大规模光伏发电经串补输电线路并网系统强迫次同步振荡机制
Mechanism of Forced Subsynchronous Oscillation of Large-Scale Photovoltaic Power Generation Grid-Connected System with Series Compensation Tranmmission Lines
通讯作者: 陈武晖,男,教授,博士生导师,电话(Tel.):0351-6010031;E-mail:chenwuhui@tyut.edu.cn.
责任编辑: 孙伟
收稿日期: 2021-10-18
基金资助: |
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Received: 2021-10-18
作者简介 About authors
林勇(1973-),男,四川省广安市人,硕士,高级工程师,研究方向为电力系统规划技术.
大规模光伏经串补并网系统存在次同步振荡失稳风险,传统研究一般基于负阻尼振荡理论对此进行解释.本文将因最大功率跟踪控制(MPPT)导致的光伏间谐波作为扰动源,大规模光伏经串补并网系统作为受迫系统,采用强迫振荡理论揭示光伏发电基于扰动式MPPT与串补并网系统相互作用的次同步振荡机制,并在PSCAD/EMTDC仿真平台进行验证.结果表明:基于扰动式MPPT的光伏逆变器因交直流侧的调制耦合作用向系统输出间谐波电流,当该间谐波频率与系统固有弱阻尼模式频率接近时,可能导致严重的强迫次同步振荡问题,对系统稳定性造成冲击;算例仿真验证了所提理论的正确性.
关键词:
There exists the subsynchronous oscillation (SSO) instability risk in large-scale photovoltaic(PV) grid-connected systems with series compensation, which is generally explained by the negative damped oscillation theory. In this paper, the inter-photovoltaic harmonics due to maximum power point tracking (MPPT) control are used as the disturbance source and the large-scale PV grid-connected system with series compensation as the forced system. The forced oscillation theory is used to reveal the SSO mechanism of PV power generation based on the interaction between the perturbed MPPT and the series compensation grid-connected system, and verified in the PSCAD/EMTDC simulation platform. The results show that the perturbed MPPT-based PV inverter outputs interharmonic currents to the system due to the modulation coupling on the AC-DC side, which may lead to serious forced SSO problems when the interharmonic frequency is close to the frequency of inherent weakly damped mode of the system, causing a shock to the system stability. The simulation results verify the correctness of the proposed theory.
Keywords:
本文引用格式
林勇, 康佳乐, 余浩, 陈鸿琳, 杨彦霁, 陈武晖.
LIN Yong, KANG Jiale, YU Hao, CHEN Honglin, YANG Yanji, CHEN Wuhui.
综上所述,本文揭示了光伏逆变器的扰动式MPPT产生的间谐波激发强迫次同步振荡机理.主要研究内容如下:首先,结合调制理论研究,基于扰动式MPPT光伏逆变器的直流电压扰动与输出间谐波之间的数学关系,分析了间谐波产生机制;其次基于小信号模型特征值分析系统振荡特性;然后揭示间谐波激励受迫系统引发强迫振荡的机理及特性;最后,在PSCAD/EMTDC仿真平台建立详细的电磁暂态模型,验证了理论的正确性.
1 大规模光伏经串补并网系统
1.1 大规模光伏并网拓扑结构
以青海某大规模光伏串补系统为研究对象,系统主要由大型光伏电站、各电压等级变压器、输电线路等构成,如图1所示.整个系统由多个100 MW的大型光伏电站升压后汇流至330 kV母线,然后经330 kV/750 kV升压变电站汇入主电网.假设每个光伏电站的330 kV线路阻抗均相同,记为Z1;750 kV线路阻抗记为Z2;750 kV变电站共包含i台330 kV/750 kV变压器,每台变压器对应m个光伏电站进行升压.
