The origin of voltage dips monitored in MV network and its effect on the evaluation of MV voltage dips performances indices

The subject addressed is the assessment of the origin (High Voltage HV / Medium Voltage MV / Low Voltage LV) of voltage dips monitored on networks and its effect on both voltage dip statistics and indices evaluation at both national and macro area level from the point of view of regulation. The method applied to assess the HV origin of the events refers to both the signal coming from the distance protection of HV lines and to the correlation of the occurrence time of those events which have been monitored on MV bus bars belonging to different HV/MV Primary Substations but underlying a common part of the HV grid. The MV bus bars monitored in this research are those of the Italian MV network monitoring system QuEEN. This method has been partially extended also to the events propagation from the MV to the LV networks thanks to the late installation of a small number of Measurements Units (MU) at specific Secondary Substations in order to realize a sort of “disturbed network tree”. The estimation of the percentage of MV events with a probable HV origin at macro area level, for the 2010÷2011 period, has shown a range of variation of about 6%÷15% in comparison with the 37% evaluated at national level. As to the voltage dips performances, indices N_2a and N_3b, their evaluation at national level for 2011 is affected respectively by the 18,6% and 14,7% by voltage dips with a HV origin. As to the events propagation from the MV to the LV networks a good correspondence between the events monitored at the two voltage levels has been observed.