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Topic Title: Generator Enclosure Earthing And Star Point Earthing - Advice Needed
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Created On: 21 March 2013 12:38 AM
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 21 March 2013 12:38 AM
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danmitchell

Posts: 6
Joined: 25 November 2009

Hi All

This is my first post on the IET forum for advice. I have searched the existing forums already and can't quite seem to find the advice I need so thought I would now post my questions. Unfortunately I need a fair few words to describe the situation.

The summary of the installation is a new 11kV/400V close-coupled packed substation is being installed supplied from a private HV cable network. The earthing within the substation will combine the HV/LV earths and seek to achieve a resistance of less than 1ohm.

The substation LV switchboard has a permanently installed 1250kVA LV generator connected to it as an alternative source of supply. The Generator has automated controls and be used in long term parallel if required as well as a standby supply in island mode. The generator is located in a metallic acoustic enclosure circa 150m away from the substation.

My questions relate to the Generator earthing and Enclosure earthing as follows:

1. To connect the generator star point to earth the proposal is to connect the 3xL and N from the generator to a connection switchboard located within the Generator Enclosure. From this switchboard 3xL, N + E will be distributed to the substation LV switchboard (150m away). Within the substation the generator N & E will be connected firstly to the LV switchboard N & E bars respectively and the switchboard earth bar will be connected the main earth bar on the wall of the substation (main earth terminal) which is connected to earth. Is this acceptable or because of the distance between the Generator and substation does a local LV earth bar and earth connection need to be provided in the vicinity of the Generator to earth the star point?

2. As the generator is to operate in parallel I understand the neutral and earthing of the star point of the Generator must be contactor controlled open when in parallel and closed if in island mode. As the neutral earthing point of the Generator has been extended from the machine to the Generator connection switchboard (approx. 2m from the machine and within the same room) could this length of cable be contactor controlled as it is providing N&E function, and alleviate the need to contactor control the generator output neutral and earthing/CPC seperately?

3. The Generator enclosure has a single phase distribution board that supplies local power and lighting etc. within the enclosure. The supply for this DB will be TN-S from the substation LV switchboard. My question then relates to the main bonding in the Generator enclosure. As long as the protective conductor of the DB supply is also big enough to satisfy the main bonding sizes required for TN-S it can provide both functions. I assume that the DB combined protective/bonding conductor can terminate on a local bar on the wall of the generator enclosure to then provide space to connect main bonds to the Generator frame, Generator enclosure, fuel pipe etc as well as a link to the DB for the final circuit c.p.c.'s? As the reference to earth will be back at the substation and the supply to the DB is TN-S can the main bonding be a maximum of 25mmsq copper? I want to check that extending the Generator star point earth to the connection switchboard in the enclosure in my opinion has not created PME, and particularly not related to the supply to the enclosure DB therefore see no reason the bonding should be related to the neutral size as would be necessary for PME.

I hope those who are knowledgeable on such subjects can provide some guidance!

Thanks in advance

Dan
 21 March 2013 07:48 AM
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dlane

Posts: 690
Joined: 28 September 2007

Good morning Dan and welcome to the forum.

To answer your questions,

1. In all the installations I have worked on , the 3 connections within the neutral chamber of the generator are shorted together to form the star point and then a single cable connected to this point is taken out to either solidly earth the star point or earth it via a neutral earthing resistor/impedance.

2. It will depend upon the requirements of your local network provider. You should contact them. In the installations I have worked on where the generator star point is switched, the single cable coming out from the generator goes to the contactor or a switched neutral earthing resistor prior to being connected to earth. This is loacted within the generator house.

3. You said that you are joining the HV and LV earthing systems together. To do this I presume you have an earth mat with appropriately spaced earth rods within the ground underneath your installation to control the step and touch potentials under HV network fault conditions. All bonding points will then eminate from this earth mat, yes you could install an earth bar connected to the mat and then bring all bonding connections to the various metal work to that.

