Impact of DFIG based offshore wind farms connected through VSC-HVDC link on power system stability

With the increased levels of offshore wind power penetration into power systems, the impact of offshore wind power on stability of power systems require more investigation. In this paper, the effects of a large scale doubly fed induction generator (DFIG) based offshore wind farm (OWF) on power system stability are examined. The OWF is connected to the main onshore grid through a voltage source converter (VSC) based high voltage direct current (HVDC) link. A large scale DFIG based OWF is connected to the New England 10-machine 39-bus test system through a VSC-HVDC. One of the synchronous generators in the test system is replaced by an OWF with an equivalent generated power. As the voltage source converter can control the active and reactive power independently, the use of the onshore side converter to control its terminal voltage is investigated. The behaviour of the test system is evaluated under both small and large grid disturbances in both cases with and without the offshore wind farm

The adequacy of the present practice in dynamic aggregated modelling of wind farm systems

Large offshore wind farms are usually composed of several hundred individual wind turbines, each turbine having its own complex set of dynamics. The analysis of the dynamic interaction between wind turbine generators (WTG), interconnecting ac cables, and voltage source converter (VSC) based High Voltage DC (HVDC) system is difficult because of the complexity and the scale of the entire system. The detailed modelling and modal analysis of a representative wind farm system reveal the presence of several critical resonant modes within the system. Several of these modes have frequencies close to harmonics of the power system frequency with poor damping. From a computational perspective the aggregation of the physical model is necessary in order to reduce the degree of complexity to a practical level. This paper focuses on the present practices of the aggregation of the WTGs and the collection system, and their influence on the damping and frequency characteristics of the critical oscillatory modes. The effect of aggregation on the critical modes are discussed using modal analysis and dynamic simulation. The adequacy of aggregation method is discussed

Fault analysis and protection for wind power generation systems

Wind power is growing rapidly around the world as a means of dealing with the world energy shortage and associated environmental problems. Ambitious plans concerning renewable energy applications around European countries require a reliable yet economic system to generate, collect and transmit electrical power from renewable resources. In populous Europe, collective offshore large-scale wind farms are efficient and have the potential to reach this sustainable goal. This means that an even more reliable collection and transmission system is sought. However, this relatively new area of offshore wind power generation lacks systematic fault transient analysis and operational experience to enhance further development. At the same time, appropriate fault protection schemes are required. This thesis focuses on the analysis of fault conditions and investigates effective fault ride-through and protection schemes in the electrical systems of wind farms, for both small-scale land and large-scale offshore systems. Two variable-speed generation systems are considered: doubly-fed induction generators (DFIGs) and permanent magnet synchronous generators (PMSGs) because of their popularity nowadays for wind turbines scaling to several-MW systems. The main content of the thesis is as follows. The protection issues of DFIGs are discussed, with a novel protection scheme proposed. Then the analysis of protection scheme options for the fully rated converter, direct-driven PMSGs are examined and performed with simulation comparisons. Further, the protection schemes for wind farm collection and transmission systems are studied in terms of voltage level, collection level wind farm collection grids and high-voltage transmission systems for multi-terminal DC connected transmission systems, the so-called “Supergrid”. Throughout the thesis, theoretical analyses of fault transient performances are detailed with PSCAD/EMTDC simulation results for verification. Finally, the economic aspect for possible redundant design of wind farm electrical systems is investigated based on operational and economic statistics from an example wind farm project

A review on DC collection grids for offshore wind farms with HVDC transmission system

