Application - ABB 615 series Technical Manual

Section 5
Protection related functions
5.4.5
792
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Figure 402: Validation of HIZ on sand

Application

HIZ is used to detect a downed conductor dropping to a very resistive ground, causing a
ground fault which is very difficult to detect by a conventional protection relay
functionality. HIZ is then targeted to be used with overhead lines. HIZ is limited to be used
in 60 Hz electrical networks with efficiently grounded or isolated neutral.
Electric power lines experience faults for many reasons. In most cases, electrical faults
manifest in mechanical damage, which must be repaired before returning the line to
service.
Most of the electrical network faults are ground 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 ground 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.
HIZ always needs sensitive IG measurement.
High-impedance fault (HIZ) detection requires a different approach than that for
conventional low-impedance faults. Reliable detection of HIZ provides safety to humans
and animals. HIZ detection can also prevent fire and minimize property damage. ABB has
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Figure 403: Validation of HIZ on grass
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615 series ANSI
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