JP2006038551A - Digital ac wattmeter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a digital AC Wattmeter of no conversion error even for an A/D converter. <P>SOLUTION: The multiplexer 3 outputs the voltage a and the current b alternatively by switching, the A/D converter 4 A/D converts the output of the multiplexer. The sine wave generator circuit 9 holds the sine waveform and the cosine waveform in phase with the voltage waveform, and the phase of sine waveform and cosine waveform are shifted by the phase difference between the voltage and current. The multiplexer 8 outputs the same phase or phase shifted sine waveform and cosine waveform synchronizing with the multiplexer by switching. The multiplier 6 multiplies the voltage digital data by the voltage in the same phase sine waveform and the cosine waveform, and multiplies the current digital data by the phase shifted sine wave and cosine wave. The integrator 11-14 integrates the result of multiplication integrates for real part and the imaginary part respectively. The complex multiplier 15 complex multiplies the result of integration so as to obtain the active power and reactive power. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a digital AC wattmeter using an A / D converter.

  This type of digital AC power meter converts each value of the voltage to be measured (hereinafter referred to as “measurement voltage”) and the current to be measured (hereinafter referred to as “measurement current”) into digital data by an A / D converter. Multiply them to find the power value. In this case, in order to reduce the cost and simplify the circuit configuration, there may be one A / D converter as shown in FIG.

  In FIG. 5, the measurement voltage is taken out by the transformer 30 and the measurement current is taken out by the current coupler 31 and led to the multiplexer 32. The multiplexer 32 switches between voltage and current in response to the square wave and alternately outputs it to the A / D converter 33. The A / D converter 33 converts the voltage and current into digital data, and these digital data are held in the latch 34 and the latch 35 in synchronization with the switching of the voltage and current of the multiplexer 32 by the action of the inverter 36, respectively. . The contents held in the latches 34 and 35 are output to the multiplier 37 and multiplied. The integrator 38 integrates the output of the multiplier 37 for a predetermined time and outputs the measured power as a display.

  Instead of multiplying the sampled voltage and current in the A / D converter as they are, the power is obtained, but the sampled voltage and current are subjected to Fourier transform (Fourier Form Transform), and the frequency of the measurement voltage and measurement current (hereinafter referred to as “the measurement voltage”). It has been proposed to obtain the power of the measurement frequency component by taking out the component (denoted as “measurement frequency”) and multiplying the voltage and current (see, for example, Patent Document 1).

  The voltage and current of the frequency individually separated by the Fourier transform are arbitrarily added after multiplication and frequency response correction, respectively. At this time, depending on the addition control, powers of a plurality of different harmonic components can be obtained independently and simultaneously, or by setting a band over a plurality of harmonic components, power in this band can be obtained.

JP-A-4-50668 (first page, FIG. 1)

  In general, in digital AC wattmeters, errors such as sampling errors and noises superimposed when sampling voltage and current, and harmonics generated by a switching power supply or the like make the indicated value of power inaccurate. In recent years, there are many devices that generate harmonics in particular, and it is desired to reduce the influence thereof.

  However, in the technique described in Patent Document 1 described above, since two A / D converters are used, there is a problem that the cost is increased and it is relatively weak against noise. Furthermore, this method seems to be used for the purpose of accurately measuring high-frequency power. For the purpose of measuring only the fundamental wave, multiplication for each frequency component is not necessary, which also causes an increase in cost. .

  On the other hand, in the example (FIG. 5) in which sampling is performed with one A / D converter, there is a problem that a phase error occurs because the voltage and current are switched and measured at different timings. This point will be described with reference to the timing chart of FIG. Here, in order to simplify the illustration and description, sampling is performed at 8 times the measurement frequency. However, in practice, sampling is often performed at a frequency of 64 to 256 times the measurement frequency.

