A method for starting a cascaded h-bridge grid-connected converter with constant regulation

Abstract

The application provides a cascaded H-bridge grid-connected converter starting method with constant modulation degree, and belongs to the technical field of power electronics. The purpose is to solve the problems of impact current and over-modulation in the starting process of the traditional starting method. The method uses current control to make the cascaded H-bridge grid-connected converter present a virtual boost resistor external characteristic, so that the DC capacitor voltage of the H-bridge module is smoothly boosted to the expected value. The method only needs to adjust the value of the virtual boost resistor in the starting process, so that the modulation degree of the cascaded H-bridge grid-connected converter is a constant value, and has certain feasibility.

Description

Technical Field

[0001] This invention relates to a starting method for a cascaded H-bridge grid-connected converter that maintains a constant modulation, belonging to the field of power electronics technology. Background Technology

[0002] Cascaded H-bridge grid-connected converters are widely used in systems such as Static Var Compensators (STATCOMs) and Active Power Filters (APFs) due to their modularity and multi-level voltage output advantages. The startup process is crucial for the stable operation of cascaded H-bridge grid-connected converters, as it charges the DC-side capacitors of each H-bridge module to provide a certain DC-side capacitor voltage to support stable system operation. However, during startup, cascaded H-bridge grid-connected converters face challenges such as overmodulation and inrush current caused by low DC voltage. Therefore, researching startup methods for cascaded H-bridge grid-connected converters has significant engineering implications.

[0003] Because the DC-side capacitors of each H-bridge module in a cascaded H-bridge grid-connected converter are independent, the externally excited starting method with an external DC power supply will increase system costs when the number of cascaded modules is large. Currently, several academic papers have researched self-excited starting methods that draw power from the grid, for example: 1) An article titled "A Slow-Set-Off Method for Suppressing Start-up Inrush Current of PWM Rectifiers" (Journal of Electrical Engineering, June 2018, Vol. 33, No. 12) proposes a startup control method that uses a slow-set-off DC-side capacitor voltage. This method involves gradually changing the reference value of the DC-side capacitor voltage to the desired value to limit the instantaneous magnitude and rate of change of the DC-side capacitor voltage, thereby suppressing the inrush current. However, this startup method cannot effectively address the inrush current generated during the first few switching cycles of the system during startup.

[0004] 2) An article titled "Research on Start-up Control of Capacitor Voltage in Active Power Filters" (Power Electronics Technology, Vol. 48, No. 2, February 2014) proposes a start-up control method using multiple proportional-integral (PI) controllers. This method effectively controls the rise rate and overshoot of the DC-side capacitor voltage by using multiple sets of different PI controller parameters at different stages of startup. However, this startup method suffers from difficulties in parameter tuning and excessive time for parameter switching.

[0005] 3) An article titled “A Novel Soft-start Method for SVG Based on D-axis Orientation”, by Shicheng Zheng, Ying Shu, Mengmeng Qi, Jiahong Lang and Xuefeng Hu, published in the Chinese Journal of Electrical Engineering, 2021, 7(1):55-6, proposes an open-loop control method for SVG startup. This method gradually reduces the modulation ratio of the sinusoidal PWM wave until the DC-side capacitor voltage reaches its rated value. However, this startup method suffers from the problem of the controller being highly sensitive to circuit parameters.

[0006] In summary, the existing startup methods have the following problems: 1. When the number of cascaded modules of the cascaded H-bridge grid-connected converter is large, the separately excited starting method with external DC power supply will increase the system cost; 2. In the existing technology research on voltage and current dual closed-loop self-excited start-up method based on proportional-integral regulator, reducing the voltage difference between the DC side capacitor voltage and its reference value cannot effectively suppress the inrush current generated in the early stage of system startup. 3. In the existing technology research on voltage and current dual closed-loop self-excitation start-up method based on proportional-integral regulator, the modification of proportional-integral regulator parameters has the problems of difficult parameter tuning and excessive parameter switching time.

