计及虚拟惯量与下垂控制的风火耦合系统小扰动稳定分析

风火耦合系统在我国北方电网中普遍存在。为了构建电网友好型风电场,主动频率支撑控制被引入风电功率外环,然而该控制可能会影响耦合系统的稳定特性。已有研究主要关注主动频率支撑控制对火电机组主导的机电振荡特性的影响,对风电机组主导的次同步振荡特性影响的研究不多。基于此,建立考虑主动频率支撑控制的风火耦合系统模型,分析在不同风电渗透率和不同锁相环阻尼系数下主动频率支撑控制对耦合系统中火电机组主导的机电振荡模态以及风电机组中锁相环主导的次同步振荡模态的影响,确定耦合系统中锁相环主导的次同步振荡模态的主要影响因素。为了探究主动频率支撑控制影响锁相环主导的次同步振荡的失稳机理,推导了耦合系统计及主动频率支撑控制时锁相环小扰动等效模型,通过复转矩法分析了主动频率支撑控制对锁相环主导的次同步振荡模态的影响。通过时域仿真验证了主动频率支撑控制参数对耦合系统稳定性的影响。     

考虑线路电阻影响的MMC-MTDC自适应下垂控制

下垂控制因其对通信的要求不高、可靠性高而在多端柔性直流输电中广泛应用。但是传统的下垂控制中下垂系数固定,尤其在系统功率波动较大时,容易导致换流站过载,并且直流线路上的电阻还会影响下垂控制换流站的有功功率分配。针对此类问题,提出了一种考虑线路电阻影响的自适应下垂控制。首先,分析在下垂控制下的多端柔性直流输电系统等效电路,推导出考虑线路电阻影响的下垂系数整定方法。然后在整定方法中用换流站的功率裕度比取代固定的功率分配比例系数,实现下垂系数的自适应调节。最后在PLECS上搭建仿真模型,对不同工况进行仿真,验证了所提控制策略的有效性。     

一种直流微电网多光伏变换器新型功率分配策略

为了解决光伏单元在运行过程中因工作模式切换造成的母线电压波动问题,提出一种新型平滑切换控制策略。根据光伏电池的输出特性曲线,将输出功率对输出电流的微分作为控制变量。通过跟踪不同的dp/di指令值来实现光伏单元最大功率点跟踪模式、恒压下垂模式的控制以及两种模式间的平滑切换。在此基础上,针对多光伏变换器工作在恒压下垂控制时因线路阻抗差异造成功率分配精度较低的问题,提出一种基于二次调节的自适应下垂控制策略。此外,构建相邻光伏单元间的稀疏通信网络,并采用动态一致性算法实现相关平均信息的全局稳定收敛。仿真和实验结果表明,所提控制策略可以实现光伏单元工作模式切换过程平滑过渡和负荷功率的精确分配。     

基于模糊单神经元PI的微电网频率自适应控制

在采用下垂控制策略的传统微电网中存在母线频率随负载增大而下降的问题,为确保母线频率不偏移标准频率,需要采取频率恢复控制策略。目前实现频率恢复的常用方法是基于比例-积分(PI)的控制策略。由于微电网网络结构和系统参数存在变化,PI控制可能无法满足频率控制快速响应、恢复的需求。为解决该问题,文中提出在第二层控制微电网中央控制器中使用单神经元自适应PI控制算法作为频率恢复算法,实现频率无差控制。为进一步增强系统的鲁棒性、加快频率的恢复,使用模糊控制器对单神经元PI控制器的神经元比例系数进行在线优化,并通过仿真与固定神经元比例系数的单神经元自适应PI控制进行对比,证明了所提改进控制策略可改善频率恢复控制的暂态性能,加快微电网的频率恢复。     

Multi-purpose droop controllers incorporating a passivity-based stabilizer for unified control of electronically interfaced distributed generators including primary source dynamics

This paper presents multi-purpose droop controllers for electronically-interfaced distributed generators (EI-DGs). These controllers allow the micro-grids to operate in grid-connected mode, islanded mode and mode transition transients with a unique control configuration. The active and reactive-power sharing among EI-DGs are satisfied by the proposed droop controllers in islanded mode. On the other hand, in the grid-connected mode, the droop controllers adjust the output active and reactive-powers of EI-DGs at the pre-programmed constant levels. The provision of sufficient damping capability and maintenance of the transient stability in all operational modes of EI-DGs are warranted by the suggested stabilizer. This stabilizer, which is designed using the passivity-based control (PBC) approach, is incorporated in the droop controllers to dampen power-angle, frequency and voltage deviations during large transients using solely local information. The primary source dynamics of EI-DGs are also considered. It is analytically proven that the presence of the primary source dynamics leads to attenuation of the damping capability of EI-DGs in transients. To compensate the adverse effect of the primary source dynamics during transients a novel compensator is inserted in the frequency-droop loop. Finally, time-domain simulations are performed on a multi-resources MG to verify the analytical results compared to those obtained, based on a recently-developed strategy.     

