SERVICE MANUAL R5888B
QUADRAMHO Chapter 1
Page 2 of 27
Section 4. CHOICE OF CHARACTERISTIC
The Type A (mho) relay is suitable for protecting most lines, where their length and
source impedances are such that the resulting shaped Zone 1 characteristics
provide the required resistive coverage. The longer the line is, the higher the
Zone 1 and Zone 2 reach settings, hence the greater the resistive coverage will be
for ground faults. Also the higher the source impedance behind the relay, the
greater the Zone 1 and Zone 2 resistive expansion will be.
For the case of a long heavily loaded line, it may be that with the Zone 3 reach
settings required, the load impedance could “encroach” upon the Zone 3
characteristics if they were purely of the offset mho type or, if being used, the load
could encroach upon the power swing blocking starter characteristic. For such a
case the Type A (mho) allows the resistive reach of the Zone 3 and power swing
blocking starter characteristics, to be reduced with respect to their forward reach
(lenticular facility). This means that desired forward reaches can be attained
without problems of load encroachment.
For very short lines, where distance relay reach settings would be low, particularly
when there is a low source impedance behind the relay, the shaped Zone 1 and
Zone 2 impedance characteristics may not provide the required ground fault
resistive coverage. In such a case, the Type B (quad) relay could be used which
has a quadrilateral ground fault characteristic. This would allow the ground fault
resistive reach to be increased or decreased independently of the forward reach,
and source impedance behind the relay so that the required ground fault resistive
coverage can be achieved.
Section 5. CHOICE OF ZONE 1 IMPEDANCE REACH
Although in most applications the reach accuracy of the relay distance
comparators is ±5%, greater errors can occur as a result of voltage and current
transformer errors and inaccuracies in line data from which the relay settings are
calculated. To prevent the possibility of relays tripping instantaneously for faults in
the next line, it is usual practice to set the Zone 1 reach of the relay to 80% of the
protected line section and rely on Zone 2 to cover the remaining 20% of the line.
With a signal aided distance protection scheme arrangement, the Zone 2 distance
comparators could provide fast triping at both ends of the line for end-zone faults.
If the Zone 1 extension scheme is used, it is usual practice to set the Zone 1
extension to 150% of the normal Zone 1 reach.
Section 6. CHOICE OF ZONE 2 IMPEDANCE REACH
As a general rule, the Zone 2 impedance reach is set to cover the protected line
plus 50% of the shortest adjacent line. The reasoning behind the value of 50% is
that Zone 2 should cover at least 20% of the adjacent line, even in the presence of
typical additional infeed at the remote terminal of the protected line.
One case of additional infeed at the remote line terminal occurs when the
protected line is paralleled by another line. When a fault occurs in the adjacent
line, approximately equal currents will flow in each of the parallel lines. The relay
on the protected line, looking towards the fault, will see an impedance which will
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