阅读说明：本技术 独立带不平衡负载的主动支撑型光伏逆变器 (Independent active support type photovoltaic inverter with unbalanced load ) 是由 李铁成 梁纪峰 夏彦卫 曾四鸣 范辉 罗蓬 周文 易皓 王振雄 于 2021-08-10 设计创作，主要内容包括：本发明提供一种独立带不平衡负载的主动支撑型光伏逆变器。该独立带不平衡负载的主动支撑型光伏逆变器包括：光伏接点、负载接点、前级变换器和后级逆变器；光伏接点用于接外部光伏,负载接点用于接外部负载；前级变换器的第一端与光伏接点连接,前级变换器的第二端与后级逆变器的第一端连接；后级逆变器的第二端与负载接点连接。本发明能够在独立带负载运行时,对电压电流进行正负序分离,独立控制正序分量与负序分量,采用恒压恒频控制策略控制正序分量维持母线电压与频率,引入负序双环控制抑制负序分量,消除功率波动,保证系统稳定运行,提高运行可靠性。(The invention provides an active support type photovoltaic inverter with an unbalanced load independently. This independent active support type photovoltaic inverter with unbalanced load includes: the photovoltaic junction, the load junction, the preceding converter and the subsequent inverter; the photovoltaic contact is used for connecting an external photovoltaic, and the load contact is used for connecting an external load; the first end of the preceding-stage converter is connected with the photovoltaic contact, and the second end of the preceding-stage converter is connected with the first end of the rear-stage inverter; and the second end of the rear-stage inverter is connected with the load contact. The invention can separate the positive sequence from the negative sequence of the voltage and the current when the independent load runs, independently control the positive sequence component and the negative sequence component, control the positive sequence component to maintain the voltage and the frequency of the bus by adopting a constant voltage and constant frequency control strategy, introduce the negative sequence double-loop control to inhibit the negative sequence component, eliminate the power fluctuation, ensure the stable running of the system and improve the running reliability.)

1. An active support type photovoltaic inverter with an unbalanced load, comprising:

the photovoltaic junction, the load junction, the preceding converter and the subsequent inverter;

the photovoltaic contact is connected with an external photovoltaic, the load contact is connected with an external load, the first end of the preceding-stage converter is connected with the photovoltaic contact, the second end of the preceding-stage converter is connected with the first end of the rear-stage inverter, and the second end of the rear-stage inverter is connected with the load contact;

the pre-stage converter is used for maintaining the direct-current voltage of the external photovoltaic to be constant;

and the rear-stage inverter is used for carrying out positive-negative sequence separation on the voltage and the current and independently controlling positive and negative sequences so as to eliminate negative sequence unbalanced voltage.

2. The active support type photovoltaic inverter with unbalanced load as recited in claim 1, wherein the pre-converter is a Boost converter;

the calculation formula of the modulation signal of the Boost switching tube corresponding to the single closed-loop control of the direct-current side capacitor voltage by the Boost converter is as follows:

wherein u is_{dc_g}For modulating signals for Boost switching tubes, k_{p_dc}Is the value of the proportional controller of the PI regulator, k_{i_dc}Integrating the value of the controller for the PI regulator, u_dcrefFor a given value of the DC-side capacitor voltage u_dcIs the measured dc side capacitance voltage.

3. The isolated unbalanced load actively supported photovoltaic inverter of claim 1, wherein the back-stage inverter comprises: the device comprises a positive and negative sequence separation module, an A/D conversion module, a positive sequence component control module, a negative sequence component control module and a D/A conversion module.

4. The isolated unbalanced-load actively-supported photovoltaic inverter as recited in claim 3, wherein the output of the A/D converter module is calculated as follows:

wherein i_a、i_b、i_cThree-phase currents, i, respectively output by inverters_d、i_qFor the value of the output three-phase current in the Dq synchronous rotation coordinate system, u_a、u_b、u_cThree-phase voltages, u, respectively output by inverters_d、u_qIs the value of the output three-phase voltage in a Dq synchronous rotation coordinate system, theta₁Is the angle between the d-axis and the phase reference axis.

