Transformer is the core equipment in grid transmission system. Therefore, safe operation of transformer plays a vital role in the grid safety. Technical Engineers of HUAYI Power explore the causes of the transformer winding deformation and the resulting hazards by analyzing the common fault winding deformation of the transformers. It is significant for the whole grid security system’s normal operation.
Our Engineers proposed that when the transformer is subjected to short-circuit current or other impact, the deformation is as follows:
The deformation of winding is caused by external forces such as impact, inclination and vibration during transportation, which lead to winding displacement. This deformed winding has the same size, but only the relative displacement to the core changed. The inductance of the winding, the inter-cake capacitance are constant, and the capacitance changes. General capacitance is reduced. In the equivalent circuit, the resonance peak shifts toward the high frequency. So compared with former spectrums, every resonance point still exists in measured spectrum, and no change occurs, but the peaks shift to the high frequency (to the right).
Partial deformation between cakes, some unfixed thread cake is squeezed under the action of the short-circuit electromagnetic force, and the others are elongated, (directly connected to the two poles of the power, and no current in electrical appliances) so that the capacitance between the cakes is changed. The deformation causes some inductances in the equivalent circuit to become larger and some become smaller; the capacitance between cakes which are in parallel with the inductance also changes. When measuring the spectrum, some resonance peak moves toward the high frequency, and the peak value decreases; some resonance point moves toward the low frequency, and the peak point rises. The deformation area and deformation degree between the cakes are judged by the change of the resonance peak.
Short circuit between the turns (directly connected to the two poles of the power, and no current in electrical appliances),theoretically speaking, after the winding occurs short circuit between the turns, the inductance value decreases, the spectrum curve changes significantly, the amplitude rises, and some resonance point peaks disappear. But this is theory, actually it is difficult to get it. Once the inter-turn short circuit occurs during operation, the coil will be burn out, the heavy gas will trip, and the pressure relief valve will start. At this time, the oil chromatography of the transformer (pressure variation) will be unqualified, and the transformer will be checked.
Lead’s displacement and deformation, because the lead is very long, when it is not firmly fixed, displacement deformation occurs during operation. When the lead is displaced, two-port capacitance will change in equivalent circuit. When the signal at the entrance of the signal is displaced, but the lead capacitance is in parallel with other circuits, so its chan0does not effect significantly to the spectral curve. However, the output lead displacement occurs, the change of the lead capacitance will significantly change the frequency response curve, especially for the range of 300 kHz to 1 MHz curves. Therefore, in the actual test, the source is injected with a neutral point to prevent the above effects. If the lead-to-ground capacitance is reduced, the amplitude in the frequency band rises, and vice versa. If the lead-to-ground capacitance becomes larger, it indicates that the lead moves toward the shell, and the lead-to-ground capacitance becomes smaller, it indicates that the lead moves toward the winding.
The winding gets radial deformation, when the winding is subjected to the radial force, inner winding is contracted inward, both diameter and inductance become smaller. At this time, the distance between the inner and outer windings becomes larger, and the capacitance becomes smaller, which will cause the resonance peak point in the spectrum to move toward the high frequency, and the amplitude increase.
The winding twists and deforms axially. When the coil space between transformers (pressure variation) is large or some of them are displaced, with the action of electromagnetic force, the winding is twisted into an S shape axially. At this time, part of the cake capacitance and ground capacitance are reduced. In the measured spectrum, some of the resonance peaks move toward the high frequency direction, and the amplitude of the resonance peak decreases in the low frequency range, the peak of the middle frequency rises slightly, and the high frequency range does not change.
Base on the measurement of internal winding characteristic parameters of transformer, Huayi Power develops RBX-H transformer winding deformation tester, and adopts internal fault frequency response analysis (FRA) method which is explored by developed countries, it can make accurate judgment on the internal fault of the transformer. The device is to quantify the transfers’ internal winding parameters with different frequency responds change, and determine the internal winding of the transformer according to the magnitude of the variation, the amplitude of the frequency response, the change of the region and the frequency response change, according to measuring result, it can be determined that whether the transformer is severely damaged and whether maintenance is required. For the running transformer, no matter whether the frequency domain characteristic map is saved before, the degree of fault can be judged by comparing the difference of the characteristic map between the coils of the faulty transformers. Of course, if the original winding characteristic map of the transformer is saved, it is easier to provide more accurate basis for the operation, post-accident analysis and maintenance of transformers.
