JPH0862254A - Direct current detector - Google Patents

Abstract

PURPOSE: To accurately detect d.c. component at a low cost by winding secon dary coil around magnetic core and short-circuiting the coil. CONSTITUTION: Secondary coil wire 36 is wound around a magnetic core 32 and is short-circuited. In this case, the value I of current to be detected 31 including a direct current component is I=A.sinΩt+IDC (A: amplitude, Ω: angular frequency, IDC: direct current component). If primary coil wire 35 for flowing of current to be detector is wound n1 turns around a magnetic core 32 and the turns of the coil wire 36 is n2 , the current I2 in the wire 36 is I2 =(n1 /n2 ) A.sinΩt. Only the magnetic flux Φ for the current difference IDC is generated in the magnetic core 32 and Φ=K.n1 .IDC (K: constant). This magnetic flux Φis detected by a hole element 33. Since the current IDC is 1% of the current I, the element 33 and an amplifier 34 can have a smaller copacitance than in the case of detecting the current I and thus the accuracy is improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a direct current detector for detecting a direct current component current contained in an alternating current.

[0002]

2. Description of the Related Art Inverters used for distributed power sources such as fuel cell power generation and solar power generation are connected to an AC power source of the grid and are operated at a frequency synchronized with the frequency of the grid in order to send generated power back to the grid power It The inverter is composed of a semiconductor switching element, and outputs the AC voltage synchronized with the power supply frequency of the system by PWM control as described above. In this PWM control, ON / OFF duty of a semiconductor switching element such as a power MOSFET, a transistor, or an IGBT is modulated by a high frequency carrier frequency, and an AC waveform synchronized with a system frequency of 50 Hz or 60 Hz is created.

The ON voltage or switching speed of the semiconductor switching element varies depending on the element, and P
During WM control, this variation may cause a DC component in the output AC voltage. The DC component causes an unbalanced current in the motor or transformer connected to the grid. As a result, torque ripples and speed fluctuations occur in the motor,
It may cause abnormal overheating, abnormal noise of the equipment, or deterioration of life. In addition, transformers cause abnormal overheating and noise due to magnetic bias. Therefore, when a DC component flows from the distributed power supply inverter to the power supply of the system, it is necessary to detect the DC component and immediately stop the distributed power supply inverter. The DC allowable value of this DC component is said to be about 1% or less of the rated current. A shunt resistor or a DC current detector (hereinafter abbreviated as DCCT) is generally used as a method of detecting a DC power source flowing out to the power source of the system.

FIG. 4 shows an example of a structure in which an inverter for a distributed photovoltaic power source is connected to a system power source. 1 in the figure
Is a solar cell, 2 is a ripple absorbing capacitor of an inverter, and 3 to 6 are semiconductor switching elements. The DC voltage generated by the solar cell 1 is applied to the semiconductor elements 3 to 6 through the ripple absorption capacitor 2. Control circuit 7
Performs PWM control for generating an inverter output synchronized with the system power supply 10, and applies ON / OFF signals to the semiconductor switching element through the drive circuit 8. Therefore, the DC voltage generated by the solar cell is converted into an AC voltage synchronized with the system frequency by turning on and off the semiconductor switching element. 9 is a current detector (here, D
Is an example of CCT), and the output current of the inverter is detected. Generally, the power frequency of the system is 50Hz or 6
0 Hz, so the output frequency of this inverter is 5
It is operated constantly at 0 Hz or 60 Hz.

The current detector 9 detects the output current of the inverter, and the processing circuit 21 shown in FIG. 5 detects the DC component. This processing circuit shows the DC component detecting section in the control circuit 7. The processing circuit 21 is a filter circuit 22,
The comparator 23 is used. The filter circuit removes the 50Hz or 60Hz AC component of the detected current,
Set the time constant to detect only the DC component. Further, the comparator generates a stop signal 24 to the implanter when the output of the film circuit, that is, the DC component exceeds an allowable value, for example, 1% of the rated current. FIG. 6A shows an inverter output current including a DC component, FIG. 6B shows a filter circuit output, and FIG. 6C shows an inverter stop signal.

[0006]

However, in the prior art, the DCCT as shown in FIG. 7 is used as the current detector. DCCT is a current detector capable of detecting a current in a wide frequency range from direct current to alternating current. In the DCCT, the magnetic flux generated by the detection current I (31) is applied to the Hall element 3 in the magnetic circuit of the magnetic core 32 having an air gap.
3 is installed to convert the magnetic flux into a voltage and detect it. The detection accuracy is typically about 5% due to the drift and offset of the Hall element and amplifier 34 due to temperature fluctuations. Therefore, in order to detect the direct current component in the alternating current by the above-mentioned method, the detection accuracy needs to be 1% or more, which is difficult to detect by the standard DCCT. Therefore, the DC used to detect the DC component in the AC current.
The CT Hall element and amplifier require highly accurate parts with low temperature fluctuations, and are extremely expensive current detectors.
It is an object of the present invention to provide a low-priced DC current detector using such expensive current detector and standard components.

[0007]

In order to solve the above problems, the present invention provides a magnetic core having an air gap, a primary coil around which an electric wire for passing a current to be measured is wound around the magnetic core, and the air gap being provided in the air gap. In the current detector including the arranged Hall elements, the secondary coil is wound around the magnetic core, and the secondary coil is short-circuited. Further, in the current detector, 2
Two secondary coils are arranged, one of the secondary coils is short-circuited, and a direct current is passed through the other secondary coil until the output signal of the Hall element becomes zero. Further, in the current detector, the output signal of the Hall element is passed through the filter circuit.

