TECHNICAL FIELD

The present invention relates to the field of three-phase power distribution networks, and more particularly to a system, device and method for cable identification in a three-phase power distribution network.

BACKGROUND OF THE INVENTION

Power distribution networks are utilized to deliver electricity from generating plants to commercial and residential customers via multiple transmission substations at which transformers step down the three-phase high voltage of the generating plants to feed multiple distribution substations. Transformers at the distribution substations further step down the power into feeder cables which provide power to local transformer stations at which a final step down to voltages of typically 120 and 240 volts for delivery to local households and industries via supply cables is performed. Each local transformer station typically feed a plurality of supply cables, each

corresponding to a respective house or other building structure at which it is connected to a respective meter cabinet. During initial installation, the identity of the electrical cables of the local transformer stations are marked or otherwise known, but over time as new customers are connected, the documentation and marking of the cables if present at all may degrade.

When performing electrical repair or modifications to existing three- phase power systems for power distribution to buildings, there is thus often need for the electrical linemen to first perform cable identification to properly determine the relevant cabling upon which to work. That is, the linemen must for instance determine which particular supply cable, amongst a plurality of supply cables in a local transformer station, leads to the meter cabinet of a particular house or other building structure. US Patent No: US 6,885,180 B2 discloses a method for identifying which live current secondary cable in a transformer, amongst a plurality of live current secondary cables in the transformer, leads to a selected meter cabinet. The method comprises the steps of engaging a transmitter to the selected meter base to generate an electrical identification signal with the transmitter. The electrical identification signal is then passed through a selected live current secondary cable leading from the selected meter base to the transformer for generating a magnetic field around the selected live current secondary cable, and a receiver is positioned within the transformer proximate to the terminal ends of the plurality of live current secondary cables in the transformer, and then moved over the terminal ends of each live current secondary cable in the transformer to sense and detect the magnetic field being generated around the terminal end of the selected live current secondary cable, thereby identifying which live current secondary cable in the transformer, amongst the plurality of live current secondary cables in the transformer, leads to the selected meter base.

SUMMARY OF THE INVENTION

In view of the above, an object of the invention is to at least provide an alternative and improved device, system and method for cable identification in a three-phase power distribution network. In particular, an object is to provide an easy to use cable identification device, system and method for on-site electrical cable identification, which is suitable for performing identification of unmarked or unidentified cables in a distribution route of a low tension network or the like.

In accordance with a first aspect of the present invention, there is provided a system for cable identification in a power network comprising a load unit which is arranged to be connectable to a first cable in the power network, where the first cable is powered via a distribution route through the power network. The system comprises a detection unit for identifying a cable belonging to the distribution route. The controllable load unit is arranged for creating an asymmetry current in the first cable. Identification of cables belonging to the distribution route is based on detection of the created asymmetry current. The present innovative concept is thus directed towards a system in which a controllable electrical load unit is connected to a first connection point in a premise like a house or other building structure and wherein the created asymmetry current is detected at a second site along the distribution route. According to an embodiment, the controllable load unit is arranged for connecting to the first cable via a wall socket (the first connection point). The wall socket is typically fed via a phase conductor L and the neutral conductor of the (first) cable feeding the wall socket. The controllable electrical load is arranged to create a predetermined asymmetry in the three- phase low tension cable associated with the first connection point. The created asymmetry is then detectable at a second site, e.g. in the first cable itself at its corresponding meter cabinet, or in a power distribution cabinet from which a feeding cable belonging to the distribution route of the first cable originates. As previously described, the power distribution cabinet typically comprises a plurality of feeding cables, but only the feeding cable of the distribution route of the first cable will contain the asymmetry current.

According to an embodiment of the invention, the detection unit comprises a current clamp, e.g. a clamp-on type ammeter. The cable identification system and corresponding method is thus applicable with no interruption of current or the continuity of the cables under test needed, and is therefore very useful when performing code marking or when performing maintenance or repairs in networks with no or limited code marking. Code marking generally contains information regarding cable addresses, fuse size, wire cross section etc. Power supply companies typically follow standards and the National Electrical Safety Board power regulations. As an example, each cable must be marked with the same code on the cable as on the fuse unit to which the cable is connected.

According to an embodiment of the system, the controllable load unit is arranged for providing at least two different electrical loads to the first cable. This is especially advantageous when performing cable identification on tension cables which are initially under an asymmetrically distributed load. By providing more than one subsequent load states, at least one detectable asymmetry current is provided, which in turn increases the expected reliability when performing cable identification of asymmetrically loaded three phase systems.

