Control method of extremely-low-frequency modular multilevel converter

Abstract

The invention provides a control method of an extremely-low-frequency modular multilevel converter, which comprises the following steps of: under the condition that common-mode voltage is injected into the extremely-low-frequency modular multilevel converter, calculating output current and output circulating current in different states, and further calculating a load current predicted value and a circulating current predicted value; constructing a value function based on the load current predicted value and the circulating current predicted value to respectively calculate the conduction time of the upper and lower bridge arm sub-modules to obtain the optimal conduction time of the upper and lower bridge arm sub-modules, and updating the conduction time of capacitor voltage balance of the single sub-module to obtain the conduction time of the single sub-module in a preset control period. Harmonic waves extracted when the common-mode voltage is injected are adjusted and then added with reference current to serve as conduction prediction time and conduction time of a single sub-module in a preset control period to generate a gate pole control signal of the extremely-low-frequency modular multi-level converter, wherein the conduction prediction time and the conduction time of the single sub-module in the preset control period; the harmonic wave of the output current can be reduced while the capacitor voltage fluctuation of the sub-module is suppressed.

Description

Technical Field

[0001] The present invention relates to the technical field of modular multi-level converter control, and in particular to a control method for an extremely low frequency modular multi-level converter. Background Art

[0002] Modular multilevel converters (MMCs), an emerging high-voltage, high-power power conversion technology, are gaining traction among researchers worldwide. Compared to traditional two- and three-level converters, MMCs offer significant advantages, including simplified structure, easy expansion, low switching losses, reduced voltage harmonics, and low installation and maintenance costs. Consequently, they are widely used in HVDC transmission. During HVDC transmission, MMC power losses account for a significant portion of system losses. Optimizing these losses not only improves overall system efficiency but also effectively reduces operating costs, making them of high research value.

[0003] In a common three-phase MMC circuit architecture, each circuit has two upper and lower bridge arms. Each bridge arm consists of N sub-modules with the same structure connected in series with an inductor. Each sub-module adopts a half-bridge topology and contains two insulated gate bipolar transistors (IGBTs) and a group of capacitors.

[0004] In AC motor drive scenarios, the capacitor voltage fluctuations of submodules in modular multilevel converters exhibit a specific pattern: the fluctuation range is positively correlated with the output current amplitude and negatively correlated with the system operating frequency. When MMC is used for motor starting or low-frequency operation control, the submodule capacitor voltage pulsates violently. This pulsation has numerous adverse effects, including significantly increasing the stress on the power devices within the bridge arms, causing distortion in the output voltage and current waveforms and subsequently degrading the output power quality. More critically, this phenomenon severely threatens the stability of the MMC system, significantly limiting its widespread application in medium- and high-voltage AC drives. Therefore, to promote the effective application of MMC in this field, it is imperative to design targeted control strategies to address the significant pulsation of submodule capacitor voltages under low-frequency conditions.

[0005] Currently, a commonly used low-frequency suppression strategy includes high-frequency signal injection. Injecting sinusoidal common-mode voltage and circulating current at a frequency significantly higher than the AC output frequency can achieve better suppression of submodule capacitor voltages. This approach has been combined with model predictive control (MPC) methods, offering a novel approach to low-frequency control. However, traditional MPC suffers from excessive computational burden and limited stability. Traditional MPC requires evaluating all switch states to determine the optimal control switch state for each control cycle, and the computational burden increases exponentially with the number of submodules. Existing methods for suppressing submodule capacitor voltage fluctuations inevitably inject high-frequency circulating currents into the MMC, which significantly distorts the output current. However, limited research and solutions addressing this issue are currently available. Summary of the Invention

[0006] The present invention provides a control method for an extremely low frequency modular multi-level converter, the purpose of which is to reduce the harmonics of the output current while suppressing the voltage fluctuation of submodule capacitors.

