JPS63274322A - Zero-phase current transformer - Google Patents

Abstract

PURPOSE:To correct the difference of the temperature characteristics of each optical current transformer by levelling zero-phase current components at every specified period and outputting a waveform subtracted from the zero-phase current component after a mean processing period ends. CONSTITUTION:A correction arithmetic circuit 11 is composed of an apparatus such as a microprocessor. One period of an output signal from an adding circuit 10 consisting of a sine waveform is divided, and sampling is conducted. Consequently, sampling signals reproducing sine waves at every one period are acquired. Signals obtained through sampling during a specified period are levelled at every dividing component, thus preparing a mean waveform composed of the sine waveform. The mean waveform acquired in this manner is memorized for the next processing period, and the previously memorized mean waveform is subtracted from the waveform of the output signal from the adding circuit 10 after the next mean processing period ends. Accordingly, zero-phase current components gradually changing by an ambient temperature are corrected to a sufficiently small value.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a zero-phase current transformer that is installed in a high-voltage three-phase distribution line to detect zero-sequence current that flows when a ground fault occurs. The present invention also relates to a safe zero-phase current transformer.

[Prior Art] FIG. 5 is a configuration diagram showing a conventional zero-phase current transformer, which is described, for example, on page 79 of "Instrument Transformer" (Denki Shoin, authored by Mihoji Ikeda).

In the figure, (1a) to (1c) are three-phase distribution lines, (2)
is one annular core provided to surround the three-phase distribution lines (1a) to (1c), (3) is a secondary winding wound around a part of the annular core (2), and (4) is a This is a secondary load such as a ground fault detection relay connected between both ends of the secondary winding (3).

Next, the operation of the conventional zero-phase current transformer shown in FIG. 5 will be explained.

During steady state, the current flowing through each phase of the three-phase distribution lines (Ia) to (fc) is only the positive phase portion and the negative phase portion.

At this time, the annular iron core (2) is induced by a magnetic field that is a composite of the magnetic fields proportional to the positive-phase components of each phase and the negative-phase bone currents, so the magnetic flux in the annular iron core (2) is zero. Therefore, no current flows through the secondary winding (3) and the secondary load (4) does not operate.

On the other hand, when a ground fault occurs in any of the three-phase distribution lines (1a) to (IC), the currents for each phase do not become zero even if they are combined, and a zero-phase current flows. Therefore, magnetic flux is induced in the annular core (2) and zero in the secondary winding (3). A secondary current proportional to the current flows, and the secondary load (4) operates.

Generally, this type of zero-phase current transformer is installed within the premises of a substation, and if a ground fault occurs, the secondary burden (4) will disconnect the three-phase distribution lines (LA) to (LC) and prevent the accident. prevents the expansion of However, even if a ground fault occurs,
In order to limit the power outage section and shorten the restoration time as much as possible, a means to quickly and accurately discover the point of failure is required. For this reason, zero-phase current transformers are installed not only inside substations, but also in the middle of three-phase distribution lines (1a) to (1c) (outdoor distribution poles).
It has become necessary to install a large number of them.

[Problems to be solved by the invention] As described above, the conventional zero-phase current transformer is connected to the three-phase distribution line (1a)
Since the zero-sequence current of the three-phase distribution lines (1a) to (1c) due to a ground fault is detected by using the ring core (2) that encloses ~ (1c) all at once, it is installed on an outdoor distribution pole. In addition, there were problems in that it became large and accidents were amplified due to lightning strikes, etc.

In addition, in order to solve these problems, the current of each phase is detected individually using a lightweight optical current transformer with high insulation reliability.
It has also been considered to synthesize and detect zero-sequence currents, but
Due to differences in temperature characteristics of each optical current transformer, secondary burden (4
) had the problem of malfunctioning.

This invention was made in order to solve the above-mentioned problems, and by combining zero-sequence currents using an optical current transformer, it is possible to create a zero-sequence current that is lightweight and does not cause the spread or trigger of an accident. The purpose is to obtain a current transformer.

