Loss optimization control method of modular multilevel converter and related equipment

Abstract

The invention relates to the technical field of converter control, and provides a loss optimization control method for a modular multilevel converter and related equipment, and the method comprises the steps: solving an expression of the average conduction loss and an expression of the average switching loss of a target modular multilevel converter; according to the expression, the positive correlation between the loss and the absolute average value of the bridge arm current is obtained through analysis, and the expression of the absolute average value of the bridge arm current is obtained; according to the positive correlation, loss optimization is equivalent to optimization of the absolute average value of the bridge arm current, and the optimal value range of the initial phase angle when the absolute average value of the bridge arm current is minimized is analyzed; according to the optimal value range of the initial phase angle, calculating an optimal initial phase angle and an optimal double frequency circulation component when the absolute average value of the bridge arm current is minimized; and controlling the target modular multi-level converter according to the optimal double-frequency circulation component and the optimal initial phase angle. According to the method, the optimization precision can be improved while loss optimization is realized.

Description

Technical Field

[0001] The present application relates to the technical field of converter control, and particularly relates to a loss optimization control method and related equipment for a modular multilevel converter. Background Art

[0002] As a new type of high-voltage and high-power electric energy conversion technology, the modular multilevel converter (MMC) has increasingly attracted the general attention of researchers in various countries. Compared with traditional two-level and three-level converters, MMC has many advantages such as simple structure, easy expansion, low switching loss, few voltage harmonics, and low installation and maintenance costs, and is widely used in the field of high-voltage direct current transmission. The power loss of MMC is the main component of the loss during high-voltage direct current transmission. Optimizing the loss can not only improve the overall efficiency of the system but also reduce the operating cost, so it has important practical value and research value.

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

[0004] The power loss of MMC mainly includes the conduction loss and switching loss of devices. The conduction loss is related to the conduction resistance of the device and the conduction current flowing through the device, while the switching loss is related to the switching speed of the device and the rate of change of current. However, it is extremely difficult to accurately calculate the power loss: the conduction resistance is affected by factors such as temperature and device aging, and its value is not constant, resulting in complexity in accurately determining the conduction resistance and calculating the conduction loss under different working conditions. The switching loss involves the switching speed and the rate of change of current. These parameters will change dynamically and non-linearly during actual operation, and it is difficult to establish an accurate mathematical model. The complexity of loss calculation further increases the difficulty of loss optimization. Since it is impossible to accurately grasp the specific influence of each parameter on the loss, it is difficult to formulate targeted optimization strategies. At the same time, multiple interrelated variables make adjusting one parameter may cause changes in other parameters, and even increase the loss of other parts. Therefore, the optimization work needs to comprehensively consider the balance of the entire system, resulting in a high difficulty in optimizing the loss of the modular multilevel converter. Summary of the Invention

[0005] The present application provides a loss optimization control method and related equipment for a modular multilevel converter, which can solve the problem of high difficulty in optimizing the loss of the modular multilevel converter.

[0006] In a first aspect, an embodiment of the present application provides a loss optimization control method for a modular multilevel converter. The loss optimization control method includes:

[0007] Obtaining an expression for the average conduction loss and an expression for the average switching loss of the target modular multilevel converter within one output current cycle;

[0008] Based on the expression for the average conduction loss and the expression for the average switching loss, it is analyzed that there is a positive correlation between the loss of the target modular multilevel converter and the absolute average value of the arm current of the target modular multilevel converter, and an expression for the absolute average value of the arm current is obtained;

[0009] According to the positive correlation, optimizing the loss of the target modular multilevel converter is equivalent to optimizing the absolute average value of the arm current. Based on the expression for the absolute average value of the arm current, the optimal value range of the initial phase angle of the target modular multilevel converter is analyzed when the absolute average value of the arm current is minimized;

[0010] According to the optimal value range of the initial phase angle, the optimal initial phase angle and the optimal double-frequency circulating current component when the absolute average value of the arm current is minimized are calculated;

[0011] Controlling the target modular multilevel converter according to the optimal double-frequency circulating current component and the optimal initial phase angle.

[0012] Optionally, obtaining the expressions for the average conduction loss and the average switching loss of the target modular multilevel converter within one output current cycle includes:

[0013] Calculating a first relational expression among the conduction voltage drop and the instantaneous conduction loss of the diode, the conduction voltage drop and the instantaneous conduction loss of the insulated gate bipolar transistor, and the arm current in the target modular multilevel converter, and a second relational expression among the turn-on loss and the turn-off loss of the insulated gate bipolar transistor, the reverse recovery loss of the diode, and the arm current;

[0014] Calculating an expression for the average conduction loss of the target modular multilevel converter within one output current cycle according to the first relational expression;

[0015] Calculating an expression for the average switching loss of the target modular multilevel converter within one output current cycle according to the second relational expression.

[0016] Optionally, the first relational expression is:

[0017]

[0018] Wherein, V CE represents the conduction voltage drop of the insulated gate bipolar transistor, V Frepresents the on - voltage drop of the diode, I C represents the on - current of the insulated - gate bipolar transistor, I F represents the on - current of the diode, f() represents the on - voltage drop V of the insulated - gate bipolar transistor CE Regarding the on - current I C Characteristic function of, f conT (), f cond (), respectively represent the on - loss calculation functions of the insulated - gate bipolar transistor and the diode, and ρ1, ρ2, ρ3, ρ4 are all fitting coefficients, p conIGBT represents the instantaneous on - loss of the insulated - gate bipolar transistor, p conDiode represents the instantaneous on - loss of the diode, i px (ωt) represents the instantaneous value of the arm current, and ωt represents the time point within one output - current cycle;

[0019] The second expression is:

[0020]

[0021] Among them, e onIGBT represents the turn - on loss of the insulated - gate bipolar transistor, e offIGBT represents the turn - off loss of the insulated - gate bipolar transistor, e recDiode represents the reverse - recovery loss of the diode, f on () represents the turn - on loss calculation function of the insulated - gate bipolar transistor, f off () represents the turn - off loss calculation function of the insulated - gate bipolar transistor, f rec () represents the reverse - recovery loss calculation function of the diode;

[0022] The expression of the average on - loss is:

[0023]

[0024] Among them, P conT represents the average on - loss of the insulated - gate bipolar transistor, P cond represents the average on - loss of the diode;

[0025] The expression of the average switching loss is:

[0026]

[0027] Among them, P on represents the average turn - on loss of the insulated - gate bipolar transistor, P off represents the average turn - off loss of the insulated - gate bipolar transistor, P rec represents the average reverse - recovery loss of the diode.

