GE 750 Instruction Manual page 224

S6 MONITORING
Note
NOTE
5 - 98
The relay calculates the distance to fault with fault resistance compensation. For the fault
location feature, a fault is defined as an event that has caused a current level greater than
the pickup threshold of an overcurrent protection feature programmed to "Trip" or "Trip &
AR" (760 only), which has remained for sufficient time to cause the relay to produce a Trip
command. After this has happened, the apparent distance to the fault is calculated in a
background mode, maintaining all other features in operation. The distance calculation is
based on the assumptions that:
1. The feeder positive and zero sequence impedances are a constant per unit
distance, and
2. Mutual compensation is not required.
If the feeder utilizes conductors of different sizes, or more than one physical arrangement
of conductors, or shares poles or towers with a parallel feeder, these assumptions are
incorrect and errors are introduced.
The algorithm uses prefault system data to reduce the error caused by variable fault
resistance, so inaccuracy is introduced for a fault which occurs when no load current was
present. Also, error is introduced if the feeder has sources at locations other than the
location of the relay, due to infeed effects.
The algorithm contains three sets of equations that are used to perform the calculations
for a specific fault type: phase-to-ground, phase-to-phase-to-ground, phase-to-phase,
and three-phase. Each of the sets (other than three-phase) consists of a subset which
covers all combinations of phases. The algorithm therefore uses a fault identification
procedure to select the appropriate equations to be used for calculation. This procedure
uses both prefault and fault current phasors from memory to identify the type of fault. The
prefault data is taken from a sample collected three power frequency cycles before the
pickup of the overcurrent element to ensure the sample contains only load current. The
after fault data is taken from samples collected 1.5 power frequency cycles after
overcurrent pickup to ensure the current had existed for at least one complete sampling
interval.
As well as the apparent distance to the fault, the locator records the feeder apparent
reactance (with fault resistance removed if prefault current was available.) This parameter
can be very useful in estimating the location of a fault on a feeder tap, where the apparent
distance can be calculated as beyond the feeder end. The date, time, type of fault, and
phases involved are also stored for the event. Non-volatile memory is provided for the past
ten events, in a FIFO queue, available under
If the feeder has a source with a grounded neutral, and is therefore capable of providing
ground fault current, the bus VTs must be both connected and selected as "Wye" in

SYSTEM SETUP BUS VT SENSING
perform the calculations properly. If the fault classification results in a phase to ground
fault, the program checks that the setpoint noted above is set to 'Wye' before the
calculation is permitted.
• LENGTH OF FEEDER: Enter the total length of the feeder, in kilometers or miles as
selected by the
UNITS OF LENGTH
• UNITS OF LENGTH: Enter the units of measurement, in kilometers or miles.
• Zpos (RESISTIVE/INDUCTIVE) OF FEEDER: Enter the total real/imaginary component
of the feeder positive sequence impedance, in actual ohms.

A1 STATUS FAULT LOCATIONS
to allow the fault locator to

VT CONNECTION TYPE
setpoint.
750 FEEDER MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER 5: SETPOINTS
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S2