Toan Phung

Frequency response analysis and short-circuit impedance measurement in detection of winding deformation within power transformers

Power transformers are in service under different environmental, electrical, and mechanical conditions [1] and may be subject to enormous hazards during the course of operation [2], [3]. They are commonly considered to be the heart of the transmission and distribution sectors of electric power systems; monitoring their condition and diagnosing faults are important parts of the maintenance function [4]. Utility engineers strive to keep power transformers in service and to prevent even shortterm outages. Failure of a transformer can cause extensive damage to equipment owned by consumers or the utility [5].

FRA vs. short circuit impedance measurement in detection of mechanical defects within large power transformer

Power transformers are supposed to be and remain in service in various environmental circumstances under different electrical and mechanical stresses. Base on failure history in power transformers obtained from four corners of the globe one of the major problems in transformers is mechanical defect. A number of monitoring and diagnostic methods have been introduced to recognize transformer active part displacement and winding deformation. Frequency response analyses and short circuit impedance measurement have been employed as two common diagnosis methods in large power transformer winding deformation recognition. On the other hand, researchers are expressing an increased concern about power transformer condition monitoring in the smart grid context. Hence, all of off-line methods need to move towards on-line applications. One of the challenges is finding reasonably accurate method which can provide sufficient information about transformer winding condition. In this study, mechanical defects of windings and their causes are investigated in detail. Frequency response analyses and short circuit impedance measurement as two popular methods in transformer winding deformation diagnosis will be employed to get insight into transformer active part condition. A large power transformer has been taken as a case in order to put the capability and sensitivity of abovementioned methods into test. Onsite test results on this giant transformer winding show that frequency response analyses method is capable to provide far more information as to the healthy or defected condition and physical movements of the transformers windings and core compared to the other method.

Practical challenges in online transformer winding deformation diagnostics

Monitoring and diagnosis of transformers have been investigated and discussed for many years. While transformer routine tests indicate electrical and also insulation weakness or defects, Frequency Response Analysis (FRA) as a modern test can indicate mechanical defects in transformer active part. This paper introduces the concepts behind online measurements in frequency domain and is striving to open the discussion doors towards advanced investigations which are all aligned with smart grid concept. This study has also concentrated partly on issues rising while online transfer function measurement techniques are going to be applied on transformers with various kinds of winding connections and accessories. In the end, crucial issues from theoretical and also practical perspectives for online transfer function implementation are discussed.

Three-dimensional vibration analysis of single-phase transformer winding under inter-disc fault

Transformer real-time monitoring systems have been studied for the last decade for developing transformer prognosis techniques. Transformer mechanical defect is one of those crucial subjects which need to be taken into account for prognosis before failure is occurd for transformer. Among all mechanical defects in transformer, transformer inter-disc short circuit is quite common in transformer fault reports. Hence, this study is practically focused on analysis the vibration spectrum of transformer under inter-disc fault. Vibration model for transformer core and winding are explained and inter-disc fault is emulated practically over a single-phase laboratory size transformer. Three-dimensional vibration results are provided and discussed in detail.

Real-time dry-type transformer aging evaluation

Dry-type transformers are quite popular in commercial and industrial applications. The IEEE and IEC standards on dry-type transformer loading provide a significant guideline and recommendation for transformer aging rate calculation. In this study, the fundamental information in these standards are used to develop a real-time dry-type transformer thermal model and evaluate transformer insulation performance in different conditions. Thermal model of the transformer can help to determine proper transformer operation condition, transformer aging rate, and approximately demonstrate transformer lifespan. Simulation studies show that increasing of transformer load and ambient temperature sampling frequency rates will lead to the increase of the accuracy of aging rate calculation and more precise lifespan estimation.

Analysis of transients in a micro-grid using wavelet transformation

Micro-grids are envisaged as single plug-and-play electrical systems that can operate either in grid-connected or islanded modes. Their main sources of power are renewable and are usually integrated through power electronic devices. These present new challenges to reliable operation of the system due to their inherent incapability to regulate the system. Transients in the micro-grid make it further vulnerable. The transient signals only occur for a few cycles and cannot be easily detected directly in the time domain. This paper utilizes wavelet transformation method to detect and analyze the transient signals in the micro-grid. The results show that the disturbances are successfully identified in both grid-connected and islanded modes, by applying wavelet transform. The variation of coefficients is found to be much smaller in grid-connected mode.

Impact of battery storage on micro-grid transient performance

The increasing penetration of distributed generation (DG) into the electrical network presents new challenges to reliable operation of the system due to the special characteristics of DG units and their low inertia. This paper investigates the dynamic behavior of the system with multiple DGs. The main aim is to study the transient caused by the sudden integration of DG. The impact of the size of the storage device used is also considered in the paper. The simulation is carried out in DIgSILENT Power Factory 14.1 and a micro-grid setup in the laboratory is used for experimental validation. The results show that it is difficult for the micro-grid to maintain stability under a disturbance in islanding mode without any battery storage. The application of battery storage will considerably improve the micro-grid transient performance and the level of the DG integrated into the system is the critical factor that will determine the appropriate capacity of the battery.

Simulation and experimental performance analysis of micro-grid based distributed energy resources

Micro-grids are becoming very popular due to the plug-and-play nature of their integration. But the high penetration of distributed generators may also create potential instability as it is unable to regulate the system. This paper compares the dynamic performance of a micro-grid when the system load demand is suddenly changed. Multiple distributed sources are used to create a flexible simulation scenario. The simulation is carried out using DIgSILENT software and the results indicate that the micro-grid may collapse when it works in islanding mode. It is found that the installation of storage device can help solve this problem, but both generation and load changes should be considered when determining its capacity. The DG penetration level also shows the impact on the system transient as it can potentially change the level of disturbance. The results of the simulation are validated using an experimental micro-grid setup, having a mix of wind, solar generation and battery storage.