[0028] Optionally, according to the expressions of the average conduction loss and the average switching loss, it is analyzed that there is a positive correlation between the loss of the target modular multilevel converter and the absolute average value of the arm current of the target modular multilevel converter, including:

[0029] According to the expressions of the average conduction loss and the average switching loss, calculate the loss relationship between the total conduction loss and the total switching loss of the target modular multilevel converter and the absolute average value of the arm current;

[0030] Analyze the loss relationship to obtain a positive correlation between the total conduction loss and the total switching loss and the absolute average value of the arm current.

[0031] Optionally, the loss relationship is:

[0032]

[0033] Among them, P con represents the total conduction loss, P sw represents the total switching loss, P conT represents the average conduction loss of the insulated gate bipolar transistor, P cond represents the average conduction loss of the diode, P on represents the average turn-on loss of the insulated gate bipolar transistor, P off represents the average turn-off loss of the insulated gate bipolar transistor, P rec represents the average reverse recovery loss of the diode, λ1, λ2 are the fitting coefficients of the average conduction loss, μ1, μ2, μ3 are the fitting coefficients of the average switching loss, I RMS represents the effective value of the arm current, I ave represents the absolute average value of the arm current:

[0034] I RMS = ξI ave

[0035] Among them, ξ represents a variable.

[0036] Optionally, the expression of the absolute average value of the arm current is:

[0037]

[0038] Among them, I ave represents the absolute average value of the arm current, α1 represents the first intersection of the arm current i px and the ωt axis.

[0039] Optionally, based on the expression of the absolute average value of the arm current, analyze the optimal value range of the initial phase angle of the target modular multilevel converter when the absolute average value of the arm current is minimized, including:

[0040] Decompose the arm current into a first function and a second function related to the DC circulating current and the double-frequency circulating current components;

[0041] Solve for the first intersection point of the first function with the ωt axis and the second intersection point of the second function with the ωt axis;

[0042] Based on the first intersection point and the second intersection point, find the derivative of the expression of the absolute average value of the arm current with respect to the initial phase angle, and analyze the derivative result to obtain the optimal value range of the initial phase angle when the absolute average value of the arm current has a minimum value.

[0043] Optionally, the first function is:

[0044] y1 = I dc + 0.5i ox

[0045] where y1 represents the value of the first function, I dc represents the DC circulating current, and i ox represents the AC side output current;

[0046] The second function is:

[0047] y2 = -I z2

[0048] where y2 represents the value of the second function, I z2 represents the double-frequency circulating current component;

[0049] The optimal value range of the initial phase angle is:

[0050]

[0051] where represents the initial phase angle, and represents the initial phase angle of the output current.

[0052] Optionally, according to the optimal value range of the initial phase angle, calculate the optimal initial phase angle and the optimal double-frequency circulating current component when the absolute average value of the arm current is minimized, including:

[0053] Within the optimal value range of the initial phase angle, analyze the variation relationship between the absolute average value of the arm current and the initial phase angle, and take the value of the initial phase angle when the absolute average value of the arm current has a minimum value as the optimal initial phase angle;

[0054] Calculate the optimal first intersection point of the first function with the ωt axis and the optimal second intersection point of the second function with the ωt axis according to the optimal initial phase angle;

[0055] Find the derivative of the expression of the absolute average value of the arm current with respect to the double-frequency circulating current component according to the optimal first intersection point and the optimal second intersection point to obtain the optimal double-frequency circulating current component.

[0056] In a second aspect, an embodiment of the present application provides a loss optimization control device for a modular multilevel converter, including:

[0057] An obtaining module, configured to obtain an expression of the average conduction loss and an expression of the average switching loss of a target modular multilevel converter within one output current cycle;

[0058] An acquisition module, configured to analyze, according to the expression of the average conduction loss and the expression of the average switching loss, that there is a positive correlation between the loss of the target modular multilevel converter and the absolute average value of the arm current of the target modular multilevel converter, and obtain an expression of the absolute average value of the arm current;

[0059] An analysis module, configured to, according to the positive correlation, equivalently optimize the loss of the target modular multilevel converter to the optimization of the absolute average value of the arm current, and based on the expression of the absolute average value of the arm current, analyze the optimal value range of the initial phase angle of the target modular multilevel converter when the absolute average value of the arm current is minimized;

[0060] A calculation module, configured to calculate the optimal initial phase angle and the optimal second-harmonic circulating current component when the absolute average value of the arm current is minimized according to the optimal value range of the initial phase angle;

[0061] A control module, configured to control the target modular multilevel converter according to the optimal second-harmonic circulating current component and the optimal initial phase angle.

[0062] In a third aspect, an embodiment of the present application provides a terminal device, including a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, the loss optimization control method of the above modular multilevel converter is implemented.

[0063] In a fourth aspect, an embodiment of the present application provides a computer-readable storage medium storing a computer program, and when the computer program is executed by a processor, the loss optimization control method of the above modular multilevel converter is implemented.

[0064] The above solution of the present application has the following beneficial effects:

[0065] In the embodiments of the present application, by obtaining the expressions of the average conduction loss and the average switching loss of the target modular multilevel converter within one output current cycle, and then based on the expressions of the average conduction loss and the average switching loss, it is analyzed that there is a positive correlation between the loss of the target modular multilevel converter and the absolute average value of the arm current of the target modular multilevel converter. The expression of the absolute average value of the arm current is obtained, and then according to the positive correlation, the loss optimization of the target modular multilevel converter is equivalent to the optimization of the absolute average value of the arm current. Based on the expression of the absolute average value of the arm current, when the absolute average value of the arm current is minimized, the optimal value range of the initial phase angle of the target modular multilevel converter is analyzed. Then, according to the optimal value range of the initial phase angle, the optimal initial phase angle and the optimal second-harmonic circulating current component when the absolute average value of the arm current is minimized are calculated. Finally, the target modular multilevel converter is controlled according to the optimal second-harmonic circulating current component and the optimal initial phase angle. Among them, by analyzing the loss of the modular multilevel converter and equating the loss optimization to the optimization of the absolute average value of the arm current, the factors considered in the optimization can be reduced, and the difficulty of loss optimization can be lowered. By obtaining the optimal initial phase angle and the optimal second-harmonic circulating current component when the absolute average value of the arm current is minimized and controlling the target modular multilevel converter, the loss optimization accuracy is effectively improved while achieving loss optimization.

