High Voltage Direct Current Applications Research Articles

Comparative Assessment of Multi-Port MMCs for High-Power Applications

Multi-port converters can interconnect different dc and ac systems while reducing semiconductor requirements and losses by eliminating redundant power conversion stages. Modular multilevel converter (MMC)-based multiport systems are well suited for application in mixed ac-dc grids containing high-voltage dc (HVDC) and medium-voltage dc (MVDC) systems. This paper conducts a detailed comparative assessment of multi-port dc-dc-ac MMCs for high power applications. Four representative topologies are chosen for study due to their contrasting internal power processing characteristics. Three different network scenarios are investigated that include HVDC and MVDC applications, covering several different power flow cases. The multi-port MMCs are compared in terms of losses, semiconductor effort, internal energy storage and magnetics requirements. The results are extensively discussed and general conclusions are summarized.

Online Health Monitoring of DC-Link Capacitors in Modular Multilevel Converters for FACTS and HVDC Applications

This article presents an online health monitoring scheme for dc capacitors in modular multilevel converters (MMCs). The health monitoring algorithm is based on detecting changes in the dc capacitance value over time. The proposed algorithm only utilizes measurements that are typically available in flexible alternating current transmission systems and high-voltage direct current applications. Hence, in the proposed estimation method, no additional sensors are used. The estimation scheme considers the presence of noise in voltage and current measurements, and utilizes a recursive least square estimator in conjunction with a special low-pass filter to minimize the estimation errors. Simulation results of a hardware replica, as well as experimental results on a low-power MMC prototype show that the proposed scheme can identify the dc-link capacitance value with a maximum error of 1%.

Formulation and Analysis of Single Switch High Gain Hybrid DC to DC Converter for High Power Applications

The necessity for DC−DC converters has been rapidly increasing due to the emergence of RES-based electrification. However, the converter designed so far exhibits the drawbacks of lower efficiency and non-compactness in size. Hence, to rectify this problem, the new topology of a flyback converter for PV application is proposed in this work. The proposed converter exhibits reduced ripple in input current and enhances the conversion efficiency. Finally, the efficiency of this proposed converter is verified using MATLAB. The results indicate that this projected topology can be suitable for high voltage DC applications.

Evaluation of Electrical Performance and Life Estimation of PPs for HVDC Power Cable

Demand and need for the application of high voltage direct current (HVDC) are increasing because of high capacity and long-distance transmission. Research on polypropylene (PP) that can increase the operation temperature compared to existing insulation is constantly being considered. This study aimed to evaluate the electrical performance and estimate the life of HVDC application of PP. In this study, a DC V-t characteristic tests were conducted on three types of PP sheets at a temperature of 110 °C. In addition, a life estimation formula based on the electrical stress was derived and the electrical performances were evaluated. The experimental results show that the life exponent of material mixed with block copolymer, homo polymer and high density polyethylene (HDPE) was 23 and the electrical performance was 17% better than block copolymer, thereby demonstrating the reliability and electrical performance for application of HVDC.

Comparative economic analysis of low frequency AC transmission system for the integration of large offshore wind farms

To fully extract the potential of offshore wind power, the wind farms are built further from mainland and the capacity is growing larger. Low Frequency AC (LFAC) transmission System is a promising solution to the integration of large offshore wind farms in long distance. This paper conducts a comparative economic analysis of LFAC. Compared to 50 Hz High Voltage AC (HVAC) transmission, LFAC can reduce charging current in AC cables to enhance the transmission ability in long distance. Compared to High Voltage DC (HVDC) transmission, LFAC eliminates the large expense of offshore converter station. In addition, the topologies of AC/AC converter in LFAC also influences the economic performance significantly. By designing typical AC/AC converters based on modular multilevel technique in a unified way, this paper performs a full-scale economic comparison in different voltage level, active power and reactive power cases. Furthermore, the economic comparison based on capital cost of HVAC, HVDC and LFAC is comprehensively studied considering different transmission capacity and the AC/AC converter topologies. The economic range of HVAC, LFAC and HVDC application is obtained, and results show that LFAC is suitable for medium distance circumstances (around 80–200 km). The economic range of LFAC expands when power rating increases.

