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?

 

[cranky108]

Have you looked at system unbalance?
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.

 

[waross]

Text book. Unbalanced primary voltages, unequal primary phase angles or both.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[MurrayBall]

cranky108 - thanks for your reply.
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?

 

[waross]

Wow!! Three posts in 26 seconds.
Check my post for your answers.

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

 

[davidbeach]

51N can't determine where the current goes, just that it flows through the neutral. Any unbalance in the primary voltages drives a circulating current in the neutral; that current then shows up in the wye windings and sums at the neutral point. If you want to trip for just transformer faults, you need to have an REF (restricted earth fault) scheme between the neutral CT and the phase CTs on the wye side. Higher settings might also be useful.

 

[cranky108]

This is probally why most companies don't allow wye/delta conections on there distribution.
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.

 

[jghrist]

Grd Wye - Delta is a ground source to the 12 kV primary system. Any fault on the 12 kV system may cause the 51N to trip. Solution is to float the primary neutral.

 

[prc]

Protection gurus, a small query. If star /delta transformers can be a source for zero sequence feeding, why this is not creating problem in generator trfs? A distance line fault sould trip the GT?

 

[cranky108]

Typically most transformers from tranmission voltage to distribution voltage are delta/wye, or delta on the trinmission voltage/wye on the distribution voltage. This in effect converts distribution zero sequence unbalance to a negative sequence unbalance on the transmission.

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.

 

[racobb]

I will second the floating primary neutral of the wye. It is standard operating practice for most.

Alan

 

[jghrist]

If star /delta transformers can be a source for zero sequence feeding, why this is not creating problem in generator trfs? A distance line fault sould trip the GT?

On a GSU, you expect a fault contribution and set the relays accordingly so that they coordinate with downstream protection.

 

[rcw retired EE]

Most power plant interties are through grounded wye transformers (Generator Step Up = GSU's)with 51N or 51G relays on the neutrals to detect ground faults in the system. The relay cannot differentiate between circulating currents caused by unbalance, winding faults in the transformer or ground faults in the utility system.

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.

 

[waross]

When the primary wye point is grounded, the voltage and phase angle of each secondary phase must be in proportion and at the same angle. When you draw the vector sketch for the delta side, based on the primary voltages and phase angles, the delta must close. If the secondary vectors don't close the triangle there will be a voltage difference. This voltage will cause a current that is limited only by the impedance of the secondary transformer secondary windings.
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

 

[prc]

Thank you Cranky and wilson.

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?

 

[waross]

Consider a single phase example:
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

 

[slavag]

Hi
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

 

[RRaghunath]

We have many 132/34.5kV Trafos Yd connected.
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.

 

[SMB1]

Hi,

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

 

[cranky108]

I don't have any good references, as all my books are old.

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).