Benefits of On-Load Tap Changers Coordinated Operation for Voltage Control in Low Voltage Grids with High Photovoltaic Penetration

The transformers that connect the Medium Voltage (MV) grid with the Low Voltage (LV) system are classically equipped with tap changers with tap positions variable only at no load. The evolution of the technologies and the increasing needs of controlling the voltage profile in the LV networks require better control capabilities. The use of MV/LV transformers with On Load Tap Changers (OLTCs) is increasing, to provide further control capabilities in LV grids with high penetration of distributed generation. In this paper, centralised voltage control is evaluated by simulating the operation of an OLTC installed inside the MV/LV transformer substation. The goal is to stabilise the voltage at the LV bus of the transformer. It is supposed that the OLTC does not communicate with other devices in the grid; thus, it does not know the voltage levels at the other nodes. At the same time, the distributed PV inverters control the voltage in their grid connection points without any information about the other nodes. The expected benefits of exploiting OLTCs in LV grids with high photovoltaic (PV) penetration are determined through indicators that assess the voltage deviations with energy flows, the global overvoltage or undervoltage persistence, and the overvoltage or undervoltage duration. The results show that the use of an OLTC can help the mitigation of voltage fluctuations, especially limiting the undervoltages. The effectiveness strongly depends on the control parameters, especially the maximum number of daily taps.