Frequency converter voltage sag treatment device and control method thereof

Abstract

The invention discloses a frequency converter voltage sag treatment device and a control method, a boost circuit of the device adopts a self-oscillation boost topology formed based on a small-capacity capacitor and a diode, and cooperates with an isolation transformer to realize low-power, low-cost and slow charging of a high-voltage energy storage capacitor when a power grid is normal, and extra control is not needed. A bidirectional switching device is introduced into the step-down circuit, so that the problem of overload of a rectifier bridge of the frequency converter caused by reverse charging of an energy storage capacitor by a direct-current bus of the frequency converter due to a fly-wheel diode in a traditional Buck circuit at the moment of voltage sag recovery is solved. The control module judges voltage sag and recovery events according to the three-phase power grid voltage and the frequency converter DC bus voltage, further controls the on and off of a bidirectional switching device in the step-down circuit, and maintains the stability of the frequency converter DC bus voltage. The problem that a frequency converter is sensitive to voltage sag in the precision manufacturing industry is effectively solved, and the frequency converter has the advantages of being low in cost, high in reliability and good in dynamic performance.

Description

Technical Field

[0001] This invention belongs to the field of power electronics, and in particular relates to a voltage sag mitigation device for frequency converters and its control method. Background Technology

[0002] With the upgrading of my country's industries, the number of precision manufacturing enterprises is increasing. Unlike traditional manufacturing, precision manufacturing enterprises have high requirements for power quality and low tolerance for voltage sags in the power grid. Voltage sags can easily cause malfunctions in precision equipment such as frequency converters on production lines, leading to defective products and even damage to the equipment, resulting in economic losses for the enterprises and hindering industrial upgrading. Since voltage sags are caused by factors such as lightning strikes, equipment short circuits, and the startup of large equipment, they are generally unavoidable. Therefore, researching the management of frequency converter voltage sags is of great significance and application value.

[0003] Currently, most inverter voltage sag mitigation devices draw power from the DC side to charge the energy storage components, requiring the capacity of the inverter's front-end rectifier diodes. However, inverters may operate under heavy load or even overload conditions, where the rectifier diodes may be carrying large currents. If the inverter voltage sag mitigation device also needs to utilize the rectifier diodes' capacity to charge its internal energy storage components, the rectifier diodes may age faster or even fail completely. Furthermore, most inverter voltage sag mitigation devices use low-voltage, high-capacity supercapacitors to store the energy required by the inverter for voltage sag mitigation devices, resulting in high system costs. Summary of the Invention

[0004] The purpose of this invention is to provide a voltage sag mitigation device and control method for frequency converters, addressing the shortcomings of existing voltage sag mitigation solutions in terms of cost, reliability, and control performance. This invention effectively solves the problem of frequency converters' sensitivity to voltage sags in the precision manufacturing industry, offering advantages such as low cost, high reliability, and good dynamic performance.

[0005] This invention is implemented according to the following technical solution:

[0006] This invention provides a voltage sag mitigation device for frequency converters, the device comprising:

[0007] A boost circuit, whose input terminal is selectively connected to the AC input side of a frequency converter, includes a circuit structure with a self-oscillating boost topology and an isolation transformer, used to boost, isolate and transform grid power and store the boosted power.

[0008] A step-down circuit includes a bidirectional switching device, the input of which is connected to the output of the boost circuit, and the output of which is connected to the DC bus of the frequency converter. The step-down circuit is configured to: when a voltage sag occurs in the power grid, step down the electrical energy stored in the boost circuit and deliver it to the DC bus of the frequency converter; when the voltage sag in the power grid recovers, cut off the reverse charging circuit from the DC bus to the boost circuit through the bidirectional switching device.

[0009] The control module is connected to the gate of the bidirectional switching device in the buck circuit, the voltage sampling point of the inverter DC bus, the voltage sampling point of the three-phase input terminals of the inverter, and the current sampling point of the filter inductor of the buck circuit, respectively, to collect electrical signals at the corresponding locations. The control module determines voltage sag and voltage sag recovery events based on the collected three-phase grid voltage signals and inverter DC bus voltage signals, and then controls the conduction and cutoff of the bidirectional switching device in the buck circuit to maintain the stability of the inverter DC bus voltage.

[0010] In one embodiment, the boost circuit includes boost diodes D7, D8, D9, and D1. 10 D 11 Small-capacity boost capacitors C2, C3, C4, and C5; high-voltage energy storage capacitor C6; isolation transformer T.

