JP3554889B2 - Double feedback current transformer - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a feedback type current transformer having a small magnetic flux and a small iron core for detecting and measuring a large current to a small current in a non-contact manner.
[0002]
[Prior art]
A feedback type current transformer is used as a current transformer for reducing the magnetic flux to reduce the exciting current. Figure 2 is an example of a conventional feedback type current transformer, to reduce the small and excitation current secondary internal voltage of the coil N ₂ of the main current transformer CT, the output of the tertiary coil N ₃ in the amplifier A ₁ amplifies and thereby fed back to the secondary load resistor R _L and the secondary coil N _2, current from 'secondary coil N _{2 of'} the feedback loop gain reduced by separately provided for stabilizing auxiliary current transformer CT Had to be supplied. Therefore, two iron cores are required, and the secondary coils N ₂ , N ₂ ′ for the primary coils N ₁ , N ₁ ′ of the main current transformer CT and the auxiliary current transformer CT ′ for flowing the primary current I ₁ . The ratios were complicated and expensive, with exactly equal ratios.
FIG. 3 shows another example in which a single iron core operates stably and is a voltage-compensated current transformer of a current transformer according to Japanese Patent No. 637920 of the present inventors. This is also fed back to amplify the voltage of the secondary load resistor R _L in the amplifier A ₂ is a kind of feedback type current transformer on the secondary series resistors R _0, wherein R _L voltage cancellation secondary coil N ₂ of This is to reduce the internal voltage. However, this had to be adjusted substantially 1 than slightly larger (secondary internal resistance of the coil N ₂ only) manually amplification factor of the amplifier A ₂ not automatic feedback. Thus such in need readjustment when the internal resistance of the secondary coil N ₂ is changed temperature, high-precision current transformer is not possible to the internal voltage of the secondary coil N ₂ to fine was not obtained.
[0003]
[Problems to be solved by the invention]
The present invention relates to an automatic control device for automatically setting the exciting current of the current transformer, that is, the internal voltage of the secondary coil to almost zero with only one iron core, and a loop for preventing the control system from oscillating and becoming unstable. It is an object to make the gain extremely low.
[0004]
[Means for Solving the Problems]
In order to achieve the above object, the present invention amplifies the voltage of the tertiary coil with one iron core, and manually negatively feeds back the voltage corresponding to the burden voltage on the secondary side to the secondary coil voltage. Reduce the feedback control loop gain to prevent oscillation.
Specifically, a voltage almost equal to the secondary burden voltage is manually compensated first by feedback of the secondary coil voltage, and a small voltage that cannot be compensated is detected and amplified by the tertiary coil voltage to perform automatic feedback compensation. In this case, the automatic control loop gain is reduced so as to perform stable operation.
[0005]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 shows an embodiment of the present invention, in which a current transformer CT includes a primary coil N ₁ , a secondary coil N ₂ , a tertiary coil N ₃ and one iron core T. _RL is a secondary load resistance, and Op1 and Op2 are operational amplifiers. Operational amplifier Op1 constitutes a first non-inverting amplifier with voltage dividing resistors _R 3, _{R 4.} Operational amplifier Op2 constitutes a second non-inverting amplifier with voltage dividing resistors _R 1, _{R 2.}
[0006]
Output voltage _{V 01} of the operational amplifier Op1 In Figure 1 are cascade connected to the lower end of the _{R 1,} it is added to the output of the operational amplifier Op2. On the other hand, the output voltage _{V 02} of the operational amplifier Op2 includes a output voltage _{V 01} of the operational amplifier Op1, which is fed back to the secondary coil _{N 2.} And Thus, it is necessary a larger voltage than the voltage of only the internal resistance of the secondary coil N ₂ secondary load resistance R _L is dividing resistor R ₂ to obtain an output V ₀₂ of the operational amplifier Op2. C is a phase adjusting capacitor. On the other hand, if the internal resistance of the secondary coil N ₂ is very small, the dividing resistors R ₂ value is extremely small. In this case, as shown in the following theoretical formula, since substantially the sum amplitude of the output voltage V ₀₁ of the operational amplifier Op1 is extremely small, the auxiliary resistor r ₀ in series with the secondary coil N ₂ was pushed, minutes allowed to increase the pressure resistance _{R 2.}
[0007]
This will be described with the following mathematical formula. (The capacitor C is not considered for simplicity.) In the figure, if the internal resistance of the secondary coil N ₂ is r ₂ and the secondary output current is I ₂ , the secondary current I ₂ is supplied to the input of the operational amplifier Op2. since not flow, the internal voltage V ₂ of the secondary coil N ₂ is expressed by the following equation.
V ₂ = I ₂ (r ₂ + r ₀ ) + I _{2 RL−} V ₀₂ (1)
However, operational amplifier Op2, dividing resistors _R 2, _{R 1} is because they constitute a non-inverting amplifier,
V ₀₂ = I ₂ R _L + I ₀₂ R ₂ (2)
I ₂ R _L = I ₀₂ R ₁ + V ₀₁ (3)
