Earth grid design LV vs HV side of stepup transformers

[Croges]

Hi, I am designing an earth grid for a water treatment plant which uses 2 x LV (415V) 1.5MW generators, then steps up to 11kV using 2 x 1500kVA 415V/11kV transformers (to reticulate rest of plant and for co-gen supply back to the grid), and I am interested in things to "look out for" when designing the earth grid within the plant? i.e. what max fault current should I design for (re max conductor size, mesh spacing etc), what complications (if any) does the step up transformer configuration present? thanks in advance, any tips would be appreciated ...

 

[davidbeach]

I don't know the answer to your question, but your 1500kVA transformers are probably undersized for a 1.5MW generator. At full power output at 80% power factor that would be 1875kVA through the transformer. You may not get quite there, but running that close to the maximum rating of the transformer is not great for the life of the transformer. At 80% power factor your transformer would only allow 1.2MW out of the generator.

 

[jghrist]

The conductors must be sized for the maximum available ground fault that will flow in the conductors. The mesh spacing is dependent on step- and touch-potentials which are proportional to the current flowing from the grid into the earth.

The difference is that for faults on the 415 volt side, current will flow in the grid conductors to the source (the generators and the transformers if the 415 volt side is grounded). Current for faults on the 415 volt side will not flow in the earth, however, because it has a much lower impedance path through equipment grounding conductors and the ground grid back to the source.

For step- and touch-potentials, the current to use is the portion of the current for an 11 kV fault that will flow through the earth. Part will flow back to the 11 kV utility source through the earth and part will flow through 11 kV neutrals and shield wires.

 

[ScottyUK]

Is this a UK installation? If so there are specific requirements for either the separation of or interconnection of the HV and LV earths.

Slight aside: I guess the transformer will be a solidly earthed star winding with an HV delta. Consider the neutral earthing arrangement of the generator so you avoid earth currents caused by multiple neutral-earth connections.


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If we learn from our mistakes I'm getting a great education!

 

[Croges]

Thanks guys ... this is an Australian installation, but I wouldn't mind the reference Scotty re the requirements for HV/LV earths. In practice I have seen common grids used. Because its a step up transformer, with the generator on the LV side, this has thrown me a bit, so thanks for your help. Conductor sizing is quite straight forward I think 1500kVA Tx with 6.25% Z gives 24MVA fault level, then fault current is just 24MVA/(1.732x415) = 33.4kA and depending on fault clearing time I can calculate min conductor size. But determining the grid current for step & touch calcs is where I am uncertain ...

 

[ScottyUK]

Croges,

Section 5 of the Electricity Supply Regulations 1988 covers earthing. No connection between an LV earth and and HV earth unless there is less than 1[Ω] between them.

http://www.opsi.gov.uk/SI/si1988/Uksi_19881057_en_1.htm

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If we learn from our mistakes I'm getting a great education!

 

[jghrist]

I find Regulations 1988 very confusing.

5.—(1) The supplier shall, in respect of his works, ensure that—
(a) every high voltage system shall be connected with earth at or as near as is reasonably practicable to the source of voltage in the System:
Provided that where there is more than one source of voltage in the System the connection with earth need only be made at one such point;
(b) every low voltage supply system shall be connected with earth in accordance with paragraphs (2), (3) and (4);
(c) so far as is reasonably practicable, no system shall become disconnected from earth in the event of a fault;
(d) no conductors which respectively connect a supply neutral conductor with earth, and any apparatus used in a high voltage system with earth—
(i) shall be interconnected unless the combined resistance to earth does not exceed 1 ohm; or,
(ii) shall be connected to separate earth electrodes unless any overlap between the resistance areas of those electrodes is not sufficient 10 (sic) cause danger;

According to 5.(1)(a), you have to connect the high voltage circuit to earth (I assume this means to connect the neutral to a grounding electrode).

According to 5.(1)(d), you cannot connect the high voltage neutral to the low voltage neutral unless the combined grounding electrode resistance is less than one ohm. You also cannot use separate electrodes if the overlap can cause danger (how to determine this?).

