Differential protection in electrical power networks
View Patent ↗A method in a three-terminal differential protection includes measuring a terminal phase current ( I T1(y) , I T2(y) , I T3(y) ) at each terminal (T 1 , T 2 , T 3 ) of a protected zone of the differential protection in each phase (L 1 , L 2 , L 3 ) to be protected, and determining a bias current (I b(y) ) of the phase on the basis of the terminal phase currents ( I T1(y) , I T2(y) , I T3(y) ). The bias current determination includes subtracting phasor values of the terminal phase currents of two terminals from the phasor value of the terminal phase current of one terminal. The terminal phase current, which is a minuend in the phasor difference computation, is selected on the basis of the phase angles or amplitudes of the terminal phase currents.
1. A method in a three-terminal differential protection in an electrical power network, the method comprising:
measuring a terminal phase current at each terminal of a protected zone of the differential protection in each phase to be protected;
determining a bias current of the phase on the basis of the terminal phase currents;
determining whether a fault exists in the protected zone based on a threshold value computed as a function of the bias current; and
sending a control signal to a terminal of a respective protected zone upon the existence of a fault,
wherein the determination of the bias current includes subtracting phasor values of the terminal phase currents of two terminals from the phasor value of the terminal phase current of a remaining one terminal, and
wherein the terminal phase current, which is a minuend in the subtraction of phasor values, is selected on the basis of a comparison of phase angles or amplitudes of the terminal phase currents among the terminals.
2. A method according to claim 1 , wherein the bias current of the phase is determined with
I
b
(
y
)
=
1
X
I
_
A
(
y
)
-
I
_
B
(
y
)
-
I
_
C
(
y
)
,
wherein I b(y) is the bias current in phase y, y=L 1 , L 2 , L 3 , I A(y) , I B(y) and I C(y) are phasor values of the terminal phase currents, and X is a scaling factor.
3. A method according to claim 2 , wherein the scaling factor X has a value of one or two.
4. A method according to claim 1 , wherein the terminal phase current, which is the minuend, is selected to be the terminal phase current whose absolute phase angle difference from the phase angles of both the remaining terminal phase currents exceeds 120 degrees.
5. A method according to claim 1 , comprising:
determining amplitudes of the terminal phase currents;
comparing the determined amplitudes with a first limit value for the amplitude of the terminal phase currents; and
if the amplitude of each terminal phase current exceeds the first limit value, determining the phase angles of the terminal phase currents, and selecting the minuend to be the terminal phase current whose absolute phase angle difference from the phase angles of both the remaining terminal phase currents exceeds 120 degrees.
6. A method according to claim 4 , wherein the terminal phase current whose absolute phase angle difference from the phase angles of both the remaining terminal phase currents exceeds 120 degrees is selected to be the minuend only if the amplitude of this terminal phase current exceeds a second limit value for the amplitude of the terminal phase currents.
7. A method according to claim 5 , wherein the terminal phase current whose absolute phase angle difference from the phase angles of both the remaining terminal phase currents exceeds 120 degrees is selected to be the minuend only if the amplitude of this terminal phase current exceeds a second limit value for the amplitude of the terminal phase currents.
8. A method according to claim 4 , wherein the minuend is selected to be the terminal phase current whose amplitude is the highest one if there is no terminal phase current whose absolute phase angle difference from the phase angles of both the remaining terminal phase currents exceeds 120 degrees.
9. A method according to claim 4 , wherein the minuend is selected to be the terminal phase current whose amplitude is the highest one if the amplitude of the terminal phase current selected to be the minuend does not exceed the second limit value for the amplitude of the terminal phase currents.
10. A method according to claim 5 , wherein the minuend is selected to be the terminal phase current whose amplitude is the highest one if there is no terminal phase current whose absolute phase angle difference from the phase angles of both the remaining terminal phase currents exceeds 120 degrees.
11. A method according to claim 5 , wherein the minuend is selected to be the terminal phase current whose amplitude is the highest one if the amplitude of the terminal phase current selected to be the minuend does not exceed the second limit value for the amplitude of the terminal phase currents.
12. A method according to claim 1 , wherein the terminal phase current, which is the minuend, is selected to be the terminal phase current whose amplitude is the highest one.
13. A method according to claim 1 , wherein a protection operation of the phase to be protected is initiated if at least a value of a differential current exceeds a corresponding threshold value.
14. A method according to claim 13 , wherein the protection operation of the phase to be protected is provided if the value of the differential current exceeds the corresponding threshold value and if other conditions for the presence of a fault inside the protected zone are also fulfilled.
