RU2281522C1 - Device for measuring short-circuit impedance of grounded-neutral three-phase transformer windings - Google Patents

Abstract

FIELD: measurement technology.

SUBSTANCE: proposed device that can be used for computer-aided measurement of short-circuit impedance in star-connected windings of grounded-neutral power transformer has three-channel common-input variable ac voltage supply and three-phase impedance meter incorporating three common-output voltage channels and three current channels. Three-phase impedance meter has analog-digital unit, nonvolatile memory unit, computing unit, record triggering unit, liquid-crystal display, and computer communication port.

EFFECT: enhanced measurement accuracy.

4 cl, 2 dwg

Description

Technical field

The invention relates to electrical measurements, in particular to measurements of the impedance of a short circuit (Z _k ) of a winding of a three-phase power transformer connected to a star and brought to the housing by neutral in order to identify possible deformations of the windings caused by through short circuits. For this, the measured value (Z _k ) is compared with the initial - basic value of this parameter, determined on a working transformer.

The invention can be used in the energy sector for testing power transformers during commissioning and maintenance work, as well as for their comprehensive examination.

In addition, the measurement of the impedance of a short circuit can be used after exposure to a short circuit of the transformer windings.

It can be successfully used for measuring the short-circuit impedance of power transformer windings at the factory and in repair organizations.

An advantageous area of application of the device is the measurement of the short circuit impedance of high and medium voltage windings of power transformers 110 kV and higher.

State of the art

A device for measuring the short circuit resistance of the windings of power transformers, containing a source of adjustable alternating voltage and a single-phase meter of total resistance of short circuit, the input of which is connected to an AC voltage source, and the output from one of the phases of the measured winding of the power transformer using connecting wires [Methodical allowances for monitoring the status of electrical equipment. Section 2. Methods for monitoring the status of power transformers, autotransformers, shunt and arc suppression reactors. Moscow, ORGRES, 1997 S. 100]. A voltmeter and an ammeter are used as a single-phase short-circuit impedance meter in this device; moreover, to find the impedance, the voltmeter reading is divided by the ammeter reading.

However, the measurement of the short-circuit impedance of the transformer windings using this device is laborious, considerable time is spent, since the short-circuit impedance of each phase is measured separately at 3 positions of the regulator under load (on-load tap-changer): at nominal and in two extreme positions. The device does not automatically detect Z _k and bring it to a frequency of 50 Hz, and it is also not possible to record the results of measurements on paper with it. The accuracy of determination of Z _k in this device is low, since all components of the harmonic current and voltage are taken into account here.

Another disadvantage of the device is the inability to transfer measurement data to a personal computer in order to use the multifunctionality of the latter.

SUMMARY OF THE INVENTION

The objective of the invention is the creation of a device that allows to simplify and automate the measurement mode, as well as to improve the accuracy of determining the short circuit impedance of each phase of the windings of a three-phase power transformer connected to a star with a neutral output to the housing, which is able to quickly save electronically the measurement data in non-volatile memory and subsequent data transfer to a personal computer to take advantage of the numerous capabilities of the latter, in particular te for creating a database and archiving. In addition, the proposed device improves the safety of measurements.

The problem is solved due to the fact that two four-wire wires are additionally introduced into the device containing a variable-voltage alternating voltage source and a single-phase impedance meter, the input of which is connected to a variable-voltage alternating voltage source, and the output - from one of the phases of the measured winding of the power transformer using connecting wires cable (voltage and current), which have terminals with color-coded phases and neutrals on their terminals. The source of adjustable AC voltage is made of a three-channel, with combined poles, common clamp, so that at all three phases of the transformer winding, AC voltage can be applied simultaneously.

The impedance meter is additionally equipped with:

- voltage sensors with galvanic isolation and protective circuits against switching overvoltages;

- current sensors with galvanic isolation in three current channels; analog-to-digital converter;

- a computing unit that allows you to calculate the effective current and voltage of the first harmonic, calculate the network frequency and bring the impedance to a frequency of 50 Hz;

- non-volatile memory;

- liquid crystal display;

- port interface with a personal computer.

