Ac / dc conversion system dc midpoint voltage balancing control apparatus and method

Abstract

This invention relates to a DC midpoint voltage balancing control device and method for AC / DC conversion systems. The device includes an inductor L, a capacitor C, a pair of series-connected semiconductor switches S1 and S2, and a control module. The capacitor C and the series circuit of semiconductor switches S1 and S2 are connected in parallel with the positive and negative terminals of the DC bus of the AC / DC conversion system. One end of the inductor L is connected to the midpoint of the DC bus, and the other end is connected to the midpoint of the series circuit of semiconductor switches S1 and S2. The control module generates a pair of complementary pulse control signals based on the ratio between the real-time detected DC bus midpoint voltage and the DC bus positive and negative terminal voltages, controlling the complementary conduction of semiconductor switches S1 and S2 to gradually adjust and balance the DC bus midpoint voltage in a set fixed step size. This invention requires few components and has a simple control method; only one hardware circuit is needed to achieve midpoint voltage balancing for single or multiple parallel AC / DC conversion devices.

Description

Technical Field

[0001] This invention relates to the field of DC midpoint voltage balancing technology for AC / DC conversion systems, and in particular to a DC midpoint voltage balancing control device and method for AC / DC conversion systems. Background Technology

[0002] Three-level converters boast numerous advantages, including large capacity, high voltage withstand rating, low output current harmonic content, and low switching losses, making them an indispensable power electronic device in modern power systems. However, due to their numerous components and complex control strategies, three-level converters also suffer from a problem unavoidable compared to two-level converters—the neutral point voltage balance issue. Maintaining neutral point voltage balance is crucial during the operation of a three-level AC / DC converter. Fluctuations in the neutral point voltage can distort the output voltage and current waveforms, increase low-order harmonics at the output, severely degrade power quality, and cause excessively high voltages across the switching devices, reducing their lifespan or even causing damage.

[0003] There are currently many methods for balancing midpoint voltage, mainly including the following:

[0004] 1. Improve the modulation algorithm: Select a reasonable vector and its action time to balance the amount of electricity flowing into and out of the midpoint voltage within one or several control cycles, thereby achieving the purpose of controlling the midpoint voltage. For example, CN119483258A discloses a midpoint voltage balance control system and method for a three-level Buck DC-DC converter. This method uses a single-stage three-level Buck circuit, but relies on a dual closed-loop PI controller to modulate the double edges of the PWM wave. The control algorithm is complex and requires the acquisition of many circuit parameters. Such methods are difficult to control the midpoint voltage balance in scenarios with large modulation intensities or low power factors.

[0005] 2. Add switching device hardware circuits across the DC side capacitors to balance the voltage of the upper and lower DC side capacitors through the operation of the switching devices; for example, CN112909919B discloses a two-stage three-level three-phase four-wire energy storage converter midpoint voltage balancing method and system, which adopts a cascaded topology structure with a three-level Buck / Boost circuit as the front stage and a T-type three-level inverter circuit as the rear stage. This method requires the introduction of multiple sets of switching transistors, inductors and complex filtering structures, resulting in high hardware costs and complex control algorithms;

[0006] 3. The DC side uses multiple independent DC power supplies to directly control the midpoint voltage at a fixed value; this method requires a large number of additional hardware circuits, which increases the circuit cost.

[0007] Therefore, adopting an economical, practical, and simple-to-control method is of great significance for balancing the midpoint voltage of the AC / DC side. Summary of the Invention

[0008] The technical problem to be solved by the present invention is to provide a DC midpoint voltage balance control device and method for AC / DC conversion systems, which can be applied to single-unit or multi-unit parallel AC / DC conversion devices, requires few components, and has a simple control method.

