Abstract:
A power line communications technique (PLC) is disclosed which allows communications across any three phase electric system by modulating the impedance through earth between any two points of differing voltage on the electrical system in order to use the power supply as the communications power source thereby using the power supply current as the communications carrier. No additional coupled communications source is required and modulation of an earth link current at the power frequency is especially suitable for remote electrical loads, such as an oil well electric submersible pump, where it is most cost effective to only have the three power conductors and no neutral nor signal wires in the power cable. Embodiments of the invention are described.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a power line communication (PLC) system, wherein information is transmitted along a power conductor. The invention is particularly suitable for oil wells and general purpose waterlift pumps, where a load such as an electric submersible pump (ESP) is installed deep in an oil well, bore hole or caisson and where the remoteness of the load results in a technical and commercial obstacle to installing a separate communication cable. 
     BACKGROUND OF THE INVENTION 
     Within power line communication systems there is provided a 3 phase power supply to power a load. If this supply were to power a perfectly balanced load the voltage between the neutral point of the supply and the neutral point of the load would be 0 volts. Subsequently, if both the supply neutral and the load neutral were connected using a neutral wire or both were connected to earth, then the current flow in this neutral or earth link would be 0 amps. 
     However, electrical systems are not perfectly balanced and for a motor powered over a long cable with various junction boxes and connectors, each phases impedance will be affected by a multitude of factors, such as: the length of the cable, the connection resistance, the position of the winding in the slot, the differences in winding lengths, the construction of the motor, the construction of the cable and the transposition of the cable. Such imbalances in a load create a voltage difference between the two neutral points and as current flow between the two neutrals is undesirable as it will cause a loss of power and they are not typically connected. 
     Power line communication systems are well known in the art. One such known system provides a means of communicating data from a remote inductive electrical load, such as a motor by modulating the motor current. However this type of system may require the need for modifying the motor windings. 
     Other existing PLC methods which use earth link currents, use a DC coupled current to act as the communication carrier. This however requires a large coupling package at both the supply and the load neutral connection, which are designed to remove signals above a few Hertz to remove the known imbalance voltage differences and as such limits the data transfer rate to a few bits per second. Furthermore, these types of coupled signals also suffer from the fact that they transfer less power to the transmitter. 
     It is an object of at least one aspect of the present invention to obviate or mitigate one or more of the aforementioned problems and disadvantages. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention there is provided a method of communicating information along a power conductor between a power supply and a load supplied by said power supply, said method comprising the steps of causing a flow of current from the power supply between two different locations within the power system through a reference conductor not required for normal operation of the power system, and modulating the flow transmitted through the reference conductor by varying an impedance placed in the path of said current flow to provide at least two current states which are used to transmit information along the power system. 
     Preferably the method uses earth as the reference conductor. 
     According to a second aspect of the present invention there is provided a power line communication system for use with a 3 phase power supply system said power line communication system comprising: 
     a 3 phase power supply, 
     a 3 phase load, incorporating a 3 phase power conductor to connect the 3 phase actual load to the 3 phase power supply, 
     a transmitter impedance means coupled between said load and a voltage reference conductor, said transmitter impedance being variable between at least a first impedance value and a second impedance value to generate a respective first and second current value, said first and second current values being used to transmit information along said reference conductor, 
     a receiver impedance means coupled between said power supply and said voltage reference conductor, said receiver impedance completing the transmission circuit and permitting the measurement of the current state. 
     Preferably, said 3 phase supply has a neutral connection and said 3 phase load has a neutral connection, and at least one transmitter is coupled at one of said neutral connections and at least one receiver is coupled to the other of said neutral connections, said at least one transmitter and said at least one receiver being powered by a normally occurring voltage differential between the supply neutral connection and the load neutral connection for providing communication signals. 
     Preferably the first and second current values differ in amplitude only. 
     Alternatively any standard modulation technique including amplitude modulation, frequency modulation or phase modulation may be implemented to achieve the variation of current flow through the reference conductor. 
     Preferably the reference conductor is earth. 
     Conveniently the reference conductor may be a conductor insulated from earth. 
     Preferably the receiver impedance is connected between a neutral connection of the power supply and the reference conductor. 
     Alternatively the receiver impedance may be connected between any point of the power supply and the reference conductor. 
     Preferably the transmitter impedance is connected between a load neutral point, such as a star connection of a three phase motor and the reference conductor. 