  In FIG. 5, when voltage / current is input to the A / D converter 33, the A / D converter 33 performs sampling at a sampling period of 8 times. However, since there is only one A / D converter 33, voltage and current cannot be sampled simultaneously. Even if the voltage and the current are sampled alternately, as shown in FIG. 6, a time lag exceeding the conversion time of each measurement data occurs.

  This deviation becomes a phase error, and if it is a time that cannot be ignored with respect to the period of the measurement frequency, it becomes an error in the measurement result. For example, if an A / D converter 33 that requires a conversion time of 20 μs is to be measured at a measurement frequency of 50 Hz, 20 μs ÷ 20 ms = 0.001, sin (0.001 × 2π) = 0.0063, and a maximum error of 0.63% is generated. If this accuracy is to be increased, it is necessary to use a higher-speed A / D converter 33, which is contrary to the purpose of reducing the cost.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a digital AC wattmeter having a function that does not cause an error due to conversion time that occurs in the prior art even if only one A / D converter is used.

  The digital AC power meter according to the present invention converts a measured voltage into a voltage that can be used in an A / D converter (4 in FIG. 1) by a transformer (1 in FIG. 1) and a measured current by a current coupler (2 in FIG. 1). Then, it is switched by a multiplexer (3 in FIG. 1) and supplied to the A / D converter. As a result, the multiplier (6 in FIG. 1) that executes the A / D converter and the Fourier transform is shared. The A / D converter samples voltage and current and converts them into digital data. The sampling frequency may be at least twice the measurement frequency according to the sampling theorem, but a higher frequency, such as 64 times or 128 times, is used. This is for high resolution.

  The multiplier performs an operation of multiplying each value of the digital data by a sine wave and a cosine wave of the measurement frequency. As is well known, in such a Fourier transform, a complex wave having a period is a combination of simple waves, and when a complex wave is multiplied by a simple wave, it has the same frequency as the simple wave. The idea is that the waves can be extracted. In digital data obtained by sampling analog voltage / current, waveform distortion has occurred and noise is also mixed in, but the sine wave and cosine wave waveforms to be multiplied are purely theoretical, mathematical and pure. As a result, the voltage / current values of the measurement frequency component can be extracted from the digital data.

  Sampling by the A / D converter is not simultaneous because the voltage and current are switched and supplied by the multiplexer. However, in the present invention, the difference between the phase when the voltage value is multiplied by the sine wave and the cosine wave and the phase when the current value is multiplied by the sine wave and the cosine wave are the same as the phase difference between the measurement voltage and the measurement current. It has the feature of setting. Thereby, even if there is a phase difference between the measurement voltage and the measurement current, it is possible to extract accurate voltage / current values following the phase difference. Also, the adverse effect of having one A / D converter can be eliminated.

Now, if the sine wave and cosine wave multiplied by the multiplier are expressed by complex numbers with the cosine wave as a real component and the sine wave as an imaginary component, the output voltage v (t) and output current i (t + Δt) of the multiplier are Can be expressed by the following formula.
v (t) = {cos (wt) + jsin (wt)} vin (1)
i (t + Δt) = {cos (w) (t + Δt) + jsin (w) (t + Δt)} iin (t + Δt) (2)
vin is a measurement voltage, iin is a measurement current, and Δt is a phase difference between the measurement voltage and the measurement current.

Next, the output voltage v (t) and output current i (t + Δt) of the multiplier are numerically integrated by the integrators (11 to 14) for a certain period of time (an integral multiple n of the period of the measurement frequency). Thus, the voltage value V (t) and the current value I (t + Δt) are obtained.
Square root of V (t) = 2 × Σv (t) ÷ n (3)
Square root of I (t + Δt) = 2 × Σi (t + Δt) ÷ n (4)
Finally, when the voltage value V (t) and the current value I (t + Δt) are multiplied by a complex multiplier (15 in FIG. 1),
P (t) = V (t) × I (t + Δt) (5)
Becomes the required power. The voltage value V (t) and the current value I (t + Δt) are complex numbers, and the real component of the power P (t) indicates active power and the imaginary component indicates reactive power.

