Effect of Series and Shunt compensation on Voltage Stability

Jun 3 • Notes • 37609 Views • 4 Comments on Effect of Series and Shunt compensation on Voltage Stability

This article discusses Series and shunt compensation and their effects on voltage stability factors which is considered to be a  very important portion of power systems of electrical department. Many questions are put up in the academics as well as the in the interviews from this portion. To know the effect of series and shunt compensation on voltage stability, first of all we will have some discussion on compensation technique.

COMPENSATION TECHNIQUE:
By artificial injection of reactive power to the loads relieves the transmission network from reactive power flow  and  improves both the transmission efficiency and  operating  power factor. But by  artificial injection of negative  reactance  in the  lines relieves  the  lines from  excessive  voltage drop and  improves the  voltage regulation. Two methods are available for the injection that are static compensation and synchronous compensation. Capacitors and reactors involves Static compensation while synchronous compensation involves synchronous phase modifier. The series and shunt compensation are described below:

SHUNT COMPENSATION:
At the buses where reactive power demand increases the bus voltage is controlled  by  connecting capacitor banks in parallel to lagging load. Capacitor banks supply a part of or full reactive power of load, which reduces magnitude of the source current needed to supply load. While the voltage drop between sending end and load gets reduced. The power factor   improves and increases active power output which is available from the source.

Depending upon the load  demand the capacitor banks can be permanently connected to the system or it can be varied by switching on or off the parallel connected capacitors either manually or automatically. Figure below shows the single line diagram of a transmission line and its phasor diagram after the addition of the shunt capacitor.

Single line diagram of a Shunt Compensation transmission line

Voltage drop is VD = I_RR + I_XX_L – I_CX_L

The expression for the difference between the voltage drops is the voltage rises due to installation of capacitor is     VR = I_CX_L

The drawbacks of using shunt capacitors are as follows:

• The Shunt capacitors does not affect current or power factor beyond their point of application
• The reactive power which is supplied by the shunt capacitor banks is directly proportional to bus voltage
• When the reactive power is required less on light load the capacitor bank output will be high.

SERIES COMPENSATION: 
When the line has the value of high reactance to resistance ratio than the inductive reactance of the transmission line could be decreased by introducing series capacitors as a results in low  voltage drop. If a load with lagging power factor is connected at the end the voltage drop in the line is given by

VD = I(Rcosό + X_Lsinό)

If a capacitance C with reactance X_c is connected in series with the line then the reactance will reduced to [XL­Xc] and the voltage drop is reduced. And also the reactive power taken by the line is reduced. Figure below shows the equivalent circuit of the line with series compensation and its phasor diagram are presented

Single line diagram of a Series Compensation transmission line

From the phasor diagram the line voltage drop is,

VD = I(Rcosό + (X_L-X_C) sinό)

Hence the series capacitors is used to reduce the voltage drop in the lines with low power factor and also improves the voltage at the receiving end specifically with low power factor loads. For variable load  conditions the  voltage is controlled  by switching  in suitable series capacitors in the line.

IMPORTANT QUESTIONS:
Following are the important questions asked on effects of series and shunt compensations on Voltage Stability-

1- What is compensation technique?

2- Describe shunt compensation?

3- What are the drawbacks of using shunt compensation?

4- Write about series compensation?

5- Discuss the effect of Series and shunt compensation on Voltage stability.

6- Compare Shunt compensation with Series compensation.

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