Power Transmission and Distribution
Principle of the Differential Protection
Station B
Station A
IA
IB
SD
SD
Protection Data
Interface
(PDI)
Protection Data
Interface
(PDI)
The 7SD5 / 7SD610 has an
other tripping characteristic:
IDiff
trip
area
forbidden area
No. 1 with
Energy Automation
Tripping
if IDiff > IRest
!!!
then TRIP
Where IRest = P-IDiff>+ I
P-IDiff>=NParameter 1210
restraint
area
I
Ii
i 0
P-IDiff>
Energy Automation
IRest
Ii = ICT-Err.+ ISignal-Err+ISync-Err
Remark:
Page 5
Power Transmission and Distribution
Principle of the Differential Protection
I-Phasor
No. 1 with
Energy Automation
The I-Phasor can be drawn as a normal phasor in a
complex area with a circle at the end. The circle with the
radius I is representing all errors of the phasor.
Im { I }
I
The I is the summation of:
Ii = ICT-Err.+ ISignal-Err.+ ISync-Err.
I =A e-j(
t+ )
Where
ICT-Err.
= CT - Errors
ISignal-Err = Error due to signal distortion
ISync-Err = Synchronization Errors
Re { I }
Energy Automation
Page 6
Power Transmission and Distribution
Principle of the Differential Protection
No. 1 with
Energy Automation
I-Phasor
Station B
Station A
IA
IB
SD
SD
Protection Data
Interface
(PDI)
Protection Data
Interface
(PDI)
Im { I }
Both relays exchange the
I-Phasor via the Protection
Data Interface (PDI). Each relay
‘combines’ the phasors
(local and remote).
IDiff = IA + IB (summation of 2 complex values)
I = IA + IB (simple summation of two values)
I
IA
IA
IB
IA
IA =A e-j(
IB
The summation is done for all
3 phases separately.
The differential protection in the
7SD is phase segregative!
IB
Energy Automation
IDiff = IA+ IB
IB =B e-j(
t+ )
Re { I }
t+ )
Page 7
Power Transmission and Distribution
Principle of the Differential Protection
No. 1 with
Energy Automation
Parameter P-IDIFF>
How to see the different components which lead to the IRest
IRest = P-IDiff>+ I
P-IDiff>= Parameter 1210
The Parameter P-IDiff> (1210) can directly be seen in the
fault record.
IA = 0 at both ends
( IRest) iS1
Energy Automation
IDiff>
Page 8
Power Transmission and Distribution
Principle of the Differential Protection
No. 1 with
Energy Automation
Parameter P-IDIFF>
How to see the different components which lead to the IRest
IRest = P-IDiff>+ I
P-IDiff>Switch On= Parameter 1213
If a Switch On is recognized by the relay the Parameter
P-IDiff>-Switch-On (1213) becomes active for the given time
parameterized in parameter 1132A.
P1210
P1213
P1132A
Energy Automation
Page 9
Power Transmission and Distribution
Principle of the Differential Protection
No. 1 with
Energy Automation
CT-Errors
How to see the different components which lead to the IRest
IRest = P-IDiff>+ I ; I = ICT-Errors + ISignal-Errors + ISync-Errors
CT – Errors:
The figure below shows a real CT error curve (blue) and
one possibility of the approximation of this curve (red)
ICT[A]
approximation of
the CT error curve
real CT error
curve at rated
burden
kscc IN-Sec
IN-Sec
Energy Automation
ICT[A]
Page 10
Power Transmission and Distribution
Principle of the Differential Protection
No. 1 with
Energy Automation
CT-Errors
How to see the different components which leads to the IRest
IRest = P-IDiff>+ I ; I = ICT-Errors + ISignal-Errors + ISync-Errors
CT – Errors:
The CT-Errors are represented by 3 parameters
CT[%]
CT
P254
[A]
Slope P254
P253
Slope P253
P251
ICT/IN-Relay-sec
IN-Relay-sec*P251
IRelay-sec[A]
The parameters 253 and 254 are defining two slopes. The
parameter 251 defines the ‘switching over’ between the two
slopes.
Energy Automation
Page 11
Power Transmission and Distribution
Principle of the Differential Protection
No. 1 with
Energy Automation
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 mA
312 mA+10% 1,5A=462 mA
Energy Automation
Page 12
Power Transmission and Distribution
Principle of the Differential Protection
No. 1 with
Energy Automation
Signal-Errors
How to see the different components which lead to the IRest
IRest = P-IDiff>+ I ;
I = ICT-Errors + ISignal-Errors + ISync-Errors
Principle:
The 7SD measures a current signal i(t) (red curve). Out of this signal the
7SD calculates the phasor from the fundamental frequency component
I = A e-j( t+ ) (blue curve) and compares both signals. The deviation
between 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.
__
__
measured signal
phasor calc. out of the measured signal
deviation between the measured signal and the calculated phasor
Energy Automation
Page 13
Power Transmission and Distribution
Principle of the Differential Protection
No. 1 with
Energy Automation
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 to
signal distortion.
Additional restraining due to
signal disturbance
Energy Automation
Page 14
Power Transmission and Distribution
Principle of the Differential Protection
No. 1 with
Energy Automation
Sync-Errors
How to see the different components which lead to the IRest
IRest = P-IDiff>+ I ;
I = ICT-Errors + ISignal-Errors + ISync-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
Energy Automation
Page 15