Fault Current and Voltage Estimation Method in Symmetrical Monopolar MMC-based DC Grids

Symmetrical monopolar configuration is the prevailing scheme configuration for modular multilevel converter based high-voltage direct current(MMC-HVDC) links, in which severe DC overvoltage or overcurrent can be caused by the DC faults. To deal with the possible asymmetry in the DC faults and the coupling effects of the DC lines, this paper analyzes the DC fault characteristics based on the phase-mode transformation. First, the DC grid is decomposed into the common-mode and the differential-mode networks. The equivalent models of the MMCs and DC lines in the two networks are derived, respectively. Then, based on the state matrices, a unified numerical calculation method for the fault voltages and currents at the DC side is proposed. Compared with the time-domain simulations performed on PSCAD/EMTDC, the accuracy of the proposed method is validated. Last, the system parameter analysis shows that the grounding parameters play an important role in reducing the severity of the pole-to-ground faults, whereas the coupling effects of the DC lines should be considered when calculating the DC fault currents associated with the pole-to-pole faults.

模块化高压大容量DC/DC变换器综述

随着新能源发电占比增加和电网规模的不断扩大,直流输电系统的容量不断提升,含有不同电压等级的直流电网是未来发展的必然趋势。在此背景下,DC/DC变换器作为直流电网关键设备,在多电压等级互联场景中发挥着重要作用。文中首先对已有的模块化高压大容量DC/DC变换器进行归纳、分类和比较,然后阐述了目前模块化高压大容量DC/DC变换器的拓扑结构、工作原理与特点,并针对其特点提出了不同变换器的适用场景,最后对未来的研究方向提出了建议。

基于限流电抗器两侧特定频段暂态能量比的MTDC电网单端量保护方案

柔性直流输电技术有利于未来大量分布式能源接入电网,而配置快速、可靠的保护方案是柔性直流技术发展的关键。基于直流电网等效模型,提出一种基于限流电抗器两侧电压特定频段暂态能量比差异性的单端量故障区域识别方法。该方法依据贝瑞隆等效模型计算出限流电抗器两侧电压比,通过广义S变换提取限流电抗器两侧特定频段电压,根据故障时正负极限流电抗器的电压幅值差的差异性实现故障选极。最后在PSCAD/EMTDC平台搭建仿真模型,分析出发生区内外故障时差异性明显,具备较强的耐过渡电阻与抗噪声干扰能力。

不平衡电网下AC-DC矩阵变换器的新型模型预测控制

为降低AC-DC矩阵变换器在输入不平衡条件下网侧有功功率波动,以及离散型模型预测控制中开关频率不固定的问题,提出一种新型模型预测控制方法。通过根据电网电流相位角选择有效矢量,避免了传统模型预测控制评估价值函数计算的负担。提出了一种二阶扩展复数卡尔曼滤波器,计算精度可以达到泰勒级数展开的二阶项,新型模型预测控制能够在不平衡电网系统中应用。仿真结果表明:所提控制策略在输入电压不平衡工况下,能够有效抑制网侧电流谐波,稳定系统输出的有功功率,并且表现出良好的性能。

基于CRS-LMD和SVD的MMC-HVDC线路故障测距方法

直流输电线路故障行波波速不确定、波头提取困难以及噪声干扰等因素制约了直流电网中故障测距技术的应用。为了降低上述因素对定位准确性的影响,提出一种基于局部特征有理样条插值均值分解(LMD based on characteristic rational spline,CRS-LMD)和奇异值分解(singular value decomposition,SVD)的故障测距方法。首先,利用特征尺度选取最优极点系数,结合有理样条插值调节拟合曲线的松紧程度,实现对故障电压行波的局部均值分解。其次,采用奇异值分解对故障行波波头进行准确提取。最后,在PSCAD/EMTDC中搭建了张北±500 kV柔性直流电网的仿真模型,模拟各种故障情况并输出故障数据,利用Matlab对故障数据进行处理并验证定位算法。最后,仿真结果表明,所提故障测距算法在不同故障距离和故障类型下均能实现故障测距,且在叠加噪声和过渡电阻的情况下也能保障较高的精确性。

Adaptive Reclosing Scheme Based on Traveling Wave Injection For Multi-Terminal dc Grids

The hybrid dc circuit breaker(HCB)has the advantages of fast action speed and low operating loss,which is an idealmethod for fault isolation ofmulti-terminal dc grids.Formulti-terminal dc grids that transmit power through overhead lines,HCBs are required to have reclosing capability due to the high fault probability and the fact that most of the faults are temporary faults.To avoid the secondary fault strike and equipment damage that may be caused by the reclosing of the HCB when the permanent fault occurs,an adaptive reclosing scheme based on traveling wave injection is proposed in this paper.The scheme injects traveling wave signal into the fault dc line through the additionally configured auxiliary discharge branch in the HCB,and then uses the reflection characteristic of the traveling wave signal on the dc line to identify temporary and permanent faults,to be able to realize fast reclosing when the temporary fault occurs and reliably avoid reclosing after the permanent fault occurs.The test results in the simulation model of the four-terminal dc grid show that the proposed adaptive reclosing scheme can quickly and reliably identify temporary and permanent faults,greatly shorten the power outage time of temporary faults.In addition,it has the advantages of easiness to implement,high reliability,robustness to high-resistance fault and no dead zone,etc.

