249 research outputs found
VSC-HVDC based Network Reinforcement
No full textThe recent development in semiconductor and control equipment has made HVDC transmission based on voltage-source converter (VSC-HVDC) feasible. VSC-HVDC has numerous advantages such as a controllable short-circuit current contribution, the rapid and independent control of active and reactive power, and good power quality. With these advantages VSC-HVDC can likely be used to solve constraints in transmission and distribution networks. In this thesis work, the technical characteristics of a coupling between two sub-transmission systems on 50 kV level through VSC-HVDC are investigated. The main objective of the VSC-HVDC link is to mitigate network constraints in one of these networks. Area 1 is a greenhouse area having a large penetration level of CHP-plants, and Area 2 feeds a 50 kV grid that is expected to be constrained in the nearby future. The dynamic reactive power support capability of the VSC-HVDC and the available distributed generation in Area 1 are used to solve the constraints in Area 2. It is shown how the VSC-HVDC link is used as an innovative solution to mitigate network constraints and postpone major reinforcements of the network infrastructure.High-voltage Components and Power SystemsElectrical Engineering, Mathematics and Computer Scienc
Optimized Control of LCL-VSC Converter With Refined s-Parameter
No full textFor the control of high-voltage dc (HVdc) systems, especially for that of the multiterminal HVdc (MTdc) systems, the voltage source converter (VSC) is a good option because of its high controllability. These days, different types of VSC converters have been realized such as the two/three level converter and modular multilevel converter. However, VSC converters are vulnerable against dc faults because the paralleled diodesmay experience large fault currents. In order to maintain the sustainability of electricity delivery, efforts have been paid on protecting the HVdc networks, such as the novel converter topologies with the capability to tolerate faults and the dc circuit breaker. Among which, the concept of the inductor-capacitor-inductor circuit (LCL)-VSC converter aims at enhancing the ability of converter to ride through dc faults, which limits currents flowing fromthe ac side to dc side. The proposed method in this paper optimizes the control of LCL-VSC for partial load so that the power loss can be drastically decreased. In addition, the preferable working range for the LCL converter is introduced to guarantee the ability of restraining fault currents. The method is verified on the PSCAD/EMTdc platform.Accepted Author ManuscriptIntelligent Electrical Power Grid
Directional Derivative-Based Method for Quasi-Stationary Voltage Support Analysis of Single-Infeed VSC-HVDC units
No full textThis paper presents an investigation of the impact of the quasi-stationary voltage support provided by a voltage source converter (VSC) connected to a single point of a power system. Based on the directional derivative concept, an analytical method is developed to quantify the sensitivities of the AC bus voltage with respect to the VSC reactive power control modes. Based on a real case study it is shown that the method is applicable to VSC units that are part of VSC-HVDC systems, which can operate in a point-to-point or multi-terminal configuration. Time-domain simulations are performed to verify the findings from the application of the analytical method on a reduced size power system
DC Cable Short Circuit Fault Protection in VSC-MTDC
No full textWith the development of offshore wind farms, Voltage Source Converter based High Voltage Direct Current or Multi-terminal High Voltage Direct Current Technology (VSC-HVDC/MTDC) is becoming promising in the field of large-capacity and long-distance power transmission. However, its extreme vulnerability to DC contingencies remains a challenge in both research and practice. DC cable short circuit faults, or cable pole-to-pole faults, though less common than DC cable ground faults, can cause the most severe damage to the VSC as well as the whole system. In this thesis work, firstly a simple 3-terminal MTDC system is built and validated in PSCAD/EMTDC. Afterwards, based on the self-built MTDC system, DC cable short circuit faults with different locations are studied and analyzed in both theory and numerical simulation. Finally, a comprehensive protection scheme is proposed against such DC cable short circuit faults in the target MTDC system, combining the sub-schemes in fault detecting/locating principles, fault isolating tools and fault current limiting technologies. The coordination among the three parts is also taken into consideration. The scheme is later proven to be fast, selective, reliable, sensitive and robust in general. Moreover, the specific design procedure is further extended into a general design philosophy for DC cable short circuit fault protection in VSC-MTDC systems.Intelligent Electrical Power GridsElectrical EngineeringElectrical Engineering, Mathematics and Computer Scienc
