阅读说明：本技术 一种有源中点钳位型三电平变流器及其调控方法 (Active neutral point clamped three-level converter and regulation and control method thereof ) 是由 李鹏程 殷嘉棋 郭英军 高星晨 霍润琦 郭伟 于 2021-08-31 设计创作，主要内容包括：本发明公开了一种有源中点钳位型三电平变流器及其调控方法,变流器包括：逆变桥、三相LCL滤波器和控制电路；逆变桥包括直流母线支撑电容C1、C2和三个结构相同且相并联的桥臂；每个桥臂由6个开关管S-(x1)-S-(x6)以及分别与开关管S-(x1)-S-(x6)反并联的二极管组成,x＝a、b或c；控制电路分别与交流电网和逆变桥连接；控制电路根据实时检测的交流电网电压,对交流电网频率进行锁相跟踪,并计算产生变流器并网运行时所需的SPWM信号,控制相应开关管的开关动作；每个桥臂的输出端作为交流输出端xC,并通过三相LCL滤波器连接到交流电网上；三相LCL滤波器用于滤除由于开关动作引起的高次谐波电流。本发明能够实现平衡开关管的导通损耗,并控制结温平衡。(The invention discloses an active neutral point clamped three-level converter and a regulation and control method thereof, wherein the converter comprises: the inverter bridge, the three-phase LCL filter and the control circuit; the inverter bridge comprises direct-current bus support capacitors C1 and C2 and three bridge arms which have the same structure and are connected in parallel; each bridge arm is composed of 6 switching tubes S x1 ‑S x6 And respectively connected with the switch tube S x1 ‑S x6 Anti-parallel diodes, x ═ a, b, or c; the control circuit is respectively connected with the alternating current power grid and the inverter bridge; the control circuit carries out phase-locked tracking on the frequency of the alternating current power grid according to the alternating current power grid voltage detected in real time, calculates and generates an SPWM signal required by the grid-connected operation of the converter, and controls the switching action of a corresponding switching tube; the output end of each bridge arm is used as an alternating current output end xC and is connected to alternating current through a three-phase LCL filterOn the net; the three-phase LCL filter is used for filtering out higher harmonic current caused by switching action. The invention can realize the conduction loss of the balance switch tube and control the junction temperature balance.)

1. An active neutral-point clamped three-level converter, comprising: the inverter bridge, the three-phase LCL filter and the control circuit;

the inverter bridge comprises direct-current bus support capacitors C1 and C2 and three bridge arms which have the same structure and are connected in parallel; each bridge arm is composed of 6 switching tubes S_x1-S_x6And respectively connected with the switch tube S_x1-S_x6Anti-parallel diodes, x ═ a, b, or c; switch tube S_x1The input end of the DC bus is connected with the anode of the DC bus, and the output end of the DC bus is connected with the S_x2、S_x5An input terminal of (1); s_x4The output end of the DC bus is connected with the negative electrode of the DC bus, and the input end of the DC bus is connected with the S_x3、S_x6An output terminal of (a); s_x5Is connected to S_x6Input terminal of, S_x2Output of (2)Termination S_x3An input terminal of (1); s_x5And S_x6The input ends of the two-way switch are connected with the midpoint O of the direct current bus; the direct-current bus supporting capacitor C1 is connected between the positive electrode of the direct-current bus and the midpoint O; the direct-current bus supporting capacitor C2 is connected between the negative electrode of the direct-current bus and the midpoint O;

the control circuit is respectively connected with an alternating current power grid and the inverter bridge; the control circuit carries out phase-locked tracking on the frequency of the alternating current power grid according to the alternating current power grid voltage detected in real time, calculates and generates an SPWM signal required by the grid-connected operation of the converter, and controls the switching action of the corresponding switching tube;

S_x2as an ac output xC and connected to the ac mains via said three-phase LCL filter; the three-phase LCL filter is used for filtering out higher harmonic current caused by switching action.

2. The active midpoint clamping type three-level converter according to claim 1, wherein the control circuit comprises a DSP main controller, a driving circuit and a signal sampling circuit; the signal sampling circuit samples voltage and current signals on the alternating current power grid at intervals of a preset time period; the DSP main controller judges whether the AC power grid voltage is in a forward zero crossing point state according to the sampling signal, and calculates and generates 18 paths of independent SPWM pulses to be transmitted to the driving circuit when the AC power grid voltage is in the forward zero crossing point state; and the driving circuit correspondingly controls the switching state of the corresponding switching tube according to 18 paths of independent SPWM pulses.

3. The active midpoint clamping type three-level converter according to claim 1, wherein the capacitance voltage of the dc bus supporting capacitors C1 and C2 has a value range of: 1.5V_dc/2～V_dcWherein V is_dcIs a dc bus voltage; the sum P of losses generated by harmonic currents of each order of the capacitor meets the following condition: p is not more than (0.6-0.8)_c·maxWherein P is_c·maxIndicating harmonic current rating I_c·max(ii) a The capacitance value C satisfies:wherein, is Δ V_C.max[ 2 ] represents the permissible voltage fluctuation on the DC side of the DC bus, I_hRepresenting the harmonic current of the capacitor, h representing the harmonic order, f_hRepresenting harmonic frequencies.

