ABB KD-10 Instruction Leaflet page 5

Type KD-10 and KD-11 Compensator Distance Relay
of the three phase-to-phase fault combinations sub-
jects the cylinder unit to a similar set of conditions.
4.0 CHARACTERISTICS
4.1 DISTANCE CHARACTERISTICS
PHASE-TO PHASE UNIT
This unit responds to all phase-to-phase faults and
most two-phase-to-ground faults. It does not re-
spond to load current, synchronizing surges, or
out-of-step conditions. While a characteristic circle
can be plotted for this unit on the R-X diagram, as
shown in Figure 10 (page 42), such a characteristic
circle has no significance except in the first quadrant
where resistance and reactance values are positive.
A small portion of the fourth quadrant, involving pos-
itive resistance values and negative reactance val-
ues, could have some significance in the event that
the transmission line includes a series capacitor.
The portion of the circle in the first quadrant is of in-
terest because it describes what the relay will do
when arc resistance is involved in the fault. The
phase-to-phase unit operating on an actual trans-
mission system is inherently directional and no sep-
arate directional unit is required.
An inspection of Figure 10 (page 42) indicates that
the circle of the phase-to-phase unit is dependent on
source impedance Z . However the circle always
S
goes through the line balance-point impedance. The
reach at the compensator (and line) angle is con-
stant, regardless of the system source impedance.
The broadening out of the characteristic circle with a
relatively high source impedance gives the
phase-to-phase unit the advantageous characteris-
tic hat for short lines, it makes a greater allowance
for resistance in the fault. Stated another way, the
characteristics approach that of a reactance relay
more and more closely as the line being protected
becomes shorter and shorter with respect to the
source impedance back of the relaying location.
4.2 SENSITIVITY: PHASE-TO-PHASE UNIT
A plot of relay reach, in percent of tap block setting,
versus relay terminal voltage and current sensitivity
is shown in Figure 12 (page 42). The unit will operate
with the correct directional sense for zero voltage
phase-to-phase faults. For this condition the fault
current must be not less than 0.015 relay amperes
with an ohm setting of 5.8 with rated voltage on the
unfaulted phase. Pick up current is proportionately
higher in S=2 and S=3 taps.
The KD-10 relay may be set without regard to possi-
ble overreach due to dc transients. Compensators
basically are insensitive to dc transients which at-
tend faults on high angle systems. The long time
constant of a high angle system provides a minimum
rate of change in flux-producing transient current
with respect to time, and therefore induces a mini-
mum of unidirectional voltage in the secondary.
Asymmetrical currents resulting from faults on
low-angle systems having a short time constant can
induce considerable voltage in the secondary, but
for the first half cycle, the transient-derived voltage
subtracts from the steady-state value. This transient
decays so rapidly that it is insignificant during the
second half cycle when it adds to the steady-state
value.
4.3 DISTANCE CHARACTERISTIC –
KD-10, 3-PHASE UNIT
The three-phase unit has a characteristic circle
which passes through the origin as shown in Figure
11 (page 42). This circle is independent of source
impedance. The three-phase unit is also inherently
directional and does not require a separate direc-
tional unit.
If a solid three-phase fault occurs right at the relay lo-
cation, the entire voltage triangle collapses to zero a
balance point condition, as shown by the relay char-
acteristic in Figure 11 which passes through the ori-
gin. However, since the YZ voltage also drops to ze-
ro, the relay would be unable to determine whether
an internal or external fault existed. To correct this
condition, a resonant circuit is added to the C-B volt-
age circuit of the relay which allows the ZY voltage
to determine whether the fault is inside the protected
line section or behind the relay.
4.4 SENSITIVITY: KD-10, 3-PHASE-UNIT
The unit will operate with the correct directional
sense for zero voltage three-phase faults when nor-
mal voltage exists at the relay terminals prior to the
fault. This operation occurs due to memory action as
described above. The unit will have zero torque or
perhaps a slight opening torque if there is zero volt-
age at the relay prior to the fault or after the memory
action has subsided. For medium and long reach re-
lays with an impedance setting of 5.8 ohms the
three-phase unit will directionally operate for faults
which produce 2 volts line-to-line and 1.0 ampere at
the relay terminals.
Sensitivity with 2 volts line-to-line for any tap is de-
fined by Equation 7:
41-490J
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