JPH067535B2 - Error compensation current transformer - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an error compensating current transformer for compensating a phase difference between a primary side and a secondary side, and more particularly to improvement of the error compensation.

[Prior Art] FIG. 3 shows a circuit of a conventional error compensation type current transformer disclosed in Japanese Patent Publication No. Sho 61-36695. A primary winding 23 is wound around the main iron core 21 and the auxiliary iron core 22. A main iron core side secondary winding 24 is wound around the main iron core 21, and an auxiliary iron core side secondary winding is wound around the auxiliary iron core 22.
25 are wound. A compensation impedance Z _C is connected to both ends of the secondary winding 25 on the auxiliary core side. The output of the main core side secondary winding 24 is given to the operational amplifier 26 via the compensation impedance Z _C.

This circuit is represented by an equivalent circuit as shown in FIG. The secondary leakage impedance of the secondary winding 24 on the main iron core side is represented by Z ₃ , and the secondary induced voltage is represented by E ₃ . Similarly, the secondary leakage impedance Z ₄ of the secondary winding 25 on the auxiliary core side and the secondary induced voltage are represented by E ₄ . Further, the virtual short-circuit line 32 is used assuming that the gain of the operational amplifier 26 is sufficiently large. If I ₂₁ is the current flowing through the virtual short-circuit wire 32 and I ₃₁ is the current flowing out from the auxiliary iron core side secondary winding 25, the following relationship is established from this equivalent circuit.

_{E 3 = I 21 · (Z} 3 + Z C) -I 31 · Z C. ． ． (1) Here, if the number of turns of the main iron core side secondary winding 24 is N ₂₁ and the number of turns of the auxiliary iron core side secondary winding 25 is N ₃₁ , then I ₃₁ = a · I ₂₁ (where a = N ₂₁ / N ₃₁ ). ． ． (2) is established. From the above equations (1) and _{(2), E 3 = I 21 · Z} 3 + (1-a) · I 21 · Z C. ． (3) Here, in order to reduce the phase difference between the primary side and the secondary side to zero, the secondary induced voltage E ₃ which is the factor may be set to zero. Therefore, when E ₃ in the equation (3) is set to zero, Z _C = Z ₃ / (a-1). ． ． ． ． ． ． ． ． The relationship of (4) can be obtained. That is, in the circuit of FIG. 3, if the value of the compensation impedance Z _C is determined so as to satisfy the above condition, error compensation can be performed.

[Problems to be Solved by the Invention] The conventional error compensating type current transformer has a problem in that the structure is complicated because it is necessary to provide the auxiliary core and the secondary winding on the auxiliary core side. Furthermore, since the structure is complicated, there is a problem that the dielectric strength between the primary winding and the secondary winding and between each winding and the iron core cannot be increased.

It is an object of the present invention to solve the above problems and provide an error compensating current transformer having a simple structure and a high dielectric strength.

[Means for Solving Problems] In the error compensating current transformer according to the present invention, the inverting input terminal of the amplifying means is connected to one end of the secondary winding of the current transformer, and the non-inverting input terminal is a compensating element. Is connected to the other end of the secondary winding.

[Operation] The compensating element has a function of canceling the voltage due to the secondary leakage impedance of the current transformer by the voltage generated across the compensating element and eliminating the influence of the secondary leakage impedance.

[Embodiment] FIG. 1 shows a circuit diagram of an error compensating current transformer according to an embodiment of the present invention. The iron core 21 includes a primary winding 23 and a secondary winding 24.
Is wound. The input terminals 10 and 11 are connected to the primary winding 23. One end of the secondary winding 24 is connected to the inverting input of an operational amplifier 42 which is an amplification means. The other end of the secondary winding 24 has a compensation impedance Z _C , which is a compensation element.
Is connected to the non-inverting input of the operational amplifier 42 via.
A first feedback resistor R ₁ is provided between the output terminal and the non-inverting input terminal of the operational amplifier, and a second feedback resistor R ₁ is provided between the output terminal and the inverting input terminal.
Is connected to the feedback resistor R ₂ . The output of the operational amplifier 42 and the other end of the secondary winding 24 form output terminals 30 and 31.

