Power Solutions Inc (‘PSI’) has integrated within its Distribution Fault Anticipation (‘DFA’) on-line monitoring device for MV power lines a patented ‘HiZ’ technology for detecting downed-conductor, high-impedance arcing faults (HiZ faults) on electric power distribution circuits.
PSI licensed commercial rights to that technology in its earlier form to General Electric some 20 years ago, and GE continues to market it as part of a protection relay platform, but no longer on an exclusive basis.
PSI’s HiZ technology originally was implemented to meet the needs of most distribution companies in the United States of America. Key observations for the HiZ implementation include the items listed below.
- Simple algorithms, such as exceeding X amps for Y seconds, are ineffective for sensitive detection of low-current earth faults.
- Arcing tends to be a dynamic process that often produces highly variable fault currents that are rich in transients and harmonics. This variability can cause magnitude-based detection to fail.
- PSI’s HiZ technology is not based primarily on current magnitude but rather uses digital signal processing and other software techniques to detect temporal and spectral characteristics unique to arcing, without regard to absolute current magnitude.
- HiZ detection must be balanced with service reliability. False operations interrupt customers needlessly and waste resources, causing crews to look for downed conductors when none exist.
- The HiZ function monitors patterns in load current to differentiate between an arcing, intact conductor in the air (e.g., tracking) and an arcing downed conductor. The HiZ function examines current levels immediately preceding the commencement of arcing, looking for either one or both of the following factors to occur just prior to commencement of arcing:
- Significant loss of load – A break in a conductor results in loss of service downstream of the break and consequently a reduction in the level of load current as measured at the substation. The HiZ function recognizes sudden loss of measurable load, immediately prior to commencement of arcing current, as an indicator that a conductor likely has broken and fallen.
- Momentary overcurrent fault – As a broken conductor falls to the earth, it often makes temporary contact with another line conductor, thereby causing a temporary overcurrent condition. The HiZ function recognizes a temporary overcurrent condition, immediately prior to commencement of arcing current, as an indicator that a conductor likely has broken and fallen.
- If the HiZ function detects arcing but does not detect loss of load or overcurrent immediately prior thereto, it will give an arcing output indication but intentionally will not give a downed-conductor output.
- Proper testing of the HiZ function therefore requires that any experimental protocol include a measureable loss of load or a temporary overcurrent condition, immediately prior to the commencement of ground contact. Otherwise, by design and intent, the HiZ function will not give a downed-conductor output.
- Even if it were known with 100% confidence that an energized conductor were on the ground, high-speed tripping often would not be desirable. Utility companies in the USA often prefer to delay tripping on a HiZ fault, often for seconds or tens of seconds, so as to allow coordination with downstream protection (e.g., reclosers, fuses). Delaying HiZ operation reduces the likelihood of interrupting an entire circuit.