A method for optimizing capacitance requirements of high modulation ratio hybrid MMC

Abstract

This invention discloses a method for optimizing the capacitor demand of a high modulation ratio hybrid MMC, relating to power electronics application technology, particularly to the field of converter technology. This application's method, based on module decoupling, specifically includes two steps: active allocation control of the output voltage commands of the full-bridge and half-bridge modules, and adjustment of the DC component of the capacitor voltage in the full / half-bridge submodules. Based on this control, the ripple voltage amplitude of the full-bridge and half-bridge submodule capacitors is kept consistent, effectively reducing the capacitance of the half-bridge submodule; thus achieving a lightweight design for the high modulation ratio hybrid MMC. Compared to existing control methods, the proposed method can shape the voltage ripple component of the half-bridge submodule capacitor, thereby effectively reducing the capacitor demand of the half-bridge module by increasing the voltage ripple coefficient.

Description

Technical Field

[0001] The present invention relates to power electronics application technology, in particular to the field of commutation technology, and specifically to a high modulation ratio hybrid MMC capacitor demand optimization method. Background Art

[0002] The modular multilevel converter (MMC) has the advantages of low switching loss and no dynamic / static voltage balancing of components. At present, the converters in high-voltage DC transmission projects at home and abroad all adopt the MMC topology. When applied to overhead line transmission, DC short circuit faults are prone to occur due to the exposed transmission line. Taking into account the DC short circuit fault ride-through capability, construction cost and operation efficiency, the full-bridge-half-bridge hybrid MMC (such as Figure 1 The hybrid MMC has become the preferred topology for high-voltage DC transmission converters, and the hybrid MMC has been successfully applied to my country's Wudongde DC project. In addition, by utilizing the negative level output capability of the full-bridge sub-module in the bridge arm, the hybrid MMC can also operate in DC step-down operating conditions and AC side step-up operating conditions to improve operating performance. The full-bridge-half-bridge hybrid MMC has the capability of DC short-circuit fault ride-through, DC side step-down operation, and AC side step-up operation, and has good application scenarios in flexible DC transmission, locomotive traction, wind power grid connection, and other occasions. In actual projects, capacitors account for more than 1 / 2 of the volume and weight of the sub-module and about 1 / 3 of the cost.

[0003] Generally, the capacitor requirement is optimized by changing the ripple component of the submodule capacitor voltage. Since the submodule capacitor voltage is related to the current flowing through the capacitor, the capacitor current mainly depends on the bridge arm output voltage and bridge arm current. To reduce the submodule capacitor requirement, the existing optimization methods mainly include the third harmonic voltage injection scheme, the double frequency circulating current injection scheme, the scheme of increasing the modulation ratio, and the hybrid method composed of the above methods. Figure 2 and Figure 3 As shown in the figure, it is a control block diagram and a control method flow chart in the prior art. For high modulation ratio operation, the full-bridge submodule needs to output a negative level, while the half-bridge submodule can only output a positive level. Figure 3 The voltage balancing algorithm shown in the figure has some problems due to the differences in the charging and discharging of the capacitors of the full-bridge and half-bridge submodules. Figure 4 The difference in the voltage fluctuation of the full / half-bridge submodule capacitors makes it difficult to reduce the capacitance of the half-bridge submodule by adjusting the fluctuation component of the voltage of the half-bridge submodule capacitor. Insufficient injection of the double-frequency circulating current will increase the current stress of the device and increase the loss. Insufficient improvement of the modulation ratio will result in the improvement effect being affected by the operating modulation ratio. Figure 3 The common disadvantage of the voltage balancing algorithms shown is that it is difficult to actively control the capacitor voltage of the half-bridge sub-module and it is difficult to reduce the capacitance of the half-bridge sub-module.

