High Voltage Direct Current Applications Research Articles

Reliability Analysis of Modular Multilevel Converters in MVDC Applications

In modular multilevel converters (MMCs) designed for medium-voltage DC (MVDC) systems, since the required sub-modules (SMs) in one arm are fewer, the converters cannot afford the same level of redundancy as observed in high-voltage DC (HVDC) applications. In order to ensure high availability of converters in MVDC systems, reliability becomes a challenge and requires different design approaches. Considering the redundancy design, operating modes, and correlation across failures of SMs in one arm, this paper analyses and compares reliability in MMCs for MVDC systems with multiple SM topologies. Under different scenarios in MVDC systems, SM topologies with better performance of reliability have been identified in this research. It is demonstrated that, for a given output voltage in converters, three-level SMs generally provide higher reliability than two-level SMs. By implementing passive and load-sharing operation schemes in converters, the MMC reliability can be improved compared with the conventional active mode. In terms of the correlation between lifetime of SMs in the same arm, the smaller disturbance introduced by one faulty SM promotes the improvement of converter reliability. Also, the correlation between failures of SMs in one arm has greater effects on the reliability of an MMC with higher level of redundancy. The outcomes of this work contribute to the optimisation of converter design and operation with high reliability and low investment for an MMC in MVDC systems.

A Novel 6500 V SiC Trench MOSFET with Integrated Unipolar Diode for Improved Third Quadrant and Switching Characteristics.

A 6500 V SiC trench MOSFET with integrated unipolar diode (UD-MOS) is proposed to improve reverse conduction characteristics, suppress bipolar degradation, and reduce switching loss. An N type base region under the trench dummy gate provides a low barrier path to suppress hole injection during the reverse conduction operation. The reverse conduction voltage VON is reduced to 1.11 V, and the reverse recovery charge (QRR) is reduced to 1.22 μC/cm2. The gate-to-drain capacitance (CGD) and gate-to-source capacitance (CGS) of the UD-MOS are also reduced to improve switching loss due to the thick oxide layer between the trench gate and dummy gate. The proposed device exhibits an excellent loss-related figure of merit (FOM). It provides a high-voltage SiC MOSFET prototype with potential performance advantages for voltage source converter-based high voltage direct current applications.

Ultra-Low Reactive Power Operation and Control Strategy of Hybrid Line Commutated Converter Based on IGCT for HVdc Application

Existing line commutated converter (LCC) based high voltage DC (HVDC) systems require large reactive power during operation and reactive power compensation equipment under construction. However, the equipment is expensive and occupies a huge area. In this letter, a novel operation of hybrid line commutated converter based on integrated gate commutated thyristor (IGCT) is proposed to achieve ultra-low reactive power (LR-LCC), and the corresponding control strategy is developed by utilizing IGCT's active blocking and bidirectional voltage withstanding capability. It can almost eliminate the system's extinguishing angle (γ), and the power factor is close to 1. This novel LR-LCC can result in advantages including a much lower manufacturing cost and volume of reactive power compensation equipment for HVDC application.

Uncertainty assessment of a resistive voltage divider for binary step-up method implementation in HVDC applications

The most used methods for determining the ratio of High Voltage Direct Current (HVDC) standard resistive dividers are described in this paper. The binary step-up method was chosen to metrologically characterize a HVDC resistive divider, in order to achieve local traceability to the SI from Josephson Voltage Standards. A Monte Carlo-based simulation was performed to estimate the uncertainty levels of each step from 1.25 kV up to 10 kV, considering that the main components are the uncertainty of the digital multimeter and the voltage source. It was found a variation up to 64-fold in the standard deviation of the voltage output, for a uniform distribution, when the voltage source uncertainty was increased up to 10-fold.

Significantly enhanced flashover voltage of epoxy in vacuum by graded surface roughness modification

AbstractThe flashover strength of epoxy (EP) insulations in the High voltage direct current applications of future energy grids can be improved by tailoring their surface condition. This work aims to improve the DC surface flashover characteristics of EP after being treated with sandpaper of different gradings. Samples with virgin EP and homogenously modified EP considering varying surface roughness (Ra = 0.54, 3.16, 5.24, and 8.35 μm) are prepared. Different experimental characterisations, such as water contact angle, surface intrinsic conductivity, surface voltages, flashover strength, and trap distributions are conducted and evaluated to analyse the difference between virgin and treated EP. Moreover, based on the obtained experimental results of homogenously treated EP and theoretical analysis, the concept of surface functionally graded materials (SFGMs) is put forward. The flashover voltages of homogenously treated EP are augmented significantly compared to virgin EP regardless of the voltage polarity and enhanced by enhancing the surface roughness. The sample TModel‐C with SFGM design shows a 45.02% and 43.75% improvement in the negative and positive flashover voltages than that of the virgin EP. In the end, COMSOL simulations are conducted to justify the experimental findings and to analyse the difference between virgin and modified samples in terms of electric field distribution.

