Techniques of Voltage Control

Techniques of Voltage Control

The electrical energy generated in the generating station is supplied to the consumers through the network of transmission and distribution. For satisfactory operation of various loads at the consumer end, it must be supplied with fairly constant voltage. In order to avoid erratic operation or malfunctioning of appliances at the consumer end, the voltage at the consumer end must be controlled and kept within permissible limits.

Because of change in load on the power system, the voltage at user end goes on changing. With increase in load, the voltage drop in alternator synchronous impedance, transmission line, transformer impedance, feeders and distributors increases. With decrease in load, these drops increase. These variations in voltages are undesirable and must be kept within proper limits. This limit is generally ± 6 % of declared voltage at consumer end. In this section we will discuss some of the methods which are used for voltage control.

Consider the two bus system shown in Fig. 7.18.1. Let the line is having negligible resistance and consists of series reactance. For fixed sending end voltage and at the given receiving end voltage, the real and reactive powers are given by,

As the real power demanded by the load must be delivered by line

P_R = P_L

The varying real power demanded by load is met by consequent changes in torque angle δ.

The reactive power of the line should remain fixed at Q^S_R when |V_S| is fixed for the specified |V^S_R|. The line will hence operate with specified receiving end voltage

for only one value of Q_L given by

Q_L = Q^S_R  

But the loads in actual practice are normally lagging in nature such that VAR demand Q_L may exceed V^S_R from above equation for V^S_R it can be seen that for Q_L > Q^S_R the receiving end voltage must change from the specified value |VR| to some value | V^S_R | to meet demanded |V_R|. Hence

The above value |V_R| is less than | V^S_R | for Q_L > Q^S_R similarly |V_R| is greater than | V^S_R | for Q_L > Q^S_R

It can be seen that under light load condition the line capacitance causes VAR demand to become negative which results in receiving end voltage exceeding the sending end voltage.

The various methods employed for voltage control include

i) Use of series capacitors

ii) Use of shunt capacitors

iii) Use of static VAR sources

iv) Use of shunt reactors

v) Tap changing of transformers.

1. Reactive Power Injection

It can be seen from the previous section that to maintain the receiving end voltage at its specified value, a fixed amount of VARs (Q^S_R) must be drawn from the line. A local VAR generator must be used for conditions of varying VAR demand Q_L. The VAR balance equation at the receiving end is given as,

Q^S_R + Q_C = Q_L

The fluctuations in Q_L are absored by local VAR generator in such a way tht total VAR drawn by the line remain fixed at Q^S_R The receiving end voltage is therefore maintained at fixed value of | V^S_R |. This is shown in Fig. 7.18.2.

This is nothing but compensation of VAR which can be made automatic by taking signal from VAR meter installed at receiving end. Normally two types of VAR generators are used in practice viz static type and rotating type.

2. Static VAR Generator

It consists of bank of 3 phase static capacitors and / or inductors. From the Fig. 7.18.3 we can see that X_C is reactance per phase of the capacitor bank.

When the load on power system is high then positive VARs are required. In this case capacitor banks are used while when light load is there on the system, negative VARs are required then inductor banks are switched on.

If it is required to have a smooth control of VAR then Silicon Controlled Rectifier (SCR) may be used. With the considerable harmonics say fifth harmonics may result in overloading of capacitors. At harmonic frequencies there is chance of series resonance to occur. The capacitors act as short circuit when they are switched on.

It can also be seen that the variation of Q_C is proportional to V²_R. So under heavy load condition when voltage decreases Q_C may not prove to be effective.

3. Rotating VAR Generator

It is nothing but a synchronous motor running at no load. The excitation of this motor can be adjusted over a wide range. In overexcited condition it supplies positive 

VARs whereas gives negative VARs when underexcited. The synchronous motor running under this condition is called synchronous condenser. It is shown in Fig. 7.18.4.

The synchronous condenser is connected to the receiving end bus bars and runs under no load condition. It takes negligibly small real power such that E_G and V_R are almost in phase. X_S is the synchronous reactance of the motor. The motor is having negligible resistance.

We have,

When |E_G | > | V_R | the machine is overexcited and provides positive VARs whereas |E_G| < | V_R |, the machine is underexcited the machine is underexcited and provides negative VARs. Thus positive and negative continuously adjustable VARs can be obtained with this method. At a given escitation the VAR injection is less sensitive to changes in |V_R , As |V_R | decreases, |E_G | - | V_R | increases with corresponding smaller reduction in Q_C.

From the above discussion it can be concluded that rotating VAR generator is more effective than static VAR generator. Hence it may be preferred. But its limiting factors are economic considerations, installation and maintainance problems.

4. Control by Tap Changing Transformer

This method is employed for narrow range of voltage control. Due to VAR demands of load, the receiving end voltage tends to decrease which can be raised by simultaneous tap changing on sending and receiving end transformers. It can be done either on no load or on load. Thus there are two types of tap changing transformers viz on load and on no load.

Consider the operation of a transmission line with a tap changing transformer at each end as shown in Fig. 7.18.5.

The impedances of the transformers are taken alongwith line impedances. These tap changing transformers do not control the voltage by controling the flow of VARs but by changing the transformation ratio, the voltage in the secondary circuit is varied  and voltage control is achieved. Let ts and tr are the fractions of the nominal transformation ratios i.e. tap ratio/nominal ratio. The product of t_s and t_r is taken as unity for ensuring uniformity in voltage level.

From the Fig. 7.18.5,

From the above equation it can be seen that for particular values of V_R and V_S and the load requirements P and Q the value of t_s can be determined.

Review Question

1. Discuss the methods of voltage control in transmission line.