CN110311407A - Dual-mode seamless switching control method for cascaded inverters based on voltage closed loop - Google Patents

Abstract

The invention discloses a double-mode seamless switching control method of a cascaded inverter based on a voltage closed loop, 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 closed-loop tracking of the modulated wave voltage is completed through amplitude synchronization and phase synchronization. On this basis, the current feedforward control is used to realize the seamless switching of the inverter from the current source mode to the voltage source mode. When the voltage source mode switches to the current source mode, the modulated wave voltage synchronous regulator is used to complete the closed-loop tracking of the modulated wave voltage. On this basis, the current feedforward control is used to realize the seamless switching of the inverter from the voltage source mode to the current source mode. The method can realize seamless switching between two control modes, and can also realize the maximum tracking of photovoltaic power generation.

Description

Dual-mode seamless switching control method for cascaded inverters based on voltage closed loop

technical field

The invention relates to a double-mode seamless switching control method for cascaded inverters based on a voltage closed loop, 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 dual-mode seamless switching control method of cascaded 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 seamless 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 seamless switching. In addition, this method is aimed at centralized inverters, and seamless 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. 32, No. 12 in June 2017 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, 2018, pp. 2144-2162) A switching method based on impedance hysteresis is proposed to improve the 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 seamless 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 seamless 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 current source and voltage source dual-mode switching of the cascading photovoltaic grid-connected inverter, a dual-mode cascading inverter based on voltage closed loop is proposed. Seamlessly switch control methods. The method adopts a voltage-based closed-loop control method, which can not only realize the seamless switching between the current source and voltage source of the cascaded 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 double-mode seamless switching control method of a cascaded inverter based on a voltage closed loop. The cascaded inverter consists of N H bridges with photovoltaic components. It is composed of a unit, a filter inductor L _S and a filter capacitor C _f , and is characterized in that, the control method includes a current source mode seamless switching voltage source mode control method and a voltage source mode seamless switching current source mode control method:

The current source mode seamless 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;

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:

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, the current source mode The reactive component V _Coq of the filter capacitor voltage obtains the total modulated wave voltage V _Cr of the inverter in the current source mode through virtual synchronous rotation inverse coordinate transformation;

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 modulated wave voltage V _Cri of each H-bridge unit in the current source mode, i=1, 2, 3...N, and its calculation formula is:

Step 11: Before 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: Before switching, calculate 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. The amplitude V _Crm and the phase θ _Cr of the total modulating wave voltage of the inverter in the current source mode are obtained, and the calculation formula is:

Step 13: 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 ;

In step 14, the actual value of the grid current I _S sampled in step 1 is converted 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 15 , according to the voltage source mode filter capacitor voltage active component V _Vod obtained in step 13 , 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 14 , 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 16, 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 17, according to the active power P _Vi of the N H-bridge units in the voltage source mode obtained in step 16, 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 18: According to the active power sum P _VT of the N H-bridge units in the voltage source mode obtained in step 17 and the average active power P _Vo of the inverter output in the voltage source mode obtained in step 15, 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;

Step 19: According to the average reactive power Q _Vo of the inverter output in the voltage source mode obtained in step 15, 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;

Step 20: According to the amplitude V _Crm and the phase θ _Cr of the total modulated wave voltage of the inverter in the current source mode obtained in step 12, the output phase angle θ Vo of the inverter in the voltage source mode obtained in step 18 and the output phase angle θ _Vo of the inverter in step 19 are obtained. The reference value of the active component of the filter capacitor voltage in the voltage source mode Through the phase synchronous regulator and the amplitude synchronous regulator in the voltage source mode, respectively, the amplitude V _Vrm and the phase θ _Vr of the total modulated wave voltage of the inverter in the voltage source mode during switching are calculated. The calculation formula is:

Among them, K _VSP1 is the proportional coefficient of the amplitude synchronous regulator in the voltage source mode, K _VSI1 is the integral coefficient of the amplitude synchronous regulator in the voltage source mode, K _VSP2 is the proportional coefficient of the phase synchronous regulator in the voltage source mode, and K _VSI2 is the voltage Integral coefficient of phase synchronization regulator in source mode;

Step 21, according to the active power P _Vi of the N H-bridge units in the voltage source mode obtained in step 16 and the active power sum P _VT of the N H-bridge units in the voltage source mode obtained in step 17 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 22, according to the actual value V _PVi of the DC side voltage of the N H-bridge units obtained in step 1, the amplitude V _Vrm and the phase θ _Vr of the total modulated wave voltage of the inverter in the voltage source mode during switching obtained in step 20, and the step 21 Obtain the power distribution coefficient Factor _Vi of N H-bridge units in the voltage source mode, and calculate the modulated wave voltage V _Vri of each H-bridge unit in the voltage source mode during switching, i=1, 2, 3...N, Its calculation formula is:

