Generator earthing

[westie101]

I've had a look through various documents (ENA ER 59/2, The Distrubution Code and internal documents) this morning to see if the answers are there!

I can find no reccomendation for the value of the customer's earth.

That it can be connected to the DNO earth is correct, all the earthing should be solid for this type of installation.
The changeover switch must also operate on the neutral (SP&N or TP&N)

It is clear from all the documents I found in some that the DNO must be consulted and approve the changeover and earthing arrangements, which is chargeable work.

From one of them

Neutral Earthing
The winding configuration and method of earthing connection shall be agreed with the DNO.
In addition, where the Generator‟s Connection Point is at Low Voltage the following shall apply
(a) Where an earthing terminal is provided by the DNO it may be used by a Generator for earthing the Generating Plant, provided the DNO earth connection is of adequate capacity. If the Generating Plant is intended to operate independently of the DNO‟s supply, the Generating Plant must include an earthing system which does not rely upon the DNO‟s earthing terminal. Where use of the DNO‟s earthing terminal is retained, it must be connected to the Generating Plant earthing system by means of a conductor at least equivalent in size to that required to connect the DNO‟s earthing terminal to the installation.
(b) Where the Generating Plant may be operated as a switched alternative only to the DNO‟s System, the Generator shall provide an independent earth electrode.
(c) Where it is intended to operate in parallel with the DNO‟s Low Voltage System with the star point connected to the neutral and/or earthing system, precautions will need to be taken to limit the effects of circulating harmonic currents. It is permissible to insert an impedance in the supply neutral of the Generating Plant for this purpose, for those periods when it is paralleled with the DNO‟s System. However, if the Generating Plant is operating in isolation from the DNO’s Distribution System it will be necessary to have the Generating Plant directly earthed.
(d) Where the Generating Plant is designed to operate independently from the DNO’s Distribution System the switchgear that is used to separate the two Systems shall break all four poles (3 phases and neutral). This prevents any phase or neutral current, produced by the Generating Plant, from flowing into the DNO’s Distribution System when it operates as a switched alternative only supply

 

[JohnD]

that all looks familiar.

I would have thought, though, that the network engineers would have one or more standard documents describing acceptable installations, to save the tedium of redrafting a new one each time.

Maybe the generator company has a template spec?

 

[JohnW2]

....That it can be connected to the DNO earth is correct, all the earthing should be solid for this type of installation.

You say that, but from the document you quote:

....(b) Where the Generating Plant may be operated as a switched alternative only to the DNO‟s System, the Generator shall provide an independent earth electrode.

Whilst I don't fully understand the meaning of "...may be operated as a switched alternative only...", I think they are probably saying that if it is used as a 'switched alternative' (which will be the case with most gennys), the DNOs earth (at least, alone) cannot be used, aren't they?

Kind Regards, John

 

[JohnD]

I thought that would be to cover the case where the network cable is cut close to the consumer, and the earth conductor might not go anywhere.

 

[JohnW2]

I thought that would be to cover the case where the network cable is cut close to the consumer, and the earth conductor might not go anywhere.

Maybe. In fact, I don't see that it would ever make any sense to allow a 'standby' generator to rely on a DNO's TN earth under any circumastances - since, as you say, a network fault (presumably the usual reason for a standby generator to be used) might be so close to the consumer that the DNO-supplied 'earth' was not an earth at all. So I don't really know what that document was trying to say!

Kind Regards, John.

 

[westie101]

Based on the other documentation that I have seen but cannot copy to here.

The DNO earth can be used, but the customer MUST supply their own earthing system if the generator is to be used for standby purposes. The two systems can be interconnected.
One of the other documents has a diagram clearly showing this.

The section of the document I quoted also applies to other forms of generator connection at LV

It doesn't need a post mortum, it doesn't need pages of theoretical discussion it is what I have quoted. Any queries to the ENA or OFGEM

 

[JohnW2]

The DNO earth can be used, but the customer MUST supply their own earthing system if the generator is to be used for standby purposes. The two systems can be interconnected.

I suppose my problem probably stems from ignorance, in not really understanding what puposes such a generator would be put to other than 'standby purposes'.

Kind Regards, John

 

[plugwash]

Presumablly the rules are written to cover both generators used for standy purposes and generators intended to be run in paralell with the grid.

 

[JohnW2]

Presumablly the rules are written to cover both generators used for standy purposes and generators intended to be run in paralell with the grid.

Yes, they clearly are - but, as I said, my ignorance is in not understanding under what circumstances one would run a generator in parallel with the grid - unless one is talking about solar-/wind-/whatever-generated electricity, which is not the sense in which I was thinking of 'generator'.

Kind Regards, John

 

[Spark123]

Generator:

1. physics
a. any device for converting mechanical energy into electrical energy by electromagnetic induction, esp a large one as in a power station
b. a device for producing a voltage electrostatically
c. any device that converts one form of energy into another form: an acoustic generator

So in essence, any of the above!

What Westie101 says backs up what the ESQCR says, the documents he refers to are probably written around them.

 

[westie101]

Three options are given for 3 types of connection, which can cover any type of generation


Running in parallel at all times

Running in either parallel or used as standby (I know of a few of these in this area). This type of system is sometimes used to allow changeover without loss of supply.

Standby use only

 

[Spark123]

Running in either parallel or used as standby (I know of a few of these in this area). This type of system is sometimes used to allow changeover without loss of supply.

I'm thinking something like a hospital? Where you could have a generator/mains feeding into a UPS to provide a guaranteed supply. Or am I barking up the wrong tree?

 

[plugwash]

I could see a few situations where runing a fossil fuel generator in paralell with the grid could make sense.

One is if the generator is part of a CHP system.

Another is if you have an overloading problem and need to add capacity but expanding the grid supply is practically difficult and the design of your sites power distribution would make it difficult to split the site into "mains areas" and "generator areas".

Another is if you have time of use pricing so that at certain times of day generating your own power becomes cheaper than buying from the grid but you don't want the disruption of the switchover (or maybe you even want to sell power from your generator).

 

[westie101]

I'm thinking something like a hospital? Where you could have a generator/mains feeding into a UPS to provide a guaranteed supply.

Any site that is susceptible to loss of supply. Technology does exist to automatically synchronise generators to the grid and pick up the load before disconnecting the grid.
Conversely it is now possible to synchronise an "islanded" generator, supplying load, to the grid and change over from generator to grid. In both directions without loss of supply

(we use them frequently)

[/quote]

 

[Spark123]

We used to do something like that years ago with 3 lamps at college