Ljubiša Čičkarić

 Thermography is a method of early diagnosis of irregular thermal conditions of power transformers. The power transformer is a complex power facility, which consists of various circuits and subsystems, which are monitored by thermography during normal operation. The methodology focuses on several critical points: lead-in (through) isolators, cooling system, transformer tank and tap changer. Defects on the lead-in (pass-through) insulators are the most common causes of failure of power transformers. They arise due to the appearance of poor connection joints and insulation defects due to moisture penetration or low oil level (for the oil type). The cooling efficiency directly determines the lifetime of the transformer insulation. The most common causes of worsening cooling are clogged radiators and pipe systems, inadequate fan operation, overheating of oil pump bearings, etc. For power transformers, temperature measurement alone is not enough to conclude about the state of internal defects. It is necessary to observe the thermal profile of the entire object. A thermal map of a transformer tank can reveal complex internal problems. The appearance of local overheating (“hot spots”) on the tank walls may indicate problems with the magnetic core or the appearance of eddy currents due to stray fluxes. The tap changer (OLTC- On-Load Tap Changer) is a device for regulating the output voltage of the power transformer under load, in order to stabilize the voltage in the transmission or distribution network. Overheating of the tap changer transmission mechanism often indicates poor contacts or excessive switching frequency, which causes excessive sparking. In general, thermography reveals failures due to poor contacts in the energy circuit and defects caused by moisture penetration and contamination, aging of the insulation system, changes in the geometry of the magnetic core and windings, and more. Paper presents examples of control of external connections of the transformer, inside the bushings, then, the iron core of the transformer, the transformer tank, assemblies and supporting equipment on the transformer, etc. The results of thermography are integrated into a complex analysis in order to reduce the risk of exploitation, which, in addition to technical diagnostics, includes reliability theory and economic analysis. The goal is to move from traditional preventive maintenance (according to the calendar) to condition-based maintenance (CBM) and risk-based maintenance (RBM). Based on the results of diagnostic tests, monitoring and work history, a transformer health index is created. The risk assessment depends on the probability of failure and the consequences that follow after failure. The result of the analysis is the creation of a risk matrix that provides recommendations for specific activities. This approach has a direct effect on increasing the reliability of the electric power network, reducing the number of unplanned power interruptions and optimizing maintenance costs.

Keywords: Thermography, Power transformer, Thermal diagnostics, Health index, Risk matrix