WO2006021030A1

WO2006021030A1 - Electrical power line sensing and sensor assembly

Info

Publication number: WO2006021030A1
Application number: PCT/AU2005/001261
Authority: WO
Inventors: Gregory James Nunn; David Russell Murray
Original assignee: Fault Detectors Pty Ltd
Priority date: 2004-08-23
Filing date: 2005-08-23
Publication date: 2006-03-02

Abstract

A sensor assembly connected between a power line to be monitored and ground as a flexible lead assembly integrated with respective input and output ends as a single encapsulated flexible unit. A V-block (101) and screw clamp (109) are attached to an input circuit (102) which includes a current sensor coil (104) which generates an optical signal and transmits the sample data over a optical waveguide (105). This sample data is received by an output circuit (106) which is connected to ground through connector (107). Circuit (106) is also connected to a resistive element in the form of a resistor chain (103) which spirals around optical waveguides (105 and 110) and connects at the input end to circuit (102) which in turn connects to V-block (101) and thence to the line. The electronic circuits on circuit boards (102) and (106) sample and digitise the instantaneous value of this current from circuit board (102 to 106) over optical waveguide (105) along with the line current sample data. The optical signal(s) are converted to electrical form at the output end. Electronic circuitry on circuit board (106) processes the current and voltage sample data to derive information about the power line such as RMS current and voltage, power flow, power factor and so on and transmits this data in digital form to other equipment via external connector (107).

Description

ELECTRICAL POWER LINE SENSING AND SENSOR ASSEMBLY

TECHNICAL FIELD OF THE INVENTION

This invention relates to electric power line sensing and in particular but not limited to a relatively low cost sensor assembly in lead form providing initial signal processing of sensed parameters where a calibrated digital data signal is produced for input to downstream devices for utilisation in a wide variety of applications.

BACKGROUND TO THE INVENTION The present invention is set against a background of costly sensor systems employing high cost transformer and insulation and service requirements. It is an object of the present invention to at least provide a useful relatively low cost alternative to the prior art. Operators of electrical networks need to measure electrical parameters for a variety of reasons including monitoring, automation, metering, fault detection and protection. The fundamental electrical parameters for power lines are current and voltage and these must be measured at suitable accuracy and speed for the application. For example for metering purposes the energy delivered to the load over a fixed interval (typically 30 minutes) is the prime requirement (MWh), this is derived from highly accurate current and voltage measurements. For basic protection purposes line current values on a cycle or sub-cycle basis are needed so that breakers may be tripped quickly in the event of a fault but the required accuracy is lower than for metering. For more sophisticated protection purposes line voltage and line current may both be required at higher accuracy. For automation purposes line current and voltage can be employed by automated switchgear controllers to automatically isolate faulted sections and restore power to un-faulted sections. Other applications such as fault detection and remote monitoring utilise either or both current and voltage measurements at different degrees of speed and accuracy to achieve their required applications.

In recent years it has become common practice to use electronic systems to sample instantaneous values of these parameters, digitise them and use well known or proprietary signal processing methods to derive the data required for the application at hand. Moreover since most electrical networks are alternating current polyphase networks it is necessary to derive characteristics of the polyphase network in addition to, or instead of, the individual phases. This relates to the well know sequence theory which is used in polyphase network analysis. The derivation of sequence currents and voltages is readily performed by digital signal processing from the current and voltage samples of individual phases.

As is well known the cost of digital signal processing and electronics system generally is falling, this, combined with falling cost of communications gives rise to the possibilities of more sophisticated and more widespread applications of electronic systems to improve electrical networks in the areas described above (fault detection, automation, monitoring and so on).

However, in many cases the limitation to the widespread use of such systems is not the cost of the electronics but is the cost of the sensors for the basic electrical parameters of voltage and current and the cost of installing those sensors. This is discussed further below.

The traditional components to measure line current and voltage are wound current transformers (CTs) and wound voltage transformers (VTs or PTs). Both these components provide alternating outputs which must be connected into the relevant equipment which must sample and digitises the signals before processing them to perform the required application. In some situations CTs can be installed at ground potential (eg. Over screened cables) and are relatively inexpensive. In other situations where the CTs have to be installed over lines at high potential expensive and bulky insulation systems have to be incorporated. Similarly all wound VT's have to incorporate expensive and bulky insulation systems in order to connect to the power lines at high potential. Many types of sensor have been invented to try to overcome the cost and size problems of wound CT's and PT's. These include: capacitive voltage sensing, resistive voltage sensing, sensors at ground potential remote from the lines which detect the electric and magnetic fields of the lines, sensors attached to the lines powered by batteries or line current which send signals by lights or by radio and so on. Many of these inventions are successful in the particular application for which they were designed (eg. overhead line fault detection) but all of them lack generality, and most of them do not have sufficient accuracy for important applications such as earth fault detection in balanced poly-phase networks. In some cases the lack of generality is not a significant disadvantage because the low cost sensor is permanently incorporated into the relevant equipment at time of manufacture. For example Automatic Circuit Reclosers (ACR's) usually have built in CT's and some have built-in capacitive VT's. In other cases this is not possible or desirable. One example is line monitoring for quality of supply purposes. In this example a power line must be monitored for a period of a few months to check on the quality of supply. Monitoring equipment must be installed at a problematic location for a few months and then taken down and installed somewhere else. The cost and disruption of installing CT's and VT's is high and so monitoring of lines in this way is expensive. In another example it is common practice in many countries to install many metal-enclosed load break switches on a network which are operated by hand. Over a period of time it becomes desirable to monitor and remote control or automate a small proportion of these switches. It would be desirable to install all switches with built-in sensors so that they could be easily converted at a later date to being monitored devices by the simple addition of the relevant electronics. However this does not happen because it is too expensive to build the sensors into the equipment. Instead either the utility installs additional expensive sensor systems external to the LBS or the utility may choose remove the manual LBS and replace with a device which has the sensing capability built in, in either case it is an expensive and disruptive exercise.

