Flexible DC Research Articles

A Flexible DC–DC Converter with Multi-Directional Power Flow Capabilities for Power Management and Delivery Module in a Hybrid Electric Aircraft

The development of fully electric and hybrid electric aircraft using fuel cells and batteries has lately received much attention since the technology can contribute to achieving an emission-free world. For hybridisation of fuel cells and batteries, the challenge of integrating different voltage sources to supply power to different loads at different voltage levels has to be met. Presently, this is achieved by using a combination of complex passive circuitry and DC–DC converters. In this paper, a new galvanically isolated DC–DC converter topology is proposed; it integrates a battery and fuel cells as input sources and supplies power to a high voltage (HV) and a low voltage (LV) load simultaneously. The converter enables power flow from the sources to the load, as well as between the sources and power flow back towards the battery during recuperation for recharging the battery during flight. The functionality of the converter design is verified using a real-time (RT) hardware-in-the-loop (HiL) test. The proposed concept is easily scalable and can be adopted in any fuel cell and battery drive train of all-electric flights of different sizes.

Flexible DC distribution line protection principle based on distance metric of clustering centres

Aiming at the issue of unsatisfactory selectivity of single-terminal protection, and poor speed and high requirements for data synchronization of double-terminal protection in flexible DC distribution networks, an artificial intelligence (AI)-based method is proposed to obtain the best clustering centres through unsupervised clustering analysis of historical data to realize the non-deterministic flexible DC distribution line protection principle. The method forms historical data samples by simulating different types of short-circuit faults at different locations of the line in advance, combining them with the actual faults, and collecting and processing the post-fault currents. The K-means clustering algorithm is then used to find the best clustering centres corresponding to different fault types, and fault identification and pole selection are realized by comparing the distance between real-time data and each clustering centre. The process relies merely on single-terminal current as the characteristic quantity, and it does not need complicated feature extraction and calculation. Thus, the cumbersome threshold setting in conventional current protections can be avoided. Finally, the case studies are carried out in PSCAD/EMTDC, and the results show that the proposed protection has good selectivity and rapidity, and the tolerance to fault resistance is improved compared with the conventional local-current-based protections.

A Review of the Development of Key Technologies for Offshore Wind Power in China

In recent years, Offshore Wind Power (OWP) has gained prominence in China’s national energy strategy. However, the levelized cost of electricity (LCoE) of wind power must be further reduced to match the average wholesale price. The cost-cutting and revenue-generating potential of offshore wind generation depends on technological innovation. The most recent studies and applications of offshore wind technology are thoroughly examined. (1) Techniques for site selection, such as site surveys, wind resource assessments, and environmental factors, are reviewed. (2) Three main technical components in offshore wind farms are discussed, including wind turbine, foundations, and booster stations. (3) The state-of-the-art method of the offshore wind farm’s construction and operation and maintenance (O&M) practices is discussed. In situ marine geological surveying, large-scale offshore wind turbine manufacturing, integrated structural design, floating foundation design, flexible DC transmission technology, shortage of specialized vessels and equipment for construction, intelligence of O&M, and other issues are challenging China’s OWP industry. A brief overview of China’s efforts in standardization, parity, and research and development are discussed. Recommendations for future development of the wind power industry are provided for China, which may be referable for other nations with comparable circumstances.

Analysis for magnetic field disturbance of modular multilevel converter based high voltage direct current (MMC‐HVDC) converter valve

The standardised design and operational reliability enhancement of the flexible DC converter valve are impending with the construction of high shares of renewables and power electronics power system. A power frequency magnetic field calculation model able to obtain the electromagnetic disturbance at the submodule (SM) drive board during normal operation of a flexible DC converter valve is presented. Hundreds of SMs with complex structures in a converter valve tower are simplified by the equivalent to the line current to fulfil calculation accuracy requirements. In addition, a random function is employed to simulate the non-sequential switching process of the SMs. By comparing the calculated results with the measured data obtained from the SM equivalent operation experiment of 2200 V/2150 A, the validity of the calculation model is confirmed. According to the final calculations, the influence of non-sequential switching cannot be ignored when recognising the time-domain distribution features of the power frequency magnetic field of the drive board. However, engineering design can be guided only by the power frequency magnetic field intensity of the drive board calculated based on SM’s full removal in the valve tower.

