Design of 66 kV dry-type step-up transformers for large capacity wind turbines

Abstract

<p indent="0mm">The development of large-scale offshore wind power has become an inevitable trend in the current renewable energy development. Offshore wind power has moved from offshore to deep-sea, and the capacity of wind turbines has reached more than <sc>8 MW.</sc> The step-up transformer increases the output voltage of the wind turbine from <sc>0.69–1.4</sc> to <sc>35 kV</sc> and above, which is the key equipment of the wind power system. The deep-sea wind power is more than <sc>50 km</sc> away, and the voltage of the step-up transformer needs to be increased to <sc>66 kV</sc> and above. At present, the <sc>66 kV</sc> step-up transformer for offshore wind power generally adopts oil-immersed transformer technology, and <sc>66 kV</sc> step-up transformer with dry-type insulation is a blank product. Oil-immersed step-up transformers have the risk of oil leakage, which may cause fire or explosion accidents, while dry-type insulation step-up transformers have low fire risk, good explosion-proof performance, and new high reliability, and have significant technical advantages in offshore wind power applications. However, limited by the design method and manufacturing process technology, the traditional epoxy casting step-up transformer cannot solve the problem of excessive partial discharge under high voltage, which is caused by high-voltage winding interface defects, restricting the application of this technology in66 kV step-up transformers. This paper breaks through the technical limitations of traditional epoxy cast transformers, innovates from the insulation design mechanism, and proposes a new design method for a <sc>66 kV</sc>/10 MVA step-up transformer based on silicone rubber insulation. By analyzing the generation mechanism of the interface defect of the epoxy casting transformer’s high-voltage winding, a design method of high-voltage insulated shielding wires incorporating semiconducting shielding layers is proposed. The outer conductor of the wire is first wrapped with the semiconducting shielding layer, and then the insulating layer is wrapped. The semiconducting shielding layer and the insulating layer use same base materials. The semiconducting shielding layer can shield the conductor interface defects, the material of the semiconducting shielding layer is designed with the same base material of the insulating layer, so the two layers can be fully compatible, which can effectively suppress the interface defects. According to a certain winding form, the high-voltage insulated shielding wire is wound into a coil structure with a certain number of turns, then integrally cast silicone rubber insulation, forming the overall structure of the high-voltage winding. The high-voltage winding is designed with sheds structure, which increases the creepage distance along the surface, and realizes the high creepage insulation design with a compact height space. The <sc>66 kV</sc>/10 MVA step-up transformer structure is designed, taking into account factors such as insulation, electric field, magnetic field, heat dissipation, and structural strength, etc. The electric field, magnetic field and thermal field of the 66 kV/10 MVA step-up transformer under steady-state operating conditions were analyzed, the electric field distribution characteristics and maximum field strength parameters of the high-voltage winding were obtained, and the magnetic field distribution and thermal field distribution characteristics of the transformer were analyzed. The rationality of the step-up transformer design is verified by multiphysics analysis. The principle prototype of 66 kV transformers was developed, and AC withstand voltage and partial discharge tests were carried out. Under the test parameters of 55 kV/<sc>30 min,</sc> the partial discharge amount was less than 5 pC. The terminal-to-ground 120 kV/<sc>1 h</sc> withstand voltage test was carried out on the high-voltage winding, during the test, partial discharge amount is less than 5 pC, the performance is good, which verifies the feasibility of the design method.