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Centralized, string inverters plus 2018 new technology iteration

2017 is the year of concentrated explosion of component technology. Before that, we only heard about monocrystalline silicon and polycrystalline silicon. Now, there are a lot of new terms in the market: PERC, half-chip, MBB, MWT, shingle, double-sided power generation.. .... Under the circumstance of new technology iterations, the production capacity of new technologies is a must. In 2017, several technologies that have matured or are about to mature are Perc technology, half-chip technology, and MBB multi-main gate technology. In theory, these technologies are compatible technologies that can be used on both monocrystalline silicon wafers and polycrystalline silicon wafers.
 
In 2017, the inverter did not have new terms like components, but it does not mean that the inverter has no technical updates. On the contrary, various manufacturers of inverters are working hard, inverters are the brain and heart of photovoltaic systems. In 2018, some new PV applications will appear in front of people, bringing new changes to photovoltaic systems. Including cost reduction and efficiency improvement, installation and maintenance will be more convenient.
 
The mainstream of inverters are centralized inverters and string inverters. Both methods have their own advantages and disadvantages. At present, in the household market, due to the small power, string inverters are used. For large power plants, both inverters can be selected. In marketing, the manufacturers of these two inverters are also playing hard, and they are saying their own advantages and the shortcomings of the other side.
 
Centralized inverter: The power is between 30kW and 2500kW. With the development of power electronics technology, the string inverters are getting bigger and bigger, and now the centralized inverters below 500KW basically return to the market. The power device adopts high-current IGBT. The system topology adopts DC-AC first-level power electronic device to transform the full-bridge inverter. The latter stage is generally connected to the double-split power-frequency boost isolation transformer. The protection level is generally IP20. Large size, indoor vertical installation.
 
The main advantages are:
 
(1) The number of inverters is small, which can be installed centrally and easy to manage;
 
(2) The number of components of the inverter is small, the number of fault points is small, and the reliability is high;
 
(3) The harmonic content is small, the DC component is small, and the power quality is high;
 
(4) The inverter has high integration degree, high power density and low cost;
 
(5) The inverter has various protection functions and the power station has high safety;
 
(6) There are power factor adjustment function and low voltage ride-through function, and the power grid is well regulated.
 
The main disadvantages are:
 
(1) The DC bus box has a high failure rate and affects the entire system.
 
(2) The centralized inverter MPPT voltage range is narrow, generally 500-820V, and the component configuration is not flexible. In the rainy days, the foggy area is short, and the power generation time is short.
 
(3) The installation and deployment of the inverter room is difficult, and a dedicated machine room and equipment are required.
 
(4) The power consumption of the inverter itself and the ventilation and heat dissipation of the equipment room are relatively complicated.
 
(5) In the centralized grid-connected inverter system, the component square matrix reaches the inverter after two convergences, and the maximum power tracking function (MPPT) of the inverter cannot monitor the operation of each component, so it is impossible to make each channel Components are at the optimum working point. When a component fails or is blocked by shadows, it will affect the power generation efficiency of the entire system.
 
(6) There is no redundancy capability in the centralized grid-connected inverter system. If there is a fault shutdown, the whole system will stop generating electricity.
 
2. String inverter:
 
The power is from 1kW to 80kW. The low-power inverter switch tube generally adopts a small current MOSFET. The medium power inverter generally adopts a power module integrating multiple discrete devices. The topology adopts DC-DC-BOOST boost and DC-AC. The full-bridge inverter two-stage power electronic device is changed, and the protection level is generally IP65. Small in size, it can be mounted on an outdoor arm. The string inverter is suitable for small and medium-sized rooftop photovoltaic power generation systems and small ground power stations.
 
The main advantages are:
 
(1) The string inverter adopts modular design, each photovoltaic string corresponds to one inverter, the DC terminal has the maximum power tracking function, and the AC terminal is connected in parallel, the advantage is that it is not subject to the difference between the modules. The effect of shadow occlusion, while reducing the optimal operating point of the PV module and the inverter does not match, maximizing the amount of power generation.
 
2) The string inverter has a wide MPPT voltage range, generally 200-800V, and the component configuration is more flexible. In the rainy days, the foggy area is long and the power generation time is long.
 
(3) The string-type grid-connected inverter is small in size, light in weight, easy to handle and install, does not require professional tools and equipment, and does not require a dedicated power distribution room. It can simplify construction in various applications. Reduce the land occupation, DC bus connection does not need DC combiner box and DC power distribution cabinet. The string type also has the advantages of low self-consumption, low impact, convenient replacement and maintenance.
 
The main disadvantages are:
 
(1) There are many electronic components, and the power device and the signal circuit are on the same board. It is difficult to design and manufacture, and the reliability is slightly poor.
 
(2) Inverters using discrete power devices have small clearances and are not suitable for high altitude areas. Inverters using power modules are not affected. Outdoor installation, wind and sun exposure can easily lead to aging of the outer casing and heat sink.