[0007] 4. In existing open-loop control-based startup methods, the controller is highly sensitive to circuit parameters. Summary of the Invention

[0008] The purpose of this invention is to provide a startup method for a cascaded H-bridge grid-connected converter that maintains a constant modulation index. This method utilizes current loop control to make the cascaded H-bridge grid-connected converter exhibit the characteristics of a virtual boost resistor. By adjusting the value of the virtual boost resistor, the active power absorbed by the cascaded H-bridge grid-connected converter from the three-phase power grid is adjusted, thereby ensuring that the DC capacitor voltage of the H-bridge module smoothly rises to the desired value. This method dynamically adjusts the relationship between the virtual boost resistor and the current-limiting resistor based on the magnitude of the DC-side capacitor voltage of the H-bridge module, thereby controlling the magnitude of the AC-side voltage of the cascaded H-bridge grid-connected converter. This ensures that the cascaded H-bridge grid-connected converter operates according to the set desired modulation index, effectively avoiding overmodulation and suppressing inrush current.

[0009] The objective of this invention is achieved as follows: This invention provides a startup method for a cascaded H-bridge grid-connected converter that maintains a constant modulation index. The circuit topology involved in this startup method includes a cascaded H-bridge grid-connected converter, a three-phase filter inductor L, a three-phase current-limiting resistor R, a three-phase grid-connected circuit breaker KM1, a three-phase current-limiting resistor circuit breaker KM2, and a three-phase power grid. The cascaded H-bridge grid-connected converter is a star-connected three-phase cascaded H-bridge grid-connected converter. Each phase of the cascaded H-bridge grid-connected converter includes n identical H-bridge modules and n identical DC-side capacitors. The n H-bridge modules are cascaded, and each H-bridge module is connected in parallel with a DC-side capacitor. The output terminal of the three-phase cascaded H-bridge grid-connected converter is connected to the three-phase power grid in series with the three-phase filter inductor L, the three-phase current-limiting resistor R, and the three-phase grid-connected circuit breaker KM1. The three-phase current-limiting resistor circuit breaker KM2 is connected in parallel with the three-phase current-limiting resistor R. This startup method includes the following steps: Step 1, Uncontrolled rectification When the three-phase grid-connected circuit breaker KM1 is closed and the three-phase current-limiting resistor circuit breaker KM2 remains open, the circuit consisting of the three-phase cascaded H-bridge grid-connected converter, the three-phase filter inductor L, the three-phase current-limiting resistor R, and the three-phase power grid is equivalent to three sets of single-phase bridge uncontrolled rectifier charging circuits. The voltage of the 3n DC-side capacitors corresponding to the 3n H-bridge modules is boosted to a steady state during the uncontrolled rectification stage. The voltages of the 3n DC-side capacitors corresponding to the 3n H-bridge modules are sampled at the instant of steady state, and the sampled values ​​are recorded as the initial DC-side capacitor voltages. ,in, For phase sequence, , This refers to the sequence number of each H-bridge module in each phase of the cascaded H-bridge grid-connected converter. ; The initial DC-side capacitor voltages of 3n The average value is denoted as the initial DC-side capacitor voltage average value. The formula for its calculation is:

[0010] Step 2, Parameter Setting

[0011] Step 2.1, Set the desired value of the DC-side capacitor voltage. Its value range is as follows:

[0012] in, This represents the peak phase voltage of the grid-connected voltage. Step 2.2, based on the initial average DC-side capacitor voltage obtained in Step 1. The expected value of the DC-side capacitor voltage set in step 2.1 Set the expected value of the adjustment system Its value range is as follows:

[0013] Step 3, Virtual boost resistor and current control

[0014] The voltages of the 3n DC-side capacitors corresponding to the 3n H-bridge modules, the current of the three-phase filter inductor L, and the grid-connected voltage are sampled in real time for N rounds. The 3n H-bridge modules are then controlled to ensure that the voltages of the 3n DC-side capacitors corresponding to the 3n H-bridge modules are smoothly boosted to the expected DC-side capacitor voltage values ​​set in step 2.1. And ensure that the modulation index of the cascaded H-bridge grid-connected converter is constant at the desired modulation index value set in step 2.2. Where N represents the voltage of all 3n DC-side capacitors reaching the expected value of the DC-side capacitor voltage. The number of real-time sampling rounds, where N is a positive integer; One round of real-time sampling and control is as follows: Step 3.1, let any round of real-time sampling in the N rounds be the th round. Round of real-time sampling, For the number of sampling rounds in real time, The voltages of the 3n DC-side capacitors corresponding to the 3n H-bridge modules are respectively subjected to the [missing information - likely a specific process or procedure]. Real-time sampling is performed, and the sampled values ​​are recorded as DC-side capacitor voltages. The current of the three-phase filter inductor L is then subjected to the first... Real-time sampling is performed, and the sampled values ​​are recorded as inductor current. The first step is to test the grid-connected voltage. Real-time sampling is performed, and the sampled values ​​are recorded as grid-connected voltage. ; , ; Step 3.2, based on the DC-side capacitor voltage obtained in Step 3.1 Calculate the average value of the 3n DC-side capacitor voltages and record it as the average DC-side capacitor voltage. The calculation formula is as follows:

