High-voltage pre-charging circuit of cascaded high-voltage frequency converter

Abstract

The utility model provides a high-voltage pre-charging circuit of a cascaded high-voltage frequency converter, which relates to the technical field of pre-charging of high-voltage frequency converters and is characterized in that a phase-shifting transformer, a power unit, a control system and a high-voltage motor are arranged in the cascaded high-voltage frequency converter; the high-voltage pre-charging circuit comprises a high-voltage bus, an incoming circuit breaker, a pre-charging resistor structure and a resistor bypass switch; the high-voltage bus is electrically connected with the incoming circuit breaker, the incoming circuit breaker is electrically connected with the resistance bypass switch, the resistance bypass switch is electrically connected with the phase-shifting transformer, the phase-shifting transformer is electrically connected with the power unit and the control system, and the power unit and the control system are electrically connected with the high-voltage motor; the pre-charging resistor structure is connected in parallel with two ends of the resistor bypass switch. According to the utility model, the configured high-voltage pre-charging circuit can pre-excite the phase-shifting transformer and can charge the direct-current capacitor in the power unit at the same time, thereby preventing voltage drop and excitation inrush current when the incoming circuit breaker is switched on.

Description

Technical Field

[0001] The utility model relates to the technical field of high-voltage frequency converter precharging, in particular to a high-voltage precharging circuit of a cascaded high-voltage frequency converter. Background Art

[0002] High-voltage inverters have been increasingly widely used in industries such as electricity, metallurgy, petrochemicals, water supply, and cement, especially cascaded high-voltage inverters, which have been favored by the domestic market for their advantages of minimal harmonic pollution to the power grid, high input power factor, good output waveform quality, and no additional heating, torque pulsation, noise, dv / dt, and common-mode voltage caused by harmonics. Cascaded high-voltage inverters are voltage source inverters, which are multi-level inverter structures with power units connected in series and powered by phase-shift transformers with multiple secondary sides on the input side.

[0003] When the phase-shifting transformer is powered on, an excitation surge current will be generated, also known as no-load closing current. This is a current generated when the transformer is put into operation at no-load or when the voltage is suddenly restored after the external fault is removed. In this case, since the magnetic flux in the iron core of the phase-shifting transformer cannot change suddenly, that is, the magnetic flux remains zero at the initial moment of applying voltage, it is similar to the physical process that occurs when the three-phase circuit is suddenly short-circuited. Two magnetic fluxes will be generated in the iron core at the same time, one is a periodic component that conforms to Ohm's law of magnetic circuit, and the other is a non-periodic component that conforms to the law of electromagnetic induction. These two magnetic flux components are equal in magnitude and opposite in polarity at the initial moment of applying voltage, making the synthetic magnetic flux zero. After half a cycle, it reaches the maximum value. At this time, the iron core is severely saturated, and the excitation current increases rapidly, which can reach 8 to 10 times the rated current, forming a current similar to a surge. This excitation surge current may cause an impact on the power grid and transformer, and will also affect the life of the DC link capacitor of the power unit.

[0004] A common suppression method is to use "low-voltage series resistance combined with phase selection closing method" to suppress the excitation inrush current of the phase-shifting transformer. The principle is: when the phase-shifting transformer is closed, an appropriate resistor is connected in series to the 380VAC input terminal set on the secondary side of the phase-shifting transformer, and the grid voltage and transformer phase sequence are calibrated to be consistent before the high voltage is closed. After a certain delay, when the impact current decays to within the no-load current, the current limiting resistor is removed.

[0005] The biggest disadvantage of this suppression method is that before closing the high-voltage side, the grid-side voltage phase sequence must be consistent with the transformer phase sequence, otherwise it will directly lead to closing failure.

