SERVICE MANUAL
R5888C
QUADRAMHO
Chapter 2
Page 47 of 74
The circuit is shown in Figure 61. Two separate ferromagnetic cores are used,
each with an air gap to minimise the reluctance of the magnetic circuit. The flux
summation occurs on core 2. The constant flux originates from the current flowing
in primary 2. The function of the windings on core 1 is to provide an additional
current source to derive the variable flux. The output voltage of the device is taken
from a coil which senses the rate of change of flux on core 2.
There are two main advantages in using a transphasor instead of a conventional
transformer-reactor (transactor) input circuit:
a) The constant amplitude phase shifting allows the relay characteristic angle to be
varied without changing the relay reach setting.
b) The transphasor has a much shorter time constant than a transactor circuit. This
is extremely advantageous when the effect of a transient dc decrement in the
input current must be removed quickly from the output voltage so as to maintain
fast comparator operating times.
For relays with quadrilateral ground fault characteristics further current input
signals are required. Since these signals do not require phase shifting,
conventional current transformers are used.
5.15
Operation with saturated CTs
Quadramho incorporates circuits which permit smaller and more econonomical
line current transformers to be used in most applications, compared with other
distance relays. These circuits are:
1) The transphasor current input circuit (an electro-magnetic coupling device),
differentiates the current input waveform to produce the
I
Z vector. The effect of
saturation is much less on the zero crossings of
I
Z than on the average current
level.
2) The signal preconditioning filters reduce the distortion in the
I
Z signals caused
by saturation.
3) The comparator can tolerate some timing errors in one or more of its input
signals and still respond correctly.
5.16
Voltage transformer supervision (fuse failure)
5.16.1 Purpose
The Voltage Transformer Supervision (VTS) feature is used to detect failure of the ac
voltage supply. Such failure can occur by short circuit to ground, or open circuit, in
the voltage transformer itself, in the secondary wiring, in the fuses, or within the
distance relay circuits. The VTS gives visual and electrical alarms and can, if
required, be set to block any unwanted operation of the distance relay
comparators arising from the failure.
Use of miniature circuit breakers (mcb) instead of fuses can be accommodated by
using a normally open contact (mcb closed) to energise an opto coupler connected
to case terminals A1/A2. When the mcb opens removing all three phases of the
VT supply the following events occur:
i) a 15ms delay in drop off of the zero sequence voltage level detector ensures
sufficient time for the opto coupler mcb to pick up and take over the blocking of
the relay from the VTS circuitry.
ii) the relay inoperative alarm contact 97Y–1 operates and the green relay
available LED is extinguished.
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