SE1651571A1 - Open current circuit detection - Google Patents

Abstract

The present disclosure relates to a method performed by a protection relay for detecting an open circuit in an electrical power system component. The method comprises calculating the import current (ip) imported to the electrical power system component. The method also comprises calculating the export current (ip) exported from the electrical power system component. The method also comprises determining that the differential current (i), being the difference between the import and export currents, is not zero, indicating a possible fault. The method also comprises determining, based on the import, export and/or differential currents, that the determined non-zero differential current is due to an open circuit rather than a fault.(Fig 3)

Description

DETEKTION Av EN ÖPPEN STRÖMKRETSOPEN CURRENT CIRCUIT DETECTIoN TECHNICAL FIELD The present disclosure relates to an open circuit detection method for current transformers for the differential protection functions.

BACKGROUND In power systems, the current transformers (CT) are used to measure thecurrents for the protection relays, Intelligent Electronic Device (IED), andmeasurement devices such as metering. The broken off current transformeroutput, normally known as “Open CT” condition, might create wrongcalculations in the protection relays. For example, a differential protectionfunction might produce “false differential current” in case of one side ofcurrent input has become zero suddenly. As a result, the protection relaymight trip incorrectly due to this condition. It is therefore needed to find a reliable way to detect the “Open CT” condition.

For the Open CT detection, different methods have been proposed before.Many of those methods are based on the zero sequence current detections.There are two issues with the zero sequence current based detection method as listed below: -The zero sequence current based detection method cannot detect the relatedOpen CT phase because it is only based on zero sequence current calculation from total three phase currents.

-The method might loose its function for high impedance fault conditions,where the single phase fault current is similar to the level of zero sequence current detection level which is used for Open CT detection.

SUMMARY In order to provide a system solution for Open CT detection for all operational conditions, a new method which is based on the detection of importing current and exporting current in a phase segmented line isprovided. The concept of reliable Open CT detection may in accordance withthe present invention be based on the calculation on importing and exportingcurrent of a protected object which could be a transmission line or other power system component(s).

Most power system components used in electrical transmission systems, suchas transmission lines or buses, may be considered as a protected object ingeneral from a power system protection point of view. If this object isprotection by differential protection, the object could also be called a differential protection zone.

According to an aspect of the present invention there is provided a methodperformed by a protection relay for detecting an open circuit in an electricalpower system component. The method comprises calculating the importcurrent (iimp) imported to the electrical power system component. Themethod also comprises calculating the export current (iexp) exported fromthe electrical power system component. The method also comprisesdetermining that the differential current (id), being the difference betweenthe import and export currents, is not zero, indicating a possible fault. Themethod also comprises determining, based on the import, export and/ ordifferential currents, that the determined non-zero differential current is due to an open circuit rather than a fault.

Embodiments of the present invention provided a general method for OpenCT detection for different power system component for differential protectionapplications. A point is that the Open CT condition may not change primarysystem conditions and it may be a secondary measuring system failurecondition. The method could enhance security of existing differentialprotection solutions used in AC power systems. For transmission lineapplication, it may give a clear difference between Open CT and high impedance fault conditions.

Generally, all terms used in the claims are to be interpreted according to theirordinary meaning in the technical field, unless explicitly defined otherwiseherein. All references to "a/ an /the element, apparatus, component, means,step, etc." are to be interpreted openly as referring to at least one instance ofthe element, apparatus, component, means, step, etc., unless explicitly statedotherwise. The steps of any method disclosed herein do not have to beperformed in the exact order disclosed, unless explicitly stated. The use of“first”, “second” etc. for different features/ components of the presentdisclosure are only intended to distinguish the features / components fromother similar features / components and not to impart any order or hierarchy to the features / components.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments will be described, by way of example, with reference to the accompanying drawings, in which: Fig 1 is a schematic illustration of an embodiment of a power systemcomponent as a differential protection zone, in accordance with the present invention.

Fig 2 is a schematic illustration of an embodiment of a power systemcomponent as a differential protection zone, under normal load conditions, in accordance with the present invention.

Fig 3 is a schematic illustration of an embodiment of a power systemcomponent as a differential protection zone, under Open CT conditions, in accordance with the present invention.

