If you are designing a grid-tie system with a single inverter, you will typically be connecting all your solar panels in series and then feeding this high-voltage DC power into an inverter.
A simplified block diagram of a typical grid-tie system using a single inverter. The residual current device (RCD) provides ground fault protection. In the United States, an RCD is known as a ground fault
interrupter (GFI)
Because of the very high DC voltages involved, additional safeguards are necessary.
The solar array must always be grounded, there must be a DC circuit breaker (also known as an isolation switch) installed between the solar array and the inverter and there must be a DC residual current device/ground fault interrupter installed to shut down the solar array in the case of a short circuit.
In the diagram below, there are sixteen 250Wp solar panels connected in series. This is a fairly typical installation for a residential property, creating a 4kWp system. Assuming each solar panel produces a 24-volt output, this system will run at a nominal 384 volts, with a peak power in the region of 640 volts and an open circuit voltage of 832 volts.
Above: A sample block diagram for a grid-tie system
This system would be legal in Europe, but breaks building regulations in the United States which disallows any system that has the potential to run at over 600V.
Consequently, this system would more typically be installed with the solar array split into two smaller strings, and running with a grid-tie inverter capable of accepting two separate solar inputs.
In Europe, voltages of up to 1,000V are allowed. However, the dangers of such high voltages means that even in Europe, it is common practice to split solar arrays to reduce voltages down to under 500-600V where possible. In general, this means that you will
not want to connect more than twenty 12-volt solar panels or ten 24-volt solar panels in series, in order to ensure that you stay well below this level.
If you are running close to the 600V limit in the United States or 1,000V limit in Europe, there are three options:
Install a multi-string system, either using an inverter that handles more than one solar feed, or by using two separate inverters
Install a micro-inverter system
Wire your solar panels in a parallel/series hybrid
For more information about open circuit voltages, refer back to page 91. To recap on multi-string systems, refer back to the section on Multiple strings on page 94.
Batteries
Batteries are wired in a similar way to your solar array. You can wire up multiple 12-volt batteries in parallel to build a 12-12-volt system with higher energy capacity, or you can wire multiple batteries in series to build a higher-voltage system.
When wiring batteries together in parallel, it is important to wire them up so that you take the positive connection off the first battery in the bank and the negative connection off the last battery in the bank.
How to wire batteries in parallel: the diagram on the left, where power feed for both positive and negative is taken off the first battery in the bank, shows how
not to do it – it will lead to poor battery performance and premature battery failure. The diagram on the right, where the positive feed is taken off the first
battery in the bank and the negative feed is taken off the last battery in the bank, is correct and will lead to a more balanced system with a significantly longer
life.
This ensures equal energy drain and charging across the entire battery bank. If you use the same battery in the bank for negative and positive connections to the controller and inverter, you drain this first battery faster than the rest of the batteries in the bank. The first battery also gets the biggest recharge from the solar array.
This shortens the life of the battery and means all the batteries in the bank end up out of balance. Other batteries in the bank never get fully charged by the solar array, as the first battery will report being fully charged first and the controller will then switch the power off rather than continuing to charge the rest of the batteries in the bank. The result is that the batteries end up with a shorter lifespan.
Controller
A controller will have connections to the solar array, to the battery bank and to DC loads. Although controllers tend not to have the same heat problems as inverters, they can get warm in use. Make sure they are installed in an area with good ventilation around them and in a location where they can be easily checked.
Inverter
Where an inverter is used in a stand-alone or grid fallback system, it is connected directly to the battery bank and not through the controller.
Make sure that you design your system so that the inverter is in a well-ventilated area.
Take into account the weight of the inverter and ensure that it is installed in a location where it can easily be checked.
Devices
Devices are connected to the inverter if they require grid-level voltage, or to the controller if they are low-voltage DC devices. They are never connected directly to the solar array or the batteries.