compensate reactive power

[wuqn]

we have 3 generators that are paralleled. Can we use capacitors to compensate the reactive power of the system? we want to reduce the current through the generators.

 

[ScottyUK]

Yes. Be careful that you do not approach the stability limit of the machine when the generator has very low excitation and is absorbing leading VAr's. You may need to switch in the capacitors once some load is established in order to avoid this condition.


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[davidbeach]

If you ever strand the generators with the capacitors and no other load you will probably let the smoke out of something. Caps out at the load to improve your power factor could work, but a cap bank on the generator bus is just asking for trouble.

 

[RAVWARD]

Hi Davidbeach,
I just recalled about an Induction Generator Project on a hydroelectric plant. Very small plant though aroubd 600 KW but it is supposed to have a cap. bank at the gen terminals for reactive power by default. So, how would you rule out the possibility of not having a problem.

Thanks

 

[davidbeach]

Induction generators are different. My comments applied to synchronous generators.

 

[wuqn]

thank everybody. But what's happen when the generator has very low excitation and is absorbing leading VAr's? Can you explain more?

 

[ScottyUK]

Very low excitation equates to a weak field. The field serves two purposes on a generator paralleled to the grid:
i) it allows the prime mover to deliver energy to the grid;
ii) it allows control of the reactive power of the machine.

The problem with operating with a weak field - i.e. absorbing reactive power - is that the ability of the field to hold the prime mover in synch with the grid is reduced. This is the reason that the capability curve is truncated at the under-excited edge at the stability limit of the machine. If you reduce the field far enough the generator will break out of synch with the grid in a very serious condition known as 'pole slipping'. A pole slip causes huge dynamic forces on the machine and must be detected and the unit tripped very rapidly to avoid serious damage. Normally the AVR is designed to prevent operation outside of the stability limit, and generate an alarm when operting near it.

----------------------------------

Sometimes I only open my mouth to swap feet...