SERVICE MANUAL R5888B
QUADRAMHO Chapter 1
Page 3 of 27
be the sum of the protected line impedance, plus, twice the impedance of the
adjacent line to the fault. If the Zone 2 reach is set to cover 50% of the adjacent
line impedance, then in this parallel infeed case, Zone 2 will effectively cover 25%
of the adjacent line.
In most situations, if the relay reaches at least 20% into the adjacent line, then
faults at the remote terminal of the protected line will be well within Zone 2 reach
and so fast operation of the Zone 2 comparators will be achieved. This is
important if signal aided tripping schemes are used.
In some situations where the protected line is long and the adjacent line is short,
then a 50% reach into the adjacent line will only be a very small overreach of the
protected line. If the protected line is paralleled by another line, then it may be that
the zero sequence mutual coupling, between the two lines, will be sufficient to
prevent the Zone 2 comparators from seeing a ground fault at the remote terminal
of the line until the remote circuit breaker trips, preventing ground fault current
flowing in the healthy parallel circuit. In such a case the Zone 2 setting may need
to be increased slightly, to avoid sequential or time delayed clearance of the fault
at the terminal remote from the fault.
In a parallel line situation, a fault on one line which is cleared sequentially can
cause a fault current reversal in the healthy line. If the Zone 2 settings are greater
than 150% of the protected line impedance and the permissive overreach or
blocking scheme is being used, then a fault current reversal in the healthy circuit
could cause that circuit to be incorrectly tripped, unless special steps are taken.
The permissive overreach and blocking schemes both have current reversal guards
incorporated to prevent such maloperations. The operation of these current reversal
guards is explained in detail later, when considering some logic timer settings.
Section 7. CHOICE OF ZONE 3 IMPEDANCE REACH
The Zone 3 forward reach should normally be set to cover the protected line
section, plus the longest adjacent section and 25% of a third section, to provide an
overall time delayed back-up protection. The reverse Zone 3 offset provides back-
up protection for the busbars behind the relay and would typically be set to 25%
of the Zone 1 setting.
When the blocking scheme or permissive overreach with weak infeed scheme is
being used, Zone 3 is required to provide a blocking function when it operates
without Zone 2, to prevent the protection scheme operating for reverse faults.
The reverse Zone 3 reach in this case must be set to reach further than Zone 2 of
the relay at the other end of the line. It must also be ensured that any resistive faults
behind the relay that are seen by Zone 2 of the remote end relay, are also seen by
Zone 3 of the local relay, to prevent tripping of healthy line for external faults.
As a general guide for most applications, it is recommended that the reverse Zone
3 reach is set to the same value as the Zone 2 setting of the remote end relay.
The forward Zone 3 reach should be set to minimum unless the power swing
blocking facility is also being used.
If the power swing blocking feature is to be used with the permissive overreach
scheme with weak infeed feature, or the blocking scheme, then the forward Zone 3
reach should be set to ≥120% of the Zone 2 reach. This could mean that the
overall diameter of the Zone 3 characteristic will be large, which could lead to
encroachment problems on the power swing blocking starter characteristic, which
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