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Out of Phase voltages?

Out of Phase voltages?

Out of Phase voltages?

(OP)
This question is mainly in regards to consequences. I have a station service throw-over scheme to put in service; and I'm wondering what happens if it's connected wrong. I'm talking phase A station service tx on the normal and accidentally using phase C station service tx on the emergency. What's the worst consequences assuming no sync check relay and no time delay?

 

RE: Out of Phase voltages?

A big explosion.  Closing in out of phase is equivalent to a short circuit.   It could cause serious injury to someone nearby.   

RE: Out of Phase voltages?

If the upstream source that is feeding both the normal and emergency bus is the same then there would no possiblity to synch out of phase.  I am assuming that is why you mentioned no synch check relay in your question.  Therefore if the basic throw-over scheme is just an under-voltage condition on that bus I do not see any issues with using two different phase sources.  I still wouldn't feel comfortable using different phases though.  Is this for a distribution substation?

RE: Out of Phase voltages?

(OP)
That's what I always thought, but I've never seen that mistake made or an explanation on paper of the consequences.

RE: Out of Phase voltages?

(OP)
yes, it's for a distribution station. someone asked me what would happen if the station service transformers fed from phases A & C respectively some how were briefly in parallel. All I could say is I think that would be bad; but I want a more informed opinion of exactly what would happen. ideally I would use the same phases; but i'm looking for worst case problem.

RE: Out of Phase voltages?

Closing out of phase can create currents larger than the maximum short circuit since the voltage difference can be greater.  The high currents can destroy switchgear and transformers that are in the path between the two out of phase sources.  (Assuming a closed transfer is used where all sources are momentarily connected together.)

If the transfer is open (first source disconnected before second source is connected) the currents will be less but still very damaging, especially if any motors are running.  The out of phase transfer essentially tries to jerk the motors ahead or behind 120 electrical degrees instantaneously, possibly causing considerable mechanical damage to the motor, the couplings, the load and the foundations.
 

RE: Out of Phase voltages?

Open transfers are commonly used for this application. Induction motors must come into phase every time they are started, they simply slip until they get there. Syncronous motors could be damaged. I haven't seen such a motor on a station service. A time delay would be advisable just so you don't have a transfer every time the voltage dips from a transient fault.

RE: Out of Phase voltages?

Hi.
You MUST check phasing of regular and emergency sources!!!
First issue is explained above. It's SC on the bus in case of paralleling of two sources. Or stress on the motors in case of fast transfer.
Second issue, if your emergency in out-of-phase , your motors will run in reverse rotation.
Best Regards.
Slava

RE: Out of Phase voltages?

At one station instead of using an automatic transfer switch we are paralleling the two services at the DC level, keeping the battery charged if either AC source goes down. A small manual transfer switch can put the lights back on.

RE: Out of Phase voltages?

As cbradley mentioned this is a Distribution Substation, a utility I assume.  I doubt that there are any motors being fed in this substation and furthermore this substation is probably comprised of outbound feeder bays only.

Now look at it this way cbradley, I am not aware of what kind of scheme you have at your distribution substation, but consider the following scenario:

If you look at a basic two transformer substation feeding two separate buses and a normally open bus tie CB between them.  You will typically not find a synch check relay involved in the closing circuit of this bus tie breaker, at least from my experience and the utility I use to work for.  Now, if you think of one station service transformer (normal) being fed from one bus and the other station service transformer (emergency) being fed from the other bus why would the secondary's of these transformers need a synch check relay if the HV primary bus tie breaker closing circuit does not?

My comments are only based on assumptions of the configuration of your distribution substation.  There could be factors that I am not aware of that would could lead to unforeseen issues that would require a more thorough throw-over scheme.
 

RE: Out of Phase voltages?

Clos... this is a reason. No motors, distribution substation.
And what with your customers????
After repair of cabels, second thing is phasing.
Sorry, I don't agree.

