RU2097864C1 - Current transformer - Google Patents

Abstract

FIELD: measurement technology. SUBSTANCE: transformer secondary winding incorporates second stage built around negative-feedback transformer. EFFECT: improved ac current measurement accuracy, reduced material input and reduced power loss due to lower resistance. 2 dwg

Description

 The invention relates to the field of electrical measurements, in particular to the measurement of alternating currents in the electric power industry.

 The invention is aimed at solving the problem of improving the accuracy of measuring alternating currents in the electric power industry and improving the operational characteristics of current transformers.

 The so-called compensation current transformers are known (see Leytman M. B. Normalizing measuring transducers of electrical signals. M. Energoatomizdat, 1986, 144 pp.). One winding of such a transformer is included in the circuit of the measured current, the other is the output, and the third is connected to the input of the amplifier. Such devices have small errors in current conversion due to negative feedback. But transformers of this type are not used in the electric power industry, since with a large primary current (thousands of amperes) a large transformation coefficient is required when using a low-power amplifier, because the output current of the transformer is determined by the output current of the amplifier (units of milliamps when using operational amplifiers). This leads to the consumption of a large amount of copper and a significant decrease in manufacturability in the manufacture of the transformer. It is possible to reduce the number of turns of the secondary winding by increasing the output current of the amplifier, but this leads to a decrease in the reliability of the transformer, and this factor is given great importance in the electric power industry.

 The closest in technical essence to the claimed one is a current transformer (see Andreev Yu.A. Abramzon GV Current transducers for measurement without breaking the circuit. L. Energy, 1979. S. 36-40, 144 S.). It contains a core made of ferromagnetic material, primary and secondary windings. Such a transformer is widely used in the electric power industry, as It is reliable and easy to manufacture. The disadvantage of the prototype is the presence of conversion errors, which, as you know, increase with increasing resistance in the secondary winding. This resistance includes the resistance of the transformer winding, connecting wires and instrumentation. In addition, the transformer and measuring devices are often separated in space, since the first is located in the switchgear, and the second on the control panel, which leads to the use of long connecting wires. This leads, on the one hand, to an increase in resistance in the secondary winding, and, consequently, to an increase in errors, and, on the other hand, to an increase in the power of losses and consumption of materials, since the secondary current of the prototype is quite significant (1A or 5A according to GOST), which is also a disadvantage of the prototype.

 The purpose of the invention is to increase the accuracy of measuring current, reducing the consumption of materials and power losses by reducing the resistance in the secondary winding of the transformer.

 To achieve this goal in a current measuring transformer, comprising: a first transformer with primary and secondary windings, the primary winding being included in the measured current circuit; a second transformer comprising three windings and an amplifier, the third winding connected between the ground bus and the input of the amplifier, the output of which through the second winding and resistor is connected to the ground bus; the first winding of the second transformer is connected between the terminals of the secondary winding of the first transformer, and the connection point of the third winding of the second transformer and resistor is the output of the device.

 A significant difference of the claimed device from the prototype is the use of a second stage based on a compensation current transformer, which allows to reduce the resistance of the secondary winding of the transformer.

 According to the information available to the authors, these essential features are not known from the prior art, which allows us to conclude that the invention meets the criterion of "novelty."

According to the authors, the essence of the claimed invention should not be obvious to specialists from the prior art, which allows us to conclude that the invention meets the criterion of "inventive step"
The features of this invention can be repeatedly used in measuring technology to obtain a technical result consisting in achieving the goal, which allows us to conclude that the invention meets the criterion of "industrial applicability".

 Functional diagram of the device is shown in FIG. 1 and comprises a first transformer with primary 1, secondary 2 windings, a second transformer with a first winding 3, an amplifier 4, a second 5, a third 6 windings, a resistor 7. The output of the device is terminal 8. In FIG. 2 shows a transformer equivalent circuit. It shows the parameters of the elements to the primary winding of the first transformer, which is shown either with a single stroke (elements of the secondary winding of the first transformer and the first winding of the second transformer), or with two strokes (elements of the second winding of the second transformer).

