What makes an oil suitable for use in ultra-high voltage electrical equipment? Having the superior properties associated with the Super Grade is essential. Jayme Nunes explains why.
What makes an oil suitable for use in ultra-high voltage electrical equipment? Having the superior properties associated with the Super Grade is essential. Jayme Nunes explains why.
“NYTRO 10XN is the oil chosen for major HVDC projects around the world, and NYTRO 4000X is the preferred choice for UHV instrument transformers,” says Jayme Nunes, transformer insulation specialist at Nynas.
Both these Nynas products belong to a select group of oils – they are Super Grade oils. Such oils display the superior properties required of insulating liquids for the ultra-high voltage (UHV) equipment used in modern High Voltage Direct Current (HVDC) transmission systems. HVDC systems are becoming increasingly popular, making it safer, easier and more efficient to transport electricity over long distances.
“However, the equipment is expensive and needs to be reliable and available at all times, and for a long period of time. The oil used for insulation and cooling has to withstand being constantly subjected to a very high level of electrical and thermal stress,” he says.
In the absence of a standardised specification for super grade oils, Jayme Nunes and his Nynas colleagues have reviewed the oil characteristics required for use in UHV equipment, grading their importance. Low viscosity and viscosity index rest firmly at the top of their list.
“Insulating fluids need to have low viscosity at high temperature in order to dissipate as much as possible of the heat generated inside the equipment. The temperature impacts significantly on the transformer lifetime, and the lower the operating temperature is, the longer the lifetime,” explains Nunes.
Electrical properties are other crucial criteria on the UHV requirement list. Here, naphthenic super grade oils perform well, especially in terms of the dielectric loss factor, the tendency for electrostatic charging, and the behaviour in partial discharges.
“Naphthenic oils enable the most efficient transformer designs, making them the best option among the insulating liquids currently available on the market,” states Jayme Nunes.
Superior solution
Naphthenic oils offer several advantages over alternative insulating fluids. One of the most important properties for use in UHV equipment is the heat transfer capacity, as the ageing of the electrical equipment is directly related to the operating temperature of the transformer. Comparative studies conducted by researchers at Nynas show that paraffinic oils and esters do not cool transformers as efficiently as naphthenic oils, and will therefore not offer the same level of thermal protection. Naphthenic oils have a low viscosity index resulting in lower viscosity at higher temperatures, enabling the insulating oil to easily move around in the interior of the transformer, efficiently dissipating the heat.