JP2007053302A - Transformer and its grounding method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress a ground fault current of an electric circuit on a low voltage side so as to develop a suitable protection cooperation, and also to prevent a malfunction in a ground fault protection relay or a ground fault interrupter due to a harmonic current or a high frequency noise. <P>SOLUTION: A contact prevention plate 4 provided between a high voltage coil 5 and a low voltage coil 3 is B-type-grounded to a structure 11 composed of a metal body such as an iron frame of an office building or the like via a ground terminal 1e. A resistor 10 is connected to between the neutral point of the low voltage coil 3 and the ground terminal 1e, and its resistance value is set to a value in a range of 10<r<(V/3<SP>1/2</SP>)/Is[Ω] (V: a rated voltage on the low voltage side, Is: an operating current of the ground fault protection relay or ground fault interrupter). When a ground fault accident in the structure 11 occurs in the electric circuit on the low voltage side, the ground fault current is limited by the resistor 10, and is suppressed to a value decided by the protection cooperation. Further, an area where a current flows when the low voltage coil 3 is short-circuited to the contact prevention plate 4, can be restricted only to the side of the low voltage coil (the side of the short proof plate) rather than a common connection point of the resistor 10 and the contact prevention plate 4. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a transformer for stepping down from a high voltage or extra high voltage to a low voltage and a grounding method thereof.

In a transformer that steps down from a high voltage or extra high voltage (hereinafter referred to as the high voltage side) to a low voltage (hereinafter referred to as the low voltage side), if the high voltage side and the low voltage side are mixed together for some reason, the ground potential of the circuit on the low voltage side increases. This can be dangerous for low pressure equipment and users. Therefore, in Non-Patent Document 1 and Non-Patent Document 2, the transformer connecting the high-voltage side and the low-voltage side is connected to the neutral point of the low-voltage side (if the low-voltage side is 300 V or less, one terminal on the low-voltage side is possible) Safety measures are required, such as performing seed grounding work, and inserting a class B ground contact prevention plate between the high and low voltage windings.
"Ministerial Ordinance for Establishing Technology Related to Electrical Equipment (March 27, 1997 Ministry of International Trade and Industry Ordinance No. 52)", Article 10, Article 11, Article 12 "Interpretation of technical standards for electrical equipment", Article 19, Article 22, Article 24, Article 25

  By the way, most of the buildings in recent years are steel-framed buildings constructed by combining large shapes. In steel-frame buildings, a method of grounding to a building structure made of steel (structure grounding) has become common. According to the electric industry, it is recognized that the steel frame of a building whose electrical resistance value with the ground is 2Ω or less is a B-grounded grounding pole applied to the low voltage side of the transformer. For this reason, structure grounding is frequently used for the B-type grounding that is performed as a safety measure when the high-pressure side and the low-pressure side are mixed.

  Usually, the electrical resistance value between the steel frame of the building and the ground is much smaller than 2Ω, so that the ground resistance of the structure ground is substantially close to 0Ω (unless positive ground resistors are inserted). For this reason, when there is a ground fault in the low-voltage side electric circuit, the circuit is almost completely short-circuited through a low electrical resistance between the steel frame or the steel frame and the ground, and a very large ground fault current flows. As a result, the same level of protection as a short circuit between electrical circuits is required despite a ground fault, and since a ground fault is more likely to occur than a short circuit between electrical circuits, an appropriate circuit breaker to protect the circuit and transformer is selected. It is difficult to take protection coordination such as.

  In addition, when harmonic current and harmonic noise from a power electronics device such as an inverter connected to the low-voltage side circuit return to the zero-phase circuit of the low-voltage side circuit, the harmonic current and high-frequency noise will be generated when grounding the structure with a small ground resistance. There is a risk that an inductive failure may cause a malfunction of a protection system such as a ground fault protection relay or a leakage breaker, or a malfunction of a measurement device or a computer system.