[0009] The technical solution adopted by this invention to solve its technical problem is as follows: An AC / DC conversion system DC midpoint voltage balance control device is provided, comprising a first semiconductor switch, a second semiconductor switch, an inductor, a capacitor, and a control module; one end of the inductor is connected to the midpoint of the DC bus of the AC / DC conversion system, and the other end is electrically connected to the source of the first semiconductor switch and the drain of the second semiconductor switch; the drain of the first semiconductor switch is electrically connected to the positive terminal of the DC bus and one end of the capacitor, and the source of the second semiconductor switch is electrically connected to the negative terminal of the DC bus and the other end of the capacitor; the control module is used to generate pulse control signals based on the real-time acquired DC bus midpoint voltage and DC bus positive and negative terminal voltages to control the complementary conduction of the first semiconductor switch and the second semiconductor switch, thereby gradually adjusting the DC bus midpoint voltage balance of the AC / DC conversion system with a fixed step size.

[0010] Furthermore, the step of generating pulse control signals based on the real-time acquired DC bus midpoint voltage and DC bus positive and negative pole voltages includes:

[0011] The ratio between the midpoint voltage of the DC bus and the positive and negative pole voltages of the DC bus is used as the modulation signal.

[0012] The amplitude of the modulation signal at the current moment is calculated based on the real-time acquisition of the DC bus midpoint voltage and the DC bus positive and negative pole voltages.

[0013] The amplitude of the modulated signal at the current moment is compared with the midpoint of the amplitude of the carrier signal, and the target amplitude of the modulated signal at the next moment is adjusted by a fixed step size based on the comparison result.

[0014] The pulse control signal is generated by comparing the target amplitude of the modulation signal with the amplitude of the carrier signal.

[0015] Furthermore, the carrier signal is a triangular carrier signal.

[0016] Furthermore, the pulse control signal includes:

[0017] A first pulse control signal is connected to the gate of the first semiconductor switch. When the triangular carrier signal is at a falling edge and its amplitude is equal to the target amplitude of the modulation signal, the first semiconductor switch is controlled to turn on. When the triangular carrier signal is at a rising edge and its amplitude is equal to the target amplitude of the modulation signal, the first semiconductor switch is controlled to turn off.

[0018] The second pulse control signal is connected to the gate of the second semiconductor switch. When the triangular carrier signal is on the rising edge and its amplitude is equal to the target amplitude of the modulation signal, the second semiconductor switch is turned on. When the triangular carrier signal is on the falling edge and its amplitude is equal to the target amplitude of the modulation signal, the second semiconductor switch is turned off.

[0019] Furthermore, adjusting the target amplitude of the modulated signal at the next moment with a fixed step size based on the comparison result includes:

[0020] If the amplitude of the modulated signal at the current moment is greater than the midpoint of the amplitude of the carrier signal, then the target amplitude of the modulated signal at the next moment is set to the amplitude of the modulated signal at the current moment minus the fixed step size.

[0021] If the amplitude of the modulated signal at the current moment is less than the midpoint of the amplitude of the carrier signal, then the target amplitude of the modulated signal at the next moment is set to the amplitude of the modulated signal at the current moment plus the fixed step size.

[0022] If the amplitude of the modulated signal at the current moment is equal to the midpoint of the amplitude of the carrier signal, then the target amplitude of the modulated signal at the next moment is set to the amplitude of the modulated signal at the current moment.

[0023] Furthermore, the AC / DC conversion system includes several AC / DC converters connected in parallel.

[0024] The present invention also provides a DC midpoint voltage balance control method for an AC / DC conversion system, applied to the device described above, comprising the following steps:

[0025] Collect the DC bus midpoint voltage and DC bus positive and negative terminal voltages of the AC / DC conversion system;

[0026] The ratio between the midpoint voltage of the DC bus and the positive and negative pole voltages of the DC bus is controlled to be one-half.

[0027] Furthermore, controlling the ratio between the DC bus midpoint voltage and the DC bus positive and negative terminal voltages to be one-half includes:

[0028] The ratio between the midpoint voltage of the DC bus and the positive and negative pole voltages of the DC bus is used as the modulation signal.

