Control and operation of multi-terminal VSC-DC networks

For the past century, ac networks have been established as the standard technology for electrical power transmission system s. However, the de technology has not disappeared completely from this picture. The capability of de systems to transmit higher power over longer distances, the possibility of interconnecting asynchronous networks, and their high efficiency has maintained the interest of both industry and academia. Historically, systems based on dc-generators and mercury valves were used for de power transmission applications, but, by the 90's, all installations were thyrsi tor-based line commutated converters (LCC). In 1999, the first system based on voltage source converters (VSC) was installed in Gotland, Sweden, marking the beginning of a new era for de transmission. Over the past 15 years, the power rating of VSC-based de transmission systems has increased from 50 to 700 MVV, the operating voltage from 120 to 500 kV, meanwhile , the covered distances have become as long as 950 km (ABB's HVDC-light installation in Namibia in 2010). The work presented in this thesis is oriented towards the control and operation of multi-terminal VSC de (MTDC) networks. The proposed approach is a hierarchical control architecture, inspired by the well-established automatic generation control strategy applied to ac networks. In the proposed architecture, the primary control of the MTDC system is decentralized and implemented using a generalized droop strategy More than analyzing the behavior of the primary control, this thesis provides a methodology for designing the various parameters that influence this behavior. The importance of correctly dimensioning the VSC's output capacitor is underlined as this element, when set in the context of a MTDC network, becomes the inertial element of the grid and it has a direct impact on the voltage overs hoots that appear during transients. Further on, an improved droop control strategy that attenuates the voltage oscillations during transients is proposed. Also part of the proposed hierarchical control, the secondary control is centralized and it regulates the operating point of the network so that optimal power flow (OPF) is achieved . Compared to other works, this thesis elaborates, both analytically and through simulations, on the coordination between the primary and secondary control layers.Durante el siglo pasado, las redes de corriente alterna se han consolidado como la tecnología estándar para los sistemas de transmisión de energía eléctrica. Sin embargo, los sistemas de transmisión en continua se han seguido utilizando en algunas aplicaciones. La capacidad de estos para transmitir mayores potencias a distancias más largas, la posibilidad de interconectar redes asincrónicas, y su alta eficiencia han propiciado que se mantuviera el interés académico, de investigación e industrial en esta tecnología . Aunque históricamente se utilizaron sistemas basados en generadores de continua y válvulas de mercurio para las redes de transmisión, en la década de los 90 todas las instalaciones ya contaban con convertidores conmutados basados en tiristores (LCC). En 1999, se instaló el primer sistema basado en convertidores en fuente de tensión (VSC) en Gotland, Suecia, marcando el comienzo de una nueva era para la transmisión en corriente continua. En los últimos 15 años, la potencia de los sistemas de transmisión en continua basados en VSC ha aumentado desde los 50 hasta los 700 MN, la tensión de servicio de 120 a 500 kV y las distancias recorridas han llegado a ser, en algunos casos, de hasta 950 kilómetros (HVDC-light de ABB en Namibia en 201 O). El trabajo presentado en esta tesis se centra en el control y operación de redes de corriente continua VSC multi-terminal (MTDC). El enfoque propuesto se basa en una arquitectura de control jerárquico, inspirada en la estrategia de control de generación automática aplicada a redes de corriente alterna. En la arquitectura propuesta, el control primario del sistema MTDC está descentralizado e implementado mediante una estrategia de 'droop' generalizada. Más allá del análisis del comportamiento del control primario, esta tesis presenta una metodología para el diseño de los diferentes parámetros que influyen en el mismo. Se destaca la importancia de dimensionar correctamente condensador de salida del VSC, ya que este elemento, cuando se encuentra en el contexto de una red MTDC, se convierte en el elemento inercial de la red y tiene un impacto directo en el comportamiento transitorio de las tensiones. Asimismo, se propone una estrategia de control de 'droop' mejorada que atenúa las oscilaciones de tensión durante los transitorios. En el marco del control jerárquico propuesto, el control secundario está centralizado y regula el punto de funcionamiento de la red de manera que se consigue un flujo de potencia óptimo (OPF). En comparación con otros trabajos, esta tesis lleva a cabo, tanto de forma analítica como a través de simulaciones, un estudio detallado sobre la coordinación entre las capas de control primario y secundario en redes MTDC

