METHOD AND APPARATUS FOR MEASURING ELECTRIC UNITS

Abstract: The invention relates to a method for measuring electric units, particularly for measuring electric power and/or energy, in which method the voltage and current signals are measured, and the corresponding electric power and/or energy is calculated on the basis thereof. According to the invention, the current signal (I) is converted to a comparable second voltage signal (V11); to the first voltage signal (V10) to be measured and to the second voltage signal (V11) comparable to the current signal there is added a DC-voltage signal (V2, the value whereof is periodically fluctuated, and to which first voltage signal (V10) the DC-voltage signal (V2) is directly added, whereas to the second voltage signal (V11) it is added in attenuated form (aV2; a<1); the summed-up signal of the second voltage signal and the attenuated DC-voltage signal (V11 + aV2) is so amplified that the product of the amplification (b) and the attenuation (a) of the above-mentioned DC-voltage signal (V2) is constant, advantageously approximately one; the first voltage signal (V10) and the second voltage signal, treated in the above-described fashion [b(V11 + aV2)] are converted from analog to digital form, so that during the conversion of one voltage signal, the value of the second voltage signal is kept unchanged, whereafter the second voltage signal is converted; and the digital voltage signals are fed to the data processing unit (5), where the power and energy values are calculated in a known fashion and written out and/or recorded in the memory for further processing.

Description: METHOD AND APPARATUS FOR MEASURING ELECTRIC UNITS The invention relates to a method for measu¬ ring electric units, defined in the introductory secti- on of patent claim 1. The invention also relates to a measuring ar¬ rangement for measuring electric units, defined in the introductory section of patent claim 5. In the prior art there are known various elect- ronic appliances for measuring electric quantities, among others voltage and current meters, where the electric unit to be measured is converted, by means of an A/D converter, from analog to digital form, and is displayed in a display unit. In such measuring ap- pliances, the A/D converter must be quick and accurate. These qualities presuppose a high-quality A/D conver¬ ter, which is expensive. Consequently the cost of the whole measuring arrangement becomes remarkably high. Another problem is, that in the course of time, the accuracy of the meter may vary. Moreover, the opera¬ tional condition of the meter is not automatically checked in any way. In the prior art there are known electronic meters for electric power, where the height of the pulse sequence is changed by means of the measured voltage, whereas the length of the pulses is modulated by means of the measured current. The time average of the pulse sequence makes a signal comparable to the active power. The above described measuring arrangement for electric power does not allow simultaneous measuring of reactive power, or of the root-mean-square values of currents or voltages. Reactive power is generally measured by means of a separate wattmeter, by employing artificial coupling. Laboratories have developed accurate digital measuring appliances for electric power by using A/D converters. A drawback with the said appliances is that for instance in the measurement of three-phase electric current, there are needed six units of quick and accurate A/D converters, which makes the apparatus extremely expensive. Moreover, the use of a rapid A/D converter also requires an effective microprocessor for processing the measured information. It is pointed out that even in serial production, the production costs of these measuring appliances become very high. Tratiotionally active power and energy is mea¬ sured by electromechanical wattmeters. The greatest weakness in conventional wattmeters is the high price of calibration; they often include mechanical regula¬ ting resistors or adjustable friction magnets. More- over, the measuring accuracy of these meters also tends to weaken in the course of time. The object of the present invention is, among others, to eliminate the above mentioned drawbacks. Another object of the invention is to introduce a new, improved method and measuring arrangement for measuring electric units. The method of the invention is characterized by the novel features enlisted in the patent claim 1. The measuring arrangement of the invention is characterized by the novel features enlisted in the patent claim 5. In the method of the invention for measuring electric units, the electric unit is sampled; the samp¬ les are converted into digital form by using an A/D converter, and written out. According to the inventi¬ on, there is provided a noise source; the root-mean- square value of the noise signals is set, so as to be in accordance with the resolution of the A/D converter as for volume; the noise signal is added to each samp- le; there are carried out several measurements of elec¬ tric units, and on the basis of these measurements, the measuring result is calculated and written out. In a preferred embodiment of the method, there is provided a number of reference voltages; the real values of the reference voltages are recorded; the reference values are measured at certain intervals, and converted to digital form by means of an A/D converter; the real and measured values of the reference voltages are compared; and the digital value of the measured electric quantities is corrected, by taking into ac¬ count the deviations between the measured and real values of the reference voltages. In another preferred embodiment of the method, on the basis of the real and measured values of the reference voltages, there is formed a conversion cor¬ respondence for the A/D converter; and the values of the measured electric quantities are corrected by means of this correspondence. In another preferred embodiment of the method, there are provided at least two DC-voltages; the real values of these DC-voltages are recorded; a DC-voltage signal is summed to the electric unit to be measured, and the value of this DC-voltage signal is periodically fluctuated in between the said DC-voltages; the diffe¬ rence of the DC-voltages is measured; the measured voltage difference and the real, recorded voltage dif- ference are compared, and on the basis of this compa¬ rison, the reliability of the measuring arrangement is evaluated. In a preferred embodiment of the method, in order to measure AC-current units, the sampling of the current is synchronized with the frequency of the said AC-current, and with the fluctuation frequency of the DC-voltages of the DC-voltage signal to be added there¬ to; the average value of the electric unit samples, taken during the half-period of the fluctuation fre- quency of the DC-voltage signal, is calculated; and this average represents the measuring value, which is written out and/or recorded for further processing. In a preferred embodiment of the method for measuring electric power and/or energy, in which method the voltage and current signals are measured and on the basis of the obtained results, the corresponding elect- ric power and/or energy is calculated, the current signal is converted to a second voltage signal, compa¬ rable thereto; to