[0015] Step 3.3, based on the expected value of the DC-side capacitor voltage set in step 2.1 Step 2.2 sets the expected value of the modulation system. The average value of the DC-side capacitor voltage obtained in step 3.2 The virtual boost resistor is obtained through calculation. The calculation formula is as follows:

[0016] in, The current-limiting resistance value of the three-phase current-limiting resistor R; Step 3.4, the inductor current obtained in real time from step 3.1 is processed. and grid voltage The d-axis inductor current in the synchronous rotating coordinate system at the fundamental frequency of the power grid is obtained by performing coordinate transformation. q-axis inductor current d-axis grid voltage and q-axis grid voltage ; Step 3.5, based on the virtual boost resistor obtained in Step 3.3 And the d-axis grid-connected voltage obtained in step 3.4 and q-axis grid voltage The reference value of the d-axis inductor current was calculated. q-axis inductor current reference value and voltage feedforward coefficient The calculation formulas are as follows:

[0017]

[0018]

[0019] Step 3.6, based on the average DC-side capacitor voltage obtained in Step 3.2 The d-axis inductor current obtained in step 3.4 q-axis inductor current d-axis grid voltage and q-axis grid voltage The reference value of the d-axis inductor current obtained in step 3.5 q-axis inductor current reference value and voltage feedforward coefficient The d-axis modulation signal is obtained through a current proportional-integral regulator. and q-axis modulation signal The calculation formula is as follows:

[0020]

[0021] in, This refers to the proportional coefficient of the current proportional-integral controller. The integral coefficient of the current proportional-integral controller. For the Laplace operator, The fundamental angular frequency of the grid-connected voltage. Let L be the inductance value of the three-phase filter inductor; d-axis modulation signal and q-axis modulation signal The upper limit is set to 1 and the lower limit is set to -1 when the d-axis modulation signal... q-axis modulation signal When the upper or lower limit value is reached, the cascaded H-bridge grid-connected converter is over-modulated; Step 3.7, modulate the d-axis signal obtained in step 3.6. and q-axis modulation signal The coordinate transformation is performed to obtain the three-phase modulation signal in the three-phase symmetrical stationary coordinate system. , Three-phase cascaded H-bridge grid-connected converter in three-phase modulation signal The individual H-bridge modules are controlled under the modulation. Step 4, denoted as the interval between each round of real-time sampling in N rounds of real-time sampling. , After time 1, the voltages of the 3n DC-side capacitors corresponding to the 3n H-bridge modules are respectively tested at the 1st time. +1 round of real-time sampling, and record the sampled value as the th... DC side capacitor voltage after wheel adjustment Examine the 3nth DC side capacitor voltage after wheel adjustment Does it meet the requirement that the DC-side capacitor voltage reaches the expected value? Requirements: If met, proceed to step 5; if not met, return to step 3.1 and perform the next round of real-time sampling and control. Step 5: Close the three-phase current-limiting resistor circuit breaker KM2 to bypass the three-phase current-limiting resistor R.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows: 1) This invention does not require an external DC power supply, thus greatly saving system costs.

[0023] 2) This invention controls the magnitude of the AC side voltage of the cascaded H-bridge grid-connected converter based on the sampled DC side capacitor voltage of the H-bridge module, so that the cascaded H-bridge grid-connected converter operates according to the set modulation expectation value, effectively avoiding over-modulation and suppressing inrush current.

[0024] 3) The present invention adaptively modifies the virtual boost resistor value and current reference value based on the sampled DC-side capacitor voltage of the H-bridge module, thereby eliminating the need to tune multiple sets of regulator parameters.

