Analyzing the development path of photovoltaic inverters from the new products of various manufacturers

In recent years, my country's photovoltaic industry has expanded rapidly. According to preliminary statistics, the total installed capacity of domestic photovoltaic power generation in 2015 was about 14.95GW, with a total installed capacity of about 43GW, surpassing Germany to become the country with the highest cumulative installed photovoltaic power generation in the world. On December 15, 2015, the relevant agencies of the National Energy Administration issued a letter on soliciting opinions on the 13th Five-Year Plan for the Development of Solar Energy Utilization. According to the plan, by the end of 2020, the installed capacity of solar power generation will reach 150GW. By 2030, the installed capacity will reach 400GW, with an average annual installed capacity of about 25GW. The photovoltaic industry will once again usher in a spring of development.

As the core equipment of the solar power generation system, the reliability of the photovoltaic inverter determines whether the entire photovoltaic system can operate stably. Therefore, any innovation or breakthrough in inverter technology attracts the attention of many industry professionals.

New products released by photovoltaic enterprises represent that the enterprise technology and the inverter field have been in deep thinking and research and development. The research and development of new products also represents the development of an innovative path for enterprises to meet the market demand for photovoltaic inverters. Photovoltaic inverter technology routes can be divided into the following categories: centralized, distributed, string, and micro inverters. The following is an analysis of photovoltaic inverter technology routes based on the characteristics of some companies' new products and future development directions.

Centralized inverters help unmanned photovoltaic power stations

Centralized photovoltaic grid-connected inverters started early and have a complete product line, and are currently the mainstream products in the photovoltaic inverter market.

Wanyin Technology launched the new ZENIT series inverters at the China Photovoltaic Technology Forum of the 2015 China International Energy Summit.

At present, most large-scale photovoltaic power stations are concentrated in areas with poor conditions and sparse population. Therefore, the cost of operating and maintaining photovoltaic power stations, especially inverters, is high. In order to save this part of the cost, Wanyin Technology designed and manufactured the ZENIT series of fully enclosed liquid-cooled inverters to help photovoltaic power stations be unmanned.

The main features of ZENIT series inverters are as follows:

Highly integrated. ZENIT is the world's largest inverter, which can reach 4MW. Currently, SMA's largest inverter is only 2.475MW. Moreover, ZENIT only uses a self-developed motherboard to control the entire device, making the overall coordination of the device better, the control more precise, and the harmonics smaller.

High reliability. Common inverter failures are usually caused by heat dissipation problems and dust. The fully enclosed built-in liquid cooling system of the ZENIT inverter perfectly solves the above problems! Experiments have shown that the operating temperature of the IGBT of the ZENIT inverter does not exceed 90 degrees Celsius at full power, which is much lower than that of conventional air-cooled inverters, fundamentally reducing the equipment failure rate and effectively extending the service life of components.

Active dust-proof design. Wanyin Technology's fully enclosed built-in liquid cooling system ensures that there is no need for a large amount of gas convection inside the device, and dust cannot remain inside the device, so there is no need to clean it as frequently as conventional inverters.

Fault warning system. The Smart HMI of the ZENIT inverter can remotely monitor the parameters of all electrical components, laying a solid foundation for subsequent big data and cloud computing.

Modular design. Such an inverter can bring customers higher power generation, lower unit investment cost, higher system efficiency, lower fault operation and maintenance cost, and less dust removal cost.

Distributed inverters help optimize the design of photovoltaic power stations

Distributed inverters are the third photovoltaic inverter technology route after centralized inverters and string inverters. They combine the respective advantages of centralized inverters and string inverters. They are known as "the best inverter solution for photovoltaic power stations with higher returns" and represent the future development direction of inverters.

At the 2016 Power Plant Design and Equipment Selection Seminar, Sineng Electric Co., Ltd. analyzed the three different technical characteristics of centralized inverters, string inverters, and distributed inverters, explained the advantages and disadvantages of the three types of inverters from the perspective of customer value, and selected the optimal inverter solution for different application scenarios.

