JPS61284669A - Digital watthour meter - Google Patents

Abstract

PURPOSE:To measure many items by one instrument simultaneously by sampling the phase currents of two phases of an electric power system and the voltage between two wires relating to them and performing specific arithmetic on the basis of digital values of the currents and voltage. CONSTITUTION:The voltage and current values of the electric power system are supplied to sample holding circuits 11 and 12 through a voltage detector PT and a current detector CT and the voltage and current value are sampled at every interval of time corresponding to, for example, an electric angle of 30 deg. and held up to a next sampling period. Those values are stored in a memory 3 through an A/D converter 3 while switched by a multiplexer 2. Then, an arithmetic processor 5 performs the specific arithmetic on the basis of stored data in the memory 4 to obtain three-phase active power and three-phase reactive power. Those values are sampled at the intervals of one second to find plus and minus three-phase active electric energy and positive and negative three-phase reactive electric energy. Thus, the various values of electric power are measured by the same instrument simultaneously, so the cost is reduced.

Description

[Detailed Description of the Invention] [Field of Industrial Application] This invention is particularly applicable to three-phase active power (W) in a power system.
; Watt), reactive power (Var).

Active energy (WH; Watt hour), reactive power jt (Var
The present invention relates to a digital watt-hour meter that digitally measures H (pearl hour).

[Conventional technology]

Traditionally, current force needle-shaped instruments have been used to measure active power (W) or reactive power (Var), and custom-made disc-shaped instruments have been used to measure active power (WH) or reactive energy (VarH). , w1-child type time-division multiplication type instruments, etc. are used, respectively.

[Problem that the invention seeks to solve]

These instruments have a long history and are technically complete, so it can be said that there are almost no major problems. In other words, the conventional measurement method basically measures one type of item: active power, reactive power, active energy, or reactive energy using a single meter, and as far as that goes, there are no particular problems. However, when trying to measure multiple items, a problem arises in that a single meter cannot handle the problem and requires as many meters as the number of items, which increases the economic burden.

Therefore, it is an object of the present invention to reduce costs by making it possible to simultaneously measure the various amounts of electric power as described above using the same device. Sampling means for sampling each phase current of at least two phases and two line voltages related thereto at a predetermined period, and analog/digital conversion means for converting each sampled current and voltage amount into a digital amount. and calculation means for performing a predetermined calculation based on the digital quantity.

[Effect]

The calculation means performs predetermined calculations using the current and voltage that have been sampled and converted into digital quantities, thereby calculating three-phase active power, three-phase reactive power, positive or negative three-phase active energy, positive or negative Measure the amount of negative reactive power for each of the three phases.

Here, the calculation formulas for determining the above quantities will be explained.

b) According to the general definition of single-phase active and reactive power, the amplitude of voltage and current is V
, I, if the phase difference between voltage and current is θ, and the angular velocity is ω, then
The instantaneous values v and i of the voltage and current of the single-phase AC circuit to be measured are expressed as v=vsla(IJt I-I, □ (ω is one θ), and therefore the single-phase active power P is P-VIc
O5θ --------(1).

To measure this digitally, it is necessary to sequentially sample the instantaneous values of voltage and current. Therefore,
If you fight, the voltage will be as shown in Figure 3.

Assuming that the instantaneous value of the current is sampled every 30" electrical angle of alternating current (1/600 seconds in the case of 50 degrees), the data is vn-Vsin (mut-nx/6)'n=l
5in(ωt −n x/ 5−〇)(n−0, 1, 2
, 3, etc.), and it is known that the single-phase active power p(w) can be expressed as shown in the following equation using these data.

P"vn in+vfL-3'n-5-vICo, θ
...Song (2) In other words, the active power P
is the voltage and current value vntin at time n, and the voltage and current value vn sampled three times earlier (9o° before).
-31in-3 by calculating the above equation (2).

Similarly, the single-phase reactive power Q is generally Q=VIs
i. θ...song (3), so if we express this in the form of sampling data, we get Qxy,, import-3+vn-3in-6 m V I sinθ...song (4
).

(Example) Three-phase active and reactive power is defined as "polyphase power is the sum of each induced power." Therefore, three-phase active power P3 (W) consisting of R phase, S phase, and
is expressed as P3-VrIr+V, I, +Vi Ii. Here, Vr# vst and vt are r phase, respectively.
The voltages of the S phase and the phase, and the currents of the r phase, S phase, and the phase, respectively.

Here, instead of the phase voltage, 5, for example, rs phase line voltage Vrs
(= Vr Vs ), ts phase line voltage Vt5 (=
By using the well-known two-wattmeter method, the three-phase active power can be calculated from the following equation, Pa = Vrs Ir + Vts It'' Vrs
Ir Vst It ”= (5) Expressing this in the form of sampling data as in equation (2), P 3-(V r, ” X 1 r n + V r s
”-3X I r n-3)-(v5t”x 't”
vst""xit"-3)... (6) Here, vrsnt vrs"3 is rs phase line voltage data, vs @"e vs t"-3 is st
Phase-line voltage data, 1r, Otori, is r-phase current data,
itn.

i and na indicate t-phase current data, respectively.

