Abstract:
There is provided a method for measuring a parameter of a power frequency current being carried by a power line. The method that includes (a) transducing a power frequency current flowing through a power line, into a power frequency voltage, via an inductive coupler that couples a communications signal from the power line, (b) separating the power frequency voltage from the communications signal, and (c) determining a value of a parameter of the power frequency current from the power frequency voltage. There is also provided a system and an apparatus for measuring the parameter.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The invention generally relates to power line communications (PLC), and more particularly, to exploiting a power line inductive coupler for measuring power frequency current on a power line, and transmitting the measurement over a PLC network of which the same inductive coupler is a component. 
         [0003]    2. Description of the Related Art 
         [0004]    In a power line communication system, power frequency is typically in a range of 50-60 Hertz (Hz) and a data communications signal frequency is greater than about 1 MHz, and typically in a range of 1 MHz-50 MHz. A data coupler for power line communications couples the data communications signal between a power line and a communication device such as a modem. 
         [0005]    An example of such a data coupler is an inductive coupler that includes a core, and a winding wound around a portion of the magnetic core. The core is fabricated of a magnetic material and includes an aperture. The inductive coupler operates as a transformer, and is situated on the power line such that the power line is routed through the aperture and serves as a primary winding of the transformer, and the winding of the inductive coupler serves as a secondary winding of the transformer. The data communications signal is coupled between the power line and the secondary winding via the core. The secondary winding is coupled, in turn, to the communication device. 
         [0006]    One technique for measuring power frequency current in the power line is to employ a current transformer coupled to the power line, where the current transformer has a secondary short-circuited through an ammeter or other current sensing device. Alternatively, in a case of an open-circuited secondary, a primary current induces a secondary voltage proportional to a primary current. 
       SUMMARY OF THE INVENTION 
       [0007]    There is provided a method for measuring a parameter of a power frequency current being carried by a power line. The method that includes (a) transducing a power frequency current flowing through a power line, into a power frequency voltage, via an inductive coupler that couples a communications signal from the power line, (b) separating the power frequency voltage from the communications signal, and (c) determining a value of a parameter of the power frequency current from the power frequency voltage. There is also provided a system and an apparatus for measuring the parameter. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1A  is an illustration of a system configured for measuring current flowing through a power line. 
           [0009]      FIG. 1B  is a schematic of the system of  FIG. 1A . 
           [0010]      FIG. 1C  is a block diagram of the system of  FIG. 1A , and provides additional detail of the operation of a communications node. 
           [0011]      FIG. 2  is a block diagram of a system that employs separate cables for PLC signals and current sense voltage signals. 
           [0012]      FIG. 3  is a block diagram of a system that senses a current in a first power line, and also sense a current in a second power line. 
           [0013]      FIG. 4  is a block diagram of another system configured for measuring current flowing through a power line. 
           [0014]      FIG. 5  is a schematic of a portion of a communications node, and shows an exemplary implementation of a high pass filter and a low pass filter. 
           [0015]      FIG. 6  is a schematic of a portion of the system of  FIG. 4 , and shows an exemplary implementation of a bypass module. 
           [0016]      FIG. 7  is a block diagram of a system for measuring phase of a current of a medium voltage power line, referenced to a phase of a power voltage. 
           [0017]      FIG. 8  is a block diagram of a portion of a power distribution network configured for measuring power parameters at various locations within the power distribution network. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0018]    Incidental to the high frequency operation of an inductive coupler, its secondary circuit output includes high frequency PLC signals and a power frequency current sense voltage, which may be separated and separately processed. 
         [0019]      FIG. 1A  is an illustration, and  FIG. 1B  is a schematic, of a system  100  configured for measuring current flowing through a power line  103 . System  100  includes an inductive coupler, i.e., coupler  105  and a communications node  112 . 
         [0020]    Coupler  105  includes a magnetic core, i.e., a core  106 , having an aperture  108  therethrough. Coupler  105  operates as a current transformer, and is situated on power line  103  such that power line  103  is routed through aperture  108  and serves as a primary winding, represented schematically in  FIG. 1B  as a primary winding  102 , for coupler  105 . Coupler  105  also includes a secondary winding  107 . Secondary winding  107  is coupled to communications node  112  via a pair of wires  110   a  and  110   b , which are collectively designated as secondary winding pair  110 . 