Over the past 25 years there have been a number of patents which describe sensors for power systems, particularly outdoor overhead electrical networks. However all have some significant weaknesses and none of these have seen widespread adoption in the industry. These include attempts by Granville in US Patents 5006846 and 5181026.

Granville describes a number of embodiments particularly for "High Voltage" (20OkV and above) employing a fibre optics cable assembly as data carrier to isolate the power line from ground and to carry data from a separate power line sensor pod to a separate data processing station. Granville uses either a flexible fibre optics cable assembly or a rigid fibre optics cable assembly between the pod and station. Where there is a direct vertical drop from the pod Granville prefers to use a rigid assembly as the carrier. Where there is a vertical and horizontal component to the drop Granville uses a flexible fibre optics cable assembly. The pod is constructed separately of the fibre optics cable assemblies which interchangeably plug into the pod at one end and into the data processing station at the other end. The fibre optics cable assembly serves only to carry the data and consequently does not provide any processing function. In addition Granville does not provide any definitive solution and in applicant's view is overly complex . For example the pod contains a whole raft of sensing and processing equipment much of which is described as optional. This means Granville suffers from a mix of disadvantages including: lack of a simple interface, lack of accuracy (particularly voltage), the need for calibration after installation, influence by contaminants such as ice and pollution, difficulty of installation and lack of reliability by requiring optical fibre connections, unreliability through use of radio systems, lack of robustness in the unforgiving pole top environment and lack of general applicability.

OUTLINE OF THE INVENTION In one aspect therefore the present invention resides in an electric power line sensor assembly having an input end including an exposed line connector, an output end including a digital data interface and a flexible lead assembly joining the ends together in operative relation by being encapsulated along with the ends in an insulating medium thereby forming a single integrated flexible unit, the input end having a sensor circuit including a line current sensor for generating a sensed line current signal and an optical signal generator adapted to generate an optical signal based on the sensed line current signal for transmission through the flexible lead assembly to the output end, the output end having an output circuit for receiving the optical signal and providing an output digital data signal indicative of line current. One or all of the signals may be processed in the assembly to provide power line measurement data, "power line measurement data" being herein defined as a meaningful power line parameter derived from raw current or voltage readings following a processing step. Preferably, the measurement data is generated at the output end but may be generated at the input end prior to delivery to the output end. Preferably, power to the input end is supplied by light from a source incorporated into the output end, the lead assembly having a light guide for delivery of light from the source.

In an alternative where line voltage is sensed through a resistive element extending from the input end to the output end power to the input end may be derived from current flowing in the resistive element.

As a further alternative power may be provided as a combination of light derived and resistive element derived power or one may be back up for the other. However, it is most preferred that a light source in the output end be the primary power source.

Preferably, the assembly may be configured for a poly-phase network by having an input end for each phase and a common interface at the output end.

Preferably, in another situation the assembly may be configured for a poly-phase network by having an input end for each phase a separate interface for at least two phases at the output end.

Preferably, in still another case the assembly may be configured for a poly-phase network by having separate assemblies for each phase.

Preferably, line voltage is sensed through a resistive element extending from the input end to the output end and for the purpose of generating a sensed voltage signal the resistive element current is sampled at the input end and at the output end and the average voltage used to derive the sensed voltage signal.

Preferably, where line voltage is sensed through a resistive element extending from the input end to the output end and for the purpose of generating a sensed voltage signal the resistive element current is sampled at the input end and at the output end and the average voltage used to derive the sensed voltage signal and sampling is synchronised using pulsed light. In another preferred form the present invention resides in an electric power line sensor assembly having an input end, an output end and a lead assembly joining the ends together in operative relation, the input end having a sensor circuit including a line current sensor for generating a sensed line current signal and there being an optical signal generator adapted to generate an optical signal based on the sensed line current signal for transmission through the lead assembly to the output end, the output end having a circuit for receiving the optical signal, processing the data received and generating an output as digital measurement data derived from the sensed line current signal for transmission to other systems via a data communications interface.

Preferably, the assembly makes provision for line voltage sensing as well as line current sensing. Therefore, it is preferable that a resistive element extend from the input end through the lead assembly to the output end for the purpose of providing a sensed voltage signal. A processing circuit is preferably provided in the output end. Signal processing takes place in the output end circuit to derive power system parameters from the line current and voltage signals for transmission as digital measurement data to other systems via a data communications interface. The sensed voltage signal may be generated at the input end or the output end of the assembly or both. Where a voltage signal is generated in the input end the optical signal is used to carry both the voltage and the current signal data to the output end.

Where a sensed voltage signal is generated at both ends as respective instantaneous input end and output end sensed voltage the two signals are averaged prior to processing to derive the output digital measurement data. In order to synchronise the sampling of the instantaneous input end and output end voltage an optical pulse generator is preferably employed to generate synchronisation pulses.

The sensor assembly is preferably in the form of a fully encapsulated assembly where the encapsulation is designed to maintain factory set geometry between components, particularly the optical components so that the assembly simply has electrical input and output connectors at the sensor and output ends respectively and the rest is encapsulated to protect from contaminants and to maintain electrical integrity.

The input connector is used to connect the assembly to a power line and the output connector is used to connect the assembly to the ground reference, a power source and to the digital interface of a downstream device. The assembly may be a "dumb" system that simply slavishly provides a stream of data or it may be an "intelligent" assembly with computing capability, bi¬ directional communication capability and storage capability so that control, data storage and common computing elements may be included in the assembly so that data and software may be uploaded or downloaded to and from the assembly.

