1. Photovoltaic Power station
Photovoltaic power station refers to a photovoltaic power generation system that uses solar energy, uses special materials such as crystalline silicon panels, inverters and other electronic components, and is connected to the grid and transmits electricity to the grid. Its advantages are: (1) no risk of exhaustion; (2) Safe and reliable, no noise, no pollution, absolutely clean (pollution-free); (3) Not limited by the distribution of resources, the advantages of building roofing can be used; (4) Local power generation and power supply without consuming fuel and erecting transmission lines; (5) High energy quality; (6) The construction cycle is short, and the time spent to obtain energy is short. Its disadvantages are: (1) the energy distribution density of irradiation is small, that is, to occupy a huge area; (2) The energy obtained is related to weather conditions such as four seasons, day and night, cloudy and sunny; (3) Bring volatility to the power grid; (4) The access of a large number of power electronic components brings harmonic pollution, and harmonic devices need to be removed.
2. Photovoltaic power station electrical design principles
The design of power station should adapt to the reform of electric power system, implement the relevant national policies and guidelines, and carry out the relevant design rules and regulations; Design based on the approved power plan, network plan and power plant output plan; The design scheme should be coordinated with the overall planning of the power grid, so as to achieve advanced technology, economic and reasonable, and fully consider the influence of uncertain factors in the power system.
3. Photovoltaic power station electrical design
3.1 Selection of electrical primary equipment
3.1.1 Selection of Power distribution devices
Elevation and site pollution level should be considered in the selection of distribution equipment. When evaluating the factors of geographical location, the fixed switchgear should be selected, and the fixed switchgear and the mobile switchgear have their own advantages. Taking the fixed switchgear as an example, its cost is low, the correction is convenient, and the maintenance space is large, but the replacement frequency is low. Taking the mobile switch cabinet as an example, its size is small, the weight is very light, and the appearance is beautiful, but it is prone to failure and other problems, increasing the workload of maintenance personnel
3.1.2 Technical Specifications of Other devices
The technical parameters of the switchgear include the following indicators: rated voltage, frequency, current, short-circuit breaking current, short-circuit closing current, dynamic stability current, thermal stability current and housing protection level. The technical parameters of the circuit breaker should be comprehensively considered as follows: withstand voltage, short-circuit breaking current, short-time withstand current, peak withstand current and so on. The technical parameters to be considered by the isolation switch include: short-time power frequency withstand voltage, lightning impulse withstand voltage, short-time withstand current, rated short-circuit closing current of the grounding switch, peak withstand current and so on.
3.2 Main wiring design of photovoltaic power station
The wiring design of photovoltaic power station mainly includes the main wiring design of booster station, the design of booster transformer wiring and the connection mode of photovoltaic power station collector line.
3.2.1 Main cable design of booster station
In the design process of the main wiring of the booster station, two key points should be paid attention to, that is, the voltage level of the booster station and the position of the power system. Therefore, an effective connection mode is selected to make it adapt to the position and function of the booster station in the system, so as to ensure the safety and reliability of the power grid.
3.2.2 Voltage Boost Cable Design
In the specific design process, affected by the capacity of the inverter, it is necessary to connect the photovoltaic module and the inverter to make it the smallest power generation unit. In the specific design process, there are three main forms, that is, the unit connection between the generator and the double-winding transformer, the expanded unit connection between the generator and the double-winding transformer, and the expanded unit connection between the generator and the double-split winding transformer. Among the three types of wiring design, the generator-double split winding transformer expansion unit wiring can not only reduce the electromagnetic interference and circulation influence between each other, but also effectively improve the quality of transmission, which is the most recommended wiring design method.
3.3 Collector circuit primary system design
3.3.1 Determine the preliminary scheme of the collector circuit
On the photovoltaic array option, a transformer with a capacity of 1000kVA can be selected to raise the output voltage of the inverter to 35kV, and then the current is concentrated to the bus bar of the switching station. Each array of the 200MWp photovoltaic array can be incorporated into the electrical line by a series connection from end to end. In the high pressure side of the box, the high pressure load switch should be set to avoid the splitting and closing of one square array to the normal operation of other square arrays.
3.3.2 Perform electrical calculation on the switch station
The switch station is generally arranged in a more gentle terrain, such as the top of the mountain with less light, which can reduce the amount of earth and facilitate the access of the line. When selecting the main electrical wiring, it should be divided into two levels of voltage for low voltage station and voltage for high voltage station. After collecting power from two collector lines, the power is sent to the switching station, and then connected to the substation through a single loop overhead outlet. In the system, it can mainly adopt the single bus main wiring mode, the bus is built by one time, and there are six cabinets above the bus, namely, overhead cable outlet cabinet, cable entry cabinet, reactive power compensation feeder cabinet, transformer outlet cabinet and bus PT cabinet, which are arranged on the six sides of the bus bar. In the side capacitor current calculation, the total length of the collector line cable has been calculated, according to the capacitor current calculation formula of the single-phase grounding of the grid on the side of the collector wire, Ic=0.1×UexL can be obtained, and the value of Ic can be calculated by digital calculation. When selecting the variable capacity of the arc suppression coil and the ground station, the capacity of the arc suppression coil and the transformer of the ground station need to be calculated. According to the design manual, after calculating the capacity, the station grounding change sleeve device should be used, arranged in the interval between the grounding change and the arc suppression coil.
3.4 Lightning Protection Grounding Design
In the electrical primary design of photovoltaic power station, because the average annual thunderstorm days in the area where the photovoltaic power station project is located are relatively few, the probability of damage by lightning is relatively low. In this case, there is basically no need to install lightning protection facilities such as lightning rods. However, in the photovoltaic power station project, in view of the large area of the battery bracket and the steel characteristics, it is very vulnerable to the damage of induction mines in the specific use process. Therefore, in the specific design process, it is necessary to reliably weld the battery bracket to ensure that in the event of thunderstorms, the bracket can achieve rapid defense through the ground network, and will not affect the entire power grid, so as to effectively ensure the safe operation of the entire power grid system. In addition, the overvoltage protector can also be placed in the bus box and AC/DC distribution cabinet to effectively reduce the damage of inductive lightning to the electrical equipment and protect the electrical equipment of the power grid.
3.5 Neutral grounding design of photovoltaic power station
In the specific design process for neutral grounding, it is necessary to ensure that there is a close relationship between the design method and the voltage level, overvoltage level, protection configuration, etc., so that it acts on the operation of the entire power system, and ensure that the power grid system can be quickly and accurately solved after the failure. In the specific design process of neutral grounding of photovoltaic power station, it mainly includes two kinds of current grounding systems, large and small. Because of the difference in grounding mode and operation characteristics of these two systems, when single-phase grounding occurs in the high-current grounding system, short-circuit current will be generated, which has an important impact on the normal power consumption of users. When single-phase grounding occurs in the small-current grounding system, the grounding current generated is not large, which will not affect the user’s normal power consumption.
4. Closing remarks
Electrical primary design is a key component of photovoltaic power generation, and it is the link that effectively combines photovoltaic power generation system and power grid. Therefore, relevant personnel need to pay more attention to the construction of photovoltaic power stations, optimize the design and research of the electrical system, constantly improve the utilization rate of photovoltaic power stations, improve the electrical level of photovoltaic power stations, promote the further development of photovoltaic power stations, and provide help for China’s green power to enter a new stage.
