no stupid questions

[bballou]

I was always told the only stupid question is the one you don't ask. Well I have asked the wrong people this question because I am getting different answers.
Question: If I have a grid tie PV system, and my service latteral is the only one connected to the load of the utility transformer on the pole outside. Is the power creadted by the PV system really feeding the utility back wards threw the transformer and supplimenting the utility's primary voltage?
Could you actually be producing PV power, recieving utility credit, and the power not even being a useable source of power to the utility?
I know I must be missing something please help!!:slaphead:

 

[GoldDigger]

If you are producing more power than you use locally, then it will be feeding back through the transformer to other customers.
If you understand how a grid tied inverter works, you should be comfortable with that statement.
If the power is not going anywhere it would not show up as power flow on the bi-directional meter.

The situation is not qualitatively different from paralleling two or more generators through distribution transformers.

Tapatalk...

 

[ggunn]

I was always told the only stupid question is the one you don't ask. Well I have asked the wrong people this question because I am getting different answers.
Question: If I have a grid tie PV system, and my service latteral is the only one connected to the load of the utility transformer on the pole outside. Is the power creadted by the PV system really feeding the utility back wards threw the transformer and supplimenting the utility's primary voltage?

Yes, transformers are bidirectional devices. You aren't changing the transformer's voltage, though, when your PV system is producing more power than you are using; you are changing the direction of current flow through the transformer.

 

[bballou]

Yes, transformers are bidirectional devices. You aren't changing the transformer's voltage, though, when your PV system is producing more power than you are using; you are changing the direction of current flow through the transformer.

thank you,
one more thing when producing more power than using would the current flow through the transformer type of set up be less efficient than if there were 3 or 4 homes on the load side of the transformer and your current was being dispersed on the low side?

 

[GoldDigger]

Not significantly. You could just as well ask whether it would be more efficient for the utility to provide everyone with their own transformer. The answer is that there will be a small power loss going back through the transformer, but so small that is does not affect POCO's design decisions.

If there is a LOT of PV generation compared to original design load there can be voltage regulation issues though. Mainly a concern where the infrastructure is already strained.

Tapatalk...

 

[bballou]

Not significantly. You could just as well ask whether it would be more efficient for the utility to provide everyone with their own transformer. The answer is that there will be a small power loss going back through the transformer, but so small that is does not affect POCO's design decisions.

If there is a LOT of PV generation compared to original design load there can be voltage regulation issues though. Mainly a concern where the infrastructure is already strained.

Tapatalk...

I can?t thank you enough for taking the time to help me understand this, I have been carrying this around for a while due to embarrassment of not knowing the answer.
I knew transformers were bidirectional but I just could not get myself to believe you could back feed to the primary.
Thank you much.

 

[GoldDigger]

I can?t thank you enough for taking the time to help me understand this, I have been carrying this around for a while due to embarrassment of not knowing the answer.
I knew transformers were bidirectional but I just could not get myself to believe you could back feed to the primary.
Thank you much.

You are more than welcome. I hope that you will feel better now.

One more point, just in case it comes up when you are thinking about this again later:

The transformer has losses internally, and one of the side effects of this is that when used in the forward direction at full load the secondary voltage will be lower than what you get by simply dividing the primary voltage by the turns ratio.
When you feed the transformer backwards at full load, on the other hand, the primary side voltage will be less than what you get by simply multiplying the applied secondary voltage by the turns ratio.

The observed effect of this loss will be that when you switch from consuming power to selling power the voltage inside your service will rise from a voltage below the no-load voltage to one which is above the no-load voltage.
The circuitry of the Grid Tie Inverter (GTI) automatically raises its output voltage as needed to smoothly make this transition. It still matches the POCO voltage, but that POCO voltage on the secondary will be higher while power is flowing back into POCO, even if the POCO primary voltage is unchanged.

If that was more confusing than informing, please ignore it until you have solidly reached your comfort level on the issue and are asking deeper questions.

PS: Since I see that your are a Fire Marshall, I just wanted to stress that if there is actually no place for the power to go (i.e. a high load impedance as seen by the GTI) the GTI simply shuts down because of the mandatory anti-islanding circuitry in the GTI.

 

[bballou]

Thank you,
I follow that, in addition to being a Fire Marshal I have been a licensed electrician for 20 years, with no solar experience and I have been trying to self educate myself on solar and that was a real wall that I just could not get over until today.

Thanks again