RE: Out of Phase voltages?

slavag,

Not sure if I am following your concerns correctly.  I think that ultimately it would require a better overview of this situation (schematics) to come to a clear understanding of this situation.

clos

RE: Out of Phase voltages?

A distribution substation will typically have motors to operate LTCs or voltage regulators, pumps and or fans on the transformer, and HVAC for the control building. Usually single phase induction motors that will spin only in one direction and all fed from station service.

RE: Out of Phase voltages?

What about three phase oil pums for the big trafos, what about three phase air-cond. systems?

RE: Out of Phase voltages?

slavag,

Note that I said usually. Always exceptions.

The scenario clos proposed is actually a good argument against a closed transition. If transformer #1 trips taking out the primary station service initiating a closed transition to the other service, during the closed part of the transition both services will be overloaded as they try to pick up the bus 1 load. If current is high enough, or closure is long enough, both fuses will blow leaving no station service.
 

RE: Out of Phase voltages?

Guys sorry!!!!!.
I'm not right, I re-read all posts again .
Sorry, my bad. Some blackout.
Clos , I don't understand you .
Best Regards.
Slava

RE: Out of Phase voltages?

The original question is what would happen if two sources that are normally in phase were accidentally misconnected out of phase with no time delay (I'm paraphrasing so excuse me if I am wrong.)  Later it was clarified by the OP as two sources "briefly in parallel"

In this case I agree with Slav and dpc and all that described a catastrophic failure

RE: Out of Phase voltages?

One way to avoid "out of phase" transfer is using an Automatic Transfer Switch (ATS) with phase rotation mismatch sensing on both Normal (utility) and Emergency (typically generator).  This controller sensing will not allow transfer is phase rotation is incorrect.

See more from ATS manfacturer, GE Zenith who provides "out of phase" transfer sensing.

RE: Out of Phase voltages?

a simple phase sequence relay (47) (timemark?) can do the same thing

RE: Out of Phase voltages?

Neither of these two solutions may help if the wiring is bulloxed, which it would have to be to physically allow this in the first place.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Out of Phase voltages?


What if the phase rotation was correct and when closing the same phases were closed together however they were slightly out of phase due to different circuit paramaters on each network.

In other words, instead of closing in phases A to C (Big Bang!) we closed in both A phases (A to A) however these A phases were out of phase by some amount due to paramaters (Network Reactances) on their two respective networks.  Would we still have a big bang?

RE: Out of Phase voltages?

Depends on what the magnitude and phase angles on both sides were just prior to closing.  Less than 15° close to generators or less than 30°further away and you probably won't have any particular problems.  Every case is somewhat different and your mileage will vary.

RE: Out of Phase voltages?

I've mentioned before the little power plant with a connection issue to the synchroscope. This was a mix of 350 kW and 600 kW diesel generators. As the load changed, there were generators going online several times a day. Some days 6 or more synchronizations.
The synchroscope was 30 degrees out. This was generators in the same plant and real power,not a reactive phase shift. Most times the installation would survive closing at 30 degrees of displacement. Once or twice a month, when the boys closed a little early and at a greater angle, a breaker would trip.
Once or twice a year they would come in a lot early and blow a coupling.
This was real power and the generator would have to make an abrupt angular shift to line the poles up with the rotating field. The equipment would survive closing at 30 degrees of displacement.
Your question is a little different. If the phase displacement is the result of reactive voltages, that implies that the lines are at differing power factors.
Closing on this would result in enough reactive current flowing through the breaker to give a weighted average to the power factor. Probably no big deal.
HOWEVER
If your 30 degree displacement is the result of an odd number of delta/wye conversions, it becomes more serious. Close in expect heavy currents possibly in excess of full load current. With a lot of miles of line expect a lot of energy to be lost heating transmission lines.
By the way, I agree completely with davidbeach. His system is many times larger than mine and not as forgiving as the small generators.

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

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