5. The isolated unbalanced-load actively-supported photovoltaic inverter as recited in claim 3, wherein the D/a conversion module output value is calculated as follows:

wherein i_a、i_b、i_cThree-phase currents, i, respectively output by inverters_d、i_qFor the value of the output three-phase current in the Dq synchronous rotation coordinate system, u_a、u_b、u_cThree-phase voltages, u, respectively output by inverters_d、u_qIs the value of the output three-phase voltage in a Dq synchronous rotation coordinate system, theta₁Is the angle between the d-axis and the phase reference axis.

6. The active-support photovoltaic inverter with unbalanced load as recited in claim 3, wherein the positive-negative sequence separation module separates the positive-negative sequence component of the voltage and the current by using a time domain detection algorithm based on trigonometric function orthogonal transformation, and expresses the voltage as a combination of the positive-sequence component and the negative-sequence component by using orthogonality of a sine function and a cosine function, and the calculation formula is as follows:

wherein, a₁Is a voltage positive sequence total component coefficient, a₂Is the coefficient of the voltage negative-sequence sinusoidal component, b₂Is the coefficient of the negative sequence cosine component of voltage, omega is the angular frequency of voltage, phi₁Is the initial phase angle of the positive sequence component of the voltage.

7. The isolated unbalanced-load actively-supported photovoltaic inverter of claim 3, wherein the positive sequence component control module controls the positive sequence component with dual voltage-current loops, and the negative sequence component control module comprises: the first voltage outer ring control module and the first current inner ring control module;

negative sequence component control module adopts constant voltage constant frequency to control negative sequence component, positive sequence component control module includes: the device comprises a capacitor voltage equalization control module, a second voltage outer ring control module, a second current inner ring control module and a voltage feedforward module.

8. The isolated unbalanced load actively supported photovoltaic inverter of claim 7, wherein the first voltage outer loop control module generates the current inner loop control module command value by the following equation:

wherein, U_{d_nref}＝0,U_{q_nref}0; in the formula i_{d_nref}Is a negative sequence d-axis current command value, i_{q_nref}Is a negative sequence q-axis current command value, k_{p_ndu}Proportional control of external ring PI regulator for negative sequence d-axis voltageValue of the system, k_{i_ndu}Integrating the value of the proportional controller, k, for a negative sequence d-axis voltage outer loop PI regulator_{p_nqu}The value, k, of the proportional controller of the outer loop PI regulator for negative sequence q-axis voltages_{i_nqu}Integrating the value of the controller for the outer loop PI regulator for negative sequence q-axis voltages, U_{d_nref}Is a negative sequence d-axis voltage command value, U_{q_nref}Is a negative sequence q-axis voltage command value, U_{d_n}And U_{q_n}The values of the negative sequence voltage on the d-axis and the q-axis, respectively;

the calculation formula of the first current inner loop control module for generating the modulation signal is as follows:

wherein m is_{d_n}And m_{q_n}Respectively d-axis negative sequence modulation quantity of inverter output voltage and q-axis negative sequence modulation quantity, k of inverter output voltage_{p_ndi}、k_{i_ndi}The value of the proportional controller and the value of the integral controller of the negative sequence d-axis voltage outer loop PI regulator are respectively_{p_nqi}、k_{i_nqi}The value of the proportional controller and the value of the integral controller, I, of the negative sequence q-axis voltage outer loop PI regulator are respectively_{d_nref}Is a negative sequence d-axis current command value, I_{q_nref}Is a negative sequence q-axis current command value, i_{d_n}And i_{q_n}The values of the negative-sequence current on the d-axis and q-axis, respectively.