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

First, before switching, start the modulated wave voltage synchronous regulator, and use the reference value of the active component of the filter capacitor voltage in the voltage source mode obtained in step 19 The d-axis PI adjustment value in the current source mode of the previous cycle obtained in step 7 and the active component V _Cod of the filter capacitor voltage in the current source mode obtained in step 5 through the modulating wave voltage synchronous regulator to obtain the reference value of the active component of the filter inductor current in the current source mode before switching Its calculation formula is:

Among them, K _CSP is the proportional coefficient of the modulated wave voltage synchronous regulator in the current source mode, and K _CSI is the integral coefficient of the modulated wave voltage synchronous regulator in the current source mode;

Secondly, after the modulation wave voltage synchronization is completed, the reference value of the active component of the filter inductor current in the voltage source mode of the previous cycle is latched and 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:

Finally, superimpose the DC voltage regulator feedforward control variable 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 present invention discloses a dual-mode seamless switching control method for cascaded inverters based on voltage closed loop, which adopts a voltage closed-loop control strategy to realize dual-mode cascaded H-bridge photovoltaic grid-connected inverters. The seamless switching of control, its beneficial effects are embodied in:

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

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 inverter of the present invention.

FIG. 2 is a general control block diagram of the dual-mode switching of the cascaded 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 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 waveforms of each H-bridge unit when the cascaded 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.

FIG. 1 is a topology structure of a cascaded inverter according to an embodiment of the present invention. As shown in the figure, the cascaded inverter consists of N _H -bridge units with photovoltaic components, filter inductors LS and filter capacitors. C _f composition. 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 seamless 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 3rd, 4th, and 5th H-bridges remains unchanged, and the illumination intensity of the 1st and 2nd H-bridges becomes E ₁ =E ₂ =800W/m ² , respectively. The DC side voltage command value V _PV1 ^* =V _PV2 ^* =30.57V of each H-bridge unit is obtained, V _PV3 ^* =V _PV4 ^* =V _PV5 ^* =30.40V.

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.

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:

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 modulated wave voltage V _Cri of each H-bridge unit in the current source mode, i=1, 2, 3...N, and its calculation formula is:

Step 11: Before 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: Before switching, calculate 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. The amplitude V _Crm and the phase θ _Cr of the total modulating wave voltage of the inverter in the current source mode are obtained, and the calculation formula is:

Step 13: 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 14, 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 15 , according to the voltage source mode filter capacitor voltage active component V _Vod obtained in step 13 , 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 14 , 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 16, 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 17, according to the active power P _Vi of the N H-bridge units in the voltage source mode obtained in step 16, 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 18: According to the active power sum P _VT of the N H-bridge units in the voltage source mode obtained in step 17 and the average active power P _Vo of the inverter output in the voltage source mode obtained in step 15, 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.

Step 19: According to the average reactive power Q _Vo of the inverter output in the voltage source mode obtained in step 15, the reactive power-voltage droop control equation is used to obtain the reference value V _V ^* _od and the voltage of the active component of the filter capacitor voltage in the voltage source mode. In the source mode, the reference value V _V ^* _oq of the reactive component of the filter capacitor voltage, 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=120V, the reactive power droop coefficient n=5e-3V/Var, and the upper-layer given reactive power command Q ^* =0Var.

Step 20: According to the amplitude V _Crm and the phase θ _Cr of the total modulated wave voltage of the inverter in the current source mode obtained in step 12, the output phase angle θ Vo of the inverter in the voltage source mode obtained in step 18 and the output phase angle θ _Vo of the inverter in step 19 are obtained. The reference value of the active component of the filter capacitor voltage in the voltage source mode Through the phase synchronous regulator and the amplitude synchronous regulator in the voltage source mode, respectively, the amplitude V _Vrm and the phase θ _Vr of the total modulated wave voltage of the inverter in the voltage source mode during switching are calculated. The calculation formula is:

Among them, K _VSP1 is the proportional coefficient of the amplitude synchronous regulator in the voltage source mode, K _VSI1 is the integral coefficient of the amplitude synchronous regulator in the voltage source mode, K _VSP2 is the proportional coefficient of the phase synchronous regulator in the voltage source mode, and K _VSI2 is the voltage Integral factor of the phase sync regulator in source mode. In this embodiment, K _VSP1 =0.5, K _VSI1 =2, K _VSP2 =40, and K _VSI2 =200.

