Voltage ratings of the various buses in the power system which includes generating station buses, switching substation buses, receiving substation buses and distribution substation buses should be within the permissible limits for satisfactory operation of all electrical equipments. The task of voltage control is closely associated with fluctuating load conditions and corresponding requirements of reactive power compensation. Therefore several voltage control methods are employed in power system to keep the voltage levels within the desirable limits. In this article some of the voltage control methods in power system are discussed.
- Excitation control and voltage regulators at the generating stations:
- Use of tap changing transformers at sending end and receiving end of the transmission lines
- Switching in shunt reactors during low loads or while energizing long EHV lines
- Switching in shunt capacitors during high loads or low power factor loads
- use of series capacitors in long EHV transmission lines and distribution lines in case of load fluctuations
- Use of tap changing transformers in industries, substations, distribution substations
- use of static shunt compensation having shunt capacitors and thyristorized control for step-less control of reactive power
- Use of synchronous condensers in receiving end substations for reactive power compensation
All the above methods are suitably applied at different parts of the power system to maintain the voltage levels within the limits
Excitation Control and Voltage Regulation in generating Stations:
The induced emf of synchronous generator (E) depends upon the excitation current (field current). The terminal voltage V of synchronous generators are given byV = E – IX The generators have excitation and automatic voltage regulation systems (AVR). The function of this systems are:
- To control the load under steady state operating conditions for operating near steady state stability limit
- To regulate voltage under fault conditions (faults in the grid system beyond generator protection zone)
- To enable sharing of reactive power. The reactive power shared by a generator depends upon its excitation level The terminal voltage of the synchronous generator is held within the permissible limits by automatic voltage regulators (AVR) systems
Voltage Control by Tap changing in transformers:
The voltage control of transmission and distribution systems is obtained basically by tap-changing Tap changers are either on-load or off load tap changers. By changing the turns ratio of the transformer the voltage ratio and the secondary voltage is changed and voltage control is obtained. Tap changing is widely used voltage control method employed at every voltage level
The voltage control of the range + 15 to -15 % can be achieved by tap changing transformers
Off load tap changing voltage control:
Adjustment of voltage ratio can be made by off-circuit tap changing. These adjustments are usually for seasonal load variations of special operational requirement of local substations and adjusting the voltage in distribution transformer at consumer end.
On-Load tap changing voltage control:
Such an arrangement of on-load tap changing is employed for changing the turn-ratio of the transformer to regulate the system voltage while the transformer is delivering load.
Voltage Control by Shunt reactors:
Shunt reactors are provided at sending end and receiving end of the long EHV and UHV transmission lines. They are switched in when the line is to be charged or during line is on low load
When the line is on no load or low load, shunt capacitance predominates and receiving end voltage is higher than the sending end voltage. This phenomenon is called Ferranti effect.
The receiving end voltage of 400kV, 1000 km long line may be as high as 800kV. The shunt capacitance of such lines is neutralized by switching in the shunt reactor. During high loads, the series inductive reactance of the line produces IXL drop and the receive end voltage drops, the shunt reactors are switched off Shunt treactors may be connected to the low voltage tertiary winding of a transformer via a suitable circuit breaker, EHV shunt reactors may be connected to the transmission line without any circuit breaker.
Voltage Control by Shunt Capacitors:
Shunt capacitors are usually switched in during high loads. Static shunt capacitors are installed near the load terminals, in industries, substations, … Most of the industrial loads (induction motors, transformers, welding sets, furnaces) draws inductive current of poor power factor (0.3 to 0.6 lag). The shunt capacitors provide leading VARs there by the total KVA loading of substation transformer and the current is reduced. Thereby IXlL drop in the line is reduced and voltage regulation is improved. shunt capacitors are switched in when KVA demand on the distribution line goes up and voltage on the bus comes down. Switching in shunt capacitor should improve the bus voltage if the compensation is effective
Voltage Control by Static Shunt Compensation:
A step-less variable compensation is possible by thyristorized control of shunt capacitor and reactors. During heavy loads, the thyristors of the capacitor control are made to conduct for longer duration in each cycle. During low loads, the thyristors in reactor circuit are made to conduct for longer duration in each cycle. Thus a step-less variation of shunt compensation is achieved by means of static shut compensation
Voltage Control by Synchronous Condensers:
Synchronous condensers are over excited synchronous motors installed in the power system to deliver the reactive power. These synchronous phase modifiers are located near the load improves the voltage profile of the power system. The main advantage of synchronous phase modifiers are the ability to deliver the reactive power can be adjusted unlike static shunt capacitors.
Voltage Control by Series Capacitors:
In Extra High Voltage (EHV) or Ultra High Voltage System (UHV) systems series capacitors are connected in series with the transmission line to reduce the effect of inductive reactance XL between the sending end and receiving end of the line. One of the major drawbacks of series capacitors is that high over voltages are produced across the capacitor terminals under short circuit condition. Series capacitors are usually employed for increasing the power transfer capability of the transmission line and not for voltage regulation
Voltage Control by Flexible AC transmission (FACT) devices:
very long high power transmission lines have high series reactance XL and shut capacitance. It is difficult to control the voltage, power flow and stability by conventional manner. FACT devices play key role in high power interconnected systems. In every intermediate substation in transmission network FACT devices are installed
- Controllable Series Capacitor banks
- Controllable shunt compensation (SVS)
Thyristors are controlled by feedback control system. Voltage power flow and voltage angle is controlled