1. Main transformer oil circuit penetration and its harm

2. Causes of mutual penetration between on load tap changer and oil circuit of main transformer

No matter what type of main transformer on load switch is widely used in the system, arc will be generated in the process of on load switching, and the insulating oil will decompose to produce free carbon, hydrogen, hydrocarbon and other gases, so that the oil insulation level and oil chamber pressure will change step by step with the increase of operation time and switching times.

According to relevant information, the change rate of internal pressure with the number of switching times is 2 times of the rated current, and the internal pressure increases by about 0.086mpa for every 100 times of switching. Generally, the sealing strength of the oil chamber of the switch is 0.05Mpa, The oil in the oil chamber of the switch will penetrate into the oil tank of the main transformer body.

In the 29th International Conference on large power grids of IEC, it is discussed that the hydrogen content and hydrocarbon content of on load tap changer body oil are higher than those of off load tap changer. Poland puts forward the allowable gas concentrations of these two transformers. If the content of acetylene and hydrogen in the oil tank of on load tap changer transformer is twice as large as that of non excitation tap changer transformer, the oil in the oil chamber of the transfer switch must enter the oil tank of the main transformer body.

According to the example analysis, the main transformer on load switch and the main body oil path penetration reasons are mainly reflected in the following three aspects: 2.1 improper design and manufacturing process of on load switch, such as 1 set of syxz series products, the assembly mismatch among eccentric flange, pressure plate and sealing ring is found during disassembly, the eccentric flange is not positioned correctly during installation, and the bolt fastening is not uniform during pressure plate installation, As a result, the seal ring is extruded unevenly, the seal is damaged and oil leakage occurs.

2.2 the sealing structure of on load tap changer is unreasonable and the sealing material is poor

In case of oil leakage of one on load switch, it is due to poor material and aging of O-ring, long-term immersion in oil, cracks and easy wear, resulting in too large gap between pressure regulating switch cylinders and seal ring being squeezed into the gap. At this time, the O-ring rolled in the groove. When the length of the squeeze in gap exceeds its elastic deformation range, it will be damaged due to deformation (generally, the fit gap of O-ring seal is 0.15m).

Syjz series composite switches are prone to leakage at the wiring base on the glass fiber insulation cylinder and the bottom steel wood chassis. Some domestic switch chassis are equipped with thread structure. The steel wood chassis is sealed with sealing ring and thread, and then dried with adhesive, or the chassis flange is fixed with flat pin after sealing ring. Due to some problems in the processing accuracy and finish of the sealing ring, as well as the aging of the material and the poor drying treatment of the adhesive, after being used in high temperature, oil immersion and vibration for a period of time, the adhesive will crack and peel off, causing oil leakage.

2.3 the installation and oil filling method of on load voltage regulating switch is improper. When vacuum oil filling is carried out on the insulating oil of main transformer body, sometimes the oil tank of main transformer body is not connected with on load switch oil chamber, but only the main transformer body is vacuumized, and the on load switch oil chamber is still under normal atmospheric pressure, resulting in large pressure difference between the two oil chambers. The seals and shaft seals of on load switch must bear this pressure difference, which causes the seals to be damaged in different degrees.

For example, the maintenance guide of power transformer (DL / t57395) stipulates that the vacuum degree (residual pressure) of 110kV, 20000kva and above main transformer should reach 0.035MPa. If the on load switch is not vacuumized, the pressure difference of its seal is 0.065mpa, which exceeds the design sealing strength of the seal by 0.05Mpa. In this case, the seal will be damaged, There is oil leakage.

3. Measures to prevent oil leakage between main transformer oil tank and on load switch oil chamber

3.1 it is necessary to prevent the occurrence of pressure difference during vacuum oil injection of transformer to avoid damaging the seal. Different methods can be adopted for different types of on load switches, such as syxz series switches. The eccentric flange, pressing plate, O-ring and other seals can not be installed first, that is, the body oil tank and switch oil chamber are connected to carry out vacuum oil injection together. When the oil is injected to the immersed iron core and coil (according to the guidelines, the oil chamber of the pressure regulating switch is 200 mm away from the top of the upper oil tank after one-time oil injection), the oil chamber of the pressure regulating switch is also filled, and then the seal is installed to separate the on load switch oil from the body oil, and then the body and on load switch oil chamber are respectively injected until the oil level is calibrated.

There are four special connecting pipes and small flange holes E2, R, Q and s on the switch cover plate of German Mr company and domestic V-type switch. E2 connects the transformer body oil tank from under the switch cover plate. When vacuumizing, just connect E2 and Q flange holes to connect switch oil chamber with transformer oil tank. If there is no pipe connection at the site, the V-type switch and the cylinder can also be placed on the support of the transformer body without installation, and the oil chamber of the switch can be closed. After vacuum oil injection, the switch cover plate can be opened, and the cylinder can be lifted by the lifting device, and then the cover plate can be installed on the support, and the connecting mechanism can be connected, and the oil can be injected to the rated oil level for the second time.

There is no special vacuum flange on the cover plate of domestic syjz series switch, so it can only be connected from the gas relay interface (25, 50) to vacuum at the same time with the body.

3.2 reduce the fit clearance between the seal ring and the switch cylinder and eccentric flange, and strengthen the sealing effect. For syxz series switches, it is recommended to reduce the fit clearance between the O-ring and the switch cylinder to less than 0.15mm, and also reduce the fit clearance of the eccentric flange, improve the surface precision, and strengthen the sealing effect (the current fit clearance is as small as 12mm and as large as more than 10mm). In addition, the bottom chassis flange can be changed from single rubber seal to double seal, and the central shaft can be changed from single shaft seal to multi shaft seal, so as to prevent shaft seal leakage caused by frequent operation.

3.3 when the main transformer is installed or overhauled on site, all seals and seal assembly conditions of on load switch shall be carefully checked, the process shall be strictly controlled, and appropriate tightening, replacement, improvement of seal structure and other measures shall be taken to improve the sealing degree.