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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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.
Distance Protection --- JAVED 2
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
Distance Protection --- JAVED 3
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
Distance Protection --- JAVED 4
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.
Distance Protection --- JAVED 5
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
Distance Protection --- JAVED 6
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).
Distance Protection --- JAVED 7
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).
Distance Protection --- JAVED 8
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