Voltage Source Converter High Voltage Direct Current (VSC-HVDC) transmission technology is widely used in electrical power transmission over very long distances. Those systems are subjected to different types of faults that have an effect on the electricity deliver continuity and reduce the device reliability. DC line faults are public in HVDC systems, it may occur in either overhead transmission lines or in submarine cables and with different configurations. In such faults, the converter stations must be blocked to isolate the transmission line and to shield the electronic switches. Therefore, fault detection, isolation and system restoring techniques are very important to development the system reliability. In this paper, DC pole to pole short circuit fault has been carefully studied. The fault behavior has been discussed from the fault moment until the fault clearance. A Matlab-Simulink model has been built to verify the mathematical analysis.

Power electronic systems are considered as one of the most important components in many applications, such as nuclear reactors, aerospace, military applications and life saving machines. In such applications the system should be high reliable, a knowledge-based expert system was developed to diagnose faults in a three phase induction motor system . A software tool called KAPPA PC 2.1 was used to develop the expert system, the system is modeled in MATLAB SIMULINK and the simulation results at normal and fault conditions was rewritten as a set of if-then rules by which the expert system can discriminate the fault.

Power Electronic Transformers (PETs) are considered as an emerging solution for power conversion due to their benefits over the low frequency power transformers (LFPTs) such as lower size, multi-port structure and different control functionalities. However, unbalanced load conditions may limit the reduction of size due to using bulky capacitors to absorb the resulting 2^nd harmonic voltage oscillations. This paper aims to provide a solution to this problem in the Dual Active Bridge (DAB)-based Modular Multilevel Converter Power Electronic Transformers (MMC-PET). In this study, a modified phase shift control strategy for the DAB isolation stage is presented. In this strategy, the duty cycle is allowed to change sinusoidally around the operating point. This consequently transfers the power oscillations to the HV side where their effect is less significant. The system is simulated using Matlab-Simulink under different unbalanced load conditions. Besides, the theoretical analysis and the simulation results are experimentally validated using a laboratory prototype. According to the obtained results, the proposed controller succeeded to reduce the voltage ripples from 7.5% to 0.2% and this helps in replacing large Aluminum electrolytic capacitors with low size film capacitors and consequently reducing the system overall size and volume which is a great benefit.

This paper presents an optimization strategy to get the optimal cross-section of induction motor with maximum induced torque. This can be obtained by using mathematical model equations, which are of main machine design parameters. The optimization strategy has been applied to reach the optimal air gap flux density, optimal inner stator diameter, optimal number of poles, stator slots and rotor bars. The optimization strategy comprises several steps. These steps are repeated using different magnetic materials types in the core and teeth of both stator and rotor. Also, the effect of variation of machine stack length and number of phases on optimized values has been studied. The optimization calculation is performed using MATLAB software. This Study leads to minimize the production costs of induction machines for the same machine outer diameter.