Abstract: Traditionally, the internal network composition of offshore wind farms consists of alternating current (AC) collection grid; all outputs of wind energy conversion units (WECUs) on a wind farm are aggregated to an AC bus. Each WECU includes: a wind-turbine plus mechanical parts, a generator including electronic controller, and a huge 50-or 60-Hz power transformer. For a DC collection grid, all outputs of WECUs are aggregated to a DC bus; consequently, the transformer in each WECU is replaced by a power converter or rectifier. The converter is more compact and smaller in size compared to the transformer. Thus reducing the size and weight of the WECUs, and also simplifying the wind farm structure. Actually, the use of offshore AC collection grids instead of offshore DC collection grids is mainly motivated by the availability of control and protection devices. However, efficient solutions to control and protect DC grids including HVDC transmission systems have already been addressed. Presently, there are no operational wind farms with DC collection grids, only theoretical and small-scale prototypes are being investigated worldwide. Therefore, a suitable configuration of the DC collection grid, which has been practically verified, is not available yet. This paper discussed some of the main components required for a DC collection grid including: the wind-turbine-generator models, the control and protection methods, the offshore platform structure, and the DC-grid feeder configurations. The key component of a DC collection grid is the power converter; therefore, the paper also reviews some topologies of power converter suitable for DC grid applications

Analysis and control of resonances in HVDC connected DFIG-based offshore wind farm

Wechselwirkungen zwischen den weit verbreiteten wechselrichtergekoppelten Netzkomponenten und den passiven Netzkomponenten können einen breiten Frequenzbereich von Resonanzen aufweisen, wodurch massive harmonische Verzerrungen hervorgerufen und sogar die Systemstabilität gefährdet werden. Ihre Folgen könnten die Trennung erneuerbarer und konventioneller Stromerzeuger vom Netz oder die physische Beschädigung empfindlicher Netzanlagen sein. Motiviert durch die Resonanzereignisse der letzten Jahre in windintegrierten Stromversorgungssystemen, untersucht diese Dissertation die resonanzinduzierten harmonischen Verzerrungs- und Stabilitätsprobleme in einem Offshore-Windpark (OWF) mit doppelt gespeisten Asynchrongeneratoren (DFIG) und Netzanschluss mittels Hochspannungsgleichstromübertragung (HGÜ). Ziel dieser Dissertation ist es, die Resonanzen genau zu charakterisieren, ihre Risiken zu bewerten und Lösungen für die Gestaltung der Minderungsstrategie bereitzustellen. Um die dynamischen Eigenschaften eines DFIG-basierten Windparks genau zu erfassen, wird eine umfassende Impedanzmodellierung unter Berücksichtigung des detaillierten PIRegelkreises und der Gleichstromdynamik der Windkraftanlage sowie der Kabelverbindungen des Mittelspannungskollektorsystems (MV) durchgeführt. Durch schrittweise Simulationsüberprüfungen hat sich die aggregierte Modellierung des MVKollektorsystems für die Breitbandresonanzanalyse als geeignet erwiesen. Auf dieser Grundlage wurden sowohl die Bode-Plot-Methode als auch der Ansatz der Resonanzmodusanalyse (RMA) angewendet, um die Resonanzprobleme unter Berücksichtigung verschiedener Betriebsbedingungen des Windparks und Änderungen der Netz-Topologie anzugehen. Ihre Auswirkungen auf die Resonanzfrequenz, die harmonische Verzerrungen und die Dämpfungen zu Resonanzen werden untersucht. Die Orte, an denen Resonanzen am einfachsten angeregt werden können, werden durch die Busbeteiligungsfaktoranalyse identifiziert. Darüber hinaus wird der Einfluss der Frequenzkopplungseffekte von Steuerungs- und Schaltvorgängen für asymmetrische Wandler auf subsynchrone Resonanz- (SSR), Mittel- und Hochfrequenzresonanzen unter Verwendung der aggregierten Modelle analysiert, die aus einem praktischen HGÜverbundenen DFIG-basierten OWF abgeleitet wurden. Für den Frequenzbereich von mehreren Hz bis zu einigen kHz werden große harmonische Verzerrungs- und Stabilitätsprobleme gezeigt. Um den negativen Einfluss von Resonanzen auf die Stromqualität und die Systemstabilität zu verhindern, wurde eine Reihe aktiver Dämpfungsmöglichkeiten untersucht und in das untersuchte windintegrierte Stromnetz implementiert, und es wird eine koordinierte Dämpfungsstrategie vorgeschlagen, mit der Breitbandresonanzen effektiv gedämpft werden können. Schließlich validieren Simulationen in MATLAB / Simulink die Ergebnisse der Impedanzmodellierung, der Resonanzanalyse sowie die Wirksamkeit der Breitbandresonanzdämpfungsstrategie.Interactions among the widely utilised converter-interfaced grid components and passive grid components can introduce wide-frequency range of resonances, thus induce massive harmonic distortions and even endanger system stability. Their consequences might be the tripping of renewable and conventional generation units or the physical damage of sensitive grid assets. Motivated by recent years’ resonance incidents in wind-integrated power systems, this study investigates the resonance-induced harmonic distortion and stability issues in doubly fed induction generator (DFIG)-based offshore wind farm (OWF) with high-voltage direct current (HVDC) grid connection. The objective of this study is to accurately characterize the resonances, evaluate their risks and provide solutions for the design of mitigation strategy. To accurately capture the dynamic characteristics of DFIG-based wind farm, a comprehensive impedance modelling considering the detailed PI control loop and DC dynamics of wind turbine as well as the cable connections of the medium-voltage (MV) collector system is conducted. Through stepwise simulation verifications, aggregated modelling of MV collector system is proved to be suitable for wideband resonance analysis. On this basis, both Bode-plot method and resonance mode analysis (RMA) approach have been adopted to address the resonance issues taking into account various wind farm operating conditions and grid topology changes. Their impacts on resonance frequency, harmonic amplification level and damping level are investigated. The locations where resonances can be most easily excited are identified through bus participation factor analysis. Moreover, the impact of the frequency-coupling effects from asymmetrial converter control and switching operations on subsynchronous resonance (SSR), middleand high-frequency resonances is analyzed using the aggregated models derived from a practical HVDC connected DFIG-based OWF. Large harmonic distortion and stability issues are demonstrated for the frequency range from several Hz to a few kHz. In order to prevent the negative impact of resonances on power quality and system stability, a series of active damping possibilites have been studied and implemented in the studied wind-integrated power system, and a coordinated damping strategy which can effectively damp wideband resonances is proposed. Finally, simulations in MATLAB/Simulink validate the results of impedance modelling, resonance analysis as well as the effectiveness of the wideband resonance damping strategy

