Current compensation method and device for power system protection
View Patent ↗The present invention relates to a method for current compensation of a protection system for protecting a zone in a power system, which zone comprises a number of transmission lines connected to power sources and a number of transmission lines connected to a number of loads where the power sources and the loads are arranged outside the zone and a number of current transformers (CT) arranged to the transmission lines, wherein the method comprises the steps of continuously measuring all the incoming currents (I in ) to the zone, continuously measuring all the outgoing currents (I out ) from the zone, continuously calculating the differential current (I d ) according to I d =I in −I out , continuously calculating q=I out /I in , continuously comparing q with set values and continuously comparing I out with a set value Set1. The invention is characterised by setting I out =I in =I comp at the instant when q changes and I out exceeds the value set1, which is an indication of the occurrence of a fault external to the protection zone due to saturation of the CT of the faulted outgoing transmission line, and using the value I comp in the protection system in order to prevent tripping of the protection system. The present invention also relates to a device and a computer program product for performing the method.
1. A method for current compensation of a protection system for protecting a zone in a power system having a number of incoming transmission lines connected to power sources and a number of outgoing transmission lines connected to a number of loads where the power sources and the loads are arranged outside the zone and a number of current transformers (CT) responsively coupled to the transmission lines, comprises the steps of:
continuously measuring all incoming currents (I in ) to the zone,
continuously measuring all outgoing currents (I out ) from the zone,
continuously calculating a differential current (I d ) according to
I d =I in −I out ,
continuously calculating q=I out /I in , and
continuously comparing q with set values,
continuously comparing I out with a set value Set 1 ,
setting I out =I in at an instant when q changes and I out exceeds a value Set 1 , which is an indication of the occurrence of a fault external to the protection zone due to saturation of the CT of the faulted outgoing transmission line, and
using the value I out as a compensated current I comp in the protection system in order to prevent tripping of the protection system due to external faults.
2. Method according to claim 1 , comprising continuously integrating said value I comp according to
I
OUTc
=
∫
t1
(
t1
+
T
)
I
comp
ⅆ
t
,
continuously putting I Rnew =I OUTc ,
continuously calculating I Dnew =I OUT −I OUTc , and
continuously comparing I Dnew and I Rnew according to I Dnew −k×I Rnew >0.
3. The method according to claim 2 , wherein the factor k is set in the range 0.3–0.8.
4. The method according to claim 1 , wherein the incoming current and outgoing current are sampled and wherein the method comprises the steps:
(1) evaluating q according to
q ( i− 2)>0.6
q ( i− 1)>0.6
q ( i )<0.8
(2) evaluating I out according to
I out ( i− 1)>Set 1 ,
(3) evaluating I in according to
I in ( i )> I in ( i− 1), and
(4) evaluating I out according to
I out ( i− 1)> I out ( i− 2)
wherein I comp is a compensated current
and
I out ( i− 1)×0.4 >I d ( i− 1),
wherein (i) is the present sampling occurrence, and setting I out =I in if all criteria (1)–(4) are fulfilled.
5. The method according to claim 4 , further comprising the steps of:
(5) evaluating I in according to
I in ( i )<Set 1
or
I in decreases, and
resetting I out to its actual value when (5) is fulfilled.
6. The method according to claim 1 , comprising maintaining I out =I in for a selected time after I out has been set to I in .
7. The method according to claim 6 , wherein the selected time is 12 ms.
8. A device for current compensation of a protection system for protecting a zone in a power system, having a number of incoming transmission lines connected to power sources and a number of outgoing transmission lines connected to a number of loads where the power sources and the loads are arranged outside the zone and a number of current transformers coupled to the transmission lines, comprising means for continuously measuring all incoming currents (I in ) to the zone, means continuously measuring all outgoing currents (I out ) from the zone, means for continuously calculating a differential current (I d ) according to
I d =I in −I out , means for continuously calculating q=I out /I in , and means for continuously comparing q with a set value S, and means for continuously comparing I out with a set value set 1 ,
means for setting I out =I in at the instant when q changes and I out exceeds the value set 1 , which is an indication of the occurrence of a fault external to the protection zone due to saturation of the CT of the faulted outgoing transmission line, and means for using the set value of I out as a compensated current I comp in the protection system in order to prevent tripping of the protection system.
9. A device according to claim 8 , further including means for continuously integrating said value I comp according to
I
OUTc
=
∫
t1
(
t1
+
T
)
I
comp
ⅆ
t
,
means for continuously putting I Rnew =I OUTc ,
means for continuously calculating I Dnew =I OUT −I OUTc , and
means for continuously comparing I Dnew and I Rnew according to I Dnew −k×I Rnew >0.
10. A computer program product comprising computer code means or software code portions for making a computer or processor perform the steps of:
continuously measuring all the incoming currents (I in ) to the zone,
continuously measuring all the outgoing currents (I out ) from the zone,
continuously calculating the differential current (I d ) according to
I d =I in −I out ,
continuously calculating q=I out /I in ,
continuously comparing q with set values,
continuously comparing I out with a set value Set 1 ,
setting I out =I in at the instant when q changes and I out exceeds the value S, which is an indication of the occurrence of a fault external to the protection zone due to saturation of the CT of the faulted outgoing transmission line, and
using the set value of I out as a compensated current I comp in the protection system in order to prevent tripping of the protection system.
11. A computer program product according to claim 10 , further comprising continuously integrating said value I comp according to
I
OUTc
=
∫
t1
(
t1
+
T
)
I
comp
ⅆ
t
,
continuously putting I Rnew =I OUTc ,
continuously calculating I Dnew =I OUT −I OUTc , and
continuously comparing I Dnew and I Rnew according to I Dnew −k×I Rnew >0.
12. A computer program product according to claim 10 , further comprising sampling the incoming current and outgoing current and performing the steps of:
(1) evaluating q according to
q ( i− 2)>0.6
q ( i− 1)>0.6
q ( i )<0.8
(2) evaluating I out according to
I out ( i− 1)>Set 1 ,
(3) evaluating I in according to
I in ( i )> I in (i−1), and
(4) evaluating I out according to
I out ( i− 1)> I out ( i− 2)
and
I out ( i− 1)×0.4 >I d ( i− 1),
wherein (i) is the present sampling occurrence, and setting I out =I in if all criteria (1)–(4) are fulfilled.
13. A computer program product according to claim 12 , further comprising performing the steps of:
(5) evaluating I in according to
I in ( i )<Set 1
or
I in decreases, and
triggering a timer at the moment when I out is set to I in , and
resetting I out to its actual value when (5) is fulfilled and the timer has ended.
14. Use of a computer program product according to claim 10 , to provide protection measures for a protection zone of a power system.
15. A computer readable medium comprising computer code means according to claim 10 .
16. Use of a device according to claim 8 to provide protection measures for a protection zone of a power system.