Transformer Primary 51N Relay Tripping
Transformer Primary 51N Relay Tripping
(OP)
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?
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?






RE: Transformer Primary 51N Relay Tripping
A wye/delta transformer should be a zero sequence source to the 12.47kv system, so a zero sequence system unbalance should be showing up on the 51N.
A better application maybe to sum the three phase currents with the neutral to form a zero sequence differential.
RE: Transformer Primary 51N Relay Tripping
To explain, remove one of the transformers (on paper) so that you have an open delta. A-B and C-A. B-C is the open side although it develops full phase voltage.
Now reconnect the secondary only of the third transformer. With full rated voltage on the secondary it will develop full primary voltage on the primary. Your vectors sketch should be the typical equal sided equal angle wye. Now draw the primary wye sketch to match. Now displace one of the phase vectors a few degrees or change the length either plus or minus about 5%.
Now scale the difference between the transformer wye and the displaced or changed line wye. Even though the voltage difference is only a few percent, it is limited only by the impedance of the transformer bank. Heavy current will circulate in the delta winding and be reflected in the primary neutral.
Possible causes: On a long distribution line, uneven single phase loading may cause unequal primary voltages. This will show up as unequal line to neutral voltages.
If this condition is corrected by single phase voltage regulators, the neutral may be displaced. This may show up as phase angle errors and unequal primary line to line voltages.
You may have both conditions.
The solution.
Step one: Determine that your transformer is suitable for operation with the primary wye point unconnected to the neutral. (This does not affect secondary system grounding in an way.)
Step two: Install a normally closed contactor between the primary neural and the wye point.
Step three: Energize the transformer with the contactor closed to avoid energization voltage transients.
Once the energization surges are past open the contactor and run with the primary neutral unconnected or high resistance connected.
The good news (for the utility, not you) is that the currents will do quite a good job of correcting the phase angle errors and the voltage errors on the utility lines adjacent to your plant. Your transformer bank will be picking up enough of any unbalanced single phase load in either direction from your plant that the meters back at the sub station will indicate that things are much better than they really are.
Bill
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"Why not the best?"
Jimmy Carter
RE: Transformer Primary 51N Relay Tripping
For system unbalance - do you mean to look at the 12.47kV voltmeters and make sure they are balanced?
Do you believe that the problem is not on the downstream side of the CT's, that is, on the transformer side but that the problem lies upstream on the utility 12.47kV line?
Could this cause significant 51N current?
Could there be some load on the 12.47kV line that is causing the problem?
RE: Transformer Primary 51N Relay Tripping
Check my post for your answers.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Transformer Primary 51N Relay Tripping
RE: Transformer Primary 51N Relay Tripping
Usually they would use a delta/delta.
The other problem is this is causing additional heating of the transformers, which should require you to derate the capacity.
RE: Transformer Primary 51N Relay Tripping
RE: Transformer Primary 51N Relay Tripping
RE: Transformer Primary 51N Relay Tripping
What is seen on the GSU neutrals is the line unbalance from untransposed lines, which typically isen't that great.
Also this zero sequence is sourced from other step-up and step-down transformers, as well as grounded capacitor banks in the grid.
The problem is on the distribution systems, all the single phase taps, if not balanced, in effect create the unbalance. And while the voltage dosen't seem that unbalanced, the currents maybe.
If it helps, here we see unbalance show up just from season to season, on different phases. Just from winter heat, summer air conditioning, and Christmas season lighting. We expect to see more if recharagable electrical cars become a reality.
RE: Transformer Primary 51N Relay Tripping
Alan
RE: Transformer Primary 51N Relay Tripping
RE: Transformer Primary 51N Relay Tripping
As David said, using a restricted earth fault (REF) scheme, which is essentially a ground fault differential current protection, can differentiate between internal and external faults and not get fooled by unbalance.
On GSU's the 51N function is still there with a long time delay as a last resort backup to utility and transformer protection.