An Identification Algorithm of Relay Protection Parameter for Flexible High-Voltage Direct Current Transmission Lines Under Synchronous Sampling

Relay protection device is a protection device when the flexible DC transmission line is long and leads to line failure. Therefore, in order to ensure the reliable identification of relay protection parameters, an identification algorithm of relay protection parameter for flexible DC transmission line under synchronous sampling is proposed. By calculating the relay protection parameters based on synchronous sampling, the algorithm obtains the positive sequence parameters and zero sequence parameters of flexible DC transmission line’s relay protection, and uses the matrix beam algorithm to identify the positive sequence parameters and zero sequence parameters, so as to obtain the subsynchronous oscillation modal parameters. In order to improve the identification effect, the extended particle swarm optimization algorithm is used to optimize the identification results. The test results show that the algorithm has the feasibility of relay protection parameter identification and the calculation function of positive sequence parameters and zero sequence parameters of relay protection device. It can reliably identify the current change and the change results of positive sequence parameters and zero sequence parameters with minimal identification error.

Обзор методов модуляции ближайшего уровня на основе MMC

In view of the relevant situation of offshore wind power research, development and construction at home and abroad, as well as the energy crisis facing the world, we have ushered in a new opportunity and outbreak period for the development of new energy. Therefore, higher requirements are placed on the power supply, and the key technology for the design of offshore wind power flexible DC transmission high-power converters is proposed. The Modular Multi-level Converter (MMC) topology can only use a lower switching frequency to obtain higher power Waveform quality. The output voltage waveform can effectively reduce the switching loss and filter capacity and improve the efficiency and economy of the converter. Among them, the method based on Level Modulation (NLM) is usually the preferred method. This paper introduces the topology and working principle of the modular multilevel converter (MMC) in detail and describes the improved modulation method for NLM in detail. The advantages, disadvantages and application occasions of different improved modulation techniques are summarized, which provides a reference for the engineering application of MMC.

Active Converter Injection-Based Protection for a Photovoltaic DC Distribution System

Due to the complex structure of flexible dc distribution systems with distributed photovoltaic power, single-ended measurement based protection cannot work well. The dc line-to-line faults are also difficult to locate due to a short time span with fault characteristics that is affected by various converter structures and control algorithms. To resolve these issues, a new fault detection and location method based on active converter injection is presented. The proposed method modifies the converter to become a frequency-controllable injection source by actively utilizing the coordinated control between local protection and the converters, which can build a clear protection boundary. Moreover, this method can locate the faulted area accurately by selecting the appropriate injected harmonic frequency, which is affected by the outlet capacitance of the dc/dc converter and the cable-distributed capacitance. Compared with current dc protection techniques, this method does not require additional injection equipment and high data sampling frequency. Hardware tests and simulation results are carried out, illustrating that the proposed mechanism is effective to achieve both control and protection through the controllability of power electronics.

Modified Voltage Control Strategy for DC Network with Distributed Energy Storage using Fuzzy Logic Controller

This research provides an extraordinary distributed energy storage (DES) technique for DC distribution networks. Under exhibited virtual inertia and capacitances, the parameters of the AC and DC networks are briefly assessed. The proposed control approach for DES is based on many interactive features, which allows the DES to respond to both DC network voltage variations and make use of AC grid frequency changes. To reduce the burden of DC network voltage degradation, a cascade droop management approach with fuzzy is proposed for DES in DC microgrid. The simulation outcomes give a response that is fruitful for improving the voltage stability in DC distributed network as compared to other existing techniques. INDEX TERMS: Distributed energy storage, flexible voltage controlstrategy, AC and DC networks, fuzzy,voltage stability.

A systematic review of robust control strategies in DC microgrids

This paper provides a comprehensive and systematic review of robust control strategies in DC microgrids. Microgrids gain worldwide attention in decades regarding its effective and feasible integration of distributed generations (DGs). Getting rid of frequency synchronization and reactive power problem, the DC microgrid is superior to its ac counterparts in stability, flexibility and complexity. The recent efforts were mainly directed toward developing the feasibility of implementing DC microgrids on kinds of specific application scenarios. DC microgrids design-related aspects such as the system architecture and topology, voltage levels, operation and control framework, stability analysis method, and protection challenge have been studied in a deep degree. In this paper, operation and control framework in both grid-connected (grid-forming) mode and islanding (grid-following) mode will be focused, to provide the guideline where we currently stand on the migration path from the overwhelming fully AC microgrids to a more flexible DC microgrids. Besides, the impediments against stability and effectiveness of DC microgrids and representatively proposed solutions will be discussed.

Multi-Frequency bands based Pole-to-Ground fault detection method for MMC-Based radial DC distribution systems

In a small-current grounding system, the pole-to-ground fault may cause the voltage drop in the fault pole and the voltage rise in the other poles. In flexible DC distribution systems, Severe voltage variation may shorten the insulation lifetime of the equipment, which leads to great concerns on the safety issue. In addition, the existence of high transition resistance degrades the accuracy of fault detection methods, thus further affecting the reliability of the system. Therefore, it is essential to explore an advanced technology for faulty feeder detection. This study proposes a faulty feeder detection method based on the characteristics of transient zero-mode current (TZMC) in multi-frequency bands. The change rate of zero-mode voltage is applied as the protection activation criterion. Then, the characteristic matrix is constructed via computing the fuzzy entropy of TZMC in each frequency band. Finally, the faulty feeder can be identified by conducting fuzzy C-means on the characteristic matrix. This proposed method is tested through simulations on the PSCAD/EMTDC platform, which successfully demonstrates its outstanding adaptability, reliability, and accuracy.