[0011] The boost diodes D7, D8, D9, and D 10 D 11 It is cross-cascaded with small-capacity boost capacitors C2, C3, C4, and C5 to form a 2x power frequency self-oscillating boost topology;

[0012] The isolation transformer T is connected in series between the input terminal of the boost circuit and the power grid to achieve isolation between the boost circuit, the high-voltage energy storage capacitor C6 and the power grid;

[0013] The high-voltage energy storage capacitor C6 is used to store electrical energy boosted by the boost circuit, and completes energy storage through a low-power slow charging mode when the power grid is supplying power normally.

[0014] In one embodiment, the step-down circuit includes a bidirectional switching device, a filter inductor L1, and a freewheeling diode D. 12 The bidirectional switching device consists of switching transistors S7 and S8 and an anti-parallel diode, with S7 and S8 connected via a common drain. The common drain terminal is connected to one end of the filter inductor L1. One source terminal of the bidirectional switching device is connected to the output terminal of the high-voltage energy storage capacitor C6 in the boost circuit, and the other source terminal is connected to the DC bus N terminal of the frequency converter. The other end of the filter inductor L1 is connected to the freewheeling diode D. 12 The anode of the inverter is connected to the DC bus P terminal of the inverter; the freewheeling diode D 12The cathode is connected to the N terminal of the DC bus of the frequency converter.

[0015] In one embodiment, the control module includes a voltage sag and recovery judgment module, a duty cycle adjustment module, and a drive module; wherein,

[0016] The voltage sag and recovery judgment module is used to judge voltage sag and voltage sag recovery events based on the collected three-phase power grid voltage signal and inverter DC bus voltage signal, and output the drive enable signal of the bidirectional switching device.

[0017] The duty cycle adjustment module is used to generate a control duty cycle signal for the bidirectional switching device based on the drive enable signal of the bidirectional switching device and the DC bus voltage deviation value of the frequency converter.

[0018] The drive module is used to amplify the duty cycle signal and drive the bidirectional switching device to perform on / off actions, thereby realizing the chopping and voltage reduction function and keeping the DC bus voltage of the frequency converter stable.

[0019] The present invention also provides a control method based on the above-described inverter voltage sag mitigation device, the method comprising:

[0020] The three-phase power grid voltage and the inverter DC bus voltage are collected; the inverter DC bus voltage value is stored as two sampling data, namely the current cycle sampling value and the previous cycle sampling value;

[0021] Calculate the grid voltage value in a two-phase orthogonal coordinate system based on the three-phase grid voltage;

[0022] Calculate the grid voltage amplitude based on the grid voltage value in a two-phase orthogonal coordinate system;

[0023] Based on the grid voltage amplitude, a grid voltage amplitude weighting coefficient is introduced. k 1 and DC bus voltage weighting factor k 2. Calculate the grid voltage sag judgment value by combining the preset three-phase voltage amplitude threshold of the power grid and the DC bus voltage threshold of the frequency converter;

[0024] Based on the calculated grid voltage sag judgment value, determine whether a grid voltage sag has occurred or whether a sag has recovered. If a voltage sag is detected, the energy of the high-voltage energy storage capacitor is transferred to the inverter's DC bus to stabilize its voltage by controlling the switching action of the bidirectional switching device, ensuring the stable operation of the inverter during the voltage sag. If the grid voltage sag is detected to have recovered, the bidirectional switching device is turned off, and the three-phase grid voltage and the inverter's DC bus voltage are continuously monitored to continuously judge grid voltage sag events.

[0025] In one embodiment, the grid voltage amplitude e magfor: , in, , In the formula, e α , e β This is the grid voltage value. e a , e b , e c This refers to the three-phase voltage of the power grid.

[0026] In one implementation, the grid voltage sag judgment value J f for: , In the formula, u dc This is the DC bus voltage. k 1 is the power grid voltage amplitude weighting coefficient, which takes a value in the range of [0,1]. Its value depends on the balance and harmonic content of the local power grid. k 2 is the DC bus voltage weighting coefficient, which takes a value in the range of [0,1]. Its value depends on the DC bus fluctuation value of the frequency converter used. e set This is the threshold value for the three-phase voltage amplitude of the power grid, and its value is the average amplitude of the voltage of the power grid in the area where it is used; u set This is the DC bus voltage threshold of the frequency converter, and its value depends on the DC bus undervoltage protection value of the frequency converter used.

[0027] In one implementation, the method for determining grid voltage sag and voltage sag recovery is as follows: If the grid voltage sag judgment value J f If the value is ≥0.7, it is determined that a voltage dip has occurred in the mains. If the grid voltage sag judgment value J f If the value is ≤0.5, it is determined to be a recovery of the grid voltage sag.

