SOLID STATE ELECTRICITY METER

Abstract: In a solid state electricity meter, a display (16) consisting of a number of juxtaposed display elements provides, in a vector form, an indication of the direction and range of power flow in the line being measured. Circuitry for driving the vector display comprises a first memory storing a plurality of power measurement references and a second memory storing corresponding vector symbols. A microprocessor (50) compares each power measurement with the references stored in the first memory to determine the range of the power measurement and, in response, reads the corresponding vector symbol from the second memory to drive the display (16).

Description: This invention relates generally toward a solid sate electricity meter having a display that simulates the rotating disk of a conventional electromechanical electricity meter to indicate the direction and approximate magnitude of power measured. The invention relates more particularly toward a solid state electricity meter, comprising measurement means coupled to a power line for obtaining voltage and current measurements; multiplier means for multiplying together said measurements to obtain the magnitude of line power; and a display comprising at least one linear series of segments. Electricity meters are commonly used by electrical utilities to measure the flow of, for example, electrical power quantities between the power source and customer loads. Within each meter there is a disk which acts as the rotor of a small electric motor coupled to the power line being measured and rotates at a speed proportional to power flowing through the meter. The amount of power applied to or obtained from a load is accumulated by a register that is coupled mechanically to the disk. The register is read periodically by the utility for customer billing. To enable the utility to determine by inspection whether the meter is operative, whether power is flowing and if so, approximately at what rate and in what direction, a portion of the disk is exposed to be viewed from outside the meter. Recently developed, microprocessor controlled meters currently becoming adopted by the electrical utilities, have capabilities to carry out numerous metering functions such as demand and time of use measurement within a single meter. One difficulty that has become apparent with such meters, however, is the inability of the meter to provide by visual inspection an indication to the utility of the direction and approximate magnitude of power flow with respect to a load as is possible in the conventional electro-mechanical electricity meter by simply observing the rotation of the disk. For instance, although the British patent GB-A-2 135 065 shows a solid state electricity meter for measuring both power and reactive power and for displaying same on a numeric display, this display does not show the direction of power flow nor does it provide a vector which indicates both power flow direction and the relative magnitude thereof. Another prior patent application, DE-A-2 939 801 discloses an apparatus with panel meter which displays an arrow comprised of LED dots to show relative magnitude and direction of the value being measured. However, this apparatus is not related to electrical power flow and thus not utilized to indicate the magnitude and direction of power flow in an electric meter. There accordingly exists a need to provide, in a solid state electricity meter, a visual display to indicate to the utility, and possibly to the customer, the direction and approximate magnitude of power flowing in the lines being measured. A need further exists in such meters to enable the display to provide to the utility the range of power flowing in the line being monitored. A general object of the invention is to provide solid state electricity meter having a visual display of power flow direction and approximate magnitude. This and other objects are obtained, in accordance with one aspect of the invention by a solid state electricity meter in conformity with claim 1. Still other objects and features of the invention will become more readily apparent to those skilled in the art from the following description when read in conjunction with the accompanying drawing, wherein there is shown and described a preferred embodiment of the invention simply by way of illustration of one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other, different, embodiments and its several details are capable of modifications in various obvious aspects, all without departing from the invention. Accordingly, the drawings and description will be regarded as illustrative in nature and not as restrictive. The accompanying drawings, incorporated in and forming a part of the specification, illustrate several aspects of the present invention, and, together with the description, serve to explain the principles of the invention, in the drawings : Figure 1 is a view of solid state meter in accordance with the invention; Figure 2 is a simplified schematic diagram of circuitry forming the solid state electricity meter; Figure 3 is a view of the display of the solid state electricity meter of Figures 1 and 2; Figures 4(a)-4(e) are displays of power having different directions and magnitudes measured by the solid state electricity meter of the invention; and Figures 5(a)-5(b) are a flow chart of programming used in the invention. In Figure l, an electricity meter 10 comprises a conventional glass electricity meter housing l2 containing a disk-shaped chassis l4 within which is located a solid state display l6 constructed in accordance with the principles of this invention. The glass housing l2, together with chassis l4, is supported on a cabinet l8 that is attached to a support at the power measurement site. Within the glass housing l2, and to the rear of plate l4, is line power measurement circuitry for driving display l6 in accordance with power in the lines being monitored. With reference to Figure 2, such line power measurement circuitry 20 for measuring three-phase power, comprises phase current transformers 22, 24 and 26 together with phase voltage transformers 28, 30 and 32 respectively applied to the three line phases to be measured. The secondary circuits of the current transformers 22, 24 and 26 are provided with burden resistors 34 