Part 5: AC Power Distribution Systems & Standards

Published by Nikola Zlatanov*

Medium Voltage Equipment Surge Protection Considerations

Transformers

If the voltage withstand/BIL rating of the transformer is less than that of the switchgear feeding the transformer, surge protection is recommended at the transformer terminals, in line with established practices. In addition, consideration should be given to using surge arresters and/or surge capacitors for transformers having equal or greater withstand/BIL ratings than that of the switchgear feeding the trans- former for distribution systems where reflected voltage waves and/or resonant conditions may occur. Typically incoming voltage surges are reflected at the transformer primary terminals (because of the change in impedance) resulting in voltages at the ends of the transformer primary terminals/windings of up to two times the incoming voltage wave. System capacitance and inductance values combined with the transformer impedance values can cause resonant conditions resulting in amplified reflected waves. Surge arresters/capacitors when required, should be located as close to the trans- former primary terminals as practical.

Motors

Surge capacitors and, where appropriate, surge arresters should be applied at the motor terminals.

Generators

Surge capacitors and station class surge arresters at the machine terminals.

Surge Protection

The distribution system can be subject to voltage transients caused by lighting or switching surges. Recognizing that distribution system can be subject to voltage transients caused by lighting or switching, the industry has developed standards to provide guidelines for surge protection of electrical equipment. Those guide- lines should be used in design and protection of electrical distribution systems independent of the circuit breaker interrupting medium. The industry standards are:

ANSI C62 Guides and Standards for Surge Protection
IEEE 242—Buff Book IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems
IEEE 141—Red Book Recommended Practice for Electric Power Distribution for Industrial Plants
IEEE C37.20.2

Surge Protection Recommendations

For circuits exposed to lightning, surge arresters should be applied in line with Industry standard practices.
Transformers:

Close-Coupled to medium voltage primary breaker: Provide transients surge protection, such as Surge Arrester in parallel with RC Snubber, or ZORC. The surge protection device selected should be located and connected at the transformer primary terminals or it can be located inside the switchgear and connected on the transformer side of the primary breaker.
Cable-Connected to medium voltage primary breaker: Provide transient surge protection, such as Surge Arrester in parallel with RC Snubber, or ZORC for transformers connected by cables with lengths up to 75 feet. The surge protection device should be located and connected at the transformer terminals. No surge protection is needed for transformers with lightning impulse withstand ratings equal to that of the switchgear and connected to the switchgear by cables at least 75 feet or longer. For transformers with lower BIL, provide surge arrester in parallel with RC Snubber or ZORC.

RC Snubber and/or ZORC damp internal transformer resonance:

The natural frequency of transformer windings can under some circumstances be excited to resonate. Transformer windings in resonance can produce elevated internal voltages that produce insulation damage or failure. An RC Snubber or a ZORC applied at the transformer terminals as indicated above can damp internal winding resonance and prevent the production of damaging elevated internal voltages. This is typically required where rectifiers, UPS or similar electronic equipment is on the transformer secondary.

Arc-Furnace Transformers— Provide Surge Arrester in parallel with RC Snubber, or ZORC at the transformer terminals.
Motors—Provide Surge Arrester in parallel with RC Snubber, or ZORC at the motor terminals. For those motors using VFDs, surge protection should be applied and precede the VFD devices as well.
Generators—Provide station class Surge Arrester in parallel with RC Snubber, or ZORC at the generator terminals.
Capacitor Switching—No surge protection is required. Make sure that the capacitor’s lightning impulse withstand rating is equal to that of the switchgear.
Shunt Reactor Switching— Provide Surge Arrester in parallel with RC Snubber, or ZORC at the reactor terminals.
Motor Starting Reactors or Reduced Voltage Auto-Transformers—Provide Surge Arrester in parallel with RC Snubber, or ZORC at the reactor or RVAT terminals.
Switching Underground Cables— Surge protection not needed.

Types of Surge Protection Devices

Generally surge protective devices should be located as closely as possible to the circuit component(s) that require protection from the transients, and connected directly to the terminals of the component with conductors that are as short and flat as possible to minimize the inductance. It is also important that surge protection devices should be properly grounded for effectively shunting high frequency transients to ground.

Figure 1.4-10. Surge Protection Devices

*Mr. Nikola Zlatanov spent over 20 years working in the Capital Semiconductor Equipment Industry. His work at Gasonics, Novellus, Lam and KLA-Tencor involved progressing electrical engineering and management roles in disruptive technologies. Nikola received his Undergraduate degree in Electrical Engineering and Computer Systems from Technical University, Sofia, Bulgaria and completed a Graduate Program in Engineering Management at Santa Clara University. He is currently consulting for Fortune 500 companies as well as Startup ventures in Silicon Valley, California.