Coordination of PV Inverters and Voltage Regulators Considering Generation Correlation and Voltage Quality Constraints for Loss Minimization

Abstract

Distributed generation sources (DGs) are widely considered as important sources of power generation in distribution systems during the last few decades. Despite the substantial benefits of DGs, increasing the penetration level of the DGs can cause dramatic voltage magnitude fluctuations. Coordination of the use of dynamic reactive power sources such as photovoltaic (PV) inverters and voltage control equipment can mitigate rapid voltage magnitude fluctuations. A coordinated volt-var control method is proposed herein to achieve the optimal expected performance (e.g., system losses) while considering the spatial correlation among PV source powers and constraining the variability of voltage magnitudes throughout the distribution network within permissible ranges. The proposed strategy formulates chance constraints on the voltage magnitude and considers the uncertainty of PV power injections over the interval of interest to maintain voltage magnitudes within acceptable limits. The proposed method has been tested on the IEEE 123-node radial distribution system for validation. Moreover, the simulation results demonstrate that the proposed method can effectively mitigate the fast voltage magnitude deviations with an acceptable reduction in system losses in the presence of intermittent renewable resources.     

Cascaded Common Nodal Point Multi-level Inverter Topology

Recent trends in the multi-level inverter (MLI) technology demand reduced number of switches, driver circuits, isolated DC sources, peak inverse voltage (PIV), appreciable number of voltage level, and lower total harmonic distortion. This paper presents an improved cascaded MLI configuration. Each module comprises ten switches, two isolated DC sources, and two capacitors; it can generate a maximum of 9-level output voltage waveform. Optimized switching sequence is developed that ensures minimum switching transitions and is implemented through single-carrier pulse width modulation for the control of the proposed topology. The classical cascaded H-bridge inverter and some recently developed MLI configurations were compared with the proposed inverter circuit. Results show that the proposed inverter configuration generates high number of output voltage levels with reduced number of power switches and PIV. It also has a lower per-unit power loss profile. Unit capacitor voltage balancing scheme is developed, which ensures proper control of the unit step voltage level in each of the cascaded modules, at extreme loading condition. For two cascaded inverter modules, simulation and experimental verifications are carried out on the proposed inverter for an R–L load. Simulation results of the output voltage waveforms and its harmonic spectrum are in conformity with experimental results.     

面向逆变器型分布式电源的快速建模与仿真方法

在利用数值仿真方法研究逆变器型分布式电源大规模接入配电网的特性和影响时,采用详细模型因涉及非线性的电力电子器件,仿真步长需选取很小,致使仿真速度缓慢,而采用过于简化的模型又不能完整反映系统的动态特性,因此有必要研究准确、高效的分布式电源建模方法。在正负序分离控制的基础上,详细分析了逆变器型分布式电源的动态相量建模方法,并且在PSCAD/EMTDC中建立了分布式电源的电磁暂态模型,同时在MATLAB/Simulink中建立了等效动态相量模型。通过仿真比较,结果表明采用的动态相量模型在配电网电压对称、不对称工况及故障情况下均能准确地反映系统运行的动态过程,并显著加快仿真速度,适合应用于逆变器型分布式电源高渗透率下配电网运行特性的快速仿真。     

基于光伏逆变器无功调节的低压配电网多模式电压控制

针对大规模户用光伏接入引起的低压配电网电压越限问题,以逆变器无功控制为手段,提出了一种多模式逆变器控制策略,以提高低压配电网对光伏的消纳能力。基于电压灵敏度理论,定义了虚拟注入有功功率的概念,实现了节点有功和无功功率之间的折算。根据节点虚拟注入功率将光伏发电的并网分为过电压抑制、欠电压抑制以及网损和功率因数的优化三种模式。当网络出现过电压(欠电压)运行风险时,以风险的抑制为目标调节逆变器无功功率;当网络运行无风险时,则以网损和功率因数的优化作为逆变器的无功调节依据。此外,为了实现就地的协调控制,结合全网电压灵敏度矩阵建立了不同节点光伏逆变器控制参数的优化模型,实现网络无通信条件下的协调电压控制。仿真结果表明,所提的多模式电压控制方法可以有效地抑制网络电压越限,同时使网络损耗和功率因数也得到优化。     

