Transformer Winding Deformation Test

What is transformer winding deformation?

Transformer winding deformation refers to effect of mechanical or electromotive force on the internal coil windings, which causes irreversible changes in the size and position of the windings, including body displacement, winding distortion, bulging and short circuits between turns, etc.

What causes of transformer winding deformation?

Transformer is affected by the load and other factors during operation, and it is inevitable that it will be affected by a certain degree of current impact. Among them, the outlet short circuit is particularly harmful to the transformer windings. Although the circuit breaker can quickly cut off the circuit, when the device fails for some reason action,  transformer coil will deform in a short time under the action of short-circuit current and electromotive force. In severe cases, it will cause a short circuit between the coils and burn out the windings.

Deformation of transformer windings and damage by external force are mainly caused by winding distortion, deformation, and displacement caused by collision and impact during installation or transportation. For the former type of failure, we usually strengthen the maintenance and protection of relay protection devices. Check to ensure the timeliness and stability of the action. After the latter is transformed into the factory, before and after installation, use the transformer winding deformation tester to check its internal consistency.

Why test transformer winding deformation?

Winding deformation is a major hidden danger that affects the safe operation of the power system. In recent years, as the capacity of the power system has increased, the short-circuit capacity has also increased. The number of accidents that cause winding damage after an outlet short-circuit has also increased. After the transformer has a winding deformation,due to the change of the insulation distance or loss of the insulation paper, when the overvoltage is encountered, the winding will break down between the cakes or between the turns, or the insulation damage will gradually expand under the action of long-term voltage, which will eventually lead to insulation damage and risk safe operation.

Secondly, after winding is deformed, the mechanical performance decreases. When it suffers from a short-circuit fault, it will fail immediately because it cannot withstand the impact force. Conventional electrical tests such as insulation resistance measurement, transformer transformation ratio measurement, capacitance measurement, etc. are difficult The deformation of the winding is found, so it becomes very important to measure the deformation of the transformer winding.

Testing method of transformer winding deformation: low voltage pulse method, impedance method and frequency response method

Low voltage pulse method:It is to inject low-voltage pulses (100V) from the end of one coil and output from the other port, and judge according to the change of the waveform.

Impedance method: to judge point resistance change at a certain frequency

Frequency response method: the frequency sweep signal is sent to one port of the coil, and the response is output from the other port, and the frequency response is drawn as a curve according to the frequency for analysis.

The low voltage pulse method and the frequency response method are actually two different aspects of one same thing from the time domain and frequency domain. Mathematically speaking, these two methods are related and equivalent. From the actual implementation method, the two methods are technically very different, from the stability of the generated waveform, the recordability In terms of resolution and the current technical level, the implementability of the low-voltage pulse method is far less than the frequency response method. Therefore, the current deformation detection technology mainly adopts the frequency response method. The practical application of the frequency response method is gradually developed and matured with the development of microcomputer technology.

Introduction of frequency response method test equipment

The GDRZ-902 power transformer winding deformation tester is a powerful and sensitive method to evaluate the mechanical integrity of core, windings and clamping structures within power transformers by measuring their electrical transfer functions over a wide frequency range. SFRA is a proven method for frequency measurements.which can make accurate judgments on the internal faults of the transformer.

How to judge for transformer winding deformation?

Among the frequency response characteristics of transformer windings, the frequency response curve of the low frequency part (10～500kHz) has abundant resonance points. The changes of these resonance points sensitively reflect the deformation of transformer windings such as broken strands, bulging, twisting, and dislocation between cakes.  And the high frequency part (above 500 kHz) can reflect the displacement of the transformer winding. For the high frequency part of the transformer winding frequency curve of 110kV and above, due to many influencing factors, it is sometimes difficult to ensure that the part of the curve overlaps well. When making judgments, the middle and low frequency parts should be paid attention to, and the high frequency part should be used as a reference when necessary.

Diagnosis steps of transformer winding deformation

a) If the difference between the phases of the three-phase winding is greater than 3.5 dB, the test result should be compared with the original test result of the turn converter. If it is significantly larger (greater than 3.5 dB), it can be judged as Deformation of the winding has occurred.

b) If there is no original test result, it can be compared with the test results of the same type and same-type transformer of the same factory. If it is significantly larger (greater than 3.5 dB), it can be judged that the winding has been deformed.

c) If it is still not possible to compare, please ask the manufacturer to explain the reason for the inconsistency of the three-phase windings, and make judgments based on short-circuit and over-current conditions.

d) If the phase difference between the three-phase windings is less than 3.5 dB, but the difference is greater than 3.5 dB compared with the original test results of the transformer, the common part of the transformer windings is deformed, or the three-phase uniform deformation occurs.

After the transformer winding is deformed, it is also necessary to know the degree of deformation. According to the aforementioned definition, the degree of deformation is divided into three types: normal, moderately deformed, and severely deformed. The diagnostic value of attention is shown in the table