Fire and Safety Aspects of High Voltage Bushings Lars Jonsson ABB, Sweden
1 INTRODUCTION The High voltage condenser bushing is a critical component found in all electrical networks and whose failure can have serious economic consequences. Although many in the electric utility industry still regard it as nothing more than a hollow piece of porcelain with a conductor running through it, despite its simple appearance, the task it performs is quite extraordinary and unfortunately often overlooked in specifications and handling. Thermal and electrical stress, ambient conditions and more are putting high demand on bushings. The worst-case scenario can result in sudden failures causing serious complications within the network. Because of the high electrical stress levels in bushings, failure mechanisms tend to result in sudden and catastrophic failure of an explosive nature. This article is aimed at reviewing certain key aspects related to fire and safety in the two main types of high voltage condenser bushings. 2 HIGH VOLTAGE CONDENSER BUSHINGS Condenser bushings facilitate electric stress control through the insertion of floating equalizer screens made of aluminium. The condenser core in which the screens are located decreases the field gradient and distributes the field along the length of the insulator. The screens are located coaxially resulting in an optimal balance between external flashover and internal puncture strength, figure 1.
Figure 1. Schematic view of a condenser core with coaxially located aluminium foils in a web of paper.
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Condenser cores are generally impregnated with transformer grade mineral oil and placed inside an insulating envelope of oil and porcelain, which prevents the bushing oil from mixing with the transformer oil. The system is called Oil Impregnated Paper (OIP) and counts for more than 80 % of all installed bushings. Many utilities have now accepted Resin Impregnated Paper (RIP) technology as being a valuable contribution in reaching better overall performance figures. The somewhat differently designed condenser core is heated, dried and vacuum impregnated by a curable epoxy resin to form a solid unit, free from oil. The outer insulation can be of two types, ceramic or polymeric; Ceramic insulators have a long history and will be used for many years to come, it is however likely to that their role will diminish in the foreseeable future as the industry seeks improved insulator performance in order to reduce overall costs, improve safety, seismic withstand and pollution performance along with lower insulator weights. 3 BUSHING FAILURE MODES IN THE FIELD There are a number of root causes of bushing failure, both internal and external. Some of the more common mechanisms are; loss of earth connection, electrical and thermal stress that exceed the design limits, mechanical stress, ingression of moisture, contamination and ageing. Factors such as over-voltages of various types, overloads, vandalism, excessive line pull, seismic events, handling errors and improperly selected ratings are often involved. The end result is not untypically followed by a catastrophic event such as violent explosion of the bushing, propelling shards of porcelain into the substation, and sometimes also fire. It is important study the sequence of event in a typical bushing failure. • • • • •
The violence (e.g. pressure pulse from an internal flashover or external forces) causes the insulator to crack or explode. There is obviously a big risk of damaging the neighbouring bushings, as well as other equipment or personnel. As the insulator cracks or explodes, the structure of the bushing collapses and the condenser body falls into the transformer and ejects oil. Depending on several factors such as level of fault current, voltage, location of the puncture and sub-station design the outcome may be a flashover that ignites the oil. In unfavourable circumstances the oil-side porcelain cracks and the residuals contaminate the windings. Moisture often has a free passage and can enter the transformer in connection with the malfunction.
Figure 2. Failure event resulting in fire
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Figure 3. Porcelain residuals and moisture ingress. Regardless of the root cause, a lot is gained if the consequences of a failure such as the type described above can be reduced and this is probably why a growing part of the utility industry today specifies dry insulated technology with outer insulation made on non-brittle materials. 4 VERIFYING TESTS Bushings, as with other electrical equipment, are bound by industry standards, which vary between international, regional and national standards for the electrical and mechanical performance of bushings. The international IEC standard has a broad global acceptance and describes in detail, among other things, testing. Equipment manufacturers have to comply with certain tests to verify the design as such, as well as routine testing to verify the quality of the individual bushing. Some examples of tests are; • • •
Electrical Mechanical Thermal
Additional tests may also be carried out to verify the long-term performance, or performance under extreme conditions. Some examples of such tests are; • • • • • • • •
Ageing Environmental Tracking and erosion Various cyclic tests e.g. temperature and bending Arcing Fire testing Seismic tests And more…
Some tests carried out by the authors company and of special interest to fire related events are described below. 4.1 Arcing test The purpose of the test was to investigate the impact on RIP bushings subject to a major external flashover and also to see if the condenser core still seals the transformer after such an event. One of the tested bushings had the condenser core purposely punctured to simulate a complete breakdown similar to that described in section 3 of this paper. Another bushing in the test was free from such defects to facilitate dielectrical testing afterwards to find out what damage the short circuit causes to the condenser core. It had been prepared to trigger an external flashover. The bushing voltage levels were 52 kV and 123 kV and the fault current was set to 50 kA for 0.3s. • The bushing with purposely a punctured condenser body was severely damaged on the air-side but the condenser body stayed in position. • The bushing with an intact condenser core was severely burned on the outside but the condenser core passed the partial discharge test as per IEC for new bushings and had a PD level below 5 pC. The conclusion is that the particular design of RIP bushing has good withstand characteristics against sudden and violent electrical loads and still seals the transformer after the simulated event, figure 4.
