27847840 Distance Protection Utility Main Protection for Transmision Lines
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Transcript of 27847840 Distance Protection Utility Main Protection for Transmision Lines
Distance Protection --- JAVED 1
DISTANCE PROTECTION
UTILITY MAIN TRANSMISSION LINE PROTECTION
(S.R. Javed Ahmed)
INTRODUCTION
Distance protections have been used universally as Short circuit Protection for almost all MV to UHV AC
Transmission lines.
In the past it was the only type of Protection used for Long EHV Overhead Transmission Lines.
This Protection was introduced in early 1920’s and has undergone continuous enhancement ever since. It is
applicable for radial lines as well as interconnected network system of lines.
Application of Differential Protection in the past was restricted due to analog technology coupled with length of
the Transmission line. For Short to medium length lines, however, Distance Protection along with Differential
Protection was best solution.
In a classical Transmission system, the Distance Protection works by utilizing the fact that the measured
Impedance from a point is directly proportional to the distance from it (which gave its name). This Protection
measures the Short circuit Impedance at its location and operates by comparing it with the setting impedance.
It is also used occasionally for protecting equipment with large inductive reactance like Power Transformers,
Shunt Reactors, Generators, and Unit Transformers. It is also useful in systems with huge variation in fault levels
from maximum to minimum where traditional Over current Protections are not quite successful.
Distance relays have undergone continuous development. Distance Protections have transformed from early
relays with Induction Disk elements to moving coil technology then to static relays with operation Amplifiers,
static electronic PCBs to Microprocessor based static with numerous discrete static electronic PCBs to fully
microprocessor based Numerical with DSPs and finally to present day digital IEDs with numerical filters,
conversion, storing & computation….and near future to Distance Protection IEDs with total automation down to
individual Logical node level with digital CT & VT connected to process bus of a typical total Automation system.
Numerical devices with advances in digital technology (A/D converters, digital filters, storing & processing data)
have become more intelligent and adaptive to system and have introduced new concepts and features like
events, disturbance & fault recording along with GPS signal reference.
I, like many Protection Engineers, am lucky enough (is it!!) to have worked with all types of Distance protections
right from early days, over the years. Wondering with awe (during early days of my career as protection
Engineer) at huge Electro-mechanical Phase distance relays, Ground distance relays in so many schemes both
switched, non switched and in combined hard wired zone-based/full-zone schemes along with scores of
ancillary devices associated with Power line carrier aided schemes.
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PROBLEMS ASSOCIATED WITH DISTANCE PROTECTIONS
From Utility Protection engineers’ point of view, the Distance Protection is the most dreaded of all Protections!!!
Right from the day of its birth, Distance Protection never stopped giving surprise problems. No matter how hard
you worked, calculated meticulously, something or the other goes wrong.
Different types of faults need different voltage & current inputs and measures different loop impedances, uses
different principles, meaning more components. Problems associated with fault resistance, transients in VT
circuits (CVTs), power swings, load encroachment, in-feeds, current reversals etc
It often puts the Utility Protection Engineers in highly embarrassing situations by tripping when it should not and
failing to trip when it is required to trip. More often it leads to huge hours spent in testing, fault analysis,
sequence of events analysis and not to mention preparing disturbance/fault & …problem-solution reports to
satisfy the ‘guys above’ ...bear in mind …it shall not repeat such an incident after a solution proposed by the
Protection Engineer is implemented!!!..to avoid a wrath....There are ‘baddie Guys’ (non-technical sort of guys
with loud mouth and well paid!!!) who just keep statistical records of ‘mal-operation, reason, date by date, line
by line’….will promptly come with a list…after another event!!! ….ok, ok, just joking…(psst…it is a fact more
often)
Worst scenario like ‘Total Blackouts’ and multiple events create havoc in the Utility and everybody right from a
shift Power Dispatcher to a ‘BIG’ Customer breaths fire…but Protection Engineer is often protected by his sound
technical knowledge….he..he…and often has last laugh.
Occasionally, it also puts the Protection Manufacturer’s Product design Engineers under constant demand of
improvement…..sometimes leads to Utilities blacklisting his product…until revised product comes out…only to
be caught again by another different incident…..it is a cycle. That’s why Distance Protection has undergone most
developments over the years compared with other protections.
A real distance protection setting nightmare problem for you…..a newly constructed Substation with about 19
(EHV & HV) lines connecting to it (major substation with two large Power Plants as main feed and heavy
interconnection from other two major power plants)…due to right of way and terrain, all lines were running
parallel for few 10s of kilometers. .. some lines were also JUST Parallel unrelated with the new substation but
interconnecting some existing EHV substations (some weak, some strong, different positive sequence sources
and zero sequence sources, with different direction of currents during fault and different level of coupling some
positive some negative)……no communication aided scheme due to terminal MUX not ready….When the
substation & Power plant was ready, most of the remote EHV Substations were not ready yet….The ‘big guys’
issued ultimatum to energize ‘some of the lines’ which were ready and run the power plant...you guessed
it…yes, that’s it … most of the parallel lines were open and grounded at both ends....now go and set the Distance
protection with optimum zero-sequence mutual impedance…and must cover at least half line length….EHV lines
with four bundle conductors….no impedance calculation software can solve so many parallel lines with bundle
conductors….remember, no false operation is allowed!!!!!..to top it all, humid & saline atmosphere in
desert….high resistivity grounds….. Well that’s challenging
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SOLUTION AT LAST?
