Ankerui staggered parallel flyback quasi-single-stage photovoltaic grid-connected micro-inverter

The micro-inverters of independent photovoltaic modules can effectively overcome the shadow problems existing in traditional photovoltaic systems. The design, analysis and control strategy of a quasi-single-stage staggered parallel micro-inverter are introduced in detail.
Ji Xiaochun 1, Wang Jianhua 2, Health Care 3, Cai Shouping 1

(1. Ankerui Electric Co., Ltd., Shanghai 201801;

2. School of Electrical Engineering, Southeast University, Nanjing 210096, China;

3. School of Automation and Electrical Engineering, Nanjing University of Technology, Nanjing 210000, China;

Interleaved Flyback Photovoltaic Grid-connected Micro Inverter

JI Xiaochun 1, WEI Shaochong 2, WANG Jianhua3, JI Baojian2,

CAI Shouping1

(1. Acrel Co., Ltd, Shanghai 201801, China;

2. School of Automation & Electrical Engineering, Nanjing University of Technology,

Nanjing Jiangsu 210009, China;

3.School of Electrical Engineering, Southeast University, Nanjing Jiangsu 210096, China;)




Abstract:To overcome the traditional photovoltaic systems have low overall output power caused by the partial mask, proposed a single PV module for a single micro-inverter topology and its control strategies. Using single-stage interleaved flyback converter, In order to overcome problem of The active circuit circuit to absorb the leakage inductance energy, achieving a zero-voltage switching tube switch, increases machine efficiency. Gives a system based on digital signal processor control process, the system uses a variable step size perturbation and Observation method to achieve the maximum power point tracking, making each photovoltaic panels working on the maximum power point. Build a experiment prototype to verify the topology and control strategy is feasible solutions.

Keywords: Micro inverter; Flyback; DCM mode; MPPT

1 Introduction

The traditional centralized and string-type photovoltaic grid-connected power generation system supplies the grid-connected inverter to the grid through the parallel connection of the photovoltaic panels to effectively increase the bus voltage. The utility model has the advantages of simple structure and high conversion efficiency, and is especially suitable for a power station system with better sunshine. However, in urban and rural areas in the east, clouds and buildings, trees, and failure of single-cell panels will seriously reduce the power generation of the entire system. The micro-inverters equipped behind each PV module work at the maximum power point by independent control of each component, greatly improving the system's ability to resist local shadows and overall power generation. Despite its relatively high cost, the modular architecture, high reliability, high power generation, and easy installation make it an important direction for distributed photovoltaic power generation.

The design, analysis and control strategies of a quasi-single-stage staggered parallel micro-inverter are introduced in detail. The high-frequency link inverter technology not only realizes the large voltage-boost ratio matching of the micro-inverter input and output voltage, but also the primary and secondary electrical isolation can not solve the leakage current problem of the non-isolated system; and realizes the switch based on the active clamp technology to absorb the leakage inductance energy. Tube ZVS. The system control block diagram and flow show that MPPT can be realized by variable step disturbance observation method, and the input voltage feedforward method can solve the quasi-single-stage micro-inverse bus voltage collapse problem.

2 main circuit topology

2.1 Topology selection

The quasi-single-stage flyback inverter has only one stage of power conversion [4], and the topology is simple, especially suitable for low-cost applications. In the discontinuous mode (DCM) and critical continuous mode (BCM), it presents the current source characteristics, the control system design is simple, and the ideal topology of the current photovoltaic micro-inverter. Due to the limited output power of the flyback converter, in the micro-inverter system structure, the staggered parallel technology shown in Figure 1 is adopted here: the two flyback converters are input in parallel, the outputs are connected in parallel, and the primary side is interleaved by 180 degrees. In order to reduce the input and output current ripple, and a common set of output polarity flipping bridges; taking into account the leakage inductance of the flyback transformer, further adopt active clamp technology to recover the leakage inductance, and realize the ZVS of the main and auxiliary tubes. , effectively reducing switching losses and improving circuit efficiency.



Figure 1 Interleaved parallel flyback micro-inverter topology

At this time, the photovoltaic module is subjected to high frequency modulation by the SPWM of the flyback converter main switch, and the output current of the uniaxial power frequency sine half wave is obtained. The power frequency reversing bridge driving timing on the AC side tracks the grid voltage, and the front unipolar power frequency sine half wave is inverted into a sine wave grid-connected current, which is in phase with the grid voltage.

2.2 Working mode analysis

According to whether the magnetic flux of the transformer is continuous, the operation mode of the flyback converter can be divided into three types: inductor current continuous mode (CCM), DCM and BCM. The reverse stability of the flyback inverter in CCM mode is poor and needs to be handled properly. At present, the mainstream flyback inverters are mainly DCM and BCM. However, since the inverter control is required in the BCM mode, the calculation and control are complicated, so DCM is used here. Compared with BCM and CCM, DCM has the advantages of constant frequency operation, simple control and elimination of secondary diode reverse recovery. The disadvantage is that the magnetizing inductance is smaller and the peak current stress of the device is larger than that of CCM.

