Problems of Voltage Stabilization in Mv and Lv Distribution Grids Due to the Operation of Renewable Energy Sources

The supply territory of the company E.ON Distribution in the Czech Republic (run by the company E.ON Czech Republic) accounts for approximately 1.5 million customers. More than 900MW of photovoltaic (PV) plants were connected to this distribution grid. It was detected the problem with overvoltage in approximately 20% low voltage distribution grids with connected PV plants and this problem has to be solved. The problem with high voltage variance is caused partially due to the operation of PV plants connected into the MV distribution grid and partially due to the operation of PV plants connected into the LV distribution grid. It is supposable, that the problem with overvoltage will occur when PV plants are connected both into the MV and into the LV distribution grid in the same part of the grid. In order to cope with the increased level of distributed generation new solutions for grid operation and new technologies are needed. In the paper are described three solutions for voltage stabilization. These proposed solutions prevent nonconforming voltage quality (overvoltage) according to the EN 50160 standard and following investments. INTRODUCTION Figure 1 shows the growing number of connected dispersed power sources. The number of connected power sources and installed capacity of these power sources are approximately ten times higher today as in year 2008. The cause of this number connected renewable energy sources (RES) is a bountiful policy of the state. The most of connected RES are PV plants but connecting of biogas plants is very popular in the last time. The experience of distribution network operator (DNO) shows the problem with the operation of these renewable energy sources (RES). In case of accumulation of sources in one part of distribution system, it is possible that overvoltage will occur respective voltage variations will not comply with the requirements of the standard EN 50 160 [1]. All 10 min r.m.s. values of the supply voltage shall be within the range of nominal voltage Un + 10%/-15%. Also when one 10 min mean r.m.s. value of the supply voltage exceeds during the week the limit Un + 10% (110% Un), it results in overvoltage and voltage quality respective voltage variations will not comply with the requirements of the standard EN 50160. Fig. 1: History of RES connected to the distribution grid E.ON in the Czech Republic OPERATION OF THE DISTRIBUTION GRIDS Grids without connected RES It is operated voltage level 23.1kV (105%Un) on the output of the HV/MV transformer with the tolerance +/-0.3kV (see Fig. 2). When no loaded medium voltage (MV) grid and no connected RES to this grid voltage can reach the value of 106. 36%Un in the MV distribution grid. When no loaded LV grid, no connected RES to this grid and no loaded MV grid at the same time voltage can reach the value of 106.36%Un in the LV distribution grid. This creates a reserve with the relation to the upper limit of voltage variations according to the EN 50 160 standard. When representative loaded MV grid and no connected RES in this grid voltage can reach the value of 99. 6%Un in the MV distribution grid. When no loaded LV grid, no connected RES to this grid and representative loaded MV grid at the same time voltage can exceed the lower limit 90%Un in the overhead LV distribution grid – see Fig. 2. Under normal operating conditions excluding the periods with interruptions, supply voltage variations should not exceed ± 10% of the nominal voltage Un when there is usual the voltage decrease 10%Un caused by load in the overhead LV distribution grid. This problem with low voltage in overhead LV distribution grids is continuously, long-term and systematic solved. C I R E D 22nd International Conference on Electricity Distribution Stockholm, 10-13 June 2013