the measured first voltage signal and to the second voltage signal, comparable to the first, there is added a DC-voltage signal, the value whereof is periodically fluctuated, and to which first voltage signal the DC-current signal is directly added, whereas to the second voltage signal it is added in attenuated form; the summed-up signal of the second voltage signal and of the attenuated DC-voltage signal is so ampli- fied, that the product of the amplification and at¬ tenuation of the above mentioned DC-voltage signal is constant, advantageously approximately one; the first voltage signal and the second voltage signal, which is processed in the fashion described above, are converted from analog to digital form, so that during the conver¬ sion of one voltage signal, the value of the second voltage signal is kept unchanged, whereafter the second voltage signal is converted; and the digital voltage signals are fed into the data processing unit, where the power and/or energy values are calculated in a fashion known as such, and are written out and/or re¬ corded for further processing. In another preferred embodiment of the method, the product of the attenuation and amplification is defined prior to the measurements proper by means of using various readings of the current signal, and re¬ corded; and the product of the attenuation and amplifi¬ cation is adjusted, according to the size of the cur¬ rent signal to be measured, so that it is constant, advantageously approximately one. The measuring arrangement of the present inven¬ tion for measuring electric units include means for successive sampling of the electric signal to be measu¬ red; an A/D converter for converting the samples from analog to digital form; and means for controlling the measurements as well as calculating and writing out the results. According to the invention, the measuring arrangement comprises a noise source, the root-mean- square value of the noise signal whereof is set so as to be in accordance with the resolution of the A/D converter in volume, and means for summing the noise signal to the electric signal to be measured. In a preferred embodiment of the measuring arrangement, the system comprises at least one DC-vol¬ tage source, wherefrom a number of reference voltages is obtained; a memory unit, where the real voltage values of the reference voltages are recorded; a switch where the reference voltages are connected to, and where the electric unit to be measured also is connec¬ ted to; comparison means, whereby the measuring values of the reference voltages, obtained from the A/D con- verter through the switch, are compared to the real values of the reference voltages, stored in the memory unit; a memory unit where the deviations between the real and measured values of the reference voltage sour¬ ces, obtained from the comparison means, are recorded; means for correcting the value of the electric unit under measurement; where the measured value of the electric unit, obtained from the A/D converter, is corrected by taking into account the deviation of the A/D converter. In another preferred embodiment of the measu¬ ring arrangement, the comparison means include a table of corrections, on the basis whereof the values of the electric quantities are corrected. In another preferred embodiment of the measu- ring arrangement, the system includes at least one DC-voltage source, wherefrom at least two DC-voltage signals are obtained; a memory unit, whereto the real values of the reference voltages are recorded; a multi¬ plexer; a summing circuit; and a control unit; whereby to the electric unit to be measured, there is added a DC-voltage signal, the voltage value whereof is fluctu- ated periodically, by means of the multiplexer, in between the DC-voltages of the DC-voltage source, and the difference of these voltages is compared in the control unit to the real values recorded in the memory unit, and the deviations from the recorded, real value are written out. In another preferred embodiment of the measu¬ ring arrangement, the DC-voltage source is formed of a voltage distributor comprising a number of preferably identical resistors coupled in series, and in parallel to these, there is arranged a DC-voltage source. The advantage of the identical resistors is that then the resistors are known, and do not have to be separately calibrated. In another preferred embodiment of the measu- ring arrangement, the second DC-voltage source is for¬ med of the first DC-voltage source connected to the measuring arrangement. In another preferred embodiment of the measu¬ ring arrangement, which is particularly designed for measuring electric power and/or energy, to the measu¬ ring arrangement there is fed both a voltage signal and a current signal, and the measuring arrangement comp¬ rises: two summing circuits, to the voltage and current signals whereof there is added, through the multiple- xer, the voltage signal of the DC-voltage source; an attenuator, wherethrough the voltage signal of the DC-voltage source is transmitted to the summing unit of the current signal; an amplifier, whereto the summed current signal and DC-voltage signal are fed, and whe- refrom they are further fed to the switch, to the A/D converter and to the calculation means of power/energy; and in which arrangement, the product of the attenuati- on and amplification of the attenuator and amplifier is constant, advantageously approximately one. Owing to the invention, a fairly inaccurate and cheap A/D converter can be used in the measurements of electric quantities. Further, owing to the invention, in the arran¬ gement for measuring electric quantities there can be used simple single-chip circuits with an integrated A/D converter. Moreover, owing to the invention the operation of the system and the accuracy of the results can be controlled in the measurement of electric quantities. In addition to this, the invention inables ver¬ satile possibilities for measuring electric quantities. Further, owing to the invention, digital mea¬ suring techniques can be economically applied to the measurement of electric quantities. Moreover, owing to the invention the measure¬ ments of electric consumption in households and other establishments can now be carried out by using a simp¬ le, operationally secure and economically advantageous digital technique. Another advantage of the invention is that the measuring arrangement can be easily coupled to external appliances. Yet another advantage of the invention is that it provides for economical, automatic distant measure¬ ments of electric consumption. Yet another advantage of the invention is that in addition to the information as for electric consump¬ tion, the system can also be used for collecting other necessary information of the location of consumption. In the following the invention is explained in more detail with reference to the appended drawings, where figure 1 is a schematical illustration of the measuring method of the invention, where noise is summed to the electric unit to be measured; figure 2 is a schematical illustration of another mea¬ suring method of the invention, where reference volta¬ ges are made use of; figure 3 is an illustration of the correspondence value of an A/D converter; figure 4 is a schematical illustration of a third mea¬ suring method of the invention, where a DC-voltage signal