A VSC-based Model for Power Flow Assessment of Multi-terminal VSC-HVDC Transmission Systems

This paper puts forward a new practical voltage source converter(VSC)based AC-DC converter model suitable for conducting power flow assessment of multi-terminal VSCbased high-voltage direct current(VSC-MTDC)systems.The model uses an advanced method to handle the operational limits and control modes of VSCs into the power flow formulation.The new model is incorporated into a unified framework encompassing AC and DC power grids and is solved by using the Newton-Raphson method to enable quadratically convergent iterative solutions.The use of complementarity constraints,together with the Fischer-Burmeister function,is proposed to enable the seamless incorporation of operational control modes of VSC and automatic enforcement of any converter’s operational limits that become violated during the iterative solution process.Thus,a dedicated process for checking limits is no longer required.Furthermore,all existing relationships between the VSC control laws and their operational limits are considered directly during the solution of the power flow problem.The applicability of the new model is demonstrated with numerical examples using various multi-terminal AC-DC transmission networks,one of which is a utility-sized power system.

A review of the protection for the multi-terminal VSC-HVDC grid

The multi-terminal VSC-HVDC grid is believed to be widely applied in the future power system. The dc line protection is the key technique for operation security and power supply reliability of the dc grid. In this paper, the single-ended protections, namely, the traveling-wave based protection and transient-variable based protection, as well as the pilot protections, mainly including the directional pilot protection and current differential protection, are discussed in detail. With the analyzed protections, the effective main and back-up protection strategy can be configured for the dc line in multi-terminal VSC-HVDC grid.

Architecture of Nan'ao Multi-terminal VSC-HVDC System and Its Multi-functional Control

This paper presents a framework of a multi-terminal HVDC transmission system and its multi-functional control strategy.The framework possesses the basic characteristics of the DC-grid and is suitable in integrating distributed power sources.The paper proposes the first architecture for a multiterminal HVDC transmission system using the VSC technology.Its control strategy offers various functionalities that include controls for operation mode,start-up and shutdown,DC voltage,and station online re-connecting,which are significantly different from the control of point-to-point VSC-HVDC systems.The framework has not only been evaluated in real-time simulation studies,but has also been implemented onsite for the first time via the China Southern Grid's Nan'ao Multi-terminal VSCHVDC(VSC-MTDC)project.This paper gives a brief review of the current research and engineering achievements in this field,which includes four aspects:the architecture of the VSCMTDC system,the structure of the control and protection system,simulation verification tests setting,and the results of real-time hardware in hardware in loop(HIL)simulation studies and onsite tests.

A Comprehensive Power Flow Approach for Multi-terminal VSC-HVDC System Considering Cross-regional Primary Frequency Responses

For the planning,operation and control of multiterminal voltage source converter(VSC)based high-voltage direct current(HVDC)(VSC-MTDC)systems,an accurate power flow formulation is a key starting point.Conventional power flow formulations assume the constant frequencies for all asynchronous AC systems.Therefore,a new feature about the complex coupling relations between AC frequencies,DC voltages and the exchanged power via VSC stations cannot be characterized if VSC-MTDC systems are required to provide cross-regional frequency responses.To address this issue,this paper proposes a comprehensive frequency-dependent power flow formulation.The proposed approach takes the frequencies of asynchronous AC systems as explicit variables,and investigates the novel bus models of the interlinking buses of VSC stations.The proposed approach accommodates different operation modes and frequency droop strategies of VSC stations,and considers the power losses of VSC stations.The effectiveness and generality of the developed approach are validated by a 6-terminal VSC-HVDC test system.The test system presents the characteristics of the coexistence of numerous VSC operation modes,the absence of slack buses in both AC and DC subsystems,and diversified grid configurations such as point-to-point integration of renewable energy sources and one AC system integrated with multiple VSC stations.

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.

基于VSC-HVDC的电力系统频率稳定控制策略

针对电力系统在典型故障与扰动下表现出的“暂态失稳”现象,提出了一种基于VSC-HVDC的电力系统频率稳定控制方法。考虑某大规模发电厂通过VSC-HVDC连接至交流电网,该方法允许此发电厂向交流电网提供频率控制服务。该控制策略包含整流器对直流电压的控制以及逆变器对频率的控制,即逆变器通过交流电网频率的变化来调节直流电压的变化,而整流器通过直流电压的变化来控制发电厂有功的投放,以补偿交流电网有功的短缺,使电网在故障恢复的初始阶段更好地提高系统稳定性。通过在Power Factory Digsilent上进行仿真,验证了该控制策略能显著增强系统的稳定性。

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.

Transport properties in a multi-terminal regular polygonal quantum ring with Rashba spin-orbit coupling

Transport properties in a multi-terminal regular polygonal quantum ring with Rashba spin-orbit coupling （SOC） are investigated analytically using quantum networks and the transport matrix metLod. The results show that conduc- tances remain at exactly the same values when the output leads are located at axisymmetric positions. However, for the nonaxisymmetrical case, there is a phase difference between the upper and lower arm, which leads to zero conductances appearing periodically. An isotropy of the conductance is destroyed by the Rashba SOC effect in the axisymmetric case. In addition, the position of zero conductance is regulated with the strength of the Rashba SOC.

