VSC-HVDC Technology Research Articles

A Novel Low-Cost DC Chopper for Offshore Wind VSC-HVDC System by Using Thyristors Instead of Full-Control Devices

VSC-HVDC technology has a broad application prospect in high-capacity offshore wind power transmission, and it is of great importance to achieve fault ride through (FRT) of offshore wind VSC-HVDC system. The existing DC chopper cannot meet the requirements of high performance and low cost at the same time, which limits its further promotion and application. In this paper, a novel low-cost DC chopper based on thyristor- <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> circuit (TLC-DCC) is proposed, which uses a low-cost semi-control device, namely the thyristor, and generates DC current zero-crossing through the cooperation of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> circuit to realize the reliable turn off of the energy consumption module, thus providing a new idea for solving the FRT problem. In addition, the TLC-DCC control strategy and key parameter coordination method are proposed. In order to verify the effectiveness of the TLC-DCC, a 4 kV/2.4 MW prototype is built. On this basis, the characteristics of the TLC-DCC are analyzed, and the simulation results show that the TLC-DCC can realize FRT under different short-circuit conditions. Compared with the existing DC chopper, TLC-DCC has the great advantage of reducing the total cost by 38% with the same performance.

A Voltage-Boosting Submodules Based Modular Multilevel Converter With Temporary Energy Storage Ability for Fault Ride Through of Offshore Wind VSC-HVDC System

VSC-HVDC technology has broad application prospects in the field of long-distance and large-capacity offshore wind system, and it is of great significance to realize the fault ride through (FRT) of the system. Aiming at the problems of high cost of adding DC chopper and low reliability of actively reducing wind turbine power, the scheme of temporary improving MMC capacitor voltage to store energy to realize FRT is considered to have the advantages of low cost and high reliability. However, due to the limit of insulated gate bipolar transistor (IGBT) operating voltage, the existing method do not have strong fault ride through ability. To solve this problem, the voltage-utilization boosting circuit composed of gap and metal oxide varistor (MOV) is proposed to ensures the safe operation of the IGBT after capacitor voltage temporary rise. In addition, a fault ride through strategy based on the cooperation of wind turbine and the voltage-boosting submodules (SMs) based modular multilevel converter with temporary energy storage ability (VBS-MMC) is proposed in this paper. Through the organic combination of new nonlinear circuit and control strategy, the system FRT under various fault conditions is realized. Furthermore, a 3.3 kV welded IGBT experimental platform is built, and the results show that the SM temporary voltage-boosting method is feasible. Finally, the simulation results of ± 320 kV/400 MW VSC-HVDC system verify the FRT capability of VBS-MMC. And the cost and performance comparisons are analysis.

VSC-HVDC Incorporated Corrective Security-Constrained Optimal Power Flow

With the rapid development and diverse engineering applications of VSC-HVDC technology, development in power industry has put forward higher economic and security requirements for the security-constrained optimal power flow. Under these circumstances, this paper proposes a security-constrained optimal power flow model for hybrid AC/DC power grids that incorporates the VSC-HVDC corrective control. The linear relationship between the pre-contingency state power flow and post-contingency state power flow is established based on the shift factor and the reliability constraints are described as the linear inequality constraints of the post-contingency state active power flow which is expressed by the pre-contingency state branch power flow. With the inherent ability of the VSC-HVDC converter station (short as VSC station) to rapidly regulate active power taken into account, this paper utilizes the linear relationship between the active power adjustment of the VSC station and the power flows of transmission lines and electrical equipment and corrects the reliability constraints with the sensitivity coefficient. Hence the correction of the post-contingency state power flow is reflected in the constraints. By solving the proposed model with the primal-dual interior-point method, the pre-contingency state optimal security operating point that considers the corrective capability of the VSC stations is obtained. Finally, the correctness and validity of the proposed model is verified based on a VSC-HVDC modified IEEE 14-bus system.

