Wye Wye Transformer Grounding/Bonding

[NJWVUGrad]

We have a customer who has 480V solar array that is backfeeding a 120/208V building service via a 750KVA Dry-Type Grnd-Wye to Grnd-Wye step down transformer. This transformer has been provided with a grounded Delta tertiary winding

We have 4 wires (A,B,C,N,G) to the High side and Low side of each transformer – which land on the X0 and H0 bushing (respectively) on the transformer – the XO and H0 are not bonded together. The neutrals are not bonded to ground in transformer on either side. The Wye-Wye transformer configuration was requested by the utility.

The issue that we are having is that when energized and under no load (The breaker supplied from the 480V side of the transformer is open) the breaker feeding the low side of the transformer is showing a current reading of 120A per phase and 360A on the neutral.

We are trying to determine what is causing these currents, it appears to be a grounding/bonding issue. Does anyone any insight into what could be causing this circulating current and the proper bonding for the transformer.

 

[jghrist]

Here is oscillography from a GSU transformer that caused the utility substation breaker to trip on ground overcurrent even though the generation plant main breaker was open. The GSU is connected grd-wye on the utility side (12.5 kV) and delta on the generator side (4.16 kV). No harmonics, only fundamental zero-sequence current. 13.4 A in each phase and 40.2 A in the neutral.

 

[stevenal]

Jghrist,

Classic ground bank action. I agree, looks like pure zero sequence current. Just to be sure, here's a bit of math:

Ia=Ib=Ic=13.4 <0deg A

I0=(Ia+Ib+Ic)/3=13.4 <0deg A

I1= (Ia+aIb+(a^2)Ic = 0 A

I2=(Ia+(a^2)Ib+aIc)= 0 A

Confirmed, positive and negative sequence values are zero.

After reflecting a bit on the notion that unbalanced voltage is needed to have unbalanced current, I don't believe this is true. The unbalanced load causes this current. If we modeled the source as an infinite bus, the single phase load would still cause an unbalance. And for a more real world scenario, we could apply single phase voltage regulation at the load location to balance the voltage drop caused by the load, and we would still see current unbalance.

 

[NJWVUGrad]

Thank for the information all

It seems that removing the neutral conductor on the XO (208V) side fixes the problem.

I have been reviewing and I can't see any reason that this would not be a suitable long term solution.

Any thoughts?

 

[stevenal]

Is this strictly grid tie, or do you wish to supply backup power when the utility side is dark? Is the transformer neutral point fully insulated?

 

[prc]

stevenal, These 750 MVA banks are in a large Aluminum smelter plant. Power is taken at 400 kV from grid and captive power plant and fed to 220 kV regulating transformers for supplying to rectifier transformers. Since 220 kV circuit was open in the particular unit, harmonics are ruled out. As mentioned above phase voltage in one phase of 400 kV is less by 5 %. So that phase of delta tertiary will have 5 % less voltage. Then as explained by Bill, will this create a circulating current in tertiary to be offset by zero sequence current in primary?

 

[waross]

No problem with the math and with the oscillography posted by jgrist.
Problem with the interpretation.
With equal fundamental currents on all phases, there will be zero sequence current and the neutral current will be zero.
With triplen harmonics the currents add on the neutral.
I correct myself as to the source of the harmonics. The source may be the system or may be the transformer itself.
Given the low impedance of the delta winding to triplen harmonic currents a fairly low harmonic content may result in a higher than expected circulating current.
A time base on the oscillography will show the currents to be 180 Hz. Rather than the phase currents being displaced 120 degrees, the third cycle of A phase is in phase with the second cycle of B phase and both are in phase with the first cycle of C phase.
The delta winding provides a stable neutral when the power-in wye side is fed with a three wire system. That is, the power-in side wye-point is floating.
Circulating currents may be replaced by neutral inversion (a neutral shift away from the expected position) and possible over voltage switching transients.
POSSIBLE ISSUES: If the solar system becomes islanded, you may not have a stable neutral at the 120/208 Volt panel. Feeding the 120/208 volt panel with no neutral connection is equivalent to feeding a wye panel from a delta system.
Your normal operation is power in on the 480 Volt side. Power out on the 120/208 Volt side. The delta will allow you to run with no neutral on the 480 Volt side.
Your solution of opening the neutral is only a good solution when the transformer is being back fed. It may have issues during normal operation.
When the transformer is back fed from the 208/120 Volt side, the combination of a four wire connection and the delta winding will cause issues.
Again the delta winding allows you to feed power in the 480 Volt side without a neutral connection but I understand that you have a four wire connection on the 480 Volt side. If that is true you do not need the delta winding and you may open the delta.

