SE1300362A1 - Method and fault detection device for detection of a fault in a protected unit included in a power system - Google Patents

Abstract

FÖRFARANDE OCH FELDETEKTERINGSANORDNING FÖR DETEKTERING AV ETT. FEL I EN SKYDDAD ENHET INNEFATTAD I ETT KRAFTSYSTEM. Ett förfarande för användning i feldetektering i en skyddad enhet inbegripen i ett kraftsystem visas. Den skyddade enheten är anordnad for överföring av kraft via åtminstone en däri anordnad strömbana. Förfarandet innefattar avkänning av ström i den åtminstone ena strömbanan i den skyddade enheten i en första ände av denna och avkänning av ström i den åtminstone ena strömbanan i den skyddade enheten i en andra ände av denna. På grundval av de avkända strömmarna bestäms en första signal och en andra signal som indikerar en förändring i ström i den åtminstone ena strömbanan i den skyddade enheten i den första änden av denna respektive den andra änden av denna. Ett polaritetsvärde hos den första signalen respektive den andra signalen bestäms. På grundval av de bestämda polaritetsvärdena bestäms det huruvida det är ett fel i den skyddade enheten.

Description

15 20 25 30 35 11562SE 2 SUMMARY OF THE INVENTION In view of the above, an object of at least some of the embodiments of the present invention is to provide an improved scheme for detection of and protection against faults, e. g. high impedance faults, that may occur in power systems.

Accordingly, a fault detection device, a method and a power system having the features included in the independent claims are provided.

According to a first aspect, there is provided a fault detection device adapted to detect whether there is a fault in a protected unit included in a power system. The protected unit is adapted to convey power via at least one current path therein. The fault detection device comprises a current sensor unit adapted to sense current in the at least one current path in the protected unit at a first end thereof and to sense current in the at least one current path in the protected unit at a second end thereof. The fault detection device comprises a processing unit which is coupled to the current sensor unit. The processing unit is adapted to, based on the sensed currents, determine a first signal and a second signal indicative of change in current in the at least one current path in the protected unit at the first end and at the second end thereof, respectively. The processing unit is adapted to determine a polarity value of the first signal and the second signal, respectively, and, based on the detennined polarity values, detennine whether there is a fault in the protected unit.

According to a second aspect, there is provided a method for use in fault detection in a protected unit included in a power system, which protected unit is adapted to convey power via at least one current path therein. The method comprises sensing current in the at least one current path in the protected unit at a first end thereof, and sensing current in the at least one current path in the protected unit at a second end thereof. Based on the sensed currents, a first signal and a second signal indicative of change in current in the at least one current path in the protected unit at the first end and at the second end thereof, respectively, are determined. A polarity value of the first signal and the second signal, respectively, is determined. Based on the detennined polarity values, there is detennined whether there is a fault in the protected unit.

According to a third aspect, there is provided a power system including a protected unit adapted to convey power via at least one current path therein and a fault detection device according to the first aspect, which fault detection device is adapted to detect whether there is a fault in the protected unit,.

Embodiments of the present invention can for example be used for DC line protection based on directional comparison and summation of signal polarities based on two end measurements for a single transmission line. However, as will be further described in the following, the same or similar principles can also been extended to multi-tenninal line 10 15 20 25 30 35 11562SE 3 protection topology or any other type of protected objects with multiple connected lines.

Embodiments of the present invention are not sensitive to load conditions, as compared to conventional line differential protection, and may cover a high range of fault impedances. By means of embodiments of the present invention, a relatively high sensitivity for high impedance fault detection and protection may be achieved. Embodiments of the present invention may be used in enhancing or strengthening existing DC line protection solutions with relatively high speed and high sensitivity as a main backup protection. Embodiments of the present invention may also be applied for AC line protection with a relatively high sensitivity.

Further objects and advantages of the present invention are described in the following by means of exemplifying embodiments.

It is noted that the present invention relates to all possible combinations of features recited in the claims. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description.

Those skilled in the art realize that different features of the present invention can be combined to create embodiments other than those described in the following.

BRIEF DESCRIPTION OF THE DRAWINGS Exemplifying embodiments of the invention will be described below with reference to the other accompanying drawings.

F ig. 1 is a schematic view of a portion of a power system in accordance with an embodiment of the present invention.

Fig. 2 is a logic diagram illustrating a method according to an embodiment of the present invention.

F ig. 3 is a logic diagram illustrating a method according to an embodiment of the present invention.

Fig. 4 is a logic diagram illustrating a method according to an embodiment of the present invention.

