Grounding Transformer In distributed wind generation 2

[rockman7892]

In relation to my other recent post I'm looking at an application where there are (3) 3MW wind turbines connected to a 12.47kV collector feeder before interconnecting to a 12.47kV distribution line (there is actually a voltage regular between turbines and utility line to bring 12kV turbine voltage up to 12.47kV.

The collector feeder is interconnected to the 12.47kV utility line through a re-closer and just prior to the recloser on the generation side there is a Delta-Wye grounding transformer. I'm trying to figure out the role of this grounding transformer. My assumption is that if there is a ground fault on collection feeder recloser will detect but once recloser opens the grounding transformer is there to provide a ground fault source to allow wind turbines to detect and shut down? Without this grounding transformer once the re-closer opens with no ground fault source the turbines may not be able to detect ground fault (assuming turbines have a wye-delta step up at each turbine with delta side facing collection feeder but trying to confirm).

Is my assumption above for purpose of grounding transformer correct? I've never seen one that operates in parallel to the utility (usually see grounding transformer on delta side of transformer).

The utility is now going to upgrade distribution line from 12.47kV to 24.9kV requiring us to put in a step-up transformer on collector feeder to step voltage up to 24.7kV. Planned location for this transformer is after re-closer existing (re-closer will remain as xfmr LV protection) and then a new 25kV rated recloser will be located on transformer HV side to connect it to utility line.

With the addition of this transformer, I'm trying to determine impact it will have on grounding transformer. Typically I'd see a Delta-Wye interconnection transformer with delta on LV collector side this in this case allowing grounding transformer to be sole source of ground fault current on collector system. I have seen however that the utility may require a wye-gnd / wye-gnd interconnection transformer and have seen a similar nearby site with a wye/wye interconnection transformer have the grounding transformer on the utility side of transformer.

Appreciate any feedback on the above as it relates to existing grounding transformer impact on delta/wye transformer as well as a potential wye/wye transformer.

Thanks

 

[waross]

12.47kV to 24.9kV requiring us to put in a step-up transformer

That will make the 12.47kV system a separately derived system.
Ground it as you wish.

Delta-Wye grounding transformer.

Are we using different conventions?
Did you mean a Wye-Delta grounding transformer?
That is: The wye sidse connected to the line and to ground and the delta with a broken corner to detect primary grounds?


--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[rockman7892]

Are we using different conventions?
Did you mean a Wye-Delta grounding transformer?
That is:The wye sidse connected to the line and to ground and the delta with a broken corner to detect primary grounds?

Apologies, yes I meant to say a Grounded wye-Delta where wye side is connected to line and ground. Delta side however is a closed delta configuration not broken. This is typically just a conventional transformer that can used as a grounding transformer.

 

[waross]

There are variations. A ground fault current limiting resistor may be placed in the wye side or in a broken delta.
Ground faults may be detected by either CTs or by the voltage rise across an open delta.
Rather than a 12.47kV to 24.9kV transformer have you considered a 12.0kV to 24.9kV transformer (or a 12.47kV to 24.9kV transformer that may be tapped down to 12.0kV) to render the regulator redundant.
If there are additional reasons for the regulator, forget that I mentioned it.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[rockman7892]

That will make the 12.47kV system a separately derived system.
Ground it as you wish.

What about if the interconnecting transformer is a wye-gnd / wye-gnd transformer that will allow zero sequence current to pass. Does it matter in that case weather the system grounding transformer is on generation side or utility side of transformer? I'd assume it needs to be on generation size of interconnecting breakers in order to source ground fault when re-closers open and utility source ground is lost?

Is there any reason why the utility would want grounding transformer on their side? Some of their interconnecting documents appear to show it that way.

 

[waross]

Is there any reason why the utility would want grounding transformer on their side? Some of their interconnecting documents appear to show it that way.

The utility may not always be right, but they're always the utility. grin
Not enough information to second guess the utility.

a wye-gnd / wye-gnd transformer that will allow zero sequence current to pass.

