SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 51 of 74
5.16.15 Scheme logic module
A general description of the scheme logic module is given in Section 4.9 and
Figure 23, shows the module in block diagram form including the associated
inputs and outputs. The general arrangement shown in Figure 23, is common to all
Quadramho relays and includes a common standard software program which
performs many functions of the distance relay such as control functions, scheme
operation and output relay control.
5.16.16 Level detector pole dead logic
The level detector pole dead logic is used to inhibit the relay comparators during
pole dead conditions (See Section 5.9) and is also utilised for other features, such
as, PSB, SOTF, weak infeed feature, band pass filter switcher, etc. Figure 64,
provides a logic equivalent of the software implementation of the pole dead logic.
5.16.17 Comparator level detector checks
As mentioned in Section 5.8, phase current level detectors are used to prevent
spurious operation of the comparator during line de-energisation. Figure 65,
Appendix A, gives the logic equivalent of this feature for each zone.
5.16.18 Voltage bandpass filter switching
Figure 66 gives the logic equivalent function. The inhibit comparator pulse of
0.15ms is used to negate any spurious edges which may occur at the instant of
switching from low pass filters to band pass filters.
5.16.19 Control of hysteresis in impedance measurement
The threshold of operation of the comparator described in Section 5.1 occurs when
the two input signals A and B are in phase or 180° out of phase. This is true
irrespective of the conditions immediately before crossing the threshold of
operation, that is, whether A was formerly lagging B (comparator operated) or
leading B (comparator reset). The inherent reset/operate ratio of impedance
measurement of the mho comparator is therefore unity.
In the practical distance relay a reset ratio of unity would give rise to discontinuous
operation of the comparator for faults on the boundary of operation in the
presence of random noise in the comparator input signals (random noise includes
high freqency components caused by clocking the input signals at discrete
intervals. A reset ratio greater than unity is therefore necessary. Too high a reset
ratio, on the other hand, is undesirable because with large reach settings
(particularly of Zone 3) the encroachment of load impedance may prevent the
comparator resetting after a fault is cleared.
In Quadramho circuits are provided to give the relay a reset ratio of nominally
105%, measured on the relay characteristic angle. Two separate controls are
involved:
1) Amplitude hysteresis, controlled by a single logic signal from the scheme logic
which is produced when the relay trips. This decreases the gain of the voltage
input module by 5%, thereby increasing the relay reach (See Section 4.2).
2) Angular hysteresis, provided to control the impedance reset ratio for highly
resistive faults on the Zone 1 or Zone 2 mho boundary under conditions of low
infeed to the fault. Under these conditions the shaped mho characteristic is
expanded in the resistive direction and decreasing the size of V has little
additional effect. Angular hysteresis is controlled by the scheme logic when a
trip occurs, resulting in the synchronous polarising signal being advanced by
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