JPS6136695B2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a current transformer for the purpose of error compensation,
In particular, the present invention relates to improvements in error-compensating current transformers that use active elements to perform error compensation with high precision.

FIG. 1 shows a conventional error compensation type current transformer using active elements. In the figure, l is an iron core, and this iron core l has a primary winding 2 with N ₁ turns on the primary side and a secondary winding with N ₂ turns on the secondary side. It has 3. 4 is an operational amplifier, the inverting input part of which is connected to one output terminal 5 of the secondary winding 3;
The non-inverting input section is 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 and 9 are the output terminals of the current transformer, and the output voltage v _p
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, the error compensation type current transformer as shown in Fig. 1 has the following equation: I ₁ /I ₂ =N ₂ /N ₁ =K (1) v _p =I ₂・R _F =−I ₁ /K・R _F ……(2) holds true. However, I ₁ is 1 of primary winding 2
The secondary current, I ₂ is the secondary current of the secondary winding 3, and R _F is the resistance value of the feedback resistor 7. By the way, the reason why the error is improved by the circuit configuration shown in FIG. 1 is as follows. Since the output terminals 5 and 6 of the secondary winding 3 are connected to the input section of the operational amplifier 4, if the gain of the amplifier 4 is sufficiently large, the potential e _5-6 between the terminals 5-6 becomes approximately zero. In other words, 2 of the secondary winding 3 of the iron core 1
The next load becomes zero and the error is improved.

However, according to experimental results, the error was not completely zero, and in current transformers that required high precision, there was a problem particularly with the phase angle error associated with the excitation current.

Examining the reason why such an error is not completely compensated, it is believed that it is due to the presence of secondary leakage impedance and excitation impedance of the secondary winding 3, and this will be explained below. Now, if Figure 1 is expressed as an equivalent circuit, it will be as shown in Figure 2. In FIG. 2, 3 corresponds to the secondary winding 3 of FIG. 1, Z ₂ represents the secondary leakage impedance of the winding 3, and E ₂ represents the secondary induced voltage due to the excitation impedance. Output terminal 5 of secondary winding 3,
This is because the terminal 5 is virtually grounded by the operation of the operational amplifier 4 due to the shorting line 10 that short-circuits the terminal 6.
Therefore, if we consider the space between terminals 5 and 6 as virtual ground, e
_5-6 = 0, and the following formula holds true.

E ₂ +I ₂・Z ₂ =0 ∴E ₂ =−I ₂ Z ₂ ...(3) However, as shown in equation (3), even if terminals 5 and 6 are shorted, 2 is a cause of error. This shows that the secondary induced voltage E ₂ does not become completely zero, but has a value corresponding to the voltage drop due to the secondary leakage impedance Z ₂ of the secondary winding 3. Therefore, this voltage E ₂
This generates a small excitation current, which causes phase angle errors. In other words, even if the secondary voltage e _5-6 is made zero, the secondary induced voltage E ₂ does not become zero, which causes an error.

The present invention has been made in view of the above circumstances, and its purpose is to connect an active load to the main core side secondary winding so that the voltage seen from the secondary winding to the load side is almost zero. In addition to ensuring that the equipment is not affected by the load, the error caused by the secondary induced voltage of the secondary winding on the main core side and the secondary leakage impedance is compensated for by inserting a compensation impedance regardless of the load. The object of the present invention is to provide an error compensation type current transformer that can convert current with high accuracy, eliminates the need to vary the compensation impedance each time an arbitrary load is connected, and can compensate with a small capacity compensation impedance. .

Hereinafter, one embodiment of the present invention will be described with reference to FIG. In the figure, 21 is the main core, 22
is an auxiliary core, and 23 is a primary winding common to cores 21 and 22, which is wound with a number of turns _N11 . 24 is the main core side 2 which is wound around the main core 21 with a number of turns N ₂₁ .
The next winding 25 is a secondary winding of the auxiliary core 22, which has the same polarity as the main core side secondary winding 24 and is wound with a number of turns _N31 . Then, the main iron core side secondary winding 24
One end of the secondary winding 24 is connected to the inverting input of the operational amplifier 26 via a compensating impedance Z _c that compensates for the secondary induced voltage and secondary leakage impedance of the secondary winding 24, and the other end of the secondary winding 24 is connected to the operational amplifier 26. It is connected to the non-inverting input part of the circuit and is grounded.
As the operational amplifier 24, one with a sufficiently large gain is used so that the voltage between the terminals 28 and 29 is approximately zero, so that the current transformer is not affected by the load side. Furthermore, the secondary winding 25 on the auxiliary core side
Both ends of are connected in parallel with the compensation impedance Zc, and the compensation impedance of the secondary winding 24 on the main iron core side
By applying a predetermined current to Zc, errors caused exclusively by the secondary induced voltage and secondary leakage impedance of the main core side secondary winding 24 are compensated for. 27 is a feedback resistor, and 30 and 31 are output terminals of the current transformer.

