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3 phase power generation

3 phase power generation

3 phase power generation

can somneone tell me the differance beteween phase rotation and phase sequence?  is it pussible to have correct rotation and wrongsequence when parrelling  generators?

RE: 3 phase power generation

The terms phase rotation and phase sequence are often used interchangably.  This has been discussed very recently on one of the EE forums.

RE: 3 phase power generation

To answer arc480, once you establish that the phase rotation is the same, the act of synchronizing the generators ensures that the phase sequence is correct.   

I agree with electricpete that these terms are often interchanged. However, on a fine point the terms may not be synonymous.

As an example, a plant I worked in had a 13.8kv switchgear lineup, with each breaker feeding a 480V unit substation. The standard practice for the 480V distribution was that cables were marked brown, orange, and yellow to show phase rotation and were taken as A, B, and C respectively. We also took any arrangement of terminals to be ABC from left to right and connected everything accordingly. Motors had corresponding colors on the leads for proper rotation and all was well.

Without going into a lot of details on the project itself, we had need to arrange a sort of tie between two of the subs. When the work was done I went to inspect it before turning it over to the plant. It was a pretty sight, with brown orange and yellow taped cables attached left to right on both the top and bottom of the switch. I instructed the chief electrician to measure the voltage line to load across each phase as a check and he balked, saying that he had checked the connections to the subs and had double checked at the switch with a phase rotation meter. A short disagreement followed, after which he went for the voltmeter. The look on his face was priceless when he measured 480V line to load across each phase of the  switch.

Using the switch terminals as the reference ABC left-right, it turned out that zero volt measurements were made from Atop to Bbottom, Btop to Cbottom, and Ctop to Abottom. Specifically, the phase rotation on both the line and load side of the switch was the same, left to right, brown-orange-yellow. However, the phase sequence was different, with ABC on the top and CAB on the bottom.  

Going back to the subs, the connections were according to the standard at the 480V bus, so it was concluded that the phase sequence must have been different at the 13.8kV primaries to the subs.  

RE: 3 phase power generation

Good discussion, rhatcher (star).  

It's easy to forget that the distinction between phase rotation and sequence as you outlined above is sometimes important (particularly when paralleling).  And another reminder about the importance of doing a phase check before paralleling or looping power.

RE: 3 phase power generation

1. Phase rotation used to be frequently used in the past. It is more related to the rotating machinery, and phase rotation meter.
2. Phase sequence is more general since it applies to rotary electrical machinery and linear electrical machinery. Also, when it comes to multiphase machines, e.g. 15-phase motor, the phase sequence appears to be more appropriate.
3. The phase rotation is also linked to the mathematics, e.g. phasors, vectors, complex variables including the complex variable angle are its rotation.
4. Since the rotation is more applicable to the mechanical reality, prime mover, the generator may have the correct rotation and the wrong phase sequence and vice-versa.

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