9. The isolated unbalanced-load actively-supported photovoltaic inverter of claim 7, wherein the second voltage outer loop control module generates the current inner loop control module command value by the following equation:

U_{d_pref}＝311,U_{q_pref}＝0

wherein, I_{d_pref}Is a positive sequence d-axis current command value, I_{q_pref}Is in positive orderq-axis current command value, k_{p_pdu}The value, k, of the proportional controller of the outer loop PI regulator for positive sequence d-axis voltages_{i_pdu}Integrating the value of the proportional controller, k, for the positive sequence d-axis voltage outer loop PI regulator_{p_pqu}Outer loop PI regulator proportional controller value, k, for positive sequence q-axis voltage_{i_pqu}Integrating the value of the controller for the outer loop PI regulator for the positive sequence q-axis voltage, U_{d_pref}For positive sequence d-axis voltage command value, U_{q_pref}For positive sequence q-axis voltage command values, U_{d_p}And U_{q_p}The values of the positive sequence voltage on the d-axis and the q-axis, respectively;

the calculation formula of the second current inner loop control module for generating the modulation signal is as follows:

wherein m is_{d_p1}And m_{q_p1}Respectively generating a modulation instruction value for a positive sequence d-axis current inner ring and a modulation instruction value, k, for a positive sequence q-axis current inner ring_{p_pdi}、k_{i_pdi}The value of the proportional controller and the value of the integral controller, k, of the outer ring PI regulator of the positive sequence d-axis voltage respectively_{p_pqi}、k_{i_pqi}The value of the proportional controller and the value of the integral controller, I, of the outer loop PI regulator of the positive sequence q-axis voltage_{d_pref}Is a positive sequence d-axis current command value, I_{q_pref}For positive-sequence q-axis current command value, i_{d_p}And i_{q_p}The values of the positive sequence current on the d-axis and the q-axis respectively, K is the proportionality coefficient of the feedforward signal, U_{d_p}And U_{q_p}The values of the positive sequence voltage on the d-axis and q-axis, respectively.

10. The actively-supported, independently unbalanced-loaded photovoltaic inverter as recited in any one of claims 1 to 9, further comprising: a filter, the load contact being connected to a first end of the filter, a second end of the filter being connected to the external load.

Technical Field

The invention relates to the technical field of new energy power generation, in particular to an active support type photovoltaic inverter with an unbalanced load independently.

Background

With the rapid development of human society, traditional energy sources such as natural gas and oil are gradually consumed, the resource shortage and the environmental problems caused by the traditional energy sources are not neglected, the development and the utilization of new energy sources go to the historical stage and become important issues which are not neglected in the global energy source field, and a photovoltaic power supply is taken as one of the main ways of supplying power by the new energy sources, and the development of the photovoltaic power supply is emphasized in various countries in the world. After the distributed energy is merged into the power grid, a fixation effect is generated on the power grid, efficient grid connection can be achieved through a reasonable control strategy, the active supporting effect on the power grid is achieved, and the operation stability is improved.

When the power grid side breaks down, the distributed photovoltaic system needs to be separated from the power grid to carry out independent loading, so that stable operation of the system is maintained, and reliability is improved. When an actual power grid runs, an asymmetric three-phase load often exists, a negative sequence component is generated in an inverter system, power fluctuation is generated, the output waveform of the inverter is seriously influenced, meanwhile, a certain negative sequence voltage drop is generated in line impedance by negative sequence current, and the power quality is reduced. The photovoltaic inverter can be provided with the capability of independently carrying three-phase unbalanced loads, has high quality and stably provides the voltage required by the load operation. When the photovoltaic inverter is independently loaded, a voltage type control strategy is generally adopted, and when three-phase loads are inconsistent, unbalanced current can be generated, so that the output voltage is unbalanced, and therefore the negative voltage is reacted to cause adverse effects on the safe use of the loads.

However, the photovoltaic inverter has a problem of poor electric energy quality when the photovoltaic inverter operates independently under a load.

Disclosure of Invention

The embodiment of the invention provides an active support type photovoltaic inverter with an unbalanced load independently, and aims to solve the problem that the photovoltaic inverter has poor electric energy quality when the photovoltaic inverter operates with the load independently.

