Advanced Earth Fault Mitigation Using Virtual Air Gap Reactors

The shift towards electrification of energy sectors and global climate change has increased the demand for reliable and safe electricity networks. This has posed a significant challenge for distribution system operators (DSOs), especially when dealing with single-line-to-earth (SLTE) faults, the most common type of faults in distribution networks. Traditional arc suppression coils (ASCs) used to limit SLTE fault currents are often limited by operational constraints. A novel type of ASC, the virtual air gap (VAG) reactor, may help to overcome this issue. These reactors use a set of auxiliary coils controlled by power electronics that can change their inductance within milliseconds. This quick inductance-changing capability could make VAG reactors a valuable asset when used as ASCs in compensated networks. To investigate the potential of VAG reactors, a dynamic time-domain model of VAG reactors was developed and validated by comparing it with a low-voltage (LV) VAG reactor. Based on the dynamic model, two distinct control approaches for VAG reactors were developed with the objective of using the reactor as an ASC in a compensated medium voltage (MV) distribution network. One controller has active harmonic mitigation, while the other requires less power but cannot control harmonics. The combination of the dynamic VAG reactor model along with the two distinct controllers was then compared to a traditional ASC in a series of time-domain MATLAB/Simulink simulations of a compensated MV distribution network. Furthermore, a series of laboratory tests were conducted to investigate the duration of an arc before its self-extinction, depending on the magnitude of the arcing current, the rate of rise of transient recovery voltage (RRTRV), and the length of the initial spark gap. The results of the simulations showed that the residual fault current can be effectively reduced during a SLTE fault by changing the inductance of the reactor to its optimal operating point. Both control approaches demonstrated the typical features of traditional ASCs, including the slowly increasing recovery voltage after fault extinction. The laboratory arc test showed that the duration of an arc before self-extinction depends more on the RRTRV than on the magnitude of the arcing current for currents at 10 A or below. Furthermore, it was found that smaller spark gaps result in longer burning arcs due to their more stable characteristics. The combination of the results of the laboratory arc test with the simulation results of the VAG reactors in the network simulation led to the conclusion that VAG reactors can be a viable alternative to traditional ASCs and help enhance the reliability of electricity networks.