Abstract: This paper concisely describes a renewable energy feedback device from the aspects of control principle characteristics, typical application scenarios, and experimental results analysis. This device features high grid-side power factor, low current harmonics, controlled current source characteristics, ease of multi-unit parallel connection, fast dynamic response, and high overall efficiency. Keywords: Energy feedback, power factor, current source I. Introduction In today's energy-scarce era, the rational use and effective conservation of energy have become a focus of attention. For example, the regenerative energy generated by the motor in the AC-DC-AC control system of a mining hoist when it is in generator mode; the regenerative energy generated during the factory testing of diesel engines and motors by diesel engine manufacturers; and the regenerative energy generated during the parking braking state in urban rail transit. Many universities and research institutes in China have made this a research hotspot. However, a review of information from recent years shows that most papers only discuss experimental prototypes, and several key parameters of the prototypes differ significantly from actual products. To address this issue, Shandong Xinfengguang Electronic Technology Development Co., Ltd. invested significant human and material resources over several years to develop and implement a megawatt-level high-power renewable energy feedback grid-connected device. II. System Principle and Characteristics The main circuit of a single unit of the high-current energy feedback device is shown in Figure 1. The main circuit topology adopts a three-level circuit. [align=center] Figure 1 Unit Main Circuit Diagram[/align] When the device power is high, several units need to be connected in parallel. When connecting in parallel, current sharing, circulating current, and their phase issues must be considered. The control system of the high-power renewable energy feedback device is a dual closed-loop control system consisting of an outer voltage loop and an inner current loop. The control system monitors the system bus in real time. When the bus value exceeds a certain set value, the system starts working. The difference between the detected bus value and the set bus value is input to the voltage regulator. After certain control calculations, the control function of the outer voltage loop is completed. The output of the voltage regulator is the output signal of the outer voltage loop, which is also the given signal of the inner current loop. The difference between its value and the real-time current detected by the system is processed by the current regulator to complete the control function of the inner current loop. The output of this block is directly fed to the PWM signal generation section, which in turn sends the generated PWM wave to the main power device circuit. Finally, multiple main power device units are connected in parallel and, after passing through current sharing and circulating current suppression circuits, are directly connected to the AC grid. The entire high-current regenerative energy feedback device control system is thus a dual closed-loop control system where the voltage outer loop and current inner loop interact. The high-power renewable energy feedback grid-connected device achieves sinusoidal grid-side current and can operate at unity power factor, exhibiting current source characteristics. It is easy to connect multiple units in parallel, has a high power factor, low total harmonic distortion (THD), fast dynamic response, can output large current in a short time, and has high overall efficiency. III. Application Scenarios Regenerative energy feedback devices can be used in many energy-saving applications. This article only briefly describes a few typical application types to encourage people to rationally utilize energy and effectively save energy. 1. Applied to mine hoist systems: Mine hoists combine regenerative energy feedback devices with specific function frequency converters. Their working principle is that when the system load is lowered or rapidly decelerated and braked, the motor enters the generator state, and the bus voltage of the mine hoist rises rapidly. The system automatically detects the bus voltage, automatically judges the feedback conditions, and automatically enters the feedback state. The feedback current waveform is sinusoidal, with low harmonics and high grid-side power factor. The system works quickly, stably, and reliably. It is applicable to various mine hoists such as single-drum, double-drum, vertical shaft, and inclined shaft. It can effectively solve the problem that a large amount of electrical energy is consumed in the slip resistance in the current mine hoist control system, which is generally used for speed regulation by winding motor rotor series resistance, resulting in serious energy waste. It also solves the problems of complex control system, high failure rate, easy damage to contactors, resistors, and winding motor carbon brushes, and large maintenance workload; low speed and crawling stages require the brake pads to rub against the drum to achieve speed control, especially when the load changes, it is difficult to achieve constant deceleration control, resulting in discontinuous speed regulation and poor speed control performance; large starting and shifting impact current, resulting in a large mechanical impact, which greatly reduces the service life of the motor and is very easy to "derail"; small starting torque in low voltage and low speed sections, poor load-carrying capacity, and inability to achieve constant torque lifting. Its control system is briefly described as follows: (1) Matching frequency converter system scheme: Three-phase AC power supply is connected to the three-phase input terminal of the mine hoist through an automatic air switch, and the three-phase output terminal of the mine hoist is connected to the motor. As shown in Figure 2. [align=center] Figure 2 Wiring diagram of the main circuit of the frequency converter[/align] The electrical control system is used in conjunction with the mine hoist. The system has provided an interface with the mine hoist and can be connected correctly according to their respective wiring instructions. (2) Modification scheme for the mine hoist system The modified mine hoist is based on the original mine hoist electrical control system. The original power frequency speed regulation system is replaced by the mine hoist speed regulation system, while the power frequency speed regulation system is retained. The two systems are used as backups for each other, increasing the reliability of the system operation. The modification requires the addition of power frequency conversion function. Before the system is running, the main circuit and control circuit switches are switched to the corresponding frequency conversion or power frequency positions. The specific connection method is as follows. Three three-pole double-throw switches (QS1, QS2, QS3) are added as the main circuit switching device. The three-phase power supply, stator coil, and rotor coil are respectively connected to the knife position of the corresponding switch. As shown in Figure 3 (a) and (b) [align=center] Figure 3 Main circuit power and frequency conversion switching principle diagram[/align] When all switches are switched to the frequency conversion position, the three-phase power supply is connected to the input terminals (R, S, T) of the mine hoist via double-throw switch QS1 and automatic air switch QA (while the neutral wire is connected to the neutral terminal N of the frequency converter). The output terminals (U, V, W) of the mine hoist are connected to the stator coil of the motor via double-throw switch QS2, and the rotor coil of the wound motor is short-circuited via double-throw switch QS3. When all switches are switched to the power frequency position, the three-phase power supply is connected to the stator coil via double-throw switches QS1 and QS2, and the rotor coil of the wound motor is connected to the original speed regulating resistor device via QS3. 