[0004] The optimization of submodule capacitance requirements is of great significance for saving the cost of the converter and reducing its volume. Therefore, it is an urgent requirement to design a high modulation ratio hybrid MMC capacitance requirement optimization method that can flexibly adjust the full-bridge and half-bridge modules, flexibly control the DC component and ripple component of the submodule capacitance voltage, and effectively reduce the capacitance of the half-bridge submodule. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a high modulation ratio hybrid MMC capacitor demand optimization method for flexibly adjusting full-bridge and half-bridge modules, flexibly controlling the DC component and ripple component of the sub-module capacitor voltage, and effectively reducing the capacitance of the half-bridge sub-module.

[0006] The technical solution to the technical problem to be solved by the present invention is: a high modulation ratio hybrid MMC capacitor demand optimization method, in which the output voltage of the doubled frequency circulating current controller output by the circulating current suppression controller and the AC internal potential output by the inner loop current controller are processed in sequence by the active distribution module of the full-bridge and half-bridge module output voltage and the flexible adjustment module of the DC component of the capacitor voltage to calculate the full-bridge and half-bridge module output voltage instructions, and then a voltage equalization algorithm is used to achieve the balance of the capacitor voltage of the full / half-bridge sub-module.

[0007] Preferably, the active allocation of the output voltage instructions of the full-bridge module and the half-bridge module includes the following processing steps:

[0008] Step 1: Calculate the interval [θ f1 ,θ f2 ] full / half-bridge module output voltage command,

[0009] in,

[0010]

[0011] Among them, F is the number of full-bridge submodules in the bridge arm, U C is the rated voltage of the submodule, m is the modulation ratio, j is a, b, c, indicating the three phases A, B and C;

[0012] Step 2: Calculate the half-bridge module in the interval [θ f1 ,θ f2 ]The energy released within

[0013]

[0014] Among them, ω1 is the fundamental angular frequency, and its value is 314.159 rad / s; i pj is the steady-state current of the upper bridge arm of one phase, where j = a, b, c; its value satisfies:

[0015]

[0016] Among them, I m is the phase current amplitude.

[0017] Step 3: Calculate [0,θ u1 ]、[θ u2 ,θ f1 ] and [θ f2 ,2π] interval half-bridge and full-bridge module output voltage instructions, where

[0018]

[0019] In the region [0,θ u1 ]、[θ u2 ,θ f1 ] and [θ f2 ,2π] The half-bridge module is put into operation, and the output voltage command of the half-bridge module is u pahref0 And full-bridge module output voltage command u pafref0 satisfy:

[0020]

[0021] u pafref0 =U dc / 2-e aref -u pahref0 θ pll ∈[0,θ u1 ]∪[θ u2 ,θ f1 ]∪[θ f2 ,2π]

[0022] In the interval [θ u1 ,θ u2 ]The output voltage instructions of the full-bridge and half-bridge modules are:

[0023]

[0024] Preferably, the closed-loop regulation method of the capacitor voltage of the half-bridge submodule includes the following steps:

[0025] Determine whether the average value of the half-bridge submodule capacitor voltage is greater than the command value of the half-bridge submodule capacitor voltage:

[0026] If yes, then determine the product of the output voltage of the double frequency circulating current controller of the phase and the current of the phase, that is, u difjref i j Is it less than zero? If so, then u pjhref =u pjhref0 ,u pjfref =u pjfref0 -u difjref, where u pjhref is the output voltage command of the j-phase upper arm half-bridge module, u pjfref is the output voltage command of the j-phase upper arm full-bridge module, if otherwise u pjhref =u pjhref0 -u difjref ,u pjfref =u pjfref0 ;

[0027] If not, then determine the product of the output voltage of the double frequency circulating current controller of the phase and the current of the phase, that is, u difjref i j Is it less than zero? If so, then u pjhref =u pjhref0 -u difjref ,u pjfref =u pjfref0 , where u pjhref is the output voltage command of the j-phase upper arm half-bridge module, u pjfref is the output voltage command of the j-phase upper arm full-bridge module, if otherwise u pjhref =u pjhref0 ,u pjfref =u pjfref0 -u difjref .