图1
图1
大规模光伏经串补并网系统拓扑结构
Fig.1
Topology of a large-scale photovoltaic system with series compensation
1.2 光伏发电单元拓扑及控制结构
每个光伏电站由n'组发电单元构成,每组发电单元由0.5 MW的光伏阵列、单级式逆变器、LC滤波器及0.48 kV/35 kV变压器构成.单组发电单元拓扑及逆变器控制结构如图2所示,采用dq坐标系双闭环控制,外环为定直流电压控制,内环为电流控制,基于同步参考坐标系的锁相环(Phase Locked Loop,PLL)提供电网电压矢量定向角度.其中,下标d、q表示d、q轴分量,上标*表示参考值,abc为三相分量,Uc和Ic为逆变器出口的电压和电流,Ut为光伏发电单元输出电压,Udc和Cdc为直流母线电压和电容,
图2
图2
光伏发电单元控制及拓扑结构
Fig.2
Control and topology of photovoltaic power generation unit
2 大规模光伏系统强迫次同步振荡机制
本节分析光伏逆变器间谐波激发强迫次同步振荡机理.强迫振荡由包含弱阻尼振荡模式的受迫系统与外部激励源两个相互作用的部分,因此强迫次同步振荡机理涉及受迫光伏系统的固有振荡特性、间谐波的产生机制以及间谐波激励光伏系统固有振荡模式的机制.
2.1 光伏间谐波产生机制
图3
光伏发电的功率极值点Pmax即为光伏电池的最大功率点,当光伏电池输出电压为Umpp时输出功率最大,即Umpp为光伏电池的最大功率点(Maximum Power Point, MPP)所对应的电压值.随外部环境温度变化,日照辐射量实时变化,MPPT控制策略能够保证光伏系统输出功率跟踪最大可能捕获的功率.单级式逆变器通过控制直流电压指令值改变逆变器直流电压大小以实现最大功率点追踪.
扰动式MPPT流程如图4所示.在一个MPPT周期内,首先检测当前光伏电池输出电压、电流(I),并计算光伏电池的输出功率;其次将第k个MPPT检测周期(本周期)内光伏电池的输出功率P(k)、输出电压U(k)与上一周期电气量P(k-1)、U(k-1)进行比较,以决定该周期逆变器直流电压的指令值
图4
图4
光伏电池P-U输出特性流程图
Fig.4
Flow chart of P-U output characteristics of photovoltaic cell
(1) 若P(k)>P(k-1)且U(k)>U(k-1),则表明当前工作点位于MPP的左侧,此时系统应保持增加参考电压的扰动方式,即
(2) 若P(k)>P(k-1)且U(k)<U(k-1),则表明当前工作点位于MPP的右侧,此时系统应保持减少参考电压的扰动方式,即
(3) 若P(k)<P(k-1)且U(k)>U(k-1),则表明当前工作点位于MPP的右侧,此时系统应采用减少参考电压的扰动方式,即
(4) 若P(k)<P(k-1)且U(k)<U(k-1),则表明当前工作点位于MPP的右侧,此时系统应采用增加参考电压的扰动方式,即
综上,扰动式MPPT通过比较各电气量的大小关系,实现最大功率点跟踪,同时上述过程可利用下式进行定量描述:
式中:sgn(X)为阶跃函数, X为正时,sgn(X)输出为1,X为负时,sgn(X)输出为-1;dP、dV分别为前后周期光伏电源输出功率差值、直流电压差值,即dP=P(k)-P(k-1),dU=U(k)-U(k-1).为获取优良的MPPT效果,扰动式MPPT扰动步长与扰动间隔应当匹配[25],较小的扰动步长一般需要较高的扰动频率,以此保证MPPT准确追踪基础上获得最大的实时输出功率.
