9.3 DISTANCE BACKUP/SUPERVISION
a) PHASE CURRENT SUPERVISION AND USE OF THE FUSE FAILURE ELEMENT
The phase-to-phase (delta) current is used to supervise the phase distance element, primarily to ensure that in a de-ener-
gized state the distance element will not be picked up due to noise or induced voltages, on the line.
However, this supervision feature may also be employed to prevent operation under fuse failure conditions. This obviously
requires that the setting must be above maximum load current and less than the minimum fault conditions for which opera-
tion is expected. This potential problem may be avoided by the use of a separate fuse fail function, which means that the
phase current supervision can be set much lower, typically 2 times the capacitance charging current of the line.
The usage of the fuse fail function is also important during double-contingency events such as an external fault during fuse
fail conditions. The current supervision alone would not prevent maloperation in such circumstances.
It must be kept in mind that the Fuse Failure element provided on the L90 needs some time to detect fuse fail conditions.
This may create a race between the Zone 2 and the Fuse Failure element. Therefore, for maximum security, it is recom-
mended to both set the current supervision above the maximum load current and use the Fuse Failure function. The current
supervision prevents maloperation immediately after the fuse fail condition giving some time for the Fuse Failure element to
take over and block the distance elements permanently. This is of a secondary importance for time-delayed Zone 2 as the
Fuse Failure element has some extra time for guaranteed operation. The current supervision may be set below the maxi-
mum load current for the time delayed zones.
Blocking distance elements during fuse fail conditions may not be acceptable in some applications and/or under some pro-
tection philosophies. Applied solutions may vary from not using the Fuse Failure element for blocking at all; through using it
and modifying – through FlexLogic™ and multiple setting groups mechanisms – other protection functions or other relays to
provide some protection after detecting fuse fail conditions and blocking the distance elements; to using it and accepting
the fact that the distance protection will not respond to subsequent internal faults until the problem is addressed.
To be fully operational, the Fuse Failure element must be enabled, and its output FlexLogic™ operand must be indi-
cated as the blocking signal for the selected protection elements.
For convenience, the current supervision threshold incorporates the square root of 3 factor.
b) PHASE DISTANCE ZONE 2
The Zone 2 is an overreaching element, which essentially covers the whole of the line length with a time delay. The addi-
tional function for the Zone 2 is as a timed backup for faults on the remote bus. Typically the reach is set to 125% of the
positive sequence impedance of the line, to ensure operation, with an adequate margin, for a fault at 100% of the line
length. The necessary time delay must ensure that coordination is achieved with the clearance of a close-in fault on the
next line section, including the breaker operating time.
a) NEUTRAL CURRENT SUPERVISION
The current supervision for the ground distance elements responds to an internally calculated neutral current (3 x I_0). The
setting for this element should be based on twice the zero-sequence line capacitance current or the maximum zero-
sequence unbalance under maximum load conditions. This element should not be used to prevent an output when the load
impedance is inside the distance characteristic on a steady state basis.
b) GROUND DISTANCE ZONE 2
To ensure that the Zone 2 can see 100% of the line, inter-circuit mutual effects must be considered, as they can contribute
to a significant under-reach. Typically this may occur on double circuit lines, when both lines may carry the same current.
An analytical study should be carried out to determine the appropriate reach setting.
The main purpose of this element is to operate for faults beyond the reach of the local Zone 1 element, and therefore a time
delay must be used similar to the phase fault case.
L90 Line Differential Relay
9 APPLICATION OF SETTINGS
9.3.2 PHASE DISTANCE
9.3.3 GROUND DISTANCE
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