[0007] In order to achieve the above object, the present invention provides a control method for an extremely low frequency modular multi-level converter, comprising:

[0008] Step 1: Obtain the capacitor voltage, output voltage, circulating current voltage, and reference current of each submodule on the bridge arm of the very low frequency modular multi-level converter;

[0009] Step 2: When a common-mode voltage is injected into the very low frequency modular multilevel converter, the output current and the output circulating current of the very low frequency modular multilevel converter under different states are calculated, and a load current prediction value and a circulating current prediction value are calculated based on the output current and the output circulating current, and a cost function is constructed based on the load current prediction value and the circulating current prediction value;

[0010] Step 3: Based on the cost function, the on-time of the upper and lower bridge arm submodules in the very low frequency modular multilevel converter is calculated to obtain the optimal on-time of the upper and lower bridge arm submodules, and the on-time of the capacitor-voltage balance of a single submodule is obtained according to the capacitor-voltage balance reference value of any submodule in the upper and lower bridge arms;

[0011] Step 4: Update the on-time of the capacitor voltage balance of a single submodule according to the volt-second principle to obtain the on-time of the single submodule within a preset control cycle, and adjust the harmonics extracted when injecting the common-mode voltage and add them to the reference current as the predicted on-time. Generate the gate control signal of the very low frequency modular multi-level converter based on the on-time of the single submodule within the preset control cycle and the predicted on-time.

[0012] Specifically, when a common-mode voltage is injected into the very low frequency modular multilevel converter, calculating the output current and output circulating current of the very low frequency modular multilevel converter under different states includes:

[0013] Obtain the bridge arm resistance, bridge arm inductance, load resistance, load inductance, control period, and load current reference values ​​of the very low frequency modular multi-level converter;

[0014] The bridge arm resistance, bridge arm inductance, load resistance, load inductance and control period are substituted into the prediction model of the very low frequency modular multi-level converter to obtain the output current value of the very low frequency modular multi-level converter at the next moment under different states.

[0015] The load current reference value and the output current value of the very low frequency modular multilevel converter at the next moment under different states are substituted into the selection function to determine the working state of each bridge arm in the very low frequency modular multilevel converter;

[0016] According to the working status of each bridge arm in the very low frequency modular multi-level converter, the inverse of the load current and the inverse of the circulating current are calculated;

[0017] Based on the inverse of the load current, the inverse of the circulating current, and the action time of each bridge arm in the working state of the very low frequency modular multilevel converter, the output current and output circulating current of the very low frequency modular multilevel converter in different states are calculated.

[0018] Furthermore, the expressions for calculating the inverse of the load current and the inverse of the circulating current are:

[0019] ;

[0020] ;

[0021] in, Indicates the inverse of the load current, represents the inverse of the circulation, Indicates the output current, Represents the output circulation, Indicates the output voltage, represents the circulating voltage, Indicates the load control voltage, represents the circulating current control voltage, represents the bridge arm resistance, represents the load resistance, represents the bridge arm inductance, Indicates the load inductance.

[0022] Furthermore, the expressions for calculating the load current prediction value and the circulating current prediction value based on the output current and the output circulating current are:

[0023] ;

[0024] ;

[0025] in, represents the load current prediction value, represents the circulation prediction value, Indicates the output current, Represents the output circulation, 、 、 They represent the inverse of the load current of the very low frequency modular multilevel converter in different states, 、 、 They represent the inverse of the circulating current of the very low frequency modular multilevel converter in different states, 、 Both indicate the action time.

[0026] More specifically, the value function is expressed as:

[0027] ;

[0028] in, Indicates value, Indicates the load current reference value, Indicates the circulating current reference value, Represents the weighting factor.

[0029] Furthermore, based on the cost function, the on-time of the upper and lower bridge arm sub-modules in the very low frequency modular multi-level converter is calculated respectively, and the calculation expression of the optimal on-time of the upper and lower bridge arm sub-modules is obtained as follows:

[0030]

[0031]

[0032] in, Indicates the optimal on-time of the upper arm submodule, Indicates the optimal on-time of the lower bridge arm submodule, Represents a binary variable.

[0033] Furthermore, the calculation expression for the on-time of the capacitor voltage balance of a single submodule is obtained based on the capacitor voltage balance reference value of any submodule in the upper and lower bridge arms:

[0034] ;

[0035] ;

[0036] in, Indicates the first The conduction time of each sub-module capacitor voltage balance, Indicates the first The conduction time of each sub-module capacitor voltage balance, Represents the bridge arm current of the upper bridge arm, Represents the bridge arm current of the lower bridge arm, It represents the proportional parameter for adjusting the voltage balance speed of the capacitor of a single submodule. Indicates the first Capacitor voltage balance reference value of each submodule, Indicates the first Capacitor voltage balance reference value of each submodule.

