Abstract:
A power meter for measuring power consumption and detecting the presence of an unsafe line condition is disclosed. The power meter has a service disconnect switch, which is interposed between load contacts and source contacts, the disconnect switch interrupts the flow of power from the source contacts to the load contacts when the disconnect switch is in an open position. The power meter also has a processor coupled to a two input signal processing circuit. The two input signal processing circuit receives a first and second voltage signal from the load contacts. The two input signal processing circuit converts the first and second voltage signal into a first and second voltage value. The processor computes the power consumption using the first and second voltage values in combination with current values. The processor uses the first or second voltage valuea to determine the presence of an unsafe line condition when either the first or second voltage value exceeds a first voltage threshold or the first or second voltage value is below a second voltage threshold when the service disconnect switch is in the open position.

Description:
FIELD OF INVENTION 
     The present invention relates generally to power systems, and more particularly to a system and a method for providing a DC voltage source within a power meter. 
     RELEVANT BACKGROUND 
     Utility companies use power meters to regulate and monitor power usage. Early power meters were electromechanical in nature converting the flow of electricity through the power meter into mechanical movement. The mechanical movement was used to turn a recording device which recorded the amount of energy being used. As technology improved over the years, the design of the power meter incorporated new innovations such as increased processing capability within the meter, elimination of mechanical parts, better accuracy and the like. 
     The power meter typically monitors and measures the amount of power that the utility consumer uses. Additionally, the utility meter may monitor the real time conditions that exist at the utility customer&#39;s site. Within the power meter, internal circuitry such as processors, microprocessors, microcontrollers or the like may perform these monitoring functions. In order to operate, the internal circuitry may require a DC (direct current) power supply. 
     Within the power meter, the DC power supply may connect directly to the utility lines that couple the utility meter to the power grid. Under normal operating conditions, the power meter may experience voltage ranging from about 70 VAC to about 288 VAC. The DC power supply converts the AC voltages to a constant DC voltage. The converted DC voltage typically ranges between 3 VDC to about 12 VDC. In addition to converting the AC voltages to a DC voltage, the power meter must be able to withstand surges in the AC voltage. Occasionally, the utility lines may experience large surges in voltage due to lightning strikes or other over voltage conditions. In order to provide a constant DC voltage as well as over voltage protection, the DC power supply may contain a multitude of discrete components. In previous power meters, the discrete components of the DC power supply required significant space on the power meter&#39;s circuit board. Some discrete components contained within the power meter&#39;s internal circuitry may include a 60 Hz transformer, various inductors, capacitors, resistors and transformers or the like. 
     As technology has advanced, the characteristics of many of the discrete components began to evolve. In previous power meters, a dedicated discrete inductor is used as an energy storage unit to provide voltage into a switching voltage regulator. Thus when the AC voltage dropped below a particular voltage threshold, the discrete inductor discharged and continued to provide voltage to the switching regulator until the incoming AC waveform returned to the voltage threshold. 
     SUMMARY 
     There exists a need in the industry to provide the same functionality of the previous switching DC power supplies while improving the efficiency and cost effectiveness of the meter design. The present disclosure addresses this need and discloses such a power meter. The present invention takes advantage of the inherent inductive characteristics of a voltage transformer and eliminates some of the discrete components necessary in the previous meter designs. The inherent inductive characteristics of the present invention provide the energy storage for a power meter&#39;s DC power supply when the input into the voltage transformer drops a predetermined voltage threshold. 
     A DC power supply for a power meter is disclosed. The DC power supply has a transformer, the transformer has an input and an output, the input receives a first AC (alternating current) voltage waveform and converts the first AC voltage to a second AC voltage. The transformer also has an internal impedance; the internal impedance further has an inductive portion. The output of the transformer is coupled to a diode bridge, the diode bridge rectifies the second ac voltage into a rectified AC voltage. The diode bridge is coupled to a first switching transistor circuit which is biased to turn on when the rectified AC voltage is greater than a first voltage threshold and turn off when the rectified AC voltage is less than the first voltage threshold. The diode bridge also coupled to a second switching transistor circuit the second switching transistor circuit is biased to turn off then the rectified AC voltage is above the first voltage threshold and turn on when the rectified AC voltage is below the first voltage threshold. The power supply also has a switching regulator circuit, the switching regulator circuit provides a DC power source, the switching regulator circuit receives energy from the inductive portion when the rectified AC voltage is below the first voltage threshold and the second transistor circuit is on. 
