Issues with Power Generation when in Sync with the Utility Supply


A bit more findings after yesterdays troubleshooting.

I suspect the phase angle is the issue. This may result from 2 reasons which can not be confirmed yet since plant is running. I will check and confirm during blackout day.

1. I will check PT secondary connection for both Generator and grid. Since I found generator PT is star-open delta connection and grid PT is 2 windings PT.

2. I will check again generator and grid PT secondary side to synchroniser voltage sensing input wiring connection.


Sorry for jumping in to this part of the thread, govyman, but I, personally, have never seen this kind of logic. I don't think you have ever posted what the reduction gear box nameplate lists for input- and output speeds. Specifically, what is the input speed for a 1500 RPM output speed? What is the 50.000 Hz RPM for the steam turbine (what is the turbine's synchronous speed)?

And, specifically, tell us what the speed is during synchronization just prior to breaker closure, as well as what happens to turbine speed just after breaker closure (as close to breaker closure as possible), and then what happens to turbine speed 0.5 seconds after breaker closure, 1 second after breaker close, 1.5 seconds after breaker closure, 2.0 seconds after breaker closure, 2.5 seconds after breaker closure, and 3.0 seconds after breaker closure.

As has been said several times, when synchronizing the turbine speed will be slightly higher than the turbine's synchronous speed, so, for example, if the turbine's synchronous speed was 5360 RPM, the turbine speed during synchronization might be 5377 RPM (for a speed differential of 100.3%). At the time of synchronization to a very, very stable grid operating at 50.00 Hz the speed would drop by exactly 17 RPM to 5360 RPM. MOST turbine control systems would hold the current speed setpoint (100.3%) which would keep the steam control valve at it's same opening, which would be admitting a little more steam than is required to keep the unit spinning at 5360 RPM (because it was spinning at 5377 RPM during synchronization prior to breaker closure)--and this "extra" steam would ensure a positive power output of the generator. The extra torque that WAS keeping the generator spinning at 1504.5 RPM (100.3% of rated speed) will be converted into armature amperes by the generator when the rotor is locked into synchronous speed (1500 RPM) when the generator breaker closes.

In addition, many turbine control systems add even a little more energy to the prime mover (steam flow in your case) to ensure a positive power output from the generator, particularly when the grid isn't so very, very stable.

Are you synchronizing to a grid with many other generators and their prime movers, or are you synchronizing to an small grid that is supplying a load that is isolated from a "large" grid? Because, that's about the only way I could understand how the turbine speed could change so much after synchronization. You have mentioned a 15 MW spike (I presume that was a positive 15 MW spike), AND you have also mentioned a -17 MW "spike" (or dip). I'm a little confused about this.

If the turbine speed was higher than grid frequency at the time of synchronization then the load (power output) "spike" at breaker closure should be positive (on a relatively stable grid). If the turbine speed was lower than grid frequency at the time of synchronization then the load (power output) "dip" would be negative--because what would happen is that the grid would have to supply power to the turbine-generator to increase the speed to match grid frequency.

If the unit is not being synchronized to a stable grid, or is being synchronized to a small, isolated grid (sometimes called an "island" grid) supplying a local load such as for a refinery or cement plant or something similar, and there is an external control system that is sending signal(s) to control frequency, that could be part of the problem. If the unit is being synchronized to a utility grid with tens or hundreds of turbines, many of which might be much larger than your steam turbine, and the grid frequency is, indeed, stable then it's very difficult to understand how, under normal circumstances, what you describe is occurring.

You mentioned in one post that the total plant load before synchronization was approximately 4 MW. Is the turbine supplying this load before it is being synchronized to a larger grid? I'm getting more confused as I re-read the previous posts. Please help to clarify your previous posts.
Dear CSA

Apologies for misunderstanding. Let me clarify.

It is a -17MW dip rather than a spike for this time. Sometimes it has 15MW spike also. My generator PT is 3 windings star-open-delta connection. Grid PT is 2 windings connection. The previous ABB synchroniser is equipped with phase angle compensation function. I am not really sure how the function is achieved.

I found that the turbine speed characteristics was not always the same. The recent time, at 1s speed is 5316, 1.5s 5345rpm, 2s 5380rpm.

I am wondering any possibility of the freq drop may because of long distance from grid. Only freq of my plant premises is dropped, and when this travels to the grid, the effect is very minimum.

Before sync, 4MW is purely supplied by grid. After sync, we will adjust the generator MW setpoint to share the load.

Thanks for your continuous support. I really appreciate it.


I have heard of plants located a long distance from a grid on a certain type of connection (can't recall the type) experiencing problems with unstable frequency, but not like this. This seems odd. How far away is your plant from the main grid? Is it a single connection to the grid? How far is your plant from the nearest generator and prime mover (other plant)?

So, I take it that there was another synchronizer before. And did the unit have the same problem when synchronizing?

And, if there was another synchronizer before, was there another turbine control system before??

You still haven't told what the turbine synchronous speed is. What is the turbine speed when the frequency is 50.0 Hz? (The reduction gear nameplate should show this information.)

I could imagine a situation where if the turbine control system closed the control valve slightly during synchronization immediately after breaker closure that the MW would dip negatively. But I would also expect the reverse power relay would trip the breaker/turbine on -17 MW, even if it was only for a split second--most reverse power relays operate such that if the reverse power is very high in magnitude the relay will trip the unit very quickly. If the reverse power increases in magnitude slowly then the relay will trip more slowly.

If the speed is dropping as much as you say it is, that's NOT good for the reduction gear or the couplings between the turbine and the reduction gear or the reduction gear and the generator.

And if the phase angle is really wrong when the breaker is closing that's not good for the couplings, the reduction gear or the turbine--or the generator rotor. The "bump" must be a BIG bump.

Very strange problem.