Feasibility and reliability analysis of LCC DC grids and LCC/VSC hybrid DC grids

Power system interconnections using high-voltage direct-current (HVDC) technologies between different areas can be an effective solution to enhance system efficiency and reliability. Particularly, the multi-terminal DC grids, that could balance and ensure resource adequacy, increase asset utilization and reduce costs. In this paper, the technical feasibility of building DC grids using the line commutated converter based (LCC) and voltage source converter based (VSC) HVDC technologies are discussed. Apart from presenting the technical challenges of building LCC DC grids and LCC/VSC hybrid DC grids, the reliability modeling and analysis of these DC grids are also presented. First, the detailed reliability model of the modular multi-level converters (MMCs) with series connected high-voltage and low-voltage bridges are developed. The active mode redundancy design is considered for the reliability model. To this end, a comprehensive whole system reliability model of the studied systems is developed. The reliability model of each subsystem is modeled in detail. Various reliability indices are calculated using this whole system reliability model. The impacts of the redundancy design of the MMCs on these indices are presented. The studies of this paper provide useful guidance for DC grid design and reliability analysis

HVDC transmission : technology review, market trends and future outlook

HVDC systems are playing an increasingly significant role in energy transmission due to their technical and economic superiority over HVAC systems for long distance transmission. HVDC is preferable beyond 300–800 km for overhead point-to-point transmission projects and for the cable based interconnection or the grid integration of remote offshore wind farms beyond 50–100 km. Several HVDC review papers exist in literature but often focus on specific geographic locations or system components. In contrast, this paper presents a detailed, up-to-date, analysis and assessment of HVDC transmission systems on a global scale, targeting expert and general audience alike. The paper covers the following aspects: technical and economic comparison of HVAC and HVDC systems; investigation of international HVDC market size, conditions, geographic sparsity of the technology adoption, as well as the main suppliers landscape; and high-level comparisons and analysis of HVDC system components such as Voltage Source Converters (VSCs) and Line Commutated Converters (LCCs), etc. The presented analysis are supported by practical case studies from existing projects in an effort to reveal the complex technical and economic considerations, factors and rationale involved in the evaluation and selection of transmission system technology for a given project. The contemporary operational challenges such as the ownership of Multi-Terminal DC (MTDC) networks are also discussed. Subsequently, the required development factors, both technically and regulatory, for proper MTDC networks operation are highlighted, including a future outlook of different HVDC system components. Collectively, the role of HVDC transmission in achieving national renewable energy targets in light of the Paris agreement commitments is highlighted with relevant examples of potential HVDC corridors

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

Control strategies of full-voltage to half-voltage operation for LCC and hybrid LCC/MMC based UHVDC systems

With the increasing demand of transmitting bulk-power over long-distance, the ultra high-voltage direct-current (UHVDC) transmission systems become an attractive option. Nowadays, not only the line commutated converter (LCC) based systems, but also the modular multilevel converter (MMC) based systems have reached UHVDC levels. The converter stations of UHVDC systems normally utilize two series-connected valve-groups to reduce the difficulties of device manufacturing and transportation. This high-voltage and low-voltage valve-group configuration allows the UHVDC systems to achieve a full-voltage to half-voltage operation which increases the flexibility of the systems. However, the existing research only focuses on the full-voltage to half-voltage control of LCC-UHVDC systems. The control strategies for hybrid LCC/MMC UHVDC systems are underresearched. Moreover, the approaches to reduce the load-shedding caused by the full-voltage to half-voltage control for both LCC and hybrid LCC/MMC based UHVDC systems have not been investigated. In this paper, full-voltage to half-voltage control strategies for both LCC and hybrid LCC/MMC based UHVDC systems have been proposed. Moreover, to avoid load-shedding caused by the half-voltage operation, a power rescheduling method that re-sets the power references of the half-voltage operating and full-voltage operating poles has been proposed. The proposed full-voltage to half-voltage control strategies and power rescheduling method can achieve a stable and fast control process with a minimum power loss. The proposed methods have been verified through the time-domain simulations conducted in PSCAD/EMTDC

An overview of HVDC technology

There is a growing use of High Voltage Direct Current (HVDC) globally due to the many advantages of Direct Current (DC) transmission systems over Alternating Current (AC) transmission, including enabling transmission over long distances, higher transmission capacity and efficiency. Moreover, HVDC systems can be a great enabler in the transition to a low carbon electrical power system which is an important objective in today’s society. The objectives of the paper are to give a comprehensive overview of HVDC technology, its development, and present status, and to discuss its salient features, limitations and applications.</jats:p

Studies of commutation failures in Hybrid LCC/MMC HVDC systems

A hybrid of line commutated converters(LCCs) and modular multi-level converters(MMCs) can provide the advantages of both the technologies. However, the commutation failure still exists if the LCC operates as an inverter in a hybrid LCC/MMC system. In this paper, the system behavior during a commutation failure is investigated. Both halfbridge and full-bridge MMCs are considered. Control strategies are examined through simulations conducted in PSCAD/EMTDC

Comparisons of MVAC and MVDC systems in dynamic operation, fault protection and post-fault restoration

One of the most significant obstacles preventing the large-scale application of direct-current (DC) technology in medium voltage (MV) distribution networks is their fault protection. The existing AC relay protection needs to be changed or redesigned to protect the future overlay MVAC and MVDC distribution networks. Therefore, a comprehensive understanding of the dynamic and fault behavior and post-fault restoration strategies of MVAC and MVDC systems are critically important. Moreover, a comparison of MVAC and MVDC systems during a fault will also contribute to designing the protection systems of hybrid MV AC/DC systems. In this paper, the challenges of protecting DC faults of MVDC systems and possible solutions are first introduced. Then, the fault characteristics and post-fault restoration of MVDC and MVAC distribution systems are compared and investigated through case studies. Time-domain simulations have been conducted in PSCAD/EMTDC. The work in this paper will be valuable for the protection design for future hybrid MV AC/DC systems

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