Abstract: This paper briefly describes a high-power renewable energy feedback grid-connected device from the aspects of control principle, system technical characteristics, and experimental results analysis. This device has the characteristics of high grid-side power factor, low current harmonics, controlled current source characteristics, easy multi-unit parallel connection, fast dynamic response, and high overall efficiency. Keywords : renewable energy, power factor, current source. I. Introduction In AC-DC-AC control systems and renewable energy control systems such as wind and solar power, renewable energy processing generally includes methods such as supercapacitor energy storage, electromagnetic bearing flywheel energy storage, pure resistive consumption, and feedback grid connection. For high-power renewable energy systems, supercapacitor energy storage has drawbacks such as high investment costs and high maintenance costs; electromagnetic bearing flywheel energy storage is still in the laboratory stage and may face certain technical challenges; pure resistive energy consumption braking has a simple structure, reliable operation, and convenient operation, but all the generated energy is converted into heat energy of the resistor, resulting in very low efficiency, making it unsuitable for most renewable energy control systems; while renewable energy feedback grid-connected systems can solve the problem of low efficiency, reduce investment costs, and improve operating efficiency, representing a "green" power electronic conversion technology in the world today. Many universities and research institutes in China have made this a research hotspot. However, a review of information from recent years shows that there are actually few operational products; most discussions only cover experimental prototypes, and several key parameters in the prototypes differ significantly from actual products. To address this issue, Shandong Xinfengguang Electronic Technology Development Co., Ltd. has invested significant human and material resources, spending several years developing a megawatt-level high-power renewable energy feedback grid-connected device. II. System Control Principle The main circuit of a single unit of the high-current renewable energy feedback device is shown in Figure 1. The main circuit topology adopts a three-level circuit (Figure 1: Unit Main Circuit Diagram). Each phase arm has four switching elements with three normal switching modes. Taking phase A as an example: when Ug1 and Ug2 are on, phase A outputs +U0/2; when Ug2 and Ug3 are on, phase A outputs zero level; when Ug3 and Ug4 are on, phase A outputs -U0/2. If Sa, Sb, and Sc represent the switching states of each arm, each arm has three switching states, as listed in Table 1. (Table 1: Unit A Phase Switching States) When the device power is high, several main circuits need to be connected in parallel. When connected in parallel, current sharing and circulating current and their phase issues need to be considered. At the same time, the three-level circuit with diode clamping needs to consider the problem of DC terminal capacitor midpoint potential imbalance. This point is especially important in the software PWM wave generation circuit. Figure 2 is the control block diagram of the high-power regenerative energy feedback device. The control system of the entire device is a double closed-loop control system with an outer voltage loop and an inner current loop. In Figure 2, 1 represents the system bus setpoint, 2 represents the voltage regulator, 3 represents the current regulator, 4 represents the PWM signal generation section, 5 represents the main power device circuit, 6 represents the current sharing and circulating current suppression circuit, 7 represents the AC power grid, 8 represents the phase current detection circuit, and 9 represents the phase voltage detection, synchronization signal generation, and bus voltage acquisition circuit. First, the control system of the device monitors the system bus in real time. When the bus value exceeds a certain setpoint, the system starts working. The difference between the detected bus value and the setpoint is input to the voltage regulator. After certain control calculations, the voltage outer loop control function is completed. The output of the voltage regulator is the output signal of the voltage outer loop, which is also the setpoint signal of the current inner loop. Its value is differed from the real-time current detected by the system, and then processed by the current regulator to complete the current inner loop control function. The output of this block is directly sent to the PWM signal generation section, which sends the generated PWM wave to the main power device circuit. Finally, multiple main power device unit circuits are connected in parallel and directly connected to the AC power grid through the current sharing and circulating current suppression circuit. The entire high-current regenerative energy feedback device control system is thus a dual closed-loop control system with interaction between the voltage outer loop and the current inner loop. [align=center]Figure 2 System Control Block Diagram[/align] III. System Technical Characteristics