CN110098637B - Dual-mode smooth switching control method for cascaded H-bridge photovoltaic grid-connected inverters - Google Patents

Abstract

The invention discloses a dual-mode smooth switching control method of a cascaded H-bridge photovoltaic grid-connected inverter, which includes a mutual switching control method between a current source mode and a voltage source mode. When the current source mode switches to the voltage source mode, the reference value of the active component of the filter inductor current and the power distribution coefficient of the N H-bridge units in the current source mode in the previous cycle of the latch switch, and the feedforward of the DC voltage regulator in each voltage source mode is calculated. The control quantity can realize the smooth switching of the inverter from the current source mode to the voltage source mode. When the voltage source mode switches the current source mode, the reference value of the active component of the filter inductor current and the power distribution coefficient of the N H-bridge units in the voltage source mode in the previous cycle of the latch switch, and the DC voltage regulator feedforward in each current source mode is calculated. The control amount can realize the smooth switching of the inverter from the voltage source mode to the current source mode. The method can realize smooth switching of two control modes, and can also realize the maximum tracking of photovoltaic power generation.

Description

Dual-mode smooth switching control method for cascaded H-bridge photovoltaic grid-connected inverters

technical field

The invention relates to a dual-mode smooth switching control method of a cascaded H-bridge photovoltaic grid-connected inverter, and belongs to the technical field of cascaded photovoltaic inverter control.

Background technique

Photovoltaic grid-connected power generation has attracted much attention because it provides clean energy and is environmentally friendly. Faced with the problems of how to improve the efficiency of photovoltaic systems and reduce the cost of power generation, the cascaded H-bridge multilevel inverter has become a research hotspot due to its advantages of modularization and easy expansion, high system efficiency, and low total harmonic distortion of grid-connected current. .

The output of the cascaded H-bridge photovoltaic inverter is directly integrated into the grid, eliminating the need for a step-up transformer at the grid-connected end and improving the overall efficiency of the photovoltaic power generation system. However, large-scale new energy power generation systems are often installed in remote areas, and the penetration rate of new energy power generation systems is also increasing. As a result, new energy power generation systems are often connected to the weak power grid at the end, and the grid impedance in this area is often affected by line impedance, The number of grid-connected units, load and system operation mode and other factors change. Under such a weak power grid with impedance change characteristics, the cascaded H-bridge photovoltaic inverter is prone to oscillation due to its small output impedance, which is connected to the grid in the traditional current source mode. At this time, the grid-connected mode of the voltage source based on droop control can be used to realize the stable control of the grid-connected inverter. Therefore, it is of great engineering significance to study the current-source-voltage-source dual-mode smooth switching control method of cascaded H-bridge photovoltaic grid-connected inverters.

At present, scholars at home and abroad have carried out relevant research on the control method of inverter current source and voltage source dual-mode smooth switching. For example, Liang Jiangang, Jin Xinmin, Wu Xuezhi and Tong Yibin published the article "Switching Technology of Microgrid Inverter VCS Mode and CCS Mode" in "Power Grid Technology" Vol. 38 No. 4 in April 2014. In this paper, for distributed power sources such as energy storage devices in microgrids, the mutual switching technology of microgrid inverters based on three-phase PWM rectifier topology between PQ-controlled current source mode and droop-controlled voltage source mode is proposed. A method of switching between closed-loop tracking between different modes. However, this method is aimed at the mutual switching problem between the inverter current source and voltage source modes caused by the inverter in grid-connected and off-grid conditions, and has not studied how the inverter is connected to the grid. Current source voltage source dual mode smooth switching. In addition, this method is aimed at centralized inverters, and the smooth switching between the current source and voltage source modes of cascaded inverters is not studied.

Shi Rongliang, Zhang Xing and Xu Haizhen, etc., published in "Power Control Strategy of Virtual Synchronous Generator Based on Adaptive Mode Switching" published in "Journal of Electrotechnical Technology" Vol. fluctuations to switch the control mode of the microgrid energy storage inverter. When the grid frequency fluctuates greatly, make the inverter work in the current source mode to prevent the overshoot or overdischarge of the energy storage battery to prolong the life of the energy storage battery. When the grid frequency is normal, the voltage source mode based on droop control is adopted to realize the current sharing of the parallel microgrid energy storage inverters. However, this method is aimed at the three-phase centralized energy storage inverter, and the cascaded inverter has not been studied. In addition, since its DC side is an energy storage battery, the maximum tracking of photovoltaic power generation power is not studied.

2018 paper "A Novel Stability Improvement Strategy for a Multi-Inverter System in a Weak Grid Utilizing Dual-Mode Control" Ming Li, Xing Zhang and Wei Zhao, "energies", 2018, 11(8), 2144-2162 ("One A novel dual-mode control strategy for improving the stability of multi-inverter systems under weak power grids", "Energy Journal", Vol. 11, No. 8, 2144-2162, 2018), and proposes a switching method based on impedance hysteresis to improve multi-inverter system stability. Inverter system stability. However, this method does not study how the inverter can achieve the maximum tracking of photovoltaic power generation in current source mode or voltage source mode. In addition, the method is aimed at three-phase centralized inverters and does not involve cascaded photovoltaic inverters.

To sum up, the existing cascading H-bridge photovoltaic grid-connected inverter current source voltage source dual-mode smooth switching control methods mainly have the following problems:

(1) The dual-mode switching control methods of inverter current source and voltage source studied in the prior art are mostly aimed at the switching control method between the inverter current source and the voltage source mode caused by the grid-connected and off-grid operating conditions of the inverter. The problem of mutual switching is less related to how to realize the smooth switching of current source and voltage source dual mode when the inverter is connected to the grid.

(2) Most of the inverter current source voltage source dual-mode switching control methods studied in the prior art are aimed at three-phase centralized energy storage inverters, and fail to involve cascaded photovoltaic inverters.

(3) The dual-mode switching control methods of inverter current source and voltage source studied in the prior art mostly aim at the stable power supply on the DC side of the inverter, and do not consider the maximum power tracking of photovoltaic power generation.