[0037] Furthermore, the on-time of the capacitor voltage balance of a single submodule is updated according to the volt-second principle, and the updated expression of the on-time of a single submodule within a preset control period is obtained as follows:

[0038]

[0039]

[0040] in, Indicates the first The conduction time of each submodule in the preset control cycle, Indicates the first The conduction time of each submodule in the preset control cycle, Indicates the preset control period.

[0041] The above solution of the present invention has the following beneficial effects:

[0042] The present invention calculates the output current and output circulating current of the very low frequency modular multilevel converter under different states when a common mode voltage is injected into the very low frequency modular multilevel converter, calculates the load current prediction value and the circulating current prediction value based on the output current and the output circulating current, constructs a value function based on the load current prediction value and the circulating current prediction value to respectively calculate the on-time of the upper and lower bridge arm submodules in the very low frequency modular multilevel converter, obtains the optimal on-time of the upper and lower bridge arm submodules, and obtains the on-time of the capacitor voltage balance of a single submodule based on the capacitor voltage balance reference value of any submodule in the upper and lower bridge arms; updates the on-time of the capacitor voltage balance of a single submodule according to the volt-second principle, obtains the on-time of the single submodule within a preset control period, and updates the on-time of the capacitor voltage balance of the single submodule within a preset control period when a common mode voltage is injected. The harmonics extracted when the common-mode voltage is injected are adjusted and added to the reference current as the conduction prediction time, and the gate control signal of the very low frequency modular multi-level converter is generated based on the conduction time and the conduction prediction time of a single sub-module within a preset control cycle; compared with the prior art, the present invention constructs a value function after injecting the common-mode voltage, thereby reducing the violent fluctuation of the sub-module capacitor voltage under the extremely low frequency working condition; the harmonics extracted when the common-mode voltage is injected are adjusted and added to the reference current as the conduction prediction time, so as to predict the reference current of the next control cycle to suppress the harmonic components, and the gate control signal of the very low frequency modular multi-level converter is generated based on the conduction time and the conduction prediction time of a single sub-module within the preset control cycle, which can achieve the goal of suppressing the fluctuation of the sub-module capacitor voltage while reducing the harmonics of the output current.

[0043] Other beneficial effects of the present invention will be described in detail in the subsequent specific implementation section. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 Schematic diagram of a process of an embodiment of the present invention;

[0045] Figure 2 A topological structure diagram of an extremely low frequency modular multi-level converter according to an embodiment of the present invention;

[0046] Figure 3 1 is a topological structure diagram of a power submodule according to an embodiment of the present invention;

[0047] Figure 4 This is a diagram showing an update sequence of the on-time of the first submodule of the upper bridge arm according to an embodiment of the present invention;

[0048] Figure 5 4 is a control block diagram for obtaining harmonics based on output current in an embodiment of the present invention. DETAILED DESCRIPTION

[0049] To make the technical problems, technical solutions, and advantages to be solved by the present invention more clear, the following is a detailed description with reference to the accompanying drawings and specific embodiments. It is obvious that the embodiments described are only some of the embodiments of the present invention, not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0050] In the description of the present invention, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings and are intended solely to facilitate and simplify the description of the present invention. They are not intended to indicate or imply that the devices or components referred to must have, be constructed, or operate in a specific orientation, and therefore should not be construed as limitations on the present invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0051] In the description of the present invention, it should be noted that, unless otherwise expressly specified or limited, the terms "mounted," "connected," and "connected" should be understood broadly. For example, they may refer to a locking connection, a detachable connection, or an integral connection; they may refer to a mechanical connection or an electrical connection; they may refer to a direct connection or an indirect connection through an intermediate medium; and they may refer to internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on the specific circumstances.

[0052] In addition, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0053] In view of the existing problems, the present invention provides a control method for an extremely low frequency modular multi-level converter.