增强并网逆变器对电网阻抗鲁棒稳定性的改进前馈控制方法

随着分布式电源并网功率的逐渐增加及接入点的广泛分布,电网越来越表现出弱电网特性,即电网阻抗相对较大,此时在并网逆变器中广泛应用的电网电压前馈控制会严重影响到系统的稳定性。以L型滤波并网逆变器为研究对象,采用框图等效变换的方法分析弱电网情况下前馈控制对并网逆变器特性的影响机理,以及电网等值电感和阻感比对系统稳定性的影响规律,并在电压前馈通道中引入一种带通滤波环节,提高弱电网下并网逆变器的鲁棒稳定性。分析表明,附加带通滤波环节的电压前馈控制可使得逆变器在短路比较小的弱电网中仍能够稳定工作。最后,搭建一台66k V·A并网逆变器实验样机,实验结果验证了理论分析的正确性和所提改进策略的有效性。     

Smoothing of Wind Power Fluctuations for Permanent Magnet Synchronous Generator-Based Wind Energy Conversion System and Fault Ride-through Consideration

Due to the increment of penetration level of wind power generation, output power fluctuation is one of the most important issue's that can destabilize the power system operation. This article mainly deals with the smoothing of the output power fluctuations of a wind energy conversion system based permanent magnet synchronous generator and fault ride-through enhancement during a grid fault. The concerned wind energy conversion system based permanent magnet synchronous generator adopts an AC-DC-AC converter system. The proposed control method limits the wind energy conversion system output power by adjusting the pitch angle of the wind turbine blades when wind speed is above the rated wind speed. In the grid-side converter, a fuzzy logic controller is used to determine the torque reference for which the kinetic energy stored by the inertia of wind turbine can smooth the output power fluctuations of the permanent magnet synchronous generator. Also, the DC-link voltage, controlled by the grid-side inverter, is adjusted in accordance with the output power fluctuations of the permanent magnet synchronous generator using a voltage smoothing index. Moreover, in this aticle, the proposed method ensures that the wind turbine stays operational during grid faults and provides fast restoration once the fault is cleared. To show the effectiveness of the proposed method, simulations under different conditions have been performed by using MATLAB/Simulink® (The Math Works, Natick, MA, USA).     

固体氧化物燃料电池发电系统模型建立及逆变器仿真研究

以固体氧化物燃料电池发电系统为研究对象,以释放变换器上电感能量,减少电容充电时间为目的,提出了在DC/DC变换器中增加一个反激式绕组。该反激式绕组的输出端、接地端分别与DC/DC变换器的输出端、输出接地端相连,构成输出回路,以释放输入电感启动时多余能量,保证输出直流母线电压的稳定性。逆变器输出电压控制采用模糊PID控制策略,可在固体氧化物燃料电池输出电压不稳定时,通过监测该系统的输出电压和给定电压差值,在线调整KP、Kd、Ki控制输出量。该控制策略与传统PID控制策略进行了比较,比较结果证明其性能优于传统PID控制策略。采用Matlab软件搭建系统模型,固体氧化物燃料电池发电系统的仿真研究验证了以上的理论分析。     

Characteristics on PWM CSI-induction motor drive system with utility interactive photovoltaic generation

