Grid tie (also known as grid-intertied, utility-interactive, or on grid) systems are intended to operate in parallel with an electric utility.
Here is its basic working principle. At night or during inclement weather all the electricity is supplied by the mains. During the daylight hours, the system generates some power, offsetting the consumption of electricity from the utility and cutting electric bills. The balance of the kWh required by your loads is automatically drawn from the input lines. If the PV panels are producing more electricity than you are using, the system will feed the surplus of the energy back to the utility. It may even spin your electric meter backward, further reducing your monthly bill.
Such configuration is the most common, simplest and less expensive than other types of grid-interactive residential PV setups. Currently, an average net cost of an installed batteryless on-grid PV generators for homes is under $5 per watt. For homeowners a large portion of this cost may be offset by various credits and rebates. There are also solar lease programs that let you get a PV system for free. The main disadvantage of on grid systems is that normally they do not provide any back-up power during blackouts even if sun is shining and the PV array is producing enough energy. For a backup you need to use a battery-based systems with special inverters.
PRINCIPLES OF OPERATION AND WIRING DIAGRAM
Here is a simplified schematic of a basic grid tie solar power system that illustrates its operation.
Several strings of solar panels are paralleled in the combiner that includes fuses or circuit breakers. It is desirable although not required to have a separate over-current interrupter for each string.
One of the two DC busses from the PV array are usually grounded. In theory, either buss can be grounded- you need to check your part's manual. Most inverters come configured for negative ground, although some panels manufacturers recommend positive ground for higher efficiency. The combiner frame or the PV arrays grounding conductor should also bonded to an earth rod. A manual DC disconnect switch is required at the place where the cables from the solar array enter the house. In U.S. the DC systems in use today can be up to 600V. Since it is hard to find UL listed single-pole breakers rated for 600VDC, installers often use a breaker with multiple poles connected in series. To reduce the possibility of a fire and to protect the system from a damage caused by lightings it is desirable to have a voltage-clamping device, such as a metal oxide varistor (MOV) across the DC buss.
Besides a manual disconnect switch there should be a DC ground fault interrupter (GFI) - a device that opens the circuit when it detects a specified leakage current to earth. Note that DC grounding should be done only in one place. If you ground a cable both before and after GFI, the GFI will go off.
The above solar power diagram shows a grounded wiring. NEC® also permits ungrounded configurations in combination with transformerless inverters. Such setups should have disconnects and fuses in both positive and negative DC lines. They should also display a proper warning.
A voltage from the PV array is converted to AC by a special utility interactive DC-AC inverter. Such a device operates as a pulse-width modulated switch mode power supply SMPS with AC output (read more). The SMPS inverter is connected directly to the main service panel. Although it includes internal disconnect switch, the utility companies usually require an additional manual AC disconnect. A grid tie inverter should meet certain power quality and synchronization requirements, and provide anti-islanding protection. Note that it will be powering your home only when utility is available. If the grid is down, the PV system has to immediately seize exporting power. The control circuit automatically synchronizes inverter's output to the mains. In order to allow the current flow back into the power lines, its output voltage has to be just slightly higher than the utility voltage. For more details see a grid tie inverter schematic.
To extract the maximum power out of the PV panels, they have to operate near peak power point of their volt-amp curve. This requires variable loading depending on the illumination and ambient temperature. Grid tie inverters for solar applications normally use maximum power tracking algorithm that helps to extract maximum power from the panels (see: I-V characteristics of solar panels). Since sunlight intensity varies during the day, it may not be simple to size your system and find the required amount of the panels.
By the way, in practice, the solar inverters often have several built-in fused pairs of DC inputs that make an external combiner unnecessary. They also usually include both internal DC interrupt switch and ground fault interrupter.
This wiring diagram provides an example of a 3-wire "split-phase" 120/240 VAC configuration typical for the wiring of most new US homes. Many inverters however provide only 2-wire 120VAC output. With such models, for 120/240 VAC you would need to buy two inverters that allow mutual synchronization and so-called "master-slave" mode. You will need to stack them with paralleled inputs and series-connected outputs. The junction between these two inverters will become the neutral. Also note that some models do not have a bulky 60Hz output transformer. They either provide isolation in a high-frequency converter stage or don't have a transformer at all.