[0066] Other beneficial effects of the present application will be described in detail in the subsequent specific implementation part. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0068] Figure 1 It is a flowchart of a loss optimization control method for a modular multilevel converter provided by an embodiment of the present application;

[0069] Figure 2 It is a structural schematic diagram of a modular multilevel converter provided by an embodiment of the present application;

[0070] Figure 3 It is a structural schematic diagram of a power sub-module provided by an embodiment of the present application;

[0071] Figure 4 It is a simulation waveform diagram provided by an embodiment of the present application;

[0072] Figure 5Structural schematic diagram of the loss optimization control device for a modular multilevel converter provided by an embodiment of the present application;

[0073] Figure 6 Structural schematic diagram of the terminal device provided by an embodiment of the present application. Detailed implementation manners

[0074] In the following description, for the purpose of illustration rather than limitation, specific details such as specific system architectures, technologies, etc. are set forth in order to provide a thorough understanding of the embodiments of the present application. However, those skilled in the art should clearly understand that the present application can also be implemented in other embodiments without these specific details. In other cases, detailed descriptions of well-known systems, devices, circuits, and methods are omitted to avoid unnecessary details from obscuring the description of the present application.

[0075] It should be understood that when used in the specification of the present application and the appended claims, the term "comprising" indicates the presence of the described features, wholes, steps, operations, elements, and / or components, but does not exclude the presence or addition of one or more other features, wholes, steps, operations, elements, components, and / or their combinations.

[0076] It should also be understood that the term "and / or" as used in the specification of the present application and the appended claims refers to any combination and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0077] As used in the specification of the present application and the appended claims, the term "if" can be interpreted as "when", "once", "in response to determining", or "in response to detecting" depending on the context. Similarly, the phrase "if determined" or "if detected [the described condition or event]" can be interpreted as meaning "once determined", "in response to determining", "once detected [the described condition or event]", or "in response to detecting [the described condition or event]" depending on the context.

[0078] In addition, in the description of the specification of the present application and the appended claims, the terms "first", "second", "third", etc. are only used for differentiating descriptions and cannot be understood as indicating or implying relative importance.

[0079] References to "one embodiment" or "some embodiments" etc. described in the specification of this application mean that specific features, structures or characteristics described in connection with that embodiment are included in one or more embodiments of this application. Thus, statements such as "in one embodiment", "in some embodiments", "in other some embodiments", "in still other embodiments" etc. that appear in different places in this specification do not necessarily all refer to the same embodiment, but mean "one or more but not all embodiments", unless otherwise specifically emphasized. The terms "comprising", "including", "having" and their variants all mean "including but not limited to", unless otherwise specifically emphasized.

[0080] Aiming at the problem of high difficulty in optimizing the losses of the existing modular multilevel converter, an embodiment of this application provides a method for controlling the loss optimization of a modular multilevel converter. The loss optimization control method obtains the expressions of the average conduction loss and the average switching loss of the target modular multilevel converter in one output current cycle, and then analyzes based on the expressions of the average conduction loss and the average switching loss to obtain a positive correlation between the loss of the target modular multilevel converter and the absolute average value of the arm current of the target modular multilevel converter, and obtains the expression of the absolute average value of the arm current. Then, according to the positive correlation, the loss optimization of the target modular multilevel converter is equivalent to the optimization of the absolute average value of the arm current, and based on the expression of the absolute average value of the arm current, the optimal value range of the initial phase angle of the target modular multilevel converter when the absolute average value of the arm current is minimized is analyzed. Then, according to the optimal value range of the initial phase angle, the optimal initial phase angle and the optimal second-harmonic circulating current component when the absolute average value of the arm current is minimized are calculated. Finally, the target modular multilevel converter is controlled according to the optimal second-harmonic circulating current component and the optimal initial phase angle. Among them, by analyzing the losses of the modular multilevel converter and making the loss optimization equivalent to the optimization of the absolute average value of the arm current, the factors considered in the optimization can be reduced, and the difficulty of loss optimization can be reduced. By obtaining the optimal initial phase angle and the optimal second-harmonic circulating current component when the absolute average value of the arm current is minimized and controlling the target modular multilevel converter, the loss optimization accuracy is effectively improved while realizing the loss optimization.

[0081] Next, an exemplary description will be given of the method for controlling the loss optimization of the modular multilevel converter provided in this application.

[0082] As Figure 1 shown, the method for controlling the loss optimization of the modular multilevel converter provided in this application includes the following steps:

[0083] Step 11, obtain the expressions of the average conduction loss and the average switching loss of the target modular multilevel converter in one output current cycle.

[0084] The target modular multilevel converter is a modular multilevel converter that needs to be loss optimized.

[0085] The structure of modular multilevel converter is as follows: Figure 2 As shown, it includes a first phase bridge arm, a second phase bridge arm, and a third phase bridge arm connected in parallel (the first phase bridge arm includes a current i pa With current i na All components between the second phase bridge arm include current i pb With current i nb All components between the third phase bridge arm include current i pc With current i nc All components between). The first phase bridge arm, the second phase bridge arm, and the third phase bridge arm are all phase bridge arms, and the phase bridge arm includes an upper bridge arm and a lower bridge arm connected in sequence, the current input end of the upper bridge arm is the current input end of the phase bridge arm, and the current output end of the lower bridge arm is the current output end of the phase bridge arm; the upper bridge arm includes a plurality of first power sub-modules ( Figure 2 Medium current i pa With current i oa Between, current i pb With current i ob Between or current i pc With current i oc A group of power sub-modules SM1, SM2 to SM N )、The first bridge arm resistance ( Figure 2 Medium power submodule SM N The second end of the resistor R is connected to the first reactor ( Figure 2 The first end of the first power submodule is the current input end of the upper bridge arm, and the second end of the first reactor is the current output end of the upper bridge arm; the lower bridge arm includes the second reactor ( Figure 2 The inductor L connected to the second end of the first reactor and the second bridge arm resistor ( Figure 2 The resistor R connected to the second reactor in the middle), multiple second power sub-modules ( Figure 2 Medium current i oa With current i na Between, current i ob With current i nb Between or current i oc With current i nc A group of power sub-modules SM1, SM2 to SM N ), the first end of the first reactor is the current input end of the lower bridge arm, and the second end of the last second power submodule is the current output end of the lower bridge arm. The current at the second end of the first reactor in the first phase bridge arm is i oa , voltage is uoa The current at the second end of the first reactor in the second-phase bridge arm is i ob and the voltage is u ob The current at the second end of the first reactor in the third-phase bridge arm is i oc and the voltage is u oc .

