Distance preotection Basics

Energy Sector© Siemens AG 2008

Distance Protectionfor transmission lines: part 1

Gustav Steynberg

Energy SectorEnergy AutomationPage 2 01/20/16© Siemens AG 2008

Why impedance protection?

Situation: Meshed network and two infeedsDirectional overcurrent time relays

0,6s

0,6s

0,3s

0,3s

0,6s

0,6s

0,3s

0,3s

non-selective trip


Localization of short-circuits by means of an impedance measurement:

fault on the protected line

fault outside the protected line

Z1

relay A

selectivity

relay A

Z2

Basic principle of impedance protection


Distance measurement (principle)

6 loops: 3 phase- phase loops and3 phase- ground loops

phase- phase -loop:

The same applies to the remaining loops

UL1-L2 = ZL ( IL1 - IL2)

Measured currentmeasured voltage

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ZL = RL + j XL

ZE = RE +j XE

IL1

IL2

IL3

IE

ZL

ZE

UL1 UL2 UL3


phase-ground-loop: UL1 = L1 · ( RL + j XL )- E · ( RE +j XE)

L1, E measured currentUL1 measured voltage

06.08.97dtgerdis3

The same applies to the remaining loops

Distance measurement (principle)

IL1

IL2

IL3

IE

ZL

ZE

UL1 UL2 UL3

ZL = RL + j XL

ZE = RE +j XE


Load and short-circuit impedances

ZL

ZLF1

ZLF2

RF RF

ZLoadDF1 F2

X

R

ZL

ZLF2

j SC1

j SC2

j L

RR

ZF1

ZF2

RR

ZLoad

ZLF1

Fault area

distance relayoperating characteristic

Increasing load

Fault in reverse direction Load area

Minimum Load Impedance:Minimum voltage 0,9 UnMaximum current 1,1 InMaximum angle 30°

Phase - Phase Fault

RR RF / 2

Phase - Earth Fault

RR RF /(1 + RE/RL)


Principle of (analog) distance relaying

ISC

E

comparator

ZL

ZSC

ZReplica (line replica impedance)(corresponds to the set zone reach)

U1= k1 USC= k1 ISCZSC.

U2=k2 ISCZReplica

ZS

Relay design:operation if

U1< U2

i.e. ZSC< ZReplica

ZReplicaX

R

Ext. fault

Internal fault

A B


Typical distance zone-characteristic

MHO-circle

shifted circle

polarisedMHO-circle quadrilateral

ZR

ZSC

ZSC'

externalfault

internalfault

X

R

X

R

ZS = 0

ZS small

ZS high ZS

RF

ZL

X

R

centre

ZSC'

ZSC

settable arc compensation

X

XA

ZSC-L Rarc

RRA


Graded distance zones

time

D1 D2 D3

t1

t2

t3

Z1

Z2

Z3

distance

t = grading time

A CB D

Z1 = 0,85 ZAB

Z2 = 0,85 (ZAB + 0,85 ZBC)

Z3 = 0,85 (ZAB + 0,85 (ZBC + 0,85 ZCD))

Safety margin is 15 %: line error CT, VT error measuring error

Grading rules:

Energy SectorEnergy Automation01/20/16© Siemens AG 2008

2nd Zone: It must initially allow the 1st zone on the neighbouring feeder(s) to clear the fault.The grading time therefore results from the addition of the following times:

operating time of the neighbouring feeder mechanical 25 - 80 msstatic: 15 - 40digital: 15 - 30

+ circuit breaker operating time HV / EHV: 60 ms (3 cycles) / 40 ms (2 cycles) MV up to about 80 ms (4 cycles)

+ distance relay reset time mechanical: approx. 60-100 ms static: approx. 30 ms digital: approx. 20 ms.

+ errors of the distance relay internal timers mechanical: 5% of the set time, minimum 60-100 msstatic: 3% of the set time, minimum 10 msdigital: 1% of the set time, minimum 10 ms

+ distance protection starting time *) mechanical: O/C starter: 10 ms, impedance starter: 25 msstatic: O/C stater: 5 ms, impedance starter: 25 msdigital: generally 15 ms

+ safety margin (ca.) grading; mechanical-mechanical: 100 ms static/digital-mechanical or vice versa: 75 ms digital-digital or static-static 50 ms

*) only relevant if the set relay times relate to the instant of fault detection / zone pick-up. This is the case with all Siemens relays. There are other relays where the time is adapted by software to relate to the instant of fault inception. In the latter case the starting time has to be dropped.