to convert a secondary circuit currents into properly scaled voltages. These scaled voltages are applied to a first sample and hold circuit (S/H) 36 through an analog switch 38 and a buffer amplifier 40. The switch 38 is controlled to sample the outputs of the three-phase current transformers 22, 24 and 26 in succession These three current measurements are passed through an analog switch 42 to an analog-to-digital (A/D) converter 44 to be digitized, stored in a current latch 46 and thereafter transferred on bus 48 to a conventional microprocessor 50. Similarly, phase voltages developed by voltage transformers 28, 30 and 32 are sampled successively by analog switch 52 to be supplied in sequence to sample and hold circuit 54 and thereafter through analog switch 42 to converter 44. The digitized phase voltage values in converter 44 are applied through latch 56 and bus 48 to the microprocessor 50. Phase lock loop (PLL) 58 is synchronized to the power line at the secondary of voltage transformer 28, although the PLL could alternatively be synchronized to any one of the other voltage phases. The output of the phase locked loop 58 controls the indexing of analog switches 38 and 52 as well as controls sampling by sample and hold circuits 36 and 54 and controls conversion by analog-to-digital converter 44. The output of PLL 58 also controls latching of the current and voltage latches 46 and 56, and controls analog switch 42 alternately between the outputs of sample and hold circuits 36 and 54. The operation of the circuitry shown in Figure 2 to develop a digital display that is a function of measured three-phase power is provided in EP-A-0 104 998, to the assignee of this invention, and incorporated herein by reference. Although it accordingly is not necessary to detail the operation of the circuitry shown, generally the phase locked loop 58 in Figure 2 generates clock signals at a rate that is approximately an integral multiple of the line frequency (e.g., 60 Hz.) at which the three-phase power waveforms are successively sampled. During sampling of the three-phase currents, current latch 46 is loaded with samples to be transferred on bus 48 to microprocessor 50. During the voltage phase samples, voltage latch 56 transfers samples to the micrprocessor 50 along the same bus 48. Divider 6l both synchronizes microprocessor 50 to the 60 Hz. power waveform and identifies the particular phase being monitored for processing by the microprocessor. Thus, the circuit 20 makes three-phase voltage and current measurements by sampling three-phase voltages and currents to develop instantaneous analog values in sample and hold circuits 36, 54. Samples converted to corresponding digital values in converter 44 are applied to microprocessor 50 for scaling and multiplication to obtain instantaneous power, and for integration and time division to obtain average power. Of particular importance, in addition to displaying digitally energy consumed in watts, VARs, watthours, VAR-hours, etc., as in EP-A-0 104 998, supra , the circuit 20 additionally displays a "vector" whose direction indicates the direction of power flow on the lines measured and whose length is an indication of the approximate instantaneous value of power. Referring to Figure 3, the display l6 (see also Figure l) consists of two rows of sixteen liquid crystal display (LCD) elements each. Of course, the number of LCD elements forming each row as well as the number of rows is arbitrary. Furthermore, light emitting diodes (LEDs) or other type of display elements, rather than LCDs, could be used to form the display l6. In any event, in the preferred embodiment, the upper row is controlled by microprocessor 50 through drivers 60 to display a register name (l6a-l6k) which may include a phase designation, a peak number (multiple peak demand registers are used) and measured quantity name (e.g., KWH). Element l6l may identify load direction (e.g., a "d" may indicate that the register only accumulates data when power is being delivered to a load whereas an "r" may indicate that the register only accumulates data when power is being received from the load) or quadrant number (e.g., a "l" indicates that the register only accumulates data when watts are being delivered to the load, i.e., the load is inductive). Ignoring LCD elements m-p temporarily, for this discussion, the microprocessor 50 controls the lower row of elements whereby LCD elements q, r may display a register ID code, such as "0l", element s may be blank and elements t-aa may display register data in, e.g., kilowatt hours. Elements bb and cc may display codes to indicate that the instrument is undergoing field or factory testing, enabled by internal switch settings, and elements dd-ff may identify the line phases active. Although the display elements are controlled to display data in the manner as set forth above in accordance with one embodiment of this invention, it is to be understood that except for display elements m-p comprising this invention, the particular data displayed in the remaining display elements are arbitrary and may be other than those proposed herein. Also, different data can be applied to the same display elements in different modes of operation of the meter by implementing default and alternate display modes, in a conventional manner. Display elements m-p, in accordance with the invention, display a vector whose direction indicates the direction of flow of power on the lines measured and whose length approximates the magnitude. Figures 4(a)-4(b) are examples of four vector displays provided by elements l6m-l6p wherein the " > " simulates counterclockwise rotation of the conventional watthour meter disk and " <" simulates clockwise rotation. The shaft of the arrow has a length formed by up to three of the four LCD elements 16m-16p. In the example of Figures 4(a)-4(e), the character forming the shaft of the vector preferably indicates the range of the power measured and the number of elements forming the shaft identifies the magnitude within that range. The hierarchy of vector shaft characters, together with vector directions, as a function of power direction and amplitude, is as follows: Vector pointers to the right (clockwise) ("> ") indicate watts delivered to the load, and vector pointers to the left (" < ") indicate watts received from