Photovoltaic micro-inverter, as a new type of photovoltaic grid-connected device, has broad development prospects. While pursuing the design goals of long-term, high-efficiency, and stable operation, the volume and cost of the equipment should be taken into consideration. According to the design requirements of micro-inverters, this paper analyzes and compares the typical main circuit topologies of photovoltaic micro-inverters, which are hot research topics, and points out the importance of soft switching technology in micro-inverters, which is of great significance to the research of photovoltaic micro-inverters. Certain reference value.
1. The background of photovoltaic micro-inverter
Under the influence of the global energy crisis, seeking efficient, sustainable and clean new energy has become one of the themes of international development today. And solar energy with its incomparable superiority. Become one of the main choices for people to relieve the energy crisis. my country is rich in solar energy resources, and as an alternative energy source to traditional energy sources, solar energy has huge economic benefits and strategic significance.
Photovoltaic grid-connected power generation is an important way for people to use solar energy at present. The traditional centralized photovoltaic grid-connected system is composed of many closely connected solar panels. These panels are first grouped in series and then connected in parallel to form a photovoltaic array. The DC power generated by the array will flow to the centralized grid-connected inverter located on the side of the battery panel, and its inverter will complete the DC/AC conversion and connect to the grid, and find the maximum power tracking point to optimize the efficiency of the photovoltaic grid-connected system . With the maturity and continuous development of technology, the problems of centralized photovoltaic grid-connected power generation system have gradually attracted attention.
2. Features of photovoltaic micro-inverters
2.1 Reliability: In the centralized photovoltaic grid-connected power generation system, the inverter is the key link and the weak link in the whole system. The failure of a single inverter may lead to the collapse of the whole system. The energy generated by the photovoltaic array during device maintenance will be wasted.
2.2 MPPT tracking efficiency: Although most centralized photovoltaic inverter manufacturers claim that the tracking efficiency can reach 99%, in fact, because their MPPT tracking is aimed at the entire photovoltaic array, it cannot take into account each photovoltaic module. Due to factors such as module matching and partial shading, the actual photovoltaic array output presents multi-peak characteristics. When the light power is uneven, the uniform maximum power tracking may make the array work at the local optimum point. In the centralized system, each photovoltaic cell module is usually connected with a bypass diode to keep the solar cells under shadow conditions. Photovoltaic cell bypass.
2.3 System scalability: The connection mode of the centralized grid-connected system determines that its system scalability is poor. In view of the problems existing in the centralized grid-connected system, many scholars have proposed various new photovoltaic grid-connected systems, among which the distributed grid-connected scheme represented by the series DC module grid-connected system and the micro-inverter grid-connected system is the current research hotspot.
3. Design requirements for photovoltaic micro-inverters
Micro-inverter design should focus on the following aspects.
3.1 Power density: The micro-inverter requires high power density, and the overall circuit should have a small volume.
3.2 Conversion efficiency: Since the energy conversion efficiency of photovoltaic cells is not high at present, every 1% increase in the efficiency of photovoltaic grid-connected equipment can bring huge economic value.
3.3 Reliability: The mean time to first failure (MTFF) of centralized grid-connected inverters is usually 5 years, and the mean time to failure (MTBF) is about 10 years. Photovoltaic cells have a lifespan of more than 20 years, so the micro-inverter life design indicators must be equivalent to photovoltaic cells in order to reflect the advantages of this solution. Its MTBF should be greater than 20 years, and MTFF should be more than 10 years.
3.4 Cost: The AC module system is equipped with a micro-inverter for each photovoltaic panel, which requires the cost of the micro-inverter to be low, and the circuit should contain fewer devices. Its controller must be able to handle all control, communication and computing tasks, but also must have a lower price.
At present, the research on the traditional photovoltaic grid-connected system has achieved a lot of results, which can be used for reference in the design of the micro-inverter grid-connected system. This paper will conduct specific research and analysis for different micro-inverter main circuits.
4. Circuit of Photovoltaic Microinverter
4.1 Circuit structure:
Due to the low output voltage of a single panel, in order to make the DC side voltage higher than the peak voltage of the grid side, the micro inverter should have a step-up link.
At present, micro-inverters mostly use high-frequency transformers. This solution has high power density, high efficiency, and can realize electrical isolation between photovoltaic cells and the grid side.
The single-stage circuit structure based on high-frequency transformers is relatively simple, while the multi-stage circuit structure is usually more complicated. According to the different ways of power conversion, it can be divided into two categories. First, the direct current is transformed into high-frequency alternating current through the front-end converter, the secondary rectification of the transformer is direct current, and finally converted into power frequency alternating current through the inverter link. If the high-frequency alternating current of the front stage is modulated according to the sinusoidal pulse width, the secondary stage can be directly converted into power frequency alternating current through the cycloconverter. Some studies have proposed a high-gain boost converter based on the combination of a Boost converter and a multiplication boost unit, which can also be used as a DC boost link of a two-stage converter. Some studies] have studied two DC/DC boost methods. The solution efficiency based on Boost and boost unit cascade is 94.5%~95.5%. The article points out that after the introduction of a passive buffer circuit, the efficiency will be further improved. The efficiency of the high-frequency transformer step-up scheme is about 96%, and the efficiency of the two schemes is equivalent. A high-frequency transformer can realize the electrical isolation between the photovoltaic panel and the grid side. At present, most micro-inverters use high-frequency transformers in the topology step-up link.
