阅读说明：本技术 一种基于最近电平逼近调制的mmc软开关实现方法 (MMC soft switch implementation method based on nearest level approximation modulation ) 是由 林磊 周雪妮 徐晨 于 2019-09-11 设计创作，主要内容包括：本发明公开了一种基于最近电平逼近调制的MMC软开关实现方法,属于电力电子技术领域,该方法包括：根据当前周期桥臂参考电压电平跳变时刻和投入的子模块个数,预测下个周期桥臂参考电压电平跳变时刻和投入的子模块个数；确定具体投入的子模块及其输出电平的变化；根据子模块输出电平的变化、电流的大小和方向,确定子模块内部将要经历的换流过程；根据换流过程,判断辅助开关管是否需要导通以及导通时刻；在电平跳变时刻使主开关管关断,完成子模块投切,并在辅助开关管导通预设时间后,将其关断。本发明解决了在最近电平逼近调制与排序算法下,由于辅助开关管需在主开关管关断之前导通,导致桥臂输出电压与参考电压之间存在相位误差的问题。(the invention discloses an MMC soft switch implementation method based on nearest level approximation modulation, belonging to the technical field of power electronics, and the method comprises the following steps: predicting the jumping time of the bridge arm reference voltage level and the number of input sub-modules according to the jumping time of the bridge arm reference voltage level of the current period and the number of input sub-modules; determining the variation of the specific input submodule and the output level thereof; determining a commutation process to be experienced in the sub-module according to the change of the output level of the sub-module and the magnitude and direction of the current; judging whether the auxiliary switching tube needs to be conducted or not and judging the conducting time according to the current conversion process; and the main switching tube is turned off at the level jump moment to complete the switching of the sub-modules, and the auxiliary switching tube is turned off after the auxiliary switching tube is conducted for a preset time. The invention solves the problem that under the recent level approximation modulation and sorting algorithm, the auxiliary switching tube needs to be conducted before the main switching tube is turned off, so that the phase error exists between the output voltage of the bridge arm and the reference voltage.)

1. A MMC soft switch implementation method based on recent level approximation modulation is characterized by comprising the following steps:

(1) According to the jump time t of the reference voltage level of the bridge arm in the current period₀and the number of the sub-modules which are put into the prediction unit, predicting the reference voltage level jump time (t) of the bridge arm in the next period₀+ T) and the number of input submodules; wherein T is a power frequency period;

(2) at the moment of level jump (t)₀Time (T) before + T)₀+ T-T) sorting the sub-module capacitance and voltage according to the current direction of the bridge arm at the moment and (T)₀+ T) the number of submodules to be put in at the moment, and determining the specific put-in submodules and the change of the output level of the submodules;

(3) the variation of the output level of the submodule switched at the level jump moment according to the next period and the set moment (t)₀The current magnitude and direction of the submodule corresponding to + T-T) are determined₀time + T-T) to (T)₀+ T) the commutation process to be undergone inside the submodule at the moment;

(4) judging whether the auxiliary switching tube needs to be conducted or not and conducting time according to the current conversion process, and conducting the auxiliary switching tube at the corresponding time;

(5) And the main switching tube is turned off at the level jump moment to complete the switching of the sub-modules, and the auxiliary switching tube is turned off after the auxiliary switching tube is conducted for a preset time.

2. MMC soft switch implementation method based on recent level approximation modulation according to claim 1, characterized in that said level jump time (t)₀the expression for time T before + T) is:

Wherein L is_rThe inductance value of the resonant inductor, i_smIs the sub-module current, I_boostThe current U flowing through the main switch tube when the main switch tube is turned off in the current conversion process of the ARCP submodule_cis the submodule dc side voltage.

