阅读说明：本技术 一种隔离型模块化多电平变换器均压控制方法及装置 (Voltage-sharing control method and device for isolated modular multilevel converter ) 是由 李琰 迟永宁 刘超 魏春霞 于 2020-05-08 设计创作，主要内容包括：本发明涉及一种隔离型模块化多电平变换器均压控制方法及装置,包括：获取隔离型变换器各子模块的电压；根据隔离型变换器各子模块的电压对隔离型变换器进行均压控制；本发明的控制方法可以实现在变换器的交直流侧传输功率的同时,维持子模块电容电压的稳定,无需采集模块的电流,方案简单易实现。(The invention relates to a voltage-sharing control method and a voltage-sharing control device for an isolated modular multilevel converter, wherein the voltage-sharing control method comprises the following steps: acquiring the voltage of each submodule of the isolated converter; carrying out voltage-sharing control on the isolated converter according to the voltage of each submodule of the isolated converter; the control method can realize the purpose of maintaining the stability of the capacitance voltage of the sub-module while transmitting power at the AC/DC side of the converter, does not need to acquire the current of the module, and has a simple and easily realized scheme.)

1. An isolated modular multilevel converter voltage-sharing control method is characterized by comprising the following steps:

acquiring the voltage of each submodule of the isolated converter;

and carrying out voltage-sharing control on the isolated converter according to the voltage of each submodule of the isolated converter.

2. The method according to claim 1, wherein the voltage-sharing control of the isolated converter according to the voltage of each submodule of the isolated converter comprises the following steps:

judging whether the voltage of each submodule of the isolated converter deviates from a preset voltage interval or not;

if yes, adjusting the phase shift angle of each submodule deviating from the preset voltage interval in the isolated converter, and if not, ending the operation.

3. The method of claim 2, wherein adjusting the phase shift angle of each submodule offset from a predetermined voltage interval in the isolated converter comprises:

sequencing each submodule in an ascending/descending order based on the voltage of each submodule deviating from a preset voltage interval in the isolated converter, acquiring an ascending/descending sequence of the submodules and numbering the submodules in the ascending/descending sequence of the submodules in sequence;

sequencing the phase shifting angles of all sub-modules deviating from a preset voltage interval in the isolated converter in a descending/ascending manner to obtain a phase shifting angle descending/ascending sequence and numbering the phase shifting angles in the phase shifting angle descending/ascending sequence in sequence;

adjusting the phase shift angle of the sub-module with the number i in the ascending/descending sequence of the sub-modules to be the phase shift angle with the number i in the ascending/descending sequence of the phase shift angle;

wherein i belongs to [1, M ], and M is the total number of the numbers.

4. The method of claim 2, wherein the predetermined voltage interval is [ 99.5% u, 100.5% u ];

and u is a standard value of the sub-module capacitor voltage.

5. The method of claim 4, wherein the sub-module capacitance voltage normalized value is determined as follows:

in the formula, U is the rated voltage of the isolated converter, and N is the total number of submodules of the isolated converter.

6. An isolated modular multilevel converter voltage-sharing control device, comprising:

the acquisition module is used for acquiring the voltage of each submodule of the isolated converter;

and the control module is used for carrying out voltage-sharing control on the isolated converter according to the voltage of each submodule of the isolated converter.

7. The apparatus of claim 6, wherein the obtaining module is specifically configured to:

the judgment unit is used for judging whether the voltage of each submodule of the isolated converter deviates from a preset voltage interval or not;

and the adjusting unit is used for adjusting the phase shift angle of each submodule deviating from the preset voltage interval in the isolated converter if the submodule deviates from the preset voltage interval, and ending the operation if the submodule does not deviate from the preset voltage interval.

8. The apparatus of claim 7, wherein the adjustment unit comprises:

the first adjusting subunit is used for performing ascending/descending sequencing on each submodule based on the voltage of each submodule deviating from a preset voltage interval in the isolated converter, acquiring an ascending/descending sequence of the submodules and numbering the submodules in the ascending/descending sequence of the submodules in sequence;

the second regulating subunit is used for performing descending/ascending sequencing on the phase shifting angles of all the submodules deviating from a preset voltage interval in the isolated converter, acquiring descending/ascending sequences of the phase shifting angles and numbering the phase shifting angles in the descending/ascending sequences of the phase shifting angles in sequence;

the third adjusting subunit is used for adjusting the phase shifting angle of the submodule numbered i in the ascending/descending sequence of the submodules to be the phase shifting angle numbered i in the descending/ascending sequence of the phase shifting angle;

wherein i belongs to [1, M ], and M is the total number of the numbers.

