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The hybrid AC/DC admittance of interlinking converters operating under AC voltage control mode may contain right half-plane (RHP) pole(s), and thus, the existing harmonic stability analyses on grid-tied interlinking converter systems may become inapplicable as well. Aiming at addressing this deficiency, this letter modifies the hybrid AC/DC admittance definition and re-studies the harmonic stability of a grid-tied interlinking converter operating under AC voltage control mode. The results reveal that when the control mode of an interlinking converter changes from $P/Q$ control to AC voltage control, the system minor-loop gain utilized to assess the AC-side stability evolves to its inverse matrix. In contrast, the stability assessment related to the DC side (and the coupling interaction between the AC and DC sides) remains unchanged. Furthermore, the DC- and AC-side stabilities can also be regulated independently and are dominated by the $d$ -axis and the $q$ -axis controllers, respectively. [ABSTRACT FROM AUTHOR]

In this article, an optimized design for a dual-coupled inductor–capacitor–capacitor-series (LCC-S)-compensated wireless power transfer (WPT) system with zero-phase angle (ZPA) input condition and load-independent constant current (CC) output is presented. To this end, based on the misalignment characteristics of the reversed-series compensation coil structure, a novel design method for determination of the compensation parameters is proposed. By introducing a stability factor, any restriction on the variation correlation of the mutual inductances between the transmitter and receiver coils, and between the compensation and receiver coils within the range of misalignment tolerant design, is eliminated, thereby increasing the degree of freedom in the coil structure. According to a predetermined misalignment tolerant range, the proposed method can limit the fluctuation of the output current within a certain range and maintain the ZPA input condition. A 200-W/85.3-kHz prototype based on the proposed method is implemented and its performance is experimentally validated. The system can operate in ZPA input and load-independent CC output with a misalignment tolerance factor of 4, as the load varies from 5 to 20 $\Omega$. [ABSTRACT FROM AUTHOR]

High-fidelity electromagnetic transient (EMT) simulation plays a critical role in understanding the dynamic behavior and fast transients involved in operation, control, and protection of multiterminal dc (MTdc) grids. This article proposes a cost-effective high-performance real-time EMT simulation platform for large-scale cross-continental MTdc grids based on graphics processing unit (GPU). Fast dynamic transients from both ac and dc networks are captured in real time with 5 μs time step, using advanced hybrid-discretized modular multilevel converter model, frequency-dependent transmission line model, and EMT-type model of synchronous generators. The proposed simulation platform i) assembles detailed EMT models of all components within an MTdc–ac grid into a single platform. This setup provides a complete simulation solution to capture fast transient signals required for high-bandwidth controller design and protection studies without any compromise; ii) implements the first GPU-based simulation architecture and corresponding algorithms for MTdc–ac grids with real-time performance at scales of 1s; iii) is highly efficient and balances the high utilization of GPU resources and low latency required for the simulation; and iv) outperforms the existing central processing unit- or digital signal processor (DSP)/field-programmable gate array-based simulators in terms of its higher scalability on large-scale MTdc–ac grids and superior price–performance ratio on the hardware. Accuracy and performance of the proposed platform are evaluated with respect to the reference results from power system computer aided design (PSCAD)/EMTdc environment. [ABSTRACT FROM AUTHOR]

Space vector modulation (SVM) is a mature modulation scheme, which is widely used for voltage source inverters. In this article, a flexible online SVM scheme is proposed for a cascaded H-bridge multilevel inverter (CHMI) under short-circuit and open-circuit switch as well as cell failure conditions. The proposed scheme directly calculates all the active redundant switching states and the dwelling duty cycles of the three space vectors enclosing the reference vector for a CHMI such that balanced line-to-line voltages and currents under failure conditions are generated. Accordingly, the resultant switching sequences are determined to meet any specific requirement, e.g., even-order harmonics elimination and minimization of switching losses. Furthermore, in order to reduce voltage stress on the semiconductors under short- and open-circuit switch conditions, an equal power distribution criterion is introduced. The proposed SVM scheme is fast and independent of the number of levels. Experimental results are presented to demonstrate performance and effectiveness of the proposed scheme. [ABSTRACT FROM AUTHOR]

Varistors are widely used in dc circuit breaker (DCCB) application as fault energy absorption elements, which is not an application the varistors are originally designed for. The fault energy stress adversely impacts the varistor lifetime, thus affecting DCCB reliability and performance. This article presents two methods for choosing the appropriate varistor type and arrangement in a DCCB by linking 1) varistor lifetime and 2) DCCB component- and system-level parameters. The first method uses a set-based design approach with random Latin hypercube sampling to narrow down viable solutions. In comparison, the second method relies on a point-based design approach and proposes an integer programming problem to optimize varistor lifetime. Both procedures are applied to the selection of varistors in 1) a 12 kV DCCB rated for 2-kA nominal current and a peak fault current of 15 kA and 2) a reduced-scale 1.2-kV DCCB hardware prototype, using publicly available varistor data. The article shows the possibility of design for varistor lifetime in DCCB applications, and concludes that both selection procedures provide consistent, reasonable, and verifiable results. However, the results also pinpoint the significant gaps and call for future work on impulse stress versus lifetime relationships for DCCB varistors. [ABSTRACT FROM AUTHOR]

