Benefits of multi-voltage-level grid control in future distribution grids

Operators of distribution grids will be confronted with new challenges caused by a high share of distributed fluctuating generators (PV, wind) together with new load profiles of a fleet of electric vehicles (EV), decentralized storage systems or electricity-heat-converters (like heat-pumps or direct heaters). Conventional grid reinforcement is an expensive and inflexible way to handle upcoming grid problems like the violation of voltage limits or the thermal overloading of operating equipment. By controlling flexible components (like on-load tap changers or reactive power generation from inverters) grid operators can counteract violations of operational limits. A grid simulation study has been performed to compare benefits of intelligent multi-voltage-level grid control with single-voltage-level control solutions and conventional grid control methods. The starting point for this work was a number of scenarios for the future grid penetration by PV, wind and EV. Representative type grids consisting of interconnected grid segments for rural and municipal distribution grids were modelled within a probabilistic load flow calculation framework. To quantify the probability of voltage violations or thermal overload situations, a probabilistic assignment of grid components (PV, wind, EV) to grid nodes was implemented. Four grid control approaches have been studied by use of the simulation, representing different distributed and centralized control methods. The results of the investigation show the benefits of multi-voltage-level grid control for distribution grids with a high share of distributed wind, PV and EV. While conventional grid operation solutions might be able to counteract local voltage problems, a combined control algorithm for the LV and MV grid segments is able to prevent the violation of given voltage and current limits in many cases quite effectively. The effectiveness of using transformers with variable on-load tap changers (OLTC) might improve significantly if communication based multi-voltage-level grid control concepts are applied. Overloading of cables resulting from solving local voltage maintenance problems by means of reactive power injection can be avoided. Multi-voltage-level grid control avoids or delays grid extension and increases grid capacity towards the installation of additional PV, wind or EV-charging units.