  The multiplication in the complex multiplier (15 in FIG. 1) multiplies the voltage and current values that are accurately extracted. As a result, although there is only one A / D converter at the time of measurement, there is no error due to conversion time that occurs in the prior art. Of course, V (t) and I (t + Δt) are out of phase by Δt, but the difference between V (t) and V (t + Δt), which is the integral value, is an instantaneous value. Since the difference between v (t) and v (t + Δt) is sufficiently gradual, the generated error is small.

  In addition, since this invention calculates | requires electric power by a Fourier transformation, the error by a harmonic is not produced like the prior art of patent document 1. FIG.

  As described above, according to the present invention, a single A / D converter is used, and the sampled voltage / current value has the same phase difference as the phase difference between the measurement voltage and the measurement current, and the sine wave / cosine of the measurement frequency. Since the configuration of multiplying the wave and integrating the multiplication result is adopted, even if the measurement time is shifted due to the single A / D converter, the error of the result due to the phase error does not occur, and the sampling error It is possible to provide a digital AC wattmeter with reduced power.

  The digital wattmeter of the present invention comprises a first switching means for alternately switching and outputting a voltage and a current to be measured, an analog / digital conversion means for converting the output of the first switching means to digital data, and the voltage / current A sine wave generating means for holding a sine waveform and a cosine waveform with respect to frequency in phase with the voltage, and a phase of the sine waveform and cosine waveform being shifted by a phase difference between the voltage and current; Alternatively, the second switching means for switching and outputting the sine waveform and the cosine waveform of the phase shift in synchronization with the first switching means, and the voltage output by the second switching means for the digital data of the voltage output from the analog / digital conversion means The digital data of the current output from the analog / digital conversion means is multiplied by the in-phase sine waveform and cosine waveform. Multiplication means for shifting the phase output from the switching means and multiplying the sine waveform and cosine waveform; integration means for integrating the multiplication result for each imaginary component and real component of the voltage and current; and complex multiplication of the integration result And a complex multiplication means for obtaining active power and reactive power.

  More specifically, the integrating means is composed of four integrators that function sequentially for each switching of a set of voltage and current output by the first switching means. The sine wave generating means is composed of a ROM table.

  In order to clarify the objects, features, and advantages of the present invention, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a block diagram showing an embodiment of the digital AC wattmeter of the present invention. This digital AC power meter includes a transformer 1, a current coupler 2, a multiplexer 3, an A / D converter 4, a latch 5, a multiplier 6, a timing generation circuit 7, a multiplexer 8, a sine wave generation circuit 9, a decoder 10, and four integrations. The circuits 11 to 14, four latches 11 a to 14 a, four latches 11 b to 14 b, and a complex multiplier 15 are included.

  The timing generation circuit 7 divides the square wave j having a frequency of 3200 Hz into a half, a half (original quarter) and a half (800 original). Generate a square wave. Here, the measurement voltage or the measurement current is sampled at a half frequency (1600 Hz) e. This corresponds to 32 times sampling when the measurement frequency is 50 Hz.

  Although the digital data obtained by sampling is integrated, a quarter-frequency (800 Hz) c is required to calculate the imaginary component and the real component for voltage and current, respectively. In addition, a 1 / 3200th frequency (1 Hz) x is generated in order to initialize each integrator at a cycle of 1 second.

  The half frequency division e is supplied to the A / D converter 4, the latch 5, the multiplexer 8 and the sine wave generation circuit 9, and the quarter frequency division c is supplied to the multiplexer 3 and the decoder 10 to be 1/3200. The divided frequency x is supplied as an initialization signal x to the integrators 11 to 14 and the latches 11b to 14b.

