RU2555524C1 - Electronic current transformer - Google Patents

Abstract

FIELD: electricity.

SUBSTANCE: electronic current transformer comprises an input measuring current transformer 2 with primary 3, measuring 4 and feedback 5 windings, a pre-amplifier 6, a phase changer 7, a controlled amplifier 8, a power amplifier 9, a current-limiting resistor 10, an output matching transformer 11 with primary 12 and secondary 13 windings. At that in the device there are also a current-setting resistor 14, an operational amplifier 15, a reference amplifier 16, a measuring resistance 17 and a comparison circuit 18.

EFFECT: reduced current and angular error, reduced weight, dimensions and costs of used basic specifications, expanding the range of measured currents, invariability of an electronic current transformer to change in the load resistance, increased temperature and timing stability.

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Description

The proposed technical solution relates to electrical engineering and is intended for precision measurements of a wide range of currents, especially in high-voltage networks and power systems.

From a wide range of measuring current transducers (IPT), a number of IPTs, called electronic current transformers (ETTs), are distinguished, the hallmark of which is the presence in them of an intermediate conversion of the measured current using electronic circuits [1].

Compensatory IPTs based on measuring current transformers (ITTs) are widely used, in which the primary magnetic flux created by the measured current is compensated by magnetic flux in the opposite direction created by the current flowing through the ITT feedback winding [2].

The closest to the proposed invention by technical essence are IPT, in which to reduce the mass, size and cost, the current of the primary windings used in them small-sized ITT is reduced hundreds of times by parallel connecting them to sections of the measured current circuit along the axis of their flow [3, 4, 5, 6].

To work with devices that have current inputs, for example, electricity and power meters, in such IPT output matching transformers (VST) are used.

The disadvantage of such IPTs is that the miniaturization of the transformers used in them significantly worsens the linearity of their magnetic characteristics, and this leads to a significant increase in the current and angular errors of IPTs, which does not allow the use of such IPTs as precision and reference current meters.

Identified shortcomings of IPT are eliminated or minimized in the proposed implementation of ETT.

The purpose of the proposed technical solution is to significantly reduce the current and angular errors of the ETT by linearizing and stabilizing the magnetic characteristics of the small-sized transformers included in their composition.

The technical result of the invention is to improve the following characteristics of ETT:

- a significant reduction in current and angular errors;

- reduction in the mass, dimensions and cost of ITT;

- expanding the range of measured currents;

- the invariance of ETT to a change in load resistance;

- increase in temperature and time stability.

The technical result is achieved in that in an electronic current transformer containing an input measuring current transformer, the primary winding of which is connected to a portion of the measured current circuit along the axis of its flow, and the measuring winding is connected to the differential inputs of the preliminary amplifier, the output of which is through a serial circuit consisting of a phase shifter , adjustable amplifier, power amplifier, limiting resistor and primary winding of the output matching transformer, connected to earth second bus, the secondary winding of the output transformer is the current output of the device, in addition, the operational amplifier is connected directly to the output of the pre-amplifier by the input amplifier, and its inverting input is connected to one of the ends of the feedback transformer of the input transformer and through the current-setting resistor to the ground bus, the second end this winding is connected to the output of the operational amplifier, in addition, the output of the adjustable amplifier is connected to the input of the reference amplifier, the output of which is through a measuring resistor connected to the ground bus and to one of the inputs of a new element of the comparison circuit, the second input of which is connected to the connection point of the limiting resistor and the primary winding of the output transformer, while the output of the comparison circuit is connected to the control input of the aforementioned adjustable amplifier.

The analysis of the existing level of electrical equipment did not reveal the presence of an analogue with features that are identical to all the features of the claimed invention, and also showed the presence of a new set of features, which, complementing the known features, allow to achieve the goal.

The drawing shows a structural diagram of ETT.

ETT contains current lead 1, input measuring current transformer 2 with primary 3, measuring 4 and feedback 5 windings, as well as preamplifier 6, phase shifter 7, adjustable amplifier 8, power amplifier 9, current limiting resistor 10, output matching transformer 11 with primary 12 and secondary 13 windings, a current setting resistor 14, an operational amplifier 15, a reference amplifier 16, a measurement resistor 17, and a comparison circuit 18.

To reduce the weight, size and cost of ETT, they use small-sized measuring current transformers (ITT). This is a useful and necessary measure, but it significantly worsens the magnetic characteristics and parameters of ITT.

A significant improvement in the magnetic characteristics of the ITT and effective stabilization of the ETT parameters as a whole is carried out by this technical solution, turns the ETT into a precision measuring tool, and this is carried out as follows.

A measured current In of the network load flows through the conductor 1. Parallel to points “a” and “b”, located along the axis of current flow Iн, the primary winding 3 of the ITT 2 is connected, so the current lead becomes the equivalent of a shunt, weakening (up to hundreds of times) the current Iп of the primary winding.

