Control of multi-terminal HVDC networks towards wind power integration: A review

© 2015 Elsevier Ltd. More interconnections among countries and synchronous areas are foreseen in order to fulfil the EU 2050 target on the renewable generation share. One proposal to accomplish this challenging objective is the development of the so-called European SuperGrid. Multi-terminal HVDC networks are emerging as the most promising technologies to develop such a concept. Moreover, multi-terminal HVDC grids are based on highly controllable devices, which may allow not only transmitting power, but also supporting the AC grids to ensure a secure and stable operation. This paper aims to present an overview of different control schemes for multi-terminal HVDC grids, including the control of the power converters and the controls for power sharing and the provision of ancillary services. This paper also analyses the proposed modifications of the existing control schemes to manage high participation shares of wind power generation in multi-terminal grids.Postprint (author's final draft

Active and reactive power control of hybrid offshore AC and DC grids

The future ‘SuperGrid’ may requires the benefit of both offshore AC network and multi-terminal DC grid. AC cable limits the power transfer capability from the larger offshore wind farm, however, HVDC transmission system is economical viable for large power wind farm integration with the grid. Another approach to develop the offshore network infrastructure is by forming an offshore AC grid connecting several offshore wind farms. Then, this offshore AC network is connected with different onshore grid using HVDC system. This enhances the trade among the countries as well as provide an economical solution for wind energy integration. In this article, operational and control concept of voltage source converter is presented to integrate an offshore AC grid with an offshore DC grid. The article presents the control principle of offshore AC network frequency and voltage with respect to active and reactive power distribution in the AC network. Later, the principle of multi-terminal HVDC system is discussed with respect to power distribution using DC voltage droop control. Power distribution criteria are defined with respect to operator power-sharing requirement and network stability. In the end, a hybrid AC/DC offshore grid is modelled and simulated in MATLAB/SIMULINK to validate the distribution criteria

An improved droop-based control strategy for MT-HVDC systems

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This paper presents an improved droop-based control strategy for the active and reactive power-sharing on the large-scale Multi-Terminal High Voltage Direct Current (MT-HVDC) systems. As droop parameters enforce the stability of the DC grid, and allow the MT-HVDC systems to participate in the AC voltage and frequency regulation of the different AC systems interconnected by the DC grids, a communication-free control method to optimally select the droop parameters, consisting of AC voltage-droop, DC voltage-droop, and frequency-droop parameters, is investigated to balance the power in MT-HVDC systems and minimize AC voltage, DC voltage, and frequency deviations. A five-terminal Voltage-Sourced Converter (VSC)-HVDC system is modeled and analyzed in EMTDC/PSCAD and MATLAB software. Different scenarios are investigated to check the performance of the proposed droop-based control strategy. The simulation results show that the proposed droop-based control strategy is capable of sharing the active and reactive power, as well as regulating the AC voltage, DC voltage, and frequency of AC/DC grids in case of sudden changes, without the need for communication infrastructure. The simulation results confirm the robustness and effectiveness of the proposed droop-based control strategy

Power Management and Protection in MT-HVDC Systems with the Integration of High-Voltage Charging Stations