     A method for supplying a DC (direct current) power source for a power meter is disclosed. The method transforms a first AC voltage to a second AC voltage by using a transformer. The transformer has an internal inductance. The method further rectifies the second AC voltage into a rectified AC voltage by using a rectifying circuit. The method receives the rectified AC voltage at a first switching transistor circuit and, the method also receives the rectified AC voltage at a second switching transistor circuit. The method further receives the rectified AC voltage at a switching regulator circuit. The method turns the first switching transistor circuit on and turns the second switching transistor circuit off when the rectified AC voltage transitions above a voltage threshold. The method turns the first switching transistor circuit off and turns the second switching transistor circuit on when the rectified AC voltage transitions below the voltage threshold. The method also sources energy from the internal inductance into the switching regulator when the rectified voltage is below the voltage threshold. 
     A more complete understanding of the present invention, as well as further features and advantages of the invention, will be apparent from the following detailed description and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  displays a 2 S single phase two element two wire power meter in accordance with another embodiment of the present invention. 
         FIG. 2  displays a DC power supply circuit in accordance to one embodiment of the present invention used by the power meter of  FIG. 1 . 
         FIG. 3  displays an exemplary voltage waveform as measured in the DC power supply of  FIG. 2 . 
         FIG. 4  displays another exemplary voltage waveform as measured in the DC power supply of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     The detailed description set forth below in connection with the appended drawings is intended as a description of various embodiments of the present invention and is not intended to represent the only embodiments in which the present invention may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the present invention. However, it will be apparent to those skilled in the art that the present invention may be practiced without these specific details. In some instances, well-known structures and components are shown in block diagram form in order to avoid obscuring the concepts of the present invention. Acronyms and other descriptive terminology may be used merely for convenience and clarity and are not intended to limit the scope of the invention. For ease of illustration, all alternating current (AC) voltage values are represented in terms of root mean squared (RMS) values unless otherwise specified. 
       FIG. 1  displays a power meter  100  similar to a 2 S single phase two wire, two element watt-hour meter. Those skilled in the art will recognize that the power meter  100  can be installed to measure power in a conventional 120/240 split phase electric system and in this configuration there is no neutral connection brought into the power meter  300 . Within the power meter  100 , a processor  110  is coupled to a current sensor  120 , a communications module  105 , a metering circuit  115 , and a service disconnect switch  125 . Providing a direct current (DC) power to the processor  110 , the communications module  105 , the metering circuit  115  as well as any other discrete logic circuitry in the power meter  100  is a power supply  140 . The power supply  140  provides a DC voltage necessary for the processor  110  and other connected circuitry to operate. As is explained in greater detail in subsequent sections, the power supply  140  receives an AC waveform and converts it to a DC voltage. In one embodiment of the present invention, the DC voltage is about 3.3 VDC. 
     The power meter  100  is designed to receive a source voltage at L 1   IN  and L 2   IN  at the source side  160  of the power meter  100 . The source voltage may be provided from the utility power grid, typically from a transformer near the subscriber site. The source voltage received at the source side  160  of the power meter  100  typically ranges between 0 and about 280 VAC, but must be able to withstand higher voltages in certain over voltage conditions. In addition, the power meter  100  may be designed to receive an AC voltage waveform that oscillates at 50 HZ (typical for European voltages) or 60 HZ (typical for North American power applications). 
     Within the power meter  100  the electrical power is routed through a current sensor  120 . The current sensor  120  measures the amount of current flowing through each source side contact (L 1   IN  and L 2   IN  ) of the power meter  100 . Coupled to the current sensor  120  is a service disconnect switch  125 . Power is supplied to the load side  170  of the power meter  100  through the service disconnect switch  125  when the service disconnect switch  125  is closed. From the service disconnect switch  125 , power is routed to the consumer via the load side contacts L 1   out  and L 2   out  . 
