Overvoltage protection is essential for ensuring the safe and reliable operation of electrical power systems.
It involves various methods to guard against transient and sustained overvoltages caused by lightning, switching, or insulation failures.
Understanding these protective measures helps prevent equipment damage and maintains system stability.
Key learnings:
Overvoltage protection involves actions taken to guard electrical systems from damage caused by too high voltage levels.
Overvoltage can result from lightning strikes, switching activities, insulation breakdown, arcing grounds, or resonance effects.
Switching impulse happens when a no-load transmission line is quickly turned on or off, creating a brief surge in voltage.
Lightning impulse refers to powerful voltage spikes caused by lightning, which are very harmful and must be controlled.
To protect against overvoltage, common methods include using earthing screens, overhead earth wires, and lightning arresters.
Electrical power systems can face abnormal overvoltages caused by factors such as sudden heavy load interruptions, lightning strikes, and switching surges.
These voltage spikes can harm the insulation of equipment and insulators. While not every overvoltage is strong enough to cause damage, preventing them is important for the reliable operation of the system.
Overvoltage protection methods are used to remove both harmful and less severe abnormal overvoltages from the system.
Overvoltages in power systems are usually short-lived and sudden. A voltage surge is a quick spike where voltage rapidly rises to a high peak and lasts only briefly.
These surges mainly come from lightning strikes and switching actions within the system. Other causes include insulation failures, arcing grounds, and resonance.
Surges from switching, insulation issues, arcing, and resonance are generally smaller, rarely going above twice the normal voltage. Proper insulation helps protect against damage from these. In contrast, lightning surges are much stronger and can cause serious harm if not properly protected against.
Therefore, overvoltage protection devices are primarily designed to defend against lightning-induced surges.
Next, we will look at each cause of overvoltage in detail.
When a no-load transmission line is suddenly switched on, its voltage can momentarily rise to twice the normal system voltage. This voltage spike is temporary. Similarly, when a loaded line is abruptly switched off, high voltage can appear across it. Current chopping during the opening of air blast circuit breakers also causes overvoltage.
Insulation failure, where a live conductor suddenly grounds, can produce sudden overvoltage as well. If an alternator’s EMF wave is distorted by higher harmonics (like the 5th), resonance may occur. At resonance, inductive and capacitive reactances cancel out, leaving only resistance, which can greatly raise system voltage.
These causes generally produce moderate overvoltages. In contrast, lightning impulses create very high and destructive voltage surges. Therefore, protecting the system from lightning-induced surges is crucial for effective overvoltage protection.
There are mainly three common methods used to protect against lightning:
An earthing screen is often used over electrical substations. It consists of a net of GI wire mounted above the substation, connected to the ground through substation structures. This offers a low-resistance path for lightning to safely reach the earth. This method is simple and cost-effective, but it cannot protect the system from traveling waves that may enter the substation via different feeders.
This protection method is similar to the earthing screen. The difference is that an earthing screen is placed over a substation, while overhead earth wires are installed over the electrical transmission network. One or two GI wires of appropriate thickness are positioned above transmission conductors and grounded at each transmission tower. These overhead earth wires direct lightning strikes safely to the ground, preventing direct hits on transmission lines.
The earthing screen and overhead earth wires protect the system from direct lightning strikes but do not guard against high voltage traveling waves that can travel along the line to substation equipment.
A lightning arrester provides a low-impedance path to ground for these high voltage traveling waves. It acts as a nonlinear resistor: its resistance falls when voltage rises above a certain level and increases as voltage drops.
Functions of a lightning arrester include:
Different types of lightning arresters used in power systems include rod gap arrester, horn gap arrester, multi-gap arrester, expulsion type LA, and valve type LA. The most commonly used lightning arrester today is the gapless ZnO lightning arrester.
Discover effective and affordable solutions to protect your electrical installations from harmful voltage surges. Check out our product lineup now!
Effective overvoltage protection is vital to shield electrical systems from destructive surges and ensure continuous, safe operation. By implementing proper devices like lightning arresters, earthing screens, and overhead wires, utilities can mitigate risk and enhance system resilience against overvoltage threats.
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