Guorui Xu

In large-capacity air-cooled synchronous condensers (ACSCs) operating under deep leading-phase conditions, significant end leakage flux induces substantial losses and heating in the stator end components. Effective electromagnetic shielding is therefore a critical design consideration and a novel shielding configuration utilizing magnetic-electrical conductive alloys is proposed in this article. Using a 300 MVar ACSC as a case study, the three-dimensional transient electromagnetic field model of the end region is first established. The effects of four configurations (copper shield+magnetic shield, Fe-16%Cu alloy shield+magnetic shield, single Fe-20%Cu alloy shield, and single magnetic shield) on the leakage flux density and losses in end components are analyzed. Subsequently, a global ventilation network and a fluid-thermal coupling model for the end region are developed for each configuration. The results from network and aforementioned losses serve as boundary conditions for the fluid-thermal coupling model. The flow fields and the temperature distributions under different configurations are then determined. The findings demonstrate that each shielding configuration exhibits specific advantages and limitations. Experimental validation based on traditional scheme confirms the accuracy of the model and methodology. The simulation results elucidate the significant potential of the Fe-Cu alloy. The single Fe-20%Cu configuration reduces the hot-spot temperature of shield, and significantly lowers weight and cost while effectively protecting the end iron core.

The reactive power absorption capacity of synchronous condensers (SC) plays a crucial role in suppressing transient overvoltage and enhancing the voltage stability of new energy power grids. To enhance the reactive power absorption capacity of the SC, the reactive power characteristics and stable operation capability under loss-of-excitation condition are analyzed and it is obtained that the air-gap length is a critical factor. The reactive power absorption of the SC is increased as the increase of the air-gap length, however, the temperature rise in the end region can restrict its reactive power absorption capability. A three-dimensional magnetic-fluid-thermal model in the end region of the SC is established to investigate the loss and temperature in the stator end structural components under different air-gap lengths. The temperature distribution in the stator end region is analyzed and the variation of the maximum temperature with the increase of the air-gap length is revealed. the maximum reactive power absorption capacity of the SC is obtained and the maximum air-gap length is determined. The experimental results on two 300-MVar SC with different air-gap length validate the accuracy of the calculations. This research provides a theoretical foundation and technical support for enhancing the reactive power absorption capacity of SC.

A gravity energy storage system (GESS) typically adopts the synchronous generator-motor (SGM) that is directly connected to the grid, in order to provide inertia and reactive power support to power systems with high penetration of renewable energy. However, due to the lack of line-starting capability, it is a critical challenge for a GESS to meet the frequent start-stop requirements in practice. To solve this problem, an SGM starting method based on mass block (MB) dragging is proposed in this study, for a chain-rail-based slope GESS (SGESS) as example. Through the analysis of the loss components of GESS, an optimal mass determination method for the starting MB is derived. The MB-dragging-based starting of 1-kW and 500-kW chain-rail-based SGESSs are then simulated and analyzed, and the dynamic grid-connection characteristics of the SGM started by different starting MBs are compared. Finally, the MB-dragging-based starting of the 1-kW chain-rail-based SGESS prototype is tested for validation, and an experiment bench of SGM-grid connection is established to emulate the starting and grid-connection processes and evaluate their impacts on the grid. The experiment results demonstrate that the proposed method suppresses the inrush current successfully and enables smooth starting of SGM.

This article investigates the design considerations of air-cooled synchronous condensers (ACSCs) in new energy power systems and proposes a many-objective optimization method combining ensemble surrogate model (ESM) with nondominated sorting genetic algorithm (NSGA)-III. The key lies in designing and optimizing the ACSC from the perspectives of electromagnetic and thermal fields to simultaneously achieve satisfactory transient parameters and thermal behaviors. First, an equivalent model considering eddy current of the ACSC is established, and the direct-axis reactance is determined using the phased amendment method. Second, a coupled model integrating the global ventilation network and local fluid-thermal field of the ACSC is developed. The results from the former were applied as boundary conditions for the latter. Subsequently, the effects of critical geometric sizes on reactance, time constants, and winding temperatures are analyzed, revealing conflicting relationships among key parameters. To rapidly and accurately determine optimal dimensions, an ESM and NSGA-III-based many-objective optimization framework is proposed and implemented, achieving a balanced compromise among optimization objectives. Finally, experimental testing on a 300 MVar ACSC validates the effectiveness of the models and optimization method.

Typical faults such as open circuit (OC), short circuit, and abnormal coupling seriously threaten the safety of conventional wireless power transfer (WPT) systems, so fault tolerance is a critical design consideration for WPT systems, especially in high-power applications. This study proposes an S/S and LCC/S-based input-series output-parallel hybrid topology, with which WPT systems can tolerate typical faults and realize zero phase angle under normal operation. Considering common misalignment conditions, a receiver-side control strategy without dual-side communication is also designed for minimum fault current ratios and power sharing between the two subtopologies under normal operation, and a system design method for constant power output is then given. A WPT prototype with the proposed hybrid topology is developed, and the experimental validation is performed. The results show that the steady-state fault current is about two times the normal value under various faults, and the system can even operate with an abnormal close coupling or OC fault at the transmitting coil branch in the LCC/S subtopology.