4.2 Power decoupling link:
When the photovoltaic cell works stably at its maximum power point, the input power Pin of the inverter is constant, but the output power Po of the inverter is transient, and there is an instantaneous power imbalance between the input and output of the inverter, which is reflected in The output side of the photovoltaic cell shows that its output voltage contains a disturbance component of double frequency. This disturbance will affect the efficiency of maximum power tracking and reduce the utilization rate of photovoltaic cells. Therefore, a power decoupling scheme should be introduced to suppress the secondary disturbance.
The traditional solution is to place a decoupling capacitor between the photovoltaic cell and the inverter, and the selection of the capacitor value can be obtained from formula (1) and formula (2).
Where: C is the capacitance of the front decoupling capacitor; Pin and Po are the input and output power respectively; Umax and Umin are the maximum and minimum values of the capacitor voltage; ΔU is the voltage fluctuation value of the capacitor; average voltage.
The output voltage of a single photovoltaic cell is usually about 23~45V, and the output power ranges from tens of watts to hundreds of watts. Due to the low output voltage of the photovoltaic cell, if the secondary disturbance is to be suppressed within a reasonable range, it can be seen from the formula (2) that the capacitance required on the photovoltaic output side must be larger, and an electrolytic capacitor with a larger capacitance is usually selected. Although this solution is simple and effective, the electrolytic capacitor is not only bulky, but also has a short life, which affects the working life and stability of the micro-inverter. It is obviously inconsistent with the design index of high reliability and long life of the micro-inverter. A major factor in the lifetime of a microinverter device.
New power decoupling schemes are the focus of current microinverter research. At present, there are many power decoupling circuits used to replace electrolytic capacitors, which can be summarized into the following three types:
4.2.1 Introduce an additional decoupling circuit to transfer the secondary power disturbance to the decoupling circuit, so that the instantaneous power on both sides of the inverter is equal.
4.2.2 It can be known from formula (2) that increasing the input voltage of the DC side or increasing the fluctuation value of the capacitor voltage can reduce the required capacitor value, and this scheme is more common in two-stage inverter circuits.
4.2.3 Three-phase micro-inverter, three-phase bridge circuit output and input instantaneous power balance, no power disturbance, only a small capacitor to filter out high-frequency ripple.
4.3 Microinverter Topology with Power Decoupling
At present, the topological types of micro-inverters are mostly single-stage and multi-stage. The traditional power decoupling scheme using electrolytic capacitors has low reliability, while the micro-inverter adopting an improved power decoupling scheme has higher reliability, which is the trend of micro-inverter research. The micro-inverter topology including the above-mentioned improved power decoupling scheme in the proposed micro-inverter circuit is mainly studied.
4.3.1 Single-stage micro-inverter:
The single-stage micro-inverter directly converts the DC power output from the photovoltaic cell into grid-side AC power through a high-frequency transformer, without other conversion links. It is simple in structure, but the control is more complicated. At present, the research on single-stage micro-inverters is mostly focused on the flyback circuit structure. This type of inverter uses fewer components, low cost, and high reliability, and is suitable for low-power applications.
Some studies have proposed a topology that functions as a decoupling circuit. This topology introduces a power decoupling circuit on the basis of the traditional flyback inverter, and transfers the double-frequency power disturbance to the decoupling capacitor through the decoupling circuit. Only a small-capacity capacitor is needed on the output side of the photovoltaic cell to filter out the high frequency ripple. The energy in the leakage inductance of the transformer can also be stored in the decoupling capacitor through the decoupling circuit. This scheme first transfers all the energy input into the magnetizing inductance to the decoupling capacitor, and then controls the on and off of the switch tube S1 through the pulse width modulation strategy, and the energy is transferred to the secondary side. The decoupling circuit needs to process all the energy, the power loss is serious, and the efficiency is low. The paper shows that the efficiency of the converter is only 70%.
4.3.2 Multi-level micro-inverter:
The two-stage inverter first increases the output voltage of the photovoltaic cell to a voltage value greater than the peak value of the grid side through the DC/DC step-up link, and performs maximum power tracking, and then converts it into grid-connected AC power through the post-stage inverter.
Some studies have explored a micro-inverter based on a full-bridge soft-switching circuit. The front side of the circuit is boosted by a full-bridge DC/DC converter, and the rear stage is a current-source inverter. The inverter The whole adopts small-capacity capacitors, and uses soft-switching technology to further improve efficiency. The paper states that the peak efficiency of the inverter is 89%.