7.7sd5 7sd61 diff principle en

7/27/2019 7.7SD5 7SD61 Diff Principle En

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Power Transmission and Distribution

Energy Automation7SD5 / 7SD61 Differential Algorithm and Principle


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No. 1 withEnergy Automation

Energy Automation Page 2

Principle of the Differential Protection Basics

Definition of the differential current:

N

i

iDiff II0

The differential current is the amplitude of the complex summation of all

currents (phasors of the fundamental frequency component) from all ends of agiven line.

Station A Station B

SDSD

IA

IB

Protection DataInterface

(PDI)

Protection Data

Interface(PDI)

IA

=A e-j(t+)

IB=B e-j(t+)


3/54





00

N

i

iDiff II

C

N

iiDiff III 0

At an ideal healthy line the

differential current is zero.

Station A Station B

SDSD

IA

IB

Protection Data

Interface

(PDI)

Protection Data

Interface

(PDI)

IA

=A e-j(t+)

IB=B e-j(t+)

On a real and healthy line the

differential current is equal to the

capacitive load current of the line (IC).

32 | LLBC

UlCfI C`B = neutral line capacity [F/km]

l = line length [km]

f = signal frequency [Hz]

ULL = Line-Line voltage [V]


4/54





N

i

istra II0

intReAt a classical differential relay therestraint current is calculated as :

Station A Station B

SDSD

IA

IB

Protection Data

Interface

(PDI)

Protection Data

Interface

(PDI)

With the differential and the

restraint current and the tripping

characteristic the relay can make a

trip decision, but..

|IA

|=A |IB|=B

CAUTION:

The 7SD5 / 7SD61 is different !

trip

area

restraint

area

IDiff

IRestraint


5/54




Principle of the Differential Protection Tripping

The 7SD5 / 7SD610 has another tripping characteristic:

if IDiff> IRest then TRIP !!!Where IRest = P-IDiff>+ IP-IDiff>= Parameter 1210

Ii = ICT-Err.+ ISignal-Err+ISync-ErrRemark:IRestraint was replaced by IRest just to have adifferent name

Station A Station B

SDSD

IA

IB

Protection Data

Interface

(PDI)

Protection Data

Interface

(PDI)

N

i

iII0

trip

area

restraint

area

IDiff

for

bidd

en

area

P-IDiff>

IRest


6/54




Principle of the Differential Protection I-Phasor

The I-Phasor can be drawn as a normal phasor in acomplex area with a circle at the end. The circle with theradius I is representing all errors of the phasor.

The I is the summation of:Ii = ICT-Err.+ ISignal-Err.+ISync-Err.

WhereICT-Err. = CT - ErrorsISignal-Err = Error due to signal distortionISync-Err = Synchronization Errors

Im { I }

Re { I }

I =A e-j(t+)

I


7/54




Principle of the Differential Protection I-PhasorStation A Station B

SDSD

IA

IB

Protection Data

Interface

(PDI)

Protection Data

Interface

(PDI)

Both relays exchange theI-Phasor via the ProtectionData Interface (PDI). Each relaycombines the phasors(local and remote).IDiff = IA + IB (summation of 2 complex values)I = IA

+ IB

(simple summation of two values)The summation is done for all3 phases separately.The differential protection in the7SD is phase segregative!

Im { I }

Re { I }

IA

IA

=A e-j(t+)

IB

=B e-j(t+)I

B

IA

IB

I IAI

B

IDiff

= IA

+ IB


8/54




Principle of the Differential Protection Parameter P-IDIFF>

How to see the different components which lead to the IRestIRest = P-IDiff>+IP-IDiff>= Parameter 1210

The Parameter P-IDiff> (1210) can directly be seen in the

fault record.

