Select the necessary knowledge of medium power string inverters

Photovoltaic inverters are fiercely competitive, hundreds of flowers are in full bloom, and one hundred schools of thought contend. All manufacturers are making every effort to differentiate their products. Some manufacturers mainly control the cost and the price is extremely low. Some manufacturers control the power density. Mainly, to minimize the size, and some manufacturers focus on improving efficiency, and some manufacturers like new devices, and some manufacturers pursue natural cooling, the fanless carry out in the end.

Photovoltaic inverters are involved in many disciplines such as power electronics, intelligent control, mechanical structure, and power quality. It is a systematic project. The parameters such as volume, weight, efficiency, noise, voltage range, and temperature of inverters are very important. If one-sided pursuit of the perfection of a certain two technical indicators, will sacrifice the other performance of the inverter, resulting in worse overall performance.

The idea of ​​the doctrine of the mediocrity runs through the 5,000-year history of civilization in China. Its core is that everything can't be too much. When it moves from day to day, it loses when the moon is full. When the instrument is full, it overflows. When water is full, it overflows. The golden mean is not to say that it is not pursuing perfection, but is pursuing perfection on a balanced basis. The development of electronic technology is changing with each passing day. New technologies have become a new bright spot in the market, but not all new technologies are perfect. Not all old technologies are outdated, and there are no absolute boundaries between new technologies and old technologies. With the principle of moderate advance, balanced development as the goal, starting from reality, adopting a combination of new and old technologies, from the user's point of view, to realize the maximization of revenue in the life cycle is the golden mean of inverter design selection. .

First, the power module

The technical breakthrough of the power module makes a string of inverters produce a qualitative leap!

Before 2013, centralized inverters were used in ground power stations, and string inverters were used in distributed power stations. Larger terrestrial power stations also began to use string inverters as the power of string inverters became larger and larger. Converters, the debate on the two options has not stopped, both options have their own advantages and disadvantages, centralized inverter advantages are less power devices, high reliability, the disadvantage is the small number of MPPT, narrow voltage range; string The advantage of the inverter is that there are many MPPTs and the voltage range is wide. The disadvantage is that there are many power devices and the reliability is low.

The power switch device is one of the most core devices of the inverter, and it is responsible for the current conversion work. It is a device that is most likely to fail when the inverter is operated at high temperature, high voltage, and high current for a long time. Each power device is a Failure point. Power switching devices in photovoltaic inverters mainly refer to discrete device power MOSFETs and power module IGBTs. In the early middle power string inverters, discrete devices were generally used. Since the power MOSFET currents are relatively small, multiple devices are generally connected in parallel. The 50KW inverter is designed using discrete devices and requires more than 60, so many switching devices are stacked together, which will cause a series of problems. Such as current sharing, electromagnetic interference and so on.

As we all know, the failure mode of the power switch tube is over-voltage, over-temperature, and over-current. Discrete components Due to the large number of devices, the distance between components is relatively long, so the stray inductance of the circuit is large, resulting in high peak voltage during operation, and components are prone to overvoltage and damage; multiple components are connected in parallel, impedance is inconsistent, and each component The current is not the same, the current of the component with low impedance is large, and it is easy to over-current; the discrete device is fixed at one end, the contact area is small, the heat dissipation is difficult to maintain the same, and it is easy to overheat.

The early mid-power string inverters using discrete components had too many faults during operation, which raised doubts about the future of the mid-power string inverters. However, this situation was not changed until Vincotech and infineon introduced power modules containing multiple components.

Vincotech's IGBT power module is designed for medium power string inverters. One module is equivalent to 8 discrete devices. The module has high flexibility.

Compared to MOSFETs, the IGBT saturation voltage is reduced, and high voltage and high current are easily achieved. The medium and high power inverters dominate. The IGBT module has a higher reliability and safe working area than the single IGBT module. The PV inverter uses the BOOST boosted IGBT power module in the front stage and the three-level IGBT power module in the rear stage. This is the best solution for the mid-power PV inverter.

The best solution for medium-power PV inverters:

(1) reduce the number of power devices, 50KW using power devices to design as long as 5, the number of centralized inverter inverter than the same size, but the use of discrete devices to design, the need for 15 steps before the boost, after Three-level inverters require 48. The overall area is reduced by more than 30%, which can increase the overall power density.