Polarization resolved analysis of phonon transport in a multi-terminal system

The atomistic Green’s function method is improved to compute the polarization resolved phonon transport in a multiterminal system. Based on the recent developments in literature, the algorithm is simplified. The complex phonon band structure of a semi-infinite periodic terminal is obtained by the generalized eigenvalue equation. Then both the surface Green’s function and phonon group velocity in the terminal are determined from the wave modes propagating away from the scattering region along the terminal. With these key ingredients, the individual phonon mode transmittance between the terminals can be calculated. The feasibility and validity of the method are demonstrated by the chain example compared with the wave packet method, and an example of graphene nanojunction with three terminals.

基于VSC-HVDC的统一附加阻尼最优控制方法

相比于传统发电机的电力系统稳定器(PSS),柔性直流输电(VSC-HVDC)系统可以有效控制区域间的低频振荡。考虑到VSC-HVDC系统有功无功均能对交流系统进行独立控制,提出了基于有功无功双环控制的统一低频振荡控制器,并基于VSC的有功功率和无功功率控制分别设计附加控制回路来增加控制器的控制效果。首先采用最小二乘旋转不变(TLS-ESPRIT)辨识方法获得系统小信号模型和振荡特性,然后基于辨识出的模型,利用基于最优控制理论对控制器进行设计,并将最终所设计控制器分别附加于有功控制回路与无功控制回路。PSCAD的仿真结果表明,相比于常规的有功附加控制器,所提统一控制方法能达到更好的控制效果,并能保证鲁棒性。

Stability Analysis of a Multi Area System with Renewable Energy and Multi-Terminal DC Tie Lines

Security constrained multi area multi objective dynamic economic dispatch (SCMAMODED) with renewable energy (RE) and all the possible MTDC stability constraints is formulated for the first time. The stability merits of multi terminal DC (MTDC) tie lines as compared to the traditional HVAC forms the main objective of this paper. Probabilistic load flow (PLF) is applied to determine the system parameters while the uncertainties are modelled using Scenario Based Method (SBM). The simulation results reveal that with the use of MTDC tie lines, the frequency and voltage stability in the MAMODED with renewable energy sources (RES) are enhanced while keeping the MTDC power exchange interface nodes at secure levels.

Analysis of Additional Damping Control Strategy and Parameter Optimization for Improving Small Signal Stability of VSC-HVDC System

The voltage source converter based high voltage direct current(VSC-HVDC)system is based on voltage source converter,and its control system is more complex.Also affected by the fast control of power electronics,oscillation phenomenon in wide frequency domain may occur.To address the problem of small signal stability of the VSCHVDC system,a converter control strategy is designed to improve its small signal stability,and the risk of system oscillation is reduced by attaching a damping controller and optimizing the control parameters.Based on the modeling of the VSC-HVDC system,the general architecture of the inner and outer loop control of the VSCHVDC converter is established;and the damping controllers for DC control and AC control are designed in the phase-locked loop and the inner and outer loop control parts respectively;the state-space statemodel of the control system is established to analyze its performance.And the electromagnetic transient simulation model is built on the PSCAD/EMTDC simulation platform to verify the accuracy of the small signal model.The influence of the parameters of each control part on the stability of the system is summarized.The main control parts affecting stability are optimized for the phenomenon of oscillation due to changes in operation mode occurring on the AC side due to faults and other reasons,which effectively eliminates system oscillation and improves system small signal stability,providing a certain reference for engineering design.

连接海上风电场的基于直接功率控制的三电平VSC-HVDC

研究了连接海上风电场的基于电压源换流器的VSC-HVDC系统。多电平逆变器应用于VSC-HVDC系统提高系统传输容量并改善电能质量问题。利用基于功率控制的方法对VSC-HVDC系统进行了仿真,实现了双侧的独立控制,风电场侧由风机带换流器组成,采用基于滞环比较的功率控制,而系统侧三电平换流器采用基于直接功率控制,三个滞环控制器分别控制有功功率、无功功率和中点电压。仿真结果证实了基于此算法的VSC-HVDC系统连接大型海上风电场,可以正常稳定地运行。系统侧三电平换流器降低谐波含量并提高功率因数。