The sensor is preferably factory calibrated for both current and voltage measurements so that no further calibration is required at the time of installation and so that the downstream systems do not require any measurement system of their own. The processing circuit in the output end utilises calibration data stored at time of calibration to correct the sensed voltage and current signals.

In one preferred embodiment there is provided an assembly connected to an electrical conductor at an input end thereof and to ground at an output end thereof for the purpose of sensing current and voltage in said electrical conductor, said assembly comprising, an electrical current sensor substantially at line voltage connected to a circuit in the form of a first electronic sub-system which encodes line current data into digital form, an optical waveguide to transmit said digital data from said first electronic sub-system to a circuit in the form of a second electronic sub-system which is substantially at ground potential, a resistive element connected between line and ground, the current through the said resistive element being proportional to the line-ground voltage and said resistive element current being monitored by the first or the second said electronic sub-system or both and being encoded into digital data form, a signal processing element in the second electronic sub-system to process said current and voltage digital data to derive measured power system parameters, an electrical data interface from the second electronic sub-system which communicates the digital data representing measured power system parameters to a client computing system for utilisation in the client system applications, the assembly having a line connection exposed at the line end and an electrical data interface exposed at the output end, the remainder being encapsulated in an insulating medium.

Preferably, the first electronic sub-system is powered by a photo-voltaic device which gets its energy via an optical waveguide from a photo-emitting device incorporated into the second electronic sub-system.

In another embodiment the first electronic sub-system is powered by current flowing in the resistive element.

Preferably, in another embodiment applied to a polyphase network the assembly has one input end per phase and respective lead assemblies merge to a common output end sub-system. Thus providing a single data interface to the client system for transmission of measured power system parameters. In another embodiment as applied to a polyphase network, the input end comprises separate sensors for each phase. Separate output ends may be provided. However, it is preferable that measured power system parameters are transmitted through a single data interface to the client system:

Preferably, in order to correct for leakage currents into and out of the resistive element for the purpose of generating the sensed voltage signal the resistive element current is sampled at the input end and at the output end, calibration correction is applied to the instantaneous data samples from the two ends and then the average of the calibrated samples is employed in the subsequent signal processing.

In a particularly preferred embodiment the invention resides broadly in a pre-calibrated electric power line sensor assembly providing calibrated digital output of line current and line voltage parameters, the assembly having an input end, a voltage sensor, an output end and a flexible lead assembly joining the ends together in operative relation, the input end having a sensor circuit including a line current sensor for generating a sensed line current signal and an optical signal generator adapted to generate an optical signal based on the sensed line current signal, the lead assembly having a data light guide for transmission of the optical signal through the lead assembly to the output end, the output end having an output circuit for receiving the optical signal and generating an output digital signal of at least one power line parameter derived from the sensed line current signal, the voltage sensor comprising a resistive element extending from the input end through the lead assembly to the output end for the purpose of providing a sensed voltage signal to the output end circuit from which at least one power line parameter may be derived, power to the sensor circuit being supplied by light from a source remote from the input end, the lead assembly having a light guide for delivery of light from the remote source, the sensor assembly having a line connection exposed at the input end and a data communications interface at the output end, the remainder being of the assembly being encapsulated in an insulating medium to maintain factory set geometry between components. The communications interface type may be electrical or optical or wireless.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a drawing showing one embodiment of a sensor assembly connectable between a power line to be monitored and ground;

Figure 2 shows a typical application of three sensors to a three phase distribution power line;

Figure 3 shows a schematic representation of the sensor of Figure 1 ;

Figures 4A and 4B are drawings depicting an outline schematic of electronic circuit which resides inside the sensor input end;

Figure 5 is a drawing illustrating input end sampling and data transmission sequence;

Figure 6 is a drawing showing, the construction of the sensor lead assembly;

Figure 7 is a drawing depicting an outline schematic of electronic circuit which resides inside the output end;

Figure 8 shows an alternative power supply arrangement for the input end of the sensor; Figure 9 is a schematic drawing illustrating an embodiment of the invention where signal processing is downstream of the output end of the sensor in a poly-phase network;

Figure 10 is a schematic drawing illustrating an embodiment whereby signal processing is carried out in a single output end shared by three input ends in a poly-phase network;

Figure 11 is a perspective drawing illustrating an implementation of the embodiment of Figure 10;

Figure 12 is a drawing illustrating one embodiment of the present invention applied to poly-phase processing carried out across three output ends;

Figure 13 is a drawing illustrating the synchronising of input end sample timing with light pulses;

Figure 14 is a drawing depicting an outline schematic of an electronic circuit to synchronise the input end sample timing with light pulses; and

Figure 15 is a schematic drawing of a sensor assembly with a build up of ice along one end of the lead assembly.

METHOD OF PERFORMANCE

Figure 1 shows one embodiment of a sensor assembly connected between a power line to be monitored and ground as a flexible lead assembly integrated with respective input and output ends as a single flexible unit. A V- block 101 and screw clamp 109 together grip and make an electrical connection to the power line (not shown). Attached to the V-block is an input circuit in the form of an electronic circuit board 102 which includes a current sensor coil 104 to sense the magnetic field from the current flowing in the power line. The electronic circuits on circuit board 102 sample and digitise the instantaneous value of the power line current on a period basis, generates an optical signal and transmits the sample data over a optical waveguide 105. This sample data is received by an output circuit in the from of electronic circuit board 106 which is connected to ground through connector 107. Circuit board 106 is also connected to a resistive element in the form of a resistor chain 103 which spirals around optical waveguides 105 and 110 and connects at the input end to circuit board 102 which in turn connects to V-block 101 and thence to the line. Current flowing in the resistor chain is proportional to the line to ground voltage. The electronic circuits on circuit boards 102 and 106 sample and digitise the instantaneous value of this current (which represents the line voltage) and the sample data is transmitted from circuit board 102 to 106 over optical waveguide 105 along with the line current sample data. The optical signal(s) are converted to electrical form at the output end. Electronic circuitry on circuit board 106 processes the current and voltage sample data to derive information about the power line such as RMS current and voltage, power flow, power factor and so on and transmits this data in digital form to other equipment via external connector 107.

