CONTROL THEORIES FOR LOAD COMPENSATION

CONTROL THEORIES FOR LOADCOMPENSATION

The loads which cause uctuations in the supply voltage unbalanced and harmonics, d.c components require compensation.

Typical loads requiring compensation arc furnaces, induction furnaces, arc welders, steel rolling mills, winders, very large motors, which start and stop frequently

Other serious loads which degrade power quality adjustable speed drives which include power electronic circuitry, all power electronics based converters rectiers cyclo converters

Important aspects to be consider to provide the load compensation for power quality improvement. 1. Type of Load (unbalance , harmonics and dc component) 2. Real and Reactive power requirements (maximum, minimum and concurrence of maximum real and reactive power requirements in multiple loads) 3. Rate of change of real and reactive power etc.

To correct load with unbalance and harmonics, instantaneous load compensation methods are used. Theory of Load Compensation-fundamental relation ship between supply system ,the load and the compensator

Fundamental Theory of Load CompensationThe supply system can be modeled as a Thevenins equivalent circuit with an open circuit voltage and a series impedance, The compensator can be modeled as variable impedance or as a variable source (or sink) of reactive current. This modeling and analysis done is on the basis of steady state and phasor quantities are used to note the various parameters in system. compensator voltage regulation for single phase systemspower factor correction for single phase systems Phase Balancing and Power Factor Correction of Unbalanced Loads

Conventional methods of load compensation can eliminate only the fundamental reactive power and unbalance in the steady state. These kinds of compensators can be realized using passive LC lters and thyristor controlled devices. However, when harmonics are present in the system, these methods fails to provide correct compensation. To correct load with unbalance and harmonics, instantaneous load compensation methods are used. The two important theories for load compensation are Instantaneous Theory of load compensation known as pq theory ( The instantaneous reactive power is defined as a unique value for arbitrary three-phase voltage and current waveforms including all distorted waveforms) Instantaneous Symmetrical Component Theorypq theory, we shall rst recall the --0 transformation. For three-phase system shown in Fig. 1, the --0 transformation for voltages and currents are given below.

Three-phase balanced system is expressed using following voltages and currents.

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The instantaneous active, p(t)

The instantaneous active, p(t)

The instantaneous reactive power, q(t)

plo "instantaneous zero-phase sequence load powerpf0.- "instantaneous zero-phase sequence compensator power