Fig 4 is a schematic graph illustrating an embodiment of Open CT conditionsimulation results where the upper window shows instantaneous signals andthe bottom window shows RMS values, in accordance with the present invention.

Fig 5 is a schematic illustration of an embodiment of a power system component as a differential protection zone under solid ground fault condition - importing current equals to differential current, in accordance with the present invention.

Fig 6 is a schematic illustration of an embodiment of a power systemcomponent as a differential protection zone under high impedance fault condition, in accordance with the present invention.

Fig 7 is a schematic graph illustrating an embodiment of high impedancefault condition simulation results where the upper window showsinstantaneous signals and the bottom window shows RMS values, in accordance with the present invention.

Fig 8 is a schematic flow chart illustrating embodiments of a method for Open CT and/ or fault detection, in accordance with the present invention.

Fig 9 is a schematic illustration of an example embodiment of a power systemcomponent as a differential protection zone, for calculation of importing current and exporting current, in accordance with the present invention.

Figs 1oa-d are schematic logic diagrams illustrating the calculation of positive sign signal in phase A, in accordance with the present invention.

Figs 11a-d are schematic logic diagrams illustrating the calculation of negative sign signal in phase A, in accordance with the present invention.

Figs 12a-b are schematic logic diagrams illustrating the calculation ofimporting current and exporting current, in accordance with the present invention.

DETAILED DESCRIPTION Embodiments will now be described more fully hereinafter with reference tothe accompanying drawings, in which certain embodiments are shown.However, other embodiments in many different forms are possible within thescope of the present disclosure. Rather, the following embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers refer to like elements throughout the description.

As shown in figure 1, two currents could be used to represent the current flowin this type of components in each phase in a three phase Alternate Current(AC) transmission system, importing current (iimp) to the component andexporting current (iexp) from the component during normal load conditions.In transmission line condition, the capacitive leakage current may becalculated as differential current id based on the importing current andexporting current. The capacitive leakage current is depending on the lengthof the line and voltage level of the line. For short lines (below 50 km) withrated voltage below 500 kV, capacitive leakage current may be neglected. Theimporting current for the transmission line may be calculated by the sum ofimporting side currents in a phase segmented way, and the exporting currentmay be calculated by the sum of load side currents exported from thecomponent. For a bus-bar, the importing current may be the sum of allsource currents feeding to the bus-bar and the exporting current may be the sum of all load currents.

For simplicity, the calculation of importing current and exporting currentmay be further given by equations (1) and (2) in the discrete time domain asbelow if we assume that there are M connected branches to the protection object (a power system component as in figure 1).

In the discrete time domain at the sampling instant k, if the measured branchcurrent samples are positive, the sum of those positive sample branch current signals may be calculated in accordance with equation 1. fr? =ï (1) In the discrete time domain at the sampling instant k, if the measured branchcurrent samples are negative, the sum of those negative sample branch current signals may be calculated in accordance with equation 2. Åfil .Åšlqså' fåi 1 __. ._ _._r'=,~'\:~i-ï -r iI. _ w» (2) Once the above values are obtained, the importing current and exportingcurrent may be obtained by using equations (3) and (4):“e _ = dldišïsf.ßrrs:f~rs.sa~rxgg :_ * * ' (3) =- ,_-=l-íz“f~r_z_rxf.tfn-f :_ J; (4) Following conditions may be considered if a short line is used as an example: 1. Normal load condition: As shown in figure 2, importing current equals toexporting current so that iimp = iexp while the differential current id is close to zero if the capacitive current can be neglected. 2. Open CT Condition: Since Open CT condition may be a secondarymeasurement condition while the primary power system condition has notbeen changed, the importing current iimp is not changed while the exportingcurrent iexp is decreased due to open CT condition. As a result, the differentialcurrent id will be increased to the level of Open CT current level. Figure 3shows this situation. Figure 4 shows that the instantaneous absolute values ofiimp, iexp and id during a normal load condition with an open CT occur at 0.4 second. 3. Fault Condition: For the internal fault conditions, two typical cases couldbe considered, solid ground fault condition and high impedance faultcondition. For the solid ground fault condition, the importing current will flow into the fault loop and the exporting current will be zero because the fault loop has shorted the power transmission route (voltage at the fault pointis almost zero). In this case, importing current is equal to the differential current and exporting current is zero. Figure 5 shows this situation.

