Improved three-level indirect matrix converter and control method thereof

Abstract

The invention discloses an improved three-level indirect matrix converter with low-voltage ride-through capability and a control method thereof, the converter is composed of an input filter inductor LF, a bidirectional current source rectifier, a T-type three-level inverter and a direct-current link filter capacitor, and through topological structure optimization, the three-level indirect matrix converter with low-voltage ride-through capability is obtained. An input filter capacitor is configured between a rectifier stage and an inverter stage to construct an equivalent voltage source, and the technical challenge that an existing three-level indirect matrix converter cannot maintain load power supply after power grid isolation is solved. According to the low-voltage ride-through control method, additional energy storage elements or hardware circuits do not need to be added, the converter can be controlled to be automatically isolated from the power grid when the low-voltage fault occurs, dynamic energy management is carried out based on inertial energy in a motor load system, and the system is kept to operate continuously within a short time until the fault ends; the motor can stably operate and keep the flux linkage constant during the fault period, and the motor can quickly recover normal operation after the fault is cleared.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power supply, in particular to an improved three-level indirect matrix converter with low voltage ride-through capability and a control method thereof. BACKGROUND

[0002] Compared with the traditional back-to-back converter, the matrix converter cancels the DC energy storage link, has higher power density and longer service life, and has gradually become a more attractive solution in the field of variable frequency speed drive. Among them, the three-level matrix converter can improve power quality, generate high-quality output waveform, reduce switching stress and common-mode voltage, and is widely concerned.

[0003] As patent application CN111277147A provides a three-level indirect matrix converter topology that reduces switching devices, which realizes high-quality input and output performance and power factor correction function through topology reconstruction, while eliminating the need for clamping circuit and significantly reducing the number of power devices. This design combines low switching loss modulation strategy and structure simplification scheme to achieve high efficiency, low du / dt and strong EMI suppression capability, and fully improves the system reliability. In view of the defects of the traditional three-level indirect matrix converter, such as input and output waveform distortion, high power device voltage stress, large loss, du / dt surge and prominent EMI problem, this patent systematically solves the problems of complex structure and control.

[0004] Patent application CN111146955A provides a T-type three-level indirect matrix converter topology structure, which realizes sinusoidal input and output, bidirectional energy flow and controllable input power factor through multi-level modulation, significantly reduces the harmonic content of output voltage and current and the voltage stress of inverter stage switching devices. This topology combines switching frequency optimization strategy to reduce switching loss, and relies on the design of no DC energy storage link to effectively improve the power density and reliability of AC-AC converter. In view of the defects of the traditional two-level indirect matrix converter, such as poor output waveform quality, high equivalent switching frequency, low efficiency, large switching device voltage stress and urgent EMI suppression requirement, the topology structure proposed by this invention systematically solves the above technical bottlenecks.

[0005] Patent application CN107204714A provides a three-level indirect matrix converter and a control method, which generates a specific third harmonic by introducing a harmonic injection circuit to ensure that the neutral point average current is zero, realizes neutral point voltage self-balancing, and significantly improves the input and output waveform quality. This method is based on the input side power factor correction requirement and the neutral point potential balance control of the inverter, generates the expected third harmonic injection inductance current through the harmonic injection circuit, not only effectively solves the problem of limited input reactive power control range, but also eliminates the constraint that the carrier layer superimposed modulation of the rectifier stage and the inverter stage must be strictly synchronized, thereby reducing the system control difficulty.

[0006] However, the inherent advantages of matrix converters make it difficult to support fault ride-through. Due to the lack of energy storage elements, any change in the grid side will directly affect the output load. If the fault ride-through method is not actively designed, the fault will cause the motor operating state to be unstable, the flux linkage to be reduced, and even cause step-out. This will seriously affect the recovery ability of the motor after the fault is cleared, and may need to be completely stopped and restarted. SUMMARY

[0007] The present application aims at the problem of insufficient grid-side disturbance rejection and lack of fault ride-through caused by small energy storage elements in traditional three-level indirect matrix converters. A low-voltage fault ride-through scheme without additional hardware is proposed. The scheme optimizes the converter structure and combines a dynamic energy management strategy based on the inertia energy of the motor load system to automatically isolate the grid and maintain the flux linkage stable when a low-voltage fault occurs in any phase. Specifically, the dynamic energy management supports motor loss and load energy demand and isolates the fault grid, significantly improving the ability of the motor to quickly recover operation after the fault is cleared, and avoiding system shutdown and restart.

