Analysis and treatment measures for multiple transformer bushing cap heating failures

introduction

The transformer bushing is the main insulation device outside the transformer box. The lead wires of the transformer winding must pass through the insulating bushing to insulate the lead wires and the lead wires from the transformer casing, and also to fix the lead wires. However, if the on-site installation and construction personnel do not strictly follow the relevant technical standards of the bushing manufacturer during the bushing installation process, the caps of the conductive rods of the winding lead wires will not be installed properly, the contact area between the caps and the conductive rods will be reduced, the connection will be loose and unstable, and the current conduction performance will be reduced, and abnormal temperature rise will eventually occur. In severe cases, the threads of the conductive rods will be damaged, the bushing seal will be destroyed, and accidents will occur in the bushings and transformers.

本文根据传奇电气(沈阳）有限公司型号为BRDLW的套管内部结构,针对220kV泷州站#2主变高套管B相发热缺陷,剖析了套管将军帽过热的原因,提出了现场处理措施,并针对我局的设备提出了反事故措施。

1. Fault phenomenon

The following are four cases of 220kV transformer bushing heating failures. According to the emergency defect standards (hot spot temperature of the bushing top column > 80℃ or relative humidity ≥ 95%), they all meet the reporting conditions for emergency defects.

(1) On September 27, 2013, the temperature of the 9-phase bushing cap outlet joint of the 220kV Xingyao Station #1 main transformer was abnormal, reaching 80°C, while phase A was 37.4°C and phase B was 37.8°C, with a maximum temperature difference of 42.6°C. At that time, the ambient temperature was 30°C and the load current was 234.8A:

(2) On March 17, 2014, the temperature of the 9-phase bushing cap outlet joint of the 220kV Ren'an Station #1 main transformer was abnormal, reaching 48.59°C, while phase A was 22.2°C and phase B was 22.7°C, with a maximum temperature difference of 26.39°C. At that time, the ambient temperature was 14°C and the load current was 243.2A:

(3) On June 28, 2016, the temperature of the high bushing B and the 9-phase general cap outlet joints of the 220kV Xingyao Station #1 main transformer was abnormal. The temperature of the B-phase bushing was 91.6°C, the temperature of the 9-phase bushing was 81.5°C, and the temperature of the A-phase was 42°C. The maximum temperature difference was 49.6°C. The ambient temperature was 30°C at that time and the load current was 253.2A:

(4) On July 25, 2016, the temperature of the general cap outlet joint of the high bushing phase B of the 220kV Longzhou Station #2 main transformer was abnormally high, reaching 85°C, while the A and 9 phases were 47°C and 49°C respectively, with a maximum temperature difference of 38°C. The ambient temperature at that time was 37°C and the load current was 175A.

2 Casing test data

The high-voltage test team conducted a DC resistance test on the defective phases of the above four faults on site (the fixing flange of the general cap is installed inside the bushing oil pillow, which makes it impossible to directly test the contact resistance of the general cap and the winding lead joint). The results showed that the numerical results of the three-phase bushings A and B in the #1 main transformer of 220kV Ren'an Station and 220kV Xingyao Station were close, and phase 9 was too large: the three-phase values ​​of the high-voltage three-phase bushings of the #1 main transformer of 220kV Xingyao Station were relatively balanced, and the value of the B phase of the high-voltage three-phase bushing of the #2 main transformer of 220kV Longzhou Station was too large.

According to the "Preventive Test Regulations for Power Equipment" (o/9sG114002-2011), for transformers above 1600kVA, the difference between the resistance of each phase winding should not be greater than 2% of the three-phase average value. From the test data, it can be seen that the difference between the phases of the direct resistance in the 220kV Xingyao Station #1 main transformer is as high as 7%, and the 220kV Longzhou Station #2 main transformer is as high as 3.5%, both exceeding the standard of the regulations. After the general cap was disassembled on site, all data were qualified, and the temperature of the main transformer was normal after the operation personnel measured it again.

3-tube general cap structure

上述四起缺陷套管都是传奇电气(沈阳）有限公司生产的,型号为BRDLW。进行原因分析前,我们先看看传奇电气(沈阳）有限公司BRDLW型油纸电容套管将军帽的内部结构,如图1所示。

Figure 1 Internal structure of the casing cap

BRDLW type bushing is a through-type bushing. The winding lead conductive rod passes through the bushing oil pillow from the bottom of the bushing through the bushing center hole, and the lead conductive rod is fixed above the oil pillow by the positioning pin. The positioning pin can ensure that the conductive rod does not move horizontally with the general cap when installing the general cap, but can continue to move upward when tightening the general cap and connect with the general cap through threads until the screw-in depth, that is, the contact area, meets the requirements.