[0025] 4) This invention is a start-up method based on closed-loop control. The controller is highly adaptable to changes in circuit parameters, which effectively improves the stability of control. Attached Figure Description

[0026] Figure 1 This is a circuit topology diagram related to the startup method of the present invention.

[0027] Figure 2 This is the single-phase equivalent circuit of the virtual boost resistor startup strategy involved in this invention.

[0028] Figure 3 This is a control block diagram of the cascaded H-bridge grid-connected converter involved in this invention.

[0029] Figure 4 The waveform of the DC-side capacitor voltage when using the traditional startup method is shown.

[0030] Figure 5 The waveform of the inductor current when using the traditional startup method is shown.

[0031] Figure 6 The waveform of the d-axis modulated signal using the traditional startup method is shown.

[0032] Figure 7 The waveform of the DC-side capacitor voltage when using the startup method of the present invention is shown.

[0033] Figure 8 The waveform of the inductor current when using the startup method of the present invention is shown.

[0034] Figure 9 The waveform of the d-axis modulation signal using the startup method of the present invention is shown. Detailed Implementation

[0035] The embodiments of the present invention will now be described with reference to the accompanying drawings and specific examples.

[0036] Figure 1 This is a circuit topology diagram related to the startup method of the present invention. Figure 1 As can be seen, the circuit topology involved in this startup method includes a cascaded H-bridge grid-connected converter, a three-phase filter inductor L, a three-phase current-limiting resistor R, a three-phase grid-connected circuit breaker KM1, a three-phase current-limiting resistor circuit breaker KM2, and a three-phase power grid. The cascaded H-bridge grid-connected converter is a star-connected three-phase cascaded H-bridge grid-connected converter, wherein each phase of the cascaded H-bridge grid-connected converter includes n identical H-bridge modules and n identical DC-side capacitors. The n H-bridge modules are cascaded, and each H-bridge module is connected in parallel with one DC-side capacitor. The output terminal of the three-phase cascaded H-bridge grid-connected converter is connected to the three-phase power grid in series with the three-phase filter inductor L, the three-phase current-limiting resistor R, and the three-phase grid-connected circuit breaker KM1. The three-phase current-limiting resistor circuit breaker KM2 is connected in parallel with the three-phase current-limiting resistor R. Figure 1 In the diagram, C represents the DC-side capacitor connected in parallel for each H-bridge module. Additionally, [the following is a list of components / functions]... Figure 1 As can be seen, in this embodiment, n=6.

[0037] Ideally, the neutral point of the cascaded H-bridge grid-connected converter can be considered to have the same potential as the neutral point of the grid voltage. Therefore, the single-phase equivalent circuit of the virtual boost resistor startup strategy involved in this invention is as follows: Figure 2 As shown. The single-phase equivalent circuit described therein is based on a three-phase system. For example, the virtual boost resistor startup strategy described above enables the cascaded H-bridge grid-connected converter to exhibit a virtual boost resistor. External characteristics, by adjusting the virtual boost resistor The size ensures smooth startup of the cascaded H-bridge grid-connected converter. Figure 2 middle, for The voltage value of the phase-connected grid voltage.

[0038] Figure 3 This is a control block diagram of the cascaded H-bridge grid-connected converter involved in this invention, including two parts: a virtual boost resistor and current control. Figure 3 As can be seen, this startup method includes the following steps: Step 1, Uncontrolled rectification When the three-phase grid-connected circuit breaker KM1 is closed and the three-phase current-limiting resistor circuit breaker KM2 remains open, the circuit consisting of the three-phase cascaded H-bridge grid-connected converter, the three-phase filter inductor L, the three-phase current-limiting resistor R, and the three-phase power grid is equivalent to three sets of single-phase bridge uncontrolled rectifier charging circuits. The voltage of the 3n DC-side capacitors corresponding to the 3n H-bridge modules is boosted to a steady state during the uncontrolled rectification stage.

[0039] The voltages of the 3n DC-side capacitors corresponding to the 3n H-bridge modules are sampled at the instant of steady state, and the sampled values ​​are recorded as the initial DC-side capacitor voltages. ,in, For phase sequence, , This refers to the sequence number of each H-bridge module in each phase of the cascaded H-bridge grid-connected converter. .

