An overview of the earthing system

Topics covered

  1. Firmly grounded system
  2. Resistance Grounded System
  3. Reason for resistance to earthing
  4. Grounding system at the EHV level

Firmly grounded system

Suppose Phase R is shorted to ground that:

  • Si = Current on short-circuited path ( Fault current )
  • In = Current flowing through the neutral connection to earth
  • Ice = Capacitive current returned via the Phase-2 network ( Phase Y ) – capacitors
  • IcB = Capacitive current returned via the Phase-1 network ( Phase B ) – capacitors

We can write:

Si = In + IcY + IcB + Ir 

Where Ir = Current returned by the network insulation resistance which is always negligible

In the event of LV, the system voltage available between phase and earth is 415 / 1.732 = 240V . The resistance of the earth plate, earth connections, etc.… is of the order of 1.5 Ohms so that the earth current is limited to approximately 240 / 1.5 = 160 amps . This magnitude is not very high and no intentional impedance is needed in a neutral connection to earth.

As in -08 equation because if you can see that if Glossy and ICB is negligible as If = What is the case in the BT system . At 415V the level capacitive earth currents are not significant and therefore we can write:

Substation-Earthing-System
Substation-Earthing-System

Si = In for solidly earthed LV system // equation-09

Resistance earthing System

In the case of an MV system ( 3.3 kV to 33 kV ) the voltage between phase and earth is high. Also capacitive load current is not large enough to compensate for the same, so the earth fault current may be excessive.

Therefore, the resistor is connected between neutral and earth. The current to neutral is limited to 100-400 amps .

Limitation of earth fault / neutral current

Although all MV level power system components are rated at full MV system fault level , for example:

  • Transformer winding,
  • Cables,
  • Bus ducts,
  • Machine rotation, etc.

What is protected by limiting the earth fault current / the current through the neutral?

The neutral of the transformer or generator are earthed by the impedance, the main element of which is resistance. This method is used when the earth current is too high if it is not restricted (for example) by MV generators. Here, a resistor is intentionally connected between neutral and earth. This is to limit the earth fault current.

Reasons to limit the earth current

The reasons for limiting the earth fault current are:

1. In rotating electrical machines like motors and generators, if the earth fault current is high, such as in the case of continuous earthing, the damage to the core would be great. To limit damage to the core, machine manufacturers only allow a limited earth fault current.

This is given in the form of a heart damage curve.

2. A typical value would be 25A-100A for 1 second . This value serves as a guide for selecting NGR and setting stator earth fault relays in generator protection.

3. Damage to windings in rotating electrical machines is not a serious problem (although windings are classified for the full fault level). Winding damage repairs can be done by the local winder. However, in the event of damage to the heart, repairs cannot be made on site. The machine must be returned to the manufacturer for repair, resulting in prolonged production losses.

Since rotating electrical machines are not present at voltage levels above 22 kV, these systems are usually solidly fused .

4. System X0 / X1 ratio also decides the type of neutral earthing. If the corresponding X0 / X1 ratio is within this predefined range. It is a choice between weatherproofing to cope with higher voltage or higher current in case of short circuit. Effectively earthing reduces the overvoltage limit of healthy phases while another phase is short-circuited to earth. But the earth fault current is very high.

This means that the system will need a large capacity circuit breaker, but the isolation system will need to have a moderate BIL rating.

But as the neutral impedance to earth increases, the earth fault current decreases, but the surge factor even increases up to 1.73 times! This therefore requires a circuit breaker with a low current capacity but a HIGH BIL for any insulation system.

Suppose Phase R ( Phase 1 in Figure 4 ) is shorted to ground that:

  • Si = Current on short-circuited path ( Fault current )
  • In = Current flowing through the neutral connection to earthing system
  • Ice = Capacitive current returned via the Phase-2 network ( Phase Y ) – capacitors
  • IcB = Capacitive current returned via the Phase-3 network ( Phase B ) – capacitors

By repeating Equation 8, we can write:

Si = In + IcY + IcB + Ir

Ignore Ir and replace it with the following:

In = -V1 / Rn ( The negative sign indicates that the capacitive charge and discharge current is in phase opposition with the current flowing through neutral )

IcY + IcB = Total capacitive load and healthy phase discharge current = j3CwV1 from equation-07

The representation of the phasor diagram will be:

So, finally, after substituting the expression In and IcY + IcB for the earth fault current in the MV system, we obtain:

If = -V1 / Rn + j3CwV1 // equation -10

The magnitude of the earth fault current will be:

| If | = | V1 | √ (I / Rn) 2+ 9C2w2

Grounding the system at EHV level

In case of HV system ( above 33kV ) the capacitive earth current is large enough to neutralize earth fault currents therefore no resistance is required in the neutral earth connection.

Strong foundations are universally adopted for the following reasons:

1. As we already understood that it is a choice between the weather conditions to deal with higher voltage or higher current in case of short circuit. At EHV level, if we go for a higher voltage than due to the high cost of insulation, selecting a higher voltage will not be a viable idea.

It is better to go for a higher current by choosing a solid ground.

2. The rotating machines are not present in the EHV system. It is therefore not useful to limit the earth fault current as in the MV system. Even though rotating machines are present due to higher voltage, the capacitive earth current is also large enough to neutralize the earth fault current.

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