Date Code 20000421
Specifications
2-31
SEL-251, -2, -3 Instruction Manual
If these fault current component magnitudes are in moderate ratios (1.5:1 or more), the relay lists
a single- or two-phase fault. If all ratios are less than 1.5, the relay lists a three-phase fault.
Explicit fault classification logic is as follows, where "I" values are uncompensated midfault
currents and "If" values are midfault currents compensated for load, yielding true fault current
components:
IF (Imax
>
4 x Imed)
THEN Single-phase,
ELSE IF (Imed
>
4 x Imin)
THEN Two-phase,
ELSE IF (Ifmax
>
1.5 x Ifmed)
THEN Single-phase,
ELSE IF (Ifmed
>
1.5 x Ifmin)
THEN Two-phase,
ELSE IF (none of the above)
THEN Three-phase.
This algorithm is largely immune to load and system grounding variations.
Once the relay determines fault type, the fault locator uses the Takagi algorithm to locate the
fault. Using prefault and fault data, it compensates for errors introduced by fault resistance in the
presence of load flow. If the event record contains no sound prefault data, the relay gives a loca-
tion based on a simple reactance measurement.
The fault locator depends on accurate distribution line parameters and instrument transformer
ratios. Pay special attention to these potential sources of difficulty:
•
Instrument transformer errors due to overburden by other devices
•
Capacitive potential transformer capacitor value
•
Distribution line parameter errors
Although the fault location computation takes several seconds, the relay can handle several faults
in quick succession. The relay stores all fault data, then processes each fault in turn. For
example, suppose three faults occur within a few seconds. The relay stores data from them as
they occur. The fault location computations begin with the first (oldest) fault and proceed until
all three fault records are processed. The relay transmits each summary event report when the
corresponding fault location is available.
The relay does not consider shunt capacitance of a line. The capacitance causes the fault
location to appear more remote by a factor of approximately 1/cos(bL), where bL is the line
length in radians. One wavelength at 60 Hz is 3100 miles. For example, the line length of a 100
mile line in radians is calculated:
(100/3100)(2)(3.14159) = 0.2027 radians
The indication neglecting capacitance is about cos (0.2027) = 0.98 times the actual location, or
about two miles short for a fault at the far end of a 100 mile line.
When a station uses shunt reactor compensation and the relay is connected to measure reactor
plus line current, the shunt reactors reduce fault locating error due to shunt capacitance.
Nomographs
The relay fault locator is designed for circuits having a constant per-unit length impedance. This
is often the case with transmission lines. However, distribution lines often have conductor
changes, resulting in different per-unit length impedances. Nomographs are used to compensate
