Common DC Bus-based Modular PV Grid-connected Generation Systems

Nowadays, in order to relieve the energy crisis and environmental pollution, the renewable and green energy sources are widely applied. In the photovoltaic (PV) generation system, the modular structure is fascinated due to the system efficiency improvement and high reliability, which becomes a research hotspot in the academic circles. In this paper, the key technologies of the modular PV grid-connected generation system based on the common DC bus are further researched. For the DC module as the front-end of the system, the built-in transformer and the built-in transformer and voltage doubler are applied to realize the high step-up and high efficiency conversion. For the parallel inverter modules as the back-end of the system, the linear power control is used to improve the dynamic performance, stablility and reliability.Firstly, by inserting the transformer into the conventional interleaved Boost converter, the non-isolated high step-up converter is derived. The built-in transformer structure is generated as the voltage gain extention cell by modifying the universal rectifier structure, which extends the voltage gain and avoids the extremely high duty-cycle of the switch. Meanwhile, the voltage stress of the switch is decreased, which makes the low on-resistance switch can be used in the converter to reduce the conduction losses. The active-clamp circuit is adopted to recycle the leakage inductance energy of the built-in transformer and depress the switch turn-off voltage spikes. The ZVS soft switching performance is obtained for all the switches to decrease the switching losses and increase the efficiency. The reverse-recovery current of the diode is controlled by the leakage inductance of the built-in transformer, which reduces the reverse-recovery losses and EMI noises. The input current ripple is decreased effectively due to the input inductors and interleaved structure of the converter to reduce the impact of the input capacitors. Furthermore, the derivation methodology and voltage step-up principle are summarized in the paper. According to the proposed principle, a series of non-isolated high step-up converters with the built-in transformer configuration are obtained, which provides a new generation law for the topology derivation.Secondly, by combining the voltage doubler rectifier circuit and the built-in transformer, the built-in transformer voltage doubler cell is derived to extend the voltage gain of the converter further. The voltage stress of the output diodes is decreased in the improved converter. Meanwhile, the higher voltage gain and the lower switch stress are realized to make it more suitable for the high step-up conversion. In order to simplify the driver circuit, the passive-clamp scheme is applied to increase the reliability of the converter. The ZCS turn-on of the switches and the nature turn-off of the clamp diodes are achieved to decrease the switching losses and increase the efficiency.Then, a50kW PV grid-connected inverter module is designed in the paper. The T-type three-level topology is adopted to increase the efficiency and reliability. The small signal model of the T-type three-level grid-connected inverter is established to obtain the control system block diagram and design the key control parameters. Moreover, reactive power perturbation is applied as the active islanding detect scheme. The experimental results have demonstrated that the excellent performance is obtained for the inverter module. Each harmonic of the grid-connected current is accord with the country standard. The active islanding detect method is effective. The multiple layers design is helpful for the heat dissipation.Finally, the modular PV grid-connected inverter system with hierarchical control is designed. The output current is controlled by the module and the DC bus voltage is controlled by the central controller in the up control layer. The linear power control scheme is adopted, which imports the bus voltage information into the reference of the current control loop. The output current can be regulated fast with the bus voltage variation, which improves the system dynamic performance. Even if the communication bus is fault, the system can maintain the operation, which increases the system reliability. Furthermore, a100kW engineering prototype of modular PV grid-connected inverter is built to demonstrate the industrial application value of the linear power control scheme.The research on the modular PV grid-connected generation system based on the common DC bus is helpful to benefit the development of the PV generation system.