Voltage Control: In power system networks, while supplying power through a transmission line, we keep the voltage constant at the sending end. The voltage at the receiving end undergoes a change that does not depend on the load and power factor. Voltage control should therefore be considered in power system networks as the voltage changes while transmitting power from sending end to end.
In an overhead transmission system, one of the most inherent problems is voltage variation and to maintain the variations within a limit, we need certain provisions for voltage control.
Here we will discuss voltage control in power systems and its methods.
Need of Voltage Control in Power System
As discussed above, there is a variation of voltage while power is transmitted from one place to another in power system networks. In order to maintain the variations in voltage within a permissible limit, we employ methods to control voltage.
The voltage must be set within the permissible limits since most electrical devices and appliances are designed to work at a specific voltage, the need for constant voltage is very important. Wide variations in voltage can cause errors in the operation and performance of electrical devices.
In the context of Nepal, the voltage variation limit is ±10% for transmission and ±5% for the distribution systems.
Illustration of Voltage and Reactive Power Control
Consider a simple power system network where active power P and reactive power Q is to be transmitted from a generation station to the load.
Here, P and Q are active and reactive power per phase respectively.
Here, VS and VR are sending end and receiving voltages and R +jX is the impedance of the transmission line.
In the transmission line, the voltage loss is Vd = VS – VR.
From the phasor diagram of the above power system network,
Where, P = VRIcosΦ and Q = VRIsinΦ
In the above expression, the second term is small compared to the first term so we can neglect the second term and we have,
In transmission lines, resistance R is negligible in comparison to the reactance X so, Vd = XQ/VR.
From this relation, we can observe that the voltage drop depends on the flow of reactive power Q through the system.
In order to keep the voltage constant, the voltage drop Vd must be kept constant. In the expression of Vd, Q is the variable term, and to make Vd constant, the reactive power Q must be adjusted.
Adjusting Reactive Power for Voltage Control
For voltage control, reactive power flow should be adjusted and its reactive VAR is adjusted locally. In other words, locally adjusting the VAR means, generating the VAR in the system locally whenever it is required.
This could be achieved with the help of shunt capacitors or synchronous capacitors and also by shunt inductors in case of capacitive loads or light loads.
Adjusting the Receiving End Voltage Constant
During the voltage control process, we observed that the voltage drop between the sending end and receiving end of a power system network depends on the reactive power Q.
One way to adjust the receiving end voltage to a constant value is to locally generating VAR through a shunt capacitor or synchronous capacitor and shunt inductor.
Another way is by keeping the product of Q and X i.e. QX in the relation of the Vd constant. This is realized by connecting a series capacitor to the transmission line. This series capacitor reduces the net reactance of the line.
For larger loads, larger will be the reactive power and larger will be the voltage variations. By switching multiple series capacitors suitably, the variations can be controlled.
Methods of Voltage Control
For voltage control in power system networks following methods are employed:
1. Using Shunt Capacitor
We use a shunt capacitor in the case of inductive load. Shunt capacitors are positioned near the receiving end of the substation or industrial loads. In the case of inductive loads, the system’s power factor reduces, and IXL drop, i.e. inductive reactance drop, increases, causing voltage fluctuations. Shunt capacitors are suitably switched to compensate for the effect of inductive loads and line voltage is regulated.
2. Using Shunt Reactor
We use a shunt reactor in the case of a capacitive load. It is generally employed in the power system networks at the receiving ends of EHV (extra high voltage) and UHV (ultra-high voltage) transmission lines.
In long transmission lines, during lightly or no-load conditions, Ferranti Effect becomes predominant. In the Ferranti effect during no-load or lightly loaded conditions, the capacitance of the transmission line is predominant, and the receiving end voltage exceeds the sending end voltage.
The shunt reactor is an inductive component. By employing a shunt reactor in the power system network, the reactor compensates for the line capacitance thereby managing the voltage control, and voltage fluctuation is maintained.
3. Using Series Capacitors
The use of a series capacitor reduces the transmission line’s net reactance, thereby reducing the voltage drop between the sending end and receiving end.
The main disadvantage of this method is the production of high voltage across the capacitor during the event of a fault or short circuit. Due to this reaction, there should be provision for additional protection of the capacitor.
4. Using Tap Changing Transformer
In transmission and distribution networks of power system networks, tap changing transformers are used for voltage control.
In a tap changing transformer, the voltage on the secondary transformer can be adjusted by adjusting the tap or in other words, adjusting the number of spins on the transformer secondary emf.
The charge in the tap of the transformer results in a change in the number of turns on the secondary side of the transformer and thereby voltage can be changed and adjusted.
OFF-load and ON-load tap changing transformers are used for this purpose.