Method and apparatus for differential protection of an electric connection
View Patent ↗A method and apparatus are provided for differential protection of a three phase electric connection of an electric system. The apparatus includes means for determining a value of an operate quantity, which operate quantity is based on a difference between current quantities at a first point and a second point of the electric connection, and means for detecting a fault in the electric connection between the first point and the second point if the value of the operate quantity exceeds a value of a predetermined threshold quantity. The operate quantity is based on a difference of residual currents at the first point and the second point, or a quantity indicative thereof, and a coefficient indicative of an angle between a residual voltage of the electric connection and the difference of residual currents at the first point and the second point.
1. A method for differential protection of a three phase electric connection of an electric system, the method comprising:
detecting current quantities at a first point and a second point of the three phase electric connection of the electric system;
determining a value of an operate quantity, the operate quantity being based on a difference between the detected current quantities at the first point and the second point of the electric connection;
determining that a fault is present in the electric connection between the first point and the second point if the value of the operate quantity exceeds a value of a predetermined threshold quantity; and
controlling, by at least one protective relay, a switching element of the electric system to activate a fault protection when a fault in the electric connection is determined to be present,
wherein the operate quantity I OP is defined by the following equation:
I OP =|d 3 Ī 0 |·gain=|3 Ī 0L +3 Ī 0R |·gain,
where
d3Ī 0 =residual differential current phasor,
3Ī 0L =residual current phasor at the first point such that a positive current flow is towards the second point,
3Ī 0R =residual current phasor at the second point such that a positive current flow is towards the first point, and
gain=cos(angle(− Ū 0 /d 3 Ī 0 )−□□ b ),
where
φ b =characteristic angle setting, and
Ū 0 =residual voltage phasor of the electric connection.
2. The method of claim 1 , wherein the characteristic angle setting is 0° for a compensated electric system, −30° for a high-resistance earthed electric system, and −90° for an unearthed electric system.
3. The method of claim 1 , wherein the characteristic angle setting is set based on an angle between a negative residual voltage and a residual current during an earth fault in the electric connection.
4. The method of claim 1 , wherein the predetermined threshold quantity is:
P 0 when I RES ≦I KNEE _ 0 , and
P 0 +S 0 *( I RES −I KNEE _ 0 ) when I RES >I KNEE _ 0 ,
where
P 0 , S 0 , I KNEE _ 0 =predetermined parameters, and
I
RES
=
I
_
aL
+
I
_
aR
2
+
I
_
bL
+
I
_
bR
2
+
I
_
cL
+
I
_
cR
2
3
where
Ī aL , Ī bL , Ī cL =phase currents of the electric connection at the first point, and
Ī aR , Ī bR , Ī cR =phase currents of the electric connection at the second point.
5. The method of claim 3 , wherein
P 0 =basic sensitivity of the protection, where
S 0 =50%, and
I KNEE _ 0 =1 p.u.
6. A non-transitory computer-readable recording medium having a computer program recorded thereon that causes a processor of a computer processing device to carry out operations for differential protection of a three phase electric connection of an electric system, the operations comprising:
detecting current quantities at a first point and a second point of the three phase electric connection of the electric system;
determining a value of an operate quantity, the operate quantity being based on a difference between the detected current quantities at the first point and the second point of the electric connection;
determining that a fault is present in the electric connection between the first point and the second point if the value of the operate quantity exceeds a value of a predetermined threshold quantity; and
controlling, by at least one protective relay, a switching element of the electric system to activate a fault protection when a fault in the electric connection is determined to be present,
wherein the operate quantity I OP is defined by the following equation:
I OP =|d 3 Ī 0 |·gain=|3 Ī 0L +3 Ī 0R |·gain,
where
d3Ī 0 =residual differential current phasor,
3Ī 0L =residual current phasor at the first point such that a positive current flow is towards the second point,
3Ī 0R =residual current phasor at the second point such that a positive current flow is towards the first point, and
gain=cos(angle(− Ū 0 /d 3 Ī 0 )−□□ b ),
where
φ b =characteristic angle setting, and
Ū 0 =residual voltage phasor of the electric connection.
7. An apparatus for differential protection of a three phase electric connection of an electric system, the apparatus comprising:
means for detecting current quantities at a first point and a second point of the three phase electric connection of the electric system;
means for determining a value of an operate quantity, the operate quantity being based on a difference between the detected current quantities at the first point and the second point of the electric connection; and
means for determining that a fault is present in the electric connection between the first point and the second point if the value of the operate quantity exceeds a value of a predetermined threshold quantity; and
means for controlling, by at least one protective relay, a switching element of the electric system to activate a fault protection when a fault in the electric connection is determined to be present,
wherein the operate quantity I OP is defined by the following equation:
I OP =|d 3 Ī 0 |·gain=|3 Ī 0L +3 Ī 0R |·gain,
where
d3Ī 0 =residual differential current phasor,
3Ī 0L =residual current phasor at the first point such that a positive current flow is towards the second point,
3Ī 0R =residual current phasor at the second point such that a positive current flow is towards the first point, and
gain=cos(angle(− Ū 0 /d 3 Ī 0 )−□□ b ),
where
φ b =characteristic angle setting, and
Ū 0 =residual voltage phasor of the electric connection.
8. The apparatus of claim 7 , wherein the characteristic angle setting is 0° for a compensated electric system, −30° for a high-resistance earthed electric system, and −90° for an unearthed electric system.
9. The apparatus of claim 7 , wherein the characteristic angle setting is set based on an angle between a negative residual voltage and a residual current during an earth fault in the electric connection.
10. The apparatus of claim 7 , wherein the predetermined threshold quantity is:
P 0 when I RES ≦I KNEE _ 0 , and
P 0 +S 0 *( I RES −I KNEE _ 0 ) when I RES >I KNEE _ 0 ,
where
P 0 , S 0 , I KNEE _ 0 =predetermined parameters
I
RES
=
I
_
aL
+
I
_
aR
2
+
I
_
bL
+
I
_
bR
2
+
I
_
cL
+
I
_
cR
2
3
where
Ī aL , Ī bL , Ī cL =phase currents of the electric connection at the first point, and
Ī aR , Ī bR , Ī cR =phase currents of the electric connection at the second point.
11. The apparatus of claim 10 , wherein P 0 =basic sensitivity of the protection, where
S 0 =50%, and
I KNEE _ 0 =1 p.u.
12. The apparatus claim 7 , comprising:
a protective relay.