Consultant Engineer, Ireland. SMF Technology, Ireland. SMF Technology, Ireland. SMF Technology, Ireland.

Transcription

1 REMOTE CURRENT SENSING ON OVERHEAD POWER LINES N P Tobin. M McCormack. E O Loughlin. K Tobin. Consultant Engineer, Ireland. SMF Technology, Ireland. SMF Technology, Ireland. SMF Technology, Ireland. SUMMARY This paper describes an innovative new technology for sensing currents in medium (MV) and high (HV) voltage power lines at a safe distance from the wires. Instrumentation can be installed outside the high voltage hot zone around the wires, e.g. half way up a pole. It measures the current in each individual conductor in real time, like a set of current transformers (CTs), and can feed this information to a range of backend data processing applications. A full description of the current behaviour in an overhead line is embedded in the magnetic fields surrounding the circuit. The magnetic fields are determined by the currents flowing in the line and the wire geometry. The magnetic fields can be monitored at a distance from the conductors. The wire geometry for a particular location is fixed and can be measured once at setup, e.g. with ultrasonics. With this data the current information can be decoded. A comprehensive and exact technique has been developed to accomplish this for any combination of current behaviour and wire geometry. The accuracy of an application is determined by the precision with which the magnetic fields are monitored and the wire geometry measured. Power utilities are challenged on several fronts. They have to achieve higher levels of performance, in terms of supply quality and reliability. They have to do so with fewer resources, particularly staff. They have to accommodate more complex connections to the networks, in terms of distributed generators, modern industrial plants, etc. There is a significant movement towards network automation to address these challenges. The major variable in network operation is the current flowing in the networks. The most exposed and problematic network element tends to be the long MV overhead lines. It is difficult to monitor currents in these networks at present due to the difficulty and expense of installing conventional sensors e.g. MV CTs. This technology addresses this problem. Electronic instruments can be installed easily without outages, live working crews, etc. The technique is independent of voltage and can be applied to any type of overhead line. This is a strategic technology that can help leverage the powerful communications and digital processing technologies now available by providing a current interface for high voltage networks. APPLICATIONS Load Recording. This application logs the current data over time. The currents for each phase are integrated over defined intervals, e.g. of 10 minutes. This data is logged in memory over a period of several weeks. It can be downloaded to a PC, directly or over a comms link, for analysis and archival. The sensors and logger are combined in a single battery powered instrument. A current profile is provided for each phase. This provides valuable information on the load shape, phase balance, etc. This information can assist with assessing spare capacity in networks for accommodating new loads, phase balancing, loss analysis, optimising network sectionalising, etc. Fault Indication. Fault indicators are a key element in network automation. They are effectively the eyes of an automation system. Fault indicators for overhead networks presently use the magnetic fields to sense fault currents. Their sensitivity however can vary with load current and wire geometry. Furthermore they can be confused by parasitic residual current effects in high impedance earthed networks. This remote current sensing technology measures the currents in a line with good accuracy. Once setup it can extract fault currents independently of wire geometry or background load current. Furthermore it provides more extensive current information, similar to a set of CTs, and does so in digital form. More sophisticated digital algorithms can be applied to differentiate extraneous effects and to cope with various forms of high impedance earthed networks. Digital fault indication can be applied to match the level of functionality of the digital source protection. The technology therefore provides a higher level of sensitivity, reliability and functionality than is presently available. Power Quality / Disturbance Recording. The current data provided by the sensors can be analysed to provide additional information on harmonics, unbalance, etc. This power quality data can be logged to memory and down loaded to a PC to support power quality monitoring applications. Furthermore the complete network current waveforms can be stored once a disturbance event is triggered. Such disturbance recording applications can provide invaluable data when investigating recurring or transient faults or protection mal-operations.

