WO2017021709A1

WO2017021709A1 - Current transformer comprising optical modulator and related method

Info

Publication number: WO2017021709A1
Application number: PCT/GB2016/052344
Authority: WO
Inventors: Haiyu Li
Original assignee: University of Manchester
Current assignee: University of Manchester
Priority date: 2015-07-31
Filing date: 2016-07-29
Publication date: 2017-02-09
Anticipated expiration: 2018-01-31
Also published as: GB201513590D0

Abstract

Example implementations relate to devices and methods for measuring an electrical characteristic, in particular, to measuring current. The devicescan use a pair of MEMS optical modulators as opposed to the more conventional coil and associated oil insulation arrangement.

Description

CURRENT TRANSFORMER COMPRISING OPTICAL MODULATOR AND RELATED METHOD

Example implementations relate to devices and methods; and, more particularly, to devices and methods for measuring current.

AC current measurement in the electric power industry has been traditionally carried out using transformers; voltage transformers and current transformers. In high-voltage environments, for example, when the line voltage is, or is greater than, 33kV such as, for example, 400 kV, electrical insulation is difficult at least in part due to the safety performance requirements of magnetic core based current sensors, which can carry thousands of amps in its primary and 1A/5A of normal current in its secondary. The current sensors can sustain large fault currents of the order of about 20 to 30 times the normal currents for a sufficiently short period to allow power system protection equipment to trip circuit breakers for the faulty line. Furthermore, significant insulation, in the form of oil, is needed for such current transformers. Consequently, current transformers for such environments are bulky and expensive. However, any moisture or small gas bubble accumulations in the current transformer can lead to a catastrophic failure such as, for example, an explosion. Optical current sensors or optical transformers can be used within such high-voltage environments. Such products are based on the Faraday rotation effect within optical fibres or within a bulk optical material in which the polarisation of an optical signal is affected by the magnetic field associated with an electric current carried by a conductor. Changes in polarisation are detected by an optical receiver. However, such products also suffer from the disadvantages that the Faraday rotation effect is, firstly, relatively weak and, secondly, that detecting changes in polarisation is relatively difficult since polarisation within optical materials varies significantly with environmental conditions. Other environmental conditions such as, for example, the vibrations caused by circuit breaker operation or weather, also adversely affect the performance of such optical current sensors. Still further, to compensate for instability, relatively large optical current transformers are required. UK patent GB 2400172 B discloses an optical AC current sensor that is based upon an electro-optic amplitude modulator having a modulation depth that has fixed relationship with the driving voltage. The driving voltage is derived from an AC current or voltage under measurement. In operation, optical power from an optical source is modulated by the driving voltage, the modulation depth has a fixed relationship with the driving voltage and the modulated optical signal is detected by an optical receiver. The electro-optic amplitude modulator is insensitive to polarisation variations due to using a diffractive MEMs based variable optical attenuator. Electro-optical amplitude modulators or variable optical attenuators, including diffractive MEMs based variable optical attenuators, require a DC bias voltage to be able to change optical attenuation in both positive and negative directions. Such an active arrangement consumes power by requiring a separate circuit to provide a biasing voltage.

Accordingly, example implementations provide a current transformer comprising a transducer responsive to a current carried by a conductor; the current transformer comprising a circuit, responsive to the transducer, having at least one optical modulator for providing a modulated optical output that varies in association with variation of the current; the circuit comprising a voltage reference line or input node that is operable as a stable reference. The stable reference can be an unbiased reference, that is, unlike the prior art, a biasing voltage is not required. Furthermore, example implementations provide a current transformer comprising a transducer responsive to a current carried, at a respective voltage, by a conductor; the current transformer comprising a circuit, responsive to the transducer, having at least one optical modulator for providing a modulated optical output that varies with the variation of the current; the circuit further presenting at least a pair of input impedances, Zi and Z₂, the input impedances having a node in common with the transducer forming a reference node. The reference node can provide a voltage reference about which the output of the transducer can vary. The reference node operates as a stable reference, from a current perspective due to the transformer inductance. Such a stable reference can provide a virtual earth. Still further example implementations provide a current transformer comprising a transducer responsive to a current carried, at a respective voltage, by a conductor; the current transformer comprising a circuit, responsive to the transducer, having at least one optical modulator for providing a modulated optical output that varies with the variation of the current. The circuit can further comprise circuitry presenting at least a pair of input impedances, Zi and Z₂. The input impedances can be arranged to be larger than the output impedance of the transducer. The circuit can have a common node formed between the input impedances. The common node provides a reference, or virtual earth, relative to which waveforms input to a pair of optical modulators can vary.

