Abstract: With the increasing complexity of mechanical equipment, in order to solve the problem of regenerative energy feedback processing in general-purpose frequency converters and improve the energy utilization efficiency of the converters, the common DC bus technology introduced in multi-motor drive systems is proposed. This technology feeds the regenerative energy of the system back to the power grid while ensuring a constant bus voltage in the AC speed control system, achieving excellent energy-saving results. Based on the basic principles of common DC bus technology, several common DC bus technology schemes are listed. Practical applications show that these schemes have good practicality in terms of energy saving and improving system control accuracy.
Keywords: general-purpose frequency converter; rectification/feedback; common DC bus technology; inverter; multi-motor drive
I. Preface
In recent years, energy conservation and emission reduction and the development of a low-carbon economy have become a global consensus. Energy conservation and consumption reduction, and building a resource-saving and environmentally friendly society have become an eternal topic in the development of society today [1]. In manufacturing production, the quality of the green performance of mechanical equipment will directly determine the impact of the product manufacturing process on the environment, and the utilization rate of energy and resources is its main assessment indicator; variable frequency speed regulation is currently recognized worldwide as an ideal energy-saving speed regulation technology, and it has been widely used in various speed regulation systems. In the fierce market competition, the development of green frequency converters has become the focus of innovation for various frequency converter manufacturers.
In the hardware design circuit, the general-purpose PWM inverter does not have the function of feeding regenerative energy back to the AC input power supply. Therefore, when the motor brakes, the motor is in the generator state, which eventually leads to an increase in the DC bus voltage in the inverter. This increased DC voltage causes the inverter to store the energy absorbed from the motor in the electrolytic capacitor of the main circuit [2]. Due to the voltage rating and capacity limitations of the electrolytic capacitor, it is impossible for it to store energy indefinitely. In order to ensure the safety of the inverter's main circuit components, the inverter must be equipped with a braking circuit and a braking resistor. When the bus voltage rises to a certain voltage value, the inverter consumes this increased electrical energy as heat by briefly connecting the braking resistor. Therefore, when designing the main circuit of the inverter, it is necessary to design a braking unit that meets the usage requirements based on the power and torque value of the load motor through detailed calculation and redundancy design, and consume electrical energy in the form of heat loss, so as to maintain the stability of the bus voltage. This working mode of braking unit + braking resistor is actually a huge waste of energy [2].
If a mechanical device has multiple drive frequency converters, the waste caused by using energy-consuming braking is huge. However, by interconnecting the DC bus of multiple frequency converters, the regenerative energy generated by one or more motors in the generator state can be transferred to other motors in the motor state for absorption and use. In the working mode of sharing the DC bus, if faster braking capability or emergency stop is required, an additional braking unit and braking resistor are required to consume the excess regenerative energy in the system through the braking resistor. For frequent braking situations, the voltage rise on the DC bus can be fed back to the grid through a feedback device. [3]
II. Composition of the Common DC Bus Control System
The common DC bus is a product of the development of power electronics technology and the continuous innovation of control theory. The principle of the common DC bus technology is based on the AC-DC-AC frequency conversion method adopted by general frequency converters. When the AC input voltage is the same, the rectified DC bus voltage is also the same. According to the function implemented by the main circuit hardware of the frequency converter, it can be divided into three parts: control unit, rectification/feedback and inverter unit. In order to reduce the system's requirements for power quality, the rectification feedback unit is usually composed of one or more single-phase or four-quadrant rectifiers. The inverter part can also be composed of one or more inverters. The rectification unit and the inverter unit are connected through the common DC bus [3].
Figure 1. Schematic diagram of AC-DC-AC control principle of frequency converter
The advantages of common DC bus technology are [5]:
a. The application of common DC bus technology can greatly reduce the need for redundant braking units, and the hardware composition is simple and reasonable;
b. A common DC bus can reduce the cost of AC input devices, such as AC input reactors and contactors;
c. A constant DC bus voltage enables controllable DC voltage and achieves pre-charging function;
d. The motors in the system operate under different conditions, recovering energy from regenerative braking, which improves system efficiency and optimizes the dynamic characteristics of the system;
e. Due to the reduction in components, the size of the electrical control cabinet can be reduced;
f. Improve the reliability of multi-drive control systems.