Resilient Operation of an MMC With Communication Interruption in a Distributed Control Architecture

Modular multilevel converters (MMCs) in high-voltage dc applications usually adopt a distributed control architecture to manage a large number of submodules (SMs) through a communication network. The communication congestion and network disconnection might lead to communication interruption (CI) and eventually cause the system to malfunction. In this article, a resilient operation strategy is proposed and studied to ride-through the CI fault, in order to prevent frequent fault SM bypassing, replacement, or even system shutdown. The analysis of the MMC distributed control system with the presence of CI indicates that the insertion index of the faulted SM might become constant, which distorts the output current and results in overvoltage of the communication interrupted SM (CI-SM). The CI-SM capacitor voltage prediction can be used to determine the MMC safe operation period after CI occurs. During the safe operation period, the CI-SM power balance is sustained by utilizing prestored phase signals to generate a sinusoidal insertion index according to its capacitor voltage tracking error. Two operation modes are proposed and analyzed to ensure the MMC stable operation under various conditions. The system protection is sensibly used only if the CI duration exceeds a safe operation period, which avoids frequent SM cut-off. Good agreement of the CI-SM capacitor voltage is achieved between the theoretical and simulation results. The effectiveness and robustness of the proposed MMC resilient operation are experimentally confirmed.

Modelling and simulation of HVDC and FACTS systems using 3DEXPERIENCE platform of Dassault Systemes

High Voltage Direct Current (HVDC) transmission is a well proven system based on power electronics devices to enhance the existing power transmission capabilities. In order to use the transmission networks more efficiently and also to increase the power transmission capability, simulation and analysis of HVDC systems are very much necessary. Flexible Alternating current Transmission system (FACTS) technology based on power electronics devices are used to enhance the existing transmission capabilities, in order to make the transmission system flexible. Simulation of these FACTS devices play an important role in analysis of power system quality. The concepts of conventional HVDC systems and application of FACTS controllers for AC transmission system has been discussed well in available literature. In this paper, the concepts are illustrated through the Dassault Systemes 3DEXPERIENCE software for the purpose of developing virtual lab module for experiments on rectifier of a HVDC system and Static Var Compensator. The simulation modules for a 12-pulse rectifier of a HVDC system and also a Static Var compensator for voltage profile improvement are developed and analysed. The performance of both the modules are validated with the theoretical equations.

Investigations on the effect of ageing on charge de‐trapping processes of epoxy–alumina nanocomposites based on isothermal relaxation current measurements

In this study, the relationship between thermal ageing and charge trapping properties of epoxy-based nanocomposites has been investigated. With ageing, any dielectric material undergoes thorough degradation. This degradation significantly affects the space charge accumulation and charge trapping behaviour of the dielectric, which are very important parameters for insulation health under high-voltage direct current (HVDC) environment. In this work, an improved model based on the isothermal relaxation current (IRC) has been developed to study the charge trapping behaviour of pure epoxy and epoxy alumina (Al2 O3) nano-composites at different ageing conditions. A methodology based on polarisation–depolarisation current (PDC) measurements has been proposed to identify the current component due to a dipolar relaxation in measured total IRC. This will help to identify the trap distribution characteristics more accurately compared to conventional IRC measurements. It was experimentally observed that the addition of nanoparticles significantly reduces trapped charge formation and reduces thermal degradation. It is observed that aging leads to the generation of deeper traps, while the addition of Al2 O3 nanoparticles mainly enhances the density of shallow traps. Results presented in this work indicate that epoxy–alumina nanocomposites are very much suitable in HVDC applications from the perspective of trapped charge accumulation.

Cybersecurity of Wide Area Monitoring, Protection, and Control Systems for HVDC Applications

The flexibility provided by High Voltage Direct Current (HVDC) systems can be further extended by Wide Area Monitoring, Protection, and Control (WAMAPC) systems. WAMPAC systems enable many HVDC applications and, on the other hand, inevitably introduce cybersecurity concerns that need to be addressed. In this work, a security domain layer and decision framework is reported to detect and mitigate the impact of false data injection (FDI) attacks targeting HVDC stations. Specifically, a rule-based cyber-attack detection method is introduced and implemented on Raspberry Pi and tested on the real-time HVDC simulation facility on the real-time digital simulator (RTDS) platform at ABB U.S. Corporate Research Center.

