Combined Planning of Medium and Low Voltage Grids

Typically, low-voltage (LV) and medium-voltage (MV) grids are planned separately. Grid planners define limits that divide the available voltage band into fixed ranges for different voltage levels. MV grids are then planned so that these limits are not violated. LV grids are planned under the worst-case assumption that the voltage at all MV connection points is equal to the voltage limit in the MV grid. This approach is likely to overestimate the demand for reinforcement, possibly leading to unnecessary investments. To analyze this effect, the capital expenditure on grid reinforcement caused by additional renewable generation are calculated for a rural MV grid in Germany and all connected LV grids. Reinforcement can be reduced if it is known which MV feeder an LV grid is connected to. On-Load Tap Changer (OLTC) transformers can lead to additional capex reductions if applied selectively. The combined MV and LV grid planning leads to the most efficient solution in terms of capital expenditure.