Controlling power factor for solar grid tie-in inverters

[bozzah]

I am trying to understand how to control power factor for grid tie-in inverters for ground mounted solar system (10MW). The solar site consists of ten 1MW inverter stations (2x500kW inverers). The AC output of the inverter is 480V at which point it transforms to intermediate voltage 27.6kV (though 1MVA transformer). Furhter, the collector circuit feeds into main substation at which point in coverts to 44kV through 10MVA transformer. The requirement is to maintain p.f. of one(1) at the 44kV utility connection. From the SCADA system, we can send commands to inverters to push kW and KVAR output to have desired p.f. (say +/- 1). Also, there is no dynamic cap banks at the substation. I am trying to undestand how to work out the formula and inverter commands (inveter p.f.) having in mind that transformers have specific reactive shifts based on the actual real time generation from solar field.

 

[pwrengrds]

KVAR consumed by each transformer is

KVAR CONSUMED = % impedance/base KVA x ACTUAL kVA x ACTUAL kVA

assume at 1MVA the TX is 7.5% it will consume 750kVAR at 1MVA. At 1.5MVA it will consume 7.5% x 2.25 = .168 MVAR.

Assume you have 10 of these at 1.2 overload
Then at the 1 MVA TX you consume 0.108 MVAR and if you have 10 then 1.08 MVAR.

assume the 10 MVA TX is 8% and assume that you overload it by 1.2x to 12 MVA. Then it will consume

8% / 10 x 12 x 12 = 1.152 MVAR

so to deliver your product at unity power factor at 44 kV you need to supply 1.08 + 1.15 = 2.23 MVAR. It's going to vary a lot depending on the load, but from this you should be able to calculate it.

This is my understanding of transformer theory. I am sure if I am wrong you will know very quickly.

David

 

[juleselec]

PF farms typically have some kind of park controller that controls the output of the individual PV inverters. Normally the inverter manufacturer will also supply the park controller, but it seems that in your case you are doing this yourself with a SCADA system.

In its simplest form, the park controller is a PI controller with a feedback loop. If you wanted to control the pf at the PCC, then the input would be the pf (or P and Q) measurement at the PCC. You'd want to adjust the proportional and integral gain (K_p and K_i) depending on the desired speed of adjustment and allowable overshoot, plus it should be small-signal stable.

I suppose you could work out formulas to dynamically adjust the reactive power output of each inverter, but I probably wouldn't bother making it so complicated.