Fault analysis and protection for wind power generation systems

Wind power is growing rapidly around the world as a means of dealing with the world energy shortage and associated environmental problems. Ambitious plans concerning renewable energy applications around European countries require a reliable yet economic system to generate, collect and transmit electrical power from renewable resources. In populous Europe, collective offshore large-scale wind farms are efficient and have the potential to reach this sustainable goal. This means that an even more reliable collection and transmission system is sought. However, this relatively new area of offshore wind power generation lacks systematic fault transient analysis and operational experience to enhance further development. At the same time, appropriate fault protection schemes are required. This thesis focuses on the analysis of fault conditions and investigates effective fault ride-through and protection schemes in the electrical systems of wind farms, for both small-scale land and large-scale offshore systems. Two variable-speed generation systems are considered: doubly-fed induction generators (DFIGs) and permanent magnet synchronous generators (PMSGs) because of their popularity nowadays for wind turbines scaling to several-MW systems. The main content of the thesis is as follows. The protection issues of DFIGs are discussed, with a novel protection scheme proposed. Then the analysis of protection scheme options for the fully rated converter, direct-driven PMSGs are examined and performed with simulation comparisons. Further, the protection schemes for wind farm collection and transmission systems are studied in terms of voltage level, collection level wind farm collection grids and high-voltage transmission systems for multi-terminal DC connected transmission systems, the so-called “Supergrid”. Throughout the thesis, theoretical analyses of fault transient performances are detailed with PSCAD/EMTDC simulation results for verification. Finally, the economic aspect for possible redundant design of wind farm electrical systems is investigated based on operational and economic statistics from an example wind farm project

Short-circuit analytical model for modular multilevel converters considering DC cable capacitance

Developing analytical short-circuit models for Modular Multilevel Converters (MMC) is not straightforward due to their switching and blocking characteristics. Short-circuit models for MMCs have been developed previously in the literature. However, there is a lack of understanding regarding the dynamics in the short-circuit model when the DC cable capacitance is taken into account. Therefore, this work proposes an analytical pole-To-pole short-circuit model for half-bridge MMCs that considers the cable capacitance and terminal capacitors and accounts their contribution to fault dynamics. An approximated analytical model has been derived separating the system solutions in different natural frequencies. The proposed model provides an excellent approximation for a vast range of realistic system parameters. The analytical model reproduced the behaviour of the variables in the time domain and provided a clear basis for interpreting the dynamics of the voltages and currents involved

Power control, fault analysis and protection of series connected diode rectifier and VSC based MTDC topology for offshore application.

A multiterminal high-voltage dc (MTDC) system is a promising method for transmitting energy generated from an offshore windfarm (OWF). The creation of MTDC systems became easier by the introduction of voltage source converter (VSC) due to the flexibility and controllability it provides. This technology is newer than the line-commutated converter technology (LCC). Power systems can include any number of windfarms together with converters for both offshore and onshore power conversion. Therefore, this thesis suggests a three-terminal MTDC model of two offshore windfarms and one onshore inverter. The electric energy generated by the two windfarms is rectified into dc and transmitted to the shore using dc cable. Although a VSC or a diode rectifier (DR) can convert ac to dc, a series connection of a VSC and two DRs was proposed at the windfarm side to convert the generated power to achieve controllability of the uncontrollable diode rectifiers and reduces the high cost of badditional VSCs. The proposed topology converts the ac power by dividing the windfarm power so that one-third is the share of the VSC and two-thirds is the share of the DRs. The same topology is used to convert the power produced from the other windfarm. Then, the dc power is transmitted via an undersea dc cable to the onshore location, and is then inverted into ac before it is supplied to the neighbouring ac grid using a grid-side VSC. The proposed topology has many advantages, including a significant save in windfarm VSC (WFVSC) capital cost and a significant reduction in the loss of power of the converter without losing the overall controllability. However, although this topology is suitable for windfarm applications, it might not be suitable for high-voltage direct current (HVDC) that requires bidirectional power flow unless making changes to the topology such as disconnecting the diode rectifiers. Furthermore, fault analyses were investigated, including dc faults and ac faults. Ac faults are categorised as symmetrical or unsymmetrical faults. For comparison purposes, a Simulink model was designed, implemented, and simulated as a reference model. The reference model can operate as VSC-, DR-based MTDC, or a mix of both in a way that any component can be added to or removed from the model at any time during the simulation run. The contribution to the dc fault current from various parts such as dc capacitor and the adjacent feeder was investigated thoroughly, and detailed mathematical formulae were developed to compute fault current from these contributors. In addition, the results of the system response due to both fault types are illustrated and discussed. Both symmetrical and unsymmetrical ac faults were initiated on the onshore grid side, and the system response results are presented for those faults. A generalised control scheme (GCS) was proposed in this thesis, which add the ability the model to control the reactive power and is suitable for both balanced and unbalanced ac faults conditions. A protection against faults was investigated and implemented using dc circuit breakers. The protection system was built to ensure safe operation and to fulfil the grid code requirements. Many grid codes are available and presented in the literature, such as Spanish, British, and Danish; however, a grid code by E.ON was chosen. The protection scheme in VSC-based MTDC networks plays a vital role during dc faults. It is vital that this protection be sensitive, selective, fast, and reliable. Specifically, it must isolate the fault reliably from the system within a short time after the fault occurrence, while maintaining the remaining components of the system in a secure operational condition. For optimal performance, the protection scheme discussed in this thesis employs solid-state circuit breakers. A literature survey relevant to the tasks mentioned above was conducted.PhD in Energy and Powe

