Introduction
Neutral Grounding Resistors are used to reduce problems such as insulation breakdown caused by transient over-voltages produced by arcing ground faults in ungrounded systems and damage to motors and switchgear caused by arcing in solidly grounded systems.
The two main methods of system neutral grounding are Low Resistance and High Resistance.
Low Resistance
The system will trip in the case of a line-to-ground fault.
The Neutral Grounding Resistor will limit the ground fault to a maximum of 100 to 1000 A (See note below). Zero-sequence Current Transformers and Ground Fault Relays will detect the fault and trip at 5 to 20% of the maximum ground fault current.
The Resistor is generally rated for 10 seconds with a maximum temperature rise of 760 °C.
The maximum ground fault current allowed by the resistor has to be large enough to positively actuate the applied ground fault relay.
200 to 400 A rated Neutral Grounding Resistors are generally used in 6.9 kV to 34.5 kV systems (Se note below).
100 to 400 A rated Neutral Grounding Resistors are generally used in 2.4 to 4.16 kV systems (See note below).
Once the current rating is determined, the Resistance or Ohmic Value of the resistor is calculated by dividing the Line to Neutral Voltage by the Current Rating.
i.e. for a 4.16 kV System Neutral Grounding Resistor rated at 400 A. The line to Neutral Voltage will be 4.16 kV /√(3) = 2400 V. The required resistance will be 2400 / 400 = 6 Ohms.
High Resistance
The system will alarm but not trip in the case of a Line-to-Ground fault. It is recommended for systems where power interruption resulting from single line-to-ground fault tripping is detrimental to the process.
The Neutral Grounding Resistor will limit the ground fault to a maximum of 5 to 10 A. Zero-sequence Current Transformers and Ground Fault Relays will detect the fault and alarm at 10 to 20% of the maximum ground fault current.
The resistor is rated for continuous duty with a maximum temperature rise of 375 °C.
The maximum ground fault current allowed by the resistor must exceed the total capacitance to ground charging current of the system and the vector sum of the system charging current plus the resistor current shall not exceed 8 A. (see System Capacitance to Ground Charging Current Calculation)
Once the current rating is determined, the Resistance or Ohmic Value of the resistor is calculated by dividing the Line to Neutral Voltage by the Current Rating.
i.e. for a 480 V System Neutral Grounding Resistor rated at 5 A. The line to Neutral Voltage will be 480 V /√(3) = 277 V. The required resistance will be 277 / 5 = 55.4 Ohms.
Note
In Medium Voltage Mine Power Systems Low Resistance is generally used with a Neutral Grounding Resistor that will limit the ground fault to a maximum of 25 to 50 A. This is necessary to limit the touch voltage to 100 V or less. Zero-sequence Current Transformers and Ground Fault Relays detect the fault and will trip at less than one third of the resistor rating. The resistor is rated for continuous duty with a maximum temperature rise of 375 °C.
Modern mine power systems can have a significant amount of distributed system capacitance and as will all Neutral Grounding Resistors the maximum ground fault current allowed by the resistor must exceed the total capacitance to ground charging current of the system and the vector sum of the system charging current plus the resistor current shall not exceed 8 A. (see System Capacitance to Ground Charging Current Calculation)
References
“Industrial Power Systems” Shoaib Khan, Sheeba Khan, Ghariani Ahmed
“System Neutral Resistance Grounding” Larry A. Pryor, P.E., GE Senior Specification Engineer
“Detrimental Effects of Capacitance on High-Resistance-GroundedMine Distribution Systems” Joseph Sottile, Senior Member, lIEEE, Steve J. Gnapragasam, Thomas Novak, Fellow, IEEE, and Jeffrey L. Kohler, Senior Member, IEEE