Fault diagnosis and analysis of a 110 kV transformer

This paper aims to provide valuable insights and assistance for ensuring the safe operation of a 110 kV main transformer by conducting an in-depth analysis of tap changer breather oil leakage defects. By employing transformer testing, disassembly analysis, and other methods, this study successfully identifies and prevents the possibility of transformer failure. The analysis reveals that the deformation of the transformer winding is caused by insufficient short-circuit resistance, which leads to the deformation of the medium-voltage windings under the continuous action of electrodynamic forces during near-area short-circuit currents. Near-area short-circuits are a significant contributing factor to transformer failures. Therefore, it is recommended to enhance operation and maintenance management for older main transformers and optimize protection settings to reduce the risk of near-area short-circuit failures, thus ensuring the safe and stable operation of the main transformer.


Introduction
Power transformer is one of the core equipment in the power grid, its safe operation of the power grid safety and stability and power supply can play a vital role.Power transformer in the operation process is often subject to a variety of external fault current impact, especially the transformer low voltage side of the near-area short-circuit impact has become an important cause of transformer damage, and fault jumping [1].As the grid system short-circuit current is increasing and the old main transformer short-circuit resistance is insufficient, other factors that are part of the main transformer cannot meet the system short-circuit fault impact requirements [2].When the main transformer suffers near-area short-circuit fault impact, there will be serious winding deformation which causes the main transformer to occur winding turn-to-turn between the cake insulation breakdown and other sudden insulation failures [3], which is a serious threat to the safe and stable operation of the power system.
Through the in-depth analysis of a 110 kV main transformer tap changer breather oil leakage defects, this paper successfully found its transformer winding deformation case, ensuring the safe operation of the transformer to provide some reference and help.Through the transformer test and disassembly analysis of two methods, this paper found, prevented the possibility of transformer failure in advance and put forward targeted preventive measures.After multi-dimensional analysis, it is concluded that the transformer winding deformation is because the transformer short-circuit resistance is insufficient, resulting in medium-voltage windings in the near-area short-circuit current under the continuous action of the electrodynamic force deformation.Near-area short-circuit is an important cause of transformer failure, the old main transformer should strengthen the operation and maintenance management, and optimize the protection value setting to reduce the risk of near-area 2 short-circuit failure and to ensure the safe and stable operation of the main transformer.

Fault process
The 110 kV substation #1 suffered from short-circuit current on January 6 and December 13, 2020, respectively, and was put into operation again on December 15 after finding and remedying faults.On the 16th, operators found that there was oil leakage at the respirator of the #1 main transformer switch, and the oil had been filled with the respirator and dripped continuously, which caused the oil level to approach the lower limit.After the power was cut off, the operators sealed the switch.On January 8, 2021, the personnel inspected the tap changer and found that there were cracks in the switch oil drum, and the switch oil drum was replaced on January 29, 2021.After February 1st, the high-voltage test was carried out on the main transformer.The two withstand voltage tests showed that the acetylene content in the oil was higher than that before.Then, the winding deformation test of the main transformer was conducted on February 23, and it was found that there was obvious deformation in the medium voltage winding of the main transformer.The main transformer on-load tap-changer breather and body oil level are shown in Figure 1.

Transformer nameplate parameters
The main transformer used in the paper is an oil-immersed power transformer, which was manufactured in 2004 and put into operation in 2005.The main nameplate parameters are shown in Table 1 below.

Oil chromatographic test
Acetylene was found in the main transformer in the pre-oil chromatographic test.On the same day, the oil chromatographic data of the main transformer were rechecked, and the values were all 0.29μL/L, which did not exceed the acetylene attention value of 5μL/L in the operation of the main transformer.The recent oil chromatographic test results are shown in Table 2 below.After two withstand voltage tests, the acetylene value in the oil chromatographic test data has increased.According to the characteristic gas method, it can be seen from the table that the main characteristic gases in the oil samples at the bottom and middle of the main transformer are H2 and C2H2, and C2H2 exceeds the reference attention value before putting into operation, so it can be preliminarily concluded that partial discharge may exist.Compared with the data at the bottom of the transformer, the concentration of each component is higher in the middle of the transformer, so it can be presumed that the fault may be in the upper and middle parts of the transformer.In summary, there may be partial discharge in the middle of the transfer.