This paper proposes a PWM current source inverter-induction motor drive system with photovoltaic generation. Solar cells are inserted in dc link to obtain constant current characteristics. They are connected to the utility system with a PWM converter. The PWM converter is controlled to obtain the maximum output photovoltaic (PV) power. The PWM inverter supplies sinusoidal currents for an induction motor, which is driven by constant V/f control. A pulsewidth control is utilized in the inverter section because of constant dc link current caused by peculiar V-I solar cell characteristics. In the system proposed here, the PV power is not only used for inverter-induction motor drive but it also flows into the utility system. The experimental results in steady state show that the proposed system has sinusoidal current with unity power factor in the utility system, the maximum output PV power and sinusoidal current/voltage for a motor. The demonstrated results for the separation of the utility system or the motor from the converter-PV-inverter system are given. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 118 (2): 79–87, 1997     

二级式光伏并网发电系统的协调控制及惯量特性分析

针对二级式光伏发电系统低惯量的问题,根据光伏-Boost和逆变器两侧分别响应频率波动的特点,提出了一种适用于二级式光伏发电系统的惯量协调控制策略,通过建立系统的静止同步发电机SSG模型分析不同参数对系统惯量的影响。光伏侧采用下垂控制响应电网频率波动时,频率下跌的最低点或上升的最高点得到改善,且电网频率的稳态偏差变小;逆变器侧采用直流电压下垂响应电网频率波动时,电网频率变化率变小;协调控制时,合理利用了光伏侧及逆变器侧响应的优势,抑制频率波动的效果最佳。仿真结果证明了所提控制策略的有效性。     

Z-源逆变器在光伏发电系统中的应用

Z-源逆变器由于采用独特的X型Z-源网络,可以利用逆变器桥臂直通状态实现升压功能,从而使单级Z-源逆变器具有与两级并网系统相类似的性能。该文论述基于Z-源逆变器的光伏并网系统在变换效率、可靠性以及成本方面的优势,提出了一种新的单级Z-源逆变器并网系统两级控制策略, 该控制策略实现了MPPT控制和逆变器并网控制,使Z-源光伏并网系统能够动态跟踪光伏电池最大功率点电压,输出电流与电网电压同相位,从而达到较高的功率因数。软件仿真和实验分析证明了该控制方法具有优良的动、静态特性,适合各种变化的天气情况。     

A suitable single-phase pulse width modulation current source inverter for utility interactive photovoltaic generation system

A suitable single-phase inverter for the utility interactive photovoltaic generation system is proposed. The single-phase Pulse Width Modulation (PWM) current source inverter has a novel circuit configuration in which an auxiliary branch is added to the normal single-phase bridge circuit. To reduce the size and weight of the dc reactor, a double frequency parallel resonance circuit (LC tank circuit) is inserted in the dc side of the inverter. As a result, the double frequency voltage appearing in the dc side of the inverter due to the pulsation of the single-phase instantaneous power is perfectly suppressed by the tank circuit. The constant dc current without pulsation is supplied from PV array to the inverter. The inverter provides a sinusoidal ac current for domestic loads and the utility line with unity power factor. The virtual maximum power of the PV array can be obtained without any feedback control. In the system, the PV array can play an important role as a current-limiter due to its V-I characteristics. Computed waveforms by simulation are shown. Excellent inverter equipment will be realized that is smaller in size and lighter in weight than is usual for a conventional inverter.     

不对称电压跌落下虚拟同步机改进低电压穿越控制策略

虚拟同步机(Virtual Synchronous Generator, VSG)是提高以新能源为主体的新型电力系统稳定性的有效途径。应用于逆变型新能源(Inverter-Interfaced Renewable Generation, IIRG)并网的虚拟同步机在不对称电压跌落情况下可能丧失VSG特性,并因低压穿越能力不足或电压电流越限而导致切机,危害电力系统安全稳定运行。对此,提出了一种新型VSG控制策略。该方法在不对称电压跌落情况下,既能持续提供系统惯性和阻尼,又能提供主动电压支撑,有效提高VSG低压穿越能力,并保证扰动下的系统稳定性。首先,分析了传统VSG在不平衡电压跌落情况下的响应特性。然后,提出了一种基于平衡电流的改进VSG控制结构,将传统VSG单电流环控制改为双电流环控制,维持VSG在电压跌落条件下的惯性阻尼特性,并实现对正负序分量的精准控制。接着,基于改进的双电流环控制拓扑,在逆变器安全运行条件下,对正负序参考电流整定方法进行优化,以实现VSG主动电压支撑和电流限幅。最后,基于Matlab/Simulink仿真平台,验证了所提控制策略在多种系统运行条件下的响应特性及有效性。     