4. The active midpoint clamp type three-level converter according to claim 1, wherein the three-phase LCL filter comprises a filter capacitor C and a filter inductor L; the filter inductor L meets the requirements of transient current tracking and harmonic current suppression, and the maximum value of the current change rate does not exceed the tracking speed of the current, and the value range is as follows:

wherein, X_LExpressed as inductance values satisfying the requirements for transient current tracking and harmonic current suppression, j ═ a, b, and c, expressed as phases, U_dcRepresents the dc bus voltage;

the filter capacitor C satisfies: the absorbed reactive power can not be more than 5% of the rated active power of the system, the resonant frequency of the three-phase LCL filter is more than 10 times of the fundamental frequency and less than 1/10 of the switching frequency, and the value range of the filter capacitor C is as follows:

5. a method for controlling an active neutral point clamped three-level converter, which is suitable for the active neutral point clamped three-level converter according to any one of claims 1 to 4, and comprises the following steps:

sampling voltage and current signals of an alternating current power grid according to a preset time interval;

carrying out positive and negative sequence decomposition on instantaneous values of the voltage and the current of the alternating current power grid to obtain positive and negative sequence voltage components and positive and negative sequence current components under a dq axis of a positive and negative sequence rotating coordinate system;

performing independent decoupling control on the obtained positive sequence current component and the obtained negative sequence current component in a positive sequence rotating coordinate system and performing independent decoupling control on the obtained positive sequence current component and the obtained negative sequence current component in a negative sequence rotating coordinate system by using park transformation and based on a double-current decoupling regulation and control method in the positive sequence rotating coordinate system and the negative sequence rotating coordinate system;

combining the positive-sequence and negative-sequence current control quantity obtained under the double-current decoupling control with the positive-sequence and negative-sequence voltage component after PI regulation to obtain a voltage control quantity;

converting the voltage control quantity under the dq axis into an alpha beta coordinate system by using park conversion to obtain a reference voltage under the alpha beta coordinate system;

carrying out park inverse transformation on the reference voltage under the alpha beta coordinate system to obtain the voltage under the abc coordinate system;

and carrying out in-phase carrier modulation on the voltage under the abc coordinate system to generate SPWM (sinusoidal pulse width modulation) pulses, and controlling the switching state of a corresponding switching tube in the converter by using the SPWM pulses.

6. The method for regulating and controlling the active neutral point clamped three-level converter according to claim 5, wherein the double-current decoupling regulation and control method based on the positive and negative sequence rotating coordinate system is used for decoupling and controlling the active power and the reactive power output by the converter and eliminating the direct-current second harmonic component generated by the unbalanced alternating-current power grid voltage, wherein the decoupling and controlling the active power and the reactive power output by the converter are performed by the following processes:

calculating active power and reactive power output by the converter according to positive and negative sequence voltage components and positive and negative sequence current components under the dq axis of the positive and negative sequence rotating coordinate system; the calculation formula is as follows:

wherein the content of the first and second substances,is the positive sequence component of the voltage in the dq axis,is the negative sequence component of the voltage under the dq axis;is the current positive sequence component in the dq axis,is the current negative sequence component under the dq axis; q₀Is the average value of the reactive power; p₀Is the average value of active power; p_c2The harmonic peak value is the cosine of the secondary active power; p_s2Is the harmonic peak of the secondary active power sine; q_c2Is the second reactive power cosine harmonic peak value; q_s2The peak value of the sine harmonic wave of the secondary reactive power is P (t), the instantaneous active power of the grid side is P (t), and the instantaneous reactive power of the grid side is Q (t);

calculating the network side apparent power of the converter when the alternating current network is in an unbalanced state according to the active power and the reactive power output by the converter, wherein the expression is as follows:

s ═ p (t) + jq (t); j represents a reactive power component coefficient;

let P_c2＝P_s2Setting the second harmonic component of the direct-current voltage to 0; let Q₀Operating the converter at unity power factor, 0; outputting the given power reference value as a positive sequence current given using:

the above formula represents a given power reference valueGiven the relationship between positive and negative sequence voltage current, where P₀ ^*Representing zero sequence active power;representing zero-sequence reactive power;a harmonic peak value given representing the cosine of the secondary active power;a harmonic peak value given representing the sine of the secondary active power;representing a d-axis positive sequence potential component;representing a q-axis positive sequence potential component;represents a d-axis negative-sequence potential component;representing a q-axis negative-sequence potential component;represents the d-axis positive sequence current given;represents a q-axis positive sequence current given;represents a d-axis negative-sequence current given;indicating a q-axis negative-sequence current given.

