JPH01274070A - Optical zero-phase current transformer - Google Patents

Abstract

PURPOSE:To improve the detecting sensitivity of a zero-phase current to a large extent, by shielding the outer surface part of a magnetic core with a magnetic shield member which is formed by laminating high-permeability magnetic materials having the different operating magnetic fields. CONSTITUTION:Magnetic fields are generated by three positive-phase currents through three-phase conductors which are provided through the inside of a zero-phase current transformer 1. The magnetic fields are attenuated through magnetic shielding layers gradually from the outside. The magnetic fields reaching a magnetic core 5 become very small. Meanwhile, when the zero-phase current flows through any one of the three-phase conductors due to the occurrence of a grounding fault, magnetic flux is induced in the magnetic core 5. A coil 6 detects this fact and sends a signal to an optical voltage sensor 4. The optical voltage sensor 4 detects the signal highly sensitively without induction and outputs an optical signal.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to an optical zero-phase current transformer.

(Prior Art) In order to detect a ground fault in a three-phase AC circuit, a zero-phase current transformer extracts a zero-phase current that does not appear in an equilibrium state but flows to the ground only in the event of a ground fault. When a ground fault occurs on a power transmission/distribution line, a zero-sequence current transformer detects the zero-sequence current flowing toward the ground and interrupts the circuit to prevent the fault from spreading. For example, in the 6.6 (KV) distribution system of a power company shown in Figure 7, if a ground fault occurs in the T phase of feeder #3 among feeders #1 to #3, A zero-sequence current of several amperes flows through the phase current transformer ZCT when a ground fault occurs! . Detects this and activates the ground fault directional relay DC to interrupt the circuit. The symbol GPT is a grounding transformer. In the case of electric power companies' overhead line distribution systems, the length of each feeder is approximately 10kl, so prompt detection of the ground fault point is essential for quick accident recovery. The basic idea is to install a zero-phase current transformer in the middle of the feeder for power distribution system operation.

Normally, a racetrack zero-phase current transformer having a racetrack magnetic core 5 whose basic structure is shown in FIG. 8 is used as the zero-phase current transformer, and is incorporated into a switch. The three-phase conductors R, S, and T of the power distribution system are inserted through the magnetic core 5 at predetermined positions. Note that high-voltage consumers such as factories also have power distribution systems with functions similar to those shown in Figure 7.
In this case as well, if the substation in FIG. 7 is replaced with substation equipment and the area outside the substation is replaced with a factory, the same idea can be obtained.

(Problems to be Solved by the Invention) However, the conventional racetrack type zero-sequence current transformer has a problem that the detectable zero-sequence current is about 200 (mA), and the detection sensitivity is poor.

The racetrack type zero-phase current transformer, as shown in Figure 8,
The winding 6 is wound approximately evenly around the magnetic core 5, and the three-phase conductor R
, S, and T generate magnetic flux in the magnetic core 5, and this magnetic flux generates an induced electromotive force in the winding that is proportional to the time differential of the magnetic flux density. This induced electromotive force is caused by the three-phase conductors of the three conductors R1S and T canceling each other over the entire circumference of the winding, so that the induced electromotive force at the output terminal becomes zero. However, in reality, due to variations in the magnetic permeability μ of the magnetic core 5 of the zero-phase current transformer and/or variations in the installation positions of the three-phase conductors R, S, and T, as shown in FIG. Since there is non-uniformity in the magnetic flux density generated in the magnetic flux density, and since the winding density of the winding 6 is non-uniform in each part of the magnetic core 5 as shown in FIG. Even with perfectly symmetrical three-phase alternating current, the output terminal voltage of a zero-phase current transformer usually does not become zero. For this reason, the conventional racetrack type zero-sequence current transformer cannot increase the detection sensitivity of zero-sequence current.

The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an optical zero-sequence current transformer with significantly improved zero-sequence current detection sensitivity.

(Means for Solving the Problems) In order to achieve the above object, according to the present invention, the periphery of a magnetic core is shielded with a magnetic shield member formed by laminating high permeability magnetic materials having different working magnetic fields. , there is provided an optical zero-phase current transformer comprising a coil wound uniformly around the magnetic core and an optical voltage sensor connected to the coil.

