AC power spring topology based on modular multilevel converter and control method

Abstract

The application discloses an AC power spring topology based on a modular multilevel converter and a control method, and belongs to the technical field of power electronics applications. The topology adopts a modular multilevel converter topology, wherein a sub-module circuit is a half-bridge converter. The level state of the modular multilevel converter is realized by different sub-modules by means of double-carrier cross-phase sinusoidal pulse width modulation, and the output end port is connected with a key load and a non-key load. The DC end of the power spring under the topology can realize miniaturization of a DC power supply by cutting a battery into small pieces, so that the output level value is diversified, and the safety and portability are better than those of a traditional structure of a battery plus an H-bridge inverter.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power electronic applications, and particularly relates to an AC power spring topology based on a modular multilevel converter and a control method. BACKGROUND

[0002] Under the background of global energy crisis and national call for energy saving and emission reduction, distributed generation technology has attracted more attention. The general distributed power such as solar cell, wind turbine and fuel cell directly connected to the grid not only has poor economic efficiency but also has negative impact on dispatching and safe operation of the main grid. Due to the intermittent and unstable nature of the distributed power, the distributed power is generally isolated from the main grid, but this method will cause a certain degree of resource waste, and the microgrid is a more effective way.

[0003] The proposal of AC power spring (ACES) has changed the traditional power distribution mode. As a device connected to the demand side load, ACES will allow the power usage to change with the power generation, solving the intermittency and instability of renewable energy power supply. At present, the most widely researched application scenario is mainly in the AC microgrid. When the critical load (CL) and non-critical load (NCL) of the terminal user are connected at the public coupling point (PCC), ACES can stabilize the PCC voltage and the power supply voltage of the CL. At present, many domestic and foreign scholars have carried out more in-depth research on ACES technology, but there are few researches on the DC side topology structure. Due to the high voltage level of the DC side, the energy storage device has a large volume and a complex structure, which reduces the volume and cost. How to further simplify the DC side structure, the energy management of the DC side battery group system and other problems cannot be ignored. SUMMARY

[0004] In view of the deficiencies of the prior art, the purpose of the present application is to provide an AC power spring topology based on a modular multilevel converter and a control method to solve the problems faced in the background art.

[0005] The purpose of the present application can be achieved by the following technical solutions:

[0006] An AC power spring topology based on a modular multilevel converter, the topology structure is composed of a modular multilevel converter and a second-order low-pass filter, the modular multilevel converter is composed of two bridge arms with the same structure, the tail parts of the two bridge arms are connected, and the head parts of the two bridge arms are connected through a load;

[0007] The second-order low-pass filter is composed of a filter inductor and a filter capacitor in series, the input end of the second-order low-pass filter is connected with the output end of the modular multilevel converter, the output end of the second-order low-pass filter is connected with the non-critical load of the end user in series, the output end of the second-order low-pass filter is also connected with the critical load in parallel and then connected with the common grid connection point, and the other side of the grid connection point is connected with the AC microgrid through a transmission line impedance.

[0008] Further, the bridge arm is composed of N sub-modules with the same structure in series.

[0009] Further, the topology structure of the sub-module is a half-bridge converter, which comprises two power switching devices, two diodes and a lithium battery.

[0010] A control method of an AC power spring topology based on a modular multilevel converter, the method comprising the following steps:

[0011] S1: modal analysis is performed on the modular multilevel converter, the working mode in a modulation period is analyzed under the condition that each bridge arm is composed of N sub-modules, and the relationship between the number of levels in each mode and the number of modules put into work of each bridge arm is listed;

[0012] S2: the effective value Vs of the voltage on the two sides of the critical load is detected, and the difference between the effective value Vs and the given value Vs_ref of the critical load voltage measured by the phase control algorithm is obtained, and then the modulation wave signal is obtained through the PR+P controller;