Fault loop impedances often fall in many zone reaches of Distance protection. To facilitate positive operation of
communication aided schemes, it is sometimes essential to set over-reaching zone considerable larger than the
line. Such settings with parallel lines, carrying huge power may cause un-faulted loop impedances to fall within
reach of a healthy parallel line at one end and correct faulted impedance loop reach at other end of healthy line.
In such a case, when common non-segregated phase communication is used, may result in tripping of healthy
line and faulted line.
In the past, due to analog technology, Protection Engineers were bound by the limitation of Line Differential
Protection. Finally there is relief to Protection Engineer… With the Numerical Technology in Protection coupled
with High-density, high-speed digital Telecommunication, and GPS clock signaling for public use, finally Line
Differential has become most suitable protection with Distance as a back-up (line length is no more a limitation
for Differential Protection)….Or is it? Distance Protection is still indispensible, no matter what, is still complex as
it was… it has to deal with many zones and has to calculate impedances for each phase-phase & phase-earth
loop on per zone basis and produce its final output as fast as ½ a cycle in a phase selective manner. Large
parallel Processors are required to perform measurement and all tasks within the required speed limit. This
requires more demand on processing.
Software, it is.
Unlike, earlier generation Protection Product Designers, new generation Protection Product designers are with
more software based knowledge compared with electrical technology based knowledge. Thereby, keep adding
feature after feature to the Protection to solve all known problems. Well, their job is done happy lot….they
are….software guys.
Now the product lands with Utility Protection Engineer to set the protection. Each and every setting value is
with selectable value and huge range!!! …and hundreds of parameters per protection….and tens of different
functions!!!
Earlier generation product design engineers were limited by static components and hence scheme settings were
more or less fixed. Product manufacturer was solely responsible for the proper operation of Protection. Now the
table has turned around, Protection Engineer has to set few hundreds of parameters each selectable in a huge
range. Again Protection Engineer is under stress. He must deal with Electrical system knowledge as well as every
manufacturer’s relay manual (to make matter worse every manufacturer has own algorithm and way of
approach). A single setting error out of thousand setting parameter might cause embarrassment to him
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GENERAL AREAS OF INTERST IN REGARD TO A DISTANCE PROTECTION:
Distance Protection has some disadvantages when compared with a Line Differential Protection. Following
points list out some points of interest in regard to a Distance Protection.
1. NEED FOR VOLTAGE INPUT: Distance Protection requires Voltage inputs (e.g. CVT, EMVT etc) in
addition to the Current inputs.
2. Fuses failed or removed: Loss of VT input or VT secondary fuses removed causes Distance Protection to
get blocked or false operate. Mho type Distance protection with no offset finds the impedance locus at
the origin of R-X plane (non-operative). Line energized without VT fuses will leave the relay without any
reference pre-fault voltage. Fuse/VT circuit supervision schemes are almost always applied in all
Distance Protection based on different principles (often dedicated external relays/high-speed auxiliary
contacts of MCB/Fuse are used additionally).
3. LOSS DIRECTIONALITY or FAILURE TO OPERATE: Distance Protection determines the direction of fault
based on the Voltage as well as current inputs (which is dependent on the phase angle between the
two). A close-in 3-phase fault removes the reference voltage which is required for directionality.
This aspect is a major drawback since a close-in fault (with small voltage signal and large noise signal
superimposed) may cause it to become non-operative or lose direction discriminating ability (to
determine a fault whether forward or backward). Most relays or schemes are provided for detection of
these ‘Zero-Volt’ faults.
4. REACH ERROR (OVER/UNDER REACHING): Distance Protection is non-unit type protection with its
boundary depends on system dynamics. Where as a Line Differential Protection is a Unit type
Protection with fixed boundary. Pre-fault load flow, errors in inputs (CTs, VTs), errors in impedance
values, errors due to earth fault loop impedance, condition of parallel line (open at at least one end or
grounded at both ends), zero-sequence mutual coupling, taps on the line etc cause the relay to
measure wrong impedance compared to actual with respect to location.
5. POWER SWINGS: With sources at both ends of a line, Distance Protections are often affected by Power
swings. Being a function of Voltage, Current & the angle between the two, Power swings cause the
same impedance which the Distance protection calculates to vary as a function of three parameters.
During a fault the impedance changes suddenly. While during a swing it changes slowly as two ends
respond based on stored energy interchange (mechanical/electrical) and associated fast acting control
systems.
6. Presence of series capacitor in compensated EHV line: Capacitive reactance being opposite in sign to
an Inductive reactance on which a distance relay reach is normally set. Thereby, the voltage & currents
measured by the relay depends on the amount of involved L & C up to the fault makes the relay
measure incorrect distance (impedance of line).
7. HIGH-SPEED DISTANCE Protection: As the distance Protection of EHV line (due to system stability
requirement) needs to be very fast, it is called up on to operate when the transients & dc components
in the primary system are at highest level. Most Distance relays require fundamental frequency voltage
& currents for determination of direction as well as impedance. When the fundamental component is
small compared to the dc & harmonic component, relay measures incorrect impedance &/or direction.
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Electromechanical relays, being slower were not as much affected as numerical relays (due to speed at
which the decision is to be made is well inside initial transient period for high-speed numerical relays).