True but the fault current does not magically appear.
A ground fault current on the wye of a delta wye transformer bank is reflected as an overloaded phase on the delta.
While it may seem counterintuitive, a single phase load or fault on a delta system will be supported by current in all three phases.
Explanation:
The easiest way to visualize this is to consider the characteristics of an open delta.
Given a firm supply, and assuming identical transformers, the virtual transformer formed by the open side of an open delta will have the same characteristics as either of the two existing transformers.
That is, the voltage drop under load and the Available Short Circuit Current will be the same.
A 100 Amp load on an open delta will cause 100 Amps at 0.5 power factor leading to flow in one transformer and 100 Amps at 0.5 power factor lagging to flow in the other transformer.
If a 100 Amp load is applied to one phase of a full delta, the in-phase transformer will contribute 50 Amps and the other transformers will each contribute 50 Amps at 0.5 PF.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[Kiribanda]

Can you please upload your SLD?

 

[wcaseyharman]

I was part of this discussion in my utility for a while before I got too busy.
Many if not most utilities use an effectively grounded distribution system. Effectively grounded is defined as X0 =< 3*X1. One of the main reasons to effectively ground a system is it allows you to connect line to neutral loads and limits the potential unfaulted phase overvoltage due to ground faults to 138% of the nominal line to neutral voltage. Utility equipment, such as surge arrestors, is all sized and specified based on the effective ground assumption. Customer over voltages are also kept to that 138% magnitude which (mostly) prevents damage to customer equipment during faults.

My guess is the utility is requiring all distributed connected generators to be effectively ground to ensure that the unfaulted phase voltages stay at or less than the 138% during faults during the brief time where the substation breaker could open before the distributed generation trips.

The 1964 Westinghouse T&D book has a good discussion around this topic.

 

[protoslash]

I agree with the TOV consideration mentioned above.

Just to directly answer the original question. The purpose of the grounding transformer is to limit TOV during fault and less to do with fault detection. Here is a sequence of events.
- Feeder SLG fault
- Upstream substation breaker trips
If all transformers are delta(12kV)-wye, and DG is not disconnected
- Entire feeder lose ground reference
- Faulted phase becomes ground potential
- Healthy phase-ground voltage goes to the full line-line voltage (TOV)
- Either DG trips on anti-islanding protection and fully clears the fault and TOV or island is actually maintained and all arrestors are blown

If you install the grounding transformer (or any ground source configuration such as wye g-delta, or wye wye delta tertiary), the system can still be effectively grounded after upstream feeder breaker trips and the TOV scenario does not appear. The fault contribution from the DG with the grounding transformer is an added benefit, but it is likely not reliable enough to be the anti islanding detection. Most DG still rely on under/over frequency/voltage to trip off.

 

[rockman7892]

Can you please upload your SLD?

Attached is SLD. The black is what is currently there today and the red is what is being added for transformation to 25.9kV. I Appreciate any feedback.

@protoslash thanks for your explanation on purpose of grounding transformer, that makes sense now.

 

[rockman7892]

For some reason SLD didn't attach to previous post so trying again here.

 

[Kiribanda]

1) You are correct. The existing grounding transformer is directly connected to the existing 12.47kV distribution system.
Per the given SLD, currently, the existing 12.47kV distribution system is ALREADY effectively grounded
(Utility will not tolerate non-effectively grounded systems) and therefore, this GND transformer is not required at all.
As a result, since its primary is solidly grounded, for all GFs on anywhere on the 12.47kV distribution system this GND
transformer is also working as a zero seq. source thereby contributing to the GF. I have seen once such a GND transformer
directly connected to Utility distribution system burnt due to a GF elsewhere on the system.
2) Once you introduce the new 12.47-25kV step up transformer, then, since your 12.47kV system now becomes ungrounded DELTA, this GND
transformer will do its job during a ground fault on the 12.47kV system. In my opinion before you introduce the new step-up transformer,
you should connect your existing DELTA of this GND transformer as a BROKEN DELTA and connect a voltage relay to measure the zero seq.
voltage during a single phase to ground fault on the new 12.47kV ungrounded system. Then you can use that zero seq. voltage to alarm
or trip the 12.47kV system

 

[rockman7892]

1) You are correct. The existing grounding transformer is directly connected to the existing 12.47kV distribution system.
Per the given SLD, currently, the existing 12.47kV distribution system is ALREADY effectively grounded
(Utility will not tolerate non-effectively grounded systems) and therefore, this GND transformer is not required at all.
As a result, since its primary is solidly grounded, for all GFs on anywhere on the 12.47kV distribution system this GND
transformer is also working as a zero seq. source thereby contributing to the GF. I have seen once such a GND transformer
directly connected to Utility distribution system burnt due to a GF elsewhere on the system.

