Generator Power Factor 2

[Cerkit]

Hi,

Can someone help me understand, in really basic terms how does a synchronous generator change the power factor at which it exports at?

I think with inverter based generation this is done by thyristors switching the current at different angles along the voltage waveform is this correct?

Any help or literature on this would be much appreciated.

 

[davidbeach]

At low field current the generator absorbs vars, at high field current the generator exports vars. Field current is controlled by the excitation system.

 

[xnuke]

To a first approximation, reactive power is controlled by the magnitude of voltage generated internally |Ea|, which is adjusted by changing the excitation current in a generator. In inverters, reactive power is adjusted by changing the magnitude of the reference sine wave in sinusoidal PWM or by changing the q-axis control loop set point in SVM. When the real power output is kept constant, varying the reactive power output allows adjustment of power factor.

To a first approximation, real power is controlled by the phase angle of voltage generated internally with respect to that of the grid (the power angle delta) which is adjusted by changing the mechanical input power to the prime mover in a generator. In inverters, real power is adjusted by changing the phase angle of the reference sine wave in sinusoidal PWM or by changing the d-axis control loop set point in SVM.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[DM61850]

A synchronous generator outputs vars based on how excited it is. Every generator generator D-curve or a rating. The D-curve shows what the generator is capable of exporting under certain conditions. What the generator is capable of doesn't have to be what the plant is capable of. You can have a generator capable of 600 MW but if the prime mover is connected to a system that can only crate 500 MW of real power, that is your limit. Vars are a little different because they don't depend on what you have turning the prime mover. There are a lot of things that can impact how many vars are exported.

1. Hydrogen pressure. Hydrogen is used on large generators to reduce windage losses and cool the rotor. Lower hydrogen pressure means less cooling which can mean less vars.
2. In plant voltage constraints. A plant can't exceed its own equipment voltage ratings. This can be more of a limiter than the D-curve. You can't have voltage greater than your equipment rating inside of your plant.
3. Var export rating is based on system assumptions. Vars move in a Vgenerator*Vgrid/Zstepuptransformer on transmission present voltage base fashion. If the grid is running high voltage wise, you'll not be able to put as many vars into the system as you would if the system voltage was low. Lagging generators are exporting vars. The convention is backwards from load convention. Some people get annoyed when they test their generators capability curve and it doesn't match the manufacture D curve. They shouldn't be getting made because you are only going to match the manufacture D-curve if the grid voltage is the same as their assumption.
4. How your step up transformer is tapped can determine the var import and export limits of your plant. If you have the low side tapped low or high side tapped high, your generator will always be trying to pull the system up by exporting vars. If you have it tapped high on the low side or low on the high side, your band will be more centered around importing vars. Transmission Planning may tell you how to tap your step up transformer so that they can get the vars plus or minus from your plant during whatever analyze contingency.


Thyrister based generation can inject current into any of the 4 quadrants. Thyristor generator is not much different from statcom with a real power source attached to it. Some wind farms with type 4 turbines (all the newer ones I beleive are no longer DFIG type 3 generators), AC to DC to AC connections have talked about using their thyristor power electronics to mitigate power quality issues. The perk with the type 4's is that since you are decoupled from the grid by using a DC bus and conversions, you don't need a gearbox since your turbine frequency is decoupled from the transmission system.

 

[waross]

A single islanded set:
The load controls the actual power factor.
Generator ratings:
One limit on a generators capacity is the ampacity of the windings.
A second limit is the maximum voltage at which the windings may be operated without saturation.
A limit on the actual capacity of a generator is the voltage at which it is rated.
So, for example, a generator with an ampacity of 200 Amps rated at 240 Volts will have a KVA rating of 48 KVA.
That is the true rating.- 24 KVA
The generator set manufacturers assume that most overall loads are somewhat inductive and apply an arbitrary rating of 80% PF.
So the true power of a 24 KVA set operating at 80% PF is 19.2 kW.
The arbitrary PF of 80% is used to justify fitting a prime mover of at least 19.2 kW plus losses rather than a prime mover of at least 24 kW plus losses.
When you see a rating of PF 0.8 on a nameplate it has no relationship to the load PF but is a justification for installing a smaller prime mover.
I am not complaining or venting.
The assumption is mostly accurate and in the real world it sizes the prime mover more closely to the load with attendant fuel savings.

When a set is islanded, the field strength or excitation controls the voltage.
When a set is grid connected, changes in field strength have little effect on the voltage and no effect on the load power factor, but change the way that the set shares VARs with the other sets in the grid.
This interaction has been described by most of the previous posters.
Is your set islanded or grid paralleled?
Take your pick of the appropriate answer.

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