[0028] In one embodiment, the step of transferring energy from the high-voltage energy storage capacitor to the DC bus of the frequency converter to stabilize its voltage by controlling the switching action of the bidirectional switching device includes: The DC bus voltage value under normal operating conditions of the inverter is used as the setpoint for the duty cycle PI regulator of the bidirectional switching device, and the difference between it and the current DC bus voltage value of the inverter is used to obtain the DC bus voltage deviation value. The DC bus voltage deviation is used as the input to the duty cycle PI regulator of the bidirectional switching device, and the output value of the PI regulator is used as the PI control term for the duty cycle of the bidirectional switching device. D 1; By employing feedforward control, the sampled DC bus voltage values ​​of the two-step inverter are used to calculate the feedforward term for the duty cycle of the bidirectional switching devices. D 2; Turn on the duty cycle PI control term of the bidirectional switching device. D 1 and feedforward terms D 2. By superimposing, the duty cycle of the bidirectional switching device is obtained. D : The control module is based on the duty cycle of the bidirectional switching device. D PWM pulses are generated to drive bidirectional switching devices for chopping and voltage reduction control, thereby transferring energy from the high-voltage energy storage capacitor to the DC bus of the frequency converter.

[0029] In one implementation, the feedforward term of the duty cycle D 2 is: , In the formula, The sampling period is This is the DC bus voltage value from the previous sampling period. The DC bus voltage value for the current sampling period, u set This is the threshold value for the three-phase voltage amplitude of the power grid.

[0030] Beneficial effects of this invention:

[0031] 1. This invention draws power directly from the three-phase power grid and employs an automatic low-power charging method, eliminating the need for additional closed-loop charging control and reducing system complexity and cost. Simultaneously, it utilizes a high-voltage energy storage solution, reducing capacitor capacity and further lowering system costs.

[0032] 2. The step-down circuit of the present invention, by introducing a bidirectional switching device, can cut off the charging circuit when the voltage recovers, without causing additional burden to the frequency converter.

[0033] 3. To improve the accuracy of voltage sag and voltage sag recovery judgment, this invention combines three-phase grid voltage information and DC bus voltage information for comprehensive judgment, and adopts feedforward control to achieve highly accurate event judgment and control. Attached Figure Description

[0034] The accompanying drawings, as part of this invention, are provided to further illustrate the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention, but do not constitute an undue limitation thereof. Clearly, the drawings described below are merely some embodiments, and those skilled in the art can obtain other drawings based on these drawings without any creative effort.

[0035] Figure 1 A topology diagram of the inverter voltage sag mitigation device provided by the present invention is provided as an embodiment of the present invention;

[0036] Figure 2 This is a flowchart of a control method for a voltage sag mitigation device according to an embodiment of the present invention;

[0037] Figure 3 A flowchart of a method for transferring energy from a high-voltage energy storage capacitor to the DC bus of a frequency converter to stabilize its voltage by controlling the switching action of a bidirectional switching device, according to an embodiment of the present invention.

[0038] Figure 4 This is a comparison chart of voltage change curves during the operation of the inverter voltage sag mitigation device provided in one embodiment of the present invention.

[0039] It should be noted that these accompanying drawings and textual descriptions are not intended to limit the scope of the invention in any way, but rather to illustrate the concept of the invention to those skilled in the art by referring to specific embodiments. Detailed Implementation

[0040] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments will be clearly and completely described below with reference to the accompanying drawings. The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0041] In the description of this invention, it should be noted that the terms "upper", "lower", "front", "rear", "left", "right", "vertical", "inner", "outer", etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limiting this invention.

[0042] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0043] To enhance understanding of the present invention, the technical solution of the present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0044] Reference Figure 1 As shown, one embodiment provides a voltage sag mitigation device for a frequency converter, the device comprising:

[0045] A boost circuit, whose input terminal is selectively connected to the AC input side of a frequency converter, includes a circuit structure with a self-oscillating boost topology and an isolation transformer, used to boost, isolate and transform grid power and store the boosted power.

[0046] A step-down circuit includes a bidirectional switching device, the input of which is connected to the output of the boost circuit, and the output of which is connected to the DC bus of the frequency converter. The step-down circuit is configured to: when a voltage sag occurs in the power grid, step down the electrical energy stored in the boost circuit and deliver it to the DC bus of the frequency converter; when the voltage sag in the power grid recovers, cut off the reverse charging circuit from the DC bus to the boost circuit through the bidirectional switching device.

[0047] The control module is connected to the gate of the bidirectional switching device in the buck circuit, the voltage sampling point of the inverter DC bus, the voltage sampling point of the three-phase input terminals of the inverter, and the current sampling point of the filter inductor of the buck circuit, respectively, to collect electrical signals at the corresponding locations. The control module determines voltage sag and voltage sag recovery events based on the collected three-phase grid voltage signals and inverter DC bus voltage signals, and then controls the conduction and cutoff of the bidirectional switching device in the buck circuit to maintain the stability of the inverter DC bus voltage.

