fundamentals related to distance relays

Knowledge Is PowerSM

Apparatus Maintenance and Power Management for Energy Delivery

Introduction to Distance Protection

Jay GosaliaVice President of Marketing

Doble Engineering Company



Objective of Relay Protection

Protect persons and equipment in the surrounding of the power system

Protect apparatus in the power system

Separate faulty parts from the rest of the power system to facilitate the operation of the healthy part of the system



Electrical Fault in Power System

Transmission lines 85%Busbar 12%Transformer/ Generator 03%

85%

12% 3%

Transmission lines Busbar TFR/Gen



Fault Statistics

Single phase to earth 80%Two phases to earth 10%Phase to phase faults 5%Three phase faults 5%

80%

10%5% 5%

Ph-G Flt Ph-Ph-G Flt Ph-Ph Flt 3 Ph Flt



Fault Type in Transmission Line

Transient faultsCommon on transmission lines, approximately 80-85%Lightnings are the most common reasonCaused by birds, falling trees, forest growth,Swinging lines, High velocity winds etc.

• Disappear after a short dead interval

Persistent faultsCaused by a broken conductor fallen downTree falling on a line

• Must be located and repaired before normal service



Protection Types

Unit ProtectionNon-Unit protection



Protection Types

Unit ProtectionDifferential protection

• Transformer differential protectionBus differential protection

• Generator differential protection• Line differential protection

Pilot protection• Transfer trip schemes• Under/over reaching pilot protection with distance

protectionNon-Unit protection

Over Current protection• Time over current or instantaneous protection

3 Zones of distance protection



Unit Protection

TFR

Protection

Unit protection provides the protection if the fault is inside the Zone of protectionIt does not provide the protection if the fault is outside the Zone of protection

Unit protection provides the protection if the fault is inside the Zone of protectionIt does not provide the protection if the fault is outside the Zone of protection



Non-Unit ProtectionA B DC

Distance

Time

Zone 1Zone 2 Zone 3

Zone 1 provides the instantaneous protection for the line AB Zone 2 and Zone 3 are the back up protection

Zone 1 provides the instantaneous protection for the line AB Zone 2 and Zone 3 are the back up protection



It’s a Non Unit Protectionbut can be modified to Unit Protection when combined with signaling channel

Non-Unit Protection



DistanceProtection

Theory of operationTheory of operation

DesignDesign

Application ExamplesApplication Examples

Basics of Distance Protection

Basics of Distance Protection



Distance Protection: Principle

Protection works on impedance seen by the protectionImpedance is directly proportional to distance so it’s a distance protectionRequires inputs for voltage and current

Using CTs and PTsCurrent is an operating force and voltage is a restraining force

Normal load condition restraining force is higher then the operating force




Current Voltage



Fault condition

Normal conditionNormal voltage & load current69 Volts and 1 A load current

Fault conditionDepressed voltage & high current20 Volts & 10 A fault current

A B DC



Fault condition

Quick isolation of the faulted sectionReduces damage caused by the faultLess stress on the electrical apparatusMaintains the flow of electricity to healthy section

Quick and fast detection of the fault condition Fast operation of the distance protection

A B DC






Distance Protection: Ph-Ph Fault

The measured impedance is equal to thePositive & Negative sequence impedance up to the fault location



Distance Protection: 3 Ph Faults

The measured impedance is equal to thePositive Sequence impedance up to the fault location



Distance Protection: Ph-G Faults

The measured impedance is equal to thePositive, Negative & Zero Sequence impedance up to the fault location



Distance Protection: Ph-G Faults

The current is Phase current + the Residual current Residual current = Iph * (Z0-Z1) / 3Z1,

KN = Zero Sequence compensation factor.The factor KN is a transmission line constant

Identical throughout the whole line length.Total loop impedance = (1+KN) Z1




A B DC

Resistance : R

Reactance : XZ

RF




IR

IX IZ

V

ZI

V

IZ

+ OutputAngleComparator

>= 900

V-IZ

Angle = 900




ZI

V

IZ


>= 900

IR

IX IZ

V

V-IZ




Angle between the two cords drawn from the diameterOf a circle is always 90 degrees.



