Theoretical and experimental studies on critical time delay of multi-DOF real-time hybrid simulation

This paper aims to investigate the critical stability of a multi-degree-of-freedom(multi-DOF)real-time hybrid simulation(RTHS).First,the critical time-delay analysis models are developed using the continuous-and discrete-time root locus(RL)techniques,respectively.A bilinear transform is introduced into the first-order Padéapproximation while conducting the discrete RL analysis.Based on this technique,the time delay can be explicitly used as the gain factor and thus the instability mechanism of the multi-DOF RTHS system can be analyzed.Subsequently,the critical time delays calculated by the continuous-and discrete-time RL techniques,respectively,are compared for a 2-DOF RTHS system.It is shown that assuming the RTHS system to be a continuous-time system will result in overestimating the critical time delay.Finally,theoretically calculated critical delays are demonstrated and validated by numerical simulation and a set of RTHS experiments.Parametric analysis provides a glimpse of the effects of time step,frequency and damping ratio in a performing partitioning scheme.The constructed analysis model proves to be useful for evaluating the critical time delay to predict stability and performance,therefore facilitating successful RTHS.

Optimum line reclosing time for the enhancement of interconnected power system stability

Automatic line reclosing schemes used in an extra-high-voltage power system is an economical and effective means to maintain transient stability. A novel method is proposed in the paper to adaptively optimize the automatic line reclosing time after a transient fault for enhancement of interconnected power system transient stability. Both the study on the transient energy over network and the structure-preserving multi-machines power system model illustrate that the excessive convergence of potential energy on the lines with a certain cutset deteriorate power system stability, and therefore, an optimum line reclosing strategy can be established by minimizing the change in transient potential energy distribution across a cutset lines in the vicinity of the faulty line as an optimization target, and the optimal reclosure time is set to the time of minimum line phase angle difference. Without any pre-determined knowledge, the method is adaptive to various power system operation modes and fault conditions, and easy to implement because only a limited number of data measured at one location on a tie-line linking sub-networks are required. Simulations have been performed with the OMIB(One Machine and Infinite Bus System) and a real inter-connected power system to verify the applicability of the method proposed.

Critical dwell time of switched linear systems

In this paper, we consider the relation between the switching dwell time and the stabilization of switched linear control systems. First of all, a concept of critical dwell time is given for switched linear systems without control inputs, and the critical dwell time is taken as an arbitrary given positive constant for a switched linear control systems with controllable switching models. Secondly, when a switched linear system has many stabilizable switching models, the problem of stabilization of the overall system is considered. An on-line feedback control is designed such that the overall system is asymptotically stabilizable under switching laws which depend only on those of uncontrollable subsystems of the switching models. Finally, when a switched system is partially controllable （While some switching models are probably unstabilizable）, an on-line feedback control and a cyclic switching strategy are designed such that the overall system is asymptotically stabilizable if all switching models of this uncontrollable subsystems are asymptotically stable. In addition, algorithms for designing switching laws and controls are presented.

Transient Stability Analysis of Grid-connected Converters in Wind Turbine Systems Based on Linear Lyapunov Function and Reverse-time Trajectory

As the proportion of converter-interfaced renewable energy resources in the power system is increasing,the strength of the power grid at the connection point of wind turbine generators(WTGs)is gradually weakening.Existing research has shown that when connected with the weak grid,the stability of the traditional grid-following controlled converters will deteriorate,and they are prone to unstable phenomena such as oscillation.Due to the limitations of linear analysis that cannot sufficiently capture the stability phenomena,transient stability must be investigated.So far,standalone time-domain simulations or analytical Lyapunov stability criteria have been used to investigate transient stability.However,the time-domain simulations have proven to be computationally too heavy,while analytical methods are difficult to formulate for larger systems,require many modelling assumptions,and are often conservative in estimating the stability boundary.This paper proposes and demonstrates an innovative approach to estimating the transient stability boundary via combining the linear Lyapunov function and the reverse-time trajectory technique.The proposed methodology eliminates the need of time-consuming simulations and the conservative nature of Lyapunov functions.This study brings out the clear distinction between the stability boundaries with different post-fault active current ramp rate controls.At the same time,it provides a new perspective on critical clearing time for wind turbine systems.The stability boundary is verified using time-domain simulation studies.

