Controlling small wind farm power factor

[ters]

Small 9 MW wind farm is connecting to a 44kV distribution line. The power utility requires that at the point of interconnection, the wind facility maintains a fixed power factor of 0.97 at all times (inductive, the wind facility imports reactive power from the utility grid).

There are 4 turbines, with full a AC/DC/AC conversion, capable of operating in the power factor regulation range of +/0.90, measured at the LV side (inverter output), and where the PF and/or voltage control takes place in the inverter itself.

The collector system is 34.5kV, so each turbine is equipped with a step up transformer and there is also common 44/34.5kV transformer at the interconnecting substation.

Some wind turbine suppliers can also provide a common controller, a sort of common AVR, which is located at the interconnecting substation, connected to station PTs and CTs, and which can be operate the wind facility in either fixed voltage mode or fixed power factor mode. The controller communicates to individual wind turbines via a communication link (usually FO) and sends signals to local inverter controllers.

However, it this particular case the wind turbine is unable to supply such “common” controller at the interconnecting substation, so a system with the same functionality would have to be designed using standard third party equipment, i.e., power factor/voltage monitoring devices, capable of sending an appropriate feedback signal to each turbine controller to adjust its own power factor to whatever is needed in order to maintain the station PF at the preset level.

Has anyone worked on something similar and can provide some hints? North American market, 60Hz. Thank you.

 

[rbulsara]

Most WTG mfr would have packages/ options to handle this. Start with them.

Rafiq Bulsara

http://www.srengineersct.com

 

[rbulsara]

All new WTG with electronic controls work at unity pf.

Rafiq Bulsara

http://www.srengineersct.com

 

[wareagle]

Is it normal for a utility to require a PF that high?

 

[dpc]

http://www.sandc.com/products/purewave/dstatcom.asp

S&C is selling a bunch of these to windfarms in this region, based on RTO requirements. But 9 MW is probably a little too small for something like this. I believe about the lowest price point for the DSTAT system is about $1M.

O.97 is pretty standard around here.

David Castor

http://www.cvoes.com

 

[ters]

Thanks for the replies. As I said, in this particular case that package is not available with the WTG supplier.

Also, this particular WTG can work at any PF between +/-0.90 including unity, but unity is not the requirement of the utility.

The utility does its own modeling and determines what constant PF is required on the imbedded generator side, and that math is based on the load on that line, other generators connected to the same line, voltage regulation at the remote TS, compensators on their side (if any), maximum allowable voltage, market rules re VAR flow, etc. The number therefore will differ from case to case. This is the standard practice of this utility, others may do it differently.

Whatever is required, unity or something else, the PF at the point of the connection is not the same PF as at any of the 4 wind turbine outputs – there are two transformers and several km of cable impedance in-between.

The utility requirement is to keep the contact PF at the point of connection, which obviously requires IT equipment and a PF controller to be located at that interconnection station. And then, some sort of communication link will feedback digital or analog signals (whatever is is suitable) to the wind turbine controllers, which are several kilometers away.

 

[ters]

Thanks dcp. But we really don't seem to need static or any compensators. The WTs are alredy compensators themselves being able to operate in the range of +/- 0.90 PF at their end, so the requirement of keeping 0.97 at the substation end is likely to be covered by the available WT PF regulation range (modeling to confirm is still to be done).

The actual question is related to selecting some hardware which will act as the group PF controller at the substation end and which will provide a feedback to individual turbine PF controllers to adjust as required so that 0.97 at the substation is maintained at all times and at any active power output. Except when there is no wind and WTs are not running and become small loads of about 10kW.

 

[dpc]

Keep in mind that any long runs of 44 kV cable will generate a lot of kVARs, and the WT does not have unlimited or instantaneous VAR capability. Power factor at each WT will not be the same as pf at the utility interface.

We see these DSTAT units even on DFIG wind farms, IIRC.


David Castor

http://www.cvoes.com

 

[ters]

dpc, our collector is 34.5kV (not 44kV) but nevertheless, what you mentioned is still quite possible. However, we have experience with a similar project (10MW) and similar utility PF factor requirements on a 27.6kV collector (35kV cable) where PF regulator range of +/-0.90 was sufficient, but the line was also 27.6kV. However, in this case WTG supplier did also provide substation PF regulator.

We will do modeling to confirm whether the turbine PF regulation range is satisfactory (it should be – cable capacitance should be partly offset by reactance of two transformers – one at each cable end), and provided that the regulation range will do, then we still need to design a system which act as station group PF regulator.

 

[panter]

TERS,
Pozdrav iz Srbije
PS Djora te ?esto pominje.
Greetings from Serbia and happy New Year . . .

 

[PHovnanian]

ters, I don't know what your utility policies are. But in many cases, utilities have arrangements for providing customers with real-time metering data. So this would reduce your equipment requirements to a controller at the interconnect point that would receive this data and delegate real/reactive power production to the individual WTG controllers.

You'll be better off using the utility metering data than installing your own equipment. Aside from the costs involved, there will be much less chance of billing disagreements if everyone is using the same numbers.