Earthing of the neutral

The neutral of a three-phase system is the star point of a transformer winding. How that point connects to earth decides the path of a current that returns through earth, and so the current of an earth fault. A pooling substation has two systems with their own neutrals: the high-voltage (HV) system of the grid and the medium-voltage (MV) collector system of the plant. This topic explains how the design earths each neutral and what the choice does to the fault currents, the equipment and the earth mat.

Read first: Symmetrical components

1The neutral and the earth-fault currentCopy link to The neutral and the earth-fault current

A fault between one phase and earth drives a current through earth back to the source. The current returns to the star points of the windings that are earthed. A winding with no earthed star point, such as a delta winding, carries no current to earth. The impedance of the return path is the zero-sequence impedance $Z_0$ of the network (Symmetrical components).

The design connects a neutral to earth in one of three ways: solidly, through a resistor, or through a reactor at a 765 kV line reactor. A solid connection gives the zero-sequence path the transformer's own impedance. The line-to-earth current is then of the order of the three-phase current. A resistor in the neutral adds three times its resistance to the zero-sequence impedance, since the three phases' zero-sequence currents flow through it together. The line-to-earth current then falls close to the resistor's rated current.

The design earths the transformers' HV neutrals solidly and their MV neutrals through a resistor. The earth mat is designed for the HV earth-fault current (Grid current). The MV earth-fault current is limited to a value the resistor is rated for.

2The design's transformer neutralsCopy link to The design's transformer neutrals

The interconnecting transformer (ICT) of a pooling substation at 66 kV to 400 kV has a star HV winding, a star MV winding and a delta tertiary. Its vector group is YNyn0 + d at the default project (Power transformers, Vector group and neutral earthing). The HV neutral is solidly earthed. The MV neutral is earthed through a neutral grounding resistor (NGR). The delta tertiary is closed, so it carries a circulating zero-sequence current, and the ICT is an earth path at the HV bus.

A 33/11 kV transformer is Dyn11. Its HV winding is a delta, with no neutral. It adds no earth path at the 33 kV bus, and the design places no transformer HV-neutral electrode. Its 11 kV star neutral is earthed through an NGR.

The design earths other neutrals solidly:

• the ICTs of a switching substation, on both sides, with no NGR
• the HV star of a generator transformer, whose generator side is a delta
• the neutral of a bus reactor

A line reactor at 765 kV is the exception at HV. Its neutral is earthed through a neutral grounding reactor, which the bill of materials (BOM) specifies to suit single-pole auto-reclosing (Reactive power equipment, The line reactor).

3The HV system: solid earthingCopy link to The HV system: solid earthing

At the HV bus, the zero-sequence impedance is the grid's path in parallel with the ICTs' earth paths. With $n$ ICTs, each of zero-sequence impedance $Z_{T0}$, the ICTs add $Z_{T0}/n$ in parallel with the grid (Short-circuit currents by IEC 60909-0, The impedances of the network). The ICTs' zero-sequence impedance is the ratio $k_0$ times the positive-sequence impedance, 0.9 by default.

The solid earth paths keep $Z_0$ near $Z_1$. At the default project's HV bus the line-to-earth current is 20.24 kA. The three-phase current of the network, without the plant's infeed, is 21.15 kA. The line-to-earth current is 96 % of it. The earth mat takes the HV design fault level as its design current, and its grid current follows from the earth-fault current (Grid current).

A 33/11 kV substation shows the effect of a delta HV winding. With Dyn11 transformers, $Z_0$ at the 33 kV bus is the grid's and the line's alone, 14.623 Ω against a $Z_1$ of 5.220 Ω. The line-to-earth current of the default 33/11 kV project is 2.51 kA, 62 % of the network's three-phase current of 4.02 kA.

4The MV system: resistance earthingCopy link to The MV system: resistance earthing

The MV bus is sectionalised, one section per transformer low-voltage winding, with the bus-section breakers normally open. Each low-voltage winding has its own NGR. A fault on one section therefore sees one winding and one NGR.

The NGR is rated by the current it passes at the MV phase voltage, $I_{\mathrm{NGR}}$, and by the time it carries that current. The design takes 1000 A for 10 s by default (Define, NGR rated current). Its resistance is the phase voltage over the rated current, $R_{\mathrm{NGR}}=(U_n/\sqrt{3})/I_{\mathrm{NGR}}$, with $U_n$ the nominal MV voltage. The BOM lists one NGR per low-voltage winding, specified as 33 kV, 1000 A for 10 s, 19.05 Ω at the default project.

The zero-sequence impedance of a MV section is the transformer's zero-sequence impedance referred to the MV side, in series with three times the resistance: $Z_{0,\mathrm{MV}}=3R_{\mathrm{NGR}}+Z_{T0}{t}^2$, with $t$ the ratio of the MV to the HV nominal voltage. The resistance dominates. In the default project $3R_{\mathrm{NGR}}$ is 57.16 Ω, while $Z_{T0}{t}^2$ is 0.744 Ω. The line-to-earth current of a MV section falls to 1097 A at $c_{\max }$, 4.5 % of its three-phase current of 24.25 kA.

The line-to-earth current is close to the rated current but above it at the maximum voltage factor, because the source voltage is $c_{\max }{U}_n/\sqrt{3}$. The checklist item on the MV neutral earthing therefore compares the calculated current at $c_{\max }$ with 1.15 times the NGR's rated current. CEA Safety Regulations 2023 permit impedance earthing to limit the fault current and set no numeric limit, so the rated current is the design's value.

5The ANSI / IEEE setCopy link to The ANSI / IEEE set

The ANSI / IEEE set studies the same networks: the ICTs' earthed HV stars with their delta tertiaries, and the MV sections with one neutral grounding resistor per low-voltage winding. Its deliverables write grounding for earthing. The station service transformers are Dyn1 in that set's convention.

IEEE C37.010-2016 calculates the currents as $E/Z$ in place of $E/X$ (§5.2.1). The resistance of the NGR would be lost from a calculation by reactance alone. In the 230/34.5 kV example at 60 Hz, the NGR of 1000 A is 19.92 Ω. The line-to-ground current on a MV section is 1055 A at $E$ = 1.058 pu. Its X/R ratio is 0.048, set by the resistor (Short-circuit duty by IEEE C37.010, The X/R ratio).

6What the neutral earthing changesCopy link to What the neutral earthing changes

The earthing of the neutrals enters the design in these places:

• the short-circuit study, through the zero-sequence networks at both buses
• the single-line diagram (SLD), which draws the HV neutral earthed and the MV neutral through the NGR
• the earth mat, which places an electrode at each ICT HV neutral, NGR and bus reactor neutral
• the BOM, with one NGR per low-voltage winding
• the checklist, with the MV earth-fault current against the NGR rating

The rated voltage of a surge arrester depends on the temporary overvoltage at its location, which follows from the neutral earthing. The design takes the arrester rating of its level table, and the checklist asks for it to be answered from the neutral earthing study (Insulation levels, The surge arrester rating).