Fault Ride-Through Strategies for VSC-Connected Wind Parks
No full textHigh-voltage direct-current transmission based on voltage-sourced converters (VSC-HVDC) is an economic connection technology for large remote wind parks. Wind power plants that are connected through a VSC-HVDC link are subjected to the same technical connection requirements as conventional generators. The requirement that these installations have to remain synchronously connected to the network during faults, is particularly challenging. Power electronic converters have no over-loading capabilities and during faults they can only supply a fault current up to the rated current. Since the wind park that is connected to the link does not directly notice the fault, a power imbalance is caused across the link that results in uncontrollability of the direct voltage. Such an uncontrollability might lead to tripping of the link and must be prevented. Three strategies are described to keep the direct voltage controllable during network faults. The first strategy is formed by overdimensioning of the GSVSC. The second is formed by dissipation of the excess wind power during the fault in a braking resistor. The third strategy is the fast reduction of the wind power production, and can be achieved by direct communication, voltage reduction and frequency increase. A reliable and costeffective solution combines over-dimensioning and/or power dissipation with one of the power reduction methods. An optimal system design can be found with the help of an optimization procedure, in which the system costs are minimized.Delft University of Technolog
Methodology for mapping operational zones in VSC-HVDC transmission sytems.
No full textSistemas de transmissão de energia elétrica em corrente contínua e alta tensão baseados em tecnologia de conversores a fonte de tensão (VSC-HVDC), ao contrário de linhas de transmissão em corrente alternada, operam como elementos de controle de variáveis elétricas, podendo ser úteis na estabilidade do sistema de potência. Mas apesar desta vantagem, sistemas VSC-HVDC possuem limitações no desempenho estável, o que enseja o desenvolvimento de uma metodologia para mapeamento de suas zonas de operação estável e possíveis regiões de instabilidade. Inicialmente estudou-se os detalhes da tecnologia VSC-HVDC tais como o funcionamento da eletrônica de potência e estratégias de controle utilizadas. Em seguida, investigou-se os modelos de geradores síncronos para interconexão com o lado CA das estações conversoras do VSC-HVDC. E, finalmente, aplicou-se a tecnologia VSC-HVDC sobre um modelo de sistema de potência com uma estação conversora localizada em um porto offshore e uma outra no continente, próxima à rede de alta tensão em corrente alternada. Simulações e análise deste sistema foram executadas considerando várias condições operacionais. O gráfico de potência gerada e consumida, obtido pela aplicação da metodologia, apresenta grande potencial de uso prático como por exemplo sua implementação na interface homem-máquina da estação de operação do porto offshore, provendo informação em tempo real de alto nível ao operador do sistema elétrico do porto offshore e consequentemente aumentando sua consciência situacional quanto a proximidade dos limites de instabilidade.High voltage direct current power transmission systems based on voltage source converters (VSC-HVDC), as opposed to alternating current ones, operates as elements of control of electrical variables, being useful for stability of power system. Besides this advantage, VSC-HVDC systems have limitations in stable performance, which instigates the development of a methodology for mapping its operational zones of stability and possible regions of instability. The author initially studied the details of the VSC-HVDC technology such as the power electronic principles and the control strategies used on this research. Subsequently, the author investigated synchronous generator models for interconnection on the AC side of the VSC-HVDC converter stations. Finally, the author applied the VSC-HVDC technology on a model of power system with two converter stations, one located on an offshore port and the other on the shore, next to an alternating current high voltage power grid. Simulations and analysis of this system were carried out considering various operational conditions. The graphic of generated and consumed power on offshore port, obtained by the application of the methodology for mapping operational zones, presents a great potential of being implemented in the man-machine interface of an operation workstation, thus providing high level online information for the operator of the offshore port electrical system and consequently improving its situational awareness of the proximity to instability limits
New Stationary Frame Control Scheme for Three Phase PWM Rectifiers Under Unbalanced Voltage Dips Conditions
