[Practical Information] Inverter Classification, Representative Companies and Technology Comparison

1. Key performance parameters of inverter: MPPT

1. Introduction to MPPT MPPT, which is the abbreviation of Maximum Power Point Tracking, means that the inverter adjusts the output power of the photovoltaic array according to different external ambient temperature, light intensity and other characteristics, so that the photovoltaic array always outputs the maximum power. MPPT is a very core technology of the inverter, and the MPPT voltage is a very critical parameter in the design of photovoltaic power stations.

Figure 1: IV and PV curves of a single photovoltaic module

In the figure above, the output voltage and current of the photovoltaic module follow the IV curve (green) and the PV curve (blue). If you want the inverter to output the maximum power, you need the DC voltage to run at the maximum point where the red dot is located. This point is the maximum power point. If the maximum power point is 550V, the power is 200W at 550V. At this time, the power running at 520V is about 190W, and at 580V is about 185W, both of which are not as large as the power at 550V. If the inverter cannot track 550V, it will lose power generation, but it will not have other effects on the system.

The PV curve changes with the light intensity, temperature and shading, and the maximum power point also changes. The maximum power point voltage may be 560V in the morning, 520V at noon, and 550V in the afternoon, so the inverter needs to constantly find this maximum power point, which is the maximum power point tracking, so as to ensure that the energy of the solar panels can be maximized throughout the day and not waste solar energy resources.

2 MPPT of photovoltaic power generation unit and its algorithm In actual engineering, a 500kW inverter is often connected to 80~90 photovoltaic strings. Due to inconsistent shading, component power deviation and other reasons, there must be output power deviation between different strings. Therefore, the output characteristic curve of the photovoltaic string connected to each inverter becomes complex and presents multiple extreme points, as shown in the figure below.

Figure 2: Output power curve of photovoltaic array

In Figure 2, the output power curve of the photovoltaic array shows multiple power peaks. To find the point with the highest power, you need to perform MPPT calculation! Currently, there are several methods for MPPT calculation:

1) MPPT algorithms for single peak power output, such as: Constant Voltage Tracking ( CVT), Perturbation And Observation method (P&O), Incremental Conductance method ( INC), incremental conductance method based on gradient variable step size, etc.

2) Multi-peak power output MPPT algorithm, such as: composite MPPT algorithm combined with conventional algorithm, Fibo nacci method, short-circuit current pulse method.

If you want to know more about inverter MPPT, you can reply "172" on the platform to view "Introduction to Inverter MPPT", the original text is "Comprehensive Analysis of Photovoltaic Inverter MPPT Algorithm" from "Sobi Photovoltaic Network", written by Zhang Zhe.

2. Classification of Inverters

According to the inverter's single-unit capacity and MPPT tracking accuracy, it can be divided into the following four categories.

3. Representative inverter companies with different technical routes

Technical route 1: large centralized inverter

Representative companies: Sungrow Power Supply, TBEA, CSR, TMEIC

Large inverters are still the mainstream of ground-based power stations and are often used as comparison samples for inverters of other technical routes.

Sungrow: After meeting the performance and quality requirements, Sungrow's box-type inverter room began to pay more attention to details such as heat dissipation and dust prevention. Sungrow and Alibaba Cloud have cooperated to establish the "Smart Light Cloud" power station operation and maintenance management platform. "Photovoltaic power generation will enter the group power station management stage from single power station management. With the help of Alibaba Cloud, Smart Photovoltaic Cloud will provide customers with standardized operation and maintenance services and investment and financing references." Sungrow will integrate nearly 20 years of solar power station operation and maintenance management experience, system design capabilities and product technologies to provide users with smart operation and maintenance services. And through Alibaba Cloud's US data center, it can provide operation and maintenance services to overseas solar power stations.

TBEA: TBEA considers how to build smart power stations from the perspective of the power grid, including how to connect the power generation, transmission and distribution ends. At the same time, it provides channels, including cloud computing platforms, real-time model mobilization and centralized control. In addition, TBEA has also launched a single-machine 2MW solution, replacing two 500kW centralized inverters with 1MW modular parallel inverters.

CSR: CSR National Power Conversion Center has the most core IGBT technology, which is not available in other domestic enterprises. The new fourth-generation model starts with high power density and reduced operation and maintenance costs after the development of the efficiency, quality and low voltage ride-through functions of the first three generations.

