phase sequence test of medium voltage sources

[sailor55]

We have a 16kV switchgear fed from mains and stand-by generator (16kV). We want them to have the same phase sequence. We want to verify the sequence at secondaries of VT's. Is there a regulation that they have to be verified from primary side, or secondary side is acceptable as well?

 

[DanDel]

Primary phasing is obviously preferred, but not usually possible when performing a phase sequence (rotation) test. The problem with checking this with the PTs is that you either must use the same switchgear/PTs (leave the phase sequence meter connected to the same PTs, first check the normal source, then check the alternate source), or you must be certain of the PT connections if using two separate switchgear/PTs so you know that you are actually verifying the correct rotation.

 

[TomG33]

Salor55.
The sequence only has to be verified once. Usually after you do this, the wiring is not disturbed, so you can be confident that for future times all will be well. We have many machines, with Synchronous motors, that we run up to synchronous speed and then actively synchronize with the Line. to do this we only use one PT from the generator and another from the line. sadly one PT is not enough to determine sequence but it is ok for phase allignment.

What we do is at the time of construction and initial Commissioning is to do what we call a DOL Bump. Where we Do a short pulse of 3 phase power on the motor and it trys to start using it's amortoser winding. This is a positive indication of which way the motor will go when powered by the electricity mains. all we need to do then when we do our "Soft Start" is to make sure we spin the motor in the same direction.

In your situation, with a standby generator, I would pick a three phase motor that is driven by the power system and goes in a known direction. I would then disconnect your local grid from the incoming line and then power it up with the standby generator. Start this known motor and verify that it is going the correct direction. This may or may not be easy for your system but it is the only way i know to be ABSOLUTELY SURE.

WE have been through the exercise of checking generator diagrams and verifying pt connections and similar exercises, however you do not really know until you do something like I suggested. This only has to be done once unless the generator connections or your local grid wiring is disturbed somehow.

Tom Grayson

 

[oldfieldguy]

You can do it on secondary voltage **IF** you're absolutely certain of your potential transformer connections. If I personally attended to the verification of the PT's, I would not mind doing such a test, **BUT** DanDel is absolutely correct in stating that tests at primary voltage are preferred.

If you have two analog high voltage meters, commonly referred to around here as "phasing sticks" you can check the rotation of the generator source in relation to the utility source without switching sources. I've done this where switching from one source to another was not an option due to operational considerations.

The voltages need to be read from two phases of the utility source to the same two phases of the generator. If rotation is correct, you will see both voltmeters rise and fall together as the generator slips slowly into and out of synchronization due to small frequency difference. If the meters swing in opposite directions, one rises and the other falls, then the generator is rotating backwards in realtion to the rest of the world.

If you can switch, then TomG33's idea works as well.

old field guy

 

[dpc]

Primary testing using phase sticks is the best way, as noted above. Anything else is less certain. However, doing this in metal-clad switchgear generally requires defeating some interlocks to allow you to open the rear shutters with the breaker removed and sticking your head (requires at least two people, really) into a cubicle with live parts. A good arc suit is recommended.

IF you can energize the **same set** of three-phase PTs from each source (one source at a time, of course), you can get a fairly good test by checking phase sequence on the secondary side of this one set of PTs.

 

[davidbeach]

My preference is, if it is possible with the gear, to heat the gear up with utility source and check phasing on the secondary of the bus VTs. Disconnect the utility and heat up the board from the generator and check phasing on the secondary of the same set of bus VTs.

 

[Snoball]

Know if phase identity is important as well as rotation could be important to the genset's voltage regulator.

 

[sailor55]

All,

Thank you for your inputs to my subject. All was very helpful indeed. The only question still needs an answer is; are there any rules, regulations and/or standards for the implementation of this verification?

 

[dpc]

There may be some testing recommendations in the NETA standards, but you're pretty much on your own. You would want to make sure you comply with OSHA requirements for arc-flash protection if you go with primary testing.

 

[elektroeng]

If your panel is equipped with capacitive voltage indicators these can also be used for phase sequence testing. Its the safest and easiest way, if your panel is not fitted with these you can allways install them.

Regards and succes.

 

[tommom]

Using primary phasing sticks - I've tried that in the past, and the best difference I could get between phases (even when I knew for a fact that the two sources were synchronized - they were the same utility feed) was about 8-9 kv on a 13.2 kv system. According to Chance, the mfr, there was too much impedance (I think they said capacitance - this was several years ago) in the cables which connected the meter to the test leads to accurately use this method.

 

[Skogsgurra]

Now, that (Chance) is a suitable name for a company in this kind of business!

If they told you that, I certainly hope they are out of business by now.

If you have solid copper (or Al or any metal) in the wires, the capacitance has no effect (well, perhaps some, in theory - but not that you would notice).

Gunnar Englund

http://www.gke.org

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

 

[oldfieldguy]

tommom--

I've used AB Chance sticks without any problems in the past.

Most of my clients had a tester made from a 120v - 5 or 10-kv ignition transformer to test the sticks before use. That's a good idea where you have difficulty accessing a live source conveniently. However, in the case we're discussing, you shouldn't have trouble testing the sticks on a live source, and if the sticks didn't measure the expected voltages, I'd be extremely disappointed.

old field guy

 

[dpc]

I've never had a problem measuring with the AB Chance voltage probes - that's interesting. It's good to have an extra pair of eyes to make sure you all agree on what you're seeing.

 

[genhead]

I agree with DavidBeach. This is also my preferred method. It is quick & simple, and you don't have to open any medium voltage switchgear

 

[rcw retired EE]

I've done it both ways and strongly prefer the method David suggests: backfeeding the same set of VT's and instruments from two sources. Usually you can do it by isolating leads at the generator and energizing the generator leads and generator VT's from the utility.

In using the Chance hot sticks or HV phasing voltmeters a couple hundred times we were lucky and never blew anything up or got anyone injured. We did notice that if one of the sources is not grounded, such as a switchgear fed from a delta transformer winding, the phasing sticks would show 7-8 kV when the 13.8kV system was in synch. I think it had to do with the capacitive function of the voltmeter.