Distance Protection

IR

IX IZ

V

ZI

V

IZ


>= 900

V-IZ Internal faultAngle >=900Internal faultAngle >=900



Distance Protection

IR

IX IZ

V

ZI

V

IZ


>= 900

V-IZ

External faultAngle < 900External faultAngle < 900



Distance Protection : Design

Set a replica impedanceReplica of line impedanceMagnitude and angle : ZRCalled reach of the Relay

Convert current I in to vector IZRDerive voltage of the system : V

Reference voltage V , Polarizing Voltage : Vpol

Calculate voltage vector Vpol - IZRMeasure the angle between Vpol and Vpol – IZROutput if the angle is 900 or greaterThis produces “MHO” Characteristic

IR

IX IZR

Vpol

Vpol - IZR



Distance Protection : Ph-G Fault

For A-G faultIZR is IA*ZR

Vpol is VA

Vpol – IZR is VA – IAZR

When polarizing voltage = Fault voltageSelf polarized RelayFor B-G fault

• Polarized voltage = Fault voltage = VB

Earlier designs were self polarized distance relays



Self Polarized Protection

LimitationsWhat happens if the fault is at the terminal of the breaker?

• Fault voltage is 0• Polarized voltage is 0• No reference voltage Vpol to compare with

V-IZ• Self polarized distance protection no good for

0 voltage phase to ground faultSolution

Use memory voltage instead of faulted phase voltage



Memory Polarized Protection

ZI

V

IZ

+ Output

AngleComparator

>= 900

Memory



Memory Polarized Protection

What memory voltage does to the “MHO”characteristic?

ZI

V

IZ

+ Output

AngleComparator

>= 900

Memory



Memory Polarization

Pre-Fault voltage at the protection before the fault is E (neglecting load current drop in Zs)

A B

G Zs

E21

E

Load Current



Memory Polarization

Pre-Fault voltage at the protection before the fault is E (neglecting load current drop in Zs)Just after the fault the E = VF + IFZs

A B

G Zs

EVF 21

Fault Current IF



Memory Polarization

IR

IX IZR

VF

V-IZ

Angle > 900Angle > 900

IZs

Vpol = V pre fault

E = VF + IFZs



Mho Ch. : Memory Polarization

IR

IX IZR

VF

V-IZ

IZs

Vpol = V pre fault



Memory Polarization Effect

IR

IX IZR

VF

V-IZ

IZsVpol = V pre fault

More fault resistanceCoverage due to memory Polarization

More fault resistanceCoverage due to memory Polarization



Memory Polarization: Summary

Provides reference voltage (Vpol) under all phase and ground faultsExpand the self polarized characteristic to cover more fault resistance

No overreaching at reach pointCircle with a diameter = Source impedance + Reach impedance

• Higher the source impedance (weak source) larger the diameter means more fault resistance coverage

Numerical protections uses memory, self, healthy phase voltages for polarizations and or different combinations of the same

Partially or fully cross polarized protections are very common now a days



Cross Polarization

It is very common to use healthy phase voltage for phase to ground fault

Provides polarizing voltage for a zero voltage faultFor A-G fault: Polarizing voltage is -(VB+VC)

• Called Cross polarizing• Same effect as Memory Polarization

VA

VC

VB

-(VB+VC)



Memory Polarization : Questions

How protection works when there is a 3 phase zero voltage faults?How the protection works when there is a permanent 3 phase zero voltage faults during reclosing?Looks like that protection can trip for a reverse faults. True?

IR

IX IZR

IZs



Distance Protection: Architecture

ZRI

V

IZ

+Output

Timer = 0.25Cycles

Memory




ZRI

V

IZ

+Output

MemoryPhaseShift- 900

PhaseDetector

V-IZ in phaseOr lagVpol




ZRI

V

IZ

+Output


PhaseDetector


ZR

V-IZ

ZsVpol




ZRI

V

IZ

+Output


PhaseDetector


ZR

V-IZ

ZsVpol

ZR

V-IZ

ZsVpol



Review Question - 1ZR

V-IZ

Zs

Vpol

How protection works for a 3 phase zero voltage faults?Typically the protection memorizes 16-20 cycles of pre fault voltages which is used when there is Zero Voltage Fault.

