Research on DC Protection Strategy in Multi-Terminal Hybrid HVDC System

Multi-terminal hybrid high-voltage direct current(HVDC)systems have been developed quickly in recent years in power transmission area.However,for voltage-source converter(VSC)stations in hybrid HVDC systems,no direct current(DC)filters are required.In addition,the DC reactor is also not installed at the line end because the DC fault can be limited by the converter itself.This means that the boundary element at the line end is absent,and the single-ended protections used in line commutated converter(LCC)based HVDC(LCC-HVDC)systems or VSC-HVDC systems cannot distinguish the fault line in multi-terminal hybrid HVDC systems.This paper proposes a novel singleended DC protection strategy suitable for the multi-terminal hybrid HVDC system,which mainly applies the transient information and active injection concept to detect and distinguish the fault line.Compared with the single-ended protections used in LCC-HVDC and VSC-HVDC systems,the proposed protection strategy is not dependent on the line boundary element and is thus suitable for the multiterminal hybrid HVDC system.The corresponding simulation cases based on power systems computer aided design(PSCAD)/electromagnetic transients including DC(EMTDC)are carried out to verify the superiority of the proposed protection.

Traveling Wave Characteristics Based Pilot Protection Scheme for Hybrid Cascaded HVDC Transmission Line

The hybrid cascaded high-voltage direct current(HVDC)transmission system has various operation modes,and some operation modes have sharply increasing requirements for protection rapidity,while the traditional pilot differential protection(PDP)has poor rapidity,and even refuses to operate when faults occur on the DC line.Therefore,a novel pilot protection scheme based on traveling wave characteristics is proposed.First,the adaptability of the traditional PDP applied in engineering is analyzed for different operation modes.Then,the expressions of the forward traveling wave(FTW)and backward traveling wave(BTW)on the rectifier side and the inverter side are derived for different fault locations.From the theoretical derivation,the difference between the BTW and FTW on the rectifier side is less than zero,and the same is true on the inverter side.However,in the event of an external fault of DC line,the difference between the BTW and FTW at nearfault terminal protection installation point is greater than zero.Therefore,by summing over the product of the difference between BTW and FTW of the rectifier side and that of the inverter side,the fault identification criterion is constructed.The simulation results show that the proposed pilot protection scheme can quickly and reliably identify the short-circuit faults of DC line in different operation modes.

Studies of commutation failures in hybrid LCC/MMC HVDC systems

A hybrid of line commutated converters(LCCs)and modular multi-level converters(MMCs)can provide the advantages of both the technologies.However,the commutation failure still exists if the LCC operates as an inverter in a hybrid LCC/MMC system.In this paper,the system behavior during a commutation failure is investigated.Both halfbridge and full-bridge MMCs are considered.Control strategies are examined through simulations conducted in PSCAD/EMTDC.Additionally,commutation failure protection strategies for multi-terminal hybrid LCC/MMC systems with AC and DC circuit breakers are studied.This paper can contribute to the protection design of future hybrid LCC/MMC systems against commutation failures.

Fault identification scheme for protection and adaptive reclosing in a hybrid multi-terminal HVDC system

A fault identification scheme for protection and adaptive reclosing is proposed for a hybrid multi-terminal HVDC system to increase the reliability of fault isolation and reclosing.By analyzing the"zero passing"characteristic of current at the local end during the converter capacitor discharge stage,the fault identification scheme is proposed.The distributed parameter-based fault location equation,which incorporates fault distance and fault impedance,is developed with the injection signal and the distributed parameter model during the adaptive reclosing stage.The fault distance is determined using a trust region reflection algorithm to identify the permanent fault,and a fault iden-tification scheme for adaptive reclosing is developed.Simulation results show that the proposed scheme is suitable for long-distance transmission lines with strong anti-fault impedance and anti-interference performance.Also,it is less affected by communication delay and DC boundary strength than existing methods.

基于故障安全域的混合级联直流输电系统后续换相失败抑制策略

混合级联直流输电系统兼顾了电网换相换流器(LCC)和模块化多电平换流器(MMC)的优势,具有良好的工程应用前景,但系统逆变侧LCC与MMC间复杂的交直流耦合特性增加了后续换相失败的抑制难度。为此,该文提出了一种应对混合级联系统后续换相失败的协调控制策略。首先,分析了控制器交互期间电气量波动和LCC无功需求对系统恢复产生的不利影响,并在考虑控制器作用和MMC动态无功支撑的基础上建立了多电气量耦合作用下的故障安全域;其次,通过对混合级联系统和基于电网换相换流器的高压直流输电故障安全域对比分析,提出了一种基于MMC和低压限流环节(VDCOL)的协调控制策略,以实现系统后续换相失败抑制和功率快速平稳恢复相协调;最后,基于PSCAD/EMTDC分别在不同严重程度交流故障、不同短路比和不同故障持续时间下进行仿真对比分析,验证了所提协调控制策略的有效性。

