Non-invasive directional earth-fault passage indicator Ricardo Energy & Environment’s* non-invasive directional earth-fault passage indicator (DEFPI) indicates the direction of an earth-fault current in meshed and radial 11kV networks. It does this by taking measurements at existing ring main units (RMUs) and switchgear, without the need for voltage transformers or outages. It can be used with existing remote terminal units (RTU) and their connections or as a standalone unit with independent low voltage (LV) measurements and communications. DEFPI can be deployed rapidly, without high voltage (HV) circuit outages, to locate faulty HV cable and line sections, and to improve the remote or automated restoration of HV faults. This reduces customer interruptions, fault duration and the potential for meshed system cascades. Connecting the DEFPI module is straightforward. It uses outputs from existing HV core balance current transformers (CTs) used for conventional earth-fault passage indicators (EFPIs) or external earth bond CTs, plus reference voltage and current inputs from the LV side of the power transformer. Background EFPIs have been used by distribution network operators (DNOs) as standard on radial circuits for many years. When it comes to meshed networks, the standard EFPI has severe limitations, which are demonstrated in the diagram shown overleaf. ee.ricardo.com EFPIs operate at all sites on the ring as fault current flows both ways from the primary substation to the fault. This means that the EFPIs only confirm that a fault is on the ring, but not where it is. Even after the section F1 to C has been isolated, the location of the fault is still not known. Therefore, no further isolation can be done remotely or automatically, even though most HV switches can be operated remotely. In contrast, the DEFPI shows the direction of the fault, as indicated by the red arrows, and locates the position of the fault to between secondary substations ‘g’ and ‘h’. Remote control of switches at ‘g’ and ‘h’ isolates fault F1, and C is closed remotely. This leaves only section ‘g’ to ‘h’ isolated. All of this is achieved well within 3 minutes, so no customer interruptions or customer minutes lost penalties are incurred. © Ricardo-AEA Ltd 2015. PPA/35/Nov15/V4 Earth fault occurs Fault current flows from feeder circuit breakers F1 and F2 DEFPI methodology Non-invasive measurements are taken of residual HV currents and LV transformer voltages and currents. HV phase voltages are calculated using a sequence component star-delta transform model. The currents and phase voltages are time stamped so their relative phase angles and complex vector quantities can be calculated for fault direction. Other DEFPI opportunities Opportunities for variants of DEFPI include: • DEFPI-GPS: fault location using multiple DEFPI units installed on the same network and timing measurements. • DEFPI-Standalone: for sub-stations without existing RTUs, LV measurements or communications. • DEFPI-OHL: overhead line variant for use at cable pole terminations. Directional protection operates circuit breaker C F1 circuit breaker trips, isolating F1 to C For more information about non-invasive directional earth-fault passage indicator, please contact one of our experts at [email protected] or +44 (0) 1235 753000 ee.ricardo.com *DEFPI was developed in association with ASH Wireless Electronics Ltd.