CHAPTER 15: THEORY OF OPERATION DISTANCE ELEMENTS
D90
PLUS
LINE DISTANCE PROTECTION SYSTEM – INSTRUCTION MANUAL 629
The voltage signals are pre-filtered using a special digital filter designed to cope with CVT
transients. This patented filter combines filtering and memory actions enabling the relay
to cope with CVT noise under high source impedance ratios (SIRs). The filter controls
underestimation of the fault voltage magnitude to less than 1% of the nominal and
prevents certain phase angle anomalies that can be encountered under heavy CVT noise
and high SIRs.
The frequency domain algorithm uses phase angle comparators to shape the operating
characteristics as described in Distance characteristics
on page 630. The voltage and
current phasors are estimated using optimized techniques as explained in the following
section.
Distance supervision
The phase and ground distance elements use memory voltage for polarization. Additional
supervising functions—different for ground and phase distance zones—complement a
classical mho characteristic to enhance directional integrity and reach accuracy.
• To avoid overreaching during resistive faults under heavy pre-fault load conditions,
the ground distance elements utilize a load-adaptive (zero-sequence polarized)
reactance characteristic to supervise the base mho characteristic
• Both negative and zero-sequence currents are compared with the memory voltage to
enhance directional integrity of the ground distance elements
• It is well-known that ground distance elements—as per the principle of distance
relaying—can have limited accuracy during double-line-to-ground faults. To prevent
maloperation in such cases, the ground elements are blocked by an extra fault-type
comparator that utilizes the phase angle between the negative-sequence and zero-
sequence currents.
• The phase distance elements use reactance and memory polarized directional
characteristics to supervise the mho characteristic
• Both ground and phase distance elements have the current supervision functions
built-in
The quadrilateral distance characteristic uses the reactance, directional, and current
supervising functions described above. Adjustable right and left blinders to both resistive
and angular positions complete the characteristic.
Additional information regarding the distance characteristics is found in Distance
characteristics on page 630. An example of analysis of the steady-state operation of the
distance elements is found in Distance elements analysis
on page 637.
The D90
Plus
provides three zones of distance protection. All zones are identical in terms of
settings. Ground zones 1 through 3 have an extra zero-sequence directional supervision
implemented for their time-delayed operation after the memory expires. Consequently,
zone 1 is recommended as an underreaching element, and zones 2 through 3 are
recommended as overreaching elements and for time-delayed tripping.
Offset ground directional overcurrent functions are used as an optional supplement of the
ground distance protection for pilot-aided schemes. The elements are described in detail in
Ground directional overcurrent theory
on page 647.
An adaptive distance reach control is provided to cope with the overreaching and sub-
synchronous oscillations when applied to, or in a near vicinity of, series compensated lines.
For more, see Series compensated lines
on page 649.
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