Abstract:This special section aims at addressing the way power electronics will help facing the big challenges arising in this upcoming paradigm, in terms of both power infrastructure (grid-related and converters architectures) and “intelligence” (supervision and control of AC-DC transmission and distribution systems), in order to keep current standards of efficiency and reliability. The call for papers invited original submissions dealing with a diversity of applications of power electronics in future power systems, with the emphasis placed preferably on the system-wide perspective. Out of a pool of 175 initial submissions, eleven papers coauthored by researchers from six different countries have successfully passed the double-blind, peer-review process and been finally accepted for publication. Taxonomically, the eleven papers accepted for this special section can be broadly classified in four groups: 1) Review papers; 2) Converter topologies; 3) Analysis and modeling issues; 4) Converter control schemes. To summarize, besides two review papers, this special section comprises two papers proposing novel converter topologies and a total of seven papers dealing with different power system applications. Those applications include fault detection and protection of HVDC systems, stability enhancement and control of grid-connected converters, including DC microgrids, power quality improvement and efficient operation of wind generators.

Abstract:The main objective of this paper is three-fold. First, to provide an overview of the current status of the power electronics technology, one of the key actors in the upcoming smart grid paradigm enabling maximum power throughputs and near-instantaneous control of voltages and currents in all links of the power system chain. Second, to provide a bridge between the power systems and the power electronic communities, in terms of their differing appreciation of how these devices perform when connected to the power grid. Third, to discuss on the role that the power electronics technology will play in supporting the aims and objectives of future decarbonized power systems. This paper merges the equipment, control techniques and methods used in flexible alternating current transmission systems (FACTS) and high voltage direct transmission (HVDC) equipment to enable a single, coherent approach to address a specific power system problem, using ‘best of breed’ solutions bearing in mind technical, economic and environmental issues.

Abstract:A few multi-terminal direct current (MTDC) systems are in operation around the world today. However, MTDC grids overlaying their AC counterpart might a reality in a near future. The main drivers for constructing such direct current grids are the large-scale integration of remote renewable energy resources into the existing alternative current (AC) grids, and the promotion and development of international energy markets through the so-called supergrids. This paper presents the most critical challenges and prospects for such emerging MTDC grids, along with a foreseeable technology development roadmap, with a particular focus on crucial control and operational issues that are associated with MTDC systems and grids.

Abstract:This paper proposes a novel hybrid multilevel converter with DC fault-blocking capability, i.e., the neutral-point clamped hybrid multilevel converter (NHMC). By employing two types of unipolar full-bridge submodules along with director switches, which are composed of series-connected insulated-gate bipolar transistors, the NHMC combines the features and advantages of the neutral-point clamped converter and the modular multilevel converter. The basic topology, operating principles, modulation scheme, and energy-balancing scheme of the NHMC are presented. The DC fault-blocking capability of the NHMC is investigated. The number of power electronic devices used by the NHMC is calculated and compared with other multilevel converters, showing that the proposed NHMC can be an economical and feasible option for medium-voltage DC transmission with overhead lines. Simulation results demonstrate the features and operating scheme of the proposed NHMC.

Abstract:Aiming to integrate the respective merits of the switched-capacitor converter and the quasi-Z-source converter. An novel high step-up quasi-Z-source DC–DC converter with a single switched-capacitor branch is proposed. Compared to other high boost DC–DC converters, the proposed converter can provide higher output voltage gain, lower current stress across the switches, and lower voltage stress across the output diodes by using the same or similar passive and active components. Therefore, the efficiency and reliability of the converter can be improved. The topological derivation, operating principle, parameter selection, and comparison with other DC–DC converters are presented. Finally, both simulations and experimental results are given to verify the characteristics of the proposed converter.

Abstract:This paper proposes a frequency domain based methodology to analyse the influence of High Voltage Direct Current (HVDC) configurations and system parameters on the travelling wave behaviour during a DC fault. The method allows us to gain deeper understanding of these influencing parameters. In the literature, the majority of DC protection algorithms essentially use the first travelling waves initiated by a DC fault for fault discrimination due to the stringent time constraint in DC grid protection. However, most protection algorithms up to now have been designed based on extensive time domain simulations using one specific test system. Therefore, general applicability or adaptability to different configurations and system changes is not by default ensured, and it is difficult to gain in-depth understanding of the influencing parameters through time domain simulations. In order to analyse the first travelling wave for meshed HVDC grids, voltage and current wave transfer functions with respect to the incident voltage wave are derived adopting Laplace domain based component models. The step responses obtained from the voltage transfer functions are validated by comparison against simulations using a detailed model implemented in PSCAD?. Then, the influences of system parameters such as the number of parallel branches, HVDC grid configurations and groundings on the first travelling wave are investigated by analysing the voltage and current transfer functions.