7. The method for controlling the active neutral point clamped three-level converter according to claim 5, further comprising:

determining the working mode of the converter according to the running state of the voltage vector in the four quadrants of the converter under the abc coordinate system;

when the voltage vector V end point is on the circular trackWhen the converter moves upwards, the current vector I lags behind the electromotive force vector E by 90 degrees, the converter operates in a rectification state, and at the moment, the converter absorbs active power and inductive reactive power from an alternating current power grid;

when the voltage vector V end point is on the circular trackWhen the converter moves upwards, the current vector I and the electromotive force vector E are parallel and in the same direction, the converter operates in a rectification state, and at the moment, the converter absorbs active and capacitive reactive power from an alternating current power grid;

when the voltage vector V end point is on the circular trackWhen the converter moves upwards, the current vector I leads the electromotive force vector E by 90 degrees, the converter operates in an active inversion state, and the converter transmits active and capacitive reactive power from an alternating current power grid;

when the voltage vector V end point is on the circular trackWhen the converter moves upwards, the current vector I and the electromotive force vector E are parallel and opposite, the converter operates in an active inversion state, and at the moment, the converter transmits active power and inductive reactive power from an alternating current power grid.

8. The method for controlling the active neutral point clamped three-level converter according to claim 5, further comprising:

when the voltage under the abc coordinate system is in a positive half period, three switching tubes in each bridge arm are defined to be in a conducting state, when the voltage is in a negative half period, the other three switching tubes in each bridge arm are defined to be in a conducting state, the size of an SPWM pulse generated when the voltage is in the positive half period or the negative half period is determined according to the defined switching state of each switching tube, and the corresponding switching tubes are controlled to be conducted or cut off under the SPWM pulse.

Technical Field

The invention relates to the technical field of multi-level converters based on direct-current buses, in particular to an active neutral-point clamped three-level converter and a regulating and controlling method thereof.

Background

The converter is a junction connecting a direct current bus and an alternating current power grid, and electric energy is converted between direct current and fixed-frequency fixed-voltage or frequency-modulation voltage-regulation alternating current to realize four-quadrant operation. Compared with the traditional two-level inverter, the three-level inverter has the advantages of small switching loss, good output waveform quality and the like, and is widely applied to distributed voltage occasions such as photovoltaic power generation and wind power generation. Compared with a reactive generator (SVG), the four-quadrant ANPC type three-level converter has the advantages of wider active and reactive power regulation freedom, wider application range and lower manufacturing cost. Compared with the rectification side unit Power Factor Correction (PFC), the method does not need to consider the current interruption condition, and is simple in design.

Compared with a traditional NPC three-level converter, the Active Neutral-point-clamped (ANPC) three-level converter topology has more vector states and higher control freedom; the power factor regulation and control capability is higher, and the power factor regulation and control device is suitable for more working conditions; the loss balance is easy to realize, the heat dissipation design is facilitated, and the system capacity is improved. However, the conventional ANPC regulation has many defects, such as inconsistent switching frequencies of the inner tube and the outer tube, inconsistent stress, difficulty in realizing higher harmonic decoupling control due to vector modulation bandwidth limitation, and the like.

Therefore, how to provide the ANPC type three-level converter and the regulation and control method thereof has important significance.

Disclosure of Invention

In view of this, the invention provides an active midpoint clamping type three-level converter and a regulation and control method thereof, which can realize conduction loss of a balanced switch tube and control junction temperature balance.

In order to achieve the purpose, the invention adopts the following technical scheme:

an active neutral point clamped three level converter comprising: the inverter bridge, the three-phase LCL filter and the control circuit;

the inverter bridge comprises direct-current bus support capacitors C1 and C2 and three bridge arms which have the same structure and are connected in parallel; each bridge arm is composed of 6 switching tubes S_x1-S_x6And respectively connected with the switch tube S_x1-S_x6Anti-parallel diodes, x ═ a, b, or c; switch tube S_x1The input end of the DC bus is connected with the anode of the DC bus, and the output end of the DC bus is connected with the S_x2、S_x5An input terminal of (1); s_x4The output end of the DC bus is connected with the negative electrode of the DC bus, and the input end of the DC bus is connected with the S_x3、S_x6An output terminal of (a); s_x5Is connected to S_x6Input terminal of, S_x2Is connected to S_x3An input terminal of (1); s_x5And S_x6The input ends of the two-way switch are connected with the midpoint O of the direct current bus; the direct-current bus supporting capacitor C1 is connected between the positive electrode of the direct-current bus and the midpoint O; the direct-current bus supporting capacitor C2 is connected between the negative electrode of the direct-current bus and the midpoint O;

the control circuit is respectively connected with an alternating current power grid and the inverter bridge; the control circuit carries out phase-locked tracking on the frequency of the alternating current power grid according to the alternating current power grid voltage detected in real time, calculates and generates an SPWM signal required by the grid-connected operation of the converter, and controls the switching action of the corresponding switching tube;

S_x2as an ac output xC and connected to the ac mains via said three-phase LCL filter; the three-phase LCL filter is used for filtering out higher harmonic current caused by switching action.