用于MMC-HVDC直流故障保护的新型拓扑及重合闸控制策略

针对柔性直流架空输电线路频发的瞬时性直流故障,提出了模块化多电平改进子模块结构和混合级联拓扑以及MMC系统重合闸控制策略。新型子模块及混合级联结构具有直流故障自动闭锁和迅速恢复的能力,能进一步降低造价及损耗,并能继承已有成熟的控制方法。系统重合闸控制可以使柔性直流系统在小电流情况下判断直流故障是否消除并恢复系统运行,能有效应对永久性、瞬时性直流双极短路故障。文章构建了等效电路来分析直流双极短路故障及重合闸过程,并通过PSCAD/EMTDC仿真对所提方案进行了有效性验证。

白鹤滩-江苏特高压混合级联直流工程数模仿真平台构建及特性研究

白(鹤滩)—江(苏)特高压直流输电工程是世界上第1个受端采用电网换相换流器(line-commutated converters,LCC)+模块化多电平换流器(modular multilevel converters,MMC)级联结构的实际工程,且受端分层接入4个交流馈入点。根据白江工程设计资料与实际参数,构建了包含与实际工程功能一致的MMC数字阀控系统、小步长现场可编程门阵列(field programmable gate array,FPGA)阀体仿真模型、直流实际控制保护装置的白江工程数模混合实时仿真平台,实现对白江工程百纳秒–微秒–毫秒级多时间尺度精准仿真。通过大量的功能性试验,以及与实际工程录波的一致性对比试验,验证了仿真平台的准确性;仿真分析了白江工程特殊结构和控制模式能在一定程度上提升对受端交流故障的抵御能力,可避免换相失败或降低换相失败的功率扰动,并改善受端故障引起送端交流过电压问题。本平台将继续应用于多回直流馈入华东的大电网数模混合仿真研究中。

混合级联直流输电系统换相失败抵御能力评估方法研究

混合级联直流输电系统适用于远距离、大容量、跨区域、多落点输电,但逆变侧换相换流器(linecommutated converter,LCC)交流母线发生故障时易发生换相失败,严重时甚至发生后续换相失败。为了评估混合级联直流输电系统换相失败抵御能力,首先给出混合级联直流输电系统准稳态模型,提出计及系统直流控制特性的交互作用因子计算方法,进而建立考虑模块化多电平变换器(modularmultilevel convertor,MMC)电气特性的混合级联系统逆变侧等值模型。然后,提出基于电压稳定因子(voltage stability factor,VSF)的混合级联系统换相失败抵御能力评估方法,并推导含有效短路比(effective short circuit ratio,ESCR)指标的换相失败免疫因子解析表达式,从理论角度分析交流系统强度对混合级联系统换相失败抵御能力的影响。最后,在PSCAD/EMTDC上搭建混合级联系统的电磁暂态详细仿真模型,验证交互作用因子计算方法、逆变侧等值模型、换相失败抵御能力评估方法的有效性,仿真结果表明,所提方法最大误差在5%左右。结合仿真,详细分析系统控制方式、逆变侧交流联络线等值耦合阻抗、MMC控制器参数对混合级联系统换相失败抵御能力的影响,为混合级联直流输电系统的安全稳定运行提供技术参考。

基于改进CUSUM算法的混合级联直流输电系统低电压保护优化策略

为提高混合级联直流输电系统直流输电线路低电压保护的速动性和可靠性,提出了一种基于改进累积和(cumulative sum control chart, CUSUM)算法的新型高压直流线路低电压保护策略。当±800 kV混合级联直流输电系统的不同位置发生故障时,保护安装处所测得电气量呈现出一定的差异性。由于传统低电压保护缺乏整定依据,并有着误动风险,本文引入CUSUM算法提取不同故障时电气量差异特征,以此保证保护的可靠性。同时为了提高保护的速动性,借助分形理论,对CUSUM算法窗口进行改进,使得窗口具有自适应性,从而提高保护的速动性。利用PSCAD/EMTDC建立混合级联直流输电模型,并借助MATLAB验证所提新保护策略。仿真结果表明:所提方案能够快速可靠动作,有着良好的快速性和可靠性;可以耐受较高过渡电阻,有良好的耐受过渡电阻能力;判据依托于单端电气量,避免了通信中噪声和数据异常等干扰,可以快速识别区内、区外故障,并可靠动作。