A Modular Multilevel Converter With Integrated Energy Dissipation Equipment for Offshore Wind VSC-HVDC System

VSC-HVDC technology has great application prospects in the field of offshore wind system. In order to realize the capability of fault ride through of offshore wind VSC-HVDC system, this paper proposes a modular multilevel converter with integrated energy dissipation equipment (IEDE-MMC), which combines the complementary advantages of MMC and DC chopper. The working process of the short-time pulse of the energy dissipation equipment provides an opportunity for the use of low current switch devices, and it shares the water-cooling system, control-protection system and energy-supply system with the MMC submodule, which eliminates the independent systems required by the traditional DC chopper, providing an economic and reliable solution for the problem of fault ride through. The simulation results of 400 MW/±150 kV VSC-HVDC system verify the fault ride through capability of IEDE-MMC, and the peak value of DC voltage is less than 1.1 p.u., which meets the requirements of the system. Finally, the manufacturing cost and floor area of IEDE-MMC are analyzed. Compared with the scheme of adding DC chopper outside MMC, the IEDE-MMC cost is reduced by 22%, and the total floor area is reduced by 35%.

Significance of fault-current-limiters and parameters optimization in HVDC circuit breakers for increased capacity of VSC-HVDC transmission networks application

The voltage and current ratings of the voltage source converter (VSC) based high voltage direct current (HVDC) transmission line projects have increased over the years. It testifies the increasing popularity of VSC-HVDC technology. Since the long-distance, VSC-HVDC transmission lines are more exposed to commonly appearing short circuit faults; thus, protection equipment with increased fault-current handling capability is required. The DC circuit breakers (DCCBs), with or without fault current limiters (FCLs), can effectively handle the fault-current. The variations in VSC-HVDC projects’ ratings and their impact for DCCB with FCL and without FCL are thoroughly analysed in this paper. The paper further includes an in-depth discussion on DCCBs with FCL and without FCL. An equivalent circuit modelling approach is used for simulation analysis of three case studies under the influence of these breakers to analyse current behaviour; the case studies include the Zhoushan HVDC Project, Xiamen HVDC Project, and Zhangbei-I HVDC Project. All these projects have different capacities in terms of their power, voltage, and current ratings. For DCCBs, this paper includes a detailed analytical model to understand the fluctuations in the size of fault-current limiting components (in DCCBs) for increased fault-current handling requirements. The simulation results using PSCAD/EMTDC and MATLAB are presented to verify different aspects of this study. A small scale laboratory prototype is also developed to validate the performance of DCCB with and without FCL. Based on theoretical, simulation, and experimental validations, it is substantiated that for increased VSC-HVDC projects’ ratings, DCCB with FCL can perform better compared to conventional DCCB without FCL.

Heuristic remedial actions in the reliability assessment of high voltage direct current networks

&lt;p&gt;Planning of high voltage direct current (HVDC) grids requires inclusion of reliability assessment of alternatives under study. This paper proposes a methodology to evaluate the adequacy of voltage source converter/VSC-HVDC networks. The methodology analyses the performance of the system using N-1 and N-2 contingencies in order to detect weaknesses in the DC network and evaluates two types of remedial actions to keep the entire system under the acceptable operating limits. The remedial actions are applied when a violation of these limits on the DC system occurs; those include topology changes in the network and adjustments of power settings of VSC converter stations. The CIGRE B4 DC grid test system is used for evaluating the reliability/adequacy performance by means of the proposed methodology in this paper. The proposed remedial actions are effective for all contingencies; then, numerical results are as expected. This work is useful for planning and operation of grids based on VSC-HVDC technology.&lt;/p&gt;

VSC-HVDC and Its Applications for Black Start Restoration Processes

System reliability is a measure of an electric grid system’s ability to deliver uninterrupted service at the proper voltage and frequency. This property of the electric system is commonly affected by critical processes, such as a total blackout. The electric system restoration is a complex process which consists of returning generators, transmission system elements, and restoring load following an outage of the electric system. However, the absence of a generator or unit of black start capabilities may worsen the duration and effects of blackouts, having severe consequences. Black start capability is important as it can reduce the interruption time, decrease the economic loss, and restart the power supply fast and efficiently. In recent years, several works have reported advances about the High Voltage Direct Current (HVDC) technology based on the Voltage-Source Converter (VSC) as an attractive and promising technology to increase black start capability. This paper is a review of the current studies of VSC-HVDC as black start power and discusses the advantages and limitations of recent methods. The major points addressed in this paper are as follows: the current theoretical approach of the black start process and the used HVDC technologies, the advantages of VSC-HVDC as black start power, a compressive review of the literature about the black start capabilities using VSC-HVDC technologies, and a description of the main methods recently used to provide an enhancement for restoration processes. Finally, this paper discusses new challenges and perspectives for VSC-HVDC links in order to provide an enhancement for restoration processes.