This imposes increased demand on performances of CTs and VTs. CVTs (or CCVTs), due to the involved
L-C circuit introduces additional transients in the secondary signals (causing secondary voltages
different from actual primary system voltage). IEC60044 introduced additional accuracy classes for CTs
& CVTs transient performances for high-speed relays. In case of Auto-reclosing, unidirectional flux in CT
due to fault before auto-reclosing and its decay over the dead time (due to decaying secondary
current) introduces additional requirement on the CT performance.
8. Fault Resistance: Fault loops normally involve a component of resistance unless it is solid fault. That
component is most times difficult to predict. Even though a distance protection is set on the basis of
reactance, fault resistance will have an effect on overall characteristics of the distance protection as
the load impedance in parallel to the fault impedance or source impedance parallel to the fault
resistance causes the reactance line to tilt causing some under/over reaching problem depending on
the location of load (in the direction of line or reverse as seen by the relay).
9. Residual compensation: Distance relay reaches are set based on positive sequence impedances.
However, a fault with earth involved will bring the zero sequence component of impedance. More
often the residual current obtained by adding phase currents (Ia, Ib & Ic) are not same as actual 3I0
(earth fault current) at the relay location. This mismatch is due to the source of zero sequence current
may be from different equipment (example a Y-grounded /D transformer or a Zig-Zag Transformer.
Secondly, the earth fault loop zero sequence impedances may not be linear all along the length. This
component is necessary to be considered. Again, based on the parallel line current & or depending on
whether parallel line is grounded at both ends or not makes this zero sequence component to either
increase or decrease. Distance relay measures incorrectly in such cases.
10. Cables are not welcome: For the same reason as the zero sequence compensation, an EHV cable being
designed with armor, sheath etc which always provide a metallic return path unlike an Overhead line.
Thus the zero sequence impedance becomes lesser than the positive sequence impedance. This calls
for negative compensation for earth faults, which becomes non practical.
11. Measured Impedances: Most often line impedances used for relay setting are not accurate. Most of
the times are assumed based on existing similar circuit. These assumed parameters of R, X
(positive=sequence, zero-sequence & and mutual) are not accurate. Self impedance of line will be
symmetrical & correct, but all other components are unsymmetrical (depends on the location of phases
with each other & above the earth). Settings are made only based on symmetrical values. For more
accurate impedances, each phase and each line is required to be measured (considered with/without
ground) to accurately estimate down to small percentage error of 3-4% of total error. Again, 5% error
for 300km line is like 15km error in fault location. To find a permanent fault in
rough/uninhibited/hostile area over huge distance of uncertainty is definitely not acceptable in many
cases.
12. Measuring loops: Faults in power systems generally fall in to two categories namely Short circuits
(Shunt faults) or Open circuits (Series faults). Short circuit themselves are with or without ground
involved. In all eleven types of faults occur…phase-phase (three..AB, BC & CA), phase-Earth (three…A-G,
B-G & C-G). Phase-phase-earth (three…AB-G, BC-G, CA-G), 3phase-G and solid 3-phase. Most common
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are Ph-G type in HV/EHV overhead systems. 3-phase faults are rarest in EHV systems due to large
clearances. Distance relay measures six measuring loops (three phase-phase & three phase-earth
loops). A Solid Three phase fault involves straight forward symmetrical calculation and fastest of all in
terms of computation time. Most complex is phase-phase-earth. Most Distance protections face
problems in these fault computation.
13. Characteristics: Distance Protection characteristics come in all shapes. Earliest electro-mechanical ones
were with simple circle (center at the origin) & straight line. These were easiest to construct with the
use of electromagnetic. Inherently they were non-directional (all working either as under or over a
setting value). With slight modification to the operating & restraining inputs, Mho circle came into
existence and has been the most common and easiest to achieve. Mho characteristics are fastest as
require one computation only. As the line length increases, Mho circle became quite large to reach to
heavy loads. This lead to clipping it with another characteristic. With static technology, a resistance
characteristic was used as it needs a solution of straight line in addition to a mho circle. With Numerical
relays, some manufacturers developed special characteristics which is applicable to a particular load
power factor limit. Each characteristic has its own merits and demerits.
14. Short line-Long line: There are two points on the Distance protection characteristics which are of
extreme importance and all distance protections are judged based on its performance at these two
points. One of them is the origin in R-X plane (relay location) and other one is the Zone-1 reach point.
These two points represent the Distance protection boundaries for instantaneous trip. The point at the
origin is close-in fault (either forward or reverse). A close in fault is important as an instantaneous
distance protection has to be stable on a reverse fault but must operate for a forward fault. The
currents could be quite large in a close-in but the voltage is zero (in extreme case of solid fault).
Without voltage signal it is not possible to determine the fault direction (whether forward or reverse).
Secondly, fault at zone-1 reach is very critical in terms of stability. Zone-1 reach is ‘definite point on the
protected line’ (even if errors added like CT/VT errors and impedance error, zone-1 reach point never
go to next line/equipment at remote substation). As the line length decreases the voltage at the relay
location during a zone-1 reach fault gets smaller. In extremely short line, the voltage becomes
significantly short to allow distance protection to operate accurately.
