Coefficient of Grounding (COG) and Ground Fault Factor (GFF)

The grounding is very important for the power system to avoid the temporary overvoltage (TOV) on the healthy phases during ground fault and also to determine the level of ground fault current during an unbalanced fault event. The effective grounding can be measured by the coefficient of grounding (COG) and Ground Fault Factor (GFF) or Earth Fault Factor (EFF).

1. Coefficient of Grounding (COG)

The Coefficient of Grounding (COG) is the ratio of the highest unfaulted phase-to-ground voltage during a ground fault to the line-to-line voltage without the fault situation.

IEEE C62.92.1 Definition: COG is the ratio of the highest RMS line-to-ground power frequency voltage (VL-G) on a healthy phase at the selected location during phase-to-ground fault on any one or more phases to the line-to-line power frequency voltage (VL-L) at the selected location without the fault situation.

The COG can be expressed in terms of percentage and is given by,

If the system has COG ≤ 80 %, then the system shall be considered as effectively grounded. The COG for the ineffective grounding system measured at the fault location can go higher than 80 %.

Alternative Method: In an alternative way, the COG can be calculated using the system impedance and the fault impedance. Consider Z1, R1 & X1 are the positive sequence impedance, resistance and inductive reactance, and Z2, R2 & X2 are the negative sequence impedance, resistance and inductive reactance, and Z0, R0 & X0 are the zero sequence impedance, resistance and inductive reactance. This calculation is only applicable if Z1 = Z2. If a single line to ground fault occurs in Phase A, the COG for Phase B and C can be calculated using equation (2). if the double line to ground fault occurs in Phase B and C, then the COG for Phase A can be calculated using equation (3). Equation (4) indicates the k value for single and double line to ground fault without fault resistance. Equation (5) indicates the k value for a single line to ground fault with fault resistance. Equation (6) indicates the k value for the double line to ground fault with fault resistance. IEEE 62.92.1 approximates that the system is effectively grounded if X0/X1 ≤ 3 & positive, R0/X1 ≤ 1 and positive.

The sequence impedance Z1 = Z2 (positive sequence impedance is equal to the negative sequence impedance of the system at the fault location) may be true for the system which includes rotating machines, lines, cables and transformers. With the high penetration of renewable energy sources, the inverter-based sources are the dominant in modern power systems which may have Z1 ≠ Z2 at the fault location. Therefore, the COG calculation given in equations (2) – (6) is not valid if the inverters are the dominant source in the power system. In such a case, equation (1) can be applied to the COG calculation. Also, COG is useful for the selection of surge arrester at the selected location.

2. Ground Fault Factor (GFF)

Ground Fault Factor (GFF) or Earth Fault Factor (EFF) is the ratio of the highest unfaulted phase to ground voltage during ground fault to the phase to ground voltage without the fault situation. If COG ≤ 80% for an effective grounding system, then the GFF are lower than 138.56 % (GFF ≤ 138.56 %) for an effective grounding system. In actual value, the GFF ≤ 1.38 for an effective grounding system. The ground fault factor (GFF) is given by,

References:

• IEEE std C62.92.1.2016 – IEEE Guide for the application of neutral grounding in electrical utility systems-Part 1: Introduction.

• IEEE Std C62.22-1997 - IEEE Guide for the Application of Metal-Oxide Surge Arresters for Alternating-Current Systems.

• NYSERDA, EPRI, Effective Grounding for Inverter-Connected DER, 2021.