WO1999043010A2

WO1999043010A2 - Self-powered current monitor

Info

Publication number: WO1999043010A2
Application number: PCT/US1999/003306
Authority: WO
Inventors: Thomas G. Edel
Original assignee: Edel Thomas G
Priority date: 1998-02-19
Filing date: 1999-02-12
Publication date: 1999-08-26
Also published as: CA2319409A1; US6018700A; AU3295199A

Abstract

A self-powered current monitor for monitoring current in electric power systems. Various data relating to input currents may be displayed, such as current magnitude, current demand, and harmonics levels. Operating power is derived from one or more of the input currents. Ordinary current transformers may be utilized as input current sources. The power supply configuration may include a burden-reducing means to reduce the burden on input current sources during sampling of the input currents. The self-powered current monitor (1) includes a power supply circuit (3), current-sensing resistors (R1 and R3), an analog-to-digital converter circuit (5), a microprocessor circuit (6), and a memory circuit (9). Optional features include a display circuit (7), a burden-reducing circuit (2), input circuit protective elements (D1, D2, D3, D4, R2, and R4), an amplifier circuit (4), a user interface circuit (10), and an interface circuit (8) for communication to other equipment.

Description

Self-Powered Current Monitor

Technical Field

This invention relates to monitonng electnc current in electric power systems, and more particularly to a current momtonng device that denves its operating power from one or more electnc current inputs

Background Art — General

Most current momtonng systems for a-c (alternating-current) electnc power systems utilize current transformers to provide input currents that are isolated from the electnc power system conductors The primary winding of a current transformer is connected in seπes with a current-carrying conductor while the secondary winding is magnetically coupled to the primary winding by a suitable magnetic core A current is induced in the secondary winding that is proportional to the pnmary current The secondary current is isolated from the pnmaiy current and is smaller than the pnmary current by the turns ratio of the primary and secondary windings The pnmary winding frequently consists of only one turn, which is often just the current-carrying conductor installed through an opening in the middle of the current transformer magnetic core The secondary winding usually consists of multiple turns wrapped around the magnetic core

In order for the secondary current generated by a current transformer to be an accurate representation of the pnmary current, the impedance of the circuit connected to the secondary winding must be kept low so that current can flow freely The impedance of the secondary circuit is often called the "burden " This burden is m seπes with the secondary current and results in a voltage that opposes the flow of secondary current This opposing voltage is normally proportional to the magmtude of the current and the magmtude of the impedance, in accordance with Ohm's Law This opposing voltage causes the secondary current to be smaller than if there were no burden (a burden of zero ohms) The accuracy of the current transformer secondary current is affected by both the magmtude of the burden of the secondary circuit and the type of cuπent transformer used Larger (more expensive) current transformers are usually less affected by secondary burden than smaller (less expensive) current transformers

The chief technical obstacle to overcome in the design of a self-powered current mom tor is the power supply itself Anv deπvation of power from an input current will cause a voltage to be induced in the input current circuit that will oppose the flow of the input current Therefore, a power supply deπving power from an input current appears to the current source to be additional burden This power supply burden effect tends to reduce the input current and may introduce nonhneaπties into it How seπously the input current is affected by this power supply burden effect depends on the characteπstics of the current source from which the input current is deπved as well as the magmtude of the power supply burden effect A good power supply design should minimize this power supply burden effect to facilitate accurate current measurement utilizing inexpensive current transformers as current sources 2 If an "ideal" cuirent momtor were possible, it may be conceived to have the following properties

(a) An "ideal" current momtor would provide all possible data regarding momtored current both for the present and any time in the past Data provided would include magmtude (RMS, average, peak), harmonic data, frequency (cycles per second), and other data to clanfy waveform characteπstics that may deviate from a pure sine wave, including a graphic representation of the waveform at any given time

(b) An "ideal" current momtor would be "self-powered," requiπng no external power source beyond the input current that it momtors

(c) An "ideal" current momtor would have an input impedance (or "burden") of zero ohms, so that the accuracy of the input current source would not be adversely affected Clearly, an ideal current momtor is not practical due to the nearly infinite amount of data that is available for any significant length of time Any practical momtor will be limited to tracking key histoncal data, as well as providing detailed information about present current Also, the "self-powered" reqmrement of the ideal current momtor appears to be in conflict with the "zero ohms" burden requirement, so some kind of compromise will be necessary in a practical current momtor

Background Art — Definitions

For the purposes of this document, the following terms are defined

(a) Average magnitude The average of the instantaneous absolute magmtude of a quantity over one cycle

(b) Crest factor The ratio of the maximum value of a waveform to the root-mean-square value of the same waveform

(c) Current demand Current magmtude averaged over a specified interval of time (such as 15, 30. or 60 minutes) Current demand is usually expressed in units of amperes

(d) Demand An electπcal load averaged over a specified interval of time (such as 15, 30, or 60 minutes) Demand may be expressed in units of amperes, watts, vars (volt-amperes reactive), or volt-amperes

(e) Distortion factor The ratio of the root-mean-square of the harmonic content of a waveform to the root- mean-square value of the fundamental quantity, expressed as a percent of the fundamental

(f) Equivalent k-factor for transformers with harmonic rating The "k-factor" value associated with a nonlinear load current calculated according to Underwπter's Laboratory standard UL 1561 "Dry Type General .Purpose and Power Transformers "

(g) Form factor The ratio of the root-mean-square magmtude to the average magmtude (h) Half-cycle A time peπod that includes only the positive or negative half of an alternating waveform In a 60-hertz alternating-current electnc power system a full cycle is l/60th of a second long (approximately 16 7 milliseconds) A "half-cvcle" is half as long, or a time peπod of approximately 8 3 milliseconds 3 (1) Harmonic A sinusoidal component of a penodic wave having a frequency that is an integral multiple of the fundamental frequency

(j) Harmonic component magnitude The root-mean-square magnitude of an individual harmonic

(k) Input Channel The signal path and related circuits associated with a particular input current

(1) Maximum current demand The greatest of all current demands that have occurred over an extended peπod of time (such as a week, a month, or indefinitely)

(m) Microcontroller A type of microprocessor in which circuit .functions that are normally external to the microprocessor integrated circuit are included as part of the microprocessor integrated circuit These functions may include such items as memoiy, communication ports, time clocks, display dπvers, and analog- to-digital converters

(n) Microprocessor A general purpose mtegrated circuit adaptable for specific applications A microprocessor generally includes anthmeUc, logic, and control circuitry required to interpret and execute instructions from a computer program

(o) Peak magnitude The maximum value of a peπodic waveform (p) Root-mean-square magmtude The square root of the average of the square of the instantaneous magmtude taken over one complete cycle

(q) RMS An abbreviation for root-mean-square magmtude

Background Art — Related Devices

Histoπcally, analog ammeters have been used extensively for many decades to momtor alternating cunent in electnc power systems One veiy common configuration utilizes a cunent transformer to reduce line cunent levels down to a range of zero to five amps, with this secondary current used to dnve an analog panel meter (with a pointer and some form of moving coil meter movement) This type of analog meter is inherently "self-powered," requiπng no separate source of power to operate Switches are frequently used to select one of several cunent transformers to be connected to a single analog panel meter, with other cunent transformer secondanes being safely shorted by the switching mechanism while not connected to the analog panel meter (cunent transformer secondary windings not properly terminated to a low resistance load are often capable of generating hazardous voltages)

While analog ammeters are very effective at showing present cunent levels and require no separate power source, they are poor at providing histoπcal information or more detailed information such as harmonic data While a second "friction pointer" may be dragged by the main pointer and record the instantaneous histoncal peak. this is not the same as maximum cunent demand

Vaπous forms of meters that measure demand have also been available for many decades They have usually been configured to momtor electnc power instead of cunent and are commonly part of utility power meters used for revenue purposes Demand meters are generally either "integrated-demand" meters or "lagged-demand" meters "Integrated-demand" meters determine maximum demand by a multi-step process First, the "demand" is found by continuously measunng cunent or power and calculating its average over a predetermined time penod called a "demand interval" (usually 15 30, or 60 minutes) Then the "demand" is determined for each successive demand interval for an extended peπod of time (such as a week, a month, or indefinitely) The greatest "demand" during this extended peπod of time is recorded by electronic or mechanical means and is said to be the "maximum demand "

A Jagged-demand" meter is similar to a standard analog meter in which the dampening of the pointer is highly accentuated Pointer movement is usually accomplished by thermal heating effects to attain the long time delays required The slow-moving demand pointer may move a friction pointer to record maximum demand Mother method of calculating demand is the "sliding window integrated-demand" method This is similar to the integrated-demand method, except that the demand interval slides through time (each successive demand interval overlaps the previous demand interval) With this method the demand is calculated more frequently (say once every minute) for the demand interval that just ended This method overcomes a weakness of the integrated- demand method The integrated-demand method can miss an occunence of maximum demand when a load peak of relatively short duration is split between two demand intervals

Looking at more recent technology, modern electnc power momtonng systems are often microprocessor-based and are able to provide a wide range of data Data is often provided not only about cunent magnitudes and harmomcs, but also about voltages and almost any combination of the two, including power, reactive power, and transient phenomena Many of these systems come close to the "ideal" cunent momtor regarding data that is provided, but the mventor is unaware of any cunent momtonng system that is self-powered (utilizing input cunent only) and that provides useful histoncal and harmonic data