As per standards (ANSI), the Transmission line is categorized as short, medium or long based on the
system impedance ratio (SIR). SIR is the ratio of source impedance (Zs) to line impedance (Zl) at the
relay location *i.e. SIR = Zs/Zl+. A Short line is a line with SIR ≥4. Medium line is a line with SIR value
which lies between 0.5 & 4. A long line is with SIR ≤0.5. Thus, a short line may be a considerable longer
in actual length but has weak source (large Zs) making Zs/Zl larger than 4. The voltage signal available
at the relay location during a fault decreases as the source gets weaker (within time shorter than
exciter response). A limit will be reached for any distance protection with reduced voltage at relay
location for a zone-1 fault to be reliable as a function of SIR. In these extreme short line cases, distance
protection becomes non operative. Further, the CT needs to be of higher quality for preventing
harmonics introduced in the secondary current (harmonic components are filtered out by the filters
making lesser fundamental current signal for relay measurement….causing the relay to incorrectly
measure the distance as larger than zone reach---under reaching occurs).
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Well, some sources of trouble for a Distance protection are seen as above. There are lots more based on
the communication schemes.
Most of the problems mentioned above are not applicable to Line differential Protection. Line Differential
Protection, however, has more serious problem associated with it than the Distance protection. It is the
requirement of phase current from remote end without addition of time delay to the local measured
current. If communication fails, differential protection totally fails unlike a distance protection (which can
work as a plain step distance protection as a backup). Some form of backup is always essential in a
differential protection. As the line length becomes significant, the demand on the communication system
speed becomes critical in a differential protection. And some communication media like a Power line carrier
(PLC) is never applied to a line protection (since the signal loss or distortion is due to the loss/problem with
wave guide (the faulted line itself!)…so when it is really required to operate it is distorted!!!! Lastly,
charging current flowing into line but not leaving it (this can happen at one end or both ends…based on
…where there is source) will conflict with fundamental of line differential protection which is based on
principle that current always enters and leaves the line when not faulted. This becomes enormous value in
long EHV systems and long cables. Sometimes, steady state charging current (sine) is larger than minimum
fault current, making the line differential insensitive to faults. To compensate for the charging current, a VT
signal is required (at one or both ends based on the source) and knowledge of positive & zero sequence
capacitance of the protected circuit.
Thanks to Numerical software guy….he puts Differential protection with built in distance protection in
it…without much addition to hardware (other than VT input).
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TRADITIONAL DISTANCE PROTECTIONS
Different principles were adopted for making distance protection based on the operating philosophies.
Historically, Static Distance protections were two types. These are Full scheme & switched schemes. A Full
scheme generally had six measuring loops for each zone. A switched scheme consists of one measuring
element per zone with inputs switched (based on type of fault as detected by some means of fault detection
scheme). Phase to earth faults require faulted phase voltage and phase current for computation. Phase to
phase requires delta voltages & delta currents (vector difference).
Rectified Bridge comparator was used by German manufacturers with Isc (short circuit current in secondary at
relay location) as operating input and Usc/R as restraining input (where Usc is short circuit loop voltage at relay
location & R is set replica impedance reach in secondary ohms). The relay operates when operating quantity
exceeds the restraining quantity. Relay boundary at zone reach is where two quantities are equal and origin is
where Usc is zero [Operation occurs when Isc > Usc/R]
Electromechanical Distance Protections due to hard wired schemes, were built as separate units one for each
type of fault and per zone. To achieve total protection, three phase-phase distance protections, three phase-
earth distance protections were used per zone with electromagnetic operation. Thus an EHV line used to have
18 units of discrete devices mounted on the panels along with ancillary for out of step blocking, communication
schemes.
Ferraris Induction cup relays were used in the US for electromechanical distance protection with Isc producing
the operating flux, Usc producing the restraining flux and a polarizing flux produced by shifted Usc. Since the
torque due to interaction of operating & restraining fluxes act on opposition on the induction cup, polarizing
flux is essential to cause rotation of cup. *operation occurs when (Usc x Isc x cos (ф-ѳ)) is ≤ (Usc²/R)
In some analog static distance measurement, angle comparison (phase) was most commonly used. Mho circle
is produced by measuring the angle between two quantities….angle between differential voltage (Isc x R – Usc)
and Usc.
Various Distance Tele-protection schemes were commonly used like Permissive (over & under reaching)
schemes, blocking schemes and accelerating schemes. Choice of scheme was based on available
communication system, most of the time and was hard wired for a particular scheme including the
communication system interfaces, channels, frequency, band width etc. Various media like Microwave, PLC,
audio-tone/voice etc were used. There was distance related errors involved. With the advancement in cheaper
optical fiber technology for digital signaling, high-speed communications become reality. Overhead metallic
ground wires are gradually replaced with metallic ground wires with optical fibers within the core (OPGW,
optical ground wire). Optical fiber communication opened up newer areas due to Higher band width and speed
and technology. Synchronous digital communication with higher bit rates like gigabits made possible extremely
reliable communication system (in addition features like packet based add/drop, ring topography made system
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self heal in event of loss of signals). Distance protection unlike differential protection requires only ‘GO/NO-GO’
status transmission and extreme accuracy is possible (line differential requires phase currents each with
specific time stamp).