Non-unit distance protection algorithm for multiterminal MMC-HVDC systems using DC capacitor resonance frequency

High-voltage DC (HVDC) transmission has been largely used to interconnect asynchronous systems and integrate renewable energy resources into electrical grids. However, the high short-circuit currents and low-tolerance of power electronics equipment impose new challenges for these systems' protection. To address these challenges, a new distance protection algorithm for HVDC grids with Modular Multilevel Converters (MMCs) is proposed in this paper. The proposed algorithm identifies the faulty line/cable using the resonance frequency of a DC capacitor installed in each terminal. The technique was tested in a four-terminal HVDC grid with five cables and ten circuit breakers. The algorithm was highly selective for faults in the lines and provided fast identification, in less than 1 ms, without communication amongst terminals. The algorithm was tested in hardware under high-noise conditions and provided reliable results

A thyristor based DC fault current limiter with inductor inserting-bypassing capability

DC fault current limiters (FCL) are becoming increasingly important for the prompt DC fault clearance of modular multilevel converter (MMC) based HVDC grid. This paper proposes a hybrid FCL topology, in which the main current limiting circuit is composed of thyristors, capacitors and an inductor. Detailed theoretical analysis of the current limiting processes was carried out to check the electrical stresses. The relationship between the voltage stress and the current limiting time was analyzed, then a design method for the FCL parameters was provided. An effective method for fast bypassing the FCL inductor was proposed to reduce the energy dissipation when fault current is interrupted by a DC Circuit Breaker (DCCB). The dynamic performance of the proposed approach has shown that the proposed FCL can effectively limit the rate of rising of the DC fault current and reduce the energy dissipation

A Signal Segmentation Approach to Identify Incident/Reflected Traveling-Waves for Fault Location in Half-Bridge MMC-HVDC Grids

This article presents a new systematic technique for identifying voltage traveling-waves (TWs) to determine the location of line faults in half-bridge modular multilevel converter-based high-voltage direct-current (HBMMC-HVDC) grids. In this technique, the buffered voltage signal frame around the fault-detection time is first scaled and then segmented via an optimization process. Finally, the incident/reflected TWs arrival times are obtained by executing a simple search algorithm on the reconstructed signal segments’ differences. This article describes how to use this technique in three forms of TW-based fault location schemes, including the single-ended scheme with known TW velocity, the double-ended scheme with known TW velocity, and the double-ended scheme with unknown TW velocity. The application results on a 4-terminal HBMMC-HVDC grid simulated with exact component models show the proposed technique’s high capability and accuracy in all the three TW-based fault-location schemes. According to these results, the average fault-location errors are less than 0.5% for all the schemes. The numerical results also confirm that the proposed technique maintains its excellent performance, even in the face of close to terminal faults with distances down to 4 km, faults with high resistances up to 450 Ω, and noisy signals with signal-to-noise ratios down to 55 dB. Moreover, the comparison results confirm that the proposed approach is more tolerant of measurement noise than the wavelet transform.©2021 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.fi=vertaisarvioitu|en=peerReviewed

Grid Forming Wind Power Plants: Black start operation for HVAC grids and Diode Rectifier-based Wind Power Plant integration

