Impedance Estimation of Photovoltaic Modules for Inverter Start-up Analysis

Starting-up of photovoltaic (PV) inverters involves pre-charging of the input dc bus capacitance. Ideally, direct precharge of this capacitance from the PV modules is possible as the PV modules are current limited. Practically, the parasitic elements of the system such as the PV module capacitance, effective wire inductance and resistance determine the startup transient. The start-up transient is also affected by the contactor connecting the PV modules to the inverter input dc bus. In this paper, the start-up current and voltages are measured experimentally for different parallel and series connections of the PV modules. These measurements are used to estimate the stray elements, namely the PV module capacitance, effective inductance and resistance. The estimation is based on a linear small-signal model of the start-up conditions. The effect of different connections of the PV modules and the effect of varying irradiation on the scaling of the values of the stray elements is quantified. The analysis of this paper can be used to estimate the expected peak inrush current in PV inverters. It can also be used to arrive at a detailed modelling of PV modules to evaluate the transient behaviour.