Surge Arrester on a Solidly Grounded Neutral? 1

[constantlylearning]

We recently had an outside consultant suggest that we add surge arresters to the H0/X0 (neutral) bushings of our autotransformers. In every case, the neutrals are solidly grounded.

For the case where we have three single-phase transformers connected in a three-phase bank, with the individual H0/X0 brought out to a neutral bus, I can see some logic in having arresters on the H0/X0 bushing, due to the REMOTE possibility of a surge or strike on the conductor between the H0/X0 and the neutral bus. Personally, I feel this is unnecessary, but of course I could be wrong.

However, for a three-phase autotransformer where the H0/X0 is jumpered directly to the ground grid, I see no benefit in a having an arrester on the H0/X0 bushing.

I would love to get some more opinions or best-practices on this.

 

[collies99]

I have never seen one also, it would be interesting to see if anyone else merits the idea.

Even in a system with NGR's, I have yet to see an SA on the neutral bushing.

 

[VTer]

You already have a surge arrester on the neutral. It is the neutral-to-ground bonding jumper. You cant' get much better than that.

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[magoo2]

Stupid is as stupid does!
Forrest Gump

 

[LionelHutz]

piece of wire = 0V surge arrestor. He wants you to put a surge arrestor in parallel with this?

 

[odlanor]

reference -Westinghouse Electrical Transmission and Distribution Reference Book-ABB - fifth edition- chapter 18 - VII surge protection for rotating machines.
At the bottom of Table 9 is defined a grounded system must have the resistance less than 50 ohm.
In my interpretation refleted waves is not important for 3Ro < 50 ohm or Ro < 16.7ohm.
So is not necessary surge arrester at neutral of solid or effectively grounded system.

 

[magoo2]

Someone is not realizing that there's a jumper between neutral and ground. Reread VTer and Lionel's post. An arrester in this situation would be like something on a bull that is not needed!

 

[prc]

I am not commenting on the best practices.But let me quote from history.Brown Boveri Review June1964(pages 377-381 Protection of Transformer Neutral) "...Where a solid earth is provided, there are occasions when the neutrals of transformers in one station are not all earthed, in order to reduce short circuit currents......Arrester in the neutral of solidly Earthed Systems.....If a fault occurs, the star point which is not earthed attains at the most 0.4 times the phase to phase voltage, provided conditions for solid earthing are observed" Also M Wellauer: Stress and overvoltage protection of the neutral point of transformers, CIGRE 1952 Report No117.
I remember a case where 132 kV neutral of transformer provided with surge arrester with a parallel line of solid earthing with isolator.I don't know how the utility used it.
Now coming to original question, if it is an auto-transformer how there can be two neutrals? In cases of auto trfs of very low co-ratio (ie H &X voltages are very close) it is a practice to provide arrester across H&X terminals.Any way providing arrester in parallel to solid earthing seems not logical.

 

[odlanor]

prc,
I guess South Korean adopt this one in the 345kV system.
both 345kV transmission line terminals grounded by neutral solidly grounded of star winding transformers.
At terminal without source can operate ungrounded in order to reduce short circuit currents.
Neutral of star winding can be grounded by an earth switch or ungrounded by surge arrester.
I guess these transformer has the 345kV BIL with full winding instead of graduated.

 

[prc]

Thank you odlanor.I don't know about Korean system. I was referring to Syrian Power system.

 

[constantlylearning]

PRC:

When I referred to the H0/XO bushings as plural, I meant it as a reference to the individual bushings on the single phase transformers. I may have mistakenly implied that there were separate H0 and X0 bushings, but of course there are not. Thank you for your input.