Nanocrystalline alloy is renowned for its excellent magnetic properties and regarded as substantial potential material for the cores of large-capacity high-frequency transformers (HFTs). At present, there are several typical core shapes being widely adopted considering the manufacturing process. However, the influences of the various core shapes on the optimal design and performance improvement of HFTs are still a challenge. This study investigates the loss and temperature distribution characteristics of the HFTs with three typical nanocrystalline core shapes. The 20-kVA HFTs with U-shaped, U-shell and triangular cores are designed, and then the transient coupling magnetic-thermal model is established for the loss and thermal analysis under different load conditions. The HFT prototypes with these core shapes are manufactured and experimental validation are also carried out. The results show that the HFT prototype with triangular core has the maximum core loss under the no-load condition, but the copper loss of the HFT with U-shaped core is slightly greater than those of the other two HFT prototypes under the short-circuit condition. As for the temperature distribution, under the full-load condition, the highest temperatures on the surfaces of the prototypes with U-shaped, U-shell and triangular cores are 108.0°C, 135.4°C and 106.5°C, respectively. Moreover, the maximum temperature differences on the surfaces of the three HFTs are approximately 55°C, 85°C and 30°C, respectively.

An AC-Excited Generator-Motor (ACEGM) is widely used in the variable speed pumped storage power station due to its flexible and powerful power regulation capability. However, the power regulation range can be restricted by the hotspot temperature in the end region of the ACEGM. In order to study the temperature distribution laws of the stator and rotor end structural components, a three-dimensional electromagnetic- fluid-thermal coupling model in the end region of the ACEGM is established. The flux densities, losses and temperatures in the end core, tooth plate, stator clamping plate and rotor retaining ring are studied under the different operating conditions. Through thermal analysis, the hotspot location in the stator and rotor end regions has been pinpointed. The hotspot temperature variations in the stator and rotor end regions along with the changes of the active and reactive powers are studied. The power regulation range of the ACEGM is determined by the thermal limits of the stator and rotor end components. The study can provide a theoretical foundation for optimizing the stator and rotor end-region structures and enhancing the power regulation capability of ACEGMs.

Doubly-Fed Induction Machines (DFIMs) are widely used in wind turbines and variable-speed hydropower plants due to their ability of flexible adjustment, however, the electromagnetic vibrations caused by the high-frequency harmonics from the converter at the rotor side still challenge their applications in industry. This study investigates the electromagnetic force (EMF) characteristics of DFIM considering the harmonics from the rotor-side converters. With the slotting and saturation effects considered, an analytical model is proposed to analyze the characteristics of the harmonic fields and the related EMFs, such as the spatial order, frequency and magnitude. The influence of these EMFs on the vibration of DFIM is also predicted. The harmonic fields and the vibration responses are analyzed by Finite Element Method (FEM) for a 30-kW, 6-pole DFIM. The conventional EMFs without the rotor-side pulse width modulation (PWM) supply and the additional EMFs caused by this supply are compared under different operating conditions to validate the proposed analytical model. The analysis indicates that the radial EMF of the 6th order in 3900 Hz and 8100 Hz significantly increases and the corresponding vibration increases by about 10%, which is attributed to the high-frequency harmonics from the rotor-side converter. A prototype of the 30-kW, 6-pole DFIM is manufactured and tested for the vibration spectrums, and the results predicted by the proposed analytical model agree well with the measured results.

The Dual-Excited Synchronous Generator (DESG) has two sets of field windings on the rotor, thus it has better stability and reliability than the traditional synchronous generator. However, the transient parameters of the DESG are different from those of the traditional synchronous generator due to the different rotor structure of the two generators. In this paper, a three-dimensional electromagnetic field model in the end region of a 300-MW DESG is established and the magnetic density and energy density in the end region of DESG are calculated and compared under the different operating conditions. The stator and rotor end leakage reactances are calculated by the energy method. Under the different operating conditions, the end leakage reactance of q-axis field winding varies by 11%, which is the largest compared to those of the d-axis field winding and stator winding. Based on that, the end leakage reactances are substituted into the two-dimensional field-circuit coupled time-stepping finite element model (FCCTSFEM), which is used to calculate the frequency characteristic of the DESG. The transient and sub-transient parameters of the DESG are calculated by combining the FCCTSFEM and the frequency domain method. The influence of the slot wedge material on the transient and sub-transient parameters are studied. Along with the increase of the rotor slot wedge conductivity, the transient and sub-transient reactances are reduced and the transient and sub-transient time constants are increased. The results can provide a theoretical basis for the parameter calculation of the DESG.