JPH0366109A - Error compensating current transformer - Google Patents

Abstract

PURPOSE:To obtain an error compensating current transformer which contrives improvement in errors by feeding an current in order to generate offsetting voltage which offsets voltage generated by the secondary leakage impedance of secondary winding to a compensating impedance that is connected to the secondary winding according to the output of an active element which makes voltage that is measured at the load side by the compensating impedance almost zero. CONSTITUTION:A current source 23 makes up a compensating means and mainly it is composed of NPN transistors 25 and 27 which are connected complementarily. Then, base terminals of both transistors are connected to the output of an operational amplifier 4 and further, emitter terminals of them are connected between the output terminal 5 of secondary winding 3 and a compensating impedance ZC and on the other hand, collector terminals are connected to a required positive current source (+VCC) as well as negative current source (-VEE) through respective resistors 29 and 31. Consequently, a feedback current IC flows through the compensating impedance ZC and then, its current and voltage I2Z2 generated by the secondary leakage impedance Z2 offset each other. In this way, error compensation is performed.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention relates to a current transformer for the purpose of error compensation, and in particular to an error compensation type current transformer that compensates for errors using active elements. Concerning vessels.

(Prior Art) FIG. 3 shows a conventional error compensation type current transformer using active elements. In the figure, 1 is an iron core, and this iron core 1 has a primary winding 2 wound with N1 turns on the primary side and a secondary winding 3 wound with N2 turns on the secondary side. We are prepared. 4 is an operational amplifier, the inverting input part of which is connected to one output terminal 5 of the secondary winding 3, and the non-inverting input part thereof being connected to the other output terminal 6 of the secondary winding 3. Further, a feedback resistor 7 is connected between the inverting input section and the output section of the operational amplifier 4.

8.9 is the output terminal of the current transformer and has an output voltage V.

occurs. Note that the non-inverting input portion of the operational amplifier 4 is commonly connected to the terminals 6 and 9 to be grounded.

Therefore, in the error compensation type current transformer as shown in FIG. 3, the following relationship holds true. However, ■1 is the primary current of primary winding 2, I2 is 2
The secondary current of the next winding 3, RF, is the resistance value of the feedback resistor 7.

In the error compensation layer current transformer configured as described above, the output terminals 5 and 6 of the secondary winding 3 are connected to the operational amplifier 40, so if the gain of the amplifier 4 is sufficiently large, then The potential e5-6 between the terminals 5-6 becomes almost zero. In other words, the secondary load on the secondary winding 3 of the iron core 1 becomes zero, and the error is improved.

(Problem to be solved by the invention) However, in reality, the error is not completely zero, and in current transformers that require high precision, there is a problem particularly with phase angle errors associated with excitation current. . This is considered to be due to the presence of secondary leakage impedance and excitation impedance of the secondary winding 3.

That is, if FIG. 3 is now expressed as an equivalent circuit, it will be as shown in FIG. 4. In FIG. 4, 3 corresponds to the secondary winding 3 in FIG. 3, I2 is the secondary leakage impedance of the winding 3, and E
2 indicates a secondary induced voltage due to excitation impedance. This is because the short-circuit wire 10 that short-circuits the output terminals 5 and 6 of the secondary winding 3 causes the terminal 5 to be virtually grounded by the operation of the operational amplifier 4. Therefore, if the terminals 5 and 6 are considered to be virtual ground, e, -6=0, and the following equation holds true.

E2+12・Z2=0,',E2=-i 2Z2...
(A) However, as shown in equation (A), even if the terminals 5 and 6 are shorted, the secondary electromotive force E2, which is the cause of the error, does not become completely zero, and the secondary electromotive force E2 of the secondary winding 3 Next leakage impedance I2
This shows that the voltage has a value equivalent to that voltage drop. Therefore, this voltage E2 generates a slight excitation current, which causes a phase angle error. That is, even if the secondary voltages e and -6 are set to zero, the secondary electromotive force E2 does not become zero, which causes an error.

The present invention has been made in view of the above, and its purpose is to provide an error compensation layer current transformer that can reliably improve errors without being affected by the secondary leakage impedance of the secondary winding. It's about doing.

[Structure of the invention] (Means for solving the problem) In order to achieve the above object, the present invention provides a primary winding and a secondary winding.
An iron core having a secondary winding, a compensation impedance connected to the secondary winding and proportional to the secondary leakage impedance of the secondary winding, and control so that the voltage seen from the compensation impedance to the load side becomes almost zero. The gist of the present invention is to include an active element and a compensating means for supplying a current to generate a canceling voltage in the compensating impedance to cancel the voltage generated by the secondary leakage impedance according to the output of the active element.

(Function) In the error compensation layer current transformer according to the present invention, a cancellation voltage is generated in the compensation impedance connected to the secondary winding to cancel the voltage generated by the secondary leakage impedance of the secondary winding. The current for this purpose is supplied in accordance with the output of the active element that makes the voltage seen from the compensation impedance to the load side approximately zero.

(Example) Hereinafter, the present invention will be explained in detail using the drawings.