Due to the significant increase of the long-distance electricity demand, effective use of Distributed Generations (DGs) in power system, and the challenges in the expansion of new transmission lines to improve the reliability of power system reliability, utilizing Multi-Terminal HVDC (MT-HVDC) technology is an applicable, reliable, and cost-effective solution in hybrid AC/DC grids. MT-HVDC systems have flexibility in terms of independent active and reactive power flow (reversible control) and voltage control. Interconnecting two AC grids with different frequencies and transmitting electricity for the long-distance with low power-losses, which leads to less operation and maintenance costs, can be done through the MT-HVDC systems. The integration of large-scale remote DGs, e.g., wind farms, solar power plants, etc., and high-voltage charging stations for Electric Vehicles (EVs) into the power grid have different issues, such as economic, technical, and environmental challenges of transmission and network expansion/operation of both AC and DC grids. In details, damping oscillation, voltage support at different buses, operation of grid-connected inverters to the off-shore and on-shore AC systems, integrating of existing converter stations in MT-HVDC systems without major changes in control system, evaluation of communication infrastructure and also reactive power and filtering units’ requirements in MT-HVDC systems are the technical challenges in this technology. Therefore, a reliable MT-HVDC system can be a possible mean of resolving all the above-mentioned issues. MT-HVDC systems need a control system that can bring stability to the power system during a certain period of the operation/planning time while providing effective and robust electricity. This thesis presents an improved droop-based control strategy for the active and reactive power-sharing on the large-scale MT-HVDC systems integrating different types of AC grids considering the operation of the hybrid AC/DC grids under normal/contingency conditions. The main objective of the proposed strategy is to select the best parameters of the local terminal controllers at the site of each converter station (as the primary controller) and a central master controller (supervisory controller) to control the Power Flow (PF) and balance the instantaneous power in MT-HVDC systems. In this work, (1) various control strategies of MT-HVDC systems are investigated to propose (2) an improved droop-based power-sharing strategy of MT-HVDC systems while the loads (e.g., high-voltage charging stations) in power systems have significant changes, to improve the frequency response and accuracy of the PF control, (3) a new topology of a fast proactive Hybrid DC Circuit Breaker (HDCCB) to isolate the DC faults in MT-HVDC grids in case of fault current interruption. The results from this research work would include supporting energy adequacy, increasing renewable energy penetration, and minimizing losses when maintaining system integrity and reliability. The proposed strategies are evaluated on different systems, and various case scenarios are applied to demonstrate their feasibility and robustness. The validation processes are performed using MATLAB software for programming, and PSCAD/EMTDC and MATLAB/Simulink for simulation

A Review on Multi-Terminal High Voltage Direct Current Networks for Wind Power Integration

With the growing pressure to substitute fossil fuel-based generation, Renewable Energy Sources (RES) have become one of the main solutions from the power sector in the fight against climate change. Offshore wind farms, for example, are an interesting alternative to increase renewable power production, but they represent a challenge when being interconnected to the grid, since new installations are being pushed further off the coast due to noise and visual pollution restrictions. In this context, Multi-Terminal High Voltage Direct Current (MT-HVDC) networks are the most preferred technology for this purpose and for onshore grid reinforcements. They also enable the delivery of power from the shore to offshore Oil and Gas (O&G) production platforms, which can help lower the emissions in the transition away from fossil fuels. In this work, we review relevant aspects of the operation and control of MT-HVDC networks for wind power integration. The review approaches topics such as the main characteristics of MT-HVDC projects under discussion/commissioned around the world, rising challenges in the control and the operation of MT-HVDC networks and the modeling and the control of the Modular Multilevel Converter (MMC) stations. To illustrate the challenges on designing the control system of a MT-HVDC network and to corroborate the technical discussions, a simulation of a three-terminal MT-HVDC network integrating wind power generation and offshore O&G production units to the onshore grid is performed in Matlab's Simscape Electrical toolbox. The results highlight the main differences between two alternatives to design the control system for an MT-HVDC network

DC Grids : Motivation, Feasibility and Outstanding Issues : Status Report for the European Commission Deliverable : D5.4

Wind energy is already a mainstay of clean power generation in Europe, with over 100GW of capacity installed so far, and another 120GW anticipated by 2020 according to various analysts. Much of this capacity is expected to be installed offshore, as it is a windier and the source is steadier compared to onshore wind energy. Hence, offshore wind has been envisaged as making a critical contribution to Europe’s demand for electrical energy and to minimising the carbon emissions associated with meeting that demand

Multi-terminal VSC-HVDC Based Offshore Wind-Farms Integration System Operation and Control