For high impedance ground fault condition, there may still be certainexporting current flowing out from the differential zone. A small increase inthe differential current may be created by the added fault loop. Under thiscondition, the importing current will feed to both the faulted loop andexporting load loop. Figure 6 shows this situation and figure 7 showssimulation results. A difference between high impedance fault condition andopen CT condition may be that the high impedance fault condition is involvedwith importing current increase because of added fault impedance loop, whilethe open CT condition is created by the secondary measurement circuitfailure which does not create the increase of importing current. This can be seen in figure 7.

If the capacitive current in a transmission line cannot be neglected, it may nenecessary to subtract the capacitive current from the importing current firstso that the calculated new importing current (iimpJww) will be equal to theexporting current during normal load condition, as given in the equation (5)below. Once this calculation is done, the described method above may still beapplied to detect Open CT condition. i :FÅ -š 'štfw s: 'tages ' 'fsï-:ïafrprstfišlfšë-æ? <ïïarrre>srñ Figure 8 illustrates a flow chart of embodiments of the Open CT detection method of the present invention.

If the protected object is a transmission line with multi-terminals, it may beneeded to calculate the importing currents and exporting currents based onall importing sides and exporting sides first and then apply the aboveprinciple to detect the “Open CT” condition.

If the protected object is a transformer, it may be necessary to make properphase angle compensation as used in the transformer differential protectionand also adjust each side currents into the same level by using turn-ratiocorrection. Once this is done, importing current and exporting current could be calculated properly based on power flow directions.

If the protected object is a generator, neutral side currents could beconsidered as importing currents and terminal side currents could be considered as exporting currents in all three phases.

The same principle may be applied to any other power system component aslong as a differential protection zone may be used such as capacitor bank, shunt reactors, motors, etc.Example - Calculation of Importing and Exporting Currents As shown above, with the equation (1) and (2), the importing and exportingcurrents may be calculated. In the discrete time domain at the samplinginstant k, if the measured branch current samples are positive, the sum ofthose positive sample branch current signals may become the total positivebranch current. In the discrete time domain at the sampling instant k, if themeasured branch current samples are negative, the sum of those negative sample branch current signals will become the total negative branch current.

Once the above values are obtained, the importing current and exporting currents can be obtained by using equations (3) and (4) as given above.

We may consider a protected zone with three current input braches as shown in figure 9.

Referring to figures 1oa-d, importing current and exporting current arecalculated by each phase as given below if we just use phase A as example.Here we assume I1As2, I2As2, I3_As2 are CT secondary currents in phase Aobtained from each CT current inputs from three CTs. Ia1_p, Ia2_p and Ia3_p are positive sign signals obtained by using positive sign comparators.

Then the sum of positive sign signal Ia_p in phase A can be obtained as shown in figures 1oa-d logic diagrams.

Similarly, the negative sign summation signal Ia_n can be obtained too, as shown in figures 11a-d.

Once the negative sign summation signal and positive sign summation signalare obtained, it is possible to calculate the total importing current and exporting current in phase A as given in figures 12a-b.

The above calculation may be extended to any number of branches connectedto a protection zone and can be used for any type of differential protectionfunction as a base with naturally phase segmented calculation. Therefore, it is not necessary to do phase selection.

Modifications and other variants of the described embodiment(s) will cometo mind to one skilled in the art having the benefit of the teachings presentedin the foregoing descriptions and the associated drawings. Therefore, it is tobe understood that the embodiment(s) is/ are not to be limited to the specificexamples disclosed and that modifications and other variants are intended tobe included within the scope of this disclosure. Although specific terms maybe employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.

Claims (1)

1. A method performed by a protection relay for detecting an open circuit in an electrical power system component, the method comprising: calculating the import current (iimp) imported to the electrical power system component; calculating the export current (iexp) exported from the electrical power SyStem COIIIPOIICIIÉ; determining that the differential current (id), being the difference between the import and export currents, is not zero, indicating a possible fault; determining, based on the import, export and/ or differential currents, thatthe determined non-zero differential current is due to an open circuit rather than a fault.