RESIDENTIAL GRID TIE SOLAR POWERED SYSTEMS

THE BASICS

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

At night or during inclement weather all the electricity is supplied by the utility. 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 utility lines. If the PV panels are producing more electricity than you are using, the system will feed the surplus of the power back to the utility. It may spin your electric meter backwards, further reducing your monthly bill.
Such configuration is the most common, simplest and less expensive then other types of residential PV systems. Currently, an average cost of an installed batteryless grid-tied systems for homes is about $8 per watt. The main disadvantage of such systems is they do not provide any back-up power when utility fails even if sun is shining and the PV array is producing electricity.

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 for each string.
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 must be grounded. In theory, either bus can be ground- you ned to check the manual. Most inverters come configured for negative ground, although some panels' manufacturers recommend positive ground for higher efficiency. The combiner's frame or the PV arrays grounding conductor should also grounded. A DC disconnect switch is required at the place where the cables from the solar array enter the house. 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.

The main DC disconnect switch is followed by a DC ground fault interrupter - a device that opens the circuit when it detects a specified leakage current to ground from an ungrounded DC bus.

A DC voltage from the PV array is then converted to AC voltage by a grid tie DC-AC inverter. The inverter operates as a pulse-width modulated switch mode power supply (SMPS) with AC output. Its AC 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 grid power lines. A grid tie inverter will operate only when utility power is available. It automatically synchronizes its voltage to the grid. If the grid is down, the inverter will immediately shut down.

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 different load points 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. The also usually include 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 the wiring of most US homes. Many models provide only 2-wire 120VAC output. With such models for 120/240 VAC connection you would need to stack two inverters with paralleled inputs and series-connected outputs. The junction between two inverters would become neutral. Also note that some models provide isolation in a high-frequency converter stage and do not have a low-frequency output transformer.

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