[0006] Therefore, there is room for improvement. The utility model provides a high-voltage pre-charging circuit for a cascaded high-voltage inverter. Utility Model Content

[0007] In view of the shortcomings of the prior art, the utility model aims to propose a high-voltage pre-charging circuit for a cascaded high-voltage inverter. The specific scheme is as follows:

[0008] A high-voltage pre-charging circuit of a cascade-type high-voltage frequency converter, wherein the cascade-type high-voltage frequency converter is provided with a phase-shifting transformer, a power unit and a control system, and a high-voltage motor, wherein the power unit and the control system include a PLC controller, and the high-voltage pre-charging circuit includes a high-voltage bus, an incoming line circuit breaker, a pre-charging resistor structure, and a resistor bypass switch;

[0009] The high-voltage busbar is electrically connected to the incoming circuit breaker, the incoming circuit breaker is electrically connected to the resistor bypass switch, the resistor bypass switch is electrically connected to the phase-shifting transformer, the phase-shifting transformer is electrically connected to the power unit and the control system, and the power unit and the control system are electrically connected to the high-voltage motor;

[0010] The pre-charging resistor structure is connected in parallel to both ends of the resistor bypass switch, and the opening and closing state of the resistor bypass switch is controlled by a PLC controller.

[0011] Therefore, the high-voltage bus, phase-shifting transformer, power unit and control system, as well as the high-voltage motor constitute the main circuit. During the pre-charging process, the incoming circuit breaker is closed first, and the resistor bypass switch is not closed. At this time, the pre-charging resistor structure is put into the main circuit, and the phase-shifting transformer, power unit and control system are pre-charged through the pre-charging resistor structure to limit the magnetizing inrush current when the phase-shifting transformer is powered on. After a period of time, the pre-charging is completed, and the phase-shifting transformer, power unit and control system control the resistor bypass switch to close. The pre-charging resistor structure is in the bypass on the side of the main circuit and does not affect the main circuit, thereby completing the power-on process of the entire cascade high-voltage inverter.

[0012] In summary, the high-voltage pre-charging circuit configured in the utility model can pre-excite the phase-shifting transformer and charge the DC capacitor in the power unit at the same time, thereby preventing voltage drop and excitation inrush current when the incoming circuit breaker is closed.

[0013] Furthermore, the high voltage bus is provided with three-phase input power supplies L1, L2, and L3, and the primary side of the phase-shifting transformer is provided with three-phase connection terminals H1, H2, and H3 correspondingly;

[0014] Corresponding to the three phases L1, L2, and L3, the resistor bypass switch adopts a three-phase bypass switch, and the pre-charging resistor junction structure includes a pre-charging resistor Ra, a pre-charging resistor Rb, and a pre-charging resistor Rc. The pre-charging resistor Ra, the pre-charging resistor Rb, and the pre-charging resistor Rc are respectively connected in parallel to the two ends of different phases of the resistor bypass switch.

[0015] Furthermore, the resistor bypass switch is configured as a high-voltage circuit breaker or a high-voltage contactor.

[0016] Furthermore, the working time of the pre-charging resistor structure is set to 4-6s.

[0017] Compared with the prior art, the beneficial effects of the utility model are as follows:

[0018] The high-voltage pre-charging circuit of the utility model is safe and effective. Based on the method of suppressing the excitation inrush current, it can effectively limit the impact current value, which is conducive to the rapid attenuation of the closing impact current. It can pre-excite the phase-shifting transformer to prevent the voltage drop and excitation inrush current when the incoming circuit breaker is closed. Compared with the prior art, the utility model does not need to be implemented by the "low-voltage series resistance combined with phase selection" method before closing the high-voltage side, which effectively avoids multiple closing failures caused by the inconsistency between the grid-side voltage phase sequence and the transformer phase sequence before closing. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is a schematic diagram of the overall principle of the utility model;

[0020] Figure 2 A schematic diagram showing the principle of the specific connection and coordination relationship of the pre-charging resistor structure.

[0021] Figure numerals: 1. high-voltage busbar; 2. incoming line circuit breaker; 3. pre-charging resistor structure; 31. pre-charging resistor Ra; 32. pre-charging resistor Rb; 33. pre-charging resistor Rc; 4. resistor bypass switch; 5. phase-shifting transformer; 6. power unit and control system; 7. high-voltage motor. DETAILED DESCRIPTION

[0022] The present invention will be further described in detail below in conjunction with the embodiments and drawings, but the implementation methods of the present invention are not limited thereto.