[0054] like Figure 1 As shown, an embodiment of the present invention provides a control method for an extremely low frequency modular multi-level converter, comprising:

[0055] Step 1: Obtain the capacitor voltage, output voltage, circulating current voltage, and reference current of each submodule on the bridge arm of the very low frequency modular multi-level converter;

[0056] Step 2: When a common-mode voltage is injected into the very low frequency modular multilevel converter, the output current and the output circulating current of the very low frequency modular multilevel converter under different states are calculated, and a load current prediction value and a circulating current prediction value are calculated based on the output current and the output circulating current, and a cost function is constructed based on the load current prediction value and the circulating current prediction value;

[0057] Step 3: Based on the cost function, the on-time of the upper and lower bridge arm submodules in the very low frequency modular multilevel converter is calculated to obtain the optimal on-time of the upper and lower bridge arm submodules, and the on-time of the capacitor-voltage balance of a single submodule is obtained according to the capacitor-voltage balance reference value of any submodule in the upper and lower bridge arms;

[0058] Step 4: Update the on-time of the capacitor voltage balance of a single submodule according to the volt-second principle to obtain the on-time of the single submodule within a preset control cycle, and adjust the harmonics extracted when injecting the common-mode voltage and add them to the reference current as the predicted on-time. Generate the gate control signal of the very low frequency modular multi-level converter based on the on-time of the single submodule within the preset control cycle and the predicted on-time.

[0059] In the embodiment of the present invention, the extremely low frequency modular multilevel converter is composed of three-phase bridge arms, each phase including upper and lower bridge arms, the upper and lower bridge arms are in series structure relative to the DC side, and in parallel structure relative to the output AC side. Each bridge arm is composed of N sub-modules SM with the same structure connected in cascade. The specific topology is as follows: Figure 2 As shown, the extremely low frequency modular multilevel converter includes a three-phase bridge arm, each phase bridge arm includes an upper bridge arm and a lower bridge arm, wherein the upper bridge arm is composed of a first reactor, a first bridge arm resistor, and a plurality of sub-modules connected in series, the input end of the first sub-module on the upper bridge arm is connected to the positive output end of the DC side, the output end of the last sub-module on the upper bridge arm is connected to the input end of the first bridge arm resistor, the ground output end of the first bridge arm resistor is connected to the input end of the first reactor, and the output end of the first reactor is respectively connected to the output end of the second reactor and the busbar on the AC side; the lower bridge arm is composed of a second reactor, a second bridge arm resistor, and a plurality of sub-modules connected in series, the input end of the second reactor is connected to the output end of the second bridge arm resistor, the input end of the second bridge arm resistor is connected to the output end of the second sub-module on the lower bridge arm, and the output end of the last sub-module on the lower bridge arm is connected to the negative output end of the DC side. Figure 2 middle, represents the parasitic resistance of the bridge arm, represents the bridge arm inductance, Indicates the DC bus voltage, and express Phase bridge arm voltage and bridge arm current, , and Indicates output voltage and output current.

[0060] Specifically, if Figure 3 As shown, each submodule adopts a half-bridge submodule topology, consisting of 2 IGBTs, 2 anti-parallel diodes and 1 capacitor. Composition, assuming that the switch signal of each sub-module is , , when one of the power switching devices is turned on, , when the other power switch device is turned on, If the capacitor voltage is , then the relationship between the submodule output voltage and the switching signal is:

[0061]

[0062] in, Indicates the output voltage of the power submodule.

[0063] Specifically, when a common-mode voltage is injected into the very low frequency modular multilevel converter, calculating the output current and output circulating current of the very low frequency modular multilevel converter in different states includes:

[0064] Obtain the bridge arm resistance, bridge arm inductance, load resistance, load inductance, control period, and load current reference values ​​of the very low frequency modular multi-level converter;

[0065] The bridge arm resistance, bridge arm inductance, load resistance, load inductance and control period are substituted into the prediction model of the very low frequency modular multi-level converter to obtain the output current value of the very low frequency modular multi-level converter at the next moment under different states.

[0066] The load current reference value and the output current value of the very low frequency modular multilevel converter at the next moment under different states are substituted into the selection function to determine the working state of each bridge arm in the very low frequency modular multilevel converter;

[0067] According to the working status of each bridge arm in the very low frequency modular multi-level converter, the inverse of the load current and the inverse of the circulating current are calculated;

[0068] Based on the inverse of the load current, the inverse of the circulating current, and the action time of each bridge arm in the working state of the very low frequency modular multilevel converter, the output current and output circulating current of the very low frequency modular multilevel converter in different states are calculated.

