Snubber circuit design for transformers in urban high rise office building

Transformer failures have in recent years led to the development of Resistor-Capacitor snubber circuits for the protection of the transformer and winding insulation from the damaging effects of highvoltage high-frequency transients. Transformer insulation may be damaged if the Basic Insulation Level (BIL) is exceeded, turn-to-turn insulation when there is excessive rate of change of voltage with time (dv/dt), and to switching devices by restrikes when the Transient Recovery Voltage (TRV) is exceeded. These transients are most often observed when dry-type transformers are close coupled to vacuum switching devices. Some manufacturers are now including snubbers in their transformer designs. This paper provides a thorough review of the causes of the transients, methods of analysis, and mitigation of the effects of these transients. An example is provided of transformers to be installed in the basement of an urban high rise office building, where the space is limited and the available fault current is high, where the transformer enclosure includes built-in snubber circuits. The strengths and weaknesses of current methods are examined. Recommendations are made for improvements in snubber circuit design and analysis.