SUMMARY OF THE INVENTION

The problem to be solved by the present invention is to overcome the limitations of the above solutions. Aiming at the problem of dual-mode switching of current sources and voltage sources of cascaded H-bridge photovoltaic grid-connected inverters, a current and voltage source of cascaded H-bridge photovoltaic grid-connected inverters is proposed. Source voltage source dual-mode smooth switching control method. The method adopts the current feedforward control method, which can not only realize the smooth switching of the current source and voltage source dual-mode of the cascaded H-bridge photovoltaic grid-connected inverter, but also realize the maximum power tracking of the photovoltaic power generation of each H-bridge unit.

In order to solve the technical problem of the present invention, the present invention provides a dual-mode smooth switching control method for a cascaded H-bridge photovoltaic grid-connected inverter. The H-bridge unit, the filter inductor L _S and the filter capacitor C _f are composed of the H bridge unit, and it is characterized in that, the control method includes a current source mode smooth switching voltage source mode control method and a voltage source mode smooth switching current source mode control method:

The current source mode smooth switching voltage source mode control method includes the following steps:

Step 1: Sampling the DC side voltage of each H-bridge unit and filtering it through a 100Hz notch filter in turn to obtain the actual value of the DC side voltage of N H-bridge units and record it as V _PVi , i=1, 2, 3.. .N; Sampling the actual value of the DC side current of N H-bridge units and denoting it as I _PVi , i=1, 2, 3...N; Sampling the actual value of the filter inductor current and denoting it as I _L ; Sampling the actual value of the filter capacitor voltage value and record it as V _o ; sample the actual value of grid current and record it as I _S ;

Step 2, by performing maximum power point tracking control on the actual DC side voltage V _PVi of each H-bridge unit, the DC side voltage command value of N H-bridge units is obtained and recorded as V _PVi ^* , where i=1,2 ,3...N;

Step 3, according to the actual DC side voltage value V _PVi of the N H-bridge units obtained in step 1 and the DC side voltage command value V _PVi ^* of the N H-bridge units obtained in step 2, through the DC voltage regulator in the current source mode , the active power P _Ci of each H-bridge unit in the current source mode is calculated, where i=1, 2, 3...N, and the calculation formula is:

Among them, K _CVP is the proportional coefficient of the DC voltage regulator in the current source mode, K _CVI is the integral coefficient of the DC voltage regulator in the current source mode, i=1, 2, 3...N, s is the Laplace operator ;

In step 4, according to the active power P _Ci of the N H-bridge units in the current source mode obtained in step 3, the sum of the active powers of the N H-bridge units in the current source mode is calculated and recorded as P _CT , and the calculation formula is:

Step 5, phase-lock the actual value V _o of the filter capacitor voltage sampled in step 1 to obtain the grid voltage amplitude V _m and phase θ _g ; transform the actual value V of the filter capacitor voltage sampled in step 1 through virtual synchronous rotation coordinate transformation. _o Convert into the active component V _Cod of the current source mode filter capacitor voltage and the current source mode filter capacitor voltage reactive component V _Coq under the rotating coordinate system; the actual value I of the filter inductor current sampled in step 1 is transformed through the virtual synchronous rotating coordinate transformation _L is converted into the current source mode filter inductor current active component I _CLd and the current source mode filter inductor current reactive component I _CLq under the rotating coordinate system;

其计算式为：Step 6: Calculate the reference value of the active component of the filter inductor current in the current source mode according to the sum P _CT of the active powers of the N H-bridge units in the current source mode obtained in step 4 and the grid voltage amplitude V _m obtained in step 5.

Its calculation formula is:

分别通过电流源模式下有功电流调节器和电流源模式下无功电流调节器，计算得到电流源模式下d轴PI调节值E_Cd和电流源模式下q轴PI调节值E_Cq，其计算式分别为：Step 7, according to the current source mode filter inductor current active component I _CLd obtained in step 5, the current source mode filter inductor current reactive component I _CLq and the current source mode filter inductor current active component reference value obtained in step 6

Through the active current regulator in the current source mode and the reactive current regulator in the current source mode, respectively, the d-axis PI regulation value E _Cd in the current source mode and the q-axis PI regulation value E _Cq in the current source mode are calculated. They are:

Among them, K _CiP is the proportional coefficient of the current regulator in the current source mode, and K _CiI is the integral coefficient of the current regulator in the current source mode;

Step 8: According to the d-axis PI adjustment value E _Cd in the current source mode obtained in step 7, the q-axis PI adjustment value E _Cq in the current source mode and the current source mode filter capacitor voltage active component V _Cod obtained in step 5 through virtual synchronous rotation Inverse coordinate transformation to obtain the total modulated wave voltage V _Cr of the inverter in the current source mode;

Step 9, according to the active power P _Ci of the N H-bridge units in the current source mode obtained in step 3 and the sum of the active powers P _CT of the N H-bridge units in the current source mode obtained in step 4. The power distribution coefficient Factor _Ci of the H-bridge unit, i=1,2,3...N, its calculation formula is:

Step 10: According to the actual value of the DC side voltage V _PVi of the N H-bridge units obtained in step 1, the total modulated wave voltage V _Cr of the inverter in the current source mode obtained in step 8 and the N voltages in the current source mode obtained in step 9 The power distribution coefficient Factor _Ci of the H-bridge unit calculates the modulation signal m _Cri of each H-bridge unit in the current source mode, i=1, 2, 3...N, and its calculation formula is:

锁存上一周期电流源模式下N个H桥单元功率分配系数记为 Factor_Cmi，计算得到每个电压源模式下直流电压调节器前馈控制量I_VFeedi， i＝1,2,3...N，其计算式为：Step 11, when switching, latch the reference value of the active component of the filter inductor current in the current source mode of the previous cycle, and record it as

The power distribution coefficient of N H-bridge units in the current source mode of the previous cycle is denoted as Factor _Cmi , and the feedforward control quantity I _VFeedi of the DC voltage regulator in each voltage source mode is calculated, i=1,2,3.. .N, which is calculated as:

Step 12: Convert the actual value V _o of the filter capacitor voltage sampled in step 1 into the voltage source mode filter capacitor voltage active component V _Vod and the voltage source mode filter capacitor voltage reactive component under the rotating coordinate system through virtual synchronous rotating coordinate transformation. V _Voq ;

Step 13, converting the actual value I _S of the grid current sampled in step 1 into the voltage source mode grid current active component I _VSd and the voltage source mode grid current reactive component I _VSq under the rotating coordinate system through virtual synchronous rotation coordinate transformation;