The question I would ask of you is why are you joining the HV and LV earthing arrangements. To do so creates a more onerous installation with regard to step and touch potentials and you should have a full earthing study carried out. You will also need to verify your earth installation with a fall of potential test.

Kind regards

Donald Lane
 21 March 2013 11:56 AM
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danmitchell

Posts: 6
Joined: 25 November 2009

Donald

Thanks for the response for points 1 & 2. I do already have a call in with the REC to check their requirements.

Any advice on point 3 regarding main bonding within the Generator Enclosure?

With regards to the combining of HV/LV earthing systems, this is a subject I have been doing a lot of reading on. In my situation the substation building is being located adjacent two exsting buildings and is in the middle of a large multi-building site.The summary from the BS EN's and other documents seems to recongnise on constrained site locations, urban locations etc it is unlikely that physical below ground space needed for the seperation of the HV/LV will be possible, therefore combined earthing will be necessary. The full design requirements of this combined arrangment are another topic entirely - which is my second headache!

Regards

Dan
 21 March 2013 02:14 PM
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alancapon

Posts: 5842
Joined: 27 December 2005

Originally posted by: danmitchell
. . . Thanks for the response for points 1 & 2. I do already have a call in with the REC to check their requirements. . .

As you are connected to a private HV network, I suspect they will not be interested, as it will not affect other customers. That aside, as you are able to operate in parallel with the DNO, albeit with an HV connection, I would expect you to have G.59 protection. You also need to take steps to ensure that you cannot supply the HV network when disconnected from the grid, unless you have arrangements in place to reference it to earth. You would also need to consider how you would either re-synchronise it or prevent closure at the DNO boundary if the generator is connected to the HV network while disconnected from the grid. These issues, the DNO will be interested in.

. . . With regards to the combining of HV/LV earthing systems, this is a subject I have been doing a lot of reading on. . .

You mentioned 1Ω earlier. Have you arrived at this figure by an earthing study? It used to be in the old Electricity Supply Regulations, but I believe the value has now been removed from all the standards. As Donald says, you need an earthing study to show what your earthing grid should look like, especially if you intend connecting the HV and LV earths together. The study will also give the maximum value for the earth, which may be 1Ω, but could be much less.

Regards,

Alan.
 26 March 2013 04:06 PM
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danmitchell

Posts: 6
Joined: 25 November 2009

Alan & Donald

Thanks for the replies to date. It is interesting you both raise the note of caution about the topic of HV/LV earthing. A Specialist will be undertaking the HV/LV earthing study & design.

All

I would still like some advice regarding my points 1 & 3, which concerns the earthing and bonding of the LV Generator and its enclosure (container). as previously described:

1. To connect the generator star point to earth the proposal is to connect the 3xL and N from the generator to a connection switchboard located within the Generator Enclosure. From this switchboard 3xL, N + E will be distributed to the substation LV switchboard (150m away). Within the substation the generator N & E will be connected firstly to the LV switchboard N & E bars respectively and the switchboard earth bar will be connected the main earth bar on the wall of the substation (main earth terminal) which is connected to earth. Is this acceptable or because of the distance between the Generator and substation does a local LV earth bar and earth connection need to be provided in the vicinity of the Generator to earth the star point?

3. The Generator enclosure has a single phase distribution board that supplies local power and lighting etc. within the enclosure. The supply for this DB will be TN-S from the substation LV switchboard. My question then relates to the main bonding in the Generator enclosure. As long as the protective conductor of the DB supply is also big enough to satisfy the main bonding sizes required for TN-S it can provide both functions. I assume that the DB combined protective/bonding conductor can terminate on a local bar on the wall of the generator enclosure to then provide space to connect main bonds to the Generator frame, Generator enclosure, fuel pipe etc as well as a link to the DB for the final circuit c.p.c.'s? As the reference to earth will be back at the substation and the supply to the DB is TN-S can the main bonding be a maximum of 25mmsq copper? I want to check that extending the Generator star point earth to the connection switchboard in the enclosure in my opinion has not created PME, and particularly not related to the supply to the enclosure DB therefore see no reason the bonding should be related to the neutral size as would be necessary for PME.

