Simplified embedded control scheme for two‐stage multistring off‐grid inverter

Abstract

This study proposes an embedded control for an off-grid single-phase multistring inverter with front-end DC-DC conversion stage followed by a synchronised H-bridge. In this inverter, the duty cycle of DC-DC converter is varied in the form of fully rectified sinusoidal fashion to produce a similar output voltage across the DC-link capacitor. This unidirectional voltage is converted into an AC voltage by the synchronised H-bridge. An application of this inverter topology with the proposed control scheme in micro-grid is investigated in the present study. The proposed control scheme not only possesses better steady-state performance with both linear and non-linear loads but also provides fast dynamic response during a step change in load. A digital controller based on field programmable gate array is utilised for the simultaneous generation of high-frequency switching pulses for DC-DC converter and synchronised fundamental frequency switching pulses for Hbridge. The entire system is simulated in MATLAB/Simulink® environment and its functioning is verified with linear and non-linear loads. The hardware prototype of inverter is also built in the laboratory and its performance is validated. Further, the better performance of the proposed system for supplying household appliances is ascertained in the laboratory. NomenclatureAC alternating current C filter capacitance C min minimum value of filter capacitance C CCM continuous conduction mode DC direct current ESR equivalent series resistance f C corner frequency f S switching frequency FF fundamental frequency HF high frequency I O output current of DC-DC converter I O max maximum value of DC load current I O min minimum value of DC load current L inductance L min minimum inductance L for CCM operation M VDC DC voltage transfer function of the DC-DC converter P O output power of DC-DC converter P O max maximum output power of DC-DC converter P O min minimum output power of DC-DC converter r C ESR of filter capacitor r C max maximum value of r C R L DC load resistance R L max maximum value of load resistance R L R L min minimum value of load resistance R L S power semiconductor switch of the DC-DC converter S 1 , S 2 , S 3 , S 4 power semiconductor switches of the H-bridge T ON conduction period of power semiconductor switch 'S' T OFF non-conduction period of power semiconductor switch 'S' THD total harmonic distortion V AC AC output voltage of H-Bridge V C voltage across C V DCinput DC voltage of DC-DC converter V r peak-to-peak value of V C ripple voltage Δi L max maximum inductor ripple current δ duty cycle η efficiency of the converter

Power decoupling strategy based on ‘virtual negative resistor’ for inverters in low‐voltage microgrids

“…In grid‐connected operation, the interfacing inverters are controlled to exchange power with the grid efficiently. To fulfil the above goals, the droop method is usually employed to control the inverters [6–8].…”