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This article presents a survey on failures, monitoring, and new trends of power transformers. There are three main types of power transformers, namely, oil immersed, gas-insulated, and dry-type transformers with or without cast coil insulation system. Operating stresses of power transformers have increased due to the load growth and the increased bulk power transactions, where recent and imminent alternating current (AC) systems are rated 1,100 kV and 1,200 kV, respectively. Failures of windings, onload tap changers (OLTC), and bushings are the main defective components as they represent about 84% of the failure statistics. Online and offline diagnostic monitoring of power transformers can be used to detect faults at an early stage, prevent degeneration into catastrophic phenomena, and monitor the aging process of the insulating systems. Many of the well-known preventive maintenance techniques are discussed. New trends of power transformers have recently taken many steps forward in different dimensions, e.g., Powerformer, Dryformer and Windformer utilizing high-voltage (HV) cross-linked polyethylene (XLPE) cables, gas-insulated transformers (GIT), converter transformers rated ±800 kV, and high-temperature super conducting (HTS) transformers. The latter type represents a key technology for future power systems engineering as they offer many advantages over the others.