Abstract:This paper proposes a simple and fast way to determine the direction of a fault in a multi-terminal high voltage direct current (HVDC) grid by comparing the rate of change of voltage (ROCOV) values at either side of the di/dt limiting inductors at the line terminals. A local measurement based secure and fast protection method is implemented by supervising a basic ROCOV relay with a directional element. This directional information is also used to develop a slower communication based DC line protection scheme for detecting high resistance faults. The proposed protection scheme is applied to a multi-level modular converter based three-terminal HVDC grid and its security and sensitivity are evaluated through electromagnetic transient simulations. A methodology to set the protection thresholds considering the constraints imposed by the breaker technology and communication delays is also presented. With properly designed di/dt limiting inductors, the ability of clearing any DC transmission system fault before fault currents exceeds a given breaker capacity is demonstrated.

Abstract:Active damped LCL-filter-based inverters have been widely used for grid-connected distributed generation (DG) systems. In weak grids, however, the phase-locked loop (PLL) dynamics may detrimentally affect the stability of grid-connected inverters due to interaction between the PLL and the controller. In order to solve the problem, the impact of PLL dynamics on small-signal stability is investigated for the active damped LCL-filtered grid-connected inverters with capacitor voltage feedback. The system closed-loop transfer function is established based on the Norton equivalent model by taking the PLL dynamics into account. Using an established model, the system stability boundary is identified from the viewpoint of PLL bandwidth and current regulator gain. The accuracy of the ranges of stability for the PLL bandwidth and current regulator gain is verified by both simulation and experimental results.

Abstract:This paper investigates the stability of LCL-filtered grid-connected inverters with capacitor current feedback (CCF) active damping. The impact of time delays in the digital controller on active damping and its equivalent virtual impedance is analyzed. The inherent relationship between these time delays and stability is illustrated. Specially, a critical value of the CCF active damping coefficient kdamp_c is proposed to define three distinct regions of stability evaluation. If kdamp_c >0, a sufficient but smaller damping coefficient (kdamp < kdamp_c) is recommended as optimum damping solution; if kdamp_c = 0, system will be unstable irrespective of active damping; and if kdamp_c <0, active damping is not necessary to design a stable system. Necessary conditions to ensure stability are identified; guidelines for controller design are then presented to optimize the performance of active damping and dynamic response. Simulation and experimental results confirm the presented analysis.

Abstract:This paper examines the interconnection of two DC microgrids (MGs) with tie-line. The voltages at respective MG buses are controlled to manage the power flow across the tie-line. Formation of such a DC MG cluster ensures higher reliability of power supply and flexibility to manage distributed energy resources and loads in the system. Two MGs consist of photovoltaic and battery units interfaced by power electronic converters. The bus voltages of two DC MGs act as an indicator for the power flow monitoring the supply-demand balance. A decentralized control approach is proposed to control each MG and bus voltage fluctuation in an allowable range. Furthermore, a mode adaptive decentralized control approach is proposed for seamless mode transition in order to assign microgrid operation modes and for the power management of DC MGs. The effectiveness of the proposed concept is validated by simulation and experimental results.

Abstract:This paper presents a comprehensive control strategy for unified power quality conditioners (UPQCs) to compensate for both voltage and current quality problems. The controllers for the series and shunt components of the UPQC are, equally, divided into three blocks: ① main controller, which deals with the fundamental-frequency issues such as active and reactive power flow; ② harmonic controller, which ensures zero-error tracking while compensating voltage and current harmonics; ③ the set-point generation block, which handles the different control objectives of the UPQC. The controller design procedure has been simplified to the selection of three parameters for each converter. Furthermore, the proposed strategy can be implemented measuring only four variables, which represents a reasonable number of sensors. In addition, a pulse width modulation (PWM)-based modulation with fixed switching frequency is used for both converters. The proposed control strategy has been validated experimentally under different conditions, including grid-frequency variations.

