Field of the Invention

The present invention concerns a current measuring shunt primarily for measuring at least a first electric parameter, where the current measuring shunt is applied a first electric potential and includes a first internal conducting pipe, and where the current measuring shunt includes a second external conducting pipe, wherein the first and the second pipes are arranged concentrically and joined at a common end plate.

The present invention also concerns a method for calculating at least a first electric parameter at a first electric potential, where the calculating of electric parameters is performed inside a coaxial current measuring shunt operating at the first electric potential.

Background of the Invention

US 5,420,504 concerns a non-inductive shunt current sensor includes a cylindrical- shaped conductive element and a concentric conductive pipe electrically connected at one end by a conductor, such as a washer. Except for the conductor, the pipe and conductive element are electrically isolated from each other by insulation, such as

Kapton™ or an air-gap. Sensing wires are electrically connected to contact points, each contact point being located on either the inner surface of the inner pipe or the outer surface of the outer pipe. The wires pass through a space where the magnetic field is substantially null, thereby reducing any mutual coupling effects.

US 5,461,307 concerns an electro -optics current sensing system for sensing and avoiding thermally induced measurement errors in a current sensor includes a beam generator, such as a light emitting diode or a laser diode, thermally coupled to the current sensor for encoding into the optical beam supplied by the beam generator both temperature and sensed current information. An electro -optics (E/O) demodulator which includes a wavelength division multiplexer (WDM) demodulates the encoded optical beam to recover the temperature and sensed current information from a pair of demodulated signals which can be further processed for filtering resistivity changes from the sensed current measurement. EP 1 028 321 A2 describes a current divider consists of a main current conductor which surrounds an auxiliary current conductor equipped with a field sensor. The main current conductor has a radial symmetric form, and its cross-section is orthogonal to the current flow direction. The main current conductor and the auxiliary current conductor are connected, both at the current input side of the current divider, as well as at its output side, and comprise contacts at those points for the current to be measured.

US 5,416,408 concerns an alternating current sensor employing a mutually-inductive current sensing scheme comprises a conductive pipe and a cylindrical-shaped conductive element at least partially surrounded by the pipe. The pipe is substantially concentric to the conductive element and a conductor electrically connects a pair of corresponding ends of the pipe and conductive element to form a connected conductive path through the sensor. The pipe and the conductive element are spaced apart a predetermined radial distance to form a magnetic field in the space between the pipe and conductive element during current flow along the conductive path.

GB 2 056 182 concerns an coaxial resistor for current measurement.

In electric network systems for production, transmission and distribution of electric energy, for current measurement is used so-called current transformers which transform high current - the primary current - to a substantially lower current - the secondary current. The secondary current is conducted to other equipment which may include protective relays, alarms, instrument displays, and energy meters for debiting etc., which are typically placed in an apparatus housing, container or similar adapted for the purpose. This way of controlling, regulating and monitoring the installation has been used for decades but provide some disadvantages. Magnet fields and thereby inductance in current transformers cause the measuring accuracy to be limited. At the same time, there is a hazard to persons by "open secondary circuits" since dangerous voltages may be induced here and present at any place where maintenance staff have direct access. Current measurement is not only performed on electric power systems for production, transmission and distribution of electric energy, but also in high voltage laboratories. Here, high, transient and exact currents are measured by means of a coaxial current measuring shunt where the shunt is free from inductance due to the coaxial design, and the current is measured as voltage difference across an known impedance. Such a measurement occurs normally with one end of the resistance connected to earth, e.g. point B in Fig. 1, providing that the measuring signal is always "bound" to earth potential.

Object of the Invention The purpose of the present invention is to perform measuring of electric parameters at a first voltage potential in an approximately field-free space, where a second purpose is to transmit measured values to a control system operating at a second voltage potential.

Description of the Invention A current measuring shunt as specified in the preamble of claim 1 may be further modified in that the first internal pipe includes a first electronics module that may operate at about the same electric potential as the first internal pipe, that the first electronics module communicates across a galvanic isolating separation with a second electronics module, and that the second electronics module is applied a second electric potential.

