JP2823423B2 - Measuring device of DC current by asymmetric component in AC current - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus for measuring a direct current due to an asymmetric component in an alternating current, and more particularly to an alternating current capable of shortening a measuring time and measuring a direct current by an asymmetric component with high accuracy. The present invention relates to an apparatus for measuring a DC current by an asymmetric component in the inside.

[0002]

2. Description of the Related Art As a conventional DC current measuring device for measuring a DC current contained in an AC current, a DC current obtained by passing an AC current to be measured through a low-pass filter and removing an AC current component is measured. There is a device to measure with. Also,
As another DC current measuring device, the zero-crossing time difference of the AC voltage signal generated by converting the AC current to be measured is measured, and the DC voltage that makes the zero-crossing time difference of the AC voltage signal zero is added. There is a device that calculates a DC current based on a DC voltage.

On the other hand, it is theoretically possible to measure a DC current due to an asymmetric component in an AC current to be measured by subtracting a DC current measured based on a zero crossing time difference from a DC current measured using a low-pass filter. it can. The asymmetric component in the measured AC current is, for example,
Published by the Institute of Electrical Engineers of Japan on January 31, 1989
The Institute of Electrical Engineers of Japan joined the joint research group on insulating materials and high voltage.
EIM-89-9-20 and HV-89-9-
As shown in FIG. 5 on page 73 of FIG.
Positive and negative peaks of the waveform of the charging current flowing through the
Appear in the vicinity, DC and many AC
It consists of. Also, based on the zero crossing time difference
The measured direct current is disclosed in JP-A-3-9267.
As shown in the figure, the zero cross point
It is something to shift.

[0004]

However, when a measuring device for measuring a DC current due to an asymmetric component due to the difference between the DC currents described above is used, it is necessary to increase the time constant of the low-pass filter when the level of the AC current is large. There is
Further, switching noise is generated when switching between the two parallel measurement circuits, an error current is generated, and the accuracy of DC current measurement is reduced. If the noise is reduced, the time required for measurement becomes longer. In the DC current measurement circuit based on the zero crossing time difference, when the ratio of the DC current is small, the DC current measurement accuracy is reduced, and the zero-phase current flowing through the cable is, for example, a fifth harmonic component. Contains a harmonic component at a level that is about 1/10 of the fundamental wave, so that the measurement error becomes large. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a DC current measuring device capable of quickly and accurately measuring a DC current due to an asymmetric component in an AC current.

[0005]

SUMMARY OF THE INVENTION According to the present invention, a fundamental wave component of a zero-phase current including an asymmetric component in an AC current is measured in order to quickly and accurately measure a DC current due to an asymmetric component in the AC current. A band-elimination filter that attenuates and removes a harmonic component to generate a first AC current signal, and a first DC current that detects a first DC current by passing the first AC current signal through a low-pass filter. Detecting means;
A second detecting means for detecting a second DC current based on a zero crossing time difference of the AC current signal, and a DC current generated from an asymmetric component based on a difference between the first and second DC currents The present invention provides a DC current measuring device provided with a calculating means for performing the following.

[0006]

According to the apparatus for measuring a direct current by an asymmetric component in an alternating current according to the present invention, there is provided at least one band rejection filter connected in series according to the levels of a fundamental component and a harmonic component of a zero-phase current. As a result, the fundamental component and the harmonic component of the zero-phase current can be attenuated. Due to this attenuation, the time constant of the low-pass filter can be reduced, whereby the circuit switching speed can be increased, and the measurement accuracy can be increased.