Worldwide, many countries direct billions of dollars into the development of renewable energy sources; especially wind generation, in an effort to relieve global warming effects and other environmental concerns. As a result of increasing numbers of remotely-located large power offshore wind farms, the AC grid faces many technical challenges in integrating such plants; such as large submarine power transmission for extended distances, power sharing and transfer, as well as remotely located induction generation reactive power support. Offshore multi-terminal VSC based HVDC (MT VSC-HVDC) transmission systems represent a possible means of dealing with those challenges. This is due to their higher capacity, flexibility and controllability than offshore AC transmission. In addition, these offshore grids provide grid integration to remote offshore wind farms leading providing additional interconnection capacity to improve the trade of electricity between different AC grids. This work presents a new centralized supervisory control strategy for controlling the power sharing and voltage regulation of MT VSC-HVDC integrating offshore wind farms. The main purpose of the proposed strategy is selecting the optimal parameters of the HVDC system VSCs' local controller. These optimal parameters are selected in order to achieve optimal system transient response and desired steady state operation. In this work, an adaptive droop-based power-sharing control strategy is proposed. The primary objective is to control the sharing of the active power transmitted by a MT VSC-HVDC network among a number of onshore AC grids or offshore loads based on the desired percentage shares. The shared power is generated by remote generation plants (e.g., offshore wind farms) or is provided as surplus of AC grids. The desired percentage shares of active power are optimized by the system operator to fulfill the active power requirements of the connected grids with respect to meeting goals such as supporting energy adequacy, increasing renewable energy penetration, and minimizing losses. The control strategy is based on two hierarchal levels: voltage-droop control as the primary controller and an optimization based secondary (supervisory) controller for selecting the optimal droop reference voltages. Based on the DC voltage transient and steady state dynamics, a methodology for choosing the droop gains for droop controlled converters has been developed. In addition, a new tuning methodology is proposed for selecting the optimum VSCs local controller gains to enhance the transient performance and the small-signal stability of the system to mitigate the change of the operating conditions, taking into consideration the overall dynamics of the MT HVDC system. The VSCs' local control loops gains are selected to maximize the system bandwidth and improve the system damping. As a part of the proposed methodology, the derivation of the aggregated linearized state-space model of a MT VSC-HVDC based offshore transmission system is provided. Based on the derived model, a small signal stability analysis was performed to show the interaction of the modes and define the dominant eigenvalues of the system. Furthermore, a communication-free DC voltage control strategy is presented for mitigating the effects of the power imbalance caused by permanent or temporary power-receiving converter outages. The proposed control strategy is targeted at fast power reduction of the wind power generation from the wind farms (WFs) in order to eliminate power imbalances in the HVDC network. This process is performed by decentralized control rules in the local controllers of the WF voltage source converter (VSC) and its wind turbines. The proposed strategy was designed to work with WFs based on both doubly fed induction generators (DFIGs) and permanent magnet synchronous generators (PMSGs). The proposed control strategies were validated on the B4 CIGRE MT VSC-HVDC test system and different case scenarios were applied to show its feasibility and robustness. The validation process was performed using Matlab software programming and Matlab/Simulink based time domain detailed model