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NUMERICAL DISTANCE PROTECTION
Modern Numerical Distance protections are multifunction devices with discreet signal processing and
numerical computation. These have numerical filter algorithm to reject non-fundamental components in signal
inputs. Furthermore, three single-phase communication aided schemes become reality which fundamentally
removed errors due to incorrect fault loops used in direction comparison at two ends. Selectivity, speed,
sensitivity and reliability increased.
Definition of Numerical Distance Protection:
A Numerical Distance Protection is a Distance Protection which utilizes microprocessor technology and analog
to digital conversion of measured currents & voltages and computes the distance.
Most Numerical distance protections have additional time domain based calculations (high-speed) to
complement frequency domain calculations.
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DISTANCE MEASUREMENT
DEVICE NUMBER:
Distance Protections, like most other relays are secondary system connected devices. ANSI/IEEE device number
21 is assigned for it. Additional letter may be added to it to distinguish a phase & ground distance protection
such as 21P & 21N respectively. There is no hard and fast rule as to which letter is added after 21 as long as it is
mentioned as abbreviation in respective drawings/documents where it is used.
SECONDARY IMPEDANCES:
Most Distance Protections require settings in terms of Secondary impedances (Zsec).
Secondary impedance (Zsec) can be calculated from primary impedance (Zprim) as below:
Zsec = Zprim x (CTR/PTR) = Zprim x k
Where;
CTR is CT ratio used (= Iprim/Isec); Iprim & Isec are CT rated primary & secondary currents at used tap
PTR is PT ratio used (Uprim/Usec); Uprim & Usec are PT rated primary & secondary voltages at used tap
Example:
A system with 115/0.115kV PT Ratio and 1000/1A CT ratio returns k as = (115/0.115)/(1000/1) = 1000/1000 = 1
And hence Zsec = Zprim in this case
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Impedance diagram:
An impedance diagram is a graphical tool used to evaluate a distance protection. It has R & X axis (resistance &
reactance) with zero at the origin and four quadrants. First quadrant with R, X positive and so on….
This diagram indicates the relay location as at the origin (reference) source impedance below and line
impedance above R- axis.
Figure below shows a Distance protection (GE-D60) with three zones in R-X plane. R-axis is horizontal & X is
vertical axis). A Load region is also seen on the same plot. Various phase & earth Faults are marked in the
diagram also. The points within the circles indicate the zone which operates. Zone-1 in this case is smallest
circle, next larger is zone-2 followed by largest zone-3. It can be seen that a point within zone-1 also happens to
be in zone-2 & zone-3 (meaning all three zones pickup for that fault)
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Rabai-Galu 21N Type=D60G__CTR=600 PTR=1200 Zone 1: Z=8.38 sec Ohm @ 80.0 deg. T=0.0sZone 2: Z=15.61 sec Ohm @ 80.0 deg. T=0.3sZone 3: Z=29.24 sec Ohm @ 80.0 deg. T=1.0sLine Z= 10.48@ 80.0 sec Ohm ( 20.96 Ohm)More details in TTY window.
Rabai-Galu 21P Type=D60P__CTR=600 PTR=1200 Zone 1: Z=8.38 sec Ohm @ 80.0 deg. T=0.0sZone 2: Z=15.61 sec Ohm @ 80.0 deg. T=0.3sZone 3: Z=29.24 sec Ohm @ 80.0 deg. T=1.0sLine Z= 10.48@ 80.0 sec Ohm ( 20.96 Ohm)
FAULT DESCRIPTION:See table in TTY window.
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Basically, two types are characteristics are used In a modern Distance protection. They are Mho circle and a
polygon (quadrilateral).
Mho circle is a circle with diameter same as the setting reach. Two ends of a Mho circle diameter being the
origin & zone reach. Since the close-in fault happens to be at the origin, Mho circle is non-operative at that
point (zero voltage at the fault). In order to cover that point, healthy phase voltage is used in some relays
(which is rotated to the faulted phase angle) as polarizing voltage. These are called cross polarized mho circles.
Advantage of cross polarization is additional resistance coverage as the diameter increases with origin shifted
in to third quadrant. There are partial cross polarized and fully cross polarized Mho circles. These are helpful for
faults other than 3-phase solid faults. In case of 3-phase solid faults all three phase voltages becomes zero and
hence makes distance protection non-operative. Some means of pre-fault voltage (from stored memory) is
normally used. In static relays, the amount of memory was limited to few cycles.
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Most of modern Pilot schemes use digital technology and Fiber optic media Traditional PLC scheme is as below:
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CRITICAL SETTINGS:
Two Zone reaches are of important for protecting a Transmission line. These are Zone-1 & Zone-2.
Setting both zone-1 & zone-2 reach is a critical item for a Protection Engineer. Once these two zones are set
correctly, the line is protected.
It is important to see that in case of lines with sources at both ends Zone-1 shall overlap. This means at least
more than half line is covered in zone-1 at both ends to have instantaneous tripping at both ends.
In case of radial line, without any parallel line, zone-1 can be longer if it feeds single line or equipment at
remote end.
Zone-1 is usually set somewhere between 80 & 85 % of the line impedance. Balance of line must be covered in
Zone-2. Thus Zone-2 must cover at least 120% in all cases.
It works well for phase faults. But with earth fault, the loop impedance comprises of all three sequence
impedances in series with Zero sequence impedance affected by the mutual coupling with parallel line. With
parallel line grounded at both ends, the net zero sequence impedance gets reduced due to mutual coupling as:
Z0act = Z0act –(Z0m²/Z0act); where Z0act is actual zero sequence impedance and Z0m is zero-sequence
mutual impedance.
