Introduction
Atmospheric lightning strikes on overhead lines, bare conductors, or metallic structures in outdoor substations, as well as overvoltages caused by switching operations of equipment and networks (switching overvoltages), pose significant hazards to electrical equipment. To protect equipment and facilitate insulation coordination, surge arresters (also known as "lightning arresters") must be installed at the incoming/outgoing points of overhead lines and in close proximity to transformers, due to their limited spatial protection zone.
Types and Characteristics of Surge Arresters
The most common surge arresters are non-linear metal oxide (MO) resistor type, housed in porcelain or silicone rubber. These are connected in parallel with the protected equipment and grounded via the earth grid. Another construction type uses silicon carbide (SiC) resistors (valve-type arresters), though these are less prevalent today.
Key Electrical Characteristics:
- Resealing Voltage: The voltage across the arrester at which the follow current is reliably interrupted after sparkover.
- Maximum Continuous Operating Voltage (MCOV): The highest power-frequency voltage (50 Hz or 60 Hz) that the arrester can withstand indefinitely.
- Rated Short-Circuit Current: The maximum short-circuit current the arrester can safely handle.
- Nominal Discharge Current: Common values include 5 kA, 10 kA, and 20 kA, indicating the arrester’s capacity to dissipate surge energy.
Surge arresters are connected between live conductors and ground. In installations with voltages above 52 kV, they may include discharge operation counters to monitor performance. An example of surge arresters is shown in Figure 1.
Additional Methods
In overhead lines and outdoor substations with voltages above 52 kV, it is common practice to install a lightning protection system comprising "lightning rods," "lightning aerial protection wires," or a combination of both.
LV Overvoltage Protection
Low Voltage (LV, where V≤1kV) equipment, particularly electronic and informatics systems, is highly susceptible to severe damage from lightning discharges that propagate through cables or building structures.
To mitigate such risks, power surge protectors (SPDs) are typically installed in LV switchboards. These devices feature standard nominal discharge currents of 5 kA, 10 kA, and 20 kA, with some advanced models capable of handling 30–70 kA.
Similar to surge arresters, SPDs are connected between live conductors and ground, as illustrated in Figure 4. This configuration diverts surge currents away from sensitive equipment, ensuring protection against overvoltage events.