VSC-HVDC Interties for Urban Power Grid Enhancement

Urban power grids are facing many operational and expansion challenges to meet further demand growth and increased reliability requirements. Advanced transmission technologies have been considered by the electric utilities to effectively increase the utilization of existing infrastructure and operational flexibility. The focus of this paper is on VSC-HVDC technologies for urban power grid enhancement and modernization. First, a potential technical scheme is proposed for converting an existing AC circuit to DC operation, which could boost the power transfer capability of the critical transmission corridor and increase network operational flexibility. Second, this paper proposes three operation modes for the VSC-HVDC interties in urban power grids corresponding to normal, emergency and island operating conditions, respectively. An integrated, adaptive emergency control strategy (AEC) is proposed that can enable adaptive power flow responses of the VSC-HVDC intertie under varying system operating conditions and critical contingencies. The flexibility and effectiveness of the proposed operational principles of urban VSC-HVDC intertie and the corresponding control strategies are verified in PSCAD/EMTDC using a realistic urban power grid in China.

Modeling and Assessment of Short-Term Electromagnetic Interference on a Railway System From Pole-to-Ground Faults on VSC-HVDC Cable Networks With Sea Electrodes

This work falls under the preliminary engineering studies that are carried out to assess the interference impact, which may be raised from the operation of the bipolar HVdc cable link between Attica and Crete. The interference impact concerns critical and public infrastructures that are located in close proximity to the intended link's on-shore routing. In this article, particular emphasis is given in modeling a specific interference situation, on a collocated railway system, that ascends from pole-to-ground fault conditions that may occur on the planned bipolar voltage source converter (VSC)-HVdc cable network. To this extent, the simulation analysis takes into account the specific characteristics of the interfering VSC-HVdc technology and the results are constructively benchmarked against the safety thresholds tabulated in international standards.

Optimal Operation Strategy of Flexible Interconnected Distribution Network Based on SES-VSC-MTDC

The increasing integration of renewable energy is challenging the secure operation of the power system. System flexibility or the capability to address the significant power fluctuations from renewable energy is becoming more and more relevant. Self-energy storage-based multiterminal back-to-back VSC-HVDC (SES-VSC-MTDC) technology is first proposed, and it can realize the power regulation on both temporal and spatial dimensions, which helps improve the power supply reliability and the capacity to accommodate renewable energy of the interconnected distribution networks. Then, to address the coordination control problem of the energy storage and back-to-back VSC-HVDC, a comprehensive control strategy of SES-VSC-MTDC is proposed based on the optimal power flow preprocessing and state of charge interval division. Then, the power regulation model and the energy-power regulation timing model of SES-VSC-MTDC are established for different control strategies. Then, we use the primal-dual interior-point method to solve the developed optimal operation model of flexible interconnected distribution network. Finally, a 33-bus system with four interconnected feeders is used to test the effectiveness of the SES-VSC-MTDC technology and its operation control strategy.

Engineering practices for the integration of large-scale renewable energy VSC-HVDC systems

With the continuous development of power electronic devices, intelligent control systems, and other technologies, the voltage level and transmission capacity of voltage source converter (VSC)-high-voltage direct current (HVDC) technology will continue to increase, while the system losses and costs will gradually decrease. Therefore, it can be foreseen that VSC-HVDC transmission technology will be more widely applied in future large-scale renewable energy development projects. Adopting VSC-HVDC transmission technology can be used to overcome issues encountered by large-scale renewable energy transmission and integration projects, such as a weak local power grid, lack of support for synchronous power supply, and insufficient accommodation capacity. However, this solution also faces many technical challenges because of the differences between renewable energy and traditional synchronous power generation systems. Based on actual engineering practices that are used worldwide, this article analyzes the technical challenges encountered by integrating large-scale renewable energy systems that adopt the use of VSC-HVDC technology, while aiming to provide support for future research and engineering projects related to VSC-HVDC-based large-scale renewable energy integration projects.