  In order to cope with such a situation, conventionally, for example, a grounding resistance is positively inserted into the grounding wire of the low-voltage side circuit or the anti-collision plate, and grounding is performed with a resistance value close to the upper limit value allowed by the electric technology. Therefore, the ground fault current is suppressed and the harmonic current is prevented from flowing back. However, in such a conventional method, the upper limit of the grounding resistance cannot be set to be equal to or greater than the allowable value of electric power, and there is a limit to the suppression of ground fault current and harmonic noise.

  The present invention has been made in view of the above circumstances, and its purpose is to suppress the ground fault current in the low-voltage side circuit in order to realize appropriate protection coordination, and to provide a ground fault protection relay by harmonic current and high frequency noise, An object of the present invention is to provide a transformer capable of preventing malfunction such as an earth leakage circuit breaker and a grounding method thereof.

In order to achieve the above object, the transformer of the present invention is a transformer that steps down from a high voltage or extra high voltage to a low voltage, and the low voltage winding is 10 <r <(V / 3 ^1/2 ) / Is [Ω] (middle (In the case of sex point grounding)
10 <r <V / Is [Ω] (when one end is grounded)
V: Rated voltage on the low voltage side [V]
Is: Operating current of ground fault protection relay or earth leakage breaker [A]
Is grounded with a grounding resistance having a resistance value r within the range, and a metal anti-intrusion prevention plate is provided between the high-voltage winding or the special high-voltage winding and the low-voltage winding. Is provided.

  According to this configuration, since the B-type grounding is applied to the anti-contact plate, the electrical regulations are completely satisfied, and the potential increase on the low-voltage side during the contact can be suppressed. In addition, since the low-voltage winding is grounded, it is possible to detect a ground fault, and since the ground resistance has a value larger than the ground resistance value required for class B grounding, for example, in the low-voltage side circuit When a ground fault occurs in a metal structure (for example, a steel frame), the ground fault current is limited by the ground resistance. In addition, an upper limit is set for the grounding resistance of the low-voltage winding, and when a ground fault occurs in the low-voltage side circuit, a ground fault current necessary for the operation of the ground fault protection relay or earth leakage breaker flows. .

  By adopting a grounding method in which one end of a grounding resistor for grounding the low-voltage winding is electrically connected to the anti-contact plate and connected to a ground bus through a common ground wire, the low-voltage winding and the anti-contact plate are used. Can be restricted to the low-voltage winding side (mixing prevention plate side) from the common connection point (current does not flow to the common ground line leading to the ground bus). ). As a result, it is possible to reduce the induction failure. In addition, the amount of ground wire used can be reduced.

  According to the present invention, it is possible to suppress an increase in potential on the low voltage side at the time of contact according to the regulations of electric technology. In addition, since the ground fault current in the low-voltage side circuit can be suppressed, it is easy to coordinate protection. It can be prevented as much as possible.

(First embodiment)
A first embodiment of the present invention will be described with reference to FIGS. 1 and 2.
FIG. 1 is an electrical configuration diagram showing a three-phase molded transformer that steps down from a high voltage to a low voltage and a grounded state thereof. FIG. 2 is a plan view (a) and a front view (b) of the transformer 1. is there. According to the regulations of electric technology, high voltage means AC voltage exceeding 600V and 7000V or less, and low voltage means 600V or less.

  The iron core 2 is a three-phase tripod iron core formed by laminating directional silicon steel sheets and the like. Each leg portion of the iron core 2 is wound with a star-connected low-voltage winding 3 which is a secondary winding on the inner peripheral side, and is a primary winding with an anti-mixing plate 4 sandwiched on the outer peripheral side. A high voltage winding 5 is wound. These low voltage winding 3 and high voltage winding 5 are molded with resin. Terminals 1U, 1V, and 1W corresponding to the start of winding of the U-phase, V-phase, and W-phase of the high-voltage winding 5 are provided at the upper front portion of the molded winding, and the rotary type is provided at the lower front portion of the winding. Tap switching terminals 6U, 6V, 6W are provided. By connecting these terminals 1U, 1V, 1W, 6U, 6V, and 6W as shown in FIG. 2B, the high-voltage winding 5 is delta-connected.

