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Rotational Direction of a Turbine Generator
Could you please help me clarify if the rotational direction of a turbine is always the same.

Hello,

I got confused those last few days while speaking with someone on a power plant. He asked me about the rotational direction of the turbine. (We were at standstill). I said I have to check in the spec but he replied "okay normally it should turn in that direction because it always turn the same." On the moment I agreed with him as I never really thought about this matter.

However, I checked with several sites where I used to work and find out that it could be the opposite direction.

So I try to go on the internet and people are saying it always turn in clock wise because we have to synchronize all the generators together and hence they have to turn the same way. I am agree with them but the turbine can turn the opposite way and we can make an inversion on the stator phases so then from grid side it is turning the same.

Anyway, I think some of you can help me.

Thanks

3 out of 4 members thought this post was helpful...

Saul,

This is a pretty common question from people at power plants, especially new employees.

You really need to recognize that the turbine is a prime mover for the generator. The generator is what gets synchronized to the grid with other generators and their prime movers, so it's the order of phasing on the generator output where it's connected to the grid (through some breaker, of course) that's really important. Generally, three-phase generators are connected such that A phase reaches it's peak first, with B phase reaching its peak next, and C phase next. That's called A-B-C rotation. There's also C-B-A rotation, which is opposite of A-B-C. This is electrical direction of rotation.

And, as you say, since the leads coming out of the generator can be connected to the breaker which will connect the generator to the grid when it's closed (during the synchronizing process) they can be connected A-B-C or C-B-A, regardless of which way the generator rotates, or is rotated by its prime mover (quite often a turbine, but could be a reciprocating engine; and turbines can be combustion turbines, or steam turbines, or hydro (water) turbines, or wind turbines).

Prime movers are generally designed to rotate in a specific direction--the mechanical direction of rotation, so the way that energy is admitted to the prime mover determines which direction the prime mover will rotate. For example, if one admits steam to a steam turbine, the direction of the nozzles and the blades will always make the steam turbine rotate in the same direction. About the only exception to this I can think of is when the steam turbine has a particular type of extraction through which steam can be admitted to the turbine and which, if admitted before steam is flowing from the main control valve can cause the turbine to rotate in the opposite direction--but that is VERY rare, and would generally mean there's a serious problem, either with valves or operating procedures.

Gas (combustion) turbines must rotate in on direction to operate properly to compress air and extract energy from the hot combustion gases flowing through the turbine section.

Some hydro turbines can actually be used a pumps, to return water back to a storage facility, and then can be used a hydro turbine driving the generator to produce electricity. In this case, the generator serves as a motor when the hydro turbine is acting as a pump, and then when the hydro turbine is acting as a prime mover the generator operates as a generator. (This is typically called pumped storage.)

Reciprocating engines used as prime movers for generators must rotate in a specific direction in order for the air and fuel to flow into the cylinders.

Generators are not that finicky, though. The same generator can be driven by a prime that rotates in the clockwise direction, or when driven by a prime mover that rotates in the anti-clockwise direction. The generator doesn't care which direction it rotates--the only thing that matters is how the output leads are connected to the breaker that will close when the unit (prime mover and generator) is being synchronized to the grid with other prime movers and generators.

One of the most important checks performed when a new prime mover and generator is first being synchronized to a grid is to confirm that the phasing of the generator matches the phasing of the grid. This check should also be performed whenever the generator phase leads are disassembled or replaced, or whenever the potential transformers and/or the wires that connect the potential transformers to the synchronizing relays are disturbed. Failure to ensure the phasing (phase rotation) of the generator matches the phasing (phase rotation) of the grid can be DISASTROUS and CATASTROPHIC.

Many generator manufacturers produce generators, and as far as the electrical output of the generator is concerned it doesn't matter which direction the generator is rotated by the prime mover--it will still convert torque to amperes (because that's what generators do, they convert torque to amperes) regardless of which way the generator rotor is turned. (The bearings will usually be installed in order to facilitate rotation in the same direction as the prime mover.) There have even been cases of generators moved from one site to another where the new site has a prime mover that rotates in the opposite direction of the old site's prime mover. In that case, all that's really necessary is to ensure the generator output leads are connected to the breaker so that the resulting phase rotation matches the grid's phase rotation. (And, in some cases the bearings have to be switched to ensure oil flows in the proper direction to develop the necessary oil wedges.)

If a prime mover driving a generator were to be capable of rotating in either direction (clockwise or anti-clockwise) then it would be necessary to change the generator output phase leads to match the prime mover rotation if it changed to ensure the phasing at the breaker matched the grid phasing. That could be done with a simple switch (which would add cost to the equipment and represent another point of possible failure), or it would have to manually done--and that's not very practical.

Some prime movers operate at speeds that are not compatible with the frequency of the grid the generators they drive are synchronized to. In that case, there are gear boxes (reduction, or load, gear boxes) between the prime mover and the generator rotor. And in some cases, the output shaft of the gear box mechanically rotates in the opposite direction of the input shaft (which is coupled to the prime mover). But, as long as the generator output leads are connected such that the electrical rotation of the generator matches the electrical rotation of the grid all is good!

And, again, prime movers are generally designed to operate in a particular direction of rotation. The direction is a matter of design, and is generally peculiar to manufacturer. Some manufacturers steam turbines always rotate clockwise, while their heavy duty gas turbines rotate anti-clockwise, and their aero-derivative gas turbines rotate clockwise. All of the internals are designed with the direction of rotation in mind, and while some steam turbine manufacturers design theirs to rotate anti-clockwise, the generator really doesn't care. As long as the generator bearings are properly oriented and the generator output phase leads are properly connected at the breaker that will be used to synchronize to a grid with other prime movers and operators. In fact, the other prime movers and generators on the grid will NOT all rotate in the same direction--HOWEVER, the phase output of the generators will always be connected to match the other generators, regardless of their, or any other prime mover, direction of rotation.