扰动观测法是广泛使用的MPPT算法,但是在外部温度及日照辐射恒定时,其在MPP附近存在三点式振荡现象[25],不断调整光伏逆变器直流电压,导致直流电压按照一定频率波动,直流电压脉动经逆变器调制到交流系统,进而产生间谐波.设扰动观测法采样周期为TMPPT,振荡中心点电压等于Umpp.在此情况下,
式中:fs为MPPT三点式振荡的基波频率,且fs=0.25TMPPT;n为谐波次数;
由式(2)可知,在外部温度及日照辐射恒定时,基于扰动式MPPT的光伏逆变器的直流电压存在持续扰动,采样周期及扰动步长将直接决定其扰动特性.在式(2)直流电源指令作用下,逆变器输出电流的d轴分量为
式中:
式中:
式中:
2.2 光伏经串补并网系统强迫振荡机理分析
式中:Δx为方程状态变量.等式左边等值光伏发电经串补系统,即受迫振荡系统,含串补输电系统/控制系统的固有振荡模式,则该固有模态角频率ωn和阻尼比ζ可以由式(6)求解,表示为
光伏交流系统中除含有基波主导分量外,还存在频率分别为
式中:F0和ω分别为次同步间谐波的幅值和角频率.等式右侧表示间谐波扰动源,此处为光伏逆变器因MPPT产生的次同步频段间谐波.
根据2阶线性常微分方程解的结构,式(8)的解由通解和特解两部分组成:
式中:Δx1(t)为通解,是与式(8)对应的齐次微分方程式(6)的解,也称自由分量;Δx2(t)为特解,即为式(8)的稳态解,也称强迫分量,代表间谐波小扰动引起的稳态分量.
求解式(6),可得式(8)的通解:
式中:B1和B2为光伏系统初始条件决定的常数;ωd为角频率,且
由式(10)可知,Δx1(t)为暂态分量,光伏系统固有参数决定其振荡形态:系统正阻尼时,该分量衰减消失;系统负阻尼时,该分量则呈现出发散振荡.
由式(8)求解可得特解:
式中:
作为间谐波小扰动引起的稳态分量,Δx2(t)的振荡频率与扰动频率相同.其幅值和相位取决于受迫光伏系统的固有特性和间谐波小扰动.结合式(12)和(13),引起光伏强迫次同步振荡的主要条件如下:
(1) 光伏逆变器输出间谐波频率与受迫系统固有频率接近.当
(2) 受迫系统的次同步振荡模式阻尼较弱.由式(14)可知,ζ与Bmax近似成反比关系,即次同步振荡模式阻尼越小,所激发的强迫振荡幅值越大.
(3) 光伏间谐波小扰动需要达到必要的幅值.由式(14)可知,F0与Bmax成正比,即F0越大,则Bmax越大.光伏电站中光伏发电单元高度集中,一般采用同厂家光伏设备,其MPPT技术相同,因此各光伏逆变器产生的间谐波频率接近,易积累出一定的幅值,从而对系统稳定性造成严重威胁.
3 算例仿真验证
表1 交流系统参数
Tab.1
设备数量 | 电压等级 | 变压器 | 输电线路 | ||||
---|---|---|---|---|---|---|---|
漏电抗(p.u.) | 励磁电流/% | 额定容量/(MV·A) | 电压等级/kV | 线路阻抗(p.u.) | |||
i=4 | 0.48 kV/35 kV | 0.065 | 0.65 | 1 | 330 | Z1=0.039+j0.297 | |
m=3 | 35 kV/330 kV | 0.140 | 0.44 | 240 | 750 | Z2=0.015+j0.196 | |
n’=200 | 330 kV/750 kV | 0.150 | 0.10 | 500 |
光伏电站中单个发电单元的直流侧电压和电容分别为0.78 kV和 7800 μF,并网点电压为0.48 kV,其他参数如表2所示.其中,kp1、ki1和kp2、ki2分别为电流内环d、q轴的比例和积分参数,kp3和ki3分别为直流电压外环的比例和积分参数,kp4和ki4分别为锁相环比例和积分参数.