9. The apparatus of claim 7, wherein the predetermined voltage interval is [ 99.5% u, 100.5% u ];

and u is a standard value of the sub-module capacitor voltage.

10. The apparatus of claim 9, wherein the sub-module capacitance voltage standard value u is determined as follows:

in the formula, U is the rated voltage of the isolated converter, and N is the total number of submodules of the isolated converter.

Technical Field

The invention relates to the technical field of power electronic commutation control, in particular to a voltage-sharing control method and device for an isolated modular multilevel converter.

Background

The Modular Multilevel Converter (MMC) topology differs from the traditional two-level or three-level topologies in that the MMC has no centralized dc side capacitance, but instead has a distributed dc capacitance in each sub-module. Therefore, how to maintain the stability of the sub-module capacitor voltage while transmitting power on the ac/dc side of the MMC becomes a problem that is not negligible in the operation process of the MMC. Different, the difference of switching loss and the difference of drive pulse of each submodule piece charge-discharge of MMC all can lead to submodule piece capacitor voltage unbalance, send the safety and stability operation that will directly influence MMC. Therefore, the sub-module capacitor voltage sharing problem becomes the key of MMC research. From an overall perspective, voltage-sharing control is closely related to the modulation strategy.

The typical strategy of the voltage-sharing control method based on the bridge arm current set in the prior art is to collect bridge arm currents, determine the charge-discharge state of each submodule according to the bridge arm currents, increase the charge time of the submodule with lower voltage and increase the discharge time of the submodule with higher voltage according to a capacitor voltage sequencing sequence, but the control can cause uneven switching frequency of the submodules, so that the submodules with similar capacitor voltages are repeatedly put in and cut off; in addition, the fundamental wave switching frequency modulation control method for periodically switching on and off and rotating each submodule does not consider the difference of physical devices of the submodules, and the difference can cause the voltage-sharing control of the submodules to be unbalanced, the periodic energy to be unbalanced and the voltage to be easily diffused.

Disclosure of Invention

Aiming at the defects in the prior art, the invention aims to provide a voltage-sharing control method for an isolated modular multilevel converter, which solves the defects in the prior art, realizes voltage-sharing control on a submodule while transmitting power on the AC/DC side of the converter, and maintains the stability of capacitance voltage of the submodule.

The purpose of the invention is realized by adopting the following technical scheme:

the invention provides an isolated modular multilevel converter voltage-sharing control method, which is improved in that the method comprises the following steps:

acquiring the voltage of each submodule of the isolated converter;

and carrying out voltage-sharing control on the isolated converter according to the voltage of each submodule of the isolated converter.

Preferably, the voltage-sharing control of the isolated converter according to the voltage of each submodule of the isolated converter includes:

judging whether the voltage of each submodule of the isolated converter deviates from a preset voltage interval or not;

if yes, adjusting the phase shift angle of each submodule deviating from the preset voltage interval in the isolated converter, and if not, ending the operation.

Further, the adjusting of the phase shift angle of each sub-module deviating from the preset voltage interval in the isolated converter includes:

sequencing each submodule in an ascending/descending order based on the voltage of each submodule deviating from a preset voltage interval in the isolated converter, acquiring an ascending/descending sequence of the submodules and numbering the submodules in the ascending/descending sequence of the submodules in sequence;

sequencing the phase shifting angles of all sub-modules deviating from a preset voltage interval in the isolated converter in a descending/ascending manner to obtain a phase shifting angle descending/ascending sequence and numbering the phase shifting angles in the phase shifting angle descending/ascending sequence in sequence;

adjusting the phase shift angle of the sub-module with the number i in the ascending/descending sequence of the sub-modules to be the phase shift angle with the number i in the ascending/descending sequence of the phase shift angle;

wherein i belongs to [1, M ], and M is the total number of the numbers.

Further, the preset voltage interval is [ 99.5% u, 100.5% u ];

and u is a standard value of the sub-module capacitor voltage.

Further, the sub-module capacitor voltage standard value is determined according to the following formula:

in the formula, U is the rated voltage of the isolated converter, and N is the total number of submodules of the isolated converter.

Based on the same inventive concept, the invention also provides an isolated modular multilevel converter voltage-sharing control device, and the improvement is that the device comprises:

the acquisition module is used for acquiring the voltage of each submodule of the isolated converter;

and the control module is used for carrying out voltage-sharing control on the isolated converter according to the voltage of each submodule of the isolated converter.