Transient Stability Simulation of 33 kV Power Grid

An example of Sandakan power grid problem is presented in this paper. Sandakan is a suburb in east coast of Sabah state of Malaysia. Stability problem occurs due to the increase in load demand, lack of generation sources and inadequate supply. The tripping disturbances occur frequently in the network which is contributing to voltage instability. In this paper dynamics stability of 33 kV power grid as related to Sandakan network is analyzed and simulated. The analysis is completed by modelling the network data in Power System Simulation for Engineering (PSS/E) software and simulate the transient stability of generator, exciter and governor during a three phase fault occurs on a far and close distance from a bus, and determine the critical clearing time as well as swing curve of rotor angle. The output values of electrical power, machine speed, rotor angle and bus voltage are observed.

Transient Stability Analysis of 33 KV Transmission Network of Egi Community, Nigeria

Transient analysis of 33 KV power transmission line stability of Egi communi-ty is considered in this research work with the aim of reducing the frequency of fault occurrence and voltage collapse in the network. The supply is taken from Egi generating station located at Total Nigeria Limited Gas Plant Obite at voltage level of 33 KV to Egi communities. This work focuses on the transient nature of network stability since transient fault is the most dangerous in elec-trical systems. The swinging of the generator rotor in the event of transient three-phase short circuit fault can be monitored by the circuit breakers and the protective relays which causes mal-functioning of the circuit breakers and pro-tective relays leading to abnormal behavior of the network. Therefore, data obtained from the power station were used as a case study of Independent Power Producer (IPP) in Nigeria. For investigation of the power angle, angular velocity, rotor angle differential changes, and angular velocity differential changes, an electrical transient analyzer tool was employed (ETap version 16.00) for circuit breaker and protective relay time setting of (0.00, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60). The work used the Trapezoidal numerical technique for data analysis. The graphs were plotted using Matlab R2015a and the results obtained showed that when a symmetrical three-phase short circuit fault occur at one or any of the feeders, the fault must be cleared as quick as possible through the coordination of the circuit breakers and protective relays. For this research work, 17 cycles corresponding to relay time setting of t = 0.34 s were recommended and at each cycle, changes in time with respect to changes in rotor angle, angular velocity, rotor differential and angular velocity differential were calculated on the power network simultaneously. The results demonstrated that the Trapezoidal method is numerically stable, accurate and has faster respond time when compared to Modified Euler and swing equation techniques in event of fault occurrence in network.

LONG-TIME BEHAVIOR OF TRANSIENT SOLUTIONS FOR CELLULAR NEURAL NETWORK SYSTEMS

By establishing concept an transient solutions of general nonlinear systems converging to its equilibrium set, long-time behavior of solutions for cellular neural network systems is studied. A stability condition in generalized sense is obtained. This result reported has an important guide to concrete neural network designs.

Transient Stability Analysis of Hu-Liao HVDC and AC Parallel Transmission System

In this paper, transient stability analysis was focused on Hu-Liao HVDC and AC parallel transmission system. The Hu-Liao HVDC project was introduced;Simulation method and mathematic models of AC and DC systems were stu-died as well as corresponding regulators and controllers. The dynamic performance and the interaction between AC and DC systems during serious disturbance were researched by detail time-domain simulation. Comparison was also made under different operation schemes. The research will bring important and significant reference for further operation and stability control of Hu-Liao HVDC and AC system.

风电不确定性输入下电力系统仿真的区间仿射算法

大规模的风电机组并网改变了以同步发电机为主的传统电力系统的暂态特性,此外风电出力的随机性会影响电力系统的暂态稳定性。为此,文中提出了基于仿射算术的电力系统不确定性时域仿真算法,以考虑风电出力的波动性。在双馈风电机组机电暂态模型的基础上,扩展电力系统机电暂态模型,建立考虑双馈风电机组输入功率波动下的电力系统机电暂态仿射模型,通过隐式梯形积分法将仿射模型转化为的仿射雅克比矩阵方程迭代求解。仿真结果表明,文中所提出的仿射方法在不假设输入不确定量概率分布的情况下,能够较蒙特卡洛仿真更快地获得电力系统暂态响应的区间边界。

考虑LVRT特性的新能源送端系统功角稳定与电压稳定交互影响分析

新能源机组的低电压穿越(LVRT)能力是高比例新能源电力系统维持电压稳定的重要保证,同时LVRT期间新能源机组的功率特性将进一步影响同步发电机的功角稳定性。为分析高比例新能源送端系统功角稳定与电压稳定的耦合机理,以风电为例,推导了LVRT期间风电机组的有功-无功耦合关系;以含风电的送端系统为例,推导了故障前、故障期间以及故障后同步发电机的机械功率和电磁功率。分析了受扰后系统功角与电压的失稳形态,揭示了含风电送端系统功角稳定和电压稳定的交互影响。基于PSASP平台,仿真分析了风电初始出力水平以及LVRT参数对电力系统功角稳定和电压稳定演化的影响。

