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.

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.

混合级联UHVDC系统换相失败抑制分析

换相失败是基于电网换相换流器的特高压直流输电系统中最常见的故障类型之一。近年来新投运的混合级联特高压直流输电系统的逆变侧仍包含电网换相换流器,因此换相失败无法避免。首先,在分析电网换相换流器触发调节过程的基础上,明确了换相失败控制的投入时刻是影响换相失败抑制效果的关键因素;进一步地,结合白鹤滩—江苏混合级联特高压直流输电受端的结构特点和接入交流系统方式,分析了混合级联拓扑下的换相失败抑制新思路;最后,提出可充分利用逆变侧电压观测点增加、模块化多电平变换器所连接母线的电压响应更为灵敏的特点,来加快换相失败控制的投入速度,提升系统的换相失败抑制性能。基于PSCAD/EMTDC仿真平台验证了该分析的正确性。

交流网严重故障引发LCC-UHVDC功率振荡机理及控制优化方法

直流输电工程在长期发展中形成相对固定的控制策略,完善该策略对提高交直流混联电网的稳定性有重要意义。近年来,使用机电–电磁混合仿真技术进行稳定分析,在大电网交流侧某些严重故障下,多条基于电网换相换流器的特高压直流(ultra-highvoltageDCbasedonline commutatedconverter,LCC-UHVDC)工程均出现了持续大幅度功率振荡,表明现有控制策略需要一步完善。该文介绍了各UHVDC工程中出现的相似功率振荡现象,并以某工程为例,深入分析直流控制器中换相裕度面积计算(commutation margin area calculation module,AMINCALC)、设置最大熄弧角输出(maximumarcanglelimiteddirect outputmodule,GAMMA0)及低压限流(voltagedependent current order limiter,VDCOL)等多个控制环节共同诱发功率振荡的机理;研究抑制该类直流振荡可能的控制措施及实用的控制优化方案。实际电网的仿真证明了该方案的有效性。

昆柳龙特高压混合多端直流阀控触发异常分析及优化措施

昆柳龙特高压混合多端直流工程是世界首个特高压混合多端直流工程,较传统两端直流运行方式更加灵活,但也增加了控制保护策略的复杂性。自投运以来多次发生运行异常,其“首台套”控制保护系统和设备的可靠性有待进一步提升。详细分析了昆柳龙直流“6·9”阀控触发异常事件,全面梳理控制保护功能配置和直流响应情况,提出了增加阀组触发异常检测保护功能并实施应用,该策略能准确迅速检测出阀控脉冲丢失或脉冲延时故障,完善了直流控制保护系统对阀组触发异常工况的风险识别和抵御能力,有效提升昆柳龙特高压多端混合直流工程运行的可靠性和稳定性,研究成果可为后续特高压混合多端直流工程控制保护系统的功能设计提供借鉴和指导。

混合级联特高压直流输电系统逆变站保护功能及闭锁策略设计

针对混合级联特高压直流输电系统的逆变站,基于其一次拓扑结构,按照保护无死区、故障易定位的原则,结合测量互感器的安装位置,提出了保护的分区建议和配置方案。根据逆变站故障发生的位置及严重程度,以快速切除故障、减少停运范围为目标,提出包括LCC(电网换相换流器)闭锁、单MMC(模块化多电平换流器)闭锁、MMC整体闭锁、顺序极闭锁和分步极闭锁的分级闭锁策略。以白鹤滩-江苏混合级联特高压直流输电工程实际的控制保护装置和按照工程成套参数建立的RTDS(实时仿真系统)模型组建了实时闭环仿真系统,通过仿真验证了所提保护方案和闭锁策略的可行性和有效性。