Step 14 , according to the voltage source mode filter capacitor voltage active component V _Vod obtained in step 12 , the voltage source mode filter capacitor voltage reactive component V _Voq and the voltage source mode grid current active component I _VSd obtained in step 13 , Voltage source mode grid current The reactive component I _VSq is calculated and filtered by a first-order low-pass filter to obtain the average active power P _Vo and average reactive power Q _Vo output by the inverter in the voltage source mode. The calculation formula is:

Among them, τ is the time constant of the first-order low-pass filter;

Step 15: According to the actual DC side voltage value V _PVi of the N H-bridge units obtained in step 1, the DC side voltage command value V _PVi ^* of the N H-bridge units obtained in step 2, and the N voltage source modes obtained in step 11 The feedforward control variable I _VFeedi of the lower DC voltage regulator is used to calculate the active power P _Vi of each H-bridge unit in the voltage source mode through the DC voltage regulator in the voltage source mode, where i=1, 2, 3... N, its calculation formula is:

Among them, K _VVP is the proportional coefficient of the DC voltage regulator in the voltage source mode, K _VVI is the integral coefficient of the DC voltage regulator in the voltage source mode, i=1, 2, 3...N;

In step 16, according to the active power P _Vi of the N H-bridge units in the voltage source mode obtained in step 15, the sum of the active powers of the N H-bridge units in the voltage source mode is calculated and recorded as P _VT , and the calculation formula is:

Step 17, according to the active power sum P _VT of the N H-bridge units in the voltage source mode obtained in step 16 and the inverter output average active power P _Vo in the voltage source mode obtained in step 14 via the active power-frequency droop control equation The output angular frequency ω _Vo of the inverter in the voltage source mode is calculated, and the output angular frequency ω _Vo in the voltage source mode is integrated to obtain the output phase angle θ _Vo of the inverter in the voltage source mode, and its active power-frequency droop control equation for:

ω _Vo =ω ^* +m(P _VT -P _Vo )

Where ω ^* is the grid synchronization angular frequency, m is the active power droop coefficient;

和电压源模式下滤波电容电压无功分量参考值

其无功功率 -电压下垂控制方程为：Step 18: According to the average reactive power Q _Vo output by the inverter in the voltage source mode obtained in step 14, the reactive power-voltage droop control equation is used to obtain the reference value of the active component of the filter capacitor voltage in the voltage source mode.

and the reference value of the reactive power component of the filter capacitor voltage in the voltage source mode

Its reactive power-voltage droop control equation is:

Among them, E is the reference electromotive force, n is the reactive power droop coefficient, and Q ^* is the upper-layer given reactive power command;

电压源模式滤波电容电压无功分量参考值

分别通过电压源模式下的有功交流电压调节器和无功交流电压调节器，计算得到电压源模式下滤波电感电流有功分量参考值

和电压源模式下滤波电感电流无功分量参考值

其计算式为：Step 19, according to the voltage source mode filter capacitor voltage active component V _Vod obtained in step 12, the voltage source mode filter capacitor voltage reactive component V _Voq and the voltage source mode filter capacitor voltage reference value obtained in step 18

Voltage source mode filter capacitor voltage reactive component reference value

Through the active AC voltage regulator and the reactive AC voltage regulator in the voltage source mode, respectively, the reference value of the active component of the filter inductor current in the voltage source mode is calculated.

and the reference value of the reactive component of the filter inductor current in the voltage source mode

Its calculation formula is:

Among them, K _VVP1 is the proportional coefficient of the AC voltage regulator in the voltage source mode, and K _VVI1 is the integral coefficient of the AC voltage regulator in the voltage source mode;

Step 20: Convert the actual value of the filter inductor current _IL sampled in step 1 into the voltage source mode filter inductor current active component I _VLd and the voltage source mode filter inductor current reactive component under the rotating coordinate system through virtual synchronous rotating coordinate transformation. _IVLq ;

电压源模式下滤波电感电流无功分量参考值

和步骤20得到的电压源模式滤波电感电流有功分量I_VLd、电压源模式滤波电感电流无功分量 I_VLq，分别通过电压源模式下有功电流调节器和无功电流调节器，计算得到电压源模式下d轴PI调节值E_Vd和电压源模式下q轴PI调节值E_Vq，其计算式分别为：Step 21, according to the reference value of the active component of the filter inductor current in the voltage source mode obtained in step 19

Reference value of filter inductor current reactive component in voltage source mode

and the voltage source mode filter inductor current active component I _VLd and the voltage source mode filter inductor current reactive component I _VLq obtained in step 20, respectively, through the active current regulator and the reactive current regulator in the voltage source mode to obtain the voltage source mode The PI adjustment value E _Vd of the lower d axis and the PI adjustment value E _Vq of the q axis in the voltage source mode are calculated as:

Among them, K _ViP is the proportional coefficient of the current regulator in the voltage source mode, and K _ViI is the integral coefficient of the current regulator in the voltage source mode;

Step 22, according to the d-axis PI adjustment value E _Vd in the voltage source mode obtained in step 21, the q-axis PI adjustment value E _Vq in the voltage source mode and the voltage source mode filter capacitor voltage active component V _Vod obtained in step 12 through virtual synchronous rotation Inverse coordinate transformation obtains the total modulated wave voltage V _Vr of the inverter in the voltage source mode;

Step 23, according to the active power P _Vi of the N H-bridge units in the voltage source mode obtained in step 15 and the active power sum P _VT of the N H-bridge units in the voltage source mode obtained in step 16 Calculate each of the voltage source modes. The power distribution coefficient Factor _Vi of the H-bridge unit, i=1,2,3...N, its calculation formula is:

Step 24, according to the actual value of the DC side voltage V _PVi of the N H-bridge units obtained in step 1, the total modulated wave voltage V _Vr of the inverter in the voltage source mode obtained in step 22 and the N voltage source mode obtained in step 23. The H-bridge unit power distribution coefficient Factor _Vi , calculates the modulation signal m _Vri of each H-bridge unit in the voltage source mode, i=1, 2, 3...N, and its calculation formula is:

The voltage source mode smooth switching current source mode control method includes:

锁存上一周期电压源模式下N个H桥单元功率分配系数记为 Factor_Vmi，计算得到每个电流源模式下直流电压调节器前馈控制量I_CFeedi， i＝1,2,3...N，其计算式为：When switching, first latch the reference value of the active component of the filter inductor current in the voltage source mode of the previous cycle, which is recorded as

