Theory Of Operation; High-Impedance (Hi-Z) Fault Detection; Description - GE F60 Instruction Manual

F60 Feeder Protection System
Chapter 9: Theory of operation

Theory of operation

This chapter outlines advanced information on operation.

9.1 High-impedance (Hi-Z) fault detection

9.1.1 Description

The Hi-Z element accomplishes high-impedance fault detection using a variety of algorithms, all coordinated by an expert
system. At the heart of the high-impedance fault-detection system is the identification of arcing on a feeder. If the Hi-Z
element detects arcing, it then determines whether or not the arcing persists for a significant period of time. If it does, the
Hi-Z element determines whether the persistent arcing is from a downed conductor or from an intact conductor and then
generates an output to indicate either the detection of a downed conductor or the detection of arcing, respectively.
Distinction between an arcing intact conductor and an arcing downed conductor is determined by looking at patterns in
the load current at the beginning of the fault. A downed conductor is indicated when a precipitous loss of load or an
overcurrent condition precedes arcing detection. Otherwise, the Hi-Z element assumes that the line is intact, even if arcing
is present. In such a case, if the detected arcing can be classified as persistent, and an output contact is configured for
'arcing detected,' the Hi-Z element closes that contact.
In some cases, arcing is determined to be present, but not persistent. For example, when it is caused by tree limb contact
or insulator degradation, arcing typically is present intermittently with relatively long periods of inactivity (such as minutes)
interspersed. In such cases, arcing can be affected by such factors as the motion of a tree limb or the moisture and
contamination on an insulator. Conditions such as these, characterized by a high number of brief occurrences of arcing
over an extended period of time (for example from a fraction of an hour to one or two hours), lead the Hi-Z element to
recognize and flag an "arcing suspected" event. None of these brief occurrences of arcing, if taken individually, are
sufficient to indicate detection of a downed conductor or to set off an alarm indicating that persistent arcing has been
detected. When considered cumulatively, however, they do indicate a need for attention. If an output contact is configured
to indicate 'arcing suspected,' the Hi-Z element recognition of such sporadic arcing closes that contact and appropriate
actions can be taken.
If the Hi-Z element determines that a downed conductor exists, oscillography and fault data are captured. In addition,
target messages and appropriate LEDs are activated on the relay faceplate.
The detection of a downed conductor or arcing condition is accomplished through the following algorithms:
• Energy algorithm
• Randomness algorithm
• Expert Arc Detector algorithm
F60 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL 9-1