[0040] The initial DC-side capacitor voltages of 3n The average value is denoted as the initial DC-side capacitor voltage average value. The formula for its calculation is:

[0041] In this example, ≈1.18kV.

[0042] Step 2, Parameter Setting

[0043] Step 2.1, Set the desired value of the DC-side capacitor voltage. Its value range is as follows:

[0044] in, This is the peak value of the phase voltage of the grid-connected voltage.

[0045] In this example, =8.16kV, =1.5kV.

[0046] Step 2.2, based on the initial average DC-side capacitor voltage obtained in Step 1. The expected value of the DC-side capacitor voltage set in step 2.1 Set the expected value of the adjustment system Its value range is as follows:

[0047] In this example, =0.81.

[0048] Adjustment system expectation value The purpose of determining the above value range is to ensure that the cascaded H-bridge grid-connected converter is not modulated during startup and that the active power absorbed by the cascaded H-bridge grid-connected converter is greater than the active power consumed by the current-limiting resistor R.

[0049] Step 3, Virtual boost resistor and current control

[0050] The voltages of the 3n DC-side capacitors corresponding to the 3n H-bridge modules, the current of the three-phase filter inductor L, and the grid-connected voltage are sampled in real time for N rounds. The 3n H-bridge modules are then controlled to ensure that the voltages of the 3n DC-side capacitors corresponding to the 3n H-bridge modules are smoothly boosted to the expected DC-side capacitor voltage values ​​set in step 2.1. And ensure that the modulation index of the cascaded H-bridge grid-connected converter is constant at the desired modulation index value set in step 2.2. Where N represents the voltage of all 3n DC-side capacitors reaching the expected value of the DC-side capacitor voltage. The number of real-time sampling rounds, where N is a positive integer.

[0051] One round of real-time sampling and control is as follows: Step 3.1, let any round of real-time sampling in the N rounds be the th round. Round of real-time sampling, For the number of sampling rounds in real time, The voltages of the 3n DC-side capacitors corresponding to the 3n H-bridge modules are respectively subjected to the [missing information - likely a specific action or procedure]. Real-time sampling is performed, and the sampled values ​​are recorded as DC-side capacitor voltages. The current of the three-phase filter inductor L is then measured. Real-time sampling is performed, and the sampled values ​​are recorded as inductor current. The first step is to test the grid-connected voltage. Real-time sampling is performed, and the sampled values ​​are recorded as grid-connected voltage. . , .

[0052] Step 3.2, based on the DC-side capacitor voltage obtained in Step 3.1 Calculate the average value of the 3n DC-side capacitor voltages and record it as the average DC-side capacitor voltage. The calculation formula is as follows:

[0053] Step 3.3, based on the expected value of the DC-side capacitor voltage set in step 2.1 Step 2.2 sets the expected value of the modulation system. The average value of the DC-side capacitor voltage obtained in step 3.2 The virtual boost resistor is obtained through calculation. The calculation formula is as follows:

[0054] in, The current-limiting resistance value of the three-phase current-limiting resistor R is given.

[0055] In this example, =500Ω.

[0056] Step 3.4, the inductor current obtained in real time from step 3.1 is processed. and grid voltage The d-axis inductor current in the synchronous rotating coordinate system at the fundamental frequency of the power grid is obtained by performing coordinate transformation. q-axis inductor current d-axis grid voltage and q-axis grid voltage .

[0057] Step 3.5, based on the virtual boost resistor obtained in Step 3.3 And the d-axis grid-connected voltage obtained in step 3.4 and q-axis grid voltage The reference value of the d-axis inductor current was calculated. q-axis inductor current reference value and voltage feedforward coefficient The calculation formulas are as follows:

[0058]

[0059]

[0060] Step 3.6, based on the average DC-side capacitor voltage obtained in Step 3.2 The d-axis inductor current obtained in step 3.4 q-axis inductor current d-axis grid voltage and q-axis grid voltage The reference value of the d-axis inductor current obtained in step 3.5 q-axis inductor current reference value and voltage feedforward coefficient The d-axis modulation signal is obtained through a current proportional-integral regulator. and q-axis modulation signal The calculation formula is as follows:

[0061]

[0062] in, This refers to the proportional coefficient of the current proportional-integral controller. The integral coefficient of the current proportional-integral controller. For the Laplace operator, The fundamental angular frequency of the grid-connected voltage. Let L be the inductance value of the three-phase filter inductor.