[0048] It should be noted that, for the sake of system signal flow continuity and simplicity, the connection between the control module and the sampling points is not specified. Figure 1 It is displayed directly in the middle.

[0049] Furthermore, the boost circuit includes boost diodes D7, D8, D9, and D1. 10 D 11 Boost capacitors C2, C3, C4, C5; high-voltage energy storage capacitor C6; isolation transformer T; boost diodes D7, D8, D9, D10. 10 D 11The boost circuit is cascaded with small-capacity boost capacitors C2, C3, C4, and C5 to form a 2x power frequency self-oscillating boost topology. An isolation transformer is connected in series between the input of the boost circuit and the power grid, realizing the isolation between the boost circuit, the high-voltage energy storage capacitor, and the power grid. The high-voltage energy storage capacitor is used to store the electrical energy boosted by the boost circuit, and completes energy storage through a low-power slow charging mode when the power grid is supplying power normally.

[0050] Furthermore, small-capacity capacitors are typically 0.47uF to 3uF, while voltage storage capacitors are generally selected at 82000uF.

[0051] Specifically, the primary side of the small isolation transformer T is connected to the grid input voltage (terminals are labeled Uin1 and Uin2), and one end of the secondary winding is connected to the lower end of the boost capacitor C2, providing the initial power input for the entire boost topology.

[0052] The upper end of capacitor C2 is connected to both the anode of diode D7 and the cathode of diode D8. The cathode of diode D7 is connected to the lower end of capacitor C3. The upper end of capacitor C3 is connected to both the anode of diode D8 and the cathode of diode D9. The cathode of diode D9 is also connected to the lower end of capacitor C4. The upper end of capacitor C4 is connected to both the anode of diode D9 and the cathode of diode D8. 10 The cathode, diode D 10 The cathode is also connected to the lower end of capacitor C5, and the upper end of capacitor C5 is connected to diode D. 10 anode and diode D 11 The cathode, diode D 11 Anode diode D 10 The cathode is connected to the high-voltage energy storage capacitor C6, and the boosted electrical energy is stored in C6.

[0053] In this embodiment, the input of the boost circuit is connected to any two phases of the three-phase input of the frequency converter, and the output is connected to the input of the buck circuit. The small isolation transformer T isolates the boost circuit, the high-voltage energy storage capacitor, and the power grid. The boost diode and boost capacitor form a double-frequency boost circuit, drawing power from the power grid through the isolation transformer and boosting it. The high-voltage energy storage capacitor stores the boosted energy. By using this boost circuit, its operating frequency is double the power frequency. By using a small-capacity boost capacitor, low-power slow charging of the high-voltage energy storage capacitor can be achieved when the power grid is supplying power normally, reducing the capacity of the isolation transformer and boost diode, and reducing the cost of the device. Furthermore, no additional control is required, avoiding the problem of current frequency converter voltage sag mitigation devices drawing power from the DC bus of the frequency converter to charge the energy storage elements, which occupies the capacity of the frequency converter's rectifier diodes, leading to overload and damage.

[0054] During voltage dips, energy storage elements are required to provide all the energy, thus necessitating the storage of a significant amount of electrical energy. Current solutions mostly employ supercapacitor modules. However, due to limitations in current supercapacitor module manufacturing processes, solutions primarily utilize low-voltage farad-level supercapacitor modules (typically below 600V), resulting in high costs. Since the energy stored by a capacitor is proportional to the square of the voltage, a high-voltage energy storage solution can drastically reduce the capacitance, allowing the use of conventional capacitors and further reducing system costs. This invention breaks with conventional design by employing a high-voltage energy storage solution, reducing capacitor capacity and further lowering system costs.

[0055] Furthermore, the step-down circuit includes a bidirectional switching device, a filter inductor L1, and a freewheeling diode D. 12 The bidirectional switching device consists of switching transistors S7 and S8 and an anti-parallel diode. S7 and S8 are connected with a common drain. The common drain terminal is connected to one end of the filter inductor L1. One source terminal of the bidirectional switching device is connected to the output terminal of the high-voltage energy storage capacitor C6 of the boost circuit, and the other source terminal is connected to the DC bus N terminal of the frequency converter. The other end of the filter inductor L1 is connected to the freewheeling diode D. 12 The anode of the inverter is connected to the DC bus P terminal of the frequency converter; the freewheeling diode D... 12 The cathode is connected to the N terminal of the DC bus of the frequency converter.

