Differences between isolated and grounded neutral systems

In electrical power systems neutral conductor can be grounded or isolated. The connection of neutral conductor has a substantial influence on line to earth and line to line with earth connection short circuits that are the most common faults in electrical systems. In this article we will describe the main differences between isolated neutral and grounded neutral systems in general, and base theory on ESMOGrid calculations in a multiple voltage levels network.

Isolated neutral system

In isolated neutral (ungrounded) systems no intentional path between neutral conductor and earth exists. On the other hand, there is a capacitive coupling between the power lines and ground, because of this, an enclosed contour to ground is established through the distributed capacitance. As line to earth short circuit current depends on the capacitive coupling, in bigger networks, especially with long underground cable connections, it can be relatively high. However, the biggest problem of the coupling capacity is overvoltage that appears during ground fault conditions.

This is because a ground fault on one line results in a full line-to-line voltage (instead of phase voltage) through the whole system and may cause insulation failure of system equipment.

The common application of the isolated neutral are medium voltage networks, where isolated neutral ensures high reliability, because it is not necessary to disconnect the whole system in case phase-to-ground short circuit appears.

Grounded neutral system

In systems with a grounded neutral a conductive path between neutral and ground is installed. Thus, because high conductivity enclosed contour is formed during line to ground fault, short circuit currents usually exceeds several kilo amps and are much greater that in isolated neutral systems. On the other hand, high short circuit current can be easily mitigated to any level by connecting a resistor in series to grounding conductor.

However, there is another important feature that makes grounded systems so widely used – neutral grounding ensures personnel safety from an electrical shock under all possible fault conditions.

This is partly because in grounded systems it is possible to use a variety of protection devices, and no potential difference between network equipment housing and ground is possible. What is more, because of high short circuit currents it is much easier to detect the fault location and to eliminate the cause.

Example on ESMOGrid

An example of multiple voltage levels system is shown in Fig. 1.

Fig. 1 An example of multiple voltage levels system in ESMOGrid. In this example winding 2 of transformer T1 is isolated and short circuit is at bus 9.

In Table 1 minimum short circuit currents under different neutral connection conditions are depicted:
Table 1 Short circuit calculations results

Neutral grounding type
Fault location
3 phase short circuit current
Line to line short circuit current
Line to ground short circuit current
Line to line with earth connection SC current
Phase voltages during line to ground fault location
Star connection with isolated neutral of transformer T1 winding 2
Bus 9
2.431 kA
2.105 kA
100.68 A
2.108 kA
Ua=127 mV;Ub=36.134 kV;Uc=36.216 kV; (normally 20 kV) 
Star connection with grounded neutral of transformer T1 winding 2
Bus 9
2.431 kA
2.105 kA
2.887 kA
2.793 kA
Ua=968 mV;Ub=18.58 kV;Uc=19.103 kV;
T1 and T3 windings 2 are grounded and T2 is with an isolated neutral of winding 2*
Bus 5
7.596 kA
6.578 kA
8.614 kA
8.382 kA
Ua=1 mV;Ub=217.83 V;Uc=224.19 V;
T3 winding 2 is grounded; T1 and T2 is with an isolated neutral of windings 2
Bus 5
7.596 kA
6.578 kA
0.003 A
6.656 kA
Ua=1 mV;Ub=404.88 V;Uc=404.88 V; (normally 230 V)

*This represents a common configuration of neutrals in power systems (when medium voltage networks are with isolated neutrals).

As one can see from Table 1 line to earth short circuit current significantly depends on neutral grounding type. For systems with an isolated neutral this current is relatively small, however, can exceed hundreds of amperes if considerable distributed capacitance between lines and ground exists.

It is also important to notice that in isolated neutral systems under phase to neutral fault conditions phase voltages become 1.73 times greater than under normal conditions.

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