[0069] It should be noted that after the common-mode voltage is injected into the very low frequency modular multilevel converter, its states include upper bridge arm on, lower bridge arm off, lower bridge arm on, upper bridge arm off, both upper and lower bridge arms on, and both upper and lower bridge arms off. The calculation methods of its output voltage and output circulating current in different states are shown in Table 1 below:

[0070] Table 1 Calculation table of output voltage and output circulating current of very low frequency modular multilevel converter in different states

[0071] In Table 1 above, state "10" means that all submodules injected into the upper bridge arm bypass the submodules in the lower bridge arm. In this state, the output voltage used for load current control is , the output circulating current used for circulating current control is , expressed as ;

[0072] State "01" means bypassing all sub-modules in the upper bridge arm and injecting the sub-module in the lower bridge arm. In this state, the output voltage used for load current control is , the output circulating current used for circulating current control is , expressed as ;

[0073] State "11" means injecting into all submodules in the upper and lower arms. In this state, the output voltage used for load current control is , the output circulating current used for circulating current control is , expressed as ;

[0074] State "00" means that all submodules in the upper and lower arms are bypassed. In this state, the output voltage used for load current control is , the output circulating current used for circulating current control is , expressed as ;

[0075] in, Represents the sum of the capacitor voltages of all submodules in the upper bridge arm, Represents the sum of the capacitor voltages of all submodules in the lower bridge arm, Indicates the common-mode voltage.

[0076] Specifically, the prediction model of the very low frequency modular multilevel converter is:

[0077] ;

[0078] in, Indicates the output current value of the very low frequency modular multi-level converter at the next moment in different states, , Indicates the last moment in a control cycle, Indicates the output current, Indicates the output voltage, represents the bridge arm resistance, represents the load resistance, represents the bridge arm inductance, represents the load inductance, Indicates the control period.

[0079] In the embodiment of the present invention, since the lower bridge arm voltage is balanced in the steady state, the upper bridge arm voltage and the lower bridge arm voltage are very small and negligible. Therefore, the impact on the load current is the same when the upper and lower bridge arms are both on or off, and their reciprocals are approximately zero. When the upper bridge arm is on and the lower bridge arm is off, the reciprocal of the load current is negative, and when the upper bridge arm is off and the lower bridge arm is on, the reciprocal of the load current is positive. Therefore, in each control cycle, the following selection function is used to select the appropriate state between the two states of upper bridge on and lower bridge arm off or upper bridge arm off and lower arm on. The expression of the selection function is:

[0080] ;

[0081] in, Indicates the selected value. Indicates the load current reference value.

[0082] The embodiment of the present invention selects the load control voltage with the smallest value to represent , and its corresponding circulating current control voltage , and the tracking errors of load current and circulating current are minimized by selecting two states: upper and lower bridge arms are both turned on and upper and lower bridge arms are both turned off. and .

[0083] Specifically, after determining the working state of the very low frequency modular multilevel converter, the expressions for calculating the inverse of the load current and the inverse of the circulating current are:

[0084] ;

[0085] ;

[0086] in, Indicates the inverse of the load current, represents the inverse of the circulation, Indicates the output current, Represents the output circulation, Indicates the output voltage, represents the circulating voltage, Indicates the load control voltage, represents the circulating current control voltage, represents the bridge arm resistance, represents the load resistance, represents the bridge arm inductance, Indicates the load inductance.

[0087] Specifically, the expressions for calculating the load current prediction value and the circulating current prediction value based on the output current and the output circulating current are:

[0088] ;

[0089] ;

[0090] in, represents the load current prediction value, represents the circulation prediction value, Indicates the output current, Represents the output circulation, 、 、 They represent the inverse of the load current of the very low frequency modular multilevel converter in different states, 、 、 They represent the inverse of the circulating current of the very low frequency modular multilevel converter in different states, 、 Both indicate the action time.

[0091] Specifically, the value function is expressed as:

[0092] ;

[0093] in, Indicates value, Indicates the load current reference value, Indicates the circulating current reference value, Represents a weighting factor that can be used to set the priority of current control.