[0063] d-axis modulation signal and q-axis modulation signal The upper limit is set to 1 and the lower limit is set to -1 when the d-axis modulation signal... q-axis modulation signal When the upper or lower limit value is reached, the cascaded H-bridge grid-connected converter is over-modulated.

[0064] In this example, =100, =6600, =314, =20mH.

[0065] Step 3.7, modulate the d-axis signal obtained in step 3.6. and q-axis modulation signal The coordinate transformation is performed to obtain the three-phase modulation signal in the three-phase symmetrical stationary coordinate system. , A three-phase cascaded H-bridge grid-connected converter with three-phase modulated signals. The individual H-bridge modules are controlled under the modulation.

[0066] In the above steps, step 3.3 is virtual boost resistor control, and steps 3.4-3.7 are current control.

[0067] Step 4, denoted as the interval between each round of real-time sampling in N rounds of real-time sampling. , After time 1, the voltages of the 3n DC-side capacitors corresponding to the 3n H-bridge modules are respectively tested at the 1st time. +1 round of real-time sampling, and record the sampled value as the th... DC side capacitor voltage after wheel adjustment Examine the 3nth DC side capacitor voltage after wheel adjustment Does it meet the requirement that the DC-side capacitor voltage reaches the expected value? Requirements: If met, proceed to step 5; if not met, return to step 3.1 to perform the next round of real-time sampling and control.

[0068] Step 5: Close the three-phase current-limiting resistor circuit breaker KM2 to bypass the three-phase current-limiting resistor R.

[0069] As can be seen from the above steps, this startup method uses the modulation unit to control the switching devices in the H-bridge modules, thereby smoothly boosting the DC-side capacitor voltage of each H-bridge module to the desired value. And keep the regulation of the cascaded H-bridge grid-connected converter constant at the expected regulation value. .

[0070] In this embodiment, the interval time It is 0.0001 seconds.

[0071] The entire startup process time is recorded as (The time is then recorded as ). The voltage values ​​of the 3n DC-side capacitors throughout the entire startup process are uniformly recorded as follows: The inductor current value throughout the entire startup process is uniformly recorded as... The magnitude of the d-axis modulation signal throughout the entire startup process is uniformly recorded as... .

[0072] Figure 4 The waveform of the DC-side capacitor voltage when using the traditional startup method is shown. Figure 5 The waveform of the inductor current when using the traditional startup method is shown. Figure 6 The waveform of the d-axis modulated signal using the traditional startup method is shown. At 0 seconds, the three-phase grid-connected circuit breaker KM1 is closed, while the three-phase current-limiting resistor circuit breaker KM2 remains open. The DC-side capacitor voltage corresponding to each H-bridge module is boosted to steady state during the uncontrolled rectification phase. At 4 seconds, the three-phase current-limiting resistor circuit breaker KM2 is closed to bypass the three-phase current-limiting resistor R, and a voltage-current dual closed-loop control algorithm based on a proportional-integral regulator is implemented. The desired value of the DC-side capacitor voltage is... The signal is set as a ramp signal from 4 seconds to 4.56 seconds, with a starting value of 1.18kV and a final value of 1.5kV. Expected DC-side capacitor voltage. It was set to 1.5kV after 4.56 seconds. From Figure 4 It can be seen that after 4 seconds of implementing the voltage-current dual closed-loop control algorithm based on a proportional-integral regulator, an inrush voltage appears on the DC-side capacitor. From Figure 5 It can be seen that an inrush current occurs in the inductor current after 4 seconds of implementing the voltage and current dual closed-loop control algorithm based on a proportional-integral regulator. From Figure 6 It can be seen that after 4 seconds of implementing the voltage and current dual closed-loop control algorithm based on the proportional-integral regulator, the d-axis modulation signal reaches the upper limit of 1, and the cascaded H-bridge grid-connected converter is over-modulated.

