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Abstract
With the growing electrical energy demand and environmental concern for fossil fuels, renewable energy has become a mainstream source of electricity. However, the grid integration of renewable energy sources (RES) brings a series of issues on both of grid-side and dc-side. In single-phase systems, the representative issues are voltage rise in the grid-side and second harmonic current (SHC) in dc-side.
Nowadays, photovoltaic (PV) inverters can provide grid-supporting functions, such as volt-watt (P(V)) response, volt-var (Q(V)) response, fixed power factor or reactive power (fixed cos φ/Q) operations and active power (Q(P) or cosφ(P)) response. Such functions are increasingly required by grid standards around the world to address voltage rise in networks. This thesis develops simulation models for six single-phase transformer-less PV inverters to study their leakage currents and evaluate the efficiency of PV inverters working with grid-supporting functions. Simulation results show that the reactive power injection/absorption in grid-supporting functions will decrease the efficiency of PV inverters. Combining with the active power curtailment (e.g. P(V) response) function, the efficiency of PV inverters varies across the range of grid voltage or the varying power factors. Experimental analysis using three commercial PV inverters verifies the simulation results but also demonstrates the impact of each proprietary controller implementation to overall efficiency.
Using Energy storage system (ESS) is another solution to address voltage rise in networks. As one of the most popular types of ESS, battery energy storage system (BESS) has experienced a rapid growth in recent years. However, SHC occurs in BESS and reduces the lifetime of battery when they are connected to the grid via single-phase inverters. This thesis takes BESS as an example to study the solutions to address SHC in dc-side. Three real-time simulation (RTS) models for BESS with the function of SHC reduction, e.g. ripple-port inverter, boost inverter and H-bridge with series-connected flyback converter, are developed. The effectiveness of three topologies is demonstrated through RTS results, which also show how the topologies and corresponding control schemes can reduce the SHC significantly. The control schemes of ripple-port inverter are also verified by control-hardware-in-loop (CHIL) simulation.