Preferably, in the active midpoint clamping type three-level converter, the control circuit includes a DSP main controller, a driving circuit and a signal sampling circuit; the signal sampling circuit samples voltage and current signals on the alternating current power grid at intervals of a preset time period; the DSP main controller judges whether the AC power grid voltage is in a forward zero crossing point state according to the sampling signal, and calculates and generates 18 paths of independent SPWM pulses to be transmitted to the driving circuit when the AC power grid voltage is in the forward zero crossing point state; and the driving circuit correspondingly controls the switching state of the corresponding switching tube according to 18 paths of independent SPWM pulses.

Preferably, in the active midpoint clamping type three-level converter, the capacitance voltage of the dc bus supporting capacitors C1 and C2 has a value range of: 1.5V_dc/2～V_dcWherein V is_dcIs a dc bus voltage; the sum P of losses generated by harmonic currents of each order of the capacitor meets the following condition: p is not more than (0.6-0.8)_c·maxWherein P is_c·maxIndicating harmonic current rating I_c·max(ii) a The capacitance value C satisfies:wherein, is Δ V_C.max[ 2 ] represents the permissible voltage fluctuation on the DC side of the DC bus, I_hRepresenting the harmonic current of the capacitor, h representing the harmonic order, f_hRepresenting harmonic frequencies.

Preferably, in the active midpoint clamping type three-level converter, the three-phase LCL filter includes a filter capacitor C and a filter inductor L; the filter inductor L meets the requirements of transient current tracking and harmonic current suppression, and the maximum value of the current change rate does not exceed the tracking speed of the current, and the value range is as follows:

wherein, X_LExpressed as inductance values satisfying the requirements for transient current tracking and harmonic current suppression, j ═ a, b, and c, expressed as phases, U_dcRepresents the dc bus voltage;

the filter capacitor C satisfies: the absorbed reactive power can not be more than 5% of the rated active power of the system, the resonant frequency of the three-phase LCL filter is more than 10 times of the fundamental frequency and less than 1/10 of the switching frequency, and the value range of the filter capacitor C is as follows:

the invention also provides a regulation and control method of the active neutral point clamped three-level converter, which is suitable for the active neutral point clamped three-level converter and is characterized by comprising the following steps:

sampling voltage and current signals of an alternating current power grid according to a preset time interval;

carrying out positive and negative sequence decomposition on instantaneous values of the voltage and the current of the alternating current power grid to obtain positive and negative sequence voltage components and positive and negative sequence current components under a dq axis of a positive and negative sequence rotating coordinate system;

performing independent decoupling control on the obtained positive sequence current component and the obtained negative sequence current component in a positive sequence rotating coordinate system and performing independent decoupling control on the obtained positive sequence current component and the obtained negative sequence current component in a negative sequence rotating coordinate system by using park transformation and based on a double-current decoupling regulation and control method in the positive sequence rotating coordinate system and the negative sequence rotating coordinate system;

combining the positive-sequence and negative-sequence current control quantity obtained under the double-current decoupling control with the positive-sequence and negative-sequence voltage component after PI regulation to obtain a voltage control quantity;

converting the voltage control quantity under the dq axis into an alpha beta coordinate system by using park conversion to obtain a reference voltage under the alpha beta coordinate system;

carrying out park inverse transformation on the reference voltage under the alpha beta coordinate system to obtain the voltage under the abc coordinate system;

and carrying out in-phase carrier modulation on the voltage under the abc coordinate system to generate SPWM (sinusoidal pulse width modulation) pulses, and controlling the switching state of a corresponding switching tube in the converter by using the SPWM pulses.

Preferably, in the above method for regulating an active midpoint clamping type three-level converter, the double-current decoupling regulation and control method based on the positive and negative sequence rotating coordinate system is used to implement decoupling control on the active power and the reactive power output by the converter, and eliminate the dc second harmonic component generated by the unbalanced ac power grid voltage, wherein the process of decoupling control on the active power and the reactive power output by the converter is as follows:

calculating active power and reactive power output by the converter according to positive and negative sequence voltage components and positive and negative sequence current components under the dq axis of the positive and negative sequence rotating coordinate system; the calculation formula is as follows:

wherein the content of the first and second substances,is the positive sequence component of the voltage in the dq axis,is the negative sequence component of the voltage under the dq axis;is the current positive sequence component in the dq axis,is the current negative sequence component under the dq axis; q₀Is the average value of the reactive power; p₀Is the average value of active power; p_c2The harmonic peak value is the cosine of the secondary active power; p_s2Is the harmonic peak of the secondary active power sine; q_c2Is the second reactive power cosine harmonic peak value; q_s2The peak value of the sine harmonic wave of the secondary reactive power is P (t), the instantaneous active power of the grid side is P (t), and the instantaneous reactive power of the grid side is Q (t);

calculating the network side apparent power of the converter when the alternating current network is in an unbalanced state according to the active power and the reactive power output by the converter, wherein the expression is as follows:

s ═ p (t) + jq (t); j represents a reactive power component coefficient;

let P_c2＝P_s2Setting the second harmonic component of the direct-current voltage to 0; let Q₀Operating the converter at unity power factor, and outputting a given power reference value as positive sequence current given by using the following formula:

the above formula represents a given power reference valueGiven the relationship between positive and negative sequence voltage currents, wherein,representing zero sequence active power;representing zero-sequence reactive power;a harmonic peak value given representing the cosine of the secondary active power;a harmonic peak value given representing the sine of the secondary active power;representing a d-axis positive sequence potential component;representing a q-axis positive sequence potential component;represents a d-axis negative-sequence potential component;representing a q-axis negative-sequence potential component;represents the d-axis positive sequence current given;represents a q-axis positive sequence current given;represents a d-axis negative-sequence current given;indicating a q-axis negative-sequence current given.

