Grounding of Transformer Primary not allowed by Utility 5

[NickParker]

Is there any time utility would not want your transformer to be grounded? If so, why?
Why the wye connected primary of transformer (grounded) should be a concern for utility?

 

[waross]

With a grounded wye primary either a delta secondary winding or a phantom delta caused by a three legged core will cause backfeeds into an open or faulted phase.
If only one primary phase is energized approximately 50% voltage will be backfed into the two open phases.
The available fault current in a single line to ground fault will be increased by the backfeed.

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

 

[cranky108]

Not grounding the primary, ensures the transformer will not contribute zero sequence on the primary.

 

[stevenal]

A grounded wye-delta creates an additional source of ground current for the utility, complicating protection. The wye point is typically grounded during single phase energization to avoid feroresonance, then ungrounded once fully energized. Grounded wye-wye transformers do not create ground sources.

 

[NickParker]

Grounded wye-wye transformers do not create ground sources

Can you please elaborate this further?

 

[davidbeach]

Compare the sequence diagrams for wye-wye and wye-delta transformers. The wye-wye (if both are grounded) will allow zero sequence to pass through the transformer but it won't source any zero sequence current. With the delta winding, the current circulating in the delta allows the transformer to source zero sequence current.

At least that's the case for ideal transformers. A bank of three single phase transformers will perform much like the ideal transformer in that regard. A three phase wye-wye can be different in a couple of ways. One is that the manufacturer may have included a delta tertiary without bringing it out, it's now a wye-delta-wye and behaves like one. A good look at the nameplate should be helpful in that regard. The other is that there can be phantom tertiary effects created through the case, creating the same type of circulating currents that a real tertiary would have.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[stevenal]

We would not allow a grounded wye-delta-wye either.
The presence of a delta allows zero sequence current to circulate around the delta. The unfaulted primary phases continue to feed the transformer, where they get transformed, circulate around the delta, and then get transformed back onto the faulted primary phase. The delta acts as a zero sequence filter. I saw a relay record once when a substation transformer was being back fed and a lg fault occurred on the wye side. It was interesting to see all three phases equal in phase and magnitude. Take away the delta (wound or phantom) and the zero sequence path back to the faulted phase no longer exists.

 

[dpc]

To their customers, electric utilities are the ultimate bureaucracy - Anything not forbidden is compulsory.

 

[cranky108]

Electric utilities are like many other medium to large companies. There are the few experts. several somewhats, and more newbies. This happens for each of the areas. Engineering, line people, legal, regulatary, field people, customer service. Each does something different, and for a reason.
If you ask the wrong people you will get the wrong answer. The answer you likely want is in the standards book, that was written by the experts.

Don't ask customer service engineering questions. And don't ask engineering customer service questions. I can't help you with your bill.

 

[davidbeach]

Any customer transformer that's a wye-delta(-wye) connected to our system requires special permission and that permission will include mandatory protection and a three-pole interrupting device. For most customers we strongly discourage going through that process. But when the customer shows up with a sufficient positive sequence source they become required to present as an effectively grounded source to our system. If they're large enough they then also get to pay for transfer trip.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[cuky2000]

The utility should be concerned if there are excessive ground fault levels. Interconnection at or near these substations naturally increases these levels and the additional ground source on the high voltage side increases them further. Below is a summary of some of the concerns identified for this subject:

^{• Solidly- and low impedance grounded systems may have high levels of ground fault currents.
• Utility may place special requirements on the connection transformer to increase the high-side grounding impedance.
• This will be done to decrease the ground fault contribution of the interconnection while still maintaining acceptable high-side grounding.
• These high levels typically require line tripping to remove the fault from the system.
• Ground overcurrent and directional overcurrent relays are the typical ground fault protection solution for such systems. However, high-impedance ground fault detection is difficult in multi-grounded four-wire systems, in which the relay measures the ground fault current combined with the unbalance current generated by line phasing and configuration and load unbalance.}

 

[Kiribanda]

If the connected custormer is only a "LOAD", then Utility does not insist
a WYE conection. But if the connected custormer is having a generation (PV, Wind, CO-GEN etc)
then the Utility is concerned about "what will be the grounding condition of their distribution or transmission network
when islanded". Therefore, when the custormer's generator is connected, the Utility wants to make sure that their distribution network is
still "Eff Grounded/ Earthed". To satisfy this condition, the transformer should have a solidly grounded WYE
on to the Utility side.But the problem is when you have such a WYE, the transformer is working as another zero seq.
source. As a result, during a ground fault, the Utlity contribution will be little lower and therefore, the
existing GF protection settings at the Utility sub have to be reviewed & re-adjusted. But if there is no generation then the transformer
can be WYE/WYE because for a ground fault WYE/WYE is equivalent to a DELTA/ WYE not a zero. seq source.