In addition, the impedance meter has three identical voltage channels and a common terminal forming four input terminals, which are connected by the first four-wire cable (voltage), respectively, to the input phase terminals of the measured winding of the power transformer and its neutral. The impedance meter also has three current channels with six input terminals, of which the top three are connected by the three wires of the second four-wire cable (current) to the terminals of the three phases of the power transformer winding, and the three lower terminals are connected to the connecting wires, respectively, with the terminals of the voltage source channels of the regulated source AC voltage, and the general output of the source of adjustable AC voltage is connected to the neutral of the transformer through the fourth wire of the second four-wire cable (current).

This ensures, firstly, the simultaneous measurement of six values: three currents of the winding phases and three voltages at the phase resistances, and secondly, the special connection of four-wire cables to a three-phase measurement object, a multi-channel power supply and a ten-clamp impedance meter provides sufficient accuracy at the same time measuring three currents and three voltages. In the computing unit of the impedance meter, the first harmonics of currents and voltages are filtered. Thanks to this connection and filtering scheme, the determination of the total resistance of all three phases of the transformer winding is achieved with sufficiently high accuracy. At the same time, the formation of the values of the total phase resistances reduced to a frequency of 50 Hz is performed.

The three-channel source of controlled alternating voltage additionally includes protective circuits that exclude overvoltage of voltage channels.

A brief description of the figures.

Figure 1 shows the structural electrical diagram of the proposed device, combined with a circuit of electrical connections. The device contains a three-channel source of controlled alternating voltage with a common terminal 1, a three-phase impedance meter 2, having three voltage channels with a common terminal and three current channels, a four-wire voltage cable 3, a four-wire current cable 4, an investigated transformer 5.

Figure 2 shows the structural diagram of a three-channel impedance meter 2, including current sensors 2.1, voltage sensors 2.2, analog-to-digital converter 2.3, non-volatile memory unit 2.4, computing unit 2.5, digital recording start unit 2.6, liquid crystal display 2.7, communication port with computer 2.8.

Disclosure of invention

The device is intended for digital measurement of the impedance of a short circuit of three phases of the windings of a power transformer connected to a star with a neutral connected to the housing. It allows you to simplify the assembly process of the measurement circuit, to automatically measure with high accuracy the short circuit impedance of each of the three phases of the transformer windings. The proposed device provides increased safety of work, and also allows you to transfer the measured values to a personal computer for archiving and creating a database.

The device, if the power three-phase transformer has a regulator under load, for example, a transformer TDN-16000-110 / 10, having an on-load tap-changer type RS-4, works as follows. The operator translates the on-load tap-changer to the “1” position. After turning on the three-channel AC voltage source 1 in the power supply network (see Fig. 1), current flows through the windings of the three-phase transformer 5, the current channels of the impedance meter 2, and the four-wire current cable 4. The voltage from the terminals of the transformer is removed using a four-wire voltage cable 3 and is supplied to the terminals of the voltage channels of the impedance meter 2.

Three-phase impedance meter 2 (see figure 2) simultaneously measures the currents and voltages of each phase with current sensors 2.1 and voltage 2.2, converts analog values to digital using the block of analog-to-digital conversion 2.3. Next, the computing unit 2.5 calculates the total resistance of the winding of the three phases by dividing the voltage by the current of the corresponding phases and outputs the obtained values to the non-volatile memory 2.4 and the LCD 2.7. Such a cycle "measurement-calculation-visualization" occurs with an interval of 1 second. Next, the operator using the digital recording start block 2.6 sends a command to write the received data to the protocol, which is recorded in the non-volatile memory block 2.4. After that, the operator with the help of the drive puts the on-load tap-changer to the position of the nominal voltage value, in our case, to the "10" position, and makes the following protocol record in the non-volatile memory block. Then, the on-load tap changer is transferred to the last stage (in our case, to position “19”) and the measured values of current, voltage and calculated values of Z _k for the first harmonic are recorded.

After measuring the short circuit impedance for the three on-load tap-changer positions, data from a non-volatile memory unit via communication port 2.8 can be transferred to a personal computer to take advantage of the multifunctional capabilities of the latter.

