Distributed Primary Frequency Control through Multi-Terminal HVDC Transmission Systems

This paper presents a decentralized controller for sharing primary AC frequency control reserves through a multi-terminal HVDC grid. By using Lyapunov arguments, the proposed controller is shown to stabilize the equilibrium of the closed-loop system consisting of the interconnected AC and HVDC grids, given any positive controller gains. The static control errors resulting from the proportional controller are quantified and bounded by analyzing the equilibrium of the closed-loop system. The proposed controller is applied to a test grid consisting of three asynchronous AC areas interconnected by an HVDC grid, and its effectiveness is validated through simulation

Distributed Secondary Frequency Control through MTDC Transmission Systems

In this paper, we present distributed controllers for sharing primary and secondary frequency control reserves for asynchronous AC transmission systems, which are connected through a multi-terminal HVDC grid. By using Lyapunov arguments, the equilibria of the closed-loop system are shown to be globally asymptotically stable. We quantify the static errors of the voltages and frequencies, and give upper bounds for these errors. It is also shown that the controllers have the property of power sharing, i.e., primary and secondary frequency control reserves are shared fairly amongst the AC systems. The proposed controllers are applied to a high-order dynamic model of of a power system consisting of asynchronous AC grids connected through a six-terminal HVDC grid.Comment: arXiv admin note: text overlap with arXiv:1409.801

Voltage stabilization in DC microgrids: an approach based on line-independent plug-and-play controllers

We consider the problem of stabilizing voltages in DC microGrids (mGs) given by the interconnection of Distributed Generation Units (DGUs), power lines and loads. We propose a decentralized control architecture where the primary controller of each DGU can be designed in a Plug-and-Play (PnP) fashion, allowing the seamless addition of new DGUs. Differently from several other approaches to primary control, local design is independent of the parameters of power lines. Moreover, differently from the PnP control scheme in [1], the plug-in of a DGU does not require to update controllers of neighboring DGUs. Local control design is cast into a Linear Matrix Inequality (LMI) problem that, if unfeasible, allows one to deny plug-in requests that might be dangerous for mG stability. The proof of closed-loop stability of voltages exploits structured Lyapunov functions, the LaSalle invariance theorem and properties of graph Laplacians. Theoretical results are backed up by simulations in PSCAD

Control of multi-terminal HVDC networks towards wind power integration: A review

© 2015 Elsevier Ltd. More interconnections among countries and synchronous areas are foreseen in order to fulfil the EU 2050 target on the renewable generation share. One proposal to accomplish this challenging objective is the development of the so-called European SuperGrid. Multi-terminal HVDC networks are emerging as the most promising technologies to develop such a concept. Moreover, multi-terminal HVDC grids are based on highly controllable devices, which may allow not only transmitting power, but also supporting the AC grids to ensure a secure and stable operation. This paper aims to present an overview of different control schemes for multi-terminal HVDC grids, including the control of the power converters and the controls for power sharing and the provision of ancillary services. This paper also analyses the proposed modifications of the existing control schemes to manage high participation shares of wind power generation in multi-terminal grids.Postprint (author's final draft

Distributed Controllers for Multi-Terminal HVDC Transmission Systems

High-voltage direct current (HVDC) is an increasingly commonly used technology for long-distance electric power transmission, mainly due to its low resistive losses. In this paper the voltage-droop method (VDM) is reviewed, and three novel distributed controllers for multi-terminal HVDC (MTDC) transmission systems are proposed. Sufficient conditions for when the proposed controllers render the equilibrium of the closed-loop system asymptotically stable are provided. These conditions give insight into suitable controller architecture, e.g., that the communication graph should be identical with the graph of the MTDC system, including edge weights. Provided that the equilibria of the closed-loop systems are asymptotically stable, it is shown that the voltages asymptotically converge to within predefined bounds. Furthermore, a quadratic cost of the injected currents is asymptotically minimized. The proposed controllers are evaluated on a four-bus MTDC system.Comment: arXiv admin note: substantial text overlap with arXiv:1406.5839, arXiv:1311.514

A System Reference Frame Approach for Stability Analysis and Control of Power Grids

During the last decades, significant advances have been made in the area of power system stability and control. Nevertheless, when this analysis is carried out by means of decentralized conditions in a general network, it has been based on conservative assumptions such as the adoption of lossless networks. In the current paper, we present a novel approach for decentralized stability analysis and control of power grids through the transformation of both the network and the bus dynamics into the system reference frame. In particular, the aforementioned transformation allows us to formulate the network model as an input-output system that is shown to be passive even if the network's lossy nature is taken into account. We then introduce a broad class of bus dynamics that are viewed as multivariable input/output systems compatible with the network formulation, and appropriate passivity conditions are imposed on those that guarantee stability of the power network. We discuss the opportunities and advantages offered by this approach while explaining how this allows the inclusion of advanced models for both generation and power flows. Our analysis is verified through applications to the Two Area Kundur and the IEEE 68-bus test systems with both primary frequency and voltage regulation mechanisms included

An improved droop control method for multi-terminal VSC-HVDC converter stations

Multi-terminal high voltage direct current transmission based on voltage source converter (VSC-HVDC) grids can connect non-synchronous alternating current (AC) grids to a hybrid alternating current and direct current (AC/DC) power system, which is one of the key technologies in the construction of smart grids. However, it is still a problem to control the converter to achieve the function of each AC system sharing the reserve capacity of the entire network. This paper proposes an improved control strategy based on the slope control of the DC voltage and AC frequency (V–f slope control), in which the virtual inertia is introduced. This method can ensure that each AC sub-system shares the primary frequency control function. Additionally, with the new control method, it is easy to apply the secondary frequency control method of traditional AC systems to AC/DC hybrid systems to achieve the steady control of the DC voltage and AC frequency of the whole system. Most importantly, the new control method is better than the traditional control method in terms of dynamic performance. In this paper, a new control method is proposed, and the simulation model has been established in Matlab/Simulink to verify the effectiveness of the proposed control method

Primary Frequency Regulation with Load-Side Participation-Part II: Beyond Passivity Approaches

We consider the problem of distributed generation and demand control for primary frequency regulation in power networks, such that stability and optimality of the power allocation can be guaranteed. It was shown in [1] that by imposing an input strict passivity condition on the net supply dynamics at each bus, combined with a decentralized condition on their steady state behaviour, convergence to optimality can be guaranteed for broad classes of generation and demand control dynamics in a general network. In this paper we show that by taking into account additional local information, the input strict passivity condition can be relaxed to less restrictive decentralized conditions. These conditions extend the classes of generation and load dynamics for which convergence to optimality can be guaranteed beyond the class of passive systems, thus allowing to reduce the conservatism in the analysis and feedback design.ER

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