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ZONE-1:
It turns out that sometimes Z0act gets so small that zone-1 becomes too short to overlap the remote end zone-1.
For a phase to ground fault the loop impedance is = (Z1+Z2+Z0)/3; where Z1=Z2 for a Transmission line]. A
Distance relay set based on Z1 (positive sequence impedance) will measure phase to ground fault the loop
impedance as = Z1 + (kn xZ1); where Kn is earth fault compensation factor given by kn = (Z0-Z1)/(3 x Z1)
From the phase to ground faulted voltage e.g. Va (in case of phase A fault to ground) and phase A current Ia,
the relay compares quantity Va/Ia with set reach of [Z1set x (1+kn)]. Relay operates if Va/Ia is less than [Z1set x
(1+kn)]
To avoid over reaching zone-1, kn is set smaller based on Z0act (affected due to Z0m; mutual).
It is required therefore, to have Z1set x (1+kn) always more than 50% of total line [(Z1+Z2+Z0)/3]
If this becomes less than 50%, it is preferred to have line differential protection or another scheme based
distance protection like pilot scheme to cover all points on the line.
ZONE-2:
When used as backup zone for line must cover 120% in the worst case. If next line happens to be too short,
then Zone-2 with 120% setting may over reach zone-2 of the next line at the remote end. Zone-2 in this case
will mis-coordinate with second line zone-2 at remote end substation. Additional time delay may become
necessary to discriminate between two Zone-2 reaches.
When the line has a tap load, and sources at both ends, the infeed current from remote end causes the reach
to become under reaching.
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SOME LITERATURES ON DISTANCE PROTECTION
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Distance Protection --- JAVED 38
Distance Protection --- JAVED 39
Distance Protection --- JAVED 40
Distance Protection --- JAVED 41
Distance Protection --- JAVED 42
Distance Protection --- JAVED 43
Distance Protection --- JAVED 44
Distance Protection --- JAVED 45
Distance Protection --- JAVED 46
Distance Protection --- JAVED 47
Distance Protection --- JAVED 48
Distance Protection --- JAVED 49
FUNDAMENTALS OF LINE PROTECTION- LITERATURE
Distance Protection --- JAVED 50
Distance Protection --- JAVED 51
Distance Protection --- JAVED 52
Distance Protection --- JAVED 53
Distance Protection --- JAVED 54
Distance Protection --- JAVED 55
Distance Protection --- JAVED 56
Distance Protection --- JAVED 57
Distance Protection --- JAVED 58
Distance Protection --- JAVED 59
Distance Protection --- JAVED 60
Distance Protection --- JAVED 61
Distance Protection --- JAVED 62
Distance Protection --- JAVED 63
Distance Protection --- JAVED 64
Distance Protection --- JAVED 65
Distance Protection --- JAVED 66
Distance Protection --- JAVED 67
Distance Protection --- JAVED 68
Distance Protection --- JAVED 69
Distance Protection --- JAVED 70
Distance Protection --- JAVED 71
Distance Protection --- JAVED 72
Distance Protection --- JAVED 73
Distance Protection --- JAVED 74
Distance Protection --- JAVED 75
Distance Protection --- JAVED 76
Distance Protection --- JAVED 77
Distance Protection --- JAVED 78
Distance Protection --- JAVED 79
Distance Protection --- JAVED 80
Distance Protection --- JAVED 81
Distance Protection --- JAVED 82
Distance Protection --- JAVED 83
Distance Protection --- JAVED 84
Distance Protection --- JAVED 85
Distance Protection --- JAVED 86
Distance Protection --- JAVED 87
Distance Protection --- JAVED 88
Distance Protection --- JAVED 89
Distance Protection --- JAVED 90
Distance Protection --- JAVED 91
Distance Protection --- JAVED 92
Distance Protection --- JAVED 93
Distance Protection --- JAVED 94
Distance Protection --- JAVED 95
Distance Protection --- JAVED 96
Distance Protection --- JAVED 97
Distance Protection --- JAVED 98
Distance Protection --- JAVED 99
Distance Protection --- JAVED 100
MORE ON DISTANCE PROTECTION
Distance Protection --- JAVED 101
Distance Protection --- JAVED 102
Distance Protection --- JAVED 103
Distance Protection --- JAVED 104
Distance Protection --- JAVED 105
Distance Protection --- JAVED 106
Distance Protection --- JAVED 107
Distance Protection --- JAVED 108
Distance Protection --- JAVED 109
Distance Protection --- JAVED 110
Distance Protection --- JAVED 111
Distance Protection --- JAVED 112
Distance Protection --- JAVED 113
Distance Protection --- JAVED 114
Distance Protection --- JAVED 115
Distance Protection --- JAVED 116
Distance Protection --- JAVED 117
Distance Protection --- JAVED 118
Distance Protection --- JAVED 119
Distance Protection --- JAVED 120
Distance Protection --- JAVED 121
Distance Protection --- JAVED 122
Distance Protection --- JAVED 123
Distance Protection --- JAVED 124
Distance Protection --- JAVED 125
Distance Protection --- JAVED 126
Distance Protection --- JAVED 127