Some momtonng systems may operate on battenes, and thus do not require external power supply connections, but these are generally mtended for temporary momtonng purposes They are generally not suitable for permanent installation due to limited battery lrfe The discussion on pnor art up to this point has focused on vanous established momtonng systems

Consideration also needs to be given to pnor art regarding power supply circuits associated with a self-powered cunent momtor, as these are not typical circuits in common momtonng systems Power supply circuits that are applicable to the invention descnbed herein find their roots in power system protection and control devices more than in power system momtonng devices Means of denving power from an a-c cunent is well established pnor art dating back to at least the 1970s

Typical pnor art power supply configurations generally include

(a) a cunent transformer as an external cunent source,

(b) a rectifying circuit,

(c) a charging capacitor that is charged by rectified cunent and provides unregulated d-c voltage, (d) a voltage-regulating circuit that provides regulated d-c (direct-cunent) power from the unregulated d-c voltage on the charging capacitor, and 5 (e) a voltage-limiting circmt to shunt input cunent away from the charging capacitor when the voltage on the charging capacitor reaches a predetermined limit

Some applications may omit the voltage regulating circuit when the voltage on the charging capacitor is stable enough for the application Also, some applications may not require a voltage-limiting circuit The following discussion regarding related patents will identify some applications that have utilized these types of power supplies Pnor art sensing circuits that may be integral to these types of power supplies will also be identified

Background Art — Related Patents

Some patents that relate to self-powered cunent momtonng or denvmg power from input cunents will be bnefly discussed

Reissued U S patent Re 28,851 to Milkovic (reissued 1976) discloses a "Cunent Transformer with Active Load Termination " This cunent-sensing configuration utilizes an operational amplifier to produce an output voltage from an input cunent with almost no burden being imposed on the cunent source

U S patent 4,027,203 to Moran and Le Court (1977) discloses a "Protective Switch Device for Electncal Distnbuhon Systems " A power supply deπved from a cunent source is part of the larger protective switch device The power supply configuration is similar to the pnor art discussed above A sensing resistor is included in the power supply circmt to sense fault cunent and initiate opemng of the protective switch by suitable controls The burden effect that the power supply circmt has on the input cunent is said to not senously affect the sensing of fault cunents U S patent 4,176,386 to Chow (1979) discloses an "Overcunent Relay" that is said to simultaneously deπve power from an input cunent and deπve an accurate information signal from the same input cunent The input circuit is configured to provide a constant impedance burden on the cunent transformer to minimize distortion of the input cunent (this is somewhat different from typical pnor art discussed above)

TJ S patent 4,422,039 to Davis (1983) discloses a "Self-Powered .Ammeter" with an integral power supply deπved from the input cunent The power supply configuration is similar to the pnor art discussed above, except that an internal transformer has been added ahead of the rectifier circuit The internal transformer reduces the burden effect that the power supply would otherwise have on the input cunent source The internal transformer also helps isolate the power supply circuit from the sensing circmt This ammeter provides a digital display, but has no means to provide histoπcal data or detailed information beyond indication of present cunent magmtude

U S patent 4,466,039 to Moran and Reis (1984) discloses an "Open Circuit Cunent Transformer Protection

Circmt " A voltage-hπuύng circuit includes a tπac connected across a cunent transformer secondary winding with suitable control means to tπgger the tnac when voltage reaches a predetermined level

U S patent 4,471,300 to Harnden and Kornrumpf (1984) and U S patent 4,559,496, to Harnden and Kornnimpf (1985) disclose "LCD Hook-on Digital Ammeters" intended for low-cost consumer use Only present 6 current magmtude is displayed Voltages generated by a cunent transformer are used to directly dπve liquid crystal display segments, so a power supply circuit is not required

U S patent 4,698,740 to Rodgers and Stacey (1987) discloses a "Cunent Fed Regulated Voltage Supply " Several power supply vanations are shown, each deπving power from an input cunent generated by a cunent transformer Each of these configurations is similar to the typical pnor art discussed above The patent appears to be claiming an improvement over pnor art by utilizing a special type of cunent transformer that has a "saturable magnetic stacked joint core" and "a lumped secondary winding " This type of cunent transformer is designed to generate reduced secondary current dunng surge conditions on the electnc power system This allows some power supply components to have smaller ratings than would otherwise be practical U S patent 4,713,598 to Smith (1987) discloses a "Power Supply Associated with AC Line Relay Switch "

Control power is denved either from a cunent transformer connected in seπes with a load cunent or from connections to an electronically-controlled switch (the switch is also connected in seπes with the load cunent) When the switch is closed, load cunent flows and power is deπved from the cunent transformer secondary When the switch is open, voltage develops across the switch and power is denved from that voltage U S patent 5,539,300 to Mathieu (1996) discloses a "Power Supply Device" that is similar to typical pnor art discussed above The claims seem to emphasize preventing the magnetic core of the cunent transformer from going into saturation and thereby preventing audible noise The basic power supply configuration shown in FIG 3 of this patent is very similar to that shown in FIG 7 of U S patent 4,698,740 (see above)

U S patent 5,552,978 to Moncorge (1996) discloses a "Device for Supplying a Voltage to an Electronic Circuit, In Particular an Electronic Circmt Associated with a Cunent Sensor Disposed on an Electncal Line " The power supply disclosed is similar to typical pnor art discussed above, but with an improvement to quickly charge the power supply upon restoration of power after an outage This facilitates q ck operation (after restoration of power) of power system protective eqmpment that is dependent upon devices that are powered by the power supply Cunent is sensed by a sensing circuit that is completely independent of the power supply input cunent U S patent 5,598,315 to Phillips (1997) discloses a "Self-Power Tnppmg Relay with Balanced Power Supply

Current and Measurement Cunent " This power supply and sensing anangement is intended pπmanly for three- phase circmt breaker tnppmg circuits Half of each current cycle is used to charge the power supply, while the other half-cycle is used to sense mput current This patent is an improvement over previous similar patents as the voltage developed dunng each half cycle has been balanced better to improve overall operation However, cunent-sensing accuracy is adversely affected by high secondary burden on the cunent transformers Enor conection curves are presented that show the difference between actual cunent and sensed cunent

U S patent 5,687,068 to Jamieson (1997) discloses a "Power Supply for In-Line Power Controllers and Two- Terminal Electronic Thermostat Employing Same " Control power is denved either from a cunent transformer connected m senes with a load cunent or from a voltage transformer connected across an electronically-controlled switch (the switch is also connected in seπes with the load cunent) When the switch is closed, load cunent flows and power is denved from the cunent transformer When the switch is open, voltage develops across the switch and power is denved from the voltage transformer

The above patents illustrate pnor art regarding self-powered cunent momtonng, power supplies with cunent sources, and input cunent-sensing circuits integral to some of the same power supplies However, none of them fulfill the objects of the self-powered cunent momtor invention descnbed herein

Disclosure of Invention

The invention descnbed herein is a "self-powered cunent momtor" that momtors the charactenstics of one or more mput currents, and denves operating power from one or more of the same input cunents Visual indication of current charactenstics may be provided by a local display, or data may be commumcated to other eqmpment for further processing The input currents will usually be generated by one or more cunent transformers external to the cunent monitor, though other types of cunent sources may also be utilized to generate input cunents The cunent transformers may be almost any type commonly utilized in electnc power systems

The current momtor's internal power supply is designed to minimize the burden effect that the power supply has on the input cunent source This is done through one or both of the following methods (a) An internal transformer may be utilized to better match the power supply input requirements to the characteπstics of typical current transformer secondary windings This reduces the burden effect of the power supply compared to the burden effect of the power supply without the internal transformer

(b) A burden-reducing circuit may be included to sharply reduce the burden effect that the power supply has on the mput current source for bπef peπods while mput current is being sensed While the input cunent is being sensed, the power supply utilizes an energy storage device (such as a capacitor) to continue supplying operating power to the rest of the cunent momtor

The current momtor penodically senses the waveform of each mput cunent While each input cunent is being sensed, the input cunent passes through a precision resistor with low resistance The voltage across this resistor is proportional to the input cunent (in accordance with Ohm's Law) This analog voltage signal is amplified (if necessary) and apphed to the mput of an analog-to-digital converter circmt The analog-to-digital converter circuit provides digital data to a microprocessor The microprocessor analyzes the digital waveform data and calculates vanous charactenstics of the input cunent Calculation results are displayed on a local display or commumcated to other eqmpment via a suitable interface circuit A memory circmt is included to store working data and program data A user interface circuit allows the user to modify operating parameters and modify how data is displayed Input current characteπstics that are calculated by the cunent momtor may include any parameter that can be calculated from one-cycle waveforms of the input cunents, including the root-mean-square (RMS) magmtude, average magmtude, peak magmtude, magmtudes of harmonic frequency components, percentage of harmonic distortion, form factor, crest factor, equivalent k-factor for transformers with harmonic rating, and graphical representation of the waveform Data mav be logged at regular intervals for future reference The cunent momtor also calculates cunent demand based on peπodic one-cycle samples of the input cunents Maximum cunent demand is stored for future reference

