CN110808607A - A photovoltaic grid-connected converter topology and its control method - Google Patents

Abstract

The invention relates to a photovoltaic grid-connected converter topological structure and a control method thereof.A resistance source network is added between a photovoltaic array and a three-phase two-level inverter, and comprises diodes D which are sequentially connected and the transformation ratio of the diodes D is N_a:N_bThe two-winding high-frequency transformer and the capacitor C; at a DC link voltage V_dc‑linkUnder certain conditions, by adjusting the through duty ratio d_stRegulating the output voltage V of a photovoltaic array_dc(ii) a DC link voltage V output according to impedance source network_dc‑linkApplying a voltage space vector modulation technology to the output voltage reference value to obtain a driving signal of each switching tube in the three-phase two-level inverter; by pair d_stAccording to d, finding the maximum power point_stAnd determining a through signal, and integrating the through signal with the driving signals of the switching tubes to obtain the final driving signal of the switching tubes of the three-phase two-level inverter. The impedance source inverter is applied to photovoltaic power generation to replace a two-stage junction of a traditional DC/DC circuit and the inverterThe structure is simpler, the cost is saved, and the loss is reduced.

Description

A photovoltaic grid-connected converter topology and its control method

technical field

The invention relates to a photovoltaic grid-connected converter topology structure and a control method thereof, in particular, to a single-stage converter topology structure used in a photovoltaic grid-connected power generation system and a maximum power tracking method for the structure .

Background technique

The environmental pollution caused by traditional power generation such as thermal power generation has become increasingly prominent. With the enhancement of people's awareness of environmental protection, various new energy power generation forms have gradually entered people's field of vision. As a clean energy, solar energy has gradually entered enterprises and thousands of households. And showing a rapid growth trend.

At present, the most widely used photovoltaic power generation is the string and centralized power generation structure, that is, multiple photovoltaic panels are connected in series and parallel, connected to the DC/DC circuit to complete the maximum power tracking, and then connected to the inverter for grid-connected control. In the traditional mode, there is usually a two-stage structure between the photovoltaic cell and the power grid, that is, the structure of DC/DC inverter and inverter combination. The two-stage structure is simple to control, but each stage is controlled by an independent controller to control the switching devices, resulting in increased loss and cost.

SUMMARY OF THE INVENTION

The invention relates to an impedance source type converter used in the field of photovoltaic power generation and a maximum power tracking strategy thereof, which is used to improve the problems of many stages, complex structure and large loss of traditional two-stage photovoltaic converters.

The technical solution adopted by the present invention to achieve the above object is: a photovoltaic grid-connected converter topology structure, an impedance source network is added between the photovoltaic array and the three-phase two-level inverter, and the impedance source network includes Sequentially connected diode D and transformation ratio are two- _winding high-frequency transformers and capacitors C of Na:Nb _; the cathode of the diode D is connected to one end of the winding Na, and the other end of the winding Na is connected to the end of the same name of the winding Nb, and the winding The other end of Nb is connected to the positive electrode of capacitor C; the anode of diode D is used as the positive input end of the impedance source network, and the negative electrode of capacitor C is used as the negative input end and negative output end of the impedance source network; the node between winding Na and winding Nb serves as the impedance source network of the positive output.

The output side voltage of the photovoltaic array is input between the positive input terminal and the negative input terminal of the impedance source network; the DC link voltage V _dc-link is output between the positive output terminal and the negative output terminal of the impedance source network to a three-phase two-level inverter. DC bus side of the unit.

A photovoltaic grid-connected converter topology control method, comprising the following steps:

When the DC link voltage V _dc-link is constant, the output voltage V _dc of the photovoltaic array is adjusted by adjusting the direct duty cycle d _st ; according to the DC link voltage V _dc-link output by the impedance source network and the output voltage reference value, the voltage Space vector modulation to obtain the drive signal of each switch tube in the three-phase two-level inverter;

Find the maximum power point by controlling d _st , determine the through signal according to d _st , and integrate the through signal with the driving signal of each switch tube to obtain the final drive signal for the three-phase two-level inverter switch tube.

Through the pass-through and cut-off of the output side of the impedance source network, the voltage across the diode D is controlled, so that the diode D works in an alternate mode of off and on, and the output side of the high-frequency transformer realizes boosting; when the output side of the impedance source network is straight-through, The output terminal is short-circuited, and the voltage is 0; when the output side of the impedance source network is turned off, the output is boosted, and the two states alternately form a DC link voltage V _dc-link .

The adjustment of the output voltage V _dc of the photovoltaic array by adjusting the through duty ratio d _st is achieved by the following formula:

Among them, the duty cycle of the shoot-through time is d _st , the transformation ratio of the high-frequency transformer is _{Na :N b ,} V _dc is the output side voltage of the photovoltaic array, and V _dc-link is the DC link voltage.

The method for finding the maximum power point is one of the disturbance observation method and the conductance increment method.

