Phasing an induction generator 5

[Rodmcm]

I am to commission an old 11kV 1000kVA induction generator. How can you reliably and easily check that the phase rotation of the rotating generator is the same as the grid?

 

[Skogsgurra]

There's always some remanence in the rotor. In an 11 kV motor, there is enough to kill someone touching the terminals while the machine is rotating.

You can use this fact to determine phase rotation. Connect an oscilloscope or a recorder to the terminals and rotate the machine by hand or using a power tool. Observe the induced voltage and see if A comes before B and B comes before C - then the rotation is positive. If not, it is negative.

See attached recording. In this recording, the machine was given a little nudge to start rotating. The rotation slows down during the recording, which can be seen in the waning amplitudes.

Gunnar Englund

http://www.gke.org

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[itsmoked]

Nice Gunnar!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[pwrengrds]

Hot sticks across the breaker a to a, b to b, c to c, when the syncroscope is at zero is very reliable. If you have PT's and relays that detect phase rotation also can be used.

You can also use a phase rotation meter on the secondary of the PT's, though be careful because some rotation meters don't work well if you have delta PT's. What I have done is use the phase rotation meter and then reverse the connection and check it again, checking to get a reversed phase reading with the leads reversed.

I have used both, when using the PT's I like to use the same set of PT's both times to double check the PT connections.

Good luck and be careful, David

 

[Skogsgurra]

There is not enough voltage on the induction machine before it has been connected to 10 kV. Remanent field usually does not produce more than a few hundred volts at nominal speed. Enough to kill a man, but not enough for hot sticks.

Has to determine direction by other means.

Gunnar Englund

http://www.gke.org

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[waross]

You can try energising with a low voltage and see which way it turns. Rotation should be the same either as a motor or as a generator.

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

 

[Skogsgurra]

Of course! Starting an 11 kV motor on LV is trivial. That's a good one, Bill!

Gunnar Englund

http://www.gke.org

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[pwrengrds]

oops, didn't pay attention to the induction generator part.

 

[edison123]

Is this a wind turbine generator ?

I would disengage the prime-mover, start the induction generator as an induction motor, direct-on-line and note that direction of rotation. If it matches the rated direction of rotation, then you are done. Else, you switch any two phases.

Does anyone see any problem with this ?

 

[itsmoked]

Just the shaft job! Er.. I mean the hassle of decoupling that large a motor from a load.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Rodmcm]

Thank you, however, some comments.
A)residual magnatism may be in a new machine from the workshop testing, but can be problematic in an old machine
B) This is a hydo machine(s) on the end of a long rural feeder. Starting DOL would take out the line, but at least I would only do it once!
C) However, what if the generator is in the region of 4MW(one was commissioned here the other week with a huge inertia, they are not unusual)
D) Often with hydo turbines the turbine is overhung on the generator shaft, so a bigger job disconnected to check direct on line
E) I have heard that there is a technigue for running the generator up to speed and closing in only two phases, measuring the third. I can't make this work in my head, any comments?

 

[cranky108]

Interesting question, as we may have one like that soon. Except not so big, and at a lower voltage.

If it's a hydro unit I would think you can spin it with the breaker open. Between the PT's om the primary, and a high voltage probe on the unit you should have enough voltage to verify the phase connection. Although the rotating frequency means you have to look at more than one phase at a time.

Connecting two phases makes the unit a single phase machine in essence, which means sending or recieving power will make the machine unbalanced.

Interesting enough closeing two phases of some capacitor banks, and phasing in the third is something that's being done now.

 

[edison123]

Rodmcm

If it is an old machine, how were they synchronizing before ?

And, residual magnetism doesn't die down with age.

BTW, I don't think synchronizing by generator residual magnetism is fool-proof. The 11 KV PT could have been wrongly wired leading to a misleading assumption that the phases are matched.

I have always perfered a single PT being fed individually from the line or the generator at anyone time and checking with the sequence meter. But, in your case, I am stumped.

 

[Rodmcm]

I have done several induction generators from 11kV to 400V, all were new and of course we had faith in the 'standards' for phase rotation wrt to the drive end. I have never had a failure, however, these are old machines being reused and I cannot necessay trust the rotation and phase markings on the windings. Hence I have been thinking about a full proof method. It also needs to be practical in that the local service supplies for a 2 MW hydro can be less than 50kVA, there is little in the way of a local service supply on the site to run up a 1000kW motor. I know of and have used the methods of Gunnar Englund, but I am also wary about residual magnetism in induction motors/generators.

 

[davidbeach]

Rent a 2MW gen set at 480V. Take to site and set up, verifying phase rotation of generators matches phase rotation of utility connection. Connect to "generator" in question and motor it. With the wicket gates closed you should get some motion out of it. Verify proper shaft direction, if not swap leads from the "generator" and try again.

 

[Rodmcm]

Yes Davisbeach, perhaps that is the only way... expensive though.....

Edison123...I do not know, they where uplifted from a decommissoned plant and are being relocated. However, we are seeing more and more small plants that are using second hand motors. I have just redone the automatic controls system on a 2 x 1000kW pelton system with old 3.3kV mining motors. Been running for 6 years.

 

[Skogsgurra]

"A)residual magnatism may be in a new machine from the workshop testing, but can be problematic in an old machine"

With what facts are you backing this?

I have used it so many times that I can guarantee it will work on every machine, from FHP to MW. Even if there are only a few volts, the sequence will always be true.

Gunnar Englund

http://www.gke.org

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

 

[Skogsgurra]

Rodmcm,

Your question "E) I have heard that there is a technigue for running the generator up to speed and closing in only two phases, measuring the third. I can't make this work in my head, any comments?"

The answer is: YES!

That is how many small workshops produce three-phase systems out of two phases. I did not have any recordings, so I set up a small machine (.55 kW/400 V/50 Hz) to get one. See attached file.

As you can see, there are three very distinct phases. Because the machine is small and run off 230 V instead of 400, the resulting three-phase system is not perfect. But there is no doubt as to phase sequemce.

If you can run the generator up to speed and if you can connect to two phases, which I think should be rather easy and also possible. Then hot sticks can be used to check rotation. You must be very close to synchronous speed to be sucessful, though.

Gunnar Englund

http://www.gke.org

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

 

[itsmoked]

So those two are in popper sequence with the network? What happens if they aren't, what would be the trace picture?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Skogsgurra]

Blue and red change place. You get A-C-B instead of A-B-C.

Gunnar Englund

http://www.gke.org

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