2 DÉTECTION DE COURANT Á DISTANCE POUR LIGNES AERIENNES ÉLECTRIQUES N P Tobin. M McCormack. E O Loughlin. K Tobin. Conseil en Ingénierie, Irlande. SMF Technology, Irlande. SMF Technology, Ireland. SMF Technology, Irlande. RÉSUMÉ Cet article décrit une nouvelle technique permettant la détection á distance de courant sur les lignes électriques á moyenne (MT) ou haute tension (HT) améliorant ainsi la sécurité. Cet équipement peut être installé en dehors de la hot zone des lignes électriques, comme par exemple á mi-distance sur un poteau électrique. Il peut mesurer le courant dans chaque conducteur en temps réel ainsi que passer cette information á des logiciels d application pour traitement. La description compléte du comportement du courant est fixée dans les domaines magnétiques qui entourent le circuit. Les domaines magnétiques sont déterminés par le courant circulant dans le ligne ainsi que par la géométrie des fils, et peuvent être contrôlés á distance des conducteurs. La géométrie des fils d un site particulier est fixe et peut être mesurée dés la mise en place. Cette mesure permettra de décoder les données obtenues sur le courant. Une technique á la fois complête et précise a été développée afin de permettre ce calcul quel que soient le comportement du courant et la géométrie des fils. Les companies électriques font face á un grand nombre de défis: elles doivent obtenir des performances toujours grandissantes au qualité et de la fiabilité de provision, cela avec de moins en moins de ressources (notamment en personnel). Il leur faut fournir des connections á des réseaux plus complexes, notamment des générateurs distributés et des sites industriels modernes. Le mouvement vers l automation des réseaux semble amené des réponses á ces défis. La variable principale dans le contrôle du réseau est la circulation du courant. Les éléments du réseau, les plus exposés et les plus problématiques, sont les longues lignes aériennes á moyenne tension. Le contrôle des courants dans de tels réseaux est difficile et coûteux avec l installation de détecteurs conventionels. Cette nouvelle technologie s attaque á ce probleme. Ces éléments électroniques peuvent être installés aisément par des équipes qui travaillent sans que des coupures de courant soient nécessaires. Cette technique fonctionne indépendement de la tension et peut s appliquer á tous les types de lignes aériennes. En fournissant un interface de courant pour les réseaux á haute tension, cette technologie peut avoir des implications stratégiques importantes grace á son impact sur les puissantes technologies digitales et de communication. APPLICATIONS Enregistrement de Charges. Cette technologie enregistre des données sur le courant. Les courants pour chaque phase sont intégrés sur des intervales définis (par exemple de 10 minutes). Cette information est enregistrée et mémorisée sur plusieurs semaines. Ces données peuvent être transférées sur ordinateur, directement ou par lien de communication pour analysis ou archives. Les détecteurs et mémoires sont intégrés dans un seul et même appareil alimenté par pile. Des détails sur le comportement du courant sont disponibles lors de chacune des phases. Cela permet d obtenir de précieuses informations sur la forme de la charge, l équilibre des phases, etc. Cette information peut être utilisée pour déterminer la sous-utilisation de réseaux, l équilibre des phases, l analyse des pertes, l optimisation de la répartition des réseaux, etc. Indication des Défauts. Les détecteurs de défauts sont des éléments cléfs de l automation des réseaux. Jusqu á présent de tels détecteurs utilisaient les domaines magnétiques afin de detecter les courants des défauts. Cependant la sensibilité de ces appareils peut fluctuer avec le courant de charge et la géometrie des fils. De plus les courants résiduels peuvent parasiter ces détecteurs conventionels dans les réseaux á mise de terre á grande impedance. Ce détecteur á distance mesure avec précision le courant circulant dans une ligne. Une fois installé il peut distinguer les courants des défauts, sans être influencé par la géométrie des fils ou par des charges. Il fournit une vaste source d information. Cette information est de plus fournie sous forme digitale. Des algorithmes digitaux plus perfectionnés peuvent être appliqués afin d isoler les effects externes et de permettre l adaptation á diverses formes de réseaux á mise de terre á grande impedance. Cette nouvelle technologie offre un degré plus élévé de sensibilité, fiabilité et fonctions qu il n était possible d obtenir jusqu á présent. Qualité de Provision / Enregistrement des Dérangements. Les données collectées par ces détecteurs, une fois analysées, fournissent un supplément d information sur les harmoniques et les déséquilibres notamment. Ces données qualitatives, après avoir été archivées sur ordinateur, permettent le soutien de logiciels de contrôle de qualité. De plus, lors de dérangements, la forme des ondes électriques est archivée, fournissant une précieuse source de données dans la lutte contre les défauts (passagéres ou répétitives) et contre les défaillances.