[0029] The amplitude of the modulation signal at the current moment is calculated based on the real-time acquisition of the DC bus midpoint voltage and the DC bus positive and negative pole voltages.

[0030] The amplitude of the modulated signal at the current moment is compared with the midpoint of the amplitude of the carrier signal, and the target amplitude of the modulated signal at the next moment is adjusted by a fixed step size based on the comparison result.

[0031] By comparing the target amplitude of the modulation signal with the amplitude of the carrier signal, a pulse control signal is generated to control the first semiconductor switch and the second semiconductor switch to conduct complementaryly.

[0032] Furthermore, adjusting the target amplitude of the modulated signal at the next moment with a fixed step size based on the comparison result includes:

[0033] If the amplitude of the modulated signal at the current moment is greater than the midpoint of the amplitude of the carrier signal, then the target amplitude of the modulated signal at the next moment is set to the amplitude of the modulated signal at the current moment minus the fixed step size.

[0034] If the amplitude of the modulated signal at the current moment is less than the midpoint of the amplitude of the carrier signal, then the target amplitude of the modulated signal at the next moment is set to the amplitude of the modulated signal at the current moment plus the fixed step size.

[0035] If the amplitude of the modulated signal at the current moment is equal to the midpoint of the amplitude of the carrier signal, then the target amplitude of the modulated signal at the next moment is set to the amplitude of the modulated signal at the current moment.

[0036] Furthermore, the carrier signal is a triangular carrier signal.

[0037] Furthermore, controlling the first semiconductor switch and the second semiconductor switch to be complementaryly turned on includes:

[0038] When the triangular carrier signal is at its falling edge and its amplitude is equal to the target amplitude of the modulation signal, the first semiconductor switch is turned on and the second semiconductor switch is turned off.

[0039] When the triangular carrier signal is at its rising edge and its amplitude is equal to the target amplitude of the modulation signal, the first semiconductor switch is turned off and the second semiconductor switch is turned on.

[0040] Furthermore, the frequency of the triangular carrier signal is fixed.

[0041] Beneficial effects

[0042] Due to the adoption of the above technical solution, the present invention has the following advantages and positive effects compared with the prior art: The present invention generates pulse control signals based on the real-time collected DC bus midpoint voltage and DC bus positive and negative voltages to control the complementary conduction of semiconductor switches S1 and S2, and gradually adjusts the DC bus midpoint voltage balance of the AC / DC conversion system with a set fixed step size. It can effectively solve the problem of DC side midpoint voltage deviation of AC / DC conversion device caused by AC unbalanced load or DC side positive and negative source load imbalance. Moreover, it has a small number of devices and a simple control method, and is especially suitable for microgrid, energy storage and flexible DC interconnection scenarios built by three-level AC / DC conversion devices, ensuring stable system operation. Attached Figure Description

[0043] Figure 1 This is a schematic diagram of the DC midpoint voltage balancing circuit according to the first embodiment of the present invention;

[0044] Figure 2 This is a schematic diagram of the pulse control signal generation principle according to the first and second embodiments of the present invention;

[0045] Figure 3 This is a flowchart of the DC midpoint voltage balance control algorithm of the first and second embodiments of the present invention. Detailed Implementation

[0046] The present invention will be further illustrated below with reference to specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the teachings of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by the appended claims.

[0047] The first embodiment of the present invention relates to a DC bus midpoint voltage balance control device for an AC / DC conversion system, comprising:

[0048] The hardware circuit, its structure is as follows Figure 1 As shown, it consists of two fully controllable semiconductor switches S1 and S2, one inductor L, and one capacitor C: semiconductor switches S1 and S2 are connected in series and then connected in parallel between the positive terminal P and the negative terminal N of the DC bus of the AC / DC conversion system; capacitor C is connected in parallel with the series circuit of semiconductor switches S1 and S2; one end of inductor L is connected to the midpoint of the series circuit of semiconductor switches S1 and S2, and the other end is connected to the midpoint O of the DC bus of the AC / DC conversion system.

