Neutral Grounding Systems

Power system neutral grounding has been practiced since the beginning of power system development to hold the phase voltages stable with respect to ground. However, the problem of system grounding is often not given the required attention. The grounding of many systems has been based on past experience, or opinion, or an extension to the grounding methods in existing installations. For this reason, system neutral grounding practice is found to vary widely in industrial plants.
Problems encountered with cable systems, such as insulation breakdown resulting from the transient overvoltages caused by arcing ground faults in ungrounded systems, and the devastating arcing ground-fault damage to switchgear and motorcontrol centers (MCCs) in solidly grounded systems resulted in the widespread applicationof high-resistance (HR) and low-resistance (LR) grounding to 480 and 600 Vsystems in the late 1960s and the general provision of ground alarms on ungrounded systems, as discussed in Nelson and Sen’s IEEE paper .

This has led to the development of preferred methods of system neutral grounding for industrial power systems, which are summarized as follows:

• 6.9 kV–34.5 kV systems: Low-resistance (LR) grounding, using a 200–400 A rated resistor, is generally used at this voltage level. Ground-fault protection is provided by using zero-sequence current transformers (CTs), and the relays are set to trip at about 5–10% of the maximum ground-fault current. Resistors rated 1000–2000 A are used for systems that utilize overhead lines with bare conductors. For such systems, ground-fault relays are residually connected and are set to trip the lines at about 10–20% of the maximum ground-fault current.  High-resistance grounding (HR) using 5–15 A rated resistors is applied for generators. HR grounding is also applied for motors fed from captive transformers. The resistor current must exceed the total system charging current of the system to which the generator or motor is directly connected.

• 4.16 kV–2.4 kV systems: LR grounding, using 100 to 400 A rated resistors is generally used, and the ground-fault protection is provided with zerosequence CTs. HR grounding is used for systems where power interruption resulting from single line-to-ground faults is detrimental to the process. Ground fault on one phase will not require removal of the faulted circuit. The resistor current rating must exceed the system charging current, and the vector sum of system charging current plus resistor current shall not exceed 8.0 A. Above this value, the ground-fault current (if maintained) will escalate to phase-to-phase or three phase.

• 600 V or 480 V systems: These networks are relatively small and form separate subsystems, so the system charging current is usually less than 5.0 A. HR grounding is generally used for such low-voltage systems. Resistors rated at 3–5 A are common, and the system can be maintained for a ground fault on one phase.

• Grounding for mine power systems: LR grounding for a medium-voltage system using a 25 A or 50 A continuously rated resistor is used to limit the touch voltage IG × RG to 100 V or less. The ground-fault relay current pickup must not exceed one-third of the resistor rating. HR grounding is used for low-voltage systems using a 5 A continuously rated resistor.