基于锁相环同步控制的双馈风机弱电网接入稳定性分析

Stability Analysis of Phase-locked Loop Synchronized DFIGs in Weak Grids

双馈感应风力发电机组(doubly-fed induction generator,DFIG)使用锁相环(phase-locked loop,PLL)跟踪端电压相位实现与电网的同步,采用该同步控制的风机接入弱电网易引发不稳定,因此,尝试解释产生该失稳现象的原因。针对弱电网并网条件下DFIG的单机无穷大系统,考虑其机侧变流器的有功控制、端电压控制及锁相控制回路之间的相互耦合关系,建立了相应的简化小信号模型。分析该模型发现,输出有功与端电压幅值之间的耦合联系会等价在锁相控制回路上引入一条额外的正反馈支路,该支路增益随接入交流电网强度的变弱而变大,进而恶化DFIG在弱电网下的稳定性。并进一步解析推导了不考虑端电压控制影响下的系统稳定边界条件。同时定性分析了不同端电压控制增益对系统稳定的附加影响。基于详细模型的时域仿真结果验证了该简化模型及分析的有效性。

Phase-locked loop(PLL)is commonly used in doubly-fed induction generator(DFIG)for its grid synchronization by tracking the phase angle of turbine terminal voltage;however a DFIG which is connected to weak grids by a synchronization technique will lead to instability.Consequently,we try to uncover the mechanism of phenomenon of instability.In non-ideal grid connection,terminal voltage phase angel is affected by the power outputs of DFIG,resulting in a dynamic coupling between PLL and DFIG power controls.Considering such a coupling effect,we propose a reduced small-signal model for a single DFIG infinite bus system.Based on this model,it is found that,owing to the coupling relation within active power and terminal voltage magnitude,an addition positive feedback branch is introduced to the PLL,and the loop gain increases as the system strength decreases,thus the stability of DFIG under weak grid operating conditions is deteriorated.Moreover,for open-loop terminal voltage control condition,the stability margin is analytically derived.Furthermore,the impact of terminal voltage control is also qualitatively analyzed.Full-order time-domain simulations are conducted to verify the effectiveness of the proposed simplified model and correlated analysis.