[0008]

Since the short-circuit winding cancels the AC current component according to the law of the number of ampere turns, the AC magnetic flux generated in the magnetic core becomes zero, and the magnetic flux has only the DC component of the AC current. The magnetic flux of only this DC component can be detected by the Hall element

[0009]

Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a configuration block diagram showing a first embodiment of the present invention. The difference from FIG. 7 of the configuration block diagram of the conventional example is that a secondary winding 36 is arranged on the magnetic core 32 and the secondary winding 36 is short-circuited. The current detection method of the present invention will be described with reference to FIG. In FIG. 1, reference numeral 31 is a current to be detected, and an alternating current containing a direct current component is expressed by equation (1). I = A · SIN (ωT) + I _DC (1) where A is the amplitude of the current, ω is the angular frequency, and I _DC is the DC component. Primary winding 35 in which current to be detected flows through magnetic body 32
Wind n ₁ turns. When the secondary winding 36 is short-circuited by turning it into n ₂ turns, the secondary winding 37 causes the current I ₂ shown by the equation (2).
Flows. I ₂ = (n ₁ / n ₂ ) A · SIN (ωT) (2) I ₂ flows in the secondary winding and cancels the magnetic flux generated by the detected current, so the magnetic substance has a DC component current I _DC Only a minute magnetic flux is generated. The generated magnetic flux Φ is given by equation (3). Φ = K · n ₁ · I _DC (3) K is a constant, n ₁ is the number of turns, and I _DC is a DC-type divided current. This magnetic flux is detected by the Hall element 33. Since the current of this DC component is about 1% with respect to the AC current, the amplification factor of the Hall element 33 and the amplifier 34 is a small-capacity (about 1%) current detector as compared with the case of detecting the AC current. Quite good. For example, in the case of an alternating current of 10 A, the direct current is about 0.1 A (1%), and the current detector used may be a DCCT with a rated current of 0.1 A. If the accuracy of this DCCT is 5% of the standard specifications, the accuracy is improved about 100 times as compared with the DCCT of 10A.

FIG. 2 is a block diagram showing the configuration of the second embodiment of the present invention. In the figure, the names of the respective parts are given the same reference numerals as in FIG. The difference from FIG. 1 is that two secondary windings are arranged in a magnetic core having an air gap. Then, one of the secondary windings is short-circuited, a DC current is passed through the other secondary winding so that the output signal of the Hall element 33 becomes zero, and the DC current component is measured. This measuring method will be described below. A current I (31) containing an AC component flows through the primary side coil to excite the magnetic core 32. Then, the current expressed by equation (2) flows in the short-circuited secondary winding,
Since the AC excitation component of the magnetic core 32 is eliminated, the magnetic flux existing in the air gap of the magnetic core 32 is only the DC magnetic flux due to the DC current component I _DC . Therefore, at this time, the Hall element 33 generates a signal voltage based on this DC current magnetic flux.
Next, a DC current is gradually applied to the other secondary winding in the direction of canceling the DC magnetic flux. If you look at the current when the signal voltage of the Hall element 33 becomes zero, this is
It becomes the direct current component of the detected current (magnetic balance type DC
CT).

When the AC component of the detection current I (31) is not completely canceled by the short-circuit voltage flowing in the short-circuit winding of the secondary winding and the AC component remains, the remaining AC component and DC After detecting the component by the hall element 33, the signal output from this hall element may be passed through a filter circuit to remove the AC component. FIG. 3 shows the characteristics of the filter that attenuates the remaining current component. Commercial frequency (5
The cutoff frequency f _c of the filter is set to about ₁ to 5 Hz in order to attenuate the frequency component f ₀ of ₀ Hz or 60 Hz). As a result, the current of the f ₀ component is attenuated to about 1/10 or less, and the direct current component can be extracted and detected.

[0012]

As described above, according to the present invention, since the current detector is configured to detect only the DC component contained in the AC current, the DC component can be accurately measured by the standard DCCT at low cost. It can be detected well. Therefore, a low-priced DC component current detector can be constructed.

[Brief description of drawings]

FIG. 1 is a configuration block diagram showing a first embodiment of the present invention.

FIG. 2 is a configuration block diagram showing a second embodiment of the present invention.

FIG. 3 is a filter characteristic according to the embodiment of the invention.

FIG. 4 is a configuration example in which an inverter for a distributed solar power source is connected to a system power source.

FIG. 5 is a processing circuit for separating only a direct current component from an alternating current.

6 is a signal waveform of each part in FIG.

FIG. 7 is a configuration block diagram of a conventional current detector.

[Explanation of symbols]

 1 Solar Battery 2 Capacitors 3, 4, 5, 6 Semiconductor Switching Element 7 Control Circuit 8 Drive Circuit 9 Current Detector 10 System Power Supply 21 Processing Circuit 22 Filter Circuit 23 Comparator 24 Stop Signal 31 Detected Current 32 Magnetic Core 33 Hall Element 34 Amplifier 35 primary winding 36 secondary winding

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication H01F 38/28 H02M 7/48 Z 9181-5H

Claims (3)

[Claims] 1. A current detector comprising a magnetic core having an air gap, a primary coil having an electric wire for passing a current to be measured wound around the magnetic core, and a Hall element arranged in the air gap, wherein: A direct current detector characterized in that a secondary coil is wound around a core and the secondary coil is short-circuited. 2. The current detector, wherein two secondary-side coils are arranged on the magnetic core, one of the secondary-side coils is short-circuited, and the other of the secondary-side coils is until the output signal of the Hall element becomes zero. A direct current detector characterized in that a direct current is passed through the secondary coil. 3. The DC current detector according to claim 1, wherein in the current detector, the output signal of the Hall element is passed through a filter circuit.