IA = 0 at both ends

(IRest) iS1 IDiff>


9/54




Principle of the Differential Protection Parameter P-IDIFF>

How to see the different components which lead to the IRestIRest = P-IDiff>+IP-IDiff>Switch On= Parameter 1213If a Switch On is recognized by the relay the ParameterP-IDiff>-Switch-On (1213) becomes active for the given timeparameterized in parameter 1132A.

P1213P1210

P1132A


10/54




Principle of the Differential Protection CT-Errors

How to see the different components which lead to the IRestIRest = P-IDiff>+I ; I = ICT-Errors + ISignal-Errors + ISync-ErrorsCT Errors:The figure below shows a real CT error curve (blue) andone possibility of the approximation of this curve (red)

real CT error

curve at ratedburden

approximation of

the CT error curve

ICT[A]

ICT

[A]

IN-Sec

kscc

IN-Sec


11/54





How to see the different components which leads to the IRestIRest = P-IDiff>+I ; I = ICT-Errors + ISignal-Errors + ISync-ErrorsCT Errors:The CT-Errors are represented by 3 parameters

CT[%]

ICT

/IN-Relay-sec

P253

P254

P251

CT

[A]

IRelay-sec

[A]IN-Relay-sec

*P251

Slope P253

Slope P254

The parameters 253 and 254 are defining two slopes. The

parameter 251 defines the switching over between the two

slopes.


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The CT Errors can also be seen in the fault record at the

restraint current (IS).Example:Parameter 251 : K_ALF/K_ALF_N = 1

Parameter 253 : E% ALF/ALF_N = 5%

Parameter 254 : E% K_ALF_N = 10%

Current thru the relay 0.5A and 1.5A secondary (P-IDiff>= 0.3 A)

312 mA+5%0,5A=336 mA312 mA+10%1,5A=462 mA


13/54




Principle of the Differential Protection Signal-Errors

How to see the different components which lead to the IRestIRest = P-IDiff>+I ; I = ICT-Errors + ISignal-Errors + ISync-Errors

__measured signal

__phasor calc. out of the measured signal

deviation between the measured signal and the calculated phasor

Principle:The 7SD measures a current signal i(t) (red curve). Out of this signal the7SD calculates the phasor from the fundamental frequency componentI = A e-j(t+) (blue curve) and compares both signals. The deviationbetween both curves (green area) is a criteria for the signal distortion(Signal Error).Important: The restraining against the signal disturbance has NO

parameters. It is an adaptive measurement.


14/54




Principle of the Differential Protection Signal-Errors

The additional restraint due to the signal disturbance can

also be seen in the fault record at the restraint current(IRest ).Example:1st violet graph : sine ordinary undisturbed current.1st green graph : disturbed current (e.g. due to CT Saturation).2nd violet graph : only restraining due to CT-Errors.2nd green graph : restraining due to CT Errors plus restraining due tosignal distortion.

Additional restraining due tosignal disturbance


15/54




Principle of the Differential Protection Sync-Errors

To get a better understanding why the synchronization

error is important it is useful to understand:

what is the root cause of this error,

how the synchronization works,

why the synchronization is needed and

what are the side effects if the

synchronization fails

the use of the GPS

How to see the different components which lead to the IRestIRest = P-IDiff>+I ; I = ICT-Errors + ISignal-Errors + ISync-Errors


16/54





Station A Station B

SDSD

IA

IB


(PDI)

Protection Data

Interface(PDI)T

A->BT

B->A

Com-Network

Synchronization errors onlyappear if the PDI of the 7SD

operates against a telecommunication network.

The reason for the synchronization error is an asymmetrical

transmission time, i.e. the transmission time of a telegram

from station A to station B (TA->B) is not the same as thetransmission time of a telegram from station B to station A

(TB->A). TA->B is unequal to TB->A .

Better: |TA->B - TB->A| > with < 50 s; measurement accuracy of the relay


17/54





How the synchronization works:

The synchronization of the 7SD is based on a simple principle

Pilot example:

A pilot starts his flight from Frankfurt to New York. While taking off he looks at the airport

clock and he notes down the local time. Landing in New York he looks at the airport clock

and he also notes down the local time.

