CN110880779A - Mmc网压不平衡状态下直流侧功率振荡抑制方法 - Google Patents
Mmc网压不平衡状态下直流侧功率振荡抑制方法 Download PDFInfo
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
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- H02J3/24—Arrangements for preventing or reducing oscillations of power in networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/26—Arrangements for eliminating or reducing asymmetry in polyphase networks
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- H—ELECTRICITY
- H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
- H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
- H02J3/00—Circuit arrangements for ac mains or ac distribution networks
- H02J3/36—Arrangements for transfer of electric power between ac networks via a high-tension dc link
- H02J2003/365—Reducing harmonics or oscillations in HVDC
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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Abstract
本发明公开了一种MMC网压不平衡状态下直流侧功率振荡抑制方法,提出了一个基于拉格朗日乘数法的约束优化问题,在减少环流和上下桥臂能量之和的振荡前提下,直接在abc坐标系下导出MMC的最优环流参考值,而且不需要进行复杂的数学变换,并根据计算最优环流参考值的解析表达式,设计了用于消除直流侧功率振荡的控制策略,由于约束优化的结果,该控制策略可以有效利用MMC内部能量缓冲能力提供的自由度,从而有效地将交流电网与直流线路解耦,防止了交流侧不平衡时,交流侧的功率振荡传播到直流系统中。
Description
技术领域
本发明涉及柔性直流输电(MMC-HVDC)系统交流侧故障下直流侧功率传输,特指一种MMC交流侧不平衡状态下直流侧功率振荡抑制新方法,属于电力技术领域。
背景技术
基于模块化多电平换流器(Modular Multilevel Converter,MMC)的高压柔性直流输电技术是以电压源换流器为核心的新一代直流输电技术,是当今世界上电力电子技术应用的制高点。相比传统直流输电技术,柔性直流输电技术不存在换相失败问题,可增加系统动态无功储备,改善电能质量,解决非线性负荷、冲击性负荷对系统的影响,具有更强的可控性和灵活性,在大规模清洁能源并网、海岛供电、交流电网同步/异步互联、构建直流电网等方面具有广泛的应用。
对于柔性直流输电系统,尤其是对于未来的多终端系统而言,重要的是要避免交流侧故障时来自交流电网的功率振荡传播到直流系统,但目前对于MMC交流侧故障控制的研究主要集中在传统VSC-HVDC控制策略的基础上进行改进,控制结构愈发复杂,主要有如下几种:瞬时有功–无功控制,对称正序控制,正负序控制,平均有功–无功控制,灵活正序和负序控制。这些控制方法大多基于经典控制理论进行设计,需要进行复杂的数学变换,多采用PI控制器,其结构虽然简单,但控制过程调节时间过长,且控制器参数依赖于精确的系统模型而不易整定,不利于系统长期运行使用,所以在一定程度上限制了其应用,如何把效果较好的控制方法合理地应用于MMC-HVDC系统中仍然值得深入讨论。
发明内容
本发明的创新之处在于,提出了一个基于拉格朗日乘数法的约束优化问题,直接在abc坐标系下得到MMC的最优环流参考值,以此可以调节上下桥臂能量的分布,实现功率区间的优化,且不需要进行复杂的数学变换,并根据计算最优环流参考值的解析表达式,设计了用于消除直流侧功率振荡的控制策略,该控制策略可以有效利用MMC内部能量缓冲能力提供的自由度,从而有效地将交流电网与直流线路解耦,实现了交流侧不平衡状态下功率的稳定传输。
本发明提供了一种MMC交流侧不平衡状态下直流侧功率振荡抑制新方法,包括:
步骤S1:根据MMC的数学模型,计算得到上下桥臂能量之和,之差的表达式;
步骤S2:根据拉格朗日乘数法的要求,确定要优化的目标即减少能量振荡和约束条件,构建目标函数表达式,并求解;
步骤S3:考虑多目标优化,引入加权系数,重新得到MMC环流的参考值;
步骤S4:考虑直流侧功率的振荡,加入新的约束条件,从而能够减少直流侧功率振荡的环流参考值,并以此设计控制结构。
有益效果
本发明直接在abc坐标系下得到MMC的最优环流参考值,以此可以合理的调节上下桥臂能量的分布,实现功率区间的优化,且不需要进行复杂的数学变换,能够将交流电网与直流线路解耦,使MMC在不平衡故障期间充当“功率振荡防火墙”或“能量缓冲器”,防止了交流侧不平衡时,交流侧的功率振荡传播到直流系统中。
附图说明
图1是本发明提供的MMC等效电路图;
图2是本发明所提供的控制框图;
具体实施方式
根据图1所示的等效电路图,我们可以得到j相环流的表达式:
其中j=a,b,c,这里我们定义上下桥臂的差模电压为udiffj,上下桥臂的共模电压ucomj,即
这里桥臂阻抗Z由电阻R0和电感L0串联组成,则桥臂环流在桥臂阻抗上的压降可以表示为:
根据已有文献,我们可知上下桥臂能量之和W∑k和能量之差WΔk:
观察得到,可以通过控制桥臂环流idiffj来控制能量的波动,为了方便对直流侧的功率进行控制,考虑到通常情况下,直流电压vdc通常为常量,我们可以通过控制vdcidiffj来控制能量的波动。
为了得到所需的目标函数,这里引入2个边界条件:
这里我们采用PI控制,可以得到如下表达式:
这样根据拉格朗日乘数法,可以得到所需的目标函数如下:
同样可以解得由此我们可以看出这是一个多目标优化的问题,为了更好的解决可能存在的目标优化间的冲突问题,引入一个加权因子α,其中α∈[0,1],这样就可以转化为减少这样同样考虑以上约束条件我们可以得到新的环流参考值
为了确保直流侧功率的稳定传输,需要在上述的基础之上,引入一个新的边界条件,
从而得到一个新的目标函数:
求解得:
由此我们根据式(11)得到的最优环流解析式,设计了如图2所示的的直流侧振荡的控制策略。
以上所述仅为本发明的实施例而已,并不用以限制本发明,凡在本发明精神和原则之内,所作任何修改、等同替换、改进等,均应包含在本发明的保护范围之内。
Claims (2)
1.一种MMC网压不平衡状态下直流侧功率振荡抑制方法,其特征在于,该方法包括以下步骤:
步骤S1:根据MMC的数学模型,计算得到上下桥臂能量之和,之差的表达式;
步骤S2:根据拉格朗日乘数法的要求,确定要优化的目标即减少能量振荡和约束条件,构建目标函数表达式,并求解;
步骤S3:考虑多目标优化,引入加权系数,重新得到MMC环流的参考值;
步骤S4:考虑直流侧功率的振荡,加入新的约束条件,从而能够减少直流侧功率振荡的环流参考值,并以此设计控制结构。
2.根据权利要求1所述的方法,其特征在于直接在abc坐标系下得到MMC的最优环流参考值,以此可以调节上下桥臂能量的分布,实现功率区间的优化,且不需要进行复杂的数学变换,并能根据计算最优环流参考值的解析表达式,可以设计用于消除直流侧功率振荡的控制策略。
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CN114336686B (zh) * | 2022-01-10 | 2023-11-07 | 武汉大学 | 一种基于自适应预测控制系统的电网区域振荡抑制方法 |
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