Power System Stabiliser 1

[JimScot]

We are purchasing 3* 35MW generators for a plant that has a load requirement of 90MW. These generators are limited by the steam consumption to overall 60MW therefore we are importing from the electrical utility 30MW. We have a 33/33 OLTC as the grid transformer.

For the generators the AVR transfer control block diagram indicated a PSS ( Power stabiliser). We are unsure whether we need this for damping of small oscillations with the grid. One scenerio we could have is one generator available producing the maximum and importing 55MW from the grid.

Do we need a PSS?

 

[Alex68]

Surely!
You need them not only to damp the little oscillations with the grid but above all among your machines.

Moreover you must set also the reactive power compound at a negative value to avoid the reactive power oscillations.

 

[dpc]

Not sure where you are located, but in the western United States, Power System Stabilizers are mandated by the regional transmission standards organization for generators above a certain size. It is the size of the generator that matters, not so much your net import or export of power.

 

[GTstartup]

I agree with dpc. I don't know what the cut off is, but 35 MW seems too small to require a PSS, which is normally required to support the grid. As for reactive power sharing, that's a different story and depends if your generators are connected at the output or after each step up transformer. if the former, you may require cross current compensation.

 

[JimScot]

thank you so far for the responses. The utility is in Qatar. Does anybody have any references to papers or practical situations?

 

[Rodmcm]

Would suggest you make contact with Basler or ABB both have PSS units

 

[Alex68]

I suggest this paper issued by GE in October 1996:

GEH-6359, "Power System Stabilizer for the EX2000 Digital Exciter".

That contains a very exaustive explanation about PSS.
In particular at page 1-4:
"Inter-unit oscillations typically involve two or more
synchronous machines at a power plant or nearby power
plants in which machines swing against each other.
Inter-unit oscillations typically occur at frequencies of
1.5 to 3 Hz."

 

[exnavynuke]

It's my understanding that on a smaller machine using a PMG type brushless exciter, a PSS is not very useful. The use of PMG excitation introduces large magnetic time constants that prevent the PSS from quickly adjusting the generator field and compensating.

Larger machines that use static exciters (such as the EX2000) can rapidly respond and compensate for system disturbances. Additionally a large machine can correct system disturbances with a PSS, where a smaller machine cannot.

I've worked at a facility that had nine 33 MW generators, equipped with PMG type exciters, and no PSS. There were no electrical instability issues between the machines. Hope this is of help.

 

[PowerfulStuff]

We have 3 30MW turbines and again no PSS and no grid connection. The PMG type exciter has a shorted winding in it to damp out transient instability. It gets a bit ugly when big loads drop off or start up, but it all holds together OK.