[0056] The input of the buck circuit (i.e., the two ends of the high-voltage energy storage capacitor) is connected to the input port of the boost circuit, and the output is connected to the positive and negative terminals of the inverter's DC bus. Unlike traditional Buck buck circuits, this buck circuit introduces a bidirectional switching device. In traditional Buck circuits, when the inverter voltage recovers, the high-voltage energy storage capacitor draws energy from the inverter's DC capacitor through the anti-parallel freewheeling diode of the unidirectional switching device in the Buck circuit, which can easily cause overload or even damage to the inverter's uncontrolled rectifier bridge. The buck circuit introduces a bidirectional switching device, solving the problem of inverter rectifier bridge overload caused by the reverse charging of the energy storage capacitor by the inverter's DC bus during voltage dip recovery due to the presence of a freewheeling diode in the traditional Buck circuit. In addition, by introducing a bidirectional switching device, the buck circuit of this invention can cut off this charging circuit when the voltage recovers, without placing an additional burden on the inverter.

[0057] Furthermore, the control module includes a voltage sag and recovery judgment module, a duty cycle adjustment module, and a drive module; among which,

[0058] The voltage sag and recovery judgment module is used to judge voltage sag and voltage sag recovery events based on the collected three-phase grid voltage signal and inverter DC bus voltage signal, and output the drive enable signal of the bidirectional switching device.

[0059] The duty cycle adjustment module is used to generate the control duty cycle signal of the bidirectional switching device based on the drive enable signal of the bidirectional switching device and the DC bus voltage deviation value of the inverter.

[0060] The drive module is used to amplify the duty cycle signal and drive the bidirectional switching device to perform on / off actions, thereby realizing the chopping and voltage reduction function and keeping the DC bus voltage of the frequency converter stable.

[0061] Specifically, the acquisition of three-phase AC grid voltage, boost circuit capacitor voltage, buck circuit current, and inverter DC bus voltage can be achieved using sensors and other accessories. The sensor installation locations are indicated by the detection signal markers in the diagram. The control module contains a control program that uses the acquired three-phase AC grid voltage and inverter DC bus voltage to determine voltage sag and recovery events. This allows for the control of the bidirectional switching devices, shutting them off during voltage sag recovery to prevent the inverter from charging the device's high-voltage energy storage capacitor. During voltage sags, the switching action of the bidirectional switching devices is controlled to maintain the inverter's DC bus voltage at a predetermined value, ensuring stable operation of the inverter during voltage sags.

[0062] When a voltage dip occurs in the mains voltage, the DC bus voltage will drop rapidly if the frequency converter is running. However, relying solely on the DC bus voltage as the criterion may present other problems, such as the frequency converter being in a faulty state, in which case the DC bus voltage may also drop rapidly. If only the mains voltage signal is used for judgment, there are extremely short periods and situations where no action is needed when the frequency converter is under light load. In these cases, activating voltage dip mitigation equipment would be of little practical benefit, and the device itself might even experience surge due to operating under light load and deviating from its design specifications. Therefore, to improve the accuracy of voltage dip and recovery judgment, this invention combines three-phase mains voltage information and DC bus voltage information for comprehensive judgment, achieving highly accurate event judgment and control.

[0063] Based on the above-mentioned inverter voltage sag mitigation device, referring to Figure 2 As shown, in one embodiment, a control method for the device is proposed. The control method based on the inverter voltage sag mitigation device includes the following steps:

[0064] Step S100: Collect the three-phase grid voltage and the inverter DC bus voltage.

[0065] The three-phase voltages of the power grid were collected as follows: e a , e b , e c DC bus voltage is u dcThe inverter's DC bus voltage value stores two sampling data points, which are the sampling values ​​for the current period. u dc (k) and the sampled value of the previous period u dc (k-1).

[0066] Step S200: Calculate the grid voltage value in the two-phase orthogonal coordinate system based on the three-phase grid voltage.

[0067] Furthermore, the grid voltage value in the two-phase orthogonal coordinate system is calculated using the following formula. e α , e β : , In the formula, e a , e b , e c This refers to the three-phase voltage of the power grid.

[0068] Step S300: Calculate the grid voltage amplitude based on the grid voltage value in the two-phase orthogonal coordinate system.

[0069] Furthermore, the grid voltage amplitude is calculated using the following formula. e mag : .

[0070] Step S400: Based on the grid voltage amplitude, introduce a grid voltage amplitude weighting coefficient. k 1 and DC bus voltage weighting factor k 2. Calculate the grid voltage sag judgment value by combining the preset three-phase voltage amplitude threshold of the power grid and the DC bus voltage threshold of the frequency converter.

[0071] Furthermore, the voltage sag judgment value is calculated using the following formula. J f : , In the formula, u dc This is the DC bus voltage. k 1 is the power grid voltage amplitude weighting coefficient, which takes a value in the range of [0,1]. Its value depends on the balance and harmonic content of the local power grid. k 2 is the DC bus voltage weighting coefficient, which takes a value in the range of [0,1]. Its value depends on the DC bus fluctuation value of the frequency converter used. e setThis is the threshold value for the three-phase voltage amplitude of the power grid, and its value is the average amplitude of the voltage of the power grid in the area where it is used; u set This is the DC bus voltage threshold of the frequency converter, and its value depends on the DC bus undervoltage protection value of the frequency converter used.

