Assessment of low frequency demand disconnection impact on network operability

This paper presents an assessment of Low Frequency Demand Disconnection (LFDD) impact on network operability. One of the issues arising from LFDD relay operation is the disconnection of Distributed Generators (DGs) along with loads during disconnection of large numbers of Extra High Voltage (EHV) substations. This could compromise the effectiveness of LFDD schemes considering the increasing presence of DGs in distribution networks. Another issue is the loss of system earth (i.e. from 132/33 kV substations) from the point of view of connected DGs following LFDD operation that might occur in some areas within distribution grids due to back-feed current from 33/11 kV primary substations. A distribution network model experiencing the above issues has been used in the PowerFactory to simulate a number of operation and fault scenarios. Different types of DGs including Photovoltaic (PV), Battery Energy Storage System (BESS), Wind Generation (WG) and Synchronous Generation (SG) have been modelled. The results provide useful insights on the LFDD schemes impact on network operation with DGs interface protection based on ENA Engineering Recommendation G99 and how the LFDD schemes based on direction of current could influence the frequency response in the network. The results also show that the loss of system earth issue on the studied LFDD scenarios occurs when the network is operating in its minimum loading condition.