Some objects and advantages of the present invention are

(a) to provide a self-powered current momtor that will provide visual indication of vanous characteπstics of one or more electnc cunents, including maximum cunent demand and other histoncal data, as well as detailed information about present cunent, including nonlinear cunent charactenstics,

(b) to provide a self-powered cunent momtor that will communicate vanous charactenstics of one or more electnc cunents to other eqmpment,

(c) to provide a self-powered current momtor that can utilize inexpensive cunent transformers as sources of input cunents and still maintain good accuracy,

(d) to provide a power supply circmt that includes a burden-reducing means to temporanly reduce the burden effect that the power supply has on input cunent sources dunng peπodic sensing of input cunents, and

(e) to provide a power supply circuit that can deπve operating power from any of several input cunents while providing means to sense these same input cunents Further objects and advantages will become apparent from a consideration of the drawings and ensuing descπption

Brief Description of Drawings

FIG. 1 shows a general schematic configuration of the self-powered cunent monitor The power supply is shown as a block Other figures will show prefened embodiments for the power supply in more detail FIG. 2A shows a power supply and cunent-sensing configuration utilizing internal transformers to facilitate deπving power from two input cunents A burden-reducing circmt is included

FIG. 2B is similar to the nght half of FIG. 1 When FIG. 2B is combined with FIG. 2A, the entire cunent momtor is shown with power supply details not shown in FIG. 1 FIG. 2B may also be laid beside FIG. 4 to show the entire cunent momtor with a different power supply and cunent-sensing configuration FIGS.3A through 31 show typical operating waveforms associated with the power supply and current-sensing configuration of FIG. 2A

FIG. 4 shows a power supply and cunent-sensing configuration that may be used instead of Fig. 2A. Denvation of power and current-sensing is accomplished without internal transformers A burden-reducing circuit is included The input cunent waveform can only be sensed while the burden-reducing circuit is actuated FIGS. 5A through 51 show typical operating waveforms associated with the power supply and cunent-sensing configuration of FIG. 4 9 FIGS. 6 A and 6B show a flowchart to illustrate the operation of the microprocessor

Modes for Carrying Out the Invention

FIG. 1 shows a block diagram illustrating the general configuration of the self-powered cunent momtor 1 A power supply circuit 3 denves power from an input cunent J2 and provides regulated d-c (direct-current) power to other circuits A burden-reducing circmt 2 may be included, shown here as an integral part of power supply 3 Power supply circmt 3 and burden-reducing circuit 2 are shown in more detail in subsequent figures and are discussed further in the discussion relating to those figures

A cunent transformer CT1 acts as a cunent source to generate input cunent J2 Cunent transformer CT1 is usually external to current momtor 1 and may be almost any type of cunent transformer (or other cunent source), but will normally be a torotdal type or split-core type of cunent transformer installed around a cunent-caπymg conductor An a-c system cunent Jl flows in the cunent-carrymg conductor as part of a larger electnc power system A-c system current Jl is alternating at the electnc power system frequency (typically 50 or 60 hertz) A-c system current Jl causes input cunent J2 to flow by the transformer action of cunent transformer CT1 Input current J2 is proportionally smaller than a-c system cunent Jl by the turns ratio of cunent transformer CT1 The secondary winding of cunent transformer CT1 is connected to cunent momtor 1 at terminals 21 and 22 by short lengths of insulated wire

Current momtor 1 senses input cunent J2 for bπef time peπods several times per minute Dunng these sensing penods, mput cunent J2 is routed through resistor Rl and causes an analog signal across resistor Rl that is a voltage signal proportional to input cunent J2 (in accordance with Ohm's Law) The cunent through resistor Rl is labeled cunent J2A because it may not be equal to input cunent J2 dunng normal power supply charging cycles Whether or not currents J2 and J2A are always equal depends on the particular power supply and cunent- sensing configuration

The voltage signal across resistor Rl is conducted by conductor VI to resistor R2 and is applied to the input of amplifier circmt 4 by conductor V9 Resistor R2 and zener diodes Dl and D2 are included to limit the maximum voltage mput to amplifier circuit 4 Without these, high voltages across resistor Rl caused by high input cunents (resulting from short-circuit cunents or other surge cunents in the electnc power system) may damage amplifier circmt 4 Zener diodes Dl and D2 will conduct only if the voltage signal across resistor Rl exceeds their reverse breakdown voltage rating Resistor R2 will limit cunent flow through zener diodes Dl and D2 to a safe operating value The resistance of resistor R2 must be relatively small compared to the input resistance of amplifier circuit 4 so that the voltage signal transferred from resistor Rl to amplifier circmt 4 will not be significantly reduced under normal operating conditions

Similar to input cunent J2, an optional second input cunent J7 is shown generated by a cunent transformer

CT2 with an a-c system current J6 Cunent transformer CT2 connects to the cunent momtor at terminals 23 and

24 A-c system current J6 causes mput current J7 to flow which is proportionally smaller than J6 by the turns ratio of current transformer CT2 As shown m FIG. 1 , this second mput is not configured to provide power to the cunent 10 monitor (FIGS. 2A and 4, to be discussed later, show alternative configurations that utilize this second input cunent as a second source of power)

A resistor R3 is m seπes with mput current J7 and thus develops a voltage signal across it that is proportional to input cunent J7 A resistor R4 and zener diodes D3 and D4 are included to limit the maximum voltage input to amplifier circmt 4 (similar to resistor R2 and zener diodes Dl and D2 for input cunent J2) The voltage signal across R3 is conducted from resistor R4 to amplifier circmt 4 by conductor V10

Circuits for additional mput cunents may be provided similar to the input configuration for input cunent J7 Only two inputs are shown for simplicity of illustration Alternatively, additional input cunents may be configured as additional inputs to power supply 3, similar to the input configuration for input cunent J2. Other types of current-sensing circuits may be used, such as an active load termination as descnbed in reissued

U S patent Re 28,851 to Milkovic (reissued 1976) (see above under "Background Art— Related Patents") While other types of current-sensing circuits may be utilized, simple resistors will be shown throughout this disclosure for simplicity of illustration and operation

Amplifier circmt 4 amplifies the voltage signals across resistors Rl and R3 to levels smtable for input to analog-to-digital converter circmt 5. Amplifier circmt 4 may be omitted if resistors Rl and R3 are sized so that the voltages across them are smtable for direct input to analog-to-digital converter circmt 5 (less expensive cunent transformers may not be rated for the increased burden resulting from higher resistances required to eliminate amplifier circmt 4) Amplifier circmt 4 has a separate amplifier for each input A detailed descnption of amplifier circmt 4 is not included herein since it involves only pnor art

Analog-to-digital converter crrcuit 5 penodically performs a senes of samples of the voltage signals across resistors Rl and R3 and converts these voltage signals into digital data when requested by microprocessor 6 The sampling frequency must be fast enough to clearly define any significant nonlineanties present in the voltage signal waveforms Approximately 100 samples for one waveform cycle is adequate for typical power system cunent waveforms The digital data is transmitted to microprocessor 6 for analysis Analog-to-digital converter circmt 5 has a separate analog mput for each input cunent The digital data may be commumcated to microprocessor 6 via senal or parallel communication A detailed descnption of analog-to-digital converter circmt 5 is not included herein since it involves only pnor art

Optionally, amplifier circuit 4 could be provided with multiple inputs and a single output connecting to analog- to-digital converter circuit 5 This requires a communication connection between the amplifier circuit 4 and microprocessor 6 so that microprocessor 6 can control which mput is selected However, analog-to-digital converter integrated circuits are commonly available with multiple inputs, so this optional configuration is not prefened

Microprocessor 6 is programmed to analyze the digital data and produce output data for visual display by display circmt 7 Microprocessor 6 may also actuate burden-reducing circuit 2 while analog-to-digital converter circuit 5 is sampling the input across resistor Rl Microprocessor 6 also interfaces to memory circmt 9, user interface circuit 10, and interface circuit 8 Operation of microprocessor 6 is explained in more detail in the 11 discussion for .FIGS. 6A and 6B A detailed descnption of microprocessor 6 is not included herein since it involves only pnor art

Display circmt 7 includes a liquid crystal display to minimize power consumption The presently prefened embodiment includes onlv texmal output to keep the cost of the cunent momtor down A graphic display mav be used if it is desired to display waveforms or other data graphically, though this will increase the cost A detailed descnption of display circuit 7 is not included herein since it involves only pnor art

User interface circmt 10 consists of pushbuttons or similar user interface means connected to microprocessor

6 to allow the user to set parameters and control what is displayed In its simplest form it consists of several pushbutton-type switches each connected to an mput terminal of microprocessor 6 In this form a direct connection to the power-distnbuting circmt may not be required A detailed descnption of user interface circmt 10 is not included herein since it involves only pnor art

