As best that I can determine the get-set stays in isochronous mode the entire time it is paralleling the utility grid; the gen-set is also trying to reduce power output to zero as it is paralleling. I can see where that could cause problems.

> I am surmising the unit isn't switching to Isochronous from Droop immediately
> after the breaker connecting the plant to the grid is closed.

 

KevinK,

> As best that I can determine the get-set stays in
> isochronous mode the entire time it is paralleling the
> utility grid; the gen-set is also trying to reduce power
> output to zero as it is paralleling. I can see where that
> could cause problems.

This seems to the root of the problem with the standby gen-set when it's operating in parallel (synchronized to) the grid. And, the business of ramping load down during the 10 seconds it's synchronized to the grid is probably some part of the design scheme/programming of the testing of the standby gen-set--which is likely why the standby gen-set is being synchronized to the grid to begin with: to test/verify the standby gen-set can produce power periodically.

It's common for standby gen-sets to be periodically started to ensure they start reliably, but many sites also synchronize them to the grid to ensure they can actually reliably and stably produce power "under load." But, ANY generator connected to a grid--especially a large or infinite grid--should be in Droop speed control mode when it is operating in parallel (not necessarily when it is being paralleled--during synchronization--but certainly when it's generator breaker is closed on to the grid). If you switched the governor of your "big" machine to Isochronous while it was operating in parallel with (synchronized to) a large or infinite grid it, too, would most likely be unstable. Maybe even more unstable if your location on the grid is truly "soft."

But, it would definitely seem that the design of the standby gen-set testing "program" should put the unit's governor in Droop speed control mode if the breaker (utility tie or tie-line breaker) which connects the facility to the grid is closed. And, if the utility tie-line breaker is open and the standby gen-set is started to provide power to the facility then the standby gen-set governor should be in Isochronous speed control mode, and, if the standby gen-set is still providing power to the facility when the utility tie-line breaker is closed to synchronize the facility back to the grid then the standby gen-set governor should switch to Droop speed control mode. (Many turbine-generators and gen-sets are started and synchronized in Isochronous speed control mode--but are immediately switched to Droop speed control mode when the generator breaker closes. Many other turbine-generators and gen-sets are started and synchronized in Droop speed control mode so when the generator breaker closes the governor is already in Droop. So, it depends on the governor and how it has to be configured whether or not it has to be in Isoch or Droop during starting and synchronizing--but it must be in Droop when the generator breaker is closed and the unit is operating in parallel with (synchronized to) other generators and their prime movers supplying power to a grid.

It's likely not the load reduction over some time (10 seconds in this case) that's the problem; it's the fact that the standby gen-set is in Isochronous speed control mode when it's paralleled with (synchronized to) the grid regardless of the time or the load reference.

There's another possible issue (or issues) if the standby gen-set is connected to the main "big" machine generator upstream of a main step-up transformer with no impedance (transformer impedance) between the two generators. This can cause problems for the generator exciters if not properly tuned or capable of operation without any interposing impedance, which can lead to fighting of the two exciters for voltage control--very similar to the way the two governors are fighting for load control when one of them is in Isochronous speed control mode.

But, it would probably be best if the only change that was made initially to try to troubleshoot, understand and resolve this issue was to make sure the standby gen-set governor was in Droop speed control mode--if not during synchronization (before the standby gen-set generator breaker is closed, then after the standby gen-set generator breaker is closed when the facility is connected to the grid. That will tell you a lot about any further steps which might be necessary--if any.

Please write back to let us know how this progresses!

 

Kevin K,

> I realize that I am keeping some of the details vague. I am hoping that by keeping the discussion generic that I and the
> readers of forum can advance our knowledge of generators and synchronizing with the grid irrespective of the particulars
> at my specific facility.

Cleverly hidden at the far right of the Menu bar of every control.com webpage is a 'Search' feature. It's recommended to use the 'Help' function the first time or when using multiple word search terms (which IS also recommended) when searching the control.com Archives for information.

The topics of synchronization, Isochronous and especially Droop speed control have all been covered many times on control.com in previous threads. And, in general, they are virtually the same for just about every power plant in the world.

<b>From the information you have provided,</b> the difference at your plant is that the standby generator-set is being periodically synchronized to the grid for a brief period of time. And that's not the purpose of a standby gen-set--it's purpose is to control facility frequency when the facility is isolated (disconnected) from the grid while providing the power for lights and motors and computers and computer monitors. When the generator breaker of the standby gen-set is closed when the facility is connected to the grid it is paralleled to (synchronized with) the grid. And it's frequency (speed) is virtually fixed by the grid. Now, if you're at a location on the grid which is considered to be "soft" then sometimes there can be frequency variations--but they aren't very big and they aren't very long, or, if they are, then breakers start tripping to prevent faults, and/or there are black-outs. (Frankly, I'm surprised the grid regulator/operator in your area puts up with the load disturbances, but every grid is different.)