电化学储能参与电网暂态功角稳定控制研究

针对电网传统紧急控制资源的日益匮乏,利用电化学储能功率控制的快速性与双向控制能力,研究将其应用于电网暂态功角稳定控制。首先推导了计及电化学储能的双机等值模型,基于扩展等面积定则分析了储能改善电网暂态稳定裕度的原理。其次,提出了一种考虑设备保护与系统保护动作时序差异的储能电站有效工作时长计算方法,改善了紧急控制储能的工作效率。提出了计及储能的基于性能代价比多类措施协调控制策略制定方法,兼顾紧急控制的安全性与经济性,为电网暂态紧急控制提供在线辅助决策支撑。进一步提出了储能电站全景监控系统架构,为实现储能电站与电网紧急控制系统的交互提供了技术实践思路。最后西北电网实际算例验证了所提方法的有效性与合理性。研究结论可为规模化储能与高比例新能源电网第二道防线的扩展与建设提供理论参考。

基于稳定流形法的构网型逆变器暂态稳定性分析

具有电流限幅环节的构网型逆变器在实际运行中,会出现类似于同步发电机功角失步的暂态稳定性问题。为了正确地判断构网型逆变器在大扰动故障下是否发生暂态失稳,需要使用准确的暂态稳定边界。因此,针对构网型逆变器单机并网系统,提出了一种基于稳定流形法的暂态稳定边界描述方法,建立了包含功角与角频率信息的稳定区域,旨在准确地判断具有电流限幅环节的构网型逆变器在电网电压发生大幅扰动后是否失稳,并将该稳定边界应用于分析系统参数对暂态稳定性的影响,同时给出对应的临界切除时间。最后,通过单级式逆变器实验平台验证了暂态稳定边界的准确性和理论分析的正确性。

Hybrid Analytical and Data-Driven Model Based Instance-Transfer Method for Power System Online Transient Stability Assessment

Data-driven methods are widely recognized and generate conducive results for online transient stability assessment.However,the tedious and time-consuming process of sample collection is often overlooked.The functioning of power systems involves repetitive sample collection due to the constant variations occurring in the operation mode,thereby highlighting the importance of collection efficiency.As a means to achieve high sample collection efficiency following the operation mode change,we propose a novel instance-transfer method based on compression and matching strategy,which facilitates the direct acquisition of useful previous samples,used for creating the new sample base.Additionally,we present a hybrid model to ensure rationality in the process of sample similarity comparison and selection,where features of analytical modeling with special significance are introduced into data-driven methods.At the same time,a data-driven method can also be integrated in the hybrid model to achieve rapid error correction of analytical models,enabling fast and accurate post-disturbance transient stability assessment.As a paradigm,we consider a scheme for online critical clearing time estimation,where integrated extended equal area criterion and extreme learning machine are employed as analytical model part and data-driven error correction model part,respectively.Derived results validate the credible efficacy of the proposed method.

基于非线性解耦方法的交直流配电系统暂态稳定性分析

随着柔性交直流配电系统的迅速发展,电力电子装置和恒功率负载等非线性元件的大规模接入使系统的暂态稳定性问题日趋严重。首先,建立了双环控制下AC/DC与DC/DC换流器并联的具有恒功率负载的交直流配电系统数学模型,并进行非线性泰勒展开。其次,根据非线性解耦理论将原始高阶系统解耦成一系列低阶子系统,确定主导解耦子系统并采用相平面法和逆轨迹法对其进行暂态稳定性分析,通过实物仿真验证理论分析的有效性。然后,基于非线性解耦原理引入非线性参与因子概念,对状态变量进行参与因子的计算,提出恒功率负载扰动下控制参数影响稳定性的分析方法,进一步给出控制参数的可行域。最后,利用PLECS软件搭建了恒功率负载下双环控制的交直流配电系统,仿真及实验验证了所提方法的有效性。

基于特征匹配的暂态稳定紧急控制策略快速生成

“在线计算、实时匹配”的紧急控制模式是降低控制策略失配风险的有效途径和技术发展趋势。为进一步提高在线紧急控制策略生成的快速性,提出了一种基于特征匹配的暂态稳定紧急控制策略快速生成方法。借助扩展等面积准则对暂态稳定性以及电网故障下轨迹时变程度的量化分析能力,综合利用电网暂态稳定轨迹特征和稳态潮流关键特征量,建立运行方式特征量匹配指标,在历史方式中匹配最接近方式,实现紧急控制策略的快速生成,通过校核验证后下发装置执行。最后,基于实际电网验证了所提方法的有效性。