Similarly, the three-phase reactive power Q3 (var-r) is calculated as above (
From formula 4L (5), Q3= (Yrs"X, rfl-3+vr, n-3X
ir"-' ) ('/s tn X l, n-3+
vstn 3 x i, n-6) (7)

c) Regarding electric power amount, "When IW power flows for one hour, it is considered as IWH." This is the basis of electric power amount. In other words, the area indicated by A in Fig. 4 is the IWH as defined, but the area indicated by B and C in the same figure is
The area represented by is also IWH. Therefore, if sampling is performed once every hour and the value at that time is IW, it may be determined that IWH has flowed, but if this is done, there is a risk that errors due to power fluctuations will become large. In other words, it should be as shown in Figure 5 (A), but it should be as shown in Figure 5 (A).
This is because there may be cases where the value fluctuates as shown in t2). However, if it is sampled once per second and all of the values are IW over an hour, it can be determined that IWH has flowed, and if the sampling values in this case are integrated for one hour, IWX3600 seconds - 3600 W seconds - 1WH, and the amount of electric power can be calculated. In reality, it is inconceivable that exactly the same amount of power as IW could flow for one hour, so
In this case, values sampled at 1-second intervals are integrated, and when the integrated value reaches 3600 W seconds, it is determined that IWH has flowed.

〔Example〕

FIG. 1 is a block diagram showing an embodiment of the invention, and FIG. 1A is a block diagram showing the structure of a memory.

In Fig. 1), 11112 is a well-known sample and hold circuit, 2 is a multiplexer for making the selection, 3 is an analog/digital converter, 4 is a memory, and 5
is a processing unit (CPU) such as a microcomputer
It is.

In other words, the voltage and current values of the power system are determined by a voltage detector (PT
), is applied to the sample and hold circuit 11 e 12 via the current detector (CT), and here, for example, the electrical angle of 30°
Voltage and current values are sampled every time corresponding to
It will be held until the next sampling period. This value is switched by a multiplexer 2, converted to a digital value by an A/D converter 3, and stored in a memory (Random Access Memory) 4. The sampled voltage and current data are stored in the RAM memory 4, for example, in the first A
The information is stored in chronological order as shown in Figures (A) and (B).

The arithmetic processing unit 5 calculates the three-phase active power P3 and the three-phase reactive power Q3 from K by calculating the equations (6) and (7) based on the data stored in the memory 4, and also calculates these values. The positive and negative three-phase active energy amounts and the positive and negative three-phase reactive energy amounts are determined by sampling at one-second intervals. In addition,
As mentioned above, the three-phase active and reactive powers are determined using the two-wattmeter method, so the data required here is #! As shown in IB diagrams (A) to (E), (rs, st phase line voltage data and r are phase current data, and therefore, these data are actually stored in the memory 4 @Next Next, a method of measuring electric energy will be explained with reference to FIG.

K for determining the three-phase active power amount is calculated by the arithmetic processing unit 5 in FIG.
The three-phase active power Pa (W) determined by is used.

Since the value of P3 is a positive or negative value depending on the direction of the tidal current, the sign is determined during calculation, and the positive data and negative data are distributed and integrated (see ■ and ■).

At this time, assuming that the IW (or IKW) power flows steadily, calculate in advance the value of measuring this power once per second and integrating it 3600 times for 1 hour. is used as the setting value (setting value) of IWH (or IKWH) (see ■). Then, the integrated values of ■ and ■ are compared with this set value, and when the integrated value exceeds the set value, the watt hour counter (WH counter;
(see ■, ■) is incremented by +1, and at the same time, the set value is subtracted from the integrated value, and the remaining values are left in ■, ■. After repeating this operation 3600 times, the value of the WH counter is output as the three-phase active energy. In this way, it is possible to simultaneously measure the amount of active power in two tidal flow directions. Note that the shorter the sampling interval is, the higher the accuracy will be, but this value should be adjusted in consideration of the processing speed of the arithmetic processing unit, the speed of power load fluctuations, etc.
In this case, we set it to 1 second.@ Also, the amount of reactive power of the three phases can be calculated in exactly the same way as above from the reactive power of the three phases Q3 obtained by equation (7). Nor.

〔Effect of the invention〕

According to this invention, a conventional active wattmeter (W meter)
, a reactive wattmeter (Var meter), an active watthour meter (WH meter), and a reactive watthour meter (VarH meter) are no longer required, and the advantage is that multiple items can be measured simultaneously with one device. It is something that is brought about.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 1A is a block diagram showing the structure of the memory, Fig. @IB is a reference diagram showing an example of data to be stored in the memory, and Fig. 2 is a diagram showing the effective energy consumption. A reference diagram to explain how to calculate the amount of electricity, Figure 3 is a waveform diagram showing sampling data of voltage and current, Figure 4 is a reference diagram to explain the amount of electricity, and Figure 5 explains how to find the amount of electricity. FIG. Code explanation 11 p 12...Sample hold circuit, 2
...Multiplexer, 3...A/D converter, 4...Memory (RAM), 5...
- Arithmetic processing unit (CPU), PT...Voltage detector, CT...Current detector. Agent Patent Attorney Akio Namiki Agent Patent Attorney Kiyoshi Matsuzaki (A) (B) Figure IIIB

Claims (1)

[Claims] Sampling means for sampling each phase current of at least two phases of the power system and two line voltages associated therewith at a predetermined period, and converting each sampled current and voltage amount into a digital amount. Analog/digital conversion means, 3-phase active power, 3-phase reactive power,
A digital watt-hour meter comprising calculation means for calculating positive or negative three-phase active energy and positive or negative three-phase reactive energy.