Most of the electrical network faults are earth faults. Conventional protection
systems based on overcurrent, impedance or other principles are suitable for
detecting relatively low-impedance faults which have a relatively large fault current.
However, a small percentage of the earth faults have a very large impedance.
They are comparable to load impedance and consequently have very little fault
current. These high-impedance faults do not pose imminent danger to power
system equipment. However, they are a considerable threat to people and property.
The IEEE Power System Relay Committee working group on High Impedance Fault
Detection Technology defines High Impedance Faults as those that 'do not produce
enough fault current to be detectable by conventional overcurrent relays or fuses.
PHIZ always needs sensitive Io measurement.
High-impedance fault (PHIZ) detection requires a different approach than that
for conventional low-impedance faults. Reliable detection of PHIZ provides safety
to humans and animals. PHIZ detection can also prevent fire and minimize
property damage. ABB has developed innovative technology for high-impedance
fault detection with over ten years of research resulting in many successful field
tests.
5.4.6
Signals
Table 788: PHIZ Input signals
Name
Type
Default
Description
Io
SIGNAL
0
Earth current meas-
ured using SEF CT
BLOCK
BOOLEAN
0=False
Block signal for acti-
vating the blocking
mode
CB_CLOSED
BOOLEAN
0=False
Circuit Breaker
Closed input
CB_OPEN
BOOLEAN
0=False
Circuit Breaker Open
input
Table 789: PHIZ Output signals
Name
Type
Description
OPERATE
BOOLEAN
Operate
5.4.7
Settings
1MRS757644 H
Protection related functions
620 series
Technical Manual
823