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REACTIVE POWER
COMPENSATION
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CONTENTS
Introduction
Reactive power
Need for reactive power
Need for reactive power compensation
Shunt compensation
Series compensation
Static VAR compensators (SVC)
Static compensators (STATCOM)
Synchronous Condensor
Conclusion
References
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INTRODUCTION
Reactive power (VAR) compensation is defined as themanagement of reactive power to improve the performanceof ac systems. There are two aspects:-
a) Load CompensationThe main objectives are to :- i)
increase the power factor of the systemii) to balance the real power drawn from the system
iii) compensate voltage regulation
iv) to eliminate current harmonics.
b) Voltage Support The main purpose is to decrease thevoltage fluctuation at a given terminal of transmission line.
Therefore the VAR compensation improves the stability of acsystem by increasing the maximum active power that can betransmitted.
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WHAT IS REACTIVE POWER ?
Power is referred as the product of voltage andcurrent
i.e. power = V x I
In an ac transmission, when the voltage and current goup and down at the same time, only real power istransmitted and when there is a time shift betweenvoltage and current both active and reactive power aretransmitted.
when the average time is calculated, the average activepower exists causing a net flow of energy whereasaverage reactive power is zero. In the case of reactivepower, the amount of energy flowing in one direction isequal to the amount of energy flowing in the opposite
direction. That means reactive power is neitherwww.8051.in
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Power Triangle
Power factor = cos
= real power / apparent power
= kW/ kVA
Whenever there is a phase shift
between V and I we have:-
a) real power (kW)
b) reactive (imaginary ) power (kVAR)
c) The combination is a complex or
apparent power (kVA)=(kW) + (kVAR)
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WHY DO WE NEED REACTIVE
POWER?
In resistive loads the current produces the heat energy
which produces the desired output but incase of
inductive loads the current creates the magnetic field
which further produces the desired work. Therefore
reactive power is the non working power caused by themagnetic current to operate and sustain magnetism in
the device .
Reactive power (vars) is required to maintain the voltage
to deliver active power (watts)through transmissionlines. When there is not enough reactive power the
voltage sags down and it is not possible to deliver the
required power to load through the lines.
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IMPORTANCE OF REACTIVE
POWER
i)It is the circulating power that does no useful work which
results from the storage elements mainly inductors and
capacitors.
ii)It has a strong effect on the power factor and system
voltages and hence must be compensated to prevent
voltage problems.
LIMITATIONS :-
Reactive power does not travel very far and is necessaryto produce it close to the location where it is required .
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Need for Reactive Power
Compensation
Reactive power generated by the ac power source is
stored in a capacitor or a reactor during a quarter of a
cycle and in the next quarter of the cycle it is sent back
to the power source. Therefore the reactive power
oscillates between the ac source and the capacitor orreactor at a frequency equals to two times the rated
value (50 or 60 Hz). So to avoid the circulation between
the load and source it needs to be compensated .
Also to regulate the power factor of the system andmaintain the voltage stability we need to compensate
reactive power .
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Methods of Reactive Power
Compensation
Shunt compensation
Series compensation
Synchronous condensers
Static VAR compensators
Static compensators
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Shunt compensation
The device that is connected in parallel with the transmissionline is called the shunt compensator. A shunt compensator isalways connected in the middle of the transmission line. Itcan be provided by either a current source, voltage source ora capacitor.
An ideal shunt compensator provides the reactive power tothe system. The voltage equation is given as :-
P + jQ = -j4V(1-cos/2)/xSince the real part of the power is 0 therefore the shunt
compensator injects only the reactive power in the system. Shunt-connected reactors are used to reduce the line over-
voltages by consuming the reactive power, while shunt-connected capacitors are used to maintain the voltage levelsby compensating the reactive power to transmission line.
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Transmission line with shunt
compensation
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Series compensation
When a device is connected in series with the
transmission line it is called a series compensator. A
series compensator can be connected anywhere in the
line.
There are two modes of operationcapacitive mode ofoperation and inductive mode of operation.
A simplified model of a transmission system with series
compensation is shown in Figure .The voltage
magnitudes of the two buses are assumed equal as V,
and the phase angle between them is .
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Transmission line with series
compensation
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Static VAR compensators
A static VAR compensator (or SVC) is an electrical device for
providing reactive power on transmission networks. The term
"static" refers to the fact that the SVC has no moving parts (other
than circuit breakers and disconnects, which do not move under
normal SVC operation).
The SVC is an automated impedance matching device, designed to
bring the system closer to unity power factor. If the power system's
reactive load is capacitive(leading), the SVC will use reactors
(usually in the form of thyristor-Controlled Reactors) to
consume vars from the system, lowering the system voltage.
Under inductive (lagging) conditions, the capacitor banks are
automatically switched in, thus providing a higher system voltage.
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ADVANTAGES
a) Static VAR compensation is not done at line voltage; a
bank of transformers steps the transmission voltage
(for example, 230 kV) down to a much lower level (for
example, 9.5 kV).This reduces the size and number of
components.
b) They are more reliable .
c) Faster in operation .
d) Smoother control and more flexibility can be provided
with the help of thyristors.
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Static Compensator
The devices use synchronous voltage sources for
generating or absorbing reactive power. A
synchronous voltage source (SVS) is constructed
using a voltage source converter (VSC). Such a
shunt compensating device is called staticcompensator or STATCOM .
A STATCOM usually contains an SVS that is driven
from a dc storage capacitor and the SVS is
connected to the ac system bus through aninterface transformer. The transformer steps the ac
system voltage down such that the voltage rating of
the SVS switches are within specified limit.
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COMPARISON OF VI
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COMPARISON OF VI
CHARACTERISTICS OF SVC AND
STATCOM
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Synchronous Condensor
A device whose main function is the improvement of pf
of the electrical system is known as the synchronous
condensor. It is installed at the receiving end of the line .
When a synchronous condensor is introduced it supplies
the kVAR to the system , and hence the current is
reduced .
Therefore the losses are reduced and provides a better
efficiency . Hence more power can be delivered to the
load and improves the pf of the system.
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CONCLUSION
From all the previous discussion we can
conclude reactive power compensation is a
must for improving the performance of the ac
system. By reactive power compensation wecan control the power factor and reduce the
consumption of electricity.
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References
[1] Juan Dixon and Luis Moran - Reactive PowerCompensation Technologies.
[2] Yongan Deng - Reactive Power Compensation OfTransmission Lines.
[3] Hong Chan- Practices of Reactive PowerManagement and Compensation.
T.J Millen- Reactive Power Control in ElectricalSystems.
Canadian Electrical Association-Static Compensators
For Reactive Power Control.
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