Analysis of storage systems for MTDC

Les sources d'électricité renouvelables sont de plus en plus intégrées dans le système électrique, posant des problèmes en termes d'inertie, de fiabilité du réseau et de qualité de l'énergie. La majeure partie de ces sources d'énergie, telles que les éoliennes, sont situées loin des systèmes électriques. Le système de transmission de courant continu haute tension (VSC-HVDC) basé sur un convertisseur de source de tension est idéal pour connecter les parcs éoliens offshore au réseau électrique CA onshore. Depuis plus de 50 ans, les systèmes à courant continu haute tension (HVDC) sont utilisés dans les systèmes de transmission d'énergie. Ce système de transport présente plusieurs avantages, notamment une distribution d'énergie active et réactive découplée, la possibilité d'inverser les flux d'énergie sans ajuster la polarité de la tension et la capacité de fonctionner dans des réseaux électriques vulnérables et indépendants. Outre les avantages mentionnés ci-dessus, les systèmes HVDC sont considérés comme une alternative viable aux systèmes de transmission conventionnels en raison de leur potentiel à transmettre de vastes volumes d'énergie sur de longues distances. En raison de la faible perte de puissance du câble, les technologies HVDC sont idéales pour transporter l'énergie électrique sur de longues distances. Ses principales utilisations comprennent l'interconnexion de réseau non synchrone, le transfert d'énergie électrique à longue distance et la transmission de câbles sous-marins et souterrains. La mise en œuvre d'un réseau hybride AC-HVDC est une étape importante dans le développement des techniques HVDC, car elle conduit à un changement dans la structure du système DC de connexions DC autonomes point à point vers un HVDC multi-terminal (MTDC) système. L'un des types les plus courants de topologies de réseau à courant continu est le VSC-HVDC multi-terminal, qui a plus de deux VSC reliés aux réseaux à courant continu. Seule la technologie VSC, et non la technologie LCC, permet ces types de réseaux HVDC maillés. Cela est dû à la capacité des IGBT à transférer le courant dans les deux sens tout en conservant la même polarité de tension. Le système MTDC est une solution appropriée pour les interconnexions d'énergie propre, et il contribuera à augmenter la stabilité, la flexibilité et les performances du système électrique. Les convertisseurs électroniques de puissance sont utilisés dans les réseaux MTDC pour communiquer avec les systèmes CA et fournir des services de contrôle. Les convertisseurs électroniques de puissance (AC / DC ou DC / DC) joueront sans aucun doute un rôle important pour garantir une stabilité, des performances et une rentabilité élevées du réseau. L'inertie globale du système diminue à mesure que les interconnexions de convertisseurs électroniques de puissance deviennent plus répandues dans le système d'alimentation. Les systèmes de génération d'interconnexion basés sur VSC, tels que les éoliennes, n'ont pas de contribution inertielle par défaut, contrairement aux générateurs synchrones. Une éolienne, par contre, peut être conçue pour fournir une assistance inertielle en ajustant la puissance de sortie pour compenser les conditions du réseau. Plusieurs solutions au manque d'inertie de ces structures à interface électronique ont été proposées. Il est indéniable que les systèmes de stockage d'énergie (SSE) basés sur des convertisseurs de puissance ont la capacité d'améliorer le comportement transitoire du système électrique. La modulation d'une fréquence d'appareil donnée est l'un des objectifs fondamentaux des ESS. L'énergie cinétique contenue dans la masse mobile des éoliennes, le stockage d'énergie par batterie, le stockage d'énergie par air comprimé, le stockage d'énergie par volant, le stockage d'énergie par supercondensateur et le stockage d'énergie magnétique supraconductrice font partie des technologies actuellement proposées. En proposant la technologie MMC pour VSC, l'utilisation de l'énergie stockée dans les stations de conversion devient plus possible car une capacité de stockage d'énergie plus capacitive est disponible dans ce type de convertisseur par rapport à un VSC traditionnel à deux niveaux. L'étude actuelle suggère que les capacités du système HVDC soient utilisées pour améliorer et sécuriser le réseau à courant alternatif du système. Les systèmes de stockage d'énergie (ESS) sont utilisés dans les réseaux MTDC pour surveiller l'électricité, la fréquence, la tension du réseau en courant continu et le partage d'énergie dans diverses conditions, y compris les pannes et les pannes de convertisseur. En résumé, les systèmes électriques sont confrontés à de nouveaux problèmes en raison de la forte pénétration des sources d'énergie renouvelables qui sont connectées au réseau par un convertisseur électronique de puissance. En conséquence, l'augmentation de la connexion de base du convertisseur affecte la fréquence et la stabilité de la tension du système d'alimentation. Les normes de liaison au réseau ont plusieurs objectifs de base, dont l'un est de maintenir la fiabilité globale du système électrique. L'étude actuelle suggère d'utiliser des systèmes de stockage d'énergie (SSE) dans les systèmes HVDC pour augmenter la stabilité du système électrique. Bien que l'utilisation de systèmes de stockage d'énergie (tels que des batteries, des volants d'inertie, des super-condensateurs ou des systèmes d'énergie magnétique supraconducteurs) ait déjà été réalisée pour augmenter l'inrtie du réseau, la combinaison de l'utilisation de systèmes de stockage d'énergie (tels que des