Multi-pole voltage source converter HVDC transmission systems

This study connects several modular multilevel converters to form multi-pole voltage source converter highvoltage dc (VSC-HVDC) links which are suited for bulk power evacuation, with increased resiliency to ac and dc network faults. The proposed arrangements resemble symmetrical and asymmetrical HVDC links that can be used for bulk power transfer over long distances with reduced transmission losses, and for the creation of multi-terminal supergrids currently being promoted for transitional dc grids in Europe. The technical feasibility of the proposed systems is assessed using simulations on symmetrical and asymmetrical tri-pole VSC-HVDC links, including the case of permanent pole-to-ground dc faults

Control and Protection of MMC-Based HVDC Systems: A Review

The voltage source converter (VSC) based HVDC (high voltage direct current system) offers the possibility to integrate other renewable energy sources (RES) into the electrical grid, and allows power flow reversal capability. These appealing features of VSC technology led to the further development of multi-terminal direct current (MTDC) systems. MTDC grids provide the possibility of interconnection between conventional power systems and other large-scale offshore sources like wind and solar systems. The modular multilevel converter (MMC) has become a popular technology in the development of the VSC-MTDC system due to its salient features such as modularity and scalability. Although, the employment of MMC converter in the MTDC system improves the overall system performance. However, there are some technical challenges related to its operation, control, modeling and protection that need to be addressed. This paper mainly provides a comprehensive review and investigation of the control and protection of the MMC-based MTDC system. In addition, the issues and challenges associated with the development of the MMC-MTDC system have been discussed in this paper. It majorly covers the control schemes that provide the AC system support and state-of-the-art relaying algorithm/ dc fault detection and location algorithms. Different types of dc fault detection and location algorithms presented in the literature have been reviewed, such as local measurement-based, communication-based, traveling wave-based and artificial intelligence-based. Characteristics of the protection techniques are compared and analyzed in terms of various scenarios such as implementation in CBs, system configuration, selectivity, and robustness. Finally, future challenges and issues regarding the development of the MTDC system have been discussed in detail