This article proposes a multiport modular single-stage current-source solid-state transformer (SST) for applications like photovoltaic, energy storage integration, electric vehicle fast charging, data center, etc. The 7.2 kV 50 kVA current-source SST consists of five input-series output-parallel modules, each based on 3.3 kV SiC reverse-blocking mosfet-plus-diode modules. The proposed SST has some unique features. First, compared with the voltage-source or matrix converter-based SSTs, the current-source SST has a unique advantage of single-stage isolated ac/dc or ac/ac conversion with an inductive dc link, but no medium-voltage (MV) ac experiments have been reported. This article for the first time demonstrates MV ac current-source SST up to 7.5 kV peak. Second, the multiport SST has a buffer port for active power decoupling (APD) or energy storage integration. The double-line-frequency power ripple from single-phase ac grid normally results in a large capacitor size in MV SSTs. The APD scheme is proposed in MV applications for the first time to enable a reduced dc link and the electrolytic capacitor-less SST with high reliability. Third, for a direct grid-connected converter without line-frequency transformer, insulation and protection are critical. A medium-frequency transformer design passes 55 kV basic-insulation level (BIL) and 60 kV high potential dielectrics withstand test with only 0.09% leakage inductance. Importantly, a lightning protection scheme is presented to protect the SST itself from 90 kV BIL impulse. Fourth, the proposed current-source SST topology is a modular soft-switching solid-state transformer (M-S4T) with full-range zero-voltage switching and controlled dv/dt for low electromagnetic interference. These concepts are verified in a three-port M-S4T prototype with forced oil cooling under single-module, stacked-module, steady-state, and dynamic operations. [ABSTRACT FROM AUTHOR]

Growing concerns with the harmonic oscillation phenomenon of multiterminal DC (MTDC) systems calls for appropriate harmonic stability analysis methods. However, the conventional ac or dc admittance-based stability assessment methods cannot effectively protect trade secret and user privacy, distinguish different system instability root causes, and put forward corresponding instability mitigation methods. To address the aforementioned issues, this article first establishes a hybrid ac/dc admittance-based stability assessment model for a MTDC system. Then, by introducing the determinant-based General Nyquist Criterion (GNC), the system stability is assessed and three types of instability root causes, including the ac side, the dc side, and the coupling interaction between the ac and dc sides are identified. Regarding the instability caused by the ac side, the dominant interlinking converter is directly identified, in which the instability can be mitigated by reshaping its q-axis admittance. In contrast, for the instability caused by the latter two root causes, the most influential interlinking converter is recognized by an additional sensitivity analysis. The instability is then mitigated by reshaping the dc-side equivalent admittance of the most influential interlinking converter. Compared with the conventional methods, the proposed method offers advantages as follows: 1) trade secret and user privacy is effectively protected, 2) different system instability root causes can be effectively distinguished, and 3) instabilities caused by diverse root causes can be mitigated by corresponding methods. Finally, the proposed method is verified by simulations in the MATLAB/Simulink environment. [ABSTRACT FROM AUTHOR]

Medium-voltage dc (MVdc) grids are attractive for electric aircraft and ship power systems, battery energy storage system (BESS), fast charging electric vehicle (EV), etc. Such EV or BESS applications need isolated bidirectional MVdc to low-voltage dc (LVdc) or LVac converters. However, the existing Si-based solutions cannot fulfill the requirements of a high-efficiency and robust converter for MVdc grids. This article presents a 5-kV SiC-based universal modular solid-state transformer (SST). This universal current-source SST can interface either an LVac or LVdc grid with an MVdc grid in single-stage power conversion, while the conventional dual-active bridge (DAB) converter needs an additional inverter. The proposed SST module using 3.3-kV SiC MOSFETs and diodes is bidirectional and can serve as a building block in series or parallel for higher voltage higher power systems. The topology of each module is based on the soft-switching solid-state transformer (S4T) with reduced conduction loss, which features reduced electromagnetic interference electromagnetic interference (EMI) through controlled dv/dt, and high efficiency with full-range zero-voltage switching for main devices and zero-current switching for auxiliary devices. Operation principle of the modular S4T (M-S4T), capacitor voltage balancing control between the cascaded modules, design of components including a medium-voltage (MV) medium-frequency transformer (MFT) to realize a 50-kVA, 5-kV dc to 600 V dc or 480 V ac M-S4T are presented. Importantly, the MV MFT prototype achieves very low leakage inductance (0.13%) and 15-kV insulation with coaxial cables and nanocrystalline cores. The proposed universal modular SST is compared against the DAB solution and verified with dc–dc and dc–ac simulation and 4-kV experimental results. Significantly, the MV experimental results of a modular dc transformer with each module at MVdc are rarely covered in the literature and reported for the first time. [ABSTRACT FROM AUTHOR]