  The lower clamp 7 that sandwiches and fixes the lower yoke portion of the iron core 2 is made of a steel material having an inverted T-shaped or L-shaped cross section, and a part thereof is placed and fixed on the base 8. The upper clamp 9 that sandwiches and fixes the upper yoke portion of the iron core 2 is made of a steel material having a U-shaped cross section that opens downward, and a resistor 10 described later is insulated on the upper clamp 9. It is attached in a state.

  On the back side of the upper clamp 9, terminals 1u, 1v, 1w, 1o on the low voltage side are provided. One end (the left end in FIG. 2) of the resistor 10 is connected to a terminal 1o connected to the neutral point o, and the other end (the right end in FIG. 2) is a ground terminal provided on the lower clamp 7, for example. Connected to 1f. Further, the lower clamp 7 is provided with a ground terminal 1e to which a lead wire drawn out from the anti-contact plate 4 is connected. The anti-contact plate 4 has a low resistance such as a steel frame of a building through the ground terminal 1e. Class B is grounded to the structure 11. Further, the lower clamp 7 is also provided with a ground terminal 1g for grounding the iron core 2, the lower clamp 7 and the upper clamp 9 in class A.

  Here, the B class grounding is grounding to prevent a potential increase on the low voltage side when the high voltage side circuit is mixed with the low voltage side circuit, and the grounding resistance is transformed in the above-mentioned electric technology. It is determined to be equal to or less than the number of ohms equal to the value obtained by dividing 150 by the amperage of the one-line ground fault current of the high-voltage side electric circuit of the device. The transformer 1 having the above configuration completely satisfies the electrical regulations because the anti-contact plate 4 is grounded to the structure body B.

Next, the operation and effect of this embodiment will be described.
In general, the anti-contact plate is provided in a transformer that uses the secondary side (low voltage side) ungrounded, and has the effect of suppressing a potential increase on the low voltage side when the high voltage winding and the low voltage winding are mixed. . Also in the transformer 1 of this embodiment, since the anti-intrusion plate 4 is grounded to the B type, the above-described action of suppressing the potential increase can be obtained.

Furthermore, the low-voltage winding 3 of the transformer 1 is grounded via a resistor 10 in order to detect a ground fault. The resistance value r of the resistor 10 is determined so as to satisfy the following equation (1).
10 <r <(V / 3 ^1/2 ) / Is [Ω] (1)
Here, V [V] is the rated voltage on the low voltage side, and Is [A] is the operating current of the ground fault protective relay or earth leakage breaker. When the low voltage side is a 420V neutral point grounding system, Is is normally set to about 1 to 2 [A], but there is a higher sensitivity type, for example, Is of about 1 [mA]. Therefore, the resistance value r of the resistor 10 needs to be set to 242 [kΩ] or less (practically 200 [kΩ] or less) by calculation shown in the following equation (2).
10 <r <(420/3 ^1/2 ) / (1 × 10 ⁻³ ) = 242 [kΩ] (2)
Also, the reason why the resistance value r of the resistor 10 exceeds 10 [Ω] is that the ground resistance of the class B grounding is defined as 10 [Ω] or less, and the grounding of the low voltage winding 3 is the class B grounding. This is because it is not necessary to satisfy the requirements.

  When a ground fault occurs with respect to the structure 11 that is a metal body in the low-voltage side electric circuit, the ground fault current is limited by the resistor 10 and is suppressed to a value (usually about 2 A) determined from the protection coordination. Thereby, malfunctions, such as a ground fault protection relay and an earth leakage circuit breaker, can be prevented, and an accident detection and protection can be normally performed as a ground fault instead of a short-circuit accident.

