Transmission and distribution (T&D) networks are a critical part of the power grid. As moving towards a smart-grid, it is essential to modernize the T&D networks and make it "Smart-grid ready". The ...concept of flexible ac transmission systems (FACTS) has been well-known for three decades. Rapid advancements in power electronics technology in the past decades have led to a new generation of FACTS devices. The Modern FACTS technology helps the transition of transmission networks to "smart". With increasing penetration of distributed generation, the distribution network is seeing unprecedented variation in terms of its fundamental operation and control, from renewable energy integration to microgrid, from active control of power quality, volt/var and frequency to self-healing and islanding operation. As a key part of smart-grid at the distribution level, we summarize the current efforts as a concept of resilient ac distribution systems (RACDS). The concepts of both FACTS and RACDS for a smart grid are introduced in this paper. Different configurations, key benefits, operating principles and world-wide installations of FACTS and RACDS devices are presented in detail. The ongoing and future direction of R&D leading to newer generations of FACTS and RACDS are also discussed.
Three-dimensional (3D) topological Dirac semimetals (TDSs) are a recently proposed state of quantum matter that have attracted increasing attention in physics and materials science. A 3D TDS is not ...only a bulk analogue of graphene; it also exhibits non-trivial topology in its electronic structure that shares similarities with topological insulators. Moreover, a TDS can potentially be driven into other exotic phases (such as Weyl semimetals, axion insulators and topological superconductors), making it a unique parent compound for the study of these states and the phase transitions between them. Here, by performing angle-resolved photoemission spectroscopy, we directly observe a pair of 3D Dirac fermions in Cd3As2, proving that it is a model 3D TDS. Compared with other 3D TDSs, for example, β-cristobalite BiO2 (ref. 3) and Na3Bi (refs 4, 5), Cd3As2 is stable and has much higher Fermi velocities. Furthermore, by in situ doping we have been able to tune its Fermi energy, making it a flexible platform for exploring exotic physical phenomena.
This paper analyzes the stability problem of the grid-connected voltage-source inverter (VSI) with LC filters, which demonstrates that the possible grid-impedance variations have a significant ...influence on the system stability when conventional proportional-integrator (PI) controller is used for grid current control. As the grid inductive impedance increases, the low-frequency gain and bandwidth of the PI controller have to be decreased to keep the system stable, thus degrading the tracking performance and disturbance rejection capability. To deal with this stability problem, an H∞ controller with explicit robustness in terms of grid-impedance variations is proposed to incorporate the desired tracking performance and the stability margin. By properly selecting the weighting functions, the synthesized H∞ controller exhibits high gains at the vicinity of the line frequency, similar to the traditional proportional-resonant controller; meanwhile, it has enough high-frequency attenuation to keep the control loop stable. An inner inverter-output-current loop with high bandwidth is also designed to get better disturbance rejection capability. The selection of weighting functions, inner inverter-output-current loop design, and system disturbance rejection capability are discussed in detail in this paper. Both simulation and experimental results of the proposed H∞ controller as well as the conventional PI controller are given and compared, which validates the performance of the proposed control scheme.
Common-mode voltage (CMV) caused by switching operations of power converters can significantly degrade the system's stability and performance. Since the effects of the CMV are more prominent in the ...wide-bandgap-based applications that require fast-switching slew rates, many research articles have been proposed to mitigate CM noise generated by the CMV. In addition, as topologies and control methods of the power converters become more complicated for better performance, more advanced CM noise attenuation methods are required accordingly. Therefore, this article proposes a CM noise attenuation method using a multilevel active CM noise power filter in a five-level inverter system. In this article, detailed control methods and considerations of the proposed multilevel system are presented, and the validity of the proposed multilevel active CM noise power filter has been verified through experimental results.
Traditional load regulation methods for a resonant converter mainly rely on frequency modulation. It is always a tradeoff between the design of the resonant network and the range of load. Especially ...for wireless power transfer (WPT) systems, the resonant network usually has a high quality factor. Small variation on frequency leads to huge drop in gain and efficiency. Due to this problem, many WPT systems are unregulated and they need one or two more front-end stages to regulate the dc bus voltage and perform power factor correction (PFC). In order to lower the cost and complexity of two- or three-stages structure, a single-stage solution with a silicon carbide (SiC) based Z-source resonant converter (ZSRC) was recently proposed. The Z-source network provides high reliability as being immune to shoot-through problems. Additionally, a ZSRC can boost the dc bus voltage while the traditional voltage-source inverter can only produce a lower voltage. However, the load regulation of this new topology has not been addressed. Two effective load regulation methods with constant frequency are presented for this SiC-based ZSRC specifically. Operation principle of the two load regulation methods are described in this paper. Experimental results based on a 200-W scale-down prototype with a full-bridge series resonant dc-dc converter are presented to illustrate the mechanism of these two methods.