It is understood that the main features of distributed inverters are centralized inverter and decentralized MPPT tracking, and the value they bring to customers is mainly reflected in three aspects:

First, decentralized MPPT reduces the power generation loss caused by various mismatches. Objectively, various factors such as the dispersion of components, obstruction of the array, and changes in the inclination of the bracket will lead to mismatch losses. In the distributed inverter solution, each intelligent MPPT controller has 4 or 8 MPPT control modules, and each MPPT control module is connected to 4 or 2 strings. Compared with the centralized inverter solution with nearly 100 strings connected to 1 MPPT, the distributed inverter greatly reduces the parallel mismatch loss of the strings.

Secondly, the distributed inverter solution reduces AC and DC line losses. The intelligent MPPT control module (corresponding to the combiner box of the centralized inverter) has a voltage boost function, which increases the DC transmission voltage over long distances (combiner box to inverter room) to about 800VDC. Compared with the 600VDC of the centralized inverter and the 480VAC of the string inverter, the line loss of the distributed inverter solution is greatly reduced. The AC output voltage of the distributed solution is also increased to about 500VAC, and the AC line loss is also reduced compared with the 315VAC of the centralized inverter.

Third, the system cost of the distributed inverter solution is basically the same as that of the centralized inverter solution, and lower than that of the string inverter solution. Using the distributed inverter solution to form 2MW and above inverter units and using the 1500V distributed inverter solution to form 4MW inverter units will have more advantages in reducing system costs and increasing power generation.

In addition, the distributed solution uses string-level electronic switches and active tripping functions of circuit breakers, which greatly improves system reliability and achieves string-level fault isolation, without affecting the normal power generation of non-faulty strings or MPPT modules. Distributed inverters are also superior to string inverters and centralized inverters in terms of flexible configuration options for capacity ratio.

Generally speaking, the distributed solution is a cost-effective solution that has been widely recognized in the industry and has begun to be used in large quantities. It is the preferred solution for the optimized design of photovoltaic power stations.

String inverters help large-scale ground power station applications

As the installed capacity of my country's photovoltaic power stations increases, power generation and maintainability will become important factors in power station design. String inverters are mature in technology, flexible in design, easy to maintain, and highly adaptable. They can be used not only in distributed rooftop power stations, but also in large ground power stations. It can be predicted that in the future, string inverters will occupy an increasingly important position in my country's photovoltaic power stations.

Sungrow won an award at the 7th China International Solar Photovoltaic Industry Summit Forum 2015, mainly for its in-depth analysis of the technical characteristics and development history of string inverters.

The characteristics of string inverters are diversified forms and high IP65 protection level. Comparison of laboratory and actual power station tests shows that forced air cooling of products above 20kW has better heat dissipation effect, and the internal temperature is more than 10℃ lower than natural cooling, ensuring long life and no derating in high temperature environments such as roofs and complex hills. Fuses are widely used in the industry as DC side short-circuit protection devices to improve system safety. In addition, the main application scenarios of string inverters are complex hills, agricultural greenhouses, roofs and other occasions, where the lighting conditions are relatively poor. At the same time, considering the power loss caused by dust and loss in the system, the inverter needs to have a certain over-matching capability. The string inverters of mainstream manufacturers in the industry can achieve over-matching of more than 1.15 times. The maximum efficiency of Sungrow's 40kW string inverter exceeds 99%, the Chinese efficiency is 98.45%, and the weight is only 39kg. It has an over-matching capability of more than 1.3 times. All indicators represent the highest level in the industry.

Regarding the future development trend of string inverters, Zhang Xianli said that component-level products (MLPE) will continue to be enriched; efficiency will continue to improve; single-machine power and power density will continue to increase; grid-connected performance requirements for inverters will continue to increase; various communication technologies will be widely used; and the application of inverters with energy storage interfaces will increase.