Memory circuit 9 contains program data, cunent demand data, and other working memory for use by microprocessor 6 Program data may be stored in permanent Read Only Memory (ROM), wlule cunent demand data may be stored m Electncally Erasable Programmable Read Only Memory (EEPROM) or Static Random Access Memory (SRAM) with battery backup Some SRAM (with or without battery backup) is used by microprocessor 6 for calculations, communication, and display functions Part or all of the memory circmt may be integral to the microprocessor circmt This is especially the case if the microprocessor is implemented with a "microcontroller" type of integrated circmt A detailed descnption of memory circmt 9 is not included herein since it involves only pnor art Interface circuit 8 is included to allow communication to external eqmpment 11 so that the cunent momtor may be installed as part of a larger momtonng system or in locations not normally accessible Part or all of this interface circmt 8 may be integral to microprocessor 6 This is especially the case if microprocessor 6 is implemented with a "microcontroller" type of integrated circuit The communication means may be any established method, including electnc signals on metallic wires or cables, radio waves (or other forms of electromagnetic radiation) through air or other medium, or light waves through air or optical fiber A detailed descnption of interface circuit 8 is not included herein since it involves only pnor art

Conductor V0 is the common bus for the power supply circuits, signal circuits, and logic circuits Conductor V5 is the power supply positive regulated voltage bus Conductor V6 is the power supply negative regulated voltage bus Together, these conductors compnse a power-distnbuting circuit to supply regulated d-c power to the vanous subsystems of the cunent momtor The power supply output voltages for the prefened embodiment are plus and minus five volts The power supply circuit may be configured for other voltages or additional voltages depending on the requirements of the vanous subsystems receiving power

Burden-reducing circmt 2 is controlled by a signal from microprocessor 6 Conductor V7 conducts the burden- reducing control signal from microprocessor 6 to actuate burden-reducing circmt 2 while input cunent J2 is being sensed Actual sensing of input cunent J2 may be delayed for a few cycles after burden-reducing circuit 2 is 12 activated to allow any transient d-c offset cunents present in input cunent J2 to decay, so that accuracy will not be adversely affected

If mput current J2 falls below a predetermined value, power supply 3 will not be able to function reliably A "charge status" signal is generated by power supply 3 to alert microprocessor 6 whenever stored energy in power supply 3 is not adequate to support continued processing Conductor V8 conducts the charge status signal from power supply 3 to microprocessor 6

To facilitate operation at low input cunent levels, the vanous circuit components of the self-powered cunent momtor are chosen to req re minimal power from power supply circuit 3

Many of the subsystem blocks shown m FIG. 1 are optional Those that are not optional include power supply circmt 3, analog-to-digital converter circmt 5, microprocessor circmt 6, and memory circuit 9 .Also, either display circmt 7 or mterface circmt 8 is necessary for the cunent momtor to be able commumcate data visually or by other means

FIG. 2A shows one possible embodiment of power supply circmt 3 and cunent-sensmg circuits in more detail than FIG. 1 In FIG. 2A, both mput currents are configured to provide power to the Self-Powered Cunent Momtor .FIG. 2B is similar to the nght side of FIG. 1 and may be laid beside FIG. 2A to show the complete cunent momtor

Components in FIGS. 2A and 2B that are also shown in FIG. 1 function as descnbed previously in the discussion for FIG. 1 The following discussion for FIG. 2A will focus on operation as it relates to input cunent J2 The conesponding circmts for input cunent J7 operate in a similar manner

An internal transformer TX1 is internal to the cunent momtor and is included to better match the input requirements of the power supply to the output characteπstics of cunent transformer CT1 Standard cunent transformers usually have zero to five amp secondary outputs with little capacity to generate sustained voltages The power supply, on the other hand, needs relatively little cunent but relatively high voltage Transformer TX1 helps keep the secondary voltage V2 of current transformer CT1 low while providing a higher voltage V3 for power supply operation

The use of an internal transformer to facilitate self-powered operation is not new A similar configuration is used as part of the "Self-Powered Ammeter" disclosed in U S Patent 4,422,039 to Davis (1983) as discussed above under "Background Art — Related Patents "

Internal transformer TX1 basically acts like a cunent transformer with excessive burden in the secondary circuit Input cunent J2 flows through the pnmary winding of internal transformer TX1 and causes secondary cunent J3 to flow Cunent J3 is smaller than cunent J2 by the turns ratio of internal transformer TX1

Field-effect transistors Ql and Q2 are part of a "burden-reducing" circmt that disables the power supply for bπef peπods of time and reduces the burden on cunent transformer CT1 Field-effect transistors Ql and Q2 are 13 enhancement-mode devices that act like open circuits when gate-to-source voltage is near zero, and start conducting as gate-to-source voltage is increased The devices used have very low on-state drain-to-source resistance to reduce the power supply burden effect dunng sensing of input cunent J2 Additionally, the devices chosen have sensitive gates for operation at low voltage levels (approximately five volts) They are configured to function as an electronic switch Resistor R7 is mcluded to provide a discharge path for charge on the gates of field-effect transistors Ql and Q2 when the burden-reducing control signal on conductor V7 goes low

When the burden-reducing control signal on conductor V7 is high, field-effect transistors Ql and Q2 are switched on (in saturation), and secondary cunent J3 is shunted through field-effect transistors Ql and Q2 The circmt connected to the secondary of internal transformer TX1 becomes a very low resistance This resistance is the sum of the on-state resistances of field-effect transistors Ql and Q2 This resistance (plus transformer TX1 winding resistance) appears from the pnmary side of the transformer to be even smaller, by a factor that is the square of the turns ratio of internal transformer TX1 This sharply reduces the burden effect of the power supply circmt on cunent transformer CT1

When the burden-reducing control signal on conductor V7 is low, field-effect transistors Ql and Q2 are switched off, and the dram-source diodes within field-effect transistors Ql and Q2 combined with diodes D5 and D6 form a full-wave bπdge rectifier that rectifies secondary cunent J3 Rectified cunent J5 charges capacitor Cl, which acts as a charge-stonng device This results in an imregulated positive d-c voltage on bus V4 (unless otherwise stated, voltages mdicated are relative to common bus VO) Voltage regulator 15 converts this unregulated voltage to a positive regulated voltage on bus V5 Capacitor C2 helps stabilize the positive regulated voltage Voltage inverter 16 generates a negative regulated voltage on bus V6 Capacitor C3 helps stabilize the negative regulated voltage

Voltage regulator 15 may be a linear type of regulator, or a switching type of regulator to improve efficiency A detailed descnption of voltage regulator 15 is not included herein since it involves only pnor art

Voltage inverter 16 may be implemented with a standard integrated circuit along with an external capacitor utilized as part of a charge pump circuit A detailed descnption of voltage inverter 16 is not included herein since it involves only pnor art

The combination of voltage regulator 15, voltage inverter 16, and capacitors C2 and C3 act together as a voltage-regulating αrcmt that provides regulated d-c power from umegulated voltage provided by a charge-stonng circuit compπsmg capacitor Cl

There are many ways to deπve regulated d-c power from an umegulated d-c voltage source The configuration shown m FIG. 2A (utilizing voltage regulator 15 and voltage inverter 16) is just one of many configurations possible utilizing pnor art This same configuration is utilized in FIG. 4 for simplicity of illustration It is not the intent of this disclosure to limit the power supply to the components illustrated

Voltage detector circmt 17 and zener diode D17 are configured to send a charge status signal to the microprocessor The charge status signal earned by conductor V8 is dπven high whenever the voltage on capacitor 14

Cl is above a predetermined value A high charge status signal indicates that stored energy in the capacitor is adequate to provide power to the cunent momtor for the next seπes of operations (see FIG. 6 flowchart) Three- terminal voltage detectors are commonly available in ratings from approximately two to six volts Zener diode D17 is mcluded to raise the effective detected voltage to a level higher than six volts A detailed descnption of voltage detector circmt 17 is not included herein since it involves only pnor art

Without some kind of voltage-limiting circmt, the voltage on unregulated voltage bus V4 could become excessively high Voltage-limiting circuit 13 has silicon-controlled rectifiers, diodes, zener diodes, and resistors configured to function as an electronic switch that limits the voltage produced by the secondary winding of internal transformer TX1 While capacitor Cl is charging, the magmtude of the transformer secondary voltage V3 is equal to the unregulated voltage on bus V4 plus the forward voltage drops of two diodes in the fiill-wave bndge rectifier (the full-wave bπdge rectifier compπsed of diodes D5 and D6, and the drain-source diodes within field-effect transistors Ql and Q2) As the unregulated voltage on bus V4 increases, a voltage level is reached that tnggers the gate circuit of silicon-controlled rectifier SCR1 or SCR2 After silicon-controlled rectifier SCR1 or SCR2 is tnggered, secondary current J3 is shunted through the silicon-controlled rectifier so secondary voltage V3 collapses to the forward voltage drop of the silicon-controlled rectifier Cunent J4 is the excess part of secondary cunent J3 that is shunted by voltage-limiting circuit 13 As soon as a silicon-controlled rectifier is tnggered, charging of capacitor Cl stops for the remainder of the half-cycle

Silicon-controlled rectifier SCR1 limits voltage dunng the positive half-cycle, while silicon-controlled rectifier

SCR2 limits voltage dunng the negative half-cycle Zener diodes D8 and D10 determine the voltage level at which the silicon-controlled rectifiers tngger Resistors R5 and R6 assure that the silicon-controlled rectifiers will not trigger on diode leakage cunents Diodes D7 and D9 prevent reverse bias conditions on the silicon-controlled rectifier gates

In the presently preferred embodiment, internal transformer TX1 compnses a toroidal saturable magnetic core with approximately ten times as many secondary winding turns as pnmary winding turns This results in secondary current J3 being approximately 10% as large as input cunent J2 More important, the voltage induced in the pnmary circmt of internal transformer TX1 is only approximately 10% of secondary voltage V3 This reduces the peak burden effect of the power supply on cunent transformer CT1 by a factor often

The ability of internal transformer TX1 to generate secondaiy voltage is proportional to the core cross sectional area and the number of secondary winding turns A good design must balance the desire for a small core against the practicality of windings with more turns The presently prefened embodiment utilizes a toroidal core made of grain-onented silicon steel with a cross sectional area of approximately two square centimeters, with ten pnmary winding turns and one hundred secondary winding turns

A failure protection circmt 28 shown m FIG. 2A has no effect on operation under normal operating conditions

However, if there is a failure of voltage-limiting circuit 13, the charging voltage peaks of secondary voltage V3 will increase and activate failure protection circuit 28 As secondary voltage V3 increases past the reverse breakdown voltage of zener diode D13. full-wave bπdge rectifier BR2 conducts cunent to charge capacitor C4 When the 15 charge on this capacitor reaches a predetermined level, the latching coil 25 of control relay CR1 closes control relay contact 27 The closure of contact 27 .safely short-circuits the secondary winding of internal transformer TX1, thus disabling the power supply The power supply remains disabled until the failure is repaired and unlatch coil 26 is energized by external means Ground connection GND is included to provide a stable voltage reference

The power supply configuration shown in FIG. 2A is configured to deπve power from input cunent J7 in addition to input cunent J2 This provides power for operation of the cunent monitor dunng times when input cunent J2 is not large enough to charge the power supply Any number of inputs may be configured in a similar way to provide charging power to the power supply circmt

Operation of power supply and sensing circuits for mput current J7 is similar to the circuits discussed for input current J2 Internal transformer TX2 functions to provide charging cunent to the power supply in a similar manner as internal transformer TX1 Voltage-limiting circuit 14 functions to limit the secondary voltage of internal transformer TX2 in the same manner that voltage-limiting circmt 13 functions to limit the secondary voltage of internal transformer TX1 Failure protection circuit 29 protects against a failure of voltage-limiting circuit 14 in the same manner that failure protection circmt 28 protects against a failure of voltage-limiting circuit 13 Rectifying and burden-reducing circmt 20 functions m a similar manner for input cunent J7 as field-effect transistors Ql and Q2 and diodes D5 and D6 function for input cunent J2

FIG. 2B, as previously mentioned, is similar to the nght side of FIG. 1 (see the discussion for FIG. 1)

FIGS. 3A through 31 illustrate typical operating waveforms of the power supply and cunent-sensing configuration shown in FIG. 2A For simplicity of illustration, a-c system cunent J6 is assumed to be zero The waveforms shown correlate to a-c system current Jl and corresponding mput cunent J2 Voltage waveforms shown are relative to common bus V0

FIG. 3A shows a typical waveform for a-c system cunent Jl A simple sine wave is shown for simplicity of illustration In actual applications the wavefoπn may show considerable harmonic distortion FIG. 3B shows mput current J2, which is the current generated by cunent transformer CT1 This shows some distortion when compared to Jl, as it is assumed that cunent transformer CT1 is adversely affected by the burden effect of the power supply in the secondaiy circuit The waveform magmtudes are normalized for clanty of illustration The magmtude of input cunent J2 is actually many times smaller than a-c system cunent Jl (proportional to the turns ratio of cunent transformer CT1) FIG. 3C shows the burden-reducing control signal on conductor V7 At time T4, the microprocessor dnves this voltage signal high to actuate the burden-reducing circuit to begin accurately sensing input cunent J2 The burden-reducing control signal remains high until the microprocessor turns it off at time T5, at which time the power supply resumes its normal charging cycle

FIG.3D shows internal transformer TX1 secondary voltage V3 As input cunent J2 begins a new half-cycle 16 at time Tl, secondary voltage V3 quickly increases At time T2 secondary voltage V3 reaches the level required to start charging capacitor Cl Between times T2 and T3 capacitor Cl is charging, and secondary voltage V3 follows the chargmg voltage on unregulated voltage bus V4 (see FIG. 3H) At time T3 silicon-controlled rectifier SCR1 tnggers, and secondary voltage V3 collapses until another half-cycle begins The charging pulses are seen to stop while the burden-reducing control signal on conductor V7 is high between times T4 and T5 Dunng this time, cunent J3 is shunted through field-effect transistors Ql and Q2 Secondary voltage V3 between times T4 and T5 is just the voltage caused by current J3 through the on-state resistance of field-effect transistors Ql and Q2

The voltage across the primary of internal transformer TX1 is similar to secondary voltage V3, but it is reduced by the turns ratio There will also be some additional sinusoidal voltage on the pnmary side of internal transformer TX1 caused by the resistance of the pnmary and secondary windings of internal transformer TX1

FIG. 3E shows internal transformer TX1 secondary cunent J3 The distortion due to the nonlinear burden effect of the power supply chargmg cycle is seen to stop between times T4 and T5 while the burden-reducing circmt is actuated

FIG. 3F shows cunent J4 into voltage-limiting circuit 13 Voltage-limiting circmt 13 is seen to be inactive while the burden-reducing circmt is actuated between times T4 and T5

FIG. 3G shows charging cunent J5 flowing to unregulated voltage bus V4 Charging cunent J5 is seen to stop while the burden-reducing circuit is actuated between times T4 and T5

FIG. 3H shows the voltages on umegulated voltage bus V4, positive regulated voltage bus V5, and negative regulated voltage bus V6 (these voltages are all relative to common bus VO) The voltage on unregulated voltage bus V4 is seen to decrease significantly while the burden-reducing circmt is actuated between times T4 and T5

This illustrates how capacitor Cl contnbutes stored energy to voltage regulator 15 to maintain regulated voltage output while the burden-reducing circuit is actuated between times T4 and T5

FIG. 31 shows the voltage signal on conductor VI (the voltage across resistor Rl, the amplifier input signal) Signal distortion is seen to be reduced while the burden-reducing circmt is actuated between times T4 and T5 FIG. 4 illustrates a power supply and cunent-sensing configuration with no internal transformers See the above discussion for FIG. 2A for operation of circuits that are common to FIGS. 2 A and 4 Operation as it relates to mput current J2 will be the focus of the following discussion Operation relating to input cunent J7 is similar

Similar to FIG. 2 A, field-effect transistors Ql and Q2 are part of a "burden-reducing" circmt that disables the power supply for bπef penods of time and reduces the burden on cunent transformer CT1 Sensing resistor Rl is now connected between field-effect transistors Ql and Q2 to sense input cunent Input cunent J2 flows through sensing resistor Rl whenever the burden-reducing circuit is actuated The resistance of resistor Rl is small, so that the voltage signal across resistor Rl does not significantly reduce the gate-source voltage of field-effect transistor Ql when the gate voltage on conductor V7 is high 17 Since cunent transformer CT1 must generate the full power supply charging voltage as configured in FIG. 4, secondary cunent J2 is subject to greater distortion than previous configurations While this configuration will work with most standard current transformers, operation is better with current transformers that have a large number of secondary winding turns The preferred embodiment utilizes external cunent transformers with secondary ratings of only one amp rather than five amps This increases the number of secondary winding turns by a factor of five For current transformers with similar cores, the ability of the cunent transformer to generate secondary voltage is also increased by a factor of five

Because of the relatively high sustained secondary voltages that cunent transformer CT1 must generate, there is a tendency for current transformer CT1 to operate at or near magnetic saturation of the core This can affect the accuracy of secondary cunent J2 even while the burden-reducing circuit is actuated To minimize this problem, the burden-reducing circmt should be actuated when the magnetic core of cunent transformer CT1 is not near saturation This may be done by actuating the burden-reducing circmt shortly after a power supply charging pulse begins, near a waveform zero crossing However, since input cunent J2 only flows through sensing resistor Rl while the burden-reducing circmt is actuated, the microprocessor has no easy way to determine the beginmng of a power supply charging pulse or a waveform zero crossing To solve this problem, resistors R8 and R9 and zener diodes Dll and D12 have been added in FIG. 4 These act to attenuate and conduct the peaks of the charging pulses to the amplifier circuit via conductor V9 These charging pulses are then commumcated to the microprocessor via the analog-to-digital converter circmt This enables the microprocessor to determine the beginmng of the power supply charging pulses, and to actuate the burden-reducing circuit at an optimal time With the charge pulse sensing circuit (resistors R8 and R9 and zener diodes Dll and D12) included as shown in FIG. 4, it is possible to eliminate voltage detector 17, with the microprocessor verifying power supply charge state from the digital data provided by the analog-to-digital circmt The peaks of the charging pulses conelate to the voltage on unregulated voltage bus V4 The added program complexity, however, may not make this change preferable

The power supply configuration shown in FIG. 4 is configured to denve power from input cunent J7 in addition to input cunent J2 This provides power for operation of the cunent momtor dunng times when input current J2 is not large enough to charge the power supply (similar to FIG. 2A) Any number of inputs may be configured in a similar way to provide charging power to the power supply circuit