2.1 Rectifier/Feedback Unit
The main function of the rectifier/feedback unit is to convert the three-phase AC input power supply into a DC power supply with a constant voltage value. It can also feed back the braking energy of the motor to the grid through the innovative power energy feedback function and keep the DC bus voltage constant within the specified range. It is generally composed of two sets of anti-parallel thyristor modules for its power section. Through a reliable control algorithm, it can realize the rectification and feedback of electrical energy between the three-phase AC input power supply and the intermediate circuit of the inverter. Its schematic diagram is shown in Figure 2. [4]
Figure 2 Schematic diagram of the rectifier/feedback unit
2.2 Inverter Module
The function of the inverter module is to convert DC voltage into three-phase AC power through advanced control theory and PWM chopping technology, and the voltage and frequency are adjustable, thereby achieving the purpose of motor speed regulation. The principle block diagram of the inverter system is shown in Figure 3 [4].
Figure 3 Schematic diagram of the inverter unit
III. Scheme Study and Characteristics of Common DC Bus System
According to the presence or absence of energy feedback, the common DC bus system can be divided into two categories: feedback type and non-feedback type. The system with feedback unit can feed the regenerated energy back to the grid, which is suitable for applications where the motor brakes frequently and the braking power is large. The non-feedback type system interconnects the frequency converters in the system through the common DC bus. The regenerated energy is absorbed and utilized by the motor in the motor state. In order to ensure the safety of the system and the braking effect, it is necessary to install a braking unit and braking resistor with appropriate power and resistance to dissipate the possible excess energy in the form of heat. In some multi-drive systems that do not require frequent braking, this non-feedback common DC bus solution has a higher cost performance. At present, the main common DC bus technical solutions are as follows [5].
3.1 Common DC Bus Technical Solution 1
In this scheme, each inverter still uses the traditional AC power supply method, without the need for an additional rectifier unit. The DC sides of the inverters are connected in parallel to realize energy transfer between the inverters. This scheme is suitable for inverters with similar power levels in the system. Even after the DC bus is disconnected, each inverter can still be used independently without affecting each other [7]. This scheme has the following characteristics:
• When the generated energy is higher than the electric energy, energy consumption braking is still required to consume the excess energy;
• Only inverters of the same power rating are allowed to be connected in DC parallel;
• Excess renewable energy cannot be fed back into the grid;
• Only conventional 6-pulse rectification can be used, with high 5th and 7th harmonics, making it impossible to configure a 12-pulse system;
• Install a braking resistor according to the braking power.
This solution can be applied to the modification of the common DC bus of centrifuges. In practical applications, it is very important to fully consider the protection of the AC input main circuit, the timing control and safety interlock of all frequency converters, system fault handling, load type, etc. when the whole system is working.
Figure 4 Schematic diagram of common DC bus technical scheme 1
3.2 Common DC Bus Technical Scheme 2
This solution uses an independent diode rectifier bridge as the rectifier unit to provide unified power to all frequency converters. The characteristics of this solution are:
• The rectifier section can be configured as a 12-pulse rectifier as needed to eliminate the 5th and 7th harmonics and reduce harmonic interference to the power grid;
• Inverters of different power ratings can share a DC bus;
• The rectifier section adopts a traditional AC-DC conversion method, which simplifies the power configuration;
• To avoid the impact of high current on the internal components of the frequency converter, a pre-charge circuit must be configured.
• Different operating states enable energy transfer between frequency converters;
• Because of the use of an uncontrollable rectifier circuit, excess regenerated energy still cannot be fed back to the grid. In order to achieve energy feedback, a separate feedback unit needs to be configured.
• A braking unit/braking resistor with matching power must be installed.
Paper-making machinery uses many guide roller drive motors, which frequently operate in braking mode. Therefore, regenerative braking must be considered in the design. Configuring a separate braking unit and braking resistor for each frequency converter would complicate the control system structure. However, using a common DC bus technology for the guide roller control system offers obvious advantages: simple system structure; energy complementarity; fewer failure points; improved system reliability; and lower costs.
Figure 5 Schematic diagram of common DC bus technical scheme 2
3.3 Common DC Bus Technical Solution 3
This design utilizes a thyristor rectifier bridge as the DC power supply unit. By controlling the conduction angle of the thyristors, the DC bus voltage is made controllable, and it also serves as a pre-charge mechanism. The application characteristics of this design are as follows:
• The rectifier circuit can be configured as a 12-pulse rectifier unit according to system requirements;
• Inverters of different power ratings can share a DC bus;
• The thyristor rectifier bridge needs to be debugged;
•Requires a braking resistor/or feedback unit;
• Single-quadrant rectification control mode with no feedback function.
