Modular Non-isloated Photovoltaic Grid-tied Inverters And Generation Systems

The initial investment and generation cost of Photovoltaic(PV) generation system are muchhigher than that of other renewable energy systems, so the efficiency improvement of both the PVgrid-tied inverters and system is a significant effort to shorten the payback time and gain the economicbenefits faster. The key technologies for high efficiency modular PV grid-tied generation systemsbased on a DC interbus are researched in this thesis.A power conversion and management control based on an improved DC bus signaling (DBS) isproposed to optimize both the energy utilization and system stability. The operations of system arecategorized into four modes: islanding with battery discharging, grid-tied with rectification, grid-tiedwith inversion and islanding with constant voltage generation. The relastionship between systemoperation modes and system power flow is analyzed in detail. The DC bus voltage level is employedas an information carrier to determine system operation mode and mode switching. Hence, there is nocentralized controller, the system reliability is enhanced with the cost reduced. Control methods forPV converter, grid-tied converter, and battery converter are proposed with power balance of thesystem under extreme conditions, such as the islanding operation with full-charged battery,considered.Current coupling among the non-isolated grid-tied inverters is researched and the currentdecoupling condition given, based upon analyzing the equivalent operation modes of the invertersmentioned under different topologies and modulations. Analysis reveals that the bipolar PWM (PulseWidth Modulation) inverters can be operated in parallel with current decoupling naturally. Unipolardouble frequency PWM inverters can be operated in parallel with harmonic current of switchingfrequency coupled and fundamental components decoupled. Unipolar PWM inverters can not beoperated in parallel with current coupling. Improved full bridge inverters with low leakage current canachieve current decoupling naturally in parallel operation. A control for modular grid-tied PVgeneration system to improve European efficiency is proposed. Two DC bus voltage regulators withdifferent output limits are employed in each inverter, based on the inherent relationship of dc busvoltage and PV generation power. The suitable one of the regulators is enabled naturally by the PVgeneration power to run the inverter at the highest efficiency point as the first choice, and at the fullload point only if the PV generation power high enough. As a result the European efficiency of themodular system is optimized with no additional hardware cost.Non-isolated grid-tied inverters feature many advantages such as higher efficiency, lower cost,smaller size and weight. However, the leakage current may induce serious safety problem. In order tofind new inverter topologies with both low leakage current and high efficiency, Two types of full bridge inverter topologies which named non-Neutral Point Clamped (NPC) and NPC are researchedrespectively. A H6non-isolated full bridge PV grid-tied inverter is proposed based on the H5topology.In the power processing period, the grid-tied current flows through three power switches during oneof half line cycles, while flows through only two power switches during another half line cycle. Theconduction losses reduced and heat-stress balance benefited with the proposed H6topology than thatwith the H5topology. The power losses and costs are fairly compared among the H5topology, Herictopology and the H6topology. As a result, the efficiency of the proposed H6topology is higher thanthat of the H5topology while slightly lower than that of the Heric topology. And the common-modeperformance of the H6topology is higher than that of Heric topology while slightly lower than that ofthe H5topology.Two types of basic switching cells, positive-NPC and negative-NPC, and a method to build NPCfull bridge PV grid-tied inverters with these two basic cells are proposed to meet the demand fornon-isolated inverter topology applied in PV generation system. A family of novel NPC non-isolatedfull bridge PV grid-tied inverters, besides existing topologies such as oH5and FB-DCBP, is derived.Operation modes of the PN-NPC topology are analyzed as an example in detail. Finally, both of theefficiency and common-mode performance of the proposed PN-NPC and Heric topologies werecompared. And the comparison results show that the proposed PN-NPC topology features betterefficiency and common-mode performance than that of the Heric topology.High reliability, high efficiency and high power density are always the goals of power electronicdevices. Two topology generation methods for the five-level full bridge inverters are summarized asfollows:①a three-level full bridge inverter combining two input split capacitors and a clampingcircuit,②a three-level half bridge inverter combining a two-level bridge leg. By using this topologygeneration methods, a family of five-level dual buck full bridge inverter (FLDBFBI) topologies isproposed with high efficiency, high power density and low EMI. The operation principle andmodulation strategy of these proposed inverters were analyzed in details. Both the power loss and costare also fairly compared among these proposed topologies. As a result, the switch-series FLDBFBItopology has the highest efficiency, while the NPC FLDBFBI has the lowest cost.Experimental platforms are built with the test results given in detail to verify the researchesmentioned, respectively.