is added to the electric unit to be measured; figure 5 illustrates a voltage signal whereto a DC-vol¬ tage fluctuating in between two levels is summed; figure 6 is a schematical illustration of a measuring arrangement of the invention for measuring electric power; figure 7 is a block and circuit diagram of a measuring arrangement of the invention for measuring power in a three-phase electric network; and figure 8 illustrates sampling from voltage and current signals. Figure 1 is a block diagram of a measuring arrangement for measuring an AC-current unit, such as AC-voltage V or AC-current I. The measuring arrange¬ ment comprises a sampling and holding circuit 1, a summing circuit 2 and an A/D converter 3. The AC-cur- rent unit V; I is fed in through the input terminal A and sampled at suitable intervals; the value of the samples is kept constant by means of the circuit 1, whereafter a noise signal V n from the noise source 3 is added thereto. The sum signal is further fed to the A/D converter 4, where the analog sum signal is conver¬ ted to digital form. From the A/D converter 4, the signal is fed to the data processing unit 5 comprising a memory unit 5a where the measuring results can be recorded and, when necessary, further processed and/or written out outside the system. The electric unit fed in through the input terminal A is submitted to several measurements, the average whereof is calculated in the data processing unit 5 and announced as the measuring result. To each measuring value, there is thus added a noise signal, which signal at each measuring value differs in time from the noise signal of another measu¬ ring value, and consequently does not correlate with it. Thus, from the same noise source there are ob¬ tained independent noise signals which are summed to the measuring signal. As an average, the noise signal does not change the measuring result, but the average thereof is zero. To the root-mean-square value of the measuring signal, the noise signal brings a constant factor which can calculated and taken into account in the final result. As the first phase of the measuring arrange¬ ment, there can advantageously be used the adaptor 6. Thereby the electric unit to be measured is adjusted to be suitable for the measuring arrangement as for volu¬ me. If the electric unit to be measured is a current signal, it is advantageously converted to a voltage signal V , in which case the noise signal is a voltage signal. The root-mean-square value of the noise signal V is advantageously set so as to be in accordance with the resolution of the A/D converter 4. This means that noise is added through the summing circuit 2 to the signal, so that its root-mean-square value is approxi¬ mately 1 bit with respect to the A/D converter. Owing to this procedure, the requirements set for the A/D converters can be alleviated. It is now possible to use a relatively inaccurate converter, which advan¬ tageously also is integrated to the data processing unit, such as a microprocessor, as a single unit in common. Figure 2 illustrates another measuring arrange¬ ment of the invention. In this case the electric unit V, I to be measured is fed, through the input terminal A, to the adaptor 6, and further to the switch 7 as a voltage signal V^ The measuring arrangement comprises a DC-voltage source 8, wherefrom a number of reference voltages is obtained: V refχ : V^, ^, V^, ..., V reflk (k = 1, 2 , 3...). The reference voltages V refi ' tne -i r number in this case being five, are connec¬ ted to the input terminals of the switch 7, parallel to the voltage signal V to be measured. The output ter¬ minal of the switch 7 is connected to the summing cir- cuit 2, whereto the noise voltage source 3 also is connected. From the summing circuit 2, the sum signal is fed to the A/D converter 4, and further to the data processing unit 5. Thus the latter part of the measu¬ ring arrangement corresponds to the arrangement of figure 1. The real values V (k = 1, 2, 3) of the refe¬ rence voltages V reflk (k = 1, 2, 3...) are stored in the memory 5a of the data processing unit 5. This can be done for instance while calibrating the measuring ar- rangement. By means of the switch 7, the reference voltages V reflk connected to its input terminals, are read at suitable intervals together with the electric signal V proper to be measured. The reference voltages V reflk are measured in this fashion at regular intervals and converted into digital form in the A/D converter 4. The real values V tk (k = 1, 2, 3...) and measu¬ red values V reflk (k = 1, 2, 3...) of the reference volta¬ ges are compared. The digital value of the measured electric quantities is corrected by taking into account the differences between the measured and real values of the reference voltages. The correcting of the measured electric quan¬ tities can be realized so that on the basis of the real values V tk (k = 1, 2, 3...) and measured values V reflk (k = 1, 2, 3... ) of the reference voltages, there is formed correspondence value for the A/D converter, and the measured electric quantities are corrected accor- ding to the said correspondence value. This procedure is illustrated in figure 3. Figure 3 represents the relations between the real and measured values in the coordinates V ,' Vref1. If these values are identical,' the situation is ideal. The ideal situation is described by the dotted line a. In reality the measured values V refi deviate from the ideal line, and the corresponding points al, a2, a3...ak (k = 1, 2, 3...) can be connec¬ ted with an interpolated curve a'. This curve can be considered as the correspondence value of the A/D con¬ verter. The correspondence value can be recorded in tables and stored in the memory 5a of the data proces¬ sing unit 5. The correspondence value can be regularly checked and adjusted, if reason for this arises on the basis of the above described measurements. In principle the correction of the value of the measured electric unit in figure 2 by means of the correspondence value a' is carried out so that on the basis of the measured value, the curve or the table is searched for the value V t , which is corrected with respect to the ideal correspondence value a; in the further processing of the measured values, this value V is treated as the final measured value. In the measu¬ ring arrangement of figure 2, the accuracy of the A/D converter is improved by regularly reading a number of predetermined reference voltages V refk (k = 1, 2, 3...). The DC-voltage source 8 can be composed for instance of a number of identical resistors which are coupled in series. The DC-voltage source 8 in turn is coupled over the serial coupling of the resistors. Because the resistors are identical, they can be built up to a very reliable voltage distributor, wherefrom the desired reference voltages V rβflk (k = 1, 2, 3...), located at equal voltage intervals from each other, are obtained. The correspondence value of the A/D converter is advantageously created so that the values of the reference voltages V re (k = 1, 2, 3...) obtained from the DC-voltage source 8 are recorded in the memory 5a by using recursive averaging. The new correspondence value is formed at regular intervals from these voltage values. By following this procedure, the changes in the temperature of the A/D converter 4, and possible effects of the aging of the