Monitoring and evaluation of transformers are essential to prevent their insulation failure. In this paper, the use of 2-furan-carboxaldehyde (2-FAL) and methanol (MeOH) concentrations as the main products of paper insulation degradation, and insulation condition markers, has been studied. In order to study the degradation process and production of degradation products, the thermal aging process of the transformer insulation system was implemented in laboratory conditions. The results of laboratory studies show that in the early stages of degradation the amount of MeOH is significant compared to 2-FAL. Also, the estimation of the degree of polymerization (DP) in the early stages of degradation (DP>800) through MeOH concentration and with decreasing DP (DP <800) through 2-FAL concentration is closer to the real value. The results of studies performed on 35 distribution transformers confirm the production of significant amounts of MeOH in the early stages of degradation. Also, the estimated DP values for the studied transformers were obtained through 2-FAL and MeOH concentration. The results show that estimating the amount of DP through MeOH concentration is associated with a probability of error of about 9% compared to estimating DP through 2-FAL concentration.

Today, the use of electricity sources is increasing as cities are growing. With the increasing use of mineral oils for transformers cooling in the distribution network, due to the problems encountered using these oils, an alternative fluid should be used inside the transformers instead of mineral oils. Therefore, mineral oils should be replaced with fluids that are more compatible with nature due to the environmental hazards and high costs. Hence, vegetable oils can be used as suitable alternatives for the mineral oils in transformers due to their low risk and the renewability. On the other hand, compared to the mineral oils that have a fire point of about 151 Celsius degrees, vegetable oils have fire points higher than 311 Celsius degrees. As a result, from this viewpoint, they are considered as harmless fluids. Vegetable oils are simply degraded in the nature, and due to their different chemical structures compared to the mineral oils, they can increase the life of the equipment. Besides, the most important point is that they improve the transformer cooling performance, in terms of thermal analysis. Thus, in this paper, the distribution transformer electromagnetic-thermal analysis and conjugate heat transfer, in presence of different types of vegetable oils, and different types of cores such as grain-oriented silicon steel, amorphous and vitroperm alloy are investigated. Afterwards, the obtained results, especially hot spot temperature, are compared with distribution transformer containing mineral oil. ANSYS software has also been used for simulations.

Recently electronic tap-changer has received more attention due to its quick response, better performance and simpler maintenance compared to the mechanical tap-changer. This paper presents the capability of the distribution transformer equipped with an electronic tap-changer for improving power quality. At this end, the analytical computation for determining the compensating limit of electronic tap-changer is given and then this system is simulated in order to show its capabilities for power quality enhancement. Meanwhile, the impact of the electronic tap-changer in power quality parameter improvement is compared with that of other custom power tools.

In this article, a multi-objective function for placement of Distributed Generation (DG) and capacitors with the tap setting of Under Load Tap Changer (ULTC) Transformer is introduced. Most of the recent articles have paid less attention to DG, capacitor placement and ULTC effects in the distribution network simultaneously. In simulations, a comparison between different modes was carried out with, and without tap setting of ULTC. Simultaneous DG, capacitor placement, and ULTC transformer tap setting improve the voltage profile of load buses globally. In addition, they can also reduce loss and increase Available Transfer Capability (ATC). The IEEE 41-bus radial distribution network is used to illustrate the effectiveness and feasibility of the proposed approach.

Unbalanced loads in the distribution grid can cause unbalanced supply voltage and reduce the power quality of the system. Besides, reactive power loads might create the voltage disturbances such as voltage sag/swell. In this paper, a four-wire distribution static synchronous compensator (DSTATCOM) integrated with Yy-transformer to compensate both the unbalanced voltage and reactive power is proposed. A DSTATCOM based on modular multilevel converter (MMC) connects to the group of taps in the primary windings of Yy-transformer to constitute an interconnected structure. The compensating currents flow through the taps. This structure is similar to an autotransformer. Compared to the conventional MMC-DSTATCOM, with proposed structure, The DSTATCOM connection point voltage decreases and one can trade-off between the capability of the power device's rated voltage and current. Therefore, it can be decreased the primary investment costs, the voltage stresses and the size of compensation system. Furthermore, a control algorithm based on measuring the point of common coupling (PCC) instantaneous voltage and current is presented that it is able to compensate the zero and negative sequence voltage due to unbalanced laod and grid. To compensate these components, voltage and current loops are used to track accurately the reference voltages of MMC-DSTATCOM by means of unbalanced algorithm. Simulation results verify the proposed method.