[0049] The control module is based on the real-time detection of the midpoint voltage of the AC / DC conversion system. With DC bus voltage The ratio between them generates a pair of complementary pulse control signals, which are respectively connected to the gates of semiconductor switch S1 and semiconductor switch S2 to control their complementary conduction, so as to balance the DC bus midpoint voltage of the AC / DC conversion system.

[0050] The pulse control signal can be generated through pulse width modulation to control the midpoint voltage. With DC bus voltage The ratio between them is used as the modulating wave U ref It is compared with the triangular carrier signal, and a pulse control signal is generated based on the comparison result.

[0051] More specifically, define Let n be the magnitude of the modulated wave. This represents the amplitude (peak value) of the triangular carrier signal. These are the positive and negative voltages of the DC bus in the AC / DC conversion system. The lower capacitor voltage of the AC / DC conversion system. The fixed step size of the modulation wave is adjusted for each control cycle. This is achieved by adjusting... To adjust for / 2 means that the voltage of the lower capacitor in the AC / DC conversion system can be half of the voltage of the positive and negative poles of the DC bus, thus maintaining the voltage balance at the midpoint of the DC bus in the AC / DC conversion system.

[0052] like Figure 2 As shown, by comparing the modulated wave with the triangular carrier signal in real time, the pulse control signals S1 and S2 can be obtained. The specific generation method is as follows:

[0053] When the triangular carrier wave is on its rising edge and its magnitude is equal to that of the modulating wave, control S2 is turned on and control S1 is turned off. At this time, the inductor L, the semiconductor switch S2, and the lower capacitor of the AC / DC conversion system form a current loop, and the lower capacitor discharges.

[0054] When the triangular carrier wave is at its falling edge and its magnitude is equal to that of the modulating wave, control S1 is turned on and control S2 is turned off. At this time, the inductor L, semiconductor switch S1, capacitor C, and the lower capacitor of the AC / DC conversion system form a current loop, and the lower capacitor is charged.

[0055] Specific control methods are as follows: Figure 3 As shown:

[0056] Step 1: Calculate the magnitude of the modulated wave signal at the current moment. ;

[0057] Step 2: Since the Uref is already labeled between 0 and 1, the triangular carrier amplitude can be set to 1. and Comparison of triangular carrier signals:

[0058] like The modulation wave needs to be gradually reduced, that is... ;

[0059] like The modulation wave needs to be gradually increased, that is... ;

[0060] like It is necessary to control the modulated wave to remain unchanged, that is ;

[0061] Step 3: Calculate the target amplitude of the modulated wave signal at the next moment. The signal is compared with a triangular carrier signal to generate pulse control signals for semiconductor switches S1 and S2, which control their on and off states to balance the DC side midpoint voltage.

[0062] The second embodiment of the present invention relates to a DC midpoint voltage balance control method for an AC / DC conversion system, applied to the device described above, and includes the following steps:

[0063] Acquire the DC bus midpoint voltage and DC bus positive and negative terminal voltages of the AC / DC conversion system;

[0064] A pulse control signal is generated based on the ratio between the DC bus midpoint voltage and the DC bus positive and negative pole voltages to control the complementary conduction of semiconductor switches S1 and S2, so as to balance the DC bus midpoint voltage of the AC / DC conversion system.

[0065] The pulse control signal can be generated through pulse width modulation to control the midpoint voltage. With DC bus voltage The ratio between them is used as the modulating wave U ref It is compared with the triangular carrier signal, and a pulse control signal is generated based on the comparison result.

