Measuring system
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 USRE31774E USRE31774E US06532347 US53234783A USRE31774E US RE31774 E USRE31774 E US RE31774E US 06532347 US06532347 US 06532347 US 53234783 A US53234783 A US 53234783A US RE31774 E USRE31774 E US RE31774E
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 variable
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 wave
 samples
 parameter
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 G—PHYSICS
 G01—MEASURING; TESTING
 G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
 G01R19/00—Arrangements for measuring currents or voltages or for indicating presence or sign thereof
 G01R19/02—Measuring effective values, i.e. rootmeansquare values

 G—PHYSICS
 G01—MEASURING; TESTING
 G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
 G01R21/00—Arrangements for measuring electric power or power factor
 G01R21/133—Arrangements for measuring electric power or power factor by using digital technique
Abstract
Description
This invention relates to a method for measuring a parameter whose value is calculable as an integral with time of a function of at least one variable. The method of this invention makes possible the use of a microprocessor when measurements such as effective (RMS) voltage, effective (RMS) current, average power, and energy are to be made by calculation from the outputs of current and potential transformers.
Use of a microprocessor for making the required measurements from current and potential transformer outputs has the advantage over previously known methods in that there is not required the use of separate measuring devices for each of the measurements and the microprocessor provides an output which can be transmitted to a central computer for use.
The method of this invention comprises a first step of sampling the value of the variable of which the parameter being measured is a function. .Iadd.other than the identity function of a single variable. .Iaddend.The sampling is done periodically at a frequency asynchronous with the wave form of the variable .Iadd.and not a harmonic or subharmonic of the waveform. .Iaddend.Periodically there is then made a calculation of the value of the function by summing the values for the function determined from the values for the variable obtained for each sample taken during the time between calculations.
In the Drawings:
FIG. 1 is a wave form showing the sampling points for the measurement.
FIG. 2 is a block diagram showing one form the microprocessor configuration can take for carrying out the method of this invention.
The theoretical basis for this measuring method is found in the fact that an exact integral equation can be approximated by a finite summation equation as follows: ##EQU1## where y is the parameter being measured and x is some function of the variable which when integrated over the period T gives the value for the parameter. N is the number of samples made between calculations (during period T) and Δt_{n} is the sample period. The period between samples becomes Δt_{n} ≈T/N. Obviously the larger the number of samples and the smaller the sample period the closer the finite sum approximates the integral.
Where the variable or variables which determine the parameter have values which follow a periodic wave form, a number of samples of the value of those variables is made during each period of the wave form with the frequency of the samples being asynchronous with the frequency of the variable. Therefore, with respect to the variable there is obtained a sliding sample so that over a period of time all parts of the wave form of the variable are sampled. It is, of course, important to use a large number of samples if the wave form of the variable is not symmetrical about the average, whereas a smaller number of samples suffices if the wave form is symmetrical.
It is convenient to utilize a microprocessor for calculating the required function of the variable to obtain a calculation of the value for the parameter being measured. Thus, it is possible to measure parameters of a 60 hertz power system by utilizing the standard current and potential transformers to provide inputs to a microprocessor through analog to digital converters so that the inputs provide the data required to calculate parameters such as RMS voltage, RMS current, average power, and energy.
If FIG. 1 shows the periodic variable, which may be current or voltage as it varies with time, it will be evident that the first series of samples A1A5 are obtained from a part of the wave form which is different from that of the samples B1B5, both of which are in turn different from the samples C1C5. Thus, as each period passes the point of sampling slips with respect to the wave form in FIG. 1 so that over a number of cycles each part of the wave is sampled.
If the frequency of the wave of FIG. 1 is 60 hertz, then the period of an ideal sine wave would be 16.667 msec. and the sample period could be 3.125 msec. for 5 samples per cycle. If, for example, an RMS calculation of the voltage is desired it may be carried out as follows: ##EQU2## Similarly, the equation for RMS current becomes: ##EQU3##
The variables e and i represent the instantaneous ac voltage and current which are taken from potential and current transformers by way of analog to digital converters. Thus in FIG. 2 the analog to digital converters 12, 14, 16, and 18 provide voltage and current inputs to the microprocessor 20.
The microprocessor 20 may advantageously have connected to it the read only memory (ROM) 22, and the random access memory (RAM) 24 as well as clock source and a power supply so the microprocessor can perform the necessary calculations.
The results of the calculated values produced by the microprocessor can be decoded directly by the decoder 28 and supplied to a digital display 30 for displaying the value of the parameter. Also, as shown in FIG. 2 the output of the microprocessor can be transmitted by transmitter 32 to a central computer which may be processing the data obtained.
Other measurements which can be made advantageously in accordance with this novel method are average power and energy.
Average power is defined as follows: ##EQU4## Thus, a product of two numbers can be evaluated by a table lookup approach with a suitable shift instruction for the factor 1/4N.
Energy is defined as follows: ##EQU5##
This relationship represents the energy for a given time period (T) which is directly related to the sample period and the number of samples.
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Cited By (13)
Publication number  Priority date  Publication date  Assignee  Title 