Tesis por compendio[ES] La Unión Europea plantea una serie de retos en el GREEN DEAL para conseguir un escenario donde sus miembros sean climáticamente neutros en 2050. Para ello se plantean unas acciones entre las que destaca la descarbonización del sector de la energía. Por otra parte, se ha puesto como objetivo conseguir una capacidad de energía eólica de 1200 GW en 2050, desde los 190 GW que se tenía en 2019. El cierre de centrales eléctricas basadas en grandes generadores síncronos junto con el aumento en la construcción de centrales eléctricas con fuentes basadas en electrónica de potencia como parques eólicos o plantas fotovoltaicas, hace necesario la incorporación de controladores grid forming en energías renovables basadas en electrónica de potencia. La integración de estrategias de control grid forming en turbinas eólicas debe considerar las funciones de reparto de potencia activa y reactiva (control droop en generadores síncronos), así como un sistema de protección ante faltas que permita una recuperación rápida cuando se despeja la falta. Por otra parte, en la transición de los actuales parques eólicos grid following que funcionan como fuentes de corriente a parques eólicos grid forming que funcionan como fuentes de tensión se debe considerar que ambas tecnologías van a coexistir conjuntamente por mucho tiempo. Por tanto, se hace necesario el estudio de estabilidad de parques eólicos con ambas tecnologías, así como el estudio de cuanta generación grid forming es necesaria para mantener el parque eólico estable en cualquier situación. Además, el uso de parques eólicos grid foming permite diferentes aplicaciones como la energización de redes eléctricas después de un apagón desde estos parques eólicos, o el uso de diodos rectificadores en enlaces HVDC para la conexión de parques eólicos marinos. Para facilitar el estudio y diseño de aerogeneradores grid forming, en la presente tesis se propone una técnica de agregación de aerogeneradores tipo-4. Esta técnica permite reducir la complejidad del parque eólico para su estudio y análisis. Esta tesis incluye las siguientes contribuciones: Para el funcionamiento de aerogeneradores grid forming en paralelo con aerogeneradores grid following se propone un método de diseño analizando la estabilidad del sistema completo. Además, se propone una estrategia de control ante faltas para asegurar una recuperación rápida y segura. Se propone el uso de técnicas de control H∞ para la sintonización de controladores grid forming. El uso de estas técnicas para el diseño de controladores puede mejorar la robustez de los controladores, así como el rendimiento de estos. Considerando la aplicación de la energización de redes HVAC desde parques eólicos, se ha propuesto como llevar a cabo dicha maniobra a partir de parque con aerogeneradores grid forming y grid following. Los resultados obtenidos validan el funcionamiento de la operación. Además, muestran que la cantidad de generación grid forming está relacionada con la dimensión de cargas que tienen que aguantar dichas turbinas más que con la estabilidad del sistema con un porcentaje bajo de generación grid forming. Finalmente, se han propuesto estrategias de protección para la integración de parque eólicos en redes malladas HVDC utilizando rectificadores de diodos. Los parques eólicos grid forming pueden ayudar a gestionar faltas disminuyendo los requisitos en las protecciones necesarias en el enlace HVDC. Esto permite una reducción del coste de la instalación, además de aumentar la robustez del sistema.[CA] La Unió Europea planteja uns reptes al GREEN DEAL per a aconseguir un escenario on els seus membres siguen climàticament neutrals al 2050. Per a aconseguir aquest objectiu es plantegen unes accions entre les quals destaca la de descarbonitzar el sector energètic. D'altra banda, s'ha posat com a objectiu aconseguir una capacitat d'energia eòlica de 1200 GW al 2050, des dels 190 GW que es tenia al 2019. El tancament de centrals elèctriques basades en grans generadors síncrons juntament amb l'augment en la construcció de centrals elèctriques basades en electrónica de potència com a parcs eòlics o plantes fotovoltaiques, fa necessari la incorporació de controladors grid forming en energies renovables que es basan en electrònica de potència. La integració d'estratègies de control grid forming per a turbines eòliques ha de considerar les funcions de repartiment de potència activa i reactiva, així com un sistema de protecció davant faltes que permeta una recuperació ràpida quan s'aïlla la falta. D'altra banda, en la transició dels actuals parcs eòlics grid following que funcionen com a fonts de corrent a parcs eòlics grid forming que funcionen com a fonts de tensió, s'ha de considerar que totes dues tecnologies coexistiran conjuntament per molt de temps. Per tant, es fa necessari l'estudi d'estabilitat d'un parc eòlic amb totes dues tecnologies, així com l'estudi del percentatge de generació grid forming necessari per a mantindre el parc eòlic estable en qualsevol situació. A més, l'ús de parcs