converter to the measuring results, are eliminated. If the coefficients of the correspondence value are remarkably different from the original coefficients, which in the measuring arrange- ment were obtained immediately after building up the arrangement, it is probable that parts of the A/D con¬ verter 4, or of some other element of the measuring arrangement, are becoming defective, and an alarm can be made. Figure 4 is a schematical illustration of a third measuring arrangement of the invention. This arrangement comprises a second DC-voltage source 9, a multiplexer 10, a summing circuit 11 and a control unit, which normally is the data processing unit 5. In addition to this, an essential element in this measuring arrangement is the system of figure 2. Like numbers are used for like parts in the measuring arrangements of figures 2 and 4. The electric unit V to be measured is fed in through the input terminal A. Through the summing circuit 11, a reference voltage V 2 is added to the electric unit V 1 . The reference voltage V 2 is obtained from the second DC-voltage source 9, which includes two DC-voltage outputs 9a, 9b, through the multiplexer 10. The multiplexer 10 couples, at predetermined intervals, the DC-voltage outputs 9a and 9b of the DC-voltage source 9 in turns to the summing circuit 11. Now the value of the reference voltage V 2 is fluctuated in between the voltage levels V ref2 : V ref21 , V rβf22 . Figure 5 illustrates the sum signal V 1 + V 2 , where the electric unit V to be measured is an AC-voltage or current signal. The real values of the voltage levels V , V rβf22 of the second DC-voltage source 9 are recorded in the memory 5a of the data processing unit 5. This is done for instance while testing or calibrating the 5. measuring arrangement. The difference V - V „ of the DC-voltage levels of the reference voltage V 2 is observed by means of the data processing unit 5, and any deviations of the recorded, real values of the voltage levels are announced. Possible deviations reflect the conditions and accuracy of the various members, such as switches, amplifiers and converters, of the system. The DC-voltage sources 8 and 9 can be coupled to form a uniform DC-voltage source 90, as is illustra- ted by dotted lines in figure 4. This DC-voltage sour¬ ce can be built of identical resistors, as was descri¬ bed above, between the terminals of which resistors there is connected a suitable DC-voltage V . The resistors form a voltage distributor, from the various channels whereof there is obtained a number of referen¬ ce voltages. Two of these are chosen and connected to the multiplexer 10, thus representing the voltages V ref21 Figure 6 illustrates a measuring arrangement of the invention particularly for measuring electric power and/or energy. To this measuring arrangement there is fed, through the input terminals A and B, a voltage signal V and a current signal I. The voltage and current signal V, I is conducted, via the adaptors 6a and 6b, further to the summing circuits 11a, lib respectively. In the adaptor 6b, the current signal I is converted to the voltage signal V . In the adaptor 6a, the voltage signal V is adapted as a voltage signal V suitable for the measuring arrangement. Furthermore, the measuring arrangement comp¬ rises a DC-voltage source 9 and a multiplexer 10, in similar fashion as in the measuring arrangement of figure 4. The DC-voltage outputs 9a, 9b of the DC-vol¬ tage source 9 are connected to the input terminals of the multiplexer 10. The reference voltage V 2 is ob¬ tained from the DC-voltage source 9 via the multiplexer 10. The voltage outputs 9a, 9b of the DC-voltage sour¬ ce 9, wherethrough the voltages rβf21 and V " rβf22 are fed out, are in turns and at predetermined intervals con¬ nected to the output terminal of the multiplexer 10 by means of the said multiplexer 10. The value of the reference voltage V 2 is in that case fluctuated in between the voltages V rβf21 and V ref22 . The output terminal of the multiplexer 10 is connected to the first summing circuit 11a, and via the attenuator 13 to the second summing circuit lib. From the first summing circuit 11a, the voltage signal V 10 + V 2 is fed to one input terminal of the switch 7. From the attenuator 13, there is obtained the voltage signal aV 2 , where a is the attenuation coefficient of the attenuator, which coefficient is smaller than 1. From the second summing circuit lib there is fed the signal V l + aV 2 to the amplifier 14. The amplification coeffi¬ cient b of the amplifier 14 is comparable to the at¬ tenuation coefficient a of the attenuator 13, so that the product ab of the said coefficients is approximate- ly 1. From the amplifier 14, the signal bV 1;L + V 2 is fed to the sampling and holding circuit 1, and further to one of the input terminals of the switch 7. The DC-voltage source 8 is connected to the input terminals of the switch 7 in similar fashion as in the embodiment of figure 4. The output terminal of the switch 7 is connected, via the summing circuit 2, further to the sampling and holding circuit 12. To the summing circuit 2 there is connected the noise voltage source 3. The signals obtained from the sampling and holding circuit 12 via the switch 7, to which signals there is added noise voltage, are fed to the A/D con¬ verter 4 and further to the data processing unit 5. 15 Thus the latter part of the measuring arrangement of figure 6 corresponds to the arrangement of figure 2. It is pointed out that the noise voltage is summed at different times both to the current and vol- tage signals. Thus the noise voltages do not correla¬ te, and do not affect the power or similar unit to be measured. In principle the measuring arrangement of figure 6 is operated as follows. The arrangement is calibrated by setting the product ab of the attenuation and amplification coefficients of the attenuator 13 and the amplifier 14 to be constant, for instance approxi¬ mately 1, within a desired area of measurement. If the current signal to be measured fluctuates remarkably, there may be several areas of measurement, and in each area the product ab of the attenuation and amplificati¬ on coefficients is set to have a suitable constant value. The purpose of this procedure is to make the measuring values suitably fall within the converting area of the DC-voltage source. Through the first input terminal A, there is measured the voltage signal V, which is adapted in the adaptor 6a to be suitable for the measuring arrange¬ ment, and to which voltage there is added, in the sum- ming circuit 11a, the reference voltage V 2 , the value whereof is fluctuated by means of the multiplexer 10 in between the voltage levels V ref21 , V ref22 of the DC-voltage source 9. The sum signal is further fed into the switch 7. Simultaneously there is measured, through the second input terminal B, the current signal I, which is adapted and converted to be suitable for the measuring arrangement in the adaptor 6b, wherefrom the voltage signal V comparable to the current I under measurement is obtained. In the second summing circuit lib, to the voltage signal V there is added the reference voltage aV 2 , obtained via the attenuator 13, the value of which reference voltage is fluctuated by means of the multi¬ plexer 10 in similar fashion and according to similar rhythm as the value of the reference voltage V 2 fed