  As described above, according to the present embodiment, in the usage mode (structure grounding) in which the transformer 1 is grounded to the low-resistance structure 11 such as a steel frame, the low-voltage side at the time of contact is satisfied by completely satisfying the electrical regulations. Can be suppressed, and the ground fault current of the low-voltage side electric circuit is limited by the resistor 10. Thereby, the current level of the ground fault can be lowered than the current level of the short circuit accident between the electric circuits. In addition, since an upper limit is set for the resistance value r of the resistor 10, when a ground fault occurs in the low-voltage side circuit, a ground fault current necessary for operating the ground fault protection relay or the earth leakage breaker flows. Therefore, it becomes easy to take protection coordination including selection of an appropriate circuit breaker for protecting the transformer 1 and the electric circuit.

  When a power electronics device such as an inverter is connected to the low voltage side electric circuit, a concentrated current flows through the power supply line leading to the power electronics device through the low voltage side electric circuit, and reaches the casing of the power electronics device through the structure 11 from the ground point to the structure 11. A distributed current flows through the path. When the distributed current flows, a region where the electromagnetic field is not canceled between the concentrated current and the distributed current is generated, so that an induction failure is likely to occur in the region. On the other hand, since the resistor 10 has the ground circuit impedance of the low-voltage side electric circuit, the amount of return of harmonic current and high-frequency noise output from the power electronics device to the low-voltage side electric circuit is reduced. As a result, it is possible to prevent malfunctions of the measuring device and the computer system due to malfunctions such as ground fault protection relays and earth leakage circuit breakers and induction faults as much as possible.

  Since the resistor 10 is provided integrally with the transformer 1, the low-voltage winding 3 can be grounded at any place without using a grounding board or the like. Even if the resistor 10 is disposed in a casing such as a transformer case or a transformer panel that houses the transformer 1, the same effect can be obtained.

(Second Embodiment)
Next, a second embodiment of the present invention will be described with reference to FIGS.
FIG. 3 is an electrical configuration diagram showing a three-phase molded transformer that steps down from a high voltage to a low voltage and a grounded state thereof, and FIG. 4 is a front view of the transformer 12. 3 and 4, the same components as those in FIGS. 1 and 2 are denoted by the same reference numerals. Terminals 12U, 12V, and 12W are terminals corresponding to the beginning of winding of the high-voltage winding 5, and terminals 12u, 12v, 12w, and 12o are terminals of the low-voltage winding 3 that are star-connected.

  The ground terminal 12e provided on the lower clamp 7 is connected to both the lead wire drawn out from the anti-contact plate 4 and one end of the resistor 10 on the ground side. Since the resistor 10 is provided integrally with the transformer 12, the anti-contact plate 4 and the resistor 10 are electrically connected at the transformer 12 (near the transformer body). For this reason, the transformer 12 of this embodiment does not have a terminal corresponding to the ground terminal 1f provided in the first embodiment. The contact prevention plate 4 is grounded to the structure 11 made of a metal body such as a steel frame of the building via the ground terminal 12e.

  In the structure grounding of steel buildings, the grounding point is required to be the first floor or the underground floor of the building. For this reason, when the transformer is installed on the middle floor or higher floor of the building, the distance between the transformer and the ground point (for example, the ground bus provided on the first floor) becomes long. In the transformer 1 of the first embodiment, both the ground line from the ground terminal 1e to the ground bus and the ground line from the ground terminal 1f to the ground bus are long. For this reason, when the low-voltage winding 3 and the incompatibility preventing plate 4 are in contact with each other, the current flows from the ground terminal 1f of the transformer 1 through the ground line to the ground point on the first floor, and from the ground point to the transformer 1 through the ground line. To the ground terminal 1e. The result is a large reflux path.

