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At present, the common solar photovoltaic power generation system grid-connected program, according to the working voltage of the solar cell matrix can be divided into low-voltage grid-connected system and high-voltage grid-connected system. Low-voltage grid-connected system often consists of 3 to 5 photovoltaic modules in series, the DC voltage is less than 120V.

The advantage of this approach is that each string of photovoltaic cells in series less, the sun shadow tolerance is relatively strong; drawback is the DC side of the current larger, in the design need to use large cross-section of the DC cable. High-voltage grid-connected system commonly used in the solar cell square rated power of the larger system, the number of components in series, DC voltage is relatively high, the shortcomings of the way the tolerance of the sun shadow is relatively small; the advantages of high voltage, Low current, the use of smaller cable diameter, and the inverter better match, making the inverter conversion efficiency is higher, the current large-scale photovoltaic power generation system to use high-pressure system.

At present, the solar photovoltaic power generation system design capacity can be from several kilowatts to several hundred kilowatts, or even on the megawatt, due to the domestic photovoltaic power generation and building a variety of forms, equipment selection according to the solar cell array installed The actual situation (such as component specifications, installation orientation, etc.) to optimize the design, solar photovoltaic grid-connected inverter system settings are divided into: centralized, master-slave, distributed and group string.

1 centralized

Centralized grid-connected way suitable for installation to the same and the same specifications of the solar cell array, in the electrical design, the use of a single inverter to achieve centralized grid-connected power generation program shown in Figure 1.

For large-scale grid-connected photovoltaic systems, if the solar cell array installation orientation, inclination and shadow, etc. are basically the same, usually using large-scale centralized three-phase inverter.

The main advantages of this approach are: the overall structure of the use of photovoltaic grid-connected inverter less installation and installation is relatively simple; use of centralized inverter power, high efficiency, usually large-scale integrated inverter efficiency ratio distribution Type inverter is about 2% higher, for the 9.3MWp photovoltaic developed system, because the number of inverters used less initial cost is relatively low; less access points, the output power quality is higher. The main drawback of this approach is that once the grid inverter failure, will cause a large area of solar photovoltaic power generation system disabled.

Centralized Inverter is generally used in large photovoltaic power plants (> 10kW) system, many parallel photovoltaic cell strings are connected to the same set of inverts DC input, the general power of the use of three-phase IGBT power module, The smaller power is used by field-effect transistors while using the DSP to improve the quality of the output power, making it very close to the sine wave current.

The biggest feature is the system of high power, low cost But by the photovoltaic cell string match and part of the impact of shadow, resulting in the efficiency of the entire photovoltaic system is not high. At the same time, the reliability of the whole photovoltaic system is affected by the poor working condition of a PV cell group. The latest research is the use of space vector modulation control, and the development of new inverter topology connection, in order to obtain some of the high load efficiency.

 On the SolarMax centralized inverter, an array of photovoltaic cells can be attached to each string of monitoring of the series of photovoltaic cells, such as one of the group of photovoltaic cells is not working properly, the system will be Will pass this information to the remote controller, while the remote control of this string of photovoltaic cells can stop working, so as not because of a string of photovoltaic battery string failure and reduce the impact of the entire photovoltaic system and energy output The

2 master-slave style

For large-scale photovoltaic power generation system can be used master-slave structure, the master-slave structure is actually a centralized type, the main feature of the structure is the use of 2 to 3 centralized inverter, the total power is divided by several inverters The In the case of low radiation, only one inverter works to improve the efficiency of the inverter in the output of low power when the solar cell is arrayed; when the solar radiation rises, the output power of the solar cell square array increases to more than one When the capacity of the inverter, the other inverter is automatically put into operation.

In order to ensure that the inverter running time equal, the master and slave inverter can automatically rotate the master and slave configuration. Master-slave grid-connected power generation principle shown in Figure 2. The initial cost of the master-slave structure will be higher, but can improve the efficiency of the PV system inverter operation, for large-scale photovoltaic systems, the efficiency of the increase can produce greater economic benefits.

3 distributed

Distributed grid-connected power generation method is suitable for the installation of different orientations or different specifications of the solar cell array, in the electrical design, the same direction and specifications of the same solar cell array through a single inverter centralized power generation, large Of the distributed system is mainly for the solar cell square direction, tilt and sun shadow is not the same situation used.

Distributed system will be the same orientation, tilt and shadowless photovoltaic cell components string into a string, by a string or a few strings constitute a solar cell square, install a grid inverter with the match. Distributed grid-connected power generation principle shown in Figure 3. In this case, you can omit the wire box, reduce costs; can also be grid-connected photovoltaic power generation system for fragmented maintenance, reduce maintenance loss of power generation.

The main drawback of distributed grid-connected power generation is the need for multiple grid-connected inverters for large and medium-sized hundreds of kilowatts or even megawatt-class PV systems, and the initial inverter costs may be relatively high The number of inverters is large, the AC side of the inverter and the access point of the public power grid are also many. It is necessary to connect the output of the inverter in parallel with the AC side of the photovoltaic power generation system, which has certain influence on the quality of the power grid.

4 groups of string

The PV grid-connected string inverter connects each photovoltaic cell module to an inverter while each PV cell module has a single maximum power peak trace, so that the PV cell assembly is better suited to the inverter. The string-string inverter has become the most popular inverter on the international market today. The string-string inverter is based on the modular concept, and each PV string (1kW ~ 5kW) passes through an inverter, End with the maximum power peak tracking, parallel in parallel with the network. Many large-scale PV power plants use string-series inverters, which are not affected by the inter-string difference and shadowing of photovoltaic cells.

The concept of "master-slave" is introduced between the strings to cause the system to group several groups of photovoltaic cells together in a single-stringed energy that can not operate a single inverter, allowing one or more of them to work Out of more power. The latest concept for several inverters to form a "team" to replace the "master - slave" concept, making the system reliability and further step. At present, no transformer series string inverter has occupied the dominant position.

Multi-string inversion is to take the advantages of centralized inverter and string inverter, to avoid its shortcomings, can be applied to several kilowatts of photovoltaic power plants. In a multi-string inverter, a separate single-power peak-tracking DC / DC converter is included, and the output of the DC / DC converter is converted into alternating current to the grid in parallel by an ordinary inverter. Because it is in parallel at the exchange, which increases the complexity of the connection side of the communication side, maintenance difficulties.

The other way to solve is how to more effectively with the grid and the net, the simple way is to directly through the ordinary AC switch and network, which can reduce the cost and equipment installation, but often around the power grid security standards may not allow this do. Another safety-related factor is the need to use isolation transformers (high or low frequency), or to allow the use of transformer-free inverters.

(Such as: different rated power, different sets of components per string, different components of the manufacturer, etc.), different sizes or different technologies of photovoltaic components, different groups of strings (such as: East, south and west), different inclinations or shadows can be connected to a common inverter while each string is working on their respective maximum power peaks. At the same time, the length of the DC cable is reduced, the shadow effect between the strings is reduced and the loss due to the difference between the strings is minimized.