Preferably, the obtaining module is specifically configured to:

the judgment unit is used for judging whether the voltage of each submodule of the isolated converter deviates from a preset voltage interval or not;

and the adjusting unit is used for adjusting the phase shift angle of each submodule deviating from the preset voltage interval in the isolated converter if the submodule deviates from the preset voltage interval, and ending the operation if the submodule does not deviate from the preset voltage interval.

Further, the adjusting unit includes:

the first adjusting subunit is used for performing ascending/descending sequencing on each submodule based on the voltage of each submodule deviating from a preset voltage interval in the isolated converter, acquiring an ascending/descending sequence of the submodules and numbering the submodules in the ascending/descending sequence of the submodules in sequence;

the second regulating subunit is used for performing descending/ascending sequencing on the phase shifting angles of all the submodules deviating from a preset voltage interval in the isolated converter, acquiring descending/ascending sequences of the phase shifting angles and numbering the phase shifting angles in the descending/ascending sequences of the phase shifting angles in sequence;

the third adjusting subunit is used for adjusting the phase shifting angle of the submodule numbered i in the ascending/descending sequence of the submodules to be the phase shifting angle numbered i in the descending/ascending sequence of the phase shifting angle;

wherein i belongs to [1, M ], and M is the total number of the numbers.

Further, the preset voltage interval is [ 99.5% u, 100.5% u ];

and u is a standard value of the sub-module capacitor voltage.

Further, determining the sub-module capacitor voltage standard value u according to the following formula:

in the formula, U is the rated voltage of the isolated converter, and N is the total number of submodules of the isolated converter.

Compared with the closest prior art, the invention has the following beneficial effects:

the invention provides a voltage-sharing control method and a voltage-sharing control device for an isolated modular multilevel converter, wherein the voltage-sharing control method comprises the following steps: acquiring the voltage of each submodule of the isolated converter; carrying out voltage-sharing control on the isolated converter according to the voltage of each submodule of the isolated converter; the control method can realize the purpose of maintaining the stability of the capacitance voltage of the sub-module while transmitting power at the AC/DC side of the converter, does not need to acquire the current of the module, and has a simple and easily realized scheme.

The voltage-sharing control method comprises the steps that voltage-sharing control is carried out on the isolated converter according to the voltage of each submodule of the isolated converter, a voltage interval is set, and the submodules with normal energy and low energy do not participate in voltage-sharing control in consideration of the difference of physical devices of the submodules, so that the defects of voltage-sharing control failure and voltage divergence are overcome, and the safety of the submodules is improved;

realize voltage-sharing control through redistributing the phase shift angle, solved the problem that the neutron module can be thrown into repeatedly and amputated in the prior art, reduced the switching frequency of submodule piece, increased system stability from another aspect.

Drawings

FIG. 1 is a flow chart of a voltage-sharing control method of an isolated modular multilevel converter according to the present invention;

FIG. 2 is a general equivalent phasor diagram for the AC side of the converter in an embodiment of the present invention;

FIG. 3 is a graph of a first variation of the sub-module capacitor voltage of the converter in an embodiment of the invention;

FIG. 4 is a second graph of variation in sub-module capacitor voltage of the converter in an embodiment of the invention;

fig. 5 is a schematic diagram of the voltage-sharing control device of the isolated modular multilevel converter.

Detailed Description

The following describes embodiments of the present invention in further detail with reference to the accompanying drawings.

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are some, but not all, embodiments of the present invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The invention provides an isolated modular multilevel converter voltage-sharing control method, as shown in fig. 1, the method comprises the following steps:

acquiring the voltage of each submodule of the isolated converter;

and carrying out voltage-sharing control on the isolated converter according to the voltage of each submodule of the isolated converter.

The process of the present invention is further explained below with reference to specific examples.

In the embodiment of the invention, voltage-sharing control is carried out on an isolated topology structure formed by connecting two sets of MMC converters through an alternating current side, and the voltage polarity of the VSC cannot be changed in the structure, so that each submodule on the VSC side adopts a half-bridge topology structure, and the voltage polarity on the LCC side can be inverted, so that each submodule on the LCC side adopts a full-bridge topology structure, and an isolation transformer is adopted in the middle to realize electrical isolation of a primary side and a secondary side.

