Abstract: This paper introduces a high-power medium-voltage frequency converter system based on a three-level topology, with a single unit capacity of 8MVA and the ability to expand by parallel connection of multiple units. The power switching device of the frequency converter adopts an integrated gate commutated thyristor (IGCT). The frequency converter adopts an AC-DC-AC back-to-back topology, and both the rectifier and inverter sections are neutral-point clamped (NPC) three-level structures. The rectifier section is designed as an active front-end (AFE), which enables the inverter to feed power back to the grid. The inverter section adopts rotor magnetic quantity oriented vector control, achieving high-performance speed regulation. The frequency converter can operate in four quadrants. The system not only has low harmonic content but also has excellent dynamic characteristics and speed regulation accuracy, making it suitable for drive systems with a wide speed range.
1 Introduction
Medium- and high-voltage high-capacity variable frequency drive systems are widely used in various industrial applications such as metallurgy, mine hoisting, papermaking, oil extraction, and ship propulsion. Developing high-performance, high-power frequency converters is of great significance for energy conservation, emission reduction, and overall industrial improvement. However, compared to the development of low-voltage, small-capacity frequency converters, the development of medium- and high-voltage high-capacity frequency converters places higher demands on the performance of power devices and circuit topology, thus presenting greater challenges.
The adoption of multi-level main circuit structures is an important development direction for medium and high voltage frequency converters. Compared with two-level structures, multi-level structures reduce the withstand voltage requirements of power devices and feature better output voltage power quality (waveform close to sinusoidal with low harmonics), lower switching losses, and smaller voltage change rate du/dt, making them very suitable for high-voltage, high-power applications. Currently, common multi-level structures include three-level netural-point-clamped voltage source converters (3L-NPCVSC), four-level flying capacitor-clamped voltage source converters (4L-FLCVSC), and series-connected H-bridge voltage source converters (SCHBVSC). Among these, the voltage source-point-clamped three-level structure has higher efficiency than other structures and is therefore more widely used in industry. Currently, most well-known foreign electrical companies have launched their own three-level frequency converter products, such as ABB's ACS6000 series and Siemens' SIMVERTML2 series, both of which have high performance indicators.
my country's research on medium- and high-voltage high-power frequency converters is relatively lagging. In the past, thyristors were mainly used as power devices to develop DC drive systems and AC-AC frequency conversion drive systems. These devices are complex in structure and high in cost, and due to their low switching frequency, their control performance and grid-side harmonics are poor. In the past fifteen years, China has made breakthrough progress in the development of large-capacity frequency converters. Companies such as Beijing Leadway Electric Technology Co., Ltd. (acquired by Schneider Electric), Beijing Hekang Yisheng Frequency Conversion Technology Co., Ltd., and Shenzhen Invt Electric Co., Ltd. have launched voltage-source type H-bridge cascaded high-voltage frequency converters using low-voltage IGBTs. Shandong Xinfengguang Electronic Technology Development Co., Ltd. and Xuzhou Zhongkuang Transmission & Automation Co., Ltd. have developed medium-voltage three-level frequency converters based on IGBTs, with voltage levels reaching 3kV. Relying on price advantages, these products can compete with foreign products to some extent. However, overall, domestic medium- and high-voltage frequency converter brands mainly focus on H-bridge series configurations, with relatively simple technology. The research and application of multi-level frequency converters are still insufficient, and the technological level lags significantly behind that of foreign countries.
To improve the level of domestically produced high-power frequency converter technology and reduce the domestic market's dependence on imported technology and equipment, Tianjin Electrical Science Research Institute Co., Ltd., based on its years of experience in high-power frequency converter technology, began research and development on a voltage source type neutral point clamping three-level frequency converter. After more than two years of dedicated research, they successfully developed a 3.3kV, 8MVA three-level frequency converter based on IGCT, as shown in Figure 1.
This frequency converter has successfully solved several key technologies, including AC motor vector control technology and active front-end rectifier control technology. It has completed a 2MW asynchronous motor drag test at the National Electrical Control and Distribution Equipment Product Quality Supervision and Inspection Center. The test results show that the device has excellent dynamic and static performance, filling the gap in domestic IGCT three-level medium-voltage frequency converter research.
