Detection of winding inter-turn faults – Part III
An inter-turn fault is one of the leading causes of power transformer failures; it can be detected using sweep frequency response analysis
1. Case studies – Detection of fault in layer windings using SFRA
Frequency response analysis can be extended
to single and multi-layer windings. At
the same time, the fault factor characteristics
are obtained for different fault percentages.
1.1. Detection of fault in single-layer winding – measurement results
Sweep frequency response analysis (SFRA) is performed on a single-layer 1000-turn winding from 10 Hz to 25 MHz. This winding has 20 sections where each section consists of 50 turns. Tappings were available at each section end in order to create inter-layer fault. For instance, 5 % fault is created by shortening one section, which corresponds to a short circuit of 50 turns. Fig. 1a and 1b show single-layer winding and a schematic diagram of single-layer winding.
Fig. 2 shows the measured impedance characteristics for healthy winding and 5 %, 10 % and 20 % faults at different locations along the winding. The first resonant frequency of the healthy winding is frh1= 40.738 kHz. The impedance characteristic shows a reduction in the impedance at this frequency depending on the location of the fault.
Sweep frequency response analysis is performed from 10 Hz to 25 MHz
Fig. 3 shows the fault factor characteristics for 5 %, 10 % and 20 % of inter-turn fault in the winding. The curve shifts upwards as the percentage of fault decreases. This curve is symmetrical with respect to the centre of the winding section and provides reliable information about fault detection.
1.2. Detection of fault in multi-layer winding – measurement results
SFRA is performed on the multi-layer windings with 10 layers from 10 Hz to 25 MHz. Each layer consists of 200 turns. Tappings were brought out at every section in order to create inter-layer short circuit faults. For instance, 10 % interlayer fault is created by shortening 2 consecutive layers in a multi-layer winding. Fig. 4a and 4b show the 10 section multi-layer winding and schematic diagram of the 10 section multi-layer winding.
The sweep frequency response of the winding changes depends on the extent of the fault. Fig. 5 shows the impedance characteristics for healthy and 10 %, 20 % and 30 % inter-layer faults along the first section in the winding.