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.

Single-ended protection method for hybrid HVDC transmission line based on transient voltage characteristic frequency band

Hybrid high-voltage direct current(HVDC)transmission has the characteristic of long transmission distance,complex corridor environment,and rapid fault evolution of direct current(DC)lines.As high fault current can easily cause irreversible damage to power devices,rapid and reliable line protection and isolation are necessary to improve the security and reliability of hybrid HVDC transmission system.To address such requirement,this paper proposes a single-ended protection method based on transient voltage frequency band characteristics.First,the frequency characteristics of the smoothing reactor,DC filter,and DC line are analyzed,and the characteristic frequency band is defined.A fault criterion is then constructed based on the voltage characteristic frequency band energy,and faulty pole selection is performed according to the fault voltage characteristic frequency band energy ratio.The proposed protection method is verified by simulation,and the results show that it can rapidly and reliably identify internal and external faults,accurately select faulty poles without data communication synchronization,and has good fault-resist-ance and anti-interference performance.

Design Aspects of Hybrid HVDC System

Hybrid high voltage direct current transmission(Hybrid HVDC)is a new type of HVDC technology developed in recent years.It combines the characteristics of large-capacity and low cost of the line commutated converters(LCC)and non-commutation failure and dynamic reactive power support of the voltage sourced converters(VSCs)in one HVDC system.It has technical advantages in the fields of unidirectional power transmission and in the application of improving the stability of multi-infeed HVDCs,giving it broad application prospects.This paper defines the basic form of hybrid HVDC first and discusses the characteristics and challenges of different hybrid HVDCs.Then,it analyzes the matching characteristics between LCC and VSC in the station-station hybrid HVDC,studies the key technologies,such as VSC topologies and control strategies for clearing the DC overhead line faults,and introduces the key technologies for analyzing and suppressing the high-frequency resonances between VSC and the power grid.In this paper,using the Wudongde UHV multi-terminal HVDC as an example,the proposed key technologies are analyzed and verified.

AC fault ride through control strategy on inverter side of hybrid HVDC transmission systems

Hybrid high-voltage direct current(HVDC)transmission systems employ a new type of HVDC transmission topology that combines the advantages of the linecommutated converter system and the voltage-source converter system.They can improve the efficiency and reliability of long-distance power transmission.However,realizing alternating-current(AC)grid-fault ride through on the inverter side of a hybrid HVDC transmission system is a challenge considering that a voltage-source converter based HVDC(VSC-HVDC)is used on the inverter side.In this study,a control strategy for an overvoltage fixed trigger angle based on the power-balance method is developed by fully utilizing the operation characteristics of a hybrid HVDC transmission system.The strategy reduces the inverter-side overvoltage of the HVDC system under a fault in the inverter-side AC system.Simulations based on Gezhou Dam are conducted to validate the effectiveness of the proposed strategy.

Sequential auto-reclosing strategy for hybrid HVDC breakers in VSC-based DC grids

The use of overhead lines for power transmission in the future high-voltage and large-capacity voltagesource converter(VSC)-based direct current(DC) grid will significantly increase the probability of temporary faults.To eliminate potential adverse impacts such as erroneous protection, line-insulation failure, and even damage to power electronic devices resulting from a DC breaker reclosing operation with the traditional sequential autoreclosing strategy, a new sequential auto-reclosing strat-egy for hybrid HVDC breakers(HHBs) in VSC-based DC grids is proposed. This strategy is based on the step-by-step operation of the transfer branch in the HHB. As a result,du/dt resulting from the HHB reclosing operation is greatly reduced, and therefore those potential negative impacts can be eliminated. Several other advantages are also presented.The feasibility and validity of the proposed strategy are verified in a four-terminal annular VSC-based DC grid electromagnetic transient model.

System security of hybrid AC-HVDC-systems challenges and new approaches for combined security assessment,preventive optimization and curative actions

The intense application of Voltage Source Converter based HVDC interconnections and grids will result in a hybrid AC-HVDC-system. The operation of such a system becomes complex regarding system security and system operation. This paper describes major challenges and proposes potential solutions, including a combined security assessment, preventive optimization and curative actions. A coordination of both systems enables an efficient application of existing transport capacity.