[0086] Figure 2 Among them, i pa represents the arm current of the first-phase bridge arm, and i pb represents the arm current of the second-phase bridge arm, and i pc represents the arm current of the third-phase bridge arm, represents the voltage across the arm of the phase bridge arm, and i na represents the output current of the first-phase bridge arm, and i nb represents the output current of the second-phase bridge arm, and i nc represents the output current of the third-phase bridge arm, and u pa represents the voltage across the series-connected multiple first power sub-modules of the first-phase bridge arm, and u pb represents the voltage across the series-connected multiple first power sub-modules of the second-phase bridge arm, and u pc represents the voltage across the series-connected multiple first power sub-modules of the third-phase bridge arm, and u na represents the voltage across the series-connected multiple second power sub-modules of the first-phase bridge arm, and u nb represents the voltage across the series-connected multiple second power sub-modules of the second-phase bridge arm, and u nc represents the voltage across the series-connected multiple second power sub-modules of the third-phase bridge arm.

[0087] Figure 2 Both the first power sub-module and the second power sub-module in are power sub-modules, and the structure of the power sub-module is as Figure 3 shown, including the first IGBT ( Figure 3 T1 in), the second IGBT ( Figure 3 T2 in), the first diode ( Figure 3 D1 in), the second diode ( Figure 3 D2 in), and the capacitor ( Figure 3 C in); the drain of the first IGBT, the positive electrode of the first diode, the source of the second IGBT, and the negative electrode of the second diode are all the first ends of the power sub-module, and the drain of the second IGBT, the positive electrode of the second diode, and the second end of the capacitor are all the second ends of the power sub-module; the source of the first IGBT is connected to the negative electrode of the first diode and the first end of the capacitor.

[0088] In some embodiments of the present application, the steps of obtaining the expressions for the average conduction loss and the average switching loss of the target modular multilevel converter within one output current cycle include:

[0089] In the first step, a first relational expression among the conduction voltage drop and instantaneous conduction loss of the diodes, the conduction voltage drop and instantaneous conduction loss of the insulated gate bipolar transistors, and the arm current in the target modular multilevel converter is calculated, and a second relational expression among the turn-on loss and turn-off loss of the insulated gate bipolar transistors, the reverse recovery loss of the diodes, and the arm current is calculated.

[0090] It should be noted that this step is directed to the upper arm of a single phase in the MMC. The conduction voltage drop and instantaneous conduction loss of the diodes mentioned above refer to the conduction voltage drop and instantaneous conduction loss of all the diodes in the upper arm. The conduction voltage drop and instantaneous conduction loss of the insulated gate bipolar transistors are the conduction voltage drop and instantaneous conduction loss of all the insulated gate bipolar transistors in the upper arm, and the arm current is the arm current of the upper arm. Correspondingly, when controlling the MMC in the subsequent steps, it is also controlled for a single upper arm.

[0091] Specifically, the first relational expression is:

[0092]

[0093] Among them, V CE represents the conduction voltage drop of the insulated gate bipolar transistor, V F represents the conduction voltage drop of the diode, I C represents the conduction current of the insulated gate bipolar transistor, I F represents the conduction current of the diode, f() represents the characteristic function of the conduction voltage drop V CE of the insulated gate bipolar transistor with respect to the conduction current I C . Considering factors such as the internal conduction resistance and carrier migration of the insulated gate bipolar transistor, it obtains the voltage drop between the collector and emitter due to internal mechanisms at different currents. f conT (), f cond () respectively represent the conduction loss calculation functions of the insulated gate bipolar transistor and the diode, characterizing the power loss model composed of the ohmic loss of the conduction resistance and the voltage drop loss of the threshold voltage. ρ1, ρ2, ρ3, ρ4 are all fitting coefficients, p conIGBT represents the instantaneous conduction loss of the insulated gate bipolar transistor, p conDiode represents the instantaneous conduction loss of the diode, i px (ωt) represents the instantaneous value of the arm current, and ωt represents the time point within one output current cycle.

[0094] The second expression is:

[0095]

[0096] Among them, e onIGBT represents the turn-on loss of the insulated gate bipolar transistor, and e offIGBT represents the turn-off loss of the insulated gate bipolar transistor, and e recDiode represents the reverse recovery loss of the diode, and f on () represents the calculation function of the turn-on loss of the insulated gate bipolar transistor, which obtains the loss by considering factors such as current change and capacitor charging and discharging during turn-on, and f off () represents the calculation function of the turn-off loss of the insulated gate bipolar transistor, which calculates the loss by taking into account factors such as current drop and parasitic inductance during turn-off, and f rec () represents the calculation function of the reverse recovery loss of the diode, which determines the loss by integrating factors such as reverse recovery current overshoot and recovery time.

[0097] It should be noted that the conduction loss and switching loss curves of the IGBT and the diode can be approximately fitted according to the data sheet to obtain the first expression and the second expression in the above text.

[0098] In the second step, the expression of the average conduction loss of the target modular multilevel converter in one output current cycle is calculated according to the first relational expression.

[0099] Specifically, the expression of the average conduction loss is:

[0100]

[0101] Among them, P conT represents the average conduction loss of the insulated gate bipolar transistor, and P cond represents the average conduction loss of the diode.

[0102] In the third step, the expression of the average switching loss of the target modular multilevel converter in one output current cycle is calculated according to the second relational expression.

[0103] Specifically, the expression of the average switching loss is:

[0104]

[0105] Among them, P on represents the average turn-on loss of the insulated gate bipolar transistor, and P off represents the average turn-off loss of the insulated gate bipolar transistor, and P rec represents the average reverse recovery loss of the diode.

[0106] Step 12: According to the expressions of the average conduction loss and the average switching loss, it is analyzed that there is a positive correlation between the loss of the target modular multilevel converter and the absolute average value of the arm current of the target modular multilevel converter, and the expression of the absolute average value of the arm current is obtained.

