66kv Transformer Fault Analysis

66kv transformer 1

For a 66kV transformer secondary winding deformation fault case, we elaborated and analyzed the whole process of 66 kv transformer from oil chromatography routine test finding abnormality to tracking and monitoring, then to power outage diagnostic test, and finally to return to the plant to locate the fault point by lifting the cover. Combining the results of high-voltage tests such as oil chromatography analysis, winding deformation and dielectric loss tests, the causes of the 66 kv transformer fault occurrence are analyzed.

A regional 66kV substation #2 main 66kv transformer, model SZ8-31500/63, rated voltage (63±106×1.25%)/10.5kV, the 66kv transformer was shipped in 1993 and returned to the factory for overhaul in 2010, and was put into use after the repair test without problems.

On March 16, 2021, the tester found that the acetylene value in the oil sample of the 66kv transformer was 3.8μL/L, which was close to the value of attention, during the routine monitoring of the oil chromatography of the 66kv transformer. The combined oil chromatography history data of the transformer and the three-ratio method determined that a high-energy discharge had occurred inside the 66kv transformer during its operation from August 21, 2020 to March 16, 2021. Since the substation is a high-load substation, the #2 66 kv transformer was temporarily put into operation after comprehensive consideration, and the oil samples were tracked and tested by oil chromatography for 8 days, and it was found that the acetylene content and total hydrocarbon content showed a slow growth trend. The detailed data of 12 oil chromatography tests of this 66 kv transformer are shown in Table 1. The testers conducted a troubleshooting test on the 66 kv transformer.

Table of Contents

Failure Analysis

66 kv transformer2

Oil Chromatography Analysis

The tracking data for two weeks is shown in Table 1. From the oil chromatography data, it can be seen that the acetylene and total hydrocarbon content of the 66 kv transformer is slowly increasing, not rapidly increasing, which indicates that the overall insulation of the 66 kv transformer is not seriously damaged and internal discharge has occurred. The amount of acetylene exceeded the standard, and the three ratios were coded as 1 2 1. Combined with the non-coded ratio method, it is known that the 66 kv transformer has a discharge and medium temperature overheating (300℃~700℃) fault.

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Winding deformation test

Winding deformation by frequency response method

The winding deformation test of #2 66 kv transformer is conducted by frequency response method, and the winding deformation frequency response pattern of this 66 kv transformer is shown in Fig. 1, the 66 kv transformer coupling group is labeled as Yd11, the primary winding has no neutral point lead, the frequency response curve of the primary winding of this type of 66 kv transformer is not high in overlap. It is difficult to find out whether there is a fault from the primary winding three phase comparison.

66 kv transformer primary winding frequency response method winding deformation data cross-sectional comparison chart
66 kv transformer primary winding frequency response method winding deformation data cross-sectional comparison chart
66 kv transformer secondary winding frequency response method winding deformation data for horizontal comparison
66 kv transformer secondary winding frequency response method winding deformation data for horizontal comparison

The measured curves are compared with the frequency response curve of the handover test on November 10, 2010, as shown in Fig. 3 and Fig. 4.

In Figure 3, the green curve on November 10, 2010 and the red curve on March 23, 2021 are shown. In Fig. 4, the green curve is shown on Nov. 10, 2010, and the red curve is shown on Mar. 23, 2021.。

66 kv transformer primary winding frequency response method winding deformation data longitudinal comparison graph
66 kv transformer primary winding frequency response method winding deformation data longitudinal comparison graph

This indicates that the longitudinal capacitance of the primary winding (i.e., the relative capacitance of the primary winding to the other windings) has changed significantly.

The frequency response curve of the secondary winding has an obvious overall left shift compared with the frequency response curve of the three phases of handover a, b and c, which indicates that the inductance of the secondary winding has become larger.

66kv transformer secondary winding frequency response method winding deformation data longitudinal comparison chart
66kv transformer secondary winding frequency response method winding deformation data longitudinal comparison chart

Comprehensive analysis of the above winding deformation mapping shows that the secondary winding of the 66kv transformer should have undergone significant deformation, resulting in a larger secondary winding inductance, which in turn affects the relative capacitance of the primary winding to the secondary winding.

