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Topic Title: Automatic Disconnection
Topic Summary: Regulation 411.3.2.6
Created On: 28 October 2013 12:33 PM
Status: Post and Reply
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 28 October 2013 12:33 PM
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GMAC

Posts: 64
Joined: 12 September 2006

I am using a Merlin Gerin Vigi unit for automatic disconnection of a fault to earth on a 3 phase supply from a wind turbine. Due to the earth loop impedance (cable 500m long) I cannot achieve enough fault current to trip the breaker without using a Vigi unit (RCD).
The turbine manufacturer believes the Vigi unit can be removed on the basis of regulation 411.3.2.6 as long as supplementary bonding is added as per regulation 415.2.
Is he interpreting the regs correctly that the RCD can be removed?
 28 October 2013 01:28 PM
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AJJewsbury

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In principle, supplementary bonding can control the risk of electric shock just as well as ADS - in some circumstances it might even be more reliable (e.g. no reliance on sticky RCDs etc). Presumably there's some disconnection that'll act eventually to mitigate the fire risk from fault currents.

I'm less convinced about how that might work with a 500m long cable - armoured cable? - where presumably a fault could occur anywhere along its length and presumably the armour considered to be an exposed-conductive-part (a lot of presuming there!)
You might need an awful lot of bonding to the cable's surroundings (whatever they are) to stay within 50V under all conditions.

I wouldn't dismiss the idea out of hand, but there's a lot questions that would need answering first.

In terms of the regs, if you could use an RCD, can you really claim that ADS cannot be achieved?

- Andy.
 29 October 2013 09:13 AM
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GMAC

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Andy,
Thanks for reply.
I think I would agree with your final statement that if we can achieve ADS with the Vigi unit then we can't claim we can't.
The turbine manufacturer has noise problems and he wants to connect his earth to the mains neutral to sink the noise (neutral giving him a better path). Unfortunately if he does this he will trip the Vigi unit. Also I believe he will need to get permission from the DNO to make the neutral mains/turbine earth connection.
Since he will trip the Vigi unit, this is the reason he wants to remove it and replace it with additional supplementary bonding.
The Merlin Gerin breaker would still be in circuit, but without the Vigi unit.
Not sure if this is an acceptable reason for claiming ADS cannot be achieved, i.e., the fact that noise is tripping the unit if the neutral earth connection is made.
 29 October 2013 09:21 AM
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Jaymack

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Originally posted by: GMAC
Due to the earth loop impedance (cable 500m long) I cannot achieve enough fault current to trip the breaker without using a Vigi unit (RCD).

I suggest that you look at CT's and an OCEF relay.

Regards
 29 October 2013 09:47 AM
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AJJewsbury

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The turbine manufacturer has noise problems and he wants to connect his earth to the mains neutral to sink the noise (neutral giving him a better path).

Wouldn't that create a combined neutral & earth conductor on the "consumer's" (non-DNO) side? Isn't that prohibited under the The Electricity Safety, Quality and Continuity Regulations 2002 - reg 8(4)? http://www.legislation.gov.uk/...2665/regulation/8/made . There was a time that the secretary of state or someone could give permission, but I thought that facility had gone now.
- Andy.
 30 October 2013 10:51 AM
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GMAC

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It does look like the turbine earth system cannot be connected to the DNO neutral, according to the regulation you quoted Andy.

I have now been advised that the turbine is star connected but the star point has no connection to earth.

This would mean, I believe, that the only return path to source (turbine generator) for the noise to earth would be through the earth electrode system at the turbine, back through the DNO's earth point at their transformer and back up through the transformer windings, through the phase cables and back into the generator.

If this is the case then it would be difficult to guarantee that the steelwork at the turbine was kept below 50V under earth fault conditions, if not using a Vigi unit to trip the earth fault.
 30 October 2013 02:07 PM
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AJJewsbury

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I have now been advised that the turbine is star connected but the star point has no connection to earth.

That would be usual for any generator connected in parallel with the mains.

This would mean, I believe, that the only return path to source (turbine generator) for the noise to earth would be through the earth electrode system at the turbine, back through the DNO's earth point at their transformer and back up through the transformer windings, through the phase cables and back into the generator.

Usually the "earth" at the generator would be the supply cable c.p.c. - presuming it's a TN system then the loop (for both noise and ADS) would be that c.p.c. back to the DNO's star point and then windings etc as you said - no electrodes required in the path.