A Switched Mid-Point Modular Multilevel Converter for HVDC Applications

The modular multilevel converter (MMC) has emerged as the industry standard for voltage source converters (VSC) based high voltage direct-current (HVDC) applications since its benefits of scalability, modularity, high efficiency and better output quality outweigh its drawback of large energy storage. However, with a growing number of offshore HVDC installations, the volume and footprint of the converter, which are largely influenced by its energy storage, are becoming increasingly critical. This has led to a rising interest in alternative converter topologies that reduce the energy storage while maintaining the benefits of the MMC. This paper introduces a switched mid-point modular multilevel converter (SMPC), that is able to generate the desired AC-side waveforms with significantly less energy storage (around <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$50\%$</tex-math></inline-formula> ) than the MMC. The DC-side is free from any distortion that requires additional filtering. Reduced energy storage and the associated reduction in volume is achieved at the expense of requiring more switches and slightly lower efficiency when compared to the MMC. Like many other MMC-style circuits, except when operating at a ‘sweet-spot’, active control of the energy balance is needed, for which a technique based on second harmonic injection is proposed. The converter operation and the proposed control scheme are verified by simulations for a medium-voltage setup and experimentally on a low-voltage lab-scale prototype.

A Low-cost Optocoupler-Based Isolator System for IoT-Based Voltage Measurement in Low and High voltage DC Applications

Internet of things (IoT) connected devices operate at extremely low voltages that are susceptible to common-mode noise and electromagnetic interference. As a result of this, integrating IoT devices with low or high-voltage direct current power sources requires galvanic isolation which is often expensive to attain. In this work, the use of a low-cost conventional optocoupler (4N35) in the galvanic isolation of an IoT voltmeter required to measure the potential difference of a low voltage direct current source with a maximum relative error of 1% was investigated and experimentally verified. The proposed isolator circuit was first simulated using NI Multism and then fabricated on a printed circuit board for experimental verification after satisfactory simulation results. Measurement results from the experimental verification process were used to fit quadratic and cubic regression equations that approximate the input signal voltage from the isolator’s output voltage measured by the IoT voltmeter. Lastly, the isolator and IoT voltmeter were connected to a variable 100-1000 VDC source via a potential divider network for performance verification at a voltage step of 100 VDC. Here, the isolator successfully achieved its primary goal of providing galvanic isolation between the voltage source and the IoT voltmeter while maintaining a maximum relative error of 1%.

Simplified Approach for Acquisition of Submodule Capacitor Voltages of the Modular Multilevel Converter Using Low Sampling Rate Sensing and Estimation

The modular multilevel converter (MMC), used in high-voltage dc applications, consists of hundreds of submodules in each arm. The operation and control of the MMC require sensing of the capacitor voltage of each submodule. The transmission of the sensed data from hundreds of submodules over individual channels leads to wiring complexity. This work suggests that time-multiplexing can be used to transmit the sensed data of multiple submodules across a common serial interface bus. Time multiplexing, however, decreases the sampling rate. Since the submodule capacitor voltage has both ac and dc components, an adequate sampling rate must be maintained to avoid the aliasing of ac components. In this article, an estimation scheme is investigated for reconstructing the ac components of the submodule capacitor voltages within the processing platform, leaving just the dc component to be transmitted over the common bus. The mathematical analysis of the data transmission system and proposed estimation scheme is performed, and the influence on the closed-loop performance is studied. The proposed scheme is experimentally verified in the laboratory.

Modulating mechanism on the temperature dependent resistivity of ceramic dielectrics and its advantage over polymer insulation

Volume resistivity of polymer dielectrics reduces significantly with the increase of temperature. As a result, in direct current (DC) power apparatus or electronic device, temperature gradient over the material can result in considerable electric field distortion which greatly threats the polymer insulation. Advanced ceramic dielectrics are promising to be alternative to polymer insulation to deal with this issue. In this paper, Al2O3 and Si3N4 ceramic materials with different ion doping concentrations have been manufactured. Temperature dependent resistivity characteristics of the ceramics are obtained and compared with typical polymer dielectrics. Conduction models of the ceramic dielectrics are applied to understand their temperature dependent resistivity and its modulating mechanism. Effect of ion doping on the temperature dependent resistivity has been obtained and analyzed which agrees well with their charge trapping property acquired by thermal stimulation depolarization current measurement, based on which the method to modulate the temperature stability of material resistivity have been proposed. Finally, a comprehensive comparison between ceramics and polymers in high voltage DC applications have been made which further proves the advantage of ceramic dielectrics.