Determination of grading times(With numerical relays 250 ms is possible)


SC

Current area forforward faults

SC

Current area for reverse faults

SC

USC

R

ZSC

Z'SC

Impedance area for forward faults

Impedance area forreverse faults

X

SC

Determination of fault direction

current / voltage diagram impedance diagram

Fault location Where is the fault ?

The impedance also shows the direction, but ....


direction may be determined together with the impedance measurementbut: problems may arise in certain cases (e.g. close-in faults)

separate directional determination required!

Why impedance measurement and directional determination separately?

line characteristic

fault with arc resistancein forward direction

fault in forward direction

fault in reverse direction

close-in fault

X

R

A B

Impedance measurement and directional determination


Alternatives for the directional measurement

faulty phase voltage

Vf

If

VL2

VL3

voltage memory(pre-fault voltage)

If

VL2VL3

VL1

healthy-phase voltage(phase to phase voltage)

If

Vf

VL2-L3 VL2VL3

~

~

~

~

~

~

~

~

~

ZlineZgrid relay

fault L1-E

Method 1 Method 2

VL1

VL1 Vf


Directional measurementSummery of all 3 methods

uRI = uL2-

L3

uf = uL1

Distance measurement

Direction measurementwith voltage memory

Direction measurementwith unfaulted voltage

if(t)uL1

if

if

if

uL2-L3

uL1

06.08.97dtgerdis9

Measuringwindow


Fault detection techniques

Over-current fault detectionVoltage dependant over-current fault detection

Voltage and angle dependantover-currentfault detection

I

U

I >>I > I >

R

X

Impedancefaultdetection

Not in 7SA522


110 kV net SCC(3)" = 1500 MVA

40 MVAuSC = 15 %20 kV

400/1 A

l

I>start = 1,5 · IN = 600 A

D

OH-line95/15 Al/StZ'L = 0,483 /km

' · l)

10 20 30 40 50 60

I>start = 600 A

0,5

1,0

1,5

2,0

2,5

ISC(2) [kA]

l [km]

ISC(2) = UN · 1,1

2 · (ZS + ZS + ZL

reach of OC starterapprox. 32 km

N T

Reach of over-current fault detection

ph-ph fault as an example

There is a limitationto the reach


II>>I>

UI>>

UI>

UN

Udigital

electro-mechanical

Powersystem

Relay

line

E

E

ZS

USC

ZSCISC

USC

SC

USC

G

G

Voltage controlled overcurrent fault detection


Voltage and angle controlled overcurrent fault detection (U-I--starting)

50 %

100 %

U/UN

I/IN1 2 3

I> I> I>>

U(I>) U(I >>)

X X

R R

2

11

2

This method is used in Germany


X

R

forwards

forwards

reverse

reverse

LoadLoad

Z1

Z2

Z4

Z3

Z1B

Z5

Line

Impedance zones of digital relays (7SA6 and 7SA52)

Distance zonesInclined with line angle Angle prevents overreach of

Z1 on faults with fault resistance that are fed from both line ends

Fault detection no fault detection polygon:

the largest zone determines the fault detection characteristic

simple setting of load encroachment area with Rmin and Load


Zone grading chart, radial feeder

D

A

D

B

D

C

>>

D

>t

ZT

Z1

Z2

Z3

Z1 = 0.85 ZA-B

Z3 = 0.85 [ ZA-B + 0.85 (ZB-C+ 0.85 ZC-D) ]

Z2 = 0.85 (ZA-B + 0.85 ZB-C)

Grading accordingthe recommendationwith the safety margin of 15%.


Ring feeder: with grading against opposite end

0.6

0.3

grading time(s)

The same grading from both sides


Grading in a branched radial system

L2

L3

L4

L1Z2

Z1

Z3

The impedances of the Z2 and Z3 must be grading with the shortest impedance

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Distance preotection Basics