The embodiment of the invention provides an independent active support type photovoltaic inverter with unbalanced load, which comprises:

the photovoltaic junction, the load junction, the preceding converter and the subsequent inverter;

the photovoltaic contact is connected with an external photovoltaic, the load contact is connected with an external load, the first end of the preceding-stage converter is connected with the photovoltaic contact, the second end of the preceding-stage converter is connected with the first end of the rear-stage inverter, and the second end of the rear-stage inverter is connected with the load contact;

the pre-stage converter is used for maintaining the constant direct-current voltage of the external photovoltaic;

and the rear-stage inverter is used for carrying out positive-negative sequence separation on the voltage and the current and independently controlling the positive sequence and the negative sequence so as to eliminate negative sequence unbalanced voltage.

In one possible implementation, the pre-converter is a Boost converter;

the modulation signal of a Boost switching tube corresponding to the single closed-loop control of the direct-current side capacitor voltage by the Boost converter is as follows:

wherein u is_{dc_g}For modulating signals for Boost switching tubes, k_{p_dc}Is the value of the proportional controller of the PI regulator, k_{i_dc}Integrating the value of the controller for the PI regulator, u_dcrefFor a given value of the DC-side capacitor voltage u_dcIs the measured dc side capacitance voltage.

In one possible implementation, the rear stage inverter includes: the device comprises a positive and negative sequence separation module, an A/D conversion module, a positive sequence component control module, a negative sequence component control module and a D/A conversion module.

In one possible implementation, the calculation formula of the output value of the a/D conversion module is as follows:

wherein i_a、i_b、i_cThree-phase currents, i, respectively output by inverters_d、i_qFor the value of the output three-phase current in the Dq synchronous rotation coordinate system, u_a、u_b、u_cThree-phase voltages, u, respectively output by inverters_d、u_qIs the value of the output three-phase voltage in a Dq synchronous rotation coordinate system, theta₁Is the included angle of the d-axis and the phase reference axis.

In one possible implementation, the calculation formula of the output value of the D/a conversion module is as follows:

wherein i_a、i_b、i_cThree-phase currents, i, respectively output by inverters_d、i_qFor the value of the output three-phase current in the Dq synchronous rotation coordinate system, u_a、u_b、u_cThree-phase voltages, u, respectively output by inverters_d、u_qIs the value of the output three-phase voltage in a Dq synchronous rotation coordinate system, theta₁Is the included angle of the d-axis and the phase reference axis.

In a possible implementation manner, the positive-negative sequence separation module adopts a time domain detection algorithm based on trigonometric function orthogonal transformation to separate positive-negative sequence components of voltage and current, and expresses the voltage as a combination of a positive sequence component and a negative sequence component by utilizing the orthogonality of a sine function and a cosine function, and the calculation formula is as follows:

wherein, a₁Is a voltage positive sequence total component coefficient, a₂Is the coefficient of the voltage negative-sequence sinusoidal component, b₂Is the coefficient of the negative sequence cosine component of voltage, omega is the angular frequency of voltage, phi₁Is the initial phase angle of the positive sequence component of the voltage.

In one possible implementation, the positive sequence component control module controls the positive sequence component using a voltage-current dual-loop, and the negative sequence component control module includes: the first voltage outer ring control module and the first current inner ring control module;

the negative sequence component control module adopts constant voltage constant frequency to control the negative sequence component, and the positive sequence component control module includes: the device comprises a capacitor voltage equipartition control module, a second voltage outer ring control module, a second current inner ring control module and a voltage feedforward module.

In one possible implementation manner, the calculation formula for the first voltage outer loop control module to generate the command value of the current inner loop control module is as follows:

wherein, U_{d_nref}＝0,U_{q_nref}0; in the formula i_{d_nref}Is a negative sequence d-axis current command value, i_{q_nref}Is a negative-sequence q-axis current command value, k_{p_ndu}The value, k, of the proportional controller of the outer loop PI regulator for the negative sequence d-axis voltage_{i_ndu}Integrating the value of the proportional controller, k, for a negative sequence d-axis voltage outer loop PI regulator_{p_nqu}Is the value, k, of the proportional controller of the negative sequence q-axis voltage outer loop PI regulator_{i_nqu}Integrating the value of the controller for the outer loop PI regulator for negative sequence q-axis voltages, U_{d_nref}Is a negative sequence d-axis voltage command value, U_{q_nref}Is a negative sequence q-axis voltage command value, U_{d_n}And U_{q_n}The values of the negative sequence voltage on the d-axis and the q-axis, respectively;

