JPH02213770A - Method for computing effective value of three-phase voltage and current and three-phase active and reactive power - Google Patents

Abstract

PURPOSE:To establish the computation method which decreases characteristic changes, has excellent responsiveness and can improve the detection accuracy of basic waves by simultaneously sampling three phases of a voltage or current at the frequency of the even times the higher harmonics to be removed per one cycle of AC. CONSTITUTION:The over sampling of the N point which is the even times the higher harmonics to be removed per one cycle is executed. Such oversampling is executed simultaneously with the three phases of the voltage or current. The instantaneous voltages Vu, Vv, Vw of the respective phases obtd. by this sampling are computed by using equation I of three phase square sum at every sampling point. The accurate effective value of the three phase AC voltage in which the error components of the higher harmonic difference components are canceled is obtd. The instantaneous currents Iu, Iv, Iw of the respective phases obtd. by the simultaneous sampling for the three phase current are also computed by equation II at every sampling point similarly with respect to the current as well. The effective value of the three phase AC current in which the error components of the higher harmonics are canceled is obtd. by taking the moving average of the one cycle of this value.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention detects the voltage and current of a three-phase AC power line, and uses the detected values to control the power or voltage of a generator. The present invention relates to a calculation method for a voltage/current effective value detection device and an active/reactive power detection device used in a control device or the like.

[Conventional technology]

In general, three-phase AC power lines contain harmonics generated from transformers, thyristor converters, etc., and when detecting the fundamental voltage and current of three-phase AC power lines, these harmonic components are part of the detected value. This is a cause of error. Conventionally, in order to avoid the effects of harmonic components, a method has been adopted in which a high-order powerful analog filter is used to extract the fundamental wave component, and then A/D conversion is performed using three-phase simultaneous sampling. Ta.

If this digital data after A/D conversion is calculated using the square root equation of the sum of squares of three phases, the effective voltage and current value of three-phase AC can be obtained, and if it is calculated using the αβ conversion method, the effective power of three-phase AC can be obtained. Reactive power can be obtained.

[Problem to be solved by the invention]

However, the method of extracting the fundamental wave using the high-order analog filter described above has the following problems.

■ Since an active filter consisting of a high-gain operational amplifier is used as an analog filter, accuracy decreases are likely to occur due to changes in the gain and offset of the operational amplifier, and therefore, changes in the characteristics of the analog filter have a large effect on fundamental wave extraction. .

■ The attenuation characteristics of an analog filter inevitably have a gradual part, and in particular cannot remove all harmonics near the fundamental wave, so the harmonic components that pass through this analog filter cause a decrease in calculation accuracy.

Another possible method is to Fourier transform the frequency sampled data, extract the fundamental wave component, and calculate the effective value etc. from this fundamental wave component, but in that case, ■ Input for at least one cycle of AC. Calculations cannot be performed unless the data is complete, and furthermore, it takes more time to perform one Fourier transform for three phases, resulting in a slow response.

■ The response cycle is rough because it does not go below at least one cycle of AC.

There is a problem.

Therefore, the purpose of the present invention is to more effectively remove harmonic error components, which conventionally relied on analog filters, by applying an oversampling method, to reduce characteristic changes, to provide excellent response, and to provide a fundamental An object of the present invention is to provide a calculation method for three-phase voltage and current effective values and three-phase active reactive power that can improve detection accuracy.

[Means for Solving Problem 8] The method of calculating the effective value of three-phase voltage and current according to the present invention calculates voltage or current at a frequency that is an even multiple of the harmonic to be removed per AC cycle of a three-phase AC voltage or current signal. The A/D conversion data obtained by simultaneous three-phase sampling of current is calculated for each point using the square root formula of the sum of squares of three phases.
The method is characterized in that a one-cycle moving average of the calculated values is calculated.

Furthermore, the method for calculating three-phase active reactive power according to the present invention includes simultaneously sampling six phases of voltage and current at a frequency that is an even number multiple of the harmonic to be removed per AC cycle of three-phase AC voltage and current signals. The method is characterized in that the obtained A/D converted data is calculated for each point each time using the formula of the αβ variation method, and a 1-cycle moving average of the calculated values is calculated.

[For production]

According to the method of calculating the three-phase voltage/current effective value and the three-phase active reactive power according to the present invention, the inclusion of aliasing noise is suppressed by the oversampling method of sampling at a frequency that is an even number multiple of the harmonic to be removed per AC cycle. Since the pre-filter for preventing this can be a low-order, simple circuit analog filter, there is no need to use a high-order, powerful analog filter at the input section as in the past, and therefore, high-speed response characteristics are improved. Furthermore, since three-phase simultaneous sampling is performed and calculations are made for each point to obtain a one-cycle moving average, harmonic error components can be effectively canceled and detection accuracy can be improved.

〔Example〕

Next, an embodiment of the method for calculating three-phase voltage and current effective values and three-phase active reactive power according to the present invention will be described in detail below with reference to the accompanying drawings.

Figure 1 is a diagram showing the sampling state for one cycle (one period) of an AC waveform of the -phase component, and per cycle, oversampling of N points that are an even multiple of the harmonic to be removed is performed. It is carried out. Performing such oversampling simultaneously for three phases of voltage or current,
FIG. 2 is a diagram showing a state in which three-phase voltages are simultaneously sampled, and the instantaneous voltages Vu and Vv of each phase obtained by this sampling.