         [0021]    Power line  103  carries (a) a power frequency current, e.g., a current of 200 amperes at a voltage of 13 kV and a frequency of 50-60 Hz, and (b) a PLC signal, also referred to herein as a data signal, e.g., a 10 volt peak-to-peak signal having a frequency in a range of 1 MHz to 50 MHz. Coupler  105  inductively couples signals between power line  103  and secondary winding pair  110  via core  106 . More particularly, coupler  105  couples the PLC signals bi-directionally between power line  103  and secondary winding pair  110 , and transduces the power frequency current signal from power line  103  into a power frequency voltage across secondary winding pair  110 . 
         [0022]    Power line  103  carries a power frequency current, i.e, I 101 , having a frequency f. Primary winding  102  has an inductance Lp, and a reactance Xp, where 
         [0000]      Xp=2πf Lp. 
         [0023]    The reactance of power line  103  passing through core  106  is less than ten milliohms at power frequency. Lp is a low value because of multiple issues relating to operation at MHz frequencies. These include the relative permeability of high frequency magnetic cores being typically in the range of 100 to 1000, and the single turn primary represented by power line  103  passing only once through the aperture  108 . For example, for f=60 Hz and Lp=3 μH, 
         [0000]      Xp=2πf Lp=1.13 milliohms. 
         [0024]    I 101  flows through primary winding  102 , and according to Ohm&#39;s Law, a magnitude of a primary voltage drop Vp across primary winding  102  is given by 
         [0000]        Vp =Xp I   101 . 
       A secondary voltage, i.e., a power frequency voltage, is induced across secondary winding  107 . The secondary voltage, also termed current sense voltage Vs, is proportional to primary voltage drop Vp, and given by 
       [0025]      Vs=k Vp, 
         [0000]    where k is a coupling coefficient for coupler  105 . Measuring Vs allows calculation of 
         [0000]        I   101   =Vp/Xp=Vs /( k Xp ). 
       Defining transadmittance as 
       [0026]        Y= 1/( k Xp ), 
       I 101  may be calculated as 
       [0027]      I 101 =Y Vs. 
         [0028]    A core magnetics&#39; B-H curve starts as a straight line, from zero current up to some value, e.g. 200 amps, and then its slope starts to decrease, as it enters a region of increasing saturation. “Low current” refers to any current below that “knee.” For low currents, Y is a constant. Core  106  begins to saturate above some value of I 101 , and causes Y to decrease as I 101  increases. This dependence of Y on I 101 , may be measured, and compensated for in the calculation 
         [0000]      I 101 =Y Vs. 
         [0029]    To illustrate the magnitudes involved, consider a power frequency f=60 Hz, and coupler  105  as having a primary inductance Lp=1 μH. Assume k=0.9. 
       Then: 
       [0030]      Xp=2πf Lp 
         [0000]        Xp =(2π)(60)(1 μH) 
         [0000]      Xp=377 micro-ohms 
         [0000]      and 
         [0000]        Y= 1/( k Xp ) 
         [0000]      Y=1/(0.9(377 micro-ohms)) 
         [0000]      Y=2950 mhos. 
         [0000]    For I 101 =200 amp power line current: 
         [0000]      Vs=k Xp I 101    
         [0000]      Vs=0.9*377 micro-ohms*200 amps 
         [0000]      Vs=68 mV. 
         [0031]    For the present example, current measuring equipment would receive this 68 mV current sense voltage, multiply the current sense voltage by transadmittance Y=2950 mhos, and calculate a power line current of I 101 =200 amps. Coupler  105 , by way of its transadmittance, transduces the power line current into a current sense voltage. 
         [0032]    Communications node  112  includes modules that sense the current sense voltage Vs. Communications node  112  also includes modules that calculate I 101 , from the current sense voltage Vs, however as an alternative, communications node  112  may transmit data representing the current sense voltage Vs to another piece of equipment that will calculate I 101 . 
         [0033]      FIG. 1C  is a block diagram of system  100 . Communications node  112  includes a filter module  115 , a modem  130 , a data processor  135 , an analog processor  140 , an r.m.s.-to-dc converter  145 , and an analog-to-digital converter (A/D)  150 . 
