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Topic Title: Overload and Fault protection of LV cables fed from 11kV Tx
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Created On: 15 June 2017 12:29 PM
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 15 June 2017 12:29 PM
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rac27

Posts: 6
Joined: 27 January 2014

Good Afternoon,

I've been an avid 'studier' of the IET forums for many years and up to now have been able to glean tons of advice from simply reading various post responses however direct advise needed please.

I recently joined a company and have 'inherited' an installation completed a few years ago where 'we' have installed LV voltage management kit between the output bars of a close-coupled 11kV Tx and the busbars feeding the original ACB.

The new cables run about 5m from the Tx to a new Schneider MCCB (1600A de-rated to 1440A), through the 1000kVA/1450A rated VO unit and back to the LV bars. A consultant has recently pulled the installation suggesting the cable run between the TX and the new MCCB is not compliant (due to the 3m/mechanical protection etc. reg.) with overload protection for this run being at the load end. What's installed is:

2x 400mm HO7 per phase in spaced trefoil config. between the Tx and the new MCCB - 5m long (Amtech calc shows c.1660A capacity). This is on high level ladder rack in a permit-only mains room. The Tx is a 1000kVA/1400kVA ANAF dry-type and the client has never and will never run it as AF as the fans don't work. With this in mind is there justifiable angle that the Tx can be treated as 1000kVA (which I suspect the original design was to?) therefore the 1600A rated installation is compliant? I've been to site and noted the MCAG and MCGG relays. Is it possible through a departure from BS7671 to utilise/re-grade these relays to offer upstream overload protection of the twin 400 HO7's?

All this seems a little futile as the cables will never be overloaded.

Any advice greatly appreciated.......

Many thanks in advance.

Edited: 16 June 2017 at 03:29 PM by rac27
 15 June 2017 06:29 PM
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AJJewsbury

Posts: 15838
Joined: 13 August 2003

All this seems a little futile as the cables will never be overloaded.

The 3m rule etc is to do with protection against faults, not overload. If the cable has the required fault protection then the overload protective device can be any distance downstream provided there aren't any branches before it etc. (433.2.2(ii) is just re-stating one method of complying with section 434).

Personally, I'd look closer at reg 434.3 - which specifically allows for fault protection to be omitted for a conductor connecting a transformer regardless of length (check the details) - I'd then argue that the conductor's fault protection is fully in accordance with the requirements of section 434 (i.e. none), so you then go down the 433.2.2 (i) option and the 3m rule doesn't come into it.

I could see that the words could be interpreted differently though - specifically "protected against fault current in accordance with the requirements stated in section 434" might be read to mean 'actually has fault protection provided - and it is in accordance with 434' rather than 'has whatever fault protection section 434 requires, even if it's none', so see what others here think.

- Andy.
 15 June 2017 07:50 PM
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rac27

Posts: 6
Joined: 27 January 2014

Hi Andy,

Thanks very much for the response. I'll get the good-book out and pull together our 'defence'.

Much obliged.

RC
 15 June 2017 07:51 PM
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mapj1

Posts: 9365
Joined: 22 July 2004

presumably there is some sort of protection somewhere back up stream on the 11kV side - it may however be equivalent to an unusably high fault current.
As a tip, to the original poster, to get this thread to catch the eyes of the right people, while logged in, go back to the top and select 'edit' for the first post - then just alter the title to something that will attract attention better, like 'fault protection scheme for 11kV TX' or something, rather than looks like a simple 'hello'.

-------------------------
regards Mike
 16 June 2017 03:21 PM
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rac27

Posts: 6
Joined: 27 January 2014

Thanks for the heads-up Mike..........
 19 June 2017 10:01 AM
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mapj1

Posts: 9365
Joined: 22 July 2004

At the risk of revealing I'm off the end of my comfort zone, can I ask what the MCAG and MCGG relays are actually operating ? Is there an HV breaker of some sort disconnecting the primary side, or is it a more complex installation than that?
Depending on what is being sensed where, and what is being interrupted and how, you already may have the protection you need -as a secondary fault that disconnects primary is fine, so long as it all holds together while doing it. (after all, what happens if the TX develops an internal fault to the case or to a winding.)