Regards

Dan
 26 March 2013 04:33 PM
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Avatar for OMS.
OMS

Posts: 19900
Joined: 23 March 2004

Originally posted by: danmitchell

Alan & Donald

Thanks for the replies to date. It is interesting you both raise the note of caution about the topic of HV/LV earthing. A Specialist will be undertaking the HV/LV earthing study & design.

All

I would still like some advice regarding my points 1 & 3, which concerns the earthing and bonding of the LV Generator and its enclosure (container). as previously described:

1. To connect the generator star point to earth the proposal is to connect the 3xL and N from the generator to a connection switchboard located within the Generator Enclosure. From this switchboard 3xL, N + E will be distributed to the substation LV switchboard (150m away). Within the substation the generator N & E will be connected firstly to the LV switchboard N & E bars respectively and the switchboard earth bar will be connected the main earth bar on the wall of the substation (main earth terminal) which is connected to earth. Is this acceptable or because of the distance between the Generator and substation does a local LV earth bar and earth connection need to be provided in the vicinity of the Generator to earth the star point?

Wouldn't you provide the N-E bond at the intake side of the generator connection switchboard and distribute 3 x phase, neutral and earth to the the facility switchboard - the generator room earth bar would be the marshalling terminal for any bonding required in the vicinity of the gen set. You may also need to be considering restricted and unrestricted earth fault protection - and you typically wouldn't want your REF extended out along the distributuion sub main


3. The Generator enclosure has a single phase distribution board that supplies local power and lighting etc. within the enclosure. The supply for this DB will be TN-S from the substation LV switchboard. My question then relates to the main bonding in the Generator enclosure. As long as the protective conductor of the DB supply is also big enough to satisfy the main bonding sizes required for TN-S it can provide both functions. I assume that the DB combined protective/bonding conductor can terminate on a local bar on the wall of the generator enclosure to then provide space to connect main bonds to the Generator frame, Generator enclosure, fuel pipe etc as well as a link to the DB for the final circuit c.p.c.'s? As the reference to earth will be back at the substation and the supply to the DB is TN-S can the main bonding be a maximum of 25mmsq copper? I want to check that extending the Generator star point earth to the connection switchboard in the enclosure in my opinion has not created PME, and particularly not related to the supply to the enclosure DB therefore see no reason the bonding should be related to the neutral size as would be necessary for PME.

There is little point worrying about the bonding as you'll have a big earth cable exiting the gen set in parallel with the DB supply. Bonding of everything in the generator room woulb be to the marshalling terminal I mentioned above (including a link from the DB to that terminal).

The consideration of a neutral earthing contactor at the gen set switch board or the main facility switchboard will be made depending on the assessment of circulating neutral currents when your in closed transition or synchronised.


Regards

Dan


OMS

-------------------------
Failure is always an option
 26 March 2013 09:26 PM
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dlane

Posts: 690
Joined: 28 September 2007

Hello Dan,

We have advised caution with connecting the HV and LV earths together because it becomes onerous to control the earth fault potentials around the site with this kind of installation including the potential onto fences and gates that members of the public may come into contact with.

It sounds like you have an existing LV installation so to install the kind of earth mat required for an HV installation around this may be very problematic.

For point 1, we earth our generator neutral points within the genrator enclosure. As OMS has pointed out if you extend the earth you will change the sensitivity of any earth protection systems you have protecting the generator.

For point 2, everything within the generator enclosure all bonds back down to the earth mat if you combine HV and LV earthing systems. If they remain seperate then you will bond back to a common earth point within the enclosure.
We would actually terminate the TN-S earth into the distribution board first and then run earth bonding out to any earth bars and from there to other metalwork that required bonding.

Kind regards

Donald Lane
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