Distance Elements (Stepped Distance Scheme); Phase Distance - GE D60 Instruction Manual

DISTANCE ELEMENTS (STEPPED DISTANCE SCHEME)

If CT saturation is an issue, such as close to a generation where long lasting dc components are likely to saturate the CTs,
note that the instantaneous overcurrent elements require 1.33 cycle of data to operate for a multiple of pickup of 1.01. For
higher multiples of pickup, the relation between the multiple of pickup and the amount of data required for operation
before complete CT saturation is approximately linear. For example, for a multiple of pickup of 4, approximately 1.33 / 4 =
0.332 of power cycle is required by the phase instantaneous overcurrent element to operate. Do not confuse the above
information with the operating time, which includes some inherent delays such as a trip rated output contact.
8.2 Distance elements (stepped distance scheme)

8.2.1 Phase distance

8.2.1.1 Phase current supervision and the fuse failure element
The phase-to-phase (delta) current is used to supervise the phase distance elements, primarily to ensure that in a de-
energized state the distance elements are not picked up due to noise or induced voltages on the line.
This supervision feature can also be employed to prevent operation under fuse failure conditions. This requires that the
setting be above maximum load current and less than the minimum fault conditions for which operation is expected. This
potential problem can be avoided by the use of a separate fuse fail function, which means that the phase current
supervision can be set much lower, typically two 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 does not prevent maloperation in such circumstances.
Keep in mind that the fuse failure element provided on the D60 needs some time to detect fuse fail conditions. This can
create a race between the instantaneous zone 1 and the fuse failure element. Therefore, for maximum security, it is
recommended 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
zones 2 and up as the fuse failure element has some extra time for guaranteed operation. The current supervision can be
set below the maximum load current for the time delayed zones.
Blocking distance elements during fuse fail conditions are not be acceptable in some applications and/or under some
protection philosophies. Applied solutions vary from not using the fuse failure element for blocking at all, to 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, and to using it and
accepting the fact that the distance protection does 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
indicated as the blocking signal for the selected protection elements.
For convenience, the current supervision threshold incorporates the
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8.2.1.2 Phase distance zone 1
As typically used for direct tripping, the zone 1 reach must be chosen so that it does not extend beyond the far end(s) of the
protected line. Zone 1 provides nominally instantaneous protection for any phase fault within a pre-determined distance
from the relay location. To ensure that no overreach occurs typically requires a setting of 80 to 90% of the line length,
which covers CT and VT errors, relay inaccuracy and transient overreach, as well as uncertainty in the line impedance for
each phase, although transposition can minimize this latter concern.
The total relay inaccuracy, including both steady state and transient overreach even when supplied from CVTs under the
source impedance ratios of up to 30, is below 5%.
8-2
CHAPTER 8: APPLICATION OF SETTINGS
factor.
D60 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL