CURRENT MEASUREMENT METHODS AND APPARATUS EMPLOYING SECOND HARMONIC SCALING

PROCEDES POUR EFFECTUER DES MESURES DE COURANT ET DISPOSITIF UTILISANT LA MESURE DE LA DEUXIEME HARMONIQUE

English Abstract

A method for measuring an AC current in a conductor (10) in which a DC current
also exists including the step of obtaining a first AC current measurement,
obtaining a measure of a power factor, fundamental frequency component and
second harmonic component (S2) and adjusting the first AC current measurement
in accordance with an error value to obtain a corrected current measurement
(S3, S4). The error value is determined as a function of the power factor,
fundamental frequency component and second harmonic component.

French Abstract

Procédé pour mesurer un courant alternatif (CA) dans un conducteur (10) dans lequel il existe également un courant continu (CC), consistant à obtenir une première mesure du CA (S1), obtenir une mesure d'un facteur de puissance, d'une composante de fréquence fondamentale et d'une deuxième composante harmonique (S2), à adapter la première mesure de CA en fonction d'une valeur d'erreur pour obtenir une mesure de courant rectifiée (S3, S4). La valeur d'erreur est calculée en tant que fonction du facteur de puissance, de la composante de fréquence fondamentale et de la deuxième composante harmonique.

Claims

-7-
I claim:
1. A method for measuring an AC current in a conductor in which a
DC current also exists, wherein a power factor is determinable from said AC and DC
currents and said AC current has at least a fundamental frequency component and a
second harmonic component, comprising the steps of:
(A) obtaining a first AC current measurement;
(B) obtaining a measure of said power factor, fundamental frequency
component, and second harmonic component; and
(C) adjusting said first AC current measurement in accordance with
an error value to obtain a corrected current measurement, said error value beingdetermined as a function of said power factor, fundamental frequency component, and
second harmonic component.
2. A method as recited in claim 1, wherein said first AC current
measurement is obtained with a current transformer operatively coupled to said
conductor, the output of said current transformer being a signal (V CT) indicative of the
AC current in said conductor.
3. A method as recited in claim 2, wherein said error value is
determined in accordance with the following equation:
(2.8 *pwr factor in degrees)
<IMG>
or other equation based on empirical data, wherein " %Error" represents the percent
error in the first current measurement, and "V2nd harm/V fund" represents a ratio of the
second harmonic component to the fundamental frequency component.
4. A method as recited in claim 1, wherein said power factor,
fundamental frequency component, and second harmonic component are employed to
access a lookup table containing said error value.

-8-
5. A system for measuring an AC current in a conductor in which a
DC current also exists, wherein a power factor is determinable from said AC and DC
currents and said AC current has at least a fundamental frequency component and a
second harmonic component, comprising:
(A) current transformer means operatively coupled to a said
conductor for obtaining a first AC current measurement; and
(B) processing means for obtaining a measure of said power factor,
fundamental frequency component, and second harmonic component, and for adjusting
said first AC current measurement in accordance with an error value to obtain a
corrected current measurement, said error value being determined as a function of said
power factor, fundamental frequency component, and second harmonic component.
6. A system as recited in claim 5, wherein an output of said current
transformer is a signal (V CT) indicative of the AC current in said conductor.
7. A system as recited in claim 5, wherein said error value is
determined in accordance with the following equation:
<IMG>
wherein "%Error" represents the percent error in the first current measurement; and
"V2nd harm/Vfund" represents a ratio of the second harmonic component to the
fundamental frequency component.
8. A system as recited in claim 5, wherein said power factor,
fundamental frequency component, and second harmonic component are employed to
access a lookup table containing said error value.

Description

W0 98/11447101520CA 02265863 2002-04-22PC!‘/US97/15081CURRENT MEASUREMENT METHODS AND APPARATUSEMPLOYING SECOND HARMONIC SCALINGCross-Reference to Related ApplicationThis is a continuation application of U.S. Patent 5,907,241.FIELD OF THE INVENTIONThe present invention relates generally to the field of current metering,and more particularly to methods and apparatus for obtaining accurate power frequencycurrent measurements when DC current is also present.BACKGROUND OF THE INVEN'I‘IONElectronic metering of electrical energy is a mature field of technology,and today's metering products must hardware cost to be competitive.Typically, one of the major cost elements in a current or Watt-hour meter is thecurrent transformer responsible for accurately reproducing the waveforms of the‘current to be Normally, the current to be measured is the currentcomponent at the power frequency, 60 Hz for systems in the United States and 50 Hzfor many international systems. However, in addition to the power frequency currents,there are harmonic currents and DC currents that can be present. These’ harmonic andDC currents can adversely affect the performance of current transformers designed tooperate at the power frequency. This is a particularly serious concern where extremelyaccurate billing information is required.Previous metering technologies have used a variety of current sensingtechniques. Generally, these techniques offer a compromise of accuracy for powerW0 98/1 14471015202530CA 02265863 l999-03- 10PCT/US97ll508l._ 2 _frequency current with and without DC current present. The DC current tends tosaturate magnetic materials, and consequently it adversely affects accuracy. Typicalcurrent sensing materials, such as Superrnalloy, tend to be expensive and are pricedproportional to weight or volume. Accordingly, a consequence of the presence of DCcurrent is the need for more magnetic material to obtain the same AC accuracy, whichin turn results in higher costs. Therefore, a need exists for a current measurementtechnique that will improve power frequency metering accuracy in the presence of DCcurrent.SUMMARY OF THE INVENTIONAccordingly, an object of the present invention is to provide a systemand method whereby the current transformer may be sized for AC current only and themeasurement result may be compensated when there is DC current present.The present invention provides a method and system for measuring anAC current in a conductor in which a DC current also exists. The invention includesthe steps of, or means for, obtaining a first AC current measurement; obtaining ameasure of a power factor, fundamental frequency component, and second harmoniccomponent; and adjusting the first AC current measurement in accordance with anerror value to obtain a corrected current measurement. The error value is determinedas a function of the power factor, fundamental frequency component, and secondharmonic component.In a presently preferred embodiment of the invention, the first ACcurrent measurement is obtained with a current transformer operatively coupled to theconductor, and the output of the current transformer is a signal (VCT) indicative of theAC current in the conductor. Further, in this preferred embodiment, the error value isdetermined in accordance with the following equation:(2.8 *pwr factor in degrees)%Error = (-68 + -------------------------- --) *(V2,..,.,a,,,,/Vf.....i)(1 + 2* (V2ndharm/Vfund))wherein “%Error” represents the percent error in the first current measurement and“Vm ,,,,,,,/Vm,,d” represents a ratio of the second harmonic component to thefundamental frequency component. Alternatively, the power factor, fundamentalfrequency component, and second hannonic component are employed to access aW0 98/ 1144710152O2530CA 02265863 l999-03- 10PCT/US97/15081_ 3 _lookup table containing the error value. Other features of the invention are disclosedbelow.BRIEF DESCRIPTION OF THE DRAWINGSFigure 1 is a block diagram of a current measuring system in accordancewith the present invention.Figure 2 is a flowchart of the operation of the current measuring systemdepicted in Figure 1.Figures 3A-3C are data plots of the performance data for a solid statemeter operating over a wide variety of different loading conditions. (These graphs areexamples of typical data that could be used for the development of error correctiondata.)DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTSAs shown in Figure 1, a presently preferred embodiment of theinvention is adapted to be coupled to a line 10 carrying a 60 Hz or 50 Hz current to bemeasured. Samples of the line current (IMEAS) and voltage (VMEAS) are obtained byconventional means, e.g., a meter 12 employing an inexpensive current transformersized to measure only AC current. According to the invention, a DSP or lookup table(memory) 14 is coupled to the meter 12, and a correction unit 16 is coupled to themeter 12 and DSP/lookup table 14. The operation of the system will be explained nextwith reference to the flowchart depicted in Figure 2.First, the meter 12 measures the line current and voltage (step S1). (Ofcourse, in a three-phase system, the components depicted in Figure 1 would be adaptedto measure all three line currents and voltages.) Next, the power factor, secondharmonic current and fundamental frequency (i.e. , 60 or 50 Hz) current are determinedand employed to generate voltage signals indicative of the respective values (step S2).Next, the percent error in the current measurement is calculated (step S3), and thenthis percent error value is employed to compensate the current measurement to obtain acorrected current measurement (step S4). Finally, the corrected current measurementis output and used for billing purposes. Of course, in a Watt—hour meter, the measuredcurrent is employed to calculate the amount of energy consumed by the customer.Thus, the disclosed concept provides correction factors for the currenttransformer output based on the amount of second harmonic signal that is present. TheW0 98/1 144710152025CA 02265863 l999-03- 10PCT/US97/15081_ 4 _invention recognizes that a correction algorithm can be determined empirically andimplemented in a DSP or memory. The result achieved by this invention is thatcurrent sensing in the presence of second harmonic current is adjusted such that themeasurement accuracy is similar to the accuracy obtained where no DC current ispresent. In this manner, core and transformer costs are maintained for AC currentswhile performance in the presence of DC current is improved.Figures 3A-3C depict the performance numbers for a solid state meteroperating over a wide variety of different loading conditions. Figure 3C relates to apower factor of unity and depicts data points relating the percent second harmoniccurrent (x-axis) to the percent error in the fundamental frequency current measurement(y-axis). A straight-line approximation of the percent error is given by,%Error = —67(V,—2/V“).Similarly, Figures 3A and 3B relate to power factors of 60° and —37°, respectively, andthe corresponding straight-line error equations are:%Error 187(V,2/Va), IDC < 50% IAC, and%Error = -16O(V,2/V“).The testing begins with no DC current and then the DC current isincreased to 14 amperes. These measurements are utilized to correlate the amount ofmeasurement error to the corresponding fundamental frequency current and DCcurrent. Figures 3A-3C provide rough correlation plots of accuracy and itsrelationship to fundamental frequency and DC current levels. This data can be madesufficiently robust to be predictive in correction of performance errors.The data files included in this disclosure are an example of the basictype of data that can be used for analysis of an algorithm to correct for DC current.The data includes the amount of error that is documented for different conditions ofAC current magnitude and phase and DC current magnitude. Using these data, anequation or a lookup table can be developed which provides correction factors for thedata. An example of such an equation follows:W0 98/1144710CA 02265863 l999-03- 10PCT/U S97] 15081.. 5 _(2.8 *pwr factor in degrees)% Error = + """""""""""""" "‘) *(V2nd harm/Vfund)(1 + 2* (V2nd hmm/Vfund))This equation allows the DSP to calculate the percent error correctionfactor using the measured power factor; the amount of second harmonic current, asrepresented by voltage signal V2,,,,,,,,,,,; and the amount of fundamental current, asrepresented by voltage signal Vfund. This particular equation may not be the finalimplementation used for production units of a given system, but it is an example of auseful equation based on empirical data taken from several units.An alternative approach is to develop a lookup table of percent errorcorrection based on measured power factor, second harmonic voltage and fundamentalvoltage. The lookup table would be determined from large amounts of productiondata. The following table, which contains the data plotted in Figures 3A—3C, could bestored in a lookup table ROM or RAM.CA 02265863 l999-03- 10W0 98/11447 PCT/US97/15081_ 6 _% Error for % Error for % Error for% PF = Unity PF = +600 PF = -370Inc Va Va/V“ Meas Calc Meas Calc Meas CalcIAC=5A 14 .22 10 -62% -5.5% 13.9% 16.1% -96.6% -18.7%VC1-=2.2mv 7 .1 4.5 -27 -2.5 31 7.2 -53 -8.43.5 .07 3.2 -15 -1.8 21 5.2 -31 -6.1.75 .079 3.5 -3.3 -1.9 9.8 5.6 -9.2 -6.6IA¢=1OA 14 .79 17.6 -62 -9.7 13 28.3 -94.3 -32.9VcT=4.5mv 7 .71 15.8 -26 -8.7 31 25.4 -52 -29.63.5 .32 7.1 -10 -3.9 18.7 11.4 -23 -13.31.75 .16 3.6 -2.7 -2. 8.4 5.8 -7.5 -6.71AC=20A 14 2.2 22 -50 -12.1 17.3 35.4 -79 -41VcT=1()mv 7 2.2 22 -18 -12.1 28 35.4 -79 -413.5 .79 7.9 -6 -4.3 14.5 12.7 -15 -14.81.75 .32 3.2 -2 -1.8 6.3 5.2 -5.5 -6.I,,C=40A 14 5. 20 -21 -11. 23 32.2 -4. -38.4VcT=25mv 7 3.5 14 -7 7.7 18 22.5 -18 -263.5 1.3 5.2 -2.4 -2.9 8.7 8.4 -7.2 -9.71.75 .32 3.2 -1 -1.1 3.7 3.2 -3.1 -3.7IAc=100A 14 7.9 11.1 -8.9 -6.1 17.7 17.9 -20 -20.8VcT=71mv 7 4.5 6.3 -3 -3.5 11 10.1 -9 -11.83.5 2.0 2.8 -1.1 -1.5 5.2 4.5 -3.8 -5.21.75 .71 1. -.6 -.6 2.2 1.6 -1.9 -1.9IAC =200A 14 8.9 6.3 -3.4 -3.4 11 10.1 -10 -11.8VCT=141mv 7 4.5 3.2 -1.2 -1.8 6.4 5.2 -4.5 -6.3.5 1.0 1.3 -.45 ~.7 3 2.1 -2 -2.41.75 .71 .53 —.35 -.3 1.3 .8 -1 -.9

Representative Drawing