Loss of excitation in Synchronous Generator connected in Grid

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Thread Starter

Santanu Das

When var (reactive power) is drawn by the synchronous generator (or Alternator) from the grid, the amount is limited by capability curve, which is a characteristics of the particular generator. But if excess var is drawn what will be the problem???

Secondly if the excitation breaker trips, and the generator connected with the grid didn't trips, what will happen? Will it still continue with the same speed of rotation (rpm ,3000 in 50 hz system), taking all var from grid system???

please Sir give me explanation
 
Santanu Das,

Please be very careful when talking about reactive current and referring to is 'power.' Some people here on control.com don't take well to this characterization.

Leading reactive current flows in the generator stator windings when the rotor field excitation is less than that required to keep the generator terminal voltage equal to or greater than the voltage of the grid with which the generator is synchronized. The reactive capability curve does NOT limit anything in any direction--that's the operator's job to use the reactive capability curve to ensure generator operation does not exceed the limits defined by the reactive capability curve.

When excessive leading VArs flow in the generator stator winding while synchronized to a grid the stator end-turns can overheat, which leads to insulation damage which can cause shorts between turns and cause large problems for the generator phase balance and operation.

Left unchecked, the rotor field strength can weaken to the point that the prime mover will cause the generator to "slip a pole" which means the generator rotor will suddenly turn as much as one complete revolution faster than it should be spinning (depending on the number of poles) and then come violently to a stop again as it tries to magnetically link up again with the stator magnetic field(s). This causes catastrophic failure of the coupling between the prime mover and the generator, and sometimes worse.

If somehow the generator doesn't slip a pole when excitation is significantly decreasing the next really bad thing that can happen is that the synchronous generator will become an induction generator, which cause the rotor to get VERY hot VERY quickly, leading to insulation damage, high vibration, rotor striking stator--in general, none of these things are desirable.

There are supposed to be protective relays monitoring the generator operation to prevent this kind of thing by tripping the generator (and turbine if necessary) before failure and damage can occur.

Hope this helps!
 
> When var (reactive power) is drawn by the synchronous generator (or
> Alternator) from the grid, the amount is limited by capability curve, which is a
> characteristics of the particular generator. But if excess var is drawn
> what will be the problem???

Normally, excitation protection limiters will not allow the system to go beyond the prescribed curve, (Underexcitation limiter). but in case of abnormal condition, entire system(excitation system,turbine and GCB) will get tripped.

> Secondly if the excitation breaker trips, and the generator connected with
> the grid didn't trips, what will happen? Will it still continue with the same
> speed of rotation (rpm ,3000 in 50 hz system), taking all var from grid system???

The above mentioned condition will not happen at all. But in case of any mechanical or electrical problem in GCB, if it happens, then synchronous generator will start working as induction generator. it will start to take 2-3 times of its rated reactive power to magnetise the rotor. during this period grid will face the major loss of voltage reduction and instability. more importantly, while running as a induction generator, stator current will be 2-3 times of its rated value based on the slip. Such a huge current will damage the stator coils badly.
 
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