Impedance Modeling and Analysis of Series-Connected Modular Multilevel Converter (MMC) and its Comparative Study With Conventional MMC for HVDC Applications

Similar to the conventional three-phase parallel-connected MMC (PC-MMC), the three-phase series-connected MMC (SC-MMC) is a strong potential solution for high-voltage direct current (HVDC) applications. The study of SC-MMC focuses on the control strategy, neglecting its small-signal dynamics, not to mention its stability when interacting with the AC grid. Work on small-signal modeling of MMC is limited to parallel-connected configurations. SC-MMC differs from PC-MMC in structure and harmonic content; hence, it is necessary to establish an accurate model of SC-MMC according to its own characteristics. This paper introduces the circuit and control of SC-MMC and compares it with PC-MMC. Multi-harmonic linearization is used to establish the AC impedance model of SC-MMC over the whole control system, including the third harmonic suppression controller. We take a PC-MMC station of the Wudongde HVDC project as a benchmark to explore the potential application of SC-MMC when constructing new HVDC stations to extract power from an existing DC transmission corridor. The strengths and weaknesses of SC-MMC in HVDC applications are discussed, and it is compared to PC-MMC with regard to the impedance characteristic.

3‐D thermal analysis for the valve‐side bushings of HVDC converter transformer and application in the evaluation of current‐carrying connections defects

To analyze the overheating problem of the valve-side bushings of high voltage direct current (HVDC) converter transformer, which is mainly caused by current-carrying connections defects at copper–aluminium transition region in the bushing, this paper set up a 3-D electromagnetic-thermal analysis model for the valve-side bushings, and a method was proposed to calculate the thermal distribution of the valve-side bushings under the influence of current-carrying connections defects. Based on this model, the details of an example were shown in this paper, and the numerical results agreed with the measured results within 1%. Then, the temperature field distribution characteristics of valve-side bushing under different working current I, defect degree k, and ambient temperature Tamb were determined. According to the characteristics, the temperature difference ΔT and working current I of the bushing were applied to evaluate contact resistance at the copper–aluminium transition region of the bushing, which can be provided by a novel method to evaluate for current-carrying connections defects in the valve-side bushings.

Integration of HTS Coils in a Lab-Scale Modular Multilevel Converter

High-temperature superconducting coils are achieving technological maturity to be integrated into medium and high-voltage direct current applications. We propose to replace the conventional air-core dry-type arm coils of a modular multilevel converter with high-temperature superconducting coils. The goal is to reduce the the size and weight of arm coils, reduce losses under normal operation and limit the current under fault conditions. To demonstrate the feasibility of this proposition, we integrate air-core superconducting pancake coils in the arms of a single-phase modular multilevel converter prototype. The pancake coils were wound with BSCCO tapes. In this article, we detail the construction of the cryo-MMC and report the first experimental results.

A Dynamic Control Methodology for DC Fault Ride Through of Modular Multilevel Converter based High Voltage Direct Current Systems

High Voltage Direct Current (HVDC) transmission has provided a variety of possibilities for renewable energy resources and regional substations to boost power supply reliability and operational flexibility. To accommodate this development, the Modular Multilevel Converter (MMC) has been pervasively selected as a potential converter solution for HVDC applications due to its modularity and scalability. However, several technical challenges can hamper their practical applications and deployment, especially in modelling, controlling, and protecting the MMC-HVDC grids, since conventional control schemes struggle to achieve DC fault protection, converter energy balance, and DC Fault Ride Through (DC-FRT) capability. As a result, a novel control structure is proposed to ensure a proper dynamic response, balance arm and leg internal energies, minimise oscillations in DC, and offer DC-FRT capability alongside Static Synchronous Compensation (STATCOM). The result analysis under normal and pole-to-pole DC fault is done in MATLAB/Simulink to validate the proposed control scheme.

