010]

deals with the regulation of the output voltage of PV arrays. They have presented a

detailed analysis of the PV voltage regulation problem using a buck converter as PV

array interface.

Bogdan et al., [1994] developed a methodology for calculation of the optimum size of

a PV array for a stand-alone hybrid wind/PV system. The least square method was

used to determine the best fit of the PV array and wind turbine to a given load. Also

an algorithm was developed to find the optimum size of the PV array in the system.

This literature reveals the investigation on hybrid energy has almost begun in 1990‟s.

The discussions on PV modelling and simulation by several authors, guided this study

for the PV park to integrate the grid along with wind integration. The combination of

PV and wind integration creates a hybrid generation which aids the existing system.

Lot of research study is going on the area of PV modelling and its efficiency

improvement.

2.6 Hybrid Energy- Wind/PV

Use of renewable energy technology to meet the energy demands has been steadily

increasing for the past few years, however, the important drawbacks associated with

renewable energy systems are their inability to guarantee reliability and their lean

nature. Presently, standalone solar photovoltaic and wind systems have been

promoted around the globe on a comparatively larger scale. These independent

systems cannot provide continuous source of energy as they are seasonal. Standalone

solar photovoltaic energy system cannot provide reliable power during non-sunny

days. The standalone wind system cannot satisfy constant load demands due to

significant fluctuations in the magnitude of wind speeds from hour to hour throughout

the year. To solve these problems energy storage systems will be required for each of

these systems in order to satisfy the power demands which are of very expensive.

Hence Hybrid power systems are better option whenever, wherever individual

renewable DG is considered.

26

Mualikrishna and Lakshminarayana [2008] proposed a hybrid system with solar and

wind sources for rural electrification. Authors compare stand alone wind and

standalone PV with Hybrid combination and concludes that hybrid would be the best

option and viable if PV module cost is below Rs 100/W and its efficiency is higher

than 20%. Nayar et al., [2007], discussed the implementation of PV/Wind/diesel

micro grid system in republic of Maldives, a remote island. They insisted the hybrid

system which has been installed was commissioned in August 2007 is able to perform

better and detail study on daily power flow of each energy has been tested practically.

From the recorded data daily energy output from wind, PV and diesel was plotted and

shown in the paper after first month of installation. They concluded that the newly

developed and installed system will provide very good opportunities to showcase high

penetration of renewable energies using wind turbines, photovoltaic modules,

advanced power electronics and control technology.

Zahedi and Kalam [2000], developed a methodology for calculating the correct size of

hybrid system and optimized the management of the same system. Main power for the

system considered is PV and wind and diesel is used as backup in this hybrid system.

The system is considered as autonomous as it is not using national grid supply. The

system was designed by calculating monthly demand of electrical energy required.

Size of the battery bank is worked out to substitute the PV array during cloudy and

non sunny days.

Dihrab et al., [2009] proposed a feasibility of using the renewable resources for power

generation by PV and wind hybrid system for grid connected applications for four

cities in Jordan. The simulation studies using Matlab solver shows that Jordan can

use the solar and wind energy to generate enough power and compared which was the

best location among four cities to install hybrid system. Yang et al., [2009] discussed

an optimal design and techno- economical analysis of a hybrid solar- wind power

generation system.

Hans et al, compared the simulation study using Matlab/SIMULINK for battery sizing

in grid connected renewable generation with the results based on the optimization tool

HOMER. They concluded that PV/Wind hybrid systems present a better solution in

terms of efficiency and energy cost as compared with pure PV or wind systems with

battery storage. Nehrir [2006] outlines the motivation for the development of an

27

alternative energy DG covering wind, PV and fuel cell power generation which has

been offered at Montana State University since 2003. Author discussed the detailed

coverage of course topics, student projects and student response on the course. It is

very interesting to know that the research area has become a course content and one

should know the importance of hybrid generation for the present era.

Kumar et al., [2011] proposed a hybrid system which includes PV/Wind/Micro-

Hydro/Diesel power generation suitable for remote area applications. They have

designed the model to provide an optimal system configuration based on hour by hour

data for energy availability and demands. They also found based on simulation results

that renewable/alternative energy sources will replace the conventional energy

sources and feasible solution for remote and distant locations.

Chang et al., [2007] discussed about the fact that if appropriate renewable energy

sources are selected and used complementarily, the overall performance and potential

supply time are anticipated to exceed those obtained by the individual use of these

resources. The study they carried out is on the complimentary operation system,

consisting of PV and Wind systems. Homer software was used to illustrate and

evaluate the technical and economic aspects of the hybrid system.