2. Design of the main circuit and control system of the frequency converter
2.1 Power Main Circuit and its Control System
The power main circuit structure of the frequency converter is shown in Figure 2.
In Figure 2, the main power circuit adopts a dual PWM back-to-back structure, allowing the inverter to operate in four quadrants. Active power can flow from the grid to the motor and back from the motor to the grid. The inverter is connected to the grid via a three-phase rectifier transformer. The three-phase AC power output from the transformer is rectified by an active front-end rectifier unit to obtain a DC voltage with positive, negative, and zero polarity. The active front-end rectifier unit uses PWM rectification technology, making its input power factor controllable over a wide range. The modulation algorithm employs Selected Harmonic Elimination (SHE) to effectively suppress harmonics flowing into the grid. The inverter uses a rotor flux-oriented vector control algorithm, improving the motor's dynamic and static performance and speed regulation accuracy. The DC link is the energy-saving braking section. When the DC voltage is high, it triggers the thyristors, discharging the DC power through a resistor. It also serves as a fast discharge circuit for the device.
The power device used in the frequency converter is ABB's patented IGCT. This device was chosen because it has low conduction and switching losses, making it very suitable for high-power medium-voltage frequency conversion technology.
2.2 Control System
To meet the requirements of high speed and real-time operation in electric drive systems, our company has successfully developed a new digital control system, TGCS, whose appearance is shown in Figure 3.
The hardware portion of TGCS uses TI's new-generation CPU and Altera's large-scale FPGA as core control chips. The TGCS software is suitable for applications requiring digital signal processors (DSPs), such as electric drives, industrial field automation control, and the manufacturing of electrical and power equipment. The software has its own basic function module library, which can be directly called as needed to achieve modular programming. Specific functions can also be written in a language and then packaged into blocks, increasing debugging flexibility. Through modular programming, the DSP in the general-purpose digital controller (TGC) completes code compilation, code download, and monitoring of the DSP program's running status.
By rationally configuring the core hardware and corresponding peripheral interfaces, the system achieves coordinated and efficient operation. The controller adopts a modular design, making hardware configuration flexible and convenient. To ensure insulation between the medium-voltage power section and the control section and to overcome electromagnetic interference during signal transmission, a new, practical, safe, and reliable 10M and 100M high-speed fiber optic communication circuit is used. Through reasonable selection and design, a high-speed and fault-tolerant encoding and decoding technology suitable for industrial operation is adopted. This digital fiber optic communication technology meets the safe, fast, stable, and reliable communication requirements of medium-voltage frequency converters.
Figure 4 shows the TGCS programming interface. When writing programs using TGCS, simply select the appropriate program function block and drag it to the graphical editing area, then connect the corresponding terminals and set the appropriate parameters. The operation process is similar to MATLAB/Simulink, making it easy for system design engineers to master and thus significantly improving programming efficiency. In addition, during program debugging, the module terminals can be monitored in real time.
The inverter has developed a new high-precision signal detection technology suitable for digital control, which mainly includes: using high-voltage direct detection technology to safely and reliably acquire various high-voltage signals in the main circuit; and using Σ/Δ analog-to-digital conversion to realize high-speed and high-precision digital acquisition circuits for various analog signals.
3. Inverters and their control
The inverter's output is connected to a load (typically a motor). Speed control is the ultimate goal of inverter control; therefore, a dual-loop control algorithm is used, consisting of a speed outer loop and a current inner loop. The asynchronous motor control system is shown in Figure 5. The control system employs a rotor flux-oriented vector control method. This algorithm features smooth torque, low ripple, and fast response, making it suitable for high-performance variable frequency control systems. Motor model prediction is a crucial part of the control. Two common modeling methods are used: one calculates the slip frequency, load angle, and flux linkage based on the current vector, known as the current model; the other calculates the flux linkage, its angular frequency, and flux linkage position angle based on the integral of the actual electromotive force, known as the voltage model.
