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Topic Title: What makes a current transformer/protection relay safe?
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Created On: 12 May 2011 12:47 PM
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 12 May 2011 12:47 PM
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wrigpm

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Joined: 25 July 2008

I have been looking at protection relays to gain some information for another product that we are designing. These relay units seem to be a box of electronics with display/controls with inputs from VT's and CT's that are connected to the HV power lines.

With regard to safety, what is providing the isolation from the HV - just the CT/VT's? presumably these also provide the isolation for spikes/surges on the lines too? Does this mean that the relay electronics then only need to meet the usual requirements of, say, 240V mains for safety?

Any help appreciated.
 12 May 2011 01:13 PM
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Jaymack

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Originally posted by: wrigpm
With regard to safety, what is providing the isolation from the HV - just the CT/VT's? presumably these also provide the isolation for spikes/surges on the lines too? Does this mean that the relay electronics then only need to meet the usual requirements of, say, 240V mains for safety?


Protection relays and other such equipment is usually supplied from a potential transformer (P.T.), This is usually placed at the switchboard position and plugged into the incoming HV supply, the secondary supply is usually 110V as I recall.

Regards
 12 May 2011 02:37 PM
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wrigpm

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Jaymack

So how is the 110V configured - does it have 2 floating ends, grounded center? Could I touch one with safety? What about CT's - do they provide enough insulation from the HV to ensure safe if accidentally touched?
 12 May 2011 06:16 PM
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Jaymack

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Originally posted by: wrigpm
So how is the 110V configured - does it have 2 floating ends, grounded center? Could I touch one with safety? What about CT's - do they provide enough insulation from the HV to ensure safe if accidentally touched?

I'm not sure about the voltage requirements of the supply authorities in the UK; but see the link for outdoor grid potential transformers by AREVA, a useful site for all things HV, judge for yourself whether touchable!
The transformers that I initially referred to were mainly for industrial applications, these are in tanks with heavy gauge steel casings and fuses for HV and LV, they sit on top of switchboards and are sometimes on wheels/rails for direct plugin.

Regards

http://www.arevausitr.com/pdf/OTEF_UK_AINSI_DRAFT.pdf
 13 May 2011 11:05 AM
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ArthurHall

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VT's are usually 110v output. They are generaly star connected but open delta is used for certain purposes.
Single phase VT's should have one end of the winding earthed. Three phase VT's traditionaly had the Yellow phase earthed the other two and neutral were fused. European practice is becoming predominent in the UK which earths the neutral.
CT's should have one leg earthed, the other leg will be at a very low voltage, however if the CT output is open circuited a very high voltage can be produced which can kill.
One of the purposes of a CT is to provide insulation from the primary circuit.
If you dont know exactly what you are doing with CT's and VT's you should not touch them or their wiring.
 13 May 2011 11:16 AM
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seeker

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Originally posted by: wrigpm

I have been looking at protection relays to gain some information for another product that we are designing. These relay units seem to be a box of electronics with display/controls with inputs from VT's and CT's that are connected to the HV power lines.



With regard to safety, what is providing the isolation from the HV - just the CT/VT's? presumably these also provide the isolation for spikes/surges on the lines too? Does this mean that the relay electronics then only need to meet the usual requirements of, say, 240V mains for safety?



Any help appreciated.


I'm making a guess here that your concern is the touch safety of downstream apparatus?
What are you envisaging the output of the units be used for? as merely metering can be installed in the same enclosure but if you are after some remote monitoring or interlock facility within a LV system perhaps you need to think about optical linkage.

Its specialist territory and you need to speak to one of the industry manufacturers I suspect.
 13 May 2011 02:19 PM
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wrigpm

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Seeker, you are sort of right! We regularly design kit to work at 240/415 - no big deal for us. However, we have been advised that one piece of kit that we are designing that has current probes on 11kV lines must have 'two safety breaks' in order to be considered safe to use. From what I can tell protection relay's do not have this even if the CT/VT's are connected/wrapped round >=11kV. I.e. they only have one isolation barrier - that of the insulation in the transformers.

Thoughts?
 13 May 2011 02:59 PM
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seeker

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Originally posted by: wrigpm

Seeker, you are sort of right! We regularly design kit to work at 240/415 - no big deal for us. However, we have been advised that one piece of kit that we are designing that has current probes on 11kV lines must have 'two safety breaks' in order to be considered safe to use. From what I can tell protection relay's do not have this even if the CT/VT's are connected/wrapped round >=11kV. I.e. they only have one isolation barrier - that of the insulation in the transformers.



Thoughts?


I am going to plead that I know my limitations as Dirty Harry would say. However, someone will hopefully be along now the job you have in mind is a little clearer. Would another transformer in the chain give the required second break perhaps or, as my first comment, a fibre optic link?
I'll be interested to learn what others suggest.
 13 May 2011 03:17 PM
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JonathanHill

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Paul,

CTs & VTs will attenuate spikes, but not totally eliminate them. Re your other post, the CTs on the 11kV line will be exposed to the effects of lightning, unless you're implementing some additional shielding from lightning.

IEC60255 is the most common European standard for protection relays. This has many parts with some focussed on the ability of the protection to work with high reliability in a fairly hostile envionment, and others aimed at general safety.

If you're designing protection equipment, you could check out the guidelines for the Energy Networks Association Protection Assessment Service in their Engineering Recommendation G79. It's worth checking this out in any case even if the application is monitoring or control if you're proposing to install your equipment on a Distribution Network Operator's network. DNOs will not allow unproven equipment to be fastened to their network without rigorous assessment.

I hope this helps.

-------------------------
Jonno
 15 May 2011 01:58 AM
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sfchew

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Conventional CTs and PTs have taken care of the insulation problems. Relays comply to IEC60255. But I think wrigpm is working on non conventional set up. He may have to set new standards or prove that his pro types are safe enough especially high voltage is concerned.

Regards
Chris Chew
 23 May 2011 02:18 PM
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wrigpm

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Chris, you are correct - this is non conventional! In essence I am trying to determine where the safety comes from. The AREVA transformers that Jaymac mentioned above (thanks BTW!) look like they demonstrate exactly my principal - i.e the 11/33/400kV is connected on one side and the other side is isolated/safe from the HV. In other words, if the secondary of one of these is 110V (they do not state the voltage on the AREVA ones) then the relay needs to be designed to withstand 110V (plus some spikes/surges)? True? Can someone comment if 60255 gives the voltage/isolation/spike levels that the relays have to withstand? i.e. what is expected out of the CT/VT's?

Hence, in my case where we only have a CT - this can provide the primary (and only) isolation/insulation from the HV and the output from the CT treated in the same way as mains/240V is (with the obvious proviso of the volts on an O/C CT)?
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