Grid Tied Question

Re: Grid Tied Question

Depending on what type of inverter, the answer to your question has two different answers.

A grid-tie only system (one without batteries), requires grid to be present to operate. The grid tie inverter syncronizes to grid and if PV array is producing power, the inverter will push current into the grid based on amount of power produced by PV array. When grid goes down the inverter shuts down.

PV arrays are a number of solar cells in series. Each cell is a current source (current based on solar flux density) with a shunting parasitic diode. If no external load is applied to the cell all the current produced is shunted away by the parasitic diode. This provides an open circuit clamping voltage on the cell which is the voltage drop of a diode (times the number of series cells in the panel). This voltage ranges from about 0.45 vdc for a hot cell to over 0.8 vdc for a cold cell. To yield the maximum power from the solar cell, the load is adjusted to just slightly below where the parasitic diode begins to conduct significant current away. This is what a MPPT controller does. If the load to PV panel is lightened then more power is shunted by the cells' parasitic diodes. There is nothing wrong with doing this if there is light load requirements.

In a hybrid, battery equipped inverter, the batteries provide the reservor tank necessary to allow the inverter to keep operating when grid goes down. Again the inverter is syncronized to the grid. The inverter is operating in parallel with the active grid. If the inverter produces a little more voltage then the grid, power is pushed from the batteries into the grid. If the inverter produces a little less voltage then the grid, power is taken from the grid and pushed into the batteries (i.e. charging).

The PV array is connected into a PV charge controller which effectively tries to push power into the batteries. The inverter charge point is set to maintain a float level on the batteries. If the PV controller/charger tries to raise the battery voltage above the float level set on the inverter, the inverter will suck off the excess and push it to the grid (or load).

Hybrid inverters like Xantrex SW and XW series do not has a transfer switch in the true sense of a conventional transfer switch. They do have two primary relays. One connects inverter output to grid. The second connects the inverter output to load. The second is primarily only used to disconnect from excessive output loading. The first relay, connected to grid will open when grid goes down.

Now to answer the final part of your question for a hybrid, battery equipped inverter system when the grid goes down. The PV charge controller will continues to push to batteries, which will, with possible help from batteries, supply power to the inverter load. Remember the relay is open that connects the inverter to the grid because grid is down.

If the load on the inverter is light, the PV array might be able to produce more power then necessary by the inverter load. It will push to batteries to point where PV controller battery charge float voltage level is set and then backs off on the charging. The voltage on the PV array rises and more current is shunted away by the solar cells parasitic diodes. Worse case the PV array rises to the open circuit voltage level of the PV panel, which again is dependent on temperature of panels and number of series connected cells in the panel.

Side point, a PV array differs from a wind generator in the wind generator needs a load to keep it from running away with itself when load is insufficient on it, perhaps to a destructive point. As with the 'old west' water pumping windmills, a govenor tail vain can turn the windmill away from the wind to keep its revolution rate within mechanically safe limits. Another way is to have an alternate load, like a water heater, to take up the excess wind generator power output if the batteries are fully charged and inverter load is light.