# Power Transformer Inrush Current Analysis: Simulation, Measurement and Effects
## 1 Introduction  
A transient condition in a power transformer is characterized by a temporary change in the current or voltage flowing through it. Transient regimes can be caused by a variety of factors, such as [1, 2, 3, 4, 5]:  
* sudden load changes--when a load is suddenly applied to a power transformer, rapid increases in current and voltage occur [1, 2, 3];
* the existence of a direct connection between the primary and secondary windings of a transformer, in this situation the transformer windings are traversed by a very high current [4];
* switching--the phenomena caused when a switch is used to connect and disconnect a load from a transformer, or even to connect and disconnect it from the mains, are characterized by the occurrence of very high currents in the power transformer [3, 5, 6, 7]. Transient phenomena/regimes cause a number of negative effects on power transformers, such as [8, 9, 10, 11]:
* overheating--high currents and voltages developed during transient phenomena cause temperature rise (overheating), which leads to damage to the transformer insulation, resulting in failure;electromagnetic interference--the high currents and voltages that can occur during transients can also cause electromagnetic interference (EMI). This may disturb other electrical equipment in the vicinity of the transformer. The most common transients are transformer to mains and short circuit [12; 13; 14; 15]: transformer to mains connection--this phenomenon occurs when a transformer is connected to the mains, the current in the primary winding increases rapidly as the magnetic field in the core builds up [12; 13]; short circuit--occurs when there is a direct connection between the primary and secondary of the transformer, this phenomenon is characterized by a rapid increase in current in the primary winding, which can cause a large voltage drop across the winding and damage the insulation of the windings [14; 15]. As the transformer is the key element in the power generation, transmission and distribution system, its operational safety requires special attention. The power transformer acts as a node to provide the connection between different voltage levels. One of the objectives of energy system managers is to keep them in an optimal and reliable operating condition by implementing a predictive maintenance system [8; 9; 10; 11; 16]. It is known that interruptions caused by repair or replacement operations on transformers cause economic losses, so the attention of network operators has been focused on eliminating the effects of transient regimes, connection/disconnection operations, which can cause faults in power transformers. The effect of inrush current on power transformers is still a major problem and can cause failure or even shutdown. The energy quality of the power supply system is disturbed by the inrush current effects and influences the operation of sensitive electrical loads. Inrush current is the transient current that occurs when the transformer is connected to the network. This current can reach high values, 100 times higher than the idle current and can generate electrical, mechanical and thermal demands on the transformer structure. The inrush current magnitude is influenced by factors such as [1; 17]: transformer core saturation; connection angle of the supply voltage; the total impedance of the circuit traversed by inrush current; the residual flow of the transformer core, generated by its multiple connections to the network; multiple instant connections. Mathematical models for predicting the maximum value of this inrush current have been presented in the literature, and parameters that directly influence the amplitude of the inrush current have been analyzed [18; 19; 20; 21; 22]. The inrush current analysis can have various reasons, such as establishing the maximum values that can be reached in order to establish the appropriate protections, the possibility of distinguishing between a normal inrush current and a fault current.