Hope this helps! Generators are really pretty dumb. I keep writing that, and it's true but not well understood or appreciated (just how dumb they really are). Governors are controlling the generator--except to the extent that governors control the amount of torque provided to the generator. Which the generator converts to amperes; more torque, more amperes; less torque, less amperes. And amperes is load. (Remember, motors convert amperes to torque--and motors are driven by generators, right? Right? Nobody seems to have a problem with how motors operate, but can't get their arms around how generators operate. Electrical transmission and distribution systems are really just a means of using wires to transmit and distribute torque. Right?)

A lot of people who write things on the World Wide Web are very well-intentioned. Even people who write text-books are very well-intentioned. But many people have very little practical experience, and when they write things they explain things the way it makes sense to them, not necessarily the way things actually work in the real world. There is the direction of rotation of the prime mover, and then there's the direction of rotation of the generator output--they're NOT the same, except for the way they are named (clockwise, or anti-clockwise). And the OUTPUT of all generators connected to (synchronized to) a grid must all be the same--regardless of the direction of rotation of the prime mover (or of the generator rotor).

Again, hope this helps! There's clockwise, and there's anti-clockwise. And, there's mechanical direction of rotation and electrical direction of rotation. And, the two don't have to match for all prime movers and generators connected to (synchronized to) a grid--except for the electrical direction of rotation.

Ain't this stuff FUN?!?!

1 out of 1 members thought this post was helpful...

First of all thank you for your detailed and clear explanation.
Just one remark, from an electrical point of view I agree that for generators it doesn't matter which direction it rotates. However, as far as I know, we can also find some blades mounted on the generator shaft in order to create an airflow and cool down the machine. Not sure it works properly in the opposite way?
____
I always wondered what would happen if by mistake someone synchronize ABC to CBA. I know for sure that it won't be good for the generator and the prime mover. Nevertheless, for sure we have electrical protections to protect us?

Don't worry I will never try, but I heard some crazy stories about it on the field. Just curiosity...

Thanks again.
Saul

3 out of 4 members thought this post was helpful...

Saul,

You are right; the internal air circulating fan blades on the generator rotor would probably work best if they were changed at the same time the generator rotation was changed. Air would still circulate, but it probably wouldn't cool as well.

Actually, most synchronization schemes use an open-delta potential transformer configuration for voltage and frequency sensing. This means that only a "single" phase from the generator output and a single phase from the grid side are used for synchronization checks.... It's presumed that someone checked during initial synchronization checks to make sure the generator phase rotation--all three phases--matches the grid phase rotation. THAT'S why it is so important to check phase rotation again any time the generator output phase leads are disturbed OR the potential transformer connections (primary or secondary) are disturbed before synchronizing the generator and prime mover to the grid. (This is done to minimize equipment and wiring which minimizes cost, and is fine--as long as proper precautions are taken any time phase leads are disturbed. And, in fact, it also applies to any time the grid phase leads are disturbed!)

Arcing and sparking and explosive sounds would be the first signs of synchronizing with opposite phase rotation. It would like damage the coupling between the generator and prime mover, also, if not the generator shaft. There are HUGE mechanical forces derived in generators from magnetic forces at work inside the generator--and we all want them to be properly balanced and aligned--and matching phase rotation is the way that happens.

It always amazes me that people know that synchronization is an important aspect of power generation, and that during synchronization speed matching is also very critical. But, once the breaker closes and the unit is synchronized they think the generator and prime mover can change speed as load changes and the energy flow-rate into the prime mover changes, in the same way that the unit changes speed during acceleration deceleration during start-up and shutdown when the energy flow-rate into the prime mover changes. That's NOT the definition of synchronism.

Alas, I digress. Hope this helped!

By Osama Arafa on 7 August, 2018 - 4:33 pm
1 out of 1 members thought this post was helpful...

This is a more than sufficient reply, I just would like to add some tiny facts:

In the stator of the generator, three-phase windings are there and distributed along the circumference of the stator in a forever fixed sequence with regard to any given sense of rotor's rotation. Meaning that, for a given generator in a given setup, if that generator is rotated clockwise, the phase sequence would be ABC (for the terminals marked A, B, and C respectively), in other words, if you use an oscilloscope to examine the waveforms, the phase A will lead the phase B and the phase B will lead phase C.

This implies that if the same generator is rotated counterclockwise, the sequence of the phases (carried out by the terminals A, B, and C respectively) would be C B A (C will lead B and B will lead A).

To conclude, there is a fixed structural link between the sense of mechanical rotation of the generator and the phase sequence exhibited by the terminals if we respect their original markings (A,B, C) for any given generator.

To rectify this permanent or structural link so that it matches the phase sequence requirements of the synchronization process we may need to swap two phaeses terminals.

In other words, we must replace any one phase terminal by another one should the inforced sense of rotation (by the prime mover) and the sequence of windings (enforced by the actual generator design w.r.t that sense of rotation enforced by the prime mover manufacturer) can't provide the desired phase sequence for synchronization. Doing that will bring us the required compliance between generator phase sequence and grid phase sequence

In phase sequence notation:
ABC == (CBA after swapping C and B) --->>> (BCA)
CBA == (ABC After swapping A and B)---->>> (BAC)

I want to add to the question:

Looking from the Aft and Forward direction of the turbogenerator, I want you to explain how someone can pick the rotational direction of turbogenerator.

Axial turbines are designed to rotate in a specific direction. It has to do with the angle of attack of the blades relative to the incoming flow.

Consult the detailed turbine drawings...of the suppliers literature files.