Mohsen Mostafaei

This paper studies the temperature distribution in the transformer bushing in the presence of Geomagnetically Induced Current (GIC). The thermal stress can significantly affect the transformer bushing reliability and lifespan. However, the existing research works have focused on thermal modeling of transformer bushing due to loading, and the impact of GIC on this valuable apparatus has not been investigated yet. In this paper, the hot-spot temperature (HST) in an oil-impregnated paper (OIP) bushing is studied during the GIC. The harmonic currents at various GIC levels are obtained from the EMTP-RV time-domain simulations using a detailed topological transformer model. Based on a frequency-dependent model of the bushing resistance, the total power loss is calculated by taking into account the GIC, load current, and harmonic currents. Subsequently, the temperature distribution in the modeled bushing is obtained from the finite-element method (FEM), considering all thermal mechanisms, including fluid flow, internal convection, and thermal conduction. In addition, a Thermal Equivalent Circuit (TEC) is developed for the bushing that replicates the FEM results. The simulation results reveal that the bushing HST may exceeds the recommended permissible temperature of the IEEE bushing standard. This situation may result in bushing insulation deterioration and reduction of the bushing lifespan.

Synchronous generator (SG) plays a vital and critical role in the power system by supplying electric power to consumers. Various faults in SGs can cause some catastrophic events such as power disruption or blackout. These faults can be classified into two electrical and mechanical faults. Short circuit in stator windings and field winding are electrical fault while bearing, static/dynamic eccentricity, and broken damper bars faults are mechanical one. Unlike the induction machines, there are no much researches in SGs condition monitoring owing to its complex behaviour against the faults. Herein, the SG modelling approaches are presented briefly to elaborate shortcoming and challenging issues in the modelling, and then a comprehensive review of various electrical and mechanical fault detection methods is presented.

Power transformer, as one of the most important apparatus in power system, must be protected against the turn-to-turn faults. Although the various methods based on the terminal currents and/or voltages have been presented for transformer protection, the flux-based methods can be used to achieve the more accurate, sensitive, and secure results. Since the symmetrical form of the magnetic flux distribution will be disturbed due to any fault occurrence in the transformer windings, it can be considered as an appropriate criterion to achieve a suitable protection algorithm, which can detect the fault occurrence and identify the faulty phase/region. In this paper, a new leakage flux sensor is introduced and then a flux-based technique is proposed to detect and identify the faulty phase in transformers to apply in online conditions. By using the proposed technique, the faulty region can be identified in offline conditions, as well. The experimental results (which are done on a distribution transformer) show that this technique is secure in the face of overfluxing and transformer energizing conditions and can detect turn-to-turn faults with high sensitivity. Moreover, tap changer operation, imbalanced loads, and/or unsymmetrical voltage sources cannot influence the proposed technique security.

Since the internal turn‐to‐turn faults (TTFs), as a common cause of the transformer failures, result in minor changes on terminal currents/voltages, they maybe undetected by the conventional protective relays. In this study, a simple, sensitive and robust linkage flux‐based technique is proposed to protect the power transformers against TTFs. In this approach, some separated multi‐turn windings as search coils (SCs) have to be wrapped around the transformer core legs to sense the related passing flux. Since passing equal flux through a transformer core leg in normal condition induces equal voltages in the related SCs, variation of the induced voltages indicates the fault occurrence in that phase. Against the traditional differential protection schemes, current transformer error and saturation, tap changer operation, energising inrush current, over‐fluxing etc. cannot impact the proposed technique performance. Therefore, it can be introduced as a comprehensive protection technique for power transformers.To confirm the proposed technique performance, it is tested on a real 50 kVA, 20 kV/400 V three‐phase distribution transformer, in various conditions, too.

Internal turn‐to‐turn faults (TTF) are considered as a common cause of transformer failures. Such defects (especially minor TTFs) may result in minor changes in terminal currents/voltages as these are undetectable by conventional current and/or voltage‐based methods. In this study, a simple, sensitive and robust method is proposed to identify the TTF locations in power transformers by using core flux based technique. In this approach, a few turns must be wrapped around the transformer core legs to sense and detect the related core flux. Passing asymmetrical flux through a transformer core leg (due to a TTF) induces different voltages in the related sensors, which are located at different places. Variation of the core flux in the corresponding sensors indicates the faulty phase and the TTF location on that phase as well. The proposed technique is verified by some simulations and experimental tests. The obtained results show that the proposed technique successfully (i) detects all the TTFs, (ii) identifies the faulty phase, and (iii) specifies the faulty region on the related phase. Although the proposed method requires a few search coils to be installed on the transformer core legs, installing these thin wires will not change the transformer design significantly.

Power transformers as an important and expensive apparatus in the power system must be protected against the faults and should be monitored permanently to increase power system reliability and to decrease maintenance costs. Internal Turn-Turn faults (TTF) are the most common failures in power transformers, which could seriously reduce the useful long life of these equipment. Although the common protection schemes such as differential relays are able to detect some of internal faults, some other ones (such as TTFs and short circuit near the neutral point) cannot be detected by them. Also, these relays may trip erroneously due to energizing inrush currents, transformer over excitation, and occurring CT saturation at one side. In this paper a novel Linkage Flux Based (LFB) scheme is proposed to detect TTF in power transforms, which uses some Search Coils (SC) located on the transformer legs to sense the related linkage flux. Any difference in induced voltage in the corresponding SCs (located in any leg) suggests passing unsymmetrical linkage fluxes through them (against the normal conditions), which stands for occurrence a fault inside the transformer. The proposed technique not only can be used to protect power transformers, but also can be employed to find the fault location during repair activities, as well.