Abstract:
A system monitors alternating current and includes a magneto-optical current transducer (MOCT) adapted to modulate an optical signal corresponding to magnitude of the alternating current. Beam splitters are in communication with the MOCT which are in turn connected to respective channels. Each channel includes an LED that is powered by a constant current source.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority from U.S. Provisional Application No. 60/874,353 filed on Dec. 12, 2006, entitled “Time Division Multiplexed Detector For A Magneto-Optical Current Transducer (MOCT)” the contents of which are relied upon and incorporated herein by reference in their entirety, and benefit of priority under 35 U.S.C. 119e is hereby claimed. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates to the sensing of high voltage currents and more particularly to the sensing of such current using a MOCT. 
       DESCRIPTION OF THE PRIOR ART 
       [0003]    Magneto-Optical Current Transducers (hereinafter “MOCT”) are used at electricity transmission substations to sense current in high voltage applications. One example of the use of a MOCT to sense such current is described in U.S. Pat. No. 4,916,387 (“the &#39;387 patent”). The &#39;387 patent describes the use of a time division multiplexed system (“TDM System”) using a MOCT for sensing the high voltage current. 
         [0004]    In the system of the &#39;387 patent, each of the PIN diodes  28  and  30  have a fixed bias current and thus the light emitted by the associated LED  24  and  26 , respectively, must be varied so that the current representative of the light detected at the associated PIN diode offsets the bias current. Thus each of the channels in the system of the &#39;387 patent do not operate at the maximum signal to noise ratio at all times regardless of the attenuation in the fiber optic loop. 
         [0005]    Thus it is desirable to have a TDM System that uses a MOCT for sensing high voltage current that substantially reduces the disturbances in the output waveform of the processing board caused by the vibration of the fiber optic cables in the system whose channels at all times operate at the maximum signal to noise ratio and whose accuracy is not affected by the light that has not passed through the MOCT, which light for ease of description will be referred to hereinafter as back-reflected light. The system of the present invention accomplishes that. 
       SUMMARY OF THE INVENTION 
       [0006]    According to one embodiment of the present invention a system is provided that monitors an alternating current. The system includes a magneto-optical current transducer adapted to modulate an optical signal corresponding to the magnitude of the alternating current. The transducer includes a first output and a second output and a first input and a second input. A first and a second beam splitter are in communication with the first and the second output respectively. A first and a second channel are in communication with the first and the second beam splitter respectively. The first channel includes a first LED and the second channel includes a second LED. The first LED is in communication with the first beam splitter and the second LED is in communication with the second beam splitter. The first channel includes a first output and the second channel includes a second output. A difference amplifier is connected to the first and the second channel outputs to subtract the first and the second channel outputs from one another to eliminate vibration induced disturbance. The first and said second LEDs are powered by a constant current source. 
     
    
     
       DESCRIPTION OF THE DRAWING 
         [0007]      FIG. 1  shows a block diagram of the system of the present invention. 
           [0008]      FIG. 2  shows a block diagram for one of the two identical channels in the system shown in  FIG. 1 . 
           [0009]      FIG. 3  shows the waveforms associated with each of the two identical channels in the system of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    Referring now to  FIG. 1 , there is shown in block diagram form the system  10  of the present invention. System  10  includes MOCT  12  which provides at outputs  12   b  and  12   c  an optical signal representative of the sensed high voltage current to an associated one of beam splitters  14   a  and  14   b . Each beam splitter  14   a  and  14   b  is connected to an associated one of opposing channels  16   a  and  16   b  of detector  16 . Each of the opposing channels  16   a  and  16   b  are identical and one example of an embodiment in accordance with the present invention of the channels  16   a  and  16   b  is shown in  FIG. 2  which is described below. For ease of illustration the back reflected light is not shown in  FIG. 1 . 
         [0011]    Each of the channels  16   a  and  16   b  pass light through the optical sensor (MOCT)  12  for a short sample period alternately in opposing directions. The physical parameter to be measured, the current through the MOCT window  12   a , modulates the intensity of the light. Since the sample period in both of the opposing channels  16   a  and  16   b  are the same, those channels have equal and opposite modulation for a given current. Vibration induced modulation appears as equal modulation on both of the opposing channels  16   a  and  16   b . Within the signal processing electronics, the output of the two opposing channels  16   a  and  16   b  are an input  18   a  and  18   b  to difference amplifier  18 . Those inputs are subtracted from one another at difference amplifier  18  to thereby eliminate the vibration induced disturbance and the original signal recovered at the difference amplifier output  18   c.    
         [0012]    Referring now to  FIG. 2 , there is shown a block diagram for one of the identical channels  16   a  and  16   b  which for ease of description hereinafter will be identified as channel  16 . 
         [0013]    Channel  16  includes a switched integrator  20  which comprises the components inside of the box shown in  FIG. 2 . More particularly the integrator  20  has an integrating amplifier  22  one of whose inputs  22   a  is connected by a switch S 1  to the junction of a PIN diode D 1  and a resistor R 1 . Input  22   a  of amplifier  22  is connected by the parallel combination of a capacitor C 1  and a switch S 2  to the amplifier output  22   b  which is the output of integrator  20 . The value of the capacitance of capacitor C 1  determines the gain of the integrating amplifier  22 . 
         [0014]    The opening and closing of switches S 1  and S 2  is controlled by timing  24 . The timing  24  also controls the driver  26  for light emitting diode (LED) D 2  which acts as a constant current source of light. This constant current source of light allows each of channels  16   a  and  16   b  to operate at a maximum signal to noise ratio at all times regardless of the attenuation in the fiber optic loop. 
