Advanced concepts in high resistance grounding

Resistance grounding is relatively simple and easy to apply in radial distribution systems at Low Voltage. When high resistance grounding is applied, using one Neutral Grounding Resistor at the supply transformer, an alarm system to detect and indicate the ground fault is then required by the installation codes. This practice has been in use and is widely applied. This paper explores the application when the distribution systems involve multiple sources operating in parallel, such as multiple transformers, multiple generators or a combination. The sizing of NGR is explored and application of hybrid grounding is suggested for situations where Low resistance grounding needs to be used. In medium voltage systems 15 kV to 36 kV the practice has been to use very low resistance grounding. The paper suggests that the criteria for NGR sizing should be based on the net distributed charging current only. Application examples are presented showing the selective instantaneous feeder tripping and concept of hybrid grounding in Low and Medium voltage systems.