According to a second aspect of the invention, there is provided a method for cable identification in a power network comprising connecting a controllable load unit to a first cable which is powered via a distribution route through the power network, providing a predetermined load to the first cable to create an asymmetry current in the first cable, and identifying a cable belonging to the distribution route based on detection of the created asymmetry current.

According to an embodiment of the method, the step of providing a predetermined load to the first cable is performed by providing at least two different predetermined loads in sequence.

According to an embodiment of the method, the sequence is performed with a predetermined period time T selected to be detectable by the human eye. In an embodiment of the invention, the predetermined period time T is selected between 0.5 s and 2 s.

Further, according to embodiments of the system and method, the detection of the created asymmetry in the three-phase cable is measured over a neutral conductor, or over all phase conductors simultaneously.

Further objectives of, features of, and advantages with, the present invention will become apparent when studying the following detailed disclosure, the drawings and the appended claims. 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

The above, as well as additional objects, features and advantages of the present invention, will be better understood through the following illustrative and non-limiting detailed description of preferred embodiments of the present invention, with reference to the appended drawings, where the same reference numerals will be used for similar elements, wherein:

Figs. 1 a - 1 c are schematic illustrations of embodiments of a system according to the present invention; Fig. 2 - 3 are schematic illustrations showing embodiments of a system according to the present invention when employed in a power distribution network; and

Fig. 4 is a schematic illustration of a load device according to an embodiment of the present invention.

All the figures are schematic, not necessarily to scale, and generally only show parts which are necessary in order to elucidate the invention, wherein other parts may be omitted or merely suggested.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A number of possible combinations of earthing systems for three-phase power distribution networks are employed all around the world. Letter classifications are used to classify earthing systems, where the first letter indicates the type of supply earthing: T - indicates that one or more points of the supply are directly earthed (for example, the earthed neutral at the transformer), and I - indicates either that the supply system is not earthed at all, or that the earthing includes a deliberately-inserted impedance, the purpose of which is to limit fault current. The second letter indicates the earthing arrangement in the installation: T - all exposed conductive metalwork is connected directly to earth, N - all exposed conductive metalwork is connected directly to an earthed supply conductor provided by the electricity supply company. The third and fourth letters indicate the arrangement of the earthed supply conductor system: S - neutral and earth conductor systems are quite separate, and C - neutral and earth are combined into a single conductor. Some systems in common use are for instance TT-, TN-S-, TN-C- S-, IT-, and TN-C systems. The exemplifying embodiments herein are described with reference to a TN-C system which is commonly used in Sweden. The TN-C system utilizes a combined neutral and earth (PEN) conductor throughout the installation as well as for the supply.

For a shielded cable, the PEN conductor is the metallic copper sheath of the cable, the sheath being the combined neutral and earth conductor, and is therefore concentric with the phase conductors (L1 -L3, three wire cable with shield). In a four wire cable one wire is dedicated as the PEN conductor (PEN, L1 -L3). The neutral conductor is used as a protective conductor. The TN-C system requires an effective equipotential environment within the installation with dispersed earth electrodes spaced as regularly as possible since the PEN conductor is both the neutral conductor and at the same time carries phase unbalance currents.

Referring now to Figs. 1 a - 1 c, an embodiment of a system when employed in a TN-C system is schematically illustrated. A mains supply cabinet 53 located at a first location 51 is illustrated, from which a low tension three-phase supply (L1 m, L2m, L3m, PENm) is distributed via a four wire supply cable 1 1 1 . Note that a plurality of supply cables are typically

