Power transformer health monitoring: A shift from off-line to on-line detection

Transformers are an important part of the electrical power system network, therefore, its fault detection is vital. Off-line methods are commonly used for their fault detection. These methods have associated costs derived from the necessity of taking the transformer out of service. The application of on-line methods reduces the expected costs and the possibility of unpredicted failures. In the present study, off-line and on-line methods are applied to the detection of short circuits in transformers, demonstrating the possibility of moving from off-line to on-line methods. Short circuits between sections in a transformer winding, between winding and core and between windings have been considered. PSPICE software is used to simulate the transformer for both detection methods. A comparison of the fault indication in both techniques proves the possibility of moving from off-line to on-line method. Sweep Frequency Response Analysis (SFRA) is considered an accurate technique for off-line tests. The changes in the windings inductance and capacitance affect the number of poles in the system response, so the number of poles will indicate the number of healthy sections. For on-line method, measurement of primary/secondary voltage/current is used to determine the measurable values which would result from a range of internal short circuit faults. From the simulated results, it is found that primary current can be used as the main indicator for the primary winding short circuits; a combination of secondary voltage and primary current is found to be useful for detecting secondary winding faults. Secondary winding voltage can be the main indicator for the cross windings short circuits.