3. The MMC soft switching implementation method based on recent level approximation modulation according to claim 1, wherein the step (3) is specifically:

When the output level is changed from a positive level to a zero level and current flows into the submodule or the output level is changed from the zero level to the positive level and current flows out of the submodule, the inside of the submodule is subjected to an ARCP commutation process;

When the output level is changed from a positive level to a zero level and a current flows out of the submodule, the current is smaller than a threshold current I_tOr the output level is changed from zero level to positive level and current flows into the submodule, and the current is smaller than the threshold current I_tThen the sub-module interior will undergo an ARCP assisted commutation process;

When the output level is changed from a positive level to a zero level and a current flows out of the submodule, the current is larger than a threshold current I_tOr the output level is changed from zero level to positive level and current flows into the submodule, and the current is larger than the threshold current I_tInside the submodule, a capacitor forced commutation process will be experienced.

4. The MMC soft switch implementation method of claim 3, wherein the threshold current I is_tthe expression of (a) is:

Wherein, C_rTo absorb the capacitance value of the capacitor, T_dIs the dead time.

5. The method according to claim 3, wherein the step (4) of determining whether the auxiliary switching tube needs to be turned on and the turning-on time according to the commutation process specifically comprises:

When the interior of the submodule undergoes an ARCP current conversion process, the conduction time t of the auxiliary switching tube_saExpressed as: t is t_sa＝t₀+T-t；

When the interior of the submodule undergoes an ARCP auxiliary current conversion process, the conduction time t of the auxiliary switching tube_saIs denoted by t_sa＝t₀+T-t+t′；

Wherein the content of the first and second substances,L_rThe inductance value of the resonant inductor, i_smIs the sub-module current, I_boostfor the current, I, flowing through the main switching tube when the main switching tube is turned off in the ARCP current conversion process_bocaFor the current U flowing through the resonant inductor when the main switching tube is turned off in the ARCP auxiliary current conversion process_cIs the direct current side voltage of the submodule;

When the interior of the submodule undergoes a capacitor forced commutation process, the auxiliary switch tube does not need to be conducted.

6. The MMC soft switch implementation method based on recent level approximation modulation of claim 5, wherein the control mode of ARCP adopts variable time control or timing control.

7. the MMC soft switch realization method based on recent level approximation modulation of claim 1, wherein the preset time of the auxiliary switch tube conducting satisfies:

Wherein, t_onFor a predetermined time for the auxiliary switching tube to be switched on, C_r1＝C_r2＝C_r，

8. The MMC soft-switching implementation method based on nearest level approximation modulation of any of claims 1-7, wherein the MMC is formed by replacing a traditional half-bridge sub-module with an auxiliary resonant ultra-soft-switching sub-module.

Technical Field

The invention belongs to the technical field of power electronics, and particularly relates to an MMC soft switch implementation method based on nearest level approximation modulation.

Background

modular Multilevel Converters (MMC) are used in medium-voltage dc power distribution networks because of their advantages of low manufacturing difficulty, good capacity-expansion property, high output voltage waveform quality, etc. But compared with the high-voltage field such as flexible direct current transmission, in a medium-voltage direct current distribution network, the number of the MMC bridge arm sub-modules is reduced to 10-30 because the direct current voltage is generally 10-50 kV. Since the number of MMC submodules is reduced, the switching frequency of the device is still higher in order to ensure the quality of the output waveform, which causes a larger switching loss. The MMC loss has direct influence on the efficiency of the converter valve, plays a crucial role in the long-term running cost and running performance of the system, and therefore certain measures are necessary to reduce the switching loss of the MMC.

the auxiliary resonance extremely-soft switch submodule replaces a traditional half-bridge submodule, and a soft switch technology is introduced into the MMC, so that a main switch tube works in a zero voltage/current switch (ZVS/ZCS) state, the turn-on loss or turn-off loss can be completely eliminated, and the switching loss is greatly reduced. However, in the auxiliary resonant very-soft switching sub-module, the auxiliary switching tube needs to be conducted before the main switching tube is turned off, so that a condition is created for realizing zero-voltage switching of the main switching tube. However, under the recent level approximation modulation and sorting algorithm, after the switching signal of the sub-module is given, the auxiliary switching tube needs to be turned on for a period of time before the main switching tube is turned off to complete the switching of the sub-module, so that a phase error exists between the output voltage of the bridge arm and the reference voltage, and the MMC cannot achieve an expected control target, thereby affecting the stability of the system.

