Abstract:
Devices and methods for measuring an electrical characteristic, in particular, for measuring current are provided. The devices can use a pair of MEMS optical modulators as opposed to the more conventional coil and associated oil insulation arrangement.

Description:
FIELD OF THE INVENTION 
       [0001]    Embodiments of the present invention relate to devices and methods; and, more particularly, to devices and methods for measuring current. 
       BACKGROUND TO THE INVENTION 
       [0002]    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 33 kV 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/small gas bubble accumulations in the current transformer can lead to a catastrophic failure such as, for example, an explosion. 
         [0003]    It is well-known to use optical current sensors or optical transformers within such high-voltage environments. These 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 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 weather, also adversely affect the performance of such optical current sensors. Still further, to compensate for instability, relatively large optical current transformers are required. 
         [0004]    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 the optical receiver. In preferred embodiments, the electro-optic amplitude modulator is insensitive to polarisation variations due to using a diffractive MEMS based variable optical attenuator. Most types of 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. One skilled in the art will appreciate that such an active arrangement consumes power by requiring a separate circuit to provide a biasing voltage. 
         [0005]    It is an object of embodiments of the present invention to at least mitigate one or more problems of the prior art. 
       SUMMARY OF INVENTION  
       [0006]    Accordingly, embodiments of the present invention provide a current transformer responsive to a current carried, at a respective voltage, by a conductor; the current transformer comprising a circuit having at least one optical modulator for providing a modulated optical output that varies with current variation of the current; the circuit comprising a voltage reference line for coupling to the conductor to bias the circuit using the respective voltage. 
         [0007]    Advantageously, devices according to embodiments of the present invention provide a passive solution that is stable, extremely reliable and that does not require a separate biasing circuit to derive a respective biasing voltage. 
         [0008]    Embodiments also provide a method of installing a current transformer or assembly having a current transducer, the method comprising electrically coupling the current transducer to a respective conductor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
           [0010]      FIG. 1  shows an embodiment of a current transformer; 
           [0011]      FIG. 2  shows an embodiment of optical modulators; 
           [0012]      FIG. 3  depicts a first embodiment of a Rogowski coil; 
           [0013]      FIG. 4  shows a current transformer assembly according to an embodiment; 
           [0014]      FIG. 5  illustrates a second embodiment of a Rogowski coil; 
           [0015]      FIG. 6  shows an embodiment of a conditioning circuit. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0016]    Referring to  FIG. 1 , there is shown a current transformer  100  according to an embodiment. The current transformer  100  comprises transducer  102  for outputting a voltage, ΔV coil , associated with the current flowing in a conductor  104 . The conductor  104  may be a busbar. The current transformer  100  also comprises a passive circuit  106  for receiving the output voltage ΔV coil  and a reference or biasing voltage V coil     —     n  and for producing half-wave rectified waveforms  108  and  110  therefrom at two pairs of output terminals  109  and  109 ′. In the illustrated embodiment, it can be appreciated that ΔV 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 . Preferred embodiments of the present invention use MEMS optical modulators such as the pair  114  and  116  of optical modulators illustrated in  FIG. 1 . 
         [0017]    Referring to  FIG. 2 , light for the optical modulators is provided by a light source  118  of an optical transceiver  119 . Light from the light source  118  is 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, are realised using a pair  122  and  124  of photodetectors such as, for example, a pair of photodiodes. Preferably, a conditioning circuit  126  is provided to process the outputs of the photodetectors to produce a signal indicative of the current carried by the conductor  104 . Preferably an amplifier  128  is used to amplify the indicative signal. Embodiments 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 . 
         [0018]    Preferably, the transducer  102  is realised using a Rogowski coil  103  having two outputs. 
         [0019]    The passive circuit  106 , in a preferred embodiment, 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  and lower  140  nodes. Preferred embodiments realise the surge protection arrangement using a pair of back-to-back Zener diodes  142  and  144 . A rectifier, preferably formed using a pair of back-to-back 
         [0020]    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  notes respectively. 
         [0021]    A reference or neutral voltage V coil     —     n  is derived directly from the conductor  104  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. 
         [0022]    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 . 
         [0023]    Preferred embodiments of the voltage optical modulators  114  and  116  are realised using MEMS mirrors having deflections associated with input voltages appearing across their inputs. Referring to  FIG. 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 photodetector or photo diodes  122  and  124 . 