to the first summing circuit 11a. The sum signal V + aV is amplified in the amplifer 14 to a suitable level, and the voltage signal bV χι + V 2 obtained from the ampli¬ fier is fed into the sampling and holding circuit 1, where the amplified voltage signal, comparable to the current signal I under measurement, and the connected DC-voltage component are held long enough for the value to be read in the switch 7, and to be transmitted furt¬ her in the measuring arrangement. The purpose of the sampling and holding circuit is to ensure that both the voltage signal V and the current signal I are read simultaneously, in order to be able to correctly calcu¬ late the power and/or energy at the sampling moment. By means of the switch 7, there are read in turns the values of the reference voltages V refk (k= 1, 2, 3...) of the DC-voltage source 8, and the voltage V + V 2 comparable to the voltage V under measurement, as well as the voltage b + V 2 comparable to the current I under measurement. To the signals obtained via the output of the switch 7, in the summing circuit 2 there is added the noise voltage signal V from the noise voltage source 3. This sum signal is fed into the second sampling and holding circuit 12, which is furt¬ her read by means of the A/D converter 4. The signal obtained from this converter 4 is further processed in the data processing unit 5. In the memory 5a of the data processing unit 5, there can be stored the values of the measured vol¬ tages V and currents I in a suitable table. From the measuring results, there are calculated active power and energy in a known fashion. When necessary, possi- ble phase differences of voltage and current can be taken into account, and for instance the value of reac¬ tive power can be calculated. The measured quantities and calculated values can be recorded in the memory 5a, for example for possible processing outside the measu¬ ring arrangement. Figure 7 is a fairly detailed illustration, partly a circuit diagram and partly a block diagram, of a preferred embodiment of a power measuring arrangement in a three-phase electrical network. In many respects this measuring arrangement corresponds to the one illu¬ strated in figure 6, and like numbers are used for like parts in the two figures. The voltages of each phase are brought to the measuring arrangement via the input terminals Al, A2, A3, and the currents of each phase through the input terminals Bl, B2, B3. The measuring arrangement is grounded through the terminal C. The input terminals Al, A2, A3 of each phase voltage under measurement are connected to their own adaptor 6a, which is formed of resistors 6al and 6a2. By means of the adaptor 6, the voltages to be measured are dropped to the level of approximately one volt. Each adaptor 6a is further connected to the input terminals of the switch 15. The current inpupt terminals Bl, B2, B3 are connected, via the adaptor 6b, to the rest of the mea¬ suring arrangement. Each adaptor 6b contains a current adaptor 6bl and resistor 6b2. By means of the adaptor 6b, the currents under measurement are adapted to be suitable for the rest of the measuring arrangement. The adaptors 6b of each phase are connected to the input terminals of the switch 16. The switches 15, 16 choose one phase at a time for measurement. The output terminals of the switches 15, 16 are further connected to the rest of the measu¬ ring arrangement, in principle in similar fashion as in the arrangement of figure 6. The measuring arrangement of figure 7 includes a microprocessor, advantageously a single-chip proces¬ sor 17. This single-chip processor 17 comprises a switch 7, a sampling and holding circuit 12, an A/D converter 4, a central or data processing unit 5, an input and output circuit 18 and a multiplexer circuit 19. The internal units of the single-chip processor are interconnected by means of a communication link 20, and respectively the input and output unit 18 is con¬ nected, by means of an external communication link 21, to the switches 15, 16, to the attenuator 13, to the amplifier 14, to the multiplexer 10 and to the sampling and holding circuit 1. In this case the noise source 3 is formed by means of a Zener diode. The noise source 3 is connec¬ ted to an input terminal of the switch 7 of the sin¬ gle-chip processor 17, which terminal is directly con- nected to the output terminal of the switch 7. The first DC-voltage source 8 is realized by means of iden¬ tical resistors 8a, which are arranged in series and over which resistors there is connected the standard DC-voltage V ref from a suitable standard voltage source, which in this case is formed by means of a second Zener diode 8b. From the first DC-voltage source 8 there are obtained five different reference voltages V refk (k= 1, 2, 3...), which are connected to the input terminals of the switch 7 of the single-chip processor 17. By means of the first DC-voltage source 8, there is also formed the second DC-voltage source 9 (cf. figure 6), i.e. they together incorporate the DC-voltage source 20. In this case two voltages, V and Vre„f2„2,' of different p t rhases, r are taken from the the DC-voltage source 8, which two voltages are connected to the input terminals of the multiplexer 10. The voltage V ref affecting over the first DC- voltage source 8, which voltage is obtained from the Zener diode 8b, is also fed to the A/D converter 4 of the single-chip processor 17. By employing the single-chip processor 17, the switches 15, 16 are controlled so that through the input terminals Al, A2, A3; Bl, B2, B3, the same phase of the three-phase current is measured simultaneously. Through the central unit 5 of the single-chip processor 17, and through the communication links 20, 21, there are controlled both the internal sampling and holding circuit 12 and the external sampling and holding cir¬ cuit 1 at the same time. From the second sampling and holding circuit 12, there is read, via the A/D conver¬ ter 4, the voltage value of one phase, whereafter the current value of the corresponding phase, converted to voltage form, is read from the sampling and holding circuit 1 via the switch 7 to the A/D converter 4. Thus the obtained digital voltage and current readings are read simultaneously at the moments tl, t2, t3... from respective measuring phases Al, A2, A3; Bl, B2, B3, as is illustrated in figure 8. The obtained values are processed in the central unit and/or recorded in the memory 5a. In this measuring arrangement, the phase vol- tages entering through the terminals Al, A2, A3 do not fluctuate remarkably, whereas the currents obtained from various phases and measured through the terminals Bl, B2, B3 may vary to a great extent. Therefore the amplification b of the amplifier 14 is adjusted in the control of the single-chip processor 17, so that the voltage signal to be fed through the sampling and hol¬ ding circuit 1 and further to the A/D converter of the single-chip processor 17 is as high as possible. Res¬ pectively, the attenuation a of the attenuator 13 is adjusted in the opposite direction, so that the product ab of the attenuation and amplification coefficients a, b of the attenuator 13 and amplifier 14 is approximate¬ ly 1 in all possible situations. To the voltage and current signals to be mea- sured from the different phases, there is added, via the multiplexer 10, the voltage V 2 , the value