Fig. 5 is a schematic view of a portion of a power system in accordance with an embodiment of the present invention.

In the accompanying drawings, the same reference numerals denote the same or similar elements throughout the views.

DETAILED DESCRIPTION The present invention will now be described more fully hereinafier With reference to the accompanying drawings, in which exemplifying embodiments of the present 10 15 20 25 30 35 11562SE 4 invention are shown. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will convey the scope of the invention to those skilled in the art. The steps of any method disclosed herein do not have to be performed in the exact order as disclosed, unless explicitly stated so.

Furthermore, like numbers refer to the same or similar elements or components throughout.

Referring now to Fig. 1, there is shown a portion of a power system 100, including a first DC source 101 and a second DC source 102 which are connected via a protected unit 103 including a transmission line 103, including at least one current path. The transmission line 103 is adapted to convey power via the at least one current path.

It is to be understood that although Fig. 1 refers to an embodiment of the present invention where the (portion of the) power system 100 is a DC power system, principles of embodiments of the present invention as described herein can be applied in AC power systems for achieving an AC transmission line protection scheme. In that case, the first DC source 101 and the second DC source 102 could be replaced by a first and a second AC source, respectively.

It is further to be understood that for the purpose of illustrating principles of embodiments of the present invention, embodiments of the present invention are described in the following generally with reference to a portion of a power system 100 including a protected unit 103, which protected unit 103 hence may be part of a power system 100. The protected unit 103 may for example comprise a DC transmission line, such as a High Voltage Direct Current (HVDC) transmission line, or several DC or HVDC transmission lines, e. g. arranged in a DC grid. The power system 100 may for example comprise a HVDC power transmission system.

The portion of the power system 100 depicted in Fig. 1 includes a first busbar 104 and a second busbar 105. As illustrated in Fig. 1, the first DC source 101 and the first busbar 104 are arranged at a first end of the protected unit 103, or transmission line 103, and the second DC source 102 and the second busbar 105 are arranged at a second end of the protected unit 103, or transmission line 103.

At the first end of the transmission line 103 there is arranged a first current sensor 106 and a first voltage sensor 107. At the second end of the transmission line 103 there is arranged a second current sensor 108 and a second voltage sensor 109. The first current sensor 106 is arranged so as to measure a current Idcpl of the transmission line 103 at the first end thereof. The second current sensor 108 is arranged so as to measure a current Idcp2 of the transmission line 103 at the second end thereof. The first voltage sensor 107 is arranged so as to sense or measure a voltage of the transmission line 103 at the first end of thereof, e.g. a voltage Udcpl between the transmission line 103 and ground at the first end of the transmission line 103, as illustrated in Fig. 1. The second voltage sensor 109 is arranged so as 10 15 20 25 30 35 11562SE 5 to sense or measure a voltage of the transmission line 103 at the second end of thereof, e. g. a voltage Udcp2 between the transmission line 103 and ground at the second end of the transmission line 103, as illustrated in Fig. 1.

The first and second voltage sensors 107, 109 are optional.

The first current sensor 106 and the second current sensor 108 may be included in a current sensor unit, which hence is adapted to sense current Idcpl in the at least one current path in the transmission line 103 at the first end thereof, and to sense current Idcp2 in the at least one current path in the transmission line 103 at the second end thereof.

The first current sensor 106 and the second current sensor 108, or the current sensor unit, may be included in a fault detection device adapted to detect whether there is a fault in the transmission line 103.

The fault detection device may further comprise a processing unit (not shown in Fig. 1) coupled to the current sensor unit.

The processing unit is adapted to, based on the sensed currents Idcpl , Idcp2, determine a first signal and a second signal indicative of change in current in the at least one current path in the transmission line 103 at the first end and at the second end, respectively, of the transmission line 103.

The processing unit is further adapted to determine a polarity value of the first signal and the second signal, respectively, and, based on the determined polarity values, determine whether there is a fault in the transmission line 103.

Principles of fault detection and/or protection according to embodiments of the present invention as described herein mainly refers to positive pole to ground faults that may occur in the protected unit 103 (e. g., a transmission line). However, the same or similar principles apply to solid ground faults, high impedance faults, and also to external faults.

Furthermore, it is to be understood that even though, for the purpose of illustrating principles of embodiments of the present invention, embodiments of the present invention are described in the following generally with reference to a single transmission line, the same or similar principles of fault detection and/or protection as described herein can be extended to multi-tenninal transmission line fault detection and/or protection systems for a topology with several interconnected transmission lines. This will be further described in the following with reference to Fig. 5.