Offshore Wind Farm-Grid Integration: A Review on Infrastructure, Challenges, and Grid Solutions

Recently, the penetration of renewable energy sources (RESs) into electrical power systems is witnessing a large attention due to their inexhaustibility, environmental benefits, storage capabilities, lower maintenance and stronger economy, etc. Among these RESs, offshore wind power plants (OWPP) are ones of the most widespread power plants that have emerged with regard to being competitive with other energy technologies. However, the application of power electronic converters (PECs), offshore transmission lines and large substation transformers result in considerable power quality (PQ) issues in grid connected OWPP. Moreover, due to the installation of filters for each OWPP, some other challenges such as voltage and frequency stability arise. In this regard, various customs power devices along with integration control methodologies have been implemented to deal with stated issues. Furthermore, for a smooth and reliable operation of the system, each country established various grid codes. Although various mitigation schemes and related standards for OWPP are documented separately, a comprehensive review covering these aspects has not yet addressed in the literature. The objective of this study is to compare and relate prior as well as latest developments on PQ and stability challenges and their solutions. Low voltage ride through (LVRT) schemes and associated grid codes prevalent for the interconnection of OWPP based power grid have been deliberated. In addition, various PQ issues and mitigation options such as FACTS based filters, DFIG based adaptive and conventional control algorithms, ESS based methods and LVRT requirements have been summarized and compared. Finally, recommendations and future trends for PQ improvement are highlighted at the end