[0107] In some embodiments of the present application, the above step of analyzing that there is a positive correlation between the loss of the target modular multilevel converter and the absolute average value of the arm current of the target modular multilevel converter according to the expressions of the average conduction loss and the average switching loss, and obtaining the expression of the absolute average value of the arm current includes:

[0108] The first step: According to the expressions of the average conduction loss and the average switching loss, calculate the loss relationship between the total conduction loss and the total switching loss of the target modular multilevel converter and the absolute average value of the arm current.

[0109] Specifically, the loss relationship is:

[0110]

[0111] Among them, P con represents the total conduction loss, P sw represents the total switching loss, P conT represents the average conduction loss of the insulated gate bipolar transistor, P cond represents the average conduction loss of the diode, P on represents the average turn-on loss of the insulated gate bipolar transistor, P off represents the average turn-off loss of the insulated gate bipolar transistor, P rec represents the average reverse recovery loss of the diode, λ1, λ2 are the fitting coefficients of the average conduction loss, μ1, μ2, μ3 are the fitting coefficients of the average switching loss, I RMS represents the effective value of the arm current, I ave represents the absolute average value of the arm current:

[0112] I RMS = ξI ave

[0113] Among them, ξ represents a variable.

[0114] The second step: Analyze the loss relationship to obtain that there is a positive correlation between the total conduction loss and the total switching loss and the absolute average value of the arm current.

[0115] It should be noted that it can be seen from the loss relationship that there is a positive correlation between the loss of the target modular multilevel converter (i.e., the total conduction loss and the total switching loss) and the absolute average value of the arm current.

[0116] Step 3: Obtain the expression of the absolute average value of the arm current.

[0117] Specifically, the expression of the absolute average value of the arm current is:

[0118]

[0119] where, I ave represents the absolute average value of the arm current, and α1 represents the first intersection point of the arm current i px with the ωt axis.

[0120] It should be noted that to achieve loss optimization, an appropriate circulating current is injected into the phase arm of the target modular multilevel converter to effectively adjust the waveform of the arm current, making it closer to the ideal state, thereby reducing I ave , and finally achieving loss optimization; according to injecting only the DC circulating current I dc into the phase arm, that is, setting the second-harmonic circulating current to 0, so that the arm finally only contains the DC circulating current I dc , obtaining the derivative of I ave with respect to the DC circulating current I dc , we can get dI ave / dI dc >0, and draw the conclusion that the absolute average value of the arm current increases with the increase of the injected circulating current, and the corresponding power loss also increases.

[0121] Step 13: According to the positive correlation relationship, equivalent the loss optimization of the target modular multilevel converter to the optimization of the absolute average value of the arm current, and based on the expression of the absolute average value of the arm current, analyze the optimal value range of the initial phase angle of the target modular multilevel converter when the absolute average value of the arm current is minimized.

[0122] In some embodiments of the present application, the step of analyzing the optimal value range of the initial phase angle of the target modular multilevel converter when the absolute average value of the arm current is minimized based on the expression of the absolute average value of the arm current includes:

[0123] Step 1: Decompose the arm current into a first function and a second function related to the DC circulating current and the second-harmonic circulating current component.

[0124] The second-harmonic circulating current component is a harmonic component in the circuit, and its frequency is twice the input signal frequency.

[0125] Specifically, the first function is:

[0126] y1 = I dc + 0.5i ox

[0127] where, y1 represents the value of the first function, I dc represents the DC circulating current, and iox Represents the AC side output current.

[0128] The second function is:

[0129] y2 = -I z2

[0130] Where y2 represents the value of the second function, and I z2 Represents the double-frequency circulating current component.

[0131] In the second step, solve for the first intersection point of the first function and the ωt axis, and the second intersection point of the second function and the ωt axis.

[0132] The above ωt axis is the axis with the time point ωt within one output current period as the coordinate.

[0133] Specifically, according to the different magnitudes of the DC circulating current and the double-frequency circulating current component, and the different value ranges of the initial phase angle, the first intersection point and the second intersection point are different.

[0134] When the value range of the initial phase angle is :

[0135]

[0136] Where α1 represents the first intersection point, α2 represents the second intersection point, Represents the initial phase angle of the output current.

[0137] When the value range of the initial phase angle is :

[0138]

[0139] When the value range of the initial phase angle is According to the different magnitudes of the DC circulating current and the double-frequency circulating current component, it is divided into two cases. When I dc < I z2 :

[0140]

[0141] When I dc > I z2 :

[0142]

[0143] It should be noted that when I dc = I z2 Both of the above two calculation methods are applicable and the results are the same, so no further discussion will be carried out.

[0144] In the third step, according to the first intersection point and the second intersection point, find the derivative of the expression of the absolute average value of the bridge-arm current with respect to the initial phase angle, and analyze the derivative result to obtain the optimal value range of the initial phase angle when the absolute average value of the bridge-arm current has a minimum value.

[0145] Specifically, substitute each group of the first intersection point and the second intersection point, and the corresponding value range of the initial phase angle into the expression of the absolute average value of the bridge-arm current, and take the derivative with respect to the initial phase angle. Analyze the derivative result of each group, and it can be analyzed that when and When, I ave has a minimum value, that is, loss optimization can be achieved.

[0146] That is, the optimal value range of the initial phase angle is:

[0147]

[0148] Among them, represents the initial phase angle, represents the initial phase angle of the output current.

[0149] Step 14: Calculate the optimal initial phase angle and the optimal second-harmonic circulating current component when the absolute average value of the bridge-arm current is minimized according to the optimal value range of the initial phase angle.

[0150] In some embodiments of the present application, the above step of calculating the optimal initial phase angle and the optimal second-harmonic circulating current component when the absolute average value of the bridge-arm current is minimized according to the optimal value range of the initial phase angle includes:

[0151] In the first step, within the optimal value range of the initial phase angle, analyze the variation relationship between the absolute average value of the bridge-arm current and the initial phase angle, and take the value of the initial phase angle when the absolute average value of the bridge-arm current has a minimum value as the optimal initial phase angle.

[0152] Specifically, according to the derivative result in step 13, within the optimal value range of the initial phase angle, analyze the variation relationship between the absolute average value of the bridge-arm current and the initial phase angle. When When, I dc <I z2 And I dc >I z2 In both cases, there is It shows that within this interval, the absolute average value of the bridge-arm current I ave Decreases with the increase of , and similarly, when There is It shows that within this interval, I ave Increases with the increase of From this, it can be concluded that when and only when I ave There is a minimum value, and loss optimization can be achieved.