Circuit board 102 operates by power received as light energy over optical waveguide 110 and converts this light energy to electrical energy with a photovoltaic device. The light energy is injected into optical waveguide 110 by a photo-emitter on circuit board 106 which in turn receives its electrical energy from an external source via external connector 107.

Those parts of the assembly including circuit boards, optical waveguides and resistor chain are covered in a moulded flexible insulating elastomeric sheath 100 which also covers part of the V-block and the connector 107 leaving only the necessary external electrical connections exposed. The exterior of the elastomeric sheath 100 mechanically protects the sensitive electronics and optical components and their connections from physical damage at time of installation and in service, it also seals these parts and protects them from moisture and other environmental hazards such as corrosive pollutants. The process of application of the sheath employs encapsulation of components so that an elastomeric interstitial matrix integral with the sheath maintains the relative positions of components throughout the life of the assembly for reliable and robust operation.

The optical waveguides are made of insulating material and the resistor chain utilises a large number of resistors to achieve a very high resistance. The resistor chain distributes the electrical potential gradient substantially evenly down the length of the lead assembly and this, combined with the sheds 108 which are moulded into the elastomeric sheath ensures that the sensor withstands the large voltage difference between its ends without electrical breakdown of the internal components or along its outer surface.

Circuit board 102, V-block 101 and clamp 109 all operate at line potential and, together with their elastomeric covering comprise the input end of the sensor. Circuit board 106 and external connector 107 operate at ground potential and, together with their elastomeric covering comprise the output end of the sensor. The optical waveguides and resistor chain together with their elastomeric covering comprise the lead assembly of the sensor.

Figure 2 shows a typical application of three sensors 201 to a three phase distribution power line 200. The three sensors are each connected to a power monitor 205 which utilises the information from each sensor separately and together to monitor the quality and stability of the power in the line and sends periodic reports to a master station by radio transmissions via antenna 207. The power monitor 205 is depicted in a typical outdoor distribution line installation and is mounted on cross-arm 202 which carries the three power line phases mounted on insulators 203. The cross-arm itself is mounted on power pole 204. The power monitor 205 is grounded by wire 206 which provides the ground reference for the sensors. Electricity supply to the monitor can be from a number of sources such as mains, solar or battery, the electricity supply is not shown.

There are numerous applications possible for the sensor such as fault detection, network protection, distribution automation or power system monitoring and the sensor can be applied equally in the outdoor environment, or the indoor environment, or incorporated into electrical equipment such as switchgear, transformers, tap changers and so on.

Figure 3 shows a schematic representation of the sensor of Figure 1. The sensor 201 is depicted by the dashed outline.

The sensor input end comprises: power line connection 101 and circuit board 102. Circuit board 102 comprises: magnetic sensor coil 104, electronic circuits 311 , photo-emitter 305 and photovoltaic device 308. Optical waveguides 105, 110 and resistor chain 103 terminate on circuit board 102.

Magnetic sensor coil 104 senses power line current and resistor 304 senses the current in the resistor chain 103 and hence senses power line voltage relative to ground. Magnetic sensor coil 104 and resistor 304 connect to electronic circuits 311 which sample and digitise their signals and so sense the power line current and voltage. These signals are encoded and transmitted as light pulses by photo-emitter 305 into optical waveguide 105 which runs down the sensor lead assembly to the output end of the sensor. At the output end opto-receiver 306 on circuit board 106 converts the light pulses into electrical signals for processing by electronic circuitry 310. Circuit board 106 also comprises photo-emitter 307 which converts electricity into light energy which is transmitted into optical waveguide 110 which runs up the sensor lead assembly to the input end at which point it is converted back into electrical energy by photovoltaic device 308. This electrical energy is used the power the input end circuitry on circuit board 102.

Circuit board 106 also comprises resistor 309 which senses the current in the resistor chain 103 and hence senses power line voltage relative to ground in a similar way to resistor 304. Resistor 309 connects to electronic circuits 310 which sample and digitises the signal from resistor 309 and processes these signals as well as the signals from the input end received by the optical waveguide 105. The signal processing derives the power line data required by the application and is described further in subsequent paragraphs.

The processed data is transmitted over data interface 312 on external connector 107 to the recipient downstream application system 303 which utilises the data for its application. Connector 107 also carries the ground connection 313 and the power connection 314 which may come from the other system 303 or from elsewhere. A plurality of sensors 201 may be connected a single application system to suit some applications, particularly in poly-phase electrical networks.

Figure 4A depicts the outline schematic of electronic circuit board 102 which resides inside the sensor input end.

Magnetic field sensor 104 feeds into high impedance operational amplifier

402 which is configured as an integrating amplifier by feedback capacitor 406.

Integration is required because the output of magnetic field sensor 104 is the time derivative of the magnetic field and hence of the line current, integration in amplifier 402 converts the signal to be proportional to the power line current itself. The output of amplifier 402 feeds into one channel of the two-channel analogue to digital converter (ADC) 403. This periodically samples and digitises the power line current signal under the control of the clock generator 407 and sends the digital sample data to photo-emitter 305 which in turn is coupled to waveguide 105.

Voltage across resistor 304 is proportional to the current in resistor chain

103 and hence is proportional to the power line voltage relative to ground. This voltage is applied to the second channel of ADC 403 and is periodically sampled and digitised and transmitted to photo-emitter 305 under the control of the clock generator 407.

