Hi, I need this fully explained on a technical level, with
information that can be followed, Thanks. High voltage Generator phase rotation conections made to switchgear.
Three-phase system have a definite phase sequence, either A-B-C or A-C-B. If you looked at the three phase voltages on an oscilloscope, this would correspond to the order or sequence of peak voltages (or zero crossings)for the three phases. A generator rotating in the proper direction will have a phase sequence, either positive (A-B-C)(normally) or negative (A-C-B). This is usually indicated on the generator nameplate or the generator instruction book.
The power system you are connecting to will also have a phase sequence. The generator phase sequence and system phase sequence must match. If they do not, the generator will be damaged or destroyed.
The only way to really know for sure is to do a phasing check with the generator energized and rotating at synchronous speed but **before** you close the generator breaker. There are various method of doing this, depending on your equipment, voltage, etc. With drawout switchgear, the normal method is take the breaker out, crank open the shutters covering bus and line connections and check across all three sets of phases with hot sticks and a voltage indicator. With the synchroscope at 0 degrees, you should have no voltage across each of the three pairs of breaker poles.
If you do have voltage, you will need to swap two of the generator leads and re-check. You must also make sure your synchroscope (a single-phase device) is connected to the proper phases.
In case of non availability of hot rods/sticks ,BUS potential transformers(PTs) and Genarator line PTs can be used for Phasing . Connect three seperate analog voltmeters (one for each phase ) across identical phases of Bus PT and Generator Line PT at switchgear end (R- R, Y- Y, B - B).
If the phase sequence is in order all the three voltmeters indicating needles will move in TANDEM. You must make sure that PT wiring is correct.
thanks !
dpc has explained the subject very well. Technically (or not) if one AC generator is connected to others, and the phase sequence, frequency and phase angle are not closely matched, expensive electrical and mechanical damage will likely result. The acceptable degree of difference immediately prior to paralleling varies depending on equipment and operating policies. You might obtain information on synchroscopes, synch-check, and automatic synchronizer relays, listed in order of sophistication. In a basic sense, incandescent lamps can be used to observe these electrical differences in real time, too.
One test that I usually perform involves energizing the same set of 3-phase VTs from both sources in turn and verifying the secondary rotation and the synchronizing instruments. This applies whether or not the generator is being switched by MV metalclad switchgear or HV open switchgear.
This test can be done by energizing the generator VTs from the bus side, with the generator leads disconnected, then opening the breaker and energizing from the generator. Alternatively, energize the bus VTs from the generator with the external supply off, then open the breaker and energize form the external source. Ensure that the phase rotation meter remains connected to the same VT secondary terminals for boith tests, to avoid any head scratching.
This confirms the phase rotation and the wiring of the synch instruments, so there are no unpleasant surprises on actual synchronizing.