A new pilot protection scheme for modular multi‐level converter‐based HVDC system based on voltage matching factor

Concerning the incorrect operation of line protection in modular multi-level converter-based multi-terminal DC system, a pilot protection method based on voltage matching factor is proposed. First, the dynamic switching process of flexible DC converter sub-module is analyzed, and the expression of DC voltage of converter station considering the influence of alternating current (AC) system is derived. On this basis, the model of DC line voltage considering the influence of AC injection current is built, and the equivalent DC line voltage is calculated according to the voltage model. Then, by constructing the distortion factor which reflects the difference between the equivalent DC line voltage and actual DC line voltage, the protection criterion for the identification of internal and external faults is established. Finally, simulation tests on real time laboratory (RT-LAB) platform verify that the proposed scheme can correctly distinguish between internal and external faults in different fault cases, with fast operation speed and strong immunity to the fault resistance.

Development and application of AC section power loss protection device

—The so-called AC section power loss refers to the loss of all AC power supplies of the system on this side. After the AC section at the receiving end of the converter station loses power, it will lead to serious AC overvoltage, and the energy of the arrester will increase sharply at the same time, which will seriously affect the safety of the equipment. In view of this situation, this paper studies the action mechanism of AC section power loss protection, and gives the design scheme of AC section power loss protection device. The developed protection device has been verified by RTDS simulation test, passed the third-party detection test of a detection and Research Institute, and is running on site. The field operation is good. The device is not only suitable for conventional DC system and flexible DC system, but also for some occasions where regional stability needs to be considered.

An active damping control method for direct-drive wind farm with flexible DC transmission system based on the remodeling of dynamic energy branches

In view of the sub-synchronous oscillation in direct-drive wind farm caused by its connection to flexible DC transmission system, an active damping control method based on the remodeling of dynamic energy branches is proposed. First, a dynamic energy model of direct-drive wind farm with flexible DC transmission system is constructed, which contains the interaction energy, dissipation energy and stored energy. The energy model depicts the energy interaction between the control links and the energy transmission process. By analyzing the role of each control link in the transmission system (whether it emits, absorbs or transmits the dynamic energy), the control link that induces the oscillation is found. Thus the source of the energy flows in the whole system is revealed, and the application locations of the compensation branches are determined. On this basis, a parameter optimization strategy for the compensation branches considering both the sub/super synchronous oscillation scenes and the fundamental-frequency characteristics is established, so that the energy distribution can be optimized under sub/super synchronous oscillations. Finally, hardware-in-loop tests are conducted on the RT-LAB platform using the parameters of a real direct-drive wind farm. Simulation results verify that, the proposed strategy can optimize the overall energy distribution in the system through coordinated optimization control. It can quickly suppress the oscillations in both sub and super synchronous frequency bands, without affecting the LVRT capability of the wind generators and the fundamental-frequency characteristics of the system.

A PLL-free control strategy for flexible DC transmission systems

The grid-connected inverter needs to know the phase information of the grid voltage. The traditional method is to use the phase-locked loop (PLL) to capture the phase of the grid. However, the PLL takes some time to capture the phase and affects the stability of the grid-connected inverter. Based on this, this paper proposes a vector control strategy without PLL. Firstly, the virtual vector axis is constructed, and the grid voltage is transformed as the reference phase angle. Then the instantaneous power formula in this case is derived. The corresponding control block diagram is constructed. The method reduces the coordinate transformation operation and simplifies the control structure while ensuring the stability of the control system. The simulation results under MATLAB/Simulink show that the control effect of this method is better than that of the PLL vector control strategy, and the method has smaller computational complexity, less resources and more cost savings.

An improved second order generalized integrators phase locked loop for flexible DC transmission

Accurate and fast phase locking is the primary condition for flexible DC transmission to realize reliable control. When the power grid is unbalanced or contains high-order harmonics, the traditional second order generalized integrator (SOGI) itself has the defect of strong dependence on the input signal frequency. Therefore, this paper proposes an improved phase-locked loops (PLL), which introduces moving average filter into the SOGI structure to filter out higher harmonics. The improved SOGI-PLL is simulated and analyzed under grid voltage unbalance and high harmonics. The results show that the improved SOGI-PLL has well grid adaptability.