Clock generator 407 consists of conventional digital oscillator and clocking circuitry to control the ADC 403 in the required manner. Typically it is desired to sample both the line current and voltage every 2ms. Accordingly the clock generator will trigger first a current sample which takes 20 microseconds to digitise by the ADC, it then triggers a sample for the line voltage and at the same time clocks out the current sample to the photo-emitter which takes approximately 200 microseconds. By the time the current sample has been sent the voltage sample has been digitised by the ADC so this is now clocked out to the photo-emitter and the sampling cycle is complete. The process will be repeated 1580 microseconds later to achieve the desired sampling rate of 2ms. The sampling and data transmission is illustrated in Figure 5.

Power supply circuit 401 converts the incoming light energy to electrical energy in photo-voltaic converter 308 which is a PIN photo-diode in this implementation. Since the typical voltage required by the rest of the circuitry is 3.3 volts and the typical output of a PIN diode is 0.4V the light source is pulsed at the output end and transformer 405 is used to step up the voltage to the required level which is then rectified and smoothed in the conventional manner. Typical pulse rate would be 10OkHz. Power supply circuit 401 also provides a mid-supply voltage Vcc/2 for the ADC and operational amplifier biasing by using a resistor divider. Many other photo-voltaic power supply designs are possible using different devices and circuits some of which will not require pulsed operation or transformers.

Feedback capacitor 406 can be replaced by a gain setting resistor network shown in Figure 4B provided that subsequent digital signal processing is carried out to digitally integrate the signal and recover the line current. This may be desirable if the stability of capacitor 406 is insufficient for some applications. Methods for digital integration are well known.

Current sensor coil 104 can be replaced by other suitable devices such as a Rogowski coil or a cored current transformer, each type of device has well known advantages and limitations which do not affect the sensor principles described above.

There are many combinations of ADC word length and accuracy and dynamic range and data transmission rate which will vary from application to application and will affect the cost of the sensor and the power consumption of the sensor but do not affect the sensor principles described above. Persons skilled in the art will be able to make many different implementations of this invention to suit their particular requirements.

Figure 6 shows the construction of the sensor lead assembly. Optical waveguides 105 and 110 and resistor chain 103 are wound helically around central strength and stiffness member 501 which was omitted from Figure 1 for clarity. Strength and stiffness member 501 is made from suitable insulating plastic and provides tensile strength to the lead assembly as well as imparting sufficient stiffness so that the lead assembly will retain its flexibility for installation purposes but will not be subject to undue wind vibration.

The lead assembly has elastomeric sheath 100 of suitable insulating material such as silicone rubber and incorporates sheds to increase the external electrical creepage distance to ensure high external electrical breakdown voltage. Elastomeric sheath 100 is preferably applied by a single moulding process to the input end, lead assembly and output end to ensure the maximum insulation integrity by avoiding any interfaces in the elastomeric which would be caused by multiple moulding processes or by joining separate components.

Figure 7 depicts the outline schematic of electronic circuit board 106 which resides inside the sensor output end. Optical waveguide 105 terminates in opto-receiver 306, a PIN diode, which is connected in turn to opto-receiver circuits 701 which sense the digital data in the waveguide, such circuits are well known and may be incorporated into the opto-receiver 306. The output of opto- receiver circuits 701 is the digital data stream 706 which originated from the ADC in the input end, the data stream is input to microprocessor 702.

Resistor chain 103 connects to sense resistor 309, the current in the resistor chain develops a voltage across the sense resistor 309 which is proportional to the power line voltage relative to ground. The sense resistor voltage is sampled and digitised by ADC 703 and the resulting digital data is input to microprocessor 702.

Microprocessor 702 processes the data samples from the power line current sensor and the two power line voltage sensors to derive the required power line parameters such as RMS current and voltage. This processing is described in more detail later. The derived data (or indeed the calibrated data sample in some applications) is sent to the application system 303 via the electrical data interface on connector 107.

Preferably data interface 312 on connector 107 provides standard electrical and protocol interfaces. Suitable electrical interfaces would be USB, 100BaseT, RS232. Suitable protocol interfaces would be DNP3, DNP3 over

TCP/IP, XML over PPP. All these protocols require bi-directional communications between the sensor and the application system, consequently

Figure 8 shows both an Rx Data line as well as a Tx Data line however specific details of the realisation of these signals are dependant on the interfaces and protocols involved and are not dealt with further in this patent. Suitably other types of interface may be provided including as wireless interfaces such as

Bluetooth™ or 802.11b or optical fibre interfaces.

Connector 107 also provides the power and ground connection to the sensor. In some cases these signals will come from the application system 303, but this does not need to be the case. Incoming power to the output end is conditioned by the power supply circuit 705 which provides regulation and surge protection in the conventional manner.

Microprocessor 702 also provides output pulses to transistor 704 which in turn pulses photo-emitter 307 as described earlier. Pulsing of the photo- emitter 307 allows the input end power supply 401 to utilise a step-up transformer as described previously. A further advantage of pulsing photo- emitter 307 is that, with the addition of suitable circuitry in the input end clock generator 407, it enables the microprocessor 702 to control the sample timing in the input end. In the preferred embodiment this is achieved by pausing the pulsing of photo-emitter 307 for 50 microseconds immediately before the required sample time. The pause is sensed by clock generator 407 which triggers the start of the sampling sequence shown in Figure 5 on the leading edge of the first pulse when the pulsing is resumed. Figure 13 shows a timing diagram and Figure 14 shows an outline of the circuitry in the clock generator 407forthis. The circuit utilises a re-triggerable monostable circuit which receives its clock input "C" 421 from the pulse transformer 405. The monostable output "Q" 422 drops out when the pulsing is paused for longer than the monostable time delay but is asserted again when the pulsing is resumed. The re-assertion of "Q" 422 forms a synchronising signal which is used by the rest of the clock generator circuitry 423 to trigger the ADC sampling sequences shown in Figure 5 which as a result are now under the timing control of the output end microprocessor 702. One advantage from controlling the sampling at the input end in this way is to synchronise the voltage samples at the input end and the output end so the samples can be averaged without any further processing to correct for timing skew between the samples. A further advantage of controlling the sample timing occurs in polyphase systems and is explained below.