Low-voltage short-circuit impedance test
In the research, this transformer was tested for low-voltage short-circuit impedance.The low-voltage impedance test data of high-to-medium voltage winding, high-to-low voltage winding, and medium-to-low voltage winding are listed in the table 3 below.The low-voltage short-circuit impedance test shows that, except for the high-to-low voltage winding, the results of high-to-middle voltage winding and medium-to-low voltage winding under low voltage do not meet the requirement that the maximum relative difference of three single-phase parameters of power transformer windings with a capacity of 100 MVA or less and a voltage of 220 kV shall not be greater than 2.5% mentioned in DL/T1093-2018: Guide for Reactance Method to Detect and Diagnose of Winding Deformation Detection of Power Transformer.As shown in Figure 1, in the mid-frequency band 100 kHz to 600 kHz, the resonance point of the medium voltage phase Am moves forward compared with the other two phases of the medium voltage.At 600 kHz, the amplitude of the resonance peak of the Am phase is abnormal, which indicates that there are some local deformation phenomena such as tilt, collapse, and warpage in the Am phase.

Winding deformation test
As can be seen from Figure 2, at 186 kHz, the number of resonance peaks of the low-voltage phase c decreases, the capacitance characteristic changes to the inductance characteristic, and the low-voltage phase c may have undergone warpage deformation.Figure 3 presents that in the mid-frequency band, the capacitance between the windings of phase A and the other phases changes to some extent, indicating that the relative position between the medium-voltage phase Am and the high-voltage phase A has changed as well.As can be seen from Figure 4, through comparative analysis of the winding deformation test and low voltage impedance test results, it is considered that the medium voltage Am phase of the #1 main transformer in the 110 kV substation has been seriously deformed, and the low voltage phase c may undergo deformation.The winding deformation condition needs to be further investigated after returning to factory service.

Disassembly
On-site disassembly found that the high-voltage winding phase A is a whole tilt situation, phase B and phase C winding are normal, as shown in Figure 5. Medium voltage winding Am phase overall is obvious deformation.There are two obvious bulging deformation phenomena.Bm phase and Cm phase winding are normal, as shown in Figure 6.Low-voltage winding A-phase as a whole is not deformed, but under the action of the deformation of the medium-voltage winding Am-phase, the low-voltage a-phase support strip is caused to be twisted.Low-voltage winding b-phase and c-phase are normal, as shown in Figure 7.Over the fault transformer dismantling inspection, it is found that the medium-voltage winding Am phase overall tilt, winding obvious deformation and low-voltage winding a phase outside the bracing strip have been distorted, other phase winding is intact.The results of dismantling and low voltage impedance test and frequency response test are the same.

Working conditions in the past (1) Operation and Maintenance
One week before the accident, no abnormalities occurred during inspections, the maintenance status was normal and the oil chromatography tests were analyzed with no problems.
(2) Impact Situations in the Past Recently, phase B and phase C of a 35 kV line in the substation were short-circuited and developed into ABC three-phase short circuit, with a maximum short-circuit current of 7022 A. After the fault, the related winding deformation and oil chromatography test were performed, and no abnormality was

