PV inverters provide reactive power to Milan’s grid at night

Researchers at the Polytechnic University of Milan have used photovoltaic inverters to supply 500 kilovolt-ampere reactive (kVAr) of inductive reactive power to Milan’s distribution network.

The trial was intended to demonstrate how existing solar assets could support voltage regulation even when they are not generating electricity.

The demonstration took place on the night of Sept. 8-9 as part of Unareti’s MindFlex pilot project. The inverters connected to the PV systems at the university’s PoliGrid site supplied 500 kVAr of inductive reactive power for four consecutive hours, from 1 a.m. to 5 a.m.

The researchers verified delivery by comparing the reactive power exchanged at the grid connection point with a baseline representing PoliGrid’s expected behavior without activation. They calculated the baseline using average operating data recorded over the preceding days.

“The particularly interesting aspect is the use of photovoltaic inverters during the night,” Filippo Bovera, a researcher at the Politecnico di Milano and PoliGrid representative, told pv magazine. “At night, PV inverters are not required to convert the active power generated by solar modules. Their unused capacity can therefore be deployed to provide reactive power services to the grid.”

In the Milan trial, this enabled PoliGrid to use existing equipment at a low delivery cost, largely limited to inverter losses and wear, while generating additional revenue for the university.

Unareti, the distribution system operator owned by local utility A2A, procured the service through the MindFlex pilot market. The mechanism remunerates capacity availability in euros per kVAr per hour during a contracted period. For the trial, the availability window corresponded to Italy’s F3 tariff band, which runs from 11 p.m. to 7 a.m. on weekdays.

The researchers said the demonstration shows how distributed energy resources, often regarded as an additional challenge for grid operators, could also support network management. When properly controlled and coordinated, inverters already connected to distribution networks can provide voltage and reactive power regulation, potentially improving power quality and reducing grid losses and overall system costs.

The ability of PV inverters to regulate reactive power is already technically established, according to the researchers. The principal obstacles to large-scale deployment are enabling the functionality across a wide asset base and ensuring that the resources can be controlled, aggregated and monetized.

Flexibility providers would need reliable communications infrastructure and monitoring and control platforms capable of coordinating large numbers of distributed devices. Distribution system operators would also require clear technical rules and market mechanisms to procure the services where they offer genuine network value. Automated dispatch systems would be needed to activate the resources in response to local grid conditions.

The researchers said that appropriate economic signals to support investment by flexibility providers and grid operators are still lacking. Reactive power is an inherently localized service, as its value depends on where the resource is connected to the network. A large aggregated inverter capacity would therefore not be sufficient on its own. Operators must also be able to activate that capacity at the times and locations where the distribution system operator needs it.

PoliGrid coordinated the PV inverters through LePo, an energy management system (EMS) developed by Maurizio Delfanti’s research group at the Politecnico di Milano’s Department of Energy in collaboration with the Energy and Environment Laboratory of Piacenza (LEAP).

The EMS uses forecasting and optimization tools for multi-energy systems to calculate the commands required to deliver the requested change in reactive power at the grid connection point. The Optimo internet-of-things architecture hosts and runs LePo, manages the resulting setpoints and transmits them to the PV control units and individual inverters.

For the trial, the inverter management systems were configured to remain operational at night, allowing the inverters to regulate reactive power when the PV arrays were not producing active power.

During activation, the system monitored reactive power at the point of delivery and continuously adjusted the inverter setpoints to deliver 500 kVAr relative to the baseline. The process involved more than setting the inverters to a fixed output of 500 kVAr, as the control system also had to account for the reactive power that the campus would have exchanged with the grid without the service.

The EMS also monitored the site’s other energy resources to assess the service’s effects on the wider energy system. According to the researchers, these effects are limited for reactive power regulation but become more significant when active power services are provided.

The participating PV systems use Huawei SUN2000 inverters controlled through Huawei SmartLogger 3000 units. Optimo’s IoT infrastructure, supported by ComAp devices, enables communication between the SmartLogger units and the EMS.

The test formed part of SIMBIOsi, a project funded by the Italian Ministry of Environment and Energy Security (MASE).