A THREE-PHASE GRID-CONNECTED PHOTOVOLTAIC SYSTEM WITH REACTIVE POWER CONTROL
Le Minh Phuong, Phan Quoc Dzung, Nguyen Minh Huy
Faculty of Electrical amp; Electronic Engineering, HCMC University of Technology.
ABSTRACT
This paper presents the control model of three phase grid connected photovoltaic generation system with a new approach for reactive power regulation. The model contains a detailed representation of the main components of the system that are the solar panels, DC/DC converter, DC-link, a grid side three phase voltage source inverter (VSI) and output filters to reduce harmonic distortion of line current. In this paper, a complex control scheme including two PI controllers and cooperated with MPPT is proposed to stabilize DC voltage. A three phase grid connected voltage source inverter synchronizes to the grid by a robust phase- locked loop (PLL). The proposed model is simulated in Matlab/Simulink Toolbox and implemented using DSP TMS320F2812. Simulation and experimental results show the high stability and high efficiency of this three-phase grid-connected PV system. It also proves the excellent performance control units as improving a flexible regulation of power factor for the interval (0.5-1).
KEYWORDS: three phase Grid-connected inverter; maximum power point tracking (MPPT); photovoltaic (PV); solar energy.
INTRODUCTION
Nowadays fossil fuel is the main energy supplier of the worldwide economy, but the recognition of it as being a major cause of environmental problems makes the mankind to look for alternative resources in power generation [1].
The control strategies applied to distributed systems become of high interest. And how to improve the performance of grid inverter, increase the switching frequency and power density to meet power quality requirements has become a research hotspot in recent years [2].
In this paper, authors focus on two main problems, as a control of dc-link voltage and a reactive power control. This paper also proposed an improved MPPT control algorithm for photovoltaic under rapidly changing solar radiation. The simulation results on Matlab/Simulink show that detection MPPT quickly in conditions of
changes in radiation and temperature. Experiments are programmed on DSP TMS320F2812 and applied solar system consists of 5 PV panels connected in series, the rate parameters of each panel is 50Wp, 22V open circuit voltage. The simulation and experiment results show good response, high stability and high efficiency of this three-phase grid-connected PV system when the grid voltage and load changes.
The main circuit block diagram of the PV grid connected system is shown in Figure 1, including:
-
- DC-DC unit: DC boost converter is used to boost the PV array voltage and track the maximum solar power.
- DC-AC unit: The three phase inverter with bridge topology converts DC to sinusoidal AC and provides to load.
- Filter circuit: AC-side filter is composed of R-L-C to ensure the quality of grid current.
Figure 1. General diagram of grid connected photovoltaic system
PROPOSED CONTROL STRUCTURE FOR GRID CONVERTER
The proposed control structure of three- phase grid-connected PV system consists of PV panels is presented in Figure 2. In this structure, the DC_Link voltage is controlled accordingly to the necessary output power. Its output is the reference for the active current controller and the reactive current must be imposed to the system to control
the power factor. The dq control structure is normally associated with PI controllers since they have a satisfactory behavior when regulating dc variables. Since the controlled current has to be in phase with the grid voltage, the phase angle used by the abc to dq transformation module has to be extracted from the grid voltages. As a solution, filtering of the grid voltages and using arc-tangent function to extract the phase angle can be a possibility [3],[4]. In addition, the Phase-Looked Loop (PLL) technique [5] became a state of the art in extracting the phase angle of the grid voltages. Moreover, to control power factor the reactive power control loop is presented in the structure, where the actual power factor (PF) is calculated using the conventional instantaneous power definition in lsquo;abcrsquo; systems.
Figure 2. Proposed control structure of three phase grid-connected PV system.
The MPPT Method
The Incremental Conductance MPPT is implemented in this control structure. Figure 3 shows a P-V characteristic of PV panel and principle of Incremental conductance algorithm.
Figure 3. Principle
of Incremental conductance algorithm.
The differential of power (dP) is expressed following:
dP
radiation (from 0.2kW/m2 to 1kW/m2), which are found by proposed Incremental conductance.
dV 0
dP 0
dV
dP 0
dV
Where
At MPPT
Left of MPPT Right of MPPT
(1)
dP d (IV ) I V dI dV dV dV
I V I
V
(2)
Replace (2) to (1), we can get:
Power (W)
dI I At MPPT
dV V
dI I Left of MPPT
dV V
dI I Right of MPPT
dV V
(3.1)
(3.2)
(3.3)
(3)
Figure 5. P-V characteristic of PV system.