As already indicated in the foregoing, it has been observed that current derivative signals A11 = d(Idcpl) / dt for positive pole current Idcpl and A12 = d(Idcp2) / dt for positive pole current Idcp2 can be utilized in transient component based directional comparison protection.

For internal faults (positive pole to ground fault) along the transmission line 103, as illustrated by Fl in Fig. 1, both of A11 and A12 will be positive, because internal faults will create travelling waves which will move from the line side toward each of the first and 10 15 20 25 30 35 11562SE 6 second busbars 104, 105 located at the respective first and second ends of the transmission line 103. For external faults, such as a busbar fault at the second end of the protected unit 103, or transmission line 103, i.e. a fault at the second busbar 105, as illustrated by F2 in F ig. 1, AI1 will be positive while A12 will be negative, because the current at the second end of the transmission line 103 (measured by current sensor 108) will flow from the line side to the busbar side of the transmission line 103. The time difference between the first wave transients of A11 and A12 is the total travelling wave time along the whole length or substantially the whole length of the transmission line 103. For example, if the length of the transmission line 103 would be 400 km and the line travelling wave speed 295 km/ms, the time difference between two first wave front transient signals would be 1.355 ms (or thereabout). According to embodiments of the present invention, the polarity and possibly also amplitude of these two signals A11 and A12 could be used to fonnulate transient based directional comparison protection. A basic concept is to check the polarity of the above-mentioned two transient current derivative signals AI1 and A12 (i.e. based on line measurements at both ends of the transmission line 103) to confinn whether there is an internal fault or not. The logic of this polarity check is illustrated in Fig. 2.

Referring now to Fig. 2, there is shown a logic diagram of a method according to an embodiment of the present invention that can be used to detect a internal fault (e. g., pole to ground fault) in the transmission line 103 (or generally, the protected unit 103). With reference to Fig. 1, there are two inputs AI1 = d(1dcp1) / dt for positive pole current Idcpl and A12 = d(ldcp2) / dt for positive pole current 1dcp2. In general, an aim of the embodiment illustrated in Fig. 2 is to detect a positive increase, if any, in each of the signals AI1 and A12 in the very beginning of the fault period, i.e. a period of e. g. a few milliseconds or less, starting when a fault occurs in the transmission line 103 and running when the fault is present in the transmission line 103.

As illustrated in Fig. 2, each of the signals A11 and AI2 are input into respective modules which check if the value of signal A11 or A12 exceeds a threshold “set1” and is less than a threshold “set2”, respectively. If the value of signal AI1 or A12 exceeds “set1”, a signal is generated after a predefined “DelayOfP time. If the value of signal AI1 or A12 is less than “set2”, a signal is generated after a predefined “DelayOfP time. As illustrated in Fig. 2, one of the respective generated signals is passed directly to an “AND”-element 201 while the other one of the respective generated signals is passed to a “NOT”-element 200, or inverter, before being passed to the “AND”-element 201. As illustrated in Fig. 2, the same logic applies to both the A11 branch and the A12 branch. The signals output from the respective “AND”-elements 201 are then passed to an “AND”-element 202, which outputs a signal DIPS. If AI1 is positive and A12 is positive during the beginning of internal fault period, the polarity of both signals A11 and A12 will become positive so that DIPS will 10 15 20 25 30 35 1l562SE 7 become one (or °high”). If AI2 is negative and AI1 is positive, DIPS will become zero (or “low”).

Values of “set1” and “set2” may for example be about 5 A/(100 ms) and -10 A/(100 ms), respectively. With such settings of “set1” and “set2” it is contemplated that high impedance faults, with impedance up to 2000 Q or more, could be detected. Deviations from the above-mentioned values of “set1” and “set2” are possible, e. g. within a few percent or possibly up to ten percent or more.

In order to make further confinnation for the fault condition, a summation signal DIPSUMS can be derived based on AI1 and AI2 as DIPSUMS=AIl + AI2. DIPSUMS will have a positive value during beginning of intemal faults, and will have a negative value during beginning of external faults.

An example of polarity detection logic based on summation of the signals AI1 and A12 is illustrated in Fig. 3, which is the combination of described signals above. An intemal fault can be confinned by observing that DIPSUMS is equal to one (or set to “high°) and an external fault can be confirmed by observing that DIPSUMS is equal to zero (or set to “low°), as illustrated in Fig. 3.