混合级联多端直流暂态过电压机理及抑制措施

混合级联多端直流采用常规直流与柔性直流混合级联的新型拓扑结构,为我国远距离、大容量、多落点的能源传输提供了新模式,但常规直流与柔性直流之间的交互作用也引发了新问题。该文首先揭示实际受端电网交流电压恢复特性导致混合级联多端直流暂态过电压机理以及低端模块化多电平换流器(modular multilevel converter,MMC)闭锁风险;然后分别从送受端两个方面,提出降功率运行、改进送端移相策略和提升MMC有功功率传输极限等暂态过电压抑制措施。最后,基于PSD-PSModel软件,搭建送受端电网机电暂态和混合级联多端直流电磁暂态的混合仿真模型,仿真验证理论分析的准确性和抑制措施的有效性。

混合级联直流输电系统整流/逆变不同控制回路动态交互作用与稳定性研究

为了研究混合级联直流输电系统整流侧和逆变侧各换流器控制系统间的交互作用,文中首先建立混合级联直流输电系统的状态空间模型,并推导得到系统的多输入多输出传递函数模型。然后针对不同的研究目标,根据单通道分析设计理论建立等效单输入单输出反馈控制模型;在此基础上,分离出不同控制回路的耦合控制通道,定量评估系统强度、逆变侧交流联络线与控制参数等对控制回路间交互作用的影响。理论分析表明,当整流侧或逆变侧交流系统强度降低时,两侧控制回路间交互作用增强;当交流联络线长度缩短时,逆变侧电网换相换流器(line commutated converter,LCC)和模块化多电平变换器(multilevel modular converter,MMC)控制回路间的动态交互增强,各控制环节参数之间存在相互制约关系。最后,在PSCAD/EMTDC上搭建混合级联直流输电系统的电磁暂态详细仿真模型,验证所建立单输入单输出反馈控制模型及上述理论分析结果的正确性。

Hierarchical Optimal Power Flow Control for Loss Minimization in Hybrid Multi-terminal HVDC Transmission System

A hierarchical control scheme is proposed for optimal power flow control to minimize loss in a hybrid multiterminal HVDC(hybrid-MTDC)transmission system.In this scheme,the lower level is the droop control,which enables fast response to power fluctuation and ensures a stable DC voltage,and the upper level is power flow optimization control,which minimizes the losses during the operation of hybrid-MTDC and solves the contradiction between minimizing losses and preventing commutation failure.A 6-terminal hybrid-MTDC is also designed and simulated in PSCAD according to the potential demand of power transmission and wind farms integration in China to verify the proposed control strategy.First,the steady state analysis is conducted and then compared with simulation results.The analysis shows that the proposed control scheme achieves the desired minimum losses while at the same time satisfying system constraints.The proposed control scheme also guarantees that the hybrid-MTDC not only has a good dynamic response,but also remains stable during communication failure.

换相失败引发送端混合级联直流系统换流母线暂态电压波动机理及抑制策略

与电网换相高压直流输电(line commutated converter HVDC,LCC-HVDC)相比,送端混合级联直流系统具有灵活性高、弱网适应性强等优势,但其换相失败后交流系统暂态电压波动机理尚不明晰。为此,首先建立了暂态电压与系统强度、功率不平衡量之间的定量关系。考虑有功、无功不平衡量对暂态电压的影响,推导了暂态电压变化率表达式。明确了换相失败及其恢复期间换流母线暂态电压特性。其次,为抑制换流母线暂态过电压,推导了LCC直流电流指令值与模块化多电平换流器(modular multilevel converter,MMC)无功指令值表达式,提出了基于多换流器的协同抑制策略。最后,基于PSCAD/EMTDC仿真平台搭建仿真模型,验证了机理分析的正确性及抑制策略的有效性。

Research on hybrid multi-terminal high-voltage DC technology for offshore wind farm integration

Multi-terminal high-voltage DC(MTDC)technology is a promising way to transmit large amounts of offshore wind power to the main grids.This paper proposes a hybrid MTDC scheme to integrate several offshore wind farms into the onshore power grids at different locations.A hybrid four-terminal HVDC system comprising two onshore line commutated converters(LCCs)and two voltage source converters(VSCs)connecting an offshore wind farm is constructed in PSCAD/EMTDC.A coordination control scheme based on the VSCs’AC voltage control and the LCCs’DC voltage droop control is designed to ensure smooth system operation and proper power sharing between onshore AC grids.The operational characteristics of the system are analyzed.In addition,a black start-up method without any auxiliary power supply for the VSCs is proposed.The transmission scheme is tested through simulations under various conditions,including start-up,wind speed variation,and the disconnection of one VSC or of one LCC.

