Earthing Transformer for 11kV line protection
Earthing Transformer for 11kV line protection
(OP)
Hi all,
This is a ittle tricky to describe a possible job coming up.
I have been tasked to come up with a design to allow an existing 11kV (1Phase) overhead line to be disconnected from the Network, then using an existing 11kV/230V (50kVA)transformer, connect a generator to the secondary (230V) side of the transformer, hence relivening the 11kV line to supply farms further down the line.
The problem I have is in providing some form of protection to the line.
I have thought of using a 50kVA single phase 11kV/230V transformer on each of the overhead 11kV phases, connecting the primary side between the 11kV line and earth. This would provide a false earth connection for any fault current further along the line to flow through. I realise the TX's would create a higher impedance path, but it is the simplest way of providing an earth connection I can think of. The other issue is what to do with the secondary of these two earthing transformers? Are they OK to leave disconnected or should they be shorted with a low Resistance.
Any help would be greatly appreciated. I have attached a picture to help better describe what I am asking.
Cheers
Peter
This is a ittle tricky to describe a possible job coming up.
I have been tasked to come up with a design to allow an existing 11kV (1Phase) overhead line to be disconnected from the Network, then using an existing 11kV/230V (50kVA)transformer, connect a generator to the secondary (230V) side of the transformer, hence relivening the 11kV line to supply farms further down the line.
The problem I have is in providing some form of protection to the line.
I have thought of using a 50kVA single phase 11kV/230V transformer on each of the overhead 11kV phases, connecting the primary side between the 11kV line and earth. This would provide a false earth connection for any fault current further along the line to flow through. I realise the TX's would create a higher impedance path, but it is the simplest way of providing an earth connection I can think of. The other issue is what to do with the secondary of these two earthing transformers? Are they OK to leave disconnected or should they be shorted with a low Resistance.
Any help would be greatly appreciated. I have attached a picture to help better describe what I am asking.
Cheers
Peter






RE: Earthing Transformer for 11kV line protection
Can your existing transformer be reconnected for 460 Volts? Two 11 kV:460 volt transformers connected to 230 Volts will each produce 5.5 kV. Put the secondaries in series and you have 11 kV and a center point for a ground connection.
Alternately and cheaper, remove one fuse and use a direct connection. Call this the neutral line and ground it at will.
As for your transformers, grounding transformers pass little current under normal operation but carry heavy current (ground fault current) when one line is grounded. There are several ways to accomplish this on three phase circuits that will not work on single phase circuits.
Your transformers will carry a current based on the secondary impedance. The current will drop to zero on one and double on the other when there is a ground fault. If you have a high enough current to be useful it will be quite wasteful and there will not be enough current difference between normal and faulted conditions to be an effective low impedance ground.
However:
Your transformers may be used with a fairly high impedance on the secondary for high impedance grounding if you so desire.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Earthing Transformer for 11kV line protection
RE: Earthing Transformer for 11kV line protection
Jghrist, unfortunately we are running all this on a single phase 11kV line (using 11kV between 2 phases) so don't have access to 3 phase. I am sure it would be easier as you suggest with 3 phase star/delta.
This wee challenge certainly isn't exactly straight forward, in ensuring the lines have some form of protection and being able to connect and disconnect relatively simply!
RE: Earthing Transformer for 11kV line protection
How many customers are affected, can you not connect a generator at each transformer pole.
RE: Earthing Transformer for 11kV line protection
In the rural area where I now reside, a single wire system is used for single phase distribution. One side of each transformer is grounded at the base of the pole and a neutral conductor is run back one span and grounded there also. Seems to work quite well.
Do you have existing grounds at the transformers?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Earthing Transformer for 11kV line protection
RE: Earthing Transformer for 11kV line protection
We have considered putting generators at each consumers premises down the line, but the distance and time taken to do this would tie up a couple of staff for a considerable portion of the job period... it was my initial suggestion to the engineers, but they want to persist with trying to find a different solution.
At the start of the work days where the line is required to be isolated from the network, the line will be broken at taps back from the one generator supply point and then at the end of the day it will be reconnected to the Network (I believe they will be having a short shutdown on the line to do the reconnecting if our liveliners are not able to attend.
It is exactly because of the protection issue as isquaredr suggested that I have asked for suggestions or possible solutions as it all was way to dodgy as soon as I heard what was going to happen originally (a couple of people who shouldn't be making suggestions said to use solid links and connect generator to LV and use the LV fusing - No 11kV fusing, No protection, Nothing!!) I am just glad to hear from others in this forum with ideas.
The Single Wire system used here is similar to the one used in Canada - the supply transformer is earthed on the secondary side and at each consumers site, the HV and LV have separate deep driven earths. I have considered turning the line into a SWER line during the day by connecting one of the HV phases to earth at the generator site and each consumers transformer site, but again that would take a lot of time as there is a reasonable number of sites and distance to travel to complete this task at each end of the day (after disconnecting from the Network and prior to reconnecting again). The constraints are awkward - make this work, do it safely and make it comply, but it must be easy!! The only fortunate thing is the generator will not be paralleled with the network at all.
RE: Earthing Transformer for 11kV line protection
1> Change one fuse at each transformer to one of double the rating Change the fuse on the line that will become the grounded line. The smaller fuse will give transformer protection and the larger fuse will protect in the event of an internal short in the transformer.
2> Connect a 230V/11kV transformer to the generator with whatever protection you feel you need on the 230V side.
3> Solidly ground one side of the 11kV transformer and connect both lines to pole mounted disconnects or fused cut-outs.
Now, the change-over is simply a matter of opening the grid connection and closing the connection to the temporary transformer.
The double size fuses on the grounded side should ensure that the hot side fuse will clear for overloads but you will still have fault level protection when on the grid or the generator.
Change the fuses back when the project is finished.
Bill
--------------------
"Why not the best?"
Jimmy Carter