An attempt was made by Rehman et al., [2011], to explore the possibility of utilizing

power of the wind and sun to reduce the dependence on fossil fuel for power

generation to meet the energy requirement of a village in Saudi Arabia. In their study

they adopted wind/PV/diesel as hybrid system with 35% renewable energy

penetration (26% wind, 9% PV) and 65% diesel power contribution as the most

economical power system. They concluded after estimation that the cost of energy of

only diesel power system was found to be more sensitive to diesel price than the cost

of energy of hybrid power system.

Milligan [2011] worked on costs of integration for wind and solar energy for large

scale studies and implications. Author presented an oversight of wind and solar cost

issues when large amount of integration is studied. Sharaf and Ozkop [2009]

proposed a novel control system for control of hybrid wind/photovoltaic farm

utilization with alternative power source for DC type loads. The proposed control

function is digitally simulated using the MATLAB /SIMULINK/ SimPower system

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software environment. The dynamic performance of the hybrid system is examined

for the control system validation under normal and abnormal operating conditions.

Soltani et al., [2008] proposed an average model of a hybrid wind photovoltaic

generating system. Model of the solar generating subsystem has been developed with

integration of power losses model involved in the power converters. It is also shown

that the model is interesting for analyzing the dynamic behaviour and for optimum

design of the hybrid system.

Karami et al., [2010] developed a hybrid topology which exhibited excellent

performance under variable load power requirement. The proposed system is only for

non interconnected remote areas. Simulation results obtained had proven their

proposed power management strategy worked properly.

Curea et al., [2004] developed a test bench for the analysis of hybrid system

behaviour which constitutes a wind generator, PV panes, storage batteries, inverter,

diesel generator and a load. First they studied about the system behaviour for the

variations of the wind speed and solar radiation. In the next step analysed power

quality issues due to balanced and unbalanced load variations. The test bench used

marked for the validation of the simulation model.

Lew et al., [2009] studied on the western wind and solar integration, which is one of

the largest regional wind and solar integration studies examining the operational

impact up to 35% wind, PV and concentrating solar power on the west connected grid

in Arizona, Colorado, Nevada, New Mexico and Wyoming. The goal was to

understand the costs and operating impacts due to the variability and uncertainty of

wind, PV, and concentrated solar power on the grid. The study did not focussed on the

cost of generating wind or solar power but rather on the operational costs and savings

due to fuel and emissions.

Jeon et al., [2007] proposed a multifunctional grid connected wind/PV/BESS hybrid

generation system. The principle of the proposed system and power control scheme

for multi operation modes were described. Fargali et al., worked on a new geothermal

space heating system, which uses PV-wind energy sources to feed the electrical loads

of the heating system in a remote area in Egypt. They also presented a complete

mathematical modelling and MATLAB SIMULINK model for the different

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components in both electrical and geothermal subsystems. The results illustrated that

the designed control technique enables the developed system to be in correct and

continuous operation.

Onar et al., [2006] in their paper focussed on the combination of wind, fuel cell and

ultra capacitor systems for sustained power generation. In their proposed system,

when wind speed is sufficient the wind turbine can meet the demand. If the available

power from wind turbine cannot satisfy the load demand the fuel cell system can meet

the excess power demand, while ultra cell can meet the excess load for short

durations. They insisted that the proposed system can tolerate the rapid changes in

wind speed and suppress the effects of these fluctuations on equipment side voltage in

a novel topology.

Chedella et al., [2010] presented the preliminary study of modelling a small stand

alone AC system with the fuel cells and solar panels as energy resources. The solar

energy is main energy source for electricity generation during the day and will be

complemented with the fuel cell and battery during night.

A new converter topology for hybrid wind/PV energy system is proposed by Jacob

and Arun [2012]. They were influenced by the authors Hui et al., [2010] who

presented a new rectifier stage topology for hybrid wind-solar energy system. The

new converter used the cuk and single ended primary inductor converter type of DC-

DC converters.

Doumbia et al., [2007] presented a dynamic simulation model using

Matlab/SIMULINK software to study the behaviour of renewable energy systems

with hydrogen storage. The complete system model is developed by integrating

individual sub units of the photovoltaic arrays, wind turbine, batteries, electrolyser,

fuel cell and power conditioning units. The state of charge control method has been

chosen to validate the developed simulation models. Their obtained results confirmed

previous experimental measurements on the test bench.