         [0034]    Filter module  115  includes a high pass filter  116  and a low pass filter  117 . High pass filter  116  passes PLC signals and blocks power frequency voltages. Low pass filter  117  passes power frequency voltages and blocks PLC signals. Low pass filter  117  outputs a filtered current sense voltage. The filtered current sense voltage is essentially the same as the current sense voltage Vs. 
         [0035]    Modem  130  is coupled to filter module  115 , and more particularly to high pass filter  116 . Modem  130  is also coupled to data processor  135 . Modem  130  conducts bi-directional communication of PLC signals with each of filter module  115  and data processor  135 . 
         [0036]    Analog processor  140  receives the filtered current sense voltage from low pass filter  117 . Analog processor  140  preferably includes a transformer and/or amplifier (not shown), and scales the filtered current sense voltage. Analog processor  140  outputs a scaled current sense voltage. 
         [0037]    r.m.s.-to-dc converter  145  receives the scaled current sense voltage from analog processor  140 . The current sense voltage Vs is an approximately sinusoidal voltage. For the measurement of current, the parameter of interest is a root mean square, or an r.m.s. value, of the current. r.m.s.-to-dc converter  145  converts the scaled current sense voltage to a dc representation of the current sense voltage. 
         [0038]    A/D  150  receives the dc representation of the current sense voltage from r.m.s.-to-dc converter  145 , and converts it to a digital output, i.e., current sense data. 
         [0039]    Data processor  135  receives the current sense data from A/D  150 , and calculates I 101 . Data processor  135  outputs data that represents a value for I 101 . 
         [0040]    Modem  130  receives the data from data processor  135 , modulates the data onto a PLC signal, and transmits the PLC signal via filter module  115 , coupler  105  and power line  103  to a remote monitoring site (not shown in  FIG. 1C ). Optionally, the current sense data and or the calculated value of I 101 , may be output to a data port (not shown) or visual display (not shown) on communications node  112 , to allow service personnel to monitor the line current on-site. 
         [0041]    In review, in system  100 , power line  103  carries a power frequency current. Coupler  105  is an inductive coupler that couples a communication signal from a power line  103 , and transduces the power frequency current into a power frequency voltage. In communications node  112 , filter module  115  separates the power frequency voltage from the communications signal, and processor  135  determines a value of a parameter of the power frequency current from the power frequency voltage. 
         [0042]      FIG. 2  is a block diagram of a system  200  that employs separate cables for PLC signals and current sense voltage signals. System  200  includes a low pass filter  205 , and a communications node  212 . Communications node  212  is similar to communications node  112 , but does not include filter module  115 . Secondary wire pair  110 , from coupler  105 , connects to modem  130  and low pass filter  205 . A cable  222  connects low pass filter  205  to analog processor  140 . 
         [0043]    Low pass filter  205  passes power frequency voltages and blocks PLC signals. Low pass filter  205  receives current sense voltage Vs via secondary pair  110 , and outputs a filtered current sense voltage to cable  222 . Thus, low pass filter  205  separates the current sense voltage Vs from the PLC signals. 
         [0044]    Analog processor  140  receives the filtered current sense voltage via cable  222 , and, as in communications node  112 , converts the filtered current sense voltage to a scaled current sense voltage. 
         [0045]    Modem  130 , data processor  135 , r.m.s.-to-dc converter  145 , and A/D  150  operate as in communications node  112 . 
         [0046]      FIG. 3  is a block diagram of a system  300  that senses I 101  in power line  103 , and also senses a current I 301  in a power line  303 . System  300  includes coupler  105  on power line  103 , and a coupler  305  on power line  303 . System  300  also includes a signal combiner  320 , a cable  330 , a secondary wire pair  311 , a low pass filter  306 , a cable  323 , and a communications node  312 . 
         [0047]    Coupler  105 , as in system  100 , (a) couples PLC signals between secondary winding pair  110  and power line  103 , and (b) couples a power signal from power line  103 , and induces a current sense voltage Vs, which is presented across secondary winding pair  110 . 
         [0048]    Coupler  305  couples PLC signals between secondary winding pair  311  and power line  303 . Additionally, coupler  305  couples a power signal from power line  303 , and induces a current sense voltage for power line  303  across a secondary winding (not shown) of coupler  305 . The current sense voltage for power line  303  is presented across secondary winding pair  311 . 