     The processor  110  is coupled to the service disconnect switch  325  and may open or close the service disconnect switch  325  by opening and closing an electromechanical solenoid switch which moves the moveable contacts  326  and  327 . Opening and closing the service disconnect switch  325  allows the processor  110  to connect or disconnect the consumer from the power grid. Power is supplied to the consumer when the service disconnect switch  325  is closed (the movable contacts  326  and  327  are engaged) via the load side  170  contacts L 1   OUT  and L 2   OUT  . 
     Also communicating with the processor  110  is a metering circuit  115 . The metering circuit  115  is coupled directly to the load side contacts L 1   OUT  and L 2   OUT  and the source side contact L 2   IN . Within the metering circuit  115  is a signal processing circuit which measures the voltage levels at the line side contacts L 1   OUT  and L 2   OUT  with respect to the source side contact L 2   IN . Those skilled in the art will recognize that the Form 2 S meter configuration does not include a connection to neutral or earth ground. The metering circuit  115  monitors the voltage levels present at the load side  170  of the service disconnect switch  325 . 
       FIG. 2  displays a schematic diagram of an exemplary power supply  140  in accordance with one embodiment of the present invention. The power supply  140  receives the AC voltage at a transformer  205 . In one embodiment, the transformer  205  may be a 60 HZ transformer capable of receiving voltage ranging between about 70 VAC and 280 VAC. In this embodiment, the transformer  205  may step the voltage down at a ratio of about 11:1. If the transformer  205  receives a 70 VAC waveform at its input, the maximum output voltage from the secondary winding of the transformer  205  may be about 8.0 VAC. Similarly, if a 120 VAC or a 240 VAC voltage is received at the primary winding of the transformer  205 , the output will be about 4.5 VAC and about 9 VAC respectively. 
     The output of the transformer  205  is directed to a diode bridge  210  comprising diodes  201 ,  202 ,  203 , and  204 . As those skilled in the art appreciate, the diode bridge  210  rectifies the stepped down AC voltage wave form. The full wave rectified voltage is in turn provided to two switching transistors circuits  220  and  240  as well as a low drop out (LDO) voltage regulator  250 . The LDO  250 , using load filter capacitor  270  provides the DC voltage to the processor and the other connected circuitry used by these circuits to operate. In one exemplary embodiment, the LDO  250  may provide a 3.3 VDC, 35 mA power source to the processor and connected circuitry. During the AC transitions when the full wave rectified voltage provided to the input of the LDO  250  drops below about 6.0 VDC, the switching transistor circuits  220  and  240  provide a means for keeping the input voltage into the LDO at or above about 6.0 VDC. 
     Within the switching transistor circuit  220 , resistor  222  is coupled to the output of the transformer  205 . The resistor  222  is also coupled with a Diode  224 . The diode  224  is connected to the base of a NPN bipolar junction transistor (BJT)  228 . Also connected to the base of the BJT  228  is resistor  226 . Connected to the collector of the transistor  228  is resistor  230 . Resistor  230  is also connected to the output of the transformer  205 . The combination of resistor  222 , diode  224  and resistor  226  bias the transistor  228  to turn on and remain on when the rectified AC voltage is at or above about 6.0 VDC. In one embodiment, resistor  222  may be a 20 kΩ ½ watt resistor, resistor  226  may be a 30 kΩ ½ watt resistor, and resistor  230  may be a 47 kΩ ½ watt resistor. The diode  224  may have a breakdown voltage of about 5.6V. 
     As is explained in subsequent sections, when the rectified voltage is above this threshold (about 6.0 VDC), switching transistor circuit  240  is off while switching transistor circuit  240  is on, and there is no charging current supplied to capacitor  234 . In the previous example, the threshold is determined by the breakdown voltage of the diode  224  and the base to emitter voltage (about 0.4V) which is about 6.0V. 
     In one embodiment, the capacitor  234  may be a 470 μF, 10V capacitor. During this period, the capacitor  234  is supplying the voltage directly to the LDO  250 . Thus, while the rectified voltage is above the previously mentioned threshold, power into the input of the LDO  250  is provided directly from the rectified AC waveform via capacitor  234 . 