Preferably, in the method for controlling an active midpoint clamping type three-level converter, the method further includes:

determining the working mode of the converter according to the running state of the voltage vector in the four quadrants of the converter under the abc coordinate system;

when the voltage vector V end point is on the circular trackWhen the converter moves upwards, the current vector I lags behind the electromotive force vector E by 90 degrees, the converter operates in a rectification state, and at the moment, the converter absorbs active power and inductive reactive power from an alternating current power grid;

when the voltage vector V end point is on the circular trackWhen the converter moves upwards, the current vector I and the electromotive force vector E are parallel and in the same direction, the converter operates in a rectification state, and at the moment, the converter absorbs active and capacitive reactive power from an alternating current power grid;

when the voltage vector V end point is on the circular trackWhen the converter moves upwards, the current vector I leads the electromotive force vector E by 90 degrees, the converter operates in an active inversion state, and the converter transmits active and capacitive reactive power from an alternating current power grid;

when the voltage vector V end point is on the circular trackWhen moving upwards, the current vector I is parallel and opposite to the electromotive force vector E, and the converter operates in an active modeAnd in an inversion state, the converter transmits active power and inductive reactive power from the alternating current power grid.

Preferably, in the method for controlling an active midpoint clamping type three-level converter, the method further includes:

when the voltage under the abc coordinate system is in a positive half period, three switching tubes in each bridge arm are defined to be in a conducting state, when the voltage is in a negative half period, the other three switching tubes in each bridge arm are defined to be in a conducting state, the size of an SPWM pulse generated when the voltage is in the positive half period or the negative half period is determined according to the defined switching state of each switching tube, and the corresponding switching tubes are controlled to be conducted or cut off under the SPWM pulse.

According to the technical scheme, compared with the prior art, the invention discloses and provides an active neutral point clamped three-level converter and a regulation and control method thereof, and the active neutral point clamped three-level converter has the following beneficial effects:

1. in each T period, the regulation and control method can enable the inner tube S in the converter to be in a state of being capable of being adjusted_x2、S_x3Symmetrical pulse shape, outer tube S_x1、S_x4Symmetrical pulse waveform, switch tube S_x5、S_x6The pulse waveform is symmetrical, which is beneficial to balancing the conduction loss of the switch tube and controlling the junction temperature balance.

2. In each T/2 period, the regulation and control method can enable the three switching tubes to perform switching action at the carrier frequency, so that the switching loss balance of the switching tubes is realized, and the average switching loss of six switching tubes of each phase is the same.

3. Under the condition of a non-ideal power grid, the invention can realize the positive and negative sequence active and reactive decoupling independent operation, the neutral current is controllable, and the grid-connected waveform is ideal.

Drawings

In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the provided drawings without creative efforts.

Fig. 1 is a schematic structural diagram of an active midpoint clamping type three-level converter provided by the invention;

FIG. 2 is a control block diagram of a conventional converter;

fig. 3 is a double-current decoupling control block diagram under a positive-negative sequence rotating coordinate system of the active midpoint clamping type three-level converter provided by the invention;

FIG. 4 is a diagram illustrating a steady-state vector relationship at the AC side of the active midpoint clamping type three-level converter provided by the present invention;

fig. 5 is a schematic diagram of pulse generation in the regulation process of the active midpoint clamping type three-level converter provided by the invention;

fig. 6 is a diagram illustrating an operation result of the grid-connected system according to the present invention.

Detailed Description

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 only a part of the embodiments of the present invention, and not all of the embodiments. 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.

As shown in fig. 1, an embodiment of the present invention discloses an active midpoint clamping type three-level converter, including: the inverter bridge, the three-phase LCL filter and the control circuit;

the inverter bridge comprises direct-current bus support capacitors C1 and C2 and three bridge arms which have the same structure and are connected in parallel; each bridge arm is composed of 6 switching tubes S_x1-S_x6And respectively connected with the switch tube S_x1-S_x6Anti-parallel diodes, x ═ a, b, or c; taking phase a as an example, switching tube S_a1The input end of the DC bus is connected with the anode of the DC bus, and the output end of the DC bus is connected with the S_a2、S_a5An input terminal of (1); s_a4The output end of the DC bus is connected with the negative electrode of the DC bus, and the input end of the DC bus is connected with the S_a3、S_a6An output terminal of (a); s_a5Is connected to S_a6Input terminal of, S_a2Is connected to S_a3An input terminal of (1); s_a5And S_a6The input ends of the two-way switch are connected with the midpoint O of the direct current bus; the direct-current bus supporting capacitor C1 is connected between the positive electrode of the direct-current bus and the midpoint O; the direct-current bus supporting capacitor C2 is connected between the negative electrode of the direct-current bus and the midpoint O; s_a2As an ac output xC and connected to the ac mains via said three-phase LCL filter; the connection mode of the b-phase bridge arm and the c-phase bridge arm is the same as that of the a-phase bridge arm.

