Contribution of MV Static Distributed Generation to Voltage Unbalance Mitigation

Focusing on medium size and large size photovoltaic plants typically connected to Medium Voltage distribution networks, their electronic power converters are usually three-phase devices. Depending on the sun irradiation profile, influenced by weather conditions and seasonal variations, these generators usually work at partial loading. As a consequence, the operating point of a PV inverter could be quite far from its current limitation, especially if a rectangular or a circular capability curve is provided. So there are margins for intentionally injecting additional currents according to network requirements without altering the converter rated size and the active power generation. Managing inverters as current controlled devices, with independent regulations on positive, negative and eventually zero sequence current components, makes supplying ancillary services a concrete opportunity, with focus on network balancing. Depending on the measurement point, the regulating action can contribute in balancing the end-user absorption (in case reducing the customer's bill) or in limiting the inverse component of current normally flowing along distribution lines. Balancing end-user absorption or currents flowing along feeders has a direct positive effect on the symmetry of voltages at the end-users' connection nodes and partially on network losses. To this aim, a control scheme has been developed and dynamically tested on a realistic network. Simulations demonstrate the approach effectiveness on network voltage symmetrisation in a Medium Voltage distribution system. It is worth noting that the proposed scheme could be implemented also in existing three-phase converters, with minor hardware and software upgrades.