Determining the through signal according to d _st and integrating the through signal with the driving signal of each switch tube is specifically as follows: obtaining the through signal of the switch tube according to d _st , and the logical relationship between the through signal and the driving signal of each switch tube is "OR".

The through signal is added at the peak and valley of the triangular carrier during the voltage space vector modulation process, and the time length is switching period/2*d _st .

The present invention has the following beneficial effects and advantages:

1. The addition of the impedance source network enables the two-level inverter to have the boost capability without adding switching devices and controllers;

2. The impedance source inverter of the present invention is applied to photovoltaic power generation, replacing the two-stage structure of the traditional DC/DC circuit and the inverter, the structure is simpler, and the cost is saved.

3. Compared with the traditional two-stage photovoltaic power generation system, the present invention realizes the functions of grid connection and maximum power tracking in the single-stage state, the switching devices are not increased, and the loss is reduced.

Description of drawings

Fig. 1 topology structure of the present invention;

Among them, 1 is the photovoltaic array, 2 is the impedance source network, 3 is the two-level inverter, and 4 is the power grid;

Fig. 2 control strategy block diagram of the present invention;

FIG. 3 is a schematic diagram of the modulation mode of the present invention, taking the first bridge arm of the inverter as an example.

Detailed ways

The present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

The invention relates to a photovoltaic power generation technology, which is used to improve the problems of traditional DC/DC topology and two-stage converters of inverters with large number of stages, complex structure and large loss. The photovoltaic power generation technology includes a structure of a single-stage inverter with boosting capability connected to a photovoltaic array and its control strategy. The converter is formed by adding an impedance source network on the DC side, and the control strategy is the maximum power tracking method for this topology.

FIG. 1 is a schematic diagram of the topology structure of the present invention, which mainly includes a photovoltaic array 1 , an impedance source network 2 , a three-phase two-level inverter 3 , and a power grid 4 . The photovoltaic array 1 is composed of multiple photovoltaic cells connected in series and parallel, and the output side is connected to the impedance source network 2 . The impedance source network 2 is composed of a fast recovery diode D, a two-winding high-frequency transformer with a transformation ratio of N _a : N _b , and a capacitor C. The output DC link voltage V _dc-link is connected to the DC of the three-phase two-level inverter 3 . bus side. The three-phase two-level inverter 3 consists of six switch tubes in pairs to form three bridge arms to complete grid-connected control, and the output side is connected to the grid through an inductor.

Through the pass-through and cut-off of the output side of the impedance source network, the voltage across the diode D is controlled to make the diode D work in the mode of turn-off and turn-on alternately. boost. When the output side of the impedance source network is connected directly, the output terminal is short-circuited and the voltage is 0; when the output side of the impedance source network is turned off, the output voltage is increased, and the two states alternately form a DC link voltage V _dc-link .

可根据负载情况人为给定一个常数，电压环中PI控制器的输出就是电流环的给定值根据给定值对d轴电流分量i_d进行控制；q轴电流i_q根据并网功率因数进行设置，若并网功率因数为1，则q轴电流给定值为0。d轴和q轴通道的PI调节器输出经过dq反变换后得到v_α、v_β，再进行电压空间矢量调制(SVPWM)，得到整合前的驱动信号S_Q1～S_Q6。Fig. 2 is a control block diagram of the present invention. First, the three-phase voltages u _A , u _B , and u _C of the grid are collected, and the grid voltages u _α and u _β in the two-phase rotating coordinate system are obtained through 3s/2s coordinate transformation, and the grid voltage amplitude is calculated. value _um , and then calculate the trigonometric function values cosθ and sinθ of the grid voltage phase. Then use the obtained grid phase to perform _dq transformation on the grid-connected currents i _A , i _B , and i _C to obtain the components id and i _q of the grid current in the two-phase stationary coordinate system. Stabilize V _dc-link at reference value via voltage loop

A constant can be artificially given according to the load situation, and the output of the PI controller in the voltage loop is the given value of the current loop The _d -axis current component id is controlled according to the given value; the q-axis current i _q is set according to the grid-connected power factor. If the grid-connected power factor is 1, the q-axis current given value is 0. The PI regulator outputs of the d-axis and q-axis channels undergo inverse dq transformation to obtain v _α and v _β , and then perform voltage space vector modulation (SVPWM) to obtain the drive signals S _Q1 to S _Q6 before integration.

After adding the impedance source network, the boost from the DC bus voltage V _dc to the DC link voltage V _dc-link can be achieved through the through-through on the inverter side. According to the volt-second balance of the excitation inductance of the high-frequency transformer, the relationship between the shoot-through time and the boost ratio is formula (1)

The duty cycle of the shoot-through time is d _st , and the transformation ratio of the high-frequency transformer is _{Na :N b .} The terminal with the same name is shown in Figure 1. V _dc is the output side voltage of the photovoltaic array, and V _dc-link is the DC link voltage.