The high-power renewable energy feedback grid-connected device realizes sinusoidal grid-side current and can operate at unity power factor, exhibiting current source characteristics. Therefore, it truly realizes the "green" power conversion technology of power electronics. The technical characteristics of the control system are briefly described as follows: (I) The control system is a dual closed-loop control system of voltage outer loop and current inner loop. The voltage outer loop is the interaction between the bus voltage detection value and the set value, as well as the synchronous signal generation circuit of the grid-side phase voltage, which outputs the given value of the current inner loop through the voltage regulator. The current inner loop is the core of the entire control system. In fact, the core of the entire control system is the control of the grid-side current, that is, the control of the current inner loop. (II) The grid side of the control system exhibits current source characteristics, making it easy to construct multi-unit parallel devices and easily scale up the power of the renewable energy feedback grid-connected device. Scale-up requires solving many practical problems. (III) The grid-side power factor is adjustable, and the renewable energy feedback grid-connected device can operate at a negative unity power factor. (iv) The grid-side current waveform is sinusoidal, and the total harmonic distortion (THD) is much less than 5%. (v) The main circuit adopts a three-level circuit, which is beneficial to reducing grid-side current harmonics and has a wide input and output voltage range from the circuit topology perspective. (vi) The control system adopts digital control, which combines modern control theory with modern power electronics technology, making the device easy to control and master. (vii) It has protection functions such as DC reverse connection, overcurrent, short circuit, temperature, and abnormal grid-side voltage. (viii) It has the advantages of fast dynamic response, large current output in a short time, and high overall efficiency. IV. Experimental Results Analysis The high-power renewable energy feedback grid-connected device made by our company has been tested and certified by the Product Quality Supervision and Inspection Center of the Ministry of Railways: it has the characteristics of high output power, good grid-side current waveform, and high overall efficiency. Table 2 is a simplified table of power factor, DC side input voltage, grid-side AC current, and overall efficiency. [align=center]Table 2 Voltage, Current, Efficiency, and Power Factor[/align] Table 2 shows that as the grid-side current increases, the power factor gets closer to 1, meaning the total harmonic distortion (THD) of the grid-side current decreases, and the overall efficiency tends to decrease. This aligns with the characteristics of modern power electronic devices; as the current increases, device losses increase, and overall efficiency decreases. [align=center]Table 3 Harmonic Content of Grid-Side Phase Currents (connected to Table 2)[/align] Table 3 shows that as the grid-side output current of high-power renewable energy feedback grid-connected devices increases, the grid-side current harmonics decrease. When the output grid-side current is above 265.4A, the total harmonic distortion (THD) of each phase current is much less than 5%, and the trend in Table 3 is consistent with that in Table 2. V. Conclusion The high-power renewable energy feedback grid-connected device developed by our company is a "green" power electronic conversion control system. It is a dual closed-loop control system with interaction between the voltage outer loop and the current inner loop. It features a high grid-side power factor, sinusoidal waveform, current loop characteristics, and ease of multi-unit parallel connection. It has applications in urban rail transit, static var compensators, active power filters, unified power flow controllers, superconducting energy storage, high-voltage direct current transmission, electric drives, and grid-connected power generation from renewable energy sources such as solar and wind power. It can be predicted that in today's era of energy crisis, this technology will receive increasing attention from industry professionals. References: 1. Zhang Chongwei, Zhang Xing. PWM rectifier and its control. Machinery Industry Press, 2003 (10): 21-23 2. Chen Boshi, Chen Minxun. AC speed control system. Machinery Industry Press, 2005 (4): 144-148 3. Yao Weizheng, Wang Zhaoan. Research on three-phase high-power PWM rectifier circuit. Journal of Hunan Institute of Engineering, 2000, VOL.11, NO.1, 1-4 4. Inspection report of the Product Quality Supervision and Inspection Center of the Ministry of Railways. (2006) JL No. W-088, 2006, 4 Hu Shunquan graduated from the School of Control Science and Engineering of Shandong University with a major in power electronics and electric drive. He is currently working in the R&D department of Shandong Xinfengguang Electronic Technology Development Co., Ltd., and is engaged in the research and development of power electronic conversion technologies such as frequency converters and renewable energy feedback devices.