Online Droop Tuning of a Multi-DG Microgrid Using Cuckoo Search Algorithm

This article presents an intelligent strategy to achieve appropriate real power sharing among distributed generators in a microgrid. The presented strategy employs two droop-based control methods and automatically adjusts their parameters. The first method is unit power control, which has specifications similar to the conventional droop method, and the second is feeder flow control, showing significant characteristics in both grid-connected and islanded modes operation of a microgrid. A combination of unit power control and feeder flow control methods is used for a multi-distributed generator microgrid. The microgrid operation mode passes from the grid-connected to the islanded through a transition. A new evolutionary algorithm called cuckoo search is employed to coordinate the power management of distributed generators within an on-line droop tuning. In comparison to the predecessor evolutionary algorithms, the cuckoo search algorithm represents more effective random processes with fewer parameters. Using the proposed control strategy, while the distributed generators contribute to load demand provision based on their rated powers, their powers are optimized in terms of overshoot and settling time. Digital time-domain simulation studies are carried out in the MATLAB/SIMULINK (The MathWorks, Natick, Massachusetts, USA) environment to verify the performance of the proposed control system.     

A fuzzy logic-based droop control for simultaneous voltage and frequency regulation in an AC microgrid

Modern power systems require increasing intelligence and flexibility in the control and optimization to ensure the capability of maintaining balance between generation and load under violent disturbances. There are several inverter interfaced distributed generations (IIDGs) with local and global control loops in a microgrid (MG). Voltage and frequency of MGs are strongly impressionable from the active and reactive load fluctuations. A change in load leads to imbalance between generation and consumption. The output voltage and frequency of the IIDGs are primarily controlled by the droop characteristics. But, in case of severe changes in load, the IIDGs may be failed and the MG is collapsed. In this paper, for optimally tuning of a generalized droop control (GDC) structure and also secondary voltage and frequency controllers, the fuzzy logic technique is utilized. It is shown that the proposed fuzzy logic controller exhibits high performance and desirable response for different scenarios of change in load.     

基于附加阻尼的微网改进下垂控制方法

采用传统有功频率、无功电压幅值下垂控制时,若下垂系数选取不合理,逆变器输出功率会出现持续振荡,当微网离网运行时,逆变器输出功率的振荡会导致微网系统频率出现波动,严重时会使整个系统出现功角稳定性问题。文中基于储能逆变器动态频率特性的概念,根据其产生正阻尼作用的相位条件和幅值条件,提出了基于附加阻尼的改进下垂控制方法。该方法可以在不影响系统稳态频率和逆变器输出功率的前提下,有效地增加系统的阻尼,从而抑制逆变器输出功率振荡,提高系统的稳定性。最后通过PSCAD仿真,验证了所提控制策略的有效性。     

配电网故障情况下多微网互联能力分析

配电网故障时,可以通过多微电网互联供电进行黑启动,恢复重要非故障区域的供电。并网时,微电网群工作在与配网同步的工作模式;配电网故障时,各微电网将分别工作在不同的孤网状态。由于微电网内部的微源特性、负荷特性等存在巨大差异,因而在多微电网互联实现配电网黑启动和非故障区域恢复供电之前,需要对多微电网的互联能力进行分析,以保证多微电网互联后微电网群的安全运行。从多微电网互联后系统的频率稳定和备用容量调节能力角度对多微电网的互联能力进行了分析,提出了相应的互联判据;搭建了仿真模型,对多微电网互联后的动态过程进行验证,结果验证为实现多微电网互联提供理论支撑。     

适用于大规模风电并网的多端柔性直流输电系统控制策略

提出了一种适用于区域大规模风电并网的六端柔性直流输电系统,设计了该系统的协调控制策略,即送端电压源型换流器(VSC)采用交流电压控制、受端VSC采用直流电压下垂控制。以直流网络损耗最小作为优化目标,计算了系统稳态运行点。通过在PSCAD/EMTDC平台上搭建仿真算例,验证了所提出的系统控制策略可以自动跟踪风电功率波动并协调受端功率分配。通过设计系统启动和风功率波动及交流侧故障和换流器停运的仿真算例,验证了该六端柔性直流输电系统具有良好的功率调控能力和运行灵活性。