FIG. 2 shows an equivalent circuit of the portion of the current transformer CT. The secondary leakage impedance of the current transformer is Z ₂ , the secondary induced voltage is _Em , the secondary current is I ₂ , the voltage at the non-inverting input terminal of the operational amplifier 42 is E ₁ , and the voltage at the output terminal is E ₀ . . When the amplification degree of the operational amplifier 42 is sufficiently large, the following formula is established.

_{E 0 = E 1 -R 2 ·} I 2. ． ． ． ． ． ． ． ． ． (5) E ₁ = Z _C · E ₀ / (R ₁ + Z _C ). ． ． ． ． ． ． (6) In addition, E _m = Z ₂ · I ₂ + E ₁ . ． ． ． ． ． ． ． ． ． ． (7) Here, from the equations (5) and (6), E ₀ = − (R ₂ / R ₁ ) · (R ₁ + Z _C ) · I ₂ .. ． (8) E1 = - (R 2 / R 1) · Z C · I 2. ． ． ． ． ． (9) is introduced. Further, from the equations (7) and (9), _Em = Z ₂ · I ₂ − (R ₂ / R ₁ ) · Z _C · I ₂ = (Z ₂ − (R ₂ / R ₁ ) · Z _C ) · I ₂ ． ． (10) is required.

In order to reduce the error of the current transformer to zero, the secondary induced voltage may be set to zero. When the condition of E _m = 0 in the equation (10) is obtained, Z _C = (R ₁ / R ₂ ) · Z ₂ . ． ． ． ． ． ． ． ． It becomes (11). Therefore, in the circuit of FIG. 1, the resistors R ₁ and R ₂ and the compensation impedance Z _C satisfy the above equation.
If is selected, a current transformer with no phase difference can be formed between the primary side and the secondary side.

Since the secondary leakage impedance Z ₂ largely depends on the DC resistance of the secondary winding, the compensation element may be a resistance instead of an impedance.

Further, in the above embodiment, the case where the secondary induced voltage is set to zero has been described, but when it is desired to make the phases of the outputs of the two current transformers the same, the secondary induced voltage is not necessarily set to zero. There is no.

[Effect of the Invention] In the error compensating current transformer according to the present invention, one end of the secondary winding is connected to the inverting input of the operational amplifier, and the other end is connected to the non-inverting input via the compensation element. Since the error compensation is performed by the voltage of this compensating element, it is not necessary to provide an auxiliary iron core or an auxiliary iron core side secondary winding. Therefore, it is possible to provide an error compensating current transformer having a simple structure and excellent dielectric strength.

[Brief description of drawings]

FIG. 1 is a circuit diagram of an error compensating current transformer according to an embodiment of the present invention, FIG. 2 is a diagram showing an equivalent circuit of a portion of the current transformer CT in FIG. 1, and FIG. 3 is a conventional error diagram. FIG. 4 is a circuit diagram showing a compensation type current transformer, and FIG. 4 is a diagram showing an equivalent circuit of FIG. CT is a current transformer, Z _C is a compensation impedance, 23 is a primary winding, 24 is a secondary winding, 30 and 31 are output terminals, and 42 is an operational amplifier. In the drawings, the same reference numerals indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A current transformer, an amplifying means in which an inverting input terminal is connected to one end of a secondary winding of the current transformer, and another end of the secondary winding of the current transformer and the amplifying means. Connected between the inverting input terminal and the secondary leakage impedance of the current transformer is Z ₂ , the first feedback resistor of the amplification means is R ₁ ,
When the second feedback resistor is R ₂ , its impedance is represented by Z _c = (R ₁ / R ₂ ) · Z ₂ , and the output of the amplifying means and the secondary winding of the current transformer. And an output means formed by the other end of the error compensating current transformer.