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

Voltage and reactive power correlation in multi-objective optimization of an offshore grid

In the last few years, many offshore wind power plants are installed at North Sea and more are under construction. Offshore grid would be vital in the integration of future offshore wind power plant with the main land grid. Both, multi-terminal DC and AC cable systems are under consideration in the concept of future offshore grid. The reactive power and voltage operating point for such a network is important for the optimized operation. This article presents an optimization criterion of voltage and reactive power control for an offshore AC grid having a parallel connecting grid-forming converter. Multi-objective optimization problem is formulated considering four reactive power management strategies. The solution of the optimization algorithm is analysed and compared with respect to active power losses in the network, voltage variation, and reactive power contribution by the sources. The research presents a methodology to apply a suitable reactive power management strategy to achieve the best optimum operating points. The solution provides the optimum operating points for offshore wind farm reactive power, frequency and voltage droop gain values for VSC-HVDC system, and reactive power sharing factor of HVDC transmission

Small signal stability analysis of proportional resonant controlled VSCs connected to AC grids with variable X/R characteristic

Capítuos 2,3 y 4 confidenciales por patente.-- Tesis completa 237.p. Tesis censurada 120 p.Para garantizar un futuro energético sostenible, es fundamental la incorporación de energías renovables en la red eléctrica. Sin embargo, con su creciente integración, las redes eléctricas AC se están volviendo cada vez más débiles, más complejas y caóticas. Por ello, se hace imprescindible el estudio de los retos técnicos que dicha integración plantea. Fenómenos como la desconexión de líneas AC, el bloqueo de convertidores, o variaciones de carga debidas a las intermitencias de la generación renovable, están comenzando a producir cambios en los valores de impedancia y en las características inductivo-resistivas de incluso las redes fuertes. Conforme una red AC se debilita su impedancia equivalente aumenta, y esto provoca cambios indeseados en las magnitudes de potencia activa y reactiva, que derivan en variaciones repentinas de tensión en diferentes puntos de la red AC. Esto también conlleva el deterioro de los convertidores y empeoramiento de la calidad de onda. Una solución parcial a este problema es limitar la potencia allí donde se genera, en perjuicio de aumentar las pérdidas locales. Otra solución es introducir controles de convertidores más robustos, para que sean capaces de sortear estos escenarios cada vez más frecuentes. En este contexto, los convertidores de fuente de tensión (VSC), y en especial los convertidores modulares multinivel, presentan una serie de prestaciones que los hacen idóneos para esta clase de escenarios, dado su mejor comportamiento dinámico frente a los convertidores de fuente de corriente, al operar a una frecuencia de conmutación mayor, y presentando capacidades LVRT y control desacoplado de potencia activa y reactiva. Entre los controles internos de corriente de los VSCs, los controladores proporcional resonantes han aparecido como alternativa a los proporcional integrales, debido a su capacidad de manejar operación tanto equilibrada como desequilibrada y a que eliminan lanecesidad de utilizar un phase-locked loop y las transformadas de Park. Muy pocos estudios se han realizado con VSCs con control proporcional resonante sujetos a cambios en la fortaleza de la red AC, y menos aun considerando la variación de su característica inductivo-resistiva. Por lo tanto, en esta tesis doctoral se propone una metodología de parametrización del control proporcional resonante de un VSC conectado a una red AC con fortaleza y característica inductivo-resistiva variables, que asegure su estabilidad en pequeña señal. Con el objetivo de caracterizar dicha estabilidad, se construye un modelo de pequeña señal del sistema compuesto por el VSC conectado a red AC. Posteriormente se valida con simulaciones EMT y se procede con el análisis de escenarios. Los resultados del análisis demuestran que tan solo una desviación del 20% en el ratio X/R de la red AC con respecto a su valor habitual puede hacer perder al sistema su estabilidad en pequeña señal cuando la red AC es débil. La metodología propone nuevas parametrizaciones del control proporcional resonante del VSC que devuelven la estabilidad al sistema en estos escenarios. La validación y verificación de la metodología se realiza a través de un caso de estudio en DIgSILENT PF: una planta de generación eólica marina que evacúa energía a la red AC por medio de un enlace de alta tensión en continua

