POWER QUALITY

Document BySANTOSH BHARADWAJ REDDYEmail: help@matlabcodes.com

Engineeringpapers.blogspot.comMore Papers and Presentations available on above site

ABSTRACT:

THE UBIQUITOUS APPLICATIONS OF POWER ELECTRONIC EQUIPMENTS

RESULTED IN POWER QUALITY PROBLEMS, SUCH AS VOLTAGE DISTORTION

DUE TO HARMONICS GENERATED IN SUPPLY CURRENTS OF VARIOUS ORDER

ENERGY EFFICIENT ELECTRONIC EQUIPMENTS AND NON LINEAR LOADS ON

VOLTAGE AND CURRENT WAVE FORMS. THE POWER QUALITY IS OF

INCREASING CONCERN OF ELECTRIC UTILITIES ,SINCE THE ILL EFFECTS OF

HARMONICS CAUSES THE DETERIORATION OF VARIOUS SENSITIVE

ELECTRONIC AND ELECTRIC EQUIPMENT DESIGNED TO OPERATE ON PURE

SINUSOIDAL WAVE FORM OF A.C. SUPPLY AT RATEDVOLTAGE AND

FREQUENCY. HARMONIC DISTORTION AND SUBSEQUENT EQUIPMENT DAMAGE

ONE OF THE MOST SERIOUS POWERQUALITY PROBLEMS NOW A DAYS .

THE THREE MAIN IMPORTANT FACTORS RELATED WITH POWER QUALITY

ARE UTILITY, CUSTOMER AND EQUIPMENT MANFACTURERS. THE AIM OF

THIS PAPER IS TO TAKE A OVERVIEW OF DIFFERENT SOURCES OF HARMONIC

GENERATION , THEIR EFFECTS, MEASUREMENT AND ELIMINATION TECHINIQUES

TO BRING THE TOTAL HARMONIC DISTORTION WITHIN PERMISSIBLE LIMIT

BY USING HARMONIC SUPPRESSOR SYSTEMS .

INTRODUCTION:

Harmonic Voltages and Currents are the integral multiples of the fundamental frequency. In

India ,for 50Hz. frequency of second harmonic is 100Hz, third harmonic is 150Hz. and so

on.

Though the applied voltage is sinusoidal because of their nonlinear characteristics the

current drawn by them is non sinusoidal. This non sinusoidal current after passing through

the low impedance of power causes distortion in sinusoidal voltage which can be split up

as sinusoids of the different frequencies, which when added together results in original

distorted waves. Advancements in power electronics caused the use of such a equipment in

power system at LV,MV,HV and EHV levels increasing at a faster rate, harmonics is

negative by product of this development. Utility of nonlinear loads resulted in large

harmonics in supply currents causing distortion of the voltage wave form resulting in

deterioration of power quality.

Deviation from sinusoidal shape is generally expressed in terms of harmonic

components. All the harmonic current deteriorates the performance and impose various

problems. Therefore , identification of sources of harmonic are important.

Sources of harmonics:

Harmonics of different orders generated when connected power system network by different

sources as below:

The non linear loads such as inverter fed adjustable speed drive.

The use of powerfactor correction capacitor creates parallel or series resonance

problems increasing the harmonic distortion.

Process control and solid state power conversion equipments.

Energy efficient compact flourescent lamps.

Use of AC and DC adjustable speed drives

Static VAR compensators.

Transformers produces very high levels of harmonics when they are initially

energized, the so called in rush current will generate harmonics of several orders.

Cycloconverters,Lift control system,Traction,AC voltage regulators, UPS, Battery

chargers.

The following are the undesirable effects of harmonic on the

operation of different equipment connected to power system network. The effect of

harmonic depends on harmonic voltages and currents are the integral multiples of the

fundamental frequency.

EFFECTS OF HARMONICS:

1. The duration presence of long duration harmonic cause more serious effects on

the various equipments connected to the power system .

2. Amplitude of harmonics ….large amplitude harmonics of short duration under

resonance condition cause dielectric breakdown due to over voltages.

Now a days various devices and equipment being measured applications are more

sensitive compared to the past.

The capacitor used for power factor correction and in different filters decreases

resulting in increasing in current drawn by capacitor beyond permissible limits. The

capacitor acts as sink for harmonic currents resultant effect of harmonics is

overloading , hence over heating increases dielectric stress and increase the power lost.

The thermal failure of capacitor may take place because of higher temperature.

Non sinusoidal power supplies results in reduction of torque of induction motor

It will increase interference with telephone , communication and logic circuits.

Error in reading of induction type energy meters which are calibrated for pure

sinusoidal A.C power.