Operation of power supply and sensing circuits for input current J7 is similar to the circuits discussed for input current J2 Voltage-limiting circuit 14 functions to limit the secondary voltage of cunent transformer CT2 in the same manner that voltage-limiting circuit 13 functions to limit the secondary voltage of cunent transformer CT1 Failure protection circmt 29 protects against a failme of voltage-limiting circmt 14 in the same manner that failme protection circuit 28 protects against a failure of voltage-limiting circuit 13 Rectifying, burden-reducing, and current-sensing circmt 19 functions in a similar manner for input cunent J7 as the equivalent circuit (compnsing field-effect transistors Ql and Q2, diodes D5 and D6, zener diodes Dll and D12, and resistors Rl, R2, R8 and R9) functions for input cunent J2 18

A single bmden-reducing control conductor (V7) is shown in FIGS. 2A, 2B and 4 to control the burden- reducing function for both input cunents The bmden-reducing function could be separately controlled for each input current with separate signals from the microprocessor Similarly, the burden-reducing function could be controlled separately or together for any number of inputs (if the microprocessor has an adequate number of outputs available for individual control)

FIGS. 5A through 51 illustrate typical operating waveforms of the power supply and cunent-sensing configuration shown in FIG. 4 Operation is similar to FIGS. 3A through 31 The present discussion will focus on waveforms that are different from previous discussions Similar to previous figures, the waveform magmtudes are normalized for clanty of illustration, and voltages are relative to common bus VO FIG. 5 A shows a typical waveform for a-c system cunent Jl (the same as FIG. 3 A)

FIG. 5B shows the bmden-reducing control signal on conductor V7, similar to FIG. 3C At time T4, the microprocessor dπves this voltage signal high to actuate the bmden-reducing circuit to begin accurately sensing mput current J2 The burden-reducing control signal remains high until the microprocessor turns it off at time T5, at which time the power supply resumes its normal charging cycle

FIG. 5C shows cunent transformer CT1 secondary voltage V2 This waveform is similar to internal transformer TXl secondary voltage V3 m FIG. 3D The chargmg pulses are seen to stop while the bmden-reducing control signal V7 is high between times T4 and T5 Dunng this time, cunent J2 is shunted through field-effect transistors Ql and Q2 Secondary voltage V2, between times T4 and T5, is just the voltage caused by input cunent J2 through resistor Rl plus the on-state resistances of field-effect transistors Ql and Q2 FIG. 5D shows mput current J2, which is the secondary cunent generated by cunent transformer CT1 This now shows considerable distortion when compared to Jl, as it is now assumed that cunent transformer CT1 is seriously affected by the burden effect of the power supply m the secondary circuit The distortion due to the nonlinear burden effect of the power supply is seen to stop between times T4 and T5 while the bmden-reducing circmt is actuated FIG. 5E shows cunent J4 into the voltage-limiting circmt 13 (similar to FIG. 3F) Voltage-limiting circuit

13 is seen to be inactive while the burden-reducing circuit is actuated between times T4 and T5

FIG. 5F shows chargmg current J5 flowing to unregulated voltage bus V4 (similar to FIG. 3G) The charging cunent is seen to stop while the bmden-reducing circuit is actuated between times T4 and T5

FIG. 5G shows the voltages on umegulated voltage bus V4, positive regulated voltage bus V5, and negative regulated voltage bus V6 (these voltages are all relative to common bus V0) Waveforms are similar to FIG. 3H

FIG. 5H shows the voltage signal on conductor VI (the voltage across resistor Rl) Input cunent J2 is seen to flow through resistor Rl whenever the burden-reducing circuit is actuated The small pulses before time T4 and after time T5 are caused by the chargmg current pulses that pass through the internal diode of field-effect transistor Ql 19 FIG. 51 shows the voltage signal on conductor V9 that is input to the amplifier circuit Between times T4 and T5 this is the same as the voltage signal on conductor VI (FIG. 5H) Before time T4 and after time T5 the peaks of the power supply charging pulses are present due to the addition of zener diodes Dll and D12 and resistors R8 and R9 (as discussed above for FIG. 4) FIGS. 6A and 6B show a flowchart that illustrates the operation of microprocessor 6 (see FIG. 1) The microprocessor automatically starts at the "power up" operation 31 when regulated d-c power at the terminals of the microprocessor is first sufficient for operation

Before proceeding, the charge state of the power supply is verified to be adequate as shown at operation 32 Adequate charge on power supply capacitor Cl is commumcated to the microprocessor via conductor V8 (see FIGS. 1, 2.A, 2B .and 4) The microprocessor waits until power supply charge is adequate before proceeding This ensures adequate power to complete the next senes of operations

Once the power supply is adequately charged, the microprocessor checks system and data integnty as indicated by operation 33 These are standard diagnostic tests based on pnor art

Operation 34 sets vanable Nl to the "input channel quantity," which is the number of input cunents to be momtored (the circmts associated with each input cunent are each assigned an input channel number) This parameter may be modified by user mput, and the microprocessor merely loads this value from nonvolatile memory

Operation 35 resets vanous demand calculation parameters This includes a demand interval timer, which is reset to zero

Operation 36 sets loop control vanable "N" to a value of 1 Vanable N controls which input cunent (or "input channel") the cunent momtor is presently sensing and performing calculations on

Operation 37 again checks the status of the power supply This ensures adequate power to complete the next senes of operations

Operation 38 branches to a user input mode if a user interface pushbutton has been activated

Operation 39 modifies vanous operational parameters as directed by user input These include such items as the number of active input channels, and winding ratios of external cunent transformers Histoncal data (such as maximum cunent demand) may be erased (or reset to zero) if so directed by user input

Operation 40 again checks the status of the power supply This ensures adequate power to complete the next senes of operations

Operation 41 displays vanous stored data as directed by user input Special calculations not routinely performed by operation 44 may also be done here by user request

At the end of the user input mode the flowchart branches back to the beginmng Demand calculations are assumed to be corrupted bv taking time for user input so demand calculations are reinitialized 20 If there is no user input as of operation 38, the microprocessor moves to operation 42 The bmden-reducing circuit is actuated, and a seπes of waveform samples is obtained via the analog-to-digital converter circmt Approximately 100 samples are taken at uniform time intervals to define a complete waveform The bmden- reducing circuit is then turned off Operation 43 again checks the status of the power supply This ensures adequate power to complete the next senes of operations

Operation 44 mvolves numerous calculations to calculate vanous data for the waveform obtained by operation 42 These are standard calculations based on pnor art They may include such items as one-cycle peak cunent magmtude, average current magmmde, RMS cunent magmtude, crest factor, distortion factor, eqmvalent k-factor for harmonic-rated transformers, and calculation of harmomc frequency components Some of these calculations (such as harmomc calculations) may be performed only by user request (as part of operation 41) to minimize the calculation time required for each loop

Operation 45 again checks the status of the power supply This ensures adequate power to complete the next seπes of operations Operation 46 displays calculation results, and saves appropπate data This operation also transmits data to external equipment if applicable

The cunent demand calculations reqmre data over the entire demand calculation peπod (typically 15 or 30 minutes) Operation 47 updates this ongoing calculation with the latest one-cycle calculation results

Operation 48 increases the loop counter by one so the current momtor will sense and perform calculations on the next (higher numbered) input channel when the loop is repeated

Operation 49 checks to see if the loop counter is still at a valid mput channel number If "yes," the loop repeats and the next mput channel is sensed and waveform calculations are performed If "no," then operation 50 checks to see whether or not the demand timer has timed out If "no," the flow branches back to operation 36 where the loop counter is reset to "one" so all input channels will be sensed and calculated again If operation 50 finds that the demand timer has timed out, then cunent demand calculations are completed for all input channels by operation 52 (after the charge on the power supply has been venfied by operation 51)

Operation 53 saves appropnate cunent demand data to nonvolatile memory, displays demand data, and transmits demand data to remote equipment (if applicable) Then flow branches back to operation 35 where the demand calculations and demand interval timer are reset The cunent momtor continues momtonng cunent indefinitely, continually updating maximum cunent demand data and displaying present data calculated for each input channel

The flowchart shown in FIGS. 6A and 6B illustrates demand calculations for operation similar to an "integrated-demand" meter (as discussed under "Background Art — Related Devices") The cunent momtor may 21 also be programmed to calculate demand like a "sliding window integrated-demand" meter, or in a way that simulates a "lagged-demand" meter Of co se, any other type of demand calculation method may also be used

Industrial Applicability

An electnc cunent flowing in a conductor causes heating of the conductor Electnc energy is converted to thermal energy everywhere that electnc cunent is flowing (except in specialized equipment utilizing superconducting matenals) Generally speaking, the amount of thermal energy produced is proportional to the resistance of the conductor and to the square of the cunent magmmde Electnc heating equipment uses this principle to produce heat for beneficial purposes However, this same pnnciple is also at work throughout electnc power systems to produce undesirable heat and related energy loss Most electnc power system components have limited cunent ratings because of this heating effect of electnc current m conductors Proper design and operation of electnc power systems depends heavily on the magnitude of current conducted by cables and eqmpment To assist with safe operation, it is common practice to install ammeters and other cunent momtonng devices at vanous locations in electnc power systems