Still further example implementations provide a current transformer comprising a transducer responsive to a current carried, at a respective voltage, by a conductor. The current transformer can comprise a circuit, responsive to the transducer, having at least one optical modulator to provide a modulated optical output that varies with the variation of the current; the circuit further comprising a series arrangement of at least a pair of impedances, Zi and Z₂. The impedances can be larger than the output impedance of the transducer. The circuit can comprise a common node formed between the impedances.

Advantageously, devices according to example implementations provide a passive solution that is stable, extremely reliable and that does not require a separate active biasing circuit to derive a respective biasing voltage or a connection to some other biasing voltage such as, for example, a voltage provided by the conductor carrying the current to be measured. Brief description of the drawings

Example implementations will now be described, by way of example only, with reference to the accompanying drawings in which: figures 1 and IB show example implementations of a current transformer; figure 2 shows example implementations of optical modulators; figure 3 depicts a one example implementation of a ogowski coil; figure 4 shows a current transformer assembly according to an example implementation; figure 5 shows an example implementation of a conditioning circuit. Detailed description of example implementations Referring to figure 1A, there is shown a current transformer 100 according to an example implementation. The current transformer 100 comprises transducer 102 for outputting a voltage, AV_coil , associated with the currents flowing in a conductor 104. The conductor 104 can be a busbar.

The current transformer 100 also comprises a circuit 106 for receiving the output voltage AV_coil and for producing half-wave rectified waveforms 108 and 110 therefrom at two pairs of output terminals 109 and 109'. In the illustrated example implementation, AV_coil varies sinusoidally. The half-wave rectified waveforms 108 and 110 are fed to an optical modulator 112. The optical modulator 112 is arranged to produce an optical output that is responsive to the half-wave rectified waveforms 108 and 110. Example implementations use MEMS optical modulators such as the pair 114 and 116 of optical modulators illustrated in figure 1. The circuit comprises a reference node 139. The reference node 139 can act as a virtual earth or other stable reference.

At least two or more of the nodes associated with the output terminals 109 and 109', that is, nodes 138, 139 and 140, are arranged to present respective input impedances Zl and Z2. Example implementations are provided in which the respective input impedances Zl and Z2 are greater than the output impedance of the transducer 102. Example implementations are provided in which the input impedances Zl and Z2 present a virtual earth or other stable reference about which the output of the transducer 102 can vary.

It will be appreciated that the example implementations described herein do not require either of a biasing circuit to provide a bias voltage for line 150, or nodes 139 and 152, unlike the prior art, or to provide a reference voltage for line 150, or nodes 139 and 152, such as coupling line 150 to the line voltage carried by the conductor 104.

Figure IB shows a view of an alternative example implementation. Reference numerals common to figure 1 and figure lb refer to corresponding elements. It can be appreciated that the circuit 106 comprises a pair of impedances Zl and Z2. The pair of impedances Zl and Z2 are arranged to present a stable voltage, that is, a reference at node 102B about which the output of the transducer can vary. Preferably, the impedances are at least one of fixed or sufficiently stable to provide such a reference. It can be appreciated that the example implementations are arranged such that the impedances Zl and Z2 bear a common node 102B that presents a stable reference about which the output of the transducer 102 can vary. The impedances present a voltage divider between nodes 138, 139 and 140.

