RESIDENTIAL GRID TIE SOLAR POWERED SYSTEMS

THE BASICS

Grid tie (also known as grid-intertied or utility-interactive) systems are intended to operate in parallel with an electric utility.

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 power 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 backwards, 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 cost of an installed batteryless on-grid PV generators for homes is about $7 per watt. For homeowners a large portion of this cost may be offset by various credits and rebates. The main disadvantage of such 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 backup you need to use battery-based systems with special inverters.

PRINCIPLES OF OPERATION AND THE SCHEMATIC

Below is a simplified electrical wiring diagram of a typical grid tie solar power system that illustrates its operation.
The strings of the solar panels are paralleled in the combiner that includes fuses or circuit breakers. It is desirable although not required to have a separate overcurrent interrupter for each group.
Wiring diagram of a grid tie solar system

Wiring diagram of a grid tie solar system

One of the two DC buses from the PV array are usually grounded. In theory, either bus can be ground- 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, you can use a breaker with mulitple 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 bus.

Besides a manual disconnect switch there should be a DC ground fault interrupter - a device that opens the circuit when it detects a specified leakage current to earth from a DC bus. 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 scheme shows a grounded setup. 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. Its output is connected to the main service panel. The utility companies usually require an additional manual AC disconnect between the inverter's AC output and the power lines. 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 service lines, the inverter's output voltage has to be 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, it is best to operate them 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 solar panels (see: I-V characteristics of solar panels).

In practice, the solar inverters often have several 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 shows an example of a 3-wire "split-phase" 120/240 VAC configuration typical for most new US homes. Many models however provide only 2-wire 120VAC output. With such models, for 120/240 VAC connection you would need to buy two inverters and stack them with paralleled inputs and series-connected outputs. The junction between two inverters would become 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.

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