Rapid Parameter-Optimizing Strategy for Plug-and-Play Devices in DC Distribution Systems under the Background of Digital Transformation

By integrating advanced digital technologies such as cloud computing and the Internet of Things in sensor measurement,information communication,and other fields,the digital DC distribution network can efficiently and reliably access DistributedGenerator(DG)and Energy Storage Systems(ESS),exhibiting significant advantages in terms of controllability and meeting requirements of Plug-and-Play(PnP)operations.However,during device plug-in and-out processes,improper systemparametersmay lead to small-signal stability issues.Therefore,before executing PnP operations,conducting stability analysis and adjusting parameters swiftly is crucial.This study introduces a four-stage strategy for parameter optimization to enhance systemstability efficiently.In the first stage,state-of-the-art technologies in measurement and communication are utilized to correct model parameters.Then,a novel indicator is adopted to identify the key parameters that influence stability in the second stage.Moreover,in the third stage,a local-parameter-tuning strategy,which leverages rapid parameter boundary calculations as a more efficient alternative to plotting root loci,is used to tune the selected parameters.Considering that the local-parameter-tuning strategy may fail due to some operating parameters being limited in adjustment,a multiparameter-tuning strategy based on the particle swarm optimization(PSO)is proposed to comprehensively adjust the dominant parameters to improve the stability margin of the system.Lastly,system stability is reassessed in the fourth stage.The proposed parameter-optimization strategy’s effectiveness has been validated through eigenvalue analysis and nonlinear time-domain simulations.

联接弱交流电网MMC-HVDC系统的直流功率传输能力提升方法

模块化多电平换流器高压直流输电(modular multilevel converter based high voltage direct current,MMC-HVDC)由于具备自换相能力,尤其适用于向弱交流电网供电。交流系统强度降低时,会制约系统的直流功率传输能力,甚至导致系统失稳。针对联接弱交流电网时MMC-HVDC系统功率传输受限的问题,建立了状态空间与直流阻抗模型,从时域、频域两方面研究了交流系统强度对直流功率传输能力的影响,明确了弱交流电网工况下功率传输受限的原因。基于参与因子定位结果,提出了在定直流电压控制环节引入直流电流反馈的功率传输能力提升方法,从时域、频域两方面对控制策略的提升作用进行了机理分析,并定量得出了控制参数的可行域及功率传输能力的最大提升水平。该方法在避免稳态误差的前提下,有效提升了MMC-HVDC系统的直流功率传输能力。

Analysis of DC Grid Prospects in China

随着化石能源的日益枯竭，中国已将发展可再生能源提升至重要地位，并逐步实现能源结构的战略性调整。分析可再生能源发电的间歇性与不稳定性特点，以及可再生能源发电集中并网带来的稳定性问题，提出通过建设广域覆盖的直流电网以有效解决可再生能源集中并网的有功波动问题。建设全国性的直流骨干输电网，不仅可更有效地利用可再生能源，也将积极推动坚强智能电网的建设。与交流系统相比，直流电网在未来城市配电、微网等领域也有较大的优势。根据中国国情，对直流电网在中国建设的可行性进行了具体分析，并提出中国直流电网主干结构设想图，指出了直流电网建设的一些关键技术问题，为中国直流电网建设提供参考。

远海风电DRU-HVDC送出系统构网控制与启动方法综述

随着海上风电开发的不断规模化和远海化,海上风电直流送出技术成为研究热点。针对低成本的二极管整流单元(DRU)型高压直流(HVDC)输电,重点指出了分布式DRU在均压、部分退出运行等方面的主要技术特点,论述了该技术与柔性直流送出、传统直流输电的差异。由于DRU的不可控性,海上电网的构建须由风电机组通过自身控制实现。因此,对现有研究的风电机组集中式和分散式构网控制进行详细综述,并对全构网型控制采用P-V和Q-f功率控制机理进行风电场-DRU层面以及风电机组层面的梳理解释。针对DRU单向潮流引起的启动问题,综述了辅助中压交流脐带电缆、辅助模块化多电平换流器、海上风电机组配置储能以及辅助低压直流电缆等启动方法,指出利用辅助中压交流脐带电缆是相对成熟的方案,而利用海上风电机组储能和全构网型控制具有较高的灵活性,但现有技术下仍面临风电机组塔筒内部安装空间不足、储能安全隐患等问题。最后,对海上风电经DRU-HVDC送出系统未来值得深入探索的研究方向进行了展望。