         [0015]    The output  22   b  of integrator  20  is connected to a sample and hold circuit  28  the output of which is connected to the resistor R 1  by an amplifier  30  which is used to remove any DC offset in the output of circuit  28 . The output of circuit  28  is also connected to an analog divider  32  which also has an input from the output of amplifier  30 . The analog divider  32  divides the output of circuit  28  by the output of amplifier  30 . The signal present on the output of circuit  28  is the ‘AC’ signal that is being sensed. The output of amplifier  30  is the ‘DC’ signal. As the attenuation in the fiber loop is varied, both the ‘AC’ and ‘DC’ signals will vary proportionally. The function of the divider  32  is to divide the ‘AC’ signal by the ‘DC’ signal such that the output of the divider  32  is the same regardless of the fiber loop attenuation. The output of divider  32  is connected to one input of the two inputs  18   a ,  18   b  to difference amplifier  18  of  FIG. 1 . 
         [0016]    The operation of channel  16  is as follows. When the channel  16  is sampling, the LED D 2  is turned on for the channel&#39;s predetermined sample period. The light emitted from the LED diode is passed through the MOCT  12  in a direction that is associated with the particular channel  16   a  or  16   b . As described above for  FIG. 1 , the light associated with channel  16   a  and the light associated with channel  16   b  are passed through the MOCT  12  in opposing directions. The current passing through MOCT window  12   a  modulates the light. 
         [0017]    During the sample period, that is, when LED D 2  is turned on, the modulated light is received by PIN diode D 1  and is converted by the diode into an electrical signal. Also during the sample period, the timing  24  closes switch S 1  and opens switch S 2  to thereby allow a charge representative of the amplitude of the light received by diode D 1  to accumulate across C 1 . 
         [0018]    Upon the expiration of the sample period for channel  16  the switch S 1  is opened and that causes the integrator  20  to hold its output at the voltage level determined by the charge accumulated in capacitor C 1 . The LED D 2  is switched off and the voltage at the output of integrator  20  is transferred to the sample and hold circuit  28 . At this point in time, both switches S 1  and S 2  are closed and the process described above can be repeated for the opposite channel. Therefore if the description given above is for channel  16   a  then once both switches S 1  and S 2  are closed, the LED D 2  of channel  16   b  can be now be turned on and switch S 1  of that channel is closed and switch S 2  of that channel remains open to thereby allow a charge to accumulate on capacitor C 1  in that channel. 
         [0019]    In one embodiment of the present invention, switch integrator  20  was a Texas Instruments IVC102 chip and sample and hold circuit  28  and divider  32  were Analog Devices AD585 and AD734 chips. 
         [0020]    Referring now to  FIG. 3 , there are shown the waveforms associated with the outputs of channels  16   a  and  16   b  and the switches S 1  and S 2  and the sample and hold circuit  28  in each of the channels. As can be seen in  FIG. 3 , at time T 1  which is the beginning of the sample period for channel  16   a , switch  51  in that channel is in a closed position and switch S 2  is opened. During the sample period of channel  16   a , that is from time T 1  to time T 2  the switches S 1  and S 2  of channel  16   b  are closed. 
         [0021]    At time T 2 , which is the end of the sampling period for channel  16   a , switch S 1  is opened and is held open until time T 4  at which time it is closed. Sample and hold circuit  26  of channel  16   a  obtains the sample from integrator  20  of channel  16   a  during the time period T 3  to T 4 . At time T 4  which is the beginning of the sample period for channel  16   b , switch S 2  of channel  16   b  is opened and switch S 1  of that channel which was previously closed remains closed. At time T 5 , the sample period of channel  16   b  ends and switch S 1  of that channel is opened. Sample and hold circuit  26  of channel  16   b  obtains the sample from integrator  20  of channel  16   b  during the time period T 6  to T 7 . It should be noted that switches S 1  and S 2  of channel  16   a  remain closed during the time period from T 4  to T 7  and at time T 7  switch S 2  of channel  16   a  closed so that channel  16   a  can start its next sample period. 
         [0022]    It should be appreciated that a channel  16  embodied in accordance with the present invention substantially eliminates back-reflections of light by holding switches S 1  and S 2  of the channel  16   a  or  16   b  that has completed its sample period closed during the sample period of the other channel  16   b  or  16   a  whose light is passing through the MOCT  12  in a direction opposite to the direction of light passage through the MOCT  12  for the channel that has just completed its sample period. By holding the switches S 1  and S 2  closed for the channel that has completed its sample period no charge can accumulate on that channel&#39;s capacitor C 1 . As described above, the capacitance of capacitor C 1  determines the gain of the integrating amplifier  20  and holding the switches S 1  and S 2  closed substantially reduces that gain and thus substantially eliminates the effect of back-reflections of light in that channel. 
         [0023]    It should also be appreciated that the sample and hold circuit  28  isolates the channel  16   a  or  16   b  that has just completed its sample period from the back-reflected light and ensures that the sample from that channel remains valid during the duration of the opposing channel&#39;s  16   b  or  16   a  sample period. 
         [0024]    It should further be appreciated that since the integrating amplifier  20  in each channel  16   a  and  16   b  samples the light from the MOCT  12  for the entire duration of the sample period the integrating amplifier  22  serves as a low pass filter that is part of the detector. It should also be further appreciated that because of the integrating amplifier  22  the signal transitions in the detector are relatively slow and this reduces the switching transients in the system. 
         [0025]    It is to be understood that the description of the foregoing exemplary embodiment(s) is (are) intended to be only illustrative, rather than exhaustive, of the present invention. Those of ordinary skill will be able to make certain additions, deletions, and/or modifications to the embodiment(s) of the disclosed subject matter without departing from the spirit of the invention or its scope, as defined by the appended claims.