Alstom SHPM 101, Service Manual

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SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 40 of 74
5.4 The lenticular characteristic
An offset lenticular characteristic is available for Zone 3 for long line applications
where load impedance may encroach on to an offset mho characteristic.
The lenticular characteristic is produced by the intersection of two circles as shown
in Figure 47. The two circles are generated by two comparators using the same
signals as for a normal offset circular characteristic, but using different phase shifts.
The inputs to the comparators then become:
A1 = V–
I
Z ) comparator Cl
B1 = (V+
I
Z') /–180
°
– Ø
°
) (main comparator)
A2 = V–
I
Z ) comparator C2
B2 = (V+
I
Z’) /–Ø
°
(inhibit comparator)
The intersections of the two circles occur on the characteristic angle of the relay
and determine the forward and reverse reach of the lenticular characteristic.
The reach remains independent of the comparison angles.
The aspect ratio, or ratio of the length of the minor and major axes of the lenticular
shape, is determined by the angle Ø. The aspect ratio can be set to 0.41, 0.67
and 1.00 (Figure 48).
The block diagram is shown in Figure 49. The comparator consists only of basic
circuitry for determining whether changes of state of the input signals constitute an
operate or a restrain sequence. There is no counter associated with C
2
, as its
purpose is only to provide a signal for the inhibit terminal of the main comparator
C
1
which therefore, only produces a trip signal for faults within the lenticular
characteristic.
5.5 The quadrilateral characteristic
The quadrilateral characteristic, available as an option for the ground fault
comparators, offers an increased coverage of fault resistance for short lines with
strong infeed, where the resistive expansion of the partially cross polarised mho
may not be sufficient to cover high tower footing or ground contact resistance.
Only a single main comparator is needed to produce a quadrilateral
characteristic, thus avoiding the race problems associated with characteristics
produced by multiple comparators. As shown in Figure 50, the main comparator
of Zone 1 produces the top or “reactance” line of the quadrilateral from inputs:
A1 = V –
I
Z and
B1 =
I
N
R, where
I
N
R = (
I
A
R +
I
B
R +
I
C
R) /–3
°
The vector
I
N
R is obtained from the line currents by three current transformers
supplying small resistive burdens. The replica signals are band pass filtered to
remove exponential and high frequency components before being mixed to
produce an
I
R signal representing the residual current component. The top line
moves with active power flow to avoid the overreach or underreach problems
associated with phase current polarised reactance characteristics.
The other three sides of the Zone 1 quadrilateral are formed by three inhibit
comparators, that is, comparators without counters, arranged to inhibit the main
comparator. The main comparator can only count up when the three inhibit
comparators all agree that the impedance is within the operating zone. The signals
used are as follows: