Stability Improvement of Series Stacked Buffer Circuit in Single Phase Solar Inverter

Small scale rooftop solar photovoltaic (PV) installations witnessed significant growth in the past decade. A single phase solar inverter is an integral part of these systems. In single phase systems, there is an instantaneous mismatch between PV power and grid power, which is conventionally supported by Aluminum Electrolytic Capacitors (AECs). These capacitors reduce the expected lifetime of PV inverter due to their rapid degradation at high operating temperatures. Series Stacked Buffer (SSB) circuit is considered as one of the best replacement for these capacitors. The SSB circuit facilitates power dense and reliable realization of solar inverter. However, the operation of SSB circuit is strongly dependent on small signal equivalent series resistance (SS-ESR) value of PV array. This dependence may lead to unstable operation when the value of SS-ESR becomes low. This also reduces the viable range of operation of PV inverter. In this work, the instability issue in the SSB circuit with a source having low SS-ESR is analytically established. Further, a control technique to improve stability of the system is proposed. The proposed controller extends the operation of PV inverter to a wide range of SS-ESR values. The controller performance is validated with the help of circuit simulations in MATLAB/Simulink.