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Guidance on Selection of Inverter for Photovoltaic Power Station Based on Local Conditions and Scientific Design


First, the classification of photovoltaic power plants and power station characteristics
According to the different installation of photovoltaic power plants, photovoltaic power plants are generally divided into desert power station, roof power station and hill power station three.

Desert power station: the use of broad flat desert ground resources development of photovoltaic power plants. This type of power station is large, generally greater than 5MW, the current single 50MW scale power station is very common; power plant inverter output after the boost directly into the 110KV, 330KV or higher voltage level of high voltage transmission network; , The PV modules are aligned and unobstructed. This type of power station is the main force of Chinas photovoltaic power plants, mainly concentrated in the western region.
Hill power station: the use of mountain, hills and other resources to develop photovoltaic power plants. The size of these types of power plants vary from several MW to hundreds of MW range; power generation into the high-voltage transmission network based; affected by the terrain, and more components are inconsistent or sooner or later to block the problem. This type of power station is mainly used in mountainous areas, mines and a large number of wasteland can not be planted.
Roof power station: the use of plant, public buildings, residential roof resources such as the development of photovoltaic power plants. The size of the type of power plant is limited by the effective roof area, the installed capacity is generally several kilowatts to tens of megawatts; power generation to encourage local consumption, direct access to low-voltage distribution network or 35KV and below the high-voltage power grid; component orientation, Shadow Shields are diverse. This type of power station is the main form of current distributed photovoltaic applications, mainly concentrated in the eastern and southern regions of China.
Second, the inverter classification and characteristics
PV inverter according to its power level, the internal circuit structure and application of different occasions, generally can be divided into centralized inverter, group string inverter and micro-inverter three types.
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Centralized Inverter: The main feature is the single power, the maximum power tracking (MPPT) a small number of low cost per watt. At present, the domestic mainstream models to 500KW, 630KW-based, Europe and North America and other regions mainstream models single power 800KW or even higher, power levels and integration is still rising, the German SMA company launched this year, stand-alone power 2.5MW reverse Variable. According to the inverter main circuit structure, the centralized inverter can be divided into the following two types:
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Centralized inverter is currently the choice of most large and medium-sized photovoltaic power plants in the world more than 5MW of photovoltaic power plants, the selection of more than 98%.
Group string inverter: stand-alone power between 3-60KW. Main models of single power 30-40KW, single or multiple MPPT, usually 6-15KW all the way MPPT. This type of inverter per watt cost is higher, mainly used in small and medium power plants, in the world 1MW capacity of the following power station selection rate of more than 50%.
Micro-inverter: single power below 1KW, single MPPT, the application of 0.25-1KW all the way MPPT, the advantage is that each piece or a few panels can be independent of the MPPT control, but the inverter per watt High cost. Currently in North America 10KW following home photovoltaic power plants have more applications.
Typical applications of several inverters are shown in the figure.
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Typical applications of several inverters
As shown in the figure, the PV module forms a series of strings in series, and the plurality of strings are connected in parallel to form a square matrix. The centralized type connects one of the strings of one square array to one or two inverters, and the number of MPPTs is relative Less string; the string will be one or several series of string access to an inverter, a square array MPPT, micro-inverter on each panel for MPPT tracking. When the components are blocked due to shadow or inconsistent, there will be series and parallel mismatch. The multi-channel MPPT can solve the parallel mismatch between strings, and the micro-inverter can solve the parallel mismatch between the strings and solve the series mismatch between the components. Therefore, from a technical point of view, the essence of several inverters is the handling of component mismatch problems.
The design of the inverter as the core of the selection, the need for PV system life cycle to find the total power generation and total cost of the balance, but also consider the power grid access, such as fault crossing capability, power quality, power grid adaptability and other aspects Claim. According to the characteristics of various inverters, combined with the actual situation of the application of photovoltaic power plants, from the grid friendly, high return on investment, easy construction and maintenance aspects of scientific and rational selection.
Third, the different power station inverter selection guide
1, desert power station - centralized advantages obvious
Centralized inverter has the following advantages, is the first choice for desert power station.
Lower initial investment. According to the comparative analysis, the centralized scheme is more than 260,000 yuan investment per kilowatt at the initial investment.
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Power generation and group string flat: desert power station in the centralized and group of string power generation is basically flat, integrated in the highest efficiency and overload capacity and other advantages, centralized power generation slightly higher than the string type. A small number of power stations appear early and late before the row of the block, the use of group string can not be overcome, need to optimize the layout of components to avoid.