基于端口能量的含VSC-HVDC的交直流混合系统暂态稳定评估

提出一种基于端口能量的含电压源换流器型高压直流输电(VSC-HVDC)的交直流混合系统暂态稳定评估方法。将端口能量描述为端口状态变量和控制变量的函数,进而借助端口能量表征VSC-HVDC系统的暂态能量聚集效应,以避免VSC-HVDC内部复杂控制过程的建模,降低能量函数的构造难度,且可扩展到网络中任意含电力电子器件的端口;进一步,构建基于端口能量的全系统能量函数,通过迭代势能边界面法(IPEBS)进行暂态稳定评估,借助点积判据在系统失稳的转子运动轨迹上搜索势能最大值作为临界能量,确定极限切除时间,实现暂态稳定的快速判别;然后,研究VSC-HVDC接入及VSC-HVDC有功功率、无功功率传输容量对系统暂态稳定性的影响,并评估了不同故障位置对所提能量函数计算精度的影响。最后,通过对修改的IEEE-39节点系统、IEEE-68节点系统和厦门电网柔直工程算例进行仿真分析,验证该方法的准确性和有效性。

基于VSC-HVDC联网的风电场故障穿越控制策略研究

交流电网发生故障扰动时,基于VSC-HVDC联网的风电场并网系统应具有一定的故障穿越能力。本文分析了基于VSC-HVDC的风电场并网系统的稳态控制策略及运行特性,对于交流电网发生故障扰动的情况,提出了一种基于风电机组惯性支持的故障穿越方法。该方法根据VSC-HVDC直流电压的波动量,调节风电场出口的频率,使风机输入的机械功率暂时以发电机转子动能的形式储存,从而减少了风电场输出的电磁功率,提高了系统的故障穿越能力。通过对由不同类型的风电机组组成的风电场经VSC-HVDC并网的系统进行仿真分析,证明了该方法能够在电网电压跌落后快速响应,防止VSC-HVDC直流过电压,提高了系统的故障穿越能力。

采用dq0坐标的VSC-HVDC稳态模型与控制器设计

基于电压源型换流器(VSC)的新型高压直流(VSC—HVDC)输电系统具有非常广阔的应用前景,目前VSC—HVDC技术在国外处于理论研究向实验系统过渡阶段,而国内则刚刚起步,许多基础理论和相关的应用基础问题有待深入研究。文中首先介绍了VSC—HVDC的基本工作原理。随后提出了一种采用dq0坐标的稳态模型,并进行了控制方案设计。通过对该控制方法原理分析与仿真实验,证明其控制效果良好,对于各种扰动都具有很快的响应速度和很好的稳定性,在不同的工作点都具有较高的稳定精度。

VSC-HVDC系统起动过程控制及仿真

对基于电压源换流器(VSC)的直流输电(VSC-HVDC)系统的起动过程控制策略进行了研究。根据VSC起动过程的特点,设计了对VSC-HVDC系统模型起动的控制器,为避免起动过程的过电流采用了一定的限流措施。仿真结果表明所设计的控制器具有良好的控制性能。

含VSC-HVDC的交直流系统电压稳定分析与控制

针对含柔性直流输电(voltage source converter-high voltage direct current,VSC-HVDC)的交直流系统电压稳定性问题,开发出一种适用于含VSC-HVDC的交直流系统连续潮流程序,以负荷裕度指标分析VSC-HVDC系统有功功率传输方向、有功功率传输容量及换流站与交流系统无功功率交换量的变化对系统电压稳定性的影响。进而利用系统电压崩溃点处雅可比矩阵,推导出系统负荷裕度对VSC-HVDC有功功率控制参数和无功功率控制参数的灵敏度,并基于该灵敏度提出一种适用于改善系统电压稳定性的VSC-HVDC调控策略。最后,将所提方法应用于修改的68节点和IEEE118节点交直流系统,仿真结果验证了所提方法的正确性和有效性。

VSC-HVDC系统控制体系框架

控制系统是VSC-HVDC系统的核心之一。目前虽已有许多研究者在该领域做了大量的工作,但多数成果集中在VSC-HVDC控制器的设计和控制方法的研究上,对整个VSC-HVDC系统控制体系框架的研究并不多见。本文在介绍VSC-HVDC系统结构的基础上,阐述了VSC-HVDC控制系统的基本功能,指出采用多重化配置和分层控制结构设计的必要性,具体提出建立四层结构的VSC-HVDC系统控制体系框架的观点,即直流系统控制层、换流器控制层、阀组控制层和独立控制层。文章在讲述各层配置原则的基础上,详细阐述了每个控制层次的控制功能和范围以及各层次之间的相互关系,并进一步介绍了控制系统和保护功能的协调配合问题,为VSC-HVDC控制系统的开发奠定了基础。

基于粒子群优化算法的VSC-HVDC系统的控制参数优化策略

建立了双端基于电压源型换流器的高压直流输电(VSC-HVDC)系统的小信号模型;利用特征根分析法,基于线性化状态空间模型,将与系统特征根直接相关的振荡模式和衰减模式的罚函数作为目标函数;提出了基于粒子群优化算法的VSC-HVDC系统的控制参数优化策略,对整个系统的控制参数同时进行整体优化。仿真结果验证了小信号模型的正确性;优化后的系统在小扰动、大扰动、潮流反转及故障情况下均具有较高的控制精度,整个系统的稳态与暂态特性均得到较大改善。