Feasible enhancement of a three‐phase soft‐switching inverter with an auxiliary resonant DC link snubber

The purpose of this paper is to improve power conversion efficiency of a three‐phase voltage source type soft‐switching inverter with a single auxiliary resonant DC link (ARDCL) snubber. First, the operating principle of an ARDCL snubber discussed here is described. Second, this paper proposes an effective pulse pattern generation method of the zero voltage space vector of the three‐phase soft‐switching inverter using IGBTs or power modules that can reduce power losses in the ARDCL snubber treated here. In particular, a zero voltage holding interval in the DC rail busline of this three‐phase soft‐switching inverter is to be regulated according to the generation method of the zero voltage space vector. Third, the maximum modulation depth M_max under the condition of correction of the instantaneous voltage space vector can be improved by using a new zero voltage space vector generation method. Finally, the feasible experimental results of this inverter are obtained confirming the operating characteristics such as power conversion actual efficiency, as well as conventional efficiency THD and RMS value of the balanced three‐phase output voltage for an experimentally built three‐phase voltage source type soft‐switching pulse modulated inverter using the latest IGBT modules and evaluated from the standpoint of practical applications in industry UPS and new energy systems. © 2003 Wiley Periodicals, Inc. Electr Eng Jpn, 146(1): 89–99, 2004; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.10234     

带超级电容的级联型光伏并网逆变器控制策略

针对由光伏电池板参数的分散性、遮挡不均以及光照强度波动等原因造成的发电功率损失及系统不稳定等问题,提出一种带超级电容储能的混合级联型光伏逆变器。首先,分析了该逆变器利用超级电容单元补偿光伏发电功率波动的工作原理。然后,提出了一种基于PI控制和重复控制的控制策略。该控制策略引入占空比修正模块对光伏单元实施分散的MPPT控制,以提高发电效率,并对超级电容单元进行功率控制以平滑逆变器输出功率的波动。最后,通过仿真和实验验证了该逆变器及其控制策略的可行性。     

电网电压不平衡下燃料电池并网逆变器功率控制

基于参考指令变更的三相并网逆变器功率控制方法,通过调节影响功率波动的参考指令内的谐波分量可以实现逆变器电流质量和功率波动间协调控制,但不能实现三相电压不平衡下负序交流分量的无静差调整。针对此问题,提出了三相电压不平衡下燃料电池三相并网逆变器功率控制方法,构建了燃料电池三相并网逆变器电路拓扑结构。在此基础上采用无锁相环直接功率控制方法,采用全通滤波器对并网逆变器电路中的电压和电流基波分量进行90°相移,消除2倍频的负序交流分量,实现并网逆变器有功功率和无功功率的有效控制。仿真结果证明,所提方法控制的并网逆变器进网电流谐波含量为0.33%,输出电流正弦度较高,电网电压不平衡状态下仍能坚持对电流进行控制。该方法功率控制效果好,具有较强的安全性。     

分布式电源对配电网电压的影响分析及其优化控制策略

针对配电网中分布式电源(DG)接入渗透率不断提高带来的电压波动、电压越限等电能质量问题,首先从理论上较全面地推导分析分布式电源接入对配电网电压的影响,包括对接入点电压和对电压分布的影响,并通过仿真对比分析了不同DG渗透率和不同DG功率因数下的电压变化情况,指出了全局电压控制的必要性。在此基础上,综合考虑配电网中分布式电源和储能的有功、无功电压调节能力,提出一种基于模型预测控制(MPC)的电压优化控制方案,通过计算各节点电压灵敏度,建立各节点电压预测模型,提前感知各节点电压变化趋势,以各节点预测电压与额定电压之间偏差最小为控制目标,实现了控制成本最低的协调电压控制策略。通过IEEE33节点配电网系统仿真算例分析,结果表明:本文提出的方法能更大限度地消除配电网中分布式可再生能源随机波动带来的影响,具有较好的电压控制灵活性和鲁棒性。     