Therefore, when FIG. 3 is expressed as an equivalent circuit, it becomes as shown in FIG. 4. In FIG. 4, Z ₃ and E ₃ indicate the secondary leakage impedance and secondary induced voltage of the secondary winding 24 on the main core side, and Z ₄ and E ₄ indicate the secondary leakage impedance and secondary induced voltage of the secondary winding 25 on the auxiliary core side. leakage impedance and secondary induced voltage. Note that the secondary leakage impedance and the secondary leakage impedance are the same concept. 32 is a virtual short-circuit line that is virtually grounded by the operation of the operational amplifier 26. Therefore, the relationship shown in FIG. 3 is established from the equivalent circuit shown in FIG. 4.

I ₂₁ =N ₁₁ /N ₂₁・I ₁₁ ...(4) I ₃₁ =N ₁₁ /N ₃₁・I ₁₁ ...(5) N ₁₁ /N ₂₁ =1/k ...(6) N ₂₁ /N ₃₁ = a ... (7) I ₃₁ = aI ₂₁ ... (8) Then, E ₃ = I ₂₁ (Z ₃ + Z _c ) - I ₃₁・Z _c ... (9) ∴E ₃ = I ₂₁・Z ₃ + (1-a) I ₂₁・Z _c ...(10). Here, in order to make the error zero, it is necessary to make E ₃ , which is a factor thereof, zero. If we find the condition for E ₃ =0 in equation (10), we get Z _c =Z ₃ /a-1 (11). However, a>1. That is, when essentially compensating for errors in the configuration shown in FIG.
The value of the compensation impedance Z _c may be Z ₃ /(a-1) as shown in equation (11). Therefore, with the above configuration, by inserting a high gain operational amplifier between the main core side secondary winding 24 and the load side, the load side is It becomes possible to reduce the voltage to almost zero, and thus it is possible to have a configuration in which the secondary winding 24 and the load side are electrically separated. Therefore, since the compensating impedance Zc only needs to compensate for the error caused by the secondary induced voltage of the secondary winding 24 on the main core side and the secondary leakage impedance, there is no need to use a variable type compensating impedance Zc. Moreover, it is not necessary to change the compensation impedance every time the load changes, and the compensation impedance and the capacity of the auxiliary core can be reduced. Furthermore, since the current obtained from the secondary winding on the main iron core side is converted into voltage by a high gain operational amplifier and output, the output of the operational amplifier can be directly supplied to the load for use.

It goes without saying that the present invention is not limited to the above embodiments. That is, in FIG. 3, Z _c is expressed by equation (11), but Z ₃ is expressed by the secondary winding 24
If |Z ₃ |≒r ₃ , then Z _c = r ₃ /a-1 _... (12), and the compensation impedance Z is
_c may be a simple resistance R _c .

In addition, in Figure 3, N ₂₁ /N ₃₁ = a, but a
If the relationship is >1, a can be any value.

Furthermore, in Fig. 3, a voltage v _p proportional to one conduction current I ₁₁ is output, but it is possible to output either a voltage or current signal proportional to I ₁₁ using a circuit that performs V/I conversion. It is a good thing.

As detailed above, according to the present invention, two iron cores
A compensation element made of a compensation impedance or pure resistance equal to the secondary leakage impedance of the secondary winding is inserted into the secondary winding, and a high gain By connecting an operational amplifier, not only can compensation be performed with high accuracy regardless of the load side, but also
Compensation can be achieved relatively easily with a small capacitance compensation element. Further, it has various effects such as being able to connect various loads and eliminating the need to change the compensation element every time the load changes.

In addition, the secondary winding on the auxiliary core side only corresponds to the secondary leakage impedance of the secondary winding on the main core side (when a = 2)
Or, let only 1/a be the load (Z in equation (11)
Since only _c )) is required, error compensation can be performed using a small iron core.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a conventional error-compensating current transformer using active elements, Fig. 2 is an equivalent circuit diagram of Fig. 1, and Fig. 3 is an implementation of an error-compensating current transformer according to the present invention. FIG. 4 is an equivalent circuit diagram of FIG. 3, and FIG. 5 is a configuration diagram showing another example of the present invention. 21... Main iron core, 22... Auxiliary iron core, 23...
Primary winding, 24...Main core side secondary winding, 25...
Auxiliary core side secondary winding, 26... operational amplifier, 27
...Feedback resistance, Z _c ... Compensation impedance, R _c
...Compensation resistance.

Claims (1)

[Claims] 1 A main core and an auxiliary core that have a common primary winding, and are connected to the main core side secondary winding, and these two
An active load that is controlled so that the voltage seen from the next winding side to the load side is almost zero, and a secondary winding of the secondary winding that is inserted between this active load and the main core side secondary winding. A compensating element consisting of a compensating impedance proportional to the leakage impedance or a compensating pure resistance is connected in parallel with the secondary winding on the auxiliary core side to both ends of this compensating element, and a predetermined current is passed through the compensating element. An error compensating current transformer comprising compensation means for generating an offset voltage equal to the voltage generated by the secondary leakage impedance of the secondary winding on the iron core side.