the calculation formula of the first current inner loop control module for generating the modulation signal is as follows:

wherein m is_{d_n}And m_{q_n}Respectively d-axis negative sequence modulation quantity of inverter output voltage and q-axis negative sequence modulation quantity, k of inverter output voltage_{p_ndi}、k_{i_ndi}The value of the proportional controller and the value of the integral controller of the negative sequence d-axis voltage outer loop PI regulator are respectively_{p_nqi}、k_{i_nqi}The value of the proportional controller and the value of the integral controller, I, of the negative sequence q-axis voltage outer loop PI regulator are respectively_{d_nref}Is a negative sequence d-axis current command value, I_{q_nref}For negative sequence q-axis current command values, i_{d_n}And i_{q_n}The values of the negative-sequence current on the d-axis and q-axis, respectively.

In one possible implementation manner, the calculation formula for the second voltage outer loop control module to generate the command value of the current inner loop control module is as follows:

U_{d_pref}＝311,U_{q_pref}＝0

wherein, I_{d_pref}Is a positive sequence d-axis current command value, I_{q_pref}Is a positive sequence q-axis current command value, k_{p_pdu}The value, k, of the proportional controller of the outer loop PI regulator for positive sequence d-axis voltages_{i_pdu}Integrating the value of the proportional controller, k, for the positive sequence d-axis voltage outer loop PI regulator_{p_pqu}Outer loop PI regulator proportional controller value, k, for positive sequence q-axis voltage_{i_pqu}Integrating the value of the controller for the outer loop PI regulator for the positive sequence q-axis voltage, U_{d_pref}For positive sequence d-axis voltage command value, U_{q_pref}For positive sequence q-axis voltage command values, U_{d_p}And U_{q_p}The values of the positive sequence voltage on the d-axis and the q-axis, respectively;

the calculation formula of the second current inner loop control module for generating the modulation signal is as follows:

wherein m is_{d_p1}And m_{q_p1}Respectively generating a modulation instruction value for a positive sequence d-axis current inner ring and a modulation instruction value, k, for a positive sequence q-axis current inner ring_{p_pdi}、k_{i_pdi}The value of the proportional controller and the value of the integral controller, k, of the outer ring PI regulator of the positive sequence d-axis voltage respectively_{p_pqi}、k_{i_pqi}The value of the proportional controller and the value of the integral controller, I, of the outer loop PI regulator of the positive sequence q-axis voltage_{d_pref}Is a positive sequence d-axis current command value, I_{q_pref}For positive-sequence q-axis current command value, i_{d_p}And i_{q_p}The values of the positive sequence current on the d-axis and the q-axis respectively, K is the proportionality coefficient of the feedforward signal, U_{d_p}And U_{q_p}The values of the positive sequence voltage on the d-axis and q-axis, respectively.

In one possible implementation manner, the method further includes: and the load contact is connected with the first end of the filter, and the second end of the filter is connected with an external load.

The embodiment of the invention provides an independent active support type photovoltaic inverter with an unbalanced load, which comprises a photovoltaic contact, a load contact, a preceding-stage converter and a subsequent-stage inverter; the photovoltaic contact is used for connecting an external photovoltaic, and the load contact is used for connecting an external load; the first end of the preceding-stage converter is connected with the photovoltaic contact, and the second end of the preceding-stage converter is connected with the first end of the rear-stage inverter; the second end of the rear-stage inverter is connected with the load contact; when the independent load-carrying operation is carried out, positive and negative sequence separation is carried out on voltage and current, a positive sequence component and a negative sequence component are independently controlled, the positive sequence component is controlled by adopting a constant-voltage constant-frequency control strategy to maintain the voltage and the frequency of a bus, negative sequence double-loop control is introduced to inhibit the negative sequence component, power fluctuation is eliminated, the stable operation of a system is ensured, and the operation reliability and the electric energy quality are improved.

