GE D90 Plus Instruction Manual page 26

DEVICE OVERVIEW
Application to series-compensated lines
In-zone transformer compensation
Load encroachment
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protection for generators, transformers, and reactors. The relay can be applied to power
systems with different earthing conditions, lines with in-zone transformers or tapped
transformer feeders and overhead lines with series compensation. Each zone element for
the phase and ground distance can be set to the quadrilateral or mho characteristic with
the flexibility of designing different characteristic shapes to suit for different power system
conditions.
The advanced comparator based distance elements provide utmost security, sensitivity
and selectivity for different types of faults. Superior digital filtering techniques provide
secure and optimum reach accuracy even under worst-case CVT transients without
compromising the operating time. Secure direction detection is achieved by using positive
sequence memory voltage polarization providing reliable directionality for worst-case
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close-in faults. The D90
provides fast, secure and reliable faulted phase identification for single pole tripping and
proper fault distance calculation for a variety of power system conditions. An additional
voltage monitoring function provides extra security to the distance element, which can be
used to block the distance elements under voltage transformer fuse failure conditions.
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The D90 provides enhanced stability and security by employing an adaptive distance
reach control to cope with the overreaching and sub-synchronous oscillations when
applied to, or in the vicinity of, series compensated lines. For directional integrity the relay
uses memory voltage polarization and a multi-input comparator is used to deal with
current inversion issues in series compensated lines.
Phase distance elements in the D90
of three-phase wye or delta transformers, allowing application of the D90
protection on generators. VTs and CTs can be installed independently of each other on
either side of the power transformer. The relay automatically compensates for transformer
connections, to guarantee accurate reach for any type of fault.
The reliable load encroachment feature offers excellent discrimination between keep
loading conditions and fault conditions especially for long lines under heavy loads by
supervising the distance elements or any overcurrent element. This prevents unwarranted
tripping under heavy line load conditions and enables optimum operation of the line while
meeting regulatory requirements for the loading.
The following diagram illustrates distance quadrilateral characteristics supervised by load
encroachment function.
Figure 9: Load encroachment characteristic example
employs a well-proven algorithm for phase selection, which
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can be used to detect faults through different types
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D90 LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL
CHAPTER 2: PRODUCT DESCRIPTION
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for backup