-------------------------
regards Mike
 19 June 2017 10:33 AM
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OMS

Posts: 22359
Joined: 23 March 2004

They will be protection relays Mike - acting hopefully on the HV breaker

Generally there will be a HV breaker protecting the Transformer and HV supply cabling (it may be close or a bit more distant) - on the LV side will be a circuit breaker that offers overload protection and quite probably restricted earth fault protection of the transformer LV tails

The HV breaker doesn't usually protect the LV tails from overload - but a load end LV breaker does this. (It may be possible to configure the HV breaker to provide protection if it's acting on a transformer temperature trip (which is a proxy for overload) or with a Limited Time Fuse (say 10A rated and sits in the circuit derived from a current transformer - so again a proxy for overload)

The LV circuit breaker usually looks back into the transformer for restricted earth faults - the HV breaker looks through the transformer for HV short circuits and earth faults and can detect LV short circuits


The HV breaker can usually provide a trip signal to the LV side and vice versa so you end up with a zone of protection that electrically isolates faulty HV cabling, faulty LV cabling and faulty transformers - and can be enhanced with all manner of other protection such as bucholz relays, unrestricted two stage earth fault protection, frame earthing etc etc

From the OP, I'm not sure there is actually a problem other than the consultant engineer getting his 3m comments a bit skewed. It sounds like a minimalist scheme, but could well be compliant with the basic principles of protection of people (rather than the equipment)

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 19 June 2017 02:57 PM
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rac27

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Thanks OMS and Andy. I too am a little outside my skill-set but I'm aware the two Alstom relays are shunting the LV ACB and/or the HV protection. I'm in the office tomorrow so will be studying AJJ's suggested reading points.

I'll update on progress....

Rich
 19 June 2017 03:51 PM
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JohnRRussell

Posts: 32
Joined: 02 December 2003

The MCAG relay is a high-impedance relay for REF (restricted earth fault) protection, probably using a neutral CT maybe in the transformer cable box and phase CTs probably in the LV ACB. This would be designed to protect the transformer LV winding and the tails up to the LV ACB, and would trip the HV CB. It is sensitive protection because it is restricted to (can only see) faults within that zone. But since there have been later modifications you should check that all of that is still in place and working. If it is in place and working then the cables are protected for faults and, as has been said, the overload protection can be provided downstream, at the LV ACB or MCCB.
 19 June 2017 05:16 PM
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OMS

Posts: 22359
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Originally posted by: rac27

Thanks OMS and Andy. I too am a little outside my skill-set but I'm aware the two Alstom relays are shunting the LV ACB and/or the HV protection. I'm in the office tomorrow so will be studying AJJ's suggested reading points.



I'll update on progress....



Rich


Can you sketch it out and put it on a file sharing site ?

OMS

-------------------------
Let the wind blow you, across a big floor.
 21 June 2017 04:12 PM
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rac27

Posts: 6
Joined: 27 January 2014

Thanks OMS.

I have a some pictures of the original schematic showing the 'REF' indicated as being connected to/monitoring the LV side of the Tx and also two symbols 'HVT' (assumed HV trip?) and 'OCE' linked to the original LV ACB. As mentioned in my first post the Tx is denoted as ANAF 1000/1400kVA but as the fans don't work are we able to add some weight to the Tx effectively being rated at 1000kVA, therefore bringing the 2x 400mm per phase at c.1600A further into 'compliance'??

Can post the picture/s if this helps?

Thanks again

Rich
 21 June 2017 07:47 PM
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OMS

Posts: 22359
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As mentioned in my first post the Tx is denoted as ANAF 1000/1400kVA but as the fans don't work are we able to add some weight to the Tx effectively being rated at 1000kVA, therefore bringing the 2x 400mm per phase at c.1600A further into 'compliance'??