Disclosure of Invention

Aiming at the defects of the prior art, the invention aims to provide a method for realizing an MMC soft switch based on recent level approximation modulation, and aims to solve the problem that when the MMC realizes the soft switch under the recent level approximation modulation and sorting algorithm, an auxiliary switch tube needs to be conducted before a main switch tube is turned off, so that a phase error exists between bridge arm output voltage and reference voltage.

In order to achieve the above object, the present invention provides an MMC soft switching implementation method based on recent level approximation modulation, including:

(1) Bridge arm reference according to current cycleMoment t of voltage level jump₀And the number of the sub-modules which are put into the prediction unit, predicting the reference voltage level jump time (t) of the bridge arm in the next period₀+ T) and the number of input submodules; wherein T is a power frequency period;

(2) At the moment of level jump (t)₀Time (T) before + T)₀+ T-T) sorting the sub-module capacitance and voltage according to the current direction of the bridge arm at the moment and (T)₀+ T) the number of submodules to be put in at the moment, and determining the specific put-in submodules and the change of the output level of the submodules;

(3) The variation of the output level of the submodule switched at the level jump moment according to the next period and the set moment (t)₀The current magnitude and direction of the submodule corresponding to + T-T) are determined₀time + T-T) to (T)₀+ T) the commutation process to be undergone inside the submodule at the moment;

(4) Judging whether the auxiliary switching tube needs to be conducted or not and conducting time according to the current conversion process, and conducting the auxiliary switching tube at the corresponding time;

(5) And the main switching tube is turned off at the level jump moment to complete the switching of the sub-modules, and the auxiliary switching tube is turned off after the auxiliary switching tube is conducted for a preset time.

Further, at the time of level jump (t)₀The expression for time T before + T) is:

Wherein L is_rThe inductance value of the resonant inductor, i_smIs the sub-module current, I_boostFor the current U flowing through the main switching tube when the main switching tube is turned off in the ARCP current conversion process_cIs the submodule dc side voltage.

Further, the step (3) is specifically:

when the output level is changed from a positive level to a zero level and current flows into the submodule or the output level is changed from the zero level to the positive level and current flows out of the submodule, the inside of the submodule is subjected to an ARCP commutation process;

When the output level is changed fromThe positive level becomes zero and a current flows out of the submodule, while the current is smaller than the threshold current I_tOr the output level is changed from zero level to positive level and current flows into the submodule, and the current is smaller than the threshold current I_tThen the sub-module interior will undergo an ARCP assisted commutation process;

When the output level is changed from a positive level to a zero level and a current flows out of the submodule, the current is larger than a threshold current I_tOr the output level is changed from zero level to positive level and current flows into the submodule, and the current is larger than the threshold current I_tinside the submodule, a capacitor forced commutation process will be experienced.

further, the threshold current I_tThe expression of (a) is:

Wherein, C_rTo absorb the capacitance value of the capacitor, T_dIs the dead time.

Further, the method for determining whether the auxiliary switching tube needs to be turned on and the turn-on time according to the commutation process in the step (4) specifically includes:

when the interior of the submodule undergoes an ARCP current conversion process, the conduction time t of the auxiliary switching tube_saExpressed as: t is t_sa＝t₀+T-t；

When the interior of the submodule undergoes an ARCP auxiliary current conversion process, the conduction time t of the auxiliary switching tube_sais denoted by t_sa＝t₀+T-t+t′；

Wherein the content of the first and second substances,L_rthe inductance value of the resonant inductor, i_smIs the sub-module current, I_boostFor the current, I, flowing through the main switching tube when the main switching tube is turned off in the ARCP current conversion process_bocafor the current U flowing through the resonant inductor when the main switching tube is turned off in the ARCP auxiliary current conversion process_cIs the direct current side voltage of the submodule;

when the interior of the submodule undergoes a capacitor forced commutation process, the auxiliary switch tube does not need to be conducted.

furthermore, the control mode of the ARCP adopts variable time control or timing control.

further, the preset time for the conduction of the auxiliary switch tube meets the following requirements:

Wherein, t_onFor a predetermined time for the auxiliary switching tube to be switched on, C_r1＝C_r2＝Cr，

And further, the MMC is formed by replacing a traditional half-bridge submodule with an auxiliary resonance extremely-soft switch submodule.