Vw is calculated for each sampling point using a three-phase sum of squares formula as shown below.

Focusing on the fact that the error fluctuates once per AC cycle, this value is calculated by taking a moving average over one cycle to obtain a highly accurate effective value of the three-phase AC voltage in which harmonic error components are canceled. be able to.

Similarly, regarding the current, as shown in Figure 3, the instantaneous current I of each phase obtained by simultaneous sampling for the three-phase current
u, Iv, and Iw are calculated for each sampling point using the following equation.

By taking the moving average of this value over one cycle, it is possible to obtain the effective value of the three-phase alternating current with harmonic error components canceled.

This calculation method can be applied to the detection device shown in FIG. 4. In FIG. Power system 161. This power is transmitted to the output line 12 of the generator 10 through the instrument transformer (three-phase) 20 and current transformer (three-phase) 18, and the output from the detection device No. 30. The three-phase voltage and three-phase current of the line 12 are monitored, the required detection amount is calculated, and the result is sent to the power fIIJ control device (not shown) for the power generator 1.
This is for controlling 0. The detection device 30 is A/
D conversion section 32, pass line 34, and detection amount calculation section 36
In the A/D converter 32, the three-phase voltage or three-phase current is simultaneously sampled per cycle at a frequency that is an even number multiple of the harmonic to be removed and converted into digital data. This digital data is sent to the detected amount calculation unit 36 via the path line 34 for each sampling, and the detected amount calculation unit 36 calculates it using, for example, the above-mentioned three-phase sum of squares formula, and converts this calculated value into an AC A moving average is taken for each cycle, and the three-phase voltage effective value or three-phase current effective value is calculated.

Here, six phases of the three-phase voltage and three-phase current are detected simultaneously through the instrument transformer 20 and the current transformer 18, and an even number of harmonics to be removed per AC cycle as shown in FIG. When oversampling of N points is performed for six phases of voltage and current, that is, the instantaneous voltage and current digital data Vu in FIGS. 2 and 3 is obtained.

When Vv, Vw, Iu, Iv, and Iw are obtained, each A/D conversion data is calculated as follows for each sampling point: P=■α・Iα×β・Iβ Q=Vβ・Iu−Vα - Calculate using the αβ conversion formula shown by Iβ and take the moving average of this value over one cycle to obtain highly accurate three-phase active power P and three-phase reactive power Q with harmonic error components canceled. be able to.

〔Effect of the invention〕

As is clear from the embodiments described above, according to the present invention,
Detecting the effective voltage and current value of a three-phase AC power line that contains harmonics The oversampling method is adopted in the calculation method of the device, and per AC cycle, multi-point sampling is performed at a frequency that is an even multiple of the harmonic to be removed. By calculating the three-phase sum of squares for each point and taking a one-cycle moving average, it is possible to obtain highly accurate three-phase voltage and current effective values with extremely low harmonic error components. It becomes possible to improve the performance of the current detection device, which has the effect of expanding its application to digital control systems. Further, by adopting this calculation method in the active reactive power detection device, it is possible to improve the performance of the digital high-speed active reactive power detection device, and the same effect can be obtained.

Furthermore, since there is no need to use a high-order active filter as an analog filter, fundamental wave extraction is not affected by changes in characteristics of the analog filter as in the conventional method.

If the calculation method according to the present invention is applied to a digital signal processor (DSP) to perform high-speed calculation, a detection device with a high-speed response time of about half a cycle in AC can be realized.

Although preferred embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above, and it goes without saying that various design changes can be made without departing from the spirit of the present invention. .

[Brief explanation of the drawing]

Figure 1 is an explanatory diagram showing oversampling for one cycle of an AC waveform for one phase, Figure 2 is an explanatory diagram showing a state in which three phases are simultaneously sampled for three-phase voltage, and Figure 3 is an explanatory diagram for three-phase current. On the other hand, FIG. 4 is an explanatory diagram showing a state in which three phases are simultaneously sampled, and FIG. 4 is an explanatory diagram showing an example of how the detection device is used. 10... Source @ machine 12... Output line 14... Circuit breaker 16... Power system 18... Current transformer (three-phase) 20... Instrument transformer (three-phase) 30 ...Detection device 32...A/D conversion section 34...Pass line 36...Detection amount calculation section FIo, 2

Claims (2)

[Claims] (1) Three-phase sum of squares for A/D conversion data obtained by performing three-phase simultaneous sampling of voltage or current at a frequency that is an even multiple of the harmonic to be removed per AC cycle of a three-phase AC voltage or current signal. A method for calculating effective values of three-phase voltage and current, characterized in that calculation is performed for each point each time using the square root formula of , and a 1-cycle moving average of the calculated values is calculated. (2) αβ conversion method is applied to A/D conversion data obtained by performing simultaneous six-phase sampling of voltage and current at a frequency that is an even multiple of the harmonic to be removed per AC cycle of three-phase AC voltage and current signals. A method for calculating three-phase active and reactive power, characterized in that calculation is performed for each point each time using a formula, and a 1-cycle moving average of the calculated values is calculated.