IEEE Std C37.114-2014 pdf download – IEEE Guide for Determining Fault Location on AC Transmission and Distribution Lines.
3. One-ended impedance-based measurement techniques 3.1 Background One-ended impedance-based fault locators calculate the fault location from the apparent impedance seen looking into the line from one end. To locate all fault types, the phase-to-ground voltages and currents in each phase must be measured. (If only line-to-line voltages are available, it is possible to locate phase-to- phase faults, and additionally, if the zero-sequence source impedance, Z 0 , is known, the location for phase- to-ground faults can also be estimated). If the fault resistance is assumed to be zero, use one of the impedance calculations in Table 1 to estimate fault location. The exact mathematical equations for fault location taking into account fault resistance and load are found in 3.3.
Voltage and current data are used to determine the impedance to the fault location, as shown in Table 1. By knowing the line impedance per unit, the distance to the fault per unit can be determined. A correct fault location estimate, unfortunately, is affected by many factors not represented by these equations: a) The combined effect of the load current and fault resistance (reactance effect). The value of the fault resistance may be particularly high for ground faults, which represent the majority of the faults on overhead lines. b) Inaccurate fault type (faulted phases) identification. c) Influence of zero-sequence mutual effects. d) Uncertainty about the line parameters, particularly zero-sequence impedance. It is often difficult to obtain an accurate zero-sequence impedance (Z 0L ) for the line. The value of Z 0L is affected by soil resistivity, which can be difficult to measure and may be changeable. A 20% error in Z 0L can introduce a 15% error in the calculated fault location. In addition, this impedance is not uniformly distributed along the line length. (100 to 1 variation in earth resistivity produces about a 2 to 1 change in Z 0 .) e) Insufficient accuracy of the line model (e.g., untransposed lines are represented as being transposed, and charging capacitance is not considered). f) Presence of shunt reactors and capacitors. g) Load flow unbalance. h) Series-compensated and multi-terminal lines. i) Measurement errors, current and voltage transformer errors, and bit resolution of analog to digital conversion (A/D) system. j) The filtering system necessary to extract the phase voltages and current phasors. For example, if the fault voltages and currents do not reach steady-state value (as in a fault with time varying resistance) or if the fault is cleared with a delay smaller than the filter nominal response time, then the estimated fault location could have substantial errors.