Out of Phase voltages? 1

[RelayDesignMan]

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?

 

[dpc]

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

 

[ACCOTTER]

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?

 

[RelayDesignMan]

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

 

[RelayDesignMan]

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.

 

[rcw retired EE]

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.

 

[stevenal]

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.

 

[slavag]

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

 

[stevenal]

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.

 

[ACCOTTER]

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.

 

[slavag]

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.

 

[ACCOTTER]

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

 

[stevenal]

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.

 

[slavag]

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

 

[stevenal]

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.

 

[slavag]

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

 

[GTstartup]

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

 

[Szalkus]

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.

 

[DTR2011]

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

 

[itsmoked]

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