eòlics grid foming permet diferents aplicacions com l'energització de xarxes elèctriques després d'una apagada des d'aquests parcs eòlics, o l'ús de díodes rectificadors en enllaços HVDC per a la connexió de parcs eòlics marins. Per a facilitar l'estudi i disseny d'aerogeneradors grid forming, en la present tesi es proposa una tècnica d'agregació d'aerogeneradors tipus-4. Aquesta técnica permet reduir la complexitat del parc eòlic per al seu estudi i anàlisi. La present tesi inclou les següents contribucions: Per al funcionament d'aerogeneradors grid forming en paral¿lel amb aerogeneradors grid following es proposa un mètode de disseny analitzant l'estabilitat del sistema complet. A més, es proposa una estratègia de control davant faltes per a assegurar una recuperació ràpida i segura. Es proposa l'ús de tècniques de control H∞ per a la sintonització de controladors grid forming. L'ús d'aquestes tècniques per al disseny de controladors pot millorar la robustesa dels controladors, a mé del rendiment d'aquestos. Considerant l'aplicació de l'energització de xarxes HVAC des de parcs eòlics, s'ha proposat com dur a terme aquesta maniobra a partir d'un parc eòlic amb aerogeneradors grid forming i grid following. Els resultats obtinguts validen el funcionament de l'operació. A més, mostren que la quantitat de generació grid forming està més relacionada amb la dimensió de les càrregues que ha d'aguantar el parc eòlic, que amb l'estabilitat del sistema que permet un menr percentatge de generació grid forming. Finalment, s'han proposat estratègies de protecció per a la integració de parcs eòlics marins en xarxes multi-punt HVDC utilitzant rectificadors de díodes. Els parcs eòlics grid forming poden ajudar a gestionar faltes disminuint els requisits de les proteccions necessàries en l'enllaç HVDC. Això permet una reducció del cost de la instal¿lació, a més d'augmentar la robustesa del sistema.[EN] The European Union GREEN DEAL aims to make its 27 members climate-neutral by 2050. The decarbonization of the energy sector stands out as one of the proposed actions. To achieve that goal, the target for wind power generation is set at 1,200 GW in 2050, from the 190 GW that was had in 2019. The closure of power plants based on large synchronous generators and the increase of power electronics based generation such as Wind Power Plants (WPPs) or photovoltaic plants, leads to the use of grid forming controllers for power electronics based renewable energy. Grid forming control strategies for wind turbines generators (WTGs) must consider active and reactive power sharing control (droop control in synchronous generators) as well as a protection system that allows a quick recovery after fault clearance. Moreover, a transition from the conventional grid following WPPs to new grid forming WPPs is required. The transition must consider the parallel operation of both technologies for a long time. Thus, it is necessary to study the stability of Wind Power Plants with both technologies, as well as the study of how much grid forming generation is required to keep a mixed grid forming and grid following WPP stable in any situation. In addition, the use of grid forming WPPs allows different applications such as the energization of HVAC and HVDC grids after a blackout from these WPPs, or the use of diode rectifiers in HVDC links for the off-shore WPPs connection. An aggregation technique for type-4 WTGs has been proposed in this thesis in order to facilitate the study and design of grid forming WPPs. The aggregation technique allows to reduce the WPP complexity for its study and analysis. The main contributions of this theses are: A design methodology has been proposed for the parallel operation of grid forming and grid following WTGs. Including the stability analysis of the complete system. Additionally, a fault control strategy is proposed to ensure a fast and safe recovery. The use of H∞ control techniques is also proposed for grid forming controller tuning. Using H∞ control techniques for controller design may improve the robustness of the controllers as well as the performance of the controllers. A procedure to carry out black start operation of HVAC grid from mixed grid forming and grid following WPPs has been proposed. The obtained results validate that the procedure works as expected. Moreover, the results show that the amount of grid-forming generation usually is determined by the load size steps as stability limits are usually less stringent. Finally, protection strategies have been proposed for the integration of off-shore WWPs in multi-terminal HVDC grids using diode rectifiers. Grid forming WPPs are able to help managing faults. Their use allow lower requirements of the HVDC protection equipment, leading to overall cost reduction and an increment of the system robustness.Martínez Turégano, J. (2022). Grid Forming Wind Power Plants: Black start operation for HVAC grids and Diode Rectifier-based Wind Power Plant integration [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/181529TESISCompendi