Fault Diagnosis with Wavelet Packet Transform and Principal Component Analysis for Multi-terminal Hybrid HVDC Network

In view of the fact that the wavelet packet transform(WPT) can only weakly detect the occurrence of fault, this paper applies a fault diagnosis algorithm including wavelet packet transform and principal component analysis(PCA) to the inverter-side fault diagnosis of multi-terminal hybrid highvoltage direct current(HVDC) network, which can significantly improve the speed and accuracy of fault diagnosis. Firstly, current amplitude and current slope are used to sample the data,and the WPT is used to extract the energy spectrum of the signal. Secondly, an energy matrix is constructed, and the PCA method is used to calculate whether the squared prediction error(SPE) statistics of various signals that can reflect the degree of deviation of the measured value from the principal component model at a certain time exceed the limit to judge the occurrence of the fault. Further, its maximum value is compared to determine the fault types. Finally, based on a large number of MATLAB/Simulink simulation results, it is shown that the PCA method using the current slope as the sampled data can detect the occurrence of a ground fault with small transition resistance within 2 ms, and identify the fault types within 10 ms,without being affected by the sampling frequency.

Research on the Voltage Supporting Capability of Multi-VSC-HVDC Subsystems Operation Strategy to Receiving-end LCC-HVDC Network in Weak AC Grid

For the hybrid multi-infeed HVDC system in which the receiving-end grid is a strong AC grid including LCC-HVDC subsystems and multiple VSC-HVDC subsystems,it has higher voltage support capability.However,for weak AC grid,the voltage support capability of the multi-VSC-HVDC subsystems to the LCC-HVDC subsystem(voltage support capability-mVSCs-LCC)can resist the risk of commutation failure.Based on this consideration,this paper proposes an evaluation index called Dynamic Voltage Support Strength Factor(DVSF)for the hybrid multi-infeed system,and uses this index to qualitatively judge the voltage support capability-mVSCs-LCC in weak AC grid.In addition,the proposed evaluation index can also indirectly judge the ability of the LCC-HVDC subsystem to suppress commutation failure.Firstly,the mathematical model of the power flow of the LCC and VSC networks in the steady-state is analyzed,and the concept of DVSF applied to hybrid multi-infeed system is proposed.Furthermore,the DVSF index is also used to qualitatively judge the voltage support capability-mVSCs-LCC.Secondly,the influence of multiple VSC-HVDC subsystems with different operation strategies on the DVSF is analyzed with reference to the concept of DVSF.Finally,the indicators proposed in this paper are compared with other evaluation indicators through MATLAB simulation software to verify its effectiveness.More importantly,the effects of multi-VSC-HVDC subsystems using different coordinated control strategies on the voltage support capability of the receiving-end LCC-HVDC subsystem are also verified.

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。

连接无源网络的LCC-FHMMC混合直流输电系统送端交流故障穿越控制策略

LCC-FHMMC型混合直流输电系统可以完全解决逆变侧换相失败问题,在向无源网络供电领域具有广阔的应用前景。建立了双端混合直流输电系统的数学模型,在分析送端交流故障机理和无源网络工作特性的基础上提出一种基于桥臂电压控制的故障穿越控制策略。通过让负荷主动参与调节换流站不平衡功率,快速恢复直流功率传输的同时有效避免切负荷,同时实现受端换流站和负荷的故障穿越。依靠全桥子模块拓扑能够输出负电平的特点扩大逆变侧直流电压的调节范围,避免子模块电容过度放电导致的换流站闭锁,有效减小了故障清除后直流冲击电流。设计了不同严重程度交流故障下相关参数的选取原则,后基于电磁暂态仿真软件搭建双端混合直流输电系统的仿真模型验证了所提控制策略的有效性。

