# A Review of Health Assessment Techniques for Distribution Transformers in Smart Distribution Grids
## 2 Distribution Transformer Failure Modes  
Different investigations and test analyses have been conducted to identify the root causes and to identify the preventive measures to avoid the breakdown of power transformers. In paper [4], the part that is indicated as the most critical to power transformer health is insulation with an incident rate of about 41%; then, components showing high failure rates are windings, 14%, bushings, 10%, and on-load tap changers at about 10%. Other components such as the cooling system, core, and operational errors do not have a significant impact. In papers [5; 6; 7], the statistical data of component failures are collected from 350 power transformers to establish a three-level model of failure mechanism, failure linkages, and failure modes. Even though power transformer and distribution transformer have the same main working principles and key components such as insulation, windings, core, etc, these transformers differ in complexity and size. In this Section, the author will reference failure modes that pertain to these common working principles and components, investigate the reasons for distribution transformer failure from literature to identify the components that are most critical to distribution transformer health. Distribution transformers can be of two types: pole mounted and substation transformers. In each of the two cases, according to the norm EN 60076, different construction features can be identified. Some of these are more common for the first type, some for the second. In particular, insulation can be either dry or oil-based. In the first case, insulation can be either oil-based, polymeric or air. In the second, insulation is typically oil-based. The basic structure of a distribution transformer is shown in Figure 1[8; 9; 10]. From Figure 1, the main components of the distribution transformers are core, windings, tank, insulation and bushings. _Core:_: The transformer's core is made of silicon steel. This core is used to transmit the power from the primary to the secondary coils of the transformer through electromagnetic induction. The core fails due to DC magnetization or displacement of the core steel during the construction of the transformer. The lamination of the core can also peel off and increase losses and heat due to eddy-current. _Windings_: Windings are the conductors wrapped around the core limb. A transformer consists of a primary winding and one or more secondary windings connected via an electromagnetic field. Windings generate magnetomotive force that is carried by the core to other windings for changing voltages. The most frequent fail in windings is due to short-circuits or transient over-voltages. _Tank:_: The tank is the physical protection for the transformer core and windings as well as is an oil container for cooling the transformer. The failure of the transformer tank can occur anywhere due to oil leakage, insulation material loses insulation function, dents... Oil leakage leads to flashover and transformer breakdown. _Insulation_: The insulation between windings in transformers is usually provided by transformer oil. Oil contamination due to the oxidation processes and increasing size of the colloidal particles can generate conducting particles, raise the temperature inside the transformer and finally damage the oil insulation. _Bushings_: are used to provide insulation while routing the winding terminals through the tank for connection with the power system. The main failure mode of the bushing is short-circuit. It may be due to material faults in the insulation or due to damage. The damage can occur due to sabotage, during shipping or airborne parts from other failed equipment. Damages, cracks in the porcelain and bad gaskets provide ingress of water inside the insulation of the bushing leading to its failure. _Tap changer._: is used to regulate the output voltage of a transformer by changing the number of turns in one winding.