Higher order harmonics causes voltage stress and carona .

Presence of harmonics in power system network can cause additional

losses in power system network, overheating of transmission lines, transformers and

generators etc.,

Malfunction or even failure of electronic or computer controls.

Hence it is clear that day by day the increase in harmonic contents will

impose new problems on operations of electronic equipment . The energy efficient

electronic equipment that will be produced in future trends result in poor

performance due to the voltage distortion. Hence it is essential to have the proper

coordination between the supply authorities and consumers regarding the power

quality problem, their causes and results and solutions available to eliminate them.

IMPORTANCE OF HARMONIC MEASUREMENTS:

Meaurement provides the real time information about the power system network

to utilities, consumer and designer .Filters designed after harmonic measurements

are technically better and economical.

Measurement provide information to analyze the combined effect of harmonic

generated by different loads on power quality.

EQUIPMENT USED FOR MEASUREMENT OF HARMONICS:

In these systems it is necessary that the signal input to the

harmonic analyzer should faithfully represents the voltage and current signals to be

analysed Hence the accuracy of Transducer used in harmonic measurement is very

important. Measurement of the harmonic current is performed by using current

transformer up to 50th harmonic without any problem.

I) Capacitive voltage transformer offers considerable error in the transformation

ratio at harmonic frequencies .The error is significant even for harmonics of

order below 25.

II) Potential transformer

III) Switchable capacitive voltage divider (CVD) . For each phase of the system with

capacity to reproduce harmonics other than 50Hz accurately .

IV)The AC system measurement in the frequency domain are carried out by a

harmonic analyzer equipped with grade signal conditioners and ADC. The

harmonic analyzer are microprocessor based high precession instrument using fast

fourier techniques to analyze the input signal.

Different types of instruments available for harmonic measurement are :

Power profiler BME 303A : It can measure and print in addition to various

electric quantities voltage and current total harmonic distortion and

frequency.

Power network analyzer DIP8000 : It is portable three phase network

analyzer designed for power surveys in which the measured data are

transformed to PC for evaluation and documentation.

Measurement setup : The measurement is done via high quality probe. The

computer loads the waveforms of graph sheets.

Memory hicorder 8840 (Hioki make) :It is used to measure three line

currents and there line to voltage.

Dynamic signal analyzer model Hp3561 : The parameters such as

fundamental frequency of A.C. current, line or phase voltage across potential

transformer.

HARMONIC ELIMINATION TECHNIQUES:

To avoid the ill effects of harmonics on the operation of sensitive

equipments, it is necessary to keep harmonic contents below safe limit by installing

filter at load end. The simplest way of eliminating harmonics of different orders is to

install filters at the location generated by different loads are connected in two ways in

power system network.

1.SERIES CONNECTED FILTERS: Such type of filters are connected in

series with power system network and offer high impedance at turning

frequencies high impedance offered by filters allow very little harmonics are

passed. The drawback of series filters are high cost, because the rating of

filter component required is rated full load current.

2.SHUNT CONNECTED FILTERS: It is most commonly used filters in A.C.

power system network and offers very low impedance path to harmonics. Shunt

type of filters are cheaper than series type because the shunt connected filters

are designed for graded insulation levels which makes the components cheaper

than the series filter components.

Following are the different techniques used to eliminate harmonics of different orders

to keep harmonic distortion within permissible limit.

PASSIVE FILTERS:

These are LC resonating or parallel resonating circuits which offer very high or low

impedance at tuning frequency. These filters are resistive at tuned frequency, capacitive

at below tuned frequencies and inductive beyond tuned frequency.

TYPES OF PASSIVE FILTERS:

1.Series passive filters: These are connected in series and offers very high

impedance to different harmonics at tuned frequency, because of its very high cost

such type of filters are not used.

2.Shunt passive filters: These are connected between line and earth and

offer very low impedance at resonant frequency. Hence particular harmonic or harmonics

directed to earth and prevented from passing further. The high pass shunt filters are

connected to the point of common coupling block all over the harmonic frequency and

passes all higher frequency.

3.Single tuned filters

4. Double tuned filters.

5. Triple tuned filters.

6. continuously tuned filters.

Passive techniques employs following devices:

1.Passive L-C shunt filters.

2.Power factor correction capacitor.

ADVANTAGES OF PASSIVE FILTERS

1.Simple in construction, less costly and efficient

2.Serves dual purpose harmonic filtration and power factor correction of

load.

DISADVANTAGES OF PASSIVE FILTERS:

1.Cannot function under saturated condition.

2.Number of passive filters installed must be equal to the number of harmonic

levels to be compensated.