  On the other hand, in the transformer 12 of the present embodiment, when the low voltage winding 3 and the anti-contact plate 4 are mixed, the current flows from the low voltage winding 3 to the resistor 10, the ground terminal 12e, and the anti-contact plate 4. The current does not flow to the ground line from the ground terminal 12e to the ground point on the first floor. That is, the region where the accident current flows when the low-voltage winding 3 and the anti-contact plate 4 are mixed is limited only to the low-voltage winding side (the anti-contact plate side) from the common connection point of the resistor 10 and the anti-contact plate 4. it can. As a result, it is possible to reduce inductive obstacles to the measuring device and the computer system. In addition, the amount of ground wire used can be halved.

(Other embodiments)
The present invention is not limited to the embodiments described above and shown in the drawings, and can be modified or expanded as follows, for example.
The present invention can be applied not only to grounding the structure 11 made of a metal body but also to grounding the earth. However, the effect of suppressing the ground fault current to prevent the malfunction of the ground fault protective relay and the earth leakage breaker and to prevent the inductive failure is more noticeable in the case of grounding the structure having a small ground resistance.

When the resistor 10 is provided in the transformer main body, it may be provided not in the upper part of the main body but in the side part of the main body or the lower part of the main body.
Moreover, in 2nd Embodiment, you may connect the end of the resistor 10, and the mixing prevention board 4 in the housing | casing in which the transformer 12 is accommodated, or an electrical room.

The low voltage winding 3 may be a delta connection. When the low voltage side is a one-end grounding system, the resistance value r of the resistor 10 may be determined so as to satisfy the following equation (3) instead of the above equation (1).
10 <r <V / Is [Ω] (3)
For example, when the low voltage side is a 210V one-end grounding system, Is is usually set to about 1 to 2 [A], but there is a higher sensitivity type, for example, Is of about 1 [mA]. Therefore, the resistance value r of the resistor 10 must be set to 210 [kΩ] or less (practically 200 [kΩ] or less) by calculation shown in the following equation (4).
10 <r <210 / (1 × 10 ⁻³ ) = 210 [kΩ] (4)
In each of the above embodiments, an example of a molded transformer is shown, but an oil-filled transformer may be used.
The same applies to a transformer that steps down from a special high voltage (which exceeds 7000 V) to a low voltage.

1 is an electrical configuration diagram showing a transformer and a grounded state thereof according to a first embodiment of the present invention. (A) is a plan view of the transformer, (b) is a front view of the transformer. FIG. 1 equivalent diagram showing a second embodiment of the present invention Front view of transformer

Explanation of symbols

  In the drawings, 1 and 12 are transformers, 12e ground terminals, 3 is a low voltage winding, 4 is an anti-contact plate, 5 is a high voltage winding, and 10 is a resistor (ground resistor).

Claims (6)

In transformers that step down from high voltage or extra high voltage to low voltage,
Low voltage winding is 10 <r <(V / 3 ^1/2 ) / Is [Ω] (in case of neutral grounding)
10 <r <V / Is [Ω] (when one end is grounded)
V: Rated voltage on the low voltage side [V]
Is: Operating current of ground fault protection relay or earth leakage breaker [A]
Is grounded with a grounding resistance having a resistance value r within the range, and a metal anti-intrusion prevention plate is provided between the high-voltage winding or the special high-voltage winding and the low-voltage winding. Transformer characterized by being given.   2. The transformer according to claim 1, wherein the resistance value r is set to a value within a range of 10 [Ω] <r ≦ 200 [kΩ].   The transformer according to claim 1 or 2, wherein the grounding resistor for grounding the low-voltage winding is disposed in a transformer main body or a casing housing the transformer main body.   The grounding terminal to which the one end of the said grounding resistor and the said incompatibility board were connected together is provided in the housing | casing which stores the said transformer main body or the said transformer main body. Transformer. In the grounding method of the transformer in any one of Claims 1 thru | or 4,
A method for grounding a transformer, comprising: electrically connecting one end of a grounding resistor that grounds the low-voltage winding to the incompatibility preventing plate, and connecting the grounding bus through a common grounding wire. 6. The transformer grounding method according to claim 5, wherein one end of the grounding resistor and the incompatibility preventing plate are electrically connected in a transformer main body or a casing housing the transformer main body.

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