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Nowadays, some Multi-terminal DC (MTDC) systems are in operation around the world. Soon, MTDC grids will be built and overlay the present AC grids. The main driver for the construction of such a grid is to facilitate large-scale integration of remote renewable energy sources to existing AC grids and to develop the energy market. This paper presents a comprehensive analogy between the control and operation aspects of the emerging MTDC grids to those of the traditional AC power grids. Similarities and difference between the two technologies are presented and highlighted. Based on the performed detailed overview, even though a three-layered control system, i.e., primary, secondary, and tertiary control layers is state-of-the-art in large-scale AC power systems, a two-layered control system will satisfy MTDC grids control and operation requirements. This paper also addresses some control and operational issues and limitations of MTDC grids.

©2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. Grid-connected converters under weak grid conditions could present instability associated with the synchronization elements and not directly with the current control loop. The impedance-based stability criterion is an effective tool to address the problem. To solve the negative effect of the synchronous reference frame phase-locked loop (SRF-PLL) on grid current control under weak grid conditions, a synchronization structure with pre-filter, based on a filtered quadrature signal generator (FQSG), is proposed in this paper to improve the dynamics of the controller's performance. The small-signal impedance model, implemented as a multi-input multi-output system in the dq-frame, characterizes the effect of the FQSG-PLL dynamics on the current control loop. From it, the impedance-based analysis shows the reduction of the stability issues of the converter. The analysis results were verified by time-domain simulations and by experimental tests in a full-scale 2 MW converter test-bench.

This study deals with impacts of the shunt conductances associated with HVDC cables and VSC-HVDC stations on optimal operation of Multi-Terminal HVDC (MT-HVDC). In this study, for the first time, shunt conductances are integrated to HVDC Optimal Power-Flow (OPF) program that is executed at the Power Dispatch Center (PDC) of the MT-HVDC grid. With the objective of losses minimization, optimal reference operation points of the VSC-HVDC stations are derived. The operating points of the power converter stations are adjusted based on the calculations performed in the dispatch center. CIGRE DCS3 MT-HVDC grid, structured by CIGRE B4 working group, is taken as the test platform. Test results have revealed the optimum voltages and loss pattern change. Moreover, the findings are compared with the case of neglecting the shunt conductances.

Future Flexible HVDC Transmission Systems will consist of several active and passive infrastructures including DC Power Flow Controller (DC-PFC) device. The addition of the DC-PFC to the future Multi-Terminal HVDC (MT-HVDC) grids arises some key concerns such as stability, system interoperability, and possible adverse interactions. Hence, a suitable model is necessary to conduct deep frequency domain analysis. In this context, this paper proposes a linearized model for small-signal stability analysis of flexible MT-HVDC grids in state space framework that can be straightforwardly utilized in the process of control design. In this paper, a spick-and-span, systematic and step-by-step process to derive the small signal model of all flexible MT-HVDC grid components is presented such that each sub-system is modeled individually and then all are integrated together. The derived model is cross-verified by time domain simulations of a nonlinear system model in a MATLAB/SIMULINK platform for CIGRE DCS3 MT-HVDC test grid.

Multiterminal dc networks are drawing a lot of interest lately in applications related to distributed generation, particularly in those that also integrate energy storage (ES). A few approaches for controlling the operation of such systems have been proposed in the literature; however, the existing structures can be significantly enhanced. This paper proposes an improved primary control layer, based on custom droop characteristics obtained by combining concepts of droop and dc-bus signaling control. This approach is designed to be generic and takes into account the various operating states of the network. Five operating bands, similar to the operating states of the ac grids, as well as various droop characteristics for different elements connected to the dc network, are defined. For the ES, the state of charge is taken into account at the primary control level and included in the droop characteristic, creating a two-variable droop surface. The proposed control strategy is validated through simulation and experimental results obtained from a case study that involves amicro dc network composed of a photovoltaic generator, a lead-acid battery, and a connection point to the ac grid.