     Alternatively the transmitter impedance may be connected between any point of the load and the reference conductor. 
     Most preferably there is provided a line powered transceiver at the load end and the supply end of the power system, both of which conveniently incorporate a fixed and variable impedance enabling duplex communication. 
     Preferably one transceiver transmits during one half cycle of a reference conductor current flow and the other transceiver on the other half cycle to achieve duplex communications. 
     Conveniently, there is provided a sensing device that enables information to be received from another transceiver by measurement of reference conductor current. 
     Advantageously an offline voltage source is provided in conjunction with one transceiver, enabling communication to be maintained when the power supply is not powered. 
     Conveniently, there is also provided a switched diode at the load transceiver, permitting insulation testing of the load and the apparatus connected at the load end of the power system. 
     Preferably the transceivers are powered by the normally occurring voltage differential between the supply neutral connection and the load neutral connection. 
     Advantageously there is provided means or a method to create a sufficient voltage differential to power the line transceivers and thereby allow communications between them. 
     Conveniently, the voltage differential between the supply neutral connection and load neutral connection may be created by use of a variable impedance placed in at least one conductor of the power line. Wherein the variable impedance is used to create an imbalance in the load, said impedance providing a means for controlling the potential difference exhibited between the supply neutral and load neutral. 
     Advantageously there is provided a control system that allows the impedance to be varied either manually, or by means of an automatic feedback system, in order to be able obtain the desired voltage differential exhibited between the supply neutral and load neutral. 
     Alternatively, the transceivers are powered by a voltage differential is created across the communication system by connecting the supply transceiver to one of the supply phases of the power source whilst the load transceiver is connected to the load neutral connection. 
     Alternatively, the transceivers are powered by a voltage differential is created across the communication system by connecting the load transceiver to one of the phases of the load whilst the supply transceiver is connected to the supply neutral connection. 
     Conveniently, the transceivers may be powered by a voltage differential is created across the communication system by connecting the supply transceiver to one of the supply phases of the power source and by connecting the load transceiver to a different phase at the load end. 
     Alternatively, the transceivers are powered by a phase conductor voltage drop, which is achieved by connecting the transceivers at either end of a conductor of the power line. 
     Advantageously the transceivers are powered by the voltage differential between any two points in the power system. 
     Advantageously the load insulation impedance may be determined by measurement of the reference conductor current flow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other aspects of the present invention will now be described by way of example with reference to the accompanying drawings, in which: 
         FIG. 1  is a three phase power system with a fixed supply neutral-earth impedance and a variable motor neutral-earth impedance; 
         FIG. 2  is a duplex transmission system using a modulated neutral current driven by a differential voltage between the supply and motor neutral points; 
         FIG. 3  is an electrical diagram of an individual inductor switch shown in  FIG. 2 ; 
         FIG. 4  is an electrical block diagram of the system shown in  FIG. 2 ; 
         FIG. 5  is an amplitude modulated current waveform showing duplex digital communications at 2 bits per Hertz; 
         FIG. 6  is an electrical diagram showing a communications system in accordance with a second embodiment of the invention with a differential voltage controlled by variable line impedance. 
         FIG. 7  is an electrical diagram of a communication system in accordance with a third embodiment of the invention showing a phase voltage used for the communications differential created by connecting the supply transceiver to one phase. 
         FIG. 8  is an electrical diagram of a communication system in accordance with a fourth embodiment of the invention showing a cable voltage drop used for the communications differential, created by connecting transceivers at either end of the cable. 
         FIG. 9  is an electrical diagram of a communication system in accordance with a fifth embodiment of the invention showing both the supply and load being delta connected, in which a phase voltage differential is used for the communications differential, created by connecting one transceiver to a supply phase and the second transceiver to a different phase at the load. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1  there is shown a three phase, power source  1  supplying a remote motor  6  through a substantial length of power cable  5 . The power source  1  having a neutral point  2  which is connected to an earth point  4  through a fixed value inductance  3 . Whilst the motor  6  is connected in a star configuration creating a motor neutral point  7  which is connected to earth via a fixed inductance  8  and a variable inductance  9 . 
     The voltage seen at the neutral point of the power supply  2  and the neutral point of the motor  7  are not equal. This is due to resistances in each conductor of the power cable  5 , and the general construction details of the motor  6 , which means the load is not perfectly balanced and hence will result in a voltage differential between the supply neutral connection and the load neutral connection. This resultant voltage is applied across the three inductors  3 , 8 , 9  via earth which results in a current flow through the inductors via earth, where Ohm&#39;s law dictates the value of current
 
