#The4thInnovationContest#Non-isolated low leakage current grid-connected inverter based on new topology

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* Brief introduction of the work:
       This design is a non-isolated low leakage current grid-connected inverter based on a new topology. It uses five power switching devices, a capacitor, and a filter to achieve sinusoidal wave inversion. The controller uses TI's TMS320F2808 16-bit fixed-point DSP, with an operating frequency of up to 100MHz. The inverter is connected to the grid by locking the grid voltage phase through software. The main advantages of the new inverter topology and its control are: 1) The negative end of the photovoltaic cell is directly connected to the zero point of the grid, completely eliminating leakage current; 2) Few devices are used and the cost is low; 3) Low loss and high efficiency; 4) It has the ability to transmit reactive power; 5) Compared with the half-bridge topology, it does not require a very high DC input; 6) It uses simple unipolar SPWM modulation, which is simple to implement; 7) A good grid-connected effect can be achieved using simple dual closed-loop control.

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* 1. Details of the work;
       This work is a non-isolated low leakage current grid-connected inverter based on a new topology, which can convert a DC voltage of about 350V into an AC voltage of 220V and a frequency of 50Hz, and realize grid-connected power supply, island operation, reactive and active power transmission and other functions. The controller uses TI's TMS320F2808 to realize sampling, control and communication functions.
The work is divided into two parts, the main board and the control board, which are connected through pin headers. The main board is mainly responsible for power conversion, signal acquisition and processing, auxiliary power supply, and off-grid control. The control board is mainly responsible for switch tube driving and DSP control. The on-grid and off-grid switching uses an AC contactor, which is controlled by a control relay.
![image.png]                New topology structure diagram
The working principle and details of the new topology are shown in the appendix document, and the new topology has been patented.
![image.png] ![image.png] Mainboard PCB
![image.png] Front of control board
![image.png] Back of control board

* 2. Describe the challenges faced by the work and the problems solved;
        Challenges faced:
        First, in order to reduce leakage current, this design adopts a new topology, and the working principle of this topology is not very well understood. Secondly, the realization of grid-connected control. Although inverters have been made before, they are all off-grid, and grid-connected is much more complicated than off-grid. Thirdly, there is the problem of power decoupling. To achieve the transmission of active and reactive power, power decoupling is required, and coordinate transformation is required. Finally, there is hardware design. DSP has just started to learn, and software and hardware are not well understood.
       Problems solved:
After the explanation of the senior brother and the reference to relevant materials, we understand the working mode of this topology and realize the inverter function. Secondly, we understand the conditions required for grid connection and the problems that need to be solved. The software phase-locked loop is used to realize the grid phase-locked grid connection. At the same time, the feasibility of the scheme and the power decoupling control problem are verified through PSIM simulation. Finally, by consulting materials and learning DSP software and hardware knowledge, I designed a DSP minimum system board and drive circuit that can work stably.

* 3. Describe the key points involved in the hardware and software parts of the work;
        there are several key points in hardware design. First, the sampling of grid voltage and current. The grid voltage is first attenuated by a precision voltage transformer and sent to the signal processing circuit. The signal is level-shifted and filtered by the op amp to control the signal between 0V and 3V, and finally sent to the DSP for software processing. The grid-connected current passes through the current Hall sensor to convert the current signal into a voltage signal, which can be sent to the DSP for processing. The above scheme has high precision, good stability, and complete isolation of high voltage and low voltage. Second, the design of the drive and auxiliary power supply. Due to the use of a new topology, the traditional drive circuit cannot be directly applied, and an isolated drive circuit is required. Therefore, an optocoupler isolation driver is used, and the optocoupler isolation driver is driven by a transistor to control the on and off of the MOSFET. At the same time, multiple sets of isolated auxiliary power supplies are required. In order to reduce the difficulty of system design and improve the stability of the auxiliary power supply, UC3842 is used as the flyback power supply controller. After the flyback power supply outputs a 12V stable voltage, it is powered by four isolated DC/DC modules to power the drive circuit respectively, ensuring the stability of the power supply while realizing the isolation of the auxiliary power supply.
        The biggest difficulty in software design is how to achieve grid connection and power decoupling. After discussion, the following method is used to achieve grid voltage phase locking and grid connection. After sampling the grid voltage, the grid voltage is first converted into two sets of orthogonal voltage signals through generalized second-order integration, and then the output signal is used as the input of the phase-locked loop. After passing through the PLL, the phase of the grid voltage is obtained. The current given value is multiplied by the grid phase to obtain the instantaneous current value. After comparing the instantaneous current value with the current value of the feedback sampling, the control signal is output through the PI regulator. After modulating the control signal with the carrier, the required SPWM can be obtained. The size of the active power is controlled by controlling the current amplitude, and the size of the reactive power is controlled by controlling the current phase. The
programming software uses CCS6.0, and the program is downloaded to the on-chip FLASH after debugging.
![image.png]
* 4. List of materials for the work;
                            List of materials for the control board:
![image.png]                                                          List of materials for the main circuit board
![image.png] ![image.png]
* 5. Upload pictures of the work; (The PCB must have the competition logo and take a photo to upload. If not, it will be deemed as giving up the competition)
![image.png] Close-up of the reverse logo
![image.png] ![image.png] Overall picture
![image.png] Front and back of the control board
![image.png] Inverter output voltage waveform

* 6. Demonstrate your work and record it into a video and upload it; (The video content must include: introduction of the work; function demonstration; performance test; close-up of the competition logo on the PCB. If not, it will be deemed as giving up the competition)
Video link: [https://www.bilibili.com/video/av68592541](https://www.bilibili.com/video/av68592541)

7. Open source documents.
See the appendix

postscript:
       This work took nearly two months. Due to limited time and energy, I also had other work to do, so I soldered the circuit and debugged it in fragmented time. During the design process, I was constantly thinking about where problems might occur and where I could optimize the design. After going through various difficulties, I finally completed the work with the cooperation of the team members, including the process of staying up all night to debug. In the early stage of debugging, the problems of pipe explosion, burning DSP control chip, driver chip, etc. caused by various reasons were very troublesome. There are also some shortcomings in the work: the controller does not have a reserved communication interface to realize the communication function. There is no LCD, buttons and other human-computer interaction devices. The efficiency of the work failed to reach the expected 94%. There will be opportunities to optimize them one by one later. (Just when the board washing water was gone, the board looked ugly, and I, who have obsessive-compulsive disorder, was uncomfortable looking at it. Please understand. If you have any questions, you can discuss them with everyone.)