Review of the Configuration and Transient Stability of Large-scale Renewable Energy Generation through Hybrid DC Transmission

Based on the complementary advantages of Line Commutated Converter(LCC)and Modular Multilevel Converter(MMC)in power grid applications,there are two types of hybrid DC system topologies:one is the parallel connection of LCC converter stations and MMC converter stations,and the other is the series connection of LCC and MMC converter stations within a single station.The hybrid DC transmission system faces broad application prospects and development potential in large-scale clean energy integration across regions and the construction of a new power system dominated by new energy sources in China.This paper first analyzes the system forms and topological characteristics of hybrid DC transmission,introducing the forms and topological characteristics of converter-level hybrid DC transmission systems and system-level hybrid DC transmission systems.Next,it analyzes the operating characteristics of LCC and MMC inverter-level hybrid DC transmission systems,provides insights into the transient stability of hybrid DC transmission systems,and typical fault ride-through control strategies.Finally,it summarizes the networking characteristics of the LCC-MMC series within the converter station hybrid DC transmission system,studies the transient characteristics and fault ridethrough control strategies under different fault types for the LCC-MMC series in the receiving-end converter station,and investigates the transient characteristics and fault ride-through control strategies under different fault types for the LCC-MMC series in the sending-end converter station.

MMC-MTDC Transmission System with Partially Hybrid Branches

This paper proposes a hybrid submodule modular multilevel converter(MMC)topology which is suitable for multi terminal direct current(MTDC)transmission systems.Each arm of the proposed MMC topology consists of a half-bridge submodule(HBSM)branch and two parallel full-bridge submodule(FBSM)branches.Comparing with the conventional MTDC transmission system,the proposed topology can selectively block the DC fault current and isolate the corresponding fault line without expensive DC circuit breakers(DCCBs).Thus,the influence range of the DC fault can be reduced and the reliability of the power supply can be improved as well.The corresponding modulation and voltage balancing strategies are developed for the proposed hybrid MMC topology.The feasibility of the proposed topology and control strategy is verified in the MATLAB/Simulink simulation.

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.

基于NLESO的交直流微电网双向AC/DC换流器改进滑模稳压控制

针对交直流混合微电网中双向AC/DC换流器在外界扰动下出现的直流母线电压波动问题,设计了一种应用于双向AC/DC换流器的非线性扩张状态观测器(nonlinear extended state observer,NLESO),以实现对分布式电源功率波动和负荷投切变化等不确定因素的快速追踪与补偿,保证了在不同扰动下交直流混合微电网的稳定性。进一步提出了基于NLESO的改进积分滑模控制方法,提高了直流母线电压的控制精度。结合非线性光滑函数设计了滑模趋近律,消除了传统滑模控制中的高频抖振现象。通过Lyapunov理论对系统的稳定性进行分析验证,仿真结果表明该控制方法响应速度快、控制精度高、抗扰动能力强并且无抖振现象。

MMC-HVDC交直流混联系统小扰动稳定域迭代求解和形态分析

随着高压直流输电技术的发展,区域电力系统将拓展为大规模交直流混联系统。因此,需要建立可以同时精确刻画交直流动态的大规模交直流混联系统模型来研究混联系统的动态与稳定性。文中考虑了交流侧和直流侧的详细动态,建立了异步联网的模块化多电平换流器型高压直流(MMC-HVDC)交直流混联系统的状态空间模型,给出了具体的解析解形式,并推导了多重迭代信赖域算法。所提算法建立了决策变量空间内的点与平衡点的映射关系,结合Hopf分岔识别边界,提出了一种多重迭代求解小扰动稳定域边界的方法。在利用Simulink仿真验证模型准确性后,以发电机出力作为决策变量绘制了交直流混联系统稳定域,利用特征根和参与因子定量研究了直流失稳和交流失稳的机理以及控制策略和控制参数对稳定域边界的影响。对比分析了多区域的混联系统与交流系统的稳定域,结果表明换流站和直流网络的引入可能使多区域系统稳定域边界呈现多约束共同决定的近似分段线性拓扑特性。

Research on power electronic transformer applied in AC/DC hybrid distribution networks

The AC/DC hybrid distribution network is one of the trends in distribution network development, which poses great challenges to the traditional distribution transformer. In this paper, a new topology suitable for AC/DC hybrid distribution network is put forward according to the demands of power grid, with advantages of accepting DG and DC loads, while clearing DC fault by blocking the clamping double sub-module(CDSM) of input stage. Then, this paper shows the typical structure of AC/DC distribution network that is hand in hand. Based on the new topology, this paper designs the control and modulation strategies of each stage, where the outer loop controller of input stage is emphasized for its twocontrol mode. At last, the rationality of new topology and the validity of control strategies are verified by the steady and dynamic state simulation. At the same time, the simulation results highlight the role of PET in energy regulation.

