GE D90 Plus Instruction Manual page 234

GROUPED PROTECTION ELEMENTS
NOTE:
224
current supervision prevents misoperation 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 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 less than the maximum load current for the time delayed zones.
Blocking distance elements during fuse fail conditions is not acceptable in some
applications or under some protection philosophies. Applied solutions vary as follows:
• Not using the fuse failure element for blocking at all
• Using the fuse failure element and modifying (through FlexLogic and multiple setting
groups mechanisms) other protection functions or other devices to provide some
protection after detecting fuse fail conditions and blocking the distance elements
• Using the fuse failure element 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 √3 factor.
Phase distance zone 1 guidelines for stepped distance
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 device inaccuracy is less than 5%. This includes both steady state and transient
overreach, even when supplied from CVTs under the source impedance ratios (SIRs) of up
to 30.
Phase distance zone 2 guidelines for stepped distance
Zone 2 is an overreaching element, which essentially covers the final 10% to 20% of the
line length with a time delay. The additional function for the zone 2 is as a timed backup for
faults on the remote bus. The reach is set typically 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.
The zone 2 time delay is typically set from 0.2 to 0.6 seconds, although this can be
reviewed more carefully if a short line terminates on the remote bus, since the two zone 2
elements can overlap and therefore not coordinate satisfactorily.
Phase distance zone 3 guidelines for stepped distance
If a remote backup philosophy is followed, then the reach of this element must be set to
account for any infeed at the remote bus, plus the impedance of the longest line that
terminates on this remote bus. The time delay must coordinate with other time-delayed
protections on any remote line. Circuit loading limitations created by a long zone reach
can be overcome by using lens or quadrilateral characteristics, a load encroachment
supervising characteristic, or both. Also consider a situation where the load impedance
can enter into the relay characteristic for a time longer than the chosen time delay, which
can occur transiently during a system power swing. For this reason, use the power swing
blocking function.
PLUS
D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 7: PROTECTION