[Means for Solving the Problems] The zero-phase current transformer according to the present invention calculates an average waveform obtained by averaging zero-sequence current components for each predetermined period, and calculates the average waveform of zero-sequence current components after the next averaging period ends. A correction calculation circuit is provided to output a waveform obtained by subtracting the average waveform from the phase current component.

[Function] In this invention, the zero-sequence current component due to the temperature characteristic difference of each optical current transformer is corrected to zero, and the zero-sequence current component due to a ground fault is output as is, thereby reducing the secondary burden. prevent malfunction.

[Example 1] An example of the present invention will be described below with reference to the drawings. 1st
The figure is a block diagram showing one embodiment of the present invention.
a) to (IC) are the same as described above.

(2&) to (2C) are three annular cores having voids, and three-phase distribution lines (1a) to (IC) are arranged so as to individually pass through the center of each annular core. (5a) - (5
C) is an optical magnetic field sensor using the Faraday effect arranged in the air gap of each annular core (2a) to (2c), (6a) to (
6C) is a light emitting element that supplies light to each magneto-optical field sensor (5a) to (5c), and (7a) to (7c) is a light emitting element that supplies light to each magneto-optical field sensor (5a) to (5c).
a light-receiving element that detects light from 5a) to (5C), (8a
) to (8C) connect the light emitting elements (6a) to (6C) and the light receiving elements (7a) to (7C) to each of the optical magnetic field sensors (5b) to (5).
It is an optical fiber for light guiding connected to C), and together with the annular cores (2a) to <2C), these constitute an optical current transformer.

(9a) to (9C) are signal processing circuits into which signals from each light receiving element (7a) to (7C) are individually input, and (10) is a signal processing circuit to which signals from each signal processing circuit (9a) to (9C) are input. The addition circuit (11) for addition is a correction calculation circuit for correcting the signal from the addition circuit (10), and these constitute a measurement circuit (12).

Although not shown, the output terminal of the measurement circuit (12) is
A secondary load (4) similar to that in FIG. 5 is connected.

Next, the operation of the embodiment of the present invention shown in FIG. 1 will be explained.

First, the annular iron cores (2a) to (2c) are connected to the three-phase distribution line (1
The currents flowing through a) to (IC) induce a magnetic field proportional to each current.

At this time, the light emitting elements (6a) to (6C) are connected to the optical magnetic field sensors (5a) to
The light reaching (5C) is modulated in proportion to the magnetic field in each gap of the annular iron cores (2a) to (2C), and the plane of polarization is rotated by the Faraday effect.

The light modulated by the optical magnetic field sensors (5a) to (5C) is detected by the light receiving elements (7a) to (7C) via the optical fibers (8a) to (8C), and converted into a signal proportional to the magnetic field. converted.

Furthermore, the signals electrically converted by the light receiving elements (7a) to (7C) are converted into current signals proportional to the current flowing through the three-phase distribution lines (1a) to (1c) by signal processing circuits (9a) to (9C). and are added by an adder circuit (10).

In a steady state, the sum of the currents of the three-phase distribution lines (1a) to (IC) becomes zero, so the output signal of the adder circuit (10) also becomes zero.

On the other hand, if a ground fault occurs in the three-phase distribution line (IC) to (1a), a zero-phase current will flow that does not become zero even when the currents are combined, so the output signal of the adder circuit (10) will be zero-phase. The value is proportional to the current, and ground faults in the three-phase distribution lines (1a) to (1c) can be detected.

However, in reality, the optical magnetic field sensors (5a) to (5c) and the signal processing circuits (9a) to (9c) have temperature characteristics, and it is normal that the temperature characteristics of each individual are different. . Therefore, the ratio of each current in the three-phase distribution lines (1a) to (IC) to each current signal from the signal processing circuits (9a) to (9c) varies between the phases due to changes in ambient temperature. Therefore, the output signal of the adder circuit (10) is
Even when zero-sequence current is not flowing in 1a) to (1c), it does not become zero, causing the secondary load (4) to malfunction.