Stability and interaction analysis in islanded power systems including VSC-HVDC and LCC-HVDC power converters

Islanded power systems are often connected to larger mainland power systems using HVDC cables. These interconnections are used to import power at lower cost compared to local generation and improve the security of supply. The increase of HVDC interconnectors in islanded systems will allow the reduction of local synchronous generation, which might lead to new interaction and stability problems due to the low inertia and short-circuit power available in the system. Traditionally LCC-HVDC technology has been used to connect island grids, but recently VSCs are presented as an alternative solution that offers more controllability to the islanded grid. Therefore, in order to increase the power transfer to the islands multi-infeed hybrid HVSC systems with VSCs and LCCs might become a common solution. The introduction of VSCs in islanded systems will allow operations in weak grids, but possible interactions with LCCs must be analysed in detail. This paper introduces the potential interactions in multi-infeed HVDC systems with LCCs and VSCs. An initial benchmark model of an islanded power system with a LCC and a VSC-HVDC link is presented to analyse new interaction phenomena between the converters and the islanded AC grid. Simulation results in PSCAD/EMTDC are presented to validate the benchmark model for voltage stability and commutation failure analysis.Postprint (published version

Power Flow Control in Multi-Terminal HVDC Grids Using a Serial-Parallel DC Power Flow Controller

© 2013 IEEE. Multi-terminal HVDC (MT-HVDC) grids have no capability of power flow control in a self-sufficient manner. To address this important issue, utilization of dc-dc high power and high-voltage converters is motivated. However, proposing suitable partial-rated dc-dc converters as well as their suitable modeling and control in both primary and secondary control layers as well as the stability analysis are the existing challenges that should be alleviated beforehand. This paper addresses the control of power flow problem through the application of a power converter with a different connection configuration, namely, serial parallel dc power flow controller (SPDC-PFC). The SPDC-PFC input is the transmission line voltage, and its output is transmission line current. Therefore, employing a full-power dc-dc converter is avoided as a merit. Additionally, in this paper, the common two-layer MT-HVDC grid control framework comprised of primary and secondary layers is efficiently modified in order to integrate the SPDC-PFC. A differential direct voltage versus active power droop control scheme is applied to the SPDC-PFC at the local control layer, guaranteeing dynamic stability, while an extended dc power-flow routine - integrating the SPDC-PFC - is developed at the secondary control layer to ensure the static stability of the entire MT-HVDC grid. The proposed control framework enables the SPDC-PFC to regulate the flow of current/power in the envisioned HVDC transmission line. From the static and dynamic simulation results conducted on the test CIGRE B4 MT-HVDC grid, successful operation of the proposed SPDC-PFC and control solutions are demonstrated by considering power flow control action. In more detail, the SPDC-PFC successfully regulates the compensated lines' power to the desired reference both in static and dynamic simulations by introducing suitable compensation voltages. In addition, good dynamic performance under both SPDC-PFC power reference and wind power-infeed change is observed

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

Topology assessment for 3 + 3 terminal offshore DC grid considering DC fault management

Peer reviewedPostprin

Recent developments in HVDC transmission systems to support renewable energy integration

The demands for massive renewable energy integration, passive network power supply, and global energy interconnection have all gradually increased, posing new challenges for high voltage direct current (HVDC) power transmission systems, including more complex topology and increased diversity of bipolar HVDC transmission. This study proposes that these two factors have led to new requirements for HVDC control strategies. Moreover, due to the diverse applications of HVDC transmission technology, each station in the system has different requirements. Furthermore, the topology of the AC-DC converter is being continuously developed, revealing a trend towards hybrid converter stations. Keywords: Direct current transmission system, Topology, Control strategy, AC-DC converte