Generator Short-Time VAR Capability

[EddyWirbelstrom]

A motor start study on a system with only local generation shows generator VAR requirement in excess of that shown on the the generator's steady-state VAR capability diagram.

I do not have any information on the generator's short-time VAR capability.
Does anyone have typical values of generator short-time VAR capability for motor starting ?

The short-circuit standard IEC60909-0 clause 4.6.1.1 shows that for a cylindrical rotor generator with an xd sat of 1.6, and an initial subtransient short-circuit current = I"k / IrG of 4, the generator can deliver a max continuous short-circuit current of 1.7 times its rated current.
Could this be used as a guide for generator VAR capability on starting motors ?

 

[motorspert]

There are two aspects to consider
(i) the thermal duty of the genetor and for thus the run up time is important - but for 10 seconds or so 170& current shouldnt cause a problem as the stator and rotor windings have a large thermal msss.
(ii) The excitation system capability - as the diodes etc are smaller devices they may not be so liberally rated. During a motor start, the load is predominantly lagging power factor, so the field will be working very hard. However, most excitation are designed for 160% overload for 10 seconds.

So for a normal run-up time the excitation system should be able to cope

 

[swn1]

Two more things to consider: there is a stability limit, beyond which the generator may or may not pull out of sync, depending on details of the transient event. One arc on the capability curve is there due to the stability limit rather than a thermal limit and it is a short-term limit.

Also, if the starting transient causes a frequency transient your generator will accumulate high-current off-frequency operating time. There is a chart you should have that shows how much time at various frequencies is allowed before (predicted) damage results.

 

[Alex68]

The stability limit of generators concerns under-excitation condition. So it doesn't involve a motor starting because it requires lagging reactive power.
During the start up, a motor requires a lot of reactive power and few active power. So no concern regarding frequency.

mnewman,
as pointed out by motorspert, you should verify the "ceiling" capability of you exciter. Every exciter is able to go beyond its own limits for a short time. Usually the ceilling can be 200% of rated current for some seconds. You should verify if it is enough for you.
Since you are testing the limits of your system, I suggest you to make a dynamical simulation including the ceiling limits of the generator and other possible components that can be affected by the voltage drop caused by the motor starting.

Regards
Alex68

http://m2ec.manunza.com

 

[tommom]

I would think the short-time var capacity of the generator would mostly be limited by the maximum voltage output of the voltage regulator. That limits the generator excitation, which is mostly responsible (speed, etc being constant) for the Var capacity.

 

[waross]

Another aspect of motor starting on smaller generators (up to several hundred KVA or more) is Under Frequency Roll Off (UFRO). This is a feature of the Automatic Voltage Regulators used on many sets in this size range. If the load exceeds the capability of the prime mover, and the speed and frequency start dropping, the AVR will start dropping the voltage. This lessens the load and allows the prime mover to recover. The voltage reduction/load shedding usually starts at 2 hz. below nominal frequency.
respectfully