Main Question or Discussion Point
What circuit design is used to ensure that the renewable energy source does not begin to absorb energy from the grid (for instance, when its terminal voltage drops below the distribution line voltage)?
Why exactly is this done? Is it just more efficient to always be feeding into a lower voltage?...
It is more common in large systems to "over panel" the PV array to ensure that the DC voltage is always above the grid voltage if there is any PV power available, and not use a boost Common to see 1000V DC PV array for 480 VAC, and 1500V DC is the hot topic today for 480 to 600VAC - usually tied directly to a Transformer to the grid.
I understand the MPPT issue - but I don't understand why you say the panel voltage must be higher than the AC peak. There are inverter designs that will step the voltage up. Is a step-up design is inherently less efficient? That's what I'm asking. A step up inverter could perform the MPPT function as well.There are a number of reasons - it starts with the need the to be able to harvest power over a wide DC supply range, the PV array voltage varies dramatically depending on solar intensity, temperature and loading. -- Looking at a MPPT tracking plot or PV Curve and you can see the first challenge.
For the inverter to work at all it needs DC voltage to be higher than the Peak Voltage of the AC tie. ( 480 V RMS ~ 670VDC) -- so for a 480V Grid, you need Vdc at 670V or higher. ...
Agree.Lastly - running at higher voltages - reduces the cabling cost ( copper) for the installation.
But a step up inverter would not allow energy to be fed back to the panel either.An interesting point that Anorlunda touches on - is that these system have the ability to do PF correction with their excess inverter capacity. This is referred to as VAR support - An optional function in this Solectria 500KW Inverter. ( LINK ) This is only possible because no energy is fed back to the PV array, due to contactors or blocking diodes.
Thanks - that's what I was questioning. If a buck converter (a step-down converter) is more efficient in practice (I didn't know if this was true or not), then yes, it makes good sense to put the solar panels in series so that their voltage is typically > peak of the AC signal they are driving.The simplest and most efficient converters are high voltage buck converters. By running cooler they can also be more reliable. A 1% improvement in efficiency increases the return on investment by more than 1% after costs. If that return can be gained by simply wiring PV cells in series rather than parallel, it would be silly not to do it that way.