Drawings

In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the embodiments or drawings used in the prior art description, and obviously, the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to these drawings without creative efforts.

Fig. 1 is a block diagram illustrating a two-stage photovoltaic independent-band unbalanced load operation control provided in an embodiment of the present invention;

fig. 2 is a structure diagram of a piconet in parallel and off-grid switching according to an embodiment of the present invention;

FIG. 3 is a block diagram of negative sequence component voltage current dual loop control provided by an embodiment of the present invention;

fig. 4 is a block diagram of positive sequence component constant voltage and constant frequency control provided in the embodiment of the present invention;

fig. 5 is a block diagram of voltage-sharing control of capacitors on the dc side of the inverter according to the embodiment of the present invention;

fig. 6 is a waveform diagram of the output voltage of the photovoltaic inverter without the power quality management function according to the embodiment of the present invention;

fig. 7 is a voltage waveform diagram of positive and negative output sequences of an active-support photovoltaic inverter without power quality management function according to an embodiment of the present invention;

fig. 8 is a waveform diagram of the output voltage imbalance of the active-support photovoltaic inverter without the power quality management function according to the embodiment of the present invention;

fig. 9 is a graph of output voltage waveforms of an independent actively-supported photovoltaic inverter with unbalanced loads according to an embodiment of the present invention;

fig. 10 is a graph of output positive and negative sequence voltage waveforms of an independent active support type photovoltaic inverter with unbalanced load according to an embodiment of the present invention;

fig. 11 is a waveform diagram of an output voltage imbalance of an independent unbalanced-load actively-supported photovoltaic inverter according to an embodiment of the present invention.

Detailed Description

In order to make the technical solution better understood by those skilled in the art, the technical solution in the embodiment of the present invention will be clearly described below with reference to the drawings in the embodiment of the present invention, and it is obvious that the described embodiment is a part of the embodiment of the present invention, and not a whole embodiment. All other embodiments obtained by a person of ordinary skill in the art based on the embodiments in the present disclosure without any creative effort shall fall within the protection scope of the present disclosure.

The terms "include" and any other variations in the description and claims of this document and the above-described figures, mean "include but not limited to", and are intended to cover non-exclusive inclusions and not limited to the examples listed herein. Furthermore, the terms "first" and "second," etc. are used to distinguish between different objects and are not used to describe a particular order.

The following detailed description of implementations of the invention refers to the accompanying drawings in which:

fig. 1 is a schematic structural diagram of an independent active-support photovoltaic inverter with unbalanced load according to an embodiment of the present invention. Referring to fig. 1, the independent unbalanced-load actively-supported photovoltaic inverter includes:

the photovoltaic junction, the load junction, the preceding converter and the subsequent inverter;

the photovoltaic contact is connected with an external photovoltaic, the load contact is connected with an external load, the first end of the preceding-stage converter is connected with the photovoltaic contact, the second end of the preceding-stage converter is connected with the first end of the rear-stage inverter, and the second end of the rear-stage inverter is connected with the load contact;

the pre-stage converter is used for maintaining the constant direct-current voltage of the external photovoltaic;

and the rear-stage inverter is used for separating positive and negative sequences of the voltage and the current and independently controlling the positive and negative sequences so as to eliminate negative sequence unbalanced voltage and ensure that the positive sequence voltage supplies power to an off-grid external load and the photovoltaic inverter outputs constant-frequency voltage.

The independent active support type photovoltaic inverter with the unbalanced load specifically comprises the following control steps:

step 1: the voltage of the direct current capacitor is controlled by the preceding stage Boost converter.

Step 1-1: the Boost converter outputs the capacitor voltage u_dcAnd a given DC capacitor voltage reference u_{dc_ref}And (3) making a difference, sending the generated difference signal into a PI (proportional-integral) regulator, and generating a PWM (pulse-width modulation) wave, wherein the calculation formula is as follows:

in the formula u_{dc_p}For modulating signals for Boost switching tubes, k_{p_dc}Is the value of the proportional controller of the PI regulator, k_{i_dc}Integrating the value of the controller for the PI regulator, u_dcrefFor a given value of the DC-side capacitor voltage u_dcIs the measured dc side capacitance voltage.

