Faults and Maintenance of Transformer Bushing

The power transformer is composed of iron core, winding, insulation, oil tank and bushing etc. Leads of power transformer winding The bushing plays a prominent role in insulating and fixing oil tank and transmitting current to outside. It needs adapting to various environmental conditions and has certain mechanical strength. There are different forms, such as pure porcelain bushing, oil-filled bushing, gas-filled bushing, capacitive bushing. There are two kinds of capacitive bushings: resin paper capacitive type and oil paper capacitive type. Here is a brief summary of faults and maintenance of oil paper capacitive bushing. 

I. Major Causes of Bushing Faults

The dirty substances on the surface of bushing absorbs water, insulation resistance reduces. As a consequence of that, it is likely to flashover and trip. Meanwhile, the flashover may damage the surface of the bushing. The conductivity improves due to water absorption. The leaked current increases, causing damage to porcelain and even breakdown.

There are two main reasons for poor sealing of the bushing. First of all, maintenance staff is lack of experience and screws are not tight enough; secondly, the bushing may continue to run regardless of its cycle or there is quality problem for rubber sheet; the structure of bushing is unreasonable and defective; the insulation oil is not qualified; the water penetrates into the bushing, which is slightly affected with damp; small grounding bushings breakdown and bushing shields generate floating potential, causing partial discharge; the oil pointer becomes dirty and it is impossible to see the oil level clearly. 

If not completely vacuumized during the overhaul process, the air would remain between shields. Under the influence of high electric field, partial discharge or even insulation breakdown happens.

II. Countermeasures against Above-mentioned Faults

1. Unqualified bushing oil sample with ethyne: examine the bushing strictly and be allowed to be put into operation on condition that all tests are up to standard to avoid breakdown. 

2. Poor sealing, water seepage or oil leakage: use high-quality rubber sheets instead of bad ones to keep tight and tighten bolts for no leakage.

3. Overheating bushing head caused by unreasonable structure: change copper aluminum transition into silver copper contact to lessen the effect of the oxidation.

4. Check whether all parts are well connected and spread conductive paste on the contact surface after polishing to reduce the contact resistance and prohibit overheating.

III. Tips that Need to Bear in Mind during Transportation, Installation and Maintenance 

1. Hoist the bushing slowly in order not to hit other objects; use the lifting lug of flange and bind the top part of bushing through hemp ropes when lifting upright; do not hoist porcelain skirt to protect porcelain bushing; any part of bushing should not touch the ground when erecting the bushing; place and fix the bushing in the exclusive box in transit. There are two supporting points at the place where the flange is installed and place soft materials under supporting points.  

Pay special attention to the following during the installation after overhaul:

(1) to prevent the bushing to be affected with damp. The environment should be clean and dry; performing the installation at 40-50℃ is the best. When the temperature of capacitive core is 10-15℃ higher than ambient temperature, the effect of damp will be reduced. Hence, it is better to heat parts and capacitive core of bushing to 70-80℃ before assembly and keep for 3-4h; then make great efforts to complete the assembly as soon as possible before the temperature drops.

(2) Sealing of the top of bushing. The sealing can be divided into two parts: bushing itself and lead. At present, the top of oil conservator of main transformer in most substations is equipped with elastic corrugated plate, which regulates the temperature together with pressure springs. When assembling the elastic corrugated plate, seal rings between positive nuts and negative ones should fit well with seal sheets on the oil conservator to achieve good effect. If the seal between top wiring board and conductive head is not tight, the rainwater will permeate into the transformer along wiring board, conductive head and cable. The water permeate into the root of transformer lead, causing breakdown and power failure. In order to avoid the mentioned fault, bolts are used to ensure good sealing.

(3) When the bushing is in service and undergoes withstand voltage test, its external grounding cover should be grounded well and corona ball is adjusted appropriately. The corona ball is at the tail of intermediate conduit and can be twisted up and down. The corona ball must be tightened; otherwise, the discharge would occur between corona ball and bushing. Adjustment of corona ball position can shorten the distance between bushing tail and the surface of oil tank and creepage distance of bushing and improve the potential distribution. If not adjusted appropriately, the sphere would generate the flash discharge and dielectric breakdown happens, causing serious damage to electric performance of bushing; the quality of oil sample valve and drain plus should be good and free from rust, and the same goes for rubber sheet; keep the residual pressure under 133.3Pa and inject the oil according to the time.

3. Focus on the position of bushing during the test, especially dielectric measurement. When the position is different, the impact of high voltage electrode and measuring electrode on the stray impedance of framework, object, wall and ground not fully grounded will have a major influence on actual results of bushing. Therefore, when measuring the dielectric loss or conducting other tests, the bushing should be vertically placed at well-grounded frame rather than placed horizontally or lifting through insulation rope to ensure correct measuring data.

4. Maintenance staff should bear the following in mind: when testers disconnect and connect leads of end-shield bushing, they should not make guide rod rotate and twist off grounding leads; everything restores to the original condition after the test; workers should tighten the sample valve after oil samples are taken; maintenance staff should observe oil level of bushing and supplement the oil in time.

5. In addition, maintenance staff should be familiar with technical requirements of bushing. First of all, it is necessary for them to be familiar with technical performance of bushing. For instance, major test items for bushing delivery include dielectric loss measurement, partial discharge measurement, power frequency withstand voltage test, leakage test, appearance and dimension inspection. Secondly, usage conditions shall be borne in mind. Ambient temperature is between -40℃ and 40℃; the angle between installation and normal axis is not more than 30°. Also, they should seize every chance of power failure to carefully inspect the bushings and eliminate hidden troubles.