[0013] S3: the double-carrier cross-phase-shift sinusoidal pulse width modulation is used to drive the switching tubes of the sub-modules in the modular multilevel converter, and two groups of carriers are staggered in phase, then the carriers and the modulation wave signal are compared respectively, the modulation signals acting on the switching tubes of the left and right bridge arms are generated, and the purpose of controlling the critical load voltage is finally realized;

[0014] S4: after each running period, the battery state of charge (SOC) of the same bridge battery group is sorted, then the pair of batteries with the largest capacity difference is formed into a group, and then the sorting and grouping are sequentially performed according to the order;

[0015] S5: the direction of the current is judged, when the sub-module is judged to be discharged, the high-capacity sub-module battery is used to replace the low-capacity sub-module to run; when the sub-module is judged to be charged, the low-capacity sub-module battery is used to replace the high-capacity sub-module to run, and the process is continuously performed until the balance state is reached.

[0016] The beneficial effects of the present application are as follows:

[0017] 1、The application can realize miniaturization of the direct current power supply by cutting the direct current end battery of the electric spring into small pieces, so that the output level value is diversified, and compared with the traditional structure of the battery plus H-bridge inverter, the application has safety and portability, and is convenient for overall planning of the battery pack according to the working state of the battery to ensure efficient and stable operation of the direct current power supply system of the alternating current electric spring.

[0018] 2、Compared with the existing battery energy management system, the application does not need additional circuit structure, and only one controller is needed to realize it. The battery capacity management strategy does not change with the number of proposed converter submodules, and is also applicable to other structures of multi-level topology such as CHB, so it has strong applicability. In addition, the strategy can be combined with digital control to make the control more intelligent and convenient.

[0019] 3、The application can be applied to the new energy grid-connected scene, and can well solve the problems of new energy generation volatility, intermittency and the like, and stabilize the microgrid grid-connected point bus voltage. BRIEF DESCRIPTION OF DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the embodiments of the application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiment or prior art description, and obviously, other drawings can be obtained by those skilled in the art without creative labor on the premise of these drawings.

[0021] Figure 1 is a novel alternating current electric spring system structure diagram of the application;

[0022] Figure 2 is a circuit topology diagram of the modular multi-level converter submodule of the application;

[0023] Figure 3 is a control strategy diagram of the double-carrier cross-phase shift of the modular multi-level converter of the application;

[0024] Figure 4 is a general control block diagram of the alternating current electric spring system of the application;

[0025] Figure 5 is a waveform diagram of the grid voltage, key load voltage, non-key load voltage and electric spring voltage when working in a pure reactive power compensation state.

[0026] Explanation of reference numerals in the drawings:

[0027] 1, modular multi-level converter; 1.1, bridge arm; 2, second-order low-pass filter; 2.1, filter inductance; 2.2, filter capacitance; 2.3, key load; 2.4, non-key load; 2.5, transmission line impedance; 2.6, alternating current microgrid. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] An AC power spring topology based on a modular multilevel converter, such as Figure 1 As shown, this topology consists of a modular multilevel converter 1 and a second-order low-pass filter 2. The modular multilevel converter 1 is composed of two identical bridge arms 1.1, connected at their tails and connected at their heads via a load. Each bridge arm 1.1 is composed of N identical sub-modules connected in series, and the topology of each sub-module is a half-bridge converter, as shown below. Figure 2 As shown, it includes two power switching devices T K1 and T K2 Two diodes D K1 and D K2 And a lithium battery E K By controlling the complementary conduction of switching devices in the control submodule, the submodule outputs zero voltage and obtains the voltages on both sides of the lithium battery. The level state of the modular multilevel converter 1 is achieved by the submodule using dual-carrier cross-phase-shift sinusoidal pulse width modulation, operating in the case where each bridge arm consists of N submodules. Within one modulation cycle, the number of output levels of the MMC is 2N+1.