connected at the main supply cabinet 53 to feed a plurality of buildings. Here however, only supply cable 1 1 1 is illustrated. The supply cable 1 1 1 is connected to the (main switch) distribution cabinet 54 of a building structure or the like, from which phase conductors L1 , L2, and L3 and the conductor to power loads of the building are accessible. In Figs. 1 a - 1 c box 1 10 is a schematic illustration of a coupling rail which is electrically connected to phase conductors L1 , L2, L3, and the PEN conductor of the fuse panel of the distribution cabinet 54. To be able to identify in the mains supply cabinet 53 which supply cable is feeding this particular distribution cabinet 54, a detection system 100 according to an embodiment of the invention is utilized. The detection system 100 comprises a load unit 101 and a detection unit 102. The load unit 101 is arranged for providing a predetermined (adaptable) single-phase load with the purpose of adding unbalance/asymmetry to the load of the system. In a symmetrical three-phase four-wire system, the three phase conductors L1 , L2 and L3 ideally have the same voltage to the system neutral, where the voltage between line conductors is 3 times the phase conductor to neutral voltage. Further, the voltage between the line conductors are mutually phase shifted 120 degrees. Since currents returning from the customers' premises to the supply transformer all share the neutral wire PEN, if the loads are evenly distributed on all three phases L1 , L2 and L3, the sum of the returning currents in the neutral wire PEN is approximately zero, while any unbalanced phase load on the three phase L1 , L2 and L3 will cause an asymmetry current l _a in the PEN conductor.

A three-phase low tension network in operation may however be (slightly) asymmetric with reference to the load thereon, due to different size of power consumption of the connected devices. The asymmetry current l _a flows through the PEN conductor (TN-C system), or the N-conductor (TN-S and TT-system) is detectable with the detection unit 102. The detection unit 102 may be for instance a clip-on type current meter. According to an embodiment of the invention, the load unit 101 is arranged to provide predetermined changes of the size of the asymmetry current l _a at

predetermined time intervals which in turn are detectable at the mains supply cabinet 53 by means of the detection unit 102. The detection unit is 102 is subsequently applied to the plurality of supply cables connected to the mains supply cabinet 53. However, only detection over a cable associated with the feeding route of cables leading to the connection point at the coupling rail 1 10, i.e. here supply cable 1 1 1 , will show the predetermined changes of the asymmetry current l _a . In Fig. 1 a, the identification of the supply cable 1 1 1 is performed over the PEN conductor PENm . The load unit may in this case be connected to any of the phases L1 - L3 of the coupling rail 1 10, since the detection is based on generated asymmetry in the PEN conductor.

Identification of the supply cable 1 1 1 may be performed over all three phase conductors L1 in-L3in simultaneously.

According to an advantageous embodiment of the invention, the load unit 101 is adapted for being connected directly to wall sockets 150 of the building. Thus, the load unit is provided with a plug contact (not shown) for connecting the predetermined load. Thereby, a person can in a convenient and intuitive manner perform cable identification at a mains supply cabinet 53 by plugging in the load unit in a wall socket at a specific room or building, and then perform identification of an associated supply cable 1 1 1 at a second location 51 by detecting the asymmetry current created by the load.

In Fig. 1 b and 1 c wall sockets 150 are connected to a respective phase conductor (L1 , L2, or L3) and neutral PEN.) Any of the wall sockets 150 can then be used when plugging in the detection unit 101 to provide the asymmetry current l _a which is then detectable by means of the detection unit 102 at the coupling rail 1 10/ the mains supply cabinet 53 in the PEN or over all phase conductors as illustrated in Fig. 1 b and Fig. 1 c.

Referring now to Fig. 2 a situation when employing the present inventive concept for cable identification in a power distribution network 200 is schematically illustrated. The power distribution network 200 comprises a transmission substation 210 which via a secondary transformer line 21 1 feeds a first distribution substation 220. From the distribution substation 220 two feeder cables 221 , 222 exit providing further stepped down power to other distribution substations, e.g. a second distribution substation 230 via feeder cable 222, which in turn provides power via a feeder cable 232 to a third distribution substation 240. The second distribution substation 230 provides power via so called service lines 233 and 231 to distribution cabinets 234 and 235, respectively, of a respective house 22 and 21 . In this case ocular inspection of the transformer cable 21 1 , the service lines 233 and 231 , and the feeding cables 221 ,222, 232 is available.

Performing code marking (litterering) in the power distribution network

200 requires identifying which cables (transformer cable, feeder cable, service line) are associated with a particular house, or even a room, facility or machinery in a building. To identify the cabling of house 22, a load unit 101 according to the present invention is plugged in and activated in a wall socket inside the house 22. The created asymmetry in the load is then detectable along a distribution route 201 of power to the house, which distribution route

201 is indicated with a dashed line through the power distribution network 200. By applying the detection unit 102 in a manner as previously described with reference to Figs. 1 a - 1 c, at the distribution cabinet 234, the second distribution substation 230, the first distribution substation 220 and/or at the secondary side of the transformer of the transmission substation, identification of which cables belong to the distribution route 201 of the particular house 22 among other service lines, feeder cables etc. can be performed. Service line 233 and feeder cable 222 may be identified at their respective end points. Service line 231 of house 21 , feeding cable 221 and the transformer cable 21 1 may here be identified by the method of elimination to complete the code marking (littering) of the entire distribution route 201 .

