Abstracts:To enhance the frequency support capability of the doubly fed induction generator (DFIG) system in microgrid while improving the control effect, a double-layer model predictive control (MPC)-based grid-forming control of DFIG (GFM-DFIG) is proposed for primary frequency regulation (PFR). Firstly, a top-layer MPC is designed to enhance the PFR capability by optimizing control coefficients and power commands of the virtual synchronous generator (VSG) controller. This layer utilizes the predicted frequency deterioration trend as reference to adaptively regulate control coefficients in both grid-connected and islanded mode. Then, the quadratic optimization algorithm is employed to calculate the additional power that can suppress frequency degradation in modifying power commands combined with the system frequency response (SFR) model. Meanwhile, a damping ratio constraint mechanism for VSG controller is designed to avoid the impacts of coefficient adjustment. Secondly, a finite-set min-beat tracking-based bottom layer MPC combined with a VSG inner electrical circuit reconstruction is designed to improve the control effect and dynamic response of DFIG's stator voltage and current. The top-layer and bottom-layer together can enhance the overall control effect and frequency support capability of the DFIG system. Finally, the effectiveness and advantages of the proposed GFM-DFIG is displayed in a simulation system.

Abstracts:The increasing penetration of inverter-based resources (IBRs) necessitates a reassessment of current grid practices. Blackstart is one specialized area that has drawn renewed attention due to the capability of grid-forming inverters (GFMIs) to emulate synchronous generators (SG). The ability of IBRs to carry out a blackstart represents a novel concept necessitating deeper investigations, particularly given the constraints of IBRs such as over-current limitations, which affect the energization of non-linear network devices. This paper presents a blackstart case study that uses a GFMI, focusing on the energization of a large unloaded plant transformer. Consequently, a unique inrush is demonstrated, known as a sympathetic inrush between the plant and the inverter medium-voltage (MV) transformer. Subsequently, an alternative method for the execution of soft energization (SE) is introduced using a power plant controller (PPC), eliminating inverter-level control modifications. Furthermore, the impact of employing the SE method on inrush currents is evaluated using the proposed method, and a qualitative analysis is lastly presented comparing SE to the hard switching (HS) blackstart method. Collectively, this paper aims to demonstrate the GFMI's ability, using both the SE and HS methods, to energize transformers which play an important role during a blackstart.

Abstracts:The substantial integration of renewable energy sources, specifically photovoltaic (PV) power into the power grid, has gradually weakened its strength. A novel switching control for a PV storage system with a GFL/GFM control structure was proposed in response to this challenge. By leveraging integrators and the state follower method, a smooth switching control strategy between these two control modes was facilitated, ensuring stable operation across varying grid strengths. Through employing the Kuramoto model and basin stability method, the parameter stability domain of the microgrid under GFM control was delineated, and the switching boundary was established based on the short-circuit ratio. Additionally, recognizing the requirements of stable operation for PV storage systems under grid faults, a fault ride through control method with a negative sequence current suppression strategy was proposed. This method effectively suppressed negative sequence current under GFM control. Subsequently, simulation-based validation confirmed the effectiveness of the proposed control strategies. It ensured the smooth operation of the PV storage system under a gamut of conditions, including symmetric and asymmetric faults, as well as islanding scenarios. The proposed control strategies ensure that the DC bus remains stable and that the current distortion rate does not exceed 5% during faults.

Abstracts:This paper proposes an energy function-based direct method for large-signal stability assessment of grid-forming (GFM) inverters leveraging an equivalent-circuit representation of all involved control- and physical-layer dynamics. Three different primary controls, a standard inner-current outer-voltage cascaded-control architecture, output $LCL$ filter, and reference-current saturation limiting are featured in the modeling and analysis framework. A composite energy function for the GFM inverter is obtained by summing up individual energy contributions gleaned from the circuit representation. The approach can readily be generalized to different primary controls, output-filter arrangements, and current limiters since it is based on a circuit-theoretic foundation. Numerical simulations validate the efficacy of the approach in estimating the critical clearing time following a large-signal disturbance.

Abstracts:In [1], a duplicate of Fig. 24 incorrectly appears as Fig. 23. The correct Fig. 23 follows. Fig. 23. Experimental verification of LGU on UL at 0.1 m/s.

