Generator, reactive power 1

[sinead99]

If a generator is at 0.95 power factor, and importing reactive power are they producing reduced active power?

 

[davidbeach]

Compared to what?

 

[sinead99]

compared to the full load rating of the generator.

 

[waross]

The prime mover sizing is based on the real power rating.
The KVA rating is generally based on a power factor of 0.80.
As long as the generator is operating above 0.80 PF full reactive power should be available.
PS. In most instances a generator operating at 0.95 power factor will be exporting reactive power.

Bill
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"Why not the best?"
Jimmy Carter

 

[rcw retired EE]

Is 0.95 power factor lag or lead? KVARs into or out of generator? I assume KVAR into the generator since you said it is importing reactive power.

The kW output is the same in either case. As long as the turbine or engine kW output does not change, the generator kW output will not change for various power factors, neglecting the minor increase in losses at lower power factors due to higher KVA.

Example, MVA at 0.95 power factor = 105.2% of the MVA at 1.0 pf for the same MW. The I^2R losses will be 10.8% higher at 0.95 power factor but still small in comparision to the output. From a chart for a 157 MVA gas turbine generator, at 150 MW 1.0 pf losses are about 1880 kW and 2020 kW at 0.95 power factor. The increase of 140 kW is less than 0.1% of MW output.

 

[dpc]

The kW output of the generator is basically determined by the prime mover and its governor. To maximize the kW output of the generator, the pf is set at unity to allow the total kVA output of the generator to be used to produce kW. But no matter what the power factor is, the throttle has to be opened up to increase kW.



David Castor

http://www.cvoes.com

 

[m3ntosan]

sinead99,

You need to reduce the active power output if you want to push the unit to its mvar leading limit and remain within the reactive capability curve. If you don't reduce the active power output you can reach the under-excited or stator winding heating limit.

May you grow up to be righteous, may you grow up to be true...

 

[ScottyUK]

Most turbo generators are not designed for operation at high leading loads. They suffer from stator end heating because the magnetic field strays into parts of the machine which are not designed to carry high flux, plus the machine will encounter stability problems in the event of a grid disturbance because the field eventually becomes too weak to hold the machine in sync. Some salient pole machines used in hydro schemes are capable of operating near rated MVA at very low leading power factors. So the answer to some extent depends on the design of the generator you're considering.


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If we learn from our mistakes I'm getting a great education!

 

[m3ntosan]

Yup, hydro units are water cooled and not subject to stator end-turn thermal limitations. The leading reactive capability of a typical hydro unit is therefore much greater than that of a thermal unit.
Also the power capability limits of a generator are mostly thermal related limits. If the system voltage is raised, the current is lowered and the power capability can be
extended.

ScottyUK,

A bit off topic but, could you please let me know why the power stations in the UK would tap GSU tx in order to modify reactive output, insted of instructing the AVR directly?
I understand the reactive output might be limited by the voltage on the 24 kV busbar where the generator is connected and by tapping you extend the margin of reactive power output, but I believe the tap changer is one of the weakest points of a tx.

May you grow up to be righteous, may you grow up to be true...

 

[dpc]

hydro units are water cooled and not subject to stator end-turn thermal limitations

That is certainly not always the case. It will depend on the unit size. I suspect the majority of hydro generators are not water-cooled.



David Castor

http://www.cvoes.com

 

[ScottyUK]

m3ntosan,

Our generating plants use the OLTC to provide coarse control of the reactive load and the AVR to provide fine control. I've never really considered why this is the case! My opinion is that the UK's Grid Code is quite demanding in requiring the machine capability to be available over a reasonably wide variation in terminal voltage, so the OLTC is a means of providing this while allowing the machine to maintain reasonably constant terminal voltage to keep away from the V/Hz limiter and the under-excitation limiter. Also the UK's grid is quite stiff because of the small geographic area and relatively large number of generating sites in that area, so the GSU transformer has a relatively high impedance to control system fault levels. A high GSU transformer impedance makes an OLTC almost mandatory to achieve a wide reactive capability.


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If we learn from our mistakes I'm getting a great education!

 

[rasevskii]

In fact hydro units are usually closed-circuit air cooled with air to water heat exchangers in the air circuit. Small units are open-circuit air cooled. By small we mean up to, shall I say, in the range of up to around 10 to 30 MVA, depending on the speed. Large units of hundreds of MVA can have direct water-cooled stator and even rotor windings.

rasevskii