The power distribution coefficient of N H-bridge units in the voltage source mode of the previous cycle is denoted as Factor _Vmi , and the feedforward control quantity I _CFeedi of the DC voltage regulator in each current source mode is calculated, i=1,2,3.. .N, which is calculated as:

Then, superimpose the DC voltage regulator feedforward control quantity I _CFeedi in the current source mode on the output of the DC voltage regulator in each current source mode, and calculate the active power P′ of each H-bridge unit in the current source mode during switching. _Ci , where i=1,2,3...N, its calculation formula is:

Compared with the prior art, the invention discloses a dual-mode smooth switching control method of a cascaded H-bridge photovoltaic grid-connected inverter, which adopts a current feedforward control strategy to realize the dual-mode of the cascaded H-bridge photovoltaic grid-connected inverter. The smooth and stable switching of control, its beneficial effects are embodied in:

1. The method proposed in the present invention can realize the smooth and stable switching of the cascaded photovoltaic inverter between the two control modes of the current source and the voltage source.

2. The method proposed by the present invention can enable the cascaded photovoltaic inverter to realize the maximum tracking of photovoltaic power generation in both current source and voltage source control modes.

3. The dual-mode switching method proposed by the present invention is simple and easy to implement in engineering.

Description of drawings

FIG. 1 is a topological block diagram of the main circuit of the cascaded H-bridge photovoltaic grid-connected inverter of the present invention.

FIG. 2 is a general control block diagram of the dual-mode switching of the cascaded H-bridge photovoltaic grid-connected inverter of the present invention.

3 shows the grid current _IS and the DC side voltage waveform of each H-bridge unit when the cascaded H-bridge photovoltaic grid-connected inverter switches from the current source mode to the voltage source mode when the control method of the present invention is adopted.

4 shows the grid current _IS and the DC side voltage waveform of each H-bridge unit when the cascaded H-bridge photovoltaic grid-connected inverter switches from the voltage source mode to the current source mode when the control method of the present invention is adopted.

Detailed ways

In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention will be further clearly and completely described below with reference to the accompanying drawings and embodiments.

1 is a topology structure of a cascaded H-bridge photovoltaic grid-connected inverter according to an embodiment of the present invention. As shown in the figure, the cascaded H-bridge photovoltaic grid-connected inverter consists of N H-bridge units with photovoltaic components. , filter inductor L _S and filter capacitor C _f . Specifically, the DC sides of the N H ^- bridge units are connected to N photovoltaic panels PV1, PV2... The voltage is 30.40V, each photovoltaic panel is connected to each H-bridge unit through a 14.1mF capacitor, and the cascade system is connected to the grid through a _1.5mH filter inductor LS and a _55uF filter capacitor Cf.

The overall control block diagram of the dual-mode switching of the present invention is shown in FIG. 2 .

The current source mode smooth switching voltage source mode control method includes the following steps:

Step 1: Sampling the DC side voltage of each H-bridge unit and filtering it through a 100Hz notch filter in turn to obtain the actual value of the DC side voltage of N H-bridge units and record it as V _PVi , i=1, 2, 3.. .N; Sampling the actual value of the DC side current of N H-bridge units and denoting it as I _PVi , i=1, 2, 3...N; Sampling the actual value of the filter inductor current and denoting it as I _L ; Sampling the actual value of the filter capacitor voltage value and record it as V _o ; sample the actual value of grid current and record it as I _S ;

In this embodiment, taking five H-bridge units as an example, the initial DC side voltage of each H-bridge unit is actually V _PV1 =V _PV2 =V _PV3 =V _PV4 =V _PV5 =35V.

Step 2, by performing maximum power point tracking control on the actual DC side voltage V _PVi of each H-bridge unit, the DC side voltage command value of N H-bridge units is obtained and recorded as V _PVi ^* , where i=1,2 ,3...N;

In this embodiment, at the initial time t=0.8s, each H-bridge unit works at the rated temperature T=25°C, and the rated light intensity E ₁ =E ₂ =E ₃ =E ₄ =E ₅ =1000W/m ² Under the conditions, the DC side voltage command value V _PV1 ^* =V _PV2 ^* =V _PV3 ^* =V _PV4 ^* =V _PV5 ^* =30.40V of each H-bridge unit is obtained. At t=1s, the temperature remains unchanged, the illumination intensity of the first and second H bridges remains unchanged, and the illumination intensity of the third, fourth, and fifth H bridges becomes E ₃ =E ₄ =E ₅ =800W/ m ² , the DC side voltage command value V _PV1 ^* =V _PV2 ^* =30.40V of each H-bridge unit is obtained, V _PV3 ^* =V _PV4 ^* =V _PV5 ^* =30.57V.

Step 3, according to the actual DC side voltage value V _PVi of the N H-bridge units obtained in step 1 and the DC side voltage command value V _PVi ^* of the N H-bridge units obtained in step 2, through the DC voltage regulator in the current source mode , the active power P _Ci of each H-bridge unit in the current source mode is calculated, where i=1, 2, 3...N, and the calculation formula is:

Among them, K _CVP is the proportional coefficient of the DC voltage regulator in the current source mode, K _CVI is the integral coefficient of the DC voltage regulator in the current source mode, i=1, 2, 3...N, s is the Laplace operator . In the current source mode, the proportional coefficient and the integral coefficient of the DC voltage regulator are designed according to the conventional grid-connected inverter. In this embodiment, K _CVP =1, K _CVI =10.

In step 4, according to the active power P _Ci of the N H-bridge units in the current source mode obtained in step 3, the sum of the active powers of the N H-bridge units in the current source mode is calculated and recorded as P _CT , and the calculation formula is:

Step 5, phase-lock the actual value V _o of the filter capacitor voltage sampled in step 1 to obtain the grid voltage amplitude V _m and phase θ _g ; transform the actual value V of the filter capacitor voltage sampled in step 1 through virtual synchronous rotation coordinate transformation. _o Convert into the active component V _Cod of the current source mode filter capacitor voltage and the current source mode filter capacitor voltage reactive component V _Coq under the rotating coordinate system; the actual value I of the filter inductor current sampled in step 1 is transformed through the virtual synchronous rotating coordinate transformation _L is converted into the active component of the current source mode filter inductor current I _CLd and the current source mode filter inductor current reactive component I _CLq under the rotating coordinate system, and the calculation formulas are:

where k ₁ is the gain coefficient. In this embodiment, k ₁ =0.5.