表2 光伏逆变器参数
Tab.2
电流控制参数 | 直流电压控制参数 | 锁相环参数 | 交流侧滤波器 |
---|---|---|---|
kp1= kp2=0.5 | kp3=3.5 | kp4=50 | Lf=0.6 mH |
ki1= ki2=12.5 | ki3=285 | ki4=10000 | Rf=0.5 Ω Cf=35 μF |
3.1 光伏经串补并网系统固有次同步振荡模式
表3 光伏系统特征值
Tab.3
模式 | 特征值 | f/Hz | ζ /% | |
---|---|---|---|---|
λ1,2 | -389.420± | j6906.484 | 1099.200 | 5.630 |
λ3,4 | -391.137± | j6293.558 | 1001.651 | 6.203 |
λ5,6 | -0.898± | j313.284 | 49.861 | 0.287 |
λ7,8 | -6.670± | j61.992 | 9.866 | 10.698 |
λ9,10 | -0.079± | j3.272 | 0.521 | 2.438 |
λ11 | -807.660 | - | - | |
λ12 | -792.464 | - | - | |
λ13 | -25.803 | - | - | |
λ14 | -25.758 | - | - |
图5
图5
次同步振荡模式特征值轨迹
Fig.5
Eigenvalue trajectory of subsynchronous oscillation mode
由图5可知,随着kp4的增加,λ7,8阻尼增加频率略有降低;随着ki4的增加,λ7,8阻尼减小频率增加.系统在特定的锁相环参数下呈现弱阻尼的次同步振荡模式,存在与光伏间谐波相互作用的强迫振荡风险.此外,在不合理的锁相环参数下,λ7,8呈现负阻尼特性,存在自激失稳风险.
3.2 光伏间谐波验证
图6
图6
扰动式MPPT光伏间谐波时域仿真及频谱分析结果
Fig.6
Time domain simulation and spectrum analysis results of disturbance MPPT photovoltaic interharmonics
3.3 光伏串补系统强迫次同步振荡
对图1所示的固有弱阻尼振荡模式与光伏间谐波之间交互影响下的强迫振荡现象进行电磁暂态仿真.需要明确的是,基于d、q轴下的小干扰动态模型所得系统固有振荡模式对应于系统的d、q轴分量,即系统的直流量;逆变器输出间谐波在d、q轴下的扰动分量频率与系统固有振荡模式相接近,并满足于式(13)推导出的3个次同步强迫振荡发生的条件.
3.3.1 间谐波频率的影响
根据2.3节研究成果,在大规模光伏经串补并网系统中,次同步振荡模式的阻尼较弱且频率段与fs较为吻合,有可能被激发出强迫振荡.同样基于PSCAD/EMTDC平台建立时域仿真模型,利用前文算例,保留变压器励磁支路的大规模光伏经串补并网系统电气参数,并调节系统锁相环参数至kp4=20、ki4=10000 时,系统固有振荡模式对应的特征值为-0.727± 9.679×2π.基于扰动式MPPT基本原理,在直流电压指令值中直接施加式(2).考虑小扰动计算误差后,以系统固有振荡频率9.769 Hz为中心的微调系统光伏并网逆变器所发出的间谐波频率,设置系统运行至40 s时施加直流电压扰动,直流电压扰动幅值(Uh)恒定为 5 V,不同扰动频率(fh)时系统并网有功功率(P')及无功功率(Q)波形如图7所示.可知,当fh=9.35 Hz 时,系统振荡幅值最大,如图7(c)所示,则 9.35 Hz 即为系统实际的固有振荡频率.当间谐波激励频率与系统固有频率一致时,强迫次同步振荡幅值最大;随着激励频率与固有频率相差越大,振荡幅值越小,与式(13)分析的条件一致.