直流断路器全工况串联型合成等效试验方法

混合式高压直流断路器作为直流电网的关键设备之一,完备的型式试验是保证其挂网运行时快速、可靠清除故障电流和故障恢复的重要手段。为了更好地等效混合式直流断路器分断及故障恢复过程中的高电压与大电流工况,研究了混合式直流断路器开断大电流和故障清除失败情况下的重合闸全过程等效性评价指标,提出了一种新型串联式的全压全流合成等效试验方法,通过适当的控制方式,方案可以在不过多增加控制复杂度,实现试验全工况等效,并通过对电容器组的高效复用,降低了试验的成本。在该方案中,试验回路由大电流回路和高电压回路串联,通过大电流回路单独提供开断电流,在断路器开断结束后投入高压试验回路,两个回路共同作用提供开断后稳态电压和重合闸的二次电流。通过对4 kV/8 kA断路器进行开断及重合闸实验,验证了该试验回路的可行性,并针对500 kV/25 kA工况下的断路器对比了该试验回路与现有并联合成试验回路,结果表明串联式的全压全流合成试验方法在等效性、二次控制简易性和经济性方面存在优势,为直流断路器全工况等效试验提供另一种可选择方案。

远距离大型DFIG风电场的混合型HVDC建模及控制

针对远距离大型双馈感应发电机(DFIG)风电场,提出了一种混合型高压直流(Hybrid HVDC)输电方式并给了出其控制策略。在该方案中,HVDC整流侧采用基于晶闸管的相控换流器(LCC),并以STATCOM作为电压支撑为DFIG提供励磁和整流器换向电压;逆变侧采用基于全控器件的电流源型逆变器(CSI),从而达到对电网提供独立无功支撑的目的。文章首先建立了DFIG风电场及混合型HVDC并网系统的动态数学模型,然后针对其中的STATCOM子系统、整流器子系统和逆变器子系统给出了相应的控制器设计方法。经Matlab/Simulink仿真验证,该系统能够顺利完成黑启动,具有精确的功率跟踪和快速的功率响应能力,还能够向电网提供独立的无功支撑。

用于大型DFIG风电场的混合型HVDC系统中整流器的建模与控制

针对DFIG风电场-混合型HVDC系统,建立了dq坐标系下整流器子系统的状态空间标准模型。根据逆变器子系统采用的不同控制模式,定量分析了整流器子系统的有功、无功电流的动态特性,由此推导出用于控制的整流器子系统近似动态模型。基于该模型,设计了整流器子系统双闭环控制器。最后,采用Matlab/Simulink进行仿真,仿真验证了整流器子系统动态特性分析的正确性,同时,基于SimPowerSystems环境对"DFIG风电场-混合型HVDC系统"进行了仿真,结果表明,所提出的整流器控制方案能有效提高HVDC系统对风电场输出电流的跟踪能力。

一种利用VSC-HVDC提高发电机阻尼的新控制策略

在交直流混合输电系统中,可以利用直流系统的功率调制来提高交流系统的运行稳定性。文章在建立含有VSC-HVDC的交直流混合输电系统模型的基础上,提出了一种新的VSC-HVDC混合功率调制方法来提高发电机阻尼,该方法利用Bang-bang控制原理设计混合调制信号,并将其加入到常规的PI调节器中。仿真结果表明,加入混合调制信号可以增强系统的阻尼并有效地抑制振荡,能够改善系统在各种扰动下的动态性能。

含VSC-HVDC的交直流混合发输电系统可靠性评估

建立了含柔性直流输电(voltage source converter based high voltage direct current,VSC-HVDC)的交直流混合发输电系统的可靠性分析模型,该模型既考虑了VSC-HVDC的稳态功率特性,还计及了发输电网络的故障率和输电线路的有功限制,采用非序惯蒙特卡罗仿真实现;在满足系统安全约束的前提条件下,对系统进行模拟调度,重点评价了与发输电网络连接的VSC-HVDC对电网可靠性的影响,给出系统和节点的风险指标,为VSC-HVDC规划和运行提供参考依据,最后算例分析证明了该算法的可行性和合理性。

基于LCC-HVDC和VSC-HVDC的混合双极直流输电系统的建模及仿真分析

近年来,我国传统换相型高压直流输电(LCC-HVDC)技术在电力系统中已经发展成熟;由于电力电子器件在实际工程应用中越来越成熟甚至达到完美状态,电压源换流器型高压直流输电(VSC-HVDC)也应用到了现代电网中。由于LCC-HVDC和VSC-HVDC有各自的优点与不足,为了发挥两者的优势,对由LCC-HVDC和VSC-HVDC两个子系统分别位于上下两极所组成的混合双极直流输电系统(Hybrid Bipolar-HVDC)进行了进一步的分析。文章介绍了Hybrid Bipolar-HVDC系统内部包含的一些基本结构,对LCC和VSC的主电路和其控制电路、在PSCAD环境下正极LCC-HVDC和负极VSC-HVDC的主电路和控制电路分别进行了具体分析,最后用PSCAD对Hybrid Bipolar-HVDC进行了仿真,并分析了其稳定运行的状态。