включается сопротивление нагрузки

состоящее из сопротивления соединительных проводов и измерительных приборов. В этом случае ток на выходе первого трансформатора равен

где

сопротивление ветви намагничивания первого трансформатора.The device operates as follows. Winding 1 is included in the measured current circuit. The number of turns on it is equal to W ₁ (often this is one turn). The output current is removed from the winding 2 having the number of turns W ₂ . The transformation ratio of the first transformer is n ₁ W ₂ / W ₁ . In the prototype between points a and b in series with the resistance of the winding 2

load resistance turns on

consisting of resistance of connecting wires and measuring instruments. In this case, the current at the output of the first transformer is

Where

resistance of the magnetizing branch of the first transformer.

, поскольку сопротивление

составляет ощутимую величину (до нескольких Ом). Погрешности уменьшаются при

,
что в прототипе недостижимо. В предлагаемом устройстве между точками a, b включается второй трансформатор, который с высокой точностью преобразовывает ток

в ток

за счет отрицательной обратной связи, что на схеме замещения отражается увеличением сопротивления ветви намагничивания

в K раз (K коэффициент усиления усилителя 4). Поэтому можно считать, что

где n₂=W₅/W₃ коэффициент трансформации второго трансформатора;
W₃, W₅ количества витков на обмотках 3 и 5, соответственно, причем обычно W₃=1.From the formula (1) it can be seen that the prototype has errors, because

since resistance

It is a tangible value (up to several ohms). Errors decrease when

,
which is unattainable in the prototype. In the proposed device, between the points a, b, a second transformer is switched on, which converts the current with high accuracy

in current

due to negative feedback, which is reflected in the equivalent circuit by an increase in the resistance of the magnetization branch

K times (K amplifier gain 4). Therefore, we can assume that

where n ₂ = W ₅ / W _{3 the} coefficient of transformation of the second transformer;
W ₃ , W _{5 the} number of turns on the windings 3 and 5, respectively, and usually W ₃ = 1.

где

сопротивления обмоток 3 и 5, соответственно:
R₇ сопротивление резистора 7.In this case, the load resistance of the first transformer is equal to:

Where

winding resistance 3 and 5, respectively:
R ₇ resistor 7 resistance.

Обычно: R₇ 2 3 кОм, n₂ 1000-10000, а обмотка 3 выполняется в виде одного витка (Z₃ _→ 0), поэтому практически

, т.е. выполняется условие (2), что способствует снижению погрешностей первого трансформатора, а, следовательно, и всего устройства, т.к. с учетом (3)

Если трансформатор и измерительные приборы разнесены в пространстве, то линия связи (провод cd на фиг. 2) в данном случае выполняется в виде провода с малым сечением, поскольку ток I_вых относительно мал (не превышает 10 mA), что способствует снижению расхода материалов и мощности потерь. Измерительные приборы подключаются к выводу 8.Since R ₇ > Z ₅ , then

Usually: R ₇ 2 3 kOhm, n ₂ 1000-10000, and winding 3 is made in the form of a single turn (Z ₃ _ → 0), therefore, practically

, i.e. condition (2) is fulfilled, which helps to reduce the errors of the first transformer, and, consequently, the entire device, because subject to (3)

If the transformer and measuring instruments are separated in space, then the communication line (wire cd in Fig. 2) in this case is made in the form of a wire with a small cross section, since the current I _{o is} relatively small (does not exceed 10 mA), which helps to reduce the consumption of materials and power losses. Measuring instruments are connected to pin 8.

 According to the results of experiments, the claimed invention can find wide application in the national economy, where it is necessary to increase the accuracy of measuring currents. It must be emphasized that this does not require any internal alteration of the first transformer, i.e. the proposed solution will allow to obtain the effect of reducing errors and material consumption on equipment that is widely used in the electric power industry, which is important.

 The claimed solution does not adversely affect the state of the environment.

Claims (1)

 A measuring current transformer comprising a first transformer with primary and secondary windings, the primary winding included in the circuit of the measured current, a second transformer containing three windings and an amplifier, the third winding connected between the ground bus and the input of the amplifier, the output of which is connected through the second winding and the resistor with an earth bus, characterized in that the first winding of the second transformer is connected between the terminals of the secondary winding of the first transformer, and the connection point of the third winding of the second t transformer and resistor is the output of the device.

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