After a few hours he starts his flight back to Frankfurt .During starting in New York and landing in Frankfurt he looks at the airport clock and he

notes down both local times.

Assuming that both flights have taken the same time it is possible to calculate the flight time

and the time zone difference between Frankfurt and New York

Takeoff Frankfurt : 6:00 Landing New York: 7:00

Takeoff New York: 10:00 Landing Frankfurt : 23:00

Time zone difference : ((6:00 7:00) + (23:00 10:00)) /2 = 6 H

Flight time : ((7:00 6:00) + (23:00 - 10:00))/ 2 = 7 H

Compared to the 7SD devices:

Time zone difference

DTO : Device Time Offset, difference between 2 local Device Time Bases (TB)

Flight time

TD : Transmission Delay = (TAB + TBA)/2


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DTO

TD

DTO

TDDevice at

Station A

Device at

Station B

send

receive

A1 A2 A3

B1 B2 B3

B1 B2 B3

send

receive A1 A2 A3

TA->B T

B->A

A4

A4

B4

B4

Sync. Function

tAA1S

(tAA1S,

tAB1R

)

tAB1R

(tAA1S,

tAB1R

)

(tBA1R,

tBB1S

)

(tBA1R,

tBB1S

)

Sync. FunctiontBB1S

tBA1R

tB

tA

Functional overview of the synchronization in the 7SD

All timestamps are taken at the receiving and transmitting of thelast bit of a telegram


19/54





Why synchronization is needed:

Two devices in StationA and B are sampling asynchronously thesame signal. Out of the sampled signal both devices are calculating

there phasors and the I with the different window times (TWindowA andTWindowB). The time difference between the windows is tWindow .

SD

TWindow A

B tWindow

A

SD

StationA Station B

i(t)

=A sin(t+)

i

t

TWindow B


20/54





After the computation of the phasors the devices exchange their

phasors via the PDI. Station B receives the phasor from stationA.

Before station B can compute the differential current it has to translate

the time base from stationA into its own time base and the device has

to rotate the phasor to eliminate the window time difference tWindow .

But what happensto the ???

Phasor received

in B from A in

time base from A

Phasor received

in B from A in

time base from B

Shifting angle to eliminate

the window difference (t)

shift

= 2fsignal

t (rad)

Im { I }

Re { I }


21/54





The handling of the I is a little more difficult. The additional error due

to the synchronization is shift|IA|. Where in shift the most importantcomponents are t and fsignal.

Phasor received

in B from A in

time base from APhasor received

in B from A intime base from B

shift

= 2fsignal

t (rad)

Im { I }

Re { I }

IAin Time of A

IA

in Time of B =IA

in Time of A+shift

*|IA

|

shift

= 2fsignal

t + 2fsignal

t (rad)

shift*|IA|


22/54




Im { I }

Re { I }

IA

in Time of A

IA

in Time of B = IA

in Time of A+shift

*|IA

|

shift

= 2fsignal

t + 2fsignal

t (rad)

shift

*|IA

|


The fsignal is the accuracy how the devise can compute the signalfrequency. Here is the reason why it is useful to connect VTs to device.

t mainly depends on the parameter 4506A for PDI 1 (4606A for PDI 2).

The user has to know the max.

transmission time difference

(0.250.. 0.500 ms is suitable

for most SDH/PDH networks).


23/54





Last question is how the user can see the additional ISync-Errordue to

the parameter 4506A in the device e.g. in the fault record.Example: -- Two 7SD devices station A P4506A = 0.4ms and station B P4506A = 0ms

-- thru flowing current = 1 A

-- all other parameters, e.g. IDIFF>; CT error etc. are assumed to be

identical in both stations.

