In liquid-immersed power and distributions transformers, the oil serves as both a dielectric insulant and a cooling medium. Owing to possible oil spillages and transformer fires, paramount to the selection of the type of oil to be used are the fire safety and the impact of oil on the surrounding environment.
Mineral oil (MO) has been the most widely used transformer oil as it is a great insulant, has good thermal conductivity and is relatively cheap. However, transformers for indoor applications are subject to more stringent fire safety requirements, which makes the use of MO a more costly option due to its lower fire point of around 180°C. Consequently, sophisticated fire suppression systems and larger fire safety clearances are required for such applications.
Unlike MO, natural ester oil (NEO) has a higher fire point of 300°C and it is self-quenching, making the need for fire suppression systems, additional real estate for larger fire safety clearances and large oil holding dams, unnecessary.
From a design and operational point-of-view, NEO’s higher kinematic viscosity is usually of great concern. For the same transformer operating temperatures, larger oil ducts are required in a NEO-filled unit to meet the cooling requirements.
Thin films of NEO left on surfaces such as pipes and core laminations can polymerise when exposed to oxygen for prolonged periods. This can be prevented by limiting surfaces that have been in contact with NEO from oxygen-exposure and by rinsing pipes with MO after use.
Overall, the fire safety and the biodegradability of the NEO make it the obvious choice for usage in transformers and other electrical equipment for indoor applications; and this is why Eskom is adopting it for its Class 0 and 1 transformers.
This article celebrates the success story of two NEO filled prototype transformers: one a 20 MVA 88/6,6 kV YNd1 power transformer for the 88 kV PPC Slurry indoor-substation and a 2,5 MVA 22/2,2 kV YNd1 medium distribution transformer (MD) transformer; both designed and manufactured locally by Powertech Transformers (PTT).
The transformers were manufactured for Eskom as part of a pilot project on the use of mineral oil alternatives. Realising these greener, safer transformers was a collaborative effort that included Cargill and M&I Materials, both NEO suppliers and the local NEO distributor Wilec. Both transformers were manufactured, tested and commissioned in late 2017. The 20 MVA transformer has since been energised.
Whilst the focus of the pilot project was fire safety and the environment, the overall project cost savings realised are worth celebrating. Though the capital cost of a NEO-filled transformer is higher than that of the equivalent MO-filled transformer, primarily because of the higher cost per litre of NEO; the overall cost of the project and the operational costs of the transformer are lower due to the simpler design of the transformer housing, fire safety reduced clearances and lower transformer maintenance costs.
In the case of the 20 MVA power transformer, the additional transformer cost was R406 080 due to the cost of the NEO alone. However, the overall project cost savings realised was R4 893 920.
Contact Abel Gilbert, Powertech Transformers, Tel 012 318-9833, email@example.com
Source: EE plublishers