         [0049]    Combiner  320  couples PLC signals between coupler  105  and communications node  312 , and couples PLC signals between coupler  305  and communications node  312 . When coupling PLC signals from couplers  105  and  305  to communications node  312 , combiner  320  combines the PLC signals to yield a combined PLC signal, and outputs the combined PLC signal onto cable  330 . When coupling PLC signals from communications node  312  to couplers  105  and/or  305 , combiner  320  receives a combined PLC signal from communications node  312 , and routes the PLC signals to couplers  105  and  305 . The use of two couplers, i.e., couplers  105  and  305 , is suitable, for example (a) to provide differential coupling onto two phases of the same three phase feeder, so as to cancel electromagnetic emissions, or (b) to couple onto two feeders going off in two directions. 
         [0050]    Modem  130  is coupled to combiner  320 , via cable  330 , for bi-directional communication of PLC signals. Modem  130 , data processor  135 , r.m.s.-to-dc converter  145 , and analog-to-digital converter  150  operate as in communications node  112 . 
         [0051]    Low pass filter  205  blocks PLC signals, and passes a current sense voltage from coupler  105 . Low pass filter  205  outputs a filtered current sense voltage corresponding to I 101 . 
         [0052]    Low pass filter  306  blocks PLC signals, and passes a current sense voltage from coupler  305 . Low pass filter  306  outputs a filtered current sense voltage corresponding to I 301 . 
         [0053]    Analog processor  340  receives the filtered current sense voltage corresponding to I 301  via cable  222 , and receives the filtered current sense voltage corresponding to I 301  via cable  323 . Analog processor  340  includes an analog multiplexer (not shown) as part of its input circuit, or another appropriate arrangement, for processing multiple input signals. 
         [0054]      FIG. 4  is a block diagram of a system  400  that, similarly to system  100 , includes a coupler  105  and a communications node  112 . Communications node  112  operates in system  400  as it does in system  100 . In contrast with system  100 , system  400  includes a circuit  405 , a bypass module  415 , and a cable  410 . Cable  410  connects each of circuit  405  and bypass module  415  to filter module  115 . 
         [0055]    Circuit  405  represents circuitry that operates in association with coupler  105 , and passes PLC signals, but blocks power frequency signals. Examples of such circuitry include a surge suppressor and/or an impedance matching transformer. 
         [0056]    Since circuit  405  blocks power frequency signals, bypass module  415  provides a path for power frequency signals from coupler  105  to communications node  112 . More particularly, bypass module  415  routes the current sense voltage from coupler  105 , around circuit  405 , to cable  410 . The PLC signals from circuit  405  and the current sense voltage are multiplexed onto cable  410 , downstream of circuit  405 . Bypass module  415  is effectively a low pass filter, and so does not appreciably affect the PLC signals at the inputs or outputs of circuit  405 . Filter module  115  receives the multiplexed signal via cable  410 , and by operations of high pass filter  116  and low pass filter  117 , demultiplexes the PLC signals and the current sensing voltage. 
         [0057]      FIG. 5  is a schematic of a portion of communications node  112  showing an exemplary implementation of high pass filter  116  and low pass filter  117 . 
         [0058]    Modem  130  includes a transformer  520  that, in turn, includes a primary winding  515 . A secondary of transformer  520  is coupled to communications circuitry within modem  130 . 
         [0059]    High pass filter  116  includes a capacitor  510  that is situated in series with primary winding  515 . Capacitor  510 , primary  515  and modem input impedance Zin  540  function together as a high pass filter. 
         [0060]    Low pass filter  117  includes chokes  525  and a capacitor  530 . Chokes  525  conduct the current sense voltage signal (i.e., a low frequency signal), and block PLC signals. Capacitor  530  filters out any residual high frequency component. 
         [0061]      FIG. 6  is a schematic of a portion of system  400 , and shows an exemplary implementation of bypass module  415 . Here, bypass module  415  is shown to include chokes  605 , a transformer  615 , and a capacitor  625 . 