[0028] Preferably, the average value of the capacitor voltage of the half-bridge submodule is

[0029]

[0030] 5. The method for optimizing capacitance requirements of a high modulation ratio hybrid MMC according to claim 3, characterized in that:

[0031] The command value of the capacitor voltage of the half-bridge submodule is

[0032]

[0033] Better, half-bridge submodule capacitance optimization method:

[0034] Calculate the voltage fluctuation of capacitors of full-bridge and half-bridge submodules

[0035]

[0036]

[0037] Among them, ε F and ε H are the fluctuation rates of the capacitor voltages of the full-bridge submodule and the half-bridge submodule respectively;

[0038] By adding ε F and ε HThe desired index values ​​are met to optimize the design of the half-bridge submodule capacitor.

[0039] The beneficial effects of the present invention are:

[0040] The technology proposed in the present invention does not need to inject third harmonic voltage and double frequency circulating current, so it does not need an isolation transformer on the AC side, and will not increase the stress of the bridge arm current and increase the loss.

[0041] The present invention flexibly adjusts the full-bridge and half-bridge modules, and can flexibly control the DC component and the ripple component of the sub-module capacitor voltage.

[0042] The present invention effectively reduces the capacitance of the half-bridge submodule while ensuring the capacitance voltage ripple components of the full-bridge and half-bridge submodules. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 Full-bridge-half-bridge hybrid MMC topology.

[0044] Figure 2 Overall control block diagram of the existing control method for reducing capacitor voltage.

[0045] Figure 3 The invention discloses a voltage balancing sorting algorithm for an existing control method for reducing capacitor voltage.

[0046] Figure 4 Schematic diagram of capacitor voltage of high modulation ratio hybrid MMC full-bridge and half-bridge sub-modules.

[0047] Figure 5 A body control block diagram of an embodiment of the present invention.

[0048] Figure 6 It is a schematic diagram of active distribution of output voltage of full-bridge and half-bridge modules of the present invention.

[0049] Figure 7 It is a schematic diagram of regulating the DC component of capacitor voltage according to the present invention. DETAILED DESCRIPTION

[0050] In order to make the technical solutions and beneficial effects of the present invention clearer, the embodiments of the present invention are further explained in detail below.

[0051] A high modulation ratio hybrid MMC capacitor demand optimization method is disclosed. Based on the existing technology, the output voltage of the doubled frequency circulating current controller output by the circulating current suppression controller and the AC internal potential output by the inner loop current controller are processed in sequence by the active distribution module of the full-bridge and half-bridge module output voltage and the flexible adjustment module of the DC component of the capacitor voltage to calculate the output voltage instructions of the full-bridge and half-bridge modules in the bridge arm, and then a voltage balancing algorithm is used to achieve the balance of the capacitor voltage of the full / half-bridge sub-modules.

[0052] Taking the A-phase upper bridge arm of the rectifier as an example, the method for processing data by the active distribution module of the output voltage command of the full-bridge module and the half-bridge module includes the following steps:

[0053] Step 1: Calculate the interval [θ f1 ,θ f2 ] full / half-bridge module output voltage command,

[0054] in,

[0055] Among them, F is the number of full-bridge submodules in the bridge arm, U C is the rated voltage of the submodule, and m is the modulation ratio. f1 ,θ f2 ] is the region where the voltage of the full-bridge sub-module in the bridge arm is greater than the output voltage of the full-bridge module.

[0056] Taking the A-phase upper bridge arm of the rectifier as an example, [θ f1 ,θ f2 ]The output voltage command of the full / half-bridge module is:

[0057]

[0058] Among them, u pafref0 and u pahref0 They are the output voltage instructions of the full-bridge and half-bridge modules of the upper bridge arm of phase A, e aref This is the instruction for outputting the internal potential of phase A.

