Background Art Conventional voltage and current transformers for high voltage application are typically very large, heavy and costly pieces of equipment that are found to be increasingly unreliable and unsafe over time.

Conventional low voltage transformers become increasingly unsafe as fault levels increase with load.

Many replacements using optical and radio transmission technologies have been developed in the past twenty years that address the problems of high voltage current transformers. Of particular interest has been the Faraday current sensor, but this has been found to experience temperature and vibration instabilities that are difficult to control or compensate. Many complex and expensive Faraday current sensors have been developed over the past twenty years and some commercial systems are available. Hybrid apparatus using optically powered remote electronics with'traditional' sensors such as current transformers and"Rogowski"coils have been commercially exploited as a low cost solution with good reliability and stability.

Non-conventional voltage sensors have been developed based on highly resistive voltage strings or electro-optic techniques such as the Pockels effect.

Summary of the Invention The invention, as currently envisaged, is a power monitoring apparatus comprising, along with other optional components, a capacitative divider having an input voltage terminal at one end to connect to a conductor and a ground terminal at the other end to connect to ground potential, and a power monitoring module which is connected to the capacitative divider to be

energised from voltages appearing across part of the divider. The power monitoring module includes ports to receive signal from a current sensor measuring current in the conductor, and from a voltage sensor measuring voltage on the conductor. The power monitoring module also includes an optical signal encoder to code the data for transmission over an optical fibre link, and a port for connection to a fibre optic cable. The signal may be split at the receiver which is not the case for conventional equipment.

Since module power is supplied from a voltage across the capacitative divider, rather than current from a powering current transformer, the monitoring module is always energised except when the conductor is'de- energised', which includes when the current is zero for extended periods.

Powering the monitoring module from the capacitive divider provides adequate power down to a small fraction of the apparatus rated voltage allowing voltage measurement well within the standards. A battery back-up, or some other energy storage such as a storage capacitor, and coordinated shut-down could allow management for out-of service periods. Some form of analog indicator such as an LED could be used to provide an additional indication of voltage on the conductor. Alternatively, an additional optical fibre and analogue transmitter can be used to provide this indication. The transmitter may use the same fibre or an additional fibre.

Commercial off-the-shelf components are available for the required electronics, optics and sensors. Sensors may have very high performance, and may require only ultra low power supply. For instance, current may be measured by means of a conventional low insulation current transformer or even a"Rogowski"coil. When a"Rogowski"coil is used the voltage and current transducers may be integrated into a single transducer with the coil acting as a voltage screen. Furthermore there need be no interaction between current and voltage sensors via field effects-a factor that complicates the design of conventional high voltage combined instrument transformers and substation layout. The design of the capacitative divider may be simplified as a result of the low power consumption and high input impedance of the monitor module. So, the size of the capacitance, or of individual capacitors, in the divider can be reduced and the possibility arises of using a composite style insulator.

The capacitative divider may conveniently take the form of a capacitor type voltage transformer. These devices usually take the form of a capacitor

bushing in which concentric and insulated cylinders/screens are placed around a conductor. A single voltage screen may be sufficient. Intermediate voltages can be tapped off the screens, and these can then be stepped down by a small voltage transformer, if desired.

Space savings result from the combined arrangement of the optically isolated current, voltage and condition monitoring sensors being located within a single apparatus, and the weight is reduced.

The need for sampling synchronisation between electrical phases to maintain polyphase measurement accuracy may be avoided by using a high sampling rate. A single fibre asynchronous apparatus transmission may be used with no up-link being required for phase synchronisation.

Correspondingly cabling costs and complexity are lowered.

Line powering from the conductor allows increased circuit consumption compared to optically powered apparatus. This offers new levels of performance giving potentially very high resolution and accuracy, relatively high sample rates, accurate signal conditioning and complex apparatus protection and self-monitoring. The increased availability of transmitter power means large optical transmission distances are possible with no error, noise or interferences with insignificant delay.

Condition monitoring auxiliary sensors may be included in the power monitoring module for fault detection and to measure other parameters, such as leakage current, gas detection, temperature, and pressure. Protection equipment may also be included along with the monitoring. The data from all the sensors may be multiplexed before transmission to the ground station where a wide range of interfacing options are available.

The potential exists for the apparatus to be installed while the conductor is energised, as there is no electrical load connection required. No disconnection is required for interchanging instrumentation saving power companies time and money.

The apparatus may integrate new electronic, material, and optical technology as it is improved.

The apparatus combines current and voltage monitoring in a single unit, and as a result the cost is expected to be a fraction of presently available optical and conventional apparatus.

Brief Description of the Drawings An example of the invention will now be described with reference to the accompanying drawing, in which : Figure 1 is a schematic representation of a power monitoring apparatus when used to monitor a high voltage conductor.

Figure 2 is a schematic illustration of a bushing embedded example of the power monitoring apparatus.

Detailed Description of the Invention The power monitoring apparatus comprises a capacitative divider. In this case the divider involves at least two capacitors connected in series to form a capacitor string 1 having a high voltage terminal 2 at one end connected to a high voltage conductor 3. A low voltage terminal 4 at the other end is connected to ground potential. A power monitoring module 5 is connected across the top capacitor 6 of the string 2 so that it is energised from voltages appearing across that capacitor 6. A resistive or capacitive divider network (not shown) may optionally be used to reduce the voltage being sensed.

The power monitoring module 5 includes a first port 8 receiving data from a current sensor 9 measuring current in the high voltage conductor 3, and a second port 10 receiving data from a voltage sensor 11 measuring voltage on the conductor 3.

The power monitoring module also includes an optical signal encoder 12 to code the data for transmission over an optical fibre link, a transmitter 13 and a port 14 for connection to a fibre optic cable 15.

The optical cable 15 carries the data to a ground station 16 where data analysis, system monitoring and management take place.

Figure 2 is a pictorial view of a bushing embedded capacitative divider 20. In this apparatus the high voltage conductor 21 enters a current transformer shield 22 at the top and passes through low insulation current transformers 23. Beneath this is a'bypass'conductor and encoder shield 24 and the encoding module 25. An optical fibre 26 is fitted between the encoder 25 to an optical connector box 27 mounted on the top of the equipment tank 28. The link may allow communication with monitoring equipment.

Equipotential cylinders 29 extend down inside the ceramic or composite insulation 30, separated from each other by screens or shields 31.

The encoder is powered by the voltage obtained from the equipotential cylinders 29. This also gives a voltage proportional to the conductor voltage as a capacitative divider voltage sensor.

There is an internal connection 32 at the lower end of the apparatus.

These bushing type capacitative dividers can be used to replace bushings on a large variety of equipment, such as power transformers, circuit breakers, and bulkheads during routine refurbishment and may be used in new equipment also.

Although the invention has been described with reference to a particular example it should be appreciated that it may be exemplified in many other ways. For instance, condition monitoring auxiliary sensors, such as a leakage current detector 17 may be incorporated into it. Also, if the capacitative divider could be fitted to a high voltage circuit breaker the whole apparatus could provide combined protection, metering and circuit interruption in a single unit.

Although two specific examples of the capacitative divider have been described in detail, it will be understood that many other options are available.

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.