current=voltage/impedance
 
where
 
impedance=fixed impedance+variable impedance.
 
     Therefore, by variation of the variable impedance  9  it is possible to vary the current through the circuit. 
     Switching a short circuit across the variable impedance  9  results in the variable impedance  9  changing between a short circuit and a fixed impedance. This results in the current changing between one of two states. These two states being used to send a single bit of information. 
     The resultant current measured through the fixed inductance  3  varies depending on the total circuit loop impedance. By measuring this current, the current can be categorised as either being low or high, indicating a low short circuited impedance or high impedance. The value of the current high or low being the state of a single bit of transmitted information. 
     Referring to  FIG. 2  there is shown a diagram for a duplex, digital communications system having a data transfer rate of 200 bits per second, for a power frequency of 50 Hertz, the bandwidth being split into 100 bits per channel. The three phase power supply  1  is connected to a star connected motor load  6  over a substantial length of power cable  5 , with a transceiver connected between the power supply neutral  2  and earth  4 , and another transceiver connected between the motor star point  7  and earth  4 . The transceiver connected at the power supply end also has an offline power supply  27  attached to it, whilst the transceiver connected at the motor end has a switched diode  11  connected in series between the motor neutral  7  and the transceiver. 
     The offline power supply  27  provides power to the communications system, that is both transceivers when the motor is not supplied with power from the power source  1 . When the motor is powered, relay  26  is closed and relay  25  is open, thereby disconnecting the offline power supply  27 . However, when the motor is off, the relays  25  and  26  are in the opposite state thereby connecting the offline power supply  27 . 
     The switched diode  11  permits insulation testing of the power supply  1 , the cable  5  and the motor  6  by presenting a diode barrier  12  to the circuit when the relay  13  is opened. However the relay  13  is closed whenever the transceiver  14  is powered. When the power supply  1  is switched on the relay  13  is open and the diode  12  does not permit ac power to flow. However, the rectified current flowing through the inductor switches  10  generates power in the inductor switch power supply  34  which supplies power to the relay  13  via a dc-dc isolator  35  and common power supply  16 . An alternative means of controlling the relay is to power the relay  13  directly from a secondary winding on the fixed inductor  8 . 
       FIG. 3  shows the construction of the inductor switch  10  shown in  FIG. 2 . Across the main power terminals  29  and  30  is a fixed inductor  32 . In parallel with this fixed inductor  32  is a diode  31  and transistor  33  which are in series with each other, this circuitry allows the current to bypass the fixed inductor  32  whenever the transistor  33  is turned on. The transistor  33  is switched on using an external signal  37  from a micro-controller  19  and the external signal  37  is connected to the transistor  33  via an opto-isolator  38 . A power supply  34  generates the power requirements for the inductor switches  10  and also supplies power to the transceivers common circuitry. Power is derived from the voltage across the fixed inductor  32  and the power supplied from the inductor switch  10  to the transceiver  14  is isolated from the inductor switch power supply  34  using a dc-dc converter  35 . 
     When the inductor switch control signal  37  is on, the inductor switch presents a nominal impedance when the applied voltage is in the direction of the modulation polarity  39 . Whilst when the control signal  37  is off, the impedance of the fixed inductor  32  is presented across the terminals  29 , 30 . This allows the control signal  37  to modulate the current flow through the inductor switch  10  such that it can transmit information. 
     The variable inductance  9  is made from a combinations of inductor switches  10 , in this embodiment four such inductor switches have been used, which can be switched to set the impedance to any one of five values as detailed below in Table 1: 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 No of 
                 Transceiver Inductance 
                   
               
             
          
           
               
                 Inductor 
                 +ve half 
                 −ve half 
                 Binary 
               
               
                 Switches ON 
                 cycle 
                 cycle 
                 State 
               
               
                   
               
               
                 0 
                 Zf + 4*Zv 
                 Zf + 4*Zv 
                 Null 
               
               
                 1 
                 Zf + 3*Zv 
                 Zf + 4*Zv 
                 00 
               
               
                 2 
                 Zf + 2*Zv 
                 Zf + 4*Zv 
                 01 
               
               
                 3 
                 Zf + 1*Zv 
                 Zf + 4*Zv 
                 10 
               
               
                 4 
                 Zf 
                 Zf + 4*Zv 
                 11 
               
               
                   
               
               
                 Zf = fixed inductor 8 value 
               
               
                 Zv = variable inductor 32 value 
               
             
          
