JP2016197614A - Instrument transformer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the following problem: Use of a wire-wound instrument transformer in an AC system formed by superposing a low-frequency voltage makes it impossible to make a correct determination as insulation monitoring.SOLUTION: The instrument transformer is formed as follows: A main impedance Zs and a first partial-pressure impedance Ze are connected in series to each phase of an alternating current and a measurement terminal is derived from each connection position. The other end of the first partial-pressure impedance Ze is subjected to three-phase star connection. A second partial-pressure impedance Zo is connected between the connection position and a ground terminal. A measurement terminal is derived from each of a three-phase star connection position and a connection position of the ground terminal to the second partial-pressure impedance Zo.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an instrument transformer, and more particularly to an instrument transformer applied to an AC system in which a low-frequency voltage is superimposed.

  In a three-phase AC circuit, measure the relative ground voltage, line voltage, zero-phase voltage, etc. of the main circuit, detect system faults and abnormalities from the measured electricity, operate the protective relay, etc. For the purpose of protection, instrument transformers and grounded instrument transformers are used.

  For example, Patent Document 1, Patent Document 2 and the like are well known when a transformer for grounding type is taken as an example of a transformer. FIG. 8 shows a voltage measurement state by a grounded-type instrument transformer. One end of the primary winding of the ground-type instrument transformer EVT is connected to the A-phase, B-phase, and C-phase of the three-phase AC separately. The other end of the primary winding is grounded by a three-phase star connection. Va, Vb, and Vc are ground voltages of the respective phases.

Patent No. 3419168 JP 62-40015

  In recent years, a method of superimposing a low-frequency voltage on a three-phase AC circuit has been proposed for the purpose of monitoring system insulation in a three-phase AC high / low voltage system. In this method, the insulation state of the system is judged by measuring and processing the current with respect to the low-frequency voltage superimposed on the three-phase AC circuit.

  When a grounded instrument transformer EVT as shown in FIG. 8 is used as a measuring instrument in the low-frequency voltage superposition method, its primary winding is equivalent to connecting an inductance between the main circuit and the ground, Corresponding to the decrease, the impedance of each phase decreases. This phenomenon is correctly determined as insulation monitoring when a low-frequency voltage is superimposed on the system for insulation monitoring, when the drop in impedance of the grounded instrument transformer EVT with respect to the low-frequency voltage is comparable to that of the system. It may not be possible. Therefore, in a system using a winding device that corresponds to the inductance between the main circuit and the ground, such as a grounded instrument transformer EVT, there is a possibility that the insulation monitoring by the low frequency voltage superposition method cannot be performed correctly. Have.

  Conventionally, as a voltage divider, a voltage divider is configured by a single capacitor, a single resistor, or a combination of a capacitor and a resistor, and the voltage of each part of the main circuit is measured by this voltage divider. At that time, a zero-phase voltage divider is used to detect the zero-phase voltage, or a voltage divider that measures each phase voltage when measuring the main-phase voltage, and a voltage divider configured separately for each measurement purpose. It is used.

  An object of the present invention is to provide an instrument transformer used in a system to which a wound-type instrument transformer cannot be applied.

The present invention is an instrument transformer for measuring various amounts of electricity connected to a three-phase AC system,
The main impedance Zs and the first divided voltage impedance Ze are connected in series to each AC phase, the measurement terminals are derived from the respective series connection positions, and the other end of each first divided voltage impedance Ze is connected in a three-phase star. And connecting a second divided impedance Zo between the connection position and the ground terminal, and deriving measurement terminals from the three-phase star connection position and the connection position of the ground terminal and the second divided impedance Zo, respectively. It is a feature.

  In the present invention, the value of the main impedance Zs, the value of the first divided impedance Ze, and the value of the second divided impedance Zo are the main impedance Zs, the first divided impedance Ze, and the second divided impedance. It is characterized in that it is selected to be a sufficiently large value compared to Zo.

  In addition, the main impedance Zs, the first divided voltage impedance Ze, and the second divided voltage impedance Zo of the present invention are configured by any one of a capacitor, a resistor alone, or a combination of a capacitor and a resistor. Is.

  Further, the instrument transformer according to the present invention is connected to a three-phase AC system on which a low-frequency voltage is superimposed, and is configured to detect a measurement voltage corresponding to a ground voltage, a zero-phase voltage, and a line voltage of each phase. It is characterized by that.

  As described above, according to the present invention, in addition to the main impedance Zs and the first divided impedance Ze that are connected in series for each phase, the second divided voltage impedance Zo is also connected in series. The voltage divider can obtain a measurement voltage corresponding to the ground voltage, the zero phase voltage of each phase of the main circuit, and the line voltage of the main circuit.

  In addition, by selecting a type of impedance that does not affect insulation monitoring even at low frequencies, such as a single capacitor, a single resistor, or a combination of a capacitor and a resistor as each impedance Zs, Ze, and Zo, low frequency voltage superposition Even in the method, voltage measurement of each part can be performed correctly without adversely affecting the determination of insulation monitoring.

  In addition, according to the configuration of the present invention, since the capacitor and the resistor are generally small and light, it is possible to obtain an effect that a small and light voltage divider can be configured as a whole compared to the conventional winding type. Is.

The block diagram of the measuring instrument transformer of this invention. The block diagram (Example 1) of the measuring instrument transformer of this invention. The block diagram of the instrument transformer of this invention (Example 2). The block diagram of the instrument transformer of this invention (Example 3). The block diagram of the instrument transformer of this invention (Example 4). The block diagram of the instrument transformer of this invention (Example 5). The block diagram of the instrument transformer of the present invention (Example 6). The use state figure of the conventional earthing type transformer for instruments.

