Secure optimal operation and control of integrated AC/MTDC meshed grids

Offshore wind energy is seen as the most promising source of electricity generation for achieving the European renewable energy targets. A number of wind farms are planned and under installation to collect the huge potential of wind energy at farther distances in the North Sea. The number of HVDC links in the North Sea is expected to increase with the development of offshore installations in Round 3 of the UK offshore windfarm programme. The increasing number of HVDC links and high power transfer control requirements leads to the formation of Multi-Terminal HVDC (MTDC) grid systems, which have become possible due to the technical advancements of VSC based HVDC systems. Additionally, a meshed MTDC grid structure can also provide interconnections for power trade across the Europe, which can help in better utilisation of power from offshore installations and can also support the AC network in tackling wind power variation issues. However, the integration of the meshed MTDC grid with the existing AC grid has more challenges to overcome alongside the added advantages. One of the major challenge is to ensure the secure and optimal operation of the combined AC/MTDC grid considering stability requirements of the AC and DC grids in different operating conditions. The behaviour of the DC grid is governed by the fast acting controllers due to the high number of power electronic equipment unlike AC grid. In combined operation the response to a disturbance of two integrated grids can be different. The power balancing, co-ordination and dispatch requirements need to be identified, to implement appropriate controls and formulate a control structure for combined operation of two grids with different characteristics under normal and disturbance conditions. In this thesis, the basic principles of well-established three-layered AC grid control is employed to identify the power balancing, coordination and dispatch requirements of the DC grid. Appropriate control methods are proposed for primary, secondary and tertiary control layers in order to accomplish the identified requirements for the secure and optimal operation of combined AC/MTDC grids. Firstly, a comparison study is performed on different power balancing controls to find the most suitable control method for the primary control of the meshed DC grid. Secondly, the combined AC/DC grid power flow method is proposed to provide updated references of the VSC station in order to maintain coordinated power flow control under secondary control layers. Finally, security constraint optimization method for combined AC/DC grid is proposed for economic dispatch under the tertiary control layer of the three-layered hierarchal control. A number of case studies are performed to implement the proposed control methods on a combined AC/DC test case network. The performance of the proposed control methods is validated in a hierarchical control structure for secure and optimal operation integrated AC/MTDC grids

Robust coordinated damping control of power systems with multi-terminal vsc-hvdc system and facts

This thesis investigates the robust and coordinated design of multiple damping controllers to ameliorate the damping characteristics of a bulky power system. A new methodology is proposed in this thesis for VSC-MTDC and FACTS damping controllers based on multiple control objectives and system multi-model. The key feature of the methodology is the robust and coordinated performance of the damping controllers. The formulated BMI-based optimization problem is solved systematically via a two- step approach. System multi-model is established in the design for the robustness of the controllers under system disturbances and changing operating conditions. The sequential design of a series of SISO controllers with properly selected feedback signals minimizes the negative interactions among the controllers. The approach is applied to a three-terminal VSC-MTDC and subsequently exerted with one terminal of VSC-MTDC and a TCSC to incorporate multiple devices and examine the generality and feasibility of the design. Given the flexible internal control configuration of VSC converter, the assessment of the impact of the d-q decoupled control modes on the effectiveness and flexibility of the damping controllers is carried out. Real-Time Digital Simulator is used to examine the effectiveness and robustness of the damping controllers under various system operating conditions and disturbances

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

Operation, control and optimization of a Meshed-HVDC system

Aquest projecte estudia el control de l'operaci o de les xarxes multi-terminals d'HVDC. Les dos estrat egies m es generalitzades, la centralitzada i droop, s on estudiades aix com el seu efecte en la operaci o del sistema. Per tal de fer aquest estudi, es presenta tamb e la modelitzaci o i el disseny i control dels convertidors VSC. Posteriorment s'han dut a terme diferents simulacions mitjan cant l'eina d'estudi PSCAD. Per tal d'analitzar aquest sistema, es presenta un m etode per calcular el ux de c arregues amb control droop. Posteriorment, es proposa un control terciari amb l'objectiu de donar una soluci o al problema de la operaci o optima en xarxes MTDC controlades distribuï dament. Finalment, per tal de validar, estudiar el seu comportament i extreure conclusions, s'han simulat diferents casos

