Parallel circulating-current restraining and neutral-point balancing method of T-type three-level photovoltaic grid-connected inverter

Abstract

The invention discloses a parallel circulating-current restraining and neutral-point balancing method of a T-type three-level photovoltaic grid-connected inverter. The method for achieving a neutral-point balance comprises the following steps: step one, converting three levels into a two-level SVPWM, figuring out a status switching time for each phase bridge arm on the basis of the SVPWM; and step two, adjusting the status switching time for each phase bridge arm, thereby acquiring a final PWM signal. After achieved the neutral-point balance, a corrected value of a small vector is figured out based on a PI controller; accordingly, a circulating-current restraint is achieved by changing a turn-on time of the small vector. The adopted neutral-point balancing method is able to deal with fluctuations of capacitors on a direct current side, and is able to improve the quality of output waveforms by a certain degree. The parallel circulating-current restraining and neutral-point balancing method of the T-type three-level photovoltaic grid-connected inverter is able to simply and conveniently achieve the restraint of the circulating current, and is simple in control. The parallel circulating-current restraining and neutral-point balancing method of the T-type three-level photovoltaic grid-connected inverter is able to achieve a neutral-point balance control and the circulating-current restraint, thereby solving the difficulty in connecting T-type inverters in parallel. Comparing to a conventional method of dealing with the circulating current by hardware, the parallel circulating-current restraining and neutral-point balancing method of the T-type three-level photovoltaic grid-connected inverter is lower in cost.

Description

technical field [0001] The invention relates to a T-type three-level photovoltaic grid-connected inverter parallel circulation suppression and midpoint balancing method based on SVPWM modulation. Background technique [0002] With the continuous development of power electronics technology, the penetration rate of power electronic converters in power systems is increasing day by day. In particular, multilevel converters have the advantages of less harmonics, high withstand voltage, small switching stress, and less electromagnetic interference (EMI). Advantages, widely used in high-voltage direct current transmission, renewable energy distributed power generation and micro-grid fields. Especially the T-type three-level topology, compared with the traditional NPC three-level topology, has the advantages of small conduction loss, small space volume, and simple protection. The topology has higher energy density and higher efficiency, so it has become the mainstream of the market...

AI Technical Summary

Problems solved by technology

[0003] However, due to the influence of switch stress, a single T-type three-level inverter cannot meet the requirements of photovoltaic high-power grid-connected inverters. Therefore, the parallel connection of multiple T-type three-level inverters has become an energy source for photovoltaic power generation systems. Change the main trend

[0004] However, the parallel connection of T-type three-level inverters will generate zero-sequence circulating current, which will increase the loss of the system, reduce the efficiency of the system, cause current stress imbalance and serious electromagnetic interference of parallel inverters, and affect IGBT Service life of switch tube

[0005] And for the T-type three-level, there is a problem of unbalanced midpoint potential

Unbalanced midpoint potential will cause output voltage waveform distortion, level number reduction, unbalanced voltage borne by inverter switching devices, and reduced life span of capacitors, etc.

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