Impact of Load Angle


Thread Starter


What is the impact of load angle change of one generator on other generators?

We have 6 nos. of 150MW generators which is connected to grid. Sometimes we observe hunting in generators without any disturbance from grid side.

Is it something to do with load angle?

So, could you be more specific, please?

Exactly what is the nature of the instability--is it speed (frequency) or load? Or is it reactive load (VArs)? Or is it generator terminal voltage, or generator field voltage/current?
Or some combination of the above?

How severe is the instability--what is the "swing" (from high to low)?

What is the load on the unit(s) which experience the instability relative to the units which are stable? (I'm presuming that not all of the units experience the instability--but, could you clarify which units experience the instability, please?)

Do each of the units have their own step-up transformer, or do any of them share a step-up transformer? If they share, do the instabilities occur on units which share a step-up transformer?

Can you relate the start of the instability to anything which happens in the plant--such as an arc furnace (I'm presuming this is a steel plant with large arc furnaces, but I may be wrong) starting, or suddenly tripping?

Here's how load angle works in my mind and understanding.... Load angle changes with load on every generator--and it is specific to every generator depending on the load being carried by that generator. The load changes as the load changes--not the other way around. There's nothing the operator can do to change the load angle to make the load change; rather, when the operator changes load the load angle changes.

I'm also presuming the generators are driven by steam turbines--but that's also not clear from your post. If they are combustion (gas) turbines, do they operate on low-BTU gas from the process, and if so, could there be a problem with pressure or flow-rate of the low-BTU gas fuel when the instability occurs?

Again, in my understanding of load angle it's just a mathematical method of explaining what is occurring when the load being carried by a synchronous generator changes--or a way of estimating the "difference" between voltages in a generator (one of which does not have a meter and is not even measured). It's more for ivory-tower types to consider when generator design and/or protection--not so much for real-world operating considerations.

You can see that there is a LOT we don't know about your plant and the circumstances surrounding the instability. The more information you can provide, the more help we may be able to offer. But, in my estimation while there may be some interaction between the units which results in varying load angles--or the problem may be analyzed using load angles--load angles are not something which can be measured (they can be calculated!) and they are the result of the load being carried, not the way that load is changed or controlled.

Load, on a synchronous generator, is a function of the torque being applied to the generator rotor by the prime mover driving the generator. Increase the torque being produced by the prime mover and the load being carried by the generator will increase.

And, since we really don't know exactly what the instability is (frequency/speed; load; reactive load (VArs); generator terminal voltage) we can't really say what might be causing the instability. Except to say that load angle is the result of the load being carried, not the reason for the load being carried. (At least not in my mind; some professors and PHd's might disagree, but I'm coming from an operational, real-world perspective--not a theoretical perspective. The theoretical perspective might be valuable in understanding the problem, but unless there are serious mechanical problems with the generator it's not likely caused by load angle differences between machines.)

There is also one more possibility that has just occurred while writing this: Is there some "external" control system being used to control the loads of some or all of the generators--using discrete logic signals or 4-20 mA signals or serial or Ethernet signals? Could the problem(s) be caused by instability of the "external" control system (sometimes called PMS, or, Power Management System)?
Same problem with us also having 55 MW LMZ Russia steam turbine. When it connected to grid, its start hunting. But as we islanded from grid at the same load, it stop hunting . what will be the reason?

We don't know anything about the governor used on the steam turbine at your site, or how it's configured during parallel operation or when islanded.

There are generally two types of governor modes: Droop and Isochronous. Isochronous is usually used on a prime mover and generator that is isolated from a grid supplying an "island" load. If there is more than one generator synchronized together on an island system/load then typically the simplest and most common set-up uses one governor in Isochronous mode and the other governors in Droop mode.

If a prime mover governor is in Isochronous mode when it's synchronized to a grid with many other generators it will usually be unstable--sometimes very unstable. We also don't know how unstable your unit is when synchronized to the grid with other generators and their prime movers. A little? A lot?

Many governors have settings called "gains" which dictate their response to changes in system variables. It could be that the gain for on-line (synchronized to a grid with other generators) is incorrect for the application, but the gain for off-line (islanded) operation is fine.

It could also be a problem with the control valve actuator and/or the design of the control valve actuator.

We don't when this problem started, or if it's been ongoing since original commissioning.

There's just not enough information to be of much more help.

Best of luck!