The DER’s Watts can be used for voltage support in two ways: a) Watt curtailment to increase the var capability to be used for voltage increase (the “Volt / var with var priority” function) and b) additional Watt injection to reduce the upstream voltage drop (e.g., from Energy Storage). The first approach implies that the additional vars released by reducing the Watt injection are more efficient for the voltage support than the reduced Watts. This approach was discussed in [1]. The second approach implies that the additional Watts are more effective for voltage support than the lost vars. This approach is addressed below.
If the inverter of DER is capable of generating vars and the voltage needs to be increased, the maximum use of available vars is preferred in most cases, unless the additional injections of Watts also serve other objectives. If the maximum use of available vars is insufficient to raise the voltage to the desired level, additional injections of Watts may help under some conditions. These conditions include the resistance to reactance ratio (R/X ratio), the initial Watt generation by the DER, the size of Watt addition, and the voltage levels from which and to which the voltage should be raised.
The available vars depend on the Watts generated by the DER and on voltage at the inverter terminals (nominal var capability). It may be further restricted by other operational limits (operational capability) [2]. It means that by increasing the Watt injections to raise the voltage, the var capability is reduced, which reduces the voltage. Which effect prevails defines the result.
Practically, the additions of Watt injections by DERs to increase the voltage may be plausible when the R/X ratio is about one or below, which characterizes the underground circuits.
In addition to the conditions mentioned above, the value of Watts for other objectives, and other available means of volt/var control should be taken into account when deciding whether to use the Watt injection for voltage support.
More information on this topic is available in [3].
References.
Nokhum Markushevich, Vars versus Watts from Distributed Energy Resources. Available: https://www.scribd.com/document/376740872/Vars-versus-Watts-from-Distributed-Energy-Resources
Nokhum Markushevich, Operations of Smart Inverters in Active Distribution Networks. Available: https://www.scribd.com/document/376627135/Operations-of-Smart-Inverters-in-Active-Distribution-Networks
Nokhum Markushevich, When to Use DER’s Watts for Voltage Support? Available: https://www.scribd.com/document/376754060/When-to-Use-DER-s-Watts-for-Voltage-Support-Watts-for-Volts-scribd