RESIDENTIAL GRID TIE SOLAR SYSTEMS WITH BATTERY BACKUP


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THE BASICS



PV systems with battery backup are similar to batteryless grid tie systems, except they can additionally provide power for critical loads in case of a utility power failure.

When sun is shining, the solar panels generate power, which reduces the consumption of electricity from the grid and cuts electric bills accordingly. If this power is not enough for your home, the required balance is automatically provided by the utility. Under normal conditions, a small portion of the energy from the grid is used to keep the storage batteries charged. If the PV system generates more electricity than your house is using, the system will feed the excess back to the grid after the batteries are fully charged and may even spin your electric meter backwards.

An obvious advantage of a solar generator backup is it provides a certain protection against short-term blackouts. Its disadvantages are larger up-front investment relative to a batteryless system and lower reliability due to the batteries' limited lifetime. Also, if "wet" batteries are selected, they need to be periodically checked for fluids.
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Sealed batteries do not need any maintenance but may not last as long as the wet types. For a comparison, a typical warranty for solar panels is 20-25 years, inverters- 3-10 years, batteries 1-3 years.

PRINCIPLES OF OPERATION AND WIRING



This simplified solar panel wiring diagram illustrates the system's operation.
Solar panel wiring diagram: grid tie system with battery backup
Note that separate earth grounds have to be provided for each AC and DC circuit. A DC ground fault interrupter disconnects the DC input when a certain leakage current from the ungrounded bus is detected.

The storage batteries should never be connected directly to solar arrays since it may result in frequent overcharging that reduces useful life of the batteries and can cause their damage. Also, with a direct connection, the battery will determines the voltage level at which the solar array will operate. This level most likely will not correspond to the peak power point of the panels (see Understanding characteristics of PV panels). That's why solar charge controllers are normally put in between the PV array leads and the battery bank. If you still chose to not use a charger, you need to connect the batteries via a diode that prevents their discharge at night due to reverse leakage current of the PV cells. Note that since practically the batteries never share loads equally, it is not recommended to run more than four parallel strings of the batteries unless you use special circuits for the current sharing.

In practice, a solar grid-interactive inverter for backup systems usually includes a DC ground fault interrupter, DC-AC inverter itself and the battery charger all in one package. It often also has several pairs of fused DC inputs, which can make an external combiner unnecessary. For a basic principals of operation see grid tie inverter schematic.

The above schematic diagram shows an example of a wiring configuration when the inverter has a low-frequency center-tapped output transformer for 120/240 VAC. More often a low-power residential-grade inverter provides only 120VAC. In this case, you need to stack in series two modules to get 120/240 output. Also note that some models provide isolation in a high-frequency DC-DC converter stage and do not have a bulky low-frequency transformer.

The shown system allows excess energy that is generated by solar panels to be exported to the grid. Under normal conditions, the grid acts as an additional energy source to keep the system's batteries charged. If the grid fails, the inverter will automatically disconnects from the grid and supplies energy from the batteries to the critical loads wired to an auxiliary panel. UL1741 standard requires a grid tie inverter to disconnect from the grid within 0.1 second when input voltage goes off or drops below 60AC. When utility voltage returns, an internal transfer relay will automatically connect the wiring system back to the utility.

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