     However, as the rectified voltage waveform drops below the predetermined threshold, the input into the LDO  250  needs to remain above about 4.0V in order for the LDO  250  to provide an output of about 3.3 VDC. In one embodiment, switching transistor circuit  220  is switched off and switching transistor circuit  240  is switched on when the rectified voltage drops below about 6.0V. When this happens, energy stored within the power supply  140  is used to provide the LDO  250  with sufficient input voltage to keep its output at about 3.3 VDC. In one embodiment of the present invention, the internal impedance of the transformer  205  acts as a storage element and supplies the energy required to keep the input into the LDO  250  at or above 4.0V. In this embodiment, when the rectified voltage transitions through the predetermined threshold, the internal impedance, and more specifically the internal inductance in the transformer  205  provides the stored energy used to supply the LDO  250 . When the switching transistor circuit  240  is on, current flows from the internal inductance through capacitor  234 . The current supplied by the internal inductance is sufficient to charge capacitor  234  and provide voltage to the LDO  250 . In this instance, capacitor  234  filters the input voltage into the LDO  250  when the rectified voltage drops below the predetermined threshold. 
     When the rectified voltage drops below the predetermined threshold, the first switching transistor circuit  220  turns off and the second switching transistor circuit  240  turns on. The second switching transistor circuit  240  comprises a transistor  242 , capacitor  236 , and resistors  232  and  238 . In this embodiment transistor  242  is a 50V MOSFET (metal oxide semiconductor field effect transistor). Those of sufficient skill in the art appreciate that when the transistor  228  of the first switching transistor circuit turns off, the voltage at the gate of transistor  242  of the second switching transistor circuit  240  increases and subsequently rises enough to turn on transistor  242 . When the transistor  242  is on, current begins to flow from the drain to the source, which is filtered by capacitor  234  and provides the LDO  250  with energy stored within the internal inductance of transformer  205 . 
     While the rectified voltage remains below the predetermined threshold, transistor  242  remains on and the internal inductance of the transformer  205  continues to provide the stored energy for the LDO  250 . As the rectified voltage begins to rise above the predetermined threshold, transistor  224  begins to turn on while transistor  242  begins to turn off. Transistor  224  may turn on before transistor  242  turns off. When transistor  242  is off, the second switching transistor circuit  240  may experience inductive flyback from the internal inductance of the transformer  205 . Those skilled in the art appreciate that when inductive flyback occurs, current being supplied by an inductive storage element may cause current to spike creating a spike in the voltage of the transformer. When the transistor  242  turns off, the internal inductance of the transformer  205  continues to supply energy into the second switching transistor circuit  240  for a brief period of time. The peak current and related peak voltage may be controlled by a voltage limiter. Without the voltage limiter, the voltage sourced by the internal inductance of the transformer  205  could rise significantly above the voltage rating of the capacitor  234  and the LDO  250 . In one embodiment, a peak voltage limiter comprised of capacitor  236  and resistors  232  and  238  helps drain off the excess current created by the inductive flyback from the transformer  205 . When transistor  242  turns off, excess current is drained through the capacitor  236  and resistor  232 . In one embodiment, resistor  232  is a 20 kΩ ½ watt resistor and resistor  238  is a 2 kΩ ½ watt resistor, and capacitor  236  is a 1000 pF 35V capacitor. 
     To better understand the operation of the inventive concepts of the present invention  FIG. 3  displays two voltage waveforms ( 310  and  320 ) measured in the power supply  140  when the power supply  140  receives a voltage of about 288 VAC at the primary winding of the transformer  205 . Voltage waveform  310  is the output voltage of the diode bridge  210 . Voltage waveform  320  is the voltage measured across the drain to source junction of the MOSFET  242 . The difference between the voltage waveforms  310  and  320  is about 6.0 VDC which is the voltage across capacitor  234 . 
     As can be seen in  FIG. 3 , at T=0, the voltage across the diode bridge is about 25 VDC, and the voltage across the transistor  242  is about 18VDC. At T=0, the MOSFET transistor  242  is off and the transistor  224  is on. As the rectified voltage  310  begins to decreases, and more specifically at about T=3.25 ms the rectified voltage drops below 6.0 VDC. At this point, the MOSFET  242  turns on and the transistor  228  turns off. During the time between T=3.25 ms to about 4.5 ms, the rectified voltage  320  is filtered by the capacitor  234 . No current is flowing through the capacitor at this time since the rectified voltage  320  is equal to or less than the capacitor  234  voltage. This non-charge condition is maintained until the rectified voltage goes through zero and then increases to about 6.0 VDC after the zero crossover. 
