Author Topic: Solar Generation with Transformer  (Read 3700 times)

zoo97

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Solar Generation with Transformer
« on: November 09, 2012, 09:29:53 PM »
We try to set up an monitoring system for solar generation and we need to use a transformer to convert 240 AC to 208 AC. I've drawn a crude single line drawing to show the scenario. Only when the CT's are placed above the transformer do I see both positive and negative readings for consumption and generation. I think the mtu is unable to determine current direction below the transformer for some reason.  The TED is running the latest firmware and the MTU is set to Standalone (Net). 

pfletch101

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Re: Solar Generation with Transformer
« Reply #1 on: November 10, 2012, 07:19:34 PM »
Where is your utility mains feed? The way you have drawn your setup, I just see (boiling everything down to essentials) solar panels feeding a load through an inverter and a transformer. In that setup, you should only see unidirectional current flow (wherever your sensors are). You certainly will only see unidirectional flow (ignoring the very low backflow that you should see when the panels are in the dark) if you put your sensors between the inverter and the first distribution panel.
Peter R. Fletcher
TED Pro Home - main MTUs monitoring utility and PV Solar feeds; 2 Spyders monitoring selected individual circuits

RussellH

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Re: Solar Generation with Transformer
« Reply #2 on: November 10, 2012, 10:26:27 PM »
Where is the load?  Where you have drawn it on the 240V side, the CTs will only see current from the PV.  They don't see a load at all.

I'm not sure why you'd be seeing any load with the CTs on the 208V side with the system as drawn.  But chances are the power readings won't be accurate becaue it will be trying to calculate power using current from the 208 side, but a 240V voltage.

zoo97

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Re: Solar Generation with Transformer
« Reply #3 on: November 10, 2012, 11:22:18 PM »
Where is your utility mains feed? The way you have drawn your setup, I just see (boiling everything down to essentials) solar panels feeding a load through an inverter and a transformer. In that setup, you should only see unidirectional current flow (wherever your sensors are). You certainly will only see unidirectional flow (ignoring the very low backflow that you should see when the panels are in the dark) if you put your sensors between the inverter and the first distribution panel.

Thank you, Peter. The utility feed is connected to the load centre, on the top right hand corner of the drawing. You are right about the low backflow.  But it is one that causes us the grief.  Currently the backflow (night time) and the solar generation (day time) all show as generation in the graph. Though it is NOT a big deal (the inverter only consumes 9 W during the nights), it sends the wrong message as our display is for educational purpose (elementary school). Hope this clarifies the issue.

zoo97

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Re: Solar Generation with Transformer
« Reply #4 on: November 10, 2012, 11:41:59 PM »
Where is the load?  Where you have drawn it on the 240V side, the CTs will only see current from the PV.  They don't see a load at all.

I'm not sure why you'd be seeing any load with the CTs on the 208V side with the system as drawn.  But chances are the power readings won't be accurate becaue it will be trying to calculate power using current from the 208 side, but a 240V voltage.

Thanks Russell. The system is a grid tied system and the load is connected to electrical panel, which is shown on the top right corner of the diagram.  To the CT, it should measure the current (along with its direction) and the voltage, then calculates the power based on the both parameters.  During daytime, the current should flow towards the transformer, as the solar panels generate electricity. During night time, the current should flow in opposite direction, as no generation and the inverter consumes small amount of electricity to keep itself alive. So the values day time and night time should be opposite, but they are NOT. Here is a screen capture to show what I meant. Hope it helps to clarify the issue. Please note that the generation in the capture is shown as positive.

RussellH

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Re: Solar Generation with Transformer
« Reply #5 on: November 11, 2012, 10:00:28 PM »
What do you have that MTU set to in the gateway?  Some settings use the absolute value of the sensed power.

zoo97

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Re: Solar Generation with Transformer
« Reply #6 on: November 12, 2012, 03:47:37 AM »
What do you have that MTU set to in the gateway?  Some settings use the absolute value of the sensed power.

It is set to Standalone (Net).

RussellH

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Re: Solar Generation with Transformer
« Reply #7 on: November 12, 2012, 09:00:03 PM »
Try Adjusted Load.


zoo97

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Re: Solar Generation with Transformer
« Reply #8 on: November 13, 2012, 05:36:49 AM »
We tried all setting, including Adjusted Load.