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Earthing 2 Parallel Distribution Transformers

11/18/2018 7:44 PM

On Dyn 11kv/415V distribution transformers in New Zealand, the standard is to earth the star (neutral) point directly using some ground rods. The aim is to achieve less than 25 ohms ground resistance, but lower is better.

Usually these transformers are physically far apart so the question never comes up, but we have a case where we want to install 2 of these transformers in parallel and physically next to each other. Each of them needs to be earthed. Question is: Is there any reason why the individual earths should not be bonded together as well? Should the 2 earths be separated and not bonded?

In my view the 2 earths systems being bonded together reduces overall ground resistance, and just helps to improve the MEN system. Lower is better.

Another argument around the office is that the 2 earths must be kept separated because "Earth faults in 1 transformer will "jump" to the other transformer and damage it as well". I don't understand how that will happen and I can't see any reason to do so.

Does anyone have some technical or practical experience and advice?

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#1

Re: Earthing 2 Parallel Distribution Transformers

11/18/2018 8:54 PM
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#2
In reply to #1

Re: Earthing 2 Parallel Distribution Transformers

11/19/2018 8:14 AM

I don't think this paper is particularly relevant to the OP's situation, because it is in reference to resistance-grounded transformers (to reduce fault current magnitudes), while his situation is directly-grounded transformer neutrals.

At the utility where I work, we will have a ground grid throughout the substation where there may be multiple transformers, and if they are solidly-grounded, both neutrals will be connected to the ground grid (but not necessarily at a common point). The ground grid, besides dissipating fault currents to earth, will also help mitigate step and touch potentials during a fault that would be a hazard to personnel.

The OP may be in a jurisdiction where they do not necessarily install a comprehensive ground grid, but just ground individual pieces of equipment. He should find out from the AHJ for his location what is required.

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#3

Re: Earthing 2 Parallel Distribution Transformers

11/19/2018 8:23 AM

If the actual grounding is well-done (very low resistance to earth), then the current from the faulted transformer will dissipate in the earth. It will NOT "jump" to the other transformer. It is common for power transformers in substations to be solidly grounded to the same, common ground grid, and they are probably the situation you would be most concerned if there were some danger of a fault in one damaging another.

There is not necessarily a requirement to join their grounds, unless the local AHJ (Authority Having Jurisdiction) specifies that they must be bonded to a common ground. But I agree with the OP's assertion that joining the grounds of the two transformers together may (I emphasize may) reduce the ground resistance, depending on how the ground rods are spaced, how much conductor is in the ground, etc. An individual analysis would need to be done to verify the ground resistance.

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#4

Re: Earthing 2 Parallel Distribution Transformers

11/19/2018 10:30 AM

The purpose of the earth return path is to ensure the circuit protection device(s) operate(s) so as to disconnect the fault current. Clearly, the lower the earth resistance the better, though correct selection and setting of the circuit protective device(s) takes the earth resistance into account.

Provided there is no connection between the transformers on the earthed side other than the earth connection it is difficult to see how <...Earth faults in 1 transformer will "jump" to the other transformer and damage it as well...> as the circuit protective device(s) will have operated before that can possibly happen, irrespective of whether or not they share an earth mat.

In the unlikely event of two faults, one on each transformer, occurring simultaneously, then something BIG is going on and there will be damage to things other than the two transformers as well.

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#5

Re: Earthing 2 Parallel Distribution Transformers

11/19/2018 2:08 PM

Thanks very much for all the replies, it is appreciated. I agree with the comments and understand it the same way. Just for interest, the only protection provided in these cases (in New Zealand) is fusing the LV side for overload & earth fault purposes; no protection relays or circuit breakers are used. The standards say that the equipment must be individually earthed, but it is unclear on whether

a) those individual earthing systems MAY be bonded together, or

b) those individual earthing systems MUST be bonded together, or

c) those individual earthing systems MAY NOT be bonded together.

Keeping the 2 earths separated would still be compliant to the standards, but from the comments I would deduce that there is certainly no technical reason NOT to bond 2 transformer neutral/earth points together if it is practical and cost-effective to do so.

Thanks

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#6
In reply to #5

Re: Earthing 2 Parallel Distribution Transformers

11/20/2018 7:06 AM

You write that transfos are parallel. Certainly, I would expect HVs to be parallel. If this is at LV, then you would parallel each phase and the star points. Since earths are connected to star points, you would inherently parallel earths.

If there is a fault to actual earth from a distribution cable, will the area of ground affected by earth currents be increased by having two transfo earths which are very close together? I think that outside the transfo compound/fence, the increase would be negligible. If the two transfos basically feed one to left of compound & one to right, I see little spread of earth current one side to the other.

The guiding principle of LV fuse protection is that, to avoid fire or electrocution, a fault must be disconnected as soon as possible. Wiring regulations do not give minimum earth loop resistance, only maximum.

Just taking 100 amp HBC fuse data table from fig. 3A1 of British Wiring regs BS7671, I find the following max times & i2t (106 amp-sec)...

1450 amp, 0.1 sec = 0.21

1230 amp, 0.2 sec = 0.3

850 amp, 1 sec = 0.72

600 amp, 5 sec = 1.8

Clearly, increasing fault current gives dramatic reduction in clearance time & i2tR = energy into a fault to start a fire. So halving earth resistance at transfo is good since it probably contributes most of the loop resistance apart from the fault earth.

In an oil/gas/industrial installation usually every electric power equipment is earthed by a core in its feed cable, conduit or armour of that feed & a bond to bedplate steel, which has its own connection back to main earth. This gives an actual earth loop resistance much less than the maximum & no-one worries about that!

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#7

Re: Earthing 2 Parallel Distribution Transformers

11/20/2018 11:58 AM
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