Analysis of the Failures in Power Transformers

In this study, the failures in Transformer Centers in Mersin Province Erdemli District have been investigated. The causes of the failures have been discussed. It is aimed to create a method for the reasons of possible failures of power transformer. The data have been collected by investigating the faults occurring at the Erdemli Substations. The failure statistics of the 200 power transformer failures that occurred between 2003 and 2019 in the Erdemli district of Mersin province have been examined. Examples of different power transformer failures have been given in fault analysis. The reasons of power transformer failures and the precautions to be taken have been explained.


INTRODUCTION
The transformer is the most expensive and integral part of electrical power systems. Power distribution transformers change the high-voltage electricity into a high-current low-voltage state with virtually no energy loss [1,2]. Transformers are the complementary elements of many electrically operated devices, and there can be also 500 tons of transformers. Also, power transformers have taken a key role in the transmission and distribution of electricity [3,4]. The system depends on the reliability of the elements that make up the system. Preventing malfunctions and operating the transformers under good operating conditions is very important in energy systems. The way to avoid very serious faults in the power transformers and to prevent them from serious financial and technical problems caused by these faults depends on providing a working environment with the tools and equipments equipped with the appropriate technical devices [10,11]. In this study, the failures in Transformer Centers in Mersin Province Erdemli District were investigated. The causes of the failures are discussed. The procedure to be used in fault analysis has been introduced in investigation of the power transformer failures used in the electrical transmission systems.

INVESTIGATION OF FAULTS IN TRANSFORMER CENTERS IN ERDEMLI DISTRICT
The life of the power transformers is 25 to 30 years according to their design. In practice, the life of a power transformer can be extended up to 50 years with appropriate maintenance. Due to the problems related to the manufacturing process in the power transformers, they fail in the first five years of operation or in life periods after twenty years depending on the maintenance and controls applied in the operation. As the transformers age, the risk of failure increases with the weakening of the material. Insulation levels may decrease to a level that cannot withstand faults such as short-circuit faults in the system or transient overvoltages. In the inspections carried out at the substations as seen in Fig. 1, a power transformer without preventive maintenance has exploded due to the deterioration of the insulation paper, and it shows a transformer that has suffered a disaster. operating information is obtained. The staff of the operating centre is interviewed, and the records and reports of the history of the transformer are analyzed in detail. Thus, the preliminary report is prepared and revised and the final report is prepared. After the power transformer is energized, the signals belonging to the protection equipment are recorded. In the stage the transformer manufacturer must be informed about the defect. Things to do during the preparation phase are the operating manual of the transformer, the transformer test reports, the factory test reports, the field tests, the routine test reports, the single line diagrams of the substation, the protection/secondary protect, the reports related to the past studies or fault conditions, the settings for the protection and measurement systems, is composed of tape measure, protective clothing, field goggles, safety glasses, lenses, searchlight, flashlight, magnet, oil sample bottle and syringe, oxygen meter.In the field review phase, the photographer should take photographs on the defective transformer, make a visual inspection, take necessary notes and obtain the information by applying the necessary tests. Before the power transformer diagnostic tests, it should be noted that the transformer is disconnected from the power system and the auxiliary source points and is properly grounded. In electrical tests, flammable gases must be removed and safety precautions taken. Insulation resistance tests were performed between the winding-winding, the winding-earth, the earth-all the windings, the cor-earth. The winding ratio was examined and the auxiliary isolation and damage point was determined.
Power factor was examined and main isolation was controlled. The winding DC resistance was examined to determine the auxiliary isolation and damage point. Dielectric strength of oil was investigated and the damage point was determined. Warning signals, low voltage impulses, impedance and frequency response analysis were examined and mechanical damage, damage detection and auxiliary isolation control were provided. Induction voltage test is examined and auxiliary isolation control is provided. The winding resistance measurements ensure that a proper connection is made. If there is an open-circuit condition or a high-resistance connection in parallel conductor windings, the detection is provided. This test is applied to determine the mechanical movement of the windings as a result of the low-voltage impulse test, short-circuittest. Partial discharge with induced voltage test is determined in isolation. With the high potential test, the basic pressures of the major insulation are determined. As seen in Fig. 2 in the power transformer windings in transformer centers, the transformer coils result in a short circuit due to the tension and radial forces.
White ash, burning housing ashes and solid remains from the burnt bushing are given in Fig. 3. The internal inspection will determine the fault location as guaranteed and determine the damage size. A low oil level causes contamination to spread, the core and the suffer into the core and the coils. If there is contamination or dirt on the surface of the oil, this pollution must be disposed of. In order to show this situation transformer tank fault is given in Fig. 4.   It has been determined that 45% of the failures in the Erdemli District, power substations are winding failures. These malfunctions; It has been understood that there is short circuit between the windings, short circuit between the windings, winding breaks, winding combustion and cooling systems failures. In the examinations made, it is understood that the reason for this malfunction is due to the fact that power transformers are used in excessive load. At the same time, it has been understood that transformers are not protected against lightning and other atmospheric external effects, there are production defects in the windings used, and deteriorations in the insulating materials used due to excessive moisture.

ANALYSIS AND PREVENTION OF FAULTS IN TRANSFORMER CENTERS OF ERDEMLI DISTRICT
In order to prevent malfunctions, healthy comments should be made. discharges are from the tip of the strands to the grounded parts of the transformer. If the loose strands are partially moist, they will form a bridge between the energized parts and the grounded parts and cause a puncture. This condition will be reduced to minimum when fixed by impregnating the filaments with resin. The wind dust and the humidity enter into the transformer during the observation and maintenance, they causes the dry oil to get dirty and to get damp. The strands in the windings absorb from dust and damp oil. As a result, it bridges the electric field and causes the oil to be perforated, and also causes the oil to oxidize.

CONCLUSIONS
When power transformers fail, they can be cause large production losses and large material losses in enterprises using the electrical energy. The power transformer failures may ocur due to incorrect design, manufacturing defects, poor quality of materials, abnormal operating conditions, unsuitable maintenance and testing techniques, improper operation and non-