Chen et al., [2007] designed a hybrid system consisting of PV power system, wind

power system and battery storage. A novel controller links each of these components

to ensure that one system will be supplying load dependant on the weather conditions.

Detailed results on tested various weather conditions and temperatures are presented.

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Kim et al., [2006] dealt in their research on power control of a wind and solar hybrid

generation system for interconnection operation with electric distribution system.

Modelling and simulation study on the entire control scheme was carried out using a

power system transient tool, PSCAD/EMTDC. The results of their simulation showed

the control performance and dynamic behaviour of the wind/PV system.

Giraud et al., [2001] reported the performance of a 4 KW grid connected residential

wind-photovoltaic system with battery storage located in Lowell, USA. This paper

also includes the discussion on system reliability, power quality, loss of supply and

effects on the randomness of the wind and solar radiation on system design.

Jain and Agarwal [2008] discussed on an integrated hybrid power supply for

distribution generation applications fed by non conventional energy sources. The

advantage of this proposed system includes low cost, compact structure and high

reliability which render the system suitable for modular assemblies and plug-n-play

type applications. They presented the analytical, simulation and experimental results

of their research study.

Phrakonkham et al., [2010] discussed the status and development of electrification as

well as the available renewable energy sources. They summarized on different micro

grid configurations and on simulation tools. They continued their case study with

respect to economic optimization using two different simulation software tools,

HOMER and HOGA.

2.7 Summary

After carrying out the literature survey the research work had one orientation towards

the importance of renewable DG in the upcoming years and the associated problems

when they work on either stand alone or grid integrated system. The solutions

suggested by several authors in their research for most of the issues caused by the DG

integration. Recent development of power electronics introduces the use of FACTS

controllers in power systems. FACTS technologies not only provides solutions for

efficiently increasing transmission system capacity but also increases available

transfer capability, relieves congestion, improve reliability and enhances operation

and control.

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Whenever renewable energy integrates the grid fluctuations in voltage, change in

power flow pattern, voltage flicker and harmonics are reported to be common

problems associated with grid. Hence maintenance of grid code becomes very

essential to achieve good power quality. For all these issues FACTS controllers are

better solution.

voltage sourced converters like STATCOM, SSSC or UPFC are more attractive as

their operation is not so strongly dependant on the grid conditions. Series devices yet

did not receive too much attention in wind power field as their role is on transmission

system not on generation site and this did not probably attract wind generation

owners. On the other hand shunt devices are normally deployed not only inside the

transmission network but at load and generation buses. UPFC has only very few

experimental applications due to its high cost.

Several investigators experimented that STATCOM play a vital role in DG

integration which in turn makes them to look like conventional power plant and helps

to achieve required grid code maintenance. Applications of STATCOM were

investigated by several researchers such as for improvement of voltage stability in

wind farm, to mitigate voltage fluctuations and harmonics reduction.

The application of FACTS devices to enhance the dynamic and transient performance

of power systems which includes large wind farm using simulation study on three

phase short circuit test at PCC on the test system using PSCAD/EMDTC has been

carried out. Such studies were limited to only three phase fault at PCC. Previous

researchers have used a combination of SSSC and STATCOM to damp oscillations.

Results showed that the FACTS devices provide an effective means in dynamic

voltage control of the wind farm, dynamic power flow control, improving power

oscillation damping and transient stability. Few researchers also investigated the

dynamic behavior of WTG during an external three phase fault with STATCOM and

without STATCOM under various wind speed changes.

All the studies were concentrated on only three phase fault occurrence at PCC and to

achieve overall power quality for fixed rating of STATCOM. Therefore, a detailed

study is necessary and has to be carried out under various fault conditions such as LG,

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LLG faults at PCC and as well as at WTG. The effect of different combinational

faults with STATCOM and without STATCOM is to be studied to achieve better

power quality. There is a need for investigating the fault ride through capability of

STATCOM when connected at PCC. Details of WTG is also not analyzed which is

important to identify each turbine behavior under different scenarios.

Based on recent publications it is found that there is a scope for further research in the

area of power oscillations damping of squirrel cage induction generator, harmonics,

flicker mitigation and fault ride through capability of various faults using STATCOM

for grid integration of wind energy. Since several studies suggested having hybrid

combination rather than individual renewable sources for better and reliable operation

of power system, this research study further carried out with PV simulation for

estimation and cost analysis of PV plant in and around the study area chosen.

Hence it was envisaged to carryout STATCOM implementation at PCC with all

possible different faults at both PCC and WTG and proposal of PV plant with the

existing wind farm.