Synchronous Generator and Power Grid Failure

S

Thread Starter

synchro

Dear all,

In case of a grid failure large generators are often obliged to be keep connected to the grid in order to help stabilizing it. The "low voltage ride through" or "fault ride through" curve defines how long a generator has to keep connected to the grid depending on the actual grid voltage during failure.

I have often read that synchronous generators get into trouble during a failure since the mechanical power seen by the generator is greater than the electrical power the generator delivers to the grid. This leads to an acceleration of the rotor angle and finally can lead to instability if the angle is getting greater than pi/2.

Now I don't understand why the electrical power the generator delivers to the grid gets smaller during failure. How does this correlate to the grid voltage (which is very low at the time)? Is the reason the lacking power supply of excitation system by the net? And what are the governing equations which can describe the effect?

Further I've read that the stability is not only influenced by the time the generator has to stay connected after failure (fault clearance time), but as well by the generator's pre-fault operating point in terms of apparent power and voltage. Maybe you could also elaborate on these dependencies.

I would be very thankful if you have any knowledge about this and if you could also suggest further literature for this problem.

Cheers,
krigl
 
krigl,

The term "grid failure" is extremely vague. That could be one of several reasons why you haven't had a response.

It might help to remember that generators are devices for converting torque (mechanical) into amperes (electrical)--just as electric motors are devices for converting amperes (electrical) into torque (mechanical).

Grid disturbance can take many forms. The grid can be overloaded because of the loss of one or more generators which have been tripped. This will result in underfrequency.

Or, the grid can have an excess of generation caused by loss of large blocks of load (neighborhoods, regions, factories, etc.). This results in overfrequency.

Or, there can be a massive earth fault of one or more phases. Or a massive short of two or more phases. In some of these types of faults the desired action may be to "feed" the fault to "burn it out." This can result in a very large load for, what is hoped will be, a very short time. And then suddenly a large loss of load. This can also result in large voltage swings depending on the types of generator exciters in use.

Or, it may be desirable to very quickly isolate the location where the failure is by opening one or breakers, resulting in a loss of load.

It's really difficult to say what will happen during any grid fault when they can be so different; hopefully you can understand.

By trying to narrow down the type of fault you might find you will get better responses. I'm no expert in grid disturbances and their effects and the desired actions, but there are frequent responders to control.com who are.

Hope this helps!
 
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