[0072] In a preferred embodiment, the method for determining grid voltage sag and voltage sag recovery is as follows: If the grid voltage sag judgment value J f If the value is ≥0.7, it is determined that a voltage dip has occurred in the mains. If the grid voltage sag judgment value J f If the value is ≤0.5, it is determined to be a recovery of the grid voltage sag.

[0073] That is, the determination of grid voltage sag and recovery is performed according to the following formula: .

[0074] Step S500: Determine whether a voltage sag has occurred or recovered based on the calculated grid voltage sag judgment value. If a voltage sag is detected, transfer the energy of the high-voltage energy storage capacitor to the inverter DC bus by controlling the switching action of the bidirectional switching device to stabilize its voltage and ensure stable operation of the inverter during the voltage sag. If a grid voltage sag is detected to have recovered, turn off the bidirectional switching device and continue to monitor the three-phase grid voltage and the inverter DC bus voltage to continuously judge the grid voltage sag event.

[0075] Specifically, when the grid voltage dip recovers, the following operations are performed: the bidirectional switching device is turned off, the process returns to step S100, and the grid voltage and the inverter's DC bus voltage continue to be monitored, with ongoing grid voltage dip event judgment. At this time, the transfer of energy from the high-voltage energy storage capacitor to the inverter's DC bus is terminated, preventing the boost circuit from being overloaded and damaged due to providing energy to the inverter. The boost circuit only charges the high-voltage energy storage capacitor, thus protecting the boost circuit. Therefore, the boost circuit used in this invention can be constructed from low-power semiconductor devices and a transformer, greatly reducing costs. Furthermore, its boost operation is achieved through self-oscillation boosting, requiring no additional control. When a grid voltage dip occurs, the process proceeds by controlling the switching action of the bidirectional switching device to maintain the inverter's DC bus voltage at a predetermined value, ensuring stable operation of the inverter during the voltage dip.

[0076] Reference Figure 3 As shown in the embodiment of this application, by controlling the switching action of the bidirectional switching device, the energy of the high-voltage energy storage capacitor is transferred to the DC bus of the frequency converter to stabilize its voltage. Specifically, the following steps are included:

[0077] Step S510: Use the DC bus voltage value under normal operating conditions of the frequency converter as the setpoint for the duty cycle PI regulator of the bidirectional switching device, and calculate the difference between it and the current DC bus voltage value of the frequency converter to obtain the DC bus voltage deviation value.

[0078] Specifically, the DC bus voltage value under normal operating conditions of the frequency converter is used as the setpoint for the duty cycle PI regulator of the bidirectional switching device. ， Current DC bus voltage value of frequency converter u dc Perform the calculation, and u dc The difference is calculated to obtain the DC bus voltage deviation value.

[0079] Step S520: Use the DC bus voltage deviation value as the input to the duty cycle PI regulator of the bidirectional switching device, and use the output value of the PI regulator as the PI control term for the on-time duty cycle of the bidirectional switching device. D 1.

[0080] Because the inverter's DC bus capacitor stores relatively little energy after a voltage sag, the DC bus voltage will drop rapidly under load. Traditional PI control alone has poor dynamic performance and easily triggers the inverter's DC bus undervoltage protection. To improve the speed of the voltage regulation device, this invention employs feedforward control, calculating the feedforward term of the bidirectional switching device's duty cycle based on the sampled two-step DC bus voltage values. D 2.

[0081] Step S530: Using feedforward control, the sampled DC bus voltage values ​​of the two-step inverter are calculated to obtain the feedforward term for the duty cycle of the bidirectional switching device. D 2.

[0082] Furthermore, the feedforward term of the duty cycle D 2 is: , In the formula, The sampling period is This is the DC bus voltage value from the previous sampling period. For the current sampling period, u set This is the DC bus voltage threshold of the frequency converter.

[0083] Step S540: Turn on the duty cycle PI control term of the bidirectional switching device. D 1 and feedforward terms D 2. By superimposing the values, the duty cycle D of the bidirectional switching device can be obtained.

[0084] Furthermore, the duty cycle of the bidirectional switching device D for: .

[0085] Step S550: The control module generates PWM pulses according to the duty cycle of the bidirectional switching device, drives the bidirectional switching device to perform chopping and voltage reduction control, and realizes the transfer of energy from the high-voltage energy storage capacitor to the DC bus of the frequency converter.

