- Overvoltage Protection Definition: Overvoltage protection is defined as measures taken to prevent electrical systems from damage due to excessive voltage levels.
- Causes of Overvoltage: Overvoltages can be caused by lightning, switching operations, insulation failure, arcing ground, and resonance.
- Switching Impulse: When a no-load transmission line is suddenly switched on or off, it can create a transient overvoltage in the system.
- Lightning Impulse: Lightning can cause very high overvoltage surges that are highly destructive and need to be prevented.
- Methods of Protection Against Overvoltage: Protection methods include earthing screens, overhead earth wires, and lightning arresters.
Electrical power system equipment can be exposed to abnormal voltages caused by lightning, switching operations, load rejection, earth faults or resonance. If the voltage magnitude and duration exceed an equipment item’s coordinated withstand level, they can damage its insulation and insulators.
Engineers use overvoltage protection to limit these stresses and coordinate them with the insulation strength of the system. Protection reduces the probability and severity of damage; it does not eliminate every overvoltage.
Voltage Surge
A voltage surge is a transient rise in voltage that reaches its peak over a short interval.
Power systems can also experience temporary overvoltages that last longer than a surge. Lightning generally produces fast-front transients, while switching operations generally produce slower-front transients. Earth faults, load rejection, resonance and ferroresonance can produce temporary or transient overvoltages, depending on the network.
In an electrical power system, the severity of an overvoltage depends on its peak, wave shape and duration. Its source impedance and location also matter. Switching and temporary overvoltages can be important, especially on high-voltage networks. Lightning surges can have a steep front and a high peak. An overvoltage protection scheme is therefore selected from an insulation-coordination study rather than from the source of the surge alone.
The following sections describe common causes of overvoltage and the main protection methods.
Switching Impulse or Switching Surge
Energising or reclosing a no-load transmission line launches travelling waves along the line. The peak voltage is not always twice the normal system voltage. It depends on factors such as the switching instant, line length, source impedance, trapped charge and breaker-pole timing. Interrupting load or chopping a small inductive current, including during some air blast circuit breaker operations, can also produce a switching surge. If insulation failure connects a live conductor to earth, the resulting fault and neutral displacement can raise the voltage on healthy phases.
A distorted EMF wave from an alternator can contain the 5th and higher harmonics. If one of these components is close to a network’s natural frequency, the interaction of inductance and capacitance can amplify the voltage.
Switching, fault and resonant overvoltages vary widely, so their magnitude must be assessed for the actual system.
Lightning can produce very steep travelling waves. Shielding, earthing and coordinated insulation work together to control the resulting risk.
Methods of Protection Against Lightning
Three common layers of lightning protection are:
- Earthing screen.
- Overhead earth wire.
- Lightning arrester or surge arrester.
Earthing Screen
An earthing screen is a network of grounded conductors placed above outdoor electrical substations. It intercepts many direct lightning strokes and carries their current into the substation earthing system.
This method of high voltage protection is simple, but it cannot limit a travelling wave that enters the substation through a feeder. Its performance also depends on the shielding geometry and the earthing system.
Overhead Earth Wire
An overhead earth wire, also called a shield wire, is installed above the phase conductors of an electrical transmission network. One or more grounded wires intercept direct strokes and carry lightning current through each transmission tower and its footing earth. Shielding reduces direct strikes to phase conductors, but poor tower earthing or a high lightning current can still cause back-flashover.
Lightning Arrester
Earthing screens and overhead earth wires help shield an electrical power system from direct lightning strokes, but they do not by themselves limit every travelling wave that reaches substation equipment.
A lightning arrester, now more commonly called a surge arrester, is connected between the line and earth near the equipment it protects.
A modern gapless metal-oxide arrester has a strongly nonlinear current-voltage characteristic. It presents high impedance at normal system voltage, conducts surge current as voltage rises, and limits the voltage across the protected equipment.
The functions of a lightning arrester or surge arrester are:
- At normal system voltage, the arrester presents high impedance and carries only a small leakage current, so it does not behave like a short circuit across the electrical insulator.
- During a surge, it conducts current to earth and limits the voltage at its terminals to its protective level.
- After the surge, the lightning arrester returns to high impedance and the power-frequency current falls to its normal leakage value.
Older power systems used rod-gap, horn-gap, multi-gap, expulsion and valve-type arresters.
For modern AC-system overvoltage protection, engineers commonly use a gapless ZnO lightning arrester. The arrester rating, energy duty, protective level, location, lead length and earth connection must suit the system and the equipment’s insulation level.