Station AStation B

SDSD

IA

=1A

in Phase AIB=1A

in Phase A


(PDI)

Protection Data

Interface

(PDI)

Com-Network

P4506A

=0msP4506A

=0.4ms

P T i i d Di t ib ti


24/54





In the fault record below the additional restraint due to the

synchronization (parameter 4506A) can be seen.

126 mA

additional restraint

Verification of the measurement:Station A P4506A= 0.4ms and Station B P4506A = 0 shift*|IA| where shift = 2fsignal t + 2fsignal t

0

|IA

| 2fsignal

t = 1A250Hz0.4ms= 0.1256 A

P T i i d Di t ib ti


25/54





Side Effects:

To calculate the differential current you must sum up twophasors (IA and IB). The phasors must have the same

frequency and the same time. A wrong synchronization (t0)

leads to a phase shift of the phasor (IB) and in the end to a

differential current (IDiff) if the phasors are added. If the 7SD

does not restrain against the sync. errors the relay will trip!

IA

=A e-j(t+)

IB=B e-j(t+)

IB=B e-j(t+t0)+)

IDiff



26/54





How to see such effect in the 7SD

Example:Station A Station B

SDSD

IA

=1A

in Phase AIB=1A

in Phase A


(PDI)


(PDI)

Com-Network

P4506A

=0msP4506A

=0.4ms

TA->B

=0 TB->A

= 0.7ms

The result of asymmetrical

transmission times is shown in the

vector diagram.

The reason for the differential current

is a result of a wrong synchronizationof the device.

CAUTION: This was a lab test. Donot try this in the real world, because

you can not distinguish between a

capacitive load current and a wrong

synchronization.



27/54





The use of GPS

In some telecommunication network where the asymmetrically

transmission time is not known, extremely high or the variation is

very high an other additional synchronization method is needed.

This method is based on an independent external clock sourcewith possibility to generate a pulse at same physical time with a

micro second accuracy of the e.g. the rising edge of the pulse

regardless of the position of the clock source.

One solution such a clock source is a GPS Clock with an 1PPS*

output. *1PPS one pulse per second

Station A Station B

SDSD

IA

IB


(PDI)


(PDI)

TA->B TB->A

Com-Network



28/54





The use of GPS

The sketch above shows the basic hardware set up for the use of

the GPS-Synchronization.

One GPS- antenna and clock is needed on every end of the

installation. The 1 PPS must be connected to the Port A of therelay.

Station A Station B

SD

IA

IB


(PDI)TA->B

TB->A

Com-NetworkGPS -

Clock

Port A

1 PPS

GPS Antenna

SD

Protection Data

Interface

(PDI)Port A

GPS -

Clock

GPS Antenna

1 PPS



29/54





How the synchronization works with GPS:

Coming back to the pilot example:

Pilot example:

A pilot starts his flight from Frankfurt to New York. While taking off he looks at the airport

clock and he notes down the local time. Landing in New York he looks at the airport clock

and he also notes down the local time.

After a few hours he starts his flight back to Frankfurt .During starting in New York and landing in Frankfurt he looks at the airport clock and he

notes down both local times.

With the GPS-Synchronization the pilot ring somebody in New York and

he asks for the local time in New York. With this New York time and his

local time he able to calculate the time zone difference between

Frankfurt and New York.

The 7SD devices are doing also. Each device capture the GPS-Pulse

and exchange the time stamp over the PDI. With this principle

asymmetrical transmission times can be eliminated.



30/54




Principle of the Differential Protection total summation

Station A Station B

SDSD

IA

IB

Protection Data

Interface

(PDI)

Protection Data

Interface

(PDI)

The figure shows the totalsummation including all

operations at Side A.

Lets go thru it in detail:

Im { I }

Re { I }

IA

IA

=A e-j(t+)

IB

=B e-j(t+)I

B

IA

IB

I = IA

+ IB

IDiff

= IA

+ IB

IB

receive

shift

Parameter:

IDIFF>



31/54





Station A Station B

SDSD

IA

IB

Protection Data

Interface

(PDI)

Protection Data

Interface

(PDI)

Side A calculates its localI-Phasor and receivesthe I-Phasor from side B.

