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 edited past article contributed to INMR by Bernhard Heil of HSP Hochspannungsgeräte in Germany, reviewed relevant parameters for such evaluations.
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.
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.
Relevant Parameters for Evaluating OIP & RIP Bushings
It is well known that power factor and DGA are both adequate parameters to evaluate operational reliability of an OIP bushing. As discussed earlier, however, the relevant parameters for an RIP bushing are different, making it important to understand these and their possible influence.
A partial breakdown results in an increase in main capacitance C1, with the extent of increase dependent on number of short-circuited compared to total number of grading layers. The higher the bushing’s voltage level, the lower will be the increase in C1 in case of a single partial breakdown. Therefore, parameters influencing capacitance C1 must be taken into account during any evaluation, especially at higher voltages.
Even if the relative importance of any increase in C1 is different when evaluating OIP versus RIP bushings, the influencing parameters are the same for both technologies. The value of C1 is influenced by stray capacitance and temperature. Since stray capacitance is a geometrical parameter, keeping the set-up constant, will keep it constant as well. Therefore, a ‘fingerprint’ measurement after installing a bushing in its final position is the best reference for subsequent measurements. The temperature dependency of C1 for OIP and RIP bushings is 0.025 %/K and 0.04 %/K, respectively and this is usually only relevant in the case of very large temperature differences. For off-line measurements, the resulting deviation is mostly minor and can be neglected. Fig. 7 shows measurements for an aged OIP bushing heated in an oven to different temperatures. A temperature dependency of 0.025%/K results in an increase of C1 of 1.75% when heating the bushing from 20°C to 90°C, i.e. from 406 pF to 413 pF. Temperature dependency for the RIP bushing is somewhat higher (0.04%/K), however can also be mostly neglected in off-line measurements.
Stray capacitance on test taps has stronger influence on C2 capacitance than on C1 capacitance. This is because C2 is composed of the condenser core as well as the ‘surroundings of the bushing’, determined by the geometry of the transformer turret and other components. As a result, higher C2 deviations can be expected, i.e. up to ±50% from the routine test value. In case of a potential tap for IEEE bushings, influence of stray capacitance is significantly reduced since capacitance C2 of the potential tap is dominated by the last two grading layers. Fig. 8 illustrates the influence of stray capacitance on C2 in the case of a potential tap and a test tap, respectively.
Power factor is an ageing parameter for OIP bushings and is also influenced by temperature. In the case of RIP bushings, an increase in power factor is an indication of humidity ingress but the value is temperature dependent as well. Table 1 summarizes the above relevant evaluation parameters and the factors that influence these values.