A New MMC Topology Which Decreases the Sub Module Voltage Fluctuations at Lower Switching Frequencies and Improves Converter Efficiency

Modular Multi-level inverters (MMCs) are becoming more common because of their suitability for applications in smart grids and multi-terminal HVDC transmission networks. The comparative study between the two classic topologies of MMC (AC side cascaded and DC side cascaded topologies) indicates some disadvantages which can affect their performance. The sub module voltage ripple and switching losses are one of the main issues and the reason for the appearance of the circulating current is sub module capacitor voltage ripple. Hence, the sub module capacitor needs to be large enough to constrain the voltage ripple when operating at lower switching frequencies. However, this is prohibitively uneconomical for the high voltage applications. There is always a trade off in MMC design between the switching frequency and sub module voltage ripple

A unidirectional hybrid HVDC transmission system based on diode rectifier and full-bridge MMC

To reduce the cost of bulk power transmission using voltage source converter HVDC technology, a unidirectional hybrid converter is proposed, where a diode rectifier and a modular multilevel converter (MMC) based on full-bridge (FB) submodules are connected in series on DC side. The FB-MMC controls its DC voltage to regulate the transmitted power. The majority of the power transmission is via the diode rectifier considering its cost and efficiency superiority and only low power rating FB-MMC is required. A thyristor valve is equipped at the DC side of the FB-MMC to prevent potential overcharge of the FB submodules during DC faults. Compared to conventional MMCs, losses can potentially be reduced by around 20%. An active power controller is proposed to regulate the DC voltage of the FB-MMC so as to control the transmitted power. With the inverter station controlling its DC terminal voltage constant, the FB-MMC increases the output DC voltage to increase the transmitted power and, vice versa. To alleviate overvoltage of the HVDC link during AC grid faults of the inverter station, a dynamic DC voltage limiter is designed to actively reduce the DC output voltage of the FB-MMC. Simulation results confirm the proposed converter operation and control

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

Flexible converters for meshed HVDC grids: From Flexible AC Transmission Systems (FACTS) to Flexible DC grids

© 2020 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 worksFlexible Alternating Current Transmission Systems (FACTS) have achieved to enhance the flexibility of modern AC power systems, by providing fast, reliable and controllable solutions to steer the power flows and voltages in the network. The proliferation of High Voltage Direct Current (HVDC) transmission systems is leading to the opportunity of interconnecting several HVDC systems forming HVDC Supergrids. Such grids can eventually evolve to meshed systems which interconnect a number of different AC power systems and large scale offshore wind (or other renewable sources) power plants and clusters. While such heavily meshed systems can be considered futuristic and will not certainly happen in the near future, the sector is witnessing initial steps in this direction. In order to ensure the flexibility and controllability of meshed DC grids, the shunt connected AC-DC converters can be combined with additional simple and flexible DC-DC converters which can directly control current and power through the lines. The proposed DC-DC converters can provide a range of services to the HVDC grid, including power flow control capability, ancillary services for the HVDC grid or adjacent grids, stability improvement, oscillation damping, pole balancing and voltage control. The present paper presents relevant developments from industry and academia in the direction of the development of these converters, considering technical concepts, converter functionalities and possible integration with other existing systems. The paper explores a possible vision on the development of future meshed HVDC grids and discusses the role of the proposed converters in such grids.Postprint (published version

Modular multilevel converter-based microgrid : a critical review

Recently, the Modular Multilevel Converter (MMC) has drawn significant attention due to its diverse merits and its applicability to a wide range of medium to high-power applications. The growing interest in the MMC can be attributed to its attractive features such as modularity, reliability, and high voltage capability. Significant research has been conducted on the MMC over the last few years to develop its operation and control in various applications. However, the application of MMCs in microgrids remains a largely unexplored topic. Therefore, this paper aims to address this research gap by offering an in-depth review of the latest developments concerning circuit topologies, control schemes, and fault-tolerance strategies of MMC within microgrid applications. This comprehensive review not only provides a synthesized overview of the current state of the art but also paves the way for future investigations in this promising field. The outcomes from this study are expected to stimulate further advancements in MMC applications in microgrid systems, thus contributing to the continuous improvement and evolution of microgrids.University of Sharjahhttps://ieeexplore.ieee.org/xpl/RecentIssue.jsp?punumber=6287639Electrical, Electronic and Computer Engineerin

Protection for submodule overvoltage caused by converter valve-side single-phase-to-ground faults in FB-MMC based bipolar HVDC systems

One of the most critical faults affecting modular multilevel converter (MMC) based bipolar high-voltage direct-current (HVDC) transmission systems is the single-phase-to-ground (SPG) faults between the converter transformer and the valve. However, half-bridge (HB) and full-bridge (FB) based MMCs exhibit a different behavior following such a fault and, thus, converter protection should be addressed in a different manner for each configuration. For HB-MMCs, an SPG fault at the valve-side leads to a severe overvoltage on the submodule (SM) capacitors in the converter upper arms and to grid-side non-zero crossing currents. Although FB-MMCs only exhibit overvoltage, these are more severe than for their HB counterparts. To address this problem, this paper presents a protection strategy considering thyristor bypass branches placed in parallel with upper arms of FB-MMCs. By employing this configuration, the upper arm overvoltage in the faulted converter is mitigated and remote converters can be quickly blocked using their local protection schemes. For completeness, the effectiveness of the strategy is verified through time-domain simulations in PSCAD/ EMTDC. The studies in this paper demonstrate the effectiveness of the presented protection scheme for station internal faults occurring in FB-MMCs in bipolar HVDC systems