[0059] Step 2: Calculate the half-bridge module in the interval [θ f1 ,θ f2 ]The energy released within

[0060]

[0061] Among them, ω1 is the fundamental angular frequency, and its value is 314.159 rad / s; i pj is the steady-state current of the upper bridge arm of one phase, where j = a, b, c; its value satisfies:

[0062]

[0063] Among them, I m is the amplitude of the AC phase current.

[0064] Taking the upper bridge arm of phase A of the rectifier as an example, calculate the half-bridge module in the interval [θ f1 ,θ f2 ]The energy released within:

[0065]

[0066] Among them, ω1 is the fundamental angular frequency, and its value is 314.159 rad / s; i pa is the steady-state current of the upper bridge arm of phase A, and its value satisfies:

[0067]

[0068] Step 3: Interval [θ f1 ,θ f2 ] is the area where the voltage of the full-bridge submodule in the bridge arm is greater than the output voltage of the full-bridge module. The calculation method of the output voltage command of the half-bridge and full-bridge modules in this range is as follows. f1 ,θ f2 ] is divided into three intervals, namely [0,θ u1 ]、[θ u2 ,θ f1 ] and [θ f2 ,2π], where

[0069]

[0070] First, in the region [0,θ u1 ]、[θ u2 ,θ f1 ] and [θ f2 ,2π], the half-bridge module can be put into use, and the half-bridge module output voltage command u pahref0 And full-bridge module output voltage command u pafref0 satisfy:

[0071]

[0072] u pafref0 =U dc / 2-e aref -u pahref0 θ pll ∈[0,θ u1 ]∪[θ u2 ,θ f1 ]∪[θ f2 ,2π]

[0073] In the interval [θ u1 ,θ u2 ]The output voltage instructions of the full-bridge and half-bridge modules are:

[0074]

[0075] As shown in the figure, the closed-loop regulation method of the capacitor voltage of the half-bridge submodule includes the following steps:

[0076] First, determine whether the average value of the capacitor voltage of the half-bridge submodule is greater than the command value of the capacitor voltage of the half-bridge submodule:

[0077] If yes, then determine the product of the output voltage of the double frequency circulating current controller of the phase and the current of the phase, that is, u difjref i j Is it less than zero? If so, then u pjhref =u pjhref0 ,u pjfref =u pjfref0 -u difjref , where u pjhref is the output voltage command of the j-phase upper arm half-bridge module, u pjfref is the output voltage command of the j-phase upper arm full-bridge module, if otherwise u pjhref =u pjhref0 -u difjref ,u pjfref =u pjfref0 ;

[0078] If not, then determine the product of the output voltage of the double frequency circulating current controller of the phase and the current of the phase, that is, u difjref i j Is it less than zero? If so, then u pjhref =u pjhref0 -u difjref ,u pjfref =u pjfref0 , where u pjhref is the output voltage command of the j-phase upper arm half-bridge module, u pjfref is the output voltage command of the j-phase upper arm full-bridge module, if otherwise u pjhref =u pjhref0 ,u pjfref =u pjfref0 -u difjref .

[0079] Among them, the average value of the capacitor voltage of the half-bridge submodule is The command value of the capacitor voltage of the half-bridge submodule is

[0080] The optimization methods for half-bridge submodule capacitance include:

[0081] First, calculate the capacitor voltage fluctuation of the full-bridge and half-bridge submodules

[0082]

[0083]

[0084] Among them, ε F and ε H They are the fluctuation rates of the capacitor voltages of the full-bridge submodule and the half-bridge submodule, respectively.

[0085] Then by F and ε H The desired index values ​​are met to optimize the design of the half-bridge submodule capacitor.

