Effect of dc-link capacitor on small signal stability of grid connected PV power plants

The intermittent nature of Photo-Voltaic (PV) systems, will excite the dc-side dynamics commonly ignored by assuming a constant dc-side voltage. The surplus oscillatory modes in grid-connected PV systems imposed by the dynamics of dc-side elements are assessed here. This issue becomes more argumentative in future power systems where distributed power sources are merely responsible for monitoring and maintaining the quality of their power supply. This paper indicates that size of dc-side parameters will affect the power quality and the dynamic stability of power system. In reality the oscillatory mode imposed to the ac side of the system introduces new issues in small signal stability of the power system. An analytical approach is presented to show that the frequency of this oscillatory mode varies subject to the output power of the PV units, and the dc-side element's size. It is founded that the frequency of this oscillatory mode can become risky if it reaches to the sub-synchronous frequency level. The newly proposed method in this paper provides the means for calculating the frequency imposed by the dc-side elements. This method is validated through the analytical analysis and time domain simulations.