[0073] Figure 7 The waveform of the DC-side capacitor voltage when using the startup method of the present invention is shown. Figure 8 The waveform of the inductor current when using the startup method of the present invention is shown. Figure 9 The image shows the d-axis modulation signal waveform using the startup method of this invention. At 0 seconds, the three-phase grid-connected circuit breaker KM1 is closed, while the three-phase current-limiting resistor circuit breaker KM2 remains open. The DC-side capacitor voltages corresponding to each H-bridge module are boosted to a steady state during the uncontrolled rectification phase. At 4 seconds, the virtual boost resistor and current control algorithm involved in the startup method of this invention are activated. At 4.56 seconds, the three-phase current-limiting resistor circuit breaker KM2 is closed to bypass the three-phase current-limiting resistor R, and the system operates stably. Figure 7 It can be seen that during startup, the DC-side capacitor voltage steadily rises to the expected value. From Figure 8 It can be seen that no inrush current occurred during the startup process. From Figure 9 It can be seen that during the startup process, the d-axis modulation signal does not reach the upper limit of 1 or the lower limit of -1, and the cascaded H-bridge grid-connected converter does not modulate. During the period from 4 seconds to 4.56 seconds, the magnitude of the d-axis modulation signal is the expected value of the modulation index, and the modulation index is constant.

[0074] In summary, this calculation method is simple to implement. It only requires using current loop control to make the cascaded H-bridge grid-connected converter exhibit the external characteristics of a virtual boost resistor, which can smoothly boost the DC capacitor voltage corresponding to the H-bridge module to the desired value and make the modulation of the cascaded H-bridge grid-connected converter a constant value, thus demonstrating its feasibility.

Claims

1. A startup method for a cascaded H-bridge grid-connected converter that maintains a constant modulation index, the startup method involving a circuit topology including a cascaded H-bridge grid-connected converter, a three-phase filter inductor L, a three-phase current-limiting resistor R, a three-phase grid-connected circuit breaker KM1, a three-phase current-limiting resistor circuit breaker KM2, and a three-phase power grid; wherein the cascaded H-bridge grid-connected converter is a star-connected three-phase cascaded H-bridge grid-connected converter, wherein... Each phase cascaded H-bridge grid-connected converter includes n identical H-bridge modules and n identical DC-side capacitors. The n H-bridge modules are cascaded, and each H-bridge module is connected in parallel with a DC-side capacitor. The output terminal of the three-phase cascaded H-bridge grid-connected converter is connected to the three-phase power grid in series with a three-phase filter inductor L, a three-phase current-limiting resistor R, and a three-phase grid-connected circuit breaker KM1. The three-phase current-limiting circuit breaker KM2 is connected in parallel with the three-phase current-limiting resistor R. The startup method is characterized by the following steps: Step 1, Uncontrolled rectification When the three-phase grid-connected circuit breaker KM1 is closed and the three-phase current-limiting resistor circuit breaker KM2 remains open, the circuit consisting of the three-phase cascaded H-bridge grid-connected converter, the three-phase filter inductor L, the three-phase current-limiting resistor R, and the three-phase power grid is equivalent to three sets of single-phase bridge uncontrolled rectifier charging circuits. The voltage of the 3n DC-side capacitors corresponding to the 3n H-bridge modules is boosted to a steady state during the uncontrolled rectification stage. The voltages of the 3n DC-side capacitors corresponding to the 3n H-bridge modules are sampled at the instant of steady state, and the sampled values ​​are recorded as the initial DC-side capacitor voltages. ,in, For phase sequence, , This refers to the sequence number of each H-bridge module in each phase of the cascaded H-bridge grid-connected converter. ; The initial DC-side capacitor voltages of 3n The average value is denoted as the initial DC-side capacitor voltage average value. The formula for its calculation is: Step 2, Parameter Setting Step 2.1, Set the desired value of the DC-side capacitor voltage. Its value range is as follows: in, This represents the peak phase voltage of the grid-connected voltage. Step 2.2, based on the initial average DC-side capacitor voltage obtained in Step 1. The expected value of the DC-side capacitor voltage set in step 2.1 Set the expected value of the adjustment system Its value range is as follows: Step 3, Virtual boost resistor and current control The voltages of the 3n DC-side capacitors corresponding to the 3n H-bridge modules, the current of the three-phase filter inductor L, and the grid-connected voltage are sampled in real time for N rounds. The 3n H-bridge modules are then controlled to ensure that the voltages of the 3n DC-side capacitors corresponding to the 3n H-bridge modules are smoothly boosted to the expected DC-side capacitor voltage values ​​set in step 2.