(2) The installation area of ​​a single power module has a large heat dissipation area compared to that of a stand-alone component, with less stress, better reliability, and many advantages in installation. In addition, a temperature sensor is integrated inside the power module, which has high measurement accuracy and can detect the junction temperature of the device more accurately and is conducive to overheat protection.

(3) The power module has a compact structure, and the connection lines between the power switching devices are much shorter than the discrete devices, which can reduce the stray inductance in the circuit and improve the reliability of the inverter.

(4) The IGBT module has a higher switching frequency, which can increase the efficiency and reduce the capacity of the inductor.

(5) Compared with discrete devices, the power module has insufficient heat consumption per unit area, and the overall heat dissipation area is small. If the power is more than 30KW, natural cooling is used to dissipate heat. When the ambient temperature is higher than 40 degrees, overheat protection occurs. . However, with forced air cooling, this problem can be completely avoided.

Second, the film capacitor

Film capacitor no longer has a short board in the string inverter!

Inverter as an electronic product, the capacitor is the most basic component, DC bus support capacitor is the main role of energy storage and filtering, to withstand high pulse current and pulse voltage, is one of the devices with the shortest inverter life, DC The bus capacitors now have two types of aluminum electrolytic capacitors and thin film capacitors, each with its own advantages. The main advantages of electrolytic capacitors are their large capacity and low price. The advantages of thin film capacitors are high cell voltages, but they also have obvious advantages:

(1) The life of an electrolytic capacitor is generally 2,000 to 3,000 hours, 5000 to 6000 hours for a long life, and it is prone to liquid leakage; the life of a film capacitor is generally 100,000 hours or more. Moreover, the film capacitor also has a self-healing effect. When a small portion of the capacitor generates a short circuit, the energy generated by the short circuit melts and evaporates the damaged electrode, so that the short-circuit point is again in an insulated state.

(2) electrolytic capacitor voltage value is generally 500-550V, film capacitor voltage value is generally 1000-1300V, the film capacitor can withstand 2 times the rated voltage surge voltage impact, can withstand long-term reverse pulse voltage.

Therefore, from the standpoint of life and reliability, the thin film capacitor can overcome the capacitance.

Third, monitoring and maintenance

Including the operation and maintenance of the monitoring system for the installer to lift the worries!

Photovoltaic plant monitoring now comes in two forms:

The first is a third-party monitoring platform, such as Amoy Branch, Miles.

The second is the monitoring platform developed by inverter manufacturers.

Both have their own advantages. They can use third-party monitoring platforms to monitor the inverters of multiple companies and adapt to installation companies with multiple manufacturers. The monitoring platform developed by inverter manufacturers can monitor only their own inverters. There are limitations, but the benefits are obvious, the interface is more targeted, the fault handling is faster, and the remote upgrade is more convenient.

Gruitt’s self-developed monitoring equipment and cloud platform are easy to operate and user-friendly. Even inexperienced people can register successfully within 5 minutes. The monitoring system can be used to check the operation of each inverter. The interface is user-friendly. More importantly, it is very convenient. The inverter's working status, real-time power, and daily power generation that the customer cares most are on the bottom of each inverter. All the data is clear at a glance. The online intelligent customer service system can also provide services on its own initiative. Problems, fault warnings, problem remote diagnosis, and processing functions.

When a photovoltaic system fails, it first checks the alarm information of the fault inverter, and then finds the corresponding fault handling method according to the information. Most of the problems can be solved on the spot. If you still can not solve, you can choose to ask questions to the customer service, the server-side online customer service will answer patiently, if it is a system software problem, you can remotely upgrade online; if it is an inverter hardware problem, online customer service will be the first time to turn to customer service center, related After the technical engineer diagnoses, he will decide to use the fastest solution for maintenance or replacement.

Fourth, summary

The string inverter with power module and bus film capacitor has the advantages of small number of centralized inverter power devices, long life of the film capacitor and high overall reliability, and the MPPT voltage range of the string inverter. The advantages of wide range, inverter small size, light weight, easy handling and other advantages. Therefore, the emergence of power modules, before the advent of new technology, can make the battle between string inverters and centralized inverters come to an end.

Grwerat's Growatt 30000TL3-S to Growatt 50000TL3-S mid-power series inverters, all using power modules and bus membrane capacitors, configure string detection, fuse protection, AC/DC lightning protection modules, and anti-PID modules. And AFCI modules. After nearly half a year of application, it has been highly evaluated by customers.

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