Modeling and Design of Cascaded h-bridge type multi-level Inverters up to Thirty-one level for the Reduction and Performance Improvement

Multilevel inverter (MLI) becomes more popular in high voltage DC (HVDC) applications, power electronic converters and drives. This paper describes the simulation of single phase multilevel cascaded H-bridge inverter. Simulation of three level, five level, thirteen level, fifteen, twenty-one, thirty-one level inverters are done in MATLAB/ Simulink. The switching schemes, and topologies are discussed in detail here up to thirty-one levels. This paper discusses the voltage level to achieve sinusoidal waveform & compare different voltage level by increasing the level through simulation. The closed loop space-vector based pulse width modulation technique is adopted for effective controlling and lower harmonic voltage conversion. The comparative results are presented for multilevel inverter up-to thirty-one level which shows the total harmonic distortion (THD) is decreased as the number of voltage level rises.

Temperature and Field Induced Variations of Electric Conductivities of HTV Silicone Rubbers Derived from Measured Currents and Surface Potential Decay Characteristics

The temperature and field dependencies of electric conductivities of two types of silicone rubber-based polymers intended for use in high voltage direct current applications are presented and discussed. The conductivities obtained with the standard method by measuring a current through the material sample placed between metallic electrodes in response to the applied voltage are compared with those deduced from the measured potential decay on pre-charged material surface in an open circuit configuration. The measurements were conducted in the range of the applied electric field strength (0.5–5) kV/mm and temperatures ranging from 22 °C to 70 °C. It is shown that the values of the conductivities obtained by the two methods are in agreement and their temperature dependences obey Arrhenius law yielding similar activation energies.

Optimized Design Method of Permanent Magnets Saturated Core Fault Current Limiters for HVDC Applications

When the fault current in high voltage direct current (HVDC) systems rises to a higher level, the DC circuit breakers will have either insufficient capacity or will require larger sizes with increased costs. Fault current limiters (FCLs) can reduce the turn-off stress and the stringent requirements on breakers, thereby also lowering their cost. The use of saturated core FCLs, which have fast response times and low power loss, is a promising approach for this issue. However, the parameter design principles for this kind of FCL has so far not been discussed in the literature. The design is often based on reactors that does not consider the transient performance. In this study, an optimized design method for permanent magnets saturated core FCLs in HVDC transmission systems, is proposed. The constraint conditions, parameter optimization and modification steps are discussed, and an optimized FCL design is obtained with the lowest cost and expected current-limiting function. Both simulations and experiments are conducted, and the results demonstrate the validation of the proposed method. Finally, a cost evaluation among main types of inductive FCLs is carried out and suggestions for scaling up saturated core FCLs to HVDC applications are given.

Energy-Based Control of a DC Modular Multilevel Converter for HVDC Grids

DC-DC converters are expected to interconnect High Voltage Direct Current (HVDC) grids with different configurations or voltage levels, which will increase reliability and operational flexibility. The dc Modular Multilevel Converter (DC-MMC) is suggested as a potential non-isolated topology for HVDC applications. This paper presents an energy-based closed-loop control for the DC-MMC. Such control structure provides a full control of the current components, while ensuring energy balancing in normal and fault operation. First, the converter configuration is described and the fundamental equations are presented. Then, the normal operation of the converter is explained from the steady state conditions and the energy and current components. A closed-loop control is designed to ensure a proper dynamic response, balance the internal energies of the converter, and minimize the ac circulating current. Also, the control structure is modified to ensure dc fault ride through capability. Simulation results in MATLAB/Simulink are used to validate the converter control under normal operation and DC faults.

