JPS61155862A - Current transformer - Google Patents

Abstract

PURPOSE:To obtain a device which is used for both AC and DC currents and transforms even a fine current with high precision by providing the 1st - the 4th windings, etc. CONSTITUTION:A transformer 1, a magnetic flux detecting circuit 4, a current amplifier 7, etc., are provided. The transformer 1 is constituted by providing primary winding 11, secondary winding 12, tertiary winding 13, and quadratic winding 14 around a core 15. The voltage induced across the winding 14 when a current I1 to be detected is zero is based upon an exciting current Iex and magnetism characteristics of the core 15, so the magnetism characteristic curve of the core 15 is symmetric between a plus and a minus side. The induced voltage of the winding 14 is also symmetric between a plus and a minus side and the asymmetric detection output of the circuit 4 is zero. When the current I1 is not zero, the core 15 is biased according to the level of the current. Consequently, positive and negative crest values of the voltage induced across the winding 14 are different. Therefore, when only a positive and a negative voltage whose amplitude are larger than some value are considered, it is known how the core 15 is biased by the current I1. For the purpose, a current I2 is flowed to the winding 12 by using an amplifier 7 so that the bias of the core 15 is zero, and then the level and polarity of the current I1 are known.

Description

[Detailed Description of the Invention] [Field of the Invention] The present invention relates to a current transformation device used for purposes such as changing the measurement range of an ammeter or isolating a detection system from a current to be detected. The present invention relates to a current transformation device that can be used for both AC and DC functions and can transform even minute currents with high precision.

[Background of the Invention] Conventionally, as this type of device, a current transformer (CT) has been known, which is used to expand the measurement range of an AC ammeter and to electrically isolate the ammeter from the circuit under test. . However, this current transformer cannot be used to measure direct current, and even if it is an alternating current, it has poor accuracy when trying to measure minute currents on the order of mA, and the secondary winding is several hundreds of meters long. This has the disadvantage that the main body of the current transformer, which is the part that extends over the turns and is connected to the circuit under test, becomes larger.

In addition, for example, if the primary winding is for three-phase AC, these three
Zero-phase current detection current transformers having phase windings are also known. In this current transformer, the output induced in the secondary winding is the zero phase i! It is used as an earth leakage detector because it is proportional to the current. However, since this zero-phase current is on the order of mA, the current transformation error is as in the general current transformer mentioned above.
30% to several 10%, and in addition to poor accuracy and linearity,
There is a disadvantage that the secondary winding has a large number of turns and is large.

Also, M, Milkovic et al.
et al.) constructed a current amplification circuit with an extremely small input impedance using an operational amplifier, and by inputting the secondary output of a current transformer to this circuit,
We are proposing a current transformation device that can transform a wide range of current as low as 0A with high precision and extremely minimizes the effects of voltage drop etc. caused by connecting a current transformer (I E E E
Transaction on M aanetic
s.

Vol, MAG-13, No, 5 Sep
tember 1977).

However, this current transformation device is only used for alternating current, and cannot be used for transforming direct current.

Furthermore, Jonathan Earle, Leahey et al.
Athan R, Leehey et al.)
proposed a DC current transformer using a so-called magnetic saturation detector (IEEE, 1982. PE5CRecord
, PP438-444). However, this current transformer
Although it is for both AC and DC use, it has a time constant in the secondary circuit, so
There are problems with detection accuracy and speed.

[Object of the Invention] The present invention has been made in view of the problems of the conventional type described above, and an object of the present invention is to provide a current transformation device that can be used for both AC and DC functions and can transform minute currents with high accuracy. shall be.

[Summary of the Invention] In order to achieve the above object, the present invention uses a transformer having at least four windings including a first winding through which a current to be detected flows, and a high-frequency current is applied to a second winding of the transformer. The magnetic flux level of the core is constantly monitored by the electromotive force of the third winding, and the current of the fourth winding is controlled so that the average level becomes equivalently zero. It is characterized in that it is detected as the current of the No. 4 winding. here,
To detect the magnetic flux level, we focus on the fact that the magnetization characteristics of the core vary depending on the DC bias. In other words, the core is excited in advance by a current source with a frequency sufficiently high for the frequency of the current to be detected, and the secondary induced voltage waveform for the exciting current is the bias component (magnetomotive force generated by the current to be detected).
Different types are used depending on the situation.

[Effects of the Invention] Therefore, according to the current transformation device of the present invention, both direct current and alternating current can be detected, and the problem of magnetic saturation is eliminated.

