Surge Protective Properties of Medium Voltage Underground Cable

Lightning and switching surges are the most common causes of electrical failures of distribution transformers. Fast rising transients that reach the transformer can cause large turn-to-turn voltages at the line end of the windings and resonances, which result in large voltages to ground elsewhere in the windings, many of which may exceed the winding insulation strength. Field experience indicates that resistor-capacitor devices are effective in preventing switching induced insulation failures by introducing significant damping into the circuit. Since shielded underground cable has the ability to absorb high frequency energy, it may be able to provide similar protection to transformers. During this investigation, impulse and switching tests were performed on representative EPR and TR-XLPE insulated cables connected to distribution transformers. This analysis was directed toward quantifying the attenuation characteristics of different types of cables vis-a-vis fast fronted lightning impulse and switching induced breaker re-ignition transients. The conclusions of this report point to an economical way for protecting transformers from fast fronted voltage transients