Hybrid AC/DC hubs for network connection and integration of renewables

High-voltage direct current (HVDC) technology has been identified as a preferred choice for long-distance power transmission, especially offshore. With the rapid development of wind energy, many point-to-point HVDC systems with different voltage levels have been built. For increased operation flexibility and reliability, and better use of the existing assets, there is a need to interconnect different AC and DC networks as part of the future transmission network infrastructure development. To address the demands of connecting wind farm converter stations with other AC/DC systems, different hybrid HVDC converters for network connection and integration of renewables are proposed and evaluated in this thesis with the consideration of converter power rating, cost, efficiency and operation flexibility including response during faults. A hybrid LCC-MMC AC/DC hub (LCC-MMC Hub) is proposed in this research, where a modular multilevel converter (MMC) and a line-commutated converter (LCC) are paralleled at the AC side to integrate onshore wind power, and connected in series at the DC sides to interconnect two DC networks with different voltages. To investigate the design requirement and performance of the hybrid AC/DC hub, power flow analysis is assessed to evaluate the converter power rating requirement. Compared to the “conventional” DC network interconnection based on a DC/DC converter, the proposed hybrid LCC-MMC Hub requires the lower power rating of a MMC with large part of the power handled by a LCC, potentially leading to higher overall efficiency and lower cost. Coordinated controls of the LCC and MMC are developed to ensure stable system operation and system safety. To ride through DC faults at either side of the interconnected DC networks, a coordinated DC fault protection method for the hybrid AC/DC hub is proposed and studied. This hybrid hub uses large AC side filters, which might be the disadvantage for certain applications. Considering the future development of offshore production platforms (e.g. oil/gas and hydrogen production plants), a diode rectifier-modular multilevel converter AC/DC hub (DR-MMC Hub) is proposed to integrate offshore wind power to onshore DC network and offshore production platforms with different DC voltage levels. In this design, the DR and MMCs are connected in parallel at the offshore AC collection network to integrate offshore wind power, and in series at the DC terminals of the offshore production platform and the onshore DC network. Compared to the parallel operation of the DR-MMC HVDC system, the required MMC power rating in the proposed DR-MMC Hub can be reduced due to the series connection, potentially leading to lower investment cost and power loss. System control of the DR-MMC AC/DC hub is designed for different operating scenarios. System behaviours and requirements during AC and DC faults are investigated. The hybrid MMCs with halfbridge and full-bridge sub-modules (HBSMs and FBSMs) are used for safe operation and protection during DC faults. Power regulation of series-connected configuration might be problematic in certain applications. To address the needs for increased DC network interconnection and the high cost of the existing F2F DC/DC converter design, a hybrid F2F DC/DC converter, as a potential option, is proposed for unidirectional applications. In the proposed DC/DC converter, the internal AC grid is established by a small MMC based STATCOM, and the active power is transferred through the DR and LCC. Compared to the conventional F2F DC/DC converters in terms of topological features and operation efficiency, the proposed DC/DC converter could offer higher power capability, higher converter efficiency and lower investment cost than those of the MMC based F2F DC/DC converters. The operation and control of the LCC and MMC-STATCOM is designed, and the system start-up procedure is presented. Detailed analysis of the behaviours and protection methods during DC faults is demonstrated. It needs to acknowledge that the converter requires large amount of passive AC filters which may lead to large footprint. In addition, the proposed DC/DC converter only support unidirectional power flow.For the three proposed topologies, extensive time-domain simulation results based on PSCAD/EMTDC software have been provided to verify the feasibilities and effectiveness (including steady state and dynamic performance) in normal operation and various fault scenarios.High-voltage direct current (HVDC) technology has been identified as a preferred choice for long-distance power transmission, especially offshore. With the rapid development of wind energy, many point-to-point HVDC systems with different voltage levels have been built. For increased operation flexibility and reliability, and better use of the existing assets, there is a need to interconnect different AC and DC networks as part of the future transmission network infrastructure development. To address the demands of connecting wind farm converter stations with other AC/DC systems, different hybrid HVDC converters for network connection and integration of renewables are proposed and evaluated in this thesis with the consideration of converter power rating, cost, efficiency and operation flexibility including response during faults. A hybrid LCC-MMC AC/DC hub (LCC-MMC Hub) is proposed in this research, where a modular multilevel converter (MMC) and a line-commutated converter (LCC) are paralleled at the AC side to integrate onshore wind power, and connected in series at the DC sides to interconnect two DC networks with different voltages. To investigate the design requirement and performance of the hybrid AC/DC hub, power flow analysis is assessed to evaluate the converter power rating requirement. Compared to the “conventional” DC network interconnection based on a DC/DC converter, the proposed hybrid LCC-MMC Hub requires the lower power rating of a MMC with large part of the power handled by a LCC, potentially leading to higher overall efficiency and lower cost. Coordinated controls of the LCC and MMC are developed to ensure stable system operation and system safety. To ride through DC faults at either side of the interconnected DC networks, a coordinated DC fault protection method for the hybrid AC/DC hub is proposed and studied. This hybrid hub uses large AC side filters, which might be the disadvantage for certain applications. Considering the future development of offshore production platforms (e.g. oil/gas and hydrogen production plants), a diode rectifier-modular multilevel converter AC/DC hub (DR-MMC Hub) is proposed to integrate offshore wind power to onshore DC network and offshore production platforms with different DC voltage levels. In this design, the DR and MMCs are connected in parallel at the offshore AC collection network to integrate offshore wind power, and in series at the DC terminals of the offshore production platform and the onshore DC network. Compared to the parallel operation of the DR-MMC HVDC system, the required MMC power rating in the proposed DR-MMC Hub can be reduced due to the series connection, potentially leading to lower investment cost and power loss. System control of the DR-MMC AC/DC hub is designed for different operating scenarios. System behaviours and requirements during AC and DC faults are investigated. The hybrid MMCs with halfbridge and full-bridge sub-modules (HBSMs and FBSMs) are used for safe operation and protection during DC faults. Power regulation of series-connected configuration might be problematic in certain applications. To address the needs for increased DC network interconnection and the high cost of the existing F2F DC/DC converter design, a hybrid F2F DC/DC converter, as a potential option, is proposed for unidirectional applications. In the proposed DC/DC converter, the internal AC grid is established by a small MMC based STATCOM, and the active power is transferred through the DR and LCC. Compared to the conventional F2F DC/DC converters in terms of topological features and operation efficiency, the proposed DC/DC converter could offer higher power capability, higher converter efficiency and lower investment cost than those of the MMC based F2F DC/DC converters. The operation and control of the LCC and MMC-STATCOM is designed, and the system start-up procedure is presented. Detailed analysis of the behaviours and protection methods during DC faults is demonstrated. It needs to acknowledge that the converter requires large amount of passive AC filters which may lead to large footprint. In addition, the proposed DC/DC converter only support unidirectional power flow.For the three proposed topologies, extensive time-domain simulation results based on PSCAD/EMTDC software have been provided to verify the feasibilities and effectiveness (including steady state and dynamic performance) in normal operation and various fault scenarios

Design, Control and Protection of Modular Multilevel Converter (MMC)-Based Multi-Terminal HVDC System

