A flexible capacitor switching current limiter

Abstract

The present invention discloses a flexible capacitor switching current limiter, which belongs to the technical field of reactive power compensation equipment. It includes power capacitors, NTC thermistors, inductors, DC fans, and bypass diodes; there are three groups of power capacitors, and the A, B, and C phases of the three-phase power supply are respectively connected to a corresponding group of power capacitors through a group of NTC thermistors, and the other ends of the three groups of power capacitors are connected together; a group of DC fans is connected between each group of NTC thermistors and the corresponding group of power capacitors, and a group of inductors are respectively connected in series at the other end of each group of DC fans, and the other ends of the three groups of inductors are connected together, and a group of bypass diodes are respectively connected in parallel with reverse polarity at both ends of each group of DC fans; the air outlet direction of each group of DC fans points to the corresponding NTC thermistor. It has the characteristics of low cost, good inrush current suppression ability, and fast recovery time.

Description

Technical Field

[0001] The invention relates to a flexible capacitor switching current limiter, belonging to the technical field of reactive power compensation equipment. Background Art

[0002] Power capacitors are essential electrical equipment for reactive power compensation, energy saving and loss reduction. With the improvement of intelligent and automated production equipment, their power consumption and reactive power are also changing dramatically, which requires reactive compensation equipment to be adjusted quickly, and power capacitors need to be switched more and more frequently. In many factories, capacitors are switched on and off many times within a minute. However, the switching of capacitors will inevitably produce a large inrush current, and sometimes an operating overvoltage, which will not only cause great damage to the capacitor itself, but also cause voltage fluctuations or oscillations in the power supply system. Therefore, how to limit the inrush current of power capacitors has always been the research focus of reactive power compensation technology.

[0003] The prior art adopts two solutions. One is the thyristor switching switch. The thyristor is used to achieve zero-point switching, which can effectively reduce the inrush current. However, the thyristor switching switch is expensive, and its own overcurrent stress resistance is weak and it is easy to be damaged. The second is the current limiting reactor. The use of reactors for current limiting has a long history, but if the inductance of the reactor is small, the inrush current suppression capability is insufficient. If the inductance is large, it will cause transient oscillation of the opening and closing, further damaging the capacitor. The overall current limiting effect is general, and the inrush current suppression capability can only reach about 5 times the rated current. Therefore, the development of a capacitor switching current limiter with low cost, good inrush current suppression capability and good reliability is of great benefit to the improvement of the switching frequency and compensation effect of power capacitors. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a flexible capacitor switching current limiter, which solves the current problems of high switching circuit cost, slow recovery time and inrush current suppression capability to be improved.

[0005] The technical problem to be solved by the present invention is achieved by adopting the following technical solutions:

[0006] A flexible capacitor switching current limiter comprises a power capacitor, an NTC thermistor, an inductor, a DC fan and a bypass diode;

[0007] There are three groups of power capacitors, and the A, B, and C phases of the three-phase power supply are connected to a corresponding group of power capacitors through a group of NTC thermistors respectively, and the other ends of the three groups of power capacitors are connected together;

[0008] A group of DC fans are connected between each group of NTC thermistors and the corresponding group of power capacitors. A group of inductors are connected in series at the other end of each group of DC fans. The other ends of the three groups of inductors are connected together. A group of bypass diodes are connected in parallel with opposite polarity at both ends of each group of DC fans.

[0009] The air outlet direction of each set of DC fans points to the corresponding NTC thermistor.

[0010] As a preferred example, the maximum current carrying capacity of the NTC thermistor is not less than the rated current of the controlled power capacitor.

[0011] As a preferred example, the cold resistance value of the NTC thermistor is not less than 50 times the hot resistance value.

[0012] Since the resistance of NTC thermistor changes with temperature and is not rigidly constant, it is called a flexible switching current limiter.

[0013] The beneficial effects of the present invention are:

[0014] (1) Low cost. This circuit structure uses low-cost NTC thermistors to achieve inrush current suppression. Its cost is several times lower than that of thyristor switches. In addition, NTC thermistors are passive ceramic devices, and their reliability, durability, and stability far exceed those of products using thyristors.

[0015] (2) Good inrush current suppression capability. The low-cost NTC thermistor in this circuit structure realizes inrush current suppression. Its essence is a dynamically changing resistor. Therefore, compared with the reactor, it will not excite oscillation or resonance, effectively improving the inrush current suppression capability of the existing technology, which also makes it possible to frequently switch on and off the power capacitor;

[0016] (3) Fast recovery time. In the prior art capacitor switching scheme, since the withdrawn capacitor needs to be discharged before it can be put back in, even if it is put in without inrush current, high-frequency switching cannot be achieved. In this circuit structure, the residual charge of the power capacitor is used to power the DC fan, so that the withdrawn power capacitor uses the residual charge to drive the fan to reset the NTC thermistor. On the one hand, compared with the discharge resistor, the residual charge is discharged very quickly, and on the other hand, the discharged charge is used to cool down and reset the NTC thermistor, thereby preparing for the second switching in a short time. By utilizing the characteristics of fast recovery time, this circuit structure can achieve a switching frequency of several times within one minute, which is of great benefit to improving the switching frequency and compensation effect of the power capacitor. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 It is a schematic diagram of the circuit structure of the present invention.

