16-10-2012, 12:39 PM
Distance Protection
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INTRODUCTION
The problem of combining fast fault clearance with
selective tripping of plant is a key aim for the protection
of power systems. To meet these requirements, highspeed
protection systems for transmission and primary
distribution circuits that are suitable for use with the
automatic reclosure of circuit breakers are under
continuous development and are very widely applied.
Distance protection, in its basic form, is a non-unit
system of protection offering considerable economic and
technical advantages. Unlike phase and neutral
overcurrent protection, the key advantage of distance
protection is that its fault coverage of the protected
circuit is virtually independent of source impedance
variations.
PRINCIPLES OF DISTANCE RELAYS
Since the impedance of a transmission line is
proportional to its length, for distance measurement it is
appropriate to use a relay capable of measuring the
impedance of a line up to a predetermined point (the
reach point). Such a relay is described as a distance relay
and is designed to operate only for faults occurring
between the relay location and the selected reach point,
thus giving discrimination for faults that may occur in
different line sections.
The basic principle of distance protection involves the
division of the voltage at the relaying point by the
measured current. The apparent impedance so
calculated is compared with the reach point impedance.
If the measured impedance is less than the reach point
impedance, it is assumed that a fault exists on the line
between the relay and the reach point.
The reach point of a relay is the point along the line
impedance locus that is intersected by the boundary
characteristic of the relay. Since this is dependent on the
ratio of voltage and current and the phase angle
between them, it may be plotted on an R/X diagram. The
loci of power system impedances as seen by the relay
during faults, power swings and load variations may be
plotted on the same diagram and in this manner the
performance of the relay in the presence of system faults
and disturbances may be studied.
RELAY PERFORMANCE
Distance relay performance is defined in terms of reach
accuracy and operating time. Reach accuracy is a
comparison of the actual ohmic reach of the relay under
practical conditions with the relay setting value in ohms.
Reach accuracy particularly depends on the level of
voltage presented to the relay under fault conditions.
The impedance measuring techniques employed in
particular relay designs also have an impact.
Operating times can vary with fault current, with fault
position relative to the relay setting, and with the point
on the voltage wave at which the fault occurs.
Depending on the measuring techniques employed in a
particular relay design, measuring signal transient errors.
Digital/Numerical Distance Relays
Digital/Numerical distance relays tend to have more
consistent operating times. They are usually slightly
slower than some of the older relay designs when
operating under the best conditions, but their maximum
operating times are also less under adverse waveform
conditions or for boundary fault conditions.
VOLTAGE LIMIT FOR ACCURATE
REACH POINT MEASUREMENT
The ability of a distance relay to measure accurately for
a reach point fault depends on the minimum voltage at
the relay location under this condition being above a
declared value. This voltage, which depends on the relay
design, can also be quoted in terms of an equivalent
maximum ZS/ZL or S.I.R.
Distance relays are designed so that, provided the reach
point voltage criterion is met, any increased measuring
errors for faults closer to the relay will not prevent relay
operation. Most modern relays are provided with healthy
phase voltage polarisation and/or memory voltage
polarisation. The prime purpose of the relay polarising
voltage is to ensure correct relay directional response for
close-up faults, in the forward or reverse direction,
where the fault-loop voltage measured by the relay may
be very small.
ZONES OF PROTECTION
Careful selection of the reach settings and tripping times
for the various zones of measurement enables correct coordination
between distance relays on a power system.
Basic distance protection will comprise instantaneous
directional Zone 1 protection and one or more timedelayed
zones. Typical reach and time settings for a 3-
zone distance protection are shown in Figure 11.6. Digital
and numerical distance relays may have up to five zones,
some set to measure in the reverse direction. Typical
settings for three forward-looking zones of basic distance
protection are given in the following sub-sections. To
determine the settings for a particular relay design or for
a particular distance teleprotection scheme, involving
end-to-end signalling, the relay manufacturer’s
instructions should be referred to.
DISTANCE RELAY CHARACTERISTICS
Some numerical relays measure the absolute fault
impedance and then determine whether operation is
required according to impedance boundaries defined on
the R/X diagram. Traditional distance relays and
numerical relays that emulate the impedance elements
of traditional relays do not measure absolute impedance.
They compare the measured fault voltage with a replica
voltage derived from the fault current and the zone
impedance setting to determine whether the fault is
within zone or out-of-zone. Distance relay impedance
comparators or algorithms which emulate traditional
comparators are classified according to their polar
characteristics, the number of signal inputs they have,
and the method by which signal comparisons are made.
The common types compare either the relative amplitude
or phase of two input quantities to obtain operating
characteristics that are either straight lines or circles
when plotted on an R/X diagram.