Review, Classification and Loss Comparison of Modular Multilevel Converter Submodules for HVDC Applications

The circuit topology of a submodule (SM) in an modular multilevel converter (MMC) defines many of the functionalities of the complete power electronics conversion system and the specific applications that a specific MMC configuration can support. Most prominent among all applications for the MMC is its use in high-voltage direct current (HVDC) transmission systems and multiterminal dc grids. The aim of the paper is to provide a comprehensive review and classification of the many different SM circuit topologies that have been proposed for the MMC up to date. Using an 800-MVA, point-to-point MMC-based HVDC transmission system as a benchmark, the presented analysis identifies the limitations and drawbacks of certain SM configurations that limit their broader adoption as MMC SMs. A hybrid model of an MMC arm and appropriate implementations of voltage-balancing algorithms are used for detailed loss comparison of all SMs and to quantify differences among multiple SMs. The review also provides a comprehensive benchmark among all SM configurations, broad recommendations for the benefits and limitations of different SM topologies which can be further expanded based on the requirements of a specific application, and identifies future opportunities.

Design, Control and Testing of a Modular Multilevel Converter with a Single Cell per Arm in Grid-Forming and Grid-Following Operations for Scaled-Down Experimental Platforms

Modular multilevel converters (MMC) can be used in several applications, especially (but not only) in high-voltage direct current (HVDC) and STATCOM. In order to develop experimental scaled-down test benches for lab validation, several projects have developed MMCs with a limited number of cells, but they need to use pulse width modulation (PWM) techniques to achieve acceptable power quality (because nearest level modulation (NLM), common in HVDC applications with hundreds of levels, cannot achieve sufficient power quality unless the number of cells is high enough). The present paper proposes a new concept which is based on designing arms with a single cell. This allows to have the simplest possible converter that maintains the structure of an MMC. While all the inner controllers of large-scale HVDC MMCs are included, the only remarkable difference is that PWM is used and NLM cannot be implemented. As this is also a limitation for other low voltage MMC, the proposed concept is suggested for scaled-down low voltage applications. The paper includes the design and construction of the converter, the definition and implementation of the converter controllers, and the converter testing, with detailed dynamic simulations and an experimental setup.

A Push–Pull Series Connected Modular Multilevel Converter for HVdc Applications

This article introduces a push–pull series connected (PPSC) ac–dc converter intended for high-voltage direct current applications. The proposed topology requires fewer submodules compared to a standard modular multilevel converter (MMC) to withstand the same dc voltage. Each converter phase unit is connected to its corresponding phase on the ac side via a center-tapped single-phase transformer which also provides the galvanic isolation needed in most applications. The required arm inductance can be merged into the transformer leakage inductance avoiding the need for additional inductors. In this article, both the design and operation of the converter are discussed in detail. Furthermore, a comparison of the PPSC and MMC in terms of energy storage requirements and efficiency is presented. The converter concept and the control strategy proposed is validated by computer simulations using the piecewise linear electrical circuit simulation (PLECS) simulation package as well as through experiments on a small-scale, 300-V, 2-kW laboratory prototype.

Overview and Assessment of HVDC Current Applications and Future Trends

High voltage direct current (HVDC) technology has begun to gather a high degree of interest in the last few decades, showing a fast evolution of achievable voltage levels, transfer capacities, and transmission lengths. All these changes occurred in a context in which power system applications are highly dependent on HVDC technologies such as energy generation from renewable sources (e.g., energy generated in offshore wind power plants), power exchanges between asynchronous networks, submarine cables, and long-length transmission overhead lines have become more common worldwide. This paper tries to summarize the current state of HVDC technologies, both voltage-source converters and current-source converters, the main components of converter substations, control strategies, key challenges arising from their use, as well as the future prospects and trends of HVDC applications. This paper represents the first step in setting the background information for analyzing the impact of a VSC-HVDC connection on the stability of the Romanian transmission network during steady-state and dynamic operation.

A Low-Loss Integrated Circuit Breaker for HVDC Applications

Hybrid dc circuit breakers (HCBs) are recognized as suitable devices for protecting high-voltage direct-current (HVDC) systems, along with other dc circuit breakers (DCCB). However, compared to mechanical circuit breakers, HCBs exhibit higher conduction losses. Such losses are inevitable under no-fault conditions as current may flow through some of the semiconductor switches. An integrated circuit breaker (ICB) minimizing these losses is presented in this paper, and this is achieved by replacing semiconductor switches by mechanical components in the current path. For completeness, the topology design, operating sequence and a mathematical analysis for component sizing of the device are provided. In addition, an estimation of the conduction losses is quantified. It is estimated that the power losses of an ICB are 2 to 30&#x0025; of an HCB only. The ICB has been implemented in PSCAD&#x002F;EMTDC to demonstrate its effectiveness for isolating dc faults, with simulations conducted on a three-terminal HVDC grid.

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%.