         [0062]    As mentioned above, circuit  405  passes PLC signals, but blocks power frequency signals. Chokes  605  provide a low impedance path for the current sense voltage signal (from secondary winding pair  110 ) to a primary  610  of transformer  615 . Transformer  615  is a low frequency transformer, and may have a non-unity turns ratio so as to scale the magnitude of the current sense voltage to match an input voltage range of analog-to-digital converter  140  or r.m.s.-to-dc  145  (see  FIG. 1C ). A secondary  620  of transformer  615  is connected in series with a wire that conducts PLC signals at the output of circuit  405 . This arrangement connects the PLC signals and current sense voltage in series, thus multiplexing the PLC signals and current sense voltage for multiplexed transmission via cable  410 . Capacitor  625  has a value in the range of nanofarads, and acts as a short circuit to conduct the (high frequency) PLC signals, while appearing as an open circuit to the (low frequency) current sense voltage. Accordingly, for low frequencies, secondary  620  is placed in series with the output of circuit  405 . 
         [0063]      FIG. 7  is a block diagram of a system  700  for measuring phase of a current of a medium voltage power line, referenced to a phase of a power voltage. System  700  includes coupler  105 , a distribution transformer  702 , and a communications node  712 . 
         [0064]    Distribution transformer  702  transforms voltage from power line  103  to a lower voltage, and provides power to loads in a premises  740 . Additionally, distribution transformer  702  provides power to communications node  712  via a low voltage power line  725 . 
         [0065]    Communications node  712  includes filter module  115 , modem  130 , analog processor  140 , r.m.s.-to-dc converter  145 , data processor  135 , and analog-to-digital converter  150 , all of which operate as described above. Communications node  712  further includes, a phase detector  730 , and an analog-to-digital converter  750 . 
         [0066]    Phase detector  730  determines the phase of I 101 , relative to a reference phase. More specifically, phase detector  730  receives a reference voltage derived from low voltage power line  725 , and also receives an amplified current sense voltage from the output of analog processor  140 . Phase detector  730  determines the phase of I 101 , based on a phase relationship between the reference voltage derived from low voltage power line  725 , and the amplified current sense voltage from the output of analog processor  140 . Phase detector  730  outputs a voltage that represents the phase of I 101  relative to the phase of the reference voltage derived from low voltage power line  725 . 
         [0067]    Analog-to-digital converter  750  has two inputs, namely (a) an input from phase detector  730 , i.e., the voltage that represents the phase of I 101 , and (b) an input from r.m.s.-to-dc converter  145 , i.e., the dc representation of the current sense voltage. Analog-to-digital converter  750  converts each of the two inputs to digital format, and outputs phase data and current sense data. 
         [0068]    Data processor  235  receives the phase data and the current sense data from analog-to-digital converter  750 , and sends the phase data and the current sense data to modem  130 . 
         [0069]    Modem  130  modulates the phase data and the current sense data onto a PCL signal, and transmits the PLC signal via filter module  115 , coupler  105  and power line  103 , to other communications nodes (not shown in  FIG. 7 ) connected by other couplers (not shown in  FIG. 7 ) on a power line grid of which power line  103  is a part. In addition, this information may be presented on a display or at a data port (neither of which is shown) installed on or adjacent to communications node  712 , for the benefit of personnel providing service at a site at which communications node  712  is located. 
         [0070]      FIG. 8  is a block diagram of a portion of a power distribution network  800  configured for measuring power parameters at various locations within power distribution network  800 . Power distribution network  800  includes a three phase medium voltage power line, i.e., power line  825 , couplers  805 ,  815  and  830 , communications nodes  840 ,  845  and  850 , and a monitor system  855 . 
         [0071]    Couplers  805 ,  815  and  830  are each similar to coupler  105 . Communications nodes  840 ,  845  and  850  are each similar to communications node  712 . Coupler  805  is situated at a location  810 , and coupled to communications node  840 . Coupler  815  is situated at a location  820 , and coupled to communications node  845 . Coupler  830  is situated at a location  835 , and coupled to communications node  850 . Monitor system  855  is also coupled to communications node  850 . 
         [0072]    A distribution transformer  837  transforms voltage from a phase line of power line  825  to a lower voltage, and provides power to loads in a premises  852 . Additionally, distribution transformer  837  provides power to communications node  840 . Communications nodes  845  and  850  are similarly powered by other distribution transformers (not shown). 
