• Energy Research

Many high power converter systems are often connected to the medium voltage network using a step-up transformer. In such systems, the converter-side windings of the transformer can be configured as an open-end and multi-level voltage waveforms can be achieved by feeding these open-end windings from both ends using the two-level dual-converter. An $LCL$ filter with separate converter-side inductors for each of the converter is commonly used to attenuate the undesirable harmonic frequency components in the grid current. The magnetic integration of the converter-side inductors is presented in this paper, where the flux in the common part of the magnetic core is completely canceled out. As a result, the size of the magnetic component can be significantly reduced. A multi-objective design optimization is presented, where the energy loss and the volume are optimized. The optimization process takes into account the yearly load profile and the energy loss is minimized, rather than minimizing the losses at a specific operating point. The size reduction achieved by the proposed inductor is demonstrated through a comparative evaluation. Finally, the analysis is supported through simulations and experimental results.

Under the development of information and communication technologies, this paper discusses a cloud-based user-side integrated energy management to improve the energy utilization efficiency in a smart community, for which a two-level model is proposed that relies on an energy hub and load aggregators. Furthermore, to address the issue of preserving confidentiality for the cloud service, an information-masking mechanism is designed based on linear mapping functions, whose information-masking requirements and processes are specified accordingly. Numerical results demonstrate the effectiveness of the proposed model and of the method for cloud computing.

As a key part in the wind turbine system, the power electronic converter is proven to have high failure rates. At the same time, the failure of the wind power converter is becoming more unacceptable because of the quick growth in capacity, remote locations to reach, and strong impact to the power grid. As a result, the correct assessment of reliable performance for power electronics is a crucial and emerging need; the assessment is essential for design improvement, as well as for the extension of converter lifetime and reduction of energy cost. Unfortunately, there still exists a lack of suitable physic-of-failure based evaluation tools for a reliability assessment in power electronics. In this paper, an advanced tool structure which can acquire various reliability metrics of wind power converter is proposed. The tool is based on failure mechanisms in critical components of the system and mission profiles in wind turbines. Potential methodologies, challenges, and technology trends involved in this tool structure are also discussed. Finally, a simplified version of the tool is demonstrated on a wind power converter based on Double Fed Induction Generator system. With the proposed tool structure, more detailed information of reliability performances in a wind power converter can be obtained before the converter can actually fail in the field and many potential research topics can also be initiated.

This paper presents a novel online technique, which simultaneously exploits the global search ability of differential evolution (DE) and rapid convergence of Newton Raphson (NR) methods (named as DE-NR) to solve intricate simultaneous transcendental trigonometric set of harmonic elimination pulse width modulation equations for modular multilevel cascade converters based power systems. Major contribution of this paper is a harmonically efficient online algorithm with rapid convergence. Switching angles for an extensive range of modulation index (M,0.86⩽M≤9.97) are computed online to demonstrate the harmonically efficient working of proposed method. Total harmonic distortion values of the filtered line-to-line output voltages during online operation are restricted to 0.35% of the fundamental component, which depict successful removal of unwanted harmonics and are significantly better than the values allowed by IEEE standard 519-2014. Comparison between DE-NR, differential evolution and Newton Raphson methods demonstrates the rapid convergence behavior of DE-NR as it requires only (on average) 4 iterations compared to 490 and 182 iterations respectively. It has also shown superior harmonic control than the recently devised online Middle-Level Selective Harmonic Elimination Pulse-Amplitude Modulation method. Successful simulation and experimental validations of DE-NR method have been done by developing three-phase modular multilevel cascade converter in MATLAB-Simulink and single-phase eleven-level modular multilevel cascade converter hardware prototype.

This article presents low-frequency ripple power steering control strategies for a power conversion topology that comprises a high frequency linked three-port converter (HFL-TPC) and a voltage source inverter (VSI) for PV-BESS/EV applications. The proposed topology has three interfaces, which can be connected with a photovoltaic (PV) port, battery energy storage systems (BESS) and/or electric vehicles (EVs) port, and a four-quadrant ac port. The proposed topology inherently exhibits double line frequency (DLF) power fluctuation that needs to be decoupled and steered away from the PV port. In this article, we have developed and demonstrated two control strategies to deal with the DLF ripple. The first control strategy utilizes loop gain of the closed loop control system to decouple the DLF power, however, a residual DLF power still exists at the ac port. To completely decouple the DLF fluctuations, an improved sequence-based control strategy is proposed where the DLF voltage components are converted into positive and negative sequence components and controlled separately on their respective synchronous reference frames. To verify the practical feasibility of the proposed low-frequency ripple power steering control strategies, a 40 V, 400 W HFL-TPC is built and tested. Experimental results show that the proposed methods decouple the DLF power fluctuation and selectively steer it to the BESS/EV port. Comparative results reveal that the sequence-based control strategy shows a higher degree of DLF ripple suppression.

Abstract Along with the increasing penetration of distributed generation with voltage-source converters (VSCs), there are extensive concerns over the potential virtual rotor angle stability, which is characterized by oscillations of power and frequency during the dynamic process of synchronization in the grid. Several control strategies have been developed for VSCs to emulate rotating inertia as well as damping of oscillations. This paper classifies these strategies and provides a small-signal modeling framework including all kinds of VSCs in different applications for virtual rotor angle stability. A unified perspective based on the famous Phillips–Heffron model is established for various VSCs. Thus, the concepts of equivalent inertia and the synchronizing and damping coefficients in different VSCs are highlighted, based on the similarities with the synchronous generator (SG) system in both physical mechanisms and mathematical models. It revealed the potentiality of various VSCs to achieve equivalence with the SG. This study helps promote the unity of VSCs and traditional SGs in both theories and methods for analyzing the dynamic behavior and enhancing the stability. Finally, future research needs and new perspectives are addressed.

Distribution systems can form islands when faults occur. Each island represents a subsection with variable boundaries subject to the location of fault(s) in the system. A subsection with variable boundaries is referred to as an island in this paper. For operation in autonomous mode, it is imperative to detect the island configurations and stabilize these subsections. This paper presents a novel scheme for the detection of island boundaries and stabilizing the system during autonomous operation. In the first stage, a boundary detection method is proposed to detect the configuration of the island. In the second stage, a dynamic voltage sensitivity factor (DVSF) is proposed to assess the dynamic performance of the system. In the third stage, a wide area load shedding program is adopted based on DVSF to shed the load in weak bus-bars and stabilize the system. The proposed scheme is validated and tested on a generic 18-bus system using a combination of EMTDC/PSCAD and MATLAB software’s.