This paper presented an overview of harmonic analysis and resonance in large wind power plant. The harmonic related problem becomes a concern as the growing numbers of nonlinear load and the increased possibility of harmonic resonances occurrence. In WPP, several conditions may contribute to the resonance phenomenon that will amplify the effect of the harmonic frequency. General overview of international standards and grid codes and basic modeling of WPP for harmonic analysis is also presented. Furthermore, an overview of the harmonic analysis methods are reviewed with emphasize given on the harmonic resonance analysis. A simple three-bus system example is evaluated applying three resonance analysis methods.

At the present time, distributed generation systems are required to disconnect from the main grid when there is an outage. In order to fulfill this requirement, photovoltaic (PV) power plants are equipped with anti-islanding algorithms, embedded in the converters controller, to avoid the island operation. However, the current trends in the development of the future electrical networks evidence that it is technically feasible and economically advantageous to keep feeding islanded systems under these situations, without cutting the power supply to the loads connected to the network. Nevertheless, commercial PV power converters are programmed as grid-feeding converters and they are unable to work in island mode if there is not an agent forming the grid. In order to overcome this problem, the synchronous power controller (SPC) is presented in this paper as a suitable alternative for controlling PV inverters. As will be further discussed, this controller permits PV plants to operate seamlessly in grid-connected and island mode, with no need of changing the control structure in either case. Moreover, the participation of SPC-based power converters integrating energy storage enables other grid-feeding systems to contribute to the grid operation in island conditions. The good results achieved with the SPC in different conditions will be shown in simulations, and also with experiments considering a real PV power plant combining SPC and commercial PV inverters.

High voltage dc (HVDC) systems act as the prevailed solution for transmitting offshore wind energy to onshore main grids. Control of the voltage source converters (VSC) in HVDC systems is decisive for the performance. This paper proposes the control of VSC-HVDC with electromechanical characteristics and unified primary strategy, as a reaction to the updated requirements of the ac grid transmission system operators. As two important aspects of VSC-HVDC control, converter control and primary control are both designed in detail. Electromechanical characteristics make the VSC capable of providing inertia to the ac networks as well as simplicity in island operation. Besides, unified primary control is given as a universal primary strategy for VSC stations, and especially takes into account frequency support and control mode transition. The proposed converter control is validated in scaled-down 10 kW laboratory setups, while the proposed primary control is endorsed by the simulation tests on a CIGRE multi-terminal HVDC model.

This paper presents a grid synchronization structure for three-phase electric power systems based on the use of a filtered quadrature signal generator (FQSG) and a phase-locked loop (PLL) structure, named Adaptive Vector Grid Synchronization system (AVGS). This system estimates the magnitude, frequency and phase of a signal, specially three-phase voltages and currents, and allows fast and accurate detection of the symmetrical components meet with the transient operating requirements imposed by grid codes. The adaptive vector based PLL (VB-PLL) permits offering a proper performance under generic grid conditions, especially under faulty scenarios. For the particular case of grid voltage synchronization of wind turbine converters, the AVGS is adjusted to provide a good response considering the most critical situation, which is the low voltage ride through (LVRT). This paper includes a detailed study of the proposed grid synchronization system and simulations with registers of real grid disturbances provided by Ingeteam Power Technology S.A.

There is a growing interest in electrical grid connection of single unit Wave Energy Converters (WECs), as well as the multiple units based Wave Power Plant (WPP) applications. Due to the increasing integration of high power converters in WPP, additional concerns on the harmonic current mitigation of WEC device arise which requires the grid filter installation. Furthermore, the identification and appropriate mitigation of additional harmonics at the WPP level is needed, as any inductances and capacitors of the WPP can interact with the resonant frequency of the entire system and lead to detrimental effects on equipment. The present paper focuses on the potential harmonic studies of a realistic proposed WPP layout, in which the system resonances are identified by performing the frequency scan and modal analysis (HRMA) methods. The later technique has the ability of determining the participation factor of each bus, finding in this way the nodes in the system that have more influence on the obtained results. In this way, the most suitable nodes in which additional filtering solutions should be implemented can be identified.