Step 21, according to the active power P _Vi of the N H-bridge units in the voltage source mode obtained in step 16 and the active power sum P _VT of the N H-bridge units in the voltage source mode obtained in step 17 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 22, according to the actual value V _PVi of the DC side voltage of the N H-bridge units obtained in step 1, the amplitude V _Vrm and the phase θ _Vr of the total modulated wave voltage of the inverter in the voltage source mode during switching obtained in step 20, and the step 21 Obtain the power distribution coefficient Factor _Vi of N H-bridge units in the voltage source mode, and calculate the modulated wave voltage V _Vri of each H-bridge unit in the voltage source mode during switching, i=1, 2, 3...N, Its calculation formula is:

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

First, before switching, start the modulated wave voltage synchronous regulator, and use the reference value of the active component of the filter capacitor voltage in the voltage source mode obtained in step 19 The d-axis PI adjustment value in the current source mode of the previous cycle obtained in step 7 and the active component V _Cod of the filter capacitor voltage in the current source mode obtained in step 5 through the modulating wave voltage synchronous regulator to obtain the reference value of the active component of the filter inductor current in the current source mode before switching Its calculation formula is:

Among them, K _CSP is the proportional coefficient of the modulated wave voltage synchronous regulator in the current source mode, and K _CSI is the integral coefficient of the modulated wave voltage synchronous regulator in the current source mode. In this embodiment, K _CSP =0.2, and K _CSI =20.

Secondly, after the modulation wave voltage synchronization is completed, the reference value of the active component of the filter inductor current in the voltage source mode of the previous cycle is latched and 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:

Finally, superimpose the DC voltage regulator feedforward control variable 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 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 switching is completed at 1.54s, the grid current has no impact, and the inverter switches seamlessly. 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 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 switch is completed at 2.04s, the grid current has no impact, and the inverter switches seamlessly. 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 kind of cascaded inverter double mode seamless switching control method based on voltage close loop, the cascaded inverter By N number of H-bridge unit with photovoltaic module, filter inductance L_SWith filter capacitor C_fComposition, which is characterized in that this control method packet Include current source mode seamless switching voltage source mode control method and voltage source mode seamless switching current source mode control method:

The current source mode seamless switching voltage source mode control method the following steps are included:

Step 1, the DC voltage of each H-bridge unit is sampled and is successively filtered by 100Hz trapper, obtain N number of H bridge list The DC voltage actual value of member is simultaneously denoted as V_PVi, i=1,2,3...N；Sample the DC side current actual value of N number of H-bridge unit simultaneously It is denoted as I_PVi, i=1,2,3...N；Sampling filter inductive current actual value is simultaneously denoted as I_L；Sampling filter capacitance voltage actual value is simultaneously It is denoted as V_o；Sampling power network current actual value is simultaneously denoted as I_S；

Step 2, by each H-bridge unit DC voltage actual value V_PViMPPT maximum power point tracking control is carried out, N number of H is obtained The DC voltage instruction value of bridge unit is simultaneously denoted as V_PVi ^*, wherein i=1,2,3...N；

Step 3, the DC voltage actual value V of the N number of H-bridge unit obtained according to step 1_PViThe N number of H bridge list obtained with step 2 The DC voltage instruction value V of member_PVi ^*, by direct current voltage regulator under current source mode, it is calculated under current source mode The active-power P of each H-bridge unit_Ci, wherein i=1,2,3...N, calculating formula are as follows:

Wherein, K_CVPFor direct current voltage regulator proportionality coefficient, K under current source mode_CVIFor DC voltage tune under current source mode Device integral coefficient is saved, i=1,2,3...N, s are Laplace operator；

Step 4, the active-power P of N number of H-bridge unit under the current source mode obtained according to step 3_CiCurrent source mode is calculated Under N number of H-bridge unit the sum of active power and be denoted as P_CT, calculating formula are as follows:

Step 5, to the filter capacitor voltage actual value V sampled in step 1_oIt carries out locking phase and obtains grid voltage amplitude V_mAnd phase θ_g；The filter capacitor voltage actual value V that will be sampled in step 1 by virtual synchronous rotating coordinate transformation_oRotation is converted into sit Current source mode filter capacitor voltage active component V under mark system_CodWith current source mode filter capacitor voltage power-less component V_Coq； The filter inductance current actual value I that will be sampled in step 1 by virtual synchronous rotating coordinate transformation_LIt is converted into rotational coordinates Current source mode filter inductance active component of current I under system_CLdWith current source mode filter inductance reactive component of current I_CLq；