  The transformer 1 takes the measurement voltage a, and the current coupler 2 takes the measurement current b and converts it into a voltage that can be used in the A / D converter 4. The measurement frequency is 50 Hz or 60 Hz for commercial power. The multiplexer 3 switches the outputs of the transformer 1 and the current coupler 2 in response to the high / low level of the quarter frequency division c and outputs it to the A / D converter 4. As a result, for example, the measurement voltage is input to the A / D converter 4 when the quarter frequency c is low, and the measurement current is input when the quarter frequency c is high.

  The A / D converter 4 alternately converts the measurement voltage a and the measurement current b from the multiplexer 3 into digital data in response to the half frequency e. This conversion is performed by sampling the analog measurement voltage a or measurement current b. Digital data obtained by sampling is input to the multiplier 6 through the latch 5. Hereinafter, when voltage and current are distinguished in digital data, the former is referred to as “voltage data” and the latter as “current data”.

  On the other hand, the sine wave generation circuit 9 holds the sine waveform sin and cosine waveform cos of the measurement frequency in a ROM table, for example. The retained sine waveform sin and cosine waveform cos are purely theoretical, mathematical and pure. The holding is performed in such a manner that the phase of the sine waveform sin and the cosine waveform cos is the same phase as the measurement voltage, and the phase of the sine waveform sin and the cosine waveform cos is shifted by the same phase difference as the measurement voltage a and the measurement current b. .

  The sine wave generation circuit 9 alternately outputs the sine waveform sin and the cosine waveform cos in response to the half frequency. The multiplexer 8 alternately outputs the sine waveform sin and the cosine waveform cos to the multiplier 6 in response to the half-frequency e. For this reason, if the state of switching between the measurement voltage a and the measurement current b in the multiplexer 3 is recalled, the sine waveform sin and cosine waveform cos when the voltage data is supplied to the multiplier 6, and the current to the multiplier 6. It will be understood that the phase of the sine waveform sin and cosine wave cos when data is supplied can be shifted by the phase difference between the measurement voltage a and the measurement current b.

  The multiplier 6 multiplies the digital data as described above by the sine waveform sin and the cosine waveform cos as described above. This is to extract a necessary measurement frequency component from the digital data. That is, the digital data input to the multiplier 6 has a waveform that is distorted and possibly noise. However, even in such a case, a desired measurement frequency component can be obtained from digital data by the effect of Fourier transform by multiplying the sine waveform sin and cosine waveform cos which are not mixed.

  The multiplier 6 multiplies the voltage data by a sine waveform sin and cosine waveform cos having the same phase, for example, and multiplies the current data by a sine waveform sin and cosine waveform cos having phase shift, for example. The former is execution of Expression (1), and the latter is execution of Expression (2). For this reason, the effect of the above-mentioned Fourier transform can be made effective. The multiplier 6 sequentially obtains the imaginary component VIm of the voltage data, the real component VRe of the voltage data, the imaginary component IIm of the current data, and the real component IRe of the current data, and outputs them on the common input lines of the latches 11a to 14a. The

  Next, the decoder 10 supplies four select signals 1 to o to the latches 11a to 14a and the integrators 11 to 14 for every input of the quarter-frequency division c. The latches 11a to 14a respectively respond to the select signals 1 to o with the imaginary component VIm of the voltage data, the actual component VRe of the voltage data, the imaginary component IIm of the current data, and the actual component IRe of the current data on the common input line. And output to the integrators 11-14.

  The integrators 11 to 14 integrate the imaginary component VIm of the voltage data, the actual component VRe of the voltage data, the imaginary component IIm of the current data, and the actual component IRe of the current data in response to the select signals 1 to o, respectively. To the latches 11b to 14b. The integration of voltage data is the execution of equation (3), and the integration of current data is the execution of equation (4).

  The latches 11 b to 14 b output the integration results from the integrators 11 to 14 to the complex multiplier 15 all at once with the initialization signal x from the timing generation circuit 7. The integrators 11 to 14 are initialized by the initialization signal x from the timing generation circuit 7, thereby completing one integration period and starting integration again from the 0 state.

