Peculiarities of longitudinal line differential protection with digital link

A.N. DONI, N.A. DONI,Research & production enterprise «EKRA», Ltd,

Russia2009

The operating principle of LDP is based on comparison of current phasors at ends of protected power line in normal and fault states. The current phasor and restraint measurements are sent from one line end to the opposite via digital communication links.

Operating principleOperating principle

B

Q1Line

A

LDP

Q2

Digital link

LDP

LDP connects to current transformers at substations with following configurations:

• one breaker per connection,• two busbar systems with bypass,• two breakers per connection (two current groups),• two breakers and reactor on power line (three current groups)

Connecting current circuits of LDPConnecting current circuits of LDP

• two synchronous communication links (CL) 64…512 Kbps,two synchronous communication links (CL) 64…512 Kbps,• automatic switching of link in case of link failure,automatic switching of link in case of link failure,• connecting to protection via multimode fiber (ST connector),connecting to protection via multimode fiber (ST connector),• admissible data deference for CL – 30 ms (in one direction),admissible data deference for CL – 30 ms (in one direction),• compensation of fixed CL asymmetry – up to 5 mscompensation of fixed CL asymmetry – up to 5 ms

Digital communication links between protectionsDigital communication links between protections

A

FO1TX

RX

B

FO2TX

RXFO1TX

RXFO2TX

RXFO2TX

RXFO1TX

RX

A B

FO2TX

RXFO1TX

RX

FO2TX

RXFO1TX

RX

A B

FO2TX

RXFO1TX

RX

ММ SM

MMSMSinglemode FO cable

9/125 μm ( up 26-100 km)

ST ST

Multimode fibre-optic cables (FO) 62.5/125 μm ( up 2 km )

Connector ST ST

ST ST

Converter 820/1300 μm Connector

FC

ConnectorFC

Converter 820/1300 μm

Using dedicated optical fiberUsing dedicated optical fiber

Using multiplexersUsing multiplexers

FO2TX

RXFO1TX

RX

A B

FO2TX

RXFO1TX

RX

FO

FOX21

ST ST

Multimode FO 62.5/125 μm ( up 2 km )

Connector ST ST

ST ST

Converter FO/X21

RXTX

CLKMX

X21RXTX

CLKMX

Digital Network

Multiplexer MultiplexerConverter FO/X21

Q1 Q2

A B87L 87L

Q1 Q2

A B

I1 I2

I1 I2

Q1 Q2

A B

I1 I2

ZH

а)

b)

c)

Rf

C1

C1

C1

C2

C2

Direction of currents for a two-end line

а – normal load current or external fault condition;b – fault on protected line; c – fault on power line in case of single-way feed. Direction of currents same as for external fault and normal load

Influence of communication link asymmetry and power flow on position of characteristic points in the complex current ratio plane

d =9000 * td,

- phase difference of equivalent EMF at line ends

a

jb

-1

Internal fault

External fault or load condition φd

-φd

-φd

φd -

a=Re(I1/I2), b=Im(I1/I2)

Dynamic errors of real CTDynamic errors of real CT

a – secondary current, Аb – secondary current fundament harmonic RMS error (per unit)c – degree error (degrees)

а)

b)

c)

-200,0

-150,0

-100,0

-50,0

0,0

50,0

100,0

150,0

200,0

250,0

300,0

-0,01 0,00 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0,10 0,11 0,12 0,13 0,14 0,15

1 - Ideal CT

2 - Real CT

1

2

0,00

0,20

0,40

0,60

0,80

1,00

1,20

-0,01 0,00 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0,10 0,11 0,12 0,13 0,14 0,15

1 - Ideal CT

2 - Real CT

1

2

-80

-70

-60

-50

-40

-30

-20

-10

0

-0,01 0,00 0,01 0,02 0,03 0,04 0,05 0,06 0,07 0,08 0,09 0,10 0,11 0,12 0,13 0,14 0,15

Phase difference of secondarycurrent for ideal and real CTs

LDP blocking and operating characteristics ||II11++II22|| > I > I0d 0d ;;11- - ||II11++II22|| > k > k||II11--II22|| –– circle zone;circle zone;22-- ||II11++II22|| > k > k(|(|II11||++||II22|)|) – cardioids; – cardioids;33-- AA22<<||II1 1 / / II22||<A<A11, , (( - - BLBL )) << arg arg ((II11 / / II22))< < (( + + BL BL ) )

