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.

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

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

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.

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

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).

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).

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

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).

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.

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

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)

-50 -40 -30 -20 -10 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 230 240

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.

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.

Most of modern Pilot schemes use digital technology and Fiber optic media Traditional PLC scheme is as below:

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.

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.

SOME LITERATURES ON DISTANCE PROTECTION

FUNDAMENTALS OF LINE PROTECTION- LITERATURE

27847840 Distance Protection Utility Main Protection for Transmision Lines

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