Figure 8 shows an alternative power supply arrangement for the input end of the sensor. The photovoltaic converter 308, optical waveguide 110, the light source 307 and transistor 704 are all omitted and the alternative input end electronics is shown in Figure 8.

In this configuration the input end gets its power from the current flowing in the resistor chain. This current is alternating and on positive half cycles the current flows through diode 801 and charges capacitor 803 which acts as a reservoir, the voltage on this capacitor is utilised as Vcc, the positive supply rail. On negative half cycles the current flows through diode 804 and charges capacitor 806 which acts as a reservoir, the voltage on this capacitor is utilised as OV, the negative supply rail. The mid-point of the two capacitors is utilised at Vcc/2 which is the mid-point bias supply and connects to the power line 201 via sense resistor 304. Zener diodes 802 and 805 regulate the power supply voltages.

The advantage of this configuration is the simplification and cost saving of omitting the previously mentioned components. One disadvantage is the small loss of accuracy in power line voltage measurement introduced by the power supply circuit which will subtract a substantially constant voltage from the sensed voltage approximately equal to the Zener values. This error can be partly compensated for in subsequent digital signal processing by simply adding the Zener voltage to all samples. Another disadvantage of this configuration is that the sensor will not work when the power line voltage is substantially reduced. For some applications these disadvantages will not be significant, for other applications they will be unacceptable.

Ideally the sensor is assembled with inherent absolute accuracy. In practice this is not possible due to manufacturing variation of the sensor and its component parts and as a consequence a method of calibration of each sensor at time of manufacture is required. The preferred method is to assemble each sensor and to calibrate the sensor by injection of known values of current and voltage and reading of the resulting sensor output. Calibration factors can then be computed for the sensor from the reading to correct for errors in ranging, offset, linearity and so on by well known methods. The calibration factors are then stored in non-volatile memory in the microprocessor 702 at time of manufacture and used by the microprocessor 702 to correct the stream of instantaneous sample data from the current sensor and the various voltage sensors. Suitably the bi-directional data interface 312 is used to instruct the microprocessor 702 to initiate calibration sequences and to store the resulting calibration factors .

The stream of calibrated instantaneous sample data available in the output end requires further processing in order to be useful in an application. For example some applications will require to know the RMS value of the line current and line voltage averaged over, say, 1 second. Transformation of the line current data samples into the RMS value is carried out by a simple, well known method involving squaring of the sample value, averaging with the other squared sample values over the 1 second interval and finding the square root of this average value. Another application may require the RMS real power flowing in the line. This can be derived by a similar well known method which involves both the current and the voltage samples. A different application may require the peak value and DC value of the line current averaged over a single cycle interval, once again persons skilled in the art of signal processing can utilise well-known methods to derive this data. In fact from the current and voltage samples it is possible to derive all possible data about the line to which the sensor is connected including real, reactive and apparent power flows, power factor and phase angle, frequency of fundamental, harmonic content, sags and surges and so on.

However in poly-phase networks it is important for many applications to utilise data about the poly-phase network as an entity rather than just each phase individually. Commonly the methods of sequence analysis are applied to achieve the required functionality in the application. For example in a ground fault protection relay application the zero-sequence current must be known. The zero sequence current can be found by summing the instantaneous line current samples from all of the phases in the poly-phase network. Derivation of the positive, negative and zero sequence voltages and currents can be carried out by well-known signal processing algorithms which are applied to the data samples from all the phases in the poly-phase network.

Figure 9 shows one architecture to achieve the signal processing required on both the individual phases and the poly-phase network. In this architecture one sensor 201 is fitted to each phase of the line and each sensor sends the calibrated instantaneous sample data stream to the client application system 303 where the required processing 901 is carried out. This architecture may be suitable for some situations where existing applications or products have the built-in signal processing capability because they are connected to traditional voltage and current transformers. The disadvantage of this method is requiring real-time signal processing capability in the client application and relatively high bandwidth data links 902 to the client application carry the sample data.

Figure 10 shows a different architecture whereby the required signal processing 901 is carried out in a single output end 906 that is common for all the sensors 905 in the poly-phase network. In this example the senor is manufactured as a single integrated device for a 3 phase network with three input ends and three lead assemblies feeding a single output end. The single output end provides a single data interface 907 for the client application 303, sending a single data stream via low bandwidth link 904 which carries only the data required by the application already processed into a suitable form.

Electronic circuitry in the output end (typically a microprocessor) carries out the required signal processing on the line current and voltage data of the poly-phase network. The processing in the output end is simplified if the current and voltage samples are taken on all phases of the electrical network at the same time. The method of pulsing the photo-emitter described above can be used to effect this synchronisation with the advantages of reduced processing and more accurate poly-phase measurements.

Figure 11 shows a possible mechanical implementation of this architecture. The advantages of this architecture are: to relieve the client application of the need for real-time signal processing and to reduce the required bandwidth and number of communications interfaces between the sensors and the client application and to reduce the number of sensor output ends to be manufactured and installed.