What about a situation when the existing recloser detects loss of utility, utility fault, or collector circuit fault and opens thus leaving the generation collector circuit ungrounded until wind turbines are shut down. Assuming that each turbine has a step-up transformer to 12kV with delta side facing collector circuit (i understand as typical) there would be a period of time where a ground fault on collector circuit could cause overvoltages if the grounding transformer was not there?

 

[rockman7892]

In looking at this application a bit further I'm not certain that I need the new 24.7kV high side recloser that I have shown on one-line. In looking at most distributed generation applications connected to utility distribution I found that there is only a breaker on low side (DG side) of interconnecting transformer but not typically on utility side.

A couple of questions here related to that:

1) Fault on 24.9kV feeder between utility breaker and interconnection transformer - Is detecting ground faults on utility feeder typically necessary or is simply monitoring voltage/frequency of utility system adequate to open/disconnect DG upon loss of utility (when utility feeder breaker opens). Is there an advantage to being able to detect utility ground faults and trip DG breaker/recloser quicker than waiting on utility feeder breaker to open? Perhaps a sensitivity issue with utility breaker not being able to see ground faults at end of line?

2) Fault on 24.9kV feeder between utility breaker and interconnection transformer - If we did want to detect ground fault on utility is easiest way just putting CT on neutral of interconnection transformer to monitor 51N? Can CT's & VT's on DG side of transformer be used to detect ground faults on utility side? What about just adding a PT to 24.7kV side to look at voltages during ground fault?

3) Fault between 12.47kV (Delta) side of interconnection transformer and existing 12.47kV recloser- With the grounding transformer in place here I would expect the recloser to see any fault between interconnection transformer and recloser with ground fault sourced from grounding transformer, and thus trip the re-closer. The issue I see here is that the utility breaker would not be able to see this ground fault so the ground fault would continue with the grounding transformer now removed ( removed when re-closer opened)causing overvoltage issue.

4) Fault between 12.47kV recloser and wind turbines (fault on collector feeder) With the grounding transformer in place here I would expect the recloser to see any fault between recloser and turbines with ground fault sourced from grounding transformer, and thus trip the re-closer. With re-closer tripped grounding transformer would still provide effective grounding of collector circuit until turbines detected and shut down.

Based on above it looks like the only case where it may make sense for a high side recloser as I have shown on one-line is case #3 for fault between interconnection transformer and 12.47kV recloser.

 

[wcaseyharman]

1) do you have transfer trip installed so your recloser trips when the utility breaker opens, or are you planning on trying to detect an island with voltage or frequency?
It seems like unless you have a communication scheme it would be wise to detect faults on the utility system to minimize the risk and time of your equipment feeding a utility fault.
2) you can see utility ground faults on the gen side, but they will be low magnitude. You will want to use something like a voltage restrained overcurrent element (51V).
Most large generators have 51N elements on the step up transformer neutral for ground fault detection on the high voltage system. Both of these will need to coordinated with the utility protection so you don’t trip off for faults on fuses laterals or on other feeders.
Voltage detection could work, but you’d want to time delay it longer than the longest fuse or breaker operating time to prevent tripping unnecessarily.

3) the recloser would have to be set pretty sensitively but should be able to detect ground faults on the delta side if it’s coordinated with the lower voltage recloser. I would think you’d probably want to use directional elements here to improve the sensitivity of the 26kV recloser to a reverse fault.
If the low voltage recloser is open, then no, there is no detecting ground faults unless you install PTs. You’ll likely want a loading resistor and a broken delta PT connection if you do that to prevent ferroresonance.


4)agreed

I doubt the utility will allow you to have a transformer connected to their system without high side protection, either a breaker, recloser or fuses, as they would not allow a fault in that transformer to drop other customers on their feeder.

 

[rockman7892]

@wcaseyharman

For detection of utility side faults would the PT's for volage detection need to be on the utility side or could they be on the gen side of transformer (delta side)? Would PT's need to be connected in wye-gnd/wye-gnd config or would this be wye-gnd/broken delta for use with 59N function?

 

[wcaseyharman]

PTs on the utility side would be the best and easiest to set. I’m guessing you could probably make due with gen side PTs if you had to, since the gen voltage will likely drop pretty low for a fault on the utility side, but you’d have to model it to be sure.

 

[cranky108]

Problem with Gen side is you don't know when the utility has returned, and you likely need sync PT anyway.
But if you need sync, you also need gen side PT.