1 Explain techniques used by power companies to correct powe

1. Explain techniques used by power companies to correct power quality.

2. Explain why energy demand is cyclic.

Solution

1 .PF Correction Equipment • Black/green boxes with capacitors in them • PF correction capacitors • Harmonic filters Other PQ Solutions • Negative sequence current reduction • Neutral blocking filter • Surge protection • Soft starters • Zig-zag reactors • Harmonic mitigating transformers (HMT) • Conservation voltage reduction (CVR) equipment • Green plug

Dynamic voltage restorer (DVR)

DVR is a series connected device designed to maintain a constant RMS voltage across a sensitive load. The DVR consists of:- An injection / series transformer. A harmonic filter. A Voltage Source Converter (VSC). An energy storage A control system

Unified Power Quality Conditioner

Unified power quality control was widely studied by many researchers as an eventual method to improve power quality of electrical distribution system . The function of unified power quality conditioner is to eliminate the disturbances that affect the performance of the critical load in power system

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4.Non adaptability to system variations .

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5.Bulky in size.

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6.Tendency to resonate with the other load.

2.ACTIVE FILTERS:

When the number of harmonics to be filtered, large no of branches of passivefilters will be required . The large no of branches of passive filters will be required. The actualnumber of branches will depend upon no of harmonic level of branches will depend upon noof harmonic level to be compensated. Hence, because of passive filter use for filtration of largeno 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 toovercome the drawbacks of passive filters.Active filters compensates voltage of current harmonic signal measured. Theinjected voltage or current harmonic signal measured. The injected voltage or current harmonicsignals in to the power system network is of same magnitude and opposite in phase of themeasured harmonic signal . It comprises power converter and control loop which controls theharmonics injection of the filter as the function of harmonic signal measure.

TYPES OF ACTIVE FILTERS :

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1.SHUNT TYPE ACTIVE FILTER:

These devises are designed to constantly monitor the harmonics in the load current and sameharmonic equal in magnitude and opposite in phase thus cancel the original harmonics. These areconnected in parallel with harmonic generating non-linear load.

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2.SERIES TYPE ACTIVE FILTER:

These are connected in series with network and non-linear load . it introduces a suitable voltagethrough a matching transformer, voltage introduce by series filter is in such a way that thesupply sees a very high impedance for harmonic and low impedance for fundamental.

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3.HYBRID FILTERS:

these are combination of passive and active filter to improve performance similar to activefilter – 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 toeliminate lower harmonic. To use the above technique the following conditions must besatisfied.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 isused.

HARMONIC BLOCKING:

When harmonic content is less, then harmonic blocking technique is usedto protect the most effected equipment e.g. capacitor bands used for power factor correctionoffers very low impedance to high frequency harmonics, may get damaged because of largecurrent drawn due to thermal break down. In this case inductor or damping resistor isconnected in series to protect the capacitor bank.

HARMONIC SUPPRESSOR SYSTEM:

Moreover a novel control method called Harmonic suppressor system is designed inwhich a new control method is used. It operates with sensing nonlinearities present inthe load. The compensation process involves novel technique which is achieved withoutany 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 currentreference whichgenerates a delay in compensation process and a low pass is alsorequired to eliminate the ripple from sensed D.C. link voltage which alsointroduces a time delay or (2)error between D.C link voltage reference and the sensed D.C. link capacitor voltageto 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 currentwithin 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 theharmonic 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 mademandatory to the

manufactures and consumers. The equipment should strictly complyto 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 customersusing equipments crossing specified limits.

Filters should made compulsory to H.T. consumers