Multi-terminal HVDC grids with inertia mimicry capability

The high-voltage multi-terminal dc (MTDC) systems are foreseen to experience an important development in the next years. Currently, they have appeared to be a prevailing technical and economical solution for harvesting offshore wind energy. In this study, inertia mimicry capability is added to a voltage-source converter-HVDC grid-side station in an MTDC grid connected to a weak ac grid, which can have low inertia or even operate as an islanded grid. The presented inertia mimicry control is integrated in the generalised voltage droop strategy implemented at the primary level of a two-layer hierarchical control structure of the MTDC grid to provide higher flexibility, and thus controllability to the network. Besides, complete control framework from the operational point of view is developed to integrate the low-level control of the converter stations in the supervisory control centre of the MTDC grid. A scaled laboratory test results considering the international council on large electric systems (CIGRE) B4 MTDC grid demonstrate the good performance of the converter station when it is connected to a weak islanded ac grid.Peer ReviewedPostprint (author's final draft

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

Analysis on impacts of the shunt conductances in multi-terminal HVDC grids optimal power-flow

This study deals with impacts of the shunt conductances associated with HVDC cables and VSC-HVDC stations on optimal operation of Multi-Terminal HVDC (MT-HVDC). In this study, for the first time, shunt conductances are integrated to HVDC Optimal Power-Flow (OPF) program that is executed at the Power Dispatch Center (PDC) of the MT-HVDC grid. With the objective of losses minimization, optimal reference operation points of the VSC-HVDC stations are derived. The operating points of the power converter stations are adjusted based on the calculations performed in the dispatch center. CIGRE DCS3 MT-HVDC grid, structured by CIGRE B4 working group, is taken as the test platform. Test results have revealed the optimum voltages and loss pattern change. Moreover, the findings are compared with the case of neglecting the shunt conductances.Peer ReviewedPostprint (author's final draft

Optimal power flow and unified control strategy for multi-terminal HVDC systems

© 2019 IEEE. This paper presents an automation strategy for multi-terminal HVDC (MT-HVDC) systems combining a dc optimal power flow (dc OPF) routine and a unified reference controller (URC). In the presented automatic framework, the dc OPF algorithm is implemented at the power dispatch center (PDC) of the MT-HVDC system to find optimal reference operation points of the power converters to minimize the losses during the operation of the MT-HVDC grid and solves the contradiction between minimizing losses and preventing commutation failure. At the local control systems, the operating points of the voltage-source converter (VSC) stations are tuned based on the calculations executed in the PDC, which enables fast response to power fluctuation and ensures a stable dc voltage. However, if the communication between the two control layers is lost, the MT-HVDC grid remains stable based on the pre-defined $V$-$P$ droop characteristics for the power converter stations till the connection establishes again, and a set of new operating points is generated and sent. The static and dynamic simulations conducted on the CIGRE B4 HVDC test grid establish the efficient and effective grid control performance with the proposed automation strategy. The analysis shows that the proposed control scheme achieves the desired minimum losses while, at the same time, satisfying the system constraints

A multi-terminal HVdc grid topology proposal for offshore wind farms

© 2020 by the authors. Although various topologies of multi-terminal high voltage direct current (MT-HVdc) transmission systems are available in the literature, most of them are prone to loss of flexibility, reliability, stability, and redundancy in the events of grid contingencies. In this research, two new wind farms and substation ring topology (2WF-SSRT) are designed and proposed to address the aforementioned shortcomings. The objective of this paper is to investigate MT-HVdc grid topologies for integrating large offshore wind farms with an emphasis on power loss in the event of a dc grid fault or mainland alternating current (ac)grid abnormality. Standards and control of voltage source converter (VSC) based MT-HVdc grids are defined and discussed. High voltage dc switch-gear and dc circuit topologies are appraised based on the necessity of dc cables, HVdc circuit breakers, and extra offshore platforms. In this paper, the proposed topology is analyzed and compared with the formers for number and ratings of offshore substations, dc breakers, ultra-fast mechanical actuators, dc circuits, cost, flexibility, utilization, and redundancy of HVdc links. Coordinated operation of various topologies is assessed and compared with respect to the designed control scheme via a developed EMTDC/PSCAD simulation platform considering three fault scenarios: dc fault on transmission link connecting the wind farm to mainland power converters, dc fault within substation ring of VSC-HVdc stations, and ultimate disconnection of grid side VSC station. Results show that 2WF-SSRT is a promising topology for future MT-HVdc grids