A DC Fault Location Method of Multiterminal Flexible DC Distribution Network

Multiterminal flexible DC distribution network is a complex system made up of many power-electronic devices. Its diversified and comprehensive fault characteristics are featured by multiperiod and strong transient. This paper starts with investigating the mechanisms of typical DC short circuit faults of the MMC-based system. Then, a typical flexible DC distribution network is built in PSCAD to analyse the characteristics of its pole-to-pole short-circuit fault and pole-to-ground fault. With the aid of the above findings, a fault detection and location method is proposed with its effectiveness evaluated through simulations.

Stability Control of Flexible High Voltage Direct Current (HVDC) Transmission Based on DC Circuit Breaker Protection

In order to control the stable operation of flexible DC transmission system and ensure the uninterrupted power supply of flexible DC transmission system, the stability control method of flexible DC transmission system based on DC circuit breaker protection is studied. Taking support vector machine as the basic classifier, the two parameters of support vector machine classifier and support vector machine classifier are optimized by bird swarm algorithm, The sample weight is adjusted in real time according to the classification error of support vector machine classifier; The corresponding kernel function is selected and the bird swarm algorithm is used to find the optimal parameters; Through the fault location method of flexible DC transmission system based on wemtr, the fault location of flexible DC transmission line is realized; According to the fault location of flexible DC transmission system, the optimal coordination strategy of DC circuit breaker based on protection line is adopted to realize the stability control of flexible DC transmission system. The simulation results show that this method can realize the stability control of flexible DC transmission in a short time. Under the control of this method, the fluctuation amplitude of voltage and current of flexible DC transmission system becomes smaller, which has application value.

Study on the oscillation transmission paths in direct‐drive wind farm transmitted via flexible dc system based on dynamic energy flow

Concerning the sub-synchronous oscillation induced by the interaction among wind power converter, flexible DC converter and multiple control links, a method to study the oscillation transmission path based on dynamic energy flow is proposed. First, the system is divided into several subsystems according to different control links, and the dynamic energy model of each subsystem is built by using the first integration method. Then, the energy flows induced by different control links are determined according to the corresponding oscillation components. On this basis, the intersection of energy flows in each dynamic energy model is defined as an energy converging point. Thus, the energy flows between different energy converging points are obtained, and the oscillation transmission path of the interconnected system between different subsystems is determined. Finally, combining the parameters of a real direct-drive wind farm transmitted via flexible DC system, a test system is built in RT-LAB for simulation verification. The results show that the proposed method can describe the oscillation transmission paths between different control links quantitatively, and reveal the control links that make against the stability of system. Besides, studying the energy distribution in the oscillation transmission paths can contribute to further research on the control parameter setting and energy suppression, and lay the theoretical foundation for the design of supplementary compensation branches.

A New Design of MP-HDCCB Topology Based on Hybrid Switching Device

Since each branch of the multiterminal DC circuit system relies on the DC circuit breaker for breaking and fault isolation, the prohibitive cost and huge volume of the Hybrid DC Circuit Breaker (HCB) limit its development and broad application in multiterminal flexible DC systems. Multiport hybrid DC circuit breaker (MP-HDCCB) based on device and branch sharing reduces the configuration cost of the circuit breaker to a certain extent. In order to further reduce the cost of MP-HDCCB, a novel MP-HDCCB topology based on hybrid switching devices is proposed, adopting full controlled switching devices to achieve rapidity of breaking fault current, and using semi-controlled switching devices in series to withstand the transient interruption voltage (TIV), so as to reduce the construction cost and technical difficulty. In this paper, the working principle and fault breaking strategy of the topology are introduced in detail, then the parameters of the major circuit are analyzed theoretically, and the parameter design of each branch is given. In the end, the rationality and validity of the proposed topology is tested and verified by simulations and experimental tests.

A coupling voltage based adaptive reclosing scheme for flexible DC grid

AbstractThe total electric field around the DC line is composed of both the ion flow field produced by coronae and the electrostatic field produced by the conductor charge. Under the effect of the total electric field, the disconnected pole will generate a coupling voltage. Therefore, based on the coupling voltage, this paper proposes an adaptive reclosing scheme for flexible DC grid to prevent power supply interruption caused by temporary fault. First, the disconnected pole coupling voltage (DPCV) under the total electric field is analysed when single pole to ground fault (SPGF) and pole to pole fault (PPF) occur respectively. Then, according to the DPCV difference between permanent fault and temporary fault, a fault property identification (FPI) criterion is constructed. The scheme is very simple and not affected by the transition resistance, fault position and wiring mode which has high reliability. Compared with the non‐selective reclosing strategy, the adaptive reclosing scheme can avoid the secondary impact on the system from the permanent fault. Extensive simulations and experiments verify the effectiveness of the proposed adaptive reclosing strategy.