The inverter plays an important role in the photovoltaic grid-connected system design. It can convert the solar energy into DC energy and then invert it to the single-phase AC energy that can be applied by various devices.
The micro inverter technology means the technology of directly connecting inverter with single PV module. Each PV module can be equipped with an inverter that can realize AC-DC conversion and can track the maximum power point. It can directly convert the power generated by PV module into the AC energy so that it can realize AC load purpose or it can be transmitted to the power grid.
What are the advantages of micro inverter?
1. High safety
- Low DC voltage
In the traditional PV system, the PV cell at DC side is connected in series and can form a high-voltage DC bus of 200V-600V or even higher voltage, and this DC bus will be connected with the inverter. The workers are faced with high electricity shock risk in the installation process. In addition, due to the insulation damage or poor contact of related connectors, the DC arc may be caused, which may cause fire disaster or personal injury because the arc temperature can reach thousands of degree. In addition, traditional PV system is also faced with many other challenges. For instance, the DC protection device is more expensive and has lower liability than AC protection device. It is difficult to break the high-voltage DC arc. The PV modules have current source characteristics and the current will not increase greatly after current passing, which makes it more difficult to provide protection. The electricity will be generated at any time when solar energy is available and it is difficult to break the energy generation. Thus, it will greatly threat the safety of rescue and fire-fighting personnel in case of device failure or fire disaster.
In the micro inverter system, each PV module is connected with the inverter separately. In this way, the highest DC voltage of the system is the open circuit open of the PV cell, which is about 40ov. The installation and application are both safe and there is no electricity shock, current arc and fire risk. It essentially solves the problems caused by the high-voltage DC of the traditional system.
- Electric isolation
To save cost, the electric isolation between the power grid and PV is not provided when connecting the traditional inverter with the power distribution network. The power grid and PV are broken by the AC relay or power electronic switch. Thus, when the inverter has failure, it is difficult to separate the AC and DC side, which will threat the safety of troubleshooting personnel. In addition, more serious failure may be caused because of the short circuit at DC side.
The micro inverter is equipped with high-frequency isolation transformer, which can effectively separate the power grid and PV side when system failure occurs. When checking the power grid failures, the PV side is in low voltage status and is isolated. When checking the PV side failures, the AC voltage will not lead to DC electrification.
All in all, due to the low DC voltage and the electric isolation, the micro inverter system can realize the safety degree that traditional system cannot achieve. Thus, it is suitable for the places with high safety requirements, such as the PV buildings.
2. High system efficiency
- Component MPPT
In the micro inverter system, each module is connected with the inverter separately, which can avoid the PV cell coupling and thus avoid the short board effect. When some PV modules are blocked or damaged, the power generation function of left system modules will not be affected.
It can greatly improve the shadow resistance ability and the environment adaptability of the system. Based on the American research results shown in figure 4, it can be seen that compared with traditional system, the efficiency of micro inverter system can improve 15% or even higher in the shadow condition.
- High dynamic MPPT efficiency
Due to the small component MPPT, the control speed and response speed of the system can reach 10 millisecond, which is tens or even hundreds of times of traditional inverter. Therefore, in cloudy weather or other situations with fast sunlight change, the efficiency of the micro inverter is 1-2% higher than the traditional inverter.
The reason why micro inverter can bring high work efficiency is introduced in the above mentioned two points. However, the benefits of the micro inverter are more than this. Different with the traditional system, the micro inverter does not have requirement on the minimum component number or on the consistency of single component. As shown in figure 6, it allows more flexible design and can maximize the land application efficiency, which is very significant when considering the land resource lack situation. The work efficiency of the micro inverter system can reach 90% and it is difficult for the traditional system to reach this level.
The data collector can collect the micro inverter data through wireless method and can upload to the server so that operation and maintenance staff as well as the owner can monitor the network situation. The micro inverter allows to monitor the situation of each component so as to know about the PV panel property. When the PV battery damage is found, it can be located accurately and can be changed in time. The problems that component damage and failure cannot be found out and located in time in traditional system can be solved, which makes the system operation and maintenance easier and can decrease the cost by 50%. In addition, through the component comparison and diagnosis, the impact of environment on the component can be analyzed, based on which the surrounding environment can be improved accordingly to ensure the system output. The micro inverter is benefit for improving safety, system efficiency, system power generation, intelligent monitor, maintenance and operation aspects. It is the best choice for the household photovoltaic system.
