Abstract:
A method is described for measuring a plurality of line voltages using a measurement circuit, the method including the steps of reducing each line voltage over a first impedance; providing a second impedance between each of the reduced voltages and a measurement circuit common reference point; coupling the common reference point to a voltage line N using a third impedance; and determining line voltage values using measurements of the reduced voltages. The above-described method provides impedance between voltage line N and measurement electronics and thus prevents transient voltages from entering the measurement circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. provisional application No. 60/114,449, filed Dec. 31, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to making voltage measurements, and more particularly, to measuring voltages on a distribution line coupled to an electricity meter. 
     A watt-hour electricity meter typically measures several current and voltage inputs. DC isolation and an impedance are often provided to scale the currents down to appropriate levels for electronic measurement. Voltage inputs, however, are most economically measured by a simple resistor divider to neutral circuit (or some other voltage line if the service type has no neutral). The neutral is one of the AC distribution lines and is connected directly into the measurement electronics of the meter. This connection to the neutral line provides a direct low impedance path for transients from the AC lines to enter the meter electronics and disrupt operation or even damage the circuits. 
     Some known meters have voltage transformers in the voltage measurement circuit. These transformers perform multiple functions including scaling the voltage down to a level appropriate for measurement by an electronic circuit, and providing both DC isolation and some level of impedance to transients entering the meter electronics from the distribution lines. The transformers, however, are costly and heavy, failure prone, introduce measurement errors, consume significant power, and require a large amount of space in the meter. 
     Other known meters include resistors to scale the voltage down to a level appropriate for measurement by an electronic circuit. The resistors avoid some of the error types present in transformers. The resistors are also lower in cost, weight, power consumed, and size. The resistor method, however, provides no DC isolation or impedance between the neutral and the measurement electronics. 
     BRIEF SUMMARY OF THE INVENTION 
     There is therefore provided, in one embodiment, a method for measuring a plurality of line voltages using a measurement circuit, the method including the steps of reducing each line voltage over a first impedance; providing a second impedance between each of the reduced voltages and a measurement circuit common reference point; coupling the common reference point to a voltage line N using a third impedance; and determining line voltage values using measurements of the reduced voltages. The above-described method provides impedance between voltage line N and measurement electronics and thus prevents transient voltages from entering the measurement circuit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 of an electronic energy meter; and 
     FIG. 2 is a circuit schematic diagram of a voltage measurement circuit in accordance with one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Although the present invention is described herein in the context of an electricity meter, it should be understood that the invention is not limited to practice in connection with such meters. The present invention can be used in connection with voltage measurement apparatus generally, and is not limited to practice in only electricity metering. 
     Referring now to the drawings, FIG. 1 is a block diagram illustration of an exemplary electronic energy meter  10  which, for example, is commercially available from General Electric Company, 130 Main Street, Somersworth, N.H. 03878, and generally referred to as the KV meter. The KV meter can be modified to incorporate the voltage measurement circuit described below in more detail. Although the present apparatus and methods are described herein in the context of an electronic electricity meter, it should be understood that the present invention is not limited to practice with any one particular meter. 
     Referring now specifically to FIG. 1, meter  10  includes voltage sensors  12  and current sensors  14 . Sensors  12  and  14 , in operation, typically receive input analog voltage and current signals from power lines (not shown). Sensors  12  and  14  are coupled to an analog-to-digital (A/D) converter  16  which converts the input analog voltage and current signals to digital signals. Digital signal output of converter  16  is provided to a digital signal processor (DSP)  18 . DSP  18  supplies microcomputer or processor  20  with digitized metering quantities, e.g., V 2 H, I 2 H. Microcomputer  20 , using metering quantities supplied by DSP  18 , performs additional metering calculations and functions. DSP  18  may, for example, be a processor commercially available as Model Number TMS320 from Texas Instruments Company, P.O. Box 6102, Mail Station 3244, Temple, Tex. 76503, modified to perform metering functions. 
     Microcomputer  20  is coupled to a liquid crystal display (LCD)  22  to control display of various selected metering quantities and to an optical communications port  24  to enable, for example, an external reader to communicate with computer  20 . Port  24  may be, e.g., an OPTOCOM™ port of General Electric Company, 130 Main Street, Somersworth, N.H. 03878, which is well known and in accordance with ANSI type II optical port specifications. Microcomputer  20  may also generate additional outputs  26  used for various other functions as is well known in the art. Microcomputer  20  may, for example, be an eight-bit microcomputer commercially available from Hitachi America, Inc., Semiconductor &amp; I.C. Division, Hitachi Plaza, 2000 Sierra Point Parkway, Brisbane, Calif. 94005-1819, modified to perform metering functions. 
     Microcomputer  20  also is coupled to an input/output (I/O) board  28  and to a function, or high-function, board  30 . DSP  18  also supplies outputs directly to high function board  30 . Microcomputer  20  further is coupled, via a control bus  32 , to an electronically erasable programmable read-only memory (EEPROM)  34 , I/O board  28  and high function board  30  also are coupled, via bus  32 , to EEPROM  34 . 
     Back-up power is supplied to meter  10  components described above by a battery  36  coupled to a wide-range power supply  38 . In normal operation when no back-up power is required, power is supplied to meter  10  components from power lines (not shown) via power supply  38 . 
     Many functions and modifications of meter  10  components described above are well understood in the metering art. The present application is not directed to such understood and known functions and modifications. Rather, the present application is directed to methods and apparatus for making voltage measurements. In addition, although methods and apparatus are described below in the hardware environment shown in connection with FIG. 1, it should be understood that such methods and apparatus are not limited to practice in such environment. The subject methods and apparatus could be practiced in many other environments. 
     Further, it should be understood that the present invention can be practiced with many alternative microcomputers, and is not limited to practice in connection with just microcomputer  20 . Therefore, and as used herein, the term microcomputer is not limited to mean just those integrated circuits referred to in the art as microcomputers, but broadly refers to microcomputers, processors, micro-controllers, application-specific integrated circuits, and other programmable circuits. 
     In accordance with one embodiment of the present invention, an impedance (not shown in FIG. 1) is coupled into a common measurement circuit voltage point (not shown in FIG.  1 ), typically neutral, to help prevent transients from entering meter  10 . In addition, compensation is made for presence of the impedance in measurement of voltages on AC lines into meter  10 . 
     More particularly, FIG. 2 is a circuit schematic diagram of a voltage measurement circuit, e.g., corresponding to voltage sensor  12  shown in FIG.  1 . Referring to FIG. 2, points A, B, C, and N represent attachment points of meter  10 , for example, to three lines A, B and C of a three-phase AC voltage line and to a neutral line N respectively. Line N, however, is not necessarily neutral and may be another line voltage, including one of phase voltage lines A, B and C. Line currents IA, IB, IC and IN flow respectively through lines A, B, C and N. Point G represents a ground or common point or reference for measurements in meter  10 . 
     Points aa, bb, and cc represent points of connection to meter  10  measurement circuit and are separated respectively from points A, B and C by very big impedances R 1 . “Very big impedances” means impedances of, for example, 1 million to 2 million ohms that reduce power into meter  10  in accordance with P=V 2 /R. Meter  10  measurement circuit load currents at points aa, bb, and cc thus are very small. Impedances R 2  couple points aa, bb and cc respectively to point G. Impedances R 1  and R 2  together reduce voltages, in accordance with ratio R 2 /(R 1 +R 2 ), respectively at points aa, bb, and cc to levels measurable by meter  10 , for example, to one volt. In one embodiment, an impedance R 3  couples neutral line N and point G. There is no return path from point G to neutral line N or to lines A, B or C, and thus no line current flows into G from meter  10  measurement circuit. 
     In accordance with the configuration shown in FIG. 2, line currents are summed in accordance with IA+IB+IC=IN. Current IN also is determined in terms of measured quantities, e.g. 
     