计及故障持续时间的电力系统暂态稳定分布式控制

针对电力系统故障持续时间会影响暂态稳定控制的问题,本文提出了一种使系统在固定时间内恢复暂态稳定运行的策略.该策略通过电力系统通信拓扑建模方法为每个发电机建立“邻居发电机”,利用发电机本地信息和其邻居发电机的信息设计分布式控制器,通过控制储能装置作用于受扰动后的系统,使其在固定时间内恢复稳定运行.其次,此控制策略还解决了一些实际挑战,如控制器的输入延时、外部的干扰、储能装置的容量限制等.通过构建李雅普诺夫函数并利用图论知识对该策略展开了稳定性分析,推导出了稳定系统的时间界限.最后,将控制器投入新英格兰39节点测试系统中,并与其它控制器进行比较,仿真结果验证了在低容量储能装置限制下和抗干扰性上控制器性能的优势.

基于实时戴维南等值参数估计的短路比分析

针对风电接入多场站电力系统暂态稳定问题,依托短路比对系统的暂态稳定性进行判定。首先,建立新能源并网系统等效模型,运用时域仿真的戴维南等值参数法来求解系统暂态的戴维南等值参数;在此基础上,计算交流系统短路容量,推导得出新能源并网系统短路比,并构建短路比与节点电压的代数关系,得到系统短路比关于节点电压的计算式;最后,基于系统最大传输功率计算得出临界短路比,通过对比实现对系统暂态稳定的实时评估。仿真结果表明,基于时域仿真的戴维南短路比计算准确,算法的适应性强。当短路比小于临界短路比时,系统处于特性不稳定区域,反之则处于稳定区域。

Modification of CaO-based sorbents prepared from calcium acetate for CO_2 capture at high temperature

CaO-based sorbent is considered to be a promising candidate for capturing CO_2 at high temperature. However,the adsorption capacity of CaO decreases sharply with the increase of the carbonation/calcination cycles. In this study, CaO was derived from calcium acetate(CaAc_2), which was doped with different elements(Mg, Al,Ce, Zr and La) to improve the cyclic stability. The carbonation conversion and cyclic stability of sorbents were tested by thermogravimetric analyzer(TGA). The sorbents were characterized by N_2 isothermal adsorption measurements, scanning electron microscopy(SEM) and X-ray diffraction(XRD). The results showed that the cyclic stabilities of all modified sorbents were improved by doping elements, while the carbonation conversions of sorbents in the 1st cycle were not increased by doping different elements. After 22 cycles, the cyclic stabilities of CaO–Al, CaO–Ce and CaO–La were above 96.2%. After 110 cycles, the cyclic stability of CaO–Al was still as high as 87.1%. Furthermore, the carbonation conversion was closely related to the critical time and specific surface area.

含构网型MMC的受端电网暂态稳定解析分析方法

针对新型电力系统中,构网型非同步机电源和同步机电源之间可能发生的暂态失稳现象,提出了适用于含构网型模块化多电平换流器(MMC)的受端电网暂态稳定解析分析方法。在故障期间和故障清除后系统中分别采用电流源和电压源对构网型MMC进行建模,并据此模型提出了临界切除角和临界切除时间的解析计算方法。基于所提解析分析方法,研究了MMC故障期间注入电流幅值和相角、故障位置、同步机和MMC对负荷的供电占比等关键因素对系统暂态稳定的影响机理。基于PSCAD/EMTDC搭建的电磁暂态仿真模型验证了所提解析分析方法的有效性、准确性和鲁棒性。

基于临界惯量和预想故障的含风电电力系统暂态功角稳定在线预警

大规模风电接入使得电力系统惯量水平降低,暂态功角失稳风险加剧,开展含风电电力系统故障前的暂态功角稳定在线预警对保障其安全稳定至关重要。提出了基于临界惯量和预想故障的含风电电力系统暂态功角稳定在线预警方法。首先,通过理论分析得到风电场在故障清除后短时间的功率恢复过程不能忽略的推论,并建立等效外特性模型,进而基于扩展等面积定则构建了计及风电场多阶段外特性的暂稳数学模型。其次,定义了含风电系统暂态功角稳定的临界等效惯量,并通过匀加速分段等效提出了实用化的求解方法,进一步提出了暂态临界惯量指数,实现稳定裕度量化分析并给出风险等级。最后,以含风电接入的四机两区域系统和IEEE39节点系统为例,仿真验证了所提方法的有效性和适用性。