3 REMOTE CURRENT SENSING ON OVERHEAD POWER LINES N P Tobin. M McCormack. E O Loughlin. K Tobin. Consultant Engineer, Ireland. SMF Technology, Ireland. SMF Technology, Ireland. SMF Technology, Ireland. INTRODUCTION This paper describes an innovative new technology for sensing currents in medium (MV) and high (HV) voltage power lines at a safe distance from the wires. Instrumentation can be installed outside the high voltage hot zone around the wires, e.g. half way up a pole. It measures the current in each individual conductor in real time, like a set of current transformers (CTs), and can feed this information to a range of backend data processing applications. I R Heights Y I Y I B Power utilities are challenged on several fronts. They have to achieve higher levels of performance, in terms of supply quality and reliability. They have to do so with fewer resources, particularly staff. They have to accommodate more complex connections to the networks, in terms of distributed generators, modern industrial plants, etc. There is a significant movement towards network automation to address these challenges. The major variable in network operation is the current flowing in the networks. The most exposed and problematic network element tends to be the long MV overhead lines. It is difficult to monitor currents in these networks at present due to the difficulty and expense of installing conventional sensors e.g. MV CTs. This technology addresses this problem. Electronic instruments to monitor or analyse current behaviour can be installed easily without outages, live working crews, etc. The technique is independent of voltage and can be applied to any type of overhead line. It can of course be used on low voltage (LV) lines, but the needs here are less compelling. TECHNOLOGY A full description of the current behaviour in an overhead line is embedded in the magnetic fields surrounding the circuit. The magnetic fields are determined by the currents flowing in the wires and the wire geometry. This relationship can be expressed as follows. H S CosθSW = I = G I 2. π. R SW W SW W The magnetic field strength, in amps/m, seen by a sensor at position S depends on the relative wire geometry G SW and the wire current I W. For a collection Measurement Spacings X Figure 1 Remote Current Sensing of wires, as with an overhead power line, the magnetic field at a sensor includes contributions from all the currents. This can be expressed in matrix form as follows. [ HS] = [ GSW] [ IW] We can recast this relationship in the following form. 1 [ IW] = [ GSW] [ HS] = [ G WS] [ HS] Thus if we can measure the magnetic fields and the wire geometry then we can derive the currents. The magnetic fields can be monitored at a distance from the conductors. The wire geometry for a particular location is fixed and can be measured once at setup, e.g. with ultrasonic or laser ranging instruments. With this data the current information can be decoded. A comprehensive and exact technique has been developed to accomplish this for any combination of current behaviour and wire geometry.

4 H S θ SW R SW Sensor Where we have a number N of independent currents we can use N independent sensors to collect the required information. In the typical case with a 3 wire overhead line there can be three independent currents. Generally there will only be two independent currents, since the 3 currents normally sum to zero. However it is necessary to allow for zero sequence current as this can have a significant impact on the magnetic fields. We therefore take the more comprehensive situation, where there can be positive, negative and zero sequence current elements. This requires the use of 3 independent magnetic field sensors to characterise the line magnetic fields. In effect with 3 wires we have 5 variables to completely describe the currents 3 current magnitudes and 2 relative phase angles. Likewise with 3 sensors we can have 3 signal magnitudes and 2 relative phase angles, so we can extract a full description of the currents. To obtain 3 independent measures of the magnetic field we use 3 sensors located at different positions and with different angular orientations. Careful selection of separation distances and angular orientations allows the set of sensors to be contained within a reasonably compact instrument. The external wire geometry is measured relative to a reference point on the instrument. This geometry is then transformed in a setup programme to the actual sensor positions and orientations within the instrument. The setup programme computes the adjusted geometric factors G SW and the inverse factors G SW. Additional minor corrections are applied for such effects as wire sag, wire deviation from a straight line / zigzag, etc. The G SW factors are then transferred to the instrument to allow it to compute the wire currents from the sensor signals. Refer to PCT (1) for a complete description of the mathematical formulation and signal and data processing. The basic instrument comprises the set of sensors, analog signal conditioning, filtering and auto ranging circuitry, an analog to digital converter and a processor to execute the transformation algorithm. This outputs a digital description of the individual wire currents at the sampling frequency in use. This sampling frequency depends on the application. Instrument functionality is Ι W Figure 2 Magnetic Field from Wire then added by applying additional algorithms to process or analyse the current data. The