CFD Analysis of an 800 kV HVDC Transformer Using Rapeseed Oil as the Cooling Fluid

This study aims to analyze rapeseed oil’s effectiveness as a coolant of a high-voltage direct current(HVDC)converter transformer while comparing it with mineral oil.We employed a numerical model,assisted by Computational Fluid Dynamics(CFD)tools,to simulate the convective heat transfer inside a HVDC converter transformer,which is an oil-directed air forced(ODAF)type.The model seeks to obtain the temperature and velocity fields at the critical points of the fluid domain.The simulation domain reproduces a 2D cross-sectional model in the center of the structure.The results show that rapeseed oil requires more time to gain heat from the core and windings due to its lower thermal conductivity.However,the average temperature of rapeseed and mineral oils will not significantly differ once the channel temperature reaches a stable value.It indicates that the rapeseed oil can effectively replace mineral oil under the same heating conditions.Also,a slower flow of the rapeseed oil allows the fluid to smoothly enter the cooling channel.Therefore,the rapeseed oil cools these channels by pushing the fluid in them and absorbs heat from the sources earlier than mineral oil.The improvement in cooling indicates that rapeseed oil is better than mineral oil as a coolant and electrical insulating fluid for power transformers.

State of the Art and Future Trends of Unconventional PD-Measurement at Power Transformers

This paper discusses the current state of the art of diagnostics at power transformers. A special focus is set on the UHF-PD-measurement （ultra-high-frequency partial discharge measurement） because at power transformers, this diagnostic method has become more important in recent years. The current state, basics and principles of operations, proceedings as well as advantages of PD-measurement methods are covered. Furthermore problems and proposed solutions are discussed. Bushings and tap changers are not discussed in detail. In many cases, one single diagnostic method does not have the ability to sufficiently evaluate a power transformer. Therefore, a variety of diagnostic methods came up over times, which are commonly used by now. To expand the evaluation opportunities of power transformers, science strives to develop new diagnostic methods as well as to improve the existing ones. Furthermore, environmentally friendly and hardly inflammable ester liquids are examined for the use at power transformers and PD-measurement at HVDC （high voltage direct current） converter transformers as well. Potential diagnostic options and respectively current developments and findings in the field of oil-paper-insulation systems are outlined conclusively.