2. Application in motor testing systems As is well known, various motors need to undergo factory testing before leaving the factory to test various performance characteristics. At present, developed countries abroad have dedicated motor testing systems that feed energy back into the power grid. In this way, only a small portion of the energy is consumed by various heat losses in the system. However, this device is expensive and technical support is inconvenient. Regenerative energy feedback devices, on the other hand, can be fully applied to motor testing systems, allowing the vast majority of energy to be fed back to the grid, with only a small portion lost in heat and other forms. The system block diagram is shown in Figure 4. [align=center] Figure 4: Structure diagram for motor testing[/align] The motor under test is driven by a voltage regulator and a frequency converter. The voltage regulator can change the voltage, and the frequency converter can change the speed. The measurement system can measure various physical quantities such as voltage, current, input power, and power factor. Speed ​​measurement and shifting can detect speed and change the generator's speed. By controlling the generator's excitation, the generator's voltage can be controlled. The regenerative energy generated by the generator does not need to be consumed by pure resistance as is currently the case in most domestic motor manufacturers. Instead, it is returned to the grid through the regenerative energy feedback device, thus saving a significant amount of electrical energy. A similar solution can also be used in diesel engine manufacturers to address the regenerative energy feedback problem during diesel engine factory testing. 3. Application in Urban Rail Transit Systems Many large and medium-sized cities in China are constructing urban rail transit systems. Urban rail systems generally use DC power supplies, such as DC750V and DC1500V. In new projects, DC 1500V systems are often used. Our company's high-power regenerative energy feedback device, employing a three-level main circuit topology, effectively solves voltage matching issues. Furthermore, due to the current source characteristics of the feedback device, multiple units can be easily connected in parallel, thus addressing the issue of high power at the DC 1500V level and easily achieving megawatt-level power. A simplified system structure diagram is shown in Figure 5: [align=center] Figure 5: Connection diagram for urban rail transit systems[/align] The regenerative energy feedback device simply requires connecting the positive and negative inputs; the three-phase output can be freely connected to the power grid without phase sequence checks. Its control system has input polarity detection and protection functions, and an output phase sequence self-retrieval function. If the input polarity is reversed, the system will not be able to connect the input, and the main circuit will not operate. The entire system can operate independently or coordinate with other systems via serial communication. It has functions for setting the main circuit connection and the start of feedback operation, and can limit the maximum feedback energy, among other technical challenges. IV. Experimental Results Analysis The regenerative energy feedback device, as verified by the Product Quality Supervision and Inspection Center of the Ministry of Railways, features high output power, good grid-side current waveform, and high overall efficiency. Table 1 is a simplified table of power factor, DC-side input voltage, grid-side AC current, and overall efficiency. [align=center] Table 1 Voltage, Current, Efficiency, and Power Factor Table[/align] As can be seen from Table 1, with the increase of grid-side current, its power factor gets closer and closer to 1, meaning that the total harmonic distortion (THD) of the grid-side current decreases, and its overall efficiency tends to decrease. This is consistent with the characteristics of modern power electronic devices; as the current increases, the device losses increase, and the overall efficiency decreases. Table 2 is a table of harmonic content of the grid-side phase currents connected to Table 1. [align=center]Table 2 Current Harmonics of Each Phase[/align] As can be seen from Table 2, with the increase of the grid-side output current of the high-power renewable energy feedback grid-connected device, its grid-side current harmonics become smaller and smaller. When the output grid-side current is above 265.4A, its total harmonic distortion (THD) of each phase current is much less than 5%. The trend in Table 2 is also consistent with that in Table 1. The voltage and current waveforms of a single unit of the renewable energy feedback device are shown in Figure 6: Channel 1 is the grid-side phase voltage waveform, and Channel 2 is the in-phase current waveform, at which point the current is approximately 120A. It can be seen from the figure that the harmonics of the current waveform are very small and the power factor is high. If multiple units are connected in parallel, the current waveform will be even better using carrier phase-shifting technology. [align=center]Figure 6 Unit Voltage and Current Waveforms[/align] V. Conclusion The regenerative energy feedback grid-connected device 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 has a high grid-side power factor, sinusoidal waveform, current loop characteristics, and is easy to connect multiple units in parallel. It can effectively save energy in mining hoists, electric motor and diesel engine factory testing, and urban rail transit. At the same time, it can also be reasonably applied in static var compensators, active power filters, unified power flow controllers, superconducting energy storage, high-voltage direct current transmission, and grid-connected power generation of renewable energy sources such as electric drives, solar energy, and wind energy. It can be predicted that in today's era of energy scarcity, this technology will be increasingly valued by 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. 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