[0030] The second-order low-pass filter 2 consists of a filter inductor L2.1 and a filter capacitor C2.2 connected in series. The input of the second-order low-pass filter 2 is connected to the output of the modular multilevel converter 1, and the output of the second-order low-pass filter 2 is connected to the non-critical load Z of the end user. NCL 2.4 In series, the output of the second-order low-pass filter 2 is also connected to the critical load Z. CL 2.3 After parallel connection, it is connected to a common grid connection point, and the other side of the grid connection point is connected to the AC microgrid 2.6 through the transmission line impedance 2.5.

[0031] In this embodiment, the critical load Z is... CL 2.3 Non-critical load Z NCL 2.4 is equivalent to a pure resistor. The modular multilevel converter 1 has a DC voltage of 112V; a pure resistor of 2.2Ω is selected for non-critical loads; a pure resistor of 43.5Ω is selected for critical loads; the filter inductor L is 6mH; the filter capacitor C has a capacitance of 50μF; the switching frequency is 1kHz; and the mains voltage is 220V.

[0032] The control method for the above-mentioned AC power spring topology based on the modular multilevel converter mainly includes two parts: the control of the modular multilevel converter and the control of the AC power spring. The modular multilevel converter is mainly realized by using double-carrier cross-phase sinusoidal pulse width modulation, and the AC power spring is phase-controlled. As shown in Figure 3 , a control cycle includes the following control steps:

[0033] S1: modal analysis is performed on the modular multilevel converter 1, the working mode in a modulation period is analyzed when each bridge arm 1.1 is composed of N sub-modules, and the relationship between the number of levels in each mode and the number of modules put into operation of each bridge arm 1.1 is listed;

[0034] S2: the effective value Vs of the voltage on both sides of the key load 2.3 is detected, and the difference between the key load voltage given value Vs_ref measured by the phase control algorithm is obtained, and then the modulation wave signal is obtained through the PR+P controller;

[0035] S3: double-carrier cross-phase sinusoidal pulse width modulation is used to drive the switching tubes of the sub-modules in the modular multilevel converter 1, and two groups of carriers are used to stagger phase shift in turn, and then the carriers and the modulation wave signal are compared respectively to generate modulation signals acting on the switching tubes of the left and right bridge arms, which can finally realize the purpose of controlling the key load voltage;

[0036] S4: after each operation cycle, the battery state of charge SOC of the same bridge arm battery pack is sorted, and then the pair of battery packs with the largest capacity difference is formed into a group, and then the sorting and grouping are sequentially performed in this order;

[0037] S5: then the direction of the current is judged, when the sub-module is judged to be discharged, the high-capacity sub-module battery is used to replace the low-capacity sub-module to operate; when the sub-module is judged to be charged, the low-capacity sub-module battery is used to replace the high-capacity sub-module to operate, and the process is repeated until the balance state is reached.

[0038] In order to reflect the function of the topology and the effectiveness of the control method proposed in the present application, the system is simulated in MATLAB / Simulink. The results Figure 4 、 Figure 5 are shown in Figs. a is the waveform diagram of the grid voltage when working in pure reactive compensation state; b is the key load voltage when working in pure reactive compensation state; c is the non-key load voltage when working in pure reactive compensation state; d is the waveform diagram of the power spring voltage when working in pure reactive compensation state.

[0039] It can be seen from the drawings that the AC power spring topology and the control strategy thereof can make the microgrid realize demand side response to supply side change, guarantee voltage stability of a key load, realize miniaturization of a direct current power supply, so that the output level value is diversified, and the coordination work of the battery pack is facilitated according to the working state of the battery, and the direct current power supply system of the AC power spring is ensured to operate efficiently and stably.

[0040] The AC power spring (ES) topology based on the MMC replaces the structure of the direct current power supply or the battery and the H-bridge inverter in the ES-2 topology with the modular multilevel converter, and adopts the control method of the double-carrier cross-phase sinusoidal pulse width modulation (DCCPS-SPWM). In addition, the topology of the MMC makes energy storage more flexible, and under the background that the energy density of the energy storage battery is low at present, the energy utilization efficiency is higher, the coordination work of the battery pack is facilitated according to the working state of the battery, and the direct current power supply system of the ACES is ensured to operate efficiently and stably and to have a fast dynamic response.

