Does anyone know what will cause Generator MW to swing. I've monitored the FSR and Actual Fuel ratio and they were not fluxuating. I've noticed the Generator MVARS were swinging with the load. Is that normal? Can the MVARS affect the MW output? The swing is typically about 1.6MW.

Please Help.

 

Power (real) is a function of torque which is a function of fuel. If power is truly fluctuating, then fuel is fluctuating.

VArs are a function of excitation. If VARs are fluctuating, then likely you will find the exciter regulator ("AVR") output is fluctuating.

The electrical power produced by a synchronous three-phase generator is:

P = Vt * Ia * (3^0.5) * pf

where P = Power in Watts
Vt is generator terminal voltage
Ia is generator armature (stator) current
3^0.5 is the square root of 3
pf is the power factor

Let's presume that the power factor is 1.0 (unity) and that, as with most generators, the generator terminal voltage is stable. The square root of 3 is a fixed number, so if you want to change the power of a generator the only variable left to change is the armature current.

A generator is a device for converting torque from a prime mover into armature current. So, to produce more armature current one needs to supply more torque input to the generator. To produce more torque from a gas turbine, one needs to increase the fuel flow.

Generator terminal voltage can have an impact on power output, but it is usually only really noticeable at very low power outputs, and because most generators can only vary their generator terminal voltage by approximately 5% more or 5% less than rated voltage (+/- 5% of rated terminal voltage) the effect isn't all that great even at low loads.

If the power is truly fluctuating, then the armature current (phase current) ammeter will also be fluctuating with the same rhythm/frequency as the power fluctuations.

If you believe FSR is stable and fuel flow is stable, then fuel supply pressure, or if the unit is running on gas fuel, the P2 pressure, is unstable. From your nom de guerre, viwapainc, I'm presuming the unit is in the Virgin Islands at the Water and Power Authority so it's likely burning liquid fuel. What is the liquid fuel forwarding pump pressure doing when the power is fluctuating?

Again, it's not very likely (not impossible but not very likely) that excitation/VAr fluctuations are causing the load swings you are seeing. That is a "small" grid, and stranger things have happened, but still not likely. It's not clear what you're using to monitor fuel flow or FSR, but something's amiss.

Do you have a GCS control system?

 

Responding to Viwapainc's 19-Jul-09 (15:38) query... before jumping the gun please provide some detail:

1) Prime mover?

2) Gen Parameters?

3) Load characteristics?

4) New or old problem?

5) Iso-operation, or in parallel w/other source?

6) If in sync with another source, is connection direct or via utility step-up xfmr or OH transmission line or combination?

7) Have you searched List Archive for like topic?

Regards, Phil Corso (cepsicon [at] aol [dot] com)

 

This may or may not be relevant but when dealing with dynamic stability issues it can be useful to consider the following equation:

Electrical Power = ((Eo.Vt)/Xs)Sin(delta)

Eo is excitation
Vt is terminal voltage
Xs is synchronous reactance
Delta is the angle between the machines magnetic field and the systems
magnetic field.

If the electrical power out does not equal the mechanical power in then the power imbalance accelerates or decelerates the rotor.

For example, if a fault on a power network occurs and reduces the terminal voltage then the electrical power output will immediately decrease. The excess mechanical power will accelerate the machine and in response to the increase in speed the governor will start to reduce mechanical power.

Also, Governors and AVR's are automatic control systems. They have to be tuned properly.

Vince