[0007]

Embodiment 1 Hereinafter, an apparatus for measuring a direct current by an asymmetric component in an alternating current according to the present invention will be described in detail with reference to the accompanying drawings. FIG. 1 shows an embodiment of a DC current measuring device using an asymmetric component in an AC current according to the present invention, in which a fundamental component of a zero-phase current of an AC current to be measured is attenuated and a harmonic component is removed. A band elimination filter 1 constituted by a band elimination filter, a low-pass filter 2 for passing a DC current component in an AC current signal attenuated by the band elimination filter 1, a minute current measurement circuit 3 for measuring a DC current, An amplifier phase shift circuit 4 for amplifying the AC current signal attenuated by the band elimination filter 1 and correcting the phase difference, and a voltage signal corresponding to the AC current signal amplified and corrected for the phase difference by the amplifier phase shift circuit 4. A zero-crossing measuring circuit 5 for measuring a zero-crossing time difference and measuring a DC current based on a DC voltage in an AC current signal based on the zero-crossing time difference; A DC current generated by an asymmetric component is measured based on the DC currents measured by the current measurement circuit 3 and the zero-cross measurement circuit 5, and the number of connection stages of the band elimination filter 1 according to the level of the zero-phase current of the AC current to be measured. And an arithmetic processing unit 6 for generating a control signal for switching the state. The changeover switch 7 switches between the contacts ab.

The arithmetic processing unit 6 determines the filter configuration of the band elimination filter 1 according to the level of the measured AC current.
A control signal for switching the band elimination filter 1 is generated. Table 1 shows the attenuation and phase characteristics of the fundamental wave component by the band elimination filter 1 configured by this switching.

The AC current signal input to the band elimination filter 1 attenuates the fundamental component as shown in Table 1 and attenuates the harmonic component of the zero-phase current contained in the AC current signal. .

First, when the switch 7 is turned on at the contact point a, the attenuated AC current signal is passed through the low-pass filter 2 to remove the AC current component, and then the DC current is measured by the minute current measuring circuit 3. .

Thereafter, the switch 7 is turned on to the contact b, and the zero crossing time difference of the alternating current signal is measured by the measuring circuit 5.
At this time, since the AC current signal has an input / output phase difference as shown in Table 1 due to the attenuation by the band elimination filter 1, the phase difference is corrected by the amplifier phase shift circuit 4, and thereafter, the zero crossing time difference measurement circuit 5 and a DC current based on the zero crossing time difference is measured.

The DC current measured based on the zero crossing time difference is input to the arithmetic processing unit 6 and is already in the small current measuring circuit.
The DC current generated by the asymmetric component of the measured current is calculated by subtracting the DC current from the DC current measured by 3 .

As described above, the attenuation of the band elimination filter is determined according to the level of the zero-phase current in the AC current to be measured. Therefore, the time constant of the low-pass filter used for measurement can be reduced, so that the When the peak value of the fundamental wave component and the value of the DC current due to the asymmetric component approach two or more digits, the measurement accuracy of the DC current due to the asymmetric component is improved. Further, the measurement of the direct current by the asymmetric component in the measured zero-sequence current can be performed quickly and with high accuracy.

[0014]

As described above, according to the apparatus for measuring a direct current based on an asymmetric component in an alternating current according to the present invention, at least one of the at least one connected in series according to the level of the zero-phase current in the input alternating current. Since two band rejection filters are provided, the measurement can be performed quickly and with high accuracy.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram showing an apparatus for measuring a direct current by an asymmetric component of an alternating current according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Band-elimination filter 2 Low-pass filter 3 Microcurrent measurement circuit 4 Amplifier phase shift circuit 5 Zero cross measurement circuit 6 Operation processing unit 7 Changeover switch

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Kazuo Kotani 5-1-1, Hidakacho, Hitachi City, Ibaraki Prefecture Inside the Power System Research Laboratories, Hitachi, Ltd. (72) Inventor Kenichiro Soma Hidakacho, Hitachi City, Ibaraki Prefecture JP-A-2-162268 (JP, A) JP-A-3-9267 (JP, A) JP-A-4-368416 (JP) , A)

Claims (1)

(57) [Claims] A band elimination filter for attenuating a fundamental component of a zero-phase current including an asymmetric component in an alternating current and removing a harmonic component to generate a first alternating current signal; A first detection means for detecting a first DC current by passing the AC current signal of the first AC current signal through a low-pass filter, based on a zero-crossing time difference of the first AC current signal,
A second detecting means for detecting a second DC current; and a calculating means for calculating a DC current generated from the asymmetric component based on a difference between the first and second DC currents. For measuring DC current due to the asymmetric component in AC current.