Step 6, the sum of active power of N number of H-bridge unit P under the current source mode obtained according to step 4_CTIt is obtained with step 5 Grid voltage amplitude V_mFilter inductance active component of current reference value under current source mode is calculatedIts calculating formula are as follows:

Step 7, the current source mode filter inductance active component of current I obtained according to step 5_CLd, current source mode filter inductance Reactive component of current I_CLqFilter inductance active component of current reference value under the current source mode obtained with step 6Lead to respectively Reactive current adjuster under watt current adjuster and current source mode, is calculated under current source mode under overcurrent source module D axis PI regulated value E_CdWith q axis PI regulated value E under current source mode_Cq, calculating formula is respectively as follows:

Wherein, K_CiPFor current regulator proportionality coefficient, K under current source mode_CiIIt is integrated for current regulator under current source mode Coefficient；

Step 8, d axis PI regulated value E under the current source mode obtained according to step 7_Cd, q axis PI regulated value E under current source mode_Cq The current source mode filter capacitor voltage active component V obtained with step 5_Cod, current source mode filter capacitor voltage power-less component V_CoqAnti- coordinate transform is rotated by virtual synchronous obtain inverter under current source mode always modulate wave voltage V_Cr；

Step 9, the active-power P of N number of H-bridge unit under the current source mode obtained according to step 3_CiThe electric current obtained with step 4 The sum of active power of N number of H-bridge unit P under source module_CTThe power partition coefficient of each H-bridge unit under calculating current source module Factor_Ci, i=1,2,3...N, calculating formula are as follows:

Step 10, the DC voltage actual value V of the N number of H-bridge unit obtained according to step 1_PVi, current source mould that step 8 obtains Inverter always modulates wave voltage V under formula_CrN number of H-bridge unit power partition coefficient under the current source mode obtained with step 9 Factor_Ci, the modulation wave voltage V of each H-bridge unit under calculating current source module_Cri, i=1,2,3...N, calculating formula are as follows:

Step 11, it before switching, latches filter inductance active component of current reference value under upper periodic current source module and is denoted as It latches N number of H-bridge unit power partition coefficient under upper periodic current source module and is denoted as Factor_Cmi, each voltage source is calculated Direct current voltage regulator feedforward control amount I under mode_VFeedi, i=1,2,3...N, calculating formula are as follows:

Step 12, before switching, d axis PI regulated value E under the current source mode obtained according to step 7_Cd, q axis PI under current source mode Regulated value E_CqThe current source mode filter capacitor voltage active component V obtained with step 5_CodCurrent source mode subinverse is calculated Become the amplitude V that device always modulates wave voltage_CrmAnd phase theta_Cr, calculating formula are as follows:

Step 13, the filter capacitor voltage actual value V that will be sampled in step 1_oIt is converted by virtual synchronous rotating coordinate transformation At the voltage source mode filter capacitor voltage active component V under rotating coordinate system_VodWith voltage source mode filter capacitor voltage power-less Component V_Voq；

Step 14, the power network current actual value I that will be sampled in step 1_SIt is converted into revolving by virtual synchronous rotating coordinate transformation Turn the voltage source mode power network current active component I under coordinate system_VSdWith voltage source mode power network current reactive component I_VSq；

Step 15, the voltage source mode filter capacitor voltage active component V obtained according to step 13_Vod, voltage source mode filtered electrical Hold voltage power-less component V_VoqThe voltage source mode power network current active component I obtained with step 14_VSd, voltage source mode power grid electricity Flow reactive component I_VSq, by calculating and filtering through low-pass first order filter, obtaining inverter output under voltage source mode averagely has Function power P_VoWith average reactive power Q_Vo, calculating formula are as follows:

Wherein, τ is low-pass first order filter time constant；

Step 16, the DC voltage actual value V of the N number of H-bridge unit obtained according to step 1_PVi, N number of H bridge list that step 2 obtains The DC voltage instruction value V of member_PVi ^*Direct current voltage regulator feedforward control amount under the N number of voltage source mode obtained with step 11 I_VFeedi, by direct current voltage regulator under voltage source mode, the wattful power of each H-bridge unit under voltage source mode is calculated Rate P_Vi, wherein i=1,2,3...N, calculating formula are as follows:

Wherein, K_VVPFor direct current voltage regulator proportionality coefficient, K under voltage source mode_VVIFor DC voltage tune under voltage source mode Section device integral coefficient, i=1,2,3...N；

Step 17, the active-power P of N number of H-bridge unit under the voltage source mode obtained according to step 16_ViVoltage source mould is calculated The sum of active power of N number of H-bridge unit and P is denoted as under formula_VT, calculating formula are as follows:

Step 18, the sum of active power of N number of H-bridge unit P under the voltage source mode obtained according to step 17_VTIt is obtained with step 15 Voltage source mode under inverter export average active power P_VoVoltage is calculated through active power-frequency droop governing equation The output angular frequency of inverter under source module_Vo, angular frequency is exported under voltage source mode_VoVoltage source mould is obtained by integral The output phase angle theta of inverter under formula_Vo, active power-frequency droop governing equation are as follows:

ω_Vo=ω^*+m(P_VT-P_Vo)

Wherein ω^*For synchronized angular frequency, m is active sagging coefficient；

Step 19, inverter exports average reactive power Q under the voltage source mode obtained according to step 15_VoThrough reactive power-electricity It depresses the governing equation that hangs down and filter capacitor voltage active component reference value under voltage source mode is calculatedAnd voltage source mode Lower filter capacitor voltage power-less component reference valueIts sagging governing equation of reactive power-voltage are as follows:

Wherein E is with reference to electromotive force, and n is idle sagging coefficient, Q^*Reactive power instruction is given for upper layer；

Step 20, inverter always modulates the amplitude V of wave voltage under the current source mode obtained according to step 12_CrmAnd phase theta_Cr, step The output phase angle theta of inverter under rapid 18 obtained voltage source modes_VoAnd filter capacitor under the obtained voltage source mode of step 19 Voltage active component reference valueRespectively by Phase synchronization adjuster and amplitude synchronous governor under voltage source mode, calculate Inverter always modulates the amplitude V of wave voltage under voltage source mode when obtaining switching_VrmAnd phase theta_Vr, calculating formula are as follows:

Wherein, K_VSP1For amplitude synchronous governor proportionality coefficient, K under voltage source mode_VSI1For the same step of amplitude under voltage source mode Save device integral coefficient, K_VSP2For Phase synchronization adjuster proportionality coefficient, K under voltage source mode_VSI2It is same for phase under voltage source mode Walk adjuster integral coefficient；

Step 21, the active-power P of N number of H-bridge unit under the voltage source mode obtained according to step 16_ViThe electricity obtained with step 17 The sum of active power of N number of H-bridge unit P under source mode_VTCalculate the power partition coefficient of each H-bridge unit under voltage source mode Factor_Vi, i=1,2,3...N, calculating formula are as follows:

Step 22, the DC voltage actual value V of the N number of H-bridge unit obtained according to step 1_PVi, when the switching that step 20 obtains Inverter always modulates the amplitude V of wave voltage under voltage source mode_VrmAnd phase theta_VrAnd N under the obtained voltage source mode of step 21 A H-bridge unit power partition coefficient Factor_Vi, the modulating wave of each H-bridge unit is electric under voltage source mode when switching is calculated Press V_Vri, i=1,2,3...N, calculating formula are as follows:

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

Firstly, start modulating wave voltage synchronous adjuster before switching, filter capacitor electricity under the voltage source mode that step 19 is obtained Press active component reference valueD axis PI regulated value under the upper periodic current source module that step 7 obtainsIt is obtained with step 5 The current source mode filter capacitor voltage active component V arrived_CodIt obtains switching preceding current source by modulating wave voltage synchronous adjuster Filter inductance active component of current reference value under modeIts calculating formula are as follows:

Wherein, K_CSPFor current source mode modulated wave voltage synchronous governor proportionality coefficient, K_CSIFor current source mode modulated Wave voltage synchronous governor integral coefficient；

Secondly, latching the filter inductance active component of current under upper periodic voltage source module after the completion of modulating wave voltage synchronous and joining Value is examined to be denoted asIt latches N number of H-bridge unit power partition coefficient under upper periodic voltage source module and is denoted as Factor_Vmi, calculate Obtain direct current voltage regulator feedforward control amount I under each current source mode_CFeedi, i=1,2,3...N, calculating formula are as follows:

Finally, by direct current voltage regulator feedforward control amount I under current source mode_CFeediIt is superimposed upon under each current source mode straight The output of current-voltage regulator, when switching is calculated under current source mode each H-bridge unit active-power P_C′_i, wherein i= 1,2,3...N, calculating formula are as follows: