Modular multilevel converter with modified half-bridge submodule and arm filter for dc transmission systems with DC fault blocking capability

Although a modular multilevel converter (MMC) is universally accepted as a suitable converter topology for the high voltage dc transmission systems, its dc fault ride performance requires substantial improvement in order to be used in critical infrastructures such as transnational multi-terminal dc (MTDC) networks. Therefore, this paper proposes a modified submodule circuit for modular multilevel converter that offers an improved dc fault ride through performance with reduced semiconductor losses and enhanced control flexibility compared to that achievable with full-bridge submodules. The use of the proposed submodules allows MMC to retain its modularity; with semiconductor loss similar to that of the mixed submodules MMC, but higher than that of the half-bridge submodules. Besides dc fault blocking, the proposed submodule offers the possibility of controlling ac current in-feed during pole-to-pole dc short circuit fault, and this makes such submodule increasingly attractive and useful for continued operation of MTDC networks during dc faults. The aforesaid attributes are validated using simulations performed in MATLAB/SIMULINK, and substantiated experimentally using the proposed submodule topology on a 4-level small-scale MMC prototype

Modified half-bridge modular multilevel converter for HVDC systems with DC fault ride-through capability

One of the main challenges of voltage source converter based HVDC systems is DC faults. In this paper, two different modified half-bridge modular multilevel converter topologies are proposed. The proposed converters offer a fault tolerant against the most severe pole-to-pole DC faults. The converter comprises three switches or two switches and 4 diodes in each cell, which can result in less cost and losses compared to the full-bridge modular multilevel converter. Converter structure and controls are presented including the converter modulation and capacitors balancing. MATLAB/SIMULINK simulations are carried out to verify converter operation in normal and faulty conditions

Simulation of Wind Power Integration with Modular Multilevel Converter-Based High Voltage Direct Current

The growing demand for large-capacity long distance transmission of wind power has boosted the development of flexible direct current (DC) transmission technology. To facilitate wind power integration, this paper designs a modular multilevel converter (MMC) for steady-state operation, using the parameters of the demonstration DC transmission project of offshore wind power in Sheyang County, eastern China\u27s Jiangsu Province. Relying on the simulation platform of PSCAD/EMTDC, the authors analyzed the proposed control theory, and verified that, under different working conditions (e.g., changing wind speed), the MMC-based high voltage direct current (MMC-HVDC) transmission system can integrate the wind power safely and efficiently. In addition, the authors discussed how to enhance the fault ride-through (FRT), a prominent problem in wind power operation, of the flexible DC system containing wind power, from the perspective of alternating current (AC) fault and DC fault

Hybrid cascaded modular multilevel converter with DC fault ride-through capability for HVDC transmission system

A new hybrid cascaded modular multilevel converter for high-voltage dc (HVDC) transmission system is presented. The half-bridge (HB) cells are used on the main power stage and the cascade full-bridge (FB) cells are connected to its ac terminals. The main power stage generates the fundamental voltages with quite low switching frequency, resulting relatively low losses. The cascaded FB cells only attenuate the harmonics generated by the main power stage, without contribution to the power transfer. Thus, the energy storage requirement of the cascaded FB cells is low and the capacitance of FB cells is reduced significantly. Due to the dc fault reverse blocking capability of the cascaded FB cells, the proposed topology can ride-through the pole-to-pole dc fault. In addition the soft restart is achieved after the fault eliminates, without exposing the system to significant inrush current. Besides, the average-value model of the proposed topology is derived, based on which the control strategy is presented. The results show the feasibility of the proposed converter

Distributed PLL-based control of offshore wind turbines connected with diode-rectifier based HVDC systems

A distributed PLL-based frequency control is proposed in this paper for offshore wind turbine converters connected with diode-rectifier based high-voltage-direct-current (HVDC) systems. The proposed control enables a large number of wind turbines to work autonomously to contribute to the offshore AC frequency and voltage regulation. The proposed control also provides automatic synchronization of the offline wind turbines to the offshore AC grid. Stability of the proposed frequency control is analyzed using root locus method. Moreover, an active dc voltage control of the onshore modular multilevel converter (MMC) is proposed to ride-through onshore AC fault, where the onshore MMC converter quickly increases the dc voltage by adding additional submodules in each phase, in order to rapidly reduce wind farm active power generation so as to achieve quick active power re-balance between the offshore and onshore sides. Thus the overvoltage of the submodule capacitor is alleviated during the onshore fault, reducing the possibility of system disconnection. Simulation results in PSCAD verify the proposed control strategy during start-up, synchronization and under onshore and offshore fault conditions

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

Management and Protection of High-Voltage Direct Current Systems Based on Modular Multilevel Converters

The electrical grid is undergoing large changes due to the massive integration of renewable energy systems and the electrification of transport and heating sectors. These new resources are typically non-dispatchable and dependent on external factors (e.g., weather, user patterns). These two aspects make the generation and demand less predictable, facilitating a larger power variability. As a consequence, rejecting disturbances and respecting power quality constraints gets more challenging, as small power imbalances can create large frequency deviations with faster transients. In order to deal with these challenges, the energy system needs an upgraded infrastructure and improved control system. In this regard, high-voltage direct current (HVdc) systems can increase the controllability of the power system, facilitating the integration of large renewable energy systems. This thesis contributes to the advancement of the state of the art in HVdc systems, addressing the modeling, control and protection of HVdc systems, adopting modular multilevel converter (MMC) technology, with focus in providing services to ac systems. HVdc system control and protection studies need for an accurate HVdc terminal modeling in largely different time frames. Thus, as a first step, this thesis presents a guideline for the necessary level of deepness of the power electronics modeling with respect to the power system problem under study. Starting from a proper modeling for power system studies, this thesis proposes an HVdc frequency regulation approach, which adapts the power consumption of voltage-dependent loads by means of controlled reactive power injections, that control the voltage in the grid. This solution enables a fast and accurate load power control, able to minimize the frequency swing in asynchronous or embedded HVdc applications. One key challenge of HVdc systems is a proper protection system and particularly dc circuit breaker (CB) design, which necessitates fault current analysis for a large number of grid scenarios and parameters. This thesis applies the knowledge developed in the modeling and control of HVdc systems, to develop a fast and accurate fault current estimation method for MMC-based HVdc system. This method, including the HVdc control, achieved to accurately estimate the fault current peak value and slope with very small computational effort compared to the conventional approach using EMT-simulations. This work is concluded introducing a new protection methodology, that involves the fault blocking capability of MMCs with mixed submodule (SM) structure, without the need for an additional CB. The main focus is the adaption of the MMC topology with reduced number of bipolar SM to achieve similar fault clearing performance as with dc CB and tolerable SM over-voltage

Simulation study of FACTS devices based on AC-AC modular multilevel hexagonal chopper

This paper proposes a new range of FACTS device based on direct AC-AC conversion, where the modular multilevel AC hexagonal chopper (M2AHC) is employed. The M2AHC is operated in quasitwo-level (Q2L) mode; and the heterodyne modulation is used to decouple voltage amplitude regulation from the phase shift; thus, independent control of active and reactive power is achieved. Then, a family of FACTS devices based on M2AHC that offers voltage, active power and reactive power flow control as both shunt and series compensators is analyzed. The use of AC cell capacitors instead of DC capacitors in M2AHC makes its footprint much smaller and lighter than conventional AC-DC or DC-AC voltage source converter (VSC) based FACTS devices; hence, high reliability and extended service life could be expected. The system modeling and controller design of the proposed FACTS devices are illustrated in a unified reference frame, considering different control modes, transient and unbalanced conditions. Simulation results are used to verify the feasibility of the proposed M2AHC based FACTS device. These FACTS devices will be preferred over conventional counterpart for confined spaced applications such as the grid access of large-scale offshore wind farms and resolution of loop flow in megacities