The control circuit is respectively connected with an alternating current power grid and the inverter bridge; the control circuit carries out phase-locked tracking on the frequency of the alternating current power grid according to the alternating current power grid voltage detected in real time, calculates and generates an SPWM signal required by the grid-connected operation of the converter, and controls the switching action of the corresponding switching tube;

the three-phase LCL filter is used for filtering out higher harmonic current caused by switching action.

Specifically, the control circuit comprises a DSP main controller, a driving circuit and a signal sampling circuit; the signal sampling circuit samples voltage and current signals on the alternating current power grid at intervals of a preset time period; in the embodiment, the sampling interval is 50us, the DSP main controller judges whether the ac grid voltage is in a forward zero-crossing state according to the sampling signal, and calculates and generates 18 paths of 20kKz independent SPWM pulses to transmit to the driving circuit when the ac grid voltage is in the forward zero-crossing state; and the driving circuit correspondingly controls the switching state of the corresponding switching tube according to 18 paths of independent SPWM pulses. When the positive zero crossing point of the grid voltage is detected, the DSP main controller detects that the voltage, amplitude and phase information of the grid-connected point accord with the grid-connected criterion, and sequentially conducts the grid-connected switch tubes of the ABC phase.

In one embodiment, the dc bus support capacitors C1 and C2 are selected in three ways:

1. the capacitor voltage. Determined by the DC bus voltage, considering a certain safety margin, generally (1.5-2) V_dc/2；

2. The capacitor heats up. Determined by the harmonic current of the capacitor, using the rated value I of the harmonic current (effective value)_c·maxTo express the maximum loss P of the capacitor_c·max. The sum of the losses generated by the harmonic currents of the capacitor should be less than the maximum loss of the capacitor, and is generally:

P≤(0.6～0.8)P_c·max。

3. capacitance value of the capacitor: voltage fluctuation DeltaV allowed by DC side_C.maxAnd/2 and capacitive harmonic current. Taken together, the range of capacitance values is determined by:

in the above formula, h represents the harmonic order, f_hRepresenting harmonic frequencies.

In another specific embodiment, the three-phase LCL filter includes a filter capacitor C and a filter inductor L; the design consideration of the filter inductor is to satisfy the requirements of transient current tracking and harmonic current suppression. In order to suppress harmonic currents generated under unbalanced grid voltages, the design principle of the inductor is as follows:

where i is the THD corresponding to the current and k represents the voltage order.

In order to make the maximum value of the current change rate not exceed the tracking speed of the current, the upper limit value of the alternating-current side inductance is as follows:

formula above combines, when adopting the unbalanced control strategy under the unbalanced grid voltage, the size of filter inductance need satisfy:

the reactive power absorbed by the filter capacitor required by the LCL type grid-connected inverter cannot be more than 5% of the rated active power of the system. Namely:

meanwhile, in order to reduce the influence of the resonance peak of the LCL filter on the circuit, the resonance frequency of the LCL should be greater than 10 times the fundamental frequency and less than 1/10, i.e., 10f, of the switching frequency₁<f_res<f_sw/10, wherein the resonant frequency f_resComprises the following steps:

obtaining by solution:

by combining the formula, the value range of the filter capacitor C can be determined as follows:

in the formula, L₁And L₂Is a filter inductance, f_resIs a resonant frequency, P_nIs rated power, u_nIs a rated voltage.

The conventional control process of the ac unit is shown in fig. 2, which is specifically represented by: and collecting three-phase voltage and current values of the power grid side, and performing an AD conversion module. The current on the network side is changed into a two-phase static coordinate system (Clack transformation) from a three-phase static coordinate system, as shown in a formula (1); conversion matrix T for converting three-phase static coordinate system into two-phase static coordinate system_cAs shown in formula (2); then, the two-phase static coordinate system is changed into a dq synchronous selection coordinate system (Park transformation), as shown in formula (3), wherein T_pIs a corresponding transformation matrix, and the specific parameters are shown as formula (4); the difference generated after decoupling and comparison with a given current reference valueThe control of grid-connected current is realized through a PI controller, a reference voltage signal for carrier modulation is obtained after Park inverse transformation, a PWM signal is produced after in-phase carrier modulation, the on-off of the IGBT is controlled through a driving circuit, and the PWM signal of the IGBT is determined according to the on-off state.

