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I have a question regarding smart energy systems. Consider a reverse power flow (where the power flows from household to grid) situation, A household has a solar installation and the energy obtained is given to the grid. So the power from solar has one phase and the grid has the other. How the phase frequency of the AC power from household gets matched with the grid's phase frequency! Do they use any relays or something

Thanks in advance.

Bhargav Ravuri
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    It's the same as when bringing a generator on-line - they have to spin up the generator to the precise frequency then connect it. It has nothing to do with solar cells. – Andy aka Jul 26 '16 at 09:53

2 Answers2

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Power from the solar cells must be connected as input to an inverter. The output of the inverter must be electronically synchronized with the grid with respect to frequency, phase and voltage.

  • Thanks for the reply. If you can help me in explaining in brief how we synchronize the frequency and phase. – Bhargav Ravuri Jul 26 '16 at 13:05
  • The answer provided by @Neil_UK looks like a reasonable one. A solar panel is a variable current source with a voltage limitation and internal impedance. To get a good understanding, you will need to study a detailed explanation. –  Jul 26 '16 at 13:27
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A grid feed inverter is a controlled current source. It senses the grid voltage, and pushes current into the grid in proportion to the sensed voltage moment to moment. Thus it is automatically always in the correct phase.

It adjusts the constant of proportionality cycle to cycle so the average power it exports is equal to the average power it receives (less losses) from the solar cells.

Neil_UK
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  • Dear @Neil_UK, your reply helped me a lot. but one more question. I have understood that grid tie inverter matches the phase and frequency with that of the grid's frequency and phase. It was easily said in all the papers but i didn't find the exact mechanism how it's gonna work. in particular after the comparator compares take a sample of grid voltage and sends it to the "grid side controller" where this match of phase & frequency happens using H-bridge. If possible can you explain me the exact mechanism or share a paper that explains it perfectly. – Bhargav Ravuri Jul 28 '16 at 10:04
  • @BhargavRavuri I've already explained it exactly in my reply. If the inverter was a spinning alternator, then there would be tricky matching to do. But it's not. Let's say the inverter wanted to send about 1kW into a 240v (rms, 340v peak) grid this cycle. It would then program the current controller to source a current of about 3A/340V = 0.0088S (1/ohms) of the line voltage measured at any instant. This gives a 100% power factor, and **automatically** matches the phase. At the zero crossing, it decides a new power level to transmit next cycle, and adjusts the output conductance accordingly. – Neil_UK Jul 28 '16 at 11:23