Power System Voltage Control

 The control of voltage levels is accomplished by controlling the production, absorption, and flow of reactive power at all levels in the system. The generating units provide the basic means of voltage control; the automatic voltage regulators control field excitation to maintain a scheduled voltage level at the terminals of the generators. Additional means are usually required to control voltage throughout the ystem 

The devices used for this purpose may be classified as follows:

1. Sources or sinks of reactive power, such as shunt capacitors, shunt reactors,synchronous condensers, and static var compensators (SVCs).                                                                                                                                                                          
2. Line reactance  compensators, such as series capacitors.
3. Regulating transformers, such as tap-changing transformers and boosters.

Shunt capacitors and reactors, and series capacitors provide passive compensation. They are either permanently connected to the transmission and distribution system, or switched. They contribute to voltage control by modifying the network characteristics.

Synchronous condensers and SVCs provide active compensation; the reactive power absorbed/supplied by them is automatically adjusted so as to maintain voltages  at other locations in the system are determined by active and reactive power flows through various circuit elements, including the passive compensating devices.

The following is a description of the basic characteristics and forms of application of devices commonly used for voltage and reactive power control.

• Shunt reactors

Shunt reactors are used to compensate for the effects of line capacitance, particularly to limit voltage rise on open circuit or light load.

They are usually required for EHV overhead lines longer than 200 km. A shorter overhead line may also require shunt reactors if the line is supplied from a weak system (low short-circuit capacity). When the far end of the line is opened, the capacitive line-charging current flowing through the large source inductive reactance (Xs) will cause a rise in voltage Es at the sending end of the line. 

A shunt reactor of sufficient size must be permanently connected to the line to limit fundamental-frequency temporary over voltages to about 1.5 pu for a duration of less than 1 second. Such line-connected reactors also serve to limit energization over voltages (switching transients). Additional shunt reactors required to maintain normal voltage under light-load conditions may be connected to the EHV bus .

During heavy loading conditions some of the reactors may have to be disconnected. This is achieved by switching reactors using circuit-breakers.

For shorter lines supplied from strong systems, there may not be a need for reactors connected to the line permanently. In such cases, all the reactors used may be switchable, connected either to the tertiary windings of transformers or to the EHV bus. in some applications, tapped reactors with on-voltage tap-change control facilities have been used, to allow variation of the reactance value. 

Shunt reactors are similar in construction to transformers, but have a single winding (per phase) on an iron core with air-gaps and immersed in oil. They may be of either single-phase or three-phase construction.

• Shunt Capacitors 

Shunt capacitors supply reactive power and boost local voltages. They are used throughout the system and are applied in a wide range of sizes.

Shunt capacitors were first used in the mid-1910s for power factor correction. The early capacitors employed oil as the dielectric. Because of their large size and weight, and high cost, their use at the time was limited. In the 1930s, the introduction of cheaper dielectric materials and other improvements in capacitor construction brought about significant reductions in price and size. The use of shunt capacitors has increased phenomenally since the late 1930s. Today, they are a very economical means of supplying reactive power. The principal advantages of shunt capacitors are their low cost and their flexibility of installation and operation. They are readily applied at various points in the system, thereby contributing to efficiency of power transmission and distribution. 

The principal disadvantage of shunt capacitors is that their reactive power output proportional to the square of the voltage. Consequently, the reactive power output reduced at low voltages when it is likely to be needed most.

• Series Capacitors 

Series capacitors are connected in series with the line conductors to compensate for the inductive reactance of the line. This reduces the transfer reactance between the buses to which the line is connected, increases maximum power that can be transmitted, and reduces the effective reactive power loss. Although series capacitors are not usually installed for voltage control as such, they do contribute to improved voltage control and reactive power balance. The reactive power produced by a series capacitor increases with increasing power transfer; a series capacitor is self-regulating in this regard.

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