[0008] To achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0009] An improved three-level indirect matrix converter with low-voltage ride-through capability mainly includes four parts: 1) input filter inductor L F ; 2) DC link filter capacitor; 3) bidirectional current source rectifier; 4) T-type three-level inverter.

[0010] Further, the input filter inductor L F , which mainly functions to filter out the switching frequency and its multiple frequency harmonics generated by the switching devices in the converter, reduce conducted electromagnetic interference, reduce input current ripple, and improve current waveform total harmonic distortion. When the system starts or the grid is in transient state (such as voltage surge / drop), the current change rate di / dt is suppressed to protect the rectifier stage switching devices from transient overcurrent damage. Together with the DC link filter capacitor, an LC filter is formed to achieve input side unity power factor under double closed-loop control (voltage outer loop + current inner loop).

[0011] Further, the DC link filter capacitor, which mainly functions to provide temporary DC voltage support for the inverter stage as an equivalent voltage source during grid isolation (such as low-voltage fault).

[0012] Further, the bidirectional current source rectifier mainly functions to realize bidirectional transmission of electric energy, rectifies at the input end of the converter, converts alternating current 1 into industrial-frequency six-pulse pulsating direct current 1, and can also work as an inverter to convert direct current 1 into alternating current 1 to support different working modes (such as fault ride-through or power regulation), thereby improving the flexibility and stability of the system.

[0013] Further, the T-type three-level inverter mainly functions to convert industrial-frequency six-pulse pulsating direct current 1 into alternating current 2, adopts a T-type three-level topology structure, realizes efficient electric energy conversion through three levels (positive voltage, negative voltage and zero voltage), thereby reducing harmonics and improving power quality and conversion efficiency.

[0014] Further, the bidirectional current source rectifier is composed of six commutating switches S i+ / - (i=a, b, c), three line-frequency bidirectional switches S iy (i=a, b, c) and four high-frequency switches S i+ / - (i=p, n); in a fault state, S k+ (k=p, n) are kept on, S k- (k=p, n) are turned off, and the commutating switches S i+ / - and the bidirectional switches S iy (i=a, b, c) are turned off; when the direct current link filter capacitor voltage exceeds the maximum line voltage amplitude of the fault power grid, the inverter in the rear stage realizes isolation from the power grid.

[0015] Further, the T-type three-level inverter is composed of three T-type branches composed of switches S i+ , S i- , S iy (i=r, s, t); in a fault state, the direct current link filter capacitor serves as an equivalent voltage source, the T-type three-level inverter collects system inertia energy and supports short-time operation of the motor.

[0016] Another object of the present application is to provide an improved control method of a three-level indirect matrix converter, which is a low-voltage fault ride-through scheme without additional hardware measures; through topology optimization, the input filter capacitor is configured between the rectification stage and the inversion stage to construct an equivalent voltage source.

[0017] Further, an improved control method of a three-level indirect matrix converter mainly includes three steps:

[0018] 1) Power grid fault detection: detect abnormal power grid voltage and activate the fault ride-through enable flag.

[0019] 2) System reconfiguration: disable the front-stage rectifier switches and bidirectional switches, isolate the back-stage inverter from the grid, and switch the control architecture from the speed control loop to the DC-link filter capacitor voltage control loop. At this stage, the front-stage rectifier works in a similar way to a three-phase diode uncontrolled rectifier, and the bidirectional switches are equivalent to back-to-back blocking diodes. The improved three-level indirect matrix converter circuit structure is equivalent to a conventional T-type three-level inverter, and the DC-link filter capacitor provides a DC voltage source for the inverter. When the DC-link filter capacitor voltage is higher than the maximum line voltage amplitude of the fault grid, the back-stage inverter is isolated from the grid through the front-stage rectifier diodes.

[0020] 3) Continuous monitoring: during the fault ride-through process, the grid voltage is monitored in real time, and after the grid-side fault is eliminated, the front-stage rectifier modulation is restored and the speed control loop is switched back to drive the motor to resume normal operation.