图7
图7
不同频率直流电压扰动下系统时域仿真结果
Fig.7
System time domain simulation results at different frequency DC voltage disturbances
3.3.2 间谐波幅值的影响
图8
图8
不同幅值直流电压扰动下系统时域仿真结果
Fig.8
Time domain simulation results of system at different amplitude DC voltage disturbances
4 结论
本文从强迫振荡的角度对大规模光伏发电经串补并网系统的次同步振荡机理及动态特性开展研究.首先分析了基于扰动式MPPT光伏逆变器间谐波特性;其次分析了光伏发电强迫次同步振荡的发生机理及特性;最后在PSCAD/EMTDC中验证了理论的正确性.主要结论如下:
(1) 基于扰动式MPPT的光伏并网逆变器在外部环境稳定时发出间谐波,该间谐波作为光伏经串补并网系统的扰动源,在光伏控制系统固有次同步振荡模式相互作用下,大规模光伏发电经串补并网系统存在强迫次同步振荡风险,当光伏并网系统固有模态为正阻尼时,仍然可能发生幅值较大的次同步振荡.
(2) 大规模光伏发电经串补并网系统作为受迫系统强迫次同步振荡的条件:间谐波幅值较大,间谐波频率与系统中某个固有次同步振荡模式的频率接近,系统存在呈现弱阻尼的固有次同步振荡模式;间谐波幅值越大、间谐波频率与系统固有次同步振荡模式越接近则引发的系统强迫振荡的振荡幅值越大.
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电力电子设备高占比电力系统电磁振荡分析与抑制综述
[J].
Overview of the analysis and mitigation methods of electromagnetic oscillations in power systems with high proportion of power electronic equipment
[J].
多光伏发电单元并联接入弱交流系统的稳定性分析
[J].
Stability analysis of multiple parallelled photovoltaic power generation units connected to weak AC system
[J].
Impact of series compensation on operation performance of large-scale PV plants
[J].
Research on clustering equivalent modeling of large-scale photovoltaic power plants
[J].There are dozens or hundreds of grid-connected inverters for large-scale photovoltaic power plants. In order to facilitate the study of the impact that large-scale photovoltaic power plants have on the power system while avoiding the need to establish a detailed model for each inverter, it is necessary to establish the equivalent model of large-scale photovoltaic power plants. Power generation units of the same type are combined by K-means clustering algorithm to reduce the simulation scale in number. According to the actual data of Shenmu large-scale grid-connected photovoltaic power station, employing RT-LAB software is used to conduct simulation verification and error analysis. The simulation results verify the proposed clustering equivalent modeling method is effective and can accurately track photovoltaic power station’s dynamic characteristics.
Subsynchronous oscillation of PV plants integrated to weak AC networks
[J].
DOI:10.1049/iet-rpg.2018.5659
[本文引用: 1]
The grid-connected photovoltaic (PV) power is booming, and large-scale PV power is mostly integrated to grid through long transmission lines; however, PV systems may face the threat of subsynchronous oscillation (SSO) when AC system strength is weak. This study establishes the sequence impedance model of grid-connected PV system; mechanism and characteristic of SSO in PV plants integrated to weak AC networks are delved into through impedance-based analysis method. The results show that, under certain conditions, the impedance of PV system is capacitive in the subsynchronous frequency domain, when the networks are weak and the PV capacity is high, the resonance may occur between the capacitive PV system and inductive AC grid. In addition, a smaller proportional gain of current loop may increase the risk of SSO; however, a larger proportional gain and integral gain of phase-locked loop can ease the SSO. Finally, time domain simulation based on PSCAD/EMTDC is conducted to validate the results of the impedance-based analysis. The research here can provide guidance to the project of scaled PV plants integrated to weak AC networks in some sense.
Stability analysis and operation control of photovoltaic generation system connected to weak grid
[C]//
Small-signal modeling and analysis of grid-connected photovoltaic generation systems
[J].
Optimization of perturb and observe maximum power point tracking method
[J].DOI:10.1109/TPEL.2005.850975 URL [本文引用: 3]
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