Im { I }

Re { I }

IA

IA

=A e-j(t+)

IB

IB

receive



32/54





Station A Station B

SDSD

IA

IB

Protection Data

Interface

(PDI)

Protection Data

Interface

(PDI)

Side A has to synchronizethe two I-Phasorstherefore it has to shiftthe received I-Phasorfrom side B with the angleshift.

Im { I }

Re { I }

IA

IA

=A e-j(t+)

IB

=B e-j(t+)I

B

IB

receive

shift



33/54





Station A Station B

SDSD

IA

IB

Protection Data

Interface

(PDI)

Protection Data

Interface

(PDI)

After the two I-Phasorsare synchronized theycan be added to acomplex IDiff-Phasor.

Im { I }

Re { I }

IA

IA

=A e-j(t+)

IB

=B e-j(t+)I

B

IDiff

= IA

+ IB

IB

receive

shift


34/54



35/54




Station A Station B

SDSD

IA

IB

Protection Data

Interface

(PDI)

Protection Data

Interface

(PDI)

To make a trip decisionthe Parameter 1210P-IDIFF> has to be added.

If the origin is part of thelast circle around theIDiff

-Phasor the relay isstable.

If the origin is not part therelay trips.

Im { I }

Re { I }

IA

IA

=A e-j(t+)

IB

=B e-j(t+)I

B

I = IA

+ IB

IDiff

= IA

+ IB

Parameter:

IDIFF>



36/54



Principle of the Differential Protection Advantages

N

i

istra II0

intRe

Comparing the classical and new

characteristic of the line differential.

Station A Station B

SDSD

IA

IB

Protection Data

Interface

(PDI)

Protection Data

Interface

(PDI)

The new huge advance of the new differential

characteristic is that every error is considered

independently.

This is an advantage if different CTs are usedin the installation, where one CT may saturate

earlier than the other (e.g. due to different

remanence).

trip

area

restraint

area

IDiff

IRestraint

trip

area

restraint

area

IDiff

f o r

b i d d

e n

a r e a

P-IDiff>

IRest

IRest = P-IDiff>+ I



37/54



Principle of the Differential Protection Advantages

N

i

istra II0

intRe

Comparing the classical and new

characteristic of the line differential.

Station A Station B

SDSD

IA

IB

Protection Data

Interface

(PDI)

Protection Data

Interface

(PDI)

Furthermore the classical characteristic is derived

from the old mech. relays.

The initial issue of the second slope was to cover

CT-saturation. Further investigation and fault

record from digital relays has shown that CT

saturation may happened even at currents belowtwo times of the nominal CT current (ext. fault

condition with large decaying e-component), were

the second slope do not effect the restraining. To

overcome this problem the differential protection

has be de-sensitive under normal operation

condition with the first slope (so the protectiongets blind for low current internal faults).

trip

area

restraint

area

IDiff

IRestraint

trip

area

restraint

area

IDiff

f o r

b i d d

e n

a r e a

P-IDiff>

IRest

IRest = P-IDiff>+ I



38/54



Principle of the Differential Protection 3 and more

Until now the differential protection was only explained at

two ended line. The question is:How the differential protection operates at a three

and more ended line.

Station A Station B

SD SD

PDI 1

Station C

SD

PDI 2

PDI 1PDI 2

PDI 1

The figure shows a three ended

installation in chain topology.

Every station measure its local

current, compute the I phosor andtransmits this phasor via the PDI.



39/54




The important thing happen at station B. Station B receives

at PDI 2 the I-Phasor from Station A and add its computedlocal I-phasor to this phasor before it sends the partialsummation of both phasor to station C via the PDI 1.Station C receives the partialsummation from station B and cancompute the total differentialand I current.This happen in both directions inparallel, so in the end every relaycan make its own trip decision.

