ABB RELION 620 Series Technical Manual page 551

1MAC504801-IB E
5.4.3
5.4.4
620 series ANSI
Technical Manual
Functionality
A small percentage of 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
substantial threat to humans and properties; people can touch or get close to conductors
carrying large amounts of energy.
ABB has developed a patented technology (US Patent 7,069,116 B2 June 27, 2006, US
Patent 7,085,659 B2 August 1, 2006) to detect a high-impedance fault.
The high-impedance fault detection function HIZ also contains a blocking functionality.
It is possible to block function outputs, if desired.
Operation principle
The function can be enabled and disabled with the Operation setting. The corresponding
parameter values are "Enable" and "Disable".
HIZ uses a multi-algorithm approach. Each algorithm uses various features of ground
currents to detect a high-impedance fault.
Although the HIZ algorithm is very sophisticated, the setting required to operate the
function is simple. The Security Level setting, with the setting range of 1 to 10, is set to
strike a balance between the extremes of security and dependability which together
constitute the reliability of any system. The setting value "10" is more secure than "1".
The higher the Security Level setting, the lower the probability of false detection, but the
system might miss out some genuine fault. On the other hand, a lower setting would make
the system operate more dependably for high-impedance faults in the line, but the
operation is more likely for other transients in the system.
It is hence recommended to set the value midway to "5" initially. Based on experience and
confidence gained in a particular application, the setting can be moved either side.
Protection-related functions
Section 5
545