Resin-impregnated paper (RIP) bushings are now an established solution for many applications. Although such bushings are often referred to as ‘maintenance-free’, some of their parameters must nevertheless be monitored prior to putting them into service and also on a regular basis thereafter. Moreover, evaluating their reliability requires understanding certain physical parameters given that unlike in oil-impregnated paper (OIP) bushings the condenser core is solid.
This article, contributed by Bernhard Heil of HSP Hochspannungsgeräte in Germany and edited by INMR, discusses relevant parameters for such evaluations. Examples from the field are also given to demonstrate how evaluating reliability can be carried out in the case of both bushing technologies.
Resin-impregnated paper (RIP) technology can now be applied for virtually all applications of bushings in the power industry. However, since the condenser core is solid and does not contain the oil found in OIP bushings, there are major differences when it comes to evaluating operational reliability.
Dissolved gas analysis (DGA), for example, is a well-known methodology to evaluate the condition of OIP bushings. Partial breakdown between grading layers, however, is more difficult to use as a diagnostic for these bushings. This is because the failure mechanism might take place between two successive off-line measurements. Therefore, it is possible that nothing unusual is detected prior to sudden and rapid complete breakdown.
By contrast, in the case of RIP bushings there is no interaction between localized partial breakdown and the rest of the condenser core. Due to the material being solid, any partial breakdown is limited to its particular location. As such, the residual life of an RIP bushing after a first partial breakdown is usually much longer than for an OIP bushing. This means that detection of partial breakdown during offline measurement might be an adequate parameter to evaluate operational reliability.
Aside from capacitance, the power factor of a bushing is also an adequate parameter for diagnostic evaluation. But due to the different design of their condenser cores, the parameters influencing power factor are different for OIP and RIP bushings. For each evaluation, the influence of environmental conditions (e.g. stray capacitances, temperature) must be taken into account. Some parameters are influenced in the same way for OIP and RIP bushings while others are influenced differently. Understanding these influencing parameters is therefore important for each evaluation.
Basic Design of Bushings
Most typically, OIP bushings are equipped with porcelain insulators while RIP bushings have a composite insulator housing. The condenser core is manufactured using paper that is wound continuously while inserting aluminum foil in precise locations to build up capacitive grading of the electric field. After winding, the core is impregnated either with epoxy resin (RIP) or oil (OIP) under vacuum and heat.
Typically the gap between the condenser core and outer composite insulator in an RIP bushing is filled with foam. Due to the hydrophobicity of silicone housing, cleaning prior to electrical measurement is usually not necessary.
Under normal operating conditions, ageing of bushing is caused by electrical stress relative to its nominal voltage compared to any overvoltages, e.g. from lightning strike or switching operations. Accelerated ageing can then be due to unacceptably high overvoltages as well as by thermal stress and humidity ingress.
OIP insulation systems are particularly sensitive to overheating. For example, an increase of only 6K above maximum operating temperature will cause normal life expectancy to be cut in half. Moreover, humidity ingress will also have significant negative influence on service life. In fact, in the event of already greatly aged insulation, the whole system might fail subsequent to any partial breakdown. This could result in catastrophic failure should the bushing catch fire and involve the transformer as well.