[0086] Figure 4 This is a comparison of voltage change curves during the operation of a voltage sag mitigation device for a 380V 150kW frequency converter. The upper graph shows the DC bus voltage change curve of the frequency converter, and the lower graph shows the high-voltage energy storage capacitor voltage change curve. This reflects the dynamic voltage relationship between the "high-voltage energy storage capacitor" and the "DC bus of the frequency converter," corresponding to the process of the high-voltage energy storage capacitor discharging and providing power to the DC bus of the frequency converter. Figure 4 As can be seen, when a voltage sag occurs in the power grid, the high-voltage energy storage capacitor releases energy to maintain the DC bus voltage stability through the step-down circuit. The decrease in the energy storage capacitor voltage and the stability of the DC bus voltage complement each other, verifying the effectiveness of the device in mitigating voltage sags. Typically, voltage sags are less than 0.5s, and as the curve shows, the device provided in this invention can meet the requirements for voltage sag mitigation.

[0087] Numerous specific details are set forth in the specification provided herein. However, it will be understood that embodiments of the invention may be practiced without these specific details. In some instances, well-known methods, structures, and techniques have not been shown in detail so as not to obscure the understanding of this specification.

[0088] Furthermore, those skilled in the art will understand that although some embodiments described herein include certain features found in other embodiments but not others, combinations of features from different embodiments are also within the scope of protection of this invention and form different embodiments. For example, in the embodiments described above, those skilled in the art can use them in combination based on known technical solutions and the technical problems to be solved by this application.

[0089] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-described technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A voltage sag mitigation device for frequency converters, characterized in that: The device includes: A boost circuit, whose input terminal is selectively connected to the AC input side of a frequency converter, includes a circuit structure with a self-oscillating boost topology and an isolation transformer, used to boost, isolate and transform grid power and store the boosted power. A step-down circuit includes a bidirectional switching device, the input of which is connected to the output of the boost circuit, and the output of which is connected to the DC bus of the frequency converter. The step-down circuit is configured to: when a voltage sag occurs in the power grid, step down the electrical energy stored in the boost circuit and deliver it to the DC bus of the frequency converter; when the voltage sag in the power grid recovers, cut off the reverse charging circuit from the DC bus to the boost circuit through the bidirectional switching device. The control module is connected to the gate of the bidirectional switching device in the buck circuit, the voltage sampling point of the inverter DC bus, the voltage sampling point of the three-phase input terminals of the inverter, and the current sampling point of the filter inductor of the buck circuit, respectively, to collect electrical signals at the corresponding locations. The control module determines voltage sag and voltage sag recovery events based on the collected three-phase grid voltage signals and inverter DC bus voltage signals, and then controls the conduction and cutoff of the bidirectional switching device in the buck circuit to maintain the stability of the inverter DC bus voltage.

2. The inverter voltage sag mitigation device according to claim 1, characterized in that: The boost circuit includes boost diodes D7, D8, D9, and D1. 10 D 11 Small-capacity boost capacitors C2, C3, C4, and C5; high-voltage energy storage capacitor C6; isolation transformer T. The boost diodes D7, D8, D9, and D 10 D 11 It is cross-cascaded with small-capacity boost capacitors C2, C3, C4, and C5 to form a 2x power frequency self-oscillating boost topology; The isolation transformer T is connected in series between the input terminal of the boost circuit and the power grid to achieve isolation between the boost circuit, the high-voltage energy storage capacitor C6 and the power grid; The high-voltage energy storage capacitor C6 is used to store electrical energy boosted by the boost circuit, and completes energy storage through a low-power slow charging mode when the power grid is supplying power normally.

3. The inverter voltage sag mitigation device according to claim 1, characterized in that: The step-down circuit includes a bidirectional switching device, a filter inductor L1, and a freewheeling diode D. 12 The bidirectional switching device consists of switching transistors S7 and S8 and an anti-parallel diode, with S7 and S8 connected via a common drain. The common drain terminal is connected to one end of the filter inductor L1. One source terminal of the bidirectional switching device is connected to the output terminal of the high-voltage energy storage capacitor C6 in the boost circuit, and the other source terminal is connected to the DC bus N terminal of the frequency converter. The other end of the filter inductor L1 is connected to the freewheeling diode D. 12 The anode of the inverter is connected to the DC bus P terminal of the inverter; the freewheeling diode D 12 The cathode is connected to the N terminal of the DC bus of the frequency converter.

4. The inverter voltage sag mitigation device according to claim 1, characterized in that, The control module includes a voltage sag and recovery judgment module, a duty cycle adjustment module, and a drive module; wherein... The voltage sag and recovery judgment module is used to judge voltage sag and voltage sag recovery events based on the collected three-phase grid voltage signal and inverter DC bus voltage signal, and output the drive enable signal of the bidirectional switching device. The duty cycle adjustment module is used to generate a control duty cycle signal for the bidirectional switching device based on the drive enable signal of the bidirectional switching device and the DC bus voltage deviation value of the frequency converter. The drive module is used to amplify the duty cycle signal and drive the bidirectional switching device to perform on / off actions, thereby realizing the chopping and voltage reduction function and keeping the DC bus voltage of the frequency converter stable.

