Safety is an important factor in the power industry due to the consequences and risks of injury from catastrophic failures of electrical equipment. This aspect must therefore always be taken into account during design – even at the inception of product development. Moreover, safety only becomes more important when talking about HV and UHV facilities and components. Bushings are no exception to this trend and new requirements as well as special tests have recently been introduced by power utilities to verify their safe performance even under extreme conditions. While all bushing technologies these days are characterized by an exceptionally high level of operating reliability deriving from long service and manufacturing experience, technical progress is nevertheless allowing new steps to further improve safety. This edited INMR article by G. Testin, P. Cardano, M. Sehovac and M. Boutlendj of GE Grid and INMR Columnist, Alberto Pigini, dealt with safety aspects of all bushings types and technologies. It discussed solutions adopted to eliminate or at the least significantly reduce the impact of any failure either in the field or during factory tests.
Specific Safety Aspects
Bushing safety covers a wide range of potential problems related to factors such as the main insulation technology used, design and manufacturing process (e.g. choice of materials, quality control and correct assembly in the factory or on site). In general, the main insulation system of HV bushings can be classified into three basic technologies or some combination thereof:
• Oil impregnated paper (OIP) bushings;
• Resin impregnated paper (RIP) bushings;
• Gas (SF6 or SF6/N2) bushing technologies.
Oil impregnated paper (OIP) bushing technology has the advantage of long service experience. Therefore, manufacturers and users both know the parameters that contribute to safety. In fact, this technology has demonstrated high reliability and safety and is still the most used on HV networks. Of course, the main disadvantage of a liquid insulation system from the safety and environmental point of view is leakage and flammability.
An internal fault followed by high fault-current and a high temperature electric arc will create enormous internal pressure that shatters all enclosures. The resulting fire can then spread to the power transformer and surrounding equipment. Although there are many possible causes for such faults, the most frequent are sealing problems (e.g. defects in gaskets/grooves, porous castings), partial or total crack of one or more components due to high thermal, mechanical or electrical stresses or inadequate design with respect to actual service conditions (e.g. pollution, major temperature changes, over-voltage stresses, seismic events, etc.).
There are two possible scenarios in the event of an internal fault on a bushing. One is that the fault occurs in the air-side of the unit and the consequence here is breakage of its porcelain housing and ejection of high velocity fragments that are dangerous to surrounding equipment and personnel. An even more dangerous scenario is if the internal fault happens in the transformer-immersed part of the bushing. The arc-fault generated in this case will be in direct contact with transformer oil and there is high probability of fire with catastrophic consequences. Should the oil ignite, in a few minutes the entire transformer will be affected with limited chance to extinguish the fire before its destruction. Even if the oil does not ignite, it still represents a major environmental problem due to the volume of mineral oil dispersed from the transformer.
Resin impregnated paper (RIP) insulation technology for bushings was developed later than OIP technology and helps resolve some of these safety risks. The bushing is dry, with no internal oil or other liquid, and therefore there is no longer risk of leakage or danger of ignition. This benefit is a key factor for bushings used in DC (indoor) applications. Yet there still exists a danger of explosion if an internal fault occurs in the air-side of the bushing should the dry type insulation be housed in porcelain. The internal pressure caused by the fault will shatter the porcelain into shards that act as projectiles towards surrounding equipment. On the other side, the transformer itself is not protected against an internal fault that occurs at the lower part of the bushing. The high temperature arc-fault can ignite the transformer oil with similar catastrophic consequences to what occurs in an OIP bushing application.
Gas (SF6) insulation system is used in bushings installed in GIS, GIL, dead-tank breakers and, most recently, in wall bushings (AC and/or DC) up to the highest voltages. The main advantage of this technology is its non-flammability and the possibility it offers for explosion-proof apparatus. The disadvantage is that apparatus is continuously under pressure during service conditions with possible leakage whose rate has to be limited to a maximum of 0.5% per year due to the powerful greenhouse effect of this gas. Still, this risk can be very well controlled by adequate design solutions, advanced sealing systems and proper management of gas operation over the bushing’s service life. The main consideration, here, from a safety point of view is if an internal fault occurs inside a porcelain-housed bushing, where the housing under high pressure could be shattered by an arc-fault due to thermal shock. The result of this kind of failure is shattering of the porcelain with serious damage to surrounding equipment. In fact, this type of failure could be even more severe than for OIP or RIP technologies due to the fairly high normal pressure of the gas. Safety precautions also have to be taken during factory tests due to the possibility that porcelain housings can rupture during voltage tests, endangering both personnel and expensive test equipment.