We had a four wire wye:delta transformer feeding electrodes to provide extra heating to a bath of molten glass. At half load the transformer temperature was near the red line. Fortunately because of the way that the electrodes were connected to the transformer, we were able to easily open the delta without disturbing the load balance.
The drop in transformer temperature was immediate and dramatic.

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

 

[MatthewDB]

It seems that removing the neutral conductor on the XO (208V) side fixes the problem.

I have been reviewing and I can't see any reason that this would not be a suitable long term solution.

Without replacing the transformer, yes that works.

Utilities dislike any delta connected transformers because they can lead to ferroresonance. Hence they prefer wye-wye transformers. But that only works if you have 3 independent iron cores or a 4 or 5 leg core. A typical 3 leg core 3 phase transformer acts as a phantom delta even if a real delta connection isn't there.

There is a small risk of ferroresonance on a single phase condition with that neutral not connected; but it isn't any more than the risk found all the time when delta-wye transformers are applied.

 

[jghrist]

Bill,

The current on the graph I posted was the grounding transformer contribution to a single phase to ground fault on the utility system. The frequency is 60 Hz. The currents are all equal but are not 120 deg apart. They are all in phase and add to 3 times the phase current. There is no positive or negative sequence current. There is no 3rd harmonic current.

The transformer could be operated with the 120/208V neutral ungrounded, but not if the 480V breaker is closed. With the 120/208V neutral ungrounded, there would be limited Ø-grd fault current on the 480V side and the breaker may not trip for a Ø-grd fault.

 

[waross]

Hi jgrist
Thank you for the explanation.



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

 

[waross]

Back to the original question. As I understand it this is a 750 KVA, 277/480:120/208 Volt transformer back fed from the 120/208 Volt side.
Full load current is 2083 Amps.
The current of issue is 120 Amps or 5.8%
If we assume for the sake of illustration that this transformer has 6% impedance, then 120 Volts x 6% = 7.2 Volts to force 20083 Amps through the windings.
The current is only 5.8% of that so we are looking at 5.8% of 7.2 Volts or 0.417 Volts. For zero sequence the impedance is x3 so we may be looking at 1.25 Volts of triplen harmonic to send that current through the delta.
That's a triplen harmonic of just over 1%
I have made some assumptions but I believe that the ballpark is reasonable.


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

 

[stevenal]

With equal fundamental currents on all phases, there will be zero sequence current and the neutral current will be zero.

Kirchhoff says otherwise. The equation for the neutral point would be Ia+Ib+Ic+In=0. If Ia=Ib=Ic=I0, then In=-3I0.

I've seen a trace similar to the one jghrist posted that included current from a neutral CT. As predicted by Kirchhoff, the neutral current was three times the magnitude of each phase and displaced by 180 degrees.

 

[stevenal]

prc,

I'm not too sure you can rule out harmonics just because one side is open. If the other rectifiers are operating the harmonics can propagate to the 400 kV side and effect the transformer in question. Yes, the voltage unbalance can cause current unbalance. And since your 220 kV side is open, the positive and negative components are effectively filtered out just like the OP's case. The delta tertiary allows I0 and any I0 like triple harmonic currents to have a low impedance path.

 

[waross]

Steven and jghrist;
Thank you for your infinite patience.
I've got my head around it now.


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