Fundamentals of open-loop and closed-loop control

The setpoints can either be implemented via open-loop control or closed-loop control. The "Open-loop control" operating mode is used when a setpoint can be met without feedback from the actual value. The "Closed-loop control" operating mode is used when a setpoint is to be met with feedback from the actual value.

Application examples:

Active power setpoints for the point of interconnection always refer to the measurement at the point of interconnection. Therefore, for pure feed-in systems, the operating mode "Open-loop control" (without feedback of the actual value) is sufficient. For systems with self-consumption, the operating mode "Closed-loop control" (with feedback of the actual value) should be used so that the excess PV energy can be converted into self-consumption and specifications for the point of interconnection are still met.

Regarding reactive power setpoints, the operating mode "Closed-loop control" (with feedback of the measured values from the point of interconnection) should be selected. This way, the influences of the line and, if applicable, of the transformer can be accounted for and controlled. This applies to both PV systems with and without self-consumption.

In systems with high self-consumption and a high reactive power demand (e.g. operation of electrical machines in commercial and industrial plants without an additional compensation system), the PV inverters might not be able to control the setpoints for reactive power or cos phi at the point of interconnection. In such systems, the PV inverters will generate almost only reactive power and hardly any active power. In such cases, the "Open-loop control" operating mode is recommended. Contact your local grid operator.