Referring to figure 2, light for the optical modulators of figures 1 and IB is provided by a light source 118 of an optical transceiver 119. In an example implementation, light from the light source 118 can be split by a light splitter 120 and fed to respective voltage optical modulators 114 and 116. The modulated light output by the modulators 114 and 116 is detected by respective light detectors that, in the present example implementation, are realised using a pair 122 and 124 of photodetectors such as, for example, a pair of photodiodes. A conditioning circuit 126 is provided to process the outputs of the photodetectors to produce a signal indicative of or associated with the current carried by the conductor 104. An amplifier 128 can be used to amplify the indicative signal. Example implementations of the optical modulators 114 and 116 can be realised using optical attenuators such as, for example, MEMS mirrors having a deflection that is associated with an applied signal such as the voltages across one or more of the inputs to the VOMs 114 and 116, which, at least insofar as concerns figure IB, correspond to voltages across the impedances Zl and Z2. The impedances present a voltage divider between nodes 138, 139 and 140.

In example implementations, the transducer 102 is realised using a ogowski coil 103 having two outputs. The circuit 106, in a preferred example implementation, comprises a voltage divider realised using first and second resistors 130 and 132. A first output 134 of the Rogowski coil is coupled to a central node 136 of the voltage divider. The other end of the first resistor 130 is connected to a respective upper node 138 whereas the other end of the second resistor 132 is connected to a respective lower node 140. A surge protection arrangement limits the voltage swings between the upper 138 node and the lower 140 node. Example implementations can realise the surge protection arrangement using a pair of back-to-back Zener diodes 142 and 144. The surge protection arranged is an example implementation of a voltage clamp or clipper. A rectifier, which can be formed using a pair of back- to-back Schottky diodes 146 and 148, is arranged to produce the half-wave rectified waveforms 108 and 110. The other output 150 from the Rogowski coil 103 is coupled to a central node 152 between the back-to-back Schottky diodes 146 and 148. The other ends of the Schottky diodes 146 and 148 are coupled to the upper 138 and lower 140 nodes respectively.

A reference or neutral voltage V_{C0ll n} is derived directly from a virtal earth connection via a suitable coupling 154. The reference voltage is coupled to the central node 152 to provide a biasing, that is, to provide a voltage about which the output from the Rogowski coil 103 can swing. Example implementations provide such a reference or neutral voltage that is one of a stable reference, or a virtual earth, about which the output of the transducer 102 can vary. It can therefore be appreciated that a biasing circuit having a specific biasing voltage is not needed.

The central node 152 and the upper node 138 form a first pair 109 of outputs from the passive circuit 106. The first pair of outputs is used as inputs to the first voltage optical modulator 116. The central node 152 and the lower node 140 form a second pair 109' of outputs from the passive circuit 106. The second pair of outputs is used as inputs to the second voltage optical modulator 114. As indicated above, the central node 152 presents an earth, actual or virtual, to the transducer about which the output of the transducer 102 can vary both positively and negatively relative to the potential of the central node 152. Example implementations of the voltage optical modulators 114 and 116 can be realised using MEMS mirrors having deflections associated with input voltages appearing across their inputs. Referring to figure 2, it can be appreciated that the voltage optical modulators 114 and 116 receive light, output by a light source 118, via respective fibre optics 156 and 158, a light splitter 120 and a respective feed fibre 157. Light is reflected by the MEMS mirrors back along the fibre optics 156 and 158 where it is detected by the photodetectors or photo diodes 120 and 124.

The conditioning circuit 126 is arranged to combine the two waveforms 108 and 110 into a single waveform. The single waveform can be a sinusoidal waveform.