考虑VSC-HVDC作用的LCC-HVDC系统短路比实用计算

准确量化LCC-HVDC(电网换相换流器型高压直流输电)及VSC-HVDC(电压源换流器型高压直流输电)多馈入系统的传输极限和电网强度是保障新型电力系统稳定运行的关键之一。通过改进传统LCCHVDC交直流系统的电网强度量化方法,提出了量化混合直流馈入系统电网强度的短路比指标,以此分析交直流系统的传输极限。首先,根据潮流方程建立混合直流馈入系统的准稳态模型;其次,通过分析雅克比矩阵奇异性与直流馈入系统功率传输极限的关系,得到用于评估混合直流馈入系统中LCC-HVDC传输极限的短路比指标,并分析指标临界值与VSC-HVDC运行方式的灵敏度关系;最后,通过PSCAD/EMTDC及MATLAB平台验证了短路比指标的有效性。分析结果表明:VSC-HVDC在受端定电压控制方式下具有更好的电压支撑能力,同时VSC-HVDC的运行方式对LCC-HVDC传输极限的影响较小,可通过所提指标进行定量分析。

Control Strategy of DC Link Voltage Flywheel Energy Storage for Non Grid Connected Wind Turbines Based on Fuzzy Control

The large-scale development of wind power is an important means to reduce greenhouse gas emissions, alleviate environmental pollution and improve the utilization rate of renewable energy. At the same time, large-scale non grid connected wind power generation theory avoids the technical difficulties of wind power integration [1]. However, due to the randomness and uncontrollability of wind energy, the output power of the wind power generation system will fluctuate accordingly [2]. Therefore, the corresponding energy storage devices are arranged in the non-grid-connected wind power generation system to ensure the power quality, and it has become the key to full utilization of renewable energy. In the case of wind speed fluctuation, the DC bus control strategy of the wind turbine is proposed in this paper. It can reduce the impact on the unit converter and the power load;this ensures safe and stable operation of non-grid connected wind turbines.

HVDC Grid Segmentation Analysis for Blackouts Reduction

Segmentation of large AC systems through DC links introduces a new concept that utilizes the advantages of direct current transmission to improve network reliability and increase power transfer capacity. Technical literature argues that the segmentation of the AC network and the introduction of DC links at these systems connection points bring benefits to system operation, once contingencies generated on one side of the DC connection point would not be reflected on the other side of DC connection, thereby reducing the likelihood of cascading shutdowns and blackouts due to load restraint on transmission lines and transformers. Amidst this scenario, this paper presents a study of the main topics regarding the use of this new network segmentation philosophy, bringing a practical point of view for the use of this concept at the electrical power system planning. The effect of DC segmentation before a contingency that would initiate major outages in an adapted electrical system model IEEE-14 bus is studied and simulations have been performed with test HVAC systems and segmented by HVDC link. The results have been compared, principally in relation to the voltage bus, reactive power generation, system losses and power flow at the lines, and demonstrated that this new concept improved the grid reliability.

直流故障下MMC-HVDC的两段式限流保护策略

如何对直流故障电流进行有效抑制是实现柔性直流电网大规模发展的关键。为此,研究模块化多电平变换器(modular multilevel converter,MMC)的控制方法,提出一种针对半桥型MMC的两段式限流保护策略,通过减少子模块的投入数目来降低换流站直流出口电压,达到限制短路电流的目的。首先,介绍MMC的拓扑结构及基于MMC的高压直流输电(MMC based high voltage direct current,MMC-HVDC)系统控制策略;其次,分析两段式限流保护策略的原理与直流故障电流特性,介绍MMC-HVDC系统的直流故障保护策略;最后,通过双端MMC-HVDC系统仿真实验,对所提限流保护策略的有效性进行验证。仿真结果表明,两段式限流保护策略可以有效降低断路器开断电流和吸收能量,节约成本。

Practice and Prospect of AC/DC Hybrid Power Grid in Southern China

China Southern Power Grid is a unique EHV AC/DC hybrid transmission network that operates in China. In its service area, the distribution of energy resources and the development of economy are extremely unbalanced, so long-distance and bulk power transmission are needed; besides, the geography and climate conditions are serious, rains, fogs, lightning and typhoon as well as high temperature are common all the year round. Facing these challenges, the power grid enhanced stability control, improved the equipment and strengthen the network structure. In the future, the power grid plans to optimize the disposition of power sources and build digitalized power system.