[0018] In the figure: power capacitor 1, NTC thermistor 2, inductor 3, DC fan 4, bypass diode 5. DETAILED DESCRIPTION

[0019] In order to make the technical means, creative features, objectives and effects of the present invention easier to understand, the present invention is further described below with reference to specific illustrations.

[0020] like Figure 1 As shown, a flexible capacitor switching current limiter includes a power capacitor 1, an NTC thermistor 2, an inductor 3, a DC fan 4, and a bypass diode 5;

[0021] There are three groups of power capacitors 1. Phases A, B, and C of the three-phase power supply are connected to a corresponding group of power capacitors 1 through a group of NTC thermistors 2 respectively, and the other ends of the three groups of power capacitors 1 are connected together.

[0022] A group of DC fans 4 are connected between each group of NTC thermistors 2 and a corresponding group of power capacitors 1, and a group of inductors 3 are connected in series at the other end of each group of DC fans 4. The other ends of the three groups of inductors 3 are connected together, and a group of bypass diodes 5 are connected in parallel with opposite polarities at both ends of each group of DC fans 4;

[0023] The air outlet direction of each group of DC fans 4 points to the corresponding NTC thermistor 2 .

[0024] The maximum current carrying capacity of the NTC thermistor 2 is not less than the rated current of the controlled power capacitor 1.

[0025] The cold resistance value of the NTC thermistor 2 is not less than 50 times the hot resistance value.

[0026] Working principle:

[0027] Inrush current suppression is achieved by using a low-cost NTC thermistor 2. When the power capacitor 1 is initially closed, the resistance of the NTC thermistor 2 is very large, so the closing inrush current is greatly weakened. As the NTC thermistor 2 itself heats up under the current, its structure becomes smaller sharply, and finally decreases to a negligible level. Almost all of the system voltage is applied to the power capacitor 1, so that the power capacitor 1 is switched on without inrush current. Since the structure of the NTC thermistor 2 changes continuously, no secondary inrush current will be generated during the entire switching process, and no oscillation or resonance will be caused. Its cost can be several times lower than that of the switching switch with a thyristor structure, and the NTC thermistor 2 is a passive ceramic device, and its reliability, durability and stability far exceed those of products using thyristors. The circuit structure adopts a low-cost NTC thermistor 2 to achieve inrush current suppression. The NTC thermistor 2 is essentially a dynamically changing resistor. Therefore, compared with the reactor, it will not excite oscillation or resonance. If the cold resistance of the NTC thermistor 2 is set relatively large, the closing inrush current of less than 2 times can be achieved, effectively improving the inrush current suppression capability of the prior art, which also makes it possible for the frequent switching of the power capacitor 1.

[0028] In addition, when the power capacitor 1 is switched on and off using the NTC thermistor 2, there is a time problem that the NTC thermistor 2 needs to cool down and recover after exiting the power capacitor 1. This circuit structure cleverly utilizes the residual charge of the power capacitor 1 to achieve a faster recovery time. The switching scheme of the power capacitor 1 in the prior art cannot achieve high-frequency switching even if the non-inrush current is achieved because the withdrawn power capacitor 1 needs to be discharged before it can be put back into operation. The circuit structure uses the residual charge of the power capacitor 1 to power the DC fan 4. When the power capacitor 1 is running, the inductor 3 has a large AC inductance, and the DC fan 4 does not work. When the power capacitor 1 is withdrawn, the residual charge drives the DC fan 4 through the inductor 3, and the DC fan 4 blows air to the NTC thermistor 2 to dissipate heat, so that the withdrawn power capacitor 1 uses the residual charge to drive the DC fan 4 to reset the NTC thermistor 2. First, the residual charge is discharged very quickly compared with the discharge resistor, and secondly, the discharged charge is used to cool and reset the NTC thermistor 2, so as to prepare for the second switching in a short time. By using the characteristics of fast recovery time, the circuit structure can achieve a switching frequency of several times within one minute, and the use of residual charge also indirectly achieves high energy saving effect. By using the cooperation of the NTC thermistor 2 and the DC fan 4, a flexible switching current limiter is realized.

[0029] The basic principles, main features and advantages of the present invention are shown and described above. It should be understood by those skilled in the art that the present invention is not limited to the above embodiments. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, which fall within the scope of the present invention. The scope of the present invention is defined by the attached claims and their equivalents.

Claims

1. A flexible capacitor switching current limiter, characterized in that: It includes a power capacitor (1), an NTC thermistor (2), an inductor (3), a DC fan (4), and a bypass diode (5); The power capacitors (1) are provided in three groups, and the A, B and C phases of the three-phase power supply are respectively connected to a corresponding group of power capacitors (1) through a group of NTC thermistors (2), and the other ends of the three groups of power capacitors (1) are connected together; A group of DC fans (4) is connected between each group of NTC thermistors (2) and a corresponding group of power capacitors (1); a group of inductors (3) are respectively connected in series at the other end of each group of DC fans (4); the other ends of the three groups of inductors (3) are connected together; and a group of bypass diodes (5) are respectively connected in parallel with opposite polarities at both ends of each group of DC fans (4); The air outlet direction of each group of DC fans (4) points to the corresponding NTC thermistor (2).

2. The flexible capacitor switching current limiter according to claim 1, characterized in that: The maximum current carrying capacity of the NTC thermistor (2) is not less than the rated current of the controlled power capacitor (1).

3. The flexible capacitor switching current limiter according to claim 1, characterized in that: The cold resistance value of the NTC thermistor (2) is not less than 50 times the hot resistance value.

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