(Function) Since the magnetic core is provided with a magnetic shield made of laminated magnetic materials with high magnetic permeability and different operating magnetic fields, the output voltage of the zero-phase current transformer is determined based on the magnetic flux induced in the magnetic core by the three-phase positive-sequence current. By eliminating fluctuations and detecting the zero-sequence current output with a non-inductive optical voltage sensor, detection sensitivity is greatly improved.

(Example) Hereinafter, an example of the present invention will be described in detail based on the accompanying drawings.

1 is a plan view and a side view showing one embodiment of the optical zero-phase current transformer of the present invention, FIG. 2 is a partial sectional view of the optical zero-phase current transformer of FIG. 1, and FIG. The figure is a sectional view taken along the line ■--■ in FIG. 2. The racetrack type optical zero-phase current transformer 1 of the present invention includes:
The magnetic core 5 is magnetically shielded in order to eliminate the induction of magnetic flux to the magnetic core 5 by the three-phase positive current and improve detection sensitivity. That is, when a phase current of 300 to 600 (A) affects the magnetic core 5, the detectable zero-sequence current does not fall below about 200 (mA), and detection sensitivity cannot be improved. Therefore, in the present invention, as will be described later, in order to complete the magnetic shield, the magnetic shield surrounding the magnetic core 5 and the coil 6 is constructed by laminating magnetic materials having various magnetic permeabilities. , the magnetic shield is the first one, as shown in FIG.
The three-layer structure includes a non-oriented silicon steel plate 7 as a layer, a grain-oriented silicon steel plate 8 as the second layer, and a permalloy 9 as the third layer. Furthermore, the coil 6 evenly wound around the magnetic core 5 is
It is connected to an optical voltage sensor 4 that detects the induced electromotive force generated in the magnetic core 5 by the zero-sequence current. A pair of optical fibers 3 are connected to the optical voltage sensor 4 . Reference numeral 2 is a mounting screw hole.

According to the inventor's analysis, the magnetic shield has a thickness of 1
The shielding effect of the shield plate (fi) is approximately equal to
It is expressed as 0.006μ. The shielding effect of a shield plate with a thickness of 15 (ms) is 0.075μ.

When a non-oriented silicon steel plate is used as a shield material in an external magnetic field of about 300^/ where a normal zero-phase current transformer is placed, its magnetic permeability μ is 2 from the H-μ characteristic shown in Figure 4.
．． It will be about 000 to 8,000. Magnetic permeability μ is the middle 4
, 000, if a non-oriented silicon steel plate with a thickness of 15 (m) is used, its shielding effect will be < K as described above.
=0.075 x 4000-300. Therefore,
Even if a non-oriented silicon steel plate with a thickness of 15 (ms) is used, the magnetic field will attenuate only to about -1 (^/II+) of 300 (A/m). For this reason, the cross section 10 (mm)
As a shielding material for magnetically shielding a magnetic core of
Even if the thickness is 15 (ms), a single non-oriented silicon steel plate is insufficient. Therefore, in the present invention, in order to perform complete magnetic shielding by using a magnetic material with an appropriate magnetic permeability μ for an appropriate magnetic field (magnetizing force) H, the magnetic shield is made of a high permeability magnetic material with a different operating magnetic field. It shall be constructed by laminating materials.

Now, assume that a magnetic shield is constructed by laminating three layers of high magnetic permeability magnetic materials, the first layer is a non-oriented silicon steel plate (average magnetic permeability μm 3,000, = 18), the second layer is The layer is made of grain-oriented silicon layer steel plate (average magnetic permeability μm 30゜000,
The third layer is made of permalloy (average magnetic permeability μ-io, ooo, K-60) 9.

As mentioned above, if the external magnetic field is 300 (A/m),
The residual magnetic field that is attenuated by the first layer of the magnetic shield and reaches the second layer is 300 + 18 = 16.7 (A/m). This residual magnetic field of 16.7 (A/m) is the maximum magnetic permeability μ as seen from the H-μ characteristics of the grain-oriented silicon steel sheet shown in Figure 5.
The residual magnetic field that is attenuated by the second N and reaches the third layer is 16.7÷180 = 92.6
(mA/m). This residual magnetic field is too small to be magnetically shielded by a silicon steel plate, so permalloy with even higher magnetic permeability is used for the third layer. Permalloy has its H-μ
From the characteristics, a magnetic permeability μ of about 105 can be ensured for a magnetizing force of 92.6 (mA/m). Therefore, the residual magnetic field inside the third layer is 92.6 (mA/m) ÷ 60 = 1.6
(mA/m). This residual magnetic field is sufficiently attenuated to detect a magnetic field of about 10 (mA/m) due to the zero-sequence current. Incidentally, the magnetic material for the magnetic shield is not limited to the above-mentioned material, nor is it limited to three layers, but other numbers of layers can be used.