         [0073]    Coupler  805  and communications node  840  operate together to sense current, phase and voltage at location  810 . Communications node  840  periodically transmits the sensed values for location  810 , in a PLC signal, via coupler  805 , power line  825 , coupler  830  and communications node  850  to monitor system  855 . 
         [0074]    Coupler  815  and communications node  845  operate together to sense current, phase and voltage at location  820 . Communications node  845  periodically transmits the sensed values for location  820 , in a PLC signal, via coupler  815 , power line  825 , coupler  830  and communications node  850  to monitor system  855 . 
         [0075]    Coupler  830  and communications node  850  operate together to sense current, phase and voltage at location  835 . Communications node  850  periodically reports the sensed values for location  835  to monitor system  855 . 
         [0076]    Monitor system  855  is at a central control location, e.g. a substation. Monitor system  855  receives the sensed data for each of locations  810 ,  820  and  835 , and develops a system-wide picture of currents and other electrical parameters throughout power distribution network  800 . Monitor system  855  includes a display, analysis equipment, recording equipment and an alarm system that allows utility personnel to monitor operation of power distribution system  800 , quickly locate and eliminate power outages when they occur, thus improving reliability and lowering the overall cost of grid maintenance of power distribution system  800 . 
         [0077]    Note that coupler  805  is attached to a lower phase line of power line  825 , and that coupler  830  is attached to a middle phase line of power line  825 . PLC signals couple from one phase to another, allowing communications relatively independent of the phase line on which a coupler is situated. 
         [0078]    While sensing is described herein as sensing parameters on the phase lines to which couplers are attached, it is possible to attach self-contained conventional current sensors (not shown) to other phases, attach the digital outputs of the current sensors to ports (not shown) on communications nodes such as communications node  840 , and provide monitor system  855  with a more complete set of data. Similarly, self-contained, conventional voltage sensors may be attached to low voltage or medium voltage power lines, and their digital data also supplied to monitor system  855 . Accordingly, if a utility company needs a complete picture of all currents and voltages at some critical electric pole, the utility company can attach the types of sensors that the utility company typically uses to the other lines (otherwise unsensed by communications nodes  840 ,  845  and  850 ), and a conveniently situated communications node  840 ,  845  or  850  can serve as a digital data relay for these extra data sources. 
         [0079]    Although the several systems described herein are described as providing bi-direction data communications in conjunction with the sensing of power line parameters, such need not be the case. For example, referring to  FIG. 1 , in system  100 , there may not be a PLC signal present on power line  103  at a time when communications node  112  is measuring I 101 . Thus, at the time of the measurement of I 101 , the signal being coupled from power line  103 , via coupler  105 , may include only the power frequency signal, and no PLC signal. Moreover, regardless of whether a PLC signal is present on power line  103 , communications node  112  need not be configured for bi-directional communication, but instead, may be configured only for PLC transmission via coupler  105 . As such, communications node  112  would sense the power frequency signal, and thereafter transmit data representing the sensed signal, in a PLC signal, via coupler  105 , to a remote monitoring station. Furthermore, communications node  112  need not be configured for any PLC via coupler  105 . For example, communications node  112  may sense, process and present a result via an interface local to communications node  112 , without necessarily transmitting any data to any other device. 
         [0080]    A system for measuring magnitude and/or phase of a current on a medium voltage power line or a high voltage power line, and communicating the measurement, preferably isolates the lethal voltages from the measuring circuit and provides protection against surges that routinely occur on such lines. The systems described herein include an inductive coupler that can provide both current measurement and communications, so the measured data is easily concentrated at a central point, for analysis, alarm, recording and fault detection. 
         [0081]    Although various exemplary embodiments of the invention have been disclosed, it should be apparent to those skilled in the art that various changes and modifications can be made that will achieve some of the advantages of the invention without departing from the true scope of the invention. For example, although high pass filter  116 , low pass filter  117  and bypass module  415  are described herein as being implemented with discrete components (i.e., discrete capacitors and discrete inductors), they may be implemented as digital circuits in which their respective operations are preformed by digital signal processing. The present invention is intended to embrace all such changes and modifications that fall within the scope of the appended claims.