In F ig. 3, the signals AI1 and AI2 are summed in summation module 301. As illustrated in Fig. 3, if the value of the signal output from module 301 exceeds “k1”, a signal - is generated afier a predefined “DelayOn” time. If the value of the signal output from module 301 is less than “k2”, a signal is generated afier a predefined “DelayOfF time. As illustrated in Fig. 3, one of the respective generated signals is passed directly to an “AND”-element 303 while the other one of the respective generated signals is passed to a “NOT”-element 302, or inverter, before being passed to the “AND”-element 303.

Values of “k1” and “k2” may for example be about 10 A/(100 ms) and -10 A/(100 ms), respectively. With such settings of “k1” and “k2” it is contemplated that high impedance faults, with impedance up to 2000 Q or more, could be detected. Deviations from the above-mentioned values of “k1” and “k2” are possible, e. g. within a few percent or possibly up to ten percent or more.

A further example of polarity detection logic is illustrated in F ig. 4, which is the combination of signals DIPS and DIPSUMS as described above. The signals DIPS and DIPSUMS are passed to an “AND”-element 401. A signal is generated afler a predefined period of time defined by a hold timer module 402, afier which a signal 403 can be generated, e.g. a trip signal indicating whether to trip circuit breakers for isolating the transmission line 103.

The logic described above with reference to Figs. 2-4 have been applied to solid ground faults and high impedance faults as well as external faults with good results.

The same principle for fault detection as discussed above with reference to a single transmission line 103 could be extended to a multi-terminal line protection system, 10 15 20 25 11562SE 8 involving a topology or an arrangement with several interconnected transmission lines, such as illustrated in Fig. 5.

In Fig. 5 there are five terrninals interconnected via transmission lines.

However, there could be two, three, four, five, or six or more terminals interconnected via transmission lines. At each terminal a current derivative signals AI = d(Idcp) / dt is determined for utilization in transient component based directional comparison protection.

According to the embodiment depicted in F ig. 5, each of modules Dl, D2, D3, D4 and DS (e. g. being constituted by or including a processing unit and/or a current sensor such as described above with reference to Fig. 1) determines or calculates a current derivative signal A11, A12, A13, A14, A15 (or in general, for an N-tenninal line protection system, A11, A12, . ..

AIN). Based on the current derivative signals A11, A12, A13, A14, A15, a signal DIPS can be determined analogously to the way illustrated in Fig. 2, where for each of the current derivative signals A11, A12, A13, A14, A15 there is a separate branch, just as there is a AI1 branch and a A12 branch in Fig. 2 for the two-terrninal case illustrated in Fig. 1.

Furthennore, from the current derivative signals A11, A12, A13, A14, A15, a signal DIPSUMS can be determined analogously to the way illustrated in Fig. 3, by inputting each of the current derivative signals A11, A12, A13, A14, A15 into a summation module (corresponding to module 301 in Fig. 3).

While the present invention has been illustrated and described in detail in the appended drawings and the foregoing description, such illustration and description are to be considered illustrative or exemplifying and not restrictive; the present invention is not limited to the disclosed embodiments. Other Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures carmot be used to advantage.

Claims (3)

10 15 20 25 30 11562SE 9 CLAIMS:

1. A fault detection device adapted to detect whether there is a fault in a protected unit included in a power system, the protected unit being adapted to convey power via at least one current path therein, the fault detection device comprising: a current sensor unit adapted to sense current in the at least one current path in the protected unit at a first end thereof and to sense current in the at least one current path in the protected unit at a second end thereof; and a processing unit coupled to the current sensor unit and adapted to: based on the sensed currents, determine a first signal and a second signal indicatíve of change in current in the at least one current path in the protected unit at the first end and at the second end thereof, respectively; determine a polarity value of the first signal and the second signal, respectively; based on the detennined polarity values, determine whether there is a fault in the protected unit.

2. A method for use in fault detection in a protected unit included in a power system, the protected unit being adapted to convey power via at least one current path therein, the method comprising: sensing current in the at least one current path in the protected unit at a first end thereof; sensing current in the at least one current path in the protected unit at a second end thereof; based on the sensed currents, determining a first signal and a second signal indicatíve of change in current in the at least one current path in the protected unit at the first end and at the second end thereof, respectively; determining a polarity value of the first signal and the second signal, respectively; based on the determined polarity values, determining whether there is a fault in the protected unit.

3. A power system including a protected unit adapted to convey power via at least one current path therein and a fault detection device according to claim 1, which fault detection device is adapted to detect whether there is a fault in the protected unit.