Compensation means the modification of the electrical characteristics of a transmission line in order to increase its power transmission capacity.
What is a Reactive power compensation device?
Any device which is connected in series or parallel with load and which is capable of supplying reactive power demanded by load is called a reactive power compensation device.
A compensation system ideally performs the following functions.
(1) It improves stability by increasing the maximum transmissible power.
(2) It helps produce a substantially flat voltage profile at all levels of power transmission
(3) It provides an economical means for meeting the reactive power requirements of the transmission system.
Compensations have been divided into four types :
(1) Passive compensation
(2) Active compensation
(3) Thyristor controlled capacitor
(4) Thyristor controlled reactor
Passive compensators include shunt reactors, capacitors and series capacitors. These devices may be either permanently connected (or) switched.
Active compensators are synchronous condensers only. These compensators are usually shunt-connected devices, which have the property of tending to maintain a substantially constant voltage at their terminals.
Reactive power compensation requirements of transmission line varies with line loading. Reactive power should be absorbed during low loads i.e. shunt reactors should be switched on.
Reactive power should be supplied during heavy loads i.e. shunt capacitors should be switched in.
Reactive power requirement increases as the length of the line.
Shunt Compensation :
Shunt capacitors and reactors are connected in parallel in the system and are used mainly for power factor improvement and in harmonic filters. It also boosts the voltage of the bus. This also known as load compensation.
Shunt Reactors :
Shunt reactors are provided at sending end and receiving end of long EHV and UHV transmission line.
When the line is on no load or lightly load the shunt capacitance predominate and receiving and voltage end is higher than the sending end voltage, which is called ferranti effect.
Shunt capacitors are switched on when VAR demands on the distribution line goes up and voltage off.
Shunt reactors are used to compensate the effects of line capacitance particularly to limit voltage rise on open circuit or light load.
The shunt reactors are used across capacitive loads or lightly loaded lines to control the voltage across the load.
Usually, oil-immersed magnetically shielded reactors with gapped cores are used for unswitched, switched, Thyristor controlled compensators.
The shunt capacitance of long transmission lines during loads or no loads are use to reduce receiving-end voltage.
The Shunt reactors are provided at sending end and receiving end of long EHV and UHV transmission lines.
For very long AC lines, shunt reactors are installed at an interval of about 250 km at intermediate substations.
When the line is on no-load or low load; the shunt capacitance predominate and receiving end voltage is higher than the sending end voltage (Ferranti effect).
The receiving end voltage of a 400 kV, 1000 km line may be as high as 800 kV. The shunt capacitance of such a line is neutralized by switching shunt reactor.
Shunt reactors may be connected to the low voltage tertiary winding of a transformer through a suitable circuit breaker.
Shunt Capacitor :
Shunt capacitor supply reactive power and increase local voltage.
Shunt capacitors are the capacitors connected in parallel with the lines.
They are installed near the load terminals, in receiving end substations, distribution substations and in switching substations.
Shunt capacitors inject reactive volt-amperes (VAr) to counteract some or all of lagging inductive VAr at the point of installation.
Normally they are arranging a three-phase bank.
The aim of power factor correction is to provide reactive power close to the point where it is being consumed, rather than supply it from a remote source.
shunt capacitors are used to compensate Blow boobs for the I2XL losses in the transmission system and to ensure better voltage levels during heavy loading conditions.
Location of Shunt Capacitor :
In transmission lines, shunt capacitors are connected either to the tertiary winding of the power transformers or to the bus bars.
Most of the industrial load draws inductive currents of poor power factor, shunt capacitors are with individual equipment to improve power factor.
The improved power factor reduces the KVA drawn from the supply, the power losses are reduced and the efficiency increased.