In order to ensure that the converter operates in DCM, its primary inductance Lp, ie the excitation inductance, is required to be less than the critical continuous inductance value. Define the power frequency period Tgrid is 2k times of the high frequency switching period, and define dp as the maximum duty ratio. Since the input current is proportional to the duty ratio, the duty cycle of each switching cycle is also the sinusoidal pulse dpsin (iπ/k). ), the average value of the primary current idc of the transformer is:



3 control system

3.1 control block diagram

The quasi-single-stage micro-inverter needs to complete MPPT, phase-locked, island detection and network current control [5][6]. As shown in FIG. 2, the reference amplitude Io of the resulting grid-connected current is provided by the MPPT calculation to ensure that the photovoltaic module transmits energy to the grid at maximum power. The phase lock provides phase information of the grid-connected current to ensure that the incoming current is in phase with the grid voltage. Islanding detection is a must-have function of grid-connected inverters. When the grid is abnormal, the inverter is turned off to ensure the safety of personnel and equipment. The incoming current control is the core control part of the grid-connected inverter. Here, the closed-loop control of the sampling output current ensures high-quality grid-connected current. (In theory, under DCM, open-loop control can realize current source grid connection, but its The total harmonic content of the grid-connected current is relatively high).



Figure 2 Control System

3.2 Quasi-single-stage system MPPT and DC bus voltage control

MPPT adjusts the output power of the inverter through the corresponding algorithm, adjusts the grid current reference, and adjusts the output power of the PV module, so that the PV module output power is the largest.

The perturbation observation method is simple and easy to implement, and is one of the most commonly used methods in the MPPT algorithm. The algorithm principle is that the current output power is compared with the previous output power. Assuming P(k+1)>P(k), the photovoltaic output voltage reference continues to be oscillated in the same direction of this change; If the output power becomes smaller, the direction of the disturbance is changed in the next cycle, so that the disturbance is repeatedly disturbed and compared until the output power of the photovoltaic system reaches the maximum. The algorithm flow is shown in Figure 3. The size of the disturbance observation method determines the speed of the algorithm tracking and the amplitude of the system oscillating back and forth around the highest point. Therefore, this paper adopts a variable step size disturbance observation method [7], in specific way when the power is less than hour, the disturbance The value of C is increased; when the power is large, the value of the disturbance value C is appropriately reduced.



Figure 3 disturbance observation algorithm flow

In the quasi-single-level grid-connected inverter system, the simple MPPT ring can not guarantee good dynamic performance and achieve system stability. When a sudden change in external conditions or a program misjudgment occurs, the DC bus voltage will violently oscillate or even collapse. As shown in Fig. 3, an input voltage loop is added on the basis of the original control to prevent the DC bus voltage from oscillating when the MPPT is misjudged, which can effectively prevent the bus voltage from collapsing and achieve stable operation of the system.

4 Experimental results

In order to verify the above-mentioned staggered parallel quasi-single-stage high-frequency photovoltaic grid-connected micro-inverter scheme, the prototype of 220W micro-inverter based on DSP28035 control was completed in the laboratory. The front-end DC input voltage Vpv=35VDC, grid-connected voltage Vo=220VAC, grid frequency fac=50Hz, the main V1 switching frequency fs=135Khz, the filter inductor L1=1mH, the photovoltaic module and the AC grid are simulated by the photovoltaic simulator and AC power. Figure 4a, b are the output waveforms of the grid-connected current io at light load and full load respectively. It can be seen that io and ug are in phase with the same frequency, and the io waveform quality is better; as can be seen from Figure 5c, V1 is open and closed, and the drain source is The voltage is zero, and the ZVS of V1 is realized. Figure 4e shows the waveforms of the transformer primary voltage up, secondary voltage us and current is, ug, which verifies the feasibility of the power frequency flip bridge.



(a) Light load output



(b) Full load output



(c) Main switch waveform



(d) Clamp tube waveform



(e) Transformer primary and secondary voltage waveforms

Figure 4 Experimental waveform

Figure 5 shows the MPPT effect of the photovoltaic simulator test with an MPPT efficiency of 99.5%.



Figure 5 IU and PU curves

The efficiency test curve of Fig. 6a further shows that the micro-inverter achieves high efficiency over the entire load range, and the maximum efficiency of full load reaches 94%. Figure 6b shows the test results of the power analyzer without considering the auxiliary power loss. The highest efficiency is 95%, and the grid-connected current THD is 1.5%, which verifies the feasibility of the micro-inverter solution.



Figure 6 Efficiency curve and THD test

5 Conclusion

The design, analysis and control strategies of a quasi-single-stage staggered parallel micro-inverter are introduced. The micro-inverter has the following characteristics: based on the high-frequency link inverter technology, the primary and secondary electrical isolation is effectively realized, and the leakage current problem of the non-isolated system is solved; the active clamping technology is used to absorb the leakage inductance energy, and the switching tube is realized. Zero-voltage switch to reduce switching loss; variable-step disturbance observation method to achieve maximum power point tracking, based on input voltage feedforward method to solve quasi-single-stage micro-inverted bus voltage collapse problem; 220W prototype machine maximum power tracking efficiency is 99.5 %, the maximum efficiency of full load reaches 94%. When the auxiliary power supply is not considered, the maximum efficiency is 95%, and the total harmonic distortion rate of the grid-connected current is less than 1.5%.

Article source: "Power Electronics Technology", 2014, issue 6

references
Renewable Energy, 2011, 36(12): 3282-3291. Technical support: Yang Junjun 18702112137 QQ http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com. Cn

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