Power factor correction and PV systems

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tallgirl

tallgirl

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Glendale, WI
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Controls Systems firmware engineer
So, we all know those whole-house "power factor" correction things are a ripoff because the utility company charges for real power, not something else.

But what if you're the one making the power, such as with a PV system? Then is there any value in having such a gadget?
 
If you are off grid then yes you will want a decent set of caps.

I?m off grid and using Outback?s products. They do not say anything about using them but if you do some looking then you will find a lot of mixed feelings about them in off grid systems.

You might look at this site. http://www.fieldlines.com/ I know that it has been talked about a few dozen times but when it comes to the search good luck.

I don?t know all the back round on adding caps but it has something to do with if motors don?t have caps start/run caps then you will need 5x the motor rating to start the motor.
With multiple inverters you have the ?master? set up to start a ?slave? at your amp set point. If you have caps in the system then you can move the set points around to run less inverter for your given load. You set points are set by looking at the largest motor on the system.
A lot of people will use soft stats (I think there called) an anything bigger than a fridge.


For questions I would suggest asking at this site http://www.fieldlines.com/ .
I will ask a few people that will know more than I do and will post them for you.
 
You have to check with your inverter mfr. Some designs could be damaged by the charging current of caps, or by having the inductive load switch off and the caps left on, creating a leading power factor. PF correction in inverter fed power systems is not something to be messed with lightly.
 
Thanks for both responses. The hardware in question is a pair of Outback GVFX3648s. The comments about poor power factor and CFLs got me wondering because the entire house is CFLs and visions of poor power factor were causing me nightmares. I'll check with Outback and see what they have to say.

Regarding the motor starting issue, I spent a lot of time and effort making sure the inverters would start HVAC blower motor. The highest 1 second line current I've seen was something like 18A.

got_nailed, you might like this link. It works like you'd expect it to.
 
PF Correction

PF Correction

Generally Power factor correction is a grid requirement, to reduce the current loading on the lines. However, your application seem to be a bit specialised, dealing with inverters and PV energy sources feeding load through inverters... I would suggest that the pf correction should be used only after studiying the exact pf of load, how much correction is needed, and how much correction can the inverter system withstand...

If you know an electrical consultant near by, it would be a good idea to have him inspect the site and give his recommendation...
 
tallgirl said:
Thanks for both responses. The hardware in question is a pair of Outback GVFX3648s. The comments about poor power factor and CFLs got me wondering because the entire house is CFLs and visions of poor power factor were causing me nightmares. I'll check with Outback and see what they have to say.

Regarding the motor starting issue, I spent a lot of time and effort making sure the inverters would start HVAC blower motor. The highest 1 second line current I've seen was something like 18A.

got_nailed, you might like this link. It works like you'd expect it to.
Click to expand...

Don't worry about the HVAC blower motor, worry about the compressor. The inrush current could be a killer on the inverters and it is much shorter than the one second run-in. You have to get that from the manufacturer unless you have a special meter that registers impulse max. Check the meter spec's as what is the averaging time.

Stay FAR away from capacitors. Period.
 
weressl said:
Don't worry about the HVAC blower motor, worry about the compressor. The inrush current could be a killer on the inverters and it is much shorter than the one second run-in. You have to get that from the manufacturer unless you have a special meter that registers impulse max. Check the meter spec's as what is the averaging time.

Stay FAR away from capacitors. Period.
Click to expand...

The compressors will never be part of the critical loads -- I think inverter cost alone to start a compressor is something like $10K, and that's probably not enough. There are water cooled AC compressors that have lower power requirements and startup demands that might work.

Don't know what you know about Outback gear, but the GVFX inverter family is limited to 2 inverters in a stack, for a total of 7.2KW continuous and 10KW peak. The non-grid interconnected inverters will support larger stacks, but not the grid interconnected ones.
 
If the HVAC system has fuel based heat (or fuel based backup heaters), then having the blower means having functioning heat in the winter, even if you don't have the compressor to provide air conditioning in the summer.

I would be horrified if someone were trying to run an central air system using a _fixed frequency_ inverter system. You have all of the power silicon needed for a VSD, but by running at fixed frequency you have all of the inrush current problems of an 'across the line' start. Talk about the worst of both worlds. Far better would be to run the motor using its own VSD supplied from the DC bus.

When operated on a VSD, the starting current to the motor can be maintained at or below full load running current. If the mechanical system is designed for fixed speed operation, then the VSD would simply act as starter and inverter, ramping the motor up to operating speed without an inrush current spike.

Working out the details of controlling such a system would not be trivial, and I don't know if inverter driven HVAC compressor systems are available for residential use, nor do I know if inverter driven HVAC systems come equipped for DC input. (Only some of the VSD systems that I've encountered have explicit instructions for how to operate them with DC power, though essentially _all_ of them have an internal DC link.)

I know that variable speed blowers for home HVAC systems are common in the highest efficiency gas furnaces. They add quite a bit to the cost, but that blower actually chews up quite a few kWh over the years. I don't know if such variable speed blowers could be easily modified for DC supply.

-Jon
 
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