X_βα＝T_cX_abc (1)；

X_qd＝T_pX_βα (3)；

In an unbalanced power grid, negative sequence and zero sequence voltage components are generated, and in the analysis of an unbalanced circuit, a symmetrical component method is usually adopted, namely any group of asymmetrical three-phase voltage or current can be represented as the sum of positive sequence, negative sequence and zero sequence components. Based on the control, the embodiment of the invention also discloses a regulation and control method of the active neutral point clamped three-level converter, which adopts double-current control based on a positive sequence rotating coordinate system and a negative sequence rotating coordinate system, and adopts the positive sequence rotating coordinate system and the negative sequence rotating coordinate system to independently control and simultaneously carry out current control, thereby eliminating direct-current voltage second harmonic generated by unbalanced power grid voltage and realizing decoupling control on the output active power and the reactive power of the inverter.

Specifically, as shown in fig. 3, the method for regulating and controlling an active midpoint clamping type three-level converter of the present invention includes the following steps:

sampling voltage and current signals of an alternating current power grid according to a preset time interval;

carrying out positive and negative sequence decomposition on instantaneous values of the voltage and the current of the alternating current power grid to obtain positive and negative sequence voltage components and positive and negative sequence current components under a dq axis of a positive and negative sequence rotating coordinate system;

performing independent decoupling control on the obtained positive sequence current component and the obtained negative sequence current component in a positive sequence rotating coordinate system and performing independent decoupling control on the obtained positive sequence current component and the obtained negative sequence current component in a negative sequence rotating coordinate system by using park transformation and based on a double-current decoupling regulation and control method in the positive sequence rotating coordinate system and the negative sequence rotating coordinate system;

combining the positive-sequence and negative-sequence current control quantity obtained under the double-current decoupling control with the positive-sequence and negative-sequence voltage component after PI regulation to obtain a voltage control quantity;

converting the voltage control quantity under the dq axis into an alpha beta coordinate system by using park conversion to obtain a reference voltage under the alpha beta coordinate system;

carrying out park inverse transformation on the reference voltage under the alpha beta coordinate system to obtain the voltage under the abc coordinate system;

and carrying out in-phase carrier modulation on the voltage under the abc coordinate system to generate SPWM (sinusoidal pulse width modulation) pulses, and controlling the switching state of a corresponding switching tube in the converter by using the SPWM pulses.

When the grid voltage is unbalanced, the grid-side instantaneous active power and the reactive power both contain second harmonic components because negative sequence voltage components exist, which results in that the direct-current output voltage also contains second harmonic components.

In order to eliminate direct-current voltage second harmonic generated by unbalanced power grid voltage and realize decoupling control on the output active power and reactive power of the converter, the invention adopts double-current control based on a positive sequence rotating coordinate system and a negative sequence rotating coordinate system, and adopts the independent control of the positive sequence rotating coordinate system and the negative sequence rotating coordinate system to be simultaneously carried out in the current control.

The active power in the unbalanced power grid is related to the average value of the active power, the harmonic peak value of the cosine of the secondary active power and the harmonic peak value of the sine of the secondary active power; the reactive power is related to the average value of the reactive power, the harmonic peak value of the cosine of the secondary reactive power, and the harmonic peak value of the sine of the secondary reactive power, and the logical relationship between the active and reactive powers of the unbalanced power grid and the positive and negative sequence voltages and currents is specifically described as follows.

When the active neutral point clamped three-level converter is in an unbalanced state at a power grid, the apparent power S of the power grid side is as follows:

j denotes a reactive power component coefficient.

In the formula:

wherein the content of the first and second substances,is the positive sequence component of the voltage in the dq axis,is the negative sequence component of the voltage under the dq axis;is the current positive sequence component in the dq axis,is the current negative sequence component under the dq axis; q₀Is the average value of the reactive power; p₀Is the average value of active power; p_c2The harmonic peak value is the cosine of the secondary active power; p_s2Is the harmonic peak of the secondary active power sine; q_c2Is the second reactive power cosine harmonic peak value; q_s2For the second harmonic peak of the reactive power sine, p (t) is the net side instantaneous active power, q (t) is the net side instantaneous reactive power.

Wherein the content of the first and second substances,

in the formula:andis the voltage under the dq axis is positiveA sequence and negative sequence component;andthe current positive and negative sequence components under the dq axis. The current control instructions with different requirements can be obtained through the formula, the obtained positive and negative sequence current instructions are independently controlled in the positive sequence rotating coordinate system, meanwhile, the obtained positive and negative sequence current instructions are also independently controlled in the negative sequence rotating coordinate system, and then positive and negative sequence double-current control is obtained.

In order to make the second harmonic component of the dc voltage 0, the second active power component on the network side is made 0, i.e. P_c2＝P_s20; in addition, only the instantaneous reactive power component is controlled to make Q₀The converter can be operated at the unit power factor only if the converter is equal to 0, and the given power reference value is set asOutputting the given power reference value as a positive sequence current given using:

the above formula represents a given power reference valueGiven the relationship between positive and negative sequence voltage currents, wherein,representing zero sequence active power;representing zero-sequence reactive power;harmonic peak representing the cosine of the second active powerA value is given;a harmonic peak value given representing the sine of the secondary active power;representing a d-axis positive sequence potential component;representing a q-axis positive sequence potential component;represents a d-axis negative-sequence potential component;representing a q-axis negative-sequence potential component;represents the d-axis positive sequence current given;represents a q-axis positive sequence current given;represents a d-axis negative-sequence current given;indicating a q-axis negative-sequence current given.