其计算式为：Step 6: Calculate the reference value of the active component of the filter inductor current in the current source mode according to the sum P _CT of the active powers of the N H-bridge units in the current source mode obtained in step 4 and the grid voltage amplitude V _m obtained in step 5.

Its calculation formula is:

分别通过电流源模式下有功电流调节器和电流源模式下无功电流调节器，计算得到电流源模式下d轴PI调节值E_Cd和电流源模式下q轴PI调节值E_Cq，其计算式分别为：Step 7, according to the current source mode filter inductor current active component I _CLd obtained in step 5, the current source mode filter inductor current reactive component I _CLq and the current source mode filter inductor current active component reference value obtained in step 6

Through the active current regulator in the current source mode and the reactive current regulator in the current source mode, respectively, the d-axis PI regulation value E _Cd in the current source mode and the q-axis PI regulation value E _Cq in the current source mode are calculated. They are:

Among them, K _CiP is the proportional coefficient of the current regulator in the current source mode, and K _CiI is the integral coefficient of the current regulator in the current source mode. In the current source mode, the proportional coefficient and the integral coefficient of the current regulator are designed according to the conventional grid-connected inverter. In this embodiment, K _CiP =4, K _CiI =20.

Step 8: According to the d-axis PI adjustment value E _Cd in the current source mode obtained in step 7, the q-axis PI adjustment value E _Cq in the current source mode and the current source mode filter capacitor voltage active component V _Cod obtained in step 5 through virtual synchronous rotation The inverse coordinate transformation obtains the total modulated wave voltage V _Cr of the inverter in the current source mode, and its calculation formula is:

V _Cr =(E _Cd +V _Cod )cosθ _g +E _Cq sinθ _g

Step 9, according to the active power P _Ci of the N H-bridge units in the current source mode obtained in step 3 and the sum of the active powers P _CT of the N H-bridge units in the current source mode obtained in step 4. The power distribution coefficient Factor _Ci of the H-bridge unit, i=1,2,3...N, its calculation formula is:

Step 10: According to the actual value of the DC side voltage V _PVi of the N H-bridge units obtained in step 1, the total modulated wave voltage V _Cr of the inverter in the current source mode obtained in step 8 and the N voltages in the current source mode obtained in step 9 The power distribution coefficient Factor _Ci of the H-bridge unit calculates the modulation signal m _Cri of each H-bridge unit in the current source mode, i=1, 2, 3...N, and its calculation formula is:

锁存上一周期电流源模式下N个H桥单元功率分配系数记为 Factor_Cmi，计算得到每个电压源模式下直流电压调节器前馈控制量I_VFeedi， i＝1,2,3...N，其计算式为：Step 11, when switching, latch the reference value of the active component of the filter inductor current in the current source mode of the previous cycle, and record it as

The power distribution coefficient of N H-bridge units in the current source mode of the previous cycle is denoted as Factor _Cmi , and the feedforward control quantity I _VFeedi of the DC voltage regulator in each voltage source mode is calculated, i=1,2,3.. .N, which is calculated as:

Step 12: Convert the actual value V _o of the filter capacitor voltage sampled in step 1 into the voltage source mode filter capacitor voltage active component V _Vod and the voltage source mode filter capacitor voltage reactive component under the rotating coordinate system through virtual synchronous rotating coordinate transformation. V _Voq , which is calculated as:

Among them, θ′ _Vo is the output phase angle of the inverter in the voltage source mode of the previous cycle, and k ₂ is the gain coefficient. In this embodiment, k ₂ =0.5.

In step 13, the actual value of the grid current I _S sampled in step 1 is transformed into the voltage source mode grid current active component I _VSd and the voltage source mode grid current reactive component I _VSq under the rotating coordinate system through virtual synchronous rotation coordinate transformation, Its calculation formula is:

Wherein k ₃ is a gain coefficient, and in this embodiment, k ₃ =0.5.

Step 14 , according to the voltage source mode filter capacitor voltage active component V _Vod obtained in step 12 , the voltage source mode filter capacitor voltage reactive component V _Voq and the voltage source mode grid current active component I _VSd obtained in step 13 , Voltage source mode grid current The reactive component I _VSq is calculated and filtered by a first-order low-pass filter to obtain the average active power P _Vo and average reactive power Q _Vo output by the inverter in the voltage source mode. The calculation formula is:

Among them, τ is the time constant of the first-order low-pass filter, and in this embodiment, τ=1e-4s.

Step 15: According to the actual DC side voltage value V _PVi of the N H-bridge units obtained in step 1, the DC side voltage command value V _PVi ^* of the N H-bridge units obtained in step 2, and the N voltage source modes obtained in step 11 The feedforward control variable I _VFeedi of the lower DC voltage regulator is used to calculate the active power P _Vi of each H-bridge unit in the voltage source mode through the DC voltage regulator in the voltage source mode, where i=1, 2, 3... N, its calculation formula is:

Among them, K _VVP is the proportional coefficient of the DC voltage regulator in the voltage source mode, K _VVI is the integral coefficient of the DC voltage regulator in the voltage source mode, i=1, 2, 3...N. In the voltage source mode, the proportional coefficient and the integral coefficient of the DC voltage regulator are designed according to the conventional grid-connected inverter. In this embodiment, K _VVP =0.05 and K _VVI =40.

In step 16, according to the active power P _Vi of the N H-bridge units in the voltage source mode obtained in step 15, the sum of the active powers of the N H-bridge units in the voltage source mode is calculated and recorded as P _VT , and the calculation formula is:

Step 17, according to the active power sum P _VT of the N H-bridge units in the voltage source mode obtained in step 16 and the inverter output average active power P _Vo in the voltage source mode obtained in step 14 via the active power-frequency droop control equation The output angular frequency ω _Vo of the inverter in the voltage source mode is calculated, and the output angular frequency ω _Vo in the voltage source mode is integrated to obtain the output phase angle θ _Vo of the inverter in the voltage source mode, and its active power-frequency droop control equation for:

ω _Vo =ω ^* +m(P _VT -P _Vo )

Where ω ^* is the grid synchronization angular frequency, m is the active power droop coefficient. In this embodiment, the grid synchronization angular frequency ω ^* =100πrad/s, and the active power droop coefficient m=6.28e-3rad/W.