[0021] Further, the grid fault detection can be achieved by detecting the change of each sequence component to determine whether a grid fault has occurred. When an asymmetric fault occurs, the decrease of positive sequence component or the appearance of negative / zero sequence component indicates that a grid fault has occurred; while in the case of a symmetric fault, only the positive sequence component changes. Although the probability of asymmetric fault is much higher than that of symmetric fault, considering the comprehensiveness of detection, the present application uses the decrease of positive sequence component as the fault judgment standard.

[0022] Further, the low-voltage fault ride-through scheme achieves energy transfer from motor inertia to capacitor by adjusting the vector control to achieve fault ride-through operation, thereby ensuring the stability of the flux linkage control while replacing the speed control loop with a capacitor voltage control loop to recover the rotational kinetic energy from the motor and help the motor quickly recover normal operation after the fault is cleared.

[0023] Further, the low-voltage fault ride-through scheme, after the grid fault occurs, the improved three-level indirect matrix converter achieves isolation of the back-stage circuit from the grid by closing the rectifier stage switches, and converts the converter into a T-type three-level inverter supported by the filter capacitor as the voltage source to support the motor operation. Under this scheme, the converter cannot exchange active power with the motor. The time range of low-voltage fault ride-through is related to the motor speed, moment of inertia and load torque at the time of fault. Different motor operating modes (load types) will produce different ride-through characteristics. When the motor is in motoring mode (load torque is positive), the speed basically presents a linear downward trend during the ride-through period, and the ride-through time is limited by the speed drop space and slope, and the ride-through process must be ended before the speed approaches 0. When the motor is in generating mode (load torque is negative), the speed basically presents a linear upward trend during the ride-through period, and the ride-through time is limited by the speed rise space and slope. In this case, the speed rise must be strictly controlled to prevent the motor from losing stability or even causing irreversible damage to its mechanical structure.

[0024] Compared with the prior art, the application has the beneficial effects that:

[0025] (1) Through the improved three-level indirect matrix converter with low voltage ride-through capability in the application, during the fault period, the rear-stage inverter is isolated from the power grid, and the DC link filter capacitor configured can provide a DC voltage source for the inverter to support the motor for a short time to support low voltage ride-through implementation.

[0026] (2) Through the low voltage fault ride-through scheme in the application, the motor is in a stable controlled state during the fault ride-through period, actively isolates the power grid, maintains the flux constant, ensures the stability of the stator current and the smoothness of the speed, and can quickly and stably restore the motor operation after the fault is cleared, effectively inhibiting the negative impact of low voltage fault on the motor operation; at the same time, the scheme does not additionally add hardware facilities, and still maintains the high power density advantage of the matrix converter.

[0027] (3) Compared with patent application CN111277147A which focuses on simplifying the number of switches of the three-level indirect matrix converter, patent application CN111146955A which focuses on the T-type three-level indirect matrix converter topology and control scheme, and patent application CN107204714A which focuses on the neutral point voltage balance and grid-side power quality optimization of the three-level indirect matrix converter, the application takes low voltage fault ride-through of the three-level indirect matrix converter as the core target, and realizes fault ride-through without adding additional hardware by improving the circuit topology of the three-level indirect matrix converter and the dynamic energy management strategy of the motor load system, thereby significantly improving the operation robustness and continuity of the system under power grid fault. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 It is a schematic diagram of the improved three-level indirect matrix converter with low voltage ride-through capability in the application.

[0029] Figure 2 It is a schematic diagram of the system power flow in the application.

[0030] Figure 3 (a) is a system control schematic diagram of the application under normal operating conditions; (b) is a system control schematic diagram of the application under low voltage fault operating conditions. DETAILED DESCRIPTION

[0031] The improved three-level indirect matrix converter with low voltage ride-through capability and the control method thereof described in the application will be further described below in combination with the drawings and specific embodiments. The present embodiment is implemented on the premise of the technical solution of the application, and gives a detailed implementation manner and specific operation process, but the protection scope of the application is not limited to the following examples.

[0032] Please refer to Figure 1 , the main circuit topology of a frequency selective radio frequency power supply is shown, including input filter inductor L F , DC link filter capacitor, bidirectional current source rectifier and T-type three-level inverter.

[0033] Specifically, the input filter inductor L F is responsible for filtering out the switching frequency and its harmonic harmonics generated by the switching device in the converter, reducing the input current ripple, and suppressing the current rate di / dt when the system starts or the power grid is transient (such as voltage surge / drop).