Modular multilevel converter based HVDC transmission system for offshore wind farms

This doctoral thesis falls within the scope of electronic power converters oriented to high voltage transmission applications, in particular the power generated in remote offshore wind farms by means of HVDC subsea cables. This research is focused on the Modular Multilevel Converter (MMC) with two level submodules but also with multilevel topology submodules such as 3L-FC (three level flying capacitors) and 3L-NPC (three level neutral point capacitors). The main contribution of this thesis is the developed PWM based modulation strategy which allows the balancing of the total amount of submodules capacitors. It is applicable to the aforementioned submodule topologies under different working conditions as evidenced by experimental results

HVDC Systems Fault Analysis Using Various Signal Processing Techniques

The detection and fast clearance of faults are important for the safe and optimal operation of HVDC systems. In HVDC systems, various types of AC faults (rectifier & inverter side) and DC faults can occur. It is therefore necessary to detect the faults and classify them for better protection and diagnostics purposes. Various techniques for fault detection and classification in HVDC systems using signal processing techniques are presented and investigated in this research work. In this research work, it is shown that the wavelet transformation can effectively detect abrupt changes in system signals which are indicative of a fault. This research has focused on DC faults at various distances along the lines and AC faults on the converter side. The DC line current is chosen as the input to the wavelet transform. The 5th level coefficients have been used to identify the various faults in the LCC-HVDC system. Moreover, the value of these coefficients has been used for the classification of the different faults. For more accurate classification of faults, the wavelet entropy principle is proposed. In LCC-HVDC systems, a different approach for fault identification and classification is proposed. In this investigation an algorithm is developed that provides the trade-off between large input data size and minimal number of neurons in the hidden layer, without compromising the accuracy. The claim is confirmed by the results provided from the investigation for various fault conditions and its corresponding ANN output which confirms the specific fault detection and its classification. A fault identification and classification strategy based on fuzzy logic for VSC–HVDC systems is proposed. Initially, the developed Fuzzy Inference Engine (FIE) detects AC faults occurring in the rectifier side and DC faults on the cable successfully. However, it could not identify the line on which the fault has occurred. Hence, to classify the faults occurring in either AC section or DC section of the HVDC system, the FIE has to be restructured with appropriate data input. Therefore, a FIE which identifies different types of fault and the corresponding line where the fault occurs anywhere in the HVDC system was developed. Initially the developed FIE with three input and seven output parameters results in an accuracy level of 99.47% being achieved. After a modified FIE was developed with five inputs and seven output parameters, 21 types of faults in the VSC HVDC system were successfully classified with 100% accuracy. The FIE was further developed to successfully classify with 100% accuracy faults in Multi-Terminal HVDC systems

Benefits and Incentives for ADS-B Equipage in the National Airspace System

Automatic Dependent Surveillance – Broadcast (ADS-B) is a technology that can replace secondary surveillance radars and enhance cockpit situational awareness. It also has the potential to enable procedures not possible with current surveillance technology that would increase the capacity of the National Airspace System (NAS) in the US. Certain forms of ADS-B also have the bandwidth to upload weather and airspace information into the cockpit. However, prior to achieving the benefits of ADS-B, operators must equip with the technology. In order to voluntarily equip, owners and operators must receive benefits from the technology that outweigh the cost or receive other incentives. Through an online survey of stakeholders, applications of ADS-B with the strongest benefits to users are identified. In-cockpit data link offerings are explored in detail, along with a detailed analysis of ADS-B benefits for Hawaiian helicopter operators. The conclusions of this study are that ADS-B should be implemented in non-radar airspace along with busy terminal areas first to gain the most benefits from non-radar separation applications and traffic awareness applications. Also, the basis for the US dual ADS-B link decision is questioned, with a single 1090-ES based link augmented with satellite data link weather recommended.The authors would like to thank all of interview and survey participants. Without their time and insights, this thesis would not be possible. Also, thanks to the FAA’s Surveillance and Broadcast Services program office for their support of this research under contract DTFA01-C-00030

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