Transformer Primary 51N Relay Tripping 4

[MurrayBall]

Transformer: 3750kVA
Primary: 12.47kV Wye solidly grounded(connected to the utility line).
Secondary: 2400V Delta
Experiencing occasional 51N trips, that is, primary neutral current. 51N relay picks up at 2.0A.
51N current all the time about 1.0A; with 300:5A CTs equates to 60A primary.
One time even experienced a trip with no load on the secondary.
Disconnected primary H0 bushing from ground which caused the 51N current to go to zero.
Transformer tests okay.
Any ideas as to where the problem lies?

 

[MurrayBall]

waross - in one of your posts on this thread, you state that 60amps of neutral current on a transformer that size(3750kVA 12.47kV primary) indicates a very good balance on the primary.
The FLC of this transformer is 174Amps, so 60amps seems relatively high to me. Can you show me how to calculate what unbalance voltage on the primary will reflect in 60amps of neutral current? Or refer me to a book that might give this detail.
Now the 51N relay is set to pick up at 120A; therefore, maybe the only time the 51N relay trips is when there is an actual fault on the utility line. So the normal neutral current due to single phase loading is probably okay.
For the record, side 2 of this substation which is fed from a different 12.47kV line has neutral current of about 18amps(identical tranformer) and has never experienced a 51N primary trip.

 

[waross]

Hello MurrayBall;
I am sorry. I mispoke. I got this thread confused with another and compared the 60 amps to a much higher current.
You are correct, 60 Amps neutral when FLC is 174 Amps is quite high.
I have looked in vain for a textbook treatment of this subject. Despite the lack of citations, I have experienced many issues concerning four wire wye delta connections in distribution service.
There are a lot of factors involved. The first factor is the amount of error voltage. By this I mean the voltage that may be measured across a break in the delta connection. This may be caused by a primary voltage unbalance or an phase angle error. The circulating current that this will produce will be limited by the impedance of the transformer secondaries only, information that is not generally available. The total impedance may be three times the impedance of a single winding, but it gets more complicated. The circulating current sums vectorilly with the load current in each winding. It may increase or lessen the phase current in each phase. The power factor of the load will also have an effect. As complicated as this is getting, it gets more complex. This assumes that the grid is firm and stable at the point of common connection. However, these issues are more common on weak or soft grids. The circulating current acts to correct the errors causing it. Now the the circulating current in the delta is causing a line current to be reflected back on the primary. This current will act to cause a voltage drop on the highest voltage phase and a voltage rise on the lowest voltage phase.
I have gained this knowledge from "On the ground" experience with actual installations.
Some of the solutions I have employed:
> Change the system to a wye/wye system.
>If the bank is oversized, remove one fuse and run open delta.
>Float the primary neutral.
> In one instance the load was such that I was able to "Break the delta". And run broken delta.
Note: North American convention. Open delta means two transformers. Broken delta means three transformers with one corner connection left unconnected.
> Special case; The customer demanded a full delta connection even though the first transformer failure was only weeks following the installation., and the transformer bank capacity was about six times the connected load. Have the toggle of a cutout type fuse holder tack welded in position so that it could be installed without a fuse link. The bank ran happily open delta while looking from the ground as if it was a full delta.
>Another special case (kill the messenger). Divest myself of my assets in anticipation of being sued for someone else's mistake. It never came to court only because it became easier to shaft the transformer vendor than to sue me. Hiding your assets is not a normal solution to transformer issues, but things are different in the turd world.
Be aware that this transformer connection has the ability to transfer real power from one or two phases to other phases. Probably the best way to measure this effect would be to connect a single phase Wattmeter in each primary phase and compare the readings to the watts supplied by the secondary. You will see at least one phase drawing less power than the average and at least one phase drawing more power than the average.
Search this site. I have written of other issues with this connection on unbalanced distribution lines in these fora.
You will find instances of this connection used for protection where the secondaries of three PTs are connected "Broken delta" (In IEC land, this is called "Open delta). Voltage across the break indicates a ground fault or leakage to ground on the wye primary lines.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

This may help. Thanks to amptramp.

amptramp (Electrical)
13 Feb 10 2:49
Maybe this will help:

http://www.basler.com/downloads/ZeroSequenceCircuit.pdf

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

The following link might be of academic interest regarding the properties of Y-D and Y-Y transformers. It's not the first time it's been mentioned on this site, but it is too good a reference to ignore.

http://www.geindustrial.com/publibrary/checkout/White Papers|GET-3388B|generic

The old timers who wrote this forty years ago knew what they were talking about.


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If we learn from our mistakes I'm getting a great education!