SYNC CHECKING (machine rotating opposite to system) 1

[UPEE]

here's the scenario what would you expect to see?

a sync check relay monitors phase-phase voltage on both the incomming and running system. the incomming system is wired incorrectly with two of the generator wires crossed.

because of the crossed wires electrically the machine is rotating opposite to the system.

what would the single phase voltages look like at the sync relay? would you ever be able to match speeds and make the sync check relay think the system was actually in sync?

or would the single phase vectors still be going in opposite directions and not allow the sync relay to see a matching speed.

 

[Skogsgurra]

A synch relay hardly ever is single-phase. Is there a RL situation behind this question?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[UPEE]

All the sync relays i've ever seen take a single phase input from the running and line side. This particluar case is actually an auto syncronizer (with sync checking) so there is some ciruitry associated with it.

 

[Skogsgurra]

Then, we are dealing with completely different systems.

Any single phase sine has two components; one positive and one negative. I think that a single phase sync relay will have a problem telling the difference between two voltages with same frequency and amplitudes if there is no phase information. The positive component in V1 will match the negative component in V2, and vice versa. I think.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ZapSib]

Commissioning stage shall confirm that there is no cross wiring.
it may be advisable ( when auto sync are used) to use another check synchro relay to monitor proper operation of the auto sync one.
in such scheme the CS relay setting ( U,f, phi)window is including the one from the auto sync.
It should prevent the event you describe to occur ( if consider that wirings are different)

 

[rcw retired EE]

The two single phase voltages could be brought into synch and the synch check relay (or autosynchronizer) would allow breaekr closure. The relay can't see the other two phases, so it can't detect the improper connections.

A detailed pe-synch test is needed to verify proper connections, proper rotations, etc. to prevent the damgea that could occur.

 

[GTstartup]

Agree with RC and also used to seeing synch check and scope with single phase. This is why it's important to get it correct.

Best way to check is to disconnect the links at the generator and backfeed both sides, then you should read in phase at synch check and scope. Feed-forward works too if you can isolate your system and close synch breaker.

 

[waross]

I agree with rcwilson on this. If phase reversal is a possibility and you are using a single phase sync check relay, it would be wise to add a phase reversal check relay.
With a phase reversal it is not possible to have three phases in sync but it is possible to have one phase in sync.
respectfully

 

[UPEE]

well what happened here is some generator cables were changed out. two cables got crossed and we missed it on our phase checks. we DID NOT actually start and close the generator. i am just trying to figure out some what ifs as to whether or not the breaker would have closed if we had started the generator.

whats confusing is if you look at the vector quantities of the line and running 3 phase (phase to phase) voltage the vectors rotate in the opposite directions. so when you look at any individual leg of this graph it appears as though the phase-phase line and phase-phase bus voltage could never come into sync because they would cross each other 120 times per second at 60 hz.

i think what i've determined and the important point to remember is there is no rotation on single phase. so two single phase sources feeding a sync device will only match speed (HZ) and could care less about rotation.

 

[Skogsgurra]

You have the answer to that question in my first post (there are two components, one positive and one negative) and they match "cross-wise". At least mathematically.

In practice, you have two single-phase voltages with no phase information. You can not tell if it is rotating CW or CCW, nor can the relay. So, when both have the same frequency, amplitudes and phase difference is zero - the relay thinks it is OK to close the breaker.

That's why I never seen a single-phase relay in my life. I think.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[ScottyUK]

Gunnar,

Here is the chance to change that, on the screen at least:

http://www.areva-td.com/static/html/TDE-AGF_Product-Product_Detail1_1055920922999.html?

The KAVS is one of the commonest check synch relays out there. It is adequate for ensuring two systems can be paralleled safely once they have been commissioned and rotation proven by other means.


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Sometimes I only open my mouth to swap feet...

 

[rcw retired EE]

Our standard design is to place the auto-synchronizer and the synch check relays on different phases. That prevents automatically closing with reverse rotation.

When we have synch lights and synch scopes on a manual panel, I try to keep them on separate phases. A good operator would notice the lights going out at 4:00 or 8:00 instead of 12:00 and hold off closing the breaker.

Some old time diesel generator panel designers use three lights and a synchscope, with one light across each phase of the synchronizing breaker. The bright/dim flashing pattern of the lights indicates phase sequence and in-synch conditions and frequency match.

Good job catching the phase swap before it became a problem!

 

[Skogsgurra]

Thanks Scotty!

I needed that refresher.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[slavag]

Hi.
From my point of view, it's not important what is a connection to synch check relay.
Are possible both of them: phase to phase or phase to earth.
Important: correct connection to relay and test before synch.
For some "safety" we are prefer used phase to phase voltages.
For big generator prefer used two phase to phase or three single phase voltages.I think it's also requrements in US.
For HV lines many times we are used single phase voltages synch check, becouse on line usally we have only one single phase VT on phase S (L2, B).
More from that , for connection block of gen-trafo w/o gen CB in the old system used on line side single phase voltages and on generator side phase to phase voltages.
Regards.
Slava

 

[stevenal]

Recently used commissioning checks:

1. With generator breaker open, energize from system. Check all meters for expected rotation.
2. With system isolated from utility further upstream, start generator and close breaker. Check meters for expected rotation. Check syncroscope and synchronizing relay for zero voltage on the monitored phase across the closed breaker.

 

[DiscoP]

I am by no means an expert in this field, this question might correct a misunderstanding of mine !

I was always led to believe that the check synch element normally has a time delay.

If there was a serious problem (eg reverse rotation) wouldn't the check element be only picked up for a very short time and not time out, therefore the check contact would not close ?

 

[davidbeach]

No, think of it this way:

Take a three phase circuit and split it into two branches. Connect each branch to a side of an open breaker but on one side swap phase B and C compared to the other side. A is opposite A, B opposite C, and C opposite B. Now apply a single phase sync check relay on phase A. The relay will see zero volts difference and locked phase, after all there is a direct connection between A and A. But you wouldn't want to close the breaker. Sync check on the second or third poles of the breaker would never permit closing.

Now split the branch and connect both branches to separate sources. There is no reason that the previous condition could not be repeated.

It is absolutely essential that it be proved that both sources are rotating the same direction before using single-phase sync check relays.

 

[DiscoP]

Thanks David but I'm still missing something.

I agree that a single phase check relay would not pick up incorrect phase sequence - I wouldn't think that a check synch device is meant to check wiring is correct.

However the part I still don't understand is even if one side had an incorrect phase sequence, wouldn't the A phase vectors be rotating in the opposite direction to each other if the generator is rotating in the wrong direction, thus only in synch for a very small period of time - much lower than the time delay.

PS - I might be talking about something different, rotation can mean phase sequence ??

 

[jghrist]

DiscoP,
Yes, you are talking about something different. Rotation in this sense does mean phase sequence. B follows A, C follows B. With only one phase, there is no rotation in this sense. The voltage increases and then decreases and then increases again, mathematically according to a sin function. I goes up and down because something is rotating (a generator rotor), but this is a different rotation.

 

[davidbeach]

CCW is A-B-C, CW is A-C-B (or C-B-A). A is A, B is B, and C is C so even if two systems are rotating in opposite directions it is possible for one phase to be synchronized between the two while the other two phases are very much not synchronized.