How protection works for a 3 phase zero voltage faults?Typically the protection memorizes 16-20 cycles of pre fault voltages which is used when there is Zero Voltage Fault.



Review Question - 2

How the protection works for a permanent 3 phase zero voltage faults during closing of the breaker?If the grounding chains were left on the breaker & breaker is closed, protection has no voltage in the memory as well as the fault voltage is zero. Protection sees only fault current.Switch On To Fault - SOTF feature is employed which trips the breaker if protection sees the current but no voltage following breaker close.

How the protection works for a permanent 3 phase zero voltage faults during closing of the breaker?If the grounding chains were left on the breaker & breaker is closed, protection has no voltage in the memory as well as the fault voltage is zero. Protection sees only fault current.Switch On To Fault - SOTF feature is employed which trips the breaker if protection sees the current but no voltage following breaker close.

A B

G Zs

21



Review Question - 3

Looks like that protection can trip for a reverse faults. True? Characteristic is true only for forward faults for reverse fault protection will not operate

Looks like that protection can trip for a reverse faults. True? Characteristic is true only for forward faults for reverse fault protection will not operate

ZR

V-IZ

Zs

Vpol



Memory Polarization : FactsA B

G

21

ZsZL

Characteristic is circle whose diameter is Zs - ZRFor faults behind the protection ZS = Zs + ZRSo characteristic should be a circle whose diameter is Zs - ZRSubstitute the value for ZsCharacteristic should be circle with a diameter equal to Zs

ZR

V-IZ

Zs

Vpol



Memory Polarization : FactsA B

G

21

ZsZL

Characteristic is circle whose diameter is Zs - ZRFor faults behind the protection ZS = Zs + ZRSo characteristic should be a circle whose diameter is Zs - ZRSubstitute the value for ZsCharacteristic should be circle with a diameter equal to Zs

ZR

Zs



Memory Polarization : ConclusionA B

G

21

ZsZL

ZR

Zs

Memory polarized MHO characteristic is very secure and not prone to operate for reverse faults

Memory polarized MHO characteristic is very secure and not prone to operate for reverse faults



Fault Res.: Memory Vs. Self Polarized A B

G

21

ZsZLZs

RF

ZR

Zs

ZR

Zs



Fault Res.: Strong Vs. Weak SourceA B

G

21

ZsZLZs

RF

ZR

Zs

ZR

ZsStrong SourceWeak Source



Fault Res.: Short Vs. Long Line

G

21

ZsZLZs

RF

ZR

Zs

ZR

ZsStrong Source/Short line Weak Source/long line



Fault Resistance CoverageStrong source & short line reduces fault resistance coverage

Use quadrilateral characteristic to improve fault resistance coverage

Weak Source & long line increases fault resistance coverageMemory polarized protection is more secure under all fault conditionsSelf polarized protection does not provide good fault resistance coverageMemory polarization increases fault resistance coverage compare to self polarized protectionHealthy phase polarization provides the same effect as memory polarized protection



Quadrilateral Characteristics

Four comparators are used to detect fault conditionsIf all four comparators produces output, protection tripsQuad characteristic provide good fault resistance coverage

Four comparators are used to detect fault conditionsIf all four comparators produces output, protection tripsQuad characteristic provide good fault resistance coverage

ZR

ZL

R

X

Load Area

Dir

ReachLoad Blinder

KR



Quadrilateral Characteristics

ZR

ZL

R

X

Dir

ReachLoad Blinder

KR

How directional line works?How directional line works?