P-V Characteristic
Voltage (V)
Figure 6 shows the output power of
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具有无功功率控制的三相并网光伏系统
Le Minh Phuong, Phan Quoc Dzung, Nguyen Minh Huy Faculty of Electrical amp; Electronic Engineering, HCMC University of Technology.
摘要:本文介绍了三相并网光伏发电系统的控制模型,提出了一种新的无功功率调节方法。该模型包含系统主要组件的详细表示,即太阳能电池板,DC / DC转换器,直流链路,电网侧三相电压源逆变器(VSI)和输出滤波器,以减少线电流的谐波失真。本文提出了一种包含两个PI控制器并与MPPT配合的复杂控制方案,以稳定直流电压。三相并网电压源逆变器通过鲁棒的锁相环(PLL)与电网同步。所提出的模型在Matlab / Simulink工具箱中进行仿真,并使用DSP TMS320F2812实现。仿真和实验结果表明,该三相并网光伏系统具有高稳定性和高效率。它还证明了优秀的性能控制单元能够在间隔(0.5-1)内灵活调节功率因数。
关键词:三相并网逆变器;最大功率点跟踪(MPPT);光伏(PV);太阳能。
1 简介
如今,化石燃料是全球经济的主要能源供应商,但认识到它是环境问题的主要原因使人类在发电中寻找替代资源[1]。
适用于分布式系统成为高利率的控制策略。 而如何提高电网逆变器的性能,提高开关频率和功率密度以满足电能质量要求已成为近年来研究的热点[2]。
在本文中,作者关注两个主要问题,即直流链路电压控制和无功功率控制。 本文还提出了一种改进的太阳辐射快速变化的光伏MPPT控制算法。 Matlab / Simulink仿真结果表明,在条件下快速检测MPPT辐射和温度的变化。 实验在DSP TMS320F2812上编程,应用太阳能系统由5个串联的PV板组成,每个面板的速率参数为50Wp,开路电压为22V。 仿真和实验结果表明,当电网电压和负载变化时,该三相并网光伏系统具有良好的响应性,高稳定性和高效率。
光伏并网系统的主电路框图如图1所示,包括:
- DC-DC单元:DC升压转换器用于提升PV阵列电压并跟踪最大太阳能。
- DC-AC单元:具有桥接拓扑结构的三相逆变器将DC转换为正弦AC并提供负载。
- 滤波器电路:交流侧滤波器由R-L-C组成,以确保电网电流的质量。
图1.并网光伏系统的总图
2 用于网格转换器的控制结构
所提出的三相并网光伏系统的控制结构由PV板组成,如图2所示。在该结构中,DC_Link电压根据必要的输出功率进行控制。其输出是有源电流控制器的参考,必须将无功电流施加到系统进行控制功率因数。 dq控制结构通常与PI控制器相关联,因为它们在调节直流变量时具有令人满意的行为。由于受控电流必须与电网电压同相,因此必须从电网电压中提取abc到dq变换模块所使用的相位角。作为一种解决方案,过滤电网电压并使用反正切函数来提取相位角是可能的[3],[4]。此外,相位环(PLL)技术[5]在提取电网电压的相位角方面成为现有技术。此外,为了控制功率因数,在结构中呈现无功功率控制环路,其中使用lsquo;abcrsquo;系统中的传统瞬时功率定义来计算实际功率因数(PF)。
图2.三相并网光伏系统的模拟控制结构
2.1 MPPT方法
增量电导MPPT在该控制结构中实现。图3显示了PV板的P-V特性和增量电导算法的原理。
图3 增量电导算法的原理
功率差(dP)表示如下:
地点:
更换(2)(1),我们可以得到:
图4 增量电导算法的流程图
在Matlab-Simulink上模拟MMPT的增量电导算法。该PV系统包括20个太阳能电池板,分成两排平行;e行由10个面板组成。每个面板的额定参数是Pn = 50Wp,开路电压Voc = 21.42V,短路电流Isc = 3.11A。图5显示了系统的特性P -V,它的MPP对应于不同的太阳辐射(从0.2kW/m2到1kW/m2),这是通过提出的增量电导发现的。
P-V Characteristic
Voltage (V)
图5.光伏系统的P-V特性
图6显示了当太阳辐射为1kW/m2时系统的输出功率,然后它降低到0.2kW/m2并输出太阳能系统的直流电压。
图6:跟踪过程的模拟(日照步骤减少)
