Figure: Stability characteristic and output phase angle of PLL. θEP ist the phase angle, at which the PLL characteristic intersects the vPCC;q = 0 line and thus defines the equilibrium point of the PLL controller. CCA denotes the critical clearing angle, which defines the maximum allowed angle deviation during a fault in order to maintain stability.
It has been noticed that renewable generation such as photovoltaics and wind generation sometimes cannot stably ride through a voltage dip even if they passed mandatory grid compliance tests. This phenomenon is related to the stability behavior of the grid side converter, which connects the unit to the grid. In this work, we present a clear analysis of the stability behavior during and after low voltage ride through. This analysis includes all components and control circuits relevant for transient stability, which includes in particular the converter‘s phase-locked loop (PLL) and the DC link voltage control. We give analytical stability measures and present also show test results of transient characteristics of the grid-side converter. We focus on the transient characteristics of converter’s PLL at the moments of fault beginning and fault clearing and the PLL’s influence on the DC link voltage after fault clearing. This transient state analysis method can obtain more stable related information than the steady-state analysis method. In addition, the present test methods are analyzed from the aspect of the PLL, which turns out to be an integral part to guaranteeing stability in large-scale fault situations. Experiments based on the controller hardware-in-the-loop test verify the aforementioned analysis results.
The paper has been published in the journal International Journal of Electrical Power and Energy Systems in a cooperation of IEAN with Prof. Robert Schuerhuber, em.Prof. Lothar Fickert as well as Dr. Katrin Friedl.