ETAP Generator and Prime Mover Rating Clarification w/ load flow analysis

[tumbleweed1]

I am attempting a load flow analysis on the following setup:

I have an oversized generator (550kVA) coupled with a gas-fired turbo engine whose output is 400kW at pf=1. Since the output of the generator is limited by the output of the engine, how is this represented in ETAP.

Specifically, when I add the properties to a generator in ETAP, on the RATING tab there is a Rating field (for the generator) and a Prime Mover Rating. If I fill in both rating fields, the Prime Mover Rating does not seem to limit the generator output when I do a load flow analysis.

Would a guy simply "derate" the generator capability to that of the engine output? Additionally, generators have efficiency relations based on loading so how would this be reflected if I simply derated the generator.

Im kind of lost here as to the proper way to make this happen.

 

[dpc]

The generator kVA rating entered must match the nameplate rating, since that is base kVA for the per unit generator reactances. If you "derate" the generator, you'll have to make sure the generator reactances are still correct. Not sure about ETAP, but you should be able to schedule a maximum kW output when running in voltage control mode. Is the generator running isochronous in a isolated mode (no grid connection).

 

[waross]

If the kilo-Watt load exceeds the prime mover capability, (400 kW) the prime mover will slow down or stall.
If the load KVA exceeds the generator KVA the generator will overheat.
The generator PF is 0.7273 (400kW/550KVA)
The PF of the prime mover is ????? I don't know what the PF of a prime mover is unless it is an electric motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[davidbeach]

PF of a prime mover is entirely undefined; probably a misunderstanding on somebody's part. Generator MVA rating should always be larger than the prime mover MW rating. Ideally I'd want a generator that won't be over dutied at full prime mover output for an power factor better than 0.80 (leading or lagging), otherwise you'll find yourself cutting back on prime mover output any time the load PF is not near unity.

 

[waross]

A PF of 0.8 is universal for diesel sets.
A prime rated set may be operated at a 10% overload for 1 hour out of 12 hours.
This implies that it may be safely operated at full load and a PF of 0.7273 for 1 hour out of 12.
I suspect that your set is prime rated and the prime mover is capable of supplying 440 kW for 1 hour out of 12.
However that information is not given and you must verify it yourself before you assume that the set is prime rated.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[tumbleweed1]

dpc- You have a good point- that's what I ended up doing, setting one generator to voltage control- 80% of full load at which point the second generator is set to swing mode to pick up the slack, all in an islanded grid.

waross- Disregard that point about the engine PF- I blindly rewrote exactly the specs that the sales guy gave me, obviously its a conversion from mechanical output to 400kWe.


Back to my question- seems like dpc and I are on the same page. Using the voltage control setting is ETAP's way of setting the output of the generator despite prime mover power.

What I have yet to understand about ETAP is what's the point of even inputting the prime mover rating as it doesnt seem to affect the generator output.

As well, where or how are the efficiency curves are instituted. The prime mover may be 400kW, but based on pf you lose a few % power, not necessarily represented by settings in ETAP.

Anyway, thanks everyone for your input! Helpful as always!

 

[waross]

The prime mover power is a limit. Up to 400 kW total the prime mover will supply the load and the losses. It may be interesting to see what ETAP does if you put a 500 kW or 600 kW load on the 400 kW prime mover.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

The voltage control settings should not have much effect on the kW output of the machine - only the kVAR. You may want to check with ETAP tech support to see if ETAP respects the prime mover rating when running power flow simulations. But you may be overthinking this. I'd just run the situation I want to analyze - if the kW output of the turbine in the simulation exceeds its rating, then you have a problem to deal with. Since the power flow solution must be iterated, we often set the generator min and max kW ratings very wide to avoid divergence problems. You just need to review the results to make sure they are reasonable. Don't expect too much of the power flow analysis - it's just a rough simulation.