Wireless power transfer (WPT) using only harmonic current for plug-in hybrid electric vehicle and electric vehicle is a fairly new concept. The frequency limitation of most high-voltage high-current ...IGBTs is around 20 kHz which also limits the system frequency of the transformer. The series resonant converter (SRC) is widely used in WPT application for its simplicity and high efficiency. Due to the bandpass filter characteristic of the resonant network, the SRC allows one frequency component to pass while the other frequency components are greatly attenuated. By deliberately selecting one harmonic current as the power carrier, the system frequency can be raised up to several times with the same switching frequency. Thus, a higher system frequency contributes to a more compact system. However, a high-order harmonic system results in high conduction loss on the switches, and it leads to a tradeoff between the system size and efficiency. This paper provides a theoretical and practical design for a system using harmonic current. Furthermore, a comprehensive comparison between different harmonic systems is carried out. A 1-kW SRC prototype with a 20-cm air gap is built to verify the proposed method.
H -bridge-based single-phase applications have second-order harmonics on the dc-link voltage. Conventionally, the second-order harmonics are filtered by bulky dc capacitors to maintain the constant ...dc-link voltage, which results in poor power density and low reliability. However, in applications, such as the solid-state variable capacitor (SSVC), the constant dc-link voltage is unnecessary since the dc link is disconnected from the load or source. This article proposes a dc capacitor-less SSVC, which removes the dc capacitors from the topology. The dc voltage is supported by the ac-side voltage directly. Only a 0.03 per unit (p.u.) capacitor is needed to filter the switching-frequency harmonics. This enables the proposed SSVC to use the snubber capacitors to achieve the function of the 0.03-p.u. filtering capacitor. Other capacitors can be removed from the dc-link busbar. The modulation, modeling, and control strategy of the proposed dc capacitor-less SSVC are provided in this article. The proposed dc capacitor-less SSVC is validated by the experiment.
This letter generalizes the fundamental limit theory in switched-capacitor (SC) converters. The fundamental limit theory proposed in 1995 sets the voltage-gain-ratio (VCR) boundaries for a specific ...number of k flying capacitors in the SC converter. Although this limit is valid for the positive VCR, another condition must be satisfied for the negative VCR. Moreover, the fundamental limit is only proved using single-input single-output SC converters, whereas multiple-input multiple-output (MIMO) SC converter implementation is desirable in some applications. Therefore, this letter extends the fundamental limit theory to include the negative VCR and the MIMO SC converter implementation.
Although gate-oxide degradation occurs in both silicon (Si) and silicon carbide (SiC) MOSFETs, it requires a special attention in SiC MOSFETs. This is because the gate oxide in SiC MOSFETs is ...comparatively thinner than the gate oxide in Si MOSFETs, and thus, a higher electric field that appears across it could push the gate oxide to its reliability limit. While several electrical parameters have been identified as precursors (indicators) for monitoring the gate-oxide degradation process in Si MOSFETs, very few have been identified for their SiC counterparts. The purpose of this article is twofold. The first objective is to demonstrate that the three gate-oxide degradation precursors identified for Si MOSFETs: 1) threshold voltage, 2) gate-plateau voltage, and 3) gate-plateau time can also be extended to SiC MOSFETs. The second objective is to demonstrate analytically and experimentally that all three precursors increase in a linear-with-log-stress-time manner during gate-oxide degradation in both planar and trench-gate SiC MOSFETs. The increasing trends of precursors and their associated logarithmic time responses were experimentally verified by inducing accelerated gate-oxide degradation in two different commercial SiC MOSFETs (650-V, 70-A trench-gate MOSFETs and 1200-V, 19-A planar MOSFETs) under high temperatures of 150 and 125 °C, respectively.
In this paper, three different inverters: conventional pulsewidth modulation (PWM) inverter, dc-dc boosted PWM inverter, and Z -source inverter were investigated and compared for fuel cell vehicle ...application. Total switching device power, passive components requirement, and constant power speed ratio of each of these inverters were calculated. For purposes of comparison, an example of the total switching device power, requirement of passive components, the constant power speed ratio, and the efficiencies of the different inverters for fuel cell vehicle powered by the same fuel cell were conducted. The comparisons show that the Z -source inverter is very promising in applications when the boost ratio is low (1-2).