The principles underlying the two models determine their applicability. The current model is suitable for control at low speeds, where the motor parameters do not change significantly and the electromotive force is relatively small, which is not conducive to model calculation. The voltage model is suitable for control at high speeds, where the model has little dependence on the motor parameters and the motor's electromotive force is relatively large, which helps improve the model's calculation accuracy. Therefore, these two models are used for control in different speed ranges.
In achieving the functionality shown in Figure 5, the transition control between the voltage and current models is crucial for the system's stable and reliable operation, with the flux linkage position angle transition being particularly important. During the inverter development process, through in-depth research into the characteristics of the voltage and current models and repeated experiments and improvements, a smooth transition between the models was achieved. Simultaneously, the transition region exhibits excellent reliability and stability, ensuring good performance of vector control.
4 Active Front-End (AFE) Rectifier and its Control
As shown in Figure 2, the rectifier section of the frequency converter adopts the same circuit structure as the inverter section, enabling both rectification and inversion functions. Compared to traditional rectification technology, active front-end rectification technology no longer passively converts AC to DC but possesses an "active" function, allowing for power control. Active front-end rectifiers not only eliminate high-order harmonics and achieve adjustable power factor, but also ensure that the rectified voltage is unaffected by grid voltage fluctuations, exhibiting superior dynamic characteristics.
Active front-end vector control principle: Using the grid voltage vector as the reference vector, the phase and amplitude of the input current can be controlled by changing the amplitude and angle of the duty cycle control signal, thereby realizing the separate control of the active and reactive components of the input current.
Figure 6 shows the overall block diagram of the vector control of the active front-end in a synchronous rotating coordinate system. As shown in the figure, the control system consists of a dual closed-loop control structure with an inner current loop and an outer DC voltage loop. The DC voltage serves as the reference command for the voltage loop, and its error with the actual feedback value is used as part of the active current setpoint via a PI regulator. The other part is the load current used as a feedforward signal. Introducing the feedforward signal is the most direct and effective way to improve the system's resistance to load disturbances. The calculation of reactive current is part of the power factor control stage and needs to be determined according to actual requirements. If the device operates in power factor rectification mode, the reactive current setpoint is 0. Furthermore, to further eliminate lower harmonics, the system employs a specific harmonic cancellation method (SHE-PWM) for modulation.
5. Inverter Test Results
To test the performance of the frequency converter, relevant experiments were conducted on the control strategy of the developed frequency converter on an existing experimental platform. The experimental system is shown in Figure 7. During the system experiment, device 1 was the main frequency converter, using speed closed-loop control, and device 2 was the auxiliary frequency converter, using torque closed-loop control and used as a load. Figure 8 records the experimental condition where the speed setpoint of device 1 was 70% and the torque setpoint of device 2 was -80%. The experimental waveforms in Figure 8 are the line voltage and phase current waveforms of the AFE rectifier unit of device 2. It can be seen from Figure 8 that the frequency converter can operate stably under energy feedback conditions, and the current sinusoidal characteristic is good.
Figure 9 shows the actual values of inverter torque current and motor speed for device 1, where device 2 experiences a sudden 3-second application of 50% load torque. The figure demonstrates good dynamic response speed and steady-state accuracy.
6 Conclusions
This article introduces a high-power medium-voltage frequency converter based on a three-level topology, developed by Tianjin Electric Power Research Institute Co., Ltd. The three-level structure represents the development trend of medium- and high-voltage high-power AC drive technology. Internationally renowned companies such as Siemens and ABB have achieved significant results in the development of three-level frequency converters, launching high-performance, serialized products. In contrast, my country's research and application of medium- and high-voltage high-power three-level devices are still insufficient, lagging significantly behind foreign counterparts. Recently, Tianjin Electric Power Research Institute Co., Ltd. successfully developed a 3.3kV, 8MVA three-level active front-end frequency converter based on IGCT. The results of a 2MW asynchronous motor drive test demonstrate that this product has successfully solved several key technologies of medium-voltage high-power three-level converters, possessing excellent dynamic and static performance, and filling the gap in domestic research on IGCT three-level medium-voltage frequency converters.