Furthermore, even minute currents can be detected with high accuracy.

[Explanation of Examples] Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 1 shows the configuration of a current transformation device according to an embodiment of the present invention. The device in the figure includes a transformer 1, a high-frequency oscillator 2, a resistor 3, a magnetic flux detection circuit 4, an adder 5, and a proportional-integral amplifier 6.
, a current amplifier 7, a current detection resistor 8, and the like.

A transformer 1 has four windings, a primary winding 11, a secondary winding 12, an excitation winding 13, and a detection winding 14, connected to a core (magnetic core) 1.
The primary winding 11 is a so-called single winding in which the conductor through which the current to be detected passes passes through the center of the core 15.
It is a tangent winding. The core 15 preferably has as little coercive force as possible from the viewpoint of current detection accuracy, and here we use an amorphous toroidal core manufactured by Allied Chemical Co., Ltd. under the trade name 2826MB (magnetic path length J -9,55cm, cross-sectional area 3 = 0). .2246 m) is used.

The high frequency oscillator 2 generates a sine wave voltage e of, for example, 10 kllz.
Here, the output of the oscillator 2 is used as a current source by supplying it to the excitation winding 13 via a resistor 3 having a relatively high resistance value. The current 1eX supplied to the excitation winding 13 is enough to sufficiently saturate the core 15.

The magnetic flux detection circuit 4 detects the magnetic flux change in the core 15 based on the voltage e4 induced in the detection winding 14 by the current i1+i@ flowing in the primary winding l1111 and the excitation winding 13, and detects the magnetic flux change in the core 15. Detection output φd according to positive/negative asymmetry
Generate ξt.

The addition calculator 5 detects the detected output voltage φda of the magnetic flux detection circuit 4.
Analog calculation is performed on t and the voltage φr=0, which is the target value of the core magnetic flux, to output a magnetic flux error signal ε(−φr−φ command t).

The summing amplifier 11, the power amplifier 72, and the current detection resistor 8 are operated as a current amplifier. Now, the magnetic flux error signal ε is converted to the current command 1 of the current amplifier via the PI amplifier.
If so, I2 flows so that the error equivalently becomes zero. Therefore, by detecting the magnitude of I2 as the IR drop of the resistor 8, the current to be detected can be detected.

Next, the operation of this current transformation device will be explained.

The operating principle of this device is based on Σφ=0.

Now, in advance, connect a relatively high resistance 3 to the high frequency oscillator 2, which is the excitation power source, and connect the excitation 11 to the winding @13 in a form close to the current source.
Let the current fex flow.

First, when the detected current 11 is zero (however, if the primary winding 11
), the voltage e4 induced in the detection winding 14 is equal to the excitation current fax and the core 1
This occurs based on the magnetization characteristics of core 1, so core 1
The magnetization characteristic curve of No. 5 is symmetrical in positive and negative directions, as shown in FIG. 2(a). In addition, the induced voltage of the detection winding 5114 is also shown in Fig. 3 (
As shown in a), it is symmetrical in positive and negative directions, and the asymmetry detection output of the magnetic flux detection circuit 4 is zero. The target value of core magnetic flux is φr
Since wO, the command value of the current amplifier becomes zero.

Next, when the current to be detected 11 is not zero, the core 15 is biased as shown in FIG. 2(b) or (C) depending on the magnitude of this current. As a result, the average value of the voltage e4 generated in the detection winding 14 remains zero, as shown in FIG. 3(b) or (C), but the positive and negative peak values are different. becomes. Therefore, by focusing only on positive and negative voltages with amplitudes above a certain level and taking the area ratio of these, it is possible to know how the core 15 is biased by the detected current 1. Therefore, in order that the bias component of the core 15 becomes zero, that is, the detected current 11
If the current amplifier 7 is used to flow a current 12 through the secondary winding 12 so as to cancel out the magnetic flux generated by .

FIG. 4 shows a more specific example of the magnetic flux detection circuit 4 in FIG. 1. The circuit in the figure includes a voltage amplifier 41, slice level generators 42, 43, and slice circuits 44, 45.
, an adder 46, a timing signal generation circuit 41, an integration circuit 48, and a sample-and-hold circuit 49.

Slice level generators 42 and 43 generate slice level signals Vsp and Vsn, which are positive and negative predetermined voltages, respectively. These slice level signals v
Sp and Vsn are usually set so that their absolute values match.

