Electricity + Control October 2018

EARTHING + LIGHTNING PROTECTION

Surge protection of substations

Jonathan Woodworth, Institute of Electrical and Electronics Engineers

Protecting substations from lightning and switching surges that lead to insulation flashover has been a key issue for as long as there have been power systems.

Take Note!

Surge sources are usually as a result of the following: 1. Direct lightning strikes to substation. 2. Switching surges on in- coming lines. 3. Switching surges created within substations 2 3

W hile this need has remained constant over 100+ years, options on how best to mitigate high voltage stresses have changed quite substantially. Considerations in substation protection • Failure rate: The acceptable number of insula- tion failures over the service life of the station. • Required Insulation Withstand Level (Urw): Given in terms of surge types such as fast, very fast, slow and temporary overvoltage. • Safety margins: Typically 15% for transformer windings and 5% for air insulation. • Separation distance: The distance between ar- rester and intended protected equipment. • Open breaker protection: While this is a low probability issue, for critical stations protection of an open breaker can be important. • Altitude: Surge performance of insulation at a substation is highly dependent on elevation since insulation withstand voltage reduces by ~11% with every 1000 m increase in altitude. • Insulation type: Some insulation can withstand flashover without damage and some cannot. Insulators in air are a self-restoring type insu- lation that depends on surrounding air for its insulation capability outside the insulator body. By contrast, transformer windings and all oil/ paper type insulation cannot recover from a disruptive discharge and are therefore consid- ered non-self-restoring insulation. Both primary and secondary transformer bush- ings are protected by default using the same ar- resters that protect transformer windings. If the transformer neutral exits the transformer through a bushing and is connected to a neutral ground- ing resistor, grounding resistor and bushing should both be considered for protection by an arrester. Assets requiring surge protection Transformer bushings

Cable terminations Cable terminations within a substation are often paired with arresters. But the protection in this case is more for the cable since the termination itself is self-recovering type insulation while the cable is not. Should cable insulation become dam- aged from a surge, repair of that asset will be a costly task. Breaker bushings and longitudinal insulation Breaker bushings and internal longitudinal insula- tion between contacts are also high value assets within a substation. Yet these are typically not as well protected as are power transformers, though they should be. More on this below. Metering equipment Voltage monitoring CCVTs and PTs are high in val- ue and, if present in a substation, their protection priority should be considered high. When design- ing substations, these devices are sometimes lo- cated where they actually become part of the pro- tection scheme due to their capacitance. Shunt capacitor banks These can absorb significant charge from a light- ning or switching surge without damage. There- fore, if a bank contains many MVar, it is not likely in need of surge protection. Smaller banks, however, may not be able to take a large surge without over charging and flashing over, so they often need pro- tection. Whether the bank is large or small, need for arrester protection of the system from the bank should also be evaluated Surge sources and mitigation methods Direct lightning strikes to substation Direct strikes to substations do occur although it is rare that a flash makes a direct hit on equipment. This is because stations are universally protected from these by overhead shield wires or masts.

30 Electricity + Control

OCTOBER 2018