By contrast, OIP bushings are still manufactured mainly with porcelain insulators partly because of lower cost but also for historic reasons since these were developed many years ago at a time of absolute domination by porcelain. Composite insulator technology was developed much later and initially not fully proven or accepted by power utilities. Even nowadays, ‘traditionalism’ remains strong. At the same time, it should be pointed out that there are some service environments where porcelain is preferable to polymeric insulators that are organic and therefore more vulnerable to premature ageing.
For example, some TSOs such as Terna in Italy have now started to use OIP bushing equipped with composite insulators. This solution is a good compromise from the safety point of view by allowing continued use of OIP insulation technology, with its well-known long-term behavior, in combination with the improved safety performance typical of composite insulators.
Safety of some porcelain-housed gas bushings has been significantly improved with introduction of a thin fiberglass tube that protects the porcelain along its entire length from excess overpressure and breakage due to thermal shocks in the event of an internal fault. This design concept also helps reduce risk of breakage or explosion of porcelain during factory dielectric tests, thereby preventing flashover that could start from the internal electrode(s) and puncture the porcelain under the sudden gas pressure.
A gas type bushing equipped with a composite insulator housing can be considered totally ‘explosion-proof’. The energy developed during an internal fault is released through pressure rise of the compressible gaseous insulation, thus activating operation of the pressure relief device installed on the bushing or GIS. In those cases where energy released during a fault is low and volume of gas insulation is relatively large, installation of a pressure relief device is not even necessary due to the relatively low pressure increase (e.g. EHV/UHV gas-insulated wall bushings).
Design Aspects Regarding Internal Arc
Another recent development has been the possibility to have bushings able to withstand the effects of an internal arc following a failure with the rated short circuit current of the network. This request is already typical for components such as current transformers where the explosive effects of an internal arc are disastrous and for which this test has been performed for years but is comparatively new for bushings. Now, it is increasingly being introduced in specifications on an experimental and eventually mandatory basis to raise safety of all electrical apparatus against violent explosion by using composite insulators and by performing the severe induced internal arc-fault test. The transformer bushing must withstand a short circuit current (e.g. 63 kA for 420 kV class bushings and 50 kA for 245 kV class bushings) for 0.5 s. The energy developed during this test is very high and the bushing design has been thoroughly reviewed to successfully meet all required customer criteria.
During the test, a bushing is mounted on a turret reproducing its service condition on a transformer. Externally, all connections are arranged in accordance with usual service geometries, i.e. same flexible conductors, same support insulator positioned at same distance as in the substation. This is intended to fully reproduce mechanical stresses that occur during a short circuit in service. The arc is generated inside the bushing by a special spark gap equipped with an arc igniting copper wire of 2.5 mm diameter (see Fig. 7). The bushing is installed on a turret, simulating the transformer tank, which is placed inside a protected area or bunker, specific for this kind of tests (Fig. 8).
Such a test was performed on an 420 kV rated OIP bushing subjected to a fault current of 63.5 kArms for 0.5 s with peak value of 148 kApk that internally generated an electric arc. The bushing passed the test without external damage, matching the safety requirement of the new technical specification that states no ejected parts are permitted outside a circular area of 3 m radius around the bushing.
Design Aspects Regarding Seismic Withstand
Seismic stresses have a specific spectrum of frequencies varying roughly from 1 to 30 Hz. Maximum peak frequency is from 2 to 10 Hz while stress is lower for other frequencies. The main problem for bushings is that their resonance frequency occurs within this same spectrum – at the lower frequency range for EHV bushings and the higher range in the case of HV bushings. During an earthquake, bushings get into resonance frequencies that can generate mechanical stresses that cause insulator breakdown. Generally, there are two possible ways to resolve this problem: one is to reinforce the bushing structure, mainly the insulator, its weakest point; the second is try to change the bushing natural resonance frequency, thereby escaping from the most dangerous frequency resonance range, as above (see Fig. 10).
Reinforcement of the bushing implies redesign, which is not always the preferred way due to economic considerations. For bushings of lower voltage class, however, this approach could be acceptable since they already have high natural frequencies. By slightly increasing thickness of components (without too high an impact on cost), the increased rigidity can move the bushing’s natural frequency out of the seismic spectrum.