Detection of transformer windings failures with frequency response analysis method obtained with different signals
AGH University of Science and Technology, Dep. of Electrical
AGH University of Science and Technology, Dep. of Electrical Engineering and Electrical Power
Power transformers are subjected to detailed diagnostic investigations. Investigations include among others mechanical condition of windings. Detection of windings failures is made with the frequency response analysis method (FRA). As a base of this method are the frequency characteristics of transfer function of transformer winding obtained with sinusoidal signals, (Sweep Frequency Response Analysis) SFRA method. SFRA method is off-line method thus test transformer must be switched off, this procedure is inconvenient while switching operations for providing constantly electrical energy for customers should be made, and frequency of this kind of measurement is rare. Usage of signals occurring in power systems during normal work can extend usage of FRA method. This signals should be wide band signals like lightning surges or steep transients. Sinusoidal signals provide constant energy for specific frequency, measurements of frequency characteristics with SFRA method have good quality, and every frequency is stimulated with the same energy, limitations of this method are due to used signal generator. Energy frequency spectrum of wide band signal depends on maximal steepness, higher frequencies are energized with lower energy, therefore frequency characteristics obtained with different measurement signals can differ in some frequency bands, limitations of usage depends on voltage shapes. Usage of random steep voltages occurring in power systems can extend FRA method to on-line method and provide constantly information’s about mechanical condition of winding.
Aim of this article is to compare frequency characteristics of transformer windings obtained with sinusoidal signal SFRA and with steep voltage surge (Low Voltage Impulse Method) LVI. Measurements of characteristics are in configuration of healthy winding and in winding with simulated failures like short circuit between turns and winding displacement, differences in frequency characteristics are caused by winding failure. Comparison of results obtained with SFRA and LVI shows similarities in characteristics, usage of LVI for FRA method is possible. Steep voltage pulses can be used as a source voltages in on-line FRA method