Conditions for Synchronization (Phase Sequence or Phase Rotation)


Thread Starter


Hi all,

As I learned for synchronization, their are five factors to be considered. Should have:

Equal line voltage
Same frequency
Phase angle
<b>phase sequence</b>

Waveform is fixed by the construction of generator. And when we connect generator to grid, do we need to consider the <b>phase sequence</b> or <b>phase rotation</b>?

Can we separately identify R-Y-B (Red-Yellow-Blue) in 3 phase waveform?

Pick a phase and named it as phase R (Red) and relative to that can identify phase Y (lead 120 degrees in balanced 3 phase waveform) and phase B (lag 120 degrees in balanced 3 phase waveform). This can be done using phase rotation meter.

So my Question is how to identify the phase R (red) in a electric power system? I saw in internet that their are separate instrument to identify phase sequence and phase rotation.

I thought that phase rotation is enough to connect a generator to grid for synchronization.

can some one please explain it?
Someone's been reading (and copying)

Generators have their phase leads marked, usually 1, 2, 3, 4, 5, & 6 (on all the generators I've been party to installing). And, they're usually designed for a specific rotation (CW or CCW; or Clockwise or Anti-Clockwise). Phase rotation is usually specified as ABC or CBA--which is the opposite of ABC as phase rotation identification goes). By convention--and unless otherwise <b>specifically</b> noted--the phase identified as 1 is A phase, the Red (R) phase. The phase identified as 2 is B phase, the Yellow (Y) phase, and it's sine wave reaches positive maximum 120 electrical degrees after A phase. The phase identified as 3 is C phase, the Blue (B) phase, and it's sine wave reaches positive maximum 120 electrical degrees after A phase, for the typical three-phase synchronous generator. The remaining three phase leads (4, 5 & 6) are tied together when the generator is connected in a wye configuration and connected to earth (ground), usually through a transformer and/or resistor of some sort.

Prior to initial synchronization the phases of the generator are physically checked against the phases of the bus to which it will be synchronized--usually with a special high-voltage meter--with the generator-set running at synchronous speed to be sure that the phases of both the generator and the bus with which it will be synchronized are indeed connected such that each phase pair is correctly terminated, A-A (or Red-Red), B-B (or, Yellow-Yellow), and C-C (or Blue-Blue). Since phase leads are usually bolted and then taped and then rarely, if ever disturbed or disassembled and the generator-set rotation never changes once verified the generator phase sequence is rarely, if ever, verified again--unless the bus bars are changed or some other situation arises which requires positive verification such as is performed prior to initial synchronization.

Having said all of the above, it would be possible to connect a synchronous, three-phase generator such that phase 2 were connected to phase RED of the bus, phase 3 to phase YELLOW of the bus and phase 1 to phase BLUE of the bus and everything would still work properly--as long as the instrumentation was also connected similarly, because presuming phase rotation was correct each generator phase's sine wave reaches positive maximum 120 electrical degrees after the other. There is no way to pick a particular phase and say IT is A or RED because one has to know it's sine wave in relation to the other two sine waves. A, or RED. phase isn't encoded somehow with some kind of signal that a device can detect to identify it as A, or RED, phase. It's all relative to the other phases. As long as the phases reach positive maximum at the required number of electrical degrees of separation then everything is good.

A generator, prior to being bolted up to a system, can be connected in any number of ways, and it's the system the generator is connected to which identifies phases as A, B or C, or Red, Yellow or Blue, or whatever the local/regional/national/continental standard for phase identification is. Once it's connected to the system--and verified--then it's phases are "identified." That's why there built as 1, 2, 3, 4, 5 & 6. Because the good thing about standards is--there are so many standards to choose from!

It would be much easier to describe with a drawing, but that's not possible--yet--on (I'm ever hopeful on this topic!)

Hope this helps!
To add to CSA's excellent answer, synchronous generators are synchronized to the grid either manually using a sync scope and voltage meters, or automatiaclly using a synchronizer. In both cases it is good practice to also have a sync check relay which is a go, no-go permissive to prevent out of phase breaker closing.

For the reasons given by CSA, only one phase is looked at to determine if a generator is in phase with the system and if the voltage levels are matched.
>... The phase identified as 3 is C phase, the Blue (B) phase, and it's sine wave reaches
> positive maximum 120 electrical degrees after A phase, ...

Ummm... sorry to seem nit picky, but it's <b>240</b> degrees after A phase.

> Ummm... sorry to seem nit picky, but it's <b>240</b> degrees after A phase.

Yes: I did a copy-and-paste and didn't correct that; thanks for pointing it out! I meant to say 120 degrees after Phase <b>B</b>. Same difference.

The upshot of all this is: The phase sequence/rotation checks are only done once, prior to initial synchronization--UNLESS the bus bar/generator phase leads or the metering PTs are disturbed--and NOT during every synchronization attempt. It's the rare prime mover (some types of hydro turbines) which can rotate in either direction, and there are protective devices to ensure the rotation is correct before allowing synchronization of those types of prime movers.