TMEIC: TMEIC is the manufacturer of the world's first MW-class inverter and the inverter leader with a market share of more than half of Japan's ground-mounted power stations. In the past two years, TMEIC has continuously increased its investment in China. In 2014, it established a factory in Shanghai. In February 2015, the Yancheng factory opened and was named "Yancheng TMEIC Power Electronics Co., Ltd." It produces two types of photovoltaic inverters: 500kW and 630kW.

Technical route 2: Distributed solution

Representative companies: Hopewind Electric, Wuxi Sineng, Han's Inverter

The distributed tracking centralized inverter photovoltaic grid-connected solution (referred to as the "distributed solution") is based on the traditional photovoltaic combiner box, adding a DC/DC boost conversion hardware unit and an MPPT control software unit to form an intelligent photovoltaic controller to achieve decentralized tracking of multiple MPPTs, which can reduce efficiency losses caused by factors such as inconsistent component parameters, local shadows, and elevation angle differences; after the output voltage of the improved photovoltaic combiner box is increased to 820V-1000V, it is centrally inverted to the inverter, which can reduce AC/DC transmission losses and inverter heating losses. While achieving accurate MPPT tracking, boosting and reducing losses, the output power quality is guaranteed through centralized inversion.

This technology originated from Satcon in the United States. Currently, there are three companies in China that use this technology, namely Hopewind Electric, Wuxi Sineng, and Han's Inverter. Among them, Hopewind Electric was the first to apply the distributed solution in China. It has been more than two years and has accumulated a total of about 100MW.

Hopewind Electric's boost module uses Sic material MOS tube to replace the traditional IGBT module, achieving high frequency, high voltage and high temperature resistance, and high conversion efficiency; from the combiner box end, one MPPT tracking is achieved for every two parallel branches. At the same time, the product has zero voltage ride-through function, which is the first in China.

Technical route three: string inverter

Representative companies of small-power string inverters: Sungrow Power Supply, Growatt, Omnik, Ginlong, SANSENG Electric, Sofar New Energy, and Samil New Energy.

At present, small-power string inverters have not been developed in China due to the distributed situation. Except for rural photovoltaic poverty alleviation projects in some areas, they are still mainly sold in overseas markets.

Sungrow: In overseas markets, including Australia and the Netherlands, where the company's small-power string inverters are widely used, Sungrow launched a new small-power single-phase string inverter to the world in 2014. The power range covers 3-5kW, and the whole machine weighs only 9kg, which can be easily installed and carried by one person. It also has a remote real-time monitoring function, which is convenient for operators to diagnose and maintain power stations. This type of product has been widely used in rural household photovoltaic systems in Anhui and other places.

In 2014, Samil New Energy vigorously developed distributors to cope with the distributed photovoltaic market.

Omnik adopts a cooperative model in China of cooperating with Zhongsheng and other companies to jointly build a photovoltaic industry chain.

At the end of 2014, Ginlong launched the world's lightest commercial 2kW grid-connected photovoltaic inverter, weighing only 5.6KG, and successfully applied its products to the Eiffel Tower, a landmark building in Paris. For the domestic market, Ginlong prefers to provide overall solutions to meet user needs. The solution has recently been applied in the photovoltaic system on the roof of Fuda Bearing Group.

The inverters of Shouhang New Energy are used in Wuxi, Henan, Anhui and other places. They are intelligent, efficient and reliable, with IP65 protection level, and can be freely installed indoors or outdoors.

SAJ Electric is one of the companies that has taken a unique approach in the industry. In addition to its great success in the Netherlands, Pakistan and India, it is also at the international leading level in photovoltaic pumping systems.

Representative companies of medium-power string inverters: Huawei, Sungrow, KOSTAL

Huawei has carried out in-depth cooperation with industry giants such as Huanghe Hydropower, Zhongli Tenghui, China Minsheng Investment, Longi Green Energy Technology, and Aikon. Huawei has established a monitoring and operation and maintenance system for Huanghe Hydropower that integrates digital technology, the Internet, and photovoltaic power generation. In the demonstration power station, a 4G wireless system dedicated to photovoltaics is used, and Wifi covers the entire power station. Unmanned aerial vehicle inspections with set flight routes are configured to view the power generation status of components in real time. The monitoring personnel at the Xining headquarters of Huanghe Hydropower found that the power generation data transmitted through the photovoltaic cloud fluctuated. They checked the problematic component array in the background, and then found the problem through high-definition cameras and called on-site operation and maintenance personnel to deal with it. At present, Huanghe Hydropower already has 200MW of "centrally operated, remotely diagnosed, and maintained in real time" intelligent photovoltaic power stations. In the future, Huawei will strengthen how to provide services more comprehensively and intelligently rather than just the products themselves.