Preface:

　　

　　The worldwide energy shortage and sustained high oil prices have become one of the important constraints to the sustainable development of the global economy. In the wave of vigorous development of renewable energy, solar power generation technology is receiving more and more attention because of its superior environmental protection, easy to large capacity, large-scale characteristics. In photovoltaic power generation system, the conversion efficiency of PV inverters is one of the most concerned technical issues in recent years in order to maximize the efficient injection of power generated by solar cells into the grid.

　　

　　With the breakthroughs in power electronics, such as power semiconductor device technology and large-scale digital control technology, photovoltaic power inductance, as an energy storage filter element, is increasingly becoming an important bottleneck affecting the power conversion efficiency of the system. In order to improve the efficiency of the system and reduce the loss of inductance, a large amount of copper and expensive high-performance magnetic materials have to be consumed, and become one of the most expensive components of the whole inverter.

　　

　　In view of the above problems, according to the basic circuit structure and working principle of various PV inverters, this paper will analyze the performance requirements of the matching inductors and the advantages and disadvantages of the inductors widely used at present. Combining with the latest achievements of the existing magnetic materials, this paper proposes the design of inductors using hybrid magnetic circuit and magnetic integration technology. Law is the way to solve this problem fundamentally.

　　

　　Classification of 1 Photovoltaic Inverters

　　

　　Because of the different power levels of grid-connected photovoltaic inverters in practical application, their circuit structure and working mechanism are not only different. According to the power level of photovoltaic inverters, grid-connected photovoltaic inverters can be generally divided into the following four categories: micro-inverters, residential photovoltaic inverters, commercial medium-power inverters and centralized power station inverters.

　　

　　2 Micro Inverter and Its Core Magnetic Components

　　

　　Because of the location of the solar module, the dark clouds, the shadows around the leaves and other factors, the power generated by each module will be discrete in varying degrees. If they are connected in series and in parallel, they will have the same bad effect as the combination of old and new batteries. When 2-3% of the battery area is covered by shadows, the total power generation often decreases by as much as 20% (1), which seriously affects the power generation efficiency of the whole system. For this reason, micro-inverters are designed to generate electricity independently from a single battery module. This problem can be avoided to the greatest extent. This scheme has been widely welcomed since its advent. However, more than a dozen or more independent inverters are needed for a household power system. Therefore, whether the inverters can achieve high efficiency and low cost has become an important restriction factor affecting the wide application of the system.

　　

　　Micro-inverters are two typical grid-connected power generation topologies. Firstly, two staggered Critical Mode Boost flyback transformers are adopted in Fig. 1. According to the sinusoidal half-wave law, the duty cycle of the boost flyback transformer is filtered by sinusoidalization, isolation boost and MPPT (Maximum Power Point Tracker) control of the single-stage circuit power, and then half-wave power frequency commutation filter of the whole bridge is used to realize the low voltage efficiently. Direct grid-connected DC power generation. This is one of the most potential working modes of micro-inverters. Fig. 2 is a general mode of two-stage circuit connected by full bridge isolation boost and filter, followed by full bridge inversion filter. The obvious disadvantage of this method is that more magnetic elements are needed, and there are too many high frequency switching devices, which have insufficient advantages in cost and efficiency. In order to simplify the circuit, hard switching driver is used without exception. Thus, the leakage inductance of the main transformer is very small, and the flat transformer structure of multi-layer circuit board is usually used. The parasitic capacitance is large, the cost is high, and EMI is difficult to deal with.

　　

　　For the mainstream CRM Interleave topology, there are two core magnetic components, flyback power transformer and AC filter electric ACL. For flyback power transformer FBT (Flyback Transformer), because it works at hundreds of KHz in critical mode, the design of such transformer must follow the following principles:

　　

　　1) Ferrite core with high Bs, high frequency and low Pcv loss is adopted.

　　

　　2) In order to reduce the loss of transformer, it is necessary to adopt the design of large effective cross-section and low magnetic circuit length to control the magnetic loss. The commonly used cores are thin or customized products with optimized shape, such as PQ and RM.

　　

　　3) Leakage inductance of transformer is controlled to the maximum extent, and a well-coupled winding structure is adopted.

　　

　　4) The inner resistance of winding wire is as small as possible. At the same time, attention must be paid to skin effect and eddy current loss of copper wire caused by air gap leakage.

　　

　　Figure 1 Topology of CRM Interleave Micro Inverter

　　

　　Figure 2 Topology of Full Bridge Micro Inverter

　　

　　For ACL with post-filter of micro-inverters, because of its relatively small ripple current, high-DC bias, high-frequency characteristics of better Highflux or better cost-effective NPF ring ferrosilicon material (3) is generally used.

　　

　　3 Residential Photovoltaic Inverter and Its Core Magnetic Components

　　

　　At present, non-isolated grid-connected residential photovoltaic inverters are widely used, and their power is basically about 1.5KW~6KW. In order to achieve high cost performance and maximize the efficiency of power supply conversion, the industry has even adopted various new technologies (4) such as SiC semiconductor, H5 topology, 3-level topology and so on, but its most basic topology is nothing more than that.

　　

　　Topology of Non-Isolated Grid-connected Inverter

　　

　　Topology of Non-Isolated Grid-connected Inverter

　　

　　The main reasons for this feature are basically the cost and efficiency considerations of the inverters and the power generation system.

　　

　　1) The MV with small power ratio (1.5~3KW) is usually controlled by a single Boost MPPT.

　　

　　2) Double Boost and double MPPT control mode is often used when the capacity of a single machine exceeds 4KW.