whereof is in this case changed every second. Thus the voltage V 2 fluctuates between two values V 2TBI2.L and V 2T£,£„22, in similar fashion as was explained above, in connection with the measuring arrangement of figure 4. By means of this rectangular signal V 2 fluctuating between two voltage values, there is controlled the condition and accuracy of the switch 7, the attenuator 13, the amplifier 14 and the A/D converter 3, by measuring the difference of the said voltage levels V rβf21 , V ref22 (cf. figure 5). The voltage V 2 is added, through the attenuator 13, to the measured current signals entering through the different phases of the input terminals Bl, B2, B3. The product of the attenuation and amplification of the attenuator 13 and the amplifier 14 is approximately 1 in all situations, so that the current signal fed into the single-chip processor 17 always contains an offset voltage as high as in the current signal. In the memo¬ ry unit provided in connection with the single-chip processor 17, there is recorded the difference of the voltage readings V ref21 and V ref22 of the voltage V 2 . From the voltage and current signals to be measured, there is separated the signal V 2 , and its value is compared to the valued recorded in the memory of the single-chip processor 17. On the basis of this comparison, the condition of the measuring arrangement is evaluated. The noise source 3 is permanently connected to the output terminal via the input terminal of the switch 7 of the single-chip processor. Thus noise can be added to each channel of the switch 7, as was desc¬ ribed above in connection to figure 1. Advantageously the employed noise source is a Zener diode 3a. A typi¬ cal feature of Zener diodes is an extremely high noise, which can be further amplified if necessary. The power value of the noise voltage fed into the switch 7 is about 1 bit, which corresponds to the resolution of the A/D converter 4. This procedure requires averaging of several measurements. Today, single-chip processors usually comprise a 10-bit A/D converter at the most. The resolution and accuracy of this type of converter as such are not sufficient for measuring current or voltage or other units calculated on the basis thereof. Consequently, the resolution of the converters is improved by feeding suitable noise voltage, together with the voltage to be measured, to the A/D converter, as was described above, in connection to figure 1. Respectively, the accuracy of the A/D converter can be improved by regularly rea- ding suitable reference voltages, as was explained above in connection to figure 2. In the measuring arrangement of figure 7, the first DC-voltage source 8 is realized as a voltage distributor containing identical resistors 8a. From the serial coupling of the identical resistors 8a, there are suitably obtained the reference voltages V , five in all, which are fed to the input terminals of the switch 7 of the single-chip processor 17, and furt¬ her to the A/D converter 4. By using the reference voltages V (k = 1, 2, 3...), there is created a cor¬ respondence value for the A/D converter 4, which cor¬ respondence value is taken into account as a correction while measuring current and voltage signals. The choice of the model for the correspondence value depends on the capacities of the A/D converter 4; the simpler the model, the fewer calculations are nee¬ ded. Most advantageously the model is created so that the reference voltage measurement values from the se¬ rial coupling of the resistors 8a are recorded in the memory by applying recursive averaging, and a new model is formed of these readings at regular intervals. This procedure eliminates the influence of possible changes in the temperature of the A/D converter 4 in the re¬ sults, as well as the effect of possible aging of the appliances. If the coefficients obtained in the model deviate remarkably from the results obtained by the manufacturer, it is probable that the converter or some other circuit is becoming defective, and an alarm can be made. The noise source 3, particularly the Zener diode 3a, is utilized in another way, too. The voltage affecting over the Zener diode 3a is read, by means of the single-chip processor 17, at regular intervals, and it is used for supervising the voltage V ref affecting over the DC-voltage source 8, i.e. in this case the accuracy of the second Zener diode 8b. If the value of the Zener diode 3a changes, with respect to the voltage V ref , more than the inaccuracy of the measuring arrange¬ ment allows, an alarm is made. The measured voltage affecting over the Zener diode is regularly compared to the reading recorded in the memory of the single-chip processor 17, which reading is stored in the memory for instance during the calibration of the measuring arran¬ gement, carried out in the factory. The accuracy of the A/D converter 4 of the single-chip processor 17 is in this case essentially based on the first DC-voltage source 8, i.e. the resis¬ tance distributor. The changes taking place in the resistance distributor can also be observed by means of the following procedure: the treshold value of the Zener diode 3a of the noise source 3 is chosen to be half of the treshold value of the voltage V arranged over the first DC-voltage source 8, i.e. the treshold value of the second Zener diode 8b. The comparison of these treshold values is carried out via the half-pe¬ riod point of the first DC-voltage source 8. If as much as one value of the reference voltage V vrefk (k = 1, 2, 3...) of this DC-voltage source 8 is changed for example 0.1 - 0.2%, it is observed immediately. The observed percentage of change depends on the equipment and the general accuracy thereof. The three-phase currents to be measured are connected to the rest of the measuring arrangement via the current adaptors 6b1 of the adaptors 6b. Thus their average must be 0. On the other hand, the volta¬ ges of the different phases are connected to the measu¬ ring arrangement without adaptors, and they may contain DC-voltage components. However, the DC-voltage com- ponents do not have any importance as for the power and/or energy to be measured. The reason for this is that the power is formed of the product of the current and voltage to be measured, and current cannot contain a DC-voltage component, apart from one that is caused by the measuring arrangement. In other words, prior to the calculation of power or energy, the DC-voltage components can be deducted from the measured readings of three-phase currents and voltages. Thus the effects of possible DC-voltage components created by the easu- ring arrangement can be totally eliminated from the measurement proper. By synchronizing the sampling both to the frequency of the three-phase network and the reproduc¬ tion frequency of the multiplexer 10, the average of the measured voltage can be accurately defined by cal¬ culating the average of all samples collected during the half-period of the voltage V 2 . In order to speed up the calculation, the average obtained during the pre¬ vious calculation cycle can be deducted from the measu- ring signals. Thus the recording of the values of single measuring points is avoided. In principle the above described synchroniza¬ tion is illustrated in figure 5; the sampling process (points e) is slid, so that during the half-periods c and d of the test voltage