3. Connection of passive filters necessities a specific analysis of each

installation.

4.Non adaptability to system variations .

5.Bulky in size.

6.Tendency to resonate with the other load.

2.ACTIVE FILTERS:

When the number of harmonics to be filtered, large no of branches of passive

filters will be required . The large no of branches of passive filters will be required. The actual

number of branches will depend upon no of harmonic level of branches will depend upon no

of harmonic level to be compensated. Hence, because of passive filter use for filtration of large

no of harmonics results in large size &more cost. Introduction of self commutated devices e.g.

MOSFETS, IGBT etc. Accelerated the research in design of active filter & resulted low cost,

high performance active filter suitable to eliminate the harmonics of different orders to

overcome the drawbacks of passive filters.

Active filters compensates voltage of current harmonic signal measured. The

injected voltage or current harmonic signal measured. The injected voltage or current harmonic

signals in to the power system network is of same magnitude and opposite in phase of the

measured harmonic signal . It comprises power converter and control loop which controls the

harmonics injection of the filter as the function of harmonic signal measure.

TYPES OF ACTIVE FILTERS :

1.SHUNT TYPE ACTIVE FILTER:

These devises are designed to constantly monitor the harmonics in the load current and same

harmonic equal in magnitude and opposite in phase thus cancel the original harmonics. These are

connected in parallel with harmonic generating non-linear load.

2.SERIES TYPE ACTIVE FILTER:

These are connected in series with network and non-linear load . it introduces a suitable voltage

through a matching transformer, voltage introduce by series filter is in such a way that the

supply sees a very high impedance for harmonic and low impedance for fundamental.

3.HYBRID FILTERS:

these are combination of passive and active filter to improve performance similar to active

filter –

with reduction in cost.

DISADVANTAGE OF ACTIVE FILTER:

Complex and costlier.

HARMONIC CANCELLATION:

The phase cancellation or cancellation technique is used in case of static power converters to

eliminate lower harmonic. To use the above technique the following conditions must be

satisfied.

1.Trasformerratio and impedance must be equal.

2.The delay angle of each pulse must be exactly same.

3.the loads must be equal.

ISOLATION TRANSFORMER;

These are used in isolating third harmonic that are common when delta to star configuration is

used.

HARMONIC BLOCKING:

When harmonic content is less, then harmonic blocking technique is used

to protect the most effected equipment e.g. capacitor bands used for power factor correction

offers very low impedance to high frequency harmonics, may get damaged because of large

current drawn due to thermal break down. In this case inductor or damping resistor is

connected in series to protect the capacitor bank.

HARMONIC SUPPRESSOR SYSTEM:

Moreover a novel control method called Harmonic suppressor system is designed in

which a new control method is used. It operates with sensing nonlinearities present in

the load. The compensation process involves novel technique which is achieved without

any complication and involves control logic. To achieve this current controlled logic.

These schemes either use

(1) a reactive volt ampere calculator to set the compensator current

reference which

generates a delay in compensation process and a low pass is also

required to eliminate the ripple from sensed D.C. link voltage which also

introduces a time delay or

(2)error between D.C link voltage reference and the sensed D.C. link capacitor voltage

to set the amplitude of the source current reference.

Single phase compensator and it’s operating principle are given below:

A single phase compensator is operated in a controlled current Boost type converter

mode.

The current drawn from the utility is made to flow a sinusoidal reference current

within a fixed hysterisis band. The height of this window determines source current

profile, it’s harmonic spectrum and switching frequency of the devices. The link

capacitor voltage is kept constant throughout the operating range of the compensator.

CONCLUSION:

Harmonic distortion is increasing day by day at a faster rate and is a matter of

concern to the utility, customer and manufacturers of different equipment. To keep the

harmonic distortion to low value, following actions are necessary.

In India it is necessary first to create awareness regarding harmonic problems, their

effects and elimination techniques among the utility, consumers and manufacturers of

different equipments to make power system less polluted.

The harmonic standards should be imposed on the equipment and should be

made mandatory to the manufactures and consumers. The equipment should strictly

comply to the harmonic standards before selling it in open market.

The utility should monitor the installation of high tension consumers periodically,

regarding the harmonic distortion and penalties should be imposed on customers

using equipments crossing specified limits.

Filters should made compulsory to H.T. consumers.

REFERENCES:

1. “Power quality issues standards &guide lines”, IEEE,Vol-32,May96

2. IEEE recommended practices and requirements for harmonic control in electric

power system standard 519-1992.

Document BySANTOSH BHARADWAJ REDDYEmail: help@matlabcodes.com

Engineeringpapers.blogspot.comMore Papers and Presentations available on above site