Modeling and Analysis of Restoration Control in Parallel Microsource Inverters: A Case Study

The paper suggests a method/strategy to compensate the deviations in the system frequency and output voltage, caused by an inverter output impedance, droop coefficients terms and amplifier gain in the RDC (robust droop control) model of single phase inverter. The compensation is acquired through adopting a TDC (transient droop characteristics) enabled droop control. The suggested technique, to achieve proportionate power sharing, nominal output voltage and frequency at steady state, is applied to a communication-less system of two parallel microsource inverters. The small signal model and eigenvalue analysis are used to characterize the virtual resistor selection criteria in the inverter control block. The damping properties of power sharing responses improve when a PI controller is used in place of an integrator in a voltage-power (V-P) loop, The effectiveness of the suggested method is evaluated against model which is not equipped with the TDC. Subsequently, the restoration function of centralized secondary control is discussed for a standard microgrid setup, consisting of two inverters connected to bus loads via feeders. The control mechanism is illustrated to fix voltage and frequency of microgrid. The synchronous reference frame, also known as the d-q reference frame, describes the submodels of inverter control, such as droop controller, voltage controller, current controller, etc., in order to derive nonlinear equations that represent the dynamics of the system. The procedural steps for obtaining the small-signal model are thoroughly described. The concept of VI (virtual impedance) is discussed followed by the control algorithm to obtain its optimum parameters. The proposed schemes/models are verified through time domain simulations in Matlab/Simulink tool.