Transformer Tank Rupture and Mitigation – Hydro-Québec Perspective: Transformer Tank Rupture and Mitigation – Hydro-Québec Perspective Marc Foata and Van Nhi Nguyen TransÉnergie Une division d'Hydro-Québec


Background: Background Early 1980s, increasing number of catastrophic failures on our 735 kV system Two major blackouts Safety threats to the workers Environment concerns due to oil spills and fire


Background: Background 1984- 1987: Study works of HQ - ABB joint teams many measures implemented; among these measures: Welded cover instead of bolted type Physical separation of radiators and conservator from main tank and with devices to prevent oil spilling in case of pipe rupture Modifications to tank reinforcement beams Fire extinguishing walls and oil retention basin AND most importantly higher test levels for more reliable performances and larger margin in insulation


Background: Background 1987-1992 Major R&D project on arc-induced explosion and tank ruptures 1992 New requirement in Hydro-Québec specifications, no minimum level is enforced but question must be adressed during the design review 1992-1993 Joint effort with ABB for a more resistant tank design 1992-2006 All our transformer suppliers have demonstrated adequate ability to take into account this new requirement in their analysis


Mitigation approaches: Mitigation approaches Protection: Improve fault interruption delay so that very low energy is released Insulation: Correct all design weaknesses so that no fault can occur Containment: Make a pressure resistant tank that can safely withstand or evacuate all energy levels


Results: Results Protection: 3.5 cycles average fault interruption time, reduced to less than 3 cycles Insulation: Several weaknesses have been identified and corrected. Yet the possibility of a major fault could not be totally discarded Containment: Very little knowledge on the subject, a R&D project is initiated


Failure statistics 735 kV (25 years): Failure statistics 735 kV (25 years) 175 failures that resulted in 111 high energy arcs causing 44 tank ruptures and finally 18 fires


Rupture vs Fault Energy 700 kV: Rupture vs Fault Energy 700 kV


"Worst" worst rupture modes: "Worst" worst rupture modes


"Less" worst rupture mode: "Less" worst rupture mode


"Least" worst rupture mode: "Least" worst rupture mode


R&D Simulation tools: R&D Simulation tools Theoretical part based on analytical and numerical models identifying fundamental parameters. Derive a straightforward method for calculating the static pressure.


R&D Arc simulation setup: R&D Arc simulation setup Experimental part involving simulation of the dynamic load factor F to be applied to the static pressure to take into account the dynamic effects.


R&D Results: R&D Results Pressure Displacement


R&D Results & Conclusions: R&D Results & Conclusions Arc induced explosions can be simulated but sophisticated tools are required Deformation, not pressure, should be used to determine tank rupture. Dynamic factor have been proposed to extrapolate static calculations Testing by high pressure gas injection is possible New tank resistance specification has been formulated


New specifications - Philosophy: New specifications - Philosophy Priority is given to the protection of the workers Worst energy levels may not always be containable by the tank First rupture point must be the cover Required calculation tools must be accessible to transformer designers Must take into account the highly dynamic phenomena involved Must be easily verified


New specifications - Objectives: New specifications - Objectives Stimulate new tank designs Exchange technical information with manufacturers Aiming in long term to see that new tank designs meet arc energy requirements


New specifications - Formula: New specifications - Formula Ps – Calculated tank pressure withstand F – Dynamic (time & location) amplification E – Fault energy level to withstand K – Arc energy conversion factor C – Tank expansion coefficient


New specifications – Dynamic factor: New specifications – Dynamic factor Time related dynamic factor (pressure and deformation) Proximity related dynamic factor Takes into account tank volume


Manufacturers' response: Manufacturers' response All transformer suppliers since 1992 have shown adequate tank withstand analysis One supplier has proposed an improved tank design (double arc containment capability to 20 MJ) Containement of the highest levels of arc energy require complicated pressure venting system with numerous rupture disk all over the tank or double-walled tanks


Where do we go from here ?: Where do we go from here ? Present design can contain up to 10 MJ for the largest tanks (735 kV) More resistant tank design can be achieved Need to implement specifications with minimum energy requirement to meet . Utilities need to agree on reasonable levels Testing by high pressure gas injection is feasible and more appealing than real arc testing