Hereby may be achieved that the first electronic circuit is provided inside a system consisting of a first internal pipe and a second external pipe, where the pipes have known impedance. The electronic circuit provided internally of the first innermost pipe is thus disposed in a largely field-free space where there is provided a particularly efficient screening against electric fields, which by always is present around the electric installation. Measured values recorded by the first electronics module may advantageously be transmitted from the almost field-free space internally of the first pipe through a galvanic separation to a second electronics module which may be included in larger control system for controlling and regulating a large electric installation. The first electronics circuit is thus well protected as it is entirely shielded at its location. Interfering fields will thereby only have very limited influence on the actual measurements performed. The first electronic circuit may thus consist of both an analog measuring circuit which performs real measurements and a digital system which may contain a microprocessor and subsequently a communication module which may communicate through the galvanic separation. Herby can be achieved, a very efficient and very reliable current measuring system.

A first electronics circuit may advantageously perform a current measurement. On the basis that the first and second pipes are designed in such a way that the impedance of at least one of the pipes is known with great accuracy, the actual voltage difference may be measured across this impedance. The measured voltage difference will thus be an expression of the current running through the pipe. Thus may be performed a current measurement directly inside the pipe in which the current is flowing.

The first electronics circuit may communicate with the second electronics circuit. It may be envisaged that communication can occur in many different ways. It will be possible to transfer data by means of a radio-based system as short-range radio communication systems may be applied which may transmit measurement data over relatively short distances, but where the radio-based communication simultaneously will imply a total galvanic separation.

Another possible form of communication may be a power line communication through the electric connection of the high voltage installation. By a power line communication it may thus be possible to move data to and from an electronic circuit disposed inside a current measuring shunt.

The first electronics circuit may calculate electric values for a protective system for an electric installation. The calculation occurring in the electronic circuit provided inside a measuring pipe, may advantageously be used for controlling a protective system. Thereby it will be possible to perform switch off of the electric system if failures occur in the electric installation. Based on the fact that any change in the electric current is detected at any time, it may quickly be determined if the current rises up to a limit critical to the installation and to perform a switch off command to the second electronic circuit. The first electronics circuit may include a transient recorder. The electronics circuit may advantageously be located inside the measuring pipe where the electronics circuit advantageously detects the electric transients possibly overlaid a supply voltage. Further it will be possible to detect all changes in the actual current flowing through the measuring system.

The first electronics circuit may include an energy meter, where the measured energy is used for billing purposes. If a voltage signal is supplied to the electronics circuit, a power measurement may be performed directly in the circuit. Another option will always be that the actually measured current is transmitted via the electronic communication means, and if in a connected control system it is possible to measure the actual electric voltage, it is simple to calculate the actual power.

The first electronics circuit may include a first transceiver circuit that may communicate with the second electronics circuit, where the second electronics circuit may include a second transceiver circuit, and where communication between the first and the second electronics circuit occurs through at least one first optical fibre. By providing a transmitter circuit which may communicate through a optical fibre, there is thus achieved a very efficient galvanic separation. Through one and the same optical fibre it will be possible to provide two-way communication, but on the basis of prior art optical fibre technique there will be no problem in using several single optical fibres. By communicating through optical fibres there is attained so great transmission speed that the amount of data capable of being transmitted far exceeds the actual need. Even if alternately transmitting up and down in a single optical fibre, there is no problem in moving any amount of data that may be envisaged in any case.

The first electronics circuit interacts with a power supply which may contain energy storage, where the power supply performs charging of the energy storage on the basis of the actual electric voltage difference across the current measuring shunt. The power supply of the measuring shunt may thus be provided by utilising the existing voltage drop which the current measuring shunt electronics is to measure in any case. This will possibly require a special electronic circuit in order to utilise this effect, but it will be possible.

The first electronics circuit is supplied with electric power through at least the first optical fibre. Another option for power supply and for charging an energy storage in the first electronic circuit is thus to use light through the optical fibre in order to charge the energy storage present in the electronic circuit.

By a method as specified in the preamble of claim 11, the method may be modified such that the electric parameter measuring shunt may include an electronics circuit operating at the first electric potential, and that the first electronics circuit communicates with a second control system through at least one optical fibre. Hereby may be achieved a very efficient measuring shunt which can be applied to all low or high- voltage installations, and where by communication through optical fibre down to a control system there maybe achieved a very efficient transmission of measuring data.

Description of the Drawing

Fig. 1 shows a section through a possible embodiment of a coaxial current measuring shunt according to the invention.

Picture 1 shows an example of an embodiment of the current measuring shunt. Picture 2 shown the same example of an embodiment as picture 1, shown with measuring wire and end plate at the opposite end. Picture 3 shows the same as picture 2, only at the opposite end.

Picture 4 shows measuring wire and an example of an insulating optical fibre for communication purposes.