       
           IN=VaaG/R   2   +VbbG/R   2 + VccG/R   2   
       
     
     where VaaG, VbbG and VccG represent respectively voltages between points aa, bb and cc and point G. Voltage VGN from point G to point N is determined in accordance with VGN=IN*R 3 . Voltage sums then are determined in accordance with: 
     
       
         
           VAN=VAG+VGN 
         
       
     
     
       
         
           VBN=VBG+VGN 
         
       
     
     
       
         
           VCN=VCG+VGN 
         
       
     
     where VAN, VBN and VCN represent respective voltages between points A, B and C and point N and VAG, VBG and VCG represent respective voltages between points A, B and C and point G. Measured phase voltages at points aa, bb and cc respectively are related to VAG, VBG and VCG in accordance with: 
     
       
           VaaG=VAG *( R   2   /R   1 + R   2 ) 
       
     
     
       
           VbbG=VBG *( R   2   /R   1 + R   2 ) 
       
     
     
       
           VccG=VCG *( R   2   /R   1 + R   2 ) 
       
     
     Voltages VAN, VBN, and VCN then are determined in accordance with: 
     
       
           VAN=VaaG *( R   1 + R   2 ) /R   2 +( R   3   /R   2 )*( VaaG+VbbG+VccG ) 
       
     
     
       
           VBN=VbbG *( R   2 + R   2 ) /R   2 +( R   3   /R   2 )*( VaaG+VbbG+VccG ) 
       
     
     
       
           VCN=VccG *( R   1 + R   2 ) /R   2 +( R   3   /R   2 )*( VaaG+VbbG+VccG ) 
       
     
     The foregoing equations are executed by DSP  18  or microcomputer  20  to determine voltages VAN, VBN, VCN from measurable quantities with impedance R 3  in place. In one embodiment impedance R 3  is selected to be small relative to impedance R 1  and to contribute minimally to meter  10  total measurement. A value for impedance R 3  is, for example, ten thousand ohms, and an exemplary range of values from which to select impedance R 3  is between one thousand and ten thousand ohms. Also, although impedances R 2  are identically designated herein, it is not necessary to select identical values for impedances R 2 . An exemplary range of values from which to select impedances R 2  is between one hundred ohms and one thousand ohms. Thus impedance R 2  is, for example, selected to be 348 ohms. 
     The above-described voltage measurement circuit provides impedance between neutral line N and measurement electronics. By using resistors to scale voltages to a level appropriate for measurement, the above-described measurement circuit provides advantages as to cost, weight, power consumption, and size. 
     While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.