Development and Research of Bypass Device to Improve DC Grid Performance

Among the factors determining the performance of the new renewable energy system, the processing part of the untreated DC power is a very important element. In fact, the area where the loss of the new renewable energy system is greatest is also the DC transmission and distribution technology. It is critical to prevent this loss and to improve the performance of the system, and analyze the performance of the resulting through the application of the resulting algorithms. In this thesis, we are discussing ways to calibrate the error in the PV distribution system within the solar power generation system using the Bypass Line Configuration Algorithm. I would like to note that if the operation of the Bypass Line device generates electrical abnormalities in the system’s circuits, it can recover the performance of the system, thereby improving performance.

Fault Identification and Islanding in DC Grid Connected PV System

Nowadays, the DC distribution system has been suggested, as a replacement for the AC power distribution system with electric propulsion. This idea signifies a fresh approach of issuing energy for low-voltage installations. It can be used for any electrical application up to 20 MW and works at a nominal voltage of 1000 V DC. The DC distribution system is just an extension of the multiple DC links that previously available in all propulsion and thruster drives, which typically comprise more than 80% of the electrical power consumption on electric propulsion vessels. A fault detection and islanding scheme for DC grid connected PV system is presented in this paper. Unlike traditional ac distribution systems, protection has been challenging for dc systems. The goals of this paper are to classify and detect the fault in the PV system as well as DC grid and to isolate the faulted section so that the system keeps operating without disabling the entire system. The results show the measured values of power at PV panel and DC grid side under different fault condition, which indicates the type of fault that occurs in the system.

Fuzzy Logic Controlled Dual Active Bridge Series Resonant Converter for DC Smart Grid Application

Over the last few years, smart grids have become a topic of intensive research, development and deployment across the world. This is due to the fact that, through the smart grid, stable and reliable power systems can be achieved. This paper presents a fuzzy logic control for dual active bridge series resonant converters for DC smart grid application. The DC smart grid consists of wind turbine and photovoltaic generators, controllable and DC loads, and power converters. The proposed control method has been applied to the controllable load＇s and the grid side＇s dual active bridge series resonant converters for attaining control of the power system. It has been used for management of controllable load＇s state of charge, DC feeder＇s voltage stability during the loads and power variations from wind energy and photovoltaic generation and power flow management between the grid side and the DC smart grid. The effectiveness of the proposed DC smart grid operation has been verified by simulation results obtained by using MATLAB and PLECS cards.

An Efficient MPPT Tracking in Solar PV System with Smart Grid Enhancement Using CMCMAC Protocol

Renewable energy sources like solar,wind,and hydro are becoming increasingly popular due to the fewer negative impacts they have on the environment.Because,Since the production of renewable energy sources is still in the process of being created,photovoltaic(PV)systems are commonly utilized for installation situations that are acceptable,clean,and simple.This study presents an adaptive artificial intelligence approach that can be used for maximum power point tracking(MPPT)in solar systems with the help of an embedded controller.The adaptive method incorporates both the Whale Optimization Algorithm(WOA)and the Artificial Neural Network(ANN).The WOA was implemented to enhance the process of the ANN model’s training,and the ANN model was developed using the WOA.In addition to this,the inverter circuit is connected to the smart grid system,and the strengthening of the smart grid is achieved through the implementation of the CMCMAC protocol.This protocol prevents interference between customers and the organizations that provide their utilities.Using a protocol known as Cross-Layer Multi-Channel MAC(CMCMAC),the effect of interference is removed using the way that was suggested.Also,with the utilization of the ZIGBEE communication technology,bidirectional communication is made possible.The strategy that was suggested has been put into practice,and the results have shown that the PV system produces an output power of 73.32 KW and an efficiency of 98.72%.In addition to this,a built-in regulator is utilized to validate the proposed model.In this paper,the results of various experiments are analyzed,and a comparison is made between the suggested WOA with the ANN controller approach and others,such as the Particle Swarm Optimization(PSO)based MPPT and the Cuckoo Search(CS)based MPPT.By examining the comparison findings,it was determined that the adaptive AI-based embedded controller was superior to the other alternatives.

直流微电网用非隔离高增益DC-DC变换器

在直流微电网中,光伏需要依靠电力电子功率变换器并网。文章提出了一种非隔离高增益DC-DC变换器,该变换器采用改进的开关电感,以提高负载的输出电压,对其连续导通模式和不连续导通模式下的稳态进行了分析,并与其它功率变换器拓扑进行了比较。最后,在Matlab仿真平台和实验样机上进行了对比,仿真和实验结果与理论分析结果基本一致。文章所提变换器与传统升压变换器相比,具有较高的电压增益,虽然该变换器使用了两个功率开关,但由于两个功率开关采用统一操作,因此控制电路简单。