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Figure 4. Result of short circuit test on bushing with completely punctured condenser core. The bushing is heavily damaged but the condenser core stayed in position. Note also how the surfaces is cracked rather than delaminated. 4.2 Fire testing The relevance of fire testing is always possible to question since factors such as shape, orientation and environment surrounding the specimen and the condition of ignition are all different from field conditions. The small scale testing detailed below is likely an underestimation of the difference between OIP and RIP from a fire resistance point of view. In reality when an oil impregnated system fails catastrophically it often exposes a delaminated surface that is easily ignited compared to the standardised test procedure used here, figure 5.
Figure 5. Typical structure in a failed OIP condenser core (left) and the relatively homogenous structure in the test set-up (right). RIP on the other hand usually cracks with a relatively clean surface structure similar to what was tested hereunder. Nevertheless the tests carried out by the authors company and described below show some important differences between the two bushing technologies. 4.2.1 Cone calorimeter testing Two important factors for how severely a fire develops are time to ignition and peak heat release of the different materials. Test samples were taken from the insulating system in a 500 kV oil impregnated bushing and from a 500 kV resin impregnated bushing, figure 6. In a Cone Calorimeter, specimens were exposed to controlled levels of radiant heating. The specimen surface is therefore heated up and an external spark ignitor ignites the pyrolysis gases from the specimen. The heat release rate is determined by measurements of the oxygen consumption derived from the oxygen concentration and the flow rate in the exhaust duct, figure 7.
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Figure 6. Cross section of OIP and RIP bushings with test specimens indicated.
Figure 7. Test set-up. 4.2.2
Time to ignition, in seconds
Time to ignition is a critical parameter in all fire testing. In a substation fire event it will, among other things, decide the likelihood of multiple seats of fire occurring simultaneously. The graph below is a summary of three test results on each material. 140 120 100 80 60 40 20 0 OIP
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Peak heat release, in kW/m
A given amount of energy can burn slowly or fast and the peak heat release is one of several parameters which describes the effect of a fire as well as the possibility to conduct efficient fire fighting. The graph below is a summary of three test results on each material. 3000 2500 2000 1500 1000 500 0 OIP
4.2.4 Energy release Energy release in MJ/kg was also analysed for the different materials. When taking into consideration the total energy, based on actual mass of different materials in the two concepts, the energy release does not show any major differences. It is however worth noting that the free oil represents more than 60 % of the total energy in an OIP bushing. 4.2.5 Ignitability If a bushing with a silicone insulator is exposed to open fire from an external source it is relevant to know if the insulator extinguishes or fuels the fire. The analysis has been done in a small scale vertical flame test according to a standardized procedure, UL 94. The silicone rubber self-extinguished immediately once the flame was removed and thus fulfilled the highest rating, V0. An observation from a real fire event confirms the self-extinguishing characteristics, figure 8. The silicone sheds burn only on the side facing the open fire.
Figure 8. Silicone insulators exposed to open fire
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4.2.6 Conclusions from the test It is clear that a resin impregnated bushing has a number of advantages related to reducing the risk of fire; time to ignition and peak heat release being the two most important factors. The total energy content does not differ markedly between OIP and RIP bushings, however a large amount of the energy in an OIP bushing is stored in oil which, in the event of an explosive failure may be sprayed over other equipment and thereby cause multiple seats of fire and exposes a very large surface to possible ignition.
5 SUMMARY High voltage bushings are one of the most stressed components in the transformer and failure mechanisms tend to result in sudden and catastrophic failure of an explosive nature. Regardless of the root cause, a lot is gained if the consequences of a failure can be reduced. Test results clearly show that the dry insulated technology has many advantages in this respect. •
RIP poses less risk of fire as the heat release is lower and time to ignition is longer compared to an oil insulated system.
Down-time in the event of major transformer failures is reduced due to the fact that no porcelain remains are left inside the transformer after a failure of explosive nature.
The transformer is sealed which means that the risk of moisture ingress to the transformer is reduced in the event of flashovers.
High mechanical strength, more flexibility built into the design and reduced weight clearly increases its ability to withstand seismic forces, which is one of the root causes of transformer failures.
Outer insulation made of non-brittle material reduces the risk of explosion and the dangerous scattering of materiel and has a high inherent fire resistance.
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