In an embodiment of the present invention, the performing voltage-sharing control on the isolated converter according to the voltage of each submodule of the isolated converter includes:

judging whether the voltage of each submodule of the isolated converter deviates from a preset voltage interval or not;

if yes, adjusting the phase shift angle of each submodule deviating from the preset voltage interval in the isolated converter, and if not, ending the operation.

The reason for realizing voltage-sharing control of the submodules is as follows by adjusting the phase shift angle of each submodule deviating from the preset voltage interval in the isolated converter:

analyzing the influence of alternating current on the capacitor voltage of each submodule, wherein the current flowing through each submodule is bridge arm current, and according to the basic principle of MMC, the bridge arm current comprises two main components: a direct current component and an alternating current component. The direct current component is a component contained by both the upper bridge arm and the lower bridge arm and belongs to a common modulus. The direct current component mainly comprises 1/2 of the direct current side current value and the ring current between the bridge arms. The ac component is a current on the ac side, and the ac side current flows through the upper and lower arms 1/2, but the direction is opposite, and the ac side current belongs to the differential mode component.

Theoretical analysis shows that the voltage-sharing control based on submodule voltage sequencing cannot avoid blind spots per se, so that the application range of the voltage-sharing control is greatly reduced. Therefore, an optimized control method is required to expand the application range. On the premise that the unit phase shift quantity is selected, the single-period energy accumulation value of the submodule is only related to the phase shift angle and is not related to the current.

By controlling the phase shift angle of the sub-module, the energy accumulation of the sub-module in a single period can be controlled. Therefore, submodule phase shift angles can be reasonably distributed based on the capacitor voltage sequencing result, and submodule voltage-sharing control is achieved. The specific theoretical analysis is as follows:

the voltage equalization of the submodules is a whole dynamic process, and in the dynamic process, the sum of the energy absorbed by the submodules is 0, and the submodules are in a balanced state. In a conventional power frequency modulation strategy, the dynamic process is in units of voltage cycles. However, in the modulation strategy of the present document, the ac voltage and the current are in the same frequency state, so that one complete energy absorption is completed in one switching cycle, and several switching cycles constitute one dynamic process, so that the analysis of the whole dynamic process is the analysis of a single switching cycle. Taking the kth sub-module of the phase-a lower bridge arm on the primary side as an example, the energy interaction of the sub-modules in a single period is analyzed. Neglecting the voltage fluctuation of the sub-module, the voltage of the sub-module is U_{pri_DC}N, N is the number of bridge arm sub-modules, U_{pri_DC}Is a primary side direct current voltage. In one switching period T_sThe energy E absorbed by the inner sub-moduleThe following equation is obtained:

in the formula, S_kThe fourier expansion of the switching function for the kth submodule is as follows:

in the formula, gamma_k,2n-1＝(2n-1)γ_k，γ_kAnd n is the phase shift angle of the kth submodule and is the point number of Fourier transform.

According to the MMC working principle, the upper and lower bridge arms equally divide an alternating current i_pCommon mode current i_cThus, the primary lower arm current i can be expressed_priAl：

According to the integral relationship shown in the above formula: in a switching period T_sThe sub-module energy will be affected by the common mode current and the alternating current. Common mode current i_cThe power supply comprises a direct current component and an even-order circulating current component provided by a direct current power supply, and is represented by the following formula:

wherein, I_priDCIs the input DC quantity of the primary power supply I_c,2mIs the amplitude of the even-order circulating component, m is the number of components in the switching period, a_2mIs the phase shift angle of the common mode current.

The switch function contains odd-numbered components in each step. According to the trigonometric function orthogonal principle, the following results are obtained: the result of integrating the even-order circulating current component with the odd-order component of the switching function is 0. And because the duty ratio of the sub-module switches is fixed 50%, the charging and discharging of the input direct current of the primary power supply to each sub-module in the bridge arm are the same, and the charging and discharging of the common-mode current of the bridge arm to all the sub-modules on the bridge arm are the same. And the alternating current components in the bridge arms will generate different energy accumulations for the sub-modules with different phase shifting angles.

Now two sub-module switching functions are determined as s_iAnd s_j，Q_iAnd Q_jObviously, when the phase shift angles are different, the electric charge quantity absorbed by the sub-modules in a single period is different, and by utilizing the difference, the energy storage difference among the sub-modules can be eliminated by controlling the phase shift angles, so that the voltage balance of the sub-modules is realized.

