"SIMULATION AND EXPERIMENTAL INVESTIGATION ON DC CURRENT CAPABILITY OF POWER TRANSFORMERS"

Abstract

Transformers serve as the vital link between electricity generation, transmission, and distribution. As an essential and fundamental component of any power system, their uninterrupted operation is critical to the modern world because its continued existence is dependent on a consistent power supply. Transformer attributes alter from their initial installation states as the transformer ages, so real-time characteristics may differ from manufacturer data. Several factors could interfere with transformer functioning in power networks. The findings of this research on one of these factors, evaluating the consequences of DC bias on the operating characteristics of power transformers, have been discussed. DC injection is a significant problem in a grid-tied electrical system, leading to decreased health and life span. The cumulative flow of direct current in the power transformer is commonly referred to as DC bias. The DC bias anomaly is an aberrant condition of the functioning in the power transformer. A current known as Geo-magnetically Induced Current (GIC) enters through the neutral link, which is normally grounded, and flows across the power networks through the transmission lines, contributing to the flow of DC bias in ferromagnetic devices. The DC bias due to the GIC and the bipolar operation in the transmission of HVDC system operated in the mono-polar mode will have similar effects. The existence of the DC current component could also be due to a mismatch in the scheme of switching power electronics switches and the variety and variation of load connected to the grid. The DC bias will lead to significant saturation of the power transformer core, resulting in nonlinear operations. This causes higher transformer reactive power absorption, generation of significant harmonics and voltage control problems. The inclusion of DC current in a transformer will result in an increase in leakage of magnetic flux in the cores, alter the magnetizing characteristics, and disrupt the transformer's normal operation, resulting in an increase in magnetizing current, vibration, noise, overheating, and dielectric breakdown. If this DC current endures in the transformer for an extended period of time, it will cause irreparable damage to the transformer. Given the multiple potential sources of DC bias in the power system, although solutions to mitigate DC bias in a transformer have been developed to reduce the extent and impact of these effects, these alterations do occur and cannot be completely eliminated. Hence, there is a research need to thoroughly understand the direct current handling capability of the power transformer and the impact of DC bias on efficiency and rating to ensure the future sustainability of the power systems. The design and rating of a transformer are critical for its proper operation. Voltage underrating of the transformer may result in saturation and poor performance. If the transformer is not properly designed to account for DC bias, the injected voltage may saturate it, resulting in improper operation. This issue can be mitigated during the transformer design stages by selecting a specific amount of allowable DC, so that the DC current withstand capability is increased without adding any additional stress to the transformer. However, oversizing/over-rating is not a good solution because it results in increased size, decreased performance, and increased production costs due to oversized conductors. Thus, the effects of DC bias on various characteristics, as well as their impact on overall performance, are thoroughly investigated, addressed, and incorporated into transformer design. This investigation performs the integrated analysis of electrical, magnetic, thermal, and mechanical effects in power transformers subjected to DC bias. The power transformer is modelled based on constructional details and examined with Ansys Multiphysics software. The finite element approach is used to study a three-phase core type 500 KVA power transformer. The power transformer is examined for diverse degrees of DC bias with different excitations. The experimental investigation is carried out through a scale-down value of a 5 KVA laboratory prototype. The multiple effects of DC bias on transformer performance are complex, and the lack of an integrated analysis of multiple parameters limits the use of these inputs in the initial design and fault diagnosis of a power transformer. However, various parameters can affect performance independently and to varying degrees, efforts are made to adopt an integrated approach that includes comprehensive and coordinated collection, analysis, and interpretation of various parameters. The interrelationships between multiple parameters and their correlations with one another, as well as how they would aid in determining the consequences of DC bias on the condition of power transformers, have been studied. It is essential to determine the impact and severity of the DC bias before a total collapse occurs. It is of the utmost importance to establish safe operating parameters to avert the risk of transformer failure. The safe limit of a transformer is defined in this research as the DC bias conditions under which the device can be expected to operate without damage. The safe limit specification combines the transformer's limitations, such as saturation and thermal breakdown, into a single curve, allowing for easy visualization of device capabilities. The analysis results provide valuable insights for the design engineer during the initial stages of designing a highperformance, cost-effective transformer.