Here, I ₀₂ is a current flowing through R ₁ and R ₂ , which is expressed by equation (3).
I ₀₂ = (I _{2 RL−} V ₀₁ ) / R ₁ (4)
Is obtained, and this is substituted into the equation (2), the output voltage of the operational amplifier Op2 becomes
_{V 02 = I 2 R L +} I 2 R L (R 2 / R 1) -V 01 (R 2 / R 1) (5)
It becomes.
[0008]
(5) third term denotes the sum of the output voltage _{V 01} of the operational amplifier _Op1, if _R 2 = 0, so that the output of the operational amplifier Op1 is not at all added. Since has become minus sign, the input of the operational amplifier Op1 must be in the negative, the tertiary coil N ₃ is the reverse wound with respect to secondary coil N _2.
To increase the addition effect by increasing the partial pressure resistors R ₂ inserts an auxiliary resistor r ₀ in series with the secondary coil N _2.
[0009]
Next, if the equation (5) is substituted into the equation (1), the internal voltage of the secondary coil becomes
_{V 2 = I 2 (r 2} + r 0) + I 2 R L - (I 2 R L + I 2 R L (R 2 / R 1) -V 01 (R 2 / R 1))
= I ₂ (r ₂ + R ₀ ) −I _{2 RL} (R ₂ / R ₁ ) + V ₀₁ (R ₂ / R ₁ ) (6)
[0010]
On the other hand, the amplification degree _{K 1} of the non-inverting amplifier consisting of an operational amplifier Op1 and voltage dividing resistors _R 3, _{R 4} is _{K 1 = V 01 / V 3} = (R 3 + R 4) / R 4 (7)
, And the if reverse winding the tertiary coil N ₃ to the secondary coil N _2, the internal voltage of the tertiary coil,
V ₃ = − (N ₃ / N ₂ ) V ₂ (8)
[0011]
Therefore, the output voltage _{V 01} of the operational amplifier Op1 _is, as _{K 0 = K 1 (N 3} / N 2),
V ₀₁ = −K ₁ (N ₃ / N ₂ ) V ₂ = −K ₀ V ₂ (9)
Next, an amplified version of the internal voltage of the secondary coil N ₂ is in the V _01. Substituting equation (9) into equation (6) and rearranging,
_{V 2 (1 + K 0 (} R 2 / R 1)) = I 2 (r 2 + r 0 -R L (R 2 / R 1)) (10)
It becomes.
[0012]
As described above, the internal voltage V ₂ of the secondary coil is
_{V 2 = (r 2 + r} 0 -R L (R 2 / R 1)) / (1 + K 0 (R 2 / R 1)) I 2 (11)
It becomes.
Now, even with a small internal resistance of the secondary coil by inserting the auxiliary resistance r ₀ with the possible to increase the voltage dividing resistors R _2, can be increased substantially loop gain _{K 0 (R 2 / R 1} ) in the denominator.
[0013]
As seen in the above (11), manually molecule by operational amplifier Op2, with as small as possible, a K ₀ in the denominator is increased by operational amplifier Op1, further automatically reduce the overall internal voltage V ₂ of the secondary coil I do. Since the numerator is small, the so-called automatic control loop gain K ₀ (R ₂ / R ₁ ) can extremely reduce the internal voltage V ₂ of the secondary coil without increasing it so much as to oscillate. The input of the operational amplifier Op2 is not necessarily a voltage of the secondary load resistance R _L directly obvious that may be a signal corresponding to the secondary current I _2.
[0014]
【The invention's effect】
The present invention is configured as described above. Since the automatic feedback is performed by the first amplifier and the manual feedback is performed by the second amplifier, the loop gain of the automatic feedback is stabilized at a low level. The coil internal voltage can be stably extremely reduced, and there is an excellent operational effect that a high precision current transformer can be obtained with a small amount of iron core.
[Brief description of the drawings]
FIG. 1 shows an embodiment of the present invention.
FIG. 2 shows a conventional example (feedback current transformer).
FIG. 3 shows a conventional example (voltage compensation current transformer of current transformer).
[Explanation of symbols]
CT, CT 'current transformer T core _{N 1, N} 1' 1 primary coil _{N 2} 2 coil _{N 3} 3 coil _A 1, _{A 2} amplifier _{R L} 2 primary burden resistor _{I 2} 2 primary current _{R 0} resistance _{V 2} The internal voltage r _{2 of the} secondary coil The internal resistance r _{0 of the} secondary coil The auxiliary resistors Op 1 and Op 2 The operational amplifiers R _{1 to} R _{4 The} resistance voltage of the capacitor V ₀₁ Op 1 The output voltage V ₀₂ Op 2 The output voltage I ₀₂ R ₂ and R ₁ of Op ₂ Flowing current

Claims (3)

A tertiary coil is provided in the current transformer, and a first amplifier for amplifying the output signal and a second amplifier for amplifying an electric signal related to the load of the current transformer are provided. A double feedback current transformer characterized in that a composite signal of the output signals of the two amplifiers is inserted and fed back in series with the secondary coil of the current transformer in a direction to reduce the internal voltage of the secondary coil. The first and second amplifiers, operational amplifiers Op1, partial pressure feedback resistor _R 3, the first non-inverting amplifier consisting of _{R 4,} and an operational amplifier Op2, partial pressure feedback resistors _R 2, _{R 1} from become second consists of a non-inverting amplifier, and the output of the first non-inverting amplifier, claim that characterized in that it is cascaded to the partial pressure feedback resistors R _2, R ₁ of the second non-inverting amplifier 2. The double feedback current transformer according to 1. Insert the auxiliary resistor r ₀ in series with the secondary coil, the double feedback current transformer according to claim 1 or 2, wherein the said raising the substantial addition amplification of the first non-inverting amplifier.

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