HVDC Circuit Breaker Development and Applications in VSC-HVDC Transmission Project

High voltage direct current (HVDC), especially voltage source based HVDC (VSC-HVDC), transmission technology has been expanding in recent decades. HVDC circuit breaker therefore attracts significant interest due to its abilities of current interruption and fault isolation. This paper aims to present the technical development, specifically the topology, control and protection technology and test technology, of HVDC circuit breaker and its applications in VSC-HVDC transmission project. The project background, system configurations, and circuit breaker topology are described for each project. Based on current circuit breaker development and applications, the challenges and associated future development are also presented.

Modular Directed Series Multilevel Converter for HVDC Applications

Modular multilevel converter is the most prominent topology for high-voltage dc (HVDC) applications. However, it requires a large number and size of its submodule (SM) capacitors, proportionally large number of semiconductor switches and has high losses. To address some of these issues, the modular directed series multilevel converter (MDSMC) is proposed in this article. It requires significantly reduced size of SM capacitors and lower number of semiconductor switches and capacitors for the same power output. It is a hybrid topology having two main circuit components, namely, the chain-links consisting of a number of series-connected SMs and the director switches realized using a number of series-connected insulated-gate bipolar transistors (IGBTs). A detailed analysis and operating principles of MDSMC are presented to highlight its important features. The control and modulation schemes of MDSMC are presented that enable this converter to be utilized most effectively. The SMs capacitor size calculation is performed and compared with the major HVDC converters to validate its lowest capacitive energy storage requirement. The performance of MDSMC is evaluated using PSCAD/EMTDC for both standalone and HVDC systems under varying system conditions. Its ability to provide wide operating range and four-quadrant operation for HVDC application are also verified. Loss computation validates the ability of MDSMC to provide higher efficiency. A detailed comparison between MDSMC and other major HVDC converters is performed on the basis of their various structural parameters. Furthermore, a three-phase 13-level MDSMC experiment prototype is developed to validate its feasibility, operating principles, and control logic.

Modeling and Voltage Control of Bidirectional Resonant DC/DC Converter for Application in Marine Power Systems

DC/DC converters have gained popularity in a number of industrial applications like electric vehicles or marine power systems, due to their high efficiency and power density levels. Pulse width modulation (PWM) and resonant converters are two main types of the DC/DC converters. Thereby, the resonant converters happen to be the preferred technology in the design of modern marine power systems since these converters are more suitable for the high and middle voltage DC applications. The resonant converters are, however, highly nonlinear systems, which limits the use of linear control methods. In this study, we propose a comprehensive analysis, modeling and control concept of a DC/DC resonant converter in marine power systems. First, a mathematical model of the DC/DC resonant converter in the so-called CLLC topology is derived based on the generalized state-space averaging method. The model is used to design a dual-loop voltage control, which aims to regulate the voltage level at the low-voltage DC bus of the resonant converter. The dual-loop voltage control consists of the primal linear controller, which directly regulates the voltage and the reference generator, which dynamically modifies the voltage reference of the primal controller. The major advantage of the suggested control concept is the improved performance of the simple controller without the need to substitute it as well as the possibility to realize, if required, a multi-rate control concept. Simulation studies under different load conditions show that the suggested modeling and control concept improves voltage control and the closed-loop system response.

DC Fault Analysis Models of Three Converter Topologies Considering Control Effects

Existing converter models are normally reduced as simplified electrical components, whereas it overlooks the impacts of fast dynamics of converter current control loops. Therefore, the dc fault current calculation with the traditional simplified converter models becomes invalid when the simulation time is longer. To overcome this problem, this article incorporates the control effects of converters during the transient process and proposes three dc fault analysis models of classical three converter topologies for the high-voltage dc (HVdc) application including line communicated converter (LCC), two-level voltage-source converter (VSC), and modular multilevel converter (MMC). For an LCC-based rectifier, the nonlinear part of original model is first linearized with the least square method due to the large dc-link voltage deviation during dc fault, and the proportional integral (PI) current regulator with the reduced linearized model can be equivalent as an RC circuit for dc fault analysis. However, for two-level VSC or MMC, the dc-link voltage deviation does not drop that much with a large dc-link capacitor discharging process during transient dynamics. As a result, the original model is linearized relying on the small-signal method based on prefault operation points, and the dynamics of fast inner current loop with the linearized model can be represented as an RL circuit for dc fault current calculation. for dc fault analysis The proposed dc fault analysis models of three converters as equivalent RLC circuits are well testified under different fault resistances and dc reactors, control parameters, and operating conditions. Moreover, dc fault simulations of a hybrid HVdc system have verified the effectiveness of the proposed converter models.

