ABB 615 series Technical Manual page 796

Section 5
Protection related functions
790
networks which can cause similar current waveforms like high-impedance faults. These
events could then be detected by the HIZ algorithm causing unnecessary detections.
Normally, electrical network operator does not know the existence of these events well
and those can also be happening very randomly. The effect is also always dependent on
event location compared to protection relay measurement location. All these facts make
the HIZ algorithm operation in certain electrical networks quite hard to measure and
forecast beforehand. There is not any direct formula which can calculate the exact right
setting based on known electrical network parameters.
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. In many
cases, it would be a good practice to use HIZ as an indicative function during a piloting
phase, until enough experience has been gathered and a suitable setting found.
GUID-91FFCBAB-470F-43DD-AC86-E673BAACCBA6 V1 EN
Figure 398: Electrical power system equipped with HIZ
Power system signals are acquired, filtered and then processed by individual high-
impedance fault detection algorithm. The results of these individual algorithms are further
processed by a decision logic to provide the detection decision. The decision logic can be
modified depending on the application requirement.
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615 series ANSI
Technical Manual