In other embodiments, as shown in fig. 4, the analysis of the ANPC four quadrant operation state is required to allow for the converter to operate in both the rectifying mode and the inverting mode. FIG. 4 is an AC steady state vector relationship diagram of the ANPC converter. The four-quadrant running state of the ANPC converter can be realized by controlling the alternating voltage vector V. Specifically, if it is assumed that the ac-side current vector | I | is constant, the ac-side inductor voltage vector | V |, is therefore constant_LThe terminal travel path of the ac voltage vector V of the ANPC converter is also constant, | ω L | I |, in which case the terminal travel path is formed by | V |_LAnd | is a circle of radius.

As shown in fig. 4(a), when the end point of the vector V is located at the point of the circular locus a, the current vector I lags behind the electromotive force vector E by 90 °, and the ANPC network side exhibits a pure inductance characteristic.

As shown in fig. 4(B), when the end point of the vector V is located at the point B of the circular trace, the current vector I is parallel and in the same direction as the electromotive force vector E, and the network side of the ANPC exhibits a positive resistance characteristic.

As shown in fig. 4(C), when the end point of the vector V is located at the point C of the circular locus, the current vector I leads the electromotive force vector E by 90 °, and the ANPC network side exhibits pure capacitance characteristics.

As shown in fig. 4(B), when the end point of the vector V is located at the point B of the circular locus, the current vector I is parallel to and opposite to the electromotive force vector E, and the network side of the ANPC exhibits negative resistance characteristics.

Namely: when the voltage vector V end point is on the circular trackWhen the ANPC converter moves upwards, the ANPC converter operates in a rectification state, and at the moment, the ANPC converter absorbs active power and inductive reactive power from a power grid. When the voltage vector V end point is on the circular trackDuring the upward movement, the ANPC converter operates in a rectification state, and at the moment, the ANPC converter absorbs active and capacitive reactive power from the power grid. When the voltage vector V end point is on the circular trackWhen the ANPC converter moves upwards, the ANPC converter operates in an active inversion state, and at the moment, the ANPC converter transmits active power and capacitive reactive power from a power grid. When the voltage vector V end point is on the circular trackWhen the ANPC converter moves upwards, the ANPC converter operates in an active inversion state, and at the moment, the ANPC converter transmits active power and inductive reactive power from a power grid.

In other embodiments, in order to achieve the switching loss balance of the switching tubes and make the average switching losses of the six switching tubes of each phase of bridge arm the same, the switching state of each phase of bridge arm is controlled as follows:

when the voltage under the abc coordinate system is in a positive half period, three switching tubes in each bridge arm are defined to be in a conducting state, when the voltage is in a negative half period, the other three switching tubes in each bridge arm are defined to be in a conducting state, the size of an SPWM pulse generated when the voltage is in the positive half period or the negative half period is determined according to the defined switching state of each switching tube, and the corresponding switching tubes are controlled to be conducted or cut off under the SPWM pulse.

In a specific embodiment, a midpoint O of the dc side capacitor is selected as a reference point, and an a-phase bridge arm is taken as an example when the power device S_a1、S_a2And S_a6When the bridge arm is conducted, the voltage u of the output end of the A-phase bridge arm to the center O_AO＝U_dc/2 (wherein U_dcIs the dc bus voltage), this state is defined as P; when power device S_a2、S_a3、S_a5Or S_a6When conducting, u_AOWhen power device S is equal to 0_a3、S_a4And S_a5When conducting, u_AO＝-U_dcAnd/2, defining the state as N, the switching sequence and states and output voltage as shown in Table 1:

TABLE 1

FIG. 5 is a schematic diagram of SPWM pulse generation at reference voltage u according to the present invention_refWhen the voltage of the power grid voltage and the current under the abc coordinate system obtained after positive and negative sequence independent decoupling control is a positive half cycle, when u is a positive half cycle_ref>v_c1Bridge arm output P state when the voltage of the triangular carrier wave is equal to_ref<v_c1The time bridge arm outputs O state, S state_a1、S_a3、S_a5Switching according to carrier frequency; at a reference voltage u_refIn the negative half period, when u_ref>v_c1The time bridge arm outputs N state, when u_ref<v_c1The time bridge arm outputs an O state, whichTime S_a2、S_a4、S_a6Switching according to the carrier frequency.

Fig. 6 is an operation result diagram of the grid-connected system provided by the invention, the switching frequencies are equal, the grid-connected voltage and current harmonics are lower, and the four-quadrant stable operation can be realized, which shows the effectiveness of the control method of the invention.

The embodiments in the present description are described in a progressive manner, each embodiment focuses on differences from other embodiments, and the same and similar parts among the embodiments are referred to each other. The device disclosed by the embodiment corresponds to the method disclosed by the embodiment, so that the description is simple, and the relevant points can be referred to the method part for description.

The previous description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.