On the other hand, if the zero-sequence current detection sensitivity is 10 (mA/m), the above-mentioned high permeability permalloy is suitable as the material for the magnetic core 5 of the zero-sequence current transformer. Since the magnetic flux density generated in the magnetic core 5 by a magnetic field of 10 (mA/m) is about 10 (C), the induced electromotive force (2) of a coil with a winding number of 10" (turns) is as follows.

V= (B-3-n) t =jω・10bow・(10-”) ”・104′= j
π10-' = 0.314 (V) Here, B: Magnetic flux density (Tesla) generated by zero-sequence current in the magnetic core, S: Core area (10aaX 10 layers m), n: Number of turns of the winding (turns) .

This small induced electromotive force can be generated from 300A to 3-phase AC.
A non-inductive optical voltage sensor 4 is required in order to perform detection without being affected by the inductive effect of the positive-sequence current of 600 (A). The optical voltage sensor 4 has a high detection impedance (element impedance), and since substantially no current flows through the winding, the winding may be made as thin as possible.

Since the zero-sequence current that flows during a ground fault is determined by the ground-fault point resistance Rg shown in Figure 6, if the zero-sequence current detection sensitivity is improved, the ground fault point resistance R, which could not be detected until now, can be reduced.
Accident points with high g can be detected.

Next, the operation of the optical zero-phase current transformer of the present invention will be explained.

The magnetic field generated by the three-phase positive-sequence current flowing through the three conductors penetrating the inside of the zero-phase current transformer 1 is sequentially attenuated by the magnetic shield layer from the outside, and the magnetic field reaching the magnetic core 5 is made extremely small. . On the other hand, when a zero-sequence current flows through any one of the three layer conductors due to the occurrence of a ground fault, magnetic flux is induced in the magnetic core 5 by this snow layer current. The coil 6 detects this and sends it to the optical voltage sensor 4, which detects it non-inductively and with high sensitivity and outputs it as an optical signal.

(Effects of the Invention) As explained above, according to the present invention, the periphery of a magnetic core is shielded with a magnetic shield member formed by laminating high permeability magnetic materials having different working magnetic fields, and a coil is attached to the magnetic core. By winding the coil uniformly and connecting a photovoltage sensor to the coil, it becomes possible to detect a zero-sequence current on the order of 10 (mA), which was previously impossible, and as a result, the detection sensitivity of the zero-phase current transformer increases. This results in a significant improvement.

[Brief explanation of the drawing]

1 is a plan view and a side view showing an embodiment of the optical zero-phase current transformer of the present invention, FIG. 2 is a partial sectional view of the optical zero-phase current transformer of FIG. 1, and FIG. 4 is a diagram showing the H-μ characteristics of a non-oriented silicon steel sheet, FIG. 5 is a diagram showing the H-μ characteristics of a grain-oriented silicon steel sheet, and FIG. Figure 7 is an electric circuit diagram showing a conventional high-voltage distribution system; Figure 8 is a diagram showing the basic configuration of a racetrack type zero-phase current transformer; Figure 9 The figure shows the non-uniformity of the magnetic flux generated in the zero-phase current transformer of FIG. 8, and the figure shows the non-uniformity of the coil winding of the zero-phase current transformer. DESCRIPTION OF SYMBOLS 1... Race track type zero-phase current transformer, 2... Screw hole, 3... Optical fiber, 4... Optical voltage sensor, 5...
- Magnetic core, 6... Coil, 7... Non-oriented silicon steel plate layer, 8... Grain-oriented silicon steel plate layer, 9... Permalloy layer.

Claims (1)

[Claims] The magnetic core is surrounded by a magnetic shielding member made of laminated high permeability magnetic materials with different working magnetic fields,
An optical zero-phase current transformer characterized in that a coil is uniformly wound around the magnetic core, and an optical voltage sensor is connected to the coil.