基于多模式分解和多支路并行残差网络的特高压三端混合直流输电线路故障区域诊断

特高压三端混合直流输电线路发生接地故障时,故障电气量变化存在很强的随机性和非线性,在噪声干扰下其规律性更弱,很难快速、准确地提取故障特征以诊断故障区域。鉴于此,提出一种集电流波形特征提取和故障区域诊断为一体的诊断模型。分析不同区域故障时的电流波形特点,利用数学机理不同的3种算法同时分解故障电流,避免一些容易混叠的模态子序列在单一分解方法中逃逸。利用多支路并行残差网络迅速地挖掘分解分量的多尺度空间耦合交互特征,并结合门控循环单元网络模块,进一步提取故障电流的时间耦合特征,使得特征得到显著增强。利用麻雀搜索算法优化该模型的关键参数,构建充分适应电网故障诊断的网络结构,实现故障区域的快速诊断。仿真结果表明,该方案有着较高的灵敏性、较强的抗干扰能力,满足直流保护的可靠性和速动性要求。

基于深度学习的特高压三端混合直流输电线路波形特征故障区域判别方法

针对将现有直流线路故障区域识别方法应用于特高压三端混合直流输电线路时,存在难以区分T区两侧故障、耐过渡电阻能力弱和阈值整定困难的问题,提出一种利用深度学习及波形特征进行特高压三端混合直流输电线路故障区域识别的方法。首先,对三端混合直流线路不同故障区域进行故障特征分析;然后,对线模电压和线模电流进行多尺度小波分解,提取线模电流中低频分量和线模电压高频分量,结合正负极电压波形特征,组成深度学习模型的输入量,并将故障区域作为输出量,构建深度学习故障区域识别模型;最后,用训练过的深度学习模型对获取的故障特征量进行处理,以实现故障区域识别的目的。通过大量仿真实验,验证了所提故障区域识别方法具有准确率高和基本不受过渡电阻影响的特性。

混合级联特高压直流输电线路纵联差动保护适应性分析

为探究现有高压直流输电线路纵联差动保护对混合级联特高压直流输电系统的适应性,结合混合级联特高压直流输电系统的运行方式,理论分析和仿真分析3种特殊运行方式下直流系统不同位置故障的故障特性和保护适应性。理论分析发现纵联差动保护适应性较好。仿真分析发现直流线路分布电容和逆变侧模块化多电平型换流器(modular multilevelconverter,MMC)控制特性影响在直流线路故障时直流线路电流延时自减值长时间处于波动状态,导致纵联差动保护长时间处于闭锁状态,此时,系统长时间承受MMC超大故障电流冲击。直流线路区外故障时整流侧与逆变侧直流电流差值的绝对值数值较大且长时间处于波动变化状态,纵联差动保护存在误动的风险。

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.

特高压混合三端直流输电系统定功率MMC交流故障穿越策略

送端采用电网换相换流器(LCC),受端采用模块化多电平换流器(MMC)的特高压混合直流输电系统是直流输电技术重要的发展方向之一。在该系统中,各换流站每极均由高、低两个阀组直流侧串联而成。当其受端MMC发生网侧故障时,现有穿越策略或将引起定有功功率MMC阀间均压环节失效,从而导致子模块电容过压的问题。针对该现象,首先梳理了系统在故障工况下面临的主要问题和矛盾;接着,重点针对阀间均压环节失效的内部机理进行了研究,并提出了一种基于有功功率限幅值的站间协调穿越策略,此外,为使其能够较好地适用于不对称故障,采用了一种零序电压注入的方法,以平衡不对称故障下相单元间的桥臂电流直流分量;最后,基于PSCAD/EMTDC仿真平台搭建了相关模型,通过仿真对比,验证了本文策略能够兼顾不同受端站的桥臂过流和子模块过压风险,实现交流故障的可靠穿越,并尽可能地提升了故障期间的功率传输能力。

Economic feasibility of large-scale hydro–solar hybrid power including long distance transmission

