Alstom SHPM 101, Service Manual

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SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 41 of 74
A2 = V –
I
R
right hand “resistance line”
B2 = –
I
Z
A3 = –
I
Z
left hand “resistance” line
B3 = V+
I
R
A4 =
I
Z
“directional” line
B4 = V
POL
Throughout, V is the faulty phase voltage, V
POL
is the partially cross polarised
voltage described under “polarising mixing circuits” and
I
Z is the residually
compensated vector (
I
Ph
Z
Ph
+
I
N
Z
N
) from the transphasors. The
I
R signal for the
resistance lines is derived from the phase current only, the absence of residual
compensation permitting good phase selection for single pole tripping purposes.
The method of producing a quadrilateral characteristic has several advantages
over other methods:
a) independent settings for reach and resistance coverage
b) relay characteristic angle can be set to line angle giving fastest operating speed
for solid faults and optimum control of reach accuracy
c) good operating speed over the whole of the characteristic as shown in
Figure 51.
5.6 Two-phase-to-ground faults (quadrilateral characteristic)
The operation of the quadrilateral characteristic during two phase to ground faults
presents special problems. This type of fault may be measured in three ways:
1) operation of the corresponding phase-phase element
2) operation of the leading ground fault element
3) operation of the lagging ground fault element
The operation of the phase-phase elements is practically independent of the fault
resistance to ground. However, the measurement of the ground fault elements
under these conditions, is affected by the resistance of the fault to ground.
The effect being that the leading phase-ground element will tend to overreach and
the lagging phase-ground element will tend to underreach. Arc resistance between
phases and to ground can also have the effect of making the leading phase-ground
element underreach and the lagging phase-ground element overreach. (See Figure
52).
The amount of overreach, or underreach depends on the arc and ground
resistances, the prefault load current and the type of polarisation used for the “top”
or reactance” line. In Quadramho the polarisation of the Zone 1 reactance line is
optimised for single phase faults and a technique is employed to inhibit the
operation of the ground fault comparators for two phase to ground faults.
The phase fault comparators, with their partially cross polarised shaped
characteristics, are allowed to operate on two phase to ground faults.
The technique used to prevent operation of the quadrilateral ground fault
comparators is as follows:
The three Zone 1 ground fault comparators each have a corresponding “guard”
zone, of the characteristic shape of which comprises the same side and directional
lines as Zone 1. The top line of the “guard “ zone has twice the reach of Zone 1