  The complex multiplier 15 multiplies the integration result of voltage data and the integration result of current data. Since each integration result consists of an imaginary component and a real component, this multiplication is a complex multiplication. That is, execution of Expression (5). The complex multiplier 15 outputs the real component obtained as a result of the complex multiplication as active power and the imaginary component as reactive power.

  Although FIG. 2 shows a detailed view of the integrator 11, the integrators 12 to 14 have the same configuration. This integrator includes an adder 20, an AND gate 21, a selector 22, and a D-type flip-flop circuit 23. Since the D-type flip-flop circuit 23 uses the select signal l as a clock, an input signal when the select signal l is input is set.

  While the initialization signal x is not input, the adder 20 adds the multiplication result from the latch 11 a to the output of the main integrator 11 held by the D-type flip-flop circuit 23. However, the selector 22 accepts the output of the main integrator 11 held by the D-type flip-flop circuit 23 and outputs it to the D-type flip-flop circuit 23 when there is no input of the select signal l. That is, the output of the adder 20 is ignored and the integrator 11 only holds the output.

  When the select signal l is input, the selector 22 receives the output of the adder 20 and outputs it to the D-type flip-flop circuit 23. Since the adder 20 adds the multiplication result from the latch 11a to the output of the D-type flip-flop circuit 23, the multiplication result from the latch 11a is reflected in the main integrator 11, and the integration is executed.

  When the initialization signal x is input, the adder 20 is cleared, and the first multiplication result from the latch 11a accepted by the subsequent first select signal l is set in the D-type flip-flop circuit 23.

  Next, the operation of the digital AC wattmeter will be described with reference to the timing charts shown in FIGS. In FIG. 1, the half frequency (1600 Hz) e generated by the timing generation circuit 7 is used as the sampling pulse e, and sampling is performed 32 times with respect to the measurement frequency 50 Hz. In view of the ease of viewing, the sampling is illustrated as 8 times.

  When the measurement voltage a and the measurement current b are supplied to the multiplexer 3, the multiplexer 3 uses the quarter frequency c as the multiplexer clock c as shown in FIG. At the level, the measurement current b is output in small slices (1/8 of the measurement cycle) (d).

  The A / D converter 4 samples the input measurement voltage a and measurement current b alternately every sampling pulse e. In FIG. 3, the A / D converter output f is shown as 0 to 3 for four sets, each set of one sampling of the measurement voltage a and the measurement current b for eight samplings. Voltage data and current data obtained by sampling are supplied to the multiplier 6 through the latch 5.

  The half frequency e is also used as the selector clock e, and the selector output i output from the multiplexer 8 outputs a sine wave g to the multiplier 6 when the selector clock e is at a high level and a cosine wave h to the multiplier 6 when the selector clock e is at a low level. . The phases of the sine wave g and the cosine wave h are shifted by the phase difference between the measurement voltage a and the measurement current b when the voltage data is supplied and when the current data is supplied. Thereby, execution of Formula (1) and Formula (2) is ensured.

  Multiplier 6 multiplies voltage data and current data by sine wave g and cosine wave h, respectively. In FIG. 4, the multiplier output k is displayed corresponding to the group displays 0 to 3 of the A / D converter output f in FIG. Therefore, the first multiplier output k in the set is the imaginary component VIm of the voltage, the second multiplier output k is the real component VRe of the voltage, the third multiplier output k is the imaginary component IIm of the current, and the fourth The multiplier output k shows the actual component IRe of the current.

  On the other hand, the decoder 10 outputs four select signals l, m, n, and o to the latches 11a, 12a, 13a, and 14a and the integrators 11, 12, 13, and 14 for each pulse of the quarter frequency c. To do. The latches 11a, 12a, 13a, and 14a respond to the select signals l, m, n, and o from the integrator output k to the voltage imaginary component p, the voltage real component q, the current imaginary component r, and the current real The component s is output to the integrators 11, 12, 13, and 14.