Through the technical scheme, compared with the prior art, the invention has the following beneficial effects:

(1) According to the method, under the condition that the latest level approaches to modulation, according to the reference voltage level jump and the submodule input condition of the period, the level jump moment of the next period and the submodule input condition are predicted, the auxiliary switch tube is conducted in advance of the predicted switching moment, so that the bridge arm output voltage can follow the reference voltage, the phase error of the bridge arm output voltage and the reference voltage caused by the fact that the auxiliary switch tube is conducted in advance is eliminated, the MMC can achieve the expected control effect, and the system operation stability is improved.

(2) the auxiliary resonance ultra-soft switch submodule replaces the traditional half-bridge submodule, so that the main switch tube works in a zero voltage/current switch (ZVS/ZCS) state, the turn-on loss or the turn-off loss can be completely eliminated, and the switch loss is greatly reduced. Therefore, the MMC soft switch implementation method provided by the invention can effectively apply the soft switch technology in the MMC, and obviously reduce the switching loss of the converter.

drawings

FIG. 1 is a main circuit topology diagram of a three-phase soft switch MMC of the present invention;

FIG. 2 is a flow chart of the MMC soft switch implementation method under recent level approximation modulation provided by the present invention;

3(a) -3 (g) are schematic diagrams of the auxiliary resonant pole module of the present invention internally undergoing an ARCP commutation process;

4(a) -4 (e) are schematic diagrams of the auxiliary resonant pole module of the present invention internally undergoing an ARCP-assisted commutation process;

5(a) -5 (c) are schematic diagrams of the auxiliary resonant pole module of the present invention undergoing a capacitor forced commutation process internally;

Fig. 6(a) -6 (c) are voltage and current waveform diagrams corresponding to the three commutation processes;

FIG. 7 is a timing diagram of the switching tube operation of the MMC soft switch implementation method under the nearest level approximation modulation according to the present invention;

FIG. 8 is a waveform of a step wave of the upper bridge arm output voltage and the bridge arm reference voltage without the method of the present invention;

fig. 9 is a waveform diagram of the step wave of the output voltage of the upper bridge arm and the reference voltage of the bridge arm when the method of the invention is adopted.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

As shown in fig. 1, the MMC structure of the present invention is formed by replacing a conventional half-bridge sub-module with an auxiliary resonant very soft switch sub-module. As shown in fig. 2, based on the structure, the present invention provides an MMC soft switching implementation method based on recent level approximation modulation, including:

(1) According to the jump time t of the reference voltage level of the bridge arm in the current period₀and the number of the sub-modules which are put into the prediction unit, predicting the reference voltage level jump time (t) of the bridge arm in the next period₀+T) And the number of the inputted sub-modules; wherein T is a power frequency period;

Specifically, the basis for deducing the time of the level jump of the reference voltage in the next period and the number of the sub-modules to be put in from the information of the current period is as follows: when the reference voltage generates the same level jump, the jump time of two adjacent periods has a power frequency period difference.

(2) At the moment of level jump (t)₀time (T) before + T)₀+ T-T) sorting the sub-module capacitance and voltage according to the current direction of the bridge arm at the moment and (T)₀+ T) the number of submodules to be put in at the moment, and determining the specific put-in submodules and the change of the output level of the submodules;

in particular, at the level jump instant (t)₀The expression for time T before + T) is:

wherein L is_rThe inductance value of the resonant inductor, i_smIs the sub-module current, I_boostfor the current U flowing through the main switching tube when the main switching tube is turned off in the ARCP current conversion process_cIs the submodule dc side voltage.