- Andy.
 30 October 2013 03:02 PM
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GMAC

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The DNO supply is a TNCS system.
I presume I can measure Ze at the DNO connection then calculate the cable impedance values for both the phase and armour of the mains to turbine cable (500m) to determine if a fault at either end of the cable would allow more than 50v to be present on either the steelwork at the DNO end (entering the consumers premises) or the steelwork at the turbine end of the cable, with the earth bonding at the turbine steelwork connected to the cable armour (armour in this case being used as the cpc to the turbine control panel beside the turbine tower.
 30 October 2013 03:26 PM
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AJJewsbury

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The DNO supply is a TNCS system.
I presume I can measure Ze at the DNO connection then calculate the cable impedance values for both the phase and armour of the mains to turbine cable (500m) to determine if a fault at either end of the cable would allow more than 50v to be present on either the steelwork at the DNO end (entering the consumers premises) or the steelwork at the turbine end of the cable, with the earth bonding at the turbine steelwork connected to the cable armour (armour in this case being used as the cpc to the turbine control panel beside the turbine tower.

If we can assume that Ze is made up of equal line and CNE resistances, then I'll predict that a L-c.p.c. fault at the DNO end of the cable will be about 115V - and that the same will be conducted out to the turbine via the c.p.c. - if that's of any help as a starting point!
- Andy.
 30 October 2013 05:56 PM
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OMS

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The generator star point would connect to the system neutral (and would be solidly earthed at the DNO PME terminal)

The system earth would bond the generator frame only (not the neutral point).

From there, it's a function of electrical design as to how you protect that 500m long cable.

I don't know how big the generator is, but at the simplest a VIGI type add on earth fault realy would do the job, from there you would be looking towards restricted and unrestricted earth fault protection and connection of OCEF triggered via a CT protection scheme

I really don't understand why the star point has no neutral connection (and that's probably the source of the "noise" as the windings are effectively floating) and either 4 pole protection and/or neutral earthing switches

Regards

OMS

-------------------------
Failure is always an option
 31 October 2013 10:25 AM
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GMAC

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The wind turbine generator is a 50kW generator.
In addition to a Merlin Gerin breaker with Vigi unit at the Turbine end of the cable, there is another with a Vigi unit at the Mains end of the cable so that a fault in the cable is cleared from both the Mains supply and the turbine supply.
Now that we know there is no earth on the turbine generator star point and that the earth fault return path is now via the cpc down to the DNO's transformer and back through the two non-faulted phases, this now creates a problem in that if the Mains Vigi unit trips first there will be no return path via the two non-faulted phases to the turbine generator source and hence no phase to earth fault current flow to trip the Vigi unit at the turbine end.
 31 October 2013 10:39 AM
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AJJewsbury

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this now creates a problem in that if the Mains Vigi unit trips first there will be no return path via the two non-faulted phases to the turbine generator source and hence no phase to earth fault current flow to trip the Vigi unit at the turbine end.

On small scale PV systems that's regarded as advantage rather than a problem - the ADS device on the DNO end of the cable disconnects all poles (including N) - as there's no N-PE link at the generator (inverter) end, the generator's output becomes separated from earth - so although the generator keeps creating a voltage between L & N etc, the shock hazard (to earth) is removed and the generator is let to shut down its own time as if for a grid failure (usually within 5s).

Given the size of your system (e.g. due to capacitance between the cable and true earth) the 'separation' of the generator from earth might be less than perfect (e.g. might still allow enough current to flow to allow a shock) - but it might be worth thinking about all the same. Presumably the turbine stops generating on grid failure.

- Andy.
 31 October 2013 11:56 AM
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GMAC

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Yes, the wind turbine generator is shut down on loss of mains supply.

Given that the turbine generator has no star point earth connection and that it shuts down in less than 5 seconds when mains lost (Mains Vigi unit tripped), could the Vigi unit at the turbine end be considered redundant and therefore removed?
Would the components in the wind turbine control panel involved in the shut down have to be compliant with standards associated with fault clearing equipment?
Would there be any companies or test houses that could review this situation and advise a course of action that is compliant with BS7671?

What takes precidence in this situation - BS7671 or BSEN60204?

Edited: 02 November 2013 at 07:18 AM by GMAC
 02 November 2013 07:25 AM
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GMAC

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Thanks all for your comments so far.
Anyone any comments on my last post?
Are there any papers written on earth fault protection in relation to small generators (50kW) in parallel with a grid supply?
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