Non-invasive directional earth

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
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.
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.
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
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
• DEFPI-Standalone: for sub-stations without
existing RTUs, LV measurements or
• DEFPI-OHL: overhead line variant for use at cable
pole terminations.
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 or +44 (0) 1235 753000
*DEFPI was developed in association with ASH Wireless Electronics Ltd.