V 2 , the measuring signal is subjected to even sampling. In other words, each point of a periodic signal is measured with similar emphasis. If 2 - 4 samples of each period are measured, and the half-period of the test voltage V 2 is about 500 ms, there are effectively obtained 50 - 100 samples per each measuring period, which is fully adequate even for measuring distorted three-phase voltage and current. Owing to the voltage V 2 , to the measuring current and voltage there are connected two DC-current voltage components V ref21 and V ref22 - By updating both averages for each measuring signal, the DC-components are eliminated from the measured signals, and the voltage value ob¬ tained to the control unit of the measuring arrangement is the deduction of the averages. Because the offset voltages caused by the circuits do not disturb the measurements, and because the amplification and at- tenuation of the amplifiers 14 and attenuators 13 can be calibrated for instance in connection with the manu¬ facturing, and the respective coefficients a, b are recorded in the memory, there is no need for adjustable resistors or the like. The elimination of adjustable components reduces the costs of the components in the measuring arrangement, and also cuts the expenses cau¬ sed by the calibration of the arrangement. While calibrating the measuring arrangement of figure 7, the respective amplification and attenuation coefficients a, b of the amplifier 14 and the at¬ tenuator 13 are in principle set with all possile va¬ lues. For each value, there is recorded a number in the memory, which number describes the transmission coefficient from the input terminals A, B as far as the A/D converter 4. Moreover, during the calibration, there is recorded the amplitude of the test voltage V 2 to be added, via the multiplexer 10, to the measured voltage and current, i.e. the difference of the refe¬ rence voltages V ref2χ and V ref22 at different values of the amplification coefficients a of the amplifier 14, with respect to further observation of the measuring arran¬ gement. Moreover, in connection with the calibration procedure, there is measured the value of the noise source 3, which value can later be used in controlling the accuracy of the total voltage V ref affecting over the first DC-voltage source 8. It is pointed out that the purpose of the various control systems in the measuring arrangement is not so much to find out which part or component of the arrangement has become damaged or changed value, but to make sure that the readings con¬ cerning power or other unit to be measured are accu- rate. The communication with the measuring arrange¬ ment can be carried out through a communication circuit 19, particularly a serial communication circuit, provi¬ ded in connection with the single-chip processor 17. All units contained in the single-chip processor 17 are controlled by means of the central unit 5. The calcu¬ lators of the measuring arrangement, as well as the synchronization of the conversions, are controlled through the input/output connection 18 of the single- chip processor. When necessary, the internal com¬ munication link 20 can be provided with memory units, or with a second processor for special measurements. The inaccuracy of the measuring arrangement of figure 7 depends essentially on the adaptors 6a, 6b. The controlling of these adaptors is possible only to some extent when using the above described self cali¬ bration and control procedure. If one of the adaptors 6a, or one of the input conductors in general, is bro¬ ken for example by a lightning, the progress of the voltage V 2 is interrupted, and it is not as such trans¬ mitted to the A/D converter 4; consequently this type of defect is easily observed. But small changes, in the class of one percent, are not detected. Other couplings and units of the measuring arrangement do, however, participate in the surveillance. Because the majority of the units, for instance the amplifier 14, are calibrated and checked regularly, absolute accuracy is not required of the single units. Owing to the various facts explained above, the measuring arrangement of the invention can be rea¬ lized with relatively low costs. Even though the mea¬ suring arrangement of figure 7, for example, can be used for simultaneously measuring a three-phase cur¬ rent, each phase separately, as well as reactive po¬ wers, root-mean-square values of currents and further root-mean-square values of voltages, in addition to the power measurements proper, the manufacturing costs of an apparatus produced according to this measuring ar¬ rangement are not remarkably increased, as compared to the production costs of conventional single-phase kilo- watthour meters. Moreover, by using the measuring arrangement of the present invention, a measuring inac¬ curacy of less than 0.2% is easily achieved. In the measuring arrangement of the invention, the A/D converter 4 can be replaced by one or two con¬ verters external to the processor 17, the resolution of the said converters being within the range 16 - 20 bits. Thus, according to the principles of the measu¬ ring arrangement illustrated in figure 6 or 7, it is possible to build an extremely accurate kilowatthour meter, which is economical in production costs and suitable for calibration purposes. It is naturally clear that in this case other critical components, such as the adaptors 6a, 6b etc., must be chosen according to the accuracy of the measuring arrangement. The invention is not limited to the above described preferred embodiments only, but many modifi¬ cations are possible within the scope of the inven- tional idea defined in the appended patent claims.

Claims: PATENT CLAIMS 1. A method for measuring electric units, in which method - the electric unit is sampled; - the samples are converted to digital form by using an A/D converter and - written out; c h a r a c t e r i z e d in that - there is provided a noise source; - the root-mean-square value of the noise signal is set to be in accordance with the resolution of the A/D converter as for volume; - a noise signal (V n ) is added to each sample; - there are carried out several measurements of the electric unit, on the basis whereof the measuring re¬ sult is calculated and written out. 2. The method of claim 1, c h a r a c t e ¬ r i z e d in that - there is provided a number of reference voltages L V rβfi (k = 1, 2, 3...)]; - the real values of the reference voltages are recor¬ ded in the memory; - the reference voltages are measured at regular inter¬ vals and converted to digital form with the A/D conver- ter; - the real and measured values of the reference volta¬ ges are compared; and - the digital value of the measured electric units is corrected by taking into account the deviations between the measured and real values of the reference voltages. 3. The method of claim 2, c h a r a c t e ¬ r i z e d in that - on the basis of the real and measured values of the reference voltages, there is created a correspondence value for the A/D converter; - the measured values of electric units is corrected on the basis of this correspondence value. 