While voltages stay relatively constant in a well-designed electnc power system, cunent magmtudes are continually varying as loads are turned on and off Of particular interest is the maximum cunent that is maintained long enough to cause significant heating of conductors and eqmpment This "maximum cunent demand" can be calculated from either detailed information about each load connected to an electncal system, or it can be determined by momtonng actual load cunents

Maximum current demand is important, as this is the magmmde of maintained cunent that eqmpment must be able to safely handle Bnef cunent surges many times normal may occur as motor loads and other loads start and stop These bnef cunents will not, however, have much impact on maximum cunent demand due to their relatively short durations

For the above reasons, it is very desirable to momtor maximum cunent demand, and not just present cunent levels It is also desirable to momtor cunent waveform distortion, in addition to cunent magmtudes and maximum current demand In recent years, nonlinear loads connected to electnc power systems have greatly increased These nonlinear loads usually are the result of electromc power supplies within electncal eqmpment Nonlinear loads result in electnc cunent waveforms that are distorted — they are no longer simple sine waves The distorted waveforms are said to contain "harmonics "

Theoretically speaking, a distorted repeating waveform can be considered to be the sum of many perfect sine waves, with the frequency of each sine wave being a different multiple of the fundamental frequency (the fundamental frequency is the frequency that the combined waveform repeats, usually the power frequency of 50 or 60 hertz) Each of these perfect sme waves (except the fundamental sine wave) is called a "harmomc " Harmonics can adversely affect an electnc power system Momtonng harmomc cunents within electnc power systems is becoming more routine as problems caused by harmonics become more common 22

Existing current momtonng systems that are able to momtor cunent demand and harmonics generally reqmre a separate voltage source for operating power This adds to the cost of installation Also, cunent momtonng at some locations is impractical due to the lack of a suitable voltage source To eliminate this problem, a cunent momtor that denves operating power from one or more input cunents is desirable

Conclusions, Ramifications, and Scope

The foregoing descπbes a self-powered current monitor for economically momtonng cunent in electnc power systems The unit requires no external power supply connections, and is capable of displaying information relating to cunent magmmde. cunent demand, and cunent distortion (harmonics)

Prefened embodiments of the self-powered cunent momtor have been descnbed and illustrated There are many other embodiments possible that will be apparent to those skilled in the art There are many vanations possible regarding the anangement of vanous subsystems descnbed herein For example, the multiple burden- reducing circmts illustrated in FIGS. 2A and 4 (one for each input) could be reconfigured into a single burden- reducing circmt connected to chargmg capacitor Cl (with an additional diode to prevent discharge of capacitor Cl) There are also many vanations possible regardmg how each subsystem is implemented For example, there are other lands of semiconductor devices that could be utilized as electromc switches to accomplish the same functions as the silicon-controlled rectifiers and field-effect transistors discussed herein Likewise, there are many possible software implementations other than the one descnbed herem relating to FIG. 6 It is not the intent of this disclosure to limit the invention to the preferred embodiments that have been illustrated The components and configurations utilized in this disclosure are intended to be illustrative only, and are not intended to limit the scope of the appended claims While only certain prefened features of the invention have been shown by way of illustration, many modifications and changes will occur to those skilled in the art It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spint of the invention

Claims

23

Claims

What is claimed is

1 A self-powered cunent momtor for providing visual indication of charactenstics of an electnc cunent, said self-powered cunent momtor compnsmg (a) a current-sensing means, providing an analog signal containing information about said charactenstics,

(b) an analog-to-digital converter means, connected to said cunent-sensing means, converting said analog signal into digital data,

(c) a display means providing visual indication of said charactenstics,

(d) a memory means stonng working data and program data, (e) a microprocessor means, connected to said analog-to-digital converter means and to said display means and to said memoiy means, receiving said digital data, utilizing said program data and said working data to determine said charactenstics, and providing output smtable for said display means, and (f) a power supply means, connected in senes with said electnc cunent, denving regulated d-c power from said electnc cunent for operation of said self-powered cunent momtor, said electnc cunent being generated by a cunent source

2 The self-powered cunent momtor of claim 1 further compnsmg an interface means, connected to said microprocessor means, communicating data to other eqmpment, said microprocessor means further providing data to said interface means for communication to other equipment

3 The self-powered cunent momtor of claim 1 wherein said electnc current is an alternating electnc current and said charactenstics of said electnc cunent compπse one or more of root-mean-square magnitude, average magmtude, peak magmtude, harmomc component magmtudes, distortion factor, form factor, crest factor, equivalent k-factor for transformers with harmomc rating, graphical representation of the waveform, cunent demand, and maximum cunent demand

4 The self-powered cunent momtor of claim 1 wherein said electnc cunent is an alternating electnc cunent, said cunent-sensing means compnses a resistor connected in seπes with said electnc cunent dunng time peπods that said electnc current is being sensed thereby producing said analog signal which is a voltage signal across said resistor proportional to said electnc cunent, and said power supply means compnses

(a) a rectifying means producing direct cunent from said alternating cunent, (b) a charge-stonng means providing an unregulated d-c voltage from said direct cunent,

(c) a voltage-regulating means producing said regulated d-c power from said unregulated d-c voltage, and

(d) a voltage-limiting means shunting excess cunent away from said charge-stonng means thereby limiting said umegulated d-c voltage to a predetermined value, said voltage-limiting means 24 compnsmg one or more electromc switch means and suitable control means, said smtable control means actuating said electronic switch means to prevent said umegulated d-c voltage from exceeding said predetermined value

The self-powered current momtor of claim 1 further compnsmg a burden-reducing means to temporanly reduce the burden effect that said power supply means may have on said cunent source while said analog-to-digital converter means is sampling said analog signal, said bmden-reducmg means being connected to said power supply means and having a control signal connection to said microprocessor means, said power supply means utilizing stored energy to maintain regulated power output while said bmden-reducmg means is actuated, said microprocessor means further controlling the time penods that said burden-reducing means is actuated

6 The self-powered cunent momtor of claim 5 wherein said electnc cunent is an alternating cunent, said cunent-sensing means compnses a resistor connected in senes with said electnc cunent dunng time penods that said electnc cunent is being sensed thereby producing said analog signal which is a voltage signal across said resistor proportional to said electnc cunent, said power supply means compnses

(a) a rectifying means producing direct cunent from said alternating cunent,

(b) a charge-stonng means providing an unregulated d-c voltage from said direct cunent,

(c) a voltage-regulating means producing said regulated d-c power from said imregulated d-c voltage, and

(d) a voltage-limiting means shunting excess cunent away from said charge-stonng means thereby limiting said unregulated d-c voltage to a predetermined value, said voltage-limiting means compnsmg one or more electromc switch means and suitable control means, said smtable control means actuating said electromc switch means to prevent said umegulated d-c voltage from exceeding said predetermined value, and said bmden-reducmg means compnses one or more electromc switch means connected to said power supply means in such a way as to temporanly shunt said electnc cunent away from said charge-stonng means when so actuated by said microprocessor means, said voltage-regulating means utilizing energy stored in said charge-stonng means to produce said regulated d-c power while said bmden-reducing means is actuated

The self-powered cunent momtor of claim 1 further providing visual indication of characteπstics of a plurality of said electnc current, further compnsmg a plurality of said cunent-sensing means producing a plurality of said analog signal each containing information about one of said electnc cunents, wherein said analog-to-digital converter means is further connected to each of said cunent-sensing means and further converts said plurality of analog signal into said digital data, said plurality of electnc cunent being generated by a plurality of said cunent source 25

8 The self-powered current momtor of claim 7 wherein said power supply means further denves said regulated d-c power from any or all of said electnc cunents, said power supply means further being connected in senes with each of said electnc cunents

9 The self-powered current momtor of claim 8 further compnsmg one or more burden-reducing means to temporanly reduce the burden effect that said power supply means may have on one or more of said current sources while said analog-to-digital converter means is sampling one of said analog signals, said bmden-reducing means being connected to said power supply means and having one or more control signal connections to said microprocessor means, said microprocessor means further controlling the time penods that said burden-reducing means is actuated

10 The combination of claim 9 wherein each said electnc cunent is an alternating cunent, each said cunent-sensing means compnses a resistor connected m senes with one of said electnc cunents dunng time penods that said electnc cunent is being sensed thereby producing one of said analog signals which is a voltage signal across said resistor proportional to said electnc cunent, said power supply means compnses

(a) a rectifying means producing direct cunent from each said alternating cunent,

(b) a charge-stonng means providing an umegulated d-c voltage from said direct cunent,

(c) a voltage-regulating means producing said regulated d-c power from said umegulated d-c voltage, and

(d) a voltage-limiting means shunting excess cunent away from said charge-stonng means thereby limiting said umegulated d-c voltage to a predetermined value, said voltage-hmiting means compnsmg one or more electromc switch means and suitable control means, said smtable control means actuating said electronic switch means to prevent said umegulated d-c voltage from exceeding said predetermined value, and said burden-reducing means compnses one or more electromc switch means connected to said power supply means in such a way as to temporanly shunt one or more of said electnc cunents away from said charge-stonng means when so actuated by said microprocessor means