–– sector or “rainbow”sector or “rainbow”AA11=-(1+=-(1+kk)/(1-)/(1-kk), ), AA22=-(1-=-(1-kk)/(1+)/(1+kk),), k – k – restraint coefficientrestraint coefficient

I0d

Operate region

Blocking region

Id

IR

k=0,5

k=1k=0,9

k=0,75

A1

k=0,75A1=-7,0A2=-0,143

Im(I1/I2)

Re(I1/I2)-1

Operate regionA2

2

3

1

Blocking region

Characteristic points upon external faults with saturation of CTs

BL

1

4

7

10

13

16

19

22

-180 -150 -120 -90 -60 -30 0 30 60 90 120 150 180

|I1|/|I2|

k=0,5

k=0,75

k=0,9

12

3

Angle characteristics of LDP blocking at different Angle characteristics of LDP blocking at different kk values values

1 - circle 2 - cardioids3 - sector

Analysis of LDP operation for internal and external faultsAnalysis of LDP operation for internal and external faults

A B u

Z C2Z C1

VT VT CTCT

LЛ ii - iC/2-( i -2iC/2) -(i - iC/2)

iC/2 iC/2СL /2 СL /2

.dt

duii L

2

CK

When installed on cables and long power lines, LDP performs equalization of currents applied to protections at both line ends during external faults. The equalization is carried out by electric “transposition” of CTs to mid-line using capacitive current compensation scheme.

Capacitive current compensationCapacitive current compensation

The additional impedance functions and zero sequence directional element immune to inrush provide for correct operation of LDP for T-line arrangement schemes.

&1

Z

ZSD

XB

LDP

TRIP

T-line applicationT-line application

LDP LDP

Additional functionsAdditional functions

• receives and sends up to 20 binary signals,receives and sends up to 20 binary signals,• fault locator two-way measurement without use of zero sequence fault locator two-way measurement without use of zero sequence components,components,• set of time-graded backup protections:set of time-graded backup protections: -5 distance zones, including distance function for ground faults,-5 distance zones, including distance function for ground faults, -5 steps of current directional zero sequence function,-5 steps of current directional zero sequence function, -power swing blocking:-power swing blocking:

current rate of change measurement,current rate of change measurement,DZ / Dt measurement,DZ / Dt measurement,

-voltage circuits failure monitoring,-voltage circuits failure monitoring, --speedup logicspeedup logic,, --current cutoff function,current cutoff function,• directional current load shedding,directional current load shedding,• breaker failure protection;breaker failure protection;

+jX

+R

XI

1 (II)

3

ZI

XII

XIII

RIII

RII

RI

2

Л

ZII

ZIII

1 (III)1 (I)

4 (I)

XI(з)ZI(з)

RI(З)

нагр

Rнагр

Load

Load

+jX

+R1(II)

XII

RII

Operate region

Swing region

RII+DR

XII+DX

-(XII+DX)

-RII-(RII+DR)

-XII

ZКЗ

(1)

(2)Zкач

ZАХ

(3)

Operating characteristics of distance functionOperating characteristics of distance function

Characteristic for DZ/Dt power swing blockingCharacteristic for DZ/Dt power swing blocking

• Line 110 kV Tolyatti Heat Station – Levoberezhnaya (Stavropolskaya-3). Length 10 km. September 2007.

• Line 110 kV Tsentralnaya – Magistralnaya (Kazan.) Length 9 km. December 2007.

• The monitoring of protections is remotely performed over GSM channels.

• In the course of monitoring some 50 external faults with currents measured at 1500 - 8000 A were recorded. One of the faults was three-phase. One of the faults was on secondary side of tapped transformer.

• No failure in optical communication links was recorded.• The experience results are positive.•The LDP with trip are installed on 11 power lines at 110 kV

voltage.

Field experience data for LDP Field experience data for LDP БЭБЭ2704v0912704v091

Real case external three-phase fault on 110 kV power lineReal case external three-phase fault on 110 kV power line

Thanks for your attention!