Gas Turbine Alternator Questions

[sheiko]

Hello,

We have a gas turbine + alternator generating less power than expected (design).

I have noticed that the actual Current and Cosphi are different from the design values.

DESIGN:
- Air temperature = 15 degC
- Cosphi = 0.75
- Current = 687.8 A
- Active power = 4.9 MW
- Alternator efficiency = 97.5 %

ACTUAL:
- Air temperature = 30 degC
- Cosphi = 0.85
- Current = 420 A
- Active power = 3.4 MW

The other alternator characteristics do not change:
- Voltage = 5.5 kV
- Frequency = 50 Hz

My questions are:
- What could explain such gaps between actual and design Current and Cosphi?
- Is it possible to correct the design active power for the actual cosphi [= 4.9*(0.85/0.75)] to be abble to compare actual and design?
- Is it possible to adjust the actual cosphi from 0.85 to 0.75? How to do that?
- What is the effect of temperature on power?

Thanks for your time

"We don't believe things because they are true, things are true because we believe them."

 

[ScottyUK]

First question is "Are you connected to a larger system?".

If you have an islanded generator then the power and reactive power are governed by the load.

If you are synchronised to a large system then: -

The active power export is ultimately controlled by the governor, which in turn is controlled by the turbine control system. You need to find out what operating mode it is in. Typically turbine controllers have some variation on: 'speed control' for acceleration and recovery from load rejection; 'isochrononous control' for islanded operation; 'load control' for operation at part-load; 'temperature control' for operation at base load conditions. Some schemes also provide 'power factor control', which is basically a modified load control scheme.

With 30°C intake air the machine may reach base load conditions before it reaches full rated output power. 'Base load' is typically defined as IGV's wide open and exhaust gas temperature on the limiter. After that there may be an option for power augmentation by injecting cooling steam into the combuster, or using an inlet chiller or fogger, although I haven't seen any of those options on such a small unit.


Reactive power export (or import) is controlled by the generator excitation. Typically an AVR will be in 'automatic' mode where it will act to regulate terminal voltage, but there is usually a 'manual' mode where the field current is held constant, and occasionally there may be a more complex automatic mode where the excitation is controlled to meet reactive despatch.