[0153] Therefore, the optimal initial phase angle is taken as

[0154] Step 2: Calculate the optimal first intersection point of the first function and the ωt axis and the optimal second intersection point of the second function and the ωt axis according to the optimal initial phase angle.

[0155] Specifically, when I dc <I z2 , the optimal first intersection point and the optimal second intersection point are:

[0156]

[0157] When I dc >I z2 , the optimal first intersection point and the optimal second intersection point are:

[0158]

[0159] Step 3: Calculate the derivative of the expression of the absolute average value of the arm current with respect to the second-harmonic circulating current component according to the optimal first intersection point and the optimal second intersection point, and obtain the optimal second-harmonic circulating current component.

[0160] Specifically, substitute the optimal first intersection point, the optimal second intersection point, and the optimal initial phase angle into the expression of the absolute average value of the arm current, and calculate the derivative with respect to the second-harmonic circulating current component. Let dI ave / dI z2 = 0, and the optimal solutions of the injected second-harmonic circulating current component I z2 in both cases are:

[0161]

[0162] Among them, I z2(opt) represents the optimal second-harmonic circulating current component, and I o represents the amplitude of the AC-side output current.

[0163] Step 15: Control the target modular multilevel converter according to the optimal second-harmonic circulating current component and the optimal initial phase angle.

[0164] Specifically, calculate the optimal arm current of the phase arm in the target modular multilevel converter according to the optimal second-harmonic circulating current component and the optimal initial phase angle, inject DC circulating current and second-harmonic circulating current into each phase arm in the target modular multilevel converter, and control the arm current of each phase arm in the target modular multilevel converter to be maintained at the optimal arm current.

[0165] It should be noted that the formula for calculating the optimal arm current is as follows:

[0166]

[0167] Among them, i px(opt) represents the optimal arm current, and ω o represents the output current angular frequency.

[0168] It is worth mentioning that by analyzing the losses of the modular multilevel converter and equating the loss optimization to the optimization of the absolute average value of the arm current, the factors considered in the optimization can be reduced, and the difficulty of loss optimization can be lowered. By obtaining the optimal initial phase angle and the optimal second-harmonic circulating current component when the absolute average value of the arm current is minimized, and controlling the target modular multilevel converter, the loss optimization is effectively achieved while improving the accuracy of loss optimization.

[0169] The method of the present application will be exemplarily described below with a specific example.

[0170] The loss optimization of the modular multilevel converter is simulated, and only injecting DC circulating current is selected as the control for the method of the present application. The obtained simulation waveform diagram is as Figure 4 shown. Figure 4 The horizontal axis represents time t, with the unit of seconds s, and the vertical axis represents the loss value, with the unit of kilowatts KW. Figure 4 a is the total power loss waveform diagram. Scheme 1 is the total power loss of only injecting DC circulating current, and Scheme 2 is the total power loss of injecting DC circulating current and second-harmonic circulating current (i.e., the method of the present application). Figure 4 b is the conduction loss waveform diagram. Scheme 1 is the conduction loss of only injecting DC circulating current, and Scheme 2 is the conduction loss of injecting DC circulating current and second-harmonic circulating current (i.e., the method of the present application). Figure 4 c is the switching loss waveform diagram. Scheme 1 is the switching loss of only injecting DC circulating current, and Scheme 2 is the switching loss of injecting DC circulating current and second-harmonic circulating current (i.e., the method of the present application).

[0171] It can be seen that the present application significantly reduces the total power loss, conduction loss, and switching loss by injecting DC circulating current and second-harmonic circulating current, fully demonstrating the significant advantage of the second-harmonic circulating current in reducing power loss, and verifying the correctness of the theoretical analysis.

[0172] The present application approximates and fits the conduction loss and switching loss of IGBTs and diodes, and obtains the average conduction loss and average switching loss within one output current cycle; according to the integral mean value theorem, the relationship between the average conduction loss, average switching loss, and the absolute average value of the arm current is obtained, and the physical meaning of the absolute average value of the arm current is further studied; the injected circulating current command is studied to simultaneously include DC circulating current I dc and second-harmonic circulating current component I z2 , and I dc<I z2 When z2 , the mathematical model of the bridge arm and the feasibility of its loss optimization; it is studied that the injected circulating current command includes both the DC circulating current I dc and the double-frequency circulating current component I z2 , and when I dc >I z2 dc , the mathematical model of the bridge arm and the feasibility of its loss optimization; calculate the optimal solutions of the double-frequency circulating current component and the initial phase angle injected for loss optimization design; while realizing power loss optimization, this application ensures the high quality of the three-phase output current and the high precision of circulating current suppression. The research results provide reliable theoretical support and technical reference for the efficient operation and engineering application of the MMC system.

[0173] Next, an exemplary description will be given of the loss optimization control device of the modular multilevel converter provided in this application.

[0174] As Figure 5 shown, an embodiment of this application provides a loss optimization control device of a modular multilevel converter. The loss optimization control device 500 of the modular multilevel converter includes:

[0175] An obtaining module 501, configured to obtain an expression of the average conduction loss and an expression of the average switching loss of the target modular multilevel converter within one output current cycle;

[0176] An acquisition module 502, configured to analyze, according to the expression of the average conduction loss and the expression of the average switching loss, that there is a positive correlation between the loss of the target modular multilevel converter and the absolute average value of the bridge arm current of the target modular multilevel converter, and obtain an expression of the absolute average value of the bridge arm current;

[0177] An analysis module 503, configured to, according to the positive correlation, equivalently optimize the loss of the target modular multilevel converter to the optimization of the absolute average value of the bridge arm current, and based on the expression of the absolute average value of the bridge arm current, analyze the optimal value range of the initial phase angle of the target modular multilevel converter when the absolute average value of the bridge arm current is minimized;

[0178] A calculation module 504, configured to calculate the optimal initial phase angle and the optimal double-frequency circulating current component when the absolute average value of the bridge arm current is minimized according to the optimal value range of the initial phase angle;

[0179] A control module 505, configured to control the target modular multilevel converter according to the optimal double-frequency circulating current component and the optimal initial phase angle.

[0180] It should be noted that for the information interaction, execution process, etc. between the above devices / units, since they are based on the same concept as the method embodiment of this application, their specific functions and the technical effects brought are specifically described in the method embodiment part, and will not be elaborated here.

