Study On Online Monitoring Of Transformer Winding Deformation Using Current Deviation Method

The power transformer is the core of the power system and has been acentral issue of the electrical equipment monitoring. It not only has asignificant impact on the safe operation of the power system but produceshuge economic losses that the power transformer is out of service due to anunexpected failure. According to statistics, more than25%of failures arecaused by the transformer winding deformation, and traditional off-linetesting methods cannot detect early and sudden failure of the windings.Therefore the online real-time monitoring of the health of the transformerwindings is an essential way to achieve the condition-based maintenance oftransformer winding deformation and improve the economy of themaintenance of the transformer, and it is vital that the early warning of theslight winding deformation to prevent major accidents.The types of the winding deformation and comparison of the differentoffline methods and online methods for monitoring the winding are analyzedin the dissertation. Based on the analysis, the current deviation method ischosen to be an adequate method for online monitoring the windingdeformation. And the current deviation method is to inject a high-frequencylow-voltage sinusoidal signal between600kHz and1.2MHz to the windingand measure the high-frequency excited signal of the line-end and responsesignal of the neutral-end of the winding. With the obtained line-end andneutral-end currents, the current offsets and current deviation coefficients are calculated to detect the extent, type and location of the winding deformation.A ladder network equivalent circuit of the transformer winding is builtto simulate the winding deformation using current deviation method. Thesimulation results show that:1.In the axial deformation, the current deviationcoefficient is always positive and in the radial deformation it is alwaysnegative;2.The current deviation coefficient is just relative to the location ofthe winding deformation regardless of the extent of the deformation and inthe same section of the winding it changes in a very narrow range wheneverthe extent of the deformation;3. The current offset has a positive correlationwith the extent of deformation.4. The change of the bushing capacitance willnot interfere with the diagnosis of winding deformation. In addition, thecomplicated winding deformation, in which there are both axial and radialdeformation, and the impact of load to diagnosis of the winding deformationare analyzed in the simulation.According to the simulation results and consideration of the actualsituation, the apparatus applied for online monitoring of transformer windingdeformation using current deviation method is designed: the high-frequencylow-voltage signal source based on DDS technology and the power amplifierconsist of discrete transistor components constitute a signal generator unit; agas discharge tube and a varistor constitute the protection circuit, and anon-invasive capacitive sensor is used to inject the excitation signal to thewinding; the high-frequency current sensor is designed with the Mn-Znferrite material as the core, and the appropriate band-pass filter for signalacquisition de-noising is designed; FPGA is the core of the control system torealize the analog signal acquisition and digital filtering, and completecommunication with the host computer.At last, the offline experiments on a small single phase220V/110Vtransformer, in which an insulation stick is embedded in the winding toanalog deformation, are made. And the online tests on a three-phasetwo-winding35000V/400V transformer are made when the deformation isdesigned with the capacitances paralleled in the line-end of the winding and the results are compared with each other while there is load or not using thecurrent deviation method.