         
       
     
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 2 
               
             
             
               
                   
               
               
                 No of 
                 Loop Inductance 
                   
               
             
          
           
               
                 Inductor 
                 +ve half 
                 −ve half 
                 Binary 
               
               
                 Switches ON 
                 cycle 
                 cycle 
                 State 
               
               
                   
               
               
                 0 
                 Zft + 4*Zvt + 
                 Zft + 4*Zvt + 
                 Null 
               
               
                   
                 Zfr + 4*Zvr 
                 Zr 
               
               
                 1 
                 Zft + 3*Zvt + 
                 Zft + 4*Zvt + 
                 00 
               
               
                   
                 Zfr + 4*Zvr 
                 Zr 
               
               
                 2 
                 Zft + 2*Zvt + 
                 Zft + 4*Zvt + 
                 01 
               
               
                   
                 Zfr + 4*Zvr 
                 Zr 
               
               
                 3 
                 Zft + 1*Zvt + 
                 Zft + 4*Zvt + 
                 10 
               
               
                   
                 Zfr + 4*Zvr 
                 Zr 
               
               
                 4 
                 Zft + Zfr + 
                 Zft + 4*Zvt + 
                 11 
               
               
                   
                 4*Zvr 
                 Zr 
               
               
                   
               
               
                 Zft = transmitter fixed inductor 8 value 
               
               
                 Zvt = transmitter variable inductor 32 value 
               
               
                 Zfr = receiver fixed inductor 8 value 
               
               
                 Zvr = receiver variable inductor 32 value 
               
               
                 Zr = receiver inductance fixed + variable 
               
             
          
         
       
     
     It can be seen from the information detailed in table 1 above that five distinct transmission currents can be generated, this is also illustrated in  FIG. 5 . Reserving the state where all switches  10  are off as a null state, this leaves 4 transmission currents which allows 2 bits of information to be transferred by the variable impedance  9 .  FIG. 5  shows a message on the positive polarity cycle of binary 00011011, a signal which has been created by sequentially, on each positive half cycle switching on 1 then 2 then 3 then 4 of the inductor switches  10 . The binary signal with a null, that is to say no inductor switches selected, preceding and following the signal, is decimal  27 . The decimal value  27  is the information that the microcontroller  19  sends to the receiver to indicate that the thermistor over-temperature detection device  15  has signalled a high value into the microcontroller  19 , or when the measurement device  20  has generated a signal that it has sent to the microcontroller. 
     The receiver for this message does not switch on the positive polarity; it is 180 degrees out of phase and the inductance of the transceiver acting as the receiver is fixed at Zf+4*Zv as listed in Table 2. Hence by measuring the lowest current through the transceiver the null signal value is known and by measuring the maximum current the changes in current between each of the transmitter states can be determined. This allows the receiver to recover the information in the current flow as transmitted by the opposing transceiver. 
     Where the voltage is insufficient to power the communications  FIGS. 6 ,  7 ,  8  and  9  show alternative approaches.  FIG. 6  adds a variable inductance  43  in one phase of the conductor to cause a load imbalance.  FIG. 7  applies a phase voltage across the communication system by connecting the supply transceiver to one phase of the supply  44 .  FIG. 8  applies a phase conductor voltage drop across the transceiver by connecting the transceivers  14  at either end  44 ,  45  of the cable  5 . 
       FIG. 9  shows delta connected supply and load where a phase to phase voltage difference is applied across the communication system by connecting one transceiver to one phase  46  of the supply  44  and the second transceiver to another phase  48  at the motor end  45 . Other approaches to generating this communications voltage are possible. It will be understood that the voltage reference conductor may be a separate conductor. 
     To send information in the opposite direction is the same as described hereinabove. 
     The primary advantages of the present invention over those of existing systems is that the present invention removes the need to run a separate communication cable to the remote load and does not require the coupling of a communication power supply or signal to the 3 phase power system. 
     Other advantages are that the modulated current is not the motor current, but a much smaller earth link current thus improving the signal to noise ratio. Also by utilising the power supply as the carrier for the transmission system, this provides a greater amount of power available for the transmitter electronics compared to systems that use a coupled supply. Likewise, because there is no coupling it is possible to modulate the earth link current at a high rate, thus allowing a high data transfer rate to be achieved. A further advantage is that with the embodiments hereinbefore described the data rate transfer is greater or equal to the motor frequency, that is 50 bits pe second for 50 Hz, and is also independent of any oscillations or changes in the motor current.