Optimal Power Flow in AC–DC Grids With Discrete Control Devices

With the increasing applications of VSC-HVDC technologies in power grids, the importance of incorporating DC grids and converter stations into the optimal power flow (OPF) problem is increasing. The modeling of a converter station involves highly nonlinear operational constraints and integer variables representing the tap settings of LTC transformers. In this paper, an efficient OPF algorithm for hybrid AC–DC grids with discrete control devices is presented. A successive linear approximation method for the power flow equations in both AC and DC grids is introduced. The operational constraints of the converter station are thoroughly investigated. The proposed linearization of the power flow equations facilitates the convexity of the VSC modeling. A fictitious branch is added to the AC grid to handle the highly nonlinear converter losses. The discrete nature of the LTC transformer in the converter station is accurately modeled. An iterative solving algorithm is proposed for the OPF model. The strict satisfaction of the power flow equations is enabled through the application of an AC feasibility recovery process based on the power flow calculation in hybrid AC–DC grids. Case studies demonstrate that the proposed algorithm outperforms several selected commercial solvers.

Estimation of investment model cost parameters for VSC HVDC transmission infrastructure

Investment model cost parameters for VSC HVDC transmission infrastructure continue to be associated with high uncertainty and their validity remains a crucial challenge. Thus, it is the key objective of this analysis to identify a new cost parameter set providing better investment cost estimates than currently available cost parameter sets. This parameter estimation is based on a previously conducted review of investment model cost parameters including its collection of existing cost parameter sets and project cost reference data. By using a particle swarm optimisation, the overall error function of the review's evaluation methodology is minimised to obtain an optimal parameter set. The results show, however, that the optimised parameter sets are far from being realistic and useful, which is why an improved overall error function is developed. Effectively penalising negative and near-zero cost parameter coefficients, this new overall error function delivers a realistic and well-performing cost parameter set when being minimised. In fact, the new parameter set produces better cost estimates for back-to-back, interconnector, and offshore wind connection projects than any of the existing cost parameter sets. Therefore, it is a valuable contribution and shall be considered in future grid investment analyses involving VSC HVDC technology.

Variable AC transmission frequencies for offshore wind farm interconnection

Non-standard AC transmission frequencies have been suggested in literature as a potential competitor to VSC-HVDC transmission for offshore wind farms integration (>100 km). Transmitting at lower than standard 50/60 Hz frequency provides the advantage of longer transmissible range for bulk AC power transfer and potentially lower losses in associated wind farm components. A higher than nominal frequency reduces the size of the required transformers and offshore platforms, potentially reducing cost substantially. This paper examines the selection of non-standard AC transmission frequencies from 1 to 100 Hz presenting a techno-economic analysis and methodology for comparing AC to standard benchmark VSC-HVDC technology in terms of power loss, size/volume of components, CAPEX and operation/maintenance metrics. It is shown at frequencies lower than 20 Hz, cost of energy is comparable to standard VSC-HVDC; due in part to lower number of cables required to carry full load power and the removal of the offshore VSC-HVDC converter station. The key contribution of this work is exploring the potential extended range of transmission capability for non-conventional AC frequency approaches that display comparable power loss and CAPEX/OPEX to VSC-HVDC based transmission for offshore wind integration.

A Review of LCC-HVDC and VSC-HVDC Technologies and Applications

High Voltage Direct Current (HVDC) systems has been an alternative method of transmitting electric power from one location to another with some inherent advantages over AC transmission systems. The efficiency and rated power carrying capacity of direct current transmission lines highly depends on the converter used in transforming the current from one form to another (AC to DC and vice versa). A well configured converter reduces harmonics, increases power transfer capabilities, and reliability in that it offers high tolerance to fault along the line. Different HVDC converter topologies have been proposed, built and utilised all over the world. The two dominant types are the line commutated converter LCC and the voltage source converter VSC. This review paper evaluates these two types of converters, their operational characteristics, power rating capability, control capability and losses. The balance of the paper addresses their applications, advantages, limitations and latest developments with these technologies.