[0023] Based on the method of suppressing the excitation inrush current by directly connecting a resistor in series in the high-voltage circuit, the utility model provides a high-voltage pre-charging circuit for a cascaded high-voltage inverter. The principle is: connecting an appropriate resistor in series between the power grid and the phase-shifting transformer 5 can limit the closing current. When the closing current decays to within the set threshold, the current-limiting resistor is removed through a switch such as a high-voltage circuit breaker.

[0024] It should be noted that, in this embodiment, the cascade high-voltage inverter is a prior art, and will not be described in detail. Only the components involved are disclosed. Specifically, the cascade high-voltage inverter is provided with a phase-shifting transformer 5, a power unit and a control system 6, and a high-voltage motor 7. Figure 1 In the figure, TM is a phase-shifting transformer 5, VFD is a power unit and control system 6, and M is a high-voltage motor 7. The power unit and control system 6 includes a PLC controller and a DC capacitor.

[0025] Based on this, the high-voltage pre-charging circuit includes a high-voltage bus 1, an incoming line circuit breaker 2, a pre-charging resistor structure 3 and a resistor bypass switch 4. Figure 1 In the figure, QF1 is the incoming circuit breaker 2, R is the pre-charging resistor structure 3, and QF2 is the resistor bypass switch 4. The high-voltage bus 1 is electrically connected to the incoming circuit breaker 2, the incoming circuit breaker 2 is electrically connected to the resistor bypass switch 4, the resistor bypass switch 4 is electrically connected to the phase-shifting transformer 5, the phase-shifting transformer 5 is electrically connected to the power unit and control system 6, and the power unit and control system 6 is electrically connected to the high-voltage motor 7.

[0026] The pre-charging resistor structure 3 is connected in parallel to two ends of the resistor bypass switch 4, and the opening and closing states of the resistor bypass switch 4 are controlled by the PLC controller.

[0027] Specifically, if Figure 2 As shown, based on the three-phase characteristics of the phase-shifting transformer 5, the high-voltage bus 1 is provided with three-phase input power supplies L1, L2, and L3, and the primary side of the phase-shifting transformer 5 is correspondingly provided with three-phase connection terminals H1, H2, and H3.

[0028] Corresponding to the three phases L1, L2, and L3, the resistor bypass switch 4 adopts a three-phase bypass switch. The resistor bypass switch 4 can select a high-voltage circuit breaker or a high-voltage contactor, or other automatic switching switches according to the current value of the main circuit.

[0029] The pre-charging resistor junction structure includes a pre-charging resistor Ra31, a pre-charging resistor Rb32, and a pre-charging resistor Rc33. The pre-charging resistor Ra31, the pre-charging resistor Rb32, and the pre-charging resistor Rc33 are respectively connected in parallel to two ends of different phases of the resistor bypass switch 4.

[0030] It should be noted that the pre-charging resistor Ra31, the pre-charging resistor Rb32, and the pre-charging resistor Rc33 are resistors that can withstand current shocks and work at high voltages, such as corrugated resistors and stainless steel resistors.

[0031] The high-voltage bus 1 is used as the medium of the power grid. When the resistor bypass switch 4 is not involved, the high-voltage bus 1, the phase-shifting transformer 5, the power unit and the control system 6, and the high-voltage motor 7 constitute the main circuit. Pre-charging is required. First, the incoming circuit breaker 2 is closed, and the resistor bypass switch 4 is not closed. At this time, the pre-charging resistor structure 3 is put into the main circuit. The phase-shifting transformer 5, the power unit and the control system 6 are pre-charged through the pre-charging resistor structure 3 to limit the magnetizing inrush current when the phase-shifting transformer 5 is powered on. After a period of time, the pre-charging is completed, and the phase-shifting transformer 5, the power unit and the control system 6 control the resistor bypass switch 4 to close. The pre-charging resistor structure 3 is in the bypass on the side of the main circuit and does not affect the main circuit, thereby completing the power-on process of the entire cascade high-voltage inverter.

