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System for mechanical condition assessment of power transformers using low voltage impulse method

– IJT 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 at the station.

The classical defectoscopy methods 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 diagnostic method (IJT) consists in applying voltage pulses of known characteristics to one of the terminals of the investigated transformer and in recording the response signals from the other terminals of the investigated transformer, analyzing the mentioned signals, respectively interpreting the obtained results.

NOVA-IJT is based on the low voltage pulse diagnosis (IJT) method and consists of: the voltage pulse generator, the digital analog oscilloscope and the “laptop” PC computer.

The connections of the devices to the windings of the investigated transformer are in accordance with the type of measurement of the response signals: direct or differential.

The generator produces rectangular pulses with a rise time of 200 ns, adjustable duration in the range of 1 – 15 microseconds, amplitude 500 Vv or 850 Vv on an equivalent output load consisting of a resistance of 75 ohms in parallel with a maximum capacity of 25 nF, repetition frequency 50 Hz.

The digital analog oscilloscope has an analog frequency range between 0 and 100 MHz, sampling rate 40 MS / s, own storage capacity 2 kB for each channel, interfaces for coupling to your own printer or to an XT / AT compatible computer.

The computer is a “laptop” Pentium 4 1000 Mb, 256 MB RAM, 40 GB HDD, color SVGA.

The software part of this new equipment consists of a software package that ensures the user interface, storing the measurement results, processing these results (by overlapping the results obtained in the same time period or in different time periods and by determining the quantitative differences between them). ), the graphic presentation of the results and of the test bulletin.

IJT - method