Development and implementation of a LabVIEW based SCADA system for a meshed multi-terminal VSC-HVDC grid scaled platform

This project is oriented to the development of a Supervisory, Control and Data Acquisition (SCADA) software to control and supervise electrical variables from a scaled platform that represents a meshed HVDC grid employing National Instruments hardware and LabVIEW logic environment. The objective is to obtain real time visualization of DC and AC electrical variables and a lossless data stream acquisition. The acquisition system hardware elements have been configured, tested and installed on the grid platform. The system is composed of three chassis, each inside of a VSC terminal cabinet, with integrated Field-Programmable Gate Arrays (FPGAs), one of them connected via PCI bus to a local processor and the rest too via Ethernet through a switch. Analogical acquisition modules were A/D conversion takes place are inserted into the chassis. A personal computer is used as host, screen terminal and storing space. There are two main access modes to the FPGAs through the real time system. It has been implemented a Scan mode VI to monitor all the grid DC signals and a faster FPGA access mode VI to monitor one converter AC and DC values. The FPGA application consists of two tasks running at different rates and a FIFO has been implemented to communicate between them without data loss. Multiple structures have been tested on the grid platform and evaluated, ensuring the compliance of previously established specifications, such as sampling and scanning rate, screen refreshment or possible data loss. Additionally a turbine emulator was implemented and tested in Labview for further testing

Operation and control of voltage source converters in transmission networks for AC system stability enhancement

The rapid expansion in power transmission for the integration of large-scale renewables is foreseen in the future. This will be complemented by infrastructure reinforcements in the form of series compensation and high-voltage direct current (HVDC) links. These changes will bring forth new operability challenges to grid operators. The stability issues pertained to such reinforcements: potential threat of subsynchronous resonance (SSR) and frequency regulation will be investigated in this thesis. Utilising the existing and future voltage source converters (VSC) based HVDC links to support the AC system by proving ancillary services will be of significant importance in the coming decades. The research work presented in this thesis is aimed to address these challenges, in particular, the technical barriers associated with AC/DC interaction and to propose measures to avoid any potential instability. The main contributions of this research work comprise of four parts, namely, (1) analysis of interactions in-terms of SSR in AC/DC grids, (2) design of SSR damping (SSRD) controllers, (3) experimental demonstration of SSRD schemes, and (4) assessment and improvement of frequency regulation in a wind-thermal bundled AC/DC grid. An VSC-HVDC connected series-compensated AC system resembling the Great Britain (GB) power system has been used as the test network to evaluate the operability challenges pertained to the reinforcements. A state-space representation has been formulated and an eigenvalue analysis has been performed to assess the impact of VSC-HVDC on the torsional modes of nearby connected thermal generation plants. This is followed by damping torque investigation for SSR screening with the results compared against time-domain simulations for testing the accuracy of the small-signal models for SSR studies. A series of SSRD schemes is presented which have been integrated with the VSC-HVDC to damp SSR in the series-compensated GB power system. In addition, this thesis proposes an adaptive SSRD method based on the real-time estimation of the subsynchronous frequency v Abstract component present in series-compensated transmission lines–key information for the optimal design of HVDC subsynchronous damping controllers. Furthermore, the combined AC/DC GB network has been implemented in a real-time digital simulator and connected to a VSCHVDC scaled-down test-rig to performhardware-in-the-loop tests. The efficacy and operational performance of the AC/DC network while providing SSR damping is tested through a series of experiments. In order to provide frequency support in a wind-thermal bundled AC/DC system a dualdroop controlmethod is presented. The scheme binds the system frequency with the DC voltage of an HVDC network. For completeness, the performance of the proposed method is compared to conventional frequency regulation schemes. Sensitivity studies and eigenvalue analyses are conducted to assess the impact that wind penetration and changes in the dual-droop coefficient have on grid stability. Experimental validation is performed using a real-time hardware-inthe- loop test-rig, with simulation and experimental results showing a good agreement and evidencing the superior performance of the proposed frequency support scheme

Real-time estimation and damping of SSR in a VSC-HVDC connected series-compensated system

Infrastructure reinforcement using high-voltage direct-current (HVDC) links and series compensation has been proposed to boost the power transmission capacity of existing ac grids. However, deployment of series capacitors may lead to subsynchronous resonance (SSR). Besides providing bulk power transfer, voltage source converter (VSC)-based HVDC links can be effectively used to damp SSR. To this end, this paper presents a method for the real-time estimation of the subsynchronous frequency component present in series-compensated transmission lines-key information required for the optimal design of damping controllers. A state-space representation has been formulated and an eigenvalue analysis has been performed to evaluate the impact of a VSC-HVDC link on the torsional modes of nearby connected thermal generation plants. Furthermore, the series-compensated system has been implemented in a real-time digital simulator and connected to a VSC-HVDC scaled-down test-rig to perform hardware-in-the-loop tests. The efficacy and operational performance of the ac/dc network while providing SSR damping is tested through a series of experiments. The proposed estimation and damping method shows a good performance both in time-domain simulations and laboratory experiments

Active distribution power system with multi-terminal DC links

A fast power restoration operational scheme and relevant stabilizing control is proposed for active distribution power systems with multi-terminal DC network in replacement of the conventional normal open switches. A 9-feeder benchmark distribution power system is established with a 4-terminal medium power DC system injected. The proposed power restoration scheme is based on the coordination among distributed control among relays, load switches, voltage source converters and autonomous operation of multi-terminal DC system. A DC stabilizer is proposed with virtual impedance method to damp out potential oscillation caused by constant power load terminals. The proposed system and controls are validated by frequency domain state space model and time domain case study with Matlab/Simulink

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