The MV and HV Primary Equipment in PV Utility-scale Plants(Part I)
Due to the floor area of large-scale ground-mounted power station, the suitable place for new construction is usually the wilderness or even desert far from the city. Therefore, the generated power needs to be boosted before it can be delivered to users through long-distance transportation. With wide application of large-scale photovoltaic ground-mounted power station, the transmission process from inverter to 110kV or 220kV cannot be separated from the support of a series of medium and high voltage primary equipment such as boost, protection, measurement and monitoring. At the same time, with the gradual increase of PV module power, subarray and oversizing ratio. The diversified demand and stability demand for medium and high voltage appliances are also gradually increasing. The medium and high voltage components and equipment involved will be introduced in detail based on the common design Solution of 3.15MW subarray as following,
1. 35kV pad-mounted substation transformer
Pad-mounted substation is usually divided into European type and American type. Generally, when there are many personnel and considering the convenience of aesthetics and safety, European type substation is adopted but the cost is high. When the environment is remote, American type can be selected.
American type substation transformer has compact structure and low cost. In terms of structure and components, American transformer usually adopts plug-in fuse, backup fuse + load switch, and most transformers are oil transformer. The European type substation transformer usually adopts SF6 load switch + combination fuse, and supports ring network power supply and terminal type. Most transformers are dry transformers. The main difference between dry transformer and oil transformer is the insulating medium. Epoxy resin is usually used as the insulating medium for dry transformer, and transformer oil is used as the insulating medium for oil transformer. According to the characteristics of photovoltaic projects, American type substation is commonly used at present.
Figure 1 American Type Substation Transformer
Figure 2 European Type Substation Transformer
The component inside the substation transformer,
Fuse: Xrnt fuse is usually used. In case of overcurrent or short circuit, the fuse will fuse, then the internal spring will jump, and the striker will be interlocked through relevant machinery to trip the load switch. Of course, we may see the protection mode of circuit breaker on other occasions. Compared with the circuit breaker, the characteristic of fuse protection is fast speed and can be removed in a short time in case of fault to protect the transformer. Generally, the short circuit can be disconnected within 10ms, while the medium voltage load switch usually needs about 100ms opening time, and the circuit breaker is about 40 ~ 80ms. Therefore, fuses are more suitable for performing short circuit protection.
Load switch: according to the difference of insulation medium, it is usually divided into air type and SF6 insulation type. No matter which case, the operation of the load switch can only disconnect the current within the rated current range. For example, for 630A load switch, the breaking current shall not be greater than 630A under rated voltage. It usually needs to be combined with fuse to achieve comprehensive protection.
Fully insulated ring network cabinet and C-GIS: in the traditional distribution network, in order to provide distribution equipment with power supply reliability, the power supply network is connected into a ring to form a ring network power supply. The utility model has the advantages of simple structure, small volume and low price. With the structural advantage of full sealing of live parts in SF6 gas, it has gradually been widely used in harsh environment and export container transformer protection circuit. The adoption of the cabinet can make the scheme more flexible.
The structure of the fully insulated ring network cabinet is compact, and there is a solution to match the fuse combination (such as unit f). It can be used as protection cabinet at high voltage side of transformer in photovoltaic power station. The action principle is still that in case of overcurrent, after the fuse is fused, the striker pushes the load switch mechanism to open, and makes the line grounding switch grounded to ensure the safety of the line.
Please click here to read Part II of this article.