The slice circuits 44 and 45 slice the magnetic flux detection signal e4 at slice levels vSp and Vsn, and output the portions whose absolute values are equal to or higher than these levels.

The timing signal generation circuit 41 generates a high frequency voltage e for excitation.
Integral reset signal P at a predetermined timing in synchronization with e&
R,! 3 and sample-and-hold signals Ps, +.

After the integration circuit 48 is reset by the integration reset signal PR, S from the timing signal generation circuit 47, the signal is output from the slice circuits 44 and 45 for about one cycle until the next signal PR, S is generated. The adder 46 integrates the combined signal (e 4sp + e 4sn in FIG. 3).

The sample-and-hold circuit 49 determines the period of the output value (Σφ in FIG. 3) of the integration circuit 48 at that time by the sample-and-hold signal 5t-1 generated from the timing signal generation circuit 47 immediately before the integral reset signal R3. The final value of is held and output as the positive/negative asymmetry detection signal φdet for about one cycle until the next signal R3 is generated.

Therefore, in the circuit of FIG. 1, this signal φdet
A current command 1zr of the secondary current I2 is generated based on the deviation between the target value φr of the core magnetic flux, and this secondary current I2
When is controlled in a negative feedback manner so that the bias component becomes zero, this secondary current value I2 and the voltage drop occurring across the resistor 8 become a voltage proportional to the detected current 1+.

FIG. 5 shows the DC characteristics of the device shown in FIG. 1, that is, the detected DC current 11 and the output voltage Vo (or m current 1 +
dat). In the same figure, (a) is
When the number of turns N2 of the secondary winding 12 of the transformer 1 is 100 turns, (b) and (C) show the characteristics when N2 is 10 turns and 1 turn, respectively.

In addition, in any case, the number of turns N1 of the primary winding 11 is 1,
Number of turns N3 and N4 of excitation winding 13 and detection winding 14
were repeated 10 times each. When any transformer was used, the linearity was extremely good, and the measurement error was less than 0.p%.

In addition, although the above-mentioned example was explained as a device for detecting direct current, this current transformation device can also be used for detecting alternating current. In this case, the detection accuracy of the alternating current decreases as the frequency of the alternating current approaches the frequency of the excitation current, but the frequency of the alternating current is sufficiently low at one-tenth or less of the excitation current, and In a range where changes in alternating current within one cycle are small enough to be considered direct current, it is possible to obtain accuracy similar to that of direct current.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the general configuration of a current transformation device according to an embodiment of the present invention, FIG. 2 is a magnetization characteristic diagram in each DC bias state of the transformer used in the current transformation device of FIG. Figure 3 is a diagram of voltage waveforms at various parts of the circuits in Figures 1 and 4.
FIG. 4 is a detailed circuit diagram of the magnetic flux detection circuit shown in FIG. 1, and FIG. 5 is an input/output characteristic diagram of the device shown in FIG. 1 Nidorance, 2: High frequency oscillator, 4: 11 Flux detection circuit, 7: Current amplifier, 44.45 Nislice circuit, 48: Integrating circuit, 49: Sample and hold circuit. Patent Applicant Mitsui Petrochemical Industries Co., Ltd. Agent Patent Attorney Tatsuo Ito Agent Patent Attorney Tetsuya Ito Figure 1 Figure 2

Claims (1)

[Claims] 1. A transformer having at least four windings including a first winding through which the current to be detected flows, and a second winding of the transformer having a frequency higher than the frequency of the current to be detected. a high-frequency current source that supplies a high-frequency constant current at a sufficiently high frequency and a peak value sufficient to saturate the magnetic core of the transformer; a magnetic flux detection circuit that generates a detection output according to the positive/negative asymmetry of high-frequency magnetic flux in the transformer; and a second current source that supplies a current to a fourth winding of the transformer according to the detection output. A current transformation device characterized in that the current is transformed into a current corresponding to the current to be detected by supplying a current to the fourth winding in order to offset the magnetic flux generated by the current to be detected. 2. The current transformation device according to claim 1, wherein the high frequency current output from the high frequency current source is a sine wave. 3. The magnetic flux detection circuit includes a slice circuit that slices the high frequency voltage induced in the third winding at a predetermined level and outputs only a portion whose absolute value is equal to or higher than the predetermined level; and an output from the slice circuit. Claim 1 or 2 comprising: an integrating circuit that integrates one period of the high-frequency voltage generated; and a sample-and-hold circuit that holds the integrated output for one period until the next integrated output is generated. Current transformation device as described in section.