batteries, des volants d'inertie, des super-condensateurs, ou systèmes d'énergie magnétique supraconducteurs est quelque peu nouvelle et fascinante dans les réseaux MTDC. Ce concept sera testé sur une variété de systèmes HVDC (point à point, MTDC) pour voir comment l'ESS affecte les différentes caractéristiques du réseau lorsqu'il est connecté via des convertisseurs.Renewable electricity sources are increasingly being integrated into the power system, posing problems in terms of inertia, grid reliability, and power quality. The bulk of these energy sources, such as wind turbines, are situated far from power systems. The voltage-source converter-based high voltage direct current (VSC-HVDC) transmission system is a good fit for connecting offshore wind farms to the onshore AC power grid. For more than 50 years, high-voltage direct current (HVDC) systems have been used in power transmission systems. This transmission system has several benefits, including decoupled active and reactive power distribution, the ability to reverse power flows without adjusting voltage polarity, and the ability to run in vulnerable and independent power networks. Aside from the benefits mentioned above, HVDC systems are seen as a viable alternative to conventional transmission systems due to their potential to transmit vast volumes of power over long distances. Because of the low cable power loss, HVDC technologies are ideal for transporting electrical power over long distances. Its key uses include nonsynchronous network interconnection, long-distance electrical energy transfer, and underwater and underground cable transmission. Implementing a hybrid AC-HVDC grid is a significant step forward in the development of HVDC techniques, as it leads to a shift in the dc system's structure from point-to-point stand-alone dc connections to a multi-terminal HVDC (MTDC) system. One of the most common types of dc grid topologies is multi-terminal VSC-HVDC, which has more than two VSC linked to the dc grids. Only VSC technology, not LCC technology, allows for these types of meshed HVDC grids. This is due to IGBTs' ability to transfer current in both directions while maintaining the same voltage polarity. The MTDC system is an appropriate solution for clean energy interconnections, and it will help to increase power system stability, flexibility, and equipment performance. Power electronic converters are used in MTDC grids to communicate with AC systems and provide control services. Power electronic converters (AC/DC or DC/DC) will undoubtedly play an important role in ensuring high grid stability, performance, and cost-effectiveness. The overall system inertia is decreasing as power electronic converter interconnections become more prevalent in the power system. VSC-based interconnection generation systems, such as wind turbines, do not have an inertial contribution by default, unlike synchronous generators. By adjusting the power output to adapt to grid circumstances, a wind turbine, on the other hand, may provide inertial support. The problem of inertia reduction in the AC/DC system has been tackled using a variety of methods. To provide frequency support for connected AC grids, these solutions include utilizing the control capability of MTDC systems and Energy Storage Systems (ESSs). It is an undeniable fact that power converter-based Energy Storage Systems (ESSs) have the ability to improve power system transient behavior. The modulation of a given device frequency is one of the basic goals of ESSs. Kinetic energy contained in the moving mass of wind turbines, battery energy storage, compressed air energy storage, flywheel energy storage, supercapacitor energy storage, and superconducting magnetic energy storage are among the technologies currently proposed. By proposing the MMC technology for VSC, using the energy stored in the converter stations is becoming more possible because more capacitive energy storage capability is available in this kind of converter in comparison with a traditional two-level VSC. The current research implies that the HVDC system's capabilities might be used to improve and safeguard the interconnected ac network. Furthermore, Energy storage systems (ESS) are used in MTDC grids to monitor electricity, frequency, dc network voltage, and power-sharing under a variety of conditions, including faults and outages. In a summary, power systems are facing new problems as a result of the high penetration of renewable energy sources that are connected to the grid by a power electronic converter. As a result, the increasing converter base connection affects the power system's frequency and voltage stability. Grid link standards have several basic goals, one of which is to maintain the overall reliability of the power system. To improve power system stability, the present study proposes utilizing the control capacity of MTDC systems and Energy Storage Systems (ESSs) in MTDC systems. The proposed approach enables the VSC converters to provide short-term frequency support for the AC side and improve the DC grid stability. While using energy storage systems (such as batteries, flywheels, super-capacitors, or superconductor magnetic energy systems) to increase grid inertia has been achieved before, the combination of using energy storage systems (such as batteries, flywheels, super-capacitors, or superconductor magnetic energy systems) in MTDC networks is somewhat new and fascinating. This concept will be tested on a variety of HVDC systems (point to point, MTDC) to see how ESS affects the network's various characteristics when connected through converters