 

[NickParker]

If a ground fault occurs between the two transformers, say between the Utility transformer and the customer transformer, will the motors (connected to the secondary of the customer transformer) contribute to this ground fault? How would the contribution be different for the cases,
1) Customer transformer being Wye grounded primary - Wye grounded secondary
2) Customer transformer being Wye ungrounded primary – Wye grounded secondary
3) Customer transformer being Delta primary – Wye grounded secondary

 

[cranky108]

Actually most motors don't contribute much to a utility fault. There are exceptions, like some large motors, and those are usually easy to identify in the customer request for service.
Most processes, now use adjustable frequency drives, which break the connection of the motors contribution.

 

[waross]

Motors will contribute more to a line to ground fault than to most line to line faults.
The back EMF will collapse quite quickly and motors with low inertia loads will slow quickly, limiting the contribution to a line to line fault to a few seconds.
In the instance of a line to ground fault, the two healthy phases will continue to drive the motor as an induction generator and although the fault contribution will be less, it will persist for considerably longer.
With a line to ground fault the motor contribution may eventually be cleared by an inverse time device. The fault may have been cleared by other protection before the motor protection operates.

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

 

[Kiribanda]

Motor contribution lasts only for maximum of 2-3cycles due to decaying trapped flux.
Therefore, its impact is not significant unless it is a very large ind. motor.
Sync. generator contribution is significant because it is feding to any type of fault.
Therefore, depending on the type of exciter (PMG etc) it feeds to the fault until it is
tripped off from protection.
1) Customer transformer being Wye grounded primary - Wye grounded secondary :
Since there is a large mag. impedance in the eqt cct, between the primary & secondary,
the zero seq. current contribution to the fault from customer transformer is almost zero.
2) Customer transformer being Wye ungrounded primary – Wye grounded secondary:
Since WYE primary is ungrounded, for a GF at that side there is no zero. seq. current contribution
to the fault from custormer transformer.
3) Customer transformer being Delta primary – Wye grounded secondary:
Because of DELTA, the primary & secondary is decoupled for zero. seq. currents. Therefore,
the zero seq. current contribution to the fault from customer transformer is zero.

That means DELTA/wye is equivelant to WYE/wye for zero seq. current contribution. As a result
Utility prefers it.

 

[waross]

One possible exception to the motor contribution time.
With a combination of a far out fault where the voltage at the motor does not drop to zero Volts,
and
A high inertia load.
The motor will self excite and may contribute to the fault for more than 2-3 cycles.
When I was teaching, I had the students couple a DC motor up to an induction motor.
A load bank was connected in parallel with the motor.
We started the induction motor.
Then we started the DC motor and increased the speed until it was over-driving the induction motor.
Then the motor-load bank was disconnected from the grid, but the load bank was left connected to the motor.
The induction motor, driven by the DC motor, continued to supply current to the load bank indefinitely.
We did not test under fault conditions.
On the other hand, the motor contribution to a far out fault would be small.

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

 

[stevenal]

That means DELTA/wye is equivelant to WYE/wye for zero seq. current contribution. As a result
Utility prefers it.

I'm not so sure I prefer it unless that three phase delta customer is served only by three phase devices. An open fuse will cause the voltage to drop to 50% on the load side of that fuse, affecting the customer in question plus anyone else served off that fuse. Wye-wye transformers will not backfeed in this manner.

 

[waross]

Hi, Stevenal.
Not sure if you mean a primary fuse or a secondary fuse.
On the primary side, one open fuse and the bank becomes an open delta and it's business as usual.
The customer typically does not see any difference.
Two open fuses on the primary and you will see the approximately 50% voltage.
A real world example.
When I started working with the little island utility, there were two major customers with wye/delta systems.
Switching was phase by phase with a hot stick and fused cutouts.
After every outage, when the first phase was energized, 2/3 of the system saw the 50% voltage that you mentioned.
All the refrigerators and freezers would try to start and would stall.
When the second phase was energized, the voltage would jump and the stalled motors would trip out on the internal thermal switch; except those that didn't.
Burned out refrigerators were an issue.
It took several years to get the industrial customers switched over to wye:wye service and the refrigerator issues went away.
I had moved back to Canada but was visiting down south.
The manager of the utility comped me an air ticket and found to come out and just walk the system.
I found a wye:delta service had been installed.
The manager explained:
"That is for the National Teleohone company and they are too big to argue with."
"Well, we have to do something."
I had the crew remove one fused cutout.
We took the fuse link out and tack welded the toggle.
We put this back in the holder and while it looked like a full delta connection it was actually an open delta connection.
The customer never knew.
Capacity?
They had a 150 KVA transformer bank.
The three phase load was 17 KVA.

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

 

[stevenal]

Warros,

I believe you are speaking of wye-deltas. I was explaining why a delta-wye would not necessarily be acceptable to a utility trying to provide reliable service to all its customers. All in all, my favorite three phase distribution connection is wye-wye regardless of ownership. Legacy stuff exists, however.