Ground resistivity of the of multipolar ground (even the natural ground) to the neutral wire of the OPL in the year must not be higher than 5Ωor 10Ω, corresponding to the line voltage is 660V or 380V (phase voltage is 380V or 220V single-phase power supply in case).

For devices with isolated neutral, resistance of ground electrical equipment must not be larger than 4 Ω.

Chapter 6-7 Calculation of Earth Fault Current

Article 569. Calculation of Earth Fault Currents

Determination of maximum earth fault current

To calculate ground resistance at a substation, earth fault current which flow through the grounding

1. Earth fault current at power gird with directly grounded neutral point.

The bigger one among the calculated current in the following two cases is selected as the earth fault current which flows into the grounding system of thesubstation in the power girdwith directly grounded neutral point.

(1) Case of earth fault outside the substation

Earth fault current which flows into the grounding system in thesubstation consists of the earth fault current which flows through the earth and the earth fault current which flows into the substation from grounding wire of the transmission line connected to the substation. (Refer toigureF 573-1). Both earth fault currents flow to the power gird through the neutral point of the transformer.

Maximum earth fault current which flows through the grounding system is calculated by the

following equation. IE = (1-η ) IN

Where

IE: Maximum earth fault current which flows through the grounding system

η : Division factor between grounding wire and grounding device of transmission tower IN: Maximum earth fault current which flows to the neutral point of the transformer

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Earthing system of Substation

Figure 569-1 Earth fault outside substation

(2) Case of earth fault inside the substation

Earth fault current which flows out of the grounding system of the substation consists of the earth fault current which flows to the grounding system, that which flows to grounding wire and that which flows to the neutral point of the transformer.

The earth fault current that flows to the grounding system, which is used designfor for grounding system, is calculated by the following equation.

IE = (1-η ) (IF-IN)

Earthing system of Substation

Figure 569-2 Earth fault inside substation

Many different types of faults may occur in the systemIt. may be difficult todetermine which fault type and location will result in the greatest flow of current between ground grid and surrounding earth because no simple rule applies.

In determining the applicable fault types, consideration shall be given to the probability ofoccurrence of the fault. Multiple simultaneous faults, even though they may result in higher ground current, need

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not be considered if their probability ofoccurrence is negligible. It is recommended, for practical reasons, that investigation be confined to single-line-to-ground and line-to-line-to-ground faults.

In the case of a line-to-line-to-ground fault, the zero sequence fault current is

I0 = ( ) [ E( (R2 + jX)]2 )( ) ( )

R1 + jX1 R0 + R2 +3Rf + j X0 + X2 + R2 + jX2 R0 +3Rf + jX0 where:

I0 is the symmetrical rms value of zero sequence fault current in A E is the phase-to-neutral voltage inV

Rf is the estimated resistance of the fault in Ω (normally it is assumed Rf = 0) R1 is the positive sequence equivalent system resistance in Ω

R2 is the negative sequence equivalent system resistance in Ω R0 is the zero sequence equivalent system resistance in Ω

X1 is the positive sequence equivalent system reactance (subtransient) in Ω X2 is the negative sequence equivalent system reactance in Ω

X0 is the zero sequence equivalent system reactance in Ω

The values R1, R2, R0, X1, X2 and X0 are computed looking into the system from the point of fault. In case of a single-to-ground fault, zero sequence fault current is

I0 = 3Rf + R1 + R2 + R0E+ j(X1 + X2 + X0 )

In many cases, the effect of the resistance terms in equation above is negligible. For practical purpose, the following simplified equations are sufficiently accurate and more convenient.

Zero sequence current for line-to-line-to-ground fault

Zero sequence current for line-to-ground fault

Effect of overhead ground wires and neutral conductors

Where transmission line overhead ground wires or neutral conductors are connected to the substation ground, substantial portion of the ground fault current is diverted away from the substation ground grid. Where this substation exists, the overhead ground wires or neutral conductors should be taken into consideration in the design of the ground grid.

Connecting the substationground to overhead ground wires or neutral conductors, or both, and through them to transmission line structures or distribution poles, will usually have the overall effect

of increasing the GPR at tower bases, while lessening it at the substation. This is because each of the nearby towers will share in each voltage rise of the substation ground mat, whatever the cause, instead of being affected only by a local insulation failure or flashover at one of the towers.Conversely, when

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such a tower fault does occur, theffect of the connectedsubstation system should decrease the magnitude of gradients near the tower bases.

2. Earth fault current at power gird with isolated neutral point.

Earth fault current at power grid with isolated neutralpoint is based on actually measured earth fault current. If this measured value is not obtained, the earth fault current is calculatedwith reference to the equations shown in Table 573.

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Table 569 [Reference] Earth fault current at power grid with isolated neutral point