Directional line : Int. Fault

IR

IZ = Signal B

VF

VF <-900 = Signal A

IX

If A lags BBy 00 – 1800

True

Dir line

Operate



Directional line : Ext. Fault

IR

IZ = Signal B

VF

VF <-900 = Signal A

IX


True



Reach line : Ext. Fault

IR

IZ VF

I*Kr = Signal B

IX


VF - IZ = Signal A



Reach line : Int. Fault

IR

IZ VF

I*Kr = Signal B

IX


VF - IZ = Signal A



Reach line

IR

IZ VF

I*Kr = Signal B

IX


VF - IZ = Signal A



Load Blinder: Internal Fault

IR

IZ

VF

I*KR

IX

If A lags BBy 00 – 1800VF - IZ = Signal B

KR Setting for the load blinder

VF-IKR=Signal A



Load Blinder: External Fault

IR

IZ

VF

I*Kr

IX


VF - IZ = Signal B

KR Setting for the load blinder

VF-IKR=Signal A

Operate



Quadrilateral Characteristic : Facts

Quad characteristic is better for short line and/or strong source as it provides better fault resistance coverageVery good for ground fault protection

Fault resistance can be high • Tower footing resistance, Tree touching the

line etc.Reactance and resistance reach can be set independent of each other for optimum fault coverage



Ph-G Faults: High Resistance Fault

High Resistance Faults•can be caused by growing trees, bushfire or objects touching a conductor• this type of high resistive faults can not be detected by impedance protection



Quad Characteristic : Reactance Line

Reactance line is not a straight line parallel to R axis.Top line has a tilt of 30

The tilt enables protection to not operate for an external faultIf the tilt is not there then protection can operate for an external fault due to effect of load on the lineHow?



Effect of Load : Quad Ch.

G G

21

ZsZLZs

Vx <00

X Y

VY <-300

Ix Iy

Ix+Iy <-Ө



Impedance Seen at Protection X

G G

21

ZsZLZs

Vx <00 VY <-300

Ix Iy

Ix+Iy <-Ө

(Ix+Iy)*R

(Ix)*ZL

Tilt of the reactance linePrevents tripping for the External faults

Tilt of the reactance linePrevents tripping for the External faults

Ix

IyIx + Iy



Effect of Load : Quad Ch.

G G

21

ZsZLZs

Vx <00

X Y

VY <+300

Ix Iy

Ix+Iy <+Ө

Ix

IyIx + Iy



Impedance Seen at Protection X

G G

21

ZsZLZs

Vx <00 VY <-300

Ix Iy

Ix+Iy <-Ө

(Ix+Iy)*R

(Ix)*ZLTilt of the reactance linePrevents operation for the Internal faults

Tilt of the reactance linePrevents operation for the Internal faults



Quad Characteristics : Ext. Fault



Quad Characteristics : Int. Fault



Quad Characteristics : Int. Fault

By polarizing the top line with –Ve or Zero sequence current, it will adapt to load ConditionBy polarizing the top line with –Ve or Zero sequence current, it will adapt to load Condition



MHO Characteristics : Zone 3

Zone 3 provides the back up protection in case Zone 1 and Zone 2 fails to operateZone 3 is typically time delayed zoneIt is used in blocking scheme to determine direction of the faultZone 3 is mostly offset characteristic that it includes the Origin in the characteristic means it can trip for a reverse faultTo avoid load encroachment due to large setting of Zone 3 it’s shape can lens



Zones of Protection

Load profile

Zone 3 : Offset MHO



OFFSET MHOIZ

-IZ’

V-IZ

V-IZ’

Z = Forward ReachZ’ = Reverse Reach




OFFSET MHOIZ

-IZ’



Load profile

Two Comparators are phase shifted by same angle in opposite direction Two Comparators are phase shifted by same angle in opposite direction



OFFSET MHOIZ

-IZ’



Load profile

Major axis to minor axis ratio is equal to tan (180 - Ө)/2Each comparator is shifted by an angle Ө

Major axis to minor axis ratio is equal to tan (180 - Ө)/2Each comparator is shifted by an angle Ө



MHO or Quad : Pros and ConsSimple and directionalLees sensitive to power swings

Reach does not extend as far along R-Axis

Limited fault resistance coverage for short lines

Simple and directionalLees sensitive to power swings

Reach does not extend as far along R-Axis

Limited fault resistance coverage for short lines

Good fault resistance coverage as char. can be set along R- Axis

Good for short line and strong source

Sensitive to power swing

Characteristic extends on R-Axis

Good fault resistance coverage as char. can be set along R- Axis

Good for short line and strong source

Sensitive to power swing

Characteristic extends on R-Axis



Documents

fundamentals related to distance relays