     When the rectified voltage  310  increases to about 6.0 VDC at about 4.5 ms until about T=6.25 ms, the transformer secondary and more specifically the internal inductance of the transformer secondary delivers increasing current to charge the capacitor  234 . Also during this time period, the rectified voltage  320  is equal to the voltage across the capacitor  234  and the balance of the rectified voltage  320  is dropped across the secondary impedance of the transformer  205 . 
     At about T=6.25 ms, the MOSFET  242  begins to turn off. During the time it takes for the transistor  242  to turn off, the inductive flyback associated with the internal impedance of the transformer  205  causes the voltage  320  to spike. The voltage spike (the peak drain to source voltage) is limited by the rate of turn off of the gate voltage of the MOSFET  242 . Once the MOSFET  242  has turned off, it remains off until the voltage  310  drops below about 6.0 VDC and the cycle repeats itself. 
     As displayed in  FIG. 3 , capacitor voltage  330  remains at about 6.0 VDC until the MOSFET turns off and the capacitor voltage  330  spikes for a brief period of time. After the peak at about T=6.25 ms, the excess current is drained by the capacitor  236 . After the excess current is drained, the power supply  140  returns to steady state where the capacitor  234  is charged to the 5.6V plus the base-emitter drop across transistor  228 . As mentioned previously, any excess voltage from the fully rectified voltage outputted from the diode bridge  210  is dropped across the MOSFET  242  so at nominal AC input voltage the waveform across the MOSFET  242  is a series of truncated half wave pulses where the filter capacitor  234  delivers load current to the LDO  250  by itself.  FIG. 4  displays two voltage waveforms  410  and  420  that may exist within the power supply  140  when the power supply receives about 104 VAC at the primary winding of the transformer  205 . Voltage waveform  410  is the output voltage of the diode bridge  210 . Voltage waveform  420  is the voltage measured across the drain to source junction of the MOSFET  242 . The difference between the voltage waveforms  410  and  420  is about 6.0 VDC which is the voltage across capacitor  234 . 
     As can be seen in  FIG. 4 , slightly after T=0, the transistors  242  and  228  have just completed a transition at  460 . As shown in  FIG. 4 , the voltage waveform  410  is about 6V below voltage waveform  420 . After the transition, the MOSFET transistor  242  is off and the transistor  228  is on. As the rectified voltage  420  begins to decreases, and more specifically at about T=3.25 ms the rectified voltage drops below 6.0 VDC. At this point, the MOSFET  242  turns on and the transistor  228  turns off. During the time between T=3.25 ms and the point where the rectified voltage returns to 6 VDC, the rectified voltage  420  is held up by the capacitor  234  and no current is flowing through the capacitor. When the rectified voltage increases above 6 VDC, charge current flows into capacitor  234 . 
     When the capacitor voltage increases above 6.0 VDC at about T=8.0 ms, transistor  242  begins to turns off, the transformer secondary and more specifically the internal inductance of the transformer secondary delivers increasing voltage in an attempt to keep current flowing. The turn off rate selected for transistor  242  limits the peak voltage delivered by the transformer inductance. 
     The voltage spike (the peak drain to source voltage) is limited by the rate of turn off of the gate voltage of the MOSFET  242 . Once the MOSFET  242  has turned off, it remains off until the voltage  420  drops below about 6.0 VDC and the cycle repeats itself. 
     The various illustrative logical blocks, modules, circuits, elements, and/or components described in connection with the embodiments disclosed herein may be implemented or performed with a general purpose processor, a digital signal processor (DSP), an application specific integrated circuit (ASIC), a field programmable gate array (FPGA) or other programmable logic component, discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. A general-purpose processor may be a microprocessor, but in the alternative, the processor may be any conventional processor, controller, microcontroller, or state machine. A processor may also be implemented as a combination of computing components, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration. 
     Although specific embodiments have been illustrated and described herein, those of ordinary skill in the art appreciate that any arrangement, which is calculated to achieve the same purpose, may be substituted for the specific embodiments shown and that the invention has other applications in other environments. This application is intended to cover any adaptations or variations of the present invention. The following claims are in no way intended to limit the scope of the invention to the specific embodiments described herein.