A38 - How to prevent Power Transformer accidents?

Accident with respect to transformers can be defined as any unsafe operation, procedure or failure mode that can result in collateral damage and /or human suffering during transport, installation, testing, operation and maintenance of transformers. Following summary is based on actual incidents reported from field.

Transport

1.????During a road route survey done for a large transformer (the heaviest package -tank with active part) an operator was electrocuted while checking the clearance from live overhead distribution lines. Use laser-based distance measurement instruments for checking clearances with live electric lines.

2.????While transporting a 315 MVA transformer over road, the driver took a deviation from the approved route to save time. An old bridge in that route collapsed along with trailer and consignment. Never deviate from surveyed and approved road route.

3.????315 MVA transformer fell down from the trailer as the vehicle was taking a bend in the newly built road inside the substation. ?The soil was not compacted properly in the newly built road and the soil on one side gave away tilting the trailer.

4.????A road trailer carrying transformer was going under 11 KV lines and due to inadequate clearances, there was a flashed over from line to tank, with current flow through tank to ground. Overall dimensions of consignment are to be checked, as loaded on trailer to make sure that dimensions of loaded consignment are not exceeding the maximum limits as per route survey. When crossing live lines or railway crossings, get line shut down if there is doubt on electrical clearances.

5.????When a large transformer with dry air filling was taken through a short tunnel in the road, the tank cover got stuck up with the structure in the tunnel. Since trailer could not be moved in reverse, tank cover stiffener removed using gas cutting. In the process internal part caught fire. Take extra care while negotiating through tunnels.

Installation

1.????Do not lift Transformer when under vacuum. The combined stresses on tank can collapse it. Before vacuuming the tank, check that tank and accessories can withstand full vacuum. Refer to the rating plate for any special caution notes on vacuum levels. Many of the medium voltage transformers may be designed only for half vacuum. This is true about conservator tank also.

2.????Avoid welding on transformer tank with active part inside but in oil drained out condition. There are cases of the welding sparks entering in to tank through the weld cracks and oil-soaked paper in active part catching fire. Welding shall be done with tank under oil filling. In case of rectifying weld cracks, maintain vacuum over the oil level to prevent oil oozing out through weld cracks and spoiling the molten metal.

3.????When transformers are stored under oil filling without conservator, make sure that there is sufficient air volume over the oil level in the tank. Leave approximately 10 % of tank height without oil. ?For every 10 C change in ambient temperature, oil will expand by 0.8 % by volume. Without expansion space over the oil level, tank can rupture from excess pressure build up during rise in ambient temperature. This can also happen in transformers fitted with air cell inside conservator. If more oil is filled in conservator than required for the corresponding ambient temperature, pressure build up can occur during loading due to oil expansion with oil temperature rise. When oil expands, there will not be sufficient space in conservator. Air cell will prevent pressure release through breather pipe. This will result in pressure build up leading to pressure relief device operation or tank /radiator welds rupturing.

4.????Jacks used for lifting the transformer shall be properly maintained and calibrated. Wood planks shall be of adequate carrying capacity. Transformers had toppled due to broken planks and failed jacks.

5.????Before starting installation work, fix the ladder as the first step.

6.????After over pressurizing (for leak check of tank), release air pressure fully before attempting to open any covers on the transformer.

7.????When working on tank top, safety railings shall be provided all around to prevent falls. Tank design shall provide necessary fixing holes and clamps to accommodate such metal or fiberglass pillars and rails.

8.????A large furnace transformer after installation at site was undergoing oil filtration. To reduce the heat dissipation, tank was covered with tarpaulin. It was peak of winter and operators used charcoal heater (sigdi) kept under the tarpaulin. At night, fire spread from heater to tarpaulin and transformer. Damaged transformer had to be taken back to factory for repair.

9.????When metallic hoses are used for oil circulation by filter machine, earth both ends of hose to drain away static charges. Keep ready a fire extinguisher near the filter machine. Avoid oil spillage during oil filling, filtering work.

10.?Accidents from bushing failures can occur if the following installation points are not looked in to.

(a)??Loose connection between terminal connector and ?bushing terminal pad. With current flow, the joint can get heated up and may damage the top oil seals of bushings from transmitted heat. Water can enter inside the bushing through these damaged oil seals causing bushing failure and fire.

(b)??The jumper connections from bushing top terminal pad to overhead catenary or bus shall not exert pull on bushing during service. If this length is short, contraction of ACSR jumper lead during winter time, can exert tremendous pull on bushing, leading to porcelain crack and oil draining out. Root cause for some of the bushing fires are such installation lapses. If the connection is made to catenary line, jumper connection point ?shall be at the ends of catenary and not at the middle. Otherwise, galloping catenary due to wind or fault current, can exert damaging pull on the bushings.