[0041] On the basis of the existing ACES topology and control, the application provides an ACES topology and a control method based on a modular multilevel converter (MMC), the modular design of the new topology makes the expansion of the level number extremely easy, has excellent output characteristics, and in addition, the voltage withstand requirement of the power device is reduced, the structure of the direct current side is simplified, the volume is small, and the portability is high. And miniaturization of the direct current power supply is realized, the reliability of the direct current power supply is improved, and conditions such as overall failure of the battery caused by the series structure of the traditional inverter direct current side are avoided. In addition, energy storage is more flexible, the coordination work of the battery pack is facilitated according to the working state of the battery, and the direct current power supply system of the ACES is ensured to operate efficiently and stably.

[0042] In the description of the present application, the description of the terms "one embodiment", "example", "specific example" and the like means that the specific features, structures, materials or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present application. In the present application, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0043] The basic principles, main features and advantages of the present application are shown and described above. It should be understood by those skilled in the art that the present application is not limited by the above embodiments, and the above embodiments and descriptions in the specification are only illustrative of the principles of the present application. Without departing from the spirit and scope of the present application, various changes and improvements can be made to the present application, and these changes and improvements all fall within the scope of the claimed present application.

Claims

1. A control method for an AC power spring topology based on a modular multilevel converter, characterized in that, The topology is composed of a modular multilevel converter (1) and a second-order low-pass filter (2), the modular multilevel converter (1) is composed of two bridge arms (1.1) with the same structure, the tail of the two bridge arms (1.1) is connected, and the head of the two bridge arms is connected through a load; The second-order low-pass filter (2) is composed of a filter inductor (2.1) and a filter capacitor (2.2) in series, the input end of the second-order low-pass filter (2) is connected with the output end of the modular multilevel converter (1), the output end of the second-order low-pass filter (2) is connected in series with a non-critical load (2.4) of an end user, the output end of the second-order low-pass filter (2) is also connected in parallel with a critical load (2.3) and then connected to a common grid connection point, the other side of the grid connection point is connected to an AC microgrid (2.6) through a transmission line impedance (2.5); The method comprises the following steps: S1: modal analysis is performed on the modular multilevel converter (1), the working mode in one modulation period is analyzed under the condition that each bridge arm (1.1) is composed of N sub-modules, and the relationship between the number of levels in each mode and the number of modules put into operation of each bridge arm (1.1) is listed; S2: the effective value Vs of the voltage on both sides of the critical load (2.3) is detected, and the difference between the effective value Vs and the given value Vs_ref of the critical load voltage measured by the phase control algorithm is obtained, and then the PR+P controller is used to obtain a modulation wave signal; S3: the double-carrier cross-phase sinusoidal pulse width modulation is used to drive the switching tubes of the sub-modules in the modular multilevel converter (1), and two groups of carriers are used to stagger and phase-shift in turn, then the carriers and the modulation wave signal are compared respectively to generate modulation signals acting on the switching tubes of the left and right bridge arms, and finally the purpose of controlling the critical load voltage is realized; S4: after each operation cycle, the battery state of charge (SOC) of the same bridge arm battery pack is sorted, then the pair of battery packs with the largest capacity difference is formed into a group, and then the sorting and grouping are sequentially performed in this order; S5: the direction of the current is judged, when the sub-module is judged to be discharged, the high-capacity sub-module battery is used to replace the low-capacity sub-module to operate; when the sub-module is judged to be charged, the low-capacity sub-module battery is used to replace the high-capacity sub-module to operate, and the process is continuously repeated until the balance state is reached.

2. A control method of an AC power spring topology based on modular multilevel converter according to claim 1, characterized in that, The bridge arm (1.1) is composed of N sub-modules with the same structure in series.

3. A control method of an AC power spring topology based on modular multilevel converter according to claim 2, characterized in that, The topology structure of the sub-module is a half-bridge converter, which includes two power switching devices, two diodes and a lithium battery.

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