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).