[0034] Specifically, the DC link filter capacitor is responsible for providing temporary DC voltage support during grid isolation (such as low voltage fault), providing an equivalent voltage source for the inverter stage.

[0035] Specifically, the bidirectional current source rectifier is composed of six commutating switches S i+ / - (i=a, b, c), three line frequency bidirectional switches S iy (i=a, b, c) and four high-frequency switches S i+ / - (i=p, n), responsible for bidirectional power transmission, converting alternating current 1 into six-pulse pulsed direct current 1, and also working as an inverter when needed, converting direct current 1 into alternating current 1 to support different working modes.

[0036] Specifically, the T-type three-level inverter is composed of three T-type branches composed of switches S i+ , S i- , S iy (i=r, s, t), responsible for converting six-pulse pulsed direct current 1 into alternating current 2, using a T-type three-level topology structure, through three levels (positive voltage, negative voltage and zero voltage) to realize efficient power conversion, thereby reducing harmonics, improving power quality and conversion efficiency.

[0037] Figure 2 The power flow of the improved three-level indirect matrix converter-motor system is shown. When operating normally, the improved three-level indirect matrix converter absorbs power from the power grid to power the load. Part of the energy is continuously lost in the electrical and mechanical systems, and part of the energy is stored as rotor rotational kinetic energy. At this time, the expression of system power balance is:

[0038]

[0039] When the power grid fails, the front-end rectifier of the improved three-level indirect matrix converter will be disconnected from the power grid, and the power transmission will be Figure 2The dashed line is interrupted, and the expression of system power balance is changed to:

[0040]

[0041] At this point, the system recovers rotational kinetic energy from the motor, using it to maintain the filter capacitor voltage above the maximum line voltage of the grid. The improved three-level indirect matrix operates as a conventional T-type three-level inverter to maintain a constant motor flux.

[0042] like Figure 3 As shown in (a) and (b), the control process of the improved three-level indirect matrix converter under normal operating conditions and low voltage fault conditions is as follows:

[0043] During normal operation, the magnetic linkage is sm Control, the speed is controlled by i st Control. When a grid fault occurs, flux control remains unaffected to maintain flux linkage, and the speed control loop is replaced with a capacitor voltage control loop to recover rotational energy from the motor. In the described low-voltage fault ride-through scheme, vector control is modified to enable ride-through operation, thereby transferring energy from motor inertia to the capacitor.

[0044] Furthermore, the specific implementation steps of the low voltage fault ride-through solution are as follows:

[0045] 1) Grid fault detection: Detects grid voltage anomalies and activates the fault ride-through enable flag.

[0046] 2) System Reconfiguration: Deactivate the front-stage rectifier switches and bidirectional switches, isolating the downstream inverter from the grid. The control architecture is switched from a speed control loop to a DC link filter capacitor voltage control loop. During this phase, the front-stage rectifier operates similarly to a three-phase diode uncontrolled rectifier, and the bidirectional switches act as back-to-back blocking diodes. The improved three-level indirect matrix converter circuit structure is equivalent to a conventional T-type three-level inverter, with the DC link filter capacitor providing the DC voltage source for the inverter. When the DC link filter capacitor voltage exceeds the maximum line voltage amplitude of the faulty grid, the downstream inverter is isolated from the grid via the front-stage rectifier diodes.

[0047] 3) Continuous monitoring: During the fault ride-through process, the grid voltage is monitored in real time. After the grid-side fault is eliminated, the front-stage rectifier modulation is promptly restored and the speed control loop is switched back to drive the motor to resume normal operation.

[0048] In the formula, the power grid fault detection can be realized by detecting the change of each sequence component. When an asymmetric fault occurs, the decrease of positive sequence component or the appearance of negative / zero sequence component indicates that the power grid fault occurs; while for a symmetric fault, only the change of positive sequence component is shown. Although the probability of asymmetric fault is much greater than that of symmetric fault, considering the comprehensiveness of detection, the decrease of positive sequence component is adopted as the fault judgment standard in the present application.

[0049] The basic principles and main features of the present application and the advantages of the present application are shown and described above. It should be understood by those skilled in the art that the present application is not limited by the above-mentioned embodiments, and the above-mentioned embodiments and descriptions in the specification are only to illustrate the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the present application. The scope of protection of the present application is defined by the appended claims and their equivalents.