Solar PV is expected to become the most cost-competitive renewable energy owing to the rapidly decreasing cost of the system. On the other hand, hydropower is a high-quality and reliable regulating power source that can be bundled with solar PV to improve the economic feasibility of long-distance transmitted power. In this paper, a quantification model is established taking into account the regulating capacity of the reservoir, the characteristics of solar generation, and cost of hydro and solar PV with long-distance transmission based on the installed capacity ratio of hydro–solar hybrid power. Results indicate that for hydropower stations with high regulating capacity and generation factor of approximately 0.5, a hydro–solar installed capacity ratio of 1:1 will yield overall optimal economic performance, whereas for hydropower stations with daily regulating capacity reservoir and capacity factor of approximately 0.65, the optimal hydro–solar installed capacity ratio is approximately 1:0.3. In addition, the accuracy of the approach used in this study is verified through operation simulation of a hydro–solar hybrid system including ultra high-voltage direct current(UHVDC) transmission using two case studies in Africa.

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.

Study on Short Circuit Current Calculation of Power System with UHVDC and New Energy Source

With the development of power system, the level of short circuit current will increase accordingly. In general, the influence of the HVDC system and the new energy source is not considered in the calculation of the short circuit current. For the power grid that short circuit current level closes to the interrupting capacity of circuit breaker, it’s necessary to fully consider all kinds of influence factors, careful checking, so as to obtain more accurate calculation results of short circuit current. In 2018, two ±800 kV high-power Ultra High Voltage Direct Current (UHVDC) transmission projects will be connected with Shaanxi power grid, accompanied by a lot of concomitant fossil-fuel generating plants as power resource, and also a large number of new energy source, includes wind power generation and photovoltaic power generation. Around one of the UHVDC converter stations, short circuit current may exceed the withstand limit of some certain circuit breakers. In order to get more accurate short circuit current calculation results, three measures are used: 1) contrastive calculation and analysis by algorithm based on schemes and algorithm based on power flow;2) analysis the influence of UHVDC by electromagnetic transient and electromechanical transient hybrid simulation;3) considered a detailed model of Doubly Fed Induction Generator (DFIG) with low voltage ride through characteristics. The calculation results shows that: in the typical operation mode of Shaanxi power grid of 2018, the original calculation results by conventional calculation method are coincident with the results by considering the influence of algorithms, UHVDC and DFIG in large, in which: the results of the algorithm based on power flow are smaller than that of the algorithm based on schemes about 2 - 8 kA;the steady values of the short circuit current provided by UHVDC converterstation (includes rectifiers and smoothing capacitors) are about 0 - 3 kA;the steady values of the short circuit current provided by DFIG are about 0 - 5 kA. The calculation results can provide reference for the selection of the circuit breaker, and it can be verified by fault recording data in the future.

特高压直流系统运行短路比检测方法及应用

弱交强直混联系统中直流系统自身故障引发的交直流连锁反应可能会影响整个区域电网的稳定运行。本文基于交直流混联系统等效电路提出交流系统等效阻抗和电源电压的计算方法,结合特高压直流换流站投切交流滤波器的过程,在直流控制器中实现对交流系统等效参数的实时检测。根据交流系统等效参数和直流换流站相关电气量,提出直流系统运行短路比的概念,用于定量分析交直流系统的强弱关系。最后针对典型弱交强直系统直流线路故障过程中的振荡问题进行分析,结合直流系统运行短路比优化直流控制参数,解决了故障过程中的振荡问题。

基于多层感知器的LCC-MMC并联型特高压三端混合直流输电线路故障检测方法探讨

针对现有故障检测方法存在阈值整定困难、特征量提取过程复杂的问题,提出一种基于多层感知器(MLP)的电网换相换流器和模块化多电平换流器(LCC-MMC)并联型特高压三端混合直流输电线路故障检测方法。首先,分析直流线路昆北侧边界、T区边界、龙门侧边界的频率特性,指出三端直流线路不同区域发生故障时的故障特征差异;其次,通过小波变换对线模电流、线模电压进行多尺度小波分解,提取小波能量,并结合正、负极电压变化量组成故障特征量。然后,将故障特征量作为MLP的输入量、故障区域作为输出量,构建基于MLP的故障区域识别模型;将测量点提取到的故障特征量输入训练完成的模型即可达到故障区域识别的目的。通过大量仿真验证了所提故障检测方法准确率高,且可耐受300Ω的过渡电阻。