A self-powered cunent momtor for communicating charactenstics of an electnc cunent, said self-powered cunent momtor compnsmg

(a) a current-sensing means, providing an analog signal contaimng information about said characteπstics,

(c) an interface means communicating said characteπstics to other equipment 26

(d) a memory means stonng worlung data and program data,

(e) a microprocessor means, conneαed to said analog-to-digital converter means and to said interface means and to said memory means, receiving said digital data, utilizing said program data and said working data to determine said characteπstics, and providing output suitable for said interface means, (f) a power supply means, connected in seπes with said electnc cunent, denying regulated d-c power from said electnc cunent for operation of said self-powered cunent momtor, said electnc cunent being generated by a cunent source

12 The self-powered current momtor of claim 11 wherein said interface means commumcates to said other equipment via electncal signals conducted by metallic conductors connected between said interface means and said other equipment

13 The self-powered current momtor of claim 11 wherein said interface means communicates to said other equipment via electromagnetic radiation traveling through air between said interface means and said other eqmpment

14 The self-powered current momtor of claim 11 wherein said interface means commumcates to said other eqmpment via light waves traveling in optical fiber connected between said interface means and said other eqmpment

15 The self-powered cunent momtor of claim 11 wherein said electnc cunent is an alternating electnc current and said charactenstics of said electnc cunent compnse one or more of root-mean-square magnitude, average magmtude, peak magmtude, harmomc component magnitudes, distortion factor, form factor, crest factor, eqmvalent k-factor for transformers with harmomc rating, graphical representation of the waveform, cunent demand, and maximum cunent demand

16 The self-powered cunent momtor of claim 11 wherein said electnc cunent is an alternating electnc cunent, said cunent-sensing means compnses a resistor connected in senes with said electnc cunent dunng time penods that said electnc current is being sensed thereby producing said analog signal which is a voltage signal across said resistor proportional to said electnc cunent. and said power supply means compnses

(a) a rectifying means producing direct cunent from said alternating cunent,

(d) a voltage-limiting means shunting excess cunent away from said charge-stonng means thereby limiting said umegulated d-c voltage to a predetermined value, said voltage-limiting means compnsmg one or more electromc switch means and suitable control means, said smtable control means actuating said electronic switch means to prevent said umegulated d-c voltage from 27 exceeding said predetermined value

The self-powered cunent momtor of claim 11 further compnsmg a bmden-reducing means to temporarily reduce the burden effect that said power supply means may have on said cunent source wlule said analog-to-digital converter means is sampling said analog signal, said burden-reducing means being connected to said power supply means and having a control signal connection to said microprocessor means, said power supply means utilizing stored energy to maintain regulated power output while said bmden-reducmg means is actuated, said microprocessor means further controlling the time penods that said burden-reducing means is actuated

18 The self-powered cunent momtor of claim 17 wherein said electnc cunent is an alternating current, said cunent-sensing means compnses a resistor connected in senes with said electnc current dunng time penods that said electnc cunent is being sensed thereby producing said analog signal which is a voltage signal across said resistor proportional to said electnc cunent, said power supply means compnses (a) a rectifying means producing direct cunent from said alternating cunent,

(d) a voltage-limiting means shunting excess cunent away from said charge-stonng means thereby limiting said unregulated d-c voltage to a predetermined value, said voltage-limiting means compnsmg one or more electromc switch means and smtable control means, said smtable control means actuating said electromc switch means to prevent said umegulated d-c voltage from exceeding said predetermined value, and said bmden-reducmg means compnses one or more electromc switch means connected to said power supply means in such a way as to temporanly shunt said electnc cunent away from said charge-stonng means when so actuated by said microprocessor means, said voltage-regulating means utilizing energy stored in said charge-stonng means to produce said regulated d-c power while said bmden-reducing means is actuated

The self-powered current momtor of claim 11 further commumcating characteπstics of a plmality of said electnc cunent, further compnsmg a plurality of said cunent-sensing means producing a plurality of said analog signal each contaimng information about one of said electnc cunents, wherein said analog- to-digital converter means is further connected to each of said cunent-sensing means and further converts said plurality of analog signal into said digital data, said plmality of electnc cunent being generated by a plurality of said cunent source

20 The self-powered cunent momtor of claim 19 wherein said power supply means further denves said regulated d-c power from any or all of said electnc cunents. said power supply means further 28 being connected in senes with each of said electnc cunents

21 The self-powered cunent momtor of claim 20 further compnsmg one or more bmden- reducing means to temporanly reduce the b den effect that said power supply means may have on one or more of said cunent sources while said analog-to-digital converter means is sampling one of said analog signals, said bmden-reducing means being connected to said power supply means and having one or more control signal connections to said microprocessor means, said microprocessor means further controlling the time penods that said bmden-reducing means is actuated

22 The combination of claim 21 wherein each said electnc cunent is an alternating cunent, each said cunent-sensing means compnses a resistor connected in senes with one of said electnc cunents dunng time penods that said electnc cunent is being sensed thereby producing one of said analog signals which is a voltage signal across said resistor proportional to said electnc cunent, said power supply means compnses (a) a rectifying means producing direct cunent from each said alternating cunent, (b) a charge-stonng means providing an umegulated d-c voltage from said direct cunent,

(d) a voltage-hnuting means shunting excess cunent away from said charge-stonng means thereby limiting said unregulated d-c voltage to a predetermined value, said voltage-limiting means compnsmg one or more electromc switch means and suitable control means, said suitable control means actuating said electromc switch means to prevent said unregulated d-c voltage from exceeding said predetermined value, and said burden-reducing means compnses one or more electromc switch means connected to said power supply means in such a way as to temporanly shunt one or more of said electnc cunents away from said charge-stonng means when so actuated by said microprocessor means

In combination (a) a power supply means receiving an electnc cunent from a cunent source and providing a regulated d-c power output denved from said electnc cunent,

(b) a current-sensing means, providing an analog signal containing information about said electnc cunent, and

(c) a burden-reducing means, connected to said power supply means, said cunent source being adversely affected by the normal bmden effect of said power supply means, said bmden-reducing means acting to temporanly reduce the burden effect that said power supply means has on 29 said cunent source while said burden-reducing means is actuated, said power supply means utilizing stored energy to maintain said regulated d-c power output while said burden-reducing means is actuated

24 The self-powered current momtor of claim 23 wherem said electnc cunent is an alternating cunent, said current-sensing means compnses a resistor connected in senes with said electnc cunent dunng time peπods that said electnc cunent is being sensed thereby producing said analog signal which is a voltage signal across said resistor proportional to said electnc cunent, said power supply means compnses

(a) a rectifying means producing direct cunent from said alternating cunent,

(d) a voltage-limiting means shunting excess cunent away from said charge-stonng means thereby limiting said umegulated d-c voltage to a predetermined value, said voltage-limiting means compnsmg one or more electromc switch means and suitable control means, said smtable control means actuating said electromc switch means to prevent said umegulated d-c voltage from exceeding said predetermined value, and said bmden-reducmg means compnses one or more electromc switch means connected to said power supply means m such a way as to temporanly shunt said electnc cunent away from said charge-stonng means when so actuated by said microprocessor means, said voltage-regulating means utilizing energy stored in said charge-stonng means to produce said regulated d-c power while said burden-reducing means is actuated

In combination

(a) a power supply means receiving a plurality of electnc cunents from a plmality of cunent sources and providing a regulated d-c power output denved from said plmality of electnc cunents, and

(b) a plurality of cunent-sensing means, connected to said power supply means, providing a plurality of analog signals each contaimng information about one of said plurality of electnc cunents

26 The combination of claim 25 wherein each said electnc cunent is an alternating cunent, each said current-sensing means compnses a resistor connected in senes with one of said electnc cunents dunng time peπods that said electnc current is being sensed thereby producing one of said analog signals which is a voltage signal across said resistor proportional to said electnc cunent. and said power supply means compnses

(c) a voltage-regulating means producing said regulated d-c power from said unregulated d-c voltage, and (d) a voltage-limiting means shunting excess cunent away from said charge-stonng means thereby limiting said unregulated d-c voltage to a predetermined value, said voltage-limiting 30 means compnsmg one or more electronic switch means and suitable control means, said smtable control means actuating said electromc switch means to prevent said umegulated d-c voltage from exceeding said predetermined value

The combmation of claim 25 further including one or more bmden-reducmg means, connected to said power supply means, temporanly reducing the burden effect that said power supply means may have on one or more of said cunent sources

28 The combination of claim 27 wherem each said electnc cunent is an alternating cunent, each said cunent-sensing means compnses a resistor connected in senes with one of said electnc cunents dunng time peπods that said electnc cunent is being sensed thereby producing one of said analog signals which is a voltage signal across said resistor proportional to said electnc cunent, said power supply means compnses

(d) a voltage-limiting means shunting excess cunent away from said charge-stonng means thereby limiting said unregulated d-c voltage to a predetermined value, said voltage-limiting means compnsmg one or more electromc switch means and smtable control means, said smtable control means actuating said electromc switch means to prevent said umegulated d-c voltage from exceeding said predetermined value, and said bmden-reducmg means compnses one or more electromc switch means connected to said power supply means in such a way as to temporanly shunt one or more of said electnc cunents away from said charge-stonng means