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.

基于交替迭代法的MMC-MTDC交直流混合系统的潮流分析

为解决大规模新型可再生能源的并网和消纳问题,多端柔性交、直流混合输电系统成为近年来研究的热点,且潮流计算分析又是研究其稳定控制及故障分析的基础.对此,建立了基于模块化多电平换流器的多端直流输电系统(MMC-MTDC)的稳态潮流计算数学模型,提出了MMC-MTDC交直流混合系统潮流计算的交替迭代求解法,对交流子系统采用传统的牛顿-拉夫逊计算方法,对直流子系统则采用节点阻抗矩阵形式的Gauss-Seidel方法.最后,在MATLAB软件平台编写交替迭代算法程序,以改进的IEEE 9和IEEE 22节点且包含三端MMC-MTDC的交直流混合系统为例进行仿真计算,结果表明所提出的交替迭代求解法具有收敛速度快、准确性高的优点.

MMC-HVDC系统结构对多端直流电网故障的影响分析

主要针对基于MMC的多端直流输电网直流故障特性进行研究。首先,对不同系统结构下换流站的稳态运行特性及短路故障电流路径进行研究,提出了不同MMC-HVDC系统结构直流接地故障电流路径的通用分析方法;然后,基于上述方法分析了不同接地方案对故障电流的影响,并提出了一种多端直流系统直流侧接地方案确定分析方法;最后,通过仿真模型对一个5端直流电网在不同系统结构下的接地与非接地直流故障响应进行了比较分析。研究结果说明了伪双极系统具有最好的单极故障电流特性,而直接接地系统受单极故障的影响最大;同时,仿真结果也说明了直流电抗对所有系统结构下的所有直流故障方式均具有良好的改善作用。

Preventing DC Over-voltage in Multi-terminal HVDC Transmission

This paper is concerned with power reduction control which is used to avoid DC over-voltage for multiterminal HVDC transmission of offshore wind power.Voltages and frequencies of offshore AC wind farm networks are used for transmitting control signals for the power reduction control.These methods do not require fast communication.Power reduction sharing among the offshore wind farms using the different control signals is analysed.The control systems are also compared against the DC chopper method to prevent a DC overvoltage.Simulation and experiments are carried out to evaluate the control systems.

Modeling,Control,and Protection of Modular Multilevel Converter-based Multi-terminal HVDC Systems:A Review

Multi-terminal direct current(MTDC)grids provide the possibility of meshed interconnections between regional power systems and various renewable energy resources to boost supply reliability and economy.The modular multilevel converter(MMC)has become the basic building block for MTDC and DC grids due to its salient features,i.e.,modularity and scalability.Therefore,the MMC-based MTDC systems should be pervasively embedded into the present power system to improve system performance.However,several technical challenges hamper their practical applications and deployment,including modeling,control,and protection of the MMC-MTDC grids.This paper presents a comprehensive investigation and reference in modeling,control,and protection of the MMC-MTDC grids.A general overview of state-of-the-art modeling techniques of the MMC along with their performance in simulation analysis for MTDC applications is provided.A review of control strategies of the MMC-MTDC grids which provide AC system support is presented.State-of-the art protection techniques of the MMCMTDC systems are also investigated.Finally,the associated research challenges and trends are highlighted.

Adaptive Reference Power Based Voltage Droop Control for VSC-MTDC Systems

Featuring low communication requirements and high reliability,the voltage droop control method is widely adopted in the voltage source converter based multi-terminal direct current(VSC-MTDC)system for autonomous DC voltage regulation and power-sharing.However,the traditional voltage droop control method with fixed droop gain is criticized for over-limit DC voltage deviation in case of large power disturbances,which can threaten stable operation of the entire VSCMTDC system.To tackle this problem,this paper proposes an adaptive reference power based voltage droop control method,which changes the reference power to compensate the power deviation for droop-controlled voltage source converters(VSCs).Besides retaining the merits of the traditional voltage droop control method,both DC voltage deviation reduction and power distribution improvement can be achieved by utilizing local information and a specific control factor in the proposed method.Basic principles and key features of the proposed method are described.Detailed analyses on the effects of the control factor on DC voltage deviation and imbalanced power-sharing are discussed,and the selection principle of the control factor is proposed.Finally,the effectiveness of the proposed method is validated by the simulations on a five-terminal VSC based high-voltage direct current(VSC-HVDC)system.

