REACTIVE POWER COMPENSATION

AND VOLTAGE CONTROL

Module II
 Introduction
 A power system engineer has been encountering in the distribution
& transmission of power, a variety of problems such as voltage
variations with load, poor power factor, large losses,
electromagnetic and electromechanical oscillations followed by
disturbances, supply voltage distortions due to harmonics
generated by non-linear loads, interference with communications
and so on.
 Their intensities may differ but all these problems exist in the main
transmission, sub-transmission and distribution networks.
 VAR Compensation:
 Reactive power compensation by appropriate means has become
the most economically attractive and effective solution technically
for both traditional and new problems at different voltage levels in
a power system.
 VAR compensation near load centre has gained more importance
in recent times. It limits the flow of load reactive current in lines
and feeders, boosts the voltage, reduces KVA demand and leads to
both energy conservation and cost savings.
VAR Compensation:
VAR Compensation:
VAR Compensation: characteristics
 Reactive power compensation of the load for power factor
improvement.
 Stepless control of reactive power continuously matching with the
prevailing load requirements from time to time.
 Maintenance of rated voltage at the point of common coupling
within a narrow range irrespective of the load acting during the
day.
 Reduction in the main line / feeder current, the losses and to
conserve energy, throughout the day.
 Capacity to absorb line charging KVAr in very high voltage system
under light load conditions.
 In case the loads introduce harmonics, the compensator should
provide bypass paths for dominant harmonics and reduce the
distortion levels.
Traditional Methods of VAR
Compensation
Shunt Capacitors:
The use of shunt capacitors in conventional way through
mechanical switches has the following advantages:
 Overall cost is very low.
 The installation is simple requiring no strong foundations.
 Incur negligible losses
 Less maintenance problems
 More reliable in service with long life.
Shunt Capacitors:
However, notable short coming are:
 Not possible to vary reactive power matching with load
demand continuously (only step variation).
 There exist a possibility for harmonics, if present, to get
amplified.
 There also exists a scope for series / parallel resonance
phenomenon to occur, which requires to be investigated
prior hand.
Series Capacitors:
A capacitor bank can be interposed is a line to partially neutralize
the line reactance. Such an arrangement has the following
attractive features.
 It automatically provides reduction in line voltage drop with
increased loads.
 It increases the power handling capacity of a line by reducing the
transfer reactance.
 It reduces voltage flicker and damp out transient oscillations.
 Quite effective in maintaining the voltage profile.
However, it poses serious problems during faults, prone for
resonance phenomenon, complexity in control and likely to give
rise to sub-synchronous oscillations. Hence the series capacitors
can be installed after careful study only. They are employed widely
in HV lines and somewhat uneconomical for distribution
networks, as the requirements in both cases differ widely.
Static VAR Compensator:
 This essentially consists of capacitor bank in suitable steps
(operated through mechanical switches / thryristors) and
thyristor controlled reactor across it of the size of minimum
step.
 This combination yields step less variation of reactive power
over the entire range.
 When SVC is applied at a receiving station it is possible to
absorb line charging KVAr produced under light load
conditions. This will enable to avoid the over voltage
phenomenon under light loads.
Static VAR Compensator:
Static VAR Compensator:
The notable features of SVC are:
 Close matching of load reactive power
 Maintenance of power factor near unity
 Voltage control and reduction in losses
However, SVC has the following limitations.
 Switching of capacitor bank steps require appropriate
coordination.
 Complexity in the control of TCR.
 Generation of harmonics through TCR control
 Harmonic Filters:
 Most loads consume reactive power, highly non-linear and
generate harmonics. The twin problems, reactive power
compensation and harmonics reduction are carried out using
shunt passive filters.
 These are tuned LC circuits to provide low impendence paths
for dominant harmonics. They are quite effective in reducing the
total harmonic distortion levels.
 An appropriately designed filter scheme can provide low
impendence paths for harmonics and inject reactive power at
fundamental frequency.
 The tuning reactor in every filter also serves the purpose of
limiting inrush / outflow currents during switching operations.
Harmonic Filters:
 It is possible to choose the appropriate filter scheme at the
point of common coupling depending on the load, its pattern
of variation, harmonics present, reactive power compensation
at fundamental frequency so as to improve the power factor,
relieve the system from adverse effects due to harmonics and
improve the quality of power supply.
Harmonic Filters:
The advantages of shunt passive filters are:
 These are of relatively low cost, less complex, easy to operate
and reliable.
 Reduction in total harmonic distortion levels and
improvement in the quality of power supply.
 These have long life compared to active filters.
 Reactive power compensation and associated benefits similar
to the use of shunt capacitors.
 Reduction in metering errors, communication interference,
and heating of electrical apparatus.
Harmonic Filters:
The limitations in their application are:
 Capacitors and Reactors are to be specially designed.
 Every filter in the scheme has to be provided with protection
and control arrangement.
 The scope for possible series / parallel resonance exists and
should be avoided by careful study before implementation.
 These do not offer 100% solution for harmonic suppression
similar to active filters.
 Their performance is subject to parameter variations, ageing
etc. and precise tuning not possible.
Advanced Compensators: Series /
Shunt Compensator
Consider a transmission line with sources at either end,
provided with shunt and series compensator separately.
Advanced Compensators: Series /
Shunt Compensator
 A shunt compensator provided at the middle of a line, if
effectively controlled can maintain the voltages Vs and Vr
equal irrespective of the directions of P & Q flows.
 This type of ideal compensator doubles the power handling
capability, improves the power factor and maintains good
voltage profiles.
 It is quite effective in providing reactive power
compensation, improves steady state performance and damps
out the transient oscillations during disturbances. It is usually
a fast acting staticVAR compensator.
Advanced Compensators: Series /
Shunt Compensator
 On the other hand a series compensator interposed in the
transmission line as shown in fig. (b) either at sending end or
somewhere in the line is quite effective to provide partial
neutralization of line impedance and to reduce the voltage drop in
the line.
 This improves the power handling capability of the line and damps
out electromagnetic oscillations.
 However, as compared to shunt compensator, series compensator
is complex to control and protect, costly and must be carefully
designed to avoid sub synchronous oscillations.
 It has been established that a combination of shunt and series
compensators called hybrid scheme.
Advanced Compensators: Series /
Shunt Compensator
Advanced Compensators: Series /
Shunt Compensator
 Under simplify conditions of operation (neglecting shunt
paths). It is well known that, the relative magnitude difference
between |Vs| and |Vr| determines the direction and
magnitude of reactive power flow in the line.
 On the other hand the relative phase angle displacement
between Vs and Vr will determine the direction and magnitude
of real power flow.
 For example, if |Vs| > |Vr| and Vs leads Vr then both P & Q
flow from source-1 to source-2.
 If |Vs| < |Vr| and still leads Vr then P flows from soucr-1 to
source-2 and Q flows from source-2 to source-1.
Advanced Compensators: Series /
Shunt Compensator
 This clearly indicates that the magnitude of P & Q and their
directions of flow depend on the voltage magnitudes and
their phase angles.
 A variety of compensating devices both in series and shunt
forms have been developed over the years to achieve
complete control on a voltage profile, the magnitude and
directions of both P and Q flows.
 The schemes in vogue are STATCOM, power conditioners,
energy sourced inverters, in phase and quadrature boosters
and so on.