Novel Double-Damped Tuned AC Filters in HVDC Systems

This paper presents a performance analysis of novel doubledampedtuned alternating current (AC) filters in high voltage direct current(HVDC) systems. The proposed double-damped tuned AC filters offer theadvantages of improved performance of HVDC systems in terms of betterpower quality, high power factor, and lower total harmonic distortion (THD).The system under analysis consists of an 878 km long HVDC transmissionline connecting converter stations at Matiari and Lahore, two major cities inPakistan. The main focus of this research is to design a novel AC filter usingthe equivalent impedance method of two single-tuned and double-dampedtuned AC filters. Additionally, the impact of the damping resistor on the ACchannel is examined. TheTHDof theHVDCsystem with and without currentAC filters was also compared in this research and a double-damped tuned ACfilter was proposed. The results of the simulation represent that the proposeddouble-damped tuned AC filter is far smaller in size, offers better powerquality, and has a much lower THD compared to the AC filters currently inplace in the converter station. The simulation analysis was carried out utilizingpower systems computer-aided design (PSCAD) software.

Recent developments in HVDC transmission systems to support renewable energy integration

The demands for massive renewable energy integration, passive network power supply, and global energy interconnection have all gradually increased, posing new challenges for high voltage direct current(HVDC) power transmission systems, including more complex topology and increased diversity of bipolar HVDC transmission. This study proposes that these two factors have led to new requirements for HVDC control strategies. Moreover, due to the diverse applications of HVDC transmission technology, each station in the system has different requirements. Furthermore, the topology of the AC-DC converter is being continuously developed, revealing a trend towards hybrid converter stations.

混合微网中双向AC/DC变换器研究与设计

在交直流混合电网中,AC/DC变换器既要实现直流微网与上级交流电网的连接,也要成为风力发电、燃气轮机发电等交流型分布式能源的并网接口。基于此,首先建立交直流混合微网中AC/DC变换器模型;其次采用PID闭环控制,实现配电系统的稳定,并在系统中增设可控出力的蓄电池双向功率变换环节,实现功率平衡;最后采用SVPWM进行逆变控制策略,降低谐波畸变率,进一步保证系统稳定性。

混合级联型HVDC系统的后续换相失败预判方法

白鹤滩—江苏±800k V特高压直流输电工程于2022年7月1日在中国投运,在世界上首次创造性地研发应用了特高压混合级联直流输电新技术方案。基于此系统背景对逆变侧电网换相换流器(line commutated converter,LCC)进行具有一定时间裕度的后续换相失败预判。分析了LCC控制器对电气量的控制作用和模块化多电平换流器(modular multilevel converters,MMC)对LCC的电压支撑作用,明确了混合级联直流输电系统逆变侧LCC发生后续换相失败的机理。推导了逆变侧LCC在首次换相失败后恢复过程末期关断角与故障后线路传输功率最小值的关系,定义了阈值功率的概念。然后建立了关断角与阈值功率的定量关系,提出一种故障后直流传输功率最小值与功率阈值相比较的后续换相失败预判方法。最后在PSCAD/EMTDC中搭建了相应的仿真模型,验证了理论分析和预测方法的准确性。

基于改进过压限制器和ACDTS的混联半波长系统故障保护策略

交直流混联半波长输电系统的暂态特性与常规的混联系统存在着很大差异。当混联系统半波长沿线发生不对称故障时,直流系统会受到沿线工频过电压和过电流的冲击。文章详细分析了混联系统半波长沿线故障的传播过程,研究了故障传播耦合机理。针对直流侧过电压问题,提出了在直流母线侧加装改进过压限制器,增设辅助限压通道,分级抑制直流侧过电压,并降低单个通道的电流值,提高了系统保护的限压水平;另一方面,对于直流侧过电流问题,提出了在换流站接口端并联接入双向晶闸管,实现交流系统等效三相短接,隔离交流侧母线馈入的故障过电流,确保换流站设备及线路的安全性。最后利用仿真验证了两种保护策略的可行性。

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.