Referring now to Fig. 3 a situation when employing the present inventive concept in cable identification in a power distribution network 300 is schematically illustrated. The power distribution network 300 comprises a transmission substation 310 which via a secondary transformer line 31 1 feeds a first distribution substation 320. From the first distribution substation 320 a feeder cable 322 exits providing power to a second distribution substation 330 via feeder cable 322. The second distribution substation 330 provides power via service lines 333 and 331 to distribution cabinets 334 and 335,

respectively, of a respective house 32 and 31 . In this case ocular inspection of the feeding cable 322 is not possible.

When performing code marking (litterering) of the power distribution network, there is a need for identifying which cables (transformer cable, feeder cable, service line) are associated with a particular house, or even a room or a facility in a building. (Including the whole distribution route for the code marking is however optional.) To identify the cabling of house 32, in a first step S1 the load unit 101 according to the present invention is plugged in and activated between a phase R, S, T (here T is connected) and PEN conductor of the coupling rails 325 at the distribution substation 320, which allows the transformer cable 31 1 to be identified by means of the detection unit 102 based on the created asymmetry in the load. In step S2, the load unit 101 is connected to coupling rails 335 at the distribution substation 330, which allows the feeder cable 322 to be identified based on the created asymmetry in the load by means of the detection unit 102 when used at either end of the feeder cable 322 (i.e. at second distribution substation 330 or at the first distribution substation 320). In step S3, the load unit 101 is connected to coupling rails 334 or preferably a wall socket at the house 32 as illustrated in Fig. 3, which allows the service line 333 to be identified based on the created asymmetry in the load by means of the detection unit 102 when used at either end of the service line 333. Service line 331 may here be identified by the method of elimination.

Referring now to Fig. 4 an exemplifying embodiment of a load unit 101 according to the present inventive concept is schematically illustrated. The load unit 101 comprises means for providing a predetermined load according to the inventive concept, which here is provided by a load circuit 151 which is connected via two switches 152 to a two-pole switch 154 which controls output terminals F, O. The switches 152 are controlled individually by a control unit 155 comprising a Central Processing Unit, CPU, and adapted software for controlling the switches 152 to, optionally with a predetermined sequencing, provide a load to the output terminals F, O. Here the load circuit 151 comprises two individually connectable resistances. The load unit 101 further comprises a cooling fan 153 for cooling the load circuit 151 (optional), and a suppression condenser 156 (optional) arranged over the output terminals F, O.

According to an embodiment of the load unit different electrical loads (with respect to magnitude and/or phase), e.g. two different resistive loads are provided in sequence T, such that at least two different readings of the asymmetry current l _a are provided at the detection unit 102. This is to minimize the effect of an initial asymmetry condition of the domestic loads connected to the low tension network at the time of performing the cable identification by means of the method and system of the current inventive concept.

According to an embodiment of the invention, the sequence time T for applying different load states are selected to be detectable to the human eye.

According to embodiment of the invention, the load unit comprises an impedance unit for providing a load of at least two power factors and magnitude utilizable in identifying the first cable. The power factor thereof is selected to vary between at least two predetermined values between substantially 90 lag and 90 lead. The impedance unit comprises a pair of terminals connectable to a first connection point (preferably a wall socket) and contains first means for forming a (variable) resistive load, a second means for forming a (variable) substantially inductive load, and third means for forming a (variable) substantially capacitive load. The controllable load of the load unit is arranged by varying the magnitude of the resistive load, the inductive load and the capacitive load.

According to an embodiment, the load unit comprises a Positive Temperature Coefficient, PTC, element.

Embodiments of the present inventive method are preferably implemented in a load unit including a software module adapted for controlling the steps of applying predetermined load states according to the system and method of the present invention (not shown). The software module may be integrated in the load unit comprising suitable processing means and memory means, or may be implemented in an external device comprising suitable processing means and memory means, and which is arranged for

interconnection with an the first connection point, that in a preferred embodiment is adapted to be directly connected into a wall socket.

The person skilled in the art realizes that the present invention by no means is limited to the embodiments described above. On the contrary, many modifications and variations are possible within the scope of the appended claims.