Optimal Power-Sharing Control for MTDC Systems

Power systems have been developing over the past few decades, especially in terms of increasing efficiency and reliability, as well as in meeting the recent rapid growth in demand. Therefore, High Voltage Direct Current (HVDC) systems are considered to be one of the most promising and important contenders in shaping the future of modern power systems. A number of trends demonstrate the need to implement Multi-terminal Direct Current (MTDC) systems, including the integration into the conventional grid of renewable energy resources such as photovoltaic (PV) and offshore wind farms. The transmission of power from or to remote areas, such as the North Sea in Europe, is another initiative that is required in order to meet the high demand for power. The interconnection between countries with different levels of frequencies over a long distance is a fundamental application of HVDC grids as well as hybrid AC/DC transmission systems. The industry has also played an essential role in the accelerated progress in power electronics devices regarding cost and quality. Consequently, Voltage Source Converter based-High Voltage Direct Current (VSC-HVDC) systems has recently attracted considerable attention in the research community. This type of HVDC systems has a significant advantage over the classic Current Source Converter based-HVDC (CSC-HVDC) in terms of the independent control of both active and reactive power. Since VSC-HVDC is now being implemented in various applications, this requires a close examination of the behavior of both the economic and operational issues of both VSC-HVDC stations and MT-HVDC systems. This thesis proposes an optimal power-sharing control of MT-HVDC systems using a hierarchical control structure. In the proposed control scheme, the primary control is decentralized and operated by a DC voltage droop control. This method regulates the voltage source converters (VSCs) and guarantees a stable DC voltage throughout the system even in the presence of sudden changes in power flow. A centralized optimal power flow (OPF) is implemented in the secondary control to set the droop gains, and voltage settings in order to fulfil a multi-objective function. This aims at minimizing the losses in DC grid lines and converter stations by an optimization algorithm, namely Semidefinite Programming (SDP). Therefore, an optimal power-sharing result is achieved taking into consideration the losses of both transmission lines and converters, as well as failure intervals of the system. The proposed control scheme was tested on a modified CIGRE B4 DC grid test system based on the PSCAD/EMTDC and MATLAB in which the primary control was designed and simulated in the former, whereas the latter was used to run the SDP algorithm

Coordinated Supervisory Control of Multi-Terminal HVDC Grids: a Model Predictive Control Approach

A coordinated supervisory control scheme for future multi-terminal High-Voltage Direct-Current (HVDC) grids is proposed. The purpose is to supervise the grid and take appropriate actions to ensure power balance and prevent or remove voltage or current limit violations. First, using DC current and voltage measurements, the power references of the various Voltage Sources Converters (VSC) are updated according to participation factors. Next, the setpoints of the converters are smoothly adjusted to track those power references, while avoiding or correcting limit violations. The latter function resorts to Model Predictive Control and a sensitivity model of the system. The efficiency of the proposed scheme has been tested through dynamic simulations of a five-terminal HVDC grid interconnecting two asynchronous AC areas and a wind farm

An improved voltage compensation approach in a droop-controlled DC power system for the more electric aircraft

This paper proposes an improved voltage regulation method in multi-source based DC electrical power system in the more electric aircraft. The proposed approach, which can be used in terrestrial DC microgrids as well, effectively improves the load sharing accuracy under high droop gain circumstance with consideration of cable impedance. Since no extra communication line and controllers are required, it is easily implemented and also increases the system modularity and reliability. By using the proposed approach the DC transmission losses can be reduced and system stability is not deteriorated for normal and fault scenarios. In this paper optimal droop gain settings are investigated and the selection of individual droop gains as well as the proportional power sharing ratio has been described. Experimental results validate the effectiveness of the proposed method

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

A generalized approach for design of contingency versatile DC voltage droop control in multi-terminal HVDC networks

The non-deterministic nature of power fluctuations in renewable energy sources impose challenges to the design of DC voltage-droop controller in Multi-Terminal High-Voltage DC (MTDC) systems. Fixed droop control does not consider converters’ capacity and system operational constraints. Consequently, an adaptive droop controller is counseled for appropriate power demand distribution. The previous adaptive droop control studies based on the converters’ Available-Headroom (AH) have lacked the demonstration of the droop gain design during consecutive power disturbances. In this paper, the design of the adaptive DC voltage droop control is investigated with several approaches, based on the permitted converters’ global and/or local AH and Loading Factor (LF). Modified adaptive droop control approaches are presented along with a droop gain perturbation technique to achieve the power-sharing based on the converters’ AH and LF. In addition, the impact of Multi-Updated (MU), Single-Updated (SU), and Irregular-Updated (IU) droop gains is investigated. The main objective of the adaptive droop control design is to minimize the power-sharing burden on converters during power variations/consecutive disturbances while maintaining the constraints of the DC grid (i.e., voltage and power rating). The presented approaches are evaluated through case studies with a 4-terminal and 5-terminal radial MTDC networks.Qatar Foundation; Qatar National Research FundScopu

Optimal power flow for resistive DC networks: a port-hamiltonian approach

This paper studies the optimal power flow problem for resistive DC networks. The gradient method algorithm is written in a port-Hamiltonian form and the stability of the resulting dynamics is studied. Stability conditions are provided for general cyclic networks and a solution, when these conditions fail, is proposed. In addition, the results are exemplified by means of numerical simulations.Postprint (published version