In some cases it may not be possible or desirable to have a single output end for mechanical reasons. In this case the required poly-phase processing can be carried out across the three output ends as shown in Figure 12. In this case output end for phase A 920 transmits its data about phase A 911 to the output end for phase B 921 which in turn transmits data about phase A and phase B 912 to the output end for phase C 922 which carries out the required processing and transmits the required data to the client system 303 over low bandwidth link '

904. The required signal processing in the output ends would be carried out by electronic circuitry, suitably a microprocessor 910 as shown. Persons skilled in the art would be able to design a number of variations on this theme with

, different amounts of processing taking place in each output end and different topologies for connection between the output ends. In all cases however the result is the delivery of the processed data 904 required by the application which yields the advantages of reduced processing in the client application and reduced bandwidth and number of communications interfaces between the sensors and the client application. Once again synchronised sampling across the phases can be employed to reduce signal processing and improve accuracy of results. In some applications a very high degree of processing may be carried out in the sensor output end or ends so reducing the amount of data sent to downstream systems still further. For example output end processing may include a fault detection and location algorithm with just the time of occurrence and the location of the fault being transmitted over the data interface. In another example the output end may include a power metering algorithm so that the data transmitted over the data interface consists of no more than the meter reading.

Contaminants on the lead assembly will distort the voltage gradient in resistor chain 103 and so affect the accuracy of the line voltage samples. The distortion occurs because the contaminants provide impedances in parallel to the voltage chain. Contaminants may be conductive (eg. Coal dust) or capacitive (eg. Ice) and are coupled to the resistor chain by the capacitance of the elastomeric sheath 100.

The contaminants will be distributed along the length of the lead assembly but are not necessarily evenly distributed. It is the unevenness which gives rise to the errors in the accuracy of the voltage samples. Figure 15 gives an example where the lower part of the lead assembly has been covered in ice 951 which has a very high relative permittivity and so couples additional currents 952 into the lower part of resistive element 103 by the sheath capacitance 950. In this example the current in the lower sensing resistor is decreased and the current in the upper sensing resistor is increased.

For some cases of contamination it is possible to reduce or eliminate the errors by utilising sample data from both the voltage sensing resistor at the input end and the output end of the sensor. By averaging samples from each end of the voltage sensing element which have been taken at the same instant a correction will be made for contamination which couples current into the resistive element from either ground or line such as the build up of ice shown in Figure 15.

Whilst the above has been given by way of illustrative examples of the present invention, many variations and modifications thereto will be apparent to those skilled in the art without departing from the broad ambit and scope of the invention as set out in the appended claims.

Claims

1. An electric power line sensor assembly having an input end including an exposed line connector, an output end including a digital data interface and a flexible lead assembly joining the ends together in operative relation by being encapsulated along with the ends in an insulating medium thereby forming a single integrated flexible unit, the input end having a sensor circuit including a line current sensor for generating a sensed line current signal and an optical signal generator adapted to generate an optical signal based on the sensed line current signal for transmission through the flexible lead assembly to the output end, the output end having an output circuit for receiving the optical signal and providing an output digital data signal indicative of line current.

2. An electric power line sensor assembly according to Claim 1 wherein at least one of the signals may be processed in the assembly to provide power line measurement data.

3. An electric power line sensor assembly according to Claim 1 wherein at least one of the signals is processed in the assembly to provide power line measurement data and the measurement data is generated at the output end.

4. An electric power line sensor assembly according to Claim 1 wherein at least one of the signals is processed in the assembly to provide power line measurement data and the measurement data is generated at the input end.

5. An electric power line sensor assembly according to Claim 1 wherein the output end has a circuit for receiving the optical signal, processing the data received and generating an output as digital measurement data derived from the sensed line current signal for transmission to other systems via a data communications interface. 6. An electric power line sensor assembly according to Claim 1 wherein the assembly further provides provision for line voltage sensing as well as line current sensing.

7. An electric power line sensor assembly according to Claim 1 wherein the assembly further provides for line voltage sensing as well as line current sensing, there being a resistive element extending from the input end through the lead assembly to the output end for the purpose of providing a sensed voltage signal.

8. An electric power line sensor assembly according to Claim 1 wherein a processing circuit is provided in the output end, and signal processing takes place in the output end circuit to derive power system parameters from the line current and voltage signals for transmission as digital measurement data.

9. An electric power line sensor assembly according to Claim 1 wherein a sensed voltage signal is generated at the input end or the output end of the assembly or both.

10. An electric power line sensor assembly according to Claim 1 wherein the assembly further provides for line voltage sensing as well as line current sensing, there being a resistive element extending from the input end through the lead assembly to the output end for the purpose of providing a sensed voltage signal generated in the input end and an optical signal is used to carry both voltage and the current signal data to the output end.

11. An electric power line sensor assembly according to Claim 1 wherein the assembly further provides for line voltage sensing as well as line current sensing, there being a resistive element extending from the input end through the lead assembly to the output end for the purpose of providing a sensed voltage signal, a sensed voltage signal is generated at both ends as respective instantaneous input end and output end sensed voltages, the two being averaged prior to processing to derive therefrom output digital measurement data.

12. An electric power line sensor assembly according to Claim 1 wherein the assembly further provides for line voltage sensing as well as line current sensing, there being a resistive element extending from the input end through the lead assembly to the output end for the purpose of providing a sensed voltage signal, a sensed voltage signal is generated at both ends as respective instantaneous input end and output end sensed voltages, the two being averaged prior to processing to derive therefrom output digital measurement data, in order to synchronise sampling of the instantaneous input end and output end voltage an optical pulse generator is employed to generate synchronisation pulses.

13. An electric power line sensor assembly according to Claim 1 wherein the sensor assembly is factory calibrated for both current and voltage measurements so that no further calibration is required at the time of installation.

14. An electric power line sensor assembly according to Claim 1 wherein the assembly is pre-calibrated and the processing circuit in the output end utilises calibration data stored in the assembly at time of calibration to correct sensed voltage and current signals.

