Research On Parallel Control System Of UPS Photovoltaic Off-grid Inverter

The independent PV off-grid power generation system can effectively solve the power supply problem of remote areas without electricity,less electricity or some special occasions.It also provides UPS(Uninterrupted Power Supply)for the power supply of important power system and avoids the significant loss of power.Failure of a single UPS power supply may cause the system to be paralyzed.Parallel redundant control can meet the needs of highly reliable power supply sites.The parallel control system is convenient for installation,design and expansion,and can reduce the current stress of power switch devices.Because the control strategy is more complex,and the inverter power supply between the active and reactive power sharing is not ideal,seriously affecting the application of parallel inverter power supply,so to solve the wireless parallel control system of active power and reactive power sharing problem is the key to realize parallel.In this paper,the single-chip microcomputer is used as the master chip to design the synchronous buck hardware circuit of the PV off-grid generator system.The working principle of the synchronous buck circuit is analyzed,and the synchronous buck parallel circuit is designed.An improved MPPT maximum power point tracking is proposed Strategy.Using DSP2809 as the main control chip,the bidirectional full bridge DC converter and bidirectional DC-AC converter hardware circuit of parallel control subsystem are designed.The working principle of bidirectional full bridge DC converter is analyzed.SG3525 is used to realize pre-stage bus boost and buck,and the working principle and mathematical model of bidirectional DC-AC converter are analyzed.The single-phase voltage type SPWM double closed-loop control strategy is proposed for the switching of rectification and inverter operation modes.The control system device is selected,and the filter,transformer,drive circuit,voltage and current detection circuit and parallel switch circuit were designed.The relationship between active and reactive power with output amplitude and phase under different output impedance conditions is analyzed.The PQ sagging property under induced impedance is deduced,and the mathematical model is established.An improved PQ sag method is proposed to calculate the sagging coefficient and the induced current.The equivalent output impedance frequency characteristics after adding the virtual impedance are analyzed.The principle and influence of the circular current in parallel are analyzed,and the method of coupling inductance suppression circular current is proposed.In the MATLAB simulation environment,the synchronous Buck is simulated under different load conditions,and the two inverters are simulated in parallel under different parameters.The system software design,including MPPT,charge management,power calculation and work mode switching.Two 3kW UPS PV inverter models were fabricated.The feasibility of MPPT charging control scheme and the improved PQ drooping method was verified by experiments.Current imbalance is not greater than 3%,with high parallel current sharing accuracy.The control system realizes the single inverter 3kW rectifier charging,loading and fast switching between commercial power and inverter power supply.The commercial power to the inverter switching time is less than 9ms,and the inverter switching time to commercial power is less than 5ms.