Even though today’s transmission grids are predominantly based on the high voltage alternating current (HVAC) scheme, interests on high voltage direct current (HVDC) are growing rapidly during the past decade, due to the increased penetration of remote renewable energy. Voltage source converter (VSC) type is preferred over the traditional line-commutated converter (LCC) for this application, due to the advantages like smaller station footprint and no need for strong interfacing ac grid. As the state-of-the-art VSC topology, modular multilevel converter (MMC) is mostly considered. Most renewable energy sources, such as wind and solar, is usually sparsely located. Multi-terminal HVDC (MTDC) provides better use of transmission infrastructure, higher transmission flexibility and reliability, than building multiple point-to-point HVDCs. This dissertation studies the MMC-based MTDC system, including design, control and protection. Passive components design methodology in MMC is developed, with practical consideration. The developed arm inductance selection criterion considers the implementation of circulating current suppression control. And the unbalanced voltage among submodule capacitor is taken into account for submodule capacitance design. Circulating current suppression control is found to impact the MMC operating range. The maximum modulation index reduction is calculated utilizing a decoupled MMC model. A four-terminal HVDC testbed is developed, with similar control and communication architectures of the practical projects implemented. Several most typical operation scenarios and controls are demonstrated or proposed. In order to allow HVDC disconnects to online trip a line, dc line current control is proposed through station control. Utilizing the dc line current control, an automatic dc line current limiting control is proposed. Both controls have been verified in the developed testbed. A systematic dc fault protection strategy of MTDC utilizing hybrid dc circuit breaker is developed, including a new fast and selective fault detection method taking advantage of the hybrid dc circuit breaker special operation mechanism. Detailed criteria and control methods to assist system recovery are presented. A novel fault tolerant MMC topology is proposed with a hybrid submodule by adding an ultra-fast mechanical switch. The converter power loss can be almost the same as the half-bridge MMC, and 1/3 reduction compared to the similar clamp-double topology