Design Aspects Regarding Safety from Explosion
The first countermeasure to resolve problems related to possible rupture/explosion of porcelain is to use composite insulators that do not explode but simply break down without sharp pieces ejected. For this reason, use of these insulators is becoming more popular in all bushing types, especially those involving gas insulation where use of porcelain is now much less than in the past. In fact, about 90% of all production of gas-insulated bushings these days is with composite insulators and only about 10% with porcelain. Bushings with RIP insulation are now also manufactured mainly with composite insulators since their development is fairly recent and at a time when awareness of safety aspects was already growing. Use of composite insulators in RIP bushings significantly improves safety and this is of prime interest for indoor applications (e.g. in DC valve halls).
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.
Lowering the natural frequency of higher voltage class bushings using rubber dampers on flanges has been done in the past. For example, this design was used mainly for Italian transmission utility, Enel, to satisfy very high transformer short circuit mechanical stresses. However, this solution has since been abandoned due to high production costs. Using extra high strength porcelains, supplied by the most competent insulator manufacturers, some bushings can be made to satisfy almost all requests in this field. By contrast, for extremely severe seismic conditions or on special request, OIP bushings can be equipped with composite insulators that, due to lower weight, higher flexibility and excellent mechanical characteristics, offer advantages compared to porcelain-housed bushings. Indeed, this design approach is prominent for GIS bushings. Seismic tests performed on 550 kV rated gas bushings at the highest seismic level in the standards demonstrated excellent performance, without any internal or external damage that could compromise service reliability.
Design Aspects for HVDC Bushings
Fire in the valve hall is the most critical event that could cause a long forced outage in a UHV DC facility. To minimize risk of such a serious event, plant designers increase project safety margins by specifying bushings based on solutions that minimize risk of transformer oil spilling into the valve hall in case of failure. This also serves to minimize risk of oil leakage from wall bushings and other equipment. In some cases, technical specifications require a bushing arrangement that allows installation of the transformer totally outside the valve hall. To comply with this requirement, transformer bushings are equipped with a long metallic extension on flanges in order to pass through the wall of the valve hall. This leads to very long bushings for UHV DC applications.
The main solutions provided for DC applications can be based on one of the following: purely solid insulation (RIP); hybrid types that are either a solid insulation combined with gas (RIP+SF6) or an oil-impregnated paper insulation combined with gas (OIP+SF6); or a purely a gas (SF6) solution. One technical solution for HVDC transformer bushings is an innovative hybrid OIP+SF6 system based on a fully-sealed OIP condenser bushing that interfaces with the transformer and a gas-filled hollow composite housing for the air side, i.e. the portion that protrudes into the valve hall (see Fig. 13)
This solution is particularly optimized from the safety point of view and its advantages include:
• the oil-impregnated paper grading capacitor is fully sealed and it ensures a complete separation between the gas compartment and the transformer turret;
• gas leakage inside the transformer is not possible since this is blocked at the level of the grading capacitor and detected by suitable oil pressure gauges;
• there is no risk of transformer oil spills from the turret to the valve hall in case of bushing failure due to the triple barrier system ensured by the composite insulator and the two sealing barriers of the grading capacitor;
• in case of damage to the barrier on the SF6 side, the risk of oil spill in the valve hall is avoided by the presence of the composite insulator and the pressure gauges;
• total free oil quantity inside the bushing is extremely low, minimizing risk of fire;
• the oil inside the grading capacitor permits better cooling of the bushing – especially useful for high current ratings up to 6300 A and for the presence of harmonics;
• the grading capacitor winding is 100% protected from risk of particle contamination during installation of the bushing onto the transformer since it is totally sealed.
A particular HVDC application that requires special attention from the safety point of view is the wall bushing. The schematic in Fig. 15 shows the different solutions applicable to wall bushings. The traditional OIP solution is no longer requested due to possible oil spills and fire risk in the valve hall. Other possibilities are RIP-SF6 technology and pure SF6 gas solutions.
The solution selected by GE Grid is the pure SF6 gas technology, which is felt to offer the following advantages:
• simple design based on a high technology solution;
• lower weight compared with equivalent solution RIP-SF6;
• less different materials, therefore lower electrical risks under DC and polarity reversal stresses;
• no ageing of the internal insulation system due to limited quantity of organic materials;
• fast production and delivery time thanks to simplicity of the solution;
• limited new investment for production;
• limited maintenance and on-line monitoring of gas status;
• excellent characteristics from the safety and pollution points of view, thanks to adoption of composite insulator housings.