Solar PV scenario

[aelectricalman]

Say you have 4 residential customers all served by the same small pole mounted XFMR. Now, say that all four homes use solar PV as a way to offset utility costs. Now, say all four homes are producing just enough power to make all their meters run backwards. Now your backfeeding the XFMR! What can happen if anything? Does it simply take a 12:1 ratio and make it a 1:12 ratio on the XFMR, or does it cause more severe problems? These questions are omitting the fact that there are net metering tariffs or Renewable energy credits. Does the transformer suffer any problems?

I have installed many small scale and medium scale systems but I am experimenting with larger scale utility grade solar PV apps. and its now a consideration that can not be overlooked. Thanks for any help..

 

[dbuckley]

It works absolutely fine; transformers are (more or less) bidirectional devices and will transform either way.

I say "more or less" as there are considerations that make transformers more suitable for going a specified way, but these considerations are insufficient to prevent the thing working.

In your case, they are generating power onto the MV distribution. Nothing wrong with that, power stations do it all the time

 

[gar]

100708-1914 EST

For the most part it does not matter what direction power flows thru a transformer. What is important is the power loss in the transformer. Much of the loss is I^2*R.

Roughly speaking if you send power based on the VA rating of the transformer in either direction you will be safe. But keep in mind it is really temperature rise that is important.

If the PV inverters do not generate pure 60 Hz sine waves, then iron losses might be greater from harmonics. Also a step down transformer will have its high voltage primary designed for just slightly higher power dissipation capability than the secondary because it has to handle the magnetizing current. When the transformer is run in the reverse direction then the magnetizing current flows in the low voltage coil. But this is probably not a big consideration.

Large transformer designs will in part be based on experimental measurement of maximum hot spot temperatures in the transformer.

At the cost of PV systems I suspect most users will not install capacity as great as their peak demand.

Even with incentives I can not calculate a reasonable time, if any, for return on investment.

.

 

[aelectricalman]

f the e o

Even with incentives I can not calculate a reasonable time, if any, for return on investment.

.[/QUOTE]

?

thanks for your XFMR help. I find it to be true. Your ROI figures are a little dated however.

 

[jonj]

The incentives depend on your location. In Maryland the State and Federal Gov. cover approx. 50%. This still gives you a long payback, So Maryland passed a Law called the renewable portfolio standard (RPS) . Basically it's a requirement Maryland placed on electric utilities to purchase Solar Renewable Energy Credits from solar generating facilities(commercial or residential). A (SREC) is issued when a Solar PV system produces 1mwh/1000kwh. These credits are currently selling for $300 dollars +. A 10kw system should produce 14 SREC'S on an annual basis 14 x 300= $4,200. As you can see this can bring the return on investment down to as little as 5 years.
The problem with this system is that this cost to the electric utility is going to increase everyone's electric bill. At the end of the day we are all going to pay for these facilities, while the system owner enjoys a 5 to 7 year return on investment. Not exactly fair is it?