(3) Without isolation transformer design, the electrical safety is slightly worse. The negative electrode grounding system of the thin film component should be equipped with an isolation transformer. The DC component is large and has a great influence on the power grid.
 
(4) When multiple inverters are connected in parallel, the total harmonics are high, and the single inverter THDI can be controlled to more than 2%. However, if more than 100 inverters are connected in parallel, the total harmonics will be superimposed and difficult. inhibition.
 
(5) The number of inverters is large, the total failure rate will increase, and the system monitoring is difficult.
 
(6) There is no DC breaker and AC breaker. If there is no DC fuse, it is not easy to disconnect when the system fails.
 
This is the comparison of the two inverters before 2014. After more than three years of development, both inverters have made great progress, and some opinions have proved to be wrong.
 
1, DC combiner box, before everyone is not very related, design and work are not good, especially heat, not a few can pass, but now basically solve the problem.
 
2, the working voltage range, before everyone's perception is that the wider the voltage range, early start late shutdown, the higher the power generation, in fact, the voltage and power of the component is not proportional, the voltage of the component is not as large as people think. It can be seen from the following two cases. This is a string of 20 270W, the working voltage of the component is 30.5V, and the operating voltage of 20 components is 660V.
 
Figure 1 is a very good weather, the inverter operating voltage is between 540-630V.
 
Figure 2 is a rainy day, no sunlight, the inverter working voltage is between 530-580V
 
Therefore, the three-phase string inverter, if it is a normal 20-23 block series, MPPT working voltage, can meet the requirements at 500-800V, only in very special cases, such as using 19 or less components, the minimum work The voltage is likely to be lower than 500V.

3. The string inverter is improved, and multiple parallels can be realized, and the total harmonics will not be superimposed.
 
There are still many improvements, but I will not list them here. However, there are two obvious differences between the two inverters. There is no way to change them. The centralized inverter is a single-stage electronic conversion. Less, high reliability, the disadvantage is that the length of the DC line is different, a 500kW inverter, the component installation area is more than 6000 square meters, the component may be more than 100 meters farthest, and only a few meters recently, the line impedance affects the MPPT function.
 
The length of the DC line before the inverter has a great influence on the MPPT efficiency.
 
MPPT-maximum power point tracking works: During a specified period, the microprocessor periodically adjusts the duty cycle D of the PWM to change the output current of the solar cell, thereby causing the output voltage of the solar cell to change, and detecting the solar cell. The output voltage and the output current are calculated, and the output power of the solar array is calculated, and then the position of the maximum power point is found according to the maximum power point tracking strategy.
 
The simple model of MPPT is as follows: There are many kinds of inverter MPPT methods, but the change is that the equivalent load impedance of the MPPT terminal is equal to the equivalent output impedance of the component end, and the output resistance of the component end includes the internal resistance of the component and the cable. Internal resistance. The longer the cable, the larger the internal resistance, and the cable of more than 100 meters may affect the efficiency by more than 1%.

The simplest equivalent model of MPPT
 
RS is the equivalent output impedance of the solar panel, and RL is the equivalent load impedance of the MPPT terminal. When the solar light changes, the RS also changes. This is to meet the maximum power output. By adjusting the load resistance of the MPPT, it matches the load. When RS = RL, the power output is the largest, which is what we call the MPP point. The method in which the RL follows the RS change in real time is called the MPPT algorithm.
 
In fact, the two inverters can be combined together, that is, a unipolar string inverter, which has only one-stage conversion DC-AC conversion, using a three-level and power module, a 50kW The inverter has only three power switching devices. Like the centralized inverter, the 50kW inverter peripheral components have a mounting area of ​​600 square meters, and the farthest distance is less than 25 meters. The DC loss is very small. At present, the monopole string inverter is also available in the market, but it is not used in large power stations. The reason is that the output voltage is 400Vac, the input voltage range is narrowed, only 580-850V, which limits the application range. The workaround can solve this problem, then the output voltage can be changed to 315V. Like the centralized inverter, the input voltage range can reach 500-850V, followed by the step-up transformer, and the application range is very wide.
 
Compared with the string inverter of the conventional two-stage structure, the string inverter has only one stage structure, the efficiency is 30% higher than the string inverter, and the working voltage range is 500-850V. It will affect the power generation. The cost of the series inverter is 30% lower than that of the string inverter, and the weight is 30% lighter. The installation is very convenient.
 
Not all monopole string inverters are called series inverters.
 
Set string inverter = string type shape + centralized core + power module +315V
 
The advantages of both the integrated inverter and the string inverter are the shortcomings of the series inverters, which are cheaper than the centralized ones. The power generation is higher than the string type, installation and operation. Vibi is more convenient and will be used in large quantities to change the current inverter landscape.

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