Abstract:This paper proposes an optimized and coordinated model predictive control (MPC) scheme for doubly-fed induction generators (DFIGs) with DC-based converter system to improve the efficiency and dynamic performance in DC grids. In this configuration, the stator and rotor of the DFIG are connected to the DC bus via voltage source converters, namely, a rotor side converter (RSC) and a stator side converter (SSC). Optimized trajectories for rotor flux and stator current are proposed to minimize Joule losses of the DFIG, which is particularly advantageous at low and moderate torque. The coordinated MPC scheme is applied to overcome the weaknesses of the field-oriented control technique in the rotor flux-oriented frame, which makes the rotor flux stable and the stator current track its reference closely and quickly. Lastly, simulations and experiments are carried out to validate the feasibility of the control scheme and to analyze the steady-state and dynamic performance of the DFIG.

Abstract:In this paper, a non-isolated stacked bidirectional DC-DC converter with zero-voltage-switching (ZVS) is introduced for the high step-up/step-down conversion systems. The extremely narrow turn-on and/or turn-off duty cycle existing in the conventional bidirectional buck-boost converters can be extended due to the stacked module configuration for large voltage conversion ratio applications. Furthermore, the switch voltage stress is halved because of the series connection of half bridge modules. The PWM plus phase-shift control strategy is employed, where the duty cycle is adopted to regulate the voltages between the input and output sides and the phase-shift angle is applied to achieve the power flow regulation. This decoupled control scheme can not only realize seam-less bidirectional transition operation, but also achieve adaptive voltage balance for the power switches. In addition, ZVS soft-switching operation for all active switches is realized to minimize the switching losses. Finally, a prototype of 1 kW operating at 100 kHz is built and tested to demonstrate the effectiveness of the proposed converter and the control strategy.

Abstract:The application of LCL filters has become popular for inverters connected to the power grid due to their advantages in harmonic current reductions. However, the power grid in a distribution system is non-ideal, presenting itself as a voltage source with significant impedance. This means that an inverter using an LCL filter may interact with other grid-connected inverters via the non-ideal grid. In this paper, damping optimization of LCL filters to reduce this interaction is studied for a three-phase voltage source inverter (VSI). Simulation results show that resonant oscillation occurs in a distributed power grid, even if the VSI with an LCL filter is well designed for standalone applications. A small-signal analysis is performed to predict this stability problem and to locate the boundary of the instability using an impedance approach. Based on these analytical results, optimized damping of the LCL filter can be designed. The oscillation phenomena and optimized damping design are verified by simulations and experimental measurements.

Abstract:Energy storage systems with multilevel converters play an important role in modern electric power systems with large-scale renewable energy integration. This paper proposes a reverse-blocking modular multilevel converter for a battery energy storage system (RB-MMC-BESS). Besides integrating distributed low-voltage batteries to medium or high voltage grids, with the inherited advantages of traditional MMCs, the RB-MMC-BESS also provides improved DC fault handling capability. This paper analyzes such a new converter configuration and its operating principles. Control algorithms are developed for AC side power control and the balancing of battery state of charge. The blocking mechanism to manage a DC pole-to-pole fault analyzed in depth. Comprehensive simulation results validate both the feasibility of the RB-MMC-BESS topology and the effectiveness of the control and fault handling strategies.

Abstract:With large-scale development of distributed generation (DG) and its potential role in microgrids, the microgrid cluster (MGC) becomes a useful control model to assist the integration of DG. Considering that microgrids in a MGC, power dispatch optimization in a MGC is difficult to achieve. In this paper, a hybrid interactive communication optimization solution (HICOS) is suggested based on flexible communication, which could be used to solve plug-in or plug-out operation states of microgrids in MGC power dispatch optimization. HICOS consists of a hierarchical architecture: the upper layer uses distributed control among multiple microgrids, with no central controller for the MGC, and the lower layer uses a central controller for each microgrid. Based on flexible communication links among microgrids, the optimal iterative information are exchanged among microgrids, thus HICOS would gradually converge to the global optimal solution. While some microgrids plug-in or plug-out, communication links will be changed, so as to unsuccessfully reach optimal solution. Differing from changeless communication links in traditional communication networks, HICOS redefines the topology of flexible communication links to meet the requirement to reach the global optimal solutions. Simulation studies show that HICOS could effectively reach the global optimal dispatch solution with non-MGC center. Especially, facing to microgrids plug-in or plug-out states, HICOS would also reach the global optimal solution based on refined communication link topology.