逆变型分布式电源控制系统的设计

分布式发电系统并网运行时外特性控制是关系到其应用的关键问题之一。根据并网分布式电源的可控拟负荷外特性,设计了采用电流正弦脉宽调制(PWM)调节方法的电压型逆变控制器,整个体系充分考虑提高电能质量水平。该控制器的核心采用含有直流电压波动前馈补偿的双环串级PI结构。第1级为功率调节器,选择公共连接点(PCC)电压、电流作为反馈量,有功、无功可以分开独立调节;第2级为电流调节器,选择逆变器输出电流作为反馈量,考虑电流反馈解耦补偿,响应速度快。直流电压前馈补偿则可以降低直流侧缺少储能引发输入电压波动对控制效果的影响。输出滤波器的考虑则可进一步提高电能质量水平。仿真结果表明电流正弦PWM的双环串级PI控制系统能够较好地维持逆变型分布式电源恒功率电压源的拟负荷外特性,简单实用、设计灵活,可以得到较好的谐波注入。     

三相光伏并网逆变器多目标优化模型预测控制

三相电压型并网逆变器广泛用于光伏发电领域。逆变控制方法用于提高并网系统效率和响应质量。模型预测控制策略使用离散时间模型预测下一个采样周期所有可能的输出值,根据评估函数选取最优电压向量。将模型预测控制用于三相电压型并网逆变器中。首先,建立三相光伏逆变器在d-q坐标系下的瞬时功率数学模型。其次,设计预测函数在线预测逆变并网参数。选择合适的目标函数控制逆变器下一采样周期的输出值。d-q坐标系下的跟踪精确迅速,所提出的控制策略计算量小,无需PWM调制,更容易实现。然后,对模型预测控制进行多目标优化。设计电流解耦控制减小系统输出有功功率,改变评估函数提高输出电流质量,修正交流侧电压参数提高预测的准确性。最后,仿真和实验结果证明提出的控制策略输出电流具有良好的动态性能和较低的谐波畸变率,可快速跟踪给定的参考值,具有无功补偿的功能。     

改进SVPWM在直驱型风电并网控制中的仿真研究

目前大功率直驱永磁同步发电系统和三相PWM逆变器控制正成为风力发电技术研究的热点。主要针对直驱风电系统中网侧逆变器控制进行研究,为减小并网逆变器输出电流中的谐波,采用空间矢量脉宽调制技术;为维持直流环节电压的恒定和使输出电流快速跟踪给定电流,实现单位功率因数并网,采用电压外环、电流内环双闭环结构控制风电并网。给出一种改进SVPWM调制方法,可以消除偶次谐波,降低输出波形畸变率,并且分别采用传统SVPWM和改进SVPWM调制方法对风电系统的有源逆变部分进行了仿真,结果验证了改进SVPWM控制方法的可行性和有效性,从而为实践中完成较高电能质量风电并网提供理论基础。     

Characteristics of vector surge relays for distributed synchronous generator protection

This work presents a detailed investigation on the performance characteristics of vector surge relays used to detect islanding of distributed synchronous generators. A detection time versus active power imbalance curve is proposed to evaluate the relay performance. Computer simulations are used to obtain the performance curves. The concept of critical active power imbalance is introduced based on these curves. Main factors affecting the performance of the relays are analyzed. The factors investigated are voltage-dependent loads, load power factor, inertia constant of the generator, generator excitation system control mode, feeder length and R/X ratio as well as multi-distributed generators. The results are a useful guideline to evaluate the effectiveness of anti-island schemes based on vector surge relays for distributed generation applications.