Calculating the AC voltage u of the primary side_pThe following were used:

primary side ac voltage u_pSecondary side ac voltage u_sThe general formula of each harmonic is:

u_p,2n-1＝(-1)ⁿ⁺¹U_p,2n-1cos(2n-1)ωt

u_s,2n-1＝(-1)ⁿ⁺¹U_s,2n-1cos[(2n-1)ωt+δ_2n-1)]

wherein, delta_2n-1(2n-1) δ, δ being the phase-shifting angle of the primary or secondary side, U_p,2n-1And U_s,2n-1Is the amplitude of the 2n-1 th harmonic of the primary and secondary side voltage. And (3) selecting the 2n-1 th voltage of the original secondary side to construct a universal equivalent phasor diagram at the AC side, as shown in figure 2. According to the phasor relationship shown in fig. 2, the general formula of each subharmonic of the primary side alternating current can be obtained:

i_p,2n-1＝(-1)ⁿ⁺¹I_p,2n-1cos[(2n-1)ωt+φ_p,2n-1]

in the formula I_p,2n-1The amplitude of each harmonic current corresponding to the primary side,is U_p,2n-1And I_p,2n-1ω is the angular frequency, and the sub-module energy E can be decomposed into:

wherein E is_constThe energy introduced for the common mode current is the same for all sub-modules. E_2n-1The energy introduced for the 2n-1 order alternating current is different for different sub-modules. Due to the orthogonal principle of the functions, energy is generated or consumed between the same voltage and current.

Therefore, the energy general formula E of each order can be obtained by multiplying and summing the alternating voltage and the current of the corresponding order_2n-1：

From the phasor relationship shown in fig. 2, the above equation can be simplified as follows.

E_2n-1＝A_2n-1f(δ_2n-1,γ_k,2n-1)

Wherein the content of the first and second substances,

f(δ_2n-1,γ_k,2n-1)＝sinδ_2n-1cosγ_k,2n-1-(Z-cosδ_2n-1)sinγ_k,2n-1，

in the formula, N₁Number of primary bridge arm sub-modules, N₂The number of sub-modules of the secondary side bridge arm is U_{sec_DC}Is a secondary side dc voltage.

According to the energy general formula of the submodule, the following conditions are shown: energy E of sub-module of each order_2n-1And phase shift angle gamma_k,2n-1And (4) correlating.

Specifically, the adjusting the phase shift angle of each sub-module deviating from the preset voltage interval in the isolated converter includes:

sequencing each submodule in an ascending/descending order based on the voltage of each submodule deviating from a preset voltage interval in the isolated converter, acquiring an ascending/descending sequence of the submodules and numbering the submodules in the ascending/descending sequence of the submodules in sequence;

sequencing the phase shifting angles of all sub-modules deviating from a preset voltage interval in the isolated converter in a descending/ascending manner to obtain a phase shifting angle descending/ascending sequence and numbering the phase shifting angles in the phase shifting angle descending/ascending sequence in sequence;

adjusting the phase shift angle of the sub-module with the number i in the ascending/descending sequence of the sub-modules to be the phase shift angle with the number i in the ascending/descending sequence of the phase shift angle;

wherein i belongs to [1, M ], and M is the total number of the numbers.

Further, the preset voltage interval is [ 99.5% u, 100.5% u ];

and u is a standard value of the sub-module capacitor voltage.

Further, the sub-module capacitor voltage standard value is determined according to the following formula:

in the formula, U is the rated voltage of the isolated converter, and N is the total number of submodules of the isolated converter.

The MMC topology is built in simulation software, in a simulation experiment of voltage-sharing control, a VSC side is connected with a +800V direct-current voltage source, an LCC side is connected with +900V, simulation experiment waveforms of capacitance and voltage of 4 sub-modules of an upper bridge arm of the VSC side A phase are used as analysis objects, as shown in figure 3, the capacitance and voltage of the 4 sub-modules are obviously diverged in a period of time after an initial moment, after the voltage-sharing control method is put into the VSC side A phase, a phase shifting angle of the voltage-sharing control method is redistributed at 0.2s, in the subsequent time, the voltage is obviously converged, and finally, the voltage and the capacitance are in a stable state. In fig. 3, the horizontal axis represents time in seconds and the vertical axis represents capacitor voltage in volts.