[0032] It should be noted that during the pre-charging process, the voltage on the input side (primary side) of the phase-shifting transformer 5 will be monitored in real time, and any unusual voltage or too slow voltage rise will generate a fault and cancel the pre-charging.

[0033] The capacitance value of a capacitor can vary depending on the circuit it is connected to and the frequency of the signal. In this case, the capacitor might be used as a sensor or filter to monitor certain parameters in an electrical system, such as voltage, current, or frequency.

[0034] In this regard, the L1 phase, the L2 phase and the L3 phase are connected to a live monitor CGQ, which is provided with a capacitor and a signal light. The live monitor CGQ combines the capacitor C and the signal light to create a composite sensor monitoring system for the L1 phase, the L2 phase and the L3 phase.

[0035] The state of capacitor C can reflect the voltage parameters of phases L1, L2 and L3, while the signal light and the live display CGQ provide user-friendly visual indications.

[0036] The pre-charging process takes about 5 seconds, and the voltage drop at the start moment is less than 1.5%. The current during the entire charging process will not exceed the rated current and will not affect the power grid and other loads of the high-voltage bus 1.

[0037] In summary, the high-voltage pre-charging circuit configured in the utility model can pre-excite the phase-shifting transformer 5 and charge the DC capacitor in the power unit at the same time, thereby preventing the voltage drop and excitation inrush current when the incoming circuit breaker 2 is closed.

[0038] The above is only a preferred embodiment of the present invention. The protection scope of the present invention is not limited to the above embodiments. All technical solutions under the concept of the present invention belong to the protection scope of the present invention. It should be pointed out that for ordinary technicians in this technical field, some improvements and modifications without departing from the principle of the present invention should also be regarded as the protection scope of the present invention.

Claims

1. A high-voltage pre-charging circuit of a cascade-type high-voltage frequency converter, wherein the cascade-type high-voltage frequency converter is provided with a phase-shifting transformer (5), a power unit and a control system (6), and a high-voltage motor (7), wherein the power unit and the control system (6) include a PLC controller, characterized in that: The high-voltage pre-charging circuit comprises a high-voltage busbar (1), an incoming line circuit breaker (2), a pre-charging resistor structure (3) and a resistor bypass switch (4); The high-voltage busbar (1) is electrically connected to the incoming line circuit breaker (2), the incoming line circuit breaker (2) is electrically connected to the resistor bypass switch (4), the resistor bypass switch (4) is electrically connected to the phase-shifting transformer (5), the phase-shifting transformer (5) is electrically connected to the power unit and control system (6), and the power unit and control system (6) is electrically connected to the high-voltage motor (7); The pre-charging resistor structure (3) is connected in parallel to two ends of a resistor bypass switch (4), and the opening and closing state of the resistor bypass switch (4) is controlled by a PLC controller.

2. The high voltage pre-charging circuit of the cascaded high voltage inverter according to claim 1, characterized in that: The high voltage busbar (1) is provided with three-phase input power supplies L1, L2 and L3, and the primary side of the phase shifting transformer (5) is correspondingly provided with three-phase connection terminals H1, H2 and H3; Corresponding to the three phases L1, L2 and L3, the resistor bypass switch (4) adopts a three-phase bypass switch, and the pre-charging resistor junction structure includes a pre-charging resistor Ra (31), a pre-charging resistor Rb (32) and a pre-charging resistor Rc (33). The pre-charging resistor Ra (31), the pre-charging resistor Rb (32) and the pre-charging resistor Rc (33) are respectively connected in parallel to two ends of different phases of the resistor bypass switch (4).

3. The high voltage pre-charging circuit of the cascaded high voltage inverter according to claim 2, characterized in that: The resistor bypass switch (4) is configured as a high-voltage circuit breaker or a high-voltage contactor.

4. The high voltage pre-charging circuit of the cascaded high voltage inverter according to claim 1, characterized in that: The working time of the pre-charging resistor structure (3) is set to 4-6s.

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