Let's say you have an 11kV cable with serving an 11kV-415v transformer. You have to connect the shield to a grounding electrode at the transformer. You have to connect the low voltage neutral to a grounding electrode. If the resistance is over one ohm, you can't connect the two together, but you can't have two electrodes close enough to each other that the voltage between the two would be dangerous. But they both have to be at the transformer.

The only way I can see how to comply with this is to make sure the ground grid has less than one ohm resistance.

 

[ScottyUK]

I agree that would be the most likely way to install the equipment.

Are you mis-interpreting the regulation slightly? I think the intent of the regulation relates more to how the functionally live parts of the system are connected to earth. For example, take the situation of an 11/0.415kV star-star transformer where both an HV and LV neutral might exist in close proximity. The regulation would apply to the HV and LV neutrals where they connect to earth, virtually enforcing the use of a combined low impedance earth electrode. A YNyn0 (or variation thereof) would be an unusual distribution transformer in the UK so it doesn't crop up too often.

The regulation would also apply where an installation contains HV and LV gear, e.g. a transmission substation with a small LV auxiliary supply within the switchyard.


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If we learn from our mistakes I'm getting a great education!

 

[cuky2000]

The design fault current recommended is the current injected in the earth which is a fraction of the maximum calculated phase-to-ground fault. The fraction could be estimated by normalized curves of earth grid resistance versus impedances and numbers of primary and secondary feeders.
The conductor size of the grid is determined to withstand the max. Phase-to-ground fault for the period to operate the backup protective device without melt or affect the mechanical characteristic such as annealing or fusing the conductor.
The mesh spacing should designed such as the maximum actual voltage calculated including the soil resistivity value is less than the allowable voltage that a person could withstand
Consult with the applicable local or applicable earthing/grounding standards applying good engineering and safety practice.

 

[7anoter4]

I have to appologize for my bad english and also as I cannot refere myself to British Standard.
Only the medium voltage system will produce a potential rise on the this area.
According to IEEE 80 or VDE 141 for isolated systems [or grounded through a high impedance]
:the maximum shortcircuit current through the ground should be the two-phase to earth fault.
_one phase grounded in the supply substation and another in your utilities.
As a coservativ solution, the 85% of rated short-time current [1sec] of the main switchgear may be used instead..
So, for a 11kv 630 A switchgear will be about 20*.85=17 kA.
As usual for a water treatment utility[ included power station] the total area may be 300*200=60000 sqr.m.
and the specific ground resistivity about100 ohm.m.
In this case according to IEEE 80 a grounding grid of 95 sqr.m copper[ lead or tin covered will be better]
should be buried[ 2 feet 6 inches under surface] and the distance between conductors will be less than 27 m[90 feet].
The GPR shoud be about 4 kV.
The all_ around fence have to be grounded and a grounding conductor will be buried one meter outside the fence.
and connected to grounding grid .Also some grounding rods of 3/4" diameter and 20 feet long .
should be implanted along the fence all 100 feet and also connected to grounding grid.
For low voltage system a 95 sqr.mm -bare or insulated- copper cable along the supply cable[close] will be enough..
I hope this may be your solution.
julgrd

 

[Marmite]

Just to pick up on Jghrists point:-

According to 5.(1)(a), you have to connect the high voltage circuit to earth (I assume this means to connect the neutral to a grounding electrode).

According to 5.(1)(d), you cannot connect the high voltage neutral to the low voltage neutral unless the combined grounding electrode resistance is less than one ohm. You also cannot use separate electrodes if the overlap can cause danger (how to determine this?).

The regulations concern the interconnection of HV and LV earthing systems. Whether the neutral is earthed on the HV side or not has no bearing. If you have an 11kV cable feeding a Dy11 11/433v Tx the transformer tank and 11kV cable screens are connected to the HV earth. The LV star point is connected to the LV neutral earth. If the combined resistance is less than 1 Ohm then the earthing systems can be connected together. This is usually the case in urban systems. If not then you have to have separate earthing systems. In practice, insulated earth cables are run in opposite directions to rods or plates.

Regards
Marmite