7.7sd5 7sd61 diff principle en
TRANSCRIPT
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Power Transmission and Distribution
Energy Automation7SD5 / 7SD61 Differential Algorithm and Principle
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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+)
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Principle of the Differential Protection Basics
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]
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Principle of the Differential Protection Basics
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
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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
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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
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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
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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>
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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
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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
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Principle of the Differential Protection CT-Errors
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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Principle of the Differential Protection CT-Errors
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
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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.
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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
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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
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Principle of the Differential Protection Sync-Errors
Station A Station B
SDSD
IA
IB
Protection DataInterface
(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
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Principle of the Differential Protection Sync-Errors
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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Principle of the Differential Protection Sync-Errors
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
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Principle of the Differential Protection Sync-Errors
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
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Principle of the Differential Protection Sync-Errors
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 }
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Principle of the Differential Protection Sync-Errors
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|
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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
|
Principle of the Differential Protection Sync-Errors
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).
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Principle of the Differential Protection Sync-Errors
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
Protection DataInterface
(PDI)
Protection Data
Interface
(PDI)
Com-Network
P4506A
=0msP4506A
=0.4ms
P T i i d Di t ib ti
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Principle of the Differential Protection Sync-Errors
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
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Principle of the Differential Protection Sync-Errors
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
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Principle of the Differential Protection Sync-Errors
How to see such effect in the 7SD
Example:Station A Station B
SDSD
IA
=1A
in Phase AIB=1A
in Phase A
Protection DataInterface
(PDI)
Protection DataInterface
(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.
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Principle of the Differential Protection Sync-Errors
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
Protection DataInterface
(PDI)
Protection DataInterface
(PDI)
TA->B TB->A
Com-Network
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Principle of the Differential Protection Sync-Errors
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
Protection DataInterface
(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
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Principle of the Differential Protection Sync-Errors
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.
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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>
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Principle of the Differential Protection total summation
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
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Principle of the Differential Protection total summation
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
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Principle of the Differential Protection total summation
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
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Principle of the Differential Protection total summation
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>
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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
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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
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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.
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Principle of the Differential Protection 3 and more
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.
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Principle of the Differential Protection 3 and more
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
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Principle of the Differential Protection 3 and more
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
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Principle of the Differential Protection 3 and more
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
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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
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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)
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Principle of the Differential Protection I-DIFF>>
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
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Principle of the Differential Protection I-DIFF>>
)()(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
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Principle of the Differential Protection I-DIFF>>
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.
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Principle of the Differential Protection I-DIFF>>
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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Principle of the Differential Protection I-DIFF>>
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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Principle of the Differential Protection I-DIFF>>
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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Principle of the Differential Protection I-DIFF>>
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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Principle of the Differential Protection I-DIFF>>
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