15. A three-terminal differential protection system for an electrical power network, the differential protection system comprising
at least one current measurement device at each terminal of a protected zone of the differential protection system in each phase to be protected for measuring terminal phase currents at each terminal; and
at least one intelligent electronic device configured to determine a bias current of the phase on the basis of the terminal phase currents obtained by measuring terminal phase currents at each terminal for each phase to be protected, and send a control signal to a respective terminal based on a value determined as a function of the bias current,
wherein the intelligent electronic device is configured for the bias current determination to subtract phasor values of terminal phase currents of two terminals from the phasor value of the terminal phase current of a remaining one terminal, and
wherein the terminal phase current, which is a minuend in the subtraction of phasor values, is selected on the basis of a comparison of phase angles or amplitudes of the terminal phase currents among the terminals.
16. A three-terminal differential protection system as claimed in claim 15 , wherein the intelligent electronic device is configured to determine the bias current of the phase with
I
b
(
y
)
=
1
X
I
_
A
(
y
)
-
I
_
B
(
y
)
-
I
_
C
(
y
)
wherein I b(y) is the bias current in phase y, y=L 1 , L 2 , L 3 , I A(y) , I B(y) and I C(y) are phasor values of the terminal phase currents, and X is a scaling factor.
17. A three-terminal differential protection system as claimed in claim 15 , wherein the intelligent electronic device is configured to select the terminal phase current, which is the minuend, to be the terminal phase current whose absolute phase angle difference from the phase angles of both the remaining terminal phase currents exceeds 120 degrees.
18. A three-terminal differential protection system as claimed in claim 15 , wherein the intelligent electronic device is configured to:
determine amplitudes of the terminal phase currents;
compare the determined amplitudes with a first limit value for the amplitude of the terminal phase currents; and
if the amplitude of each terminal phase current exceeds the first limit value, determine the phase angles of the terminal phase currents, and select the minuend to be the terminal phase current whose absolute phase angle difference from the phase angles of both the remaining terminal phase currents exceeds 120 degrees.
19. A three-terminal differential protection system as claimed in claim 17 , wherein the intelligent electronic device is configured to select the terminal phase current whose absolute phase angle difference from the phase angles of both the remaining terminal phase currents exceeds 120 degrees to be the minuend only if the amplitude of this terminal phase current exceeds a second limit value for the amplitude of the terminal phase currents.
20. A three-terminal differential protection system as claimed in claim 18 , wherein the intelligent electronic device is configured to select the terminal phase current whose absolute phase angle difference from the phase angles of both the remaining terminal phase currents exceeds 120 degrees to be the minuend only if the amplitude of this terminal phase current exceeds a second limit value for the amplitude of the terminal phase currents.
21. A three-terminal differential protection system as claimed in claim 16 , wherein the intelligent electronic device is configured to select the minuend to be the terminal phase current whose amplitude is the highest one if there is no terminal phase current whose absolute phase angle difference from the phase angles of both the remaining terminal phase currents exceeds 120 degrees.
22. A three-terminal differential protection system as claimed in claim 16 , wherein the intelligent electronic device is configured to select the minuend to be the terminal phase current whose amplitude is the highest one if the amplitude of the terminal phase current selected to be the minuend does not exceed the second limit value for the amplitude of the terminal phase currents.
23. A three-terminal differential protection system as claimed in claim 17 , wherein the intelligent electronic device is configured to select the minuend to be the terminal phase current whose amplitude is the highest one if there is no terminal phase current whose absolute phase angle difference from the phase angles of both the remaining terminal phase currents exceeds 120 degrees.
24. A three-terminal differential protection system as claimed in claim 17 , wherein the intelligent electronic device is configured to select the minuend to be the terminal phase current whose amplitude is the highest one if the amplitude of the terminal phase current selected to be the minuend does not exceed the second limit value for the amplitude of the terminal phase currents.
25. An intelligent electronic device including a processor and a non-transitory computer-readable recording medium, which has recorded thereon a computer program that, when executed by the processor, causes the intelligent electronic device to execute a method in a three-terminal differential protection in an electrical power network, the method comprising:
measuring a terminal phase current at each terminal of a protected zone of the differential protection in each phase to be protected;
determining a bias current of the phase on the basis of the terminal phase currents;
determining whether a fault exists in the protected zone based on a threshold value computed as a function of the bias current; and
sending a control signal to a terminal of a respective protected zone upon the existence of a fault,
wherein the determination of the bias current includes subtracting phasor values of the terminal phase currents of two terminals from the phasor value of the terminal phase current of a remaining one terminal, and
wherein the terminal phase current, which is a minuend in the subtraction of phasor values, is selected on the basis of a comparison of phase angles or amplitudes of the terminal phase currents among the terminals.