Any grounded wye transformer or load is a zero sequence source that will see current flow during a utility line fault and possibly give spurious trips. For example, a 13.8 kV electric boiler's zero-sequence ground fault relay at a local hospital trips every time the utility has an 13.8 kV cable failure. For safety reasons, the sensitive ground fault can't be set higher or delayed. A 67N directional relay might work better, if a reliable polarizing signal was available.
RE: Transformer Primary 51N Relay Tripping
Impedance measurements include both primary and secondary windings. Although the impedance of three windings is three times the impedance of one winding, the secondary may have much lower impedance than the primary winding so hat the circulating current may be even greater than would be estimated from the transformer impedance rating.
On a transmission line the load balance may be much better than the load balance on many distribution circuits.
Transformers for transmission voltages typically have much higher pu impedances than transformers used for distribution service.
60amps of neutral current on a transformer that size indicates a very good balance on the primary. But the 60 amps still represents I2R loses in your transformers.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Transformer Primary 51N Relay Tripping
Cranky108, Can you explain a little more on"This in effect converts distribution zero sequence unbalance to a negative sequence unbalance on the transmission."or any text book chapter where I can read more.You are mentioning that transmission to distribution trfs shall be delta/star.I remeber in India, at least when giving supply to a factory it used to be delta/star.Say 110 kV to 11 Kv. But nowadays it is no longer followed anywhere,at least here.All are star/star as a delta on HV side (esp with OLTC)makes trfs costly.
waross-I am not getting the impedance of secondary winding limiting the circulating current.You mean winding resistance +self inductance? Will the leakage impedance of transformer come in to picture?
RE: Transformer Primary 51N Relay Tripping
Connect a 100 Volt transformer in parallel with a 110 Volt transformer. You have 10 Volts driving a circulating current. The current circulates in the two transformer windings which appear to the circulating current to be in series.
The circulating current is limited by the sum of the impedances of the two windings.
P.U. impedance ratings report the pu sum of both the primary and secondary windings of a transformer. Of the two the lower voltage winding will have the lower pu impedance.
The point is that voltage errors in the order of the pu impedance ratings of the transformers may cause circulating currents in the order of full load current, and possibly greater.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Transformer Primary 51N Relay Tripping
Im with David and RCWilson, change setting ( current and time).
Or put this function in "Disable mode", we do it in transmission, where lot of Xfrs wye-G/delta, but not for GSU
Best Regards.
Slava
RE: Transformer Primary 51N Relay Tripping
The practice is to set directional earth fault protection on 132kV side of transformer with HV Stand by Earth fault protection (Neutral CT supplied)set to operate with long time delay to prevent trips for faults in 132kV OHL system.
RE: Transformer Primary 51N Relay Tripping
because the Xfmr primary is Y grounded and secondary is Delta, any L-G Fault in the utility network your Xfmr will feed the fault by Zero sequence current which cause your relay operation regardless the load.
to solve this set your ground relay similar setting of your phase relay. this normal practice
thanks
SMB
RE: Transformer Primary 51N Relay Tripping
As most people in this industry know, a phase to ground fault will develop positive, negative, and zero sequence currents. The positive and negative sequence current can be carried through a delta/wye transformer. However the zero sequence current can't pass through a delta/wye transformer. The transformer will become the ground path for the zero sequence current. (there maybe a better way to explain it).
Auto transformers are cheeper than delta/wye transformers, that could be a reason to purchase them over a two winding transformer.
I've seen the trend to buy the cheepest thing rather than the right thing, and frankly the people who make those desisions either won't admit they created the problem, and usually get promoted. While the rest of us are left to fix the problems (sort of irritating).
RE: Transformer Primary 51N Relay Tripping
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.
RE: Transformer Primary 51N Relay Tripping
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
RE: Transformer Primary 51N Relay Tripping
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Transformer Primary 51N Relay Tripping
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