accuracy of an application is determined by the precision with which the magnetic fields are monitored and the wire geometry measured. The technique itself is exact. The nominal accuracy for applications is 5%. However this can be improved with careful installation. In practice the greatest source of inaccuracy arises from measuring the external wire geometry relative to the instrument. This can be measured in rectangular coordinates, as the height of the wires above the instrument and the wire spacings based on standard cross-arm constructions. Alternatively the wire geometry can be measured in polar co-ordinates relative the instrument. High levels of accuracy could be achieved here using a special set-up instrument combining a laser range finder and a digital clinometer. The setup software can accommodate a wide range of geometry input specifications. APPLICATIONS Load Recording. This application logs the current data over time. The currents for each phase are integrated over defined intervals, e.g. of 10 minutes. This data is logged in memory over a period of several weeks. It can be downloaded to a PC, directly or over a comms link, for analysis and archival. The sensors and logger are combined in a single battery powered instrument. This instrument can stand alone for up to a month or for longer periods with supplemental power from such as a solar panel. The sampling is not continuous to conserve battery power. It switches on and off to sample defined windows of time within each integration interval. This timing is a compromise between power conservation and tracking current variation accurately. The instrument can be connected to a PC/laptop for setup and downloading using a standard RS 232 port. Alternatively for more permanent installations a communications link, e.g. radio, GSM, WAP, can be installed to provide remote monitoring capabilities from a control centre or engineering office. The instrument can be pre-programmed with a set of wire spacings for standard cross-arm configurations and for standard placements below the wires, e.g. 2.5m or 3m from the wires. This considerably simplifies installation. It is then only necessary to mount the instrument at the standard height, to select the appropriate cross-arm arrangement with a rotary switch and then switch on the instrument. A current profile is logged for each phase. This provides valuable information on the load shape, phase balance,

5 MV Network Currents Amps - R Amps - Y Amps - B Amps /09 08/09 09/09 10/09 11/09 12/09 13/09 14/09 15/09 Date / Time Figure 3 Load Recorder Profile time and duration of peaks, etc. This information can assist with assessing spare capacity in networks for accommodating new loads, phase balancing, loss analysis, optimising network sectionalising, etc. Fault Indication. Fault indicators are a key element in network automation and fault management. They help locate faults on a feeder so that remote switching can be used to sectionalise a fault and restore supply quickly to the majority of customers on the affected feeder. They are effectively the eyes of an automation system. They also help crews on the ground to locate faults. Key performance requirements are reliability and sensitivity. False indications can seriously mislead operators and crews and be more of a hindrance than a help. Fault management is a stressful operation with operators and crews working under pressure often coping with hazards, inclement weather, darkness and rural terrain. There is very little tolerance for maloperating equipment. The majority of faults on overhead networks, typically 80%, are earth faults. Their behaviour depends on the fault impedance and on the method of system neutral earthing. Behaviour can be quite variable and complex. Low current / high impedance faults can arise with fallen conductors and can present significant hazards. It is desirable that fault indicators should have sufficient sensitivity to detect such faults without resorting to repeated trial sectionalising. Fault indicator sensitivity should be similar to that of the source sensitive earth fault protection. There is no point in being better than the source protection as this protection is the first line of defence. Fault indicators for overhead networks presently use the magnetic fields to sense fault currents. Their sensitivity however can vary with load current and wire geometry. Furthermore they can be confused by parasitic residual current effects in high impedance earthed networks. Many utilities are presently spending a lot of money on network automation systems, to improve supply reliability performance and increase operational efficiency. Fault indicators are often the weak link in these systems. Many utilities desire a higher level of performance from their indicators than they are presently experiencing. This remote current sensing technology addresses these issues. It actually measures the currents in a line with a good degree of accuracy. It can therefore extract fault currents independently of wire geometry or background load current. Furthermore it provides more extensive current information, similar to a set of CTs, and does so in digital form. Advanced digital filtering and digital signal processing (DSP) techniques can therefore be applied similar to modern digital protection. More sophisticated digital algorithms can also be used to differentiate extraneous effects and to cope with various forms of high impedance earthed networks.