During the container lowering process and the braking process of the trolley and gantry crane, the motor is in a generating state. The potential energy converted into electrical energy cannot be fed back to the grid. Therefore, the remaining electrical energy can only flow to the braking resistor through the DC side braking unit and be consumed as heat energy, resulting in a huge energy waste. However, by adopting common DC bus technology and installing single-phase limited controllable rectifier and energy feedback device, this part of the regenerated energy can be returned to the grid, further reducing the system's energy consumption and achieving green energy saving.
Figure 6. Technical Scheme 3 for Common DC Bus
3.4 Common DC Bus Technical Solution 4
This scheme adopts a four-quadrant fully controlled rectification method. The main problem with the single-phase frequency converters introduced earlier is that the rectifier circuit components cannot be fully controlled, and energy cannot be fed back to the grid. This scheme, however, truly realizes rectification and regenerative feedback functions. This scheme has the following characteristics:
• Using thyristors as rectifier devices, the DC bus voltage is controllable, enabling pre-charging of the electrolytic capacitors;
• According to the design requirements, the rectifier circuit can be set as a 12-pulse rectifier system to reduce the 5th and 7th harmonics;
• Inverters of different power ratings can share a DC bus;
• It has a feedback function that enables a large regeneration capacity;
• The output capacity on the DC side is limited by the device;
• Controllable rectification and energy feedback provide high dynamic response, and the DC bus voltage is unaffected by load fluctuations;
Figure 7. Technical Scheme 4 for Common DC Bus
The above lists four common DC bus technology schemes and elaborates on the characteristics of each scheme. As introduced earlier in this article, the common DC bus technology is the optimal solution for multi-motor drive technology. In the same drive system, each frequency converter will be in a different working state at different times. The common DC bus technology solves the problem of energy conversion between the generator and motor states of the motor. The rectification and feedback functions of the rectifier feedback unit in the system not only provide a stable DC voltage supply for the system, but also realize the pre-charging function and feed the regenerated energy back to the grid, thereby improving the system efficiency [2].
The multi-drive system using common DC bus technology is more compact in structure, and reduces equipment failure points and improves equipment reliability by eliminating unnecessary rectifier and braking units. [7]
4. Conclusion
Common DC bus technology is currently widely used in multi-motor drive applications such as cranes, roller conveyors, and centrifuges. It represents an innovative development in variable frequency speed control systems. Practical applications show that this method saves up to 25% more energy than traditional energy-consuming braking methods. While improving the system's dynamic performance, it can also feed the energy generated during braking back to the power grid, increasing energy utilization. Its application prospects are very broad. Furthermore, due to the reduction in energy-consuming resistance in the system, the temperature rise of the electrical control cabinet is greatly reduced, and the rate of equipment downtime due to thermal failures is almost zero. Common DC bus technology has excellent prospects for application and promotion.
References
[1] Peng Jinxin. Global Green and Low-Carbon Development and Its Implications for China. Green Leaf. No. 7, 2011.
[2] Li Fangyuan. Design and application of common DC bus scheme for general frequency converter. Electrical Engineering Technology Magazine. No. 6, 2004.
[3] Wang Wanxin. Application of common DC bus in AC drive. Electrical Drive. No. 5, 2002.
[4] Yu Hongliang, Gu Chunlei, Wang Jianzhong. Design principle and application of SPDMR with shared DC bus. Electrical Drive. 2009, No. 1
[5]http://wenku.baidu.com/view/4a2263f7ba0d4a7302763ac8.html
[6] Bai Xiangang, Xia Naixue, Gong Junpeng, Xiong Yanmei. Application of common DC bus of general frequency converter in centrifuge. Electrical Age. No. 9, 2010.
[7] Zhao Ruilin. Design of a shared DC bus frequency converter scheme. Automation Technology and Application. 2013-11
Author : Ding Yunfei, born in 1978, is a senior engineer with a Bachelor of Engineering degree. He has been engaged in the research and development project management and quality management of high-end CNC machine tools, all-digital bus CNC systems and servo drives.