15. An electric power line sensor assembly according to Claim 1 wherein the an assembly is connected to a power line at the input end thereof and to ground at the output end thereof for the purpose of sensing current and voltage in said power line, said assembly comprising, an electrical current sensor substantially at line voltage connected to a circuit in the form of a first electronic sub-system which encodes line current data into digital form, an optical waveguide to transmit said digital data from said first electronic sub-system to a circuit in the form of a second electronic sub-system which is substantially at ground potential, a resistive element connected between line and ground, the current through the said resistive element being proportional to the line-ground voltage and said resistive element current being monitored by the first or the second said electronic sub-system or both and being encoded into digital data form, a signal processing element in the second electronic sub-system to process said current and voltage digital data to derive measured power system parameters, an electrical data interface from the second electronic sub-system which communicates the digital data representing measured power system parameters to a client computing system for utilisation in client system applications.

16. An electric power line sensor assembly according to Claim 1 wherein the an assembly is connected to a power line at the input end thereof and to ground at the output end thereof for the purpose of sensing current and voltage in said power line, said assembly comprising, an electrical current sensor substantially at line voltage connected to a circuit in the form of a first electronic sub-system which encodes line current data into digital form, an optical waveguide to transmit said digital data from said first electronic sub-system to a circuit in the form of a second electronic sub-system which is substantially at ground.potential, a resistive element connected between line and ground, the current through the said resistive element being proportional to the line-ground voltage and said resistive element current being monitored by the first or the second said electronic sub-system or both and being encoded into digital data form, a signal processing element in the second electronic sub-system to process said current and voltage digital data to derive measured power system parameters, an electrical data interface from the second electronic sub-system which communicates the digital data representing measured power system parameters to a client computing system for utilisation in client system applications, the first electronic sub-system being powered by a photo-voltaic device which gets its energy via an optical waveguide from a photo-emitting device incorporated into the second electronic sub-system.

17. An electric power line sensor assembly according to Claim 1 wherein the an assembly is connected to a power line at the input end thereof and to ground at the output end thereof for the purpose of sensing current and voltage in said power line, said assembly comprising, an electrical current sensor substantially at line voltage connected to a circuit in the form of a first electronic sub-system which encodes line current data into digital form, an optical waveguide to transmit said digital data from said first electronic sub-system to a circuit in the form of a second electronic sub-system which is substantially at ground potential, a resistive element connected between line and ground, the current through the said resistive element being proportional to the line-ground voltage and said resistive element current being monitored by the first or the second said electronic sub-system or both and being encoded into digital data form, a signal processing element in the second electronic sub-system to process said current and voltage digital data to derive measured power system parameters, an electrical data interface from the second electronic sub-system which communicates the digital data representing measured power system parameters to a client computing system for utilisation in the client system applications, the first electronic sub-system being powered by current flowing in the resistive element.

18. An electric power line sensor assembly according to Claim 1 applied to a polyphase network the assembly having one input end per phase and respective said lead assemblies merge to a common output end providing a single data interface to a client system for transmission of measured power system parameters.

19. An electric power line sensor assembly according to Claim 1 applied to a polyphase network, the input end comprises separate sensors for each phase.

20. An electric power line sensor assembly according to Claim 1 applied to a polyphase network, the input end comprises separate sensors for each phase and measured power system parameters being transmitted through a single data interface to the client system.

21. An electric power line sensor assembly according to Claim 1 wherein line voltage is sensed through a resistive element extending from the input end to the output end and in order to correct for leakage currents into and out of the resistive element for the purpose of generating the sensed voltage signal the resistive element current is sampled at the input end and at the output end, calibration correction is applied to the instantaneous data samples from the two ends and then the average of the calibrated samples is employed in subsequent signal processing.

22. An electric power line sensor assembly according to Claim 1 wherein the assembly comprises a pre-calibrated electric power line sensor assembly providing calibrated digital output of line current and line voltage parameters, the assembly having a sensor circuit in the input end and a voltage sensor comprising a resistive element extending from the input end through the lead assembly to the output end for the purpose of providing a sensed voltage signal to the output end circuit from which at least one power line parameter may be derived, power to the sensor circuit being supplied by light from a source incorporated into the output end, the lead assembly having a light guide for delivery of light from the source. 23. An electric power line sensor assembly according to Claim 1 wherein power to the input end is supplied by light from a source incorporated into the output end, the lead assembly having a light guide for delivery of light from the source.

24. An electric power line sensor assembly according to Claim 1 wherein line voltage is sensed through a resistive element extending from the input end to the output end and power to the input end is derived from current flowing in the resistive element.

25. An electric power line sensor assembly according to Claim 1 wherein the assembly is configured for a poly-phase network by having an input end for each phase and a common interface at the output end.

26. An electric power line sensor assembly according to Claim 1 wherein the assembly is configured for a poly-phase network by having an input end for each phase a separate interface for at least two phases at the output end.

30. An electric power line sensor assembly according to Claim 1 wherein the assembly is configured for a poly-phase network by having separate assemblies for each phase.

31. An electric power line sensor assembly according to Claim 1 wherein line voltage is sensed through a resistive element extending from the input end to the output end and for the purpose of generating a sensed voltage signal the resistive element current is sampled at the input end and at the output end and the average voltage used to derive the sensed voltage signal.

32. An electric power line sensor assembly according to Claim 1 wherein line voltage is sensed through a resistive element extending from the input end to the output end and for the purpose of generating a sensed voltage signal the resistive element current is sampled at the input end and at the output end and the average voltage used to derive the sensed voltage signal and sampling is synchronised using pulsed light.

33. An electric power line sensor assembly according to Claim 1 wherein power to the input end is primarily supplied by light from a source incorporated into the output end, the lead assembly having a light guide for delivery of light from the source, line voltage being sensed through a resistive element extending from the input end to the output end and power to the input end is derived from current flowing in the resistive element as a secondary power source.