[0094] In the embodiment of the present invention, before calculating the optimal on-time of the upper and lower bridge arm submodules, it is necessary to first establish the condition for minimizing the cost function, which is expressed as follows:

[0095] ;

[0096] Solving the above equation, we get:

[0097] ;

[0098] in,

[0099] ;

[0100]

[0101]

[0102] like 、 and As shown in the expression, is eliminated, which means that this control scheme does not require weighting factor adjustment. and If the value is negative, it is zero. and Due to physical limitations, Then .

[0103] Specifically, the on-time of the upper and lower bridge arm submodules in the very low frequency modular multilevel converter is calculated based on the cost function, and the optimal on-time of the upper and lower bridge arm submodules is obtained. The optimal on-time is then applied to the very low frequency modular multilevel converter through phase-shifted carrier modulation. The calculation expression is:

[0104]

[0105]

[0106] in, Indicates the optimal on-time of the upper arm submodule, Indicates the optimal on-time of the lower bridge arm submodule, Represents a binary variable. When the upper bridge arm is turned on and the lower bridge arm is turned off Equal to 1, when the upper bridge arm is turned off and the lower bridge arm is turned on Equal to 0.

[0107] In the embodiment of the present invention, although the submodule capacitor voltage is normal when the very low frequency modular multilevel converter is operating normally, a separate submodule capacitor balancing solution is still required. Therefore, it is necessary to obtain the on-time of the capacitor voltage balance of a single submodule based on the capacitor voltage balance reference value of any submodule in the upper and lower bridge arms. The calculation expression is:

[0108] ;

[0109] ;

[0110] in, Indicates the first The conduction time of each sub-module capacitor voltage balance, Indicates the first The conduction time of each sub-module capacitor voltage balance, Represents the bridge arm current of the upper bridge arm, Represents the bridge arm current of the lower bridge arm, It represents the proportional parameter for adjusting the voltage balance speed of the capacitor of a single submodule. Indicates the first Capacitor voltage balance reference value of each submodule, Indicates the first Capacitor voltage balance reference value of each submodule.

[0111] Each submodule in the chicken house is balanced, and the submodule capacitor voltage All for , using the bridge arm voltage Calculate the conduction time of each bridge. In phase-shift carrier modulation, set the carrier frequency to , at each sampling point, only one submodule’s on-time will change. Therefore, the on-time of the capacitor voltage balance of a single submodule can be updated according to the volt-second principle. The formula is:

[0112] ;

[0113] ;

[0114] in, , ;

[0115] Taking into account the voltage balance of each submodule, the updated expression for the on-time of a single submodule within the preset control cycle is obtained as follows:

[0116] ;

[0117] ;

[0118] in, Indicates the first The conduction time of each submodule in the preset control cycle, Indicates the first The conduction time of each submodule in the preset control cycle, Indicates the preset control period.

[0119] In the embodiment of the present invention, the on-time of each submodule is updated at the peak and valley of the triangular carrier. The on-time update sequence diagram of the first submodule of the upper bridge arm is as follows: Figure 4 As shown, the update frequency is , the triangular carrier phase shift of a single bridge arm submodule is , there is no need for phase shift between the upper and lower bridge arms, so that the output voltage can be at a level of , the equivalent switching frequency is ,in is the carrier frequency.

[0120] In the embodiment of the present invention, the output voltage includes fundamental and harmonic waves. Taking the a-phase bridge arm of the very low frequency modular multi-level converter as an example, the fundamental and harmonic waves can be divided into the following items:

[0121] Fundamental voltage of the drive motor , obtained from the motor control loop.

[0122] The injected common mode voltage is used to suppress the voltage fluctuation of the sub-module capacitor at low frequency. , The same in three phases, so there is no zero-sequence harmonic current in the three-phase three-wire topology.

[0123] Due to the difference in voltage ripple between the upper and lower bridge arms , The leading components of are derived as follows:

[0124] ;

[0125]

[0126] in, is the angular frequency of the injected high-frequency component, which is usually higher than the angular frequency of the fundamental frequency component. Much bigger, is the phase angle of the output current, is the number of submodules of a bridge arm, is the amplitude of the output current, M is the modulation index, is the capacitance of the submodule.