[0066] More specifically, define Let n be the magnitude of the modulated wave. This represents the amplitude (peak value) of the triangular carrier signal. These are the positive and negative voltages of the DC bus in the AC / DC conversion system. The lower capacitor voltage of the AC / DC conversion system. The fixed step size of the modulation wave is adjusted for each control cycle. This is achieved by adjusting... for / 2 means that the voltage of the lower capacitor in the AC / DC conversion system can be half of the voltage of the positive and negative poles of the DC bus, thus maintaining the voltage balance at the midpoint of the DC bus in the AC / DC conversion system.

[0067] like Figure 2 As shown, by comparing the modulated wave with the triangular carrier signal in real time, the pulse control signals S1 and S2 can be obtained. The specific generation method is as follows:

[0068] When the triangular carrier wave is on its rising edge and its magnitude is equal to that of the modulating wave, control S2 is turned on and control S1 is turned off. At this time, the inductor L, the semiconductor switch S2, and the lower capacitor of the AC / DC conversion system form a current loop, and the lower capacitor discharges.

[0069] When the triangular carrier wave is at its falling edge and its magnitude is equal to that of the modulating wave, control S1 is turned on and control S2 is turned off. At this time, the inductor L, semiconductor switch S1, capacitor C, and the lower capacitor of the AC / DC conversion system form a current loop, and the lower capacitor is charged.

[0070] Specific control methods are as follows: Figure 3 As shown:

[0071] Step 1: Calculate the magnitude of the modulated wave signal at the current moment. ;

[0072] Step 2: and Comparisons are made, and pulse control signals are generated for gradual adjustment. for ;

[0073] like The modulation wave needs to be gradually reduced, that is... ;

[0074] like The modulation wave needs to be gradually increased, that is... ;

[0075] like It is necessary to control the modulated wave to remain unchanged, that is ;

[0076] Step 3: Calculate the target amplitude of the modulated wave signal at the next moment. The signal is compared with a triangular carrier signal to generate pulse control signals for semiconductor switches S1 and S2, which control their on and off states to balance the DC side midpoint voltage.

Claims

1. A DC midpoint voltage balance control device for an AC / DC conversion system, characterized in that, The system includes a first semiconductor switch, a second semiconductor switch, an inductor, a capacitor, and a control module. One end of the inductor is connected to the midpoint of the DC bus of the AC / DC conversion system, and the other end is electrically connected to the source of the first semiconductor switch and the drain of the second semiconductor switch. The drain of the first semiconductor switch is electrically connected to the positive terminal of the DC bus and one end of the capacitor. The source of the second semiconductor switch is electrically connected to the negative terminal of the DC bus and the other end of the capacitor. The control module generates pulse control signals based on the real-time collected DC bus midpoint voltage and DC bus positive and negative terminal voltages to control the complementary conduction of the first and second semiconductor switches, thereby gradually adjusting the DC bus midpoint voltage balance of the AC / DC conversion system with a fixed step size. The step of generating pulse control signals based on the real-time acquired DC bus midpoint voltage and DC bus positive and negative pole voltages includes: The ratio between the midpoint voltage of the DC bus and the positive and negative pole voltages of the DC bus is used as the modulation signal. The amplitude of the modulation signal at the current moment is calculated based on the real-time acquisition of the DC bus midpoint voltage and the DC bus positive and negative pole voltages. The amplitude of the modulated signal at the current moment is compared with the midpoint of the amplitude of the carrier signal, and the target amplitude of the modulated signal at the next moment is adjusted by a fixed step size based on the comparison result. The pulse control signal is generated by comparing the target amplitude of the modulation signal with the amplitude of the carrier signal. The step of adjusting the target amplitude of the modulated signal at the next moment with a fixed step size based on the comparison result includes: If the amplitude of the modulated signal at the current moment is greater than the midpoint of the amplitude of the carrier signal, then the target amplitude of the modulated signal at the next moment is set to the amplitude of the modulated signal at the current moment minus the fixed step size. If the amplitude of the modulated signal at the current moment is less than the midpoint of the amplitude of the carrier signal, then the target amplitude of the modulated signal at the next moment is set to the amplitude of the modulated signal at the current moment plus the fixed step size. If the amplitude of the modulated signal at the current moment is equal to the midpoint of the amplitude of the carrier signal, then the target amplitude of the modulated signal at the next moment is set to the amplitude of the modulated signal at the current moment.