从2010国际大电网会议看直流输电技术的发展方向

根据2010国际大电网会议与高压直流输电和电力电子技术对应的B4技术委员会发表的论文,探讨了直流输电技术的发展方向以及代表性的工程,论述了基于两端直流输电技术的多端直流输电技术、±1000kV及以上的直流输电技术、模块化多电平电压源换流器型直流输电技术、基于两电平级联式换流器型直流输电技术、混合式电压源换流器型直流输电技术以及基于电压源换流器的多端直流输电技术。

混合级联型直流输电系统直流故障特性及恢复控制策略

整流侧采用LCC、逆变侧采用MMC与LCC串联的混合级联型直流输电系统可实现直流故障穿越、换相失败抑制和大容量功率传输。建立混合级联型直流输电系统模型,设计系统整体控制策略,并利用PSCAD/EMTDC仿真软件研究系统功率阶跃时的动态特性,验证控制策略的有效性。对系统的直流故障特性进行仿真分析,发现若不采取合适措施,系统发生直流故障时会出现由于并联MMC之间的电流分配不均衡而产生过电流现象以及故障清除后系统恢复过程波动大的问题,为此,提出系统故障期间及故障清除后的恢复控制策略,仿真验证了该控制策略的有效性。

级联型混合直流输电系统的自适应下垂控制策略研究

文中研究了受端级联型混合高压直流(简称混合直流)输电系统的控制特性,分析了受端柔性直流在下垂控制模式下对级联型混合直流控制特性的影响,明确了混合级联直流的总体伏安特性曲线。相比于主从控制,采用下垂控制的模块化多电平换流器(MMC)具有同时控制直流电压和直流功率的能力,不会出现功率反送现象。但在下垂控制作用下,MMC无法实现直流电压的准确控制。因此,文中提出一种受端级联型混合直流输电系统的自适应下垂控制策略,该策略可根据系统直流电流的变化实时调节下垂特性,避免MMC的直流电压随直流电流的变化而产生波动。最后,基于PSCAD/EMTDC的仿真结果验证了所提控制策略的有效性,该策略可以实现混合直流输电系统直流电压的准确控制,提高了混合直流输电系统的稳定性。

混合级联特高压直流输电系统旁通对过负荷保护优化

为解决混合级联特高压直流输电系统中存在的旁通对过负荷保护误动问题,本文分析研究故障后的电流回路和保护误动机理,提出采用换流器高、低压侧实测电流的保护判据。针对站间通信中断下执行极闭锁时逆变站旁通对过负荷保护动作的情况,提出逆变站电网换相换流器(LCC)合旁通开关(BPS)的策略,并研究相应的保护定值整定方法。实时数字仿真试验表明,提出的方案简单可行,避免了旁通对过负荷保护误动,提高了直流输电系统的可靠性。

一种适用于大规模新能源远距离外送的分层混联输电系统

为实现内陆大规模风电等新能源的直流汇集与远距离输送,该文提出一种适用于不同规模风电基地分层接入的混合级联输电系统。首先介绍分层接入–混合级联输电系统的拓扑结构与运行特性。该系统为双层结构,高压阀组采用电网换相换流器,多个模块化多电平换流器并联作为低压阀组,可实现不同规模风电基地的分层接入。随后分析系统的数学模型,并在此基础上设计协调控制方案。针对风电基地出力波动,为系统设计阀组电压自适应控制,保障系统的稳定运行。最后在PSCAD/EMTDC中搭建分层接入–混合级联输电系统仿真模型,验证控制策略的有效性。研究结果表明,该系统可为大规模风电远距离传输提供一种新选择。

特高压混合级联系统受端交流故障问题分析

受端混合级联直流输电系统具有经济性高、灵活性强等诸多优势,应用前景十分广泛。当其受端VSC发生交流系统故障时,换流阀功率输送能力减弱,此时,整流站功率持续输出会加剧直流侧的功率盈余,造成VSC换流器电压急剧升高。故障结束后,系统需要较长时间恢复功率正常输送,严重影响系统的正常运行和稳定性。针对特高压混合级联系统受端换流器发生交流故障时直流侧过电压问题及故障结束后的功率恢复问题,提出了电压-功率协同控制策略及基于受端交流电压变化的交流低压限流控制策略。最后采用真实控制保护装置搭建基于RTDS仿真系统的硬件在环仿真平台,验证了所提策略的可行性。