Balanced Development - The Necessary Knowledge for Selecting 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 system project. The parameters such as volume, weight, efficiency, noise, voltage range, and temperature of the inverter are all 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. It is pursuing perfection on the basis of balance. The development of electronic technology is changing with each passing day, and new technologies have become a new bright spot in the market. However, not all new technologies are perfect. All old technologies are outdated, and there are no absolute boundaries between new technologies and old technologies.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. .

The 50/60KW power class inverter is the most widely used inverter for domestic large-scale roofs, hills, ground and other photovoltaic power plants. The design focus of domestic manufacturers is different, and the route style is also varied.

First, the choice of heat dissipation scheme and volume weight

There are two types of heat dissipation methods for string inverters: natural cooling and forced cooling. The differences in the heat dissipation capabilities of the string inverters for the 50 kW power class have been found to be different:

1. Natural cooling: The cooling effect is poor, derating operation occurs in high temperature environment, resulting in loss of power generation, affecting the return of investors.

2. Strong cooling air: excellent heat dissipation effect, which helps to extend the service life of the inverter and ensure efficient work.

The fan used for strong cooling air is divided into two types: high-speed fan and medium-speed fan:

(1) High speed fan:

Advantages: Reduce the size and weight of the radiator

Disadvantages: Increased noise, shorter fan life

(2) Medium speed fan:

Advantages: long fan life

Disadvantages: larger radiator

At present, the domestic inverter manufacturers have the following solutions:

A, a large area of ​​natural heat. Using discrete devices, the heat source is dispersed, the heat sink is very bulky and heavy, and the size of the inverter is very large. The weight of the 60KW inverter can reach about 70kg.

The disadvantage of this solution is high cost, inconvenient installation and maintenance, if it is installed on the roof, handling is a very difficult thing.

B. Natural heat dissipation. At the same time, in order to reduce the volume, a derating design is adopted, and the actual maximum output power is lower than the model's declared power. This scheme generally starts to reduce the output power when the ambient temperature of the inverter reaches 50 degrees or more.

Disadvantages of this solution: First, there are few components and the second is during the peak hours of summer power generation, the inverter is installed outdoors, the ambient temperature easily exceeds 50 degrees, and the derating output will cause a loss of power generation, seriously affecting Customer benefits.

C, forced high speed air cooling. In order to achieve the world's smallest light weight inverter, the industry's most high-speed cooling fan. The advantages of this solution in terms of installation and maintenance are obvious, but at the same time, the disadvantages are obvious:

1. The fan's operating sound is particularly loud and particularly sharp;

2. The life of the fan is difficult to guarantee. The selection of high-speed fan is very elegant. The general fan specification defines how many hours the ambient temperature can work. You need to select a high-performance fan to ensure a variety of harsh environments. long term work;

3. In order to dissipate heat, the output current is limited by the output current and the output voltage is designed to be too high. If the voltage tolerance of the components is insufficient, the components will often work in an overloaded state, affecting the life of the components.

D, smart air cooling. The use of medium-speed cooling fan, natural heat dissipation when working at low power, forced air cooling at high power, this solution takes into account the advantages of natural cooling sound and small volume of forced air cooling, but also overcome the natural heat dissipation bulk weight and force The disadvantage of high-speed air-cooled noise is the most appropriate solution at present.

Second, MPPT way and power generation and efficiency

Many tender documents now clearly require MPPT to be at least x roads. Is that actually more MPPT roads better? This requires a dialectical view of the issue from actual conditions.

If the components are inconsistent and the components are installed at different angles or orientations, from the perspective of resolving mismatch problems, the more MPPTs are more advantageous. But the fundamental solution to the problem of inconsistent components is to choose high-quality components rather than adding all the way to MPPT. The installation angle and orientation of the components are determined according to the application scenario.

On the other hand, in terms of stability and efficiency, the greater the number of MPPTs, the higher the system cost; and the smaller the MPPT current per path at a fixed rated power, the worse the stability of the maximum power tracking, the more losses, and the worse the power generation. So how do you understand the fixed output power rating, the more MPPT, the greater the loss?

A. Functional loss: There are many MPPT algorithms, such as interference observation method, incremental conductance method, conductance increment method, etc. No matter which kind of algorithm, it is through constantly changing the DC voltage to judge the intensity change of sunlight. There will be errors, for example, when the voltage is actually at the best operating point, the inverter will still try to change the voltage, making the actual work in the non-maximum power point, more than one MPPT, there will be more loss.

B. Measurement Loss: In the MPPT operation, the inverter needs to measure current and voltage. In general, the greater the current, the greater the anti-jamming capability and the smaller the error. The 2-channel MPPT is 1 times larger than the 4-channel MPPT and the error is twice as small. If a company's 50KW inverter uses an open-loop DC current sensor HLSR20-P, the current is 20A and the error is 1%. When the input current is less than 0.5A, the error often occurs. When the input current is less than 0.2A, Basically not working.

C. Circuit Loss: The MPPT main circuit has an inductor and a switch that also generate losses during operation. In general, the higher the current, the smaller the inductance, the less the loss.

The diversity of inverter MPPT technology has brought great convenience to the power station design. PV power plant designers should select different inverters in light of actual, scientific design, different terrain, and different lighting conditions, thereby reducing the cost of the power plant and increasing economic efficiency.

A. The complex hilly power station and multi-faceted roof power station have the phenomenon of inconsistent orientation and partial shading, and different shading characteristics of different hills lead to mismatch of components. It is recommended to select two or more MPPT inverters, which can increase morning and evening. Power generation time

B. For relatively flat hill power stations and medium to large-sized roof power stations, it is recommended to select two MPPT inverters that can take into account system stability and power generation.

C. In areas where there is no obstruction on the flat ground and good lighting conditions, it is recommended to select a single-channel MPPT and a single-stage inverter to improve system reliability and reduce system cost.

It is the best principle for inverter design to follow the golden path, develop in a balanced manner, and moderately ahead!

Shenzhen Guruwatt New Energy Co., Ltd. was established in May 2010 with a registered capital of 100 million yuan. It is a national high-tech company that specializes in providing new energy system solutions such as photovoltaic inverters, energy storage systems and home smart energy management systems. enterprise. Since its establishment, Guruwatt inverters have been exported overseas to more than 100 countries and regions in Europe, the United States, Australia, Asia, Africa, and Latin America, gaining recognition and favor from global customers. Guruwat has always maintained its leading position in China's home inverter market and has always been committed to being the world's largest provider of user-side smart energy solutions.

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