Cause analysis
(1) It is preliminarily considered that the reason may be that rivets in the oil chamber of the on-load tap-changer of the main transformer are loose and the seal has failed due to factors such as mechanical vibration, environmental impact, and material aging during long-term operation.The oil tank of the main transformer is connected to the oil chamber of the tap changer.Because the oil level of the main transformer, that is, the oil conservator is higher than the tap changer, the pressure makes the oil leak outward from the tap changer respirator.
(2) According to the winding deformation and low-voltage impedance test data, it can be preliminarily inferred that the medium-voltage A-phase winding of the main transformer is significantly deformed because it suffered a short-circuit impact in the near area.The latest shock happened on January 6, 2020, and the maximum value of ABC three-phase short-circuit current is about 7022 A. After the withstand voltage test, the position where acetylene is produced in the oil is not clear, so it is necessary to make further inspection of the transformer after returning to the factory.
(3) Since the transformer is short-circuited in the nearby area, excessive current will flow in the A-phase winding, and then, a leakage magnetic field will be generated.Under the action of the magnetic field, the electromotive force will be produced.The winding bears radial force, the low-voltage winding is unstable under pressure, and the mechanical strength of the transformer becomes weak.Therefore, the thermal power generated by huge short-circuit currents may directly damage the solid insulation of transformer wires or may reduce the mechanical strength of wires, leading to the destruction of thermal power and electromotive force.

Suggested measures
This paper focuses on the fault analysis of a 110 kV transformer whose on-load switch leaks oil and fails to pass the test after overhaul.The possibility and suspicious faults of the main transformer are verified by routine tests, and then, the following suggestions are put forward for transformer operation and maintenance.
(2) After transformers suffer from short circuit faults in the near area, windings may deform under the action of electrodynamic force, which seriously affects the safe operation of transformers.Therefore, transformers should be inspected as soon as possible, and different treatment methods should be applied accordingly [5].
(3) It is recommended to strengthen the verification of short-circuit resistance of transformers of 110 kV and above in combination with the operating life of transformers.Old transformers with an operating life of more than 15 years and the same batch of products from similar manufacturers with faults caused by insufficient short circuit resistance are supposed to be focused [6].
(4) In transformer operation and maintenance, we should actively explore new science and technology, strengthen online monitoring of transformer health, and discover hidden faults in time.
(5) When a transformer is tripped by a short circuit impact, the winding deformation evaluation should be carried out according to the standard requirements [7].
(6) For an old transformer that has been running for a long time, when it has not tripped after short-circuit impact, but the current impact has reached more than 65% of its tolerance (allowable) short-circuit current, the winding deformation assessment should also be carried out to dynamically grasp the winding deformation so that timely measures can be taken to prevent accidents [8] [9] (7) The main factors affecting the repeatability of the frequency response test are the tap position of a transformer, the connection of non-test winding, temperature and humidity, signal injection point, wiring technology, etc.Therefore, the test instruments, signal injection points, and wiring technology before and after the fault should be consistent as far as possible.
(8) In the field test, it is advisable to adopt two or more methods for assessment to improve the accuracy of the evaluation results [10][11] (9) When the low-voltage impedance method is used for assessment, three-phase measurement, and single-phase measurement shall be adopted respectively, which is convenient for longitudinal and transverse comparison [12] [13] (10) When the frequency response method is used for evaluation, not only should the correlation coefficient be considered, but the changes in the frequency response curves at different positions, as well as the changes in winding capacitance and inductance properties should be analyzed.The axial and radial deformation trends of windings can be evaluated according to left-right movement amplitudes of frequency response curves and the change of resonance peaks and valleys [14][15][16]

Conclusion
This paper utilizes experimental testing and analysis methods to effectively analyze and prevent transformer faults caused by inadequate short-circuit resistance and winding deformation based on field operation and maintenance defects.Moreover, this study proposes a set of comprehensive management measures, considering factors such as near-area short-circuits, operating age, condition monitoring, and testing technology to ensure the safe, stable, reliable, and cost-effective operation of the transformer.These measures provide valuable insights for reference in transformer management.

Figure 1 .
Figure 1.Mains tap changer breather and body oil level.
A winding deformation test was conducted on the main transformer.The test charts of medium voltage winding, low voltage winding, and high-to-medium voltage winding are shown in Figure 1, Figure 2, and Figure 3 respectively.

Table 2 .
Oil chromatography test results.

Table 3 .
Low-voltage short-circuit impedance test results.