昆柳龙特高压三端混合直流输电线路边界频率特性研究

为研究电网换相换流器和模块化多电平换流器并联型特高压三端混合直流线路行波边界保护,有必要研究电网换相换流器和模块化多电平换流器并联型特高压三端混合直流线路边界频率特性。特高压三端混合直流输电线路边界不对称,分析了昆北侧边界、柳北侧边界、昆柳段线路末端边界、柳龙段线路首端边界、龙门侧边界的拓扑结构。建立特高压多端混合直流输电线路边界频域模型,分析线路边界的频率特性,研究高频和中低频暂态信号在输电线路边界作用下的衰减特性。研究结果表明:昆北、柳北、龙门侧边界,对高频暂态信号呈高阻特性,有较强的衰减作用;昆柳段线路末端边界和柳龙段线路首端边界,对中低频暂态信号具有一定衰减作用,对高频暂态信号衰减较弱。

LCC-MMC混合直流输电系统单阀投入过程定有功MMC站电压平衡策略

送端采用常规直流(LCC)受端采用柔性直流(MMC)的混合特高压直流输电系统,在单阀在线投入过程中,定有功功率MMC站两串联阀组间电压可能出现失衡现象。分析了该不平衡现象的产生机理,随后提出一种基于调整阀间电压偏差值的电压平衡策略。该策略通过设定一个与预升压速率相同的函数分别对阀间电压的偏差值进行调整,并将其转化为有功功率偏差量。进而将该值与初始有功参考量相加后作为有功外环的参考值,最终实现阀组的在线平稳投入并过渡至稳态。基于PSCAD/EMTDC仿真平台搭建了相关模型,通过对比三种不同阀间电压平衡策略对系统动态特性的影响验证了文章所提策略的有效性。

FHMMC特高压直流输电系统中串联换流阀组启动充电策略

800k V昆柳龙直流工程在两个受端换流站采用对称双极高低端阀组串联的全半桥子模块混合型模块化多电平换流器。在不同的充电工况下,因全桥子模块(fullbridgesub-module,FBSM)和半桥子模块(halfbridge sub-module,HBSM)器件及其模块化串联的桥臂支路充电特性不同,造成两种子模块电容电压难以平衡,导致阀组及系统无法启动解锁。提出了特高压柔直系统中全半桥子模块混合式模块化多电平换流器(full and half bridge hybrid modular multilevel converter,FHMMC)串联阀组在直流侧不短接和短接两种方式下的启动充电策略,针对不同工况下相应导通FBSM T_(3)、T_(4)管和HBSM T_(2)管,强制转换充电路径和充电对象,实现两种子模块电容均衡充电。针对直流侧不短接时后充电阀组FBSM感应带负压问题,研究并提出了极充电顺控策略,通过缩短阀组充电时间间隔来减少后充电阀组负压幅值。上述启动充电策略经过现场试验验证并用于工程实际。

大规模风电接入的特高压混合级联直流系统送端频率控制策略

针对特高压混合直流系统的送端电网由于高比例风电接入带来的频率易越限的问题,提出了风电与混合直流系统协调参与的送端电网频率控制策略。首先,介绍了混合直流系统的拓扑,其中送、受端均采用电网换相换流器串联模块化多电平换流器的结构;其次,分别在风机和传统直流换流站中引入考虑一次调频特性与惯性特性的附加频率控制策略,在柔性直流换流站中引入虚拟惯性控制策略;设计了风电与混合直流系统各站协调参与送端电网调频的时序动作规则与各换流站之间的电压协调策略;同时,基于风电与直流并网系统的频率响应模型,定性分析了风电与混合直流系统参与调频的机理,提出了相关控制参数的设计方法;最后,通过PSCAD/EMTDC中建立模型进行仿真验证。仿真结果表明所提频率协调控制策略可以提高送端系统的惯量与一次调频能力,有效改善交流系统在不同运行工况下的频率稳定问题。