a. a. abdoos

Numerous methods exist to distinguish between inrush current and internal faults, but these approaches have not yet become practical due to their inherent limitations. As a result, conventional methods, despite their well-known drawbacks, continue to be widely used in practice. In this paper, a new method based on time-frequency analysis is presented for detecting inrush current situations. To do this, a diverse array of scenarios involving a power transformer switching ON and internal fault cases are simulated using the PSCAD/EMTDC software package. Then, a hyperbolic S-transformer is employed to extract a determining index from the simulation results. Finally, a suitable threshold value for this index is computed so that inrush current can be distinguished from fault current by comparing the index with its threshold.  Evaluation of the efficiency of the proposed method using simulation and real data confirms its excellent accuracy. Therefore, it can be used in algorithms for power transformer differential protection to improve their stability during inrush current transients.

The ground differential (GD) relay is used to detect internal ground faults on the power transformer windings and terminals. In fact, this relay is employed as a complement to the phase differential (PD) relay. However, its stability is affected by magnetic saturation in current transformers (CTs) cores so that the spurious differential current caused by CT saturation may lead to the relay malfunction. In this paper, by modifying the restraining current definition in a conventional method, an adaptive restrain method is proposed. To evaluate the proposed method performance compared with conventional method, a real 230/63 kV power transformer under a large number of internal fault, external fault and in rush current cases are simulated in PSCAD/EMTDC environment. It is worth noting that Jiles-Atherton (JA) model is utilized to simulate CTs. MATLAB software is used to implement and analyze these methods using obtained simulation. Obtained results show that the proposed method is quite superior to the conventional method, despite its simplicity.

The restricted earth fault (REF) protection is provided for electrical power transformer in order to sense internal earth faults, mainly because it is more sensitive than the main differential protection. However, The REF relay may maloperate when current transformer (CT) saturation happens following a severe external fault. In this paper, a new simple algorithm is proposed for REF protection scheme which is realized by considering four fundamental conditions. These conditions are defined based on the differential and neutral currents as well as sum of phase currents. When these conditions are simultaneously satisfied, the relay detects the internal ground fault. This algorithm is implemented and evaluated by MATLAB program based on obtained result data from simulation of a real power system using PSCAD/EMTDC software package. The well-known Jiles-Atherton (JA) model is used to simulate the transient behavior of CTs. The satisfactory results obtained from exhaustive investigation justify the high security of the proposed protection scheme.

High power permanent magnet synchronous generators (PMSGs) are suitable for wind power applications because of their high efficiency. According to the electromagnetic machine design principles, the main disadvantages of low-speed and high-power generators are large size, heavy weight and high manufacturing cost. The main objective of this paper is to optimize the exterior-rotor PMSG for direct-drive wind turbine applications in order to reduce the generator system cost under design constraints. At first, a multidisciplinary and accurate model is proposed for optimal designing of exterior-rotor permanent magnet wind generator system. Next, the design variables that affect the generator system cost are investigated and specified. Furthermore, the impact of these variables on generator efficiency as one of the main design constraints, are investigated. At last, the unified particle swarm optimization (UPSO) technique is used to optimize the design variables based on the presented analytical model. By comparison the optimal design results of this study with two 500-kW inner-rotor PMSGs and one 15-kW prototype exterior-rotor PM wind generator, it is shown that the proposed method yields an optimal design with lower total volume, lower generator system cost and higher efficiency. Moreover, 3-D finite element analysis is employed to verify the obtained results of the proposed optimal design of 500-kW exterior-rotor PMSG.