The correction calculation circuit (11) is for preventing malfunctions as described above, and is composed of, for example, a microprocessor, and includes three optical magnetic field sensors (5a) to (5c) and signal processing circuits (9a) to (9c). ), only the zero-sequence current component caused by the difference in temperature characteristics between the two is always corrected to zero. In other words, the zero-sequence current component due to differences in temperature characteristics is caused by changes in the ambient temperature and therefore changes slowly, whereas the zero-sequence current component due to a ground fault occurs instantaneously at the same time as the fault occurs, so the change is rapid. Focusing on this, correction calculations are performed according to the flowchart shown in FIG.

First, the output signal of the adder circuit (10) having a sine waveform is divided and sampled, for example, every 30 degrees of one cycle (step si).

As a result, 12 signals reproducing a sine waveform are obtained every cycle. Also, the sampling period at this time is not limited to every 30° of the output waveform, but may be subdivided into 30" or less if the calculation processing speed can be increased, or conversely, if high precision is not required, the sampling period may be divided into 30" or more. In< may be used.

Next, the signals obtained by sampling during a predetermined period are averaged for each divided component to create an average waveform consisting of a sine waveform (step S2).

The predetermined period at this time is set to an appropriate time of 0.1 seconds or more in accordance with the time during which the secondary burden (4) operates (usually 0.1 to 0.5 seconds).

Next, the average waveform thus obtained is stored for the next processing period (step S3), and after the end of the next average processing period, the previously stored average waveform is subtracted from the waveform of the output signal of the adder circuit (10) (step S3). s4).

That is, the average waveform computed last time is subtracted from the output waveform of the adder circuit (1o) obtained this time. At this time, the current output waveform is also averaged at the same time, and the average waveform used for the next subtraction is stored. Therefore, an average waveform obtained by averaging the zero-sequence current components for each predetermined period is calculated, and a waveform obtained by subtracting the average waveform from the zero-sequence current component after the next averaging processing period ends is output.

In this way, by subtracting the previously calculated average waveform from the sampling waveform signal of the output signal of the adder circuit (10), the correction calculation circuit (11 ) is always corrected to zero or a value sufficiently lower than the operating level of secondary burden (4).

On the other hand, if the zero-sequence current changes suddenly due to a ground fault, the subtraction process is not performed during the average processing period, that is, at least the time during which the secondary burden (4) operates, so the zero-sequence current component is corrected as is. For example, if a ground fault occurs just before the end of the averaging processing period, and a zero-sequence current component occurs in the output signal of the adding circuit (10), then the next averaging processing period During the middle (predetermined period), the subtraction process is not performed, and the zero-sequence current component is output for a time sufficient for the secondary load (4) to operate. Therefore, it is possible to reliably detect a ground fault, operate the secondary load (4), and disconnect the three-phase distribution lines (1a) to (1c).

In the above embodiment, the correction calculation circuit (11) is replaced by an addition circuit (
10), but they may be integrated and configured using a microprocessor that simultaneously performs addition and correction operations.

In addition, the optical current transformer is made of annular cores (2a) to (2) having voids.
c) and optical magnetic field sensors (5a) to (5c) using the Faraday effect placed in each gap, but as shown in FIG.
Winding wires (13) each wound around a part of a) to (2c)
m) to (13c) and these windings (13a) to (13c
) are connected across the resistors (14a) to (14e
), and an optical voltage sensor (15a) connected in parallel to these resistors (14a) to (14c) and using the Pockels effect to measure the voltage across each resistor (14a) to (14c). ~(15c) may be formed into an ellipse. Here, the portions not shown have the same structure as in FIG. 1.

In this case, a secondary current proportional to the negative current of the three-phase distribution lines (1a) to (1c) is obtained by the windings (13m) to (13e), and these secondary currents are connected to the resistor (14a). ~(
14e) into voltage, and a photovoltage sensor (15m)
~(15c).

