Neutral earthing

[lyledunn]

Why are liquid earthing resistors used on powerstaion generator earths and at other parts of the network where the voltage is relatively low, yet solid earths are used on 275Kv etc?

 

[gordonl]

In generating stations and industrial settings ground faults often destroy motors, generators, switchgear, etc. By limiting the ground fault current the damage is limited making repair or recovery faster and cheaper. I'm not into HV but I suspect solidly grounded is used because a typical HV ground fault would be high impedance over a long distance, so solid earthing would be needed to detect and clear the fault.

 

[camos]

Has this got anything to do with negative sequence currents? Ground faults are obviously unbalanced faults and therefore have negative sequence currents. Negative sequence currents overheat motors magnetic circuit very quickly.

 

[gordonl]

Negative sequence currents are different from ground or zero sequence currents. Negative sequence currents are returning unbalanced currents, where ground currents return through a ground path. Ground currents generally damage motors by burning core steel as the current returns through the core and frame. A neutral grounding resistor will have no affect on negative sequence currents.

 

[busbar]

One reason is that the two different insulation and grounding schemes evolved from the very different needs. For grounded-wye high-voltage windings, the coil insulation can be graded; i.e., less insulation is needed at the grounded end. Grounding at both ends of an HV line make switching surges and lightning hits easier to dissipate with less damage susceptibility.

On the LV/station-auxiliary side, typically resistance-grounded design limits ground-fault and circulating-current flow and it’s easier to make lower-voltage windings with proportionately higher BIL design. The lowside exposure to overvoltage is limited by having only relatively short-run bus/localized loads.

Negative-sequence currents can flow on both sides of the transformer; where zero-sequence current will only exist on the high [line] side. Rotating machinery is more easily damaged by heating from negative-sequence current when compared to transformers.

All boils down to economics.

 

[jwerthman]

Gordonl is correct to the extent that the neutral grounding impedance has no effect on the negative sequence current in a motor or generator during normal running operation. However, the grounding impedance has a significant effect on the negative sequence currents in a system during a ground fault (either single-line-to-ground or double-line-to-ground). This is because the negative and zero sequence circuits are interconnected for both of these unbalanced faults. Changes in the zero sequence circuit will affect the distribution of current in the negative sequence circuit. This is not usually an issue unless negative sequence relaying is being used.

High voltage systems are generally solidly grounded to minimize the transient line to ground voltages on unfaulted phases during ground faults and to permit reduced insulation at the neutral of large transformers. It is not uncommon in many Asian countries (and maybe Europe as well) to operate some large generator step-up transformer neutrals ungrounded (or resistance grounded) to reduce the ground fault duty on the high voltage system. A grounding switch is usually provided to permit selectively grounding only certain transformers. This practice results in very expensive transformers at the higher voltages because the neutrals have to be fully insulated. I have not seen this done in North America.