Station A Station B

SD SD

PDI 1

Station C

SD

PDI 2

PDI 1PDI 2

PDI 1

IC

IC

IB+I

C

IB

IA

+IB

IA

IA

IA

+IB+I

CI

A+I

B+I

C

IA

+IB+I

C

If an other station

is added to the

constellation, the

added station only

do the partial

summation.



40/54




The figure shows a 4 ended ring topology. If all other links are

healthy the connection from station D PDI 1 to station A PDI 2 is in

hot standby i.e. it is not needed for the differential protection. If one

of the other links fail the information are rerouted and the link get

into differential service.

Station A Station B

SD SD

PDI 1

Station C

SD

PDI 2

PDI 1PDI 2

PDI 1

IC

IB

IA

IA

IA

+IB+I

C+I

C

SD

PDI 1 PDI 2

Station D

ID

PDI 2

IA

+IB+I

C+I

CIA+IB+IC+ID

IA

+IB+I

C+I

D

IB+I

C+I

DI

A+I

B

IC+I

DI

A+I

B+I

C

Hotstandby

Hotst

andby

Hotstandby

IC



41/54




The figure below shows the reaction of the 7SD5 is one link fails in

a 4 ended ring topology.

The reaction is similar to every ring topology regardless of the

numbers of ends.

Station A Station B

SD SD

PDI 1

Station C

SD

PDI 2

PDI 1PDI 2

PDI 1

IC

IB

IA

IA

IA

+IB+I

C+I

C

SD

PDI 1 PDI 2

Station D

ID

PDI 2

IA

+IB+I

C+I

D

IA

+IB+I

C+I

D

IA

+IB+I

C+I

D

IB

IA

+IC+I

DI

B+I

CIC

broken link broken link broken link

IB+I

C

+ID

IA

+ID



42/54




Compared to a 3 ended chain topology the 3 ended ring topology

works a little bit different. Here all devices talk directly to each

other and can exchange the I-phasors directly. If one link fails thedevices switch back to a chain topology with the partial summation.

Station A Station B

SD SD

PDI 1

Station C

SD

PDI 2

PDI 1PDI 2

PDI 1

IC

IC

IB

IB

IB

IA

IA

IA

+IB+I

CI

A+I

B+I

C

IA

+IB+I

C

PDI 2

IC

IA



43/54



Principle of the Differential Protection I-DIFF>

Right now the first differentialprotection algorithm (I-DIFF>)

has been explained.

So it is time to start with the

second one (I-DIFF>>)

Station A Station B

SDSD

IA

IB

Protection Data

Interface

(PDI)

Protection Data

Interface

(PDI)

trip

area

restraint

area

IDiff

for

bid

den

area

P-IDiff> IRest



44/54



Principle of the Differential Protection I-DIFF>>

The second differential protection algorithm (I-DIFF>>) isbased on a charge summation. The principle here is that

you can apply the 2nd Kirchhoffs Law not only to phasors.

The 2nd Kirchhoffs Law is also applicable to charges

(the time integral of the current).

Both algorithms run absolutely in depended and in parallelin the 7SD. --- Lets see how the second algorithm works:

Station A Station B

SDSD

IA

IB

Protection Data

Interface

(PDI)

Protection Data

Interface

(PDI)



45/54




The figure below shows the principle of the charge

computing. The charge window length is 5ms (integrationwindow). The offset between two windows is 2.5 ms. The

start and the end time of a window must not be synchronous

to the samples of the device. In general a charge window

consist of three sub windows Q1, Q2 and Q3.