4. Failure Cause Analysis

The heating of the bushing cap of the high B phase of the 220kV Longzhou Station #2 main transformer is taken as an example for analysis.

After we loosened and removed the cap of the B-phase bushing, we found that the inner thread of the cap and the outer thread of the winding lead wire conductive rod each had an obvious and matching hot spot. The number of inner threads a of the cap was measured, and there were 30 turns in total, while the number of inner threads b from the hot point of the cap to the outer end of the thread was 5 turns in total, and the number of outer threads c from the hot point of the conductive rod to the top was 15 turns in total, as shown in Figures 2 and 3.

From this, we can see that the number of threads of the conductive rod screwed into the general cap is b+c=15+5=20 turns, that is, the contact surface length of the general cap and the conductive rod is the length of 20 turns of thread, and the total number of threads of the general cap is 30, that is, the length of the matching threads of the general cap and the conductive rod accounts for 20/30=2/3 of the total thread length, which reduces the contact area between the two by nearly 1/3.

Insufficient screwing depth may lead to the following two problems, causing the device to heat up.

(1) Insufficient contact area. When the screw-in depth does not meet the requirements, the matching pressure between the conductive rod and the general cap is insufficient. Under light load conditions, insufficient contact area will not lead to overheating. However, as the main transformer runs longer and the load increases, the problem of insufficient contact area will become apparent, causing the general cap to heat up.

(2) Insufficient matching pressure. Under normal full screw-in depth, the conductive rod will rise to the highest point and connect with the general cap. At this time, the positioning pin passes through the lower end of the conductive rod and the conductive rod positioning tube, and is stuck at the uppermost end of the movable groove of the positioning tube to prevent the conductive rod from continuing to rise, so that the contact area between the general cap and the conductive rod is kept at the maximum. Moreover, the matching pressure of the conductive rod and the general cap can be increased by the state that the positioning pin is stuck in the conductive rod. When the screw-in depth is insufficient, the positioning pin will not rise to the highest point, which will lead to insufficient matching pressure. When the main transformer is running at high load, the vibration of the main transformer increases. At this time, the conductive rod will shake fatally as the vibration of the main transformer increases, resulting in heat generation.

The main reason for the insufficient matching area between the general cap and the conductive rod thread is that the construction unit did not have enough awareness of this problem when using a belt wrench to install the sleeve. They thought that it would be fine as long as the general cap could be installed, but ignored the fact that the number of turns of the general cap must meet the requirements to ensure the entry and exit depth of the lead conductive rod.

5. Anti-accident measures

由于多起将军帽发热故障的套管都是传奇电气(沈阳）有限公司生产的,型号为BRDLW,该产品在我局的使用率非常高,这就造成我市电网的安全可靠运行存在较大隐患,故应采取以下措施:

(1) Strengthen temperature measurement. Strengthen infrared temperature measurement of main transformer bushings. Any abnormality should be reported and handled as soon as possible. This time, the operation personnel promptly discovered and reported the heating defect of the 220kV Longzhou Station #2 main transformer, and immediately arranged a power outage to eliminate the defect, which avoided the expansion of the heating point and reduced the difficulty of fault elimination.

(2) Install an online temperature measuring device. When the temperature measuring personnel measure the temperature on the ground, the casing oil pillow just blocks the view of the general cap, which makes it difficult to determine the heating point from the heating diagrams of multiple casing heating failures. It is recommended to install an online temperature measuring device at the general cap to ensure that it can be discovered in time when it is hot.

(3) Pay attention to analyzing the test data. For the same type of bushings, combined with the pre-test power outage, it is found that although the DC resistance does not exceed the standard, the imbalance rate is close to 2%. The bushing cap should also be removed for inspection.

(4）咨询厂家相关数据。针对BRDLW型套管,可咨询传奇电气(沈阳）有限公司,各电压等级的将军帽分别在旋多少圈时才能使其与导电杆接触面积满足要求。

(5) Prepare spare parts. Purchase related sealing rings and other materials as spare parts, because basically every time the general's cap is removed, the original sealing ring will be damaged and need to be replaced.