In installing a current transformer according to an example implementation, an engineer can derive the reference or biasing voltage from a suitably stable reference node, which can be derived from stable input impedances Zl and Z2 of the optical modulators 114 and 116. Therefore, a couple 154 can be used to form a direct electrical connection between an earth potential or earthed connection and the circuit 106. Example implementations can realise the foregoing by providing a direct electrical connection to an output of the current transducer, such as, for example, the ogowski coil. Suitably, example implementations provide a current transducer, such as, for example, a Rogowski coil, comprising a pair of outputs for providing a voltage to the circuit, and a means of electrically coupling the current transducer, such as, for example, the Rogowski coil, to a stable reference such as, for example, a virtual earth or other reference, about which the output of the Rogowski coil can vary. Such as stable reference can be provided by the common node between the impedances Zl and Z2. Example implementations provide a method of installing a current transformer comprising the step of installing a current transducer about a conductor; and coupling an output of the current transducer to a stable node associated with a pair of impedances having a common node about which the output of the current transducer can vary to provide a voltage to a circuit for driving at least one optical modulator or coupling an input of a circuit, for driving at least one optical modulator, to the conductor to derive a biasing voltage therefrom. It will be appreciated that the biasing voltage will be at a stable potential such as, for example, a virtual earth, or a floating potential.

Referring to figure 3, there is shown a Rogowski coil 103 according to an example implementation comprising windings 302 having a first output 134 and a second output 150 for providing a voltage V_{coll assoc}j_ate yyjj-h _th_e f|_{ux 0}f _a conductor (not shown). The Rogowski coil 103 also comprises an input 204 having the couple 154 at a free end thereof. Although the example implementation of the Rogowski coil 103 is shown as having a separate electrical connector for coupling to the conductor, alternative example implementations can be realised. For example, a conductive cap at one end of the coil, or preferably conductive caps at both ends of the coil, can be integrally formed with the coil to provide a unitary structure for deriving the biasing voltage from an earth. It will be appreciated that the caps will be electrically connected to the circuit or, preferably, to an output of the current transducer 102 intended for coupling to the passive circuit to provide the biasing voltage.

Referring to figure 4 there is shown an example implementation of a current transformer assembly 400 according to an example implementation. The assembly comprises a Rogowski coil 102 coupled to the circuit 106 and optical modulator 112, which are supported by an insulating tower 402. The insulating tower 402 is mounted on a mount 404. The mount 404 houses electro-optical equipment for communicating with a monitoring station (not shown). The electro-optical equipment comprises the optical transceiver 119. The electro-optical equipment also comprises communication electronics compliant with, for example, IEC61850-9-2 to support digital communications between substations and monitoring equipment. The insulating tower 402 can be hollow and houses the fibre optic cables referred to above, but referenced collectively as 406 in figure 4. Example implementations, in the form of such an assembly 400, can be pre-fabricated at a site that is remote from a substation or the like where the assembly will be installed, which facilitates ease of installation at the substation or the like. The current transducer 102 can be coupled to a busbar at the substation or the like. Although example implementations have been described with reference to the output of the current transformer assembly being digital, example implementations are not limited to such an arrangement. Example implementations can be realised in which the output is an analogue signal. The analogue signal can be applied to a relay that actuates a circuit breaker that is in-line at least electrically with the conductor such that opening the circuit breaker prevents current flow within the conductor. Similarly, example implementations using the digital communications described above can forward data relating to the current in the conductor to a merging unit. The merging unit can then take appropriate action such as, for example, actuating a circuit breaker to prevent current flow within the conductor.

Example implementations provide a method of installing a current transformer 100 or assembly 400 having a current transducer 102 as described herein. The method comprises electrically coupling the current transducer 102 to an earth potential. One skilled in the art will appreciate that an example implementation can be or is provided per phase to be monitored.