Highly Efficient Fault-Tolerant Modular Embedded Thyristor Directed Converter for HVDC Applications

Modular multilevel converter containing half-bridge submodules (SMs) is the most preferred topology for high voltage dc (HVDC) applications. However, the inability to block/limit dc fault current and large number of capacitors requirement are some of its limitations. Recently, hybrid topologies utilizing thyristors for such applications have gained significant interest as they offer lower on-state voltage drop, higher reliability, and power handling capability. In this article, the modular embedded thyristor directed converter (METDC) is proposed, which uses series-connected thyristors as the high voltage switches and chain-links comprising of IGBT based half and full-bridge SMs. A control technique is proposed for force-commutation of thyristors by using full-bridge SMs for lagging loads, while they are naturally commutated for leading loads. The METDC offers wide operating range besides being fault tolerant. The results obtained from the METDC-HVDC model developed in PSCAD substantiate its ability to provide independent active and reactive power control required for HVDC application. The fault studies carried out validate the dc fault tolerant capability of METDC. The results obtained from the various ac fault/unbalanced case studies substantiate the efficacy of proposed controller to force-commutate the thyristors even during contingencies. Loss computation validates the ability of converter to provide higher efficiency.

A Physics-Based Transient Electrothermal Model of High-Voltage Press-Pack IGBTs Under HVdc Interruption

The wide use of press-pack insulated-gate bipolar transistors (IGBTs) in high-voltage dc (HVdc) applications makes the accurate modeling of high-voltage press-pack IGBTs more urgent. Based on the mechanism of the switching transient, a physics-based electrothermal transient model of the high-voltage press-pack IGBT is proposed. Considering the wider base width of high-voltage IGBTs (HVIGBTs), a transient model of the HVIGBT with buffer layer is presented taking into account the carrier recombination in the base region and the injection level in the buffer layer. Besides, a modified thermal network of press-pack IGBTs is implemented considering the double-sided heat transfer structure. An electrothermal coupling model of high-voltage press-pack IGBTs is then obtained combining the proposed electro and thermal models. Finally, simulations of key dynamic performances of the proposed HVIGBT model match well with the testing results. Furthermore, the HVdc interruption process of a solid breaker with different number of series-connected IGBTs is simulated using the proposed IGBT model and tested. The testing results show good consistency between the simulated and measured voltage and current of the series-connected press-pack IGBTs, which brings strong support to the accurate design of solid breakers in the HVdc transmissions.

Balancing Algorithm for a Self-Powered High-Voltage Switch Using Series-Connected IGBTs for HVDC Applications

This paper presents a balancing algorithm for the so-called Director Switch, a controlled, reverse conducting, self-powered, high-voltage switch, using series-connected insulated gate bipolar transistors (IGBTs). This is a key component on two new voltage-source, high-voltage direct current (HVDC) converter topologies. These, which promise efficiency, cost, and size reductions, are an alternate arm converter and a series bridge converter. One of the main design challenges of the director switch is how to supply the IGBT gate drivers, and other auxiliary electronics while overcoming the high-voltage insulation barrier to ground level. In this paper, this is achieved by using the energy stored in a capacitive clamp snubber circuit, which also helps with the overvoltage mitigation during rare hard-switching events. However, the clamp capacitor and by proxy the IGBT voltages become highly unbalanced during operation, compromising the integrity of the switch. This paper presents a balancing strategy that consists in applying a controlled delay in the switching of the IGBT levels, in a way that forces a voltage balance on all the switch levels. The paper explains the unbalance. Then, the balancing strategy is explained and validated on simulation and on a high-voltage experimental rig, on switches comprising two and three series-connected IGBTs, respectively.