Transient overvoltage rating and BIL of the transformerless intelligent power substation

The overvoltage impulse protection of the transformerless intelligent power substation (TIPS) is explored in this paper. Two failure mechanisms, excess current causing thermal failure of the devices and excess component to ground voltage causing failure of the insulation, are considered. A variety of grounding, protection, and filter circuits were analyzed. It was found that the current and voltage do not exceed the limits of safety if the midpoint of the TIPS DC bus capacitor bank is grounded. A recommended protection scheme was proposed based on these results, consisting of a MOV at the input to the TIPS to divert surge currents and a reactive connection to ground to limit the device currents without resulting in a large voltage between the midpoint and ground. A large voltage was found to fall across the input filter, which should be designed with sufficient BIL to withstand this voltage.