FIG. 1 is a circuit diagram of an error compensation layer current transformer according to an embodiment of the present invention. In FIG. 1, Zc is a compensation impedance, and 23 is a feedback current I for generating a later-described offset voltage for the compensation impedance ZC in accordance with the output of the operational amplifier 4 constituting an active element. a current source supplying 9,
10 is an input terminal of the operational amplifier 4. In FIG. 1, the same components as those in FIG. 3 are given the same reference numerals, and detailed explanations are omitted.

The current source 23 constitutes a compensation means, and mainly includes an NPN transistor 25 and a PNP transistor connected in a complementary manner.
) consists of a transistor 27. That is, the transistors 25 and 27 have their base terminals connected to the output of the operational amplifier 4, and their emitter terminals connected to the secondary winding 3.
output terminal 5 and compensation impedance Z. while its collector terminals are connected to the required positive power supply (+V cc) and negative power supply (VER) via resistors 29 and 31, respectively.

Next, the operation of this embodiment will be explained using the equivalent circuit of FIG. 1 shown in FIG. In FIG. 2, the same parts as those in FIG. 4 are designated by the same reference numerals, and detailed description thereof will be omitted.

From FIG. 2, the following equation holds true.

Io=aI, ・・・・・・
・Assuming (2), E2=I2 Z2 + (I2 IC) ZC+Z9-1
0...(3) Assuming that the gain of operational amplifier 4 is sufficiently large, E 9-1°=0...
・(4) becomes, and formula <3> becomes E2 = I2 Z2 + (I2 IC) ZC-(
5). Note that a is a constant.

Here, in order to make the error zero, the factor E2
needs to become zero. In formula (5), the condition for E2=0 is determined as follows. However, a>1. That is, when essentially compensating for errors in the configuration shown in FIG. 1, the value of the compensation impedance Zc is Z2/(
a-1) is sufficient.

However, the condition for a is that equation (2) holds true.

In the configuration shown below, the compensation impedance Zc
The feedback current I. By causing the secondary leakage impedance Z2 to cancel the voltage l2Z2 generated by the secondary leakage impedance Z2 and making the secondary electromotive force E2, which is the cause of the error, zero, essential error compensation becomes possible.

That is, by inserting a high-gain operational amplifier 4 between the secondary winding 3 and the load side, it is possible to reduce the voltage seen from the secondary winding 3 side to the load side to almost zero. 2
The secondary winding 3 and the load side can be electrically separated from each other. As a result, the compensation impedance Zc only needs to compensate for the error caused by the secondary electromotive force E2 of the secondary winding 3 and the secondary leakage impedance Z2.
In particular, there is no need to use a variable type, and there is no need to vary the compensation impedance every time the load changes. Further, since the current obtained from the secondary winding 3 is converted into a voltage by the high gain operational amplifier 4 and output, the output of the operational amplifier 4 can be directly supplied to the load for use.

Furthermore, since the secondary induced electric sweat E2 can be reduced to zero, the magnetic flux inside the iron core 1 is reduced, contributing to downsizing of the current transformer.

Moreover, since essential compensation is performed, it is effective even when the negative order current 11 is a distorted wave.

It goes without saying that the present invention is not limited to the above embodiments. That is, the compensation impedance Z in FIG. is expressed by equation (6), but the secondary leakage impedance Z2 is considered as the DC resistance Y2 of the secondary winding 3, and l
If Z21-'=r2, then compensation impedance Z. may be a simple resistor RC. Further, in FIG. 1, I is set to =aI2 as in equation (2), but a may be any value as long as a>1.

Furthermore, in this embodiment, the output is a voltage output, but
A voltage/current conversion circuit may be used to output a current proportional to the primary current (1).

[Effects of the Invention] 2 As explained above, according to the present invention, a compensating impedance connected to a secondary winding generates a canceling voltage that cancels the voltage generated by the secondary leakage impedance of the secondary winding. Since the current is supplied according to the output of the active element that makes the voltage seen from the compensation impedance to the load side almost zero, the error can be improved without being affected by the secondary leakage impedance of the secondary winding. can be reliably achieved.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of an error compensation type current transformer according to an embodiment of the present invention, FIG. 2 is an equivalent circuit diagram of FIG. 1, and FIG. 3 is a circuit diagram of a conventional error compensation type current transformer. FIG. 4 is an equivalent circuit diagram of FIG. 3. 1... Iron core 2... Primary winding 3.
...Secondary winding 4...Operation amplifier 7...
・Feedback resistance 23...Current source zo...Compensation impedance Z2...Secondary leakage impedance E2...Secondary induced voltage

Claims (1)

[Claims] An iron core having a primary winding and a secondary winding, a compensation impedance connected to the secondary winding and proportional to the secondary leakage impedance of the secondary winding, and a voltage from which the voltage seen from the compensation impedance to the load side is approximately zero. and compensating means for supplying a current to generate a canceling voltage in the compensation impedance to cancel the voltage generated by the secondary leakage impedance according to the output of the active element. Features an error compensation type current transformer.