Photovoltaic system architectures

General Rules

Where photovoltaic installations are connected to the network and energy is sold, it is necessary to optimise efficiency and reduce installation costs. With this in mind, a relatively high DC operating voltage of between 200 and 500 V is often used for residential applications, with up to 1000 V (600V in North America for non-"behind-the-fence" applications) being used for applications requiring a higher level of power.

All the modules in a PV array should be identical (same brand and same type) and selected to supply the same level of power. For example, the modules should all be 180 W, even though there exists other power levels in the same PV modules product range (170 W, 180 W and 190 W).

In practice, the protection units (DC and AC units) should be positioned close to the inverters for ease of maintenance.

PV array with a single string of modules

This is the simplest configuration (see Fig. P18). It is used for small PV arrays with peak power of up to 3 kWp depending on the modules deployed. In most cases, it is used for residential PV operations.

Fig. P18 – Diagram showing a single-string photovoltaic array

Modules are connected in series, supplying direct current of between 200 and 500 VDC in this instance. Optimal efficiency is obtained from the inverter within this voltage range.

A single DC line is fed through to the inverter. The PV array can be isolated from the inverter by means of a load break switch near the inverter.

PV array with several module strings in parallel

This configuration (see Fig. P19), mainly deployed on buildings or in small PV power plants on the ground, is used for PV installations of up to thirty strings in parallel with power output of some 100 kWp. This limit is imposed for technological and financial reasons. If exceeded, the required width of the main DC cable would be impractical.

Direct current can be determined based on the number of modules in series per string and in this instance is between 300 and 600 VDC. By paralleling identical strings, the power required for the installation can be attained. The strings are paralleled in a PV array box (or DC combiner box). This box includes the safety devices required for paralleling the strings and appliances used to measure the strings’ current. A single DC cable connects these boxes to the inverter. The PV array can be isolated from the inverter by means a load break switch near the inverter.

Fig. P19 – Diagram showing a multi-string photovoltaic array with one inverter

As a variation on this diagram, several single-phase inverters can be installed in a three-phase arrangement (see Fig. P20).

Fig. P20 – Diagram showing a multi-string photovoltaic array with several single-phase inverters connected in a three-phase arrangement

PV array with several strings divided into several groups

When power levels exceed 50 or 100 kW, photovoltaic arrays are split into subgroups (see Fig. P21) to make it easier to connect the various components. Strings are paralleled on two levels.

• Strings in each subgroup are paralleled in subgroup PV array boxes (DC sub-combiner boxes). These boxes are fitted with safety devices, the necessary measuring equipment and monitoring devices.
• The outputs of these boxes are paralleled in a PV array box (DC master-combiner box) near the inverter. This box is also fitted with the required safety devices as well as the measuring and monitoring equipment necessary for paralleling the subgroups.

The array can be isolated from the inverter using a load block switch which may or may not be fitted in the PV array box. The array’s direct current is approximately 1000 VDC.

Fig. P21 – Diagram showing a photovoltaic array consisting of several groups