Areva MiCOM P441, Technical Manual

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Application Notes P44x/EN AP/E33
MiCOM P441/P442 & P444 Page 23/220
•
The zone 1 elements of a distance relay should be set to cover as much of the
protected line as possible, allowing instantaneous tripping for as many faults as
possible. In most applications the zone 1 reach (Z1) should not be able to respond to
faults beyond the protected line. For an underreaching application the zone 1 reach
must therefore be set to account for any possible overreaching errors. These errors
come from the relay, the VTs and CTs and inaccurate line impedance data. It is
therefore recommended that the reach of the zone 1 distance elements is restricted to
80 - 85% of the protected line impedance (positive phase sequence line impedance),
with zone 2 elements set to cover the final 20% of the line. (Note: Two of the channel
aided distance schemes described later, schemes POP Z1 and BOP Z1 use
overreaching zone 1 elements, and the previous setting recommendation does not
apply).
•
The zone 2 elements should be set to cover the 20% of the line not covered by zone
1. Allowing for underreaching errors, the zone 2 reach (Z2) should be set in excess of
120% of the protected line impedance for all fault conditions. Where aided tripping
schemes are used, fast operation of the zone 2 elements is required. It is therefore
beneficial to set zone 2 to reach as far as possible, such that faults on the protected
line are well within reach. A constraining requirement is that, where possible, zone 2
does not reach beyond the zone 1 reach of adjacent line protection. Where this is not
possible, it is necessary to time grade zone 2 elements of relays on adjacent lines.
For this reason the zone 2 reach should be set to cover
≤
50% of the shortest adjacent
line impedance, if possible. When setting zone 2 earth fault elements on parallel
circuits, the effects of zero sequence mutual coupling will need to be accounted for.
The mutual coupling will result in the Zone 2 ground fault elements underreaching. To
ensure adequate coverage an extended reach setting may be required, this is covered
in Section 2.7.7.
•
The zone 3 elements would usually be used to provide overall back-up protection for
adjacent circuits. The zone 3 reach (Z3) is therefore set to approximately 120% of the
combined impedance of the protected line plus the longest adjacent line. A higher
apparent impedance of the adjacent line may need to be allowed where fault current
can be fed from multiple sources or flow via parallel paths.
•
Zone P is a reversible directional zone. The setting chosen for zone P, if used at all,
will depend upon its application. Typical applications include its use as an additional
time delayed zone or as a reverse back-up protection zone for busbars and
transformers. Use of zone P as an additional forward zone of protection may be
required by some users to line up with any existing practice of using more than three
forward zones of distance protection. Zone P may also be useful for dealing with some
mutual coupling effects when protecting a double circuit line, which will be discussed
in section 2.7.7.
•
The zone 4 elements would typically provide back-up protection for the local busbar,
where the offset reach is set to 25% of the zone 1 reach of the relay for short lines
(<30km) or 10% of the zone 1 reach for long lines. Setting zone 4 in this way would
also satisfy the requirements for Switch on to Fault, and Trip on Reclose protection, as
described in later sections. Where zone 4 is used to provide reverse directional
decisions for Blocking or Permissive Overreach schemes, zone 4 must reach further
behind the relay than zone 2 for the remote relay. This can be achieved by setting:
Z4
≥
((Remote zone 2 reach) x 120%) minus the protected line impedance.