Control and operation of wind power plants connected to DC grids

Remote offshore wind power plants (WPPs) are being linked through high-voltage de voltage-source converter (VSC-HVdc) transmission to the main grids. The current deployments of HVdc grid connections for offshore WPPs are point-to-point transmission systems. Moreover, WPPs connected to the offshore VSC-HVdc form an offshore ac grid which operates non­ synchronously to the main grids. lt is characterized by extensive submarine cabling and, in the case offull-scale power converter-based wind turbines, by being purely converter-based. This thesis goes into two main aspects regarding the operation of HVdc-connected WPPs: i) reactive power and voltage control and ii) fault ride through (FRT) in the ac offshore grids. Optimization-based reactive power control strategies are enhanced to the application of an ac grid consisting ofone grid-forming and several grid-connected converters. A reactive power and voltage control method is introduced which aims to increase the annual energy production from a single WPP. In the industrial application, several WPPs might be clustered which leads to multi-layered controllers and operation boundaries. Taking this into account, an operation strategy with reasonable communication requirements is suggested and evaluated against conventional methods . The work further propases a control framework for the grid-form ing offshore VSC-HVdc. Special emphasis is put on the FRT of unbalanced faults in the offshore grid and the provision of controlled currents for ease of fault detection. Furthermore, the internal variables of the offshore modular multi-level VSC-HVdc are analyzed. Moreover, tour FRT strategies for the grid­ connected converters are evaluated for unbalanced faults in the offshore grid. This consequently implies that control strategies in symmetrical components are considered. Furthermore, the reduction of over-modulation and over-voltages by the power converters in the offshore grid is dealt with.Los parques eólicos marinos suelen conectarse a redes eléctricas terrestres a través de corriente continua de alta tensión (siglas en inglés: HVdc) utilizando convertidores de fuente de tensión (siglas en inglés: VSC) cuando la corriente alterna de alta tensión (siglas en inglés: HVac) resulta tecnológicamente e económicamente desfavorable. Los parques eólicos conectados al convertidor HVdc marino crean redes eléctricos marinas de corriente alterna que operan asíncronamente a las redes terrestres. Dichas redes se caracterizan por tener cables submarinos, y, en el caso de aerogeneradores con convertidores de plena potencia, resultan en redes constituidas únicamente por convertidores de potencia. Esta tesis investiga dos de los aspectos principales de la operación de parques eólicos marinos conectados en corriente continua de alta tensión: i) la regulación de potencia reactiva y tensión y ii) la operación durante faltas eléctricas en las redes marinas. Se han propuesto estrategias de optimización del control de reactiva para su aplicación a una red ac con varios convertidores conectados. Se ha introducido un método de regulación de potencia reactiva y tensión cuyo objetivo es incrementar la generación eléctrica del parque eólico. En la implementación práctica, varios parques eólicos podrían pertenecer a la misma red lo cual conduce a reguladores multicapas y a la consideración las interfaces entre los operadores. Teniendo esto en cuenta, se propone una estrategia de regulación de potencia reactiva asumiendo unos tiempos de comunicación razonables, y se compara a conceptos convencionales. La segunda parte de la tesis sugiere un método de control para el convertidor marino en secuencia directa e inversa. Está diseñado para la operación normal y la operación durante faltas asimétricas y permite la inyección de corrientes reguladas para la detección de la falta. Además, se analizan las variables internas del convertidor modular multinivel (siglas en inglés: MMC) en estas situaciones. Asimismo, se han evaluado cuatro estrategias de respuesta a faltas asimétricas por parte de los convertidores de los aerogeneradores. Estas estrategias también incluyen el control en secuencia directa e inversa. Finalmente, se investiga la reducción de sobremodulación en los convertidores y sobretensiones en la red marina.Hochspannungs–Gleichstrom–Übertragung (HGÜ) stellt eine effiziente Lösung zur Netzanbindung weit entfernter Offshore–Windkraftanlagen dar. Die derzeit verwendeten Punkt–zu–Punkt–Anbindungen basieren dabei auf spannungsgeführten Umrichtertopologien. Das seeseitige Wechselstromnetz verbindet die Windkraftanlagen mit der netzbildenden HGÜ–Umrichterstation. Es charakterisiert sich im Vergleich zu gewöhnlichen Netzen durch das ausschließliche Verwenden von Seekabeln und, im Fall einer Verwendung von Windkraftanlagen mit Vollumrichtern, durch das Fehlen gewöhnlicher, direkt gekoppelter Synchrongeneratoren. Die vorliegende Dissertation behandelt zwei Kernaspekte bezüglich dem Betrieb HGÜ–angebundener Windparks: i) die kontinuierliche Regelung der Blindleistung und Spannung und ii) das Umrichterverhalten bei Spannungseinbrüchen aufgrund von Netzkurzschlüssen [engl. fault ride through (FRT)] im seeseitigen Wechselspannungsnetz. Hierfür werden Blindleistungsoptimierungsverfahren präsentiert, die für die Anwendung in Wechselstromnetzen mit einem netzbildenden Umrichter und weiteren netzsynchronen Umrichtern geeignet sind. Die vorgeschlagene Blindleistung– und Spannungsregelungsmethode verringert die Energieverluste im seeseitigen Netz und erhöht damit die Energieausbeute des Systems. Häufig werden verschiedene Windparks zu Clustern zusammengeschlossen, die mehrschichtige Regelungsansätze fordern. Hierfür wird ein weiteres Verfahren vorgeschlagen, das ähnliche Kommunikationsanforderungen wie herkömmliche Betriebsverfahren aufweist, jedoch geringere Verluste verursacht. Die Arbeit untersucht ferner ein dynamisches Regelungsverfahren für den seeseitigen HGÜ–Umrichter. Dabei wird speziell das Verhalten während unsymmetrischer Kurzschlüsse im seeseitigen Netz berücksichtigt. Darüber hinaus wird der Betrieb des modularen Mehrpunktumrichters (engl. MMC) für diese Anwendung analysiert. Bezüglich des Verhaltens netzsynchroner Umrichter während asymmetrischer Spannungseinbrüche im seeseitigen Netz werden weiterhin vier Verfahren untersucht. Diese zielen unter anderem auf die Verringerung von möglicher Übermodulation der Umrichter und Überspannungen im seeseitigen Netz ab

A Review on Multi-Terminal High Voltage Direct Current Networks for Wind Power Integration

With the growing pressure to substitute fossil fuel-based generation, Renewable Energy Sources (RES) have become one of the main solutions from the power sector in the fight against climate change. Offshore wind farms, for example, are an interesting alternative to increase renewable power production, but they represent a challenge when being interconnected to the grid, since new installations are being pushed further off the coast due to noise and visual pollution restrictions. In this context, Multi-Terminal High Voltage Direct Current (MT-HVDC) networks are the most preferred technology for this purpose and for onshore grid reinforcements. They also enable the delivery of power from the shore to offshore Oil and Gas (O&G) production platforms, which can help lower the emissions in the transition away from fossil fuels. In this work, we review relevant aspects of the operation and control of MT-HVDC networks for wind power integration. The review approaches topics such as the main characteristics of MT-HVDC projects under discussion/commissioned around the world, rising challenges in the control and the operation of MT-HVDC networks and the modeling and the control of the Modular Multilevel Converter (MMC) stations. To illustrate the challenges on designing the control system of a MT-HVDC network and to corroborate the technical discussions, a simulation of a three-terminal MT-HVDC network integrating wind power generation and offshore O&G production units to the onshore grid is performed in Matlab's Simscape Electrical toolbox. The results highlight the main differences between two alternatives to design the control system for an MT-HVDC network