抢先看 | CES TEMS 2024年第2期目次及摘要

Abstract—Based on the complementary advantages of Line Commutated Converter (LCC) and Modular Multilevel Converter (MMC) in power grid applications, there are two types of hybrid DC system topologies: one is the parallel connection of LCC converter stations and MMC converter stations, and the other is the series connection of LCC and MMC converter stations within a single station. The hybrid DC transmission system faces broad application prospects and development potential in large-scale clean energy integration across regions and the construction of a new power system dominated by new energy sources in China. This paper first analyzes the system forms and topological characteristics of hybrid DC transmission, introducing the forms and topological characteristics of converter-level hybrid DC transmission systems and system-level hybrid DC transmission systems. Next, it analyzes the operating characteristics of LCC and MMC inverter-level hybrid DC transmission systems, provides insights into the transient stability of hybrid DC transmission systems, and typical fault ride-through control strategies. Finally, it summarizes the networking characteristics of the LCC-MMC series within the converter station hybrid DC transmission system, studies the transient characteristics and fault ride-through control strategies under different fault types for the LCC-MMC series in the receiving-end converter station, and investigates the transient characteristics and fault ride-through control strategies under different fault types for the LCC-MMC series in the sending-end converter station.

Abstract—High-frequency oscillation (HFO) of grid-connected wind power generation systems (WPGS) is one of the most critical issues in recent years that threaten the safe access of WPGS to the grid. Ensuring the WPGS can damp HFO is becoming more and more vital for the development of wind power. The HFO phenomenon of wind turbines under different scenarios usually has different mechanisms. Hence, engineers need to acquire the working mechanisms of the different HFO damping technologies and select the appropriate one to ensure the effective implementation of oscillation damping in practical engineering. This paper introduces the general assumptions of WPGS when analyzing HFO, systematically summarizes the reasons for the occurrence of HFO in different scenarios, deeply analyses the key points and difficulties of HFO damping under different scenarios, and then compares the technical performances of various types of HFO suppression methods to provide adequate references for engineers in the application of technology. Finally, this paper discusses possible future research difficulties in the problem of HFO, as well as the possible future trends in the demand for HFO damping.

Abstract—Virtual synchronous generators (VSGs) are widely introduced to the renewable power generation, the variable-speed pumped storage units, and so on, as a promising grid-forming solution. It is noted that VSGs can provide virtual inertia for frequency support, but the larger inertia would worsen the synchronization stability, referring to keeping synchronization with the grid during voltage dips. Thus, this paper presents a transient damping method of VSGs for enhancing the synchronization stability during voltage dips. It is revealed that the loss of synchronization (LOS) of VSGs always accompanies with the positive frequency deviation and the damping is the key factor to remove LOS when the equilibrium point exists. In order to enhance synchronization stability during voltage dips, the transient damping is proposed, which is generated by the frequency deviation in active power loop. Additionally, the proposed method can realize seamless switching between normal state and grid fault. Moreover, detailed control design for transient damping gain is given to ensure the synchronization stability under different inertia requirements during voltage dips. Finally, the experimental results are presented to validate the analysis and the effectiveness of the improved transient damping method.

Abstract—High-voltage direct current (HVDC) transmission is a crucial way to solve the reverse distribution of clean energy and loads. The line commutated converter-based HVDC (LCC-HVDC) has become a vital structure for HVDC due to its high technological maturity and economic advantages. During the DC fault of LCC-HVDC, such as commutation failure, the reactive power regulation of the AC grid always lags the DC control process, causing overvoltage in the AC sending grid, which brings off-grid risk to the wind power generation based on power electronic devices. Nevertheless, considering that wind turbine generators have fast and flexible reactive power control capability, optimizing the reactive power control of wind turbines to participate in the transient overvoltage suppression of the sending grid not only improves the operational safety at the equipment level but also enhances the voltage stability of the system. This paper firstly analyses the impact of wind turbine's reactive power on AC transient overvoltage. Then, it proposes an improved voltage-reactive power control strategy, which contains a reactive power control delay compensation and a power command optimization based on the voltage time series prediction. The delay compensation is used to reduce the contribution of the untimely reactive power of wind turbines on transient overvoltage, and the power command optimization enables wind turbines to have the ability to regulate transient overvoltage, leading to the variation of AC voltage, thus suppressing the transient overvoltage. Finally, the effectiveness and feasibility of the proposed method are verified in a ±800kV/5000MW LCC-HVDC sending grid model based on MATLAB/Simulink.

Abstract—As the core component of energy conversion for large wind turbines, the output performance of doubly-fed induction generators (DFIGs) plays a decisive role in the power quality of wind turbines. To realize the fast and accurate design optimization of DFIGs, this paper proposes a novel hybrid-driven surrogate-assisted optimization method. It firstly establishes an accurate subdomain model of DFIGs to analytically predict performance indexes. Furthermore, taking the inexpensive analytical dataset produced by the subdomain model as the source domain and the expensive finite element analysis dataset as the target domain, a high-precision surrogate model is trained in a transfer learning way and used for the subsequent multi-objective optimization process. Based on this model, taking the total harmonic distortion of electromotive force, cogging torque, and iron loss as objectives, and the slot and inner/outer diameters as parameters for optimizing the topology, achieve a rapid and accurate electromagnetic design for DFIGs. Finally, experiments are carried out on a 3MW DFIG to validate the effectiveness of the proposed method.