Step 1-2: the on-off of the switching tube in the boost circuit is controlled by the generated duty ratio signal.

Step 2: and the later-stage inverter part controls the photovoltaic inverter to output stable voltage to maintain the constant frequency of the bus voltage.

Step 2-1: the system bus voltage u_abcAnd output inductor current i_abcPositive and negative sequence components U are obtained through conversion_{d_n}、U_{q_n}And i_{d_n}、i_{q_n}. The voltage (flow) is represented as a combination of positive and negative sequence components:

wherein, subscript 1 represents the positive sequence component, and subscript 2 represents the negative sequence component, for the convenience of calculation, the positive sequence component is represented as the sum of the sine component and the cosine component, and the orthogonality of the sine function and the cosine function can be used for calculating the positive and negative sequence components:

step 2-2: the negative sequence component is controlled by using a voltage-current double loop, and the control block diagram is shown in FIG. 3:

step 2-2-1: and generating a current inner ring command value by using the voltage outer ring, wherein the calculation formula is as follows:

in the formula I_{d_nref}Is a negative sequence d-axis current command value, I_{q_nref}Is a negative sequence q-axis current command value, k_{p_ndu}The value, k, of the proportional controller of the outer loop PI regulator for the negative sequence d-axis voltage_{i_ndu}Integrating the value of the proportional controller, k, for a negative sequence d-axis voltage outer loop PI regulator_{p_nqu}The value, k, of the proportional controller of the outer loop PI regulator for negative sequence q-axis voltages_{i_nqu}Integrating the value of the controller for the outer loop PI regulator for negative sequence q-axis voltages, U_{d_nref}Is a negative sequence d-axis voltage command value, U_{q_nref}Is a negative sequence q-axis voltage command value, U_{d_n}And U_{q_n}The values of the negative sequence voltage on the d-axis and q-axis, respectively.

Wherein the content of the first and second substances,

U_{d_nref}＝0,U_{q_nref}＝0 (5)

step 2-2-2: the current inner loop control module generates a modulation signal, and the calculation formula is as follows:

in the formula m_{d_n}And m_{q_n}The d-axis negative sequence modulation quantity and the q-axis negative sequence modulation quantity, k of the output voltage of the inverter are_{p_ndi}、k_{i_ndi}The value of the proportional controller and the value of the integral controller of the negative sequence d-axis voltage outer loop PI regulator are respectively_{p_nqi}、k_{i_nqi}The value of the proportional controller and the value of the integral controller, I, of the negative sequence q-axis voltage outer loop PI regulator are respectively_{d_nref}Is a negative sequence d-axis current command value, I_{q_nref}Is a negative sequence q-axis current command value, i_{d_n}And i_{q_n}The values of the negative-sequence current on the d-axis and q-axis, respectively.

And step 3: the positive sequence component is controlled by adopting a constant-voltage constant-frequency control strategy to provide voltage frequency support required by normal operation for off-grid loads, and a control block diagram is shown in fig. 4:

step 3-1: the zero sequence current injection method is used for controlling the voltage equalizing of the capacitor on the direct current side of the inverter, and the control block diagram is shown in fig. 5. And (3) making a difference between the direct current side capacitor voltage and the voltage difference value, sending the voltage difference value to a PI (proportional integral) regulator, and making a difference between the voltage difference value and a zero sequence current component to generate an instruction signal:

in the formula, m_p0Respectively, the generated modulation signals u₁、u₂Respectively, the DC side upper and lower capacitor voltages, k_{p_pdc}、 k_{i_pdc}Respectively the value of the proportional controller and the value of the integral controller, I, of the PI regulator₀Is the current zero sequence component.