I'm going to go through some very basic fundamentals here to get the terminology and concepts clear and understood. Basically, synchronization and paralleling are the same thing: the act (process) of connecting a generator and it's prime mover (a "gen-set" or a turbine-generator) with other generators and their prime movers.

Parallel operation, or synchronized operation, is when the generator and it's prime mover are connected to (synchronized to; in parallel with) other generators and prime movers.

On an AC system, the speed of the generator and it's prime mover are directly related to frequency. And, no single generator can run faster or slower than the frequency of the grid with which it is synchronized. Full stop. Period. (Except under transient and abnormal circumstances--which does not include the situation you are vaguely describing). A two-pole generator on a 60.0 Hz system will have to spin at 3600 RPM--and all two-pole generators paralleled with (synchronized to) a grid are all operating at 3600 RPM (when the frequency is 60.0 Hz), and are ALL operating at what's called synchronous speed, which is the speed which is proportional to frequency.

(If you plug your AC-powered device (drill motor for example) into an outlet at your home or at the facility you--and the device--are expecting the frequency to be rated (I'm presuming 60 Hz from your post). If it isn't, then the speed of your drill motor isn't going to be what it's supposed to be (for non-variable speed drill motors!). The electric generators providing power to the grid, they all have to operate (and they do!) at the same frequency; no one generator can operate at a different frequency (for very long under normal circumstances). One generator can't be running at 60.7 Hz when synchronized to a 60.0 Hz grid, and another generator can't be running at 59.8 Hz when synchronized to a 60.0 Hz grid. They all have to be running at 60.0 Hz when connected to a 60.0 Hz grid--and they are when synchronized.)

For generators that are directly coupled to their prime mover, the prime mover speed is the same as the generator speed (regardless of the number of poles).

There are two basic types of speed control: Isochronous and Droop. Isochronous speed control is when the prime mover governor is very closely trying maintain a constant speed--which is directly related to frequency--and as load changes, and frequency would tend to change, the governor will adjust the energy flow-rate into the prime mover very quickly to keep the frequency (speed) constant.

Prime mover governors being operated in Isochronous speed control do not like it when other governors in Isochronous speed control are paralleled to a grid. Or, if the grid is large, or infinite, a prime mover governor being operated in Isochronous speed control when paralleled to (synchronized to) the grid will usually behave very badly--because it will be trying to control any grid frequency excursions (and no grid always operates an 100.00% of rated frequency). So, therefore unless a special type of Isochronous load sharing is implemented generators and their prime movers are not operated in Isochronous speed control when paralleled to a large or infinite grid.

It's very important to note that (except for machines operated with an Isochronous load sharing scheme) it's not possible to control the load of a generator and it's prime mover when it's being operated in Isochronous speed control mode. An operator can only change the speed (frequency) setpoint, and it's usually desirable to operate at a constant speed (frequency) so changing the speed setpoint and, again, when being synchronized with other generators and their prime movers the machine has to operate at the same frequency as the other machines, and the governor does not respond to load changes from an operator in the way that operators think it should respond.

Droop speed control is the mode that allows multiple generators to be paralleled together (synchronized together) to supply a load that is MUCH larger than any single generator could supply by itself--and to all operate and behave very well when synchronized (paralleled) together. So, it is the mode that virtually all generators and their prime movers are operated in when synchronized to (paralleled to) other generators and their prime movers on a large or infinite grid.

Generators whose prime movers are being operated in Droop speed control mode when synchronized together operate stably with each other, and the load can be increased or decreased by the operator(s). And, prime mover governors in Droop speed control are not really interested in controlling frequency--they leave that up to grid operators, and for smaller grids that can even be a single machine operating in Isochronous speed control mode.

In fact a lot of small "grids" ("islanded" systems) usually have one machine in Isoch and others in Droop.

But, to stably contribute to a grid and "share" the load of the grid with other generators and their prime movers, the governors of generator prime movers are almost always operated in Droop speed control when synchronized to other generators and their prime movers. And when one is connected to (paralleled with; synchronized to) a grid in Isochronous mode (unless it has Isochronous load sharing--which wouldn't apply to your site) it will NOT be stable and will behave very badly.

When the generator breaker of your gen-set is closed, it is in parallel with other generators and their prime movers--it is synchronized with other generators and their prime movers. Paralleling the gen-set is the process of matching speed (frequency) and voltage in preparation for closing the generator breaker.

Hope this helps!

 

Kevin...

If left to its own devices (excuse the pun) do the swings worsen, or lessen?

Phil

 

KevinK,

Realizing it's the holiday season in many parts of the world, has there been an opportunity to try switching the stand-by gen-set to Droop mode when synchronized to the grid to see if that has had any effect on improving operations?

Even if there hasn't been an opportunity, can you let us know what the status of this issue is--are you planning to try changing the synchronized operation mode of the stand-by gen-set? Are you thinking of some other changes/modifications?

Thanks!