(3) Determining (T) according to the output level change of the submodule switched at the level jump moment in the next period and the current size and direction of the submodule corresponding to the set moment (T0+ T-T)₀time + T-T) to (T)₀+ T) the commutation process to be undergone inside the submodule at the moment;

Specifically, the step (3) is specifically as follows:

As in fig. 3(a) -3 (g), when the output level changes from a positive level to a zero level and current flows into the sub-module, or the output level changes from a zero level to a positive level and current flows out of the sub-module, the sub-module interior will undergo an ARCP commutation process;

When the output level changes from positive level to zero level and the current flows out of the sub-module, the current is less than the threshold current I as shown in FIG. 4(a) -FIG. 4(e)_tOr the output level is changed from zero level to positive level and current flows into the submodule while current is less thanThreshold current I_tThen the sub-module interior will undergo an ARCP assisted commutation process; wherein the threshold current I_tThe expression of (a) is:

wherein, C_rTo absorb the capacitance value of the capacitor, T_dIs the dead time.

when the output level changes from positive level to zero level and the current flows out of the sub-module, the current is larger than the threshold current I as shown in FIG. 5(a) -FIG. 5(c)_tor the output level is changed from zero level to positive level and current flows into the submodule, and the current is larger than the threshold current I_tinside the submodule, a capacitor forced commutation process will be experienced. The voltage and current waveform diagrams corresponding to the three commutation processes are respectively shown in fig. 6(a) -6 (c).

(4) according to the current conversion process, whether the auxiliary switch tube needs to be conducted or not and the conducting time t_saAnd the auxiliary switch tube is conducted at the corresponding time;

In particular, t_sa＝t₀+ T-T + T ', for different commutation situations, T' has different values: for the ARCP current conversion process, the auxiliary switch tube needs to be immediately conducted after the switched sub-module is selected, namely t' is 0; for the ARCP auxiliary current conversion process, the auxiliary switch tube needs to pass through (delta t) after the switching submodule is selected_boost-Δt_boca) The power-on state is carried out,Wherein L is_rthe inductance value of the resonant inductor, i_smIs the sub-module current, I_boostFor the current, I, flowing through the main switching tube when the main switching tube is turned off in the ARCP current conversion process_bocaFor the current U flowing through the resonant inductor when the main switching tube is turned off in the ARCP auxiliary current conversion process_cfor sub-module DC-side voltages, i.e.For the forced commutation process of the capacitor, auxiliary switchNo action is required for closing the tube. When Δ t is reached_boostAnd Δ t_bocawhen the ARCP changes along with the change of the load current in real time, the control mode of the ARCP adopts time-varying control; when Δ t is reached_boostand Δ t_bocawhen the ARCP does not change along with the change of the load current in real time, the control mode of the ARCP adopts timing control. The embodiment of the invention adopts timing control, and the main parameters are shown in the table 1:

TABLE 1

(5) And the main switching tube is turned off at the level jump moment to complete the switching of the sub-modules, and the auxiliary switching tube is turned off after the auxiliary switching tube is conducted for a preset time.

Specifically, the preset time for the conduction of the auxiliary switch tube meets the following requirements:

wherein, t_onFor a predetermined time for the auxiliary switching tube to be switched on, C_r1＝C_r2＝C_r，

The timing diagram of the switching tube action of the MMC soft switch implementation method under the recent level approximation modulation provided by the invention is shown in figure 7. Fig. 8 is a waveform diagram of step waves of the output voltage of the upper bridge arm and the reference voltage of the bridge arm when the method of the present invention is not used, wherein a dotted line represents the step wave corresponding to the reference voltage of the upper bridge arm, and a solid line represents the waveform of the actual output voltage of the upper bridge arm. Comparing the two waveforms, that is, when the level of the reference voltage step wave changes, the output voltage of the bridge arm does not change immediately, and a phase error exists between the output voltage of the bridge arm and the reference voltage step wave; fig. 9 is a waveform diagram of step waves of the output voltage of the upper bridge arm and the reference voltage of the bridge arm when the method of the present invention is adopted, wherein a dotted line represents the step wave corresponding to the reference voltage of the upper bridge arm, and a solid line represents the waveform of the actual output voltage of the upper bridge arm.

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.