Utility Plant Transformer configuration for Backfeeds

[RobsVette]

Hi Guys,

Here in NYC I am working on a cogen project for a college and the topic of power export to the utility came up. The concept that was mentioned was to export power to the utility via existing feeders into a large campus substation, via existing transformers, just with power flowing in reverse. It goes without saying that you would need to change out all teh protective relaying, as this is probably all set up for power flow into the facility. However would the transformer configuration need to change?

The current transformers are 26KV Delta configuration on the utility side and 4160 Wye configuration on the load side. I asked about this and was told that this would not be a problem to reverse the power flow across this transformer. I assume this would only work if the 26KV side is a delta configuration to begin with. As far as I understood, if you reverse the power flow through a Delta to Wye transformer, you would be created a delta source on the utility side. If the utility side is a Wye source with a Delta connected load, wouldnt this create a major problem (due to the normal reasons of 30 degree ofset, line to ground voltage, etc...)

My question then extends further to utility plants in general. How are backfeeds done to connect eletrical generator to utility grids through transformers?

From what I understand (please correct me if I'm wrong here) most generators are Wye so that they can be grounded. They would then go through a synronization breaker and feed a GSU type transformer that would have a delta x wye configuration (Delta on the generator side, since this is the power in side, and wye on the utility grid side, since this is the power out side.) At least this is how all the ones I remember seeing in the past are (I might be remembering wrong.)

If thats the case, when you first energize the transformer and try to bring power to teh sync breaker, would you have a delta source trying to sync with a wye generator?

I know this is done very often, but the more I understand, it seems like the more questions I have.

I was hoping you guys could clarify my understand on how typical utility plants are configured to make these connections work.

Thanks in advance for any help.

Rob

 

[collies99]

Consider the supply transformer,as a black box, power goes in and power goes out. It does not matter which side is which.

Synchronizing to an existing system will work.

There may be some design considerations if you contemplate operating in island mode.

yes, protection may need looking into.

 

[wroggent]

Is 4160 low for the high side of a GSU?

 

[dpc]

Transformer works equally well in either direction, regardless of connection. However, the utility may have some requirements for ground fault detection that make the delta on the utility side a bit of a problem. The utility needs to make sure your generator does not become islanded with a part of their system if their substation feeder breaker opens. So you need to have protection that can detect faults of all types on the utility system and trip off your generators, or open a tie breaker to island your system - they don't care which generally - that's your problem. This will all be easier if the utility side of the transformer is a grounded wye (in general). But this will also depend on the size of your generator relative to the load on the feeder and other factors. I'd check with your friendly local utility if this is under serious consideration. The delta primary can work, but you'll have to do whatever the utility says is required.

 

[waross]

A wye:delta connection may work well but there are some unique pitfalls. If the primary neutral is floating, you can get serious transient over voltages upon energization. You can avoid this issue by never energizing the transformer by a switch closure on the wye side.
That may be easier than it sounds. If the transformer is already energized on the delta side the co-gen may be connected safely. If the utility fails while the co-gen is on-line no problem.
In the event of a power failure when the co-gen is off-line, open the primary feed, close the co-gen breaker and then start the co-gen. You will probably have to do this in any event to deal with inrush issues.
If the transformer wye point must be grounded to service single phase loads, or for code compliance, Then the delta will back feed into a fault and the available fault current may be double what is indicated by a calculation which does not consider the back feed.
Any voltage unbalance on the wye voltages will cause a circulating current in the delta that will be limited by three times the impedance. For example, a 15% unbalance on a transformer with 5% impedance voltage will cause full load current to circulate in the delta. (15% unbalance. 3 x 5% Z = 15%)
This is not to say that you cannot use a wye:delta transformer, it is done. Just find an EE who is experienced with this type of issue.

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

 

[Kiribanda]

RobsVette,
The connection symbol of the transformer doesnot matter for you to export power. But there is a protection & safety issue.
The Utility is always effectively grounded and therfore by adding your generator also the Utility has to be effectively grounded.
If it is not then you may have to make provision in your generator grounding method. Therefore, if the transformer is having
DELTA on the Utility side, then when there is a ground fault at the Utility side only the upstream Utility breaker will trip but
your generator is continuing to supply power to the fault which makes a serious safety issue unless & until you trip the generator
breaker from a transfer trip (TT) signal from the upstream Utility breaker.

 

[RobsVette]

Hi guys,

Thanks for the reponses. The only thing I dont understand at this point is

If the source on the utility side is a wye and all the transformers are connected as delta on teh utility side and wye on the load side and the customer tries to export power, wouldnt you have a wye source in parrallel with a delta source?

As I understood, it this is not a good idea due to the fact that there would be a 30 degree phase difference from the delta to the wye. I am thinking of this as teh equivalent of parrelleling a delta wye transformer to as delta delta transformer.

If the utility is a delta configured system them I would think it would work just fine. However, I assume it is a wye connection with a grounded neutral for the distribution to customers.

Any further insight is appreciated.

thanks,
Rob

 

[davidbeach]

If A phase on the utility is zero degrees, then A phase on the cogen is at -30 degrees and the transformer nicely lines them up.

 

[waross]

You can not parallel a wye:wye transformer with a wye:delta transformer if both are fed from the same source.
But
With a single transformer between a co-generator and the utility supply, there is no problem. Generally the co-gen is started and run up slightly above synchronous speed. Sometimes less than one cycle above synchronous speed. The phase angle between the utility source and the co-generator is then constantly changing from 0 degrees up to 360 degrees (which is back to 0 degrees). When the synchro-scope sees the source in phase with the co-gen, the breaker is closed locking the two sources together. The slight advance of the throttle picks up a little load on the co-gen to avoid a reverse power trip. As The co-gen is now locked to the utility. There is a 30 degree difference between the utility and the co-gen which the transformer accommodates as David points out.

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

 

[collies99]

If RobsVette can provide us with a single line sketch, it would alleviate the discussion.

 

[RobsVette]

Hi Guys,

Thanks very much for the help. My misunderstanding has been cleared up. I now understand what I did not before.

What I was misunderstanding was that when you parrallel to the utility grid through a delta wye transformer, you are parralleling your second source to a first source. So the transformer configuration wouldnt make a difference. As opposed to using a delta wye and wye wye transformer from the same source.

Thank you also for the insight on the grounding and utility issues. I now understand the potential problems that can be created with this type of connection.

I can consider the issue closed at this time. Thanks for the help and understanding.

Rob