Voltage control and reactive power control are interrelated and need to be therefore considered together. One of the most troublesome features associated with the operation of overhead transmission system is the inherent variation of voltage at the receiving end, due to variations in load. These fluctuations of voltage are to be kept within the reasonable limits fixed by IE rules.
When power is supplied to a load through a transmission line keeping sending-end voltage constant, the receiving-end (or load) voltage undergoes variations depending upon the magnitude of the load and power factor of the load. The higher the load with smaller power factor the greater is the voltage variation. The voltage variation at a load is an indication of the unbalance between the reactive power generated and absorbed by that load. When the reactive power generated exceeds the reactive power absorbed, the voltage goes up and vice versa.
This problem is illustrated below:
Let P be the power to be transferred per phase in watts, Q the reactive power to be transferred per phase in VARs, Vs the sending-end voltage per phase, VR the receiving-end voltage per phase and (R + jX) be the impedance of transmission line per phase in ohms.
From phasor diagram shown is Fig. 13.31, we have:
Since in this the only variable quantity is Q, it is these reactive VARs which must be locally adjusted to keep this quantity fixed. The local generation of VARs can be obtained by connecting shunt capacitors or synchronous capacitors and or shunt inductors (for light loads or capacitive loads).
Alternatively the receiving-end voltage VR can also be kept constant for a given sending-end voltage Vs by keeping the product QX constant. This is achieved by introducing series capacitors which will reduce the net reactance of the system. Since the voltage variation will be more for larger loads (larger reactive power), the variation could be controlled by switching- in suitable series capacitors.
Methods of Voltage Control:
The methods usually employed for voltage control are by use of:
ADVERTISEMENTS:
(i) Shunt capacitors and reactors
(ii) Series capacitors
Earlier the voltage control used to be affected by adjusting the excitation of the generator (alternator) at the generating (or sending) end. The larger the reactive power required by the load the more is the excitation to be provided at the generating end. This method proved useful in small isolated system where there was no local load at the generating end.
Also there are limits for the excitation as well. Excitation below a certain limit may cause instability of the system in case the machine is connected to a synchronous load. The excitation above a certain level will result in overheating of the rotor. Thus the voltage regulation by excitation control is restricted by the permissible voltage rise at the sending end and by the difficulty of designing efficient generating plant for a wide range of excitation.
ADVERTISEMENTS:
1. Shunt Capacitors and Reactors:
Shunt capacitors are employed across an inductive load whereas reactors are employed across capacitive loads or lightly loaded line. In both cases the effect is to supply the requisite reactive power to maintain receiving-end voltage constant (shunt capacitors across an inductive load supply part of the reactive VARs required by the load whereas shunt reactors connected across capacitive loads or lightly loaded lines absorb some of the lagging VARs).
Capacitors are connected either directly to a bus-bar or through a tertiary winding of the main transformer and are arranged along the route to minimize the losses and voltage drops. The drawback of this method of voltage control is that as the voltage falls the correction VARs also falls i.e., when it is most needed, its effectiveness falls. Similarly, on light loads when the corrective VARs requirement is comparatively less, the capacitor output is large.
2. Series Capacitors:
ADVERTISEMENTS:
Series capacitors are connected in series with the line conductors. They reduce the effect of inductive reactance between the sending end and receiving end of the line. One drawback of this method is that high voltage is produced across the capacitor terminals when short circuit current flows through them. Therefore special protective devices (such as spark gaps with high speed contactors) are used for the protection of the capacitors under such condition.
If the load VAR requirement is small, series capacitors are of little use. With series capacitors the reduction in line current is small. Hence if thermal considerations limit the current, little advantage is obtained and shunt compensation should be employed. The shunt capacitors improve the power factor of the system whereas the series capacitors have little effect on the power factor.
For long transmission lines where the total reactance is high, series capacitors are effective in improving stability of the system.