EP0256183A1 (en) *  19860815  19880224  Dynamic Systems Inc.  RMS current determination by sampling current differential 
US4977513A (en) *  19840820  19901211  Power Solutions, Inc.  Circuit breaker current monitoring 
WO1991019201A2 (en) *  19900606  19911212  M.T. Mcbrian Company, Inc.  System and method for monitoring and analyzing energy characteristics 
WO1994003818A1 (en) *  19911213  19940217  The Dow Chemical Company  High speed power analyzer 
US5736847A (en) *  19941230  19980407  Cd Power Measurement Limited  Power meter for determining parameters of muliphase power lines 
US5828576A (en) *  19941230  19981027  Cd Power Measurement Limited  Power monitor apparatus and method with object oriented structure 
US6186842B1 (en)  19990809  20010213  Power Measurement Ltd.  Revenue meter bayonet assembly and method of attachment 
US6397155B1 (en)  19990809  20020528  Power Measurement Ltd.  Method and apparatus for automatically controlled gain switching of monitors 
US6493644B1 (en)  19990809  20021210  Power Measurement Ltd.  Abase revenue meter with power quality features 
US6611922B2 (en)  19990809  20030826  Power Measurement, Ltd.  Power system time synchronization device and method for sequence of event recording 
US6615147B1 (en)  19990809  20030902  Power Measurement Ltd.  Revenue meter with power quality features 
US20040066311A1 (en) *  19990809  20040408  Power Measurement Ltd.  Interactive user interface for a revenue meter 
US6798191B1 (en)  19990809  20040928  Power Measurement Ltd.  Revenue meter with a graphic user interface being operative to display scalable objects 
Citations (2)
Publication number  Priority date  Publication date  Assignee  Title 

GB1475818A (en) *  19740419  19770610  Sartorius Werke Gmbh  Method and apparatus for conditioning analogue data in balances 
US4139896A (en) *  19750103  19790213  Curtis Instruments, Inc.  Method and apparatus for producing nonlinear integral functions 
Patent Citations (2)
Publication number  Priority date  Publication date  Assignee  Title 

GB1475818A (en) *  19740419  19770610  Sartorius Werke Gmbh  Method and apparatus for conditioning analogue data in balances 
US4139896A (en) *  19750103  19790213  Curtis Instruments, Inc.  Method and apparatus for producing nonlinear integral functions 
Cited By (22)
Publication number  Priority date  Publication date  Assignee  Title 

US4977513A (en) *  19840820  19901211  Power Solutions, Inc.  Circuit breaker current monitoring 
EP0256183A1 (en) *  19860815  19880224  Dynamic Systems Inc.  RMS current determination by sampling current differential 
WO1991019201A2 (en) *  19900606  19911212  M.T. Mcbrian Company, Inc.  System and method for monitoring and analyzing energy characteristics 
WO1991019201A3 (en) *  19900606  19920319  M T Mcbrian Company Inc  System and method for monitoring and analyzing energy characteristics 
WO1994003818A1 (en) *  19911213  19940217  The Dow Chemical Company  High speed power analyzer 
US5736847A (en) *  19941230  19980407  Cd Power Measurement Limited  Power meter for determining parameters of muliphase power lines 
US5828576A (en) *  19941230  19981027  Cd Power Measurement Limited  Power monitor apparatus and method with object oriented structure 
US6185508B1 (en)  19941230  20010206  Power Measurement, Ltd.  Power meter for determining parameters of multiphase power lines 
US6611922B2 (en)  19990809  20030826  Power Measurement, Ltd.  Power system time synchronization device and method for sequence of event recording 
US6397155B1 (en)  19990809  20020528  Power Measurement Ltd.  Method and apparatus for automatically controlled gain switching of monitors 
US6493644B1 (en)  19990809  20021210  Power Measurement Ltd.  Abase revenue meter with power quality features 
US6186842B1 (en)  19990809  20010213  Power Measurement Ltd.  Revenue meter bayonet assembly and method of attachment 
US6615147B1 (en)  19990809  20030902  Power Measurement Ltd.  Revenue meter with power quality features 
US6687627B1 (en)  19990809  20040203  Power Measurement Ltd.  Electric charge storage apparatus and method for an intelligent electronic device 
US20040066311A1 (en) *  19990809  20040408  Power Measurement Ltd.  Interactive user interface for a revenue meter 
US6792364B2 (en)  19990809  20040914  Power Measurement Ltd.  Revenue meter with power quality features 
US6798191B1 (en)  19990809  20040928  Power Measurement Ltd.  Revenue meter with a graphic user interface being operative to display scalable objects 
US6825776B2 (en)  19990809  20041130  Power Measurement Ltd.  External I/O and communications interface for a revenue meter 
US6983211B2 (en)  19990809  20060103  Power Measurement, Ltd.  Revenue meter bayonet assembly and method of attachment 
US7006934B2 (en)  19990809  20060228  Power Measurement Ltd.  Revenue meter with power quality features 
US7158050B2 (en)  19990809  20070102  Power Measurement Ltd.  Revenue meter with fiber connection 
US7478003B2 (en)  19990809  20090113  Cowan Peter C  Revenue meter bayonet assembly and method of attachment 
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