Features of Huayi power transformer winding deformation tester
The hardware movement adopts DDS digital high-speed sweep- frequency technique to accurately diagnose faults, such as twisting, bulging, displacement, tilting, turn-to-turn short- circuit deformation and phase-to-phase contact short-circuit.
Acquisition control uses high-integration microprocessor, selects components with precision and high-stability, high- speed dual-channel 16-bit A/D sampling (field test alters tap- changer, waveform curve changes obviously), repeat test for the same phase, the repetitive rate of measurement is above 99.5%.
The instrument has two measuring system, linear sweep frequency measurement and segmented sweep frequency measurement, which is compatible with the domestic two technical genre measurement modes at present.
Only the connecting bus of the transformer needs to be removed in testing process, and all tests are completed without lifting the cover and disassembling the transformer.
When measuring the transformer, wiring personnel can randomly arrange the signals to input and output leads, which has no effect on measurement results. The wiring personnel can stay on the transformer tank to have a break, so as to reduce the labor intensity.
The amplitude-frequency characteristics conform to the national technical indicators of the amplitude-frequency characteristic tester. The abscissa (frequency) is composed of linear indexing and logarithmic indexing. Therefore, the printed curve can be a linear indexing curve or a logarithmic indexing curve, and the users can select according to their needs.
The instrument is highly intelligent, and the scale of signal output amplitude is automatically adjusted by software. The maximum amplitude is ±10V, and the sampling frequency is automatically adjusted.
The instrument has the function to measure various linear sweep- frequency systems, linear sweep measurement scan frequency up to 2MHz, frequency scan intervals are 0.25kHz, 0.5kHz and 1kHz, so that provides more analysis for transformer deformation.
Provide historical curve comparison analysis, and load multiple historical curve observations at the same time, it can select arbitrary curves and do horizontal and vertical analysis. Equipped with expert intelligent analysis and diagnosis system, it can automatically diagnose the state of transformer winding, load 6 curves at the same time, automatically calculate the relevant parameters of each curve, automatically diagnose the deformation of the winding, and give the reference conclusion of diagnosis.
The software management function is very powerful. The product design engineers of Huayi Power fully consider the needs of field service, the software automatically saves environmental condition parameters to provide basis for transformer winding deformation diagnosis. The measurement data is automatically analyzed and saved, the electronic document (Word) is saved, and the color printing function is provided, so that the user can output the test report.
Measurement and Use Method
The usual method of detecting transformer is as above. Huayi Power transformer winding deformation tester is mainly composed of a main measuring unit and a notebook computer, and also there are three special measuring cables, measuring clip and grounding wire. The main measuring unit system and the test sample are connected by 50 high frequency coaxial cable, sweep frequency signal goes through the output port (excitation output), and the signal clip (yellow) injects signal into the tested object by connecting cable; the signal is taken from the tested object by signal clip (green), and transmitted by cables (response input); the synchronous reference signal is obtained from the tested object injection point by the signal measurement, and transmitted to the input (reference input) via cables. The shield of the tested case and the testing cable must be reliably connected and grounded. The wire and fuel tank of large transformer are usually connected by iron-core earthing bushing, as common ground point, transformer case is grounded.
Judging principle
1. Waveform comparison
Normally operating transformer windings are well correlated with three-phase characteristic. If an accident does not cause winding deformation, the curves before and after the accident basically coincide.
After the winding is deformed, the curves before and after the accident obviously deviate and do not coincide, and the correlation is poor. During the deformation, the peak value of the curve in the lower frequency band of 0.5-200kHz will be shifted, the frequency is added or reduced. The number of decibels corresponding to the peak value will also change, and the number of peak points will generally decrease.
2. Judging theory
Compared with the frequency response curve of transformer before accident, it requires the transformer to have the original frequency response curve data.
Compare with products of the same model and manufacturers at the same time.
Phase-to-phase comparisons are possible for three-phase or single-phase transformers.
The judgment is based on two characteristic values: mean square error Exy and the correlation coefficient Pxy, the smaller the Exy is, the closer the better for two curves. The closer Pxy is to 1, the higher the similarity between the two curves. If two or more frequency bands have Pxy<0.98 and Exy>3.0; or Pxy<0.9 and Exy>1.5, or Exy>4.5 and Pxy is only 0.99, it will cause deformation for the transformer. Combined with other methods, such as "oil chromatographic analysis", "winding DC resistance measurement" "short-circuit impedance" "no-load test" and other methods to comprehensively judge whether the transformer winding is deformed.
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