Grid-connected renewable energy conversion systems (RECSs) are usually required by grid codes to possess the low voltage ride through (LVRT) and reactive power support capabilities so as to cope with grid voltage sags. During LVRT, RECS's terminal voltage becomes sensitive and changeable with its output current, which brings a great challenge for the RECS to resynchronize with the grid by means of phase-locked loops (PLLs). This paper indicates that loss of synchronism (LOS) of PLLs is responsible for the transient instability of grid-connected RECSs during LVRT, and the LOS is essentially due to the transient interaction between the PLL and the weak terminal voltage. For achieving a quantitative analysis, an equivalent swing equation model is developed to describe the transient interaction. Based on the model, the transient instability mechanism of RECSs during LVRT is clarified. Furthermore, a transient stability enhancement method is proposed to avoid the possibility of transient instability. Simulations performed on the New England 39-bus test system verify the effectiveness of the method.

中文翻译：

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LVRT过程中可再生能源转换系统的暂态稳定性分析和增强

电网规范通常要求并网可再生能源转换系统（RECS）具有低压穿越（LVRT）和无功功率支持功能，以应对电网电压骤降的问题。在LVRT期间，RECS的端电压变得敏感并且随其输出电流而变化，这给RECS通过锁相环（PLL）与电网重新同步带来了巨大挑战。本文指出，在LVRT期间，PLL的同步丢失（LOS）导致了并网RECS的瞬态不稳定性，而LOS主要是由于PLL与弱端电压之间的瞬态相互作用引起的。为了实现定量分析，开发了等效的摆动方程模型来描述瞬态相互作用。根据该模型，阐明了RCRT期间RECS的瞬态不稳定机制。此外，提出了一种暂态稳定性增强方法，以避免暂态不稳定性的可能性。在新英格兰39总线测试系统上进行的仿真验证了该方法的有效性。