5. A control method based on the inverter voltage sag mitigation device according to any one of claims 1-4, characterized in that, The method includes: The three-phase power grid voltage and the inverter DC bus voltage are collected; the inverter DC bus voltage value is stored as two sampling data, namely the current cycle sampling value and the previous cycle sampling value; Calculate the grid voltage value in a two-phase orthogonal coordinate system based on the three-phase grid voltage; Calculate the grid voltage amplitude based on the grid voltage value in a two-phase orthogonal coordinate system; Based on the grid voltage amplitude, a grid voltage amplitude weighting coefficient is introduced. k 1 and DC bus voltage weighting factor k 2. Calculate the grid voltage sag judgment value by combining the preset three-phase voltage amplitude threshold of the power grid and the DC bus voltage threshold of the frequency converter; Based on the calculated grid voltage sag judgment value, determine whether a grid voltage sag has occurred or whether a sag has recovered. If a voltage sag is detected, the energy of the high-voltage energy storage capacitor is transferred to the inverter's DC bus to stabilize its voltage by controlling the switching action of the bidirectional switching device, ensuring the stable operation of the inverter during the voltage sag. If the grid voltage sag is detected to have recovered, the bidirectional switching device is turned off, and the three-phase grid voltage and the inverter's DC bus voltage are continuously monitored to continuously judge grid voltage sag events.

6. The control method for a frequency converter voltage sag mitigation device according to claim 5, characterized in that, The power grid voltage amplitude e mag for: ， in, , In the formula, e α , e β This is the grid voltage value. e a , e b , e c This refers to the three-phase voltage of the power grid.

7. The control method for a frequency converter voltage sag mitigation device according to claim 6, characterized in that, The grid voltage sag judgment value J f for: ， In the formula, u dc This is the DC bus voltage. k 1 is the power grid voltage amplitude weighting coefficient, which takes a value in the range of [0,1]. Its value depends on the balance and harmonic content of the local power grid. k 2 is the DC bus voltage weighting coefficient, which takes a value in the range of [0,1]. Its value depends on the DC bus fluctuation value of the frequency converter used. e set This is the threshold value for the three-phase voltage amplitude of the power grid, and its value is the average amplitude of the voltage of the power grid in the area where it is used; u set This is the DC bus voltage threshold of the frequency converter, and its value depends on the DC bus undervoltage protection value of the frequency converter used.

8. The control method for a frequency converter voltage sag mitigation device according to claim 7, characterized in that, The method for determining grid voltage sags and voltage sag recovery is as follows: If the grid voltage sag judgment value J f If the value is ≥0.7, it is determined that a voltage dip has occurred in the mains. If the grid voltage sag judgment value J f If the value is ≤0.5, it is determined to be a recovery of the grid voltage sag.

9. The control method for a frequency converter voltage sag mitigation device according to claim 5, characterized in that, The method of transferring energy from the high-voltage energy storage capacitor to the DC bus of the frequency converter to stabilize its voltage by controlling the switching action of the bidirectional switching device includes: The DC bus voltage value under normal operating conditions of the inverter is used as the setpoint for the duty cycle PI regulator of the bidirectional switching device, and the difference between it and the current DC bus voltage value of the inverter is used to obtain the DC bus voltage deviation value. The DC bus voltage deviation is used as the input to the duty cycle PI regulator of the bidirectional switching device, and the output value of the PI regulator is used as the PI control term for the duty cycle of the bidirectional switching device. D 1; By employing feedforward control, the sampled DC bus voltage values ​​of the two-step inverter are used to calculate the feedforward term for the duty cycle of the bidirectional switching devices. D 2; Turn on the duty cycle PI control term of the bidirectional switching device. D 1 and feedforward terms D 2. By superimposing, the duty cycle of the bidirectional switching device is obtained. D : The control module is based on the duty cycle of the bidirectional switching device. D PWM pulses are generated to drive bidirectional switching devices for chopping and voltage reduction control, thereby transferring energy from the high-voltage energy storage capacitor to the DC bus of the frequency converter.

10. The control method for a frequency converter voltage sag mitigation device according to claim 9, characterized in that, The feedforward term of the duty cycle D 2 is: ， In the formula, The sampling period is This is the DC bus voltage value from the previous sampling period. This is the DC bus voltage value for the current sampling period. u set This is the threshold value for the three-phase voltage amplitude of the power grid.

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