Step 3-2: and generating a current inner ring command value by using the voltage outer ring, wherein the calculation formula is as follows:

wherein:

U_{d_pref}＝311,U_{q_pref}＝0 (9)

in the formula I_{d_pref}Is a positive sequence d-axis current command value, I_{q_pref}Is a positive sequence q-axis current command value, k_{p_pdu}The value, k, of the proportional controller of the outer loop PI regulator for positive sequence d-axis voltages_{i_pdu}Integrating the value of the proportional controller, k, for the positive sequence d-axis voltage outer loop PI regulator_{p_pqu}Outer loop PI regulator proportional controller value, k, for positive sequence q-axis voltage_{i_pqu}Integrating the value of the controller for the outer loop PI regulator for the positive sequence q-axis voltage, U_{d_pref}For positive sequence d-axis voltage command value, U_{q_pref}For positive sequence q-axis voltage command values, U_{d_p}And U_{q_p}The values of the positive sequence voltage on the d-axis and q-axis, respectively.

Step 3-3: the current inner loop control module is used for generating a modulation signal, and the calculation formula is as follows:

in the formula, m_{d_p1}And m_{q_p1}Generating a modulation command value for the positive sequence d-axis current inner loop and a modulation command value, k, for the positive sequence q-axis current inner loop_{p_pdi}、k_{i_pdi}The value of the proportional controller and the value of the integral controller, k, of the outer ring PI regulator of the positive sequence d-axis voltage respectively_{p_pqi}、k_{i_pqi}The values of the proportional controller and the integral controller of the outer loop PI regulator of the positive sequence q-axis voltage are respectively_{d_pref}Is a positive sequence d-axis current command value, I_{q_pref}For positive-sequence q-axis current command value, i_{d_p}And i_{q_p}The values of the positive sequence current on the d-axis and the q-axis respectively, K is the proportionality coefficient of the feedforward signal, U_{d_p}And U_{q_p}The values of the positive sequence voltage on the d-axis and q-axis, respectively.

Step 3-4: m is to be_{d_p1}And m_{q_p1}Carrying out dq/abc inverse transformation to generate a driving PWM signal, and adding a DC offset modulation signal m_p0And generating a positive sequence PWM signal. A/D (i.e., abc/dq) and D/A (i.e., dq/abc) refer to the switching of digital and analog circuits, which shall be referred to herein as hardware circuits, and dq refers to the software component.

Step 3-5: and superposing the positive-sequence PWM signal and the negative-sequence PWM signal to obtain the inverter switching tube driving PWM wave.

In order to test the invention, a simulation model of an independent active support type photovoltaic inverter with unbalanced load is built in MATLAB/Simulink.

Fig. 6, 7, and 8 show waveforms of the photovoltaic inverter without the power quality control function with a three-phase unbalanced load. Fig. 6 is a voltage waveform diagram of the output voltage of the photovoltaic inverter without the function of managing the power quality, fig. 7 is a waveform diagram of the positive and negative sequence components of the output voltage, and fig. 8 is a waveform diagram of the imbalance degree of the output voltage. It can be seen from the comparison of the three oscillograms that the negative sequence component contained in the output voltage of the photovoltaic inverter without the function of managing the electric energy quality is larger when the three-phase load is unbalanced, the voltage unbalance degree is higher, and the stable operation of the system is not facilitated.

Fig. 9, 10 and 11 show waveforms of independent unbalanced-load active-support photovoltaic inverters with three-phase unbalanced loads. Fig. 9 is a waveform diagram of an output voltage of an independent active support type photovoltaic inverter with an unbalanced load, fig. 10 is a waveform diagram of positive and negative sequence components of the output voltage, and fig. 11 is a waveform diagram of an unbalanced degree of the output voltage. As can be seen from the figure, through the control strategy, the negative sequence component of the output voltage of the photovoltaic inverter is greatly reduced, the unbalance degree of the three phases of the voltage is very low, the electric energy quality of the load voltage is guaranteed, and the reliability and the stability of the active supporting power supply of the photovoltaic inverter are improved.

The above examples are only intended to illustrate the technical solution of the present invention, but not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be understood by those of ordinary skill in the art that: the technical solutions described in the foregoing embodiments may still be modified, or some technical features may be equivalently replaced; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions of the embodiments of the present invention.