Distance Protection --- JAVED 128
Distance Protection --- JAVED 129
Distance Protection --- JAVED 130
Distance Protection --- JAVED 131
Distance Protection --- JAVED 132
Distance Protection --- JAVED 133
Distance Protection --- JAVED 134
Distance Protection --- JAVED 135
Distance Protection --- JAVED 136
Distance Protection --- JAVED 137
Distance Protection --- JAVED 138
Distance Protection --- JAVED 139
Distance Protection --- JAVED 140
Distance Protection --- JAVED 141
Distance Protection --- JAVED 142
Distance Protection --- JAVED 143
Distance Protection --- JAVED 144
Distance Protection --- JAVED 145
Distance Protection --- JAVED 146
Distance Protection --- JAVED 147
Distance Protection --- JAVED 148
Distance Protection --- JAVED 149
Distance Protection --- JAVED 150
Distance Protection --- JAVED 151
Distance Protection --- JAVED 152
Distance Protection --- JAVED 153
Distance Protection --- JAVED 154
Distance Protection --- JAVED 155
Distance Protection --- JAVED 156
Distance Protection --- JAVED 157
Distance Protection --- JAVED 158
Distance Protection --- JAVED 159
Distance Protection --- JAVED 160
Distance Protection --- JAVED 161
Distance Protection --- JAVED 162
Distance Protection --- JAVED 163
Distance Protection --- JAVED 164
Distance Protection --- JAVED 165
Distance Protection --- JAVED 166
Distance Protection --- JAVED 167
Distance Protection --- JAVED 168
Distance Protection --- JAVED 169
Distance Protection --- JAVED 170
Distance Protection --- JAVED 171
Distance Protection --- JAVED 172
Distance Protection --- JAVED 173
Distance Protection --- JAVED 174
Distance Protection --- JAVED 175
Distance Protection --- JAVED 176
Distance Protection --- JAVED 177
Distance Protection --- JAVED 178
Distance Protection --- JAVED 179
Distance Protection --- JAVED 180
Distance Protection --- JAVED 181
Distance Protection --- JAVED 182
Distance Protection --- JAVED 183
Distance Protection --- JAVED 184
Distance Protection --- JAVED 185
Distance Protection --- JAVED 186
Distance Protection --- JAVED 187
Distance Protection --- JAVED 188
Distance Protection --- JAVED 189
Distance Protection --- JAVED 190
Distance Protection --- JAVED 191
Distance Protection --- JAVED 192
Distance Protection --- JAVED 193
Distance Protection --- JAVED 194
Distance Protection --- JAVED 195
Distance Protection --- JAVED 196
Distance Protection --- JAVED 197
Distance Protection --- JAVED 198
Distance Protection --- JAVED 199
Distance Protection --- JAVED 200
Distance Protection --- JAVED 201
Distance Protection --- JAVED 202
Distance Protection --- JAVED 203
Distance Protection --- JAVED 204
Distance Protection --- JAVED 205
Distance Protection --- JAVED 206
Distance Protection --- JAVED 207
Distance Protection --- JAVED 208
Distance Protection --- JAVED 209
Distance Protection --- JAVED 210
Distance Protection --- JAVED 211
Distance Protection --- JAVED 212
Distance Protection --- JAVED 213
Distance Protection --- JAVED 214
Distance Protection --- JAVED 215
Distance Protection --- JAVED 216
Distance Protection --- JAVED 217
Distance Protection --- JAVED 218
Distance Protection --- JAVED 219
Distance Protection --- JAVED 220
Distance Protection --- JAVED 221
Distance Protection --- JAVED 222
Distance Protection --- JAVED 223
Distance Protection --- JAVED 224
Distance Protection --- JAVED 225
Distance Protection --- JAVED 226
Distance Protection --- JAVED 227
Distance Protection --- JAVED 228
Distance Protection --- JAVED 229
Distance Protection --- JAVED 230
Distance Protection --- JAVED 231
Distance Protection --- JAVED 232
Distance Protection --- JAVED 233
Distance Protection --- JAVED 234
Distance Protection --- JAVED 235
Distance Protection --- JAVED 236
Distance Protection --- JAVED 237
Distance Protection --- JAVED 238
Distance Protection --- JAVED 239
Distance Protection --- JAVED 240
Distance Protection --- JAVED 241
Distance Protection --- JAVED 242
Distance Protection --- JAVED 243
Distance Protection --- JAVED 244
Distance Protection --- JAVED 245
Distance Protection --- JAVED 246
Distance Protection --- JAVED 247
Distance Protection --- JAVED 248
Distance Protection --- JAVED 249
Distance Protection --- JAVED 250
Distance Protection --- JAVED 251
Distance Protection --- JAVED 252
Distance Protection --- JAVED 253
Distance Protection --- JAVED 254
Distance Protection --- JAVED 255
Distance Protection --- JAVED 256
Distance Protection --- JAVED 257
Distance Protection --- JAVED 258
Distance Protection --- JAVED 259
Distance Protection --- JAVED 260
Distance Protection --- JAVED 261
Distance Protection --- JAVED 262
Distance Protection --- JAVED 263
Distance Protection --- JAVED 264
Distance Protection --- JAVED 265
Distance Protection --- JAVED 266
Distance Protection --- JAVED 267
Distance Protection --- JAVED 268
Distance Protection --- JAVED 269
Distance Protection --- JAVED 270