和电压源模式下滤波电容电压无功分量参考值

其无功功率 -电压下垂控制方程为：Step 18: According to the average reactive power Q _Vo output by the inverter in the voltage source mode obtained in step 14, the reactive power-voltage droop control equation is used to obtain the reference value of the active component of the filter capacitor voltage in the voltage source mode.

and the reference value of the reactive power component of the filter capacitor voltage in the voltage source mode

Its reactive power-voltage droop control equation is:

Among them, E is the reference electromotive force, n is the reactive power droop coefficient, and Q ^* is the upper-layer given reactive power command. In this embodiment, the reference electromotive force E=100V, the reactive power droop coefficient n=5e-3V/Var, and the upper-layer given reactive power command Q ^* =0Var.

电压源模式滤波电容电压无功分量参考值

分别通过电压源模式下的有功交流电压调节器和无功交流电压调节器，计算得到电压源模式下滤波电感电流有功分量参考值

和电压源模式下滤波电感电流无功分量参考值

其计算式为：Step 19, according to the voltage source mode filter capacitor voltage active component V _Vod obtained in step 12, the voltage source mode filter capacitor voltage reactive component V _Voq and the voltage source mode filter capacitor voltage reference value obtained in step 18

Voltage source mode filter capacitor voltage reactive component reference value

Through the active AC voltage regulator and the reactive AC voltage regulator in the voltage source mode, respectively, the reference value of the active component of the filter inductor current in the voltage source mode is calculated.

and the reference value of the reactive component of the filter inductor current in the voltage source mode

Its calculation formula is:

Among them, K _VVP1 is the proportional coefficient of the AC voltage regulator in the voltage source mode, and K _VVI1 is the integral coefficient of the AC voltage regulator in the voltage source mode. In the voltage source mode, the proportional coefficient and the integral coefficient of the AC voltage regulator are designed according to the conventional grid-connected inverter. In this embodiment, K _VVP1 =0.05 and K _VVI1 =40.

Step 20: Convert the actual value of the filter inductor current _IL sampled in step 1 into the voltage source mode filter inductor current active component I _VLd and the voltage source mode filter inductor current reactive component under the rotating coordinate system through virtual synchronous rotating coordinate transformation. I _VLq , which is calculated as:

Wherein k ₄ is a gain coefficient, and in this embodiment, k ₄ =0.5.

电压源模式下滤波电感电流无功分量参考值

和步骤20得到的电压源模式滤波电感电流有功分量I_VLd、电压源模式滤波电感电流无功分量 I_VLq，分别通过电压源模式下有功电流调节器和无功电流调节器，计算得到电压源模式下d轴PI调节值E_Vd和电压源模式下q轴PI调节值E_Vq，其计算式分别为：Step 21, according to the reference value of the active component of the filter inductor current in the voltage source mode obtained in step 19

Reference value of filter inductor current reactive component in voltage source mode

and the voltage source mode filter inductor current active component I _VLd and the voltage source mode filter inductor current reactive component I _VLq obtained in step 20, respectively, through the active current regulator and the reactive current regulator in the voltage source mode to obtain the voltage source mode The PI adjustment value E _Vd of the lower d axis and the PI adjustment value E _Vq of the q axis in the voltage source mode are calculated as:

Among them, K _ViP is the proportional coefficient of the current regulator in the voltage source mode, and K _ViI is the integral coefficient of the current regulator in the voltage source mode. In the voltage source mode, the proportional coefficient and the integral coefficient of the current regulator are designed according to the conventional grid-connected inverter. In this embodiment, K _ViP =0.05, and K _ViI =0.

Step 22, according to the d-axis PI adjustment value E _Vd in the voltage source mode obtained in step 21, the q-axis PI adjustment value E _Vq in the voltage source mode and the voltage source mode filter capacitor voltage active component V _Vod obtained in step 12 through virtual synchronous rotation The inverse coordinate transformation obtains the total modulated wave voltage V _Vr of the inverter in the voltage source mode, and its calculation formula is:

V _Vr =(E _Vd +V _Vod )cosθ′ _Vo +E _Vq sinθ′ _Vo

Step 23, according to the active power P _Vi of the N H-bridge units in the voltage source mode obtained in step 15 and the active power sum P _VT of the N H-bridge units in the voltage source mode obtained in step 16 Calculate each of the voltage source modes. The power distribution coefficient Factor _Vi of the H-bridge unit, i=1,2,3...N, its calculation formula is:

Step 24, according to the actual value of the DC side voltage V _PVi of the N H-bridge units obtained in step 1, the total modulated wave voltage V _Vr of the inverter in the voltage source mode obtained in step 22 and the N voltage source mode obtained in step 23. The H-bridge unit power distribution coefficient Factor _Vi , calculates the modulation signal m _Vri of each H-bridge unit in the voltage source mode, i=1, 2, 3...N, and its calculation formula is:

The voltage source mode smooth switching current source mode control method includes:

锁存上一周期电压源模式下N个H桥单元功率分配系数记为 Factor_Vmi，计算得到每个电流源模式下直流电压调节器前馈控制量I_CFeedi， i＝1,2,3...N，其计算式为：When switching, first latch the reference value of the active component of the filter inductor current in the voltage source mode of the previous cycle, which is recorded as

The power distribution coefficient of N H-bridge units in the voltage source mode of the previous cycle is denoted as Factor _Vmi , and the feedforward control quantity I _CFeedi of the DC voltage regulator in each current source mode is calculated, i=1,2,3.. .N, which is calculated as:

Then, superimpose the DC voltage regulator feedforward control quantity I _CFeedi in the current source mode on the output of the DC voltage regulator in each current source mode, and calculate the active power P′ of each H-bridge unit in the current source mode during switching. _Ci , where i=1,2,3...N, its calculation formula is:

3 shows the grid current _IS and the DC side voltage waveforms of each H-bridge unit when the cascaded H-bridge photovoltaic grid-connected inverter switches from the current source mode to the voltage source mode when the control method of the present invention is adopted. The voltage source mode is switched from the current source mode at 1.5s, and the switchover is completed at 1.6s, the grid current has no impact, and the inverter switches smoothly. After the switching is completed, the DC side voltage of each H-bridge unit is controlled at its DC side voltage command value V _PVi ^* , i=1, 2, 3...N, that is, at the maximum power point of each H-bridge unit.