In this case, the current Ip, passing through the inductive reactance of the primary winding, is shifted in phase relative to the current In by an amount reaching several tens of degrees, and induces a voltage Uin in the measuring winding 4, which is amplified by a preamplifier (PU) 6, the output voltage of which Upu is supplied to the input of the phase shifter 7, providing compensation for the phase shift of the voltage of the primary winding 3 ITT 2.

Then, the voltage UпУ is supplied to the direct input of the operational amplifier (ОУ) 15, the output and inverse input of which is connected to the feedback winding (OS) 5. Such switching on the ОУ 15 allows it to simultaneously perform three very important functions, namely:

- voltage-current conversion;

- stabilization of the parameters and characteristics of ITT 2;

- effective negative feedback.

OS 15 is turned on according to the voltage-current converter (PNT) circuit, the current load of which is the OS 5 winding, the current of which depends only on the variation of two values, namely the voltage Uпу and the value of the current-setting resistor 14 and practically does not depend on the variation in the instability of the characteristics of the OS winding itself 5.

Due to this unique property, the magnetic field created by the OS 5 winding, acting on the magnetic field created by the primary winding 3, creates a resulting magnetic field that linearizes and stabilizes the parameters and characteristics of ITT 2 and suppresses (not even taken into account) the conversion errors.

It is known that the stability of the parameters and characteristics of any device increases with increasing depth of the negative OS (OOS) surrounding it, therefore ITT 2, PU 7 and the winding of OS 5 are turned on in such a way that they form a loop of deep OOS, in which the OS acts as an effective regulator of the tracking OOS , in other words, serves as an additional stabilizer of the parameters and characteristics of ITT 2.

Further, the work of ETT is as follows. The voltage Uf from the output of the phase shifter 7 is supplied to the signal input of an adjustable amplifier (RU) 8, the output voltage Upy of which is supplied to the input of a power amplifier 9, the output of which through a current-limiting resistor 10 is supplied to the primary winding 12 of the output matching transformer (BST) 11, the secondary winding 13 which is the current output of the ETT.

A significant reduction in errors and an increase in the stability of the characteristics and parameters of the TSA 11 can be realized only by including it in the loop of the deep OOS, and for this it is necessary to assess the level of error of the TSA 11.

This assessment is carried out by the differential-zero method, for this the voltage Uout (point "c") of the primary winding of VST 11 is supplied to one of the inputs of the comparison circuit (CC) 18, and its other input receives the reference (model) voltage Ue formed by the reference amplifier (EU) 16 at its load with a measuring resistor 17 (point d).

The output voltage of SS 18, carrying information about the deviation of the voltage Uout from the voltage of Ue, is fed to the control input of RU 8 and, thereby, combines the deep OOS of RU 8, UM 9, VST 11 and SS 18 into a loop.

The action of this OOS loop linearizes and increases the stability of the characteristics and parameters of the TSB 11. Thus, both transformers (ITT 2 and TSB 11) are covered by a deep OOS.

In all current measuring devices, the current and angular (phase) errors, which are part of the ITT and other transformers, have maximum values at the initial section of the measured currents.

It is in this area that the rigid OOS introduced into it effectively works in the ETT, due to which the current and angular (phase) errors are reduced by 3–5 times, and the temperature and long-term stability of the ETT parameters are improved in the whole range due to the action of the OOS.

These affordable and inexpensive measures allow you to achieve your goal and turn the ETT into a small-sized precision instrument for measuring a wide range of currents with high accuracy (minimizing current and angular errors) and the stability of the measurement results.

Literature

1. GOST R IEC 60044-8-2010. Electronic current transformers (part 8).

2. Leytman M.B. Normalizing measuring transducers of electrical signals. M .: Energoatomizdat, 1986.

3. Rozhnov E.I. RF patent No. 2176088. Measuring transducer.

4. Rozhnov E.I. RF patent No. 2224262. Measuring current transducer.

5 Rozhnov E.I. RF patent No. 2224266. Meter of metrological characteristics of current transformers.

6. Rozhnov E.I. RF patent No. 2300774. Measuring transducer.

Claims (1)

An electronic current transformer containing a measuring current transformer, the primary winding of which is connected to a portion of the measured current circuit along its axis, and the measuring winding is connected to the differential inputs of the pre-amplifier, the output of which is through a serial circuit consisting of a phase shifter, an adjustable amplifier, a power amplifier, a limit amplifier the resistor and the primary winding of the output transformer, connected to the earth bus, while the secondary winding of the output transformer is the current output of the device, characterized in that the output of the pre-amplifier is connected to the direct input of the operational amplifier, the inverting input of which is connected to one of the ends of the feedback winding of the measuring current transformer and through the current-setting resistor to the ground bus, the second end of this winding is connected to the output of the operational amplifier In addition, the output of the adjustable amplifier is connected to the input of the reference amplifier, the output of which is connected through the measuring resistor to the ground bus and to one of the input in the comparison circuit, the second input of which is connected to the connection point of the limiting resistor and the primary winding of the output matching transformer, while the output of the comparison circuit is connected to the control input of the adjustable amplifier.