         [0024]    A conditioning circuit  126  is arranged to combine the two waveforms  108  and  110  into a single waveform. In a preferred embodiment, the single waveform is a sinusoidal waveform. 
         [0025]    It will be appreciated that installing a current transformer according to an embodiment of the invention will require an engineer to derive the reference or biasing voltage directly from transmission line. Therefore, a couple  154  is used to form a direct electrical connection between the conductor  104  and the passive circuit  106 . Embodiments realise the foregoing by providing a direct electrical connection to an output of the current transducer, such as, for example, the Rogowski coil. Suitably, embodiments of the present invention provide current transducer, such as, for example, a Rogowski coil, comprising a pair of outputs for providing a voltage to the passive circuit, and a means of electrically coupling the current transducer, such as, for example, the Rogowski coil, to the transmission line to derive a reference voltage. 
         [0026]    Embodiments 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 the conductor to provide a biasing 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 the same as the voltage of the line or conductor  104 . 
         [0027]    Referring to  FIG. 3 , there is shown a view  300  of a Rogowski coil  103  according to an embodiment comprising windings  302  having a first output  134  and a second output  150  for providing a voltage ΔV coil  associate with the flux of a conductor (not shown). The Rogowski coil  103  also comprises an input  204  having the couple  154  at a free end thereof. Although the embodiment of the Rogowski coil  103  is shown as having a separate electrical connector for coupling to the conductor, alternative embodiments can be realised. For example, a conductive cap at one end of the coil, or conductive caps at both ends of the coil, can be integrally formed with the coil to provide a unitary structure for driving the biasing voltage from the conductor. It will be appreciated that the caps will be electrically connected to the passive circuit or to an output of the current transducer  102  intended for coupling to the passive circuit to provide the biasing voltage. 
         [0028]    Referring to  FIG. 4  there is shown an embodiment of a current transformer assembly  400  according to an embodiment. The assembly comprises a Rogowski coil  103  coupled to the passive 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  can house electro-optical equipment for communicating with a monitoring station (not shown). The electro-optical equipment can comprise the optical transceiver  119 . The electro-optical equipment can also comprise communication electronics compliant with, for example, IEC61850-9-2 to support digital communications between substations and monitoring equipment. It can be appreciated that the insulating tower  402  is hollow and houses the fibre optic cables referred to above, but referenced collectively as  406  in  FIG. 4 . Embodiments of the present invention, 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  is coupled to a busbar at the substation or the like. 
         [0029]    Although embodiments of the invention have been described with reference to the output of the current transformer assembly being digital, embodiments are not limited to such an arrangement. Embodiments 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, embodiments 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. 
         [0030]    Embodiments of the invention comprise 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 the conductor  104 . One skilled in the art will appreciate that an embodiment of the present invention is provided per phase to be monitored. 
         [0031]    Referring to  FIG. 5 , there is shown a current transducer  500  according to an embodiment. The current transducer comprises a Rogowski coil  502  having a pair  504  and  506  of conductive end caps arranged, in used, to be in electrical connection with a conductor  104 . The conductive caps  504  and  506  are both coupled to an output line of the coil, such as, for example, line  150 . 
         [0032]    Referring to  FIG. 6 , there is shown a view  600  of an embodiment of a conditioning circuit  126 . The conditioning circuit  126  comprises an op-amp  126 - 1  configured as an adder to add together the two waveforms  108  and  110  to produce a combined waveform  602 . Embodiments can be realised in which a characteristic of the combined waveform  602  is associated with a characteristic of the current  104 . The combined waveform can have an amplitude that is proportional to the current  104 . However, other characteristics could equally well be used as alternatives to amplitude. The conditioning circuit can use a capacity or  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  602 . Preferred embodiments achieve balance, that is, a 1:1 proportion. In the embodiment shown, the resistance is realised using a series arrangement of two resistors  126 - 3  and  126 - 4 . One of the resistors, such as, for example,  126 - 3  can be 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 . 
         [0033]    Although embodiments of the invention have been described with reference to the conductor  104  being a busbar, they are not limited thereto. Embodiments can be realised in which the conductor is a conductive entity other than a busbar. 
         [0034]    Furthermore, although embodiments of the present invention have been described within the context of monitoring current within a power distribution system, embodiments are not limited thereto. Embodiments can be realised for monitoring currents in other conductors such as those supplying heavy motors or furnaces.