  The integrators 11, 12, 13, and 14 respond to the select signals l, m, n, and o from the integrator output k to the voltage imaginary component p, the voltage real component q, the current imaginary component r, and the current. Are integrated, and an imaginary component integral t of the voltage, an integral u of the real component of the voltage, an integral v of the imaginary component of the current, and an integral w of the real component of the current are output.

  These integrations are continued until the initialization signal x having a frequency of 1/3200 is input from the timing generation circuit 7. Accordingly, the integral t of the imaginary component of the voltage, the integral u of the real component of the voltage, the integral v of the imaginary component of the current, and the integral w of the real component of the current are the imaginary component p of the voltage for 800 times and the real component of the voltage, respectively. q, the imaginary component r of the current, and the real component s of the current are added.

  When the initialization signal x is input to the integrators 11 to 14 and the latches 11b to 14b, the integrators 11 to 14 integrate the voltage imaginary component t, the voltage real component integral u, the current imaginary component integral v, and the current. The real component integral w is output to the complex multiplier 15 via the latches 11b to 14b. The complex multiplier 15 performs complex multiplication on these to output reactive power y and active power z.

  In the present invention, even if sampling is performed using only one A / D converter, the voltage data and current data obtained by sampling are multiplied by the sine wave and cosine wave of the measurement frequency, and the result is obtained. Since each integration is performed, noise and the like can be removed and a necessary frequency component can be extracted. The voltage data and current data are multiplied by a sine wave / cosine wave of the measurement frequency with the same phase difference as the phase difference between the measurement voltage and the measurement current, and the multiplication result is integrated. An error due to a long conversion time does not occur.

The block diagram which shows one Example of the digital AC meter of this invention Detailed view of integrator 11 in FIG. Timing chart showing the operation of the first half of the digital AC meter according to the present invention Timing chart showing the latter half of the operation of the digital AC meter according to the present invention Block diagram showing an example of a conventional digital AC meter Timing chart of the conventional example of FIG.

Explanation of symbols

1 Transformer 2 Current Coupler 3 Multiplexer 4 A / D Converter 5 Latch 6 Multiplier 7 Timing Generation Circuit 8 Multiplexer 9 Sine Wave Generation Circuit 10 Decoder
11a-14a latch
11-14 integrator
11b-14b Latch 15 Complex multiplier 20 Adder 21 AND gate 22 Selector 23 D-type flip-flop circuit

Claims (3)

First switching means for alternately switching and outputting the voltage and current to be measured;
Analog / digital conversion means for converting the output of the first switching means into digital data;
A sine wave generating means for holding a sine waveform and a cosine waveform with respect to the frequency of the voltage / current in phase with the voltage, and shifting a phase of the sine waveform and cosine waveform by a phase difference between the voltage and current;
Second switching means for switching and outputting the same-phase or phase-shifted sine waveform and cosine waveform from the sine wave generating means in synchronization with the first switching means;
The digital data of the voltage output from the analog / digital conversion means is multiplied by a sine waveform and a cosine waveform in phase with the voltage output from the second switching means, and the digital data of the current output from the analog / digital conversion means. Includes a multiplying means for shifting the phase output from the second switching means and multiplying the sine waveform and the cosine waveform,
Integrating means for integrating the result of the multiplication for each imaginary component and real component of the voltage and current;
A digital wattmeter comprising complex multiplication means for obtaining active power and reactive power by complex multiplication of the result of integration. 2. The digital wattmeter according to claim 1, wherein the integrating unit is configured by four integrators that function sequentially for each switching of a set of voltage and current output from the first switching unit. The digital wattmeter according to claim 1, wherein the sine wave generating means is constituted by a ROM table.

Images