Abstract—With the continuous improvement of permanent magnet (PM) wind generators' capacity and power density, the design of reasonable and efficient cooling structures has become a focus. This paper proposes a fully enclosed self-circulating hydrogen cooling structure for a originally forced-air-cooled direct-drive PM wind generator. The proposed hydrogen cooling system uses the rotor panel supports that hold the rotor core as the radial blades, and the hydrogen flow is driven by the rotating plates to flow through the axial and radial vents to realize the efficient cooling of the generator. According to the structural parameters of the cooling system, the Taguchi method is used to decouple the structural variables. The influence of the size of each cooling structure on the heat dissipation characteristic is analyzed, and the appropriate cooling structure scheme is determined.

Abstract—For electric vehicles (EVs), it is necessary to improve endurance mileage by improving the efficiency. There exists a trend towards increasing the system voltage and switching frequency, contributing to improve charging speed and power density. However, this trend poses significant challenges for high-voltage and high-frequency motor controllers, which are plagued by increased switching losses and pronounced switching oscillations as consequences of hard switching. The deployment of soft switching technology presents a viable solution to mitigate these issues. This paper reviews the applications of soft switching technologies for three-phase inverters and classifies them based on distinct characteristics. For each type of inverter, the advantages and disadvantages are evaluated. Then, the paper introduces the research progress and control methods of soft switching inverters (SSIs). Moreover, it presents a comparative analysis among the conventional hard switching inverters (HSIs), an active clamping resonant DC link inverter (ACRDCLI) and an auxiliary resonant commuted pole inverter (ARCPI). Finally, the problems and prospects of soft switching technology applied to motor controllers for EVs are put forward.

Abstract—Cascaded H-bridge inverter (CHBI) with supercapacitors (SCs) and dc-dc stage shows significant promise for medium to high voltage energy storage applications. This paper investigates the voltage balance of capacitors within the CHBI, including both the dc-link capacitors and SCs. Balance control over the dc-link capacitor voltages is realized by the dc-dc stage in each submodule (SM), while a hybrid modulation strategy (HMS) is implemented in the H-bridge to balance the SC voltages among the SMs. Meanwhile, the dc-link voltage fluctuations are analyzed under the HMS. A virtual voltage variable is introduced to coordinate the balancing of dc-link capacitor voltages and SC voltages. Compared to the balancing method that solely considers the SC voltages, the presented method reduces the dc-link voltage fluctuations without affecting the voltage balance of SCS. Finally, both simulation and experimental results verify the effectiveness of the presented method.

Abstract—In recent years, motor drive systems have garnered increasing attention due to their high efficiency and superior control performance. This is especially apparent in aerospace, marine propulsion, and electric vehicles, where high performance, efficiency, and reliability are crucial. The ability of the drive system to maintain long-term fault-tolerant control (FTC) operation after a failure is essential. The likelihood of inverter failures surpasses that of other components in the drive system, highlighting its critical importance. Long-term FTC operation ensures the system retains its fundamental functions until safe repairs or replacements can be made. The focus of developing a FTC strategy has shifted from basic FTC operations to enhancing the post-fault quality to accommodate the realities of prolonged operation post-failure. This paper primarily investigates FTC strategies for inverter failures in various motor drive systems over the past decade. These strategies are categorized into three types based on post-fault operational quality: rescue, remedy, and reestablishment. The paper discusses each typical control strategy and its research focus, the strengths and weaknesses of various algorithms, and recent advancements in FTC. Finally, this review summarizes effective FTC techniques for inverter failures in motor drive systems and suggests directions for future research.

Abstract—The magnetic flux in a permanent magnet transverse flux generator (PMTFG) is three-dimensional (3D), therefore, its efficacy is evaluated using 3D magnetic field analysis. Although the 3D finite-element method (FEM) is highly accurate and reliable for machine simulation, it requires a long computation time, which is crucial when it is to be used in an iterative optimization process. Therefore, an alternative to 3D-FEM is required as a rapid and accurate analytical technique. This paper presents an analytical model for PMTFG analysis using winding function method. To obtain the air gap MMF distribution, the excitation magneto-motive force (MMF) and the turn function are determined based on certain assumptions. The magnetizing inductance, flux density, and back-electro-magneto-motive force of the winding are then determined. To assess the accuracy of the proposed method, the analytically calculated parameters of the generator are compared to those obtained by a 3D-FEM. The presented method requires significantly shorter computation time than the 3D-FEM with comparable accuracy.

Abstract—Increasing attention has been paid to the efficiency improvement of the induction traction system of high-speed trains due to the high demand for energy saving. In emergency self-propelled mode, however, the dc-link voltage and the traction power of the motor are significantly reduced, resulting in decreased traction efficiency due to the low load and low speed operations. Aiming to tackle this problem, a novel efficiency improved control method is introduced to the emergency mode of high-speed train traction system in this paper. In the proposed method, a total loss model of induction motor considering the behaviors of both iron and copper loss is established. An improved iterative algorithm with decreased computational burden is then introduced, resulting in a fast solving of the optimal flux reference for loss minimization at each control period. In addition, considering the parameter variation problem due to the low load and low speed operations, a parameter estimation method is integrated to improve the controller's robustness. The effectiveness of the proposed method on efficiency improvement at low voltage and low load conditions is demonstrated by simulated and experimental results.