[0181] Those skilled in the art can clearly understand that, for the convenience and conciseness of description, only the above division of each functional unit and module is used as an example. In actual applications, the above functions can be allocated to different functional units and modules according to needs, that is, the internal structure of the device is divided into different functional units or modules to complete all or part of the functions described above. Each functional unit and module in the embodiment can be integrated into a processing unit, or each unit can exist physically alone, or two or more units can be integrated into one unit. The above integrated unit can be implemented in the form of hardware or in the form of a software functional unit. In addition, the specific names of each functional unit and module are only for the convenience of mutual distinction and do not limit the protection scope of this application. The specific working processes of the units and modules in the above system can refer to the corresponding processes in the foregoing method embodiments and will not be elaborated here.

[0182] As Figure 6 shown, an embodiment of the present application provides a terminal device. The terminal device D10 in this embodiment includes: at least one processor D100 ( Figure 6 only one processor is shown in the figure), a memory D101, and a computer program D102 stored in the memory D101 and executable on the at least one processor D100. When the processor D100 executes the computer program D102, the steps in any of the foregoing method embodiments are implemented.

[0183] Specifically, when the processor D100 executes the computer program D102, by obtaining the expressions of the average conduction loss and the average switching loss of the target modular multilevel converter in one output current cycle, and then based on the expressions of the average conduction loss and the average switching loss, it is analyzed that there is a positive correlation between the loss of the target modular multilevel converter and the absolute average value of the arm current of the target modular multilevel converter, and the expression of the absolute average value of the arm current is obtained. Then, according to the positive correlation, optimizing the loss of the target modular multilevel converter is equivalent to optimizing the absolute average value of the arm current. Based on the expression of the absolute average value of the arm current, when the absolute average value of the arm current is minimized, the optimal value range of the initial phase angle of the target modular multilevel converter is analyzed. Then, according to the optimal value range of the initial phase angle, the optimal initial phase angle and the optimal second-harmonic circulating current component when the absolute average value of the arm current is minimized are calculated. Finally, the target modular multilevel converter is controlled according to the optimal second-harmonic circulating current component and the optimal initial phase angle. Among them, by analyzing the loss of the modular multilevel converter and equating the loss optimization to the optimization of the absolute average value of the arm current, the factors considered in the optimization can be reduced, and the difficulty of loss optimization can be lowered. By obtaining the optimal initial phase angle and the optimal second-harmonic circulating current component when the absolute average value of the arm current is minimized and controlling the target modular multilevel converter, the loss optimization accuracy is effectively improved while achieving loss optimization.

[0184] The so-called processor D100 may be a central processing unit (CPU, Central Processing Unit), and this processor D100 may also be other general-purpose processors, digital signal processors (DSP, Digital Signal Processor), application specific integrated circuits (ASIC, Application Specific Integrated Circuit), field-programmable gate arrays (FPGA, Field-Programmable Gate Array) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor may be a microprocessor or this processor may also be any conventional processor, etc.

[0185] In some embodiments, the memory D101 may be an internal storage unit of the terminal device D10, such as a hard disk or memory of the terminal device D10. In some other embodiments, the memory D101 may also be an external storage device of the terminal device D10, such as a plug-in hard disk equipped on the terminal device D10, a Smart Media Card (SMC), a Secure Digital (SD) card, a Flash Card, etc. Further, the memory D101 may also include both the internal storage unit of the terminal device D10 and the external storage device. The memory D101 is used to store an operating system, application programs, a BootLoader, data, and other programs, such as program codes of the computer program. The memory D101 may also be used to temporarily store data that has been output or is to be output.

[0186] An embodiment of the present application further provides a computer-readable storage medium storing a computer program, and when the computer program is executed by a processor, the steps in the above method embodiments can be implemented.

[0187] An embodiment of the present application provides a computer program product, and when the computer program product runs on a terminal device, the terminal device is caused to execute the steps in the above method embodiments.

[0188] If the integrated unit is implemented in the form of a software functional unit and sold or used as an independent product, it may be stored in a computer-readable storage medium. Based on such an understanding, to implement all or part of the processes in the above method embodiments of the present application, a computer program may be used to instruct relevant hardware to complete, and the computer program may be stored in a computer-readable storage medium. When the computer program is executed by a processor, the steps in the above method embodiments can be implemented. Among them, the computer program includes computer program code, and the computer program code may be in the form of source code, object code, an executable file, or some intermediate form, etc. The computer-readable medium may at least include: any entity or device capable of carrying the computer program code to the loss optimization control method device / terminal device of the modular multilevel converter, a recording medium, a computer memory, a Read-Only Memory (ROM), a Random Access Memory (RAM), an electrical carrier signal, a telecommunication signal, and a software distribution medium. For example, a USB flash drive, a mobile hard disk, a magnetic disk, or an optical disc, etc. In some jurisdictions, according to legislation and patent practice, the computer-readable medium may not be an electrical carrier signal and a telecommunication signal.

[0189] In the above embodiments, the descriptions of the respective embodiments have their own focuses. For the parts not described in detail or recorded in a certain embodiment, reference may be made to the relevant descriptions of other embodiments.

[0190] Those of ordinary skill in the art can realize that the units and algorithm steps of the examples described in combination with the embodiments disclosed herein can be implemented by electronic hardware or a combination of computer software and electronic hardware. Whether these functions are executed in a hardware or software manner depends on the specific application and design constraints of the technical solution. Professional technicians can use different methods to implement the described functions for each specific application, but such implementation should not be considered to exceed the scope of this application.

[0191] The above is the preferred implementation manner of this application. It should be noted that for those of ordinary skill in the art of this technology, without departing from the principle described in this application, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of this application.

Claims

1. A loss optimization control method for a modular multilevel converter, characterized in that Including: Obtaining the expressions of the average conduction loss and the average switching loss of the target modular multilevel converter within one output current cycle; According to the expressions of the average conduction loss and the average switching loss, analyzing that there is a positive correlation between the loss of the target modular multilevel converter and the absolute average value of the arm current of the target modular multilevel converter, and obtaining the expression of the absolute average value of the arm current; According to the positive correlation, optimizing the loss of the target modular multilevel converter is equivalent to optimizing the absolute average value of the arm current, and based on the expression of the absolute average value of the arm current, analyzing the optimal value range of the initial phase angle of the target modular multilevel converter when the absolute average value of the arm current is minimized; According to the optimal value range of the initial phase angle, calculating the optimal initial phase angle and the optimal double-frequency circulating current component when the absolute average value of the arm current is minimized; Controlling the target modular multilevel converter according to the optimal double-frequency circulating current component and the optimal initial phase angle.