Referring to figure 5, there is shown a view 500 of an example implementation of a conditioning circuit 126. The conditioning circuit 126 comprises an op-amp 126-1 configured as an adder circuit to add together the two waveforms 108 and 110 to produce a combined waveform 502. Example implementations can be realised in which a characteristic of the combined waveform 502 is associated with a characteristic of the current 104. Preferably, the combined waveform has an amplitude that is proportional to the current 104. However, other characteristics could equally well be used as alternatives to amplitude. The conditioning circuit uses a capacitor 126-2 in parallel with a resistance. The resistance is used to scale the two waveforms 108 and 110 relative to one another to achieve a desire proportion between the two waveforms in the combined waveform 502. Preferred example implementations achieve balance, that is, a 1:1 proportion. In the example implementation shown, the resistance is realised using a series arrangement of two resistors 126-3 and 126-4. Preferably, one of the resistors, such as, for example, 126-3 is a variable resistor, which facilitates achieving a desired proportion. It can be seen that the positive input of the op-amp 126-1 is earthed via terminal 126-5.

Although example implementations have been described with reference to the conductor 104 being a busbar, they are not limited to such an arrangement. Example implementations can be realised in which the conductor is a conductive entity other than a busbar.

Example implementations can be realised in which the Rogowski coil are rated such that the normal current through the coil gives respective voltage variations as follows: 100A/+1V, 500A/+1V, 1000A/+1V, 2000A/+1V, 3000A/+1V or 5000A/+1V, the resistors can have values of 100 to 200 ohms; the Zener diodes can have low reverse voltages such as, for example, 0.3V or less; the diodes SD1 and SD2 can have a reverse breakdown voltage of not less than 30V, the impedances can present 10k-20k ohms and the optical modulators can be any MEMS based Fast Voltage Optical Attenuators (FVOA) such as, for example, FVOA2000 or FVOA5000 available from, for example, www.lightconnect.com or www.neophotonics.com respectively; all taken jointly and severally in any and all permutations.

Furthermore, although example implementations have been described within the context of monitoring current within a power distribution system, example implementations are not limited thereto. Example implementations can be realised for monitoring currents in other conductors such as those supplying heavy motors or furnaces.

Claims

1. A current transformer comprising a transducer responsive to a current carried, at a respective voltage, by a conductor; the current transformer comprising a circuit, responsive to the transducer, for providing a modulated optical output that varies with the current of the conductor via at least one optical modulator; the circuit comprising a reference node operable as a reference about which the output of the transducer can vary; the reference node being formed at a common node of a pair of impedances.

2. The current transformer of claim 1, in which the circuit comprises a rectifying arrangement adapted to produce a pair of rectified waveforms from a voltage variation derived from any such current variation.

3. The current transformer of claim 2, wherein the rectifying arrangement comprises a pair of resistors in parallel with at least a pair of diodes arranged, in use, to be operationally biased oppositely to produce two driving waveforms for driving respective optical modulators adapted to have an output that varies with the driving voltages.

4. The current transformer of any preceding claim, wherein the respective optical modulators comprise MEMS mirrors adapted to deflect received light to produce the output that varies with the driving voltages.

5. The current transformer of any preceding claim, wherein the circuit comprises upper and lower nodes having a voltage divider, a voltage clamp or clipper and a rectifying arrangement spanning the upper and lower nodes.

6. A circuit for a current transformer of any preceding claim, further comprising a conditioning circuit for producing an output waveform associated with the current.

7. A method of installing a current transformer; the method comprising installing a current transducer about a conductor; and coupling an output of the current transducer to a reference node associated with a pair of impedances to provide a voltage about which the output of the transducer can vary for driving an optical modulator.

8. A current transformer assembly; the assembly comprising a current transformer of any of claims 1 to 5, mounted on an insulating tower for separating the current transformer from a mount for housing.

9. A current transformer substantially as described herein with reference to and/or as illustrated in the accompanying drawings.

10. A current transformer assembly substantially as described herein with reference to and/or as illustrated in the accompanying drawings.

11. A method substantially as described herein with reference to and/or as illustrated in the accompanying drawings.