在V_dc-link一定的情况下，根据公式(1)，通过调节直通占空比d_st可以调节阻抗源网络的输入电压V_dc，光伏电池的输出电压是V_PV，输出电流是I_PV，根据V_PV和I_PV可以得到光伏电池输出的总功率，因此通过调节d_st就可以调节光伏电池输出的总功率。根据该原理，通过对d_st的控制寻找最大功率点，并通过寻优得到的d_st确定直通信号，直通信号得到方法如图3，下一段进行详细介绍。将直通信号与SVPWM的输出信号整合，通过“或”运算得到逆变器最终的开关管驱动信号

此处最大功率追踪的具体算法有多种选择，如扰动观察法、电导增量发等，本发明不指定具体方法。The DC link voltage V _dc-link is stabilized at the

In the case of a certain V _dc-link , according to formula (1), the input voltage V _dc of the impedance source network can be adjusted by adjusting the through duty cycle d _st , the output voltage of the photovoltaic cell is V _PV , and the output current is I _PV , According to V _PV and I _PV , the total power output by the photovoltaic cell can be obtained, so the total power output by the photovoltaic cell can be adjusted by adjusting d _st . According to this principle, find the maximum power point through the control of d _st , and determine the through signal through the d _st obtained by optimization. Integrate the straight-through signal with the output signal of SVPWM, and obtain the final switch tube drive signal of the inverter through the "OR" operation

There are various options for the specific algorithm of the maximum power tracking here, such as the disturbance observation method, the conductance increment generation, etc. The present invention does not specify a specific method.

Fig. 3 is the modulation mode of the present invention: for the three-phase SVPWM modulation mode, there is a zero vector at the peaks and troughs of the triangular carrier, and the zero vector is an invalid vector, and adding through time in this time period does not affect the normal inversion. According to this principle, the shoot-through time is added at the peak and valley of each carrier. The shaded part in Figure 3 is the shoot-through time added on the basis of traditional SVPWM, and the time length is switching period/2×d _st .

Claims (8)

1. A topological structure of a photovoltaic grid-connected converter is characterized in that an impedance source network is added between a photovoltaic array and a three-phase two-level inverter, and the impedance source network comprises diodes D and a transformation ratio N which are sequentially connected_a:N_bThe two-winding high-frequency transformer and the capacitor C; the cathode of the diode D is connected with one end of a winding Na, the other end of the winding Na is connected with the end of the same name of a winding Nb, and the other end of the winding Nb is connected with the anode of the capacitor C; the anode of the diode D is used as the positive input end of the impedance source network, and the cathode of the capacitor C is used as the negative input end and the negative output end of the impedance source network; the node between the windings Na and Nb serves as the positive output of the impedance source network.

2. The photovoltaic grid-connected converter topology structure according to claim 1, wherein an output side voltage of the photovoltaic array is input between a positive input end and a negative input end of the impedance source network; a DC link voltage V is output between the positive output end and the negative output end of the impedance source network_dc-linkTo the dc bus side of a three-phase two-level inverter.

3. A topology control method of a photovoltaic grid-connected converter is characterized in that,

at a DC link voltage V_dc-linkUnder certain conditions, by adjusting the through duty ratio d_stRegulating the output voltage V of a photovoltaic array_dc(ii) a DC link voltage V output according to impedance source network_dc-linkAnd obtaining each switching tube in the three-phase two-level inverter by voltage space vector modulation with the output voltage reference valueThe drive signal of (1);

by pair d_stAccording to d, finding the maximum power point_stAnd determining a through signal, and integrating the through signal with the driving signals of the switching tubes to obtain a final driving signal for the switching tubes of the three-phase two-level inverter.

4. The topology control method of the photovoltaic grid-connected converter according to claim 3, characterized in that the voltage across the diode D is controlled by the switch-through and switch-off of the output side of the impedance source network, so that the diode D works in a mode of alternating switch-off and switch-on, and the output side of the high-frequency transformer realizes boosting; when the output side of the impedance source network is straight, the output end is short-circuited and the voltage is 0; when the output side of the impedance source network is turned off, the boosted voltage is output, and the two states alternately form a direct current link voltage V_dc-link。

5. The photovoltaic grid-connected converter topology control method according to claim 3, wherein the direct-connection duty ratio d is adjusted_stRegulating the output voltage V of a photovoltaic array_dcIs achieved by the following formula:

wherein the duty ratio of the through time is d_stThe transformation ratio of the high-frequency transformer is N_a:N_b，V_dcIs the photovoltaic array output side voltage, V_dc-linkIs a dc link voltage.

6. The photovoltaic grid-connected converter topology control method according to claim 3, wherein the method for finding the maximum power point is one of a disturbance observation method and a conductance increment method.

7. The method according to claim 3, wherein the method is based on d_stDetermining a through-signal, comparing the through-signal with eachThe integration of the driving signals of the switching tube is specifically as follows: according to d_stAnd obtaining a through signal of the switch tube, wherein the logical relation between the through signal and the driving signal of each switch tube is OR.

8. The topology control method of the photovoltaic grid-connected converter according to claim 7, wherein the through signal is added at a peak valley of a triangular carrier in the voltage space vector modulation process, and the time length is a switching period/2 x d_st。

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