Claims

1. An improved three-level indirect matrix converter, characterized in that: It mainly includes four parts: (1) Input filter inductor L F ; (2) DC link filter capacitor; (3) bidirectional current source rectifier; (4) T-type three-level inverter.

2. The improved three-level indirect matrix converter according to claim 1, characterized in that: The bidirectional current source rectifier consists of six commutation switches S i+ / - (i=a, b, c), three line frequency bidirectional switches S iy (i=a, b, c) and four high-frequency switches S i+ / - (i=p, n); in the fault state, maintain S k+ (k=p,n) conduction, S k- (k=p, n) turns off and closes the reversing switch S i+ / - and bidirectional switch S iy (i=a, b, c); when the DC link filter capacitor voltage exceeds the maximum line voltage amplitude of the faulty grid, the subsequent inverter is isolated from the grid.

3. The improved three-level indirect matrix converter according to claim 1, characterized in that: The T-type three-level inverter includes a switch S i+ , S i- , S iy The system consists of three T-shaped branches consisting of (i=r, s, t); in the fault state, the DC link filter capacitor acts as an equivalent voltage source, and the T-shaped three-level inverter collects the system inertia energy and supports the motor to run for a short time.

4. A control method for an improved three-level indirect matrix converter, characterized in that: Through topology optimization, the input filter capacitor is configured between the rectifier stage and the inverter stage to build an equivalent voltage source.

5. The control method of the improved three-level indirect matrix converter according to claim 4, characterized in that: It mainly includes three steps: (1) Grid fault detection: Detects grid voltage anomalies and activates the fault ride-through enable flag; (2) System reconfiguration: disable the front-stage rectifier switch and the bidirectional switch, isolate the rear-stage inverter from the grid, and switch the control architecture from the speed control loop to the DC link filter capacitor voltage control loop; in this stage, the front-stage rectifier works in a manner similar to a three-phase diode uncontrolled rectifier, and the bidirectional switch is equivalent to a back-to-back blocking diode; the circuit structure of the improved three-level indirect matrix converter is equivalent to a conventional T-type three-level inverter, and the DC link filter capacitor provides a DC voltage source for the inverter; when the DC link filter capacitor voltage is higher than the maximum line voltage amplitude of the fault grid, the rear-stage inverter is isolated from the grid through the front-stage rectifier diode; (3) Continuous monitoring: During the fault ride-through process, the grid voltage is monitored in real time. After the grid-side fault is eliminated, the front-stage rectifier modulation is promptly restored and the speed control loop is switched back to drive the motor to resume normal operation.

6. The control method of the improved three-level indirect matrix converter according to claim 5, characterized in that: The live grid fault detection determines whether a grid fault has occurred by detecting changes in each sequence component. When an asymmetric fault occurs, a decrease in the positive sequence component or the appearance of a negative / zero sequence component indicates that a grid fault has occurred. A symmetric fault, on the other hand, manifests itself only as a change in the positive sequence component. Although the probability of an asymmetric fault is much greater than that of a symmetric fault, considering the comprehensiveness of the detection, the decrease in the positive sequence component is used as the fault judgment criterion.

7. The control method of the improved three-level indirect matrix converter according to claim 4, characterized in that: Fault ride-through operation is achieved by adjusting vector control, thereby realizing energy transfer from motor inertia to the capacitor. When a grid fault occurs, the flux control remains unchanged to ensure flux stability. At the same time, the speed control loop is replaced with a capacitor voltage control loop to recover rotational kinetic energy from the motor, helping the motor to quickly resume normal operation after the fault is cleared.

8. The control method of the improved three-level indirect matrix converter according to claim 4, characterized in that: After a grid fault occurs, the improved three-level indirect matrix converter isolates the subsequent circuit from the grid by turning off the rectifier stage switch, and transforms the converter into a T-type three-level inverter with the filter capacitor as the voltage source to support the motor operation.

9. The control method of the improved three-level indirect matrix converter according to claim 8, characterized in that: When the motor is in electric mode, that is, the load torque is positive, the speed basically decreases linearly during the ride-through period. The ride-through time is subject to the space and slope of the speed drop, and the ride-through process must be completed before the speed approaches 0; when the motor is in power generation mode, that is, the load torque is negative, the speed basically increases linearly during the ride-through period, and the ride-through time is subject to the space and slope of the speed increase.

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