Analysis and Control of Modular Multi-terminal DC Power Flow Controller with Fault Current Limiting Function

In order to overcome the problems of power flow control and fault current limiting in multi-terminal high voltage direct current(MTDC)grids,this paper proposes a modular multi-terminal DC power flow controller(MM-DCPFC)with fault current limiting function.The topology structure,operation principle,and equivalent circuit of MM-DCPFC are introduced,and such a structure has the advantages of modularity and scalability.The power balance mechanism is studied and a hierarchical power balance control strategy is proposed.The results show that MM-DCPFC can achieve internal power exchange,which avoids the use of external power supply.The fault characteristics of MM-DCPFC are analyzed,fault current limiting and self-protection methods are proposed,and the factors affecting the current limiting capability are studied.The simulation models are established in PLECS,and the simulation results verify the effectiveness of MM-DCPFC in power flow control,fault current limiting,and scalability.In addition,a prototype is developed to validate the function and control method of MM-DCPFC.

不同调度模式下VSC-MTDC系统的协调控制策略

针对电力系统3种调度模式对直流电网实时的潮流分布要求,在多端柔性直流输电(voltage source converter multiterminal direct current,VSC-MTDC)系统中设计了相应的协调控制策略,通过改变下垂系数的值和切换控制模式,以满足直流电网中的潮流分布。为避免控制模式的频繁切换,在受端换流站控制器中加入了滞环比较器。通过在PSCAD/EMTDC仿真软件中搭建的四端柔性直流输电系统,对直流电网处在不同的调度模式下分别进行仿真,结果表明,与定下垂系数的控制方式相比,该协调控制策略可以满足电力系统调度对潮流实时的分布要求,直流电网的输电灵活性得到提高。

多端直流架构的交直流混合配电网三相不平衡潮流计算

在配电网向交直流混合供电的方向发展的新形势下,提出一种能够计及交流三相不平衡的交直流配电网潮流计算方法。首先建立了电压源型换流器的三相稳态潮流模型,推导了换流器的三相潮流以及直流配电网潮流计算方程,并对不平衡系统进行了补偿。考虑了不同的控制方式及适用于配电网的多端直流控制策略,在此基础上,提出了适合配电网的三相不平衡交直流交替迭代潮流计算方法,该方法采用了模块化思想,不需要全局迭代,可以从交、直流侧任意方向开始潮流计算,自动修正越限变量并调用控制策略,实现了交直流潮流计算的解耦。最后,对修改的IEEE 37节点配电系统进行仿真计算,仿真结果证明了算法的有效性、快速性和准确性。

直流潮流控制器对多端直流系统的运行网损分析

直流潮流控制器可有效增强复杂直流系统中的潮流可控性,对直流电网运行网损具有一定的调节能力。为统一描述和比较各类直流潮流控制器的外特性,建立了基于等效变比M的外特性模型。首先以三端环网直流系统场景为例,结合上述变比模型,合理分析并比较不同直流潮流控制器对该直流系统运行损耗的影响。分析表明,通过对直流潮流控制器合理选型并设定优化控制目标,单个直流潮流控制器和多个直流潮流控制器分别可使系统运行损耗最多降低0.0002%和1.81%。并针对一个四端直流输电系统场景,建立反应特定三线间直流潮流控制器对系统运行网损影响的定量模型,该场景下网损最大降幅为3.35%。最后,针对具体的两个典型场景在PLECS环境下搭建仿真模型,验证了上述模型的有效性和稳定性。

A novel control strategy based on a look-up table for optimal operation of MTDC systems in post-contingency conditions

Multi terminal VSC-HVDC systems are a promising solution to the problem of connecting offshore wind farms to AC grids.Optimal power sharing and appropriate control of DC-link voltages are essential and must be maintained dur-ing the operation of VSC-MTDC systems,particularly in post-contingency conditions.The traditional droop control methods cannot satisfy these requirements,and accordingly,this paper proposes a novel centralized control strategy based on a look-up table to ensure optimal power sharing and minimum DC voltage deviation immediately during post-contingency conditions by considering converter limits.It also reduces destructive effects(e.g.,frequency devia-tion)on onshore AC grids and guarantees the stable operation of the entire MTDC system.The proposed look-up table is an array of data that relates operating conditions to optimal droop coefficients and is determined according to N-1 contingency analysis and a linearized system model.Stability constraints and contingencies such as wind power changes,converter outage,and DC line disconnection are considered in its formation procedure.Simulations performed on a 4-terminal VSC-MTDC system in the MATLAB-Simulink environment validate the effectiveness and superiority of the proposed control strategy.