Auxiliary equipment: Auxiliary equipment required for cooling, such as fans. oil pumps, control devices, and wiring, should be checked on an annual basis. The equipment should be cleaned and damaged parts replaced.
External inspection: The transformer should be given an external inspec- tion on a semiannual basis. The inspection should include checking the tank, radiators, auxiliary equipment, gasket leakage, and metal parts for corrosion. Also, the electrical connection should be checked for tightness and overheating. Transformer bushings should be checked for mechanical damage, cleanness, and leakage. Bushings should be wiped clean on a regu- lar basis to minimize flashovers.
Internal inspection: This inspection involves the internal investigation of the tank and core. On liquid-filled transformers, the covers can be removed to examine for evidence of moisture or rust around the bushing supports and transformer top cover. To examine the tank and core, the liquid can be drained out. Examination of the core should be made to check for sludge deposits, loose connections, and any damage to the transformer parts. Evidence of carbon may indicate internal problems. The winding inspection should be checked for damage to terminal panels, barriers, loose connec- tions, and overall connection of the winding. Obviously, such things as untanking the transformer for internal inspection would have to be judi- ciously made and would depend on the age of the transformer and its over- loading and trouble history. The frequency of this inspection should be 5 to
10 years or more.
Transformer fluid: All transformer fluids are subject to deterioration, and the main contaminants are air, moisture, and heat. These contaminants react with transformer fluid and produce acids and sludge. The acid, in turn, attacks the winding insulation, and sludge deposits tend to decrease cool- ing. Moisture in the transformer fluid tends to lower the dielectric strength of the fluid, which combined with sludge, will lower the flashover value of insulators and terminal boards inside the transformer tank. As discussed earlier, regular inspection of the transformer is needed to maintain the fluid in a contaminant-free state. For proper maintenance of insulating fluids refer to Chapter 4.
5.7.3.5 Drying-Out Methods
Similar to dry-type transformers, the liquid insulating transformer can be considered as consisting of core and coil assembly, except that the assembly is immersed in an insulating fluid. Elaborate measures are taken to prevent and detect the infiltration and increase of moisture con- tent in the transformer. Before the transformer liquid becomes saturated with water, the paper insulation of the winding in a transformer has already absorbed a concentration of moisture because of its great affinity for water. The water in the paper insulation accelerates the degradation of the insulation and lowers its electrical integrity.
We have discussed in the previous sections several tests to judge the dryness of the transformer, such as IR, PI, and PF. One simple method for detecting the water content in the transformer oil can be made by an approximate method known as the cloud test. It consists of cooling a test tube sample of oil in an ice bath. If a cloud appears in the test tube above 0°C, the transformer contains excessive moisture. Confirmation of the water content in the transformer can be made by a laboratory test.
The distribution of moisture in the transformer is always in a state of unequilibrium. Through the cooler range of temperatures, the solid insulation of the transformer winding will tend to absorb more moisture than the trans- former liquid. However, as the transformer is loaded, the rise in winding tem- perature will release this moisture. This change due to varying loads and temperature is constant, regardless of whether there is an excess of water or only a very small quantity of moisture in the transformer. Also, transformer liquids such as oil tend to hold more water with an increase in temperature. In other words, there will be more moisture in the transformer oil when it is carrying a load than when it is unloaded. Other factors, such as decomposition of paper insulation and contaminants, will tend to generate more moisture in the trans- former liquid. When it becomes necessary to dry out liquid transformers, the following methods can be used: (1) heat alone, or (2) heat followed by vacuum.
Heat alone: This method involves application of heat to the transformer alone. One form of heat application is oven drying, which can be done at any of the service shops of major manufacturers. When the transformer is oven dried in the service shop, it is important to monitor the winding resistance to see when the transformer, reaches oven temperature (100°C–120°C). PF and IR measurements should be made at about 6 h intervals to see when drying is achieved, that is, when at least four readings are of the same value.
Heat followed by vacuum: The heating of the transformer with liquid can be performed by applying short circuit to the transformer or by circulating hot oil by means of an external system. As in the previous method, PF and IR measurement should be made at about 6 h intervals. Completed drying is indicated by at least four readings that are the same. The field drying methods may involve heating the transformer liquid, removing the liquid, and immediately applying high vacuum. Another method may involve removing the liquid and heating the transformer by circulation of hot air. Once the winding reaches 90°C–100°C a high vacuum of about 0.5 Torr is applied. When the temperature drops below 50°C, drying is stopped. The normal length of time to apply heat and vacuum may be a week or more, depending on the size of the transformer. Once the transformer is dried and the vacuum pulled, clean transformer liquid can be introduced into the transformer. Precautions to observe during this process are as follows:
Before the vacuum is pulled, make sure the tank is braced for full vacuum
The air temperature for drying should not exceed 100°C