[0127] In order to suppress the current harmonics caused by the above voltage harmonics, the embodiment of the present invention controls the measurement of the output harmonic current through coordinate transformation with the LPF. The control block diagram is as follows: Figure 5 Considering The three-phase output current is converted into dq rotation coordinates under the fundamental frequency ω through ABC to dq transformation. The dominant fundamental frequency component becomes the DC term, while the harmonics that need to be suppressed are still AC terms. Then, through the dq to ABC inverse transformation, the fundamental component without harmonics can be restored. Finally, the fundamental current is subtracted from the original output current to obtain the harmonics.

[0128] After acquiring the harmonics, the embodiment of the present invention uses a feedback control loop with a proportional controller to suppress the harmonics. The reference value of the three-phase harmonics is zero. The proportional controller is used to track the effectiveness, reliability, and availability of the harmonic current. Considering that the harmonic components are complex AC terms with time-varying amplitude and frequency, advanced controllers such as proportional-integral and proportional-resonant controllers are likely to produce poor design results. The proportional controller is more suitable for this situation in terms of satisfactory dynamic performance, stability, and simplicity. Therefore, the output current of the proportional controller is added to the current reference value as the actual output current reference value. Then, the on-time of each sub-module after the harmonic suppression link is added is calculated, and the corresponding IGBT trigger pulse signal is generated to control the operation of the extremely low frequency modular multi-level converter, ultimately achieving the goal of suppressing the sub-module capacitor voltage fluctuation under low-frequency conditions while suppressing the output current harmonic components caused by common-mode voltage injection.

[0129] The present invention calculates the output current and output circulating current of the very low frequency modular multilevel converter under different states when a common mode voltage is injected into the very low frequency modular multilevel converter, calculates the load current prediction value and the circulating current prediction value based on the output current and the output circulating current, constructs a value function based on the load current prediction value and the circulating current prediction value to respectively calculate the on-time of the upper and lower bridge arm submodules in the very low frequency modular multilevel converter, obtains the optimal on-time of the upper and lower bridge arm submodules, and obtains the on-time of the capacitor voltage balance of a single submodule based on the capacitor voltage balance reference value of any submodule in the upper and lower bridge arms; updates the on-time of the capacitor voltage balance of a single submodule according to the volt-second principle, obtains the on-time of the single submodule within a preset control period, and updates the on-time of the capacitor voltage balance of the single submodule within a preset control period when a common mode voltage is injected. The harmonics extracted when the common-mode voltage is injected are adjusted and added to the reference current as the conduction prediction time, and the gate control signal of the very low frequency modular multi-level converter is generated based on the conduction time and the conduction prediction time of a single sub-module within a preset control cycle; compared with the prior art, the present invention constructs a value function after injecting the common-mode voltage, thereby reducing the violent fluctuation of the sub-module capacitor voltage under the extremely low frequency working condition; the harmonics extracted when the common-mode voltage is injected are adjusted and added to the reference current as the conduction prediction time, so as to predict the reference current of the next control cycle to suppress the harmonic components, and the gate control signal of the very low frequency modular multi-level converter is generated based on the conduction time and the conduction prediction time of a single sub-module within the preset control cycle, which can achieve the goal of suppressing the fluctuation of the sub-module capacitor voltage while reducing the harmonics of the output current.

[0130] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A control method for an extremely low frequency modular multi-level converter, characterized in that: include: Step 1: Obtain the capacitor voltage, output voltage, circulating current voltage, and reference current of each submodule on the bridge arm of the very low frequency modular multi-level converter; Step 2: When a common-mode voltage is injected into the very low frequency modular multi-level converter, the output current and the output circulating current of the very low frequency modular multi-level converter under different states are calculated, and a load current prediction value and a circulating current prediction value are calculated based on the output current and the output circulating current, and a cost function is constructed based on the load current prediction value and the circulating current prediction value. Step 3: Calculate the on-time of the upper and lower bridge arm submodules in the very low frequency modular multilevel converter based on the cost function to obtain the optimal on-time of the upper and lower bridge arm submodules, and obtain the on-time of the capacitor-voltage balance of a single submodule based on the capacitor-voltage balance reference value of any submodule in the upper and lower bridge arms; Step 4: Update the on-time of the capacitor voltage balance of a single submodule according to the volt-second principle to obtain the on-time of the single submodule within a preset control period, adjust the harmonics extracted when injecting the common-mode voltage, and add the adjusted harmonics to the reference current as the predicted on-time, and generate the gate control signal of the very low frequency modular multi-level converter based on the on-time of the single submodule within the preset control period and the predicted on-time.

