Research On Modular Control Of Three-level Photovoltaic Grid-connected Inverters

Three-level modular photovoltaic grid-connected inverters basically combine the positive aspects of the three-level topology such as low voltage distortion rate, low THD, and high efficiency with the advantages of the modular photovoltaic structure such as exponential system capacity, low production cycle and cost, and high light-load efficiency on the condition that the current press of the power switches is not added.However, the corresponding theories and technologies of the three-level modular photovoltaic grid-connected inverters have not been formed, such as mechanism of the three-level zero-sequence circulating current (ZSCC), three-level modulation strategies, eliminating strategies of the three-level ZSCC and, neutral-point (NP) voltage balancing and rapid reactive current compensation in low-voltage ride-through (LVRT) operation, which restrict its application in large-scale photovoltaic power generation system.In this paper, according to the three-level modular photovoltaic grid-connected inverters, the researches on ZSCC, modulation strategies, NP voltage balancing and rapid reactive current compensation in LVRT operation have been done, which lays the foundation for large-scare use in the photovoltaic system. The main content and innovation of this paper are as follows:1) According to the ZSCC in the three-level modular photovoltaic grid-connected inverters, this paper points out that the main component of circulating current is the ZSCC. Paths of the ZSCC are analyzed and equivalent model of the ZSCC is built. Then, the ZSCC is divided into three types (type-1 ZSCC, type-2 ZSCC, and type-3 ZSCC), according to the exciting sources, which lays the foundation for the following sections.2) According to the influence on the ZSCC from the three-level modulation strategies, the conditions of the modulation strategies are analyzed. Then, advantages and disadvantages of common three-level modulation strategies are compared. Finally, large-middle-zero-vector modulation (LMZVM) is selected for the three-level modular photovoltaic grid-connected inverters. Meanwhile, the unity between vector-based modulation and carrier-based modulation for LMZVM are given.3) According to the type-1 ZSCC in the three-level modular photovoltaic grid-connected inverters, an eliminating scheme sharing neutral bus is proposed. According to the high-frequency component of the type-2 ZSCC, an eliminating scheme based on improved LCL filter is proposed. According to the low-frequency component of the type-2 ZSCC, an eliminating scheme based on quasi resonance regulator quasi resonis proposed. The proposed schemes are also proven to be effective to elimination of the type-3 ZSCC.4) According to NP voltage unbalanced problem of the three-level inverter in LVRT operation, the influence on the NP voltage from positive-active, positive-reactive, negative-active, and negative-reactive factors are analyzed based on symmetrical component method. Then, an original balancing strategy, which replaces the zero vector with two opposite P-type or N-type small vectors of equal duty cycle, is put forward to. According to reactive current compensation, a "phase locked loop (PLL) while compensating" mode and a quick detect solution based on grid voltage difference are proposed to reduce the reactive current compensation time.In order to verify the correctness of the theories and control strategies in this paper, a prototype of the three-level modular photovoltaic grid-connected inverters rated 30kW is constructed. Through the prototype, the mechanism of the three-level ZSCC, three-level modulation strategies, eliminating strategies of the three-level ZSCC and, NP voltage balancing and rapid reactive current compensation in LVRT operation are all effectively verified.