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Operation and maintenance more convenient and more economical. By comparing the operation and maintenance data of the centralized and group-type mainstream models in the 100MW power plant, the loss of power generation is quite the same. Since the string-type equipment is the whole machine maintenance, and the centralized equipment is the maintenance of the device and the equipment maintenance cost, Centralized advantage is very obvious. At the same time, in a large number of acres of 100-megawatt-class large-scale power station, the complete dispersion of the string of string inverter replacement, maintenance personnel spent on the road will be much higher than the time to replace the equipment, which is Group of large-scale power plant applications one of the unfavorable factors.
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2, hill power station - multi-MPPT centralized program-based, but also consider the string of string-type program. Centralized solution more in line with the power grid access requirements. High-voltage transmission network on the grid-connected photovoltaic power generation in the scheduling response, fault crossing, limit hair, hair, smooth, harmonic limit, power rate of change, emergency start and stop, and so have strict requirements. Fault crossing refers to the grid short circuit, surge, lack of circumstances, the inverter must be able to 625 milliseconds to a few seconds still output a certain amount of active and reactive power, to ensure that the power system relay protection to normal action , Because the centralized inverter in the power station in the small number of units, stand-alone powerful, simple communication control, failure to pass the probability of failure is much greater than the string of string inverter. In mid-June 2013, the State Grid organizations field of low-voltage through the test, a number of models of inverter in varying degrees also appear off the network situation, imagine if there are thousands of small inverters running in large power plants, once the power grid failure , Due to many complex control equipment, power grid coupling, the probability of resonance increases, the string string inverter is bound to a large number of off the net, or even damage to the equipment itself, endanger the safe operation of the grid. In addition, because the number of inverters too much, can not ensure that within 30ms response to reactive power command instructions.
Hill power station can be seen as terrain is not flat desert power station, but also feed into the main transmission network, the scale of more than 5MW. In the hill power station project, usually a coordinate system planning more than 100 KW capacity components (such as 125KW components laid out the same direction), to achieve the optimal amount of power generation and investment and maintenance costs. For more than 100 KW components per MPPT for each MPPT module model developed for this application, the design area of the same component towards the component is reduced to about 1000 square meters, greatly improving the construction convenience and effective To solve the problem of facing and occlusion, while a total of AC bus output, with centralized inverter grid friendly features, is the preferred scheme of the mountain power station.
If the selected hill power station terrain is very complex, it is difficult to achieve more than 100 KW components towards the laying construction, and the string-type inverter can be considered as a supplement.
3, roof power station - recommended group string type, can also choose a centralized program
Roof power plant design is relatively complex, by the roof size, layout, material load, and shadow shading and other effects, through the need for component laying and inverter selection planning to maximize the benefits. At the same time components installed on the roof, need to consider fire prevention and other security issues. Access to the distribution network, directly close to the user load, need to consider the user power security, power quality to meet the requirements, and with the original distribution between the relay protection coordination. After access to the user distribution network, the impact of the power factor is very obvious, the inverter in addition to output power, but also need to quickly according to the real-time photovoltaic power generation system, the user real-time load data and user power distribution room SVC, SVG inputs are calculated to determine the real-time reactive power output of the inverter. Therefore, the choice of rooftop PV system program needs to be in the safety, power grid friendly, return on investment, maintenance and other factors to seek a balance.
Roof structure is complex, there is blocking or inconsistent, it is recommended to use group string inverter. Roof structure is complex, in order to simplify the design, it is recommended to use string-type inverter, and according to the actual roof and the location of the grid and the dot voltage level, select the inverter. Group string inverter need to have the arc monitoring and shutdown ability to effectively prevent the occurrence of fire, with PID elimination function, with high-precision leakage current protection and island protection functions. The same time as the above-
Large plant, taking into account the roof load and maintenance convenience, you can choose a centralized program. Industrial plant roof flat, large scale, less shadow cover, towards a simple, mostly 10KV medium voltage distribution network and network. Taking into account the majority of the plant for the color steel roof, load-bearing can not install string-type inverter, as well as routine maintenance, does not affect the normal production and other practical conditions, the choice of centralized inverter.
Summary: The inverter as a bridge between components and the grid is the core component of the PV system. According to the size of the power plant, as well as different applications, select the appropriate inverter, the system cost and power generation are of great benefit. In the large-scale, terrain of the mountain power station, suitable for the use of multi-MPPT centralized inverter; in the relatively small size, the layout of diverse Of the roof power station, suitable for selection of string-type inverter. In accordance with local conditions, the scientific choice of photovoltaic power plant inverter, PV power plants in the investment decision-making stage to ensure that detours in the latter part of the operation and maintenance phase more reliable and efficient operation.