Mitigation of Inrush Current in Three Phase Power Transformer by Prefluxing Technique

Transformers are major component for electrical energy transfer in power system. Sta¬bility and security of the transformer protection are important to system operation. At the time of transformer energization, a high current will be drawn by the transformer. The mentioned current is called transient inrush current and it may rise to ten times the nominal full load current of transformer during operation. Energization transients can produce me-chanical stress to the transformer, cause protection system malfunction and it often affects the power system quality and may disrupt the operation of sensitive electrical loads such as computers and medical equipment connected to the system. Re¬duction and the way to control of energization transient currents have become im-portant concerns to the power industry for engineers. One of the methods to reduce inrush current is use of point on wave switching at the time transformer is initially connected to supply. It is called controlled switching or point-on-wave switching. In the point on wave switching, the energization of three phases is controlled ac-cording to the residual flux which remains in the transformer. Conven¬tionally, controlled switching or point on wave switching was the method being used to counter this problem, but this method required the knowledge of residual fluxes of transformer before energization which is quite tedious to get. So a technique has been pro-posed to mitigate inrush current in three phase transformer, by a process called pre-fluxing. After setting the in-itial fluxes of transformer it is energized by conventional controlled switching. A system of power transformer of specified rating is simulated in MATLAB simulink and results were obtained. This Paper describes the mod-eling of inrush current of 3- phase, 300 MVA, 11/400 KV, 50 Hz transformer, and mitigation of inrush current with both techniques using point on wave switching and prefluxing. The simulation is done in MATLAB..

Reduced Switch Technique for Solar PV System Based Multilevel Inverter for PQ Improvement

This paper describes reduced number of switches technique for Solar Photo Voltaic (SPV) voltage source based 31-level Multi Level Inverter (MLI) for Power Quality (PQ) improvement. The proposed topology reduces number of IGBT switches to ten, which is very less compared to diode clamped, capacitor and cascaded multilevel inverter. The desired multistep AC output voltage with minimum number of switches is obtained using this topology which reduces cost of system. The proposed MLI reduces number of harmonics at the AC output voltage which improves the PQ. The square wave switching used in this topology reduces the switching losses compared to pulse width modulation technique. The performance of four solar PV voltage sources with Buck/Boost converter has been discussed in this paper. The proposed MLI is mainly used in this paper for off grid system. The results of the Solar PV System based 31-level MLI have been simulated in MATLAB R2013a environment and hardware results have been validated using Opal-RT real time simulator OP-5142.