Furthermore, as shown in FIG. 4, optical magnetic field sensors (16a) to (16c) using the Faraday effect are arranged in an elliptical ring shape, and three-phase power distribution lines (1a) to (1c) are surrounded by the center of each ring. The optical current transformer may be constructed in such a manner that

In this case, the light incident through the optical fibers (8a) to (8c) circulates through the three-phase distribution lines (1a) to (1c), and then passes through the optical fibers (8a) to (8c) again to the light receiving element. (
7a) to (7c) and the measurement circuit (12).

Therefore, each of the optical magnetic field sensors (16a) to (16c) repeats reflection.

[Effects of the Invention] As described above, according to the present invention, the average waveform obtained by averaging the zero-sequence current components for each predetermined period is calculated, and the average waveform is calculated from the zero-sequence current component after the next averaging processing period ends. A correction calculation circuit is provided to output a waveform obtained by subtracting , and the zero-sequence current component due to differences in temperature characteristics of each optical current transformer is corrected to zero, and the zero-sequence current component due to a ground fault is output as is. As a result, it is possible to prevent malfunction due to the secondary load, and there is an effect that a lightweight, accurate, and safe zero-phase current transformer can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing one embodiment of this invention, FIG. 2 is a flowchart for explaining the operation of FIG. 1, and FIG. 3 is a configuration diagram showing the main parts of a second embodiment of this invention. , 4th
The figure is a configuration diagram showing the main part of the third embodiment of this invention,
FIG. 5 is a configuration diagram showing a conventional zero-phase current transformer. (1a) to (1c)... Three-phase distribution line (2a) to (2c)... Annular iron core (5a) to (5c)... Optical magnetic field sensor (6a) to (6
c)...Light emitting element (7a) to (7c)...Light receiving element (8a) to (8c)...Optical fiber (11)...Correction calculation circuit (12)...Measuring circuit (13a) )~(13c)-・Winding (14a)~(
14c) - Resistors (15a) to (15c)... Optical voltage sensors (16a) to (16c)... Optical magnetic field sensor In the drawings, the same reference numerals indicate the same or corresponding parts. Figure 1 Figure 2 Figure 3 Figure 4 To 12 Figure 5

Claims (5)

[Claims] (1) A zero-phase system equipped with three optical current transformers that individually measure the current in each phase of a three-phase distribution line, and a measurement circuit that combines the signals from these optical current transformers and detects the zero-phase current component. In the phase current transformer, the measurement circuit calculates an average waveform by averaging the zero-sequence current components for each predetermined period, and subtracts the average waveform from the zero-sequence current component after the next averaging period ends. A zero-phase current transformer including a correction calculation circuit for outputting a waveform. (2) The zero-phase current transformer according to claim 1, wherein the predetermined period is 0.1 seconds or more. (3) The optical current transformer includes annular cores each having a gap, each of which is individually provided so that the three-phase distribution line passes through the center, and an optical magnetic field sensor using the Faraday effect placed in each gap. , a light-emitting element, a light-receiving element, and an optical fiber for connecting the light-emitting element and the light-receiving element to each of the optical magnetic field sensors. Phase current transformer. (4) An optical current transformer consists of an annular core that is individually installed so that a three-phase distribution line passes through the center, windings wound around each of these annular cores, and a connection between each end of each of these windings. A connected resistor, a photovoltage sensor using the Pockels effect for measuring the voltage across each end of these resistors, a light emitting element and a light receiving element, and the light emitting element and the light receiving element being connected to each of the above photovoltage sensors. 3. The zero-phase current transformer according to claim 1, further comprising an optical fiber for connection to a zero-phase current transformer. (5) An optical current transformer consists of an annular optical magnetic field sensor using the Faraday effect, which is individually provided so that a three-phase distribution line passes through the center, a light emitting element and a light receiving element, and a light emitting element and a light receiving element. 3. The zero-phase current transformer according to claim 1, further comprising an optical fiber for connecting the optical magnetic field sensor to each of the optical magnetic field sensors.