采用VSC-HVDC改善LCC-HVDC直流侧暂态特性的无功协调控制

为了抑制常规直流输电换相失败引起的整流侧直流过电流和交流暂态电压,提出了一种适用于柔性直流输电和常规直流输电整流侧并联情况下的混合多馈入直流系统的暂态无功协调控制策略。首先分析了换相失败后混合多馈入直流系统的暂态特性,然后基于常规直流输电的定直流电流控制和柔性直流输电的定交流电压控制的控制特点,设计了混合多馈入直流系统的暂态无功协调控制模块,包括故障判断模块、快速触发角响应模块和快速无功响应模块,抑制换相失败引起的过电流和暂态电压。在PSCAD/EMTDC中搭建混合多馈入直流系统的模型并进行仿真对比分析,结果验证了所提暂态无功协调控制策略的控制效果优于柔性直流输电采用定无功功率控制和定交流电压控制。

接入MMC-HVDC的交流系统线路自适应距离保护研究

接入MMC-HVDC的交直流混合系统中,当与柔性直流线路直接相连的下级交流线路保护不能正确切除区内故障时,将由上级交流线路的距离Ⅱ段或Ⅲ段动作切除故障。柔性直流输电线路运行方式的变化将导致上级线路距离保护Ⅱ段的保护范围缩小,以很大的概率发生拒动,此时只能依靠距离保护Ⅲ段动作,故障清除时间延长。针对这一问题开展研究,深入分析了MMC-HVDC运行方式对交流系统距离保护的影响,提出了一种基于柔性直流输电公共连接点处量测信息的自适应距离保护方法。该方法能够有效降低MMC-HVDC对上级交流线路距离保护的影响,保证距离Ⅱ段的正确动作,减小了故障的清除时间。在PSCAD/EMTDC仿真环境中搭建含MMC-HVDC的交直流混合系统,仿真验证了所做分析和所提出方法的有效性。

送端交流不对称故障下LCC-MMC-HVDC穿越控制策略研究

混合直流输电系统将LCC-HVDC和VSC-HVDC进行优势互补,其发展面临的一个问题是:当送端交流系统发生不对称故障时,LCC-MMC混合直流输电系统将面临输送功率跌落甚至中断和直流侧二倍频波动的问题。首先阐述了LCC-MMC-HVDC的拓扑结构、数学模型和基本控制策略,在分析系统在送端交流不对称故障情况下暂态特性的基础上,提出集功率续传和二倍频波动抑制为一体的穿越控制策略:基于主动降压控制的功率续传策略通过改变逆变侧MMC运行点减小输送功率跌落幅度;三次谐波注入法增大了逆变侧直流电压的可调节范围;直流电压波动抑制策略中,逆变侧直流电压作为唯一控制变量,有效降低送端交流系统负序分量对系统逆变器及受端系统造成的影响。最后,在PSCAD/EMTDC中建立仿真模型,算例仿真结果验证了所提出穿越控制策略的有效性。

MMC-HVDC混合旁路直流故障保护及重合闸控制策略

直流线路故障保护是基于模块化多电平的柔性直流输电(MMC-HVDC)系统必须解决的关键问题。首先分析了基于混合旁路直流故障隔离的原理与隔离步骤,通过旁路桥臂转移故障电流,在零电流下断开快速机械开关,实现换流器与直流线路的物理隔离并清除直流故障电流。系统重合闸无需解锁子模块,通过旁路桥臂判断故障是否消除,并在零电压下闭合快速机械开关恢复系统运行。当判断直流故障为永久性故障时,换流器能够重启作为STATCOM运行。混合旁路直流故障保护方案能有效应对永久性、瞬时性直流双极短路故障。最后通过在PSCAD/EMTDC建立双端柔性直流输电仿真模型验证该方案的可行性。