2. The control method of the very low frequency modular multi-level converter according to claim 1, characterized in that: When a common mode voltage is injected into the very low frequency modular multilevel converter, calculating the output current and the output circulating current of the very low frequency modular multilevel converter in different states includes: Obtaining a bridge arm resistance, a bridge arm inductance, a load resistance, a load inductance, a control period, and a load current reference value of the very low frequency modular multi-level converter; Substituting the bridge arm resistance, bridge arm inductance, load resistance, load inductance and control period into the prediction model of the very low frequency modular multi-level converter to obtain the output current value of the very low frequency modular multi-level converter at the next moment under different states; Substituting the load current reference value and the output current value of the very low frequency modular multilevel converter at the next moment under different states into a selection function to determine the working state of each bridge arm in the very low frequency modular multilevel converter; Calculating the inverse of the load current and the inverse of the circulating current according to the operating state of each bridge arm in the very low frequency modular multi-level converter; Based on the inverse of the load current, the inverse of the circulating current, and the action time of the working state of each bridge arm in the very low frequency modular multilevel converter, the output current and the output circulating current of the very low frequency modular multilevel converter in different states are calculated.

3. The control method of the very low frequency modular multi-level converter according to claim 2, characterized in that: The expressions for calculating the inverse of the load current and the inverse of the circulating current are: ； ； in, Indicates the inverse of the load current, represents the inverse of the circulation, Indicates the output current, Represents the output circulation, Indicates the output voltage, represents the circulating voltage, Indicates the load control voltage, represents the circulating current control voltage, represents the bridge arm resistance, represents the load resistance, represents the bridge arm inductance, Indicates the load inductance.

4. The control method of the very low frequency modular multi-level converter according to claim 3, characterized in that: The expressions for calculating the load current prediction value and the circulating current prediction value according to the output current and the output circulating current are: ； ； in, represents the load current prediction value, represents the circulation prediction value, Indicates the output current, Represents the output circulation, 、 、 They represent the inverse of the load current of the very low frequency modular multilevel converter in different states, 、 、 They represent the inverse of the circulating current of the very low frequency modular multilevel converter in different states, 、 Both indicate the action time.

5. The control method of the very low frequency modular multi-level converter according to claim 4, characterized in that: The expression of the value function is: ； in, Indicates value, Indicates the load current reference value, Indicates the circulating current reference value, Represents the weighting factor.

6. The control method of the very low frequency modular multi-level converter according to claim 5, characterized in that: Based on the cost function, the conduction time of the upper and lower bridge arm submodules in the very low frequency modular multi-level converter is calculated respectively, and the calculation expression of the optimal conduction time of the upper and lower bridge arm submodules is obtained as follows: in, Indicates the optimal on-time of the upper arm submodule, Indicates the optimal on-time of the lower bridge arm submodule, Represents a binary variable.

7. The control method of the very low frequency modular multi-level converter according to claim 5, characterized in that: The calculation expression for the on-time of the capacitor voltage balance of a single submodule is obtained according to the capacitor voltage balance reference value of any submodule in the upper and lower bridge arms: ； ； in, Indicates the first The conduction time of each sub-module capacitor voltage balance, Indicates the first The conduction time of each sub-module capacitor voltage balance, Represents the bridge arm current of the upper bridge arm, Represents the bridge arm current of the lower bridge arm, Indicates the proportional parameter for adjusting the capacitor voltage balancing speed of a single submodule. Indicates the first Capacitor voltage balance reference value of each submodule, Indicates the first Capacitor voltage balance reference value of each submodule.

8. The control method of the very low frequency modular multi-level converter according to claim 7, characterized in that: The on-time of the capacitor voltage balance of a single submodule is updated according to the volt-second principle, and the update expression of the on-time of a single submodule within the preset control period is obtained as follows: in, Indicates the first The conduction time of each submodule in the preset control cycle, Indicates the first The conduction time of each submodule in the preset control cycle, Indicates the preset control period.

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