Earth Fault Protection on Low Voltage System

[arvinolga]

Got a query,

We have a medium voltage system (13.8kV) stepped down to 0.415kV via an oil immersed transformer. The LV system has a Main ACB of 3200Amp and Sub-breaker of 4 x 800Amp distributed to 4 tenants. It is currently installed with an Earth leakage relay ("ELR") for each sub-breaker. Problem is we are getting nuisance tripping via the ELR so we decided to have it checked. The recommendation is to replace the protection with a Earth Fault Relay ("EFR") instead of the ELR for reason that the ELR is too sensitive (commissioning tests show that the sub-breaker trips with less than 1A current injected) and that ELR is usually used for final circuits to protect personnel.

The consultant recommended replacement so that the protection setting may be made higher (15%-20% of the sub-breaker rating). Can anyone suggest what brand of EFR to use including the ZCT's compatible to the relay? I can provide the single line diagram if anyone is interested to help me.

Thanks
Arvin

 

[ScottyUK]

For a typical LV earth fault relay, have a look at Merlin Gerin's Vigirex relays. They use an external sensor so they are quite flexible in terms of the current you can measure. You have the option to set the tripping current and any delay to coordinate with downstream devices if necessary.

http://www.merlingerin.com/MG/en/products/index_fon11_fam71_ABT_Vigirex.htm

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I don't suffer from insanity. I enjoy it...

 

[rbulsara]

Your best bet would be to contact the manufacturer of the switchboard, provide them with the equipment data and they should be able to help you as how best to implement a EFR in the existing equipment. There are more than one ways to do it and actual equipment construction could determine the most practical way. It mostly has to do with feasibiliy of installing a ZCT or individual CTs for each phase and neutral.

 

[rbulsara]

On the second thought, why not ask the consultants who recommended the change to help you out. They seem to know what they are doing. (The ELR was wrong to install to begin with.)

 

[arvinolga]

Thank you very much for the help scottyUK and rbulsara. Will look into the site you forwarded. As for the consultant, we actually did not engage them, it was more of a loose talk than anything else.

 

[nightfox1925]

I would also recommend that you install additional transformer neutral earth fault relay (51N) connected to the transformer neutral (that is if the neutral CT is available and the transformer is solidly grounded). This relay will be set to trip the main circuit breaker and coordinate with the downstream 51Gs. The instantaneous settings (50 or 50G element) of the incomers should be disabled. The subfeeder 50 or 50G elements maybe set if directly connected to a transformer, otherwise should also be disabled and only the 51 or 51G elements are set.

Yours truly

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[nightfox1925]

I would like to add a supplementary to my earlier proposal: "The subfeeder 50 or 50G elements maybe set if directly connected to a transformer, otherwise should also be disabled and only the 51 or 51G elements are set."

At the subfeeder breakers, you may set the 50G elements if they are directly connected to transformers and the primary transformer winding is connected delta.


GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[arvinolga]

Thank you 02101972, but that's exactly what we want to eliminate, the main breaker tripping wherein the fault is only caused by the load in one of the sub-breakers.

ScottyUK, have browsed the site you mentioned but it seems that they offer ELRs and not EFRs

 

[ScottyUK]

arvin,

I am wondering what you are using as a distinguishing factor between the two tyes of relay. For example, a 20A fault to earth for 5 seconds on a solidly earthed LV system is significantly more than 'leakage' - that is well within the range of adjustment of the Vigirex 'earth leakage relay'. Perhaps you can clarify your requirements in terms of time and current? Are you looking for a relay simlar to the 50N/51N on the transformer to use in a dedicated role for the sub-distribution?


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I don't suffer from insanity. I enjoy it...

 

[nightfox1925]

Is your subfeeder circuit breakers feeding a transformer directly or a sub-distribution panelboard equippped with a main MCCB with ground fault unit?

As long as the 51N relay at the incomer circuit breaker is have time graded with the ground fault element of the downstream subfeeder circuit breakers by 0.35~0.50 second, it will be acceptable and the 51N will serve as a backup protection for the sub-feeder ground protection. Applying a proper current tripping interval (CTI) between these protection elements will provide you the selectivity and reliable protection you need.

GO PLACIDLY, AMIDST THE NOISE AND HASTE-Desiderata

 

[RalphChristie]

Arvin:

A lot depends on your earthing arrangement of your transformer - or the value of your earthfault currents. There are a lot of different methods, like an earthfault relay in the residual connection, sensitive earth fault protection, standby earth fault protection, etc. Each one have some advantages, but also some disadvantages, some may be better in solid earth connections, while other may be better if used with resistance earthed methods.

Another factor is your working environment and regulations. An example is like in Industrial and Mining installations, safety codes specify (at least in South Africa) that any voltage above the range of 25 to 40V is considered to be unsafe - and must have earth leakage protection. (for touch potentials)

To answer your question - it depends. You should consult someone with some practical experience and knowledge of your country's regulations to assists you.

Regards




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[Electrifier]

Please provide as below for a better analysis of problem;
• specification/Manufacturer/model/rating of Incomer and feeder breakers.
• Specification, setting, input sensing(ZCT or residual connected CT), CT/sensor ratios
• What type of loads are fed by feeder breakers (Motor/transformer/LV distribution board)

In normal practice, Protection of low voltage feeder breakers may be provided by following means;
• Integral solid state breaker trip devices. They shall be supplied with time delayed ground fault trips. Instantaneous trips shall only be provided for dedicated feeders where there is no downstream protective device.
• Residually connected, Time delay ground O/C relays. The time element shall be coordinated with downstream and upstream ground protection relays.
• Zero sequence CT connected, Instantaneous ground O/C relays. Normally used where no co-ordination is required with downstream system. Applicable to motors and transformers with Delta connected primary winding.

Hope this help

 

[arvinolga]

Scotty - as long as the protection would cover at least 10-15% of the breaker rating, that is according to the consultant.

02101972 - ours is the latter, but enclosed in one Main Distribution panel, and the sub-breakers are MCCB type with ground fault unit. Understood regarding the protective relaying coordination, what really needs to be addressed is the current ELR of the MCCB's being too sensitive.

RalphChristie - thank you for the inputs, yes, we are currently sourcing the proper contractor to do the work. Its just that I need to verify it from my end.

Electrifier - hope someone can tell me how to provide additional details by submitting some one-line diagrams as well as pictures here at the site so that everybody can have a better understanding of the system

 

[waross]

Hello arvinolga
This how to post pictures in your post. faq238-1161 itsmoked
respectfully