Time to breakdown of high voltage winding insulations with respect to microscopic properties and manufaturing qualities

High voltage rotating machines remain to play a significant role in generating electrical energy as the demand for new or refurbishment of power stations continues to increase. However, one of the main causes for down times in high voltage rotating machines is related to problems with the winding insulation. The deregulated and liberalised energy market leads to cost pressure on the producers of the insulation materials and on the utilities. Manufacturers are requesting winding insulations that can withstand a higher electric field than the 2 – 3 kV/mm common today while keeping the costs low. The utilities want to reduce costs through longer maintenance intervals and a higher lifetime of the machines. These demands create a challenge for the producers of winding insulations, the manufacturers of high voltage rotating machines and the utilities and were therefore a reason to carry out this project. Although, electrical degradation and breakdown in polymers has been studied extensively, the processes that lead to failure in high voltage winding insulations have still not been conclusively described. In particular, the influences of the numerous combinations of different raw materials, mica tapes, manufacturing qualities and service loads on the material properties and on time to breakdown are not fully understood. Since the material effort for testing insulations with the standard test method (IEEE 1043 Std) is very high, only few results of tests of winding insulations are available, each having a low number of samples and therefore a high statistical uncertainty. Therefore, the main factors that influence time to breakdown of high voltage winding insulations were analysed in this work. It was concentrated on describing the influences of raw materials, mica tapes, manufacturing qualities and selected service loads on the microscopic properties and on time to breakdown of the insulations. The analysis of microscopic properties of mica tapes in winding insulations show that the tapes are circularly bent at the edges of the conductor, at voids or within the insulation. In addition, the mica tapes are bent at regions of tape overlappings. Investigations in model needle-plane arrangements show that the electrical tree cannot penetrate a mica tape if it is undistorted. In contrast, the tree can penetrate circularly bent tapes, whereby the Ruben Vogelsang, Dissertation ETH No.15656 ISBN: 3-89649-965-3 iv probability for penetrating the tapes decreases with increasing radius. For tape overlappings, the probability of the tree penetrating the tapes decreases with increasing apex angle. Time to breakdown can significantly be reduced when the tree penetrates the tape. The cause for the tree penetrating the bent tape is a higher degree of separation of the mica flakes at stronger bending. Thereby, the tree propagates more easily through the separated flakes, which leads to a shorter breakdown path and to a significantly reduced time to breakdown. Tapes with glass fabric as a support material are more flexible, which resulted in less tree penetrations than tapes with PETP-film. For manufacturers of winding insulations, it can be concluded that tape bends should be minimised (e.g. by applying tapes with a higher width). If bends cannot be avoided, such as at the conductor edge, their radius should be as high as possible. As the tree could penetrate all tapes at radii of ≤ 1mm, the value for tape bends in the insulations must have a minimum of > 1 mm. For winding insulations in which more tape bends are expected (e.g. poor manufacturing qualities), tapes with glass fabric as support material are recommended for application. Electrical tree propagation is the main process that leads to breakdown in high voltage winding insulations. Due to a significantly enhanced electric field in voids at the inner conductor or at the outer coating, the time interval for tree inception in the bars is negligibly low. After inception, the tree propagates through the insulation along the mica tapes at the interfaces mica-support material-binder resin. Tree propagation is reduced in insulations with a higher number of tape layers and higher mica content. In order to increase time to breakdown (or lifetime) of winding insulations, it is concluded that tree inception should be delayed and tree propagation must be slowed down. Tree inception can be delayed by preventing voids at the conductor (e.g. by using a tape with higher resin content in the first layer) or by applying a conductive layer at the inner conductor. To slow down tree propagation, the mica content and the number of tape layers of the main wall insulation must be as high as possible. Therefore, more layers of thinner mica tapes are recommended for use. In order to measure the tree propagation through the material, a test was developed that requires much less material than the conventional test according to IEEE 1043 