When a three-channel AC voltage source is disconnected from the mains supply, its built-in protective circuits do not interrupt the phases and ensure a smooth decrease in the current of the transformer winding, thereby eliminating switching overvoltages caused by disconnection of circuits with a large inductance of the transformer winding, and protect the three-channel AC voltage source and channels voltage of the impedance meter from damage.

In case of accidental current interruption in the phase of the transformer winding (for example, the clamp of the four-wire current cable was disconnected from the hardware clamp of the transformer input due to a gust, or the operator mistakenly began to disassemble the current circuits before turning off the AC voltage source and reducing the current to zero) the voltage circuit of the voltage impedance meter eliminate overvoltage on their channels, thereby preserving them from damage in emergency situations.

The common connection of four-wire voltage and current cables to the hardware terminals of the inputs of the power three-phase transformer allows to increase the measurement accuracy, since this eliminates the influence on the measurement of the total phase resistances of the transformer windings of the resistance of the wires of the four-wire current cable. The value of the input resistance of the voltage channels of the impedance meter is selected so that it exceeds hundreds of thousands of times the resistance of the connecting wires of the four-wire voltage cable and the impedance of the phases of the transformer winding. As a result, the resistance of the four-wire voltage cable and the input resistance of the voltage channels of the impedance meter practically do not affect the measurement accuracy. This connection allows the use of voltage and current cables with internal wires of any cross section, without compromising the accuracy of the measurement.

The cable connection is complemented by color coding, which simplifies the process of assembling the circuit and connecting the device to the measurement object in the field at the substation.

In addition, the connection using a long cable allows you to place a workplace (where a three-channel impedance meter and a three-channel source of adjustable AC voltage are located) for making measurements at ground level in the immediate vicinity of the on-load tap-changer, which eliminates work at height, resulting in increased work safety.

Execution example

To measure the short-circuit impedance of a power three-phase transformer of the TDN-16000/110 type with a regulator under a load of the RS-4 type, two 15 × 5 PVA cables are used. The impedance meter includes voltage sensors with an input impedance of 500 kOhm, a high-speed microprocessor, non-volatile memory on an ATmel AT45DB161 chip, a PowerTyp type PG320240 graphic liquid crystal display, and an RS-232 serial port.

The AC voltage source provides regulation of the output voltage in the range of 5 ÷ 380 V.

Claims (4)

1. A device for measuring the short-circuit impedance of the windings of a three-phase power transformer and a common point of connection connected to the housing - a neutral, containing a frequency meter, a variable voltage source and a single-phase voltage meter, the input of which is connected to a variable voltage source, and the output is from from the phases of the measured winding of the power transformer using connecting wires, characterized in that the device circuit is additionally introduced two four-wire cables, the impedance meter has three identical voltage channels and a common terminal forming four input terminals, which are connected by the first four-wire cable (voltage) to the phase terminals of the measured winding of the power transformer and its neutral, respectively, in addition, three current channels with six input clamps, of which the top three are connected by the three wires of the second four-wire cable (current) with the clamps of the three phases of the power transformer winding, and the bottom three clamps by the connector bubbled wires - respectively to the output terminals a controlled AC voltage source voltage channels, the total output power controlled variable voltage connected to the neutral of the transformer via the fourth wire of the second four-wire cable (DC). 2. The device according to claim 1, characterized in that the impedance meter is additionally equipped with voltage sensors with galvanic isolation, protective circuits against switching overvoltages, current sensors with galvanic isolation in three current channels, an analog-to-digital converter, a computing unit that allows to calculate the effective the value of the current and voltage of the first harmonic, the network frequency and bring the impedance of the short circuit to a frequency of 50 Hz, non-volatile memory, a liquid crystal display and a port for interfacing with a personal computer. 3. The device according to claim 1, characterized in that protective circuits that exclude overvoltage of voltage channels are additionally included in the three-channel AC voltage source. 4. The device according to claim 1, characterized in that the four-wire cables (voltage and current) at their terminals have terminals with color-coded phases and neutrals.

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