2nd-order lowpass filter HVDC PV generation autonomous microgrid coordinated control current source mode (CSM) data testing diesel generator differential feedforward distributed generations dynamic frequency support frequency control frequency stability grid impedance grid-connected inverter grid-connection/island switching process grid-tied inverter harmonic suppression hybrid energy storage impedance enhancement line commutated converter microgrid modeling method multi-inverter system multi-time scale nonlinear dynamic system optimal virtual resistor output impedance parameter identification power quality