(c)??Poor quality joints between bushing bottom tail pad and winding leads can cause local ?overheating and bushing failure. This is true for the lead joint inside the draw lead tube of the bushing. Some of the bushing failures and consequent fires were from such poor-quality current carrying joints by bolting or brazing

?Commissioning /Testing

1.????Tests (insulation resistance, winding resistance, ratio) involving application of voltage shall not be done when the tank is under vacuum. Even though full vacuum is a good insulator, under partial vacuum, break down voltage of air comes down drastically. This can cause insulation breakdown of windings.

2.????All electrical safety measures shall be taken during testing as high voltages can be developed by induction at winding terminals.

3.????Major transformer failures can be wit arc flash over in oil that may release huge amounts of inflammable carbon gases. No electrical tests shall be done immediately after such failures as combustible gases in tank can catch fire from electric arcs across broken electric paths during voltage application. A 450 MVA 400 kV auto bank met with an internal failure during a 3 L fault on tertiary terminals. After 2 days, when low voltage tests were done, PRD operated, spilling out oil. A 400 kV shunt reactor (after an LL fault inside the tank) was under LV tests in oil drained out condition. Smoke came out of tank as oil-soaked paper caught fire.

Operation

1.????Earthing of tank and winding neutrals shall be done properly to avoid any dangerous touch potentials in service. Grounding resistance shall be below 1 ohm.

2.????When transformer is under energised condition, work shall not be done on the tank earthing or neutral earthing bus work. Fatalities were reported during theft of such grounding copper leads in unattended substations. Painting of such copper bus is a good practice to camouflage copper.

3.????Tank body grounding connection may carry current of the order 10-100 A and any break in it can result in arc flash and fatality.

Maintenance

1.????Slope provided on tank cover for draining out water shall not be excessive to prevent slipping when people walk on cover. Such areas should be given anti-skid painting or provide over laid chequered plate.

2.????Monitoring /protective accessories shall be easily accessible. Buchholz relay shall be accessible from tank top.

3.????Ladder design shall be appropriate and easy for climbing. A protective, lockable cover shall be provided at entry of ladder to prevent unauthorized access on energized transformer.

4.????Before working on switched off transformer, ground all terminals to drain away static charges (especially, terminals of delta winding)

5.????Stop all oil leaks immediately and clean tank and radiator surfaces of oil traces. Fires had started from leaked oil soaking nearby dried grass and other vegetation. Keep ?all inflammable materials away from transformer.

Failure

Some of the transformer failures also may lead to accidents. Failure may result in tank rupture and oil flowing out to water sources. Bushing failures may shatter porcelain causing collateral damages and oil fire. ?Transformer fires in mineral oil filled transformers can be from two modes – tank rupture or OIP bushing explosion.

1.?????In case of low impedance faults inside transformer (eg line end flash over to ground) arc flash will carry high current and massive energy flow generates huge amount of gas inside the tank. This in turn, results in a dynamic pressure wave inside the tank, shattering tank wall and releasing hot carbon gases in to atmosphere. When these hot gases come in to contact with oxygen in air, fire starts. Oil gushing out from tank feeds the fire, leading to an inferno. Some of the solutions considered are flexible tank, fast acting pressure relieving valves spread around the tank and cover. IEEE Guide C57.156-2016 “Guide for tank Rupture mitigation of liquid immersed transformers and reactors” is a good source for calculation of inside pressure rise from arc energy and guide lines for tank rupture mitigation.

2.????In case of distribution transformers, there is no low oil level monitoring (alarm) as in case of power transformers. Hence chances of fire are high when continuous oil leak is not attended to or oil is drained out through theft. In such conditions, when the live winding get exposed to air, arcing to ground may cause oil impregnated paper to catch fire. Oil fire and tank explosion may follow. A solution to this is fitting of metal or concrete panels around the transformer to prevent hot oil splashing around. Some years back, in Kolkata, a distribution transformer exploded and burning oil reached inside a nearby apartment through window resulting in fatalities. A metal panel near the tank, could have prevented fatality. Current practice is to go for dry cast resin transformers wherever vulnerability is high- inside of high-rise buildings, auditoriums and stadia for sports and games. Ester oil having higher fire point than mineral oil, is another alternative insulation in transformers for thickly populated areas.

3.????Oil impregnated condenser bushings are used at voltages of 52 kV and above. These bushings contain oil. Any arcing inside the bushing (from aged insulation, moisture entry or any other defect) release high energy resulting in breakdown of oil in to high volume hot gas. Dynamic pressure rise shatters the porcelain and flying porcelain pieces can cause damage to nearby equipment and personnel. Released hot carbon gases can catch fire starting an oil fire. Bushings shall be replaced as soon as the incipient defect is detected through continual measurement of capacitance and power factor of bushing insulation. Continuous on-line monitors are available for checking bushing lots having suspected insulation integrity. Installation mistakes as explained earlier shall be avoided to prevent bushing failures and consequent fire. Future trend will be to use resin impregnated dry condenser bushings with silicone rubber casing. These dry bushings are fire resistant and will fail safely.

Conclusion

This is an attempt to collect and collate possible reasons for accidents in distribution and power transformers based on author’s experience of more than ?five decades. Preventive measures are proposed to prevent such accidents.