基于切换模型的双向AC-DC变换器控制策略

双向AC-DC变换器作为交直流混合微电网的重要组成部分,对系统的稳定运行和功率的协调分配有着重要作用。本文基于切换系统理论,提出一种双向AC-DC切换控制方法。该方法首先建立双向AC-DC切换动态模型,选取系统的储能函数作为共同Lyapunov函数。基于此,设计了系统最优切换律并分析了该切换律条件下系统在切换平衡点处的稳定性。为了便于控制器数字化实现,建立了切换系统单步预测模型并对切换策略离散化。仿真和实验结果验证了本文所采用的建模方法和控制策略的有效性。

AC/DC混合输电系统分散协调控制

针对电压源型高压直流输电(VSC-HVDC)在应用中带来的AC/DC交直流输电系统的协调控制问题,建立了AC/DC数学模型,并对AC/DC提出了一种新的分散最优协调控制模式。在该模式中,逆系统线性化理论被应用于直流输电系统的控制。文中为了与交流电网相配合,将标幺制引入了直流输电系统,并以Lyapunov间接法证明了该闭环非线性系统的稳定性。AC/DC混合输电系统的数值仿真结果表明了该控制策略的有效性和鲁棒性。

基于双向DC/AC变换器的混合储能系统动态控制策略

针对风电和光伏并网发电系统的功率波动问题,研究了一种基于双向DC/AC变换器的混合储能系统的动态控制策略。对含有超级电容器与蓄电池组的混合储能系统,通过双闭环控制器对变换器内部的电压电流进行控制,把波动变化较快的电流分量分配给超级电容器,由蓄电池来响应波动变化较慢的电流分量。同时,控制系统将超级电容器的电压稳定在预设范围内。基于扩展卡尔曼滤波(Extended Kalman Filter,EKF)对蓄电池的荷电状态(State of Charge,SOC)进行控制,使其SOC值稳定在安全范围内并延长了蓄电池的使用寿命。通过仿真实验,验证了控制方法的有效性。

电流畸变时AC/DC双向功率变换器的控制策略

在交直流混合微电网中,通过电力电子接口接入的可再生能源和非线性负载会使交流母线电流发生畸变。因此,要求AC/DC双向功率变换器不仅能可靠变换功率以保证交直流子网功率平衡,而且能抑制交流母线谐波电流。该文基于Fryze-Buchholz-Depenbrock(FBD)功率理论的谐波电流检测法,提出一种可以治理交流母线谐波电流的AC/DC双向功率变换器控制策略。该控制策略在实现交直流子网功率传输及母线电压稳定的基础上,还可以通过AC/DC双向功率变换器治理交流母线谐波电流,保证微电网具有良好的电能质量。最后通过仿真和实验验证了该控制策略的有效性。

交直流混合微电网中双向AC/DC变换器模型预测控制策略

基于模型预测控制理论,提出一种应用于交直流混合微电网的双向AC/DC变换器控制策略,用来平衡交直流子网间的能量流动,使系统能够安全可靠运行。为此,建立双向AC/DC变换器的动态模型,在变换器电压空间矢量有限的条件下,根据系统离散时间模型来预测系统下一采样时刻的输出值,利用价值函数确定开关管的状态。仿真结果表明,所提出的控制策略可行、有效。

交直流微电网AC/DC双向功率变换器控制策略

为了实现交直流混合微电网的可靠并网,基于微电网中AC/DC双向功率变换器下垂控制策略和预同步工作原理,提出一种适用于混合微电网中连接交直流子网的AC/DC双向功率变换器的控制策略。孤岛/并网模式时采用双向下垂控制实现双向功率流动。在由孤岛模式转为并网模式时,利用消除dq轴电压偏差实现幅值与相位同步,无需通过锁相环获取相位信息,实现平滑并网。同时,针对微电网中由于不平衡负载导致的三相不平衡工况,采用正负序分别控制的方法实现了非理想工况下微电网的同步互联。仿真结果验证了该方案的可行性。

孤立交直流混合微电网双向AC/DC换流器功率控制与电压波动抑制策略

在孤立交直流混合微电网中,对双向AC/DC换流器进行合理控制可以有效实现微网功率的协调分配并提高系统的抗外界干扰能力。将孤立混合微电网中的交流微网和直流微网等效为整合电源,分别给出双向AC/DC换流器交流侧和直流侧的有功功率下垂控制方程,进而提出基于全网功率成比例分配原则的双向AC/DC换流器外环功率控制策略,实现孤立混合微电网的功率平衡和自主分配;同时针对常规比例-积分(proportionalintegral,PI)内环控制无法获得理想的电压动态响应的问题,在对双向AC/DC换流器进行电压波动分析的基础上,提出一种抑制电压波动的双向AC/DC换流器改进内环控制策略,实现换流器电压对参考信号的无静差跟踪,提高下垂控制微电网系统的鲁棒稳定性。仿真结果验证了该控制策略的正确性和有效性。