Abstracts:The direct-current (DC) excitation source flux regulation hybrid excitation motor (FRHEM) is proposed to solve the problem of difficult flux regulation of permanent magnet synchronous motor (PMSM). However, the problem of output torque drop of FRHEM at the flux-weakening state has not been solved. Therefore, the alternating-current (AC) excitation source flux-torque regulation hybrid excitation motor (FTRHEM) is studied and the problem of low output torque at the flux-weakening state is solved. To clarify the influence of different excitation sources on the axial-radial hybrid excitation motor, combined with the development process of the hybrid excitation motors (HEMs), the electromagnetic performance of PMSM, FRHEM, and FTRHEM is compared and analyzed in this paper. Firstly, the topology of the three motors, flux regulation principle, and torque generation mechanism of different topologies are dynamically compared. Secondly, the comprehensive electromagnetic performances are analyzed by the three-dimensional finite-element method (3D-FEM), which includes the magnetic flux regulation performance and torque performance. On this basis, the speed-torque curve of the three motors is obtained and the advantages and disadvantages of different motors are given. Finally, the FTRHEM prototype is fabricated and tested, which verifies the results of 3D-FEM and gives the conclusion.

Abstracts:This paper proposes a short-circuit current (SCC) reduction method for dual three-phase (DTP) permanent-magnet machines by asymmetric pitch technique. The conventional DTP winding configuration with symmetric pitch is used as a benchmark. The above two windings are named as asymmetric and symmetric configurations, respectively. Firstly, the asymmetric configuration with physical isolation is introduced and distinguished based on the spatial distribution. Secondly, the stator magnetomotive forces generated by different configurations are analyzed, and the effects of asymmetric configuration on harmonic order and content are studied with emphasis. Afterwards, the SCC and braking torque of different configurations are analyzed and compared under fault condition. The asymmetric configuration has great significance to reduce the SCC and braking torque. Moreover, it exhibits comparable torque ripple compared to the symmetric counterpart, although with a reduction slightly in average torque. Finally, the 48-slot/8-pole DTP permanent-magnet machines with different configurations are manufactured. The experiments are conducted and compared to validate theoretical analysis and reliability improvement design.

Abstracts:The diagnosis of inter-turn short circuit (ITSC) fault in permanent magnet synchronous machine (PMSM) servo drive has been extensively studied. However, its application in low-cost drive systems without a position sensor under varying speed conditions requires further investigation. The 2nd harmonics in dq-axis signals are widely employed for diagnosis in servo drive, but they are inaccessible without a position sensor. Additionally, extracting fault features under variable speed conditions remains challenging without instantaneous frequency information. This paper proposes a frequency mapping method based on maximum harmonic amplitude extraction in a time-frequency plane to obtain the instantaneous phase (IP) from two-phase currents. A pseudo synchronous coordinate is established to integrate both the amplitude and frequency fault information of the phase currents using the extracted IP. Subsequently, the order tracking method is applied to signals in the proposed pseudo coordinate to extract the fault feature. Experimental validation on a PMSM demonstrates the method's effectiveness in ITSC fault detection under variable speed conditions without requiring speed signals.

Abstracts:High-purity aluminum (Al) Litz wires have become increasingly attractive as conductors in cryogenic electrical machines (CEMs) and “passives” such as chokes and transformers for power electronics due to their light weight, high thermal conductivity, and potential to reduce both DC and AC (eddy) losses at cryo-temperatures. However, these losses are interrelated: a decrease in one results in an increase in the other with changing cryo-temperatures. Thus, defining optimal operating conditions as a function of Al Litz wire parameters is essential to minimize overall losses, a domain yet unexplored. This study investigated the DC and AC losses of two bespoke Al Litz coils at diverse cryo-temperatures and frequencies based on numerical modeling and experiments, elucidating the prerequisite for achieving minimal loss. New analytical equations were formulated to calculate the minimum total loss and minimum-loss resistivities. The minimum-loss resistivities were used to quantify optimal operating temperatures and residual-resistance ratio (RRR) values for distinct Al Litz wire diameters. The research reveals that relentlessly pursuing ultra-low temperatures or high-purity conductors does not consistently contribute to a high efficiency in CEMs. This work enhances comprehension of electromagnetic losses in Litz wires and serves as a pivotal reference for cryogenic electric component design.