2. The apparatus according to claim 1, characterized in that, The carrier signal is a triangular carrier signal, and the pulse control signal includes: A first pulse control signal is connected to the gate of the first semiconductor switch. When the triangular carrier signal is at a falling edge and its amplitude is equal to the target amplitude of the modulation signal, the first semiconductor switch is controlled to turn on. When the triangular carrier signal is at a rising edge and its amplitude is equal to the target amplitude of the modulation signal, the first semiconductor switch is controlled to turn off. The second pulse control signal is connected to the gate of the second semiconductor switch. When the triangular carrier signal is on the rising edge and its amplitude is equal to the target amplitude of the modulation signal, the second semiconductor switch is turned on. When the triangular carrier signal is on the falling edge and its amplitude is equal to the target amplitude of the modulation signal, the second semiconductor switch is turned off.

3. The apparatus according to claim 1, characterized in that, The AC / DC conversion system includes several AC / DC converters connected in parallel.

4. A method for DC midpoint voltage balance control in an AC / DC conversion system, characterized in that, Applied to the apparatus as described in any one of claims 1-3, comprising the following steps: Collect the DC bus midpoint voltage and DC bus positive and negative terminal voltages of the AC / DC conversion system; The ratio between the midpoint voltage of the DC bus and the positive and negative pole voltages of the DC bus is controlled to be one-half.

5. The method according to claim 4, characterized in that, The control of the ratio between the DC bus midpoint voltage and the DC bus positive and negative terminal voltages to be one-half includes: The ratio between the midpoint voltage of the DC bus and the positive and negative pole voltages of the DC bus is used as the modulation signal. The amplitude of the modulation signal at the current moment is calculated based on the real-time acquisition of the DC bus midpoint voltage and the DC bus positive and negative pole voltages. The amplitude of the modulated signal at the current moment is compared with the midpoint of the amplitude of the carrier signal, and the target amplitude of the modulated signal at the next moment is adjusted by a fixed step size based on the comparison result. By comparing the target amplitude of the modulation signal with the amplitude of the carrier signal, a pulse control signal is generated to control the first semiconductor switch and the second semiconductor switch to conduct complementaryly.

6. The method according to claim 5, characterized in that, The step of adjusting the target amplitude of the modulated signal at the next moment with a fixed step size based on the comparison result includes: If the amplitude of the modulated signal at the current moment is greater than the midpoint of the amplitude of the carrier signal, then the target amplitude of the modulated signal at the next moment is set to the amplitude of the modulated signal at the current moment minus the fixed step size. If the amplitude of the modulated signal at the current moment is less than the midpoint of the amplitude of the carrier signal, then the target amplitude of the modulated signal at the next moment is set to the amplitude of the modulated signal at the current moment plus the fixed step size. If the amplitude of the modulated signal at the current moment is equal to the midpoint of the amplitude of the carrier signal, then the target amplitude of the modulated signal at the next moment is set to the amplitude of the modulated signal at the current moment.

7. The method according to claim 5, characterized in that, The carrier signal is a triangular carrier signal.

8. The method according to claim 7, characterized in that, The control of the first semiconductor switch and the second semiconductor switch to be complementaryly turned on includes: When the triangular carrier signal is at its falling edge and its amplitude is equal to the target amplitude of the modulation signal, the first semiconductor switch is turned on and the second semiconductor switch is turned off. When the triangular carrier signal is at its rising edge and its amplitude is equal to the target amplitude of the modulation signal, the first semiconductor switch is turned off and the second semiconductor switch is turned on.

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