The Q1 is the first

interpolation

between the window

start time and the

first sample. Q2 is

the integration of the

full samples in thewindow and Q3 is

interpolation from

the last full sample

to the window end

time .

i(t)

=A sin(t+)

t0

t1

t2

t3

t4

t5

t6 t7 t8 t9

i

1 sample

charge integration

window (5ms)

2,5mswindow offset

t10

t11

Q3

Q2

Q1

tstart

tend



46/54




)()(2

1

)(

)_()1(11

)0(0

01

)0()1(

)_(

Startttstart

tStart

tt

Startt

iittQ

itttt

iii

The interpolation between the start time and the first sample

is done like:

t0

t1

t2

Q2

Q1

i(t start)

i(1)

tstart

Similar for Q3Q2 is calculate to:

)5()4()3()2()1(2 22221

tttttSample iiiiiTQ

The total charge is the

summation of all three

partial charges

Qtotal=Q1+Q2+Q3



47/54




N

i

iDiff QQ0

After the computation of the charge the devices exchangetheir charges via the PDI. The differential charge and the

restraint charge (QDIFF and Q) is calculation in analogy tothe differential current.

Station A Station B

SDSD

IA

(QA

)

Protection Data

Interface

(PDI)

Protection Data

Interface

(PDI)

IB

(QB)

and

N

i

iQQ0

But take care the tripping characteristic is different.



48/54




QQDiff

The tripping decision of the charge summation is made as

described in the following formula:

Station A Station B

SDSD

IA

(QA

)

Protection Data

Interface

(PDI)

Protection Data

Interface

(PDI)

IB

(QB)

Trip if:

DiffDiff QPQand

Where Q is the restraint chargeand P-QDiff>> is the Parameter 1233

trip

area

restraint

area

QRest

QDIFF

P-IDIFF>>

(P-QDIFF>>

)



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How the I-DIFF>> is restraint (Q):The I-DIFF>> is restraint against CT errors andsynchronization errors.For the CT errors only the parameter 254 is relevant.I-DIFF>> was designed to have fast clearance on high current fault thereforeonly the high error is needed.The handling of the synchronization error in I-DIFF>> is equalto the handling of the synchronization error in I-DIFF>.CAUTION: I-DIFF>> block if parameter 4506A (PDI1 and 4606APDI2) exceeds 0.85 ms.

Station A Station B

SDSD

IA

(QA

)

Protection Data

Interface

(PDI)

Protection Data

Interface

(PDI)

IB

(QB)


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The last question regarding the I-DIFF>> algorithm is:

Can I-DIFF>> also handle installation with more than 2 ends??

The answer is : YES

Station A Station B

SD SD

PDI 1

Station C

SD

PDI 2

PDI 1PDI 2

PDI 1

So lets see how this works !



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The I-DIFF>> algorithm uses the same principle as the I-

DIFF> algorithm the partial summation.

The figure below gives an overview how the partial

summation forI-DIFF>> works.

Station A Station B

SD SD

PDI 1

Station C

SD

PDI 2

PDI 1PDI 2

PDI 1

IC

QC

QB+Q

C

IB

QA

IA

QA+QB+QC

PDI 2

QB+Q

A

QA+QB+QC

QA

+QB+Q

C



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Station A Station B

SDSD

IA

(QA

)

Protection Data

Interface

(PDI)

Protection Data

Interface

(PDI)

IB

(QB)

High-Speed Charge summation offers high speed tripping

and a fast decision for internal or external fault condition. Charge summation doesn't suppress DC-components

and harmonics. (simple integration)

Therefore recommended setting is > ILoad,max (1.2 - 2 IN).

Charge summation decides in 5 ms for internal or external

faults (5 ms window)

Internal: Immediate trip command (trip time typical 12 ms for2 or 3 end topology) for differential currents IDiff > 1.2 - 2 IN

External: If IFault > 2.5P-IDiff>> setting: immediate blocking of the

charge summation. Reason:

CT-saturation possible. Avoids any risk for stability due to

differential current from current comparison.



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Principle of the Differential Protection

THE END

7.7sd5 7sd61 diff principle en

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