*2.2. PV System Capabilities for Voltage Optimization*

This paper deals with PV prosumers with reactive power capability, i.e., other prosumer types are not considered. DSOs have the main responsibility for voltage optimization in DNs. Traditionally, the available variables are limited to the capacitor bank placement, tap changing transformer, network reconfiguration, cross-section enhancement, etc. However, some researchers [30,31] suggest that the aforementioned strategies may not be effective for prosumer-based DNs due to their slow response. The application of PV inverters represents a promising solution and in combination with already present control mechanisms can give results, so several studies have proposed their use for voltage optimization [32–36]. Different modes of operation are possible for PV inverters and the authors of [30] distinguish the following:


In the fixed power factor mode, the power factor is maintained at a constant value and thus voltages are directly affected. Voltage control in the volt–watt control mode is achieved by active power from PVs. In the power rate limit mode, the rate of active power output from the PV inverter is limited. In volt–VAR control, reactive power from the PV inverter is used for voltage optimization. The general operating principle of volt–VAR control is described using a volt–VAR curve shown in Figure 2 [37]. The volt–VAR curve represents a relation between a voltage value at the point of common coupling (PCC) and reactive power from the PV inverter. If the voltage value on PCC is lower than the specific threshold, the PV inverter injects reactive power. On the other hand, in the case of the higher voltage value on PCC, reactive power is absorbed.

Reactive power capability determines the amount of reactive power available from the inverter [38]. Figure 3 represents reactive power capability determined with vectors of apparent power *S* and active power *P* [8,38,39]. The reactive power of the PV inverter depends on the active power and can be determined as:

$$Q\_1^2 \le \sqrt{S\_1^2 - P\_1^2} \tag{1}$$

**Figure 3.** Power capability curve of PV inverter.