4 shows the grid current _IS and the DC side voltage waveform of each H-bridge unit when the cascaded H-bridge photovoltaic grid-connected inverter switches from the voltage source mode to the current source mode when the control method of the present invention is adopted. The current source mode is switched from the voltage source mode at 2s, and the switchover is completed at 2.04s, the grid current has no impact, and the inverter switches smoothly. After the switching is completed, the DC side voltage of each H-bridge unit is controlled at its DC side voltage command value V _PVi ^* , i=1, 2, 3...N, that is, at the maximum power point of each H-bridge unit.

Claims (1)

1. A dual-mode smooth switching control method for a cascaded H-bridge photovoltaic grid-connected inverter is characterized in that the cascaded H-bridge photovoltaic grid-connected inverter comprises N H-bridge units with photovoltaic components and a filter inductor L_SAnd a filter capacitor C_fThe control method comprises a current source mode smooth switching voltage source mode control method and a voltage source mode smooth switching current source mode control method:

the control method for the current source mode smooth switching voltage source mode comprises the following steps:

step 1, sampling the direct current side voltage of each H-bridge unit, filtering the direct current side voltage by a 100Hz wave trap in sequence to obtain the actual direct current side voltage values of N H-bridge units, and recording the actual direct current side voltage values as V_PViN, · 1,2,3,. N; sampling the actual DC side current values of N H-bridge units and recording as I_PViN, · 1,2,3,. N; sampling the actual value of the filter inductance current and recording the actual value as I_L(ii) a Sampling the actual value of the voltage of the filter capacitor and recording the actual value as V_o(ii) a Sampling the actual value of the current of the power grid and recording the actual value as I_S；

Step 2, the actual value V of the direct-current side voltage of each H-bridge unit is obtained_PViCarrying out maximum power point tracking control to obtain direct current side voltage instruction values of N H-bridge units and recording the direct current side voltage instruction values as V_PVi ^*Wherein i is 1,2,3.. N;

step 3, obtaining the actual value V of the direct-current side voltage of the N H-bridge units according to the step 1_PViAnd the direct current side voltage instruction values V of the N H-bridge units obtained in the step 2_PVi ^*Calculating the active power P of each H-bridge unit in the current source mode through the direct-current voltage regulator in the current source mode_CiWherein i ═ 1,2,3.. N, calculated as:

wherein, K_CVPIs the proportional coefficient, K, of a DC voltage regulator in a current source mode_CVIThe integral coefficient of the direct-current voltage regulator in the current source mode is 1,2,3.. N, and s is a Laplace operator;

step 4, obtaining the active power P of the N H-bridge units in the current source mode according to the step 3_CiCalculating the sum of the active power of N H-bridge units in the current source mode and recording as P_CTThe calculation formula is as follows:

step 5, the actual value V of the voltage of the filter capacitor sampled in the step 1 is processed_oPhase locking is carried out to obtain the voltage amplitude V of the power grid_mAnd phase theta_g(ii) a Converting the actual value V of the filter capacitor voltage sampled in the step 1 through virtual synchronous rotation coordinate_oConverting the voltage into the active component V of the current source mode filter capacitor under the rotating coordinate system_CodAnd the voltage reactive component V of the current source mode filter capacitor_Coq(ii) a The actual value I of the filter inductance current sampled in the step 1 is converted through a virtual synchronous rotation coordinate_LConversion into active component I of current source mode filter inductor current under rotating coordinate system_CLdAnd current source mode filtering inductive current reactive component I_CLq；

Step 6, obtaining the active power of the N H-bridge units in the current source mode according to the step 4And P_CTAnd step 5, obtaining the voltage amplitude V of the power grid_mCalculating to obtain a reference value of active components of the filter inductor current in the current source mode

The calculation formula is as follows:

and 7, filtering the active component I of the inductor current in the current source mode according to the current source mode obtained in the step 5_CLdCurrent source mode filtering inductive current reactive component I_CLqAnd 6, obtaining the reference value of the active component of the filter inductance current in the current source mode

Respectively calculating to obtain a d-axis PI regulating value E in the current source mode through an active current regulator in the current source mode and a reactive current regulator in the current source mode_CdQ-axis PI regulation value E in sum current source mode_CqThe calculation formula is respectively:

wherein, K_CiPFor the current regulator proportionality coefficient in current source mode, K_CiIIs the current regulator integral coefficient in the current source mode;

step 8, obtaining a d-axis PI regulating value E under the current source mode according to the step 7_CdQ-axis PI regulation value E in current source mode_CqAnd step 5, obtaining the active component V of the current source mode filter capacitor voltage_CodObtaining the total modulation wave voltage V of the inverter in the current source mode through the virtual synchronous rotation inverse coordinate transformation_Cr；

Step 9, obtaining the active power P of the N H-bridge units in the current source mode according to the step 3_CiAnd N H bridge units in the current source mode obtained in the step 4Sum of active power P of elements_CTCalculating the power distribution coefficient Factor of each H-bridge unit in the current source mode_Ci1,2,3.. N, calculated as:

step 10, obtaining the actual value V of the direct-current side voltage of the N H-bridge units according to the step 1_PViAnd 8, obtaining the total modulation wave voltage V of the inverter under the current source mode_CrAnd the power distribution coefficients Factor of the N H-bridge units in the current source mode obtained in the step 9_CiCalculating the modulation signal m of each H-bridge unit in the current source mode_Cri1,2,3.. N, calculated as:

step 11, during switching, latching the reference value of the active component of the filter inductor current in the last period current source mode as

Latching the power distribution coefficients of N H-bridge units in the last periodic current source mode and recording the power distribution coefficients as Factor_CmiAnd calculating the feedforward control quantity I of the direct-current voltage regulator in each voltage source mode_VFeedi1,2,3.. N, calculated as:

step 12, the actual value V of the voltage of the filter capacitor obtained by sampling in the step 1 is used_oConverting the virtual synchronous rotating coordinate into a voltage active component V of a voltage source mode filter capacitor under a rotating coordinate system_VodVoltage reactive component V of sum voltage source mode filter capacitor_Voq；