Detailed Description of Embodiments of the Invention

Fig. 1 shows a section through a possible embodiment of a coaxial current measuring shunt 2. The current measuring shunt 2 includes the electrically conducting and concentrically arranged pipes 6, 8 which are closed with end plates 7 and 9. The measuring, communicating and calculating electronics 14 is found within the pipe 6. The calculating electronics communicate with the optical fibre 16, 18. Picture 1 shows an example of a design of the current measuring shunt with connection points A and B, but without optical fibre connection at C, and thus shown in a version for contact-free/wireless signal transmission.

Picture 2 shows the same example of an embodiment as picture 1, only shown with measuring wire and end plate at the opposite end of C. Measuring wire and end plate are not joint to pipe A.

Picture 3 shows the same as picture 2, only at the opposite end. The electronics are indicated by a rectangular piece of cardboard, but may assume other design configurations.

Picture 4 shows measuring wire and end plate at the joint C and an example of an insulating optical fibre for communication purposes.

The term "inductance-free current measuring shunt" is often used, when using concentrically arranged conductors. However, the case is always that the mutual inductance is so small that it is negligible, but the self-inductance across the measuring pipe can be significant if the resistance across which the voltage difference to be measure is very small. If necessary, the self-inductance of the measuring pipe in a coaxially designed current measuring shunt may be calculated by Ampere's Law and corrected for self-inductance by the calculating electronics. In the following, the common expression "coaxially designed resistive current measuring shunt" is used; bearing in mind that correction for self-inductance of the measuring pipe possibly has to be made, depending of the choice of material.

With reference to Fig. 1 and the pictures 1-4, shows a pipe 6 with defined impedance. Placing the electronic circuit 14 inside pipe 6, both ends of pipe 6, are joined with an end plate 7,9, respectively. Hereby is achieved a field-free volume in the inner pipe 6, forming a housing for a digital and/or analog measuring, communicating and calculating electronic circuit, which with knowledge of the impedance of pipe 6, the electronic circuit can calculate the current through the pipes from the measured electric voltage difference in the measurement connection points 8 and 10. The electric and magnetic shielding of the electronic circuit 14 may be established inside pipes 6, and 8, B by selecting material which is both electrically and magnetically conducting, e.g. μ- metal. Alternatively, it will also be possible to let pipes 6, and 8, constitute the screening against electric fields, and then to shield the electronics only against magnetic fields. The measuring, communicating and calculating electronics may, besides making the current value accessible to internal use, transform the calculated electric parameters to a optical signal in the optical fibre 16, 18. Through the optical fibre, the measuring, communicating and calculating electronics may transmit as well as receive data, making data communication with the surroundings possible.

The measuring conductors 8 and 10, which is connected in endpoints 8 and 10 respectively, to the electronic circuit 14. The measuring conductors may be disposed at the centre of the pipes and be carried to the measuring, communicating and calculating electronics 14 as circular placed conductors. By a possible embodiment, the measuring wire 22 can be designed with a optical fibre 16 at the centre of measuring wire. Hereby, the measuring, communicating and calculating electronics are then connected optical to the connection point 19. The connection 19 connects the transmitted/received light of the measuring, communicating and calculating electronics to optical fibre 18 which is dimensioned for sufficient electric insulation for the electric voltage level at which the current measuring shunt is mounted. The signal or signals may now be used apart from the voltage level at which the current is measured. Besides being an insulator, the optical fibre 18 can also act as a possible power supply for the measuring, communicating and calculating electronics 14. A third task of the optical fibre 16, 18 is to transmit other measured values, e.g. the voltage in the installation where the current measuring shunt is located, from the surroundings to the measuring, communicating and calculating electronics. With the knowledge of the measuring, communicating and calculating electronics of both actual current and actual voltage, almost all electric values can be calculated and used for internal and/or external purposes.

An alternative may be designed by using other contact-free signal transmission methods. It will thus be possible to transmit data from the current measuring shunt via a radio signal, e.g. over a wireless network for computer communication. Another way communication is to use "power line communication" where a winding around one of the electric conductors may transmit a high-frequency signal across the existing electric supply.

In high voltage installations high voltage components can be placed inside a tank, which tank contains an insolating gas. Also a current measuring shunt as previously described can be placed together with a transformer inside a tank. Communication up and down towards the current measuring shunt can be performed through the isolating gas by one ore more beams of light generated by modulated light sources towards light receiving components. The light sources can be formed by one or more lasers.