And, control VSC side direct current to 43A, in order to simulate the condition of load sudden change. As shown in fig. 4, it can be seen that the fluctuation range of the capacitor voltage becomes large, the fluctuation value thereof is less than 1.4V, and the direct current rises. But the capacitor voltage of each sub-module still shows a convergence trend. For a sub-module with a nominal operating voltage of 200V, a voltage fluctuation of 1.4V is still within an acceptable range. Therefore, the voltage-sharing control method has certain applicability, can keep a good voltage-sharing control effect when the load is large or small, and has no failure in voltage-sharing control at the moment of sudden change of the load. In fig. 4, the horizontal axis represents time in seconds, the left vertical axis represents DC current in amperes a, and the right vertical axis represents capacitor voltage in volts.

Based on the same inventive concept, the present invention further provides an isolated modular multilevel converter voltage-sharing control apparatus, as shown in fig. 5, the apparatus includes:

the acquisition module is used for acquiring the voltage of each submodule of the isolated converter;

and the control module is used for carrying out voltage-sharing control on the isolated converter according to the voltage of each submodule of the isolated converter.

Preferably, the obtaining module is specifically configured to:

the judgment unit is used for judging whether the voltage of each submodule of the isolated converter deviates from a preset voltage interval or not;

and the adjusting unit is used for adjusting the phase shift angle of each submodule deviating from the preset voltage interval in the isolated converter if the submodule deviates from the preset voltage interval, and ending the operation if the submodule does not deviate from the preset voltage interval.

Further, the adjusting unit includes:

the first adjusting subunit is used for performing ascending/descending sequencing on each submodule based on the voltage of each submodule deviating from a preset voltage interval in the isolated converter, acquiring an ascending/descending sequence of the submodules and numbering the submodules in the ascending/descending sequence of the submodules in sequence;

the second regulating subunit is used for performing descending/ascending sequencing on the phase shifting angles of all the submodules deviating from a preset voltage interval in the isolated converter, acquiring descending/ascending sequences of the phase shifting angles and numbering the phase shifting angles in the descending/ascending sequences of the phase shifting angles in sequence;

the third adjusting subunit is used for adjusting the phase shifting angle of the submodule numbered i in the ascending/descending sequence of the submodules to be the phase shifting angle numbered i in the descending/ascending sequence of the phase shifting angle;

wherein i belongs to [1, M ], and M is the total number of the numbers.

Further, the preset voltage interval is [ 99.5% u, 100.5% u ];

and u is a standard value of the sub-module capacitor voltage.

Further, determining the sub-module capacitor voltage standard value u according to the following formula:

in the formula, U is the rated voltage of the isolated converter, and N is the total number of submodules of the isolated converter.

In summary, the voltage-sharing control method and device for the isolated modular multilevel converter provided by the present invention include: acquiring the voltage of each submodule of the isolated converter; carrying out voltage-sharing control on the isolated converter according to the voltage of each submodule of the isolated converter; the control method can maintain the stability of the capacitance voltage of the sub-module while transmitting power at the AC/DC side of the converter, does not need to acquire the current of the module, and has a simple and easily realized scheme;

the voltage-sharing control method comprises the steps that voltage-sharing control is carried out on the isolated converter according to the voltage of each submodule of the isolated converter, a voltage interval is set, and the submodules with normal energy and low energy do not participate in voltage-sharing control in consideration of the difference of physical devices of the submodules, so that the defects of voltage-sharing control failure and voltage divergence are overcome, and the safety of the submodules is improved; and, realize voltage-sharing control through redistributing the phase shift angle, solved the problem that the neutron module can be thrown into and amputated repeatedly in prior art, reduced the switching frequency of submodule piece, increased system stability from another aspect.

As will be appreciated by one skilled in the art, embodiments of the present application may be provided as a method, system, or computer program product. Accordingly, the present application may take the form of an entirely hardware embodiment, an entirely software embodiment or an embodiment combining software and hardware aspects. Furthermore, the present application may take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, and the like) having computer-usable program code embodied therein.

The present application is described with reference to flowchart illustrations and/or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the application. It will be understood that each flow and/or block of the flow diagrams and/or block diagrams, and combinations of flows and/or blocks in the flow diagrams and/or block diagrams, can be implemented by computer program instructions. These computer program instructions may be provided to a processor of a general purpose computer, special purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create means for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.

These computer program instructions may also be stored in a computer-readable memory that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable memory produce an article of manufacture including instruction means which implement the function specified in the flowchart flow or flows and/or block diagram block or blocks.

These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions specified in the flowchart flow or flows and/or block diagram block or blocks.

Finally, it should be noted that: the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting the same, and although the present invention is described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that: modifications and equivalents may be made to the embodiments of the invention without departing from the spirit and scope of the invention, which is to be covered by the claims.