[0086] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Through the above description, relevant staff can make various changes and modifications without departing from the technical concept of the present invention. The technical scope of the present invention is not limited to the contents of the specification. All so-called equal changes and modifications of the shapes, structures, features and spirits described in the scope of the claims of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A method for optimizing capacitance requirements of a high modulation ratio hybrid MMC, characterized in that: The output voltage of the double frequency circulating current controller output by the circulating current suppression controller and the AC internal potential output by the inner loop current controller are processed by the active distribution module of the full-bridge and half-bridge module output voltage and the flexible adjustment module of the DC component of the capacitor voltage in turn, and the output voltage instructions of the full-bridge and half-bridge modules are calculated, and then the voltage balancing algorithm is used to achieve the balance of the capacitor voltage of the full / half-bridge sub-modules; The active distribution of the full-bridge and half-bridge module output voltage instructions includes the following processing steps: Step 1: Calculate the interval according to the steady-state bridge arm voltage output instruction The full / half-bridge module output voltage command, in, Among them, F is the number of full-bridge submodules in the bridge arm, U C is the rated voltage of the submodule, m is the modulation ratio, j is a, b, c, indicating the three phases A, B and C; Step 2: Calculate the half-bridge module in the range The energy released within Among them, ω1 is the fundamental angular frequency, and its value is 314.159 rad / s; i pj is the steady-state current of the upper bridge arm of one phase, where j = a, b, c; its value satisfies: Among them, I m is the phase current amplitude; Step 3: Calculate [0,θ u1 ]、[θ u2 ,θ f1 ] and [θ f2 ,2π] interval half-bridge and full-bridge module output voltage instructions, where In the region [0,θ u1 ]、[θ u2 ,θ f1 ] and [θ f2 ,2π] The half-bridge module is put into operation, and the output voltage command of the half-bridge module is u pahref0 And full-bridge module output voltage command u pafref0 satisfy: you pafref0 =U dc / 2-e aref -u pahref0 ;θ pll ∈[0,θ u1 ]∪[θ u2 ,i f1 ]∪[θ f2 ,2π] In the interval [θ u1 ,θ u2 ]The output voltage instructions of the full-bridge and half-bridge modules are: Optimization method of half-bridge submodule capacitance: Calculate the voltage fluctuation of capacitors of full-bridge and half-bridge submodules Among them, ε F and ε H are the fluctuation rates of the capacitor voltages of the full-bridge submodule and the half-bridge submodule respectively; By adding ε F and ε H The desired index values ​​are met to optimize the design of the half-bridge submodule capacitor.

2. The method for optimizing capacitance requirements of a high modulation ratio hybrid MMC according to claim 1, characterized in that: The closed-loop regulation method of the capacitor voltage of the half-bridge submodule includes the following steps: Determine whether the average value of the half-bridge submodule capacitor voltage is greater than the command value of the half-bridge submodule capacitor voltage: If yes, then determine the product of the output voltage of the double frequency circulating current controller of the phase and the current of the phase, that is, u difjref i j Is it less than zero? If so, then u pjhref =u pjhref0 ,u pjfref =u pjfref0 -u difjref , where u pjhref is the output voltage command of the j-phase upper arm half-bridge module, u pjfref is the output voltage command of the j-phase upper arm full-bridge module, if otherwise u pjhref =u pjhref0 -u difjref ,u pjfref =u pjfref0 ; If not, then determine the product of the output voltage of the double frequency circulating current controller of the phase and the current of the phase, that is, u difjref i j Is it less than zero? If so, then u pjhref =u pjhref0 -u difjref ,u pjfref =u pjfref0 , where u pjhref is the output voltage command of the j-phase upper arm half-bridge module, u pjfref is the output voltage command of the j-phase upper arm full-bridge module, if otherwise u pjhref =u pjhref0 ,u pjfref =u pjfref0 -u difjref .

3. A high modulation ratio hybrid MMC capacitance requirement optimization method according to claim 2, characterized in that: The average value of the capacitor voltage of the half-bridge submodule is 4. The method for optimizing capacitance requirements of a high modulation ratio hybrid MMC according to claim 2, characterized in that: The command value of the capacitor voltage of the half-bridge submodule is

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