Hybrid AC-High Voltage DC Grid Stability and Controls

abstract: The growth of energy demands in recent years has been increasing faster than the expansion of transmission facility construction. This tendency cooperating with the continuous investing on the renewable energy resources drives the research, development, and construction of HVDC projects to create a more reliable, affordable, and environmentally friendly power grid. Constructing the hybrid AC-HVDC grid is a significant move in the development of the HVDC techniques; the form of dc system is evolving from the point-to-point stand-alone dc links to the embedded HVDC system and the multi-terminal HVDC (MTDC) system. The MTDC is a solution for the renewable energy interconnections, and the MTDC grids can improve the power system reliability, flexibility in economic dispatches, and converter/cable utilizing efficiencies. The dissertation reviews the HVDC technologies, discusses the stability issues regarding the ac and HVDC connections, proposes a novel power oscillation control strategy to improve system stability, and develops a nonlinear voltage droop control strategy for the MTDC grid. To verify the effectiveness the proposed power oscillation control strategy, a long distance paralleled AC-HVDC transmission test system is employed. Based on the PSCAD/EMTDC platform simulation results, the proposed power oscillation control strategy can improve the system dynamic performance and attenuate the power oscillations effectively. To validate the nonlinear voltage droop control strategy, three droop controls schemes are designed according to the proposed nonlinear voltage droop control design procedures. These control schemes are tested in a hybrid AC-MTDC system. The hybrid AC-MTDC system, which is first proposed in this dissertation, consists of two ac grids, two wind farms and a five-terminal HVDC grid connecting them. Simulation studies are performed in the PSCAD/EMTDC platform. According to the simulation results, all the three design schemes have their unique salient features.Dissertation/ThesisDoctoral Dissertation Electrical Engineering 201

Mitigating the effects of low-inertia on HVDC-rich AC grids

The integration of large-scale power from renewable energy sources (RESs) via high voltage direct current (HVDC) transmission will contribute to the achievement of energy targets made by the government of several nations. This will help to reduce greenhouse gas emissions and combat climate change. With variable-speed wind turbines (VSWTs), maximum wind energy can be captured; additionally, multi-terminal HVDC grids (MTDC) can help to connect offshore wind farms (OWFs), solar farms and several countries, thus, aiding cross border trading, balancing services and RES integration. A major component of these technologies is power electronics which makes them not contribute to the system inertia. The research work presented in this thesis is aimed at investigating the effects of reduced system inertia in an ac grid rich in power electronics (i.e. HVDC and VSWTs) and proposing measures to mitigate these effects. The main contributions of this research work are: (1) investigating the effects of large-scale connection of VSWTs to the GB power system, (2) analysis of inertial contribution of VSWTs, (3) coordination of fast frequency support from MTDC grids and (4) experimental validation of frequency control schemes, including a proposed auxiliary dead-band controller (ADC). To investigate the inertial contribution from VSWTs, a test system consisting of a three-machine Great Britain (GB) power system connecting full power converterbased VSWTs was modelled. In this test system, the wind generation capacity was varied and the effect on the system frequency response was studied. It was observed that the system frequency deviation and rate of change of frequency (RoCoF) increased with the wind penetration increase. This study was followed by analysing the VSWT synthetic inertia capability. The temporary overproduction strategy which allows the release of stored kinetic energy during power imbalances was used. HVDC grids may provide fast frequency support to ac grids with the aid of supplementary control algorithms. Three fast frequency control schemes are presented. These supplementary control schemes are fitted with all the converters within a four-terminal HVDC grid connecting an OWF and three onshore ac grids. During periods of frequency support, undesirable power flows and reduced power transfers occurred within the grid. To prevent these issues, an ADC algorithm was proposed. The results show that the ADC improves the performance of the supplementary frequency controllers. An experimental test platform was designed to validate the fast frequency control algorithms and the ADC performance. The three-machine GB power system was implemented in a real-time digital simulator which was connected to a meshed three-terminal HVDC test rig. With this system, the frequency control schemes and ADC were validated