High Impedance Fault Detection; High Impedance Fault Protection Implementation; Fundamental Analysis - GE MiCOM P40 Agile Technical Manual

P14D
18

HIGH IMPEDANCE FAULT DETECTION

A High Impedance Fault, also known as a Downed Conductor, happens when a primary conductor makes
unwanted electrical contact with a road surface, pathway, tree etc., whereby due to the high impedance of the
fault path, the fault current is restricted to a level below that which can be reliably detected by standard
overcurrent devices. Even in cases where the instantaneous fault current may exceed the thresholds, the duration
of this transient is usually so small that the standard overcurrent IED will not pick up. It is quite a challenging
problem to detect such faults, and it requires a special method combining multiple techniques.
Due to the high impedance and transient nature of such faults it is not possible to derive the fault calculation from
short-circuit computing. HIF detection therefore relies on the detection of the fault current and voltage waveform
signatures. These waveforms may be very different from fault to fault, but they often have commonalities typified
by:
● Third harmonic content
The transient bursting (intermittent change of amplitude)
●
We can use these phenomena to detect the fault.
We may need to establish the direction of the fault. For this, we can use instantaneous power measurement.
Hence we can see there are three components necessary to provide a reliable HIF detection function:
● Component harmonic Analysis (CHA)
Fundamental Analysis (FA) (with or without directional analysis (DIR)
●
18.1

HIGH IMPEDANCE FAULT PROTECTION IMPLEMENTATION

18.1.1

FUNDAMENTAL ANALYSIS

Fundamental Analysis (FA) captures the intermittent characteristics associated with a fault current. Generally, the
system current is fairly stable and it tracks the load conditions. An average of this current is calculated by
continually averaging the latest samples, and this value is stored in a buffer. This value is being continually
compared with latest current value. If there is a sudden increase in current, its value will significantly exceed the
average value. It is this increment that is used to start the fault evaluation process.
The averaged current load tracks system load conditions using an averaging process. A discrepancy between the
actual amplitude and the average amplitude starts the fault evaluation process. If the increment is greater than a
start threshold, determined by the setting FA> Start Thresh, the FA will start fault evaluation. A Burst Valid (BV)
threshold, determined by the setting FA> Burst Thresh, is used to judge whether the increment indicates
conduction of fault current. By counting the changes of the BV states within a time window, an event is issued and
it is possible to establish whether an intermittent fault has been detected.
FA detection can be triggered by any sudden increase of the amplitude. However, only those sustained series of
changes within a specified time-window can be evaluated as a High Impedance Fault (HIF). Fault classification
criteria can be determined using timing and counting of these bursts. The following table shows the classification
criteria.
Counter Status
BV state changes exceed count limit
BV stage changes do not exceed count limit, but
are more than two
Less than two changes
Others
P14D-TM-EN-8
Timer Status
Within time window of one FA section
Within time window of one FA section
While fundamental amplitude remains above BV threshold
within time window
Chapter 6 - Current Protection Functions
HIF
Transient Event
Steady Event
Noises
Result
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