  FIG. 1 shows a configuration diagram of an instrument transformer according to the present invention. In FIG. 1, Zs is a main impedance, and is connected to the A phase, B phase, and C phase of the AC circuit. A first divided impedance Ze is connected in series to the main impedance Zs of each phase, and measurement terminals a, b, and c are derived from the connection points, respectively. Further, the other end of each first divided impedance Ze is connected in common (three-phase star), and the measurement terminal n is derived. A second divided impedance Zo is connected between the three-phase star-connected portion and the ground terminal, and a measurement terminal g is derived from the ground terminal side of the second divided impedance Zo. Va, Vb, and Vc are ground voltages of respective phases of the main circuit.

Here, the magnitude of the main impedance Zs is selected to be sufficiently larger than Ze and Zo. That is, by selecting each impedance so that Zs >> Ze, Zs >> Zo, it is possible to measure the ground voltage of each phase of the main circuit and measure the main circuit zero-phase voltage as follows.
(1) Measurement of the ground voltage of each phase of the main circuit When the ground voltage Va of the main circuit A phase is taken as an example, the ground voltage Va is divided by the respective impedances Zs, Ze, and Zo and output between the terminals an. Is done. The voltage is output in accordance with the following equation from the condition of Zs >> Ze, Zs >> Zo.
(Ze / (Zs + Ze + Zo)) Va≈ (Ze / Zs) Va
Similarly, the voltage divided by Ze / Zs is output between the terminals b and n, and the voltage Vc of the main circuit C phase is output between the terminals c and n, respectively. .
(2) Measurement of main circuit zero-phase voltage If the symmetrical voltage of each phase of the main circuit is Ea, Eb, Ec, and the zero-phase voltage included in each phase of the main circuit is V0, each relative ground voltage Va, Vb, Vc is expressed as follows.
Va = Ea + V0
Vb = Eb + V0
Vc = Ec + V0
Each of these voltages is divided by the respective impedances Zs, Ze, Zo and outputted between the terminals ng, and the voltage division ratio at this time is determined by measuring the ground voltage of each phase of the main circuit in (1). Similarly, Zo / Zs is obtained from the conditions of Zs >> Ze, Zs >> Zo, and voltages obtained by dividing the ground voltage of each phase are added in vector and output. That is, the output value is as follows.

Va (Zo / Zs) + Vb (Zo / Zs) + Vc (Zo / Zs)
= (Ea + V0) (Zo / Zs) + (Eb + V0) (Zo / Zs) +
(Ec + V0) (Zo / Zs)
= (Ea + Eb + Ec) (Zo / Zs) + 3V0 (Zo / Zs)
Here, since Ea, Eb, and Ec are symmetrical voltage divisions, the vector sum is zero, and the zero-phase voltage V0 of each phase of the main circuit is divided between terminals n and g at a voltage division ratio of (3Zo / Zs). Output voltage is output.

  As described above, when the results of (1) and (2) are summarized, voltages as shown in the following table are output between the measurement terminals.

  FIGS. 2 to 7 show examples of instrument transformers using capacitors C alone, resistors R alone, or combinations of capacitors C and resistors R as the respective impedances Zs, Ze, and Zo. Even in the combination, by selecting a kind of impedance that does not affect the insulation monitoring, the insulation monitoring at a low frequency can be performed correctly. The subscripts s, e, and o attached to the capacitor C and the resistor R in FIGS. 2 to 7 indicate main, first, and second, respectively, that are the same as those in FIG.

  The instrument transformer shown in FIGS. 2 to 7 may be used for a normal AC system other than the low-frequency voltage superposition method. In an AC system that does not superimpose the low-frequency voltage, an inductance is included as an impedance. It may be configured. Therefore, conventionally, when the voltage of each part of the main circuit is measured by a single capacitor, a single resistor, or a combination of a capacitor and a resistor, a zero-phase voltage divider and a voltage divider for each phase of the main circuit are configured separately. In the present invention, in addition to the main impedance Zs and the first divided voltage impedance Ze connected in series for each phase, the second impedance Zo is also connected in series by a simple configuration. With the pressure device, a measurement voltage corresponding to the ground voltage of each phase of the main circuit, the zero-phase voltage, and the line voltage of the main circuit can be obtained.

Rs ... Main impedance Re ... First divided voltage impedance Ro ... Second divided voltage impedance a, b, c, n, g ... Measurement terminals Va, Vb, Vc ... Ground voltage of each phase

Claims (4)

In an instrument transformer that is connected to a three-phase AC system and measures various types of electricity,
The main impedance Zs and the first divided voltage impedance Ze are connected in series to each AC phase, the measurement terminals are derived from the respective series connection positions, and the other end of each first divided voltage impedance Ze is connected in a three-phase star. And connecting a second divided impedance Zo between the connection position and the ground terminal, and deriving measurement terminals from the three-phase star connection position and the connection position of the ground terminal and the second divided impedance Zo, respectively. A characteristic instrument transformer. The value of the main impedance Zs, the value of the first voltage dividing impedance Ze, and the value of the second voltage dividing impedance Zo are such that the main impedance Zs is compared with the first voltage dividing impedance Ze and the second voltage dividing impedance Zo. 2. The instrument transformer according to claim 1, wherein a sufficiently large value is selected. 2. The main impedance Zs, the first divided impedance Ze, and the second divided impedance Zo are configured by any one of a capacitor, a resistor, and a combination of a capacitor and a resistor. 2. An instrument transformer according to 2. The instrument transformer is connected to a three-phase AC system on which a low-frequency voltage is superimposed, and is configured to detect a measurement voltage corresponding to the ground voltage, zero-phase voltage, and line voltage of each phase. The instrument transformer according to any one of claims 1 to 3, wherein the instrument transformer is characterized.

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