4. The method of claim 1, 2, or 3, c h a ¬ r a c t e r i z e d in that - there is provided at least two DC-voltages (V r , v r e f22 ) ; - the real voltage values (V tl , V ) of the DC-voltage sources are recorded in the memory; - to the electric unit to be measured, there is added a DC-voltage signal (V 2 ), the value whereof is periodical¬ ly fluctuated in between the said DC-voltages; - the difference of the DC-voltages is measured; - the measured voltage difference and the real, recor¬ ded voltage difference is compared, and on the basis of this comparison, the realiability of the measuring arrangement is evaluated. 5. The method of claim 4 for measuring AC- current units, c h a r a c t e r i z e d in that - the sampling of the electric unit is synchronized with the frequency of the AC-current under measurement, and with the fluctuation frequency of the DC-voltages ( V ref2 i' V ref22 ^ of the D C ~ voltage signal to be added thereto; - the average of the electric unit samples taken during the half-period (c, d) of the fluctuation frequency of the DC-voltage signal is calculated; - this average represents a measuring value which is written out and/or recorded for further processing. 6. The method of claim 4 or 5 for measuring electric power and/or energy, in which method the vol¬ tage and current signals are measured, and on the basis thereof there is calculated the corresponding electric power and/or energy, c h a r a c t e r i z e d in that - the current signal (I) is converted to a comparable second voltage signal (V n ) ; - to the first voltage signal (V 10 ) to be measured, and to the second voltage signal (V ) comparable to the current signal, there is added a DC-voltage signal (V ), the value whereof is periodically fluctuated, and to which first voltage signal (V 10 ) the DC-voltage signal (V 2 ) is directly added, whereas to the second voltage signal (V ) it is added in attenuated form (aV ; a<l); - the summed-up signal of the second voltage signal and the attenuated DC-voltage signal (V u + aV 2 ) is at¬ tenuated, so that the product of the amplification (b) and the attenuation (a) of the above mentioned DC-vol¬ tage signal (V 2 ) is constant, advantageously approxima- tely one; - the first voltage signal (V ) and the second voltage signal [t>( + aV 2 ) ] treated in the above described fashion are converted from analog to digital form, so that during the conversion of one voltage signal, the value of the second voltage signal is kept unchanged, whereafter this voltage signal is converted; - the digital voltage signals are fed into the data processing unit (5), where the power and/or energy values are calculated in a known fashion, and written out and/or recorded in the memory for further proces¬ sing. 7 . The method of claim 6 , c h a r a c t e ¬ r i z e d in that - the product (ab) of the attenuation and amplification is defined prior to the measurements proper at various readings of the current signal, and stored in the memo¬ ry; - the product of the attenuation and amplification is set to be constant, advantageously approximately one, according to the size of the current signal under mea¬ surement. 8. A measuring arrangement for measuring electric units, the said measuring arrangement comp¬ rising: - means for successive sampling of the electric signal under measurement (1); - an A/D converter (4) for converting the samples from analog to digital form; and - means for controlling the measuring procedure, for calculating the results and writing them out (5) ; c h a r a c t e r i z e d in that the measuring arran- gement comprises: - a noise source (3), the root-mean-square value of the noise signal (V n ) whereof is set according to the reso¬ lution of the A/D converter (4) as for volume; and - means (2) for adding the noise signal (V ) to the electric signal under measurement. 9. The measuring arrangement of claim 8, c h a r a c t e r i z e d in that the measuring arran¬ gement comprises: - at least one DC-voltage source (8) which produces a number of reference voltages: [V refk (k = 1, 2, 3...)]; - a memory unit (5a) where the real voltage values [V (k = 1, 2, 3...)] of the reference voltages are sto¬ red; - a switch (7), where the reference voltages are con- nected to, and where the electric unit to be measured also is connected to; - comparison means (5) whereby the measured values from the reference voltage sources, obtained from the A/D converter through the switch (7) , are compared to the real values of the reference voltages, recorded in the memory unit, which comparison means most advantageously include a table of corrections, on the basis of which table the measured values of electric units are corrected; - a memory unit (5a), where the deviations between the real and measured values of the reference voltages, obtained from the comparison means (5), are recorded; - means (5) for correcting the value of the electric unit under measurement, where the measured value of the electric unit, obtained from the A/D converter (4), is corrected by taking into account the deviations of the A/D converter from the table of corrections. 10. The measuring arrangement of claim 8, c h a r a c t e r i z e d in that the arrangement comprises - at least one DC-voltage source (9), which produces at least two DC-voltage signals (V ref21 , ref22 ) ; - a memory unit (5a), whereto the real values (V 2 ) of the reference voltages (V ref21 , V ref22 ) are recorded; - a multiplexer (10); - a summing circuit (11; 11a, lib); - a control unit (5); whereby a DC-voltage signal (V 2 ) is added to the elect¬ ric unit under measurement, the voltage value of which signal is periodically fluctuated by means of the mul¬ tiplexer (11) between the voltages (V r , V ref22 ) of the DC-voltage source (9), and the difference of these voltages is compared in the control unit (5) to the real values stored in the memory unit (5a), and the deviations from the recorded, real values are announ¬ ced. 11. The measuring arrangement of claim 9, c h a r a c t e r i z e d in that the DC-voltage sour¬ ce (8) is formed of a voltage distributor comprising a number of serially coupled, most advantageously identi¬ cal resistors (8), and in parallel to these there is installed a DC-voltage source (8b). 12. The measuring arrangement of claim 10, c h a r a c t e r i z e d in that the second DC-volta¬ ge source (9) is formed of the first DC-voltage source (8) connected to the measuring arrangement. 13. The measuring arrangement of claim 10, particularly for measuring electric power and/or ener¬ gy, to which measuring arrangement there is fed both voltage and current signal, c h a r a c t e r i z e d in that the measuring arrangement comprises: - two summing circuits (11a, lib), whereby to the vol¬ tage and current signals there is added the voltage signal from the DC-voltage source (9) via the multiple- xer (10); - an attenuator (13), wherethrough the voltage signal from the DC-voltage source (9) is transmitted to the summing circuit (lib) of the current signal; - an amplifier (14), whereto the summed-up current signal and DC-voltage signal are fed; and wherefrom they are further fed to the switch (7) , to the A/D converter (4) and to the means (5) for calculating the power/energy; and - in which arrangement the product (ab) of the at¬ tenuation (a) and amplification (b) of the attenuator (13) and amplifier (14) is constant, advantageously approximately one.

Inventors: SEPPAE HEIKKI, SEPPAE, HEIKKI
Applicants: VALTION TEKNILLINEN, VALTION TEKNILLINEN TUTKIMUSKESKUS
Application year: 1990
Main classification: G01R21
IPC codes: G01R21