6 Power S e n s o r s Current Extraction Digital Filtering 50Hz Quantities Indicator Logic Remote Comms Indications Dip Sws, IrDA Port Geometry Settings Fault Data Figure 4 Fault Indicator The prototype digital fault indicator comprises a set of sensors similar to the load recorder. However the sensors can be arranged to fit within a smaller instrument package since the accuracy demands for sensing positive and zero sequence currents are less demanding than for sensing the currents in each wire individually. The sensed currents are digitally filtered to extract fundamental frequency quantities. This digital filtering suppresses harmonic inrush currents and arcing noise to provide more reliable indication. These currents are analysed in relation to pre-programmed set points similar to a protection relay. The indicator logic identifies a transient fault when one of the set points is exceeded. If the fault is followed by a loss of line voltage of sufficient duration to indicate recloser lock out then the fault is upgraded to a permanent status. The indicator logic also identifies the type of fault, earth fault or short circuit, and the faulted phase(s). This information in addition to the date and time of fault occurrence is stored in a list of the 5 most recent faults for permanent faults. The indicator is pre-programmed with a set of standard cross-arm configurations and placement heights for ease of installation. At installation the specific configuration is selected using DIP switches. Set point values can also be pre-programmed and selected from DIP switches at installation. There is a standard RS232 port for interfacing with a communications channel. This facilitates remote interrogation as part of a network automation scheme. The indications and fault data can be uploaded to a computer in an operations centre. This port can also be used in conjunction with the setup programme to remotely re-programme the indicator with new set points, to synchronise the clocks or even to modify the standard wire geometries. There is also the option of installing a local infra-red IrDA port to facilitate local wireless interrogation and reprogramming in the field. Local indication is provided by a lamp for permanent faults and an LED for transient faults. The fault type and faulted phases are indicated by LEDs. Permanent indications are reset by voltage restoration and by a programmable time out interval. Transient indications are reset by a separately programmable time out interval. Power is provided by a battery with provision for supplementing this with an external supply such as from a solar panel. The digital logic is normally dormant to conserve power. It is activated in the event of a disturbance by an analog trigger. This remote current sensing technology enables digital fault indication that approaches the level of performance and functionality of the digital source protection. It provides a higher level of sensitivity, reliability and functionality than is presently available. It therefore has a significant role to play in network automation technology. Power Quality / Disturbance Recording. Power quality is becoming an increasingly significant issue for supply utilities and for major customers such as manufacturing facilities. These are commercially dependent on their power supply. Modern electronic and computer automation equipment are both critically dependent on their power supply and sensitive to disturbances. Where disturbances or maloperations arise these need to be investigated. This often requires the use of monitoring instrumentation. The current data provided by the remote sensing technology can be analysed to provide additional information on harmonics, phase unbalance, etc. This power quality data can be integrated over defined time intervals and logged to memory.

7 Figure 5 Power Quality Applications It can be down loaded to a PC, locally or over a comms link, to support power quality monitoring applications. Supplementary power, e.g. from a solar panel, is generally needed to support these extended functionality instruments. Furthermore the complete network current waveforms are available and can be stored to implement disturbance recording functionality. The waveform data can be pipelined and logged in memory once a disturbance event is triggered. Such disturbance recording applications can provide invaluable data when investigating recurring or transient faults or protection mal-operations. The sensor front end is similar to the load recorder. However the sampling frequency is considerably higher to capture the harmonic detail. A more powerful processor is also needed to implement the analysis algorithms. In practice power quality analysis and disturbance recording functionality are already well developed in existing instrumentation. These instruments however rely on instrument transformer inputs. They can only be used where there is insulated access to conductors. Application to overhead networks can now be achieved by interfacing the remote current sensing technology with existing power quality instrument functionality. Normally power quality and disturbance recording instruments are installed in substations or customer premises, where there is access to conventional instrument transformers. However utilities are often reluctant to interfere with CT wiring particularly in customer substations for fear of tripping a circuit or plant or compromising its protection. If there is a facility to monitor supply performance externally on the overhead supply line without interfering with existing equipment this can be a very convenient and attractive option. There are also needs to monitor disturbances along MV lines, particularly when tracing a recurring transient fault. Remote current sensing technology enables these applications. SUMMARY This remote current sensing technology provides a neat solution to the problem of monitoring currents on MV and HV overhead networks. It is a strategic technology that can help leverage the powerful communications and digital processing technologies now available by providing a current interface for HV/MV overhead networks. REFERENCES 1. PCT/IE99/00114, Apparatus and Method for Remotely Measuring Current.