Problems with bushings are one of the leading causes of transformer failure and that means their proper selection can significantly impact transformer and substation safety. Traditional bushing technologies such as oil-impregnated paper bushings (OIP) and resin-impregnated paper bushings (RIP) are long established but still come with certain disadvantages. In the case of OIP bushings, for example, there are safety, reliability and environmental risks. For RIP bushings, moisture ingress during storage is a threat.
This edited article, based on a paper at the 2015 INMR WORLD CONGRESS by Gobi Kannan & Wong Fook Ming of Tenaga Nasional Berhad (TNB) in Malaysia, Daniel Egger & Thomas Schütte of ABB Switzerland and Jan Czyzewski of ABB Poland, outlined production and testing experience with new resin-impregnated synthetic (RIS) bushings. The first part summarized tests performed to assess long-term behavior under stress; the second reviewed early field experience based on a pilot installation in Malaysia.
With RIS bushings, a synthetic polymer fibre replaces the dense paper utilized in OIP and RIP technologies. This open mesh fabric can be impregnated by alumina or silica-powder filled epoxy resin whose viscosity is much higher than that of traditional impregnation liquids such as oil and pure resin. Powder-filled resins are a proven insulating material with decades of experience across several MV & HV applications and this ensures good electrical and mechanical performance when applied to bushings. Moreover, by using these resins, the condenser core can be moulded directly into its final shape and hardened in a short time. In addition, core drying prior to impregnation is no longer necessary since, unlike paper, polymer fibres absorb virtually no humidity. The silicone elastomer that forms the external part of the insulator can also be directly moulded onto the air side of the condenser body. Finally, the combination of advanced material selection and improved processing capabilities allows RIS bushings uncommonly short production lead times versus other styles of HV bushings.
Oil-to-air transformer RIS bushings with silicone external insulation have now been developed and are commercially available. They fulfil all the specifications required in IEC 60137 (2008) and also the electrical, thermal and mechanical properties of the relevant IEEE standards. These bushings are characterized by a very low dielectric loss factor (tan δ typically below 0.35%), while their electrical design and void-free impregnation process allows partial discharge (PD) free operation up to twice the maximum phase-to-ground operating voltage, specified at a PD measurement background noise of 2 pC.
Such performance, equivalent to stateof- the-art RIP bushings, is significantly better than the minimum specified by the IEC standard (i.e. a loss factor < 0.7% and PD levels of 10 pC and 5 pC at 1.5 and 1.05 times maximum phase-toground operating voltage, respectively). At the same time, the creepage distance of the silicone insulators on RIS bushings allows them to be installed even in environments with pollution severity class ‘e’ (i.e. very heavy) according to IEC 60815-1 (2008). Mounting angle can be from vertical to horizontal (0° to 90°) due to the bushing’s dry structure.
International standards such as IEC require type tests for new product designs and also establish routine test procedures. However, when it comes to new technologies such as RIS, standards cannot provide clear guidance about the testing needed to demonstrate performance. Rather, it is up to manufacturers to establish appropriate test procedures while recognizing that their utility customers may require additional tests in order to approve new equipment installed on power networks.
Boiling Water Test
The silicone of the bushing’s external air side insulation does not itself prevent moisture from creeping through to the condenser body. For this reason, an RIP bushing typically features a moisture barrier added between silicone housing and condenser core. For RIS bushings, however, where the silicone is molded directly onto the core, this issue is not a concern. Mechanical integrity of the silicone housing as well as prevention of moisture ingress is verified by means of water immersion testing (e.g. boiling tests). Such testing must demonstrate that there is no de-lamination of the external silicone insulation from the condenser core under stress. Moreover, the test also has to demonstrate that there is no moisture ingress, even though the complete bushing is being boiled with no special protection. A boiling test was performed on three 24 kV RIS bushings in order to test the interface between condenser core and silicone housing. One bushing was pre-stressed thermomechanically according to IEC 60099-4 (see Fig. 1). The bushing was bent in line with specified long-term load while direction of load was altered every 24 h. Temperature cycles were performed in parallel with the long-term test load such that two cycles from -40°C to +60°C were applied with 48 h duration.
After pre-stressing, all three bushings were kept fully immersed for 42 hours in a vessel of boiling de-ionized water having 1 kg/m3 NaCl. All three bushings again passed the full dielectric test according to IEC 60137 type test specifications, including lightning impulse, capacitance and tan δ measurement. Afterwards, adhesion of the silicone to the condenser core was tested by cutting into the insulation layer and trying to peel it off. This test demonstrated that the cohesive bonding characteristic of the silicone remained unchanged.