the load. Thus, the vector shown in Figure 4(a) corresponds to 100% of class delivered to the load. In Figure 4(b), from one to three percent of class is delivered to the load, and in Figure 4(c), .1 to .3% of class is delivered to the load. On the other hand, in Figure 4(d), 3 to 10% of class is received from the load, and in Figure 4(e), at least 100% of class is received from the load. The displays generated in Figures 4(a) and 4(e) are developed by microprocessor 50 and applied to the LCD array l6 via a conventional display driver 60 (Figure 2). Microprocessor 50 is preferably programmed in firmware stored in memory 64 wherein a first portion M₁ of the memory contains predetermined ranges of power that generate the various vector symbols as set forth above, and a second portion M₂ contains the vector symbols themselves. Thus, in response to a voltage and current sample pair on bus 48, as shown in the circuit of Figure 2, the microprocessor 50 obtains the product of the pair to determine instantaneous power and compares that power with the contents of memory portion M₁ to determine within which range the sampled power lies. The microprocessor 50 also detects the polarity of the sample product to determine the direction of power flow. Based upon the direction of power flow and the power magnitude, determined by comparison of measured power and power magnitudes stored in M₁, a corresponding vector symbol is read from memory portion M₂, to be applied through drivers 60 to LCD l6. During the next sample, the display l6 is updated in accordance with the outputs of memory portions M₁, M₂ of read only memory 64. Programming of the microprocessor 50 to generate vector displays in response to samples on bus 48 is shown in Figures 5(a) and 5(b) wherein power is first measured at step 64 in the manner set forth above and then polarity tested (step 66). If the power flow is negative, the microprocessor 50 executes the program branch set forth in the right-hand portion of the Figure; otherwise, it executes the branch in the left-hand portion. Assuming, for example, that the power is negative, the microprocessor 50 determines whether the magnitude of power is less than .36 of l% of class (step 68). If so, the symbol "----" is displayed by LCD l6 This display indicates that the power flow on the line measured is virtually at zero. The constant 0.36 is the magnitude below which the power reading is too noise contaminated to be meaningful. Similarly, if power flow measured by microprocessor 50 is positive and the power magnitude is less than .36% of class (step 70), the display "----" is applied to LCD l6. If power flow is positive, and the power measured is greater than 0.36% of class, microprocessor 50 determines by comparison with references stored in portion M₁ of memory 64, whether the power measured is less than 3.6% of class. If so, the symbol " > " is displayed in LCD l6 (step 72). If the power measured is greater than 3.6% of class, the microprocessor 60 next determines whether the power measured is less than l0.8% of class. If so, LCD l6 is controlled to display "- > " (step 74). Similar steps are executed by microprocessor 50 throughout the remainder of the flow chart of Figures 5(a) and 5(b) to display each particular vector symbol stored in M₂ that corresponds to the direction and approximate magnitude of line power measured by circuit 20. There accordingly has been described a solid state electricity meter display formed of a number of successive display elements arranged linearly and controlled by microprocessor to form a vector symbol whose direction indicates the direction of power flow measured and whose length indicates approximate power magnitude. The display, which simulates the rotating disk of a conventional electromechanical electricity meter, enables the utility to determine, upon inspection, whether a load being measured by the electricity meter is using or generating power, and indicates the approximate power amplitude. The solid state display, in addition to its inherent improved reliability compared to the conventional electromechanical rotating disk, provides a more accurate estimation of power amplitude than is possible in electromechanical electricity meters. In this disclosure, there is shown and described only the preferred embodiments of the invention, but, as aforementioned, it is to be understood that the invention is capable of use in various other combinations and environments and is capable of changes or modifications within the scope of the claims.

Claims: A solid state electricity meter (10), comprising measurement means (20) coupled to a power line for obtaining voltage and current measurements; multiplier means (50) for multiplying together said measurements to obtain the magnitude of line power; and a display (16) comprising at least one linear series of segments, characterized in that said multiplier means (50) also obtains the direction of flow of said line power, and by first memory means (M1) storing a plurality of reference line powers having predetermined magnitudes; second memory means (M2) storing a plurality of different non-alphanumeric indicia; comparator means (50) for comparing a line power obtained from said multiplier means (50) with said plurality of references stored in said first memory means (M1), and control means (60) responsive to said comparator means (50) for applying to said display (16) selected ones of said non-alphanumeric indicia to form a vector representation of the magnitude and direction of said measured line power. The electricity meter of claim 1, characterized in that said stored indicia consist of the symbols "-", "*", ">" and "<". The electricity meter of claim l or 2, characterized in that said control means (60) comprises a microprocessor and wherein said first (Ml) and second (M2) memory means comprise read only memories. The electricity meter of claim 2 or 3, characterized in that said display (16) consists of at least three display elements and wherein said control means (60) includes means responsive to direction of line power flow to apply selectively a ">" or a "<" to the first or last element of said display (16).

Inventors: PERRY, DUANE, STEPHENS, RAY
Applicants: SCHLUMBERGER INDUSTRIES, INC.
Application year: 1987
Main classification: G01R21
IPC codes: G01R21, G01R13