Guest Editorial Special Section on HVDC Systems for Large Offshore Wind Power Plants

The papers in this special section focus on HVDC systems for large offshore wind power plants. These plants have experienced an important growth in the last decade and are becoming a key energy source worldwide. Offshore wind presents clear advantages due to high and stable wind speeds offshore, enabling the use of larger wind turbines than those commonly used on inland wind power plants. However, transmitting power to load centres also brings a number of technical challenges. Depending on the distance to the shore and the power rating, AC or DC technology may be the preferable option for integrating offshore wind power to onshore grids. There are a number of issues which have to be considered requiring substantial research and development. For DC connections, (Voltage Source Converter) VSC-HVDC technology has shown advantages over (Line Commutated Converter) LCC-HVDC. The development of multiterminal HVDC grids though brings further challenges related to system operation, control, and protection and coordination between HVDC converters, wind power plants, and the onshore system. The development of power electronic switches has reached a point where the size, rating, and the system efficiency can meet the needs of commercial HVDC. Wind power plants connected to HVDC transmission system require specific control and protection coordination between HVDC converters and wind turbines.

Synthesis of a hybrid model of the VSC FACTS devices and HVDC technologies

The motivation of the presented research is based on the need for development of new methods and tools for adequate simulation of FACTS devices and HVDC systems as part of real electric power systems (EPS). The Research object: An alternative hybrid approach for synthesizing VSC-FACTS and -HVDC hybrid model is proposed. The results: the VSC- FACTS and -HVDC hybrid model is designed in accordance with the presented concepts of hybrid simulation. The developed model allows us to carry out adequate simulation in real time of all the processes in HVDC, FACTS devices and EPS as a whole without any decomposition and limitation on their duration, and also use the developed tool for effective solution of a design, operational and research tasks of EPS containing such devices.

Improvement of Stable Operation Ability for Large-Scale Wind Power System Based on VSC-HVDC Technology

Rigorous transmission technology is important when large-scale wind farm is connected to the power grid. Hence, a power transmission topological framework based on VSC-HVDC is proposed. The mathematical model is built in the dq synchronous frame, and the related direct current control strategy of VSC converters are designed. The doubly fed induction generator wind farm with VSC-HVDC system are modelled in DIgSILENT/PowerFactory. Ultimately, the conditions of the short-circuit fault at the receiving-end VSC AC bus is simulated. The control scheme is proved to be effective.

Variable Frequency Operation for Future Offshore Wind Farm Design: A Comparison with Conventional Wind Turbines

Recent developments of offshore power grid demonstrations in European waters at substantial distances from the onshore grid have accelerated the demand to look at high voltage DC power transmission (HVDC) as an alternative topology to traditional high voltage AC methods [1]. Current VSC-HVDC technology can realise the integration of a centralised power converter to control the speed of a cluster of turbines, thus reducing the number of power converters from the farm and therefore potentially increasing reliability and lowering power loss. This paper will primarily concentrate on the benefits of variable frequency operation of a wind farm using a centralised VSC-HVDC converter and comparing its performance to 50Hz Type 4 wind turbines. The analysis will incorporate measured wind data form Irish offshore locations and will provide a detailed power loss methodology for the wind farm components with annual energy capture results for both variable and fixed frequency approaches.

Integrating Offshore Wind Power and Multiple Oil and Gas Platforms to the Onshore Power Grid Using VSC-HVDC Technology

AbstractThis paper investigates the possibilities of integrating oil and gas platforms and offshore wind power to the onshore power grid. The main motivation for this proposal is to reduce the large greenhouse gas emissions associated with onsite power generation on traditional oil and gas platforms and to improve the economy of offshore wind projects. The feasibility of a hypothetical power system in the North Sea consisting of five oil and gas platforms and one offshore wind power plant with a common connection to the onshore power grid is studied. This combined grid integration concept has not been given much attention in similar previous publications. The connection to the onshore grid is realized through a high-voltage direct current transmission system based on voltage source converter technology. A dynamic simulation model of the system and a control system are presented, and simulations are performed, using SimPowerSystems in MATLAB/Simulink, on system disturbances that are thought to be critical for the operation of the system. The simulation cases represent large disturbances that the system should be designed to withstand. It was concluded that the system handles variations in the load very well and that the system configuration studied in this paper is regarded as a feasible way of integrating oil and gas platforms and offshore wind power to the onshore grid.