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

A cascaded converter interfacing long distance HVDC and back-to-back HVDC systems

This paper proposes a cascaded converter dedicated to long-distance HVDC infeed and asynchronous back-to-back interconnection of receiving grids. The cascaded converter is consisted of MMCs in series and parallel connection, meeting the high DC voltage and power demand of HVDC system. It realizes hierarchical feeding and asynchronous interconnection of receiving grids, optimizing the multi-infeed short circuit ratio and improving the flexibility of the receiving grids. The topology and operating characteristics of the cascaded converter are introduced in detail. The multi-infeed short-circuits ratio (MISCR) and the maximum power infeed of the cascaded converter based HVDC systems are analyzed. Various feasible operating modes with online switching strategies of the cascaded converter are studied to improve the operational flexibility of the system. The simulation results verify the effectiveness of the control strategy of the HVDC system embedding the cascaded converter. The DC faults clearing strategy and operating modes switching strategies are also validated

Modular multilevel converters: Recent applications [History]

The story of the modular multilevel converter (MMC) started with its invention by Prof. Rainer Marquardt in 2001. Since then, this new concept has been recognized as a milestone achievement in power electronics. MMCs have revolutionized the capabilities of power conversion technologies, particularly in high-voltage dc (HVdc) transmission systems

Operation Analysis of Thyristor Based Front-to-Front Active-Forced-Commutated Bridge DC Transformer in LCC and VSC Hybrid HVDC Networks

The active-forced-commutated (AFC) bridge employs a symmetrical thyristor-bridge with auxiliary self-commutated full-bridge chain-link (FB-CL) circuit to assist its soft transition and forced commutation. This combination can form a thyristor based voltage source converter (VSC) with significantly reduced on-state losses and dc-fault blocking capability. Due to the full topological symmetry of the AFC-bridge, either current direction or dc-link voltage polarity can be reversed for power flow reversal as for the full-bridge modular multilevel converter (FB-MMC). Thus, the AFC-bridge is compatible with both line-commutated-converter (LCC) and VSC terminals in a multi-terminal high voltage direct current (MT-HVDC) network. This paper investigates its front-to-front (F2F) dc-dc application for matching the regional dc grids in a LCC and VSC hybrid HVDC network. Simulation studies are carried out to demonstrate its potentials as a high efficiency multi-functional solution for dc-dc conversion

Dynamic performance of voltage balancing and circulating current suppression control for modular multilevel converter

Ph.D. ThesisGlobal power consumption has increased by approximately 3% each year over the past 15 years. The growing demand for energy has stimulated the spread of clean and reliable renewable energy networks and power grid interconnections throughout the world. For example, in Europe, there are 23 High Voltage Direct Current (HVDC) Transmission lines under construction which are scheduled for completion before 2024. The Modular Multilevel Converter (MMC) is one of the most attractive candidates for the HVDC transmission system converter technology. Its high flexibility and controllability make it an attractive option for HVDC transmission. However, the higher initial investment and the unfavourable conditions for using associated DC circuit breakers have always been a barrier to further installations. Since ABB successfully developed the HVDC DC circuit breakers in 2012, there is increasing interest in DC grids using the MMC HVDC transmission system. However, one of the common problems existing in the HVDC transmission system is the control of the capacitor volt-age in each submodule of the MMC. However, in the transmission systems, especially in the renewable energy systems, there are disturbances existing. The conventional voltage balancing control is weak to the disturbances, such as power and sampling frequency changes. Therefore, the proposed voltage balancing control in this thesis has improved the responding time and precision of the control. It determines the charging state of each submodule by deriving the capacitor voltage variations, thereby ensuring the voltage of each capacitor is within pre-defined range regardless the disturbance. In later study, both simulation and experimental results have shown the proposed control approach has strong immunity to the sampling frequency noise compared to the conventional control. However, even with the proposed voltage balancing control, the capacitor voltage difference cannot be eliminated entirely. They will cause circulating current flowing among the phases of the circuit. Therefore, causing unnecessary pressures to the affected components. The circulating current suppression control pro-posed in this thesis can eliminate the AC component of the circulating current, by regulating it according to the power going through the converter. It gets rid of the two PID controllers and abc-dq transformation which are commonly used in conventional circulating current control approach. The simulation and experiment results have shown the suppression of the proposed control approach regarding the AC components in the circulating current, and the fast response time taking effect within one control cycle. In this thesis, both proposed control approaches are presented with simulation results and validated with the scaled down experiment model

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 Converters in Hybrid Multi-Terminal HVDC Systems

High-voltage direct current (HVDC) systems are becoming commonplace in modern power systems. Line commutated converters (LCCs) are suitable for bulk power and ultra-HVDC (UHVDC) transmission, while with inflexible power reversal capability and possible commutation failures. However, voltage source converters (VSCs) possess flexible power reversal capability and provide immunity to commutation failures. Modular VSC topologies offer improved performance compared to conventional 2 level/3 level VSC-based HVDC. The family of modular VSCs includes the well-established modular multilevel converter (MMC) and other emerging modular VSC topologies such as the DC-fault tolerant alternate arm converter (AAC) that share topological and operational similarities with the MMC. It is noteworthy that the integration of LCC and modular VSCs leads to unique benefits despite the challenges of different HVDC configurations. Hence, it is necessary to explore the system performance of different HVDC converter topologies, especially more complex hybrid multiterminal HVDC (MTDC) systems and DC-grids combining different converters. This thesis focuses on the combination of the LCC, MMC and AAC to constitute different hybrid HVDC transmission systems. It is of significance to provide a common platform where the proper comparison and evaluation of different HVDC systems and control methods can be completed and independently validated. Therefore, this thesis also provides an overview of current HVDC benchmark models available in the existing literature. In addition, the detailed modeling methods of HVDC systems are discussed in this thesis. For ensuring the static security of HVDC systems especially the future DC-grids, this thesis proposes a generalized expression of DC power flow under mixed power/voltage (P/V) and current/voltage (I/V) droop control, considering the DC power flow for normal operation and after converter outage. Detailed simulation models are established in PLECS-Blockset and Simulink to study the hybrid HVDC/MTDC systems and DC grid combining the LCC with the MMC and (or) AAC. The detailed sets of results demonstrate the functionalities of developed hybrid HVDC systems and validate the performance of systems complying with widely accepted HVDC operating standards. The developed LCC/AAC-based HVDC/MTDC systems and LCC/MMC/AAC-based DC grid in this thesis are prime steps towards the study of more complex MTDC systems and a key element in the development of future DC super grids

Active-forced-commutated bridge using hybrid devices for high efficiency voltage source converters

Peer reviewedPublisher PD

A unidirectional DC-DC autotransformer for DC grid application

Conventional unidirectional DC-DC converters for DC grid application employ DC-AC-DC two-stage conversion technology and suffer from high converter cost and power loss. To solve these issues, a unidirectional step-up DC-DC autotransformer (UUDAT) and a unidirectional step-down DC-DC autotransformer (DUDAT) are studied. The UUDAT and DUDAT are composed of a series connection of diode bridges and voltage source converters. Topologies of UUDAT and DUDAT are detailed. The harmonic and un-controllability issues are discussed. Control and possible application scenarios for UUDAT and DUDAT are depicted. DC fault isolation mechanism and the methods of dimensioning the voltage and power ratings of the components in UUDAT and DUDAT are studied. Extensive simulations on power system level and experiments on a UUDAT and DUDAT prototype verified their technical feasibility