There is always a possibility of electric power system being affected due to abnormal over voltages. These abnormal overvoltages can occur due to various reasons such as sudden interruption of heavy load, lightening impulses, switching impulses etc. These overvoltage stresses can damage the insulation of various equipment and power system insulators. Although, not all overvoltage stresses are strong enough to damage system insulation, even these overvoltages must be avoided to ensure smooth operation of electrical power systems.
All these types of catastrophic and non-destructive abnormal over voltages are eliminated from the system by over voltage protection.
Over voltage stresses on a power system are usually transient in nature. A voltage transient or voltage surge is defined as a sudden increase in voltage to a high peak in a very short period of time.
Voltage surges are transient in nature, meaning they exist for a very short period of time. The main cause of these voltage surges in the power system is lightning and system switching impulses. But it can also be caused by over voltage in the power system, insulation failure, arcing ground and resonance etc.
Switching surges, insulation breakdowns, arcing grounds and resonances not too high in magnitude show voltage surges in the power system. These over voltages hardly exceed twice the normal voltage level. Generally, proper insulation of various power system equipment is sufficient to prevent any damage due to these over voltages. But due to over voltage in the power system, lightning is very high. If the power system is not protected from over voltage, the possibility of serious damage can be high. Therefore, all over voltage protection devices used in power systems are primarily due to lightning strikes.
Switching Impulse or Switching Surge:
When a no-load transmission line is switched on suddenly, the voltage on the line doubles the normal system voltage. This voltage is transient in nature. When a loaded line is suddenly closed or interrupted, the voltage across the line also increases significantly.Cutting the current in the system, mainly during the opening of the air blast circuit breaker, causes an over voltage in the system. Is. During an insulation failure, a live conductor is suddenly earthed. It can also cause sudden over voltage in the system.
Methods of Protection Against Lightning:
These are basically three main methods that are commonly used for lightning protection.
- Earthing screen.
- Overhead ground wire.
- A lightning arrest-or or surge divider.
Earthing screens are commonly used at electrical substations. In this arrangement GI wire mesh is laid at the substation. The GI wires, used for earthing screen, are properly grounded through various substation structures. This network of grounded GI wire at an electrical substation provides a low-resistance path to earth for electric shock.
This method of high voltage protection is very simple and economical but its main drawback is that it cannot protect the system from traveling wave which may reach the substation through various feeders.
Overhead Earth Wire:
This technique for over voltage insurance is like an earthing screen. That’s what the main distinction is, an earthing screen is put at an electrical substation, while an above earth wire is put at an electrical transmission organization. A couple of abandoned GI wires of reasonable cross-segment are put on the transmission directors. These GI wires are appropriately grounded at every transmission tower. These above ground wires or earth wires redirect all lightning strokes to the ground as opposed to permitting them to strike straightforwardly on the transmission transmitters.
The two techniques talked about before for example earthing screen and above earth wire are truly appropriate for shielding the electrical framework from coordinated lighting stroke however these strategies for safeguarding the framework from coordinated lighting stroke give no insurance against high voltage voyaging wave. Can’t give that can spread. through the line to the substation gear.
A lighting arrester is a gadget that gives a low-impedance way to ground for high-voltage voyaging waves.
The idea of a lightning strike is extremely straightforward. This gadget acts like a nonlinear electrical opposition. Obstruction diminishes as the voltage increments as well as the other way around, after a specific voltage level.
The functions of lightning arresters or surge dividers can be listed below.
Under normal voltage levels, these devices easily withstand the system voltage as electrical insulators and provide no path for system current to flow.
In the event of a voltage surge in the system, these devices provide a low-impedance path for excess charge to ground.
After the surge charges are carried to ground, the voltage returns to its normal level. The lightning rod then properly regains its insulation and prevents it from regaining its insulating properties and further conduction of current to ground.
There are different types of lightning arresters used in power systems, such as rod gap arresters, horn gap arresters, multi-gap arresters, exclusion type LA, value type LA.
Apart from these, the most widely used lighting arrestor for over-voltage protection today is the gapless ZnO lighting arrestor.