仿真结果表明,在隔离阶跃变化40%的情况下,系统可以保持稳定。 MPPT方法的高稳定性还将通过在不同日照条件下从PV板获取最大功率来确保系统的高效率。
2.2 Dc-Link控制
直流电压控制器用于产生电流控制器的参考电流值。
PI控制器用于DC电压,其输出被前馈到PF控制器的输出以获得有效电流id和无功电流iq的参考。
2.3 无功功率控制
对于具有正弦电压和正弦电流的三相电力系统,通常在平均概念上定义诸如有功功率,无功功率,有功电流和无功电流的量。但对于具有不平衡和失真电流的系统,平均概念并不合适[7]。
在这种并网光伏系统中,提供了有效的方法来计算和补偿三相系统的无功功率。实际功率P和Q使用(abc)系统中的传统瞬时功率定义通过(4)和(5)计算。
2.4 三相Pll结构
PLL用于确定相位角theta;和网格的频率,在本文中,使用传统的同步参考帧PLL。所用PLL的框图如图7所示。调节器PI用于控制该变量,该调节器的输出是电网频率。以这种方式,检测到电网电压的相位角,这是算法的输出。
图7. PLL的控制结构
3 控制方案的模拟
按照所提出的控制方案在Matlab/Simulink Toolbox中实现,如图8所示。仿真结果证明了所提出的控制方案的优异性能。该DC/DC转换器结合MPPT算法可以提升高达600V的电压和三相并网的DC/AC逆变器,额定电网参数为380V,50Hz。该光伏系统包括20块太阳能电池板分成两排平行; e行由10个面板组成。每个面板的额定参数是Pn = 50Wp,开路电压Voc = 21.42V,短路电流Isc = 3.11A。
模拟从以下条件开始:振荡为1kW/m2,温度为25℃,在t=0.27s时,升压DC/DC的输出电压达到580V,这三相逆变器与电网同步; 在时间t = 0.4s时,内部减少到0.6kW / m 2。功率因数受到控制,在模拟过程中给出1。
仿真结果如图8所示,显示系统在时间t = 0.27s处连接到网格并且始终是即使辐射下降40%,也能保持稳定。
图8.模拟三相并网光伏系统的仿真波形
4 使用DSP 2812实现硬件实现
在早期理论分析的基础上,在DSP TMS320LF2812上设计并实现了实验系统。它安装在胡志明市科技大学电力电子研究实验室。使用Tektronix TDS2024B示波器和Fluke 345 Power Quality Clamp Meter测量实验结果。光伏系统包括5个串联的50 Wp PV板,并通过三相Staco可变变压器连接到50-120V三相电网。
图9:实验模型
图9显示了在越南胡志明市大学电子研究实验室安装的拟议光伏系统中的逆变器和光伏电池板的照片。 实验实施在以下三个案例研究的越南气候的实际情况中,温度为30°C:
4.1案例研究1:
功率因数(PF)受控制并给出1.相电压波形和三相电流如图10所示。
图10:电压和电流的实验波形(PF = 1)
4.2案例研究2:
功率因数(PF)受控制并给出0.9。相电压波形和三相电流如图11所示。
图11:电压和电流的实验波形(PF = 0.9)
4.3案例研究3:
功率因数(PF)受控制并给出0.9。 相电压波形和三相电流如图12所示。
图12:电压和电流的实验波形(PF = 0.5)
5 结论
本文介绍了三相并网光伏系统的实施。在系统中应用新的控制方法可以在快速变化的日照过程中显着提高系统的稳定性。由于其在动态响应方面的改进,直流链路电压几乎保持恒定,它允许逆变器与电网同步并稳定系统,即使日照减少到40%。在日射的阶跃变化之后,控制器可以保持直流链路电压并使其保持接近MPP。可以控制系统的输出无功功率。PF的实际值非常接近参考值。它表明了该控制系统的效率。
6 参考文献
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- A. Lohner, T. Meyer, and A. Nagel, “A new panels-integratable inverter concept for grid connected photovoltaic systems,” in Proc. IEEE Int. Symp. Ind. Electron., Warsaw, Poland, vol. 2, Jun. 17–20, 1996, pp. 827–831.
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