Distance Protection --- JAVED 271
Distance Protection --- JAVED 272
Distance Protection --- JAVED 273
Distance Protection --- JAVED 274
Distance Protection --- JAVED 275
Distance Protection --- JAVED 276
Distance Protection --- JAVED 277
Distance Protection --- JAVED 278
Distance Protection --- JAVED 279
Distance Protection --- JAVED 280
Distance Protection --- JAVED 281
Distance Protection --- JAVED 282
Distance Protection --- JAVED 283
Distance Protection --- JAVED 284
Distance Protection --- JAVED 285
Distance Protection --- JAVED 286
Distance Protection --- JAVED 287
Distance Protection --- JAVED 288
Distance Protection --- JAVED 289
Distance Protection --- JAVED 290
Distance Protection --- JAVED 291
Distance Protection --- JAVED 292
Distance Protection --- JAVED 293
Distance Protection --- JAVED 294
Distance Protection --- JAVED 295
Distance Protection --- JAVED 296
Distance Protection --- JAVED 297
Distance Protection --- JAVED 298
Distance Protection --- JAVED 299
Distance Protection --- JAVED 300
Distance Protection --- JAVED 301
Distance Protection --- JAVED 302
Distance Protection --- JAVED 303
Distance Protection --- JAVED 304
Distance Protection --- JAVED 305
Distance Protection --- JAVED 306
Distance Protection --- JAVED 307
Distance Protection --- JAVED 308
Distance Protection --- JAVED 309
Distance Protection --- JAVED 310
Distance Protection --- JAVED 311
Distance Protection --- JAVED 312
Distance Protection --- JAVED 313
Distance Protection --- JAVED 314
Distance Protection --- JAVED 315
Distance Protection --- JAVED 316
Distance Protection --- JAVED 317
Distance Protection --- JAVED 318
Distance Protection --- JAVED 319
Distance Protection --- JAVED 320
Distance Protection --- JAVED 321
Distance Protection --- JAVED 322
Distance Protection --- JAVED 323
Distance Protection --- JAVED 324
Distance Protection --- JAVED 325
Distance Protection --- JAVED 326
Distance Protection --- JAVED 327
Distance Protection --- JAVED 328
Distance Protection --- JAVED 329
Distance Protection --- JAVED 330
Distance Protection --- JAVED 331
Distance Protection --- JAVED 332
Distance Protection --- JAVED 333
Distance Protection --- JAVED 334
Distance Protection --- JAVED 335
Distance Protection --- JAVED 336
Distance Protection --- JAVED 337
Distance Protection --- JAVED 338
Distance Protection --- JAVED 339
Distance Protection --- JAVED 340
Distance Protection --- JAVED 341
Distance Protection --- JAVED 342
Distance Protection --- JAVED 343
Distance Protection --- JAVED 344
Distance Protection --- JAVED 345
Distance Protection --- JAVED 346
Distance Protection --- JAVED 347
Distance Protection --- JAVED 348
Distance Protection --- JAVED 349
Distance Protection --- JAVED 350
Distance Protection --- JAVED 351
Distance Protection --- JAVED 352
Distance Protection --- JAVED 353
Distance Protection --- JAVED 354
Distance Protection --- JAVED 355
Distance Protection --- JAVED 356
Distance Protection --- JAVED 357
Distance Protection --- JAVED 358
Distance Protection --- JAVED 359
Distance Protection --- JAVED 360
Distance Protection --- JAVED 361
Distance Protection --- JAVED 362
Distance Protection --- JAVED 363
Distance Protection --- JAVED 364
Distance Protection --- JAVED 365
Distance Protection --- JAVED 366
Distance Protection --- JAVED 367
Distance Protection --- JAVED 368
Distance Protection --- JAVED 369
Distance Protection --- JAVED 370
Distance Protection --- JAVED 371
Distance Protection --- JAVED 372
Distance Protection --- JAVED 373
Distance Protection --- JAVED 374
Distance Protection --- JAVED 375
Distance Protection --- JAVED 376
Distance Protection --- JAVED 377
Distance Protection --- JAVED 378
Distance Protection --- JAVED 379
Distance Protection --- JAVED 380
Distance Protection --- JAVED 381
Distance Protection --- JAVED 382
Distance Protection --- JAVED 383
Distance Protection --- JAVED 384
Distance Protection --- JAVED 385
Distance Protection --- JAVED 386
Distance Protection --- JAVED 387
Distance Protection --- JAVED 388
Distance Protection --- JAVED 389
Distance Protection --- JAVED 390
Distance Protection --- JAVED 391
Distance Protection --- JAVED 392
Distance Protection --- JAVED 393
Distance Protection --- JAVED 394
Distance Protection --- JAVED 395
Distance Protection --- JAVED 396
Distance Protection --- JAVED 397
Distance Protection --- JAVED 398
Distance Protection --- JAVED 399
Distance Protection --- JAVED 400
Distance Protection --- JAVED 401
Distance Protection --- JAVED 402
Distance Protection --- JAVED 403
Distance Protection --- JAVED 404
Distance Protection --- JAVED 405
Distance Protection --- JAVED 406
Distance Protection --- JAVED 407
Distance Protection --- JAVED 408
Distance Protection --- JAVED 409
Distance Protection --- JAVED 410
Distance Protection --- JAVED 411
Distance Protection --- JAVED 412