OK - the transformer can deliver 1000kVA when naturally air cooled and up to 1400kVA when force cooled (and probably more if running in short time emergency rating)

So basically what sets the rating on your LV unloading tails is the actual building load and/or the set point of the LV circuit breaker

So, if your building only demands 1250A and your ACB set point is 1440A then your tails only need to be rated to 1250A if you are certain no overload beyond that point can be applied or 1440A if you want to ensure than little or no thermal risk applies

In an equivalent set up where we stipulate that the LV switchboard has a diversity of 1, I have a 1250A Form 4 Type 7 switchboard supplied from a 2500A switchboard supplied by a close coupled 1500kVA ANAF transformer (so realistically about 1750kVA)

The ACB feeding the 1250A switchboard is a 1600A unit to contend with the temperature rise requirements and is set at 1250A (not surprisingly) - The supply cables are rated at 1250A (4 x 1 x 4C185mm2 XLPE/SWA/MDPE

2x 400mm HO7 per phase in spaced trefoil config


OK - I don't have HO7 data to hand but I suspect you have just a little shy of 1600A per phase capability at about 90C - the TX spill box should be good for 105C and the ACB for 90C

So, based on that, the tails are entirely protected (and then some) by the 1440A set point of the ACB

In summary, the tails are fine based on the LV CB setting, you don't need to consider the Transformer rating further - it is compliant (based on my assumptions)

Given that you have a REF scheme with a known relay you almost certainly are compliant for earth fault protection unless someone has dialled in some silly settings of restraint

The short circuit protection is almost certainly assured as well from the description (same caveat regarding "silly settings") as you have an inter trip send and receive scheme as well

It sounds like you've inherited a typical voltage optimisation scheme with the attendant confusion of what is protecting which element of the distribution. You do have a risk that the original direct connected busbars between the transformer and LV switchboard were not protected as they would be considered as a fault free zone in some applications - but from your description I think you have a classic REF protection scheme with the attendant short circuit protection being provided by the HV CB

Regards

OMS

-------------------------
Let the wind blow you, across a big floor.
 21 June 2017 10:51 PM
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alancapon

Posts: 6881
Joined: 27 December 2005

Originally posted by: rac27
. . . I have a some pictures of the original schematic showing the 'REF' indicated as being connected to/monitoring the LV side of the Tx and also two symbols 'HVT' (assumed HV trip?) . . .

Correct. "HVT" is the trip to the HV circuit breaker / fuse switch. It would be usual for the REF CTs to be physically located as close as possible to the LV ACB, with the single neutral - earth bond located between the neutral CT and the ACB, and likely to be made on the incoming neutral connection bolt on the ACB. This connection must never be removed with the transformer energised.

. . . and 'OCE' linked to the original LV ACB. . .

I am assuming that OCE is written in a circle, with a line linking it to the LV ACB. This denotes that the LV ACB has "overcurrent" protection and also "earth fault" protection settings enabled.


Regards.

Alan.
 26 June 2017 09:35 AM
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rac27

Posts: 6
Joined: 27 January 2014

Thanks OMS and alancapon. Some very interesting items raised.
 26 June 2017 10:35 AM
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statter

Posts: 145
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What sort of Voltage Optimiser Unit is it? As I understand the installation you are relying on the HV protection to disconnect supply if there is a phase to phase fault in the VO unit (I assume that the VO control circuits will have their own OC protection). As has been said its accepted practice for even quite long transformer tails but there must come a point when 'interposed equipment' such as a VO unit makes this unviable. Some VO units have much more potential for phase-phase faults than a set of transformer tails and its important to understand this. The manufacturers are not always very good at explaining how they should be installed so it may be that some form of overcurrent protection is needed before the VO unit.

I think you should also check and see if anything has been done by way of adding extra CT's into the REF protection associated with the VO unit installation. I have seen all sorts of 'helpful modifications' which end up defeating REF protection. It should provide some sensitive protection for earth faults within the VO device (not unlikely) provided that is in the zone covered but may not if additional CTs have been added.
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