Generator Capability with additional Cooling

[mrg397cs]

Most large generators can provide increased output with additional cooling providing the prime mover has the ability to provide the additional power. The generator capability curve is expanded to show the new limits.

Question. Can the generator supply additional motor starting kva at the increased rating?

 

[Kiribanda]

Hi mrg397cs,

It is similar to different ratings of a transformer with ONAN/ONAF cooling. If the transformer has got 20/ 31.5 MVA/ 12 %, then 12 % is defined at the machine basic rating i.e. 20 MVA. As such for 31.5 MVA the impedance is 19%.

Similarly, the sub-transient reactance xd’’ ( which is responsible for short circuit current on the gen terminals ) of the generator is defined for the machine basic MVA rating. So naturally, if the MVA rating is enhanced by better cooling then the value of xd” increases for the new MVA rating. Finally the short circuit current at the generator terminal increases. That means your motor starting capability increases on the generator bus.

You can simply calculate the value of xd” for the new machine rating and then the short circuit current at the generator bus!!

Regards!
Kiribanda :->

 

[RAMConsult]

Assuming no other changes beyond higher coolant flow and prime mover input, the answer is a qualified yes- up to a point. That point is the limit imposed by the excitation system, the field current and where it meets the stator current limit on the generator capability curve.

On a generator capability curve the right hand circular portion shows the constant MVA output limited by the stator current, the upper portion shows the MVAR capability limited by the field current. Increasing the prime mover input only increases the amount of real power available but does little for the reactive power. The reactive power generation capability will depend on the excitation system capacity and its tuning parameters. Therefore the answer to this question depends upon how linearly the MVAR (top) portion of the generator capability curve tracks the MVA (right) portion.

The attached paper provides an excellent discussion of the excitation system's role in prime mover and cooling upgrades:

http://www.transmission.bpa.gov/orgs/opi/Power_Stability/FldCurPanelGE.pdf

 

[JODIGAL]

mrg397cs
Dear mrg
What is acheived by improving/adding the cooling is simply changing the thermodynamic scenario or altering the rate of evacuation of heat and hence permitting more I square heating and hence loading.
Where as in the case of MOTOR STARTING ( A LOW POWER FACTOR CONDITION ) there is an excessive demand of reactive power or KVAR . Unless the generator bank has enough quota of KVAR in its kitty no overdrafting is possible by improving the cooling efficiency ( which has no connection with catering to increased KVAR demand .
Hence this increase in cooling may not be of any use in catering to additional motor starting KVA.
the only other way is AVR RESPONSE to increase the excitation to cater to the excessive demand of KVAR and try to maintain the Voltage enough to sustain starting conditions and make sure the motor accelerates.

 

[JODIGAL]

mrg397cs
Dear mrg
What is acheived by improving/adding the cooling is simply changing the thermodynamic scenario or altering the rate of evacuation of heat and hence permitting more I square heating and hence loading.
Where as in the case of MOTOR STARTING ( A LOW POWER FACTOR CONDITION ) there is an excessive demand of reactive power or KVAR . Unless the generator bank has enough quota of KVAR in its kitty no overdrafting is possible by improving the cooling efficiency ( which has no connection with catering to increased KVAR demand .
Hence this increase in cooling may not be of any use in catering to additional motor starting KVA.
the only other way is AVR RESPONSE to increase the excitation to cater to the excessive demand of KVAR and try to maintain the Voltage enough to sustain starting conditions and make sure the motor accelerates.

Regards
Jodigal

 

[ScottyUK]

mrg397cs,

Most generator capability curves have a flat line at the top indicating the mechanical limit of the prime mover. For power factors better than about 0.9, the limiting factor is the prime mover rating, not the generator stator heating limit. On that basis, there is a theoretical ability to increase the MW output up to the rated MVA of the set at unity PF, provided the prime mover is sufficiently powerful. If the prime mover is rated so that MW rating = MVA rating, then for any normal operating PF - typically 0.85 to 0.9 - the prime mover will be under-utilised.

The ability (or inability) to provide adequate cooling for the rotor may well limit the ability to increase the MVA rating of the set beyond original design parameters if the set is to operate at non-unity lagging (VAr exporting) power factors. The point on the capability diagram at which the rotor heating limit line crosses the stator heating limit line will occur at a higher PF as the MVA rating is pushed higher.

 

[brupp]

A lot of good information is imparted above, but some key points are missing. Improving the cooling will improve the thermal characteristics of a generator, which is important for steady state and any long term operational parameters. But if you're concerned about occasional motor starting, this is a short term event and the generator capability curve (representing the thermal performance) is not entirely relevant. What is relevant is that the excitation limiters are typically set to points on the capability curve so they may prevent the generator from short term excursions which could provide the vars needed without negative effects on the generator. The paper referenced by RAMconsult offers some interesting solutions to that with newer, more flexible excitation limiters.

More importantly, unless the generator is already operating near the limits of the capability curve your motor starting will be unlikely to push an edge of the curve. That's easy enough to calculate.

And most important to understand is that the internal impedances probably did not change due to any cooling or nameplate changes. A synchronous machine during starting is best modeled as a constant kVA device. The generator needs to provide that needed kVA and the only ways to provide more are to increase voltage or to reduce impedance. Chances are, neither of these are happening and there will be no effect on motor starting.