Std. Using that treeing test with an embedded electrode, significant differences in the insulation properties can be identified. A further advantage is that the electrode can be implanted in a determined region of the insulation, which allows testing insulation properties at certain sections of the bar. As the test can also be applied at bars that were already produced, it is very easily applicable and can be recommended for tests where only a low number of samples are accessible, which might be due to time, cost or material reasons. At manufacturing, impregnation and the type of taping play an important role towards the insulation quality. A lower impregnation quality causes voids and delaminations in the material, which results in significantly reduced time to breakdown values. The degree of reduction depends on the material, whereby mica tapes with glass fabric are less sensitive for impregnation failure, than tapes with PETP-film as a support material. The influence of taping on time to breakdown was investigated on industrially-manufactured insulations directly from the production line of different companies. Insulations that were taped by machine have a structure without any voids or delaminations and therefore no significant differences in the time to breakdown values, compared to the reference material. In contrast, insulations that were taped by hand tend to have a lower insulation quality, which may lead to a dramatic decrease in time to breakdown compared to the reference material. The 63%-quantile of time to breakdown of a hand-taped VPI-insulation was reduced by more than two orders of magnitude and that of a resin rich insulation by more than four orders of magnitude! Both insulation systems showed an internal quality with many voids and delaminations between the mica tapes. Electrical trees could therefore propagate very Ruben Vogelsang, Dissertation ETH No.15656 ISBN: 3-89649-965-3 v quickly along the tapes, thus leading to the early breakdown. Considering the materials, resin rich insulations are more sensitive to manufacturing failures than VPI-insulations as their time to breakdown values were vastly reduced at both, poor impregnation and poor taping quality. In conclusion, machine taping is strongly recommended for the wrapping of mica tapes. If hand taping shall be used, VPI-tapes with glass fabric as support material are less sensitive for poor taping quality and should therefore be applied. In order to ensure good quality during production, electrical treeing tests with the embedded electrode are recommended for checking the insulation quality in the production line at regular intervals. Although time to breakdown values of insulations with poor quality is significantly lower than the values for the reference material, most of the bars would have passed the standard “One minute proof test” according to IEC 60243-1 Std and IEC 1212.2 Std. At three times the rated voltage, breakdown in poor insulations occurred within a few minutes, whereas in good insulations it occurred in some tens of hours up to about 5,000 hours. It is therefore recommended to extend the time interval of the “One minute proof test” to > 10 min. Insulations with a poor quality would thereby be tested out and insulations with a good quality would experience no significant degradation. At elevated temperatures, the VPI-insulation showed, compared to 20 °C, an increase in time to breakdown at 160 °C and a decrease at 180 °C. In contrast, the resin rich insulation showed, compared to 20 °C, a decrease of time to breakdown at both, 160 and 180 °C. The different breakdown behaviour of the materials can be related to the relief of mechanical strain of the bars, the different materials of the mica tapes and the expansion of the material. At temperatures of 180 °C, PETP-film as support material is critical since it degrades and leads to faster tree propagation, which finally causes a significant decrease of time to breakdown. For the operation of the insulations it is concluded that elevated temperatures are only critical if they are close or above the glass transition temperature of the material. Operating at that temperature level, or hot spots in the insulation, should therefore be avoided or minimised. Since there is an increase of time to breakdown, the operation at slightly elevated temperatures should be considered. At mechanical vibrations between ± 0.15 and ± 0.80 mm, time to breakdown is mainly dependent on the vibration frequency and the material used rather than on the total number of vibrations. For both materials, the highest reduction of time to breakdown was measured at the highest vibration frequency of the bars of 50 Hz. The reason for the decrease is that the epoxy resin in the insulation becomes porous, which gives way to fast tree propagation and finally results in significantly reduced time to breakdown values. For both, VPI and resin rich insulations, it can be concluded that mechanical vibrations are very critical and must therefore be absolutely avoided in the mac