Typical design scheme of photovoltaic power station (inverter part)-EEWORLD

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The PV Leader Program was launched in 2015 with the original intention of promoting the advancement of PV power generation technology, industrial upgrading, market application and cost reduction. Through market support and experimental demonstration, the transformation of technological achievements into market applications will be accelerated, backward technologies and production capacity will be eliminated, and the goal of grid parity for PV power generation will be achieved in 2020. The first batch of 1GW projects of the Leader Program were located in the coal mining subsidence area of Datong, Shanxi Province. There were 7 100MW projects and 6 50MW projects, which were connected to the grid in late June 2015. In 2016, the PV Leader Program had 8 base projects with a total of 5.5GW, adding the content of bidding for grid access and taking electricity price as the main competitive condition. In 2017, there were 6.5GW, 10 application leader bases and 3 technology leader bases, which were required to be completed and connected to the grid by the end of 2018 and the first half of 2019 respectively.

1. Topographical situation of photovoltaic leaders

Most of the photovoltaic leader bases are mainly coal mining and oil mining subsidence areas, and some are saline-alkali land, copper mines, or other mining areas, with various terrains. Most places are hilly, with complex and changeable terrain; there are also slightly flat areas, and after taking remediation measures, the land has restored its original suitability and productivity, and can be used as photovoltaic agricultural greenhouses; a small number of places are relatively flat. The photovoltaic leader base generally has a capacity of more than 50MW, and is connected to the grid for power generation with a high voltage of 220KV.

2. Selection of components

The technical indicators of the leaders were further improved in 2017. Among them, the conversion efficiency of polycrystalline silicon cells and modules reached 19.5% and 17% respectively, and the conversion efficiency of monocrystalline silicon cells and modules reached 21% and 18% respectively. For 60-cell modules, the polycrystalline must be above 275W and the monocrystalline must be above 290W. For 72-cell modules, the polycrystalline must be above 335W and the monocrystalline must be above 350W to meet the requirements.

3. Use weather-resistant steel bracket

Weathering steel refers to a type of low alloy steel that has a small amount of alloying elements added to significantly improve its atmospheric corrosion resistance. Its surface can form a protective rust layer, which effectively blocks the penetration and transmission of corrosive media. The longer it is used, the more prominent its weathering effect is.

Weathering steel has a weather resistance 2 to 8 times that of ordinary carbon steel. It is also rust-resistant, making components corrosion-resistant and life-extending, thinning and reducing consumption, saving labor and energy, etc. It is mainly used for steel structures such as railways, vehicles, bridges, towers, etc. that are exposed to the atmosphere for a long time. It is also used to manufacture containers, railway vehicles, oil derricks, seaport buildings, oil production platforms, and chemical petroleum equipment containing hydrogen sulfide corrosive media and other structural parts. Compared with stainless steel, weathering steel has only trace amounts of alloying elements, and the total amount of these alloying elements accounts for only a few percent, unlike stainless steel, which reaches more than ten percent, so the price is relatively low.

Compared with galvanized steel, weathering steel has several advantages: First, it will not pollute the soil due to the shedding of the zinc layer, and it is more energy-saving and environmentally friendly. Second, it is inexpensive. Each ton of weathering steel brackets saves 1,000 yuan compared to galvanized brackets. The preliminary design of the spiral pipe piles and brackets of the CGN Yangquan Leader project requires 8,000 tons of steel. The use of weathering steel reduces the cost by 8 million yuan, which is about 0.8 cents per watt. Third, weathering steel does not need to repair the coating like galvanized steel, so the subsequent maintenance cost is lower.

4. Inverter selection and system solution

(1) Multi-channel MPPT string solution

If the mountain has multiple slopes and multiple azimuths, there is little flat land, and it is impossible to level the land. The terrain facing due south is also limited. The power of each square array is less than 60kW. In order to ensure the capacity, the southeast, southwest, east and west slopes must be fully utilized. At this time, the installation direction of the solar panels cannot be completely arranged facing south. Multi-channel MPPT string inverters can be used to accurately track every 1~2 strings, fully tap the maximum output power of each solar panel, and greatly reduce the component mismatch loss caused by distance and shading. The string solution can well adapt to factors such as shadow shading and inconsistent component orientation in mountains and hills.

The system uses multiple string inverters in parallel, and then boosts the voltage to enter the high-voltage grid. The design uses 80KW inverters, and every 12 inverters are merged once, and then connected to a high-efficiency 35KV step-up transformer (0.48/35KV, 1000KVA) to connect to the local 35KV medium-voltage grid to achieve grid-connected power generation.

The solar cell modules used in the photovoltaic power station have a power of 335W, 21 in series and 12 in parallel. Each inverter is connected to 252 modules, with a total power of 84.42KW.

(2) Single-channel MPPT string solution

In some areas with relatively more flat land and good lighting conditions, the power of each array is less than 100kW and greater than 60kW. It is recommended to choose a single-channel MPPT, single-stage structure inverter, which can improve system reliability and reduce system costs. The system uses multiple string inverters in parallel and then boosts the voltage to enter the high-voltage grid. The design uses a 60KW inverter, and every 16 inverters are merged once, and then connected to a high-efficiency 35KV step-up transformer (0.4/35KV, 1000KVA) to connect to the local 35KV medium-voltage grid to achieve grid-connected power generation.

The solar cell modules used in the photovoltaic power station have a power of 335W, 19 in series and 10 in parallel. Each inverter is connected to 190 modules, with a total power of 63.65kW.

(3) Centralized inverter:

The centralized grid-connected solution is suitable for power stations with a power greater than 100kW in a relatively flat area with a relatively concentrated component layout and a square array with consistent component orientation and no obstructions. This type of power station uses 1.7MW as a subsystem. The entire power station is composed of several 1.7MW subsystems. The output of each subsystem is centralized and grid-connected after convergence. Each 1.7MW subsystem is an outdoor inverter room, equipped with two 850KW grid-connected inverters, with an output rated voltage of three-phase 400V and a frequency of 50Hz; it is connected to the local 35KV medium-voltage power grid through a high-efficiency 35KV double-split step-up transformer (0.4/0.4/35KV, 1700KVA) T to achieve grid-connected power generation function.

The solar cell modules used in the photovoltaic power station have a power of 340W, with 20 strings in each group. The output power of the string is 6.8kW. 16-way DC lightning protection combiner boxes are used. Each combiner box connects 320 modules, with a total power of 108.8KW. Each inverter is connected to 8 combiner boxes. A total of 2560 modules are connected to the system, with a total power of 870.4KW.

The inverter adopts an internal integrated DC power distribution design with confluence and reverse flow prevention functions. The DC power distribution cabinet can be configured according to the user. The DC input circuit is equipped with a disconnectable DC circuit breaker and DC voltmeter. According to user requirements, fuses, lightning arresters, reverse flow protection diodes, etc. can be selected.

The megawatt-class outdoor inverter unit is to install two 850KW inverters in a special machine room to complete the grid-connected control, data collection and remote transmission functions of the photovoltaic power generation system. Compared with the traditional on-site built inverter room, the box-type photovoltaic inverter station has the characteristics of low cost, simple installation and debugging, and beautiful appearance. No machine room is needed, which can save costs and shorten the construction period.

Array power and inverter solutions:

Reference address：Typical design scheme of photovoltaic power station (inverter part)

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