System for evaluating the mechanical state of the windings of power transformers by the method of frequency response analysis
– ARF method –
One of the most common failure regimes in the power system is the short circuit regime when the power transformers (along with other equipment) are traversed during the short circuit current failure and are subjected to mechanical stresses created by these overcurrents. Transformers can be damaged due to short-circuit stresses. Problems can occur especially with high power transformers as their short circuit performance is rarely demonstrated by testing. Even though the technology that ensures the resistance of transformers to short circuit has progressed a lot, the problem remains, especially for transformers whose operation time is longer. A significant weakening of the windings can occur due to the thermal aging of the insulation, which determines the reduction of the pressing forces and the short-circuit resistance capacity.
Defects caused by external short circuits are very destructive, manifesting themselves in the form of winding deformation or very often by short circuits between turns. The most common occasion of these short circuits in the system is the ground phase defect produced as a result of direct lightning strikes. In such cases the transformer can be disconnected from the system as a result of the operation of its protections, being necessary the evaluation of the real state and the establishment of the decision regarding the possibility of its reconnection to the system. Visual inspections are very expensive and very often inconclusive. In addition, in the vast majority of cases these inspections are difficult to perform in the station. The classical methods of defectoscopy specified in the operating instructions are insensitive for detecting the deformation of the windings of the transformers in operation.
Methods based on the application of low voltage pulses (IJT) and frequency response analysis (ARF) respectively are recognized as much more sensitive.
The two defectoscopy methods consist of the application of low voltage pulses and respectively the analysis of the frequency response is based on the modification of the inductors and the local capacities of the winding structure, determined by the deformation of the coils or other defects. The values of inductors and capacitances in this equivalent circuit depend on the properties of the materials and the geometry of the windings. They change due to deformation or other defects caused by short circuit loads.
The method of frequency response analysis (ARF) consists in applying a signal of sinusoidal shape of variable frequency to one of the transformer terminals, recording the response signals at the other terminals, processing and interpreting the results.
The NOVA ARF diagnostic system uses the diagnostic method based on frequency response analysis (ARF) and consists of: computer-controlled sinusoidal signal generator with frequency synthesis, power amplifier, a computer-controlled signal analyzer and a type computer “laptop”.
The generator provides a sinusoidal signal with a value of 1 V on a load impedance of 75 ohms, for a frequency range between 20 kHz to 1 MHz. The frequency step, the number of steps and the frequency band limits are controlled by the computer. The minimum step can be 1 kHz and the frequency band scan can be done linearly (equal steps).
The signal analyzer is basically a digital electronic voltmeter of alternating current, connected to the computer. It ensures the synchronous quadrature detection of the measured signals, thus minimizing the influence of electromagnetic disturbances. An interface ensures the connection of the voltmeter to the computer and controls the attenuation level of the measured signals.
The operation of the NOVA ARF system is completely dependent on the computer. The specific program package ensures the intimate control of the equipment to be investigated, the functional sequencing of the various blocks, the processing of the data from the measurements, the graphic presentation of the results and of the test bulletin. The user interface allows viewing and programming commands.