The design of DC distribution device
1 Photovoltaic DC lightning protection bus box
Photovoltaic lightning junction box is mainly used in the distributed power generation system using centralized photovoltaic inverters, the design of the junction box mainly considers the following factors: 1) the junction box should choose the commonly used specifications: 8, 12 and 16, the specific should be determined according to the layout of the photovoltaic array. 2) The bus box should have the function of isolating short circuit fault. Photovoltaic series should be protected by photovoltaic special DC fuse; The rated voltage should not be less than the open circuit voltage Voc when the PV string is at the lowest local temperature. The current carrying capacity of the DC cable used in the photovoltaic series should not be less than the rated current of the protection fuse. 3) The outlet side of the bus box should be isolated by a photovoltaic DC circuit breaker. 4) The bus box should be equipped with a photovoltaic lightning protection device, and the positive and negative poles should have lightning protection function. 5) The bus box should be equipped with a monitoring device, which can monitor and upload the DC current, total output current, bus voltage and total output power of each battery string in real time, the position and fault state of the DC circuit breaker or the load switch, the status of the DC lightning arrester and the abnormal alarm of the battery string.
2 Photovoltaic DC surge protection bus cabinet
Photovoltaic DC lightning protection bus cabinet is mainly used in the distributed power generation system with a centralized photovoltaic inverter and a single inverter capacity ≥250kW. The design of photovoltaic DC lightning protection bus cabinet mainly considers the following factors: 1) Each inverter corresponds to a DC distribution cabinet, and each DC distribution cabinet is configured with several fixed photovoltaic DC circuit breakers; 2) The rated current of the DC circuit breaker is selected according to the rated current of the DC circuit breaker or the DC load switch of the bus box with the largest number of bus routes in the actual project. 3) The switch position state and fault state of the DC circuit breaker are led to the terminal block and connected to the measurement and control device. When there are special requirements, it should have the function of measuring bus voltage, current, output power and electric energy. 4) The cabinet should be equipped with a special photovoltaic lightning protection device, and the positive and negative poles should have lightning protection function.
AC distribution device design
Ac distribution device design principles: 1) 0.4kV distribution device should be selected easy to install and operate, with obvious breakpoints, with the ability to break fault current AC distribution equipment. Circuit breakers can choose micro, plastic-case or frame type circuit breakers, according to the short-circuit current level to choose the equipment breaking capacity, and need to leave a certain margin. When the altitude is above 1000m, the coefficient should be corrected. 2) 10/35kV distribution device should be easy to install, easy to operate, can be locked, with obvious breakpoint, grounding function, can be broken fault current breaking equipment. Circuit breakers can be vacuum circuit breakers or SF6 circuit breakers, according to the level of short circuit current to choose the device breaking capacity, and must leave a certain margin. When the altitude is above 1000m, the coefficient should be corrected.
Booster transformer design
Step-up transformers are mostly used in the distributed photovoltaic power generation project of step-up and grid-connected, which are commonly used in indoor dry transformers and box transformers. The design of the booster transformer mainly considers the following factors: 1) the selection principle of the booster transformer: for the booster grid-connected distributed generation system using a series of photovoltaic inverters, the booster transformer can choose a double-winding booster transformer; For the booster grid-connected distributed generation system using a centralized photovoltaic inverter, the selection of the booster transformer should be determined according to the characteristics of the selected photovoltaic inverter, which can be divided into double winding dry booster transformer and double split winding booster transformer. The box-type transformer should be able to carry out remote monitoring, should be equipped with a box transformer measurement and control device, and have an intelligent interface, can communicate with the data acquisition device. 2) The high-voltage side of the box-type transformer should be equipped with a load switch-current limiting fuse combination as a short-circuit protection, because the photovoltaic power station is designed according to the principle of “unattended or few people on duty”, the load switch should be able to operate electrically; Low voltage circuit breaker should be installed on the low voltage side, and with long delay, short delay, instantaneous overcurrent protection. 3) Transformer voltage level: the high voltage side of the booster transformer is 10/35kV, and the low voltage side voltage level should be determined according to the outlet voltage of the selected photovoltaic inverter. 4) Short-circuit current level: the short-circuit current should be calculated according to the relevant conditions during the specific engineering design, and the equipment should be selected and verified according to the short-circuit current calculation results. 5) Transformer material: In order to reduce transformer self-loss and improve the reliability of the system, the transformer winding material can be made of copper core material. The specific engineering design can be determined according to the actual situation. 6) Transformer cooling method: Distributed photovoltaic power generation transformer should have natural air cooling (AN) and forced air cooling (AF) cooling method. 7) Insulation level: the insulation level of each winding of the transformer meets the requirements of relevant standards, such as the short-time power frequency withstand voltage of 35kV on the 10kV side and the short-time power frequency withstand voltage of 3kV on the 0.4kV side.
Cable selection and laying
1 Cable Selection
Cable selection to follow the principles are: 1) to meet the loop capacity requirements, reduce cable loss, reduce cable consumption, save investment costs. 2) The rated voltage of the cable should be determined according to the grounding mode of the system. 3) Armored cables should be selected for direct buried cables. According to the local hydrological and meteorological conditions, waterproof, anti-corrosion or cold-resistant cables are used if necessary. 4) The rated voltage of the cable selected for the DC part of the photovoltaic power generation system should be 1000V. 5) The selection of the minimum cross section of the cable conductor should meet the requirements of thermal stability when carrying current and the maximum short-circuit current through the system. 6) The calculation of cable current-carrying capacity should consider different ground temperature, different soil thermal resistance coefficient and the current-carrying correction coefficient when multiple cables are laid in parallel. 7) According to the requirements of GB 50217 and the “Twenty-five Key Requirements for Preventing Major Accidents in Power Production” for cable selection, the cable in the photovoltaic power station should be flame retardant and fireproof.
2 Cable Laying
Cable laying design to follow the principles: 1) according to local conditions, to achieve advanced technology, economic and reasonable, safe application, easy to construction and maintenance. 2) According to the characteristics of distributed photovoltaic power generation projects, cable laying methods can be used cable trench, cable bridge, cable shaft, direct burial, pipe, etc. The actual project design should fully consider the original cable channel and reduce the amount of construction. 3) When the cable is laid in the direct burial mode, the cable burial depth should be determined according to the depth of the local soil and frozen soil and frost heaving, and the cable path should be the shortest and as far as possible the same path to reduce the amount of excavation.
Conclusion
From the current situation, photovoltaic power stations are the most advanced means, with the advantages of energy conservation and emission reduction, can effectively convert solar energy, so as to meet the needs of society for electricity energy. In this process, the distributed photovoltaic power station system has high environmental and economic benefits, and can effectively convert light energy into point energy, so as to ensure People’s Daily life electricity. In the process of system design, technicians should be combined with the radiation range of the power supply system, supply demand, economic investment, infrastructure distribution and other conditions, do a good job in the electrical design of DC distribution devices, AC distribution devices, boost transformers and cables, laying the foundation for the stable, efficient and safe use of photovoltaic power supply systems.