2. The loss optimization control method according to claim 1, characterized in that The obtaining of the expressions of the average conduction loss and the average switching loss of the target modular multilevel converter within one output current cycle includes: Calculating the first relational expressions among the conduction voltage drop and the instantaneous conduction loss of the diodes, the conduction voltage drop and the instantaneous conduction loss of the insulated gate bipolar transistors, and the arm current in the target modular multilevel converter, and the second relational expressions among the turn-on loss and the turn-off loss of the insulated gate bipolar transistors, the reverse recovery loss of the diodes, and the arm current; Calculating the expression of the average conduction loss of the target modular multilevel converter within one output current cycle according to the first relational expression; Calculating the expression of the average switching loss of the target modular multilevel converter within one output current cycle according to the second relational expression.

3. The loss optimization control method according to claim 2, wherein The first relational expression is: Among them, V CE represents the on-state voltage drop of the insulated gate bipolar transistor, V F represents the on-state voltage drop of the diode, I C represents the on-state current of the insulated gate bipolar transistor, I F represents the on-state current of the diode, f() represents the on-state voltage drop V CE of the insulated gate bipolar transistor with respect to the on-state current I C characteristic function, f conT (), f cond () respectively represent the on-state loss calculation functions of the insulated gate bipolar transistor and the diode, ρ1, ρ2, ρ3, ρ4 are all fitting coefficients, p conIGBT represents the instantaneous on-state loss of the insulated gate bipolar transistor, P conDiode represents the instantaneous on-state loss of the diode, i px (ωt) represents the instantaneous value of the arm current, ωt represents the time point within one output current cycle; The second expression is: Among them, e onIGBT represents the turn-on loss of the insulated gate bipolar transistor, and e offIGBT represents the turn-off loss of the insulated gate bipolar transistor, and e recDiode represents the reverse recovery loss of the diode, and f on () represents the calculation function of the turn-on loss of the insulated gate bipolar transistor, and f off () represents the calculation function of the turn-off loss of the insulated gate bipolar transistor, and f rec () represents the calculation function of the reverse recovery loss of the diode; The expression of the average conduction loss is: Among them, P conT represents the average conduction loss of the insulated gate bipolar transistor, and P cond represents the average conduction loss of the diode; The expression of the average switching loss is: Among them, P on represents the average turn-on loss of the insulated gate bipolar transistor, P off represents the average turn-off loss of the insulated gate bipolar transistor, P rec represents the average reverse recovery loss of the diode.

4. The loss optimization control method according to claim 1, wherein The analysis that there is a positive correlation between the loss of the target modular multilevel converter and the absolute average value of the arm current of the target modular multilevel converter according to the expressions of the average conduction loss and the average switching loss includes: Calculating the loss relational expressions between the total conduction loss and the total switching loss of the target modular multilevel converter and the absolute average value of the arm current according to the expressions of the average conduction loss and the average switching loss; Analyzing the loss relational expressions to obtain a positive correlation between the total conduction loss and the total switching loss and the absolute average value of the arm current.

5. The loss optimization control method according to claim 4, wherein The loss relational expression is: Among them, P con represents the total conduction loss, P sw represents the total switching loss, P conT represents the average conduction loss of the insulated gate bipolar transistor, P cond represents the average conduction loss of the diode, P on represents the average turn-on loss of the insulated gate bipolar transistor, P off represents the average turn-off loss of the insulated gate bipolar transistor, P rec represents the average reverse recovery loss of the diode, λ1 and λ2 are the fitting coefficients of the average conduction loss, μ1, μ2, and μ3 are the fitting coefficients of the average switching loss, I RMS represents the effective value of the arm current, I ave represents the absolute average value of the arm current: I RMS = ξI ave Where ξ represents a variable.

6. The loss optimization control method according to claim 1, wherein The expression of the absolute average value of the arm current is: Among them, I ave represents the absolute average value of the arm current, and α1 represents the first intersection point of the arm current i px with the ωt axis.

7. The loss optimization control method according to claim 6, characterized in that The analysis of the optimal value range of the initial phase angle of the target modular multilevel converter when the absolute average value of the arm current is minimized based on the expression of the absolute average value of the arm current includes: Decomposing the arm current into a first function and a second function related to the DC circulating current and the double-frequency circulating current component; Solve for the first intersection point of the first function and the ωt axis and the second intersection point of the second function and the ωt axis; According to the first intersection point and the second intersection point, find the derivative of the expression of the absolute average value of the arm current with respect to the initial phase angle, and analyze the derivative result to obtain the optimal value range of the initial phase angle when the absolute average value of the arm current has a minimum value.

8. The loss optimization control method according to claim 7, wherein The first function is: y1 = I dc + 0.5i ox Among them, y1 represents the value of the first function, and I dc represents the DC circulating current, and i ox represents the AC side output current; The second function is: y2 = -I z2 where y2 represents the value of the second function, and I z2 represents the double-frequency circulating current component; The optimal value range of the initial phase angle is: and Among them, represents the initial phase angle, represents the initial phase angle of the output current.

9. The loss optimization control method according to claim 8, wherein According to the optimal value range of the initial phase angle, calculate the optimal initial phase angle and the optimal second-harmonic circulating current component when the absolute average value of the arm current is minimized, including: Within the optimal value range of the initial phase angle, analyze the variation relationship between the absolute average value of the arm current and the initial phase angle, and take the value of the initial phase angle when the absolute average value of the arm current has a minimum value as the optimal initial phase angle; Calculate the optimal first intersection point of the first function and the ωt axis and the optimal second intersection point of the second function and the ωt axis according to the optimal initial phase angle; Find the derivative of the expression of the absolute average value of the arm current with respect to the second-harmonic circulating current component according to the optimal first intersection point and the optimal second intersection point to obtain the optimal second-harmonic circulating current component.

10. A terminal device, comprising a memory, a processor, and a computer program stored in the memory and executable on the processor, characterized in that, When the processor executes the computer program, it implements the loss optimization control method of the modular multilevel converter according to any one of claims 1 to 9.

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