Overvoltage Protection

Electrical power system can experience abnormal overvoltages due to various reasons like sudden interruption of heavy loads, lightning impulses, and switching impulses. These overvoltage stresses can damage the insulation of equipment and insulators. Even if not all overvoltages are strong enough to cause damage, they should still be avoided to ensure the smooth operation of the electrical power system.
These all types of destructive and non destructive abnormal over voltages are eliminated from the system by means of over voltage protection.

Voltage Surge

The over voltage stresses applied upon the power system, are generally transient in nature. Transient voltage or voltage surge is defined as sudden sizing of voltage to a high peak in very short duration.
The voltage surges are transient in nature, that means they exist for very short duration. The main cause of these voltage surges in power system are due to lightning impulses and switching impulses of the system. But over voltage in the power system may also be caused by, insulation failure, arcing ground and resonance etc.

Voltage surges in the electrical power system from switching surges, insulation failures, arcing ground, and resonance are usually not very large. These overvoltages rarely exceed twice the normal voltage level. Proper insulation can generally prevent damage from these surges. However, overvoltages from lightning are much higher and can cause severe damage if protection is not provided. Thus, overvoltage protection devices are mainly used to guard against lightning surges.

Let us discuss different causes of over voltages one by one.

Switching Impulse or Switching Surge

When a no load transmission line is suddenly switched on, the voltage on the line becomes twice of normal system voltage. This voltage is transient in nature. When a loaded line is suddenly switched off or interrupted, voltage across the line also becomes high enough current chopping in the system mainly during opening operation of air blast circuit breaker, causes over voltage in the system. During insulation failure, a live conductor is suddenly earthed. This may also caused sudden over voltage in the system.

If the EMF wave produced by an alternator is distorted, resonance can occur due to 5^th or higher harmonics. At these frequencies, the system’s inductive and capacitive reactances cancel each other out, making the system purely resistive. This phenomenon, known as resonance, can significantly increase the system voltage.
But all these above mentioned reasons create over voltages in the system which are not very high in magnitude.
But over voltage surges appear in the system due to lightning impulses are very high in amplitude and highly destructive. The affect of lightning impulse hence must be avoided for over voltage protection of power system.

Methods of Protection Against Lightning

These are mainly three main methods generally used for protection against lightning. They are

1. Earthing screen.
2. Overhead earth wire.
3. Lighning arrester or surge dividers.

Earthing Screen

An earthing screen is commonly used over electrical substations. This involves mounting a net of GI wire over the substation, with these wires grounded through the substation structures. This creates a low-resistance path to the ground for lightning strokes.

This method of high voltage protection is very simple and economic but the main drawback is, it can not protect the system from travelling wave which may reach to the sub-station via different feeders.

Overhead Earth Wire

This method of over voltage protection is similar as earthing screen. The only difference is, an earthing screen is placed over an electrical sub-station, whereas, overhead earth wire is placed over electrical transmission network. One or two stranded GI wires of suitable cross-section are placed over the transmission conductors. These GI wires are properly grounded at each transmission tower. These overhead ground wires or earth wire divert all the lightning strokes to the ground instead of allowing them to strike directly on the transmission conductors.

Lightning Arrester

The previously discussed two methods, i.e. earthing screen and over-head earth wire are very suitable for protecting an electrical power system from directed lightning strokes but system from directed lightning strokes but these methods can not provide any protection against high voltage travelling wave which may propagate through the line to the equipment of the sub-station.
The lightning arrester is a devices which provides very low impedance path to the ground for high voltage travelling waves.
The concept of a lightning arrester is very simple. This device behaves like a nonlinear electrical resistance. The resistance decreases as voltage increases and vice-versa, after a certain level of voltage.

The functions of a lightning arrester or surge dividers can be listed as below.

1. Under normal voltage level, these devices withstand easily the system voltage as electrical insulator and provide no conducting path to the system current.
2. On occurrence of voltage surge in the system, these devices provide very low impedance path for the excess charge of the surge to the ground.
3. After conducting the charges of surge, to the ground, the voltage becomes to its normal level. Then lightning arrester regains its insulation properly and prevents regains its insulation property and prevents further conduction of current, to the ground.

There are different types of lightning arresters used in power system, such as rod gap arrester, horn gap arrester, multi-gap arrester, expulsion type LA, value type LA.
In addition to these the most commonly used lightning arrester for over voltage protection now-a-days gapless ZnO lightning arrester is also used.