Step 13, sampling in step 1The obtained actual value I of the current of the power grid_SConverting the virtual synchronous rotating coordinate into a voltage source mode power grid current active component I under a rotating coordinate system_VSdAnd reactive component I of voltage source mode grid current_VSq；

Step 14, obtaining the active component V of the voltage source mode filter capacitor according to the step 12_VodVoltage reactive component V of voltage source mode filter capacitor_VoqAnd step 13, obtaining the active component I of the voltage source mode power grid current_VSdVoltage source mode network current reactive component I_VSqThe average active power P output by the inverter in the voltage source mode is obtained by calculation and filtering through a first-order low-pass filter_VoAnd average reactive power Q_VoThe calculation formula is as follows:

wherein τ is a first order low pass filter time constant;

step 15, obtaining the actual value V of the direct-current side voltage of the N H-bridge units according to the step 1_PViAnd D, obtaining the direct-current side voltage instruction values V of the N H-bridge units obtained in the step 2_PVi ^*And step 11, the feedforward control quantity I of the direct-current voltage regulator under the N voltage source modes is obtained_VFeediCalculating the active power P of each H-bridge unit in the voltage source mode through the direct-current voltage regulator in the voltage source mode_ViWherein i ═ 1,2,3.. N, calculated as:

wherein, K_VVPIs the proportional coefficient of a DC voltage regulator in voltage source mode, K_VVIThe integral coefficient of the direct-current voltage regulator in a voltage source mode is 1,2,3.. N;

step 16, obtaining the active power P of the N H-bridge units in the voltage source mode according to the step 15_ViCalculating to obtain a voltage sourceThe sum of the active powers of N H-bridge units in the mode is recorded as P_VTThe calculation formula is as follows:

step 17, according to the sum P of the active powers of the N H-bridge units in the voltage source mode obtained in step 16_VTAnd step 14, obtaining the average active power P of the inverter output under the voltage source mode_VoCalculating to obtain the output angular frequency omega of the inverter in the voltage source mode through an active power-frequency droop control equation_VoAngular frequency ω output in voltage source mode_VoObtaining the output phase angle theta of the inverter in the voltage source mode through integration_VoThe active power-frequency droop control equation is as follows:

ω_Vo＝ω^*+m(P_VT-P_Vo)

wherein ω is^*The power grid synchronous angular frequency is adopted, and m is an active droop coefficient;

step 18, outputting the average reactive power Q of the inverter under the voltage source mode according to the obtained result in the step 14_VoThe reference value of the active component of the voltage of the filter capacitor under the voltage source mode is obtained through calculation of a reactive power-voltage droop control equation

Voltage reactive component reference value of filter capacitor in voltage source mode

The reactive power-voltage droop control equation is as follows:

wherein E is reference electromotive force, n is reactive droop coefficient, and Q^*Giving a reactive power instruction for an upper layer;

step 19, obtaining according to step 12Voltage source mode filter capacitor voltage active component V_VodVoltage reactive component V of voltage source mode filter capacitor_VoqAnd 18, obtaining the reference value of the active component of the voltage source mode filter capacitor

Reference value of voltage reactive component of voltage source mode filter capacitor

Calculating to obtain the reference value of the active component of the filter inductance current in the voltage source mode through the active alternating current voltage regulator and the reactive alternating current voltage regulator in the voltage source mode respectively

Reference value of reactive component of filter inductance current in voltage source mode

The calculation formula is as follows:

wherein, K_VVP1Is the proportional coefficient of the AC voltage regulator in voltage source mode, K_VVI1Is the integral coefficient of the AC voltage regulator in the voltage source mode;

step 20, filtering the actual value I of the inductor current sampled in the step 1_LConverting the virtual synchronous rotating coordinate into active component I of voltage source mode filter inductor current under a rotating coordinate system_VLdAnd voltage source mode filtering inductive current reactive component I_VLq；

Step 21, obtaining the reference value of the active component of the filter inductor current in the voltage source mode according to the step 19

Voltage sourceReference value of reactive component of filter inductance current under mode

And the active component I of the voltage source mode filter inductor current obtained in the step 20_VLdVoltage source mode filtering inductive current reactive component I_VLqRespectively calculating to obtain a d-axis PI regulation value E in the voltage source mode through an active current regulator and a reactive current regulator in the voltage source mode_VdQ-axis PI regulation value E in voltage source mode_VqThe calculation formula is respectively:

wherein, K_ViPIs the current regulator proportionality coefficient in voltage source mode, K_ViIIs the integral coefficient of the current regulator in the voltage source mode;

step 22, according to the d-axis PI regulating value E in the voltage source mode obtained in the step 21_VdQ-axis PI regulation value E in voltage source mode_VqAnd 12, obtaining the active component V of the voltage source mode filter capacitor_VodObtaining the total modulation wave voltage V of the inverter in the voltage source mode through the virtual synchronous rotation inverse coordinate transformation_Vr；

Step 23, obtaining the active power P of the N H-bridge units in the voltage source mode according to the step 15_ViAnd the sum P of the active powers of the N H-bridge units in the voltage source mode obtained in the step 16_VTCalculating the power distribution coefficient Factor of each H-bridge unit in the voltage source mode_Vi1,2,3.. N, calculated as:

step 24, obtaining the actual value V of the direct-current side voltage of the N H-bridge units according to the step 1_PViThe total modulation wave voltage V of the inverter in the voltage source mode obtained in step 22_VrAnd the electricity obtained in step 23N H-bridge unit power distribution coefficient Factor in voltage source mode_ViCalculating the modulation signal m of each H-bridge unit in the voltage source mode_Vri1,2,3.. N, calculated as:

the control method for smoothly switching the current source mode in the voltage source mode comprises the following steps:

when switching, firstly, latching the reference value of the active component of the filter inductance current in the last period voltage source mode as

Latching the power distribution coefficients of N H-bridge units in the last periodic voltage source mode and recording the power distribution coefficients as Factor_VmiAnd calculating the feedforward control quantity I of the direct-current voltage regulator under each current source mode_CFeedi1,2,3.. N, calculated as:

then, the feedforward control quantity I of the direct current voltage regulator in the current source mode is obtained_CFeediThe output of the direct-current voltage regulator in each current source mode is superposed, and the active power P 'of each H-bridge unit in the current source mode during switching is calculated'_CiWherein i ═ 1,2,3.. N, calculated as:

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