Earthing Transformer for 11kV line protection 2

[arrow123]

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

 

[waross]

Not knowing where in the world you are and what standard transformers are available gives an extra challenge.
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

 

[jghrist]

I don't think that you can establish an earth with two single phase transformers. I think you need to use a three-phase bank connected grounded wye-delta.

 

[arrow123]

Thanks to you both for responding. I am in New Zealand. Waross, I did like your suggestion about turning it into a Single wire earth return line, which I had earlier considered as an option. The only issue with this is the setup we need to do is to be used on maybe alternate days over a month period, then it all goes back to normal. At each of the transformer sites along the line one leg of the overhead would need to be earthed as well to meet our SWER requirements. The suggestion for a centre tapped supply has some merit - I will investigate if this is possible (allowed in NZ).
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!

 

[isquaredr]

I dont think that I would consider going down this method of supply, all sounds far too risky. What would happen to the engineer responsible if a line came down and remained live due to poor earthing / inadiquate protection with a high possibility of injuring someone or worse, I know what would happen in the UK...porridge.
How many customers are affected, can you not connect a generator at each transformer pole.

 

[waross]

In Canada and in with two systems I was familiar with in Central America, each transformer secondary was earthed already. The distribution was wye, so single phase branches were line to grounded neutral. The primary neutral was tied to the transformer secondary ground at each transformer.
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

 

[jghrist]

What is the source of the 11 kV line before disconnecting and powering from generators? Is it two phases from a grounded wye power transformer secondary? This might affect possible ways of connecting the grounding transformers. Will the line have to be reconnected to the utility source if the generators are off-line? Will the generators ever have to be paralleled with the utility source?

 

[arrow123]

The system here is 11kV 3 phase typically, though this line is run on two phases only. The LV side of all the 11kV/400V transformers are all connected MEN, with driven earths at the base of the pole and at each consumers premises. The 11kV side is Delta connected, with the supply transformer (33kV/11kV) connected Delta - Star, so there is normally an earth reference at the point of supply. As this line will be disconnected from the normal supply (it is the end of a spur line), there will not be an earth reference as all the transformers are Delta connected.
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.

 

[waross]

Off the wall but it may be acceptable.
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