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Short-circuit impedance method winding deformation

Low voltage short-circuit impedance method for winding deformation test, the measured value of 8.73% under the rated tap (11 gears), compared with the 2010 handover test value of 8.44% increased by 3.41%, exceeding the standard ± 2% of the specified value, so the test item was determined to be unqualified. Combined with the frequency response curve, this 66kv transformer winding may have internal deformation.

Dissipation test

pad mounted transformer reactive power loss calculation formula

In addition to the excess capacity in the dielectric loss test, there is no abnormality in the other insulation tests, and the dielectric loss data of secondary/ground, secondary/primary and ground capacity vary greatly, reaching 17.16% and 10.30%, which is unqualified. The low-voltage winding as a whole may have deformation and displacement.

The 66 kv transformer secondary/ground capacity is 8942pF, compared with the capacity of 7632pF in 2010 handover test, the difference is 17.2%, more than the standard requirement of ± 5%.

Thus, it is judged that the insulation performance of the 66kv transformer has not changed, but the relative displacement between the secondary winding and the iron core may have occurred.

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Return to the plant to check the situation of the lifting cover

Inspection branch in a 66kv transformer plant to the 66 kv transformer product testing and disassembly inspection analysis.

The 66 kv transformer primary and secondary winding removal found: the 66 kv transformer primary winding is relatively intact, no obvious problems.

The inner side of the 66kv transformer winding was deformed
The inner side of the 66kv transformer winding was deformed

Secondary winding b, c two phases occurred a serious bulge, part of the cake line insulation off, as shown in Figure 5, Figure 6. This latent fault is a major hidden danger for the safe operation of the electric power 66kv transformer. Long-term operation will cause the mechanical properties of the winding to degrade and even lead to partial discharge and insulation breakdown.

66 kv transformer secondary winding hood (from left to right for phase a, b, c
66 kv transformer secondary winding hood (from left to right for phase a, b, c
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Analysis of fault causes

Comprehensive historical maintenance records, protection actions, distribution line fault points, hood inspection and other types of information, the cause of the failure is analyzed as follows.

The 66 kv transformer was produced in 1994 and was overhauled on site in 2010. After that, each routine overhaul and test results are qualified, not exceeding the overhaul cycle.

Returned to the factory crane disassembly situation, and on-site test data analysis results basically coincide.

The main reason for the failure is that the secondary winding of 66kV#2 66 kv transformer suffered from short-circuit current impact during operation (verified with the dispatcher that there were two 10kV line short-circuit trips on the 10kV II section bus of the substation between August 21, 2020 and March 16, 20221), and the huge electric force caused serious radial deformation of the secondary winding.

The relative position of the winding and the core changed, resulting in a change in the inductance of the winding and a change in the relative capacitance of the 66kv transformer secondary winding to other parts. At the same time of the impact, a short-time arc discharge occurred inside the 66kv transformer and acetylene was generated in the oil.

But this shock did not affect the overall insulation of the 66kv transformer, the internal overvoltage ended after the arc extinguished, so the acetylene content did not appear to continue to grow rapidly.

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Conclusion

(1) The 66 kv transformer fault occurred because the 66 kv transformer was short-circuited shock, the secondary winding deformation, the secondary / ground capacitance value changes; 66 kv transformer internal discharge, acetylene value has increased.

2) Active oil chromatography test. The abnormal acetylene value in the oil chromatography, combined with the diagnostic test items, judged that the 66kv transformer winding has been seriously deformed, the short-circuit shock resistance has been very weak, do not have the conditions to continue a long time in operation. The final 66 kv transformer timely decommissioning and replacement also avoided the occurrence of grid failure

(3) the operation of 66 kv transformer subject to short-circuit shock should be recorded in detail, accumulate data.

In addition, actively carry out 66 kv transformer winding deformation test work, timely detection of problematic 66 kv transformer, and planned to carry out lifting cover verification and overhaul.

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