1. And ensure that the modulation index of the cascaded H-bridge grid-connected converter is constant at the desired modulation index value set in step 2.

2. Where N represents the voltage of all 3n DC-side capacitors reaching the expected value of the DC-side capacitor voltage. The number of real-time sampling rounds, where N is a positive integer; One round of real-time sampling and control is as follows: Step 3.1, let any round of real-time sampling in the N rounds be the th round. Round of real-time sampling, For the number of sampling rounds in real time, The voltages of the 3n DC-side capacitors corresponding to the 3n H-bridge modules are respectively subjected to the [missing information - likely a specific process or procedure]. Real-time sampling is performed, and the sampled values ​​are recorded as DC-side capacitor voltages. The current of the three-phase filter inductor L is then subjected to the first... Real-time sampling is performed, and the sampled values ​​are recorded as inductor current. The first step is to test the grid-connected voltage. Real-time sampling is performed, and the sampled values ​​are recorded as grid-connected voltage. ; , ; Step 3.2, based on the DC-side capacitor voltage obtained in Step 3.1 Calculate the average value of the 3n DC-side capacitor voltages and record it as the average DC-side capacitor voltage. The calculation formula is as follows: Step 3.3, based on the expected value of the DC-side capacitor voltage set in step 2.1 Step 2.2 sets the expected value of the modulation system. The average value of the DC-side capacitor voltage obtained in step 3.2 The virtual boost resistor is obtained through calculation. The calculation formula is as follows: in, The current-limiting resistance value of the three-phase current-limiting resistor R; Step 3.4, the inductor current obtained in real time from step 3.1 is processed. and grid voltage The d-axis inductor current in the synchronous rotating coordinate system at the fundamental frequency of the power grid is obtained by performing coordinate transformation. q-axis inductor current d-axis grid voltage and q-axis grid voltage ; Step 3.5, based on the virtual boost resistor obtained in Step 3.3 And the d-axis grid-connected voltage obtained in step 3.4 and q-axis grid voltage The reference value of the d-axis inductor current was calculated. q-axis inductor current reference value and voltage feedforward coefficient The calculation formulas are as follows: Step 3.6, based on the average DC-side capacitor voltage obtained in Step 3.2 The d-axis inductor current obtained in step 3.4 q-axis inductor current d-axis grid voltage and q-axis grid voltage The reference value of the d-axis inductor current obtained in step 3.5 q-axis inductor current reference value and voltage feedforward coefficient The d-axis modulation signal is obtained through a current proportional-integral regulator. and q-axis modulation signal The calculation formula is as follows: in, This refers to the proportional coefficient of the current proportional-integral controller. The integral coefficient of the current proportional-integral controller. For the Laplace operator, The fundamental angular frequency of the grid-connected voltage. Let L be the inductance value of the three-phase filter inductor; d-axis modulation signal and q-axis modulation signal The upper limit is set to 1 and the lower limit is set to -1 when the d-axis modulation signal... q-axis modulation signal When the upper or lower limit value is reached, the cascaded H-bridge grid-connected converter is over-modulated; Step 3.7, modulate the d-axis signal obtained in step 3.

6. and q-axis modulation signal The coordinate transformation is performed to obtain the three-phase modulation signal in the three-phase symmetrical stationary coordinate system. , Three-phase cascaded H-bridge grid-connected converter in three-phase modulation signal The individual H-bridge modules are controlled under the modulation. Step 4, denoted as the interval between each round of real-time sampling in N rounds of real-time sampling. , After time 1, the voltages of the 3n DC-side capacitors corresponding to the 3n H-bridge modules are respectively tested at the 1st time. +1 round of real-time sampling, and record the sampled value as the th... DC side capacitor voltage after wheel adjustment Examine the 3nth DC side capacitor voltage after wheel adjustment Does it meet the requirement that the DC-side capacitor voltage reaches the expected value? Requirements: If met, proceed to step 5; if not met, return to step 3.1 and perform the next round of real-time sampling and control. Step 5: Close the three-phase current-limiting resistor circuit breaker KM2 to bypass the three-phase current-limiting resistor R.

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