No full textA new stationary frame control scheme for three-phase pulsewidth-modulation (PWM) rectifiers operating under unbalanced voltage dips conditions is proposed in this paper. The proposed control scheme regulates the instantaneous active power at the converter poles to minimize the harmonics of the input currents and the output voltage ripple. This paper's novelty is the development of a new current-reference generator implemented directly in stationary reference frame. This allows using proportional sinusoidal signal integrator (P-SSI) controllers for simultaneous compensation of both positive and negative current sequence components. No phase-locked loop (PLL) strategies and coordinate transformations are needed for the proposed current-reference generator. Experimental results are presented for a 20-kV A alternative current (ac)/direct current (dc) converter prototype to demonstrate the effectiveness of the proposed control scheme. A comparison with two other existing control techniques is also performed. Fast dynamic performance with small dc-link voltage ripple and input sinusoidal currents are obtained with this control scheme, even under severe voltage dips operating condition
Offshore wind power plants with VSC-HVDC transmission: Grid code compliance optimization and the effect on high voltage ac transmission system
No full textThe development of large offshore wind power generation in the North Sea has been significantly accelerated in the last years. The large distance from shore in combination with the need for large transmission capacity has raised the interest for the voltage source converter high voltage direct current technology (VSC-HVDC). Transmission system operators in order to ensure high degree of the power system security of supply, impose strict grid connection requirements to offshore wind power plants and their HVDC transmission. Based on these boundary conditions, the overall research objectives that have been assessed in the context of this thesis include the following. Assessment of the state of the art coordinated fault-ride through strategies for offshore wind power plants with VSC-HVDC transmission. Analysis of unbalanced grid faults for wind power plants with VSC-HVDC transmission. Investigation of the effect of negative sequence current control for onshore and offshore AC faults. Analysis of the effect of typical grid codes on the power system voltage and rotor angle stability. The developed methodologies, models and control schemes proposed within the context of this thesis could facilitate the analysis and stable operation of transmission systems with VSC-HVDC connected offshore wind power plants.Intelligent Electrical Power Grid
A soft start-up method for DC micro-grid based on improved two-level VSC with DC fault ride-through capability
No full textFor the black start-up of DC micro-grids, three-phase charging resistors are required to limit the uncontrollable surge current. The main drawback of this start-up method is the difficulty in determining the appropriate resistance value to achieve a rapid start-up and limit the surge current with the change of grid parameters. To address this problem, this article proposes a soft start-up method for the DC micro-grid based on an improved two-level voltage source converter (VSC). Specifically, an silicon controlled rectifier and anti-parallel diode are added in each up-bridge-arm in the improved VSC. By conducting a dynamic control strategy of the firing angle on the SCRs, the start-up current can always be maintained near a given value to achieve rapid start-up. Moreover, the improved VSC has DC fault ride-through capability. The simulation results based on PSCAD/EMTDC are provided to validate the feasibility of the proposed start-up method.Intelligent Electrical Power Grid
Offshore VSC-HVDC Networks: Impact on Transient Stability of AC Transmission Systems
No full textThe transition towards a sustainable society calls for the massive deployment of renewable energy sources such as large wind parks located far offshore. High-voltage direct current transmission based on voltage sourced converter technology (VSC-HVDC) offers a wide range of technological benefits that foster the grid integration of offshore wind parks. Coupling AC and HVDC grids comes with significant challenges. Control and system functions, which were formerly separated, interact, especially during faults in the transmission system. Classical (transient stability) modelling and simulation does not suffice and must be made ready for VSC-HVDC.This Ph.D. thesis answers two questions to master these challenges. First, what is the impact of the operation and control of a, possibly multi-terminal, offshore grid based on VSC-HVDC on the transient stability of the onshore power system? Second, how can we model and simulate these impacts while maintaining the desired simulation accuracy and speed? The results of this thesis facilitate fast and accurate assessment of stability impacts of large transmission systems with a significant proportion of converter-interfaced generation
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