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A PRESENTATION ON REACTIVE
K.S.S VENKATA SATYA NAVEEN
Need for reactive power
Need for reactive power compensation
Static VAR compensators (SVC)
Static compensators (STATCOM)
Reactive power (VAR) compensation is defined as the
management of reactive power to improve the performance
of ac systems. There are two aspects:-
a) Load Compensation – The main objectives are to :-
i) increase the power factor of the system
ii) 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 the
voltage fluctuation at a given terminal of transmission line.
Therefore the VAR compensation improves the stability of ac
system by increasing the maximum active power that can be
WHAT IS REACTIVE POWER ?
Power is referred as the product of voltage and
i.e. power = V x I
The portion of electricity that establishes and sustains
the electric and magnetic fields of alternating-current
equipment. Reactive power must be supplied to most
types of magnetic equipment, such as motors and
In an ac transmission, when the voltage and
current go up and down at the same time, only real
power is transmitted and when there is a time shift
between voltage and current both active and reactive
power are transmitted.
ANALOGY OF REACTIVE POWER
Why an analogy? Reactive Power is an essential aspect of
the electricity system, but one that is difficult to comprehend
by a lay man. The horse and the boat analogy best describe
the Reactive Power aspect. Visualize a boat on a canal, pulled
by a horse on the bank of the canal.
In the horse and boat analogy, the horse’s objective (real power)
is to move the boat straightly.
The fact that the rope is being pulled from the flank of the
horse and not straight behind it, limits the horse’s capacity to
deliver real work of moving straightly.
Therefore, the power required to keep the boat steady in
navigating straightly is delivered by the rudder movement
Without reactive power there can be no transfer of real power,
likewise without the support of rudder, the boat cannot move in a
Power factor = cosø
= 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)² + (kVAR)²
WHY DO WE NEED REACTIVE
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 the
magnetic current to operate and sustain magnetism in
the device .
Reactive power (vars) is required to maintain the voltage
to deliver active power (watts)through transmission
lines. 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.
Need for Reactive Power
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 or
reactor 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 and
maintain the voltage stability we need to compensate
reactive power .
Methods of Reactive Power
Static VAR compensators
The device that is connected in parallel with the
transmission line is called the shunt compensator. A
shunt compensator is always connected in the middle of
the transmission line. It can be provided by either a
current source, voltage source or a capacitor.
An ideal shunt compensator provides the reactive power
to 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 levels by compensating the reactive power to
Transmission line with shunt
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
There are two modes of operation – capacitive mode of
operation 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 δ.
Transmission line with series
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.
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
b) They are more reliable .
c) Faster in operation .
d) Smoother control and more flexibility can be provided
with the help of thyristors.
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 static
compensator 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 an
interface transformer. The transformer steps the ac
system voltage down such that the voltage rating of
the SVS switches are within specified limit.
Structure of STATCOM
is comprised of
Set of coupling
reactors or a step-
COMPARISON OF VI
CHARACTERISTICS OF SVC AND
Advantages of STATCOM
The reactive components used in the STATCOM are
much smaller than those in the SVC.
The characteristics of STATCOM are superior.
The output current of STATCOM can be controlled up to
the rated maximum capacitive or inductive range.
Reduction of the capacity of semiconductor power
converter and capacitor bank to one half of those for the
Better transient response of the order of quarter cycle.
Reduction of harmonic filter capacity.
Reduction of size of high value air-cored reactor.
Reduction of equipment volume and foot-print.
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
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.
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 we
can control the power factor and reduce the
consumption of electricity.
 Juan Dixon and Luis Moran -“ Reactive Power
 Yongan Deng -“ Reactive Power Compensation Of
 Hong Chan- “ Practices of Reactive Power
Management and Compensation”.
T.J Millen- “ Reactive Power Control in Electrical
Canadian Electrical Association-Static Compensators
For Reactive Power Control.