Online assessment of winding deformation based on optimised excitation

Current Status and Future Trends in Frequency-Response Analysis With a Transformer in Service

Abstract: This paper presents the current status and future trends in the application of the frequency-response analysis (FRA) technique with the transformer in service (online) through bibliographic review and analysis. As a result, three basic stages of the online FRA test have been identified and defined: injection and excitation signal measurement; recording, filtering and processing of measured signals; and curve analysis and interpretation. This work presents an overview of the online FRA technique, useful for subsequent research in this area.

The application of sweep frequency response analysis for the online monitoring of power transformers

Abstract: Frequency response analysis (FRA) is a technique used to diagnose the mechanical integrity of a transformer winding; such diagnostic tools can be of enormous value since power transformers are a critical asset within any electrical network. To minimize the probability of an unexpected outage, or prevent a catastrophic failure, maintenance and monitoring of power transformers is essential for utilities. Over the past couple of decades, FRA has been utilized as an off-line diagnosis method. However, with the recent development in smart grid systems, there is now a growing interest in the development of on-line FRA techniques. This paper proposes a technique for in-service monitoring of power transformer winding deformation, which uses a broad frequency sine wave voltage excitation signal and high frequency Current Transformers (CT) in conjunction with the bushings test taps. Experiments using this system were conducted and then validated on a single-phase 22kV/110V voltage transformer.

Online monitoring of power transformers using impulse frequency response analysis

Abstract: With the recent development in smart grid systems, there is now a growing interest in the development of on-line monitoring techniques. This is to reduce the maintenance costs as well as minimizing the probability of unexpected outages, or potentially to prevent catastrophic failures in the electrical network.

Winding deformation analysis in power transformers using Finite Element Method

Abstract: Winding deformation in power transformers affects its mechanical integrity. Diagnosis of winding deformation is a necessary step towards preventing its permanent failure. Sweep Frequency Response Analysis (SFRA) is widely used to analyze the nature and extent of deformation. However, the location of deformation needs the signature from the deformed transformer at various locations, which is practically not feasible. In this paper, Finite Element Method (FEM) and SFRA are used to model deformations on a benchmark winding and obtain corresponding signatures. Different parameters derived from SFRA are used as indices to distinguish type and location of deformations.

Simulation and Analysis of Transformer Winding Inter-disk and Inter-turn Faults for Online Diagnosis

Abstract: Transformers are considered as the key components in power system network as they are employed in generation, transmission and distribution sectors. Hence electricity providers are highly concerned about proper operation of these valuable assets and specifically normal performance of the transformer active parts. One of the most reliable techniques to monitor the active part of the transformer is the Frequency Response Analysis (FRA). Given the fact that FRA technique has been successfully utilized in offline application for transformer diagnosis; it has been recently studied to apply the FRA once the transformer is in service, called online FRA. In this study, the transformer winding inter-disk and inter-turn faults are studied along with the effect of the bushing on frequency response signature in online FRA setup. Transformer winding physical model is created, simulated and numerically analyzed. It is revealed that the winding inter-disk and inter-turn faults lead to FRA anti-resonance frequency shift and response magnitude alteration in online FRA. Emulation of transformer bushing in the form of two paralleled capacitors unveils the significant influence of this component over the low frequency band of FRA spectrum. Destructive effect of the bushing reactive impedance should be taken into consideration during online FRA test measurements.

Procedures for detecting winding displacements in power transformers by the transfer function method

Abstract: The paper investigates three different ways of using the transfer function method for detecting mechanical winding displacements in power transformers. The most reliable approach is time-based comparison , which requires finger print data from a previous measurement. Such information is, however, usually not available. For multilegged transformers without zigzag-connected windings the results of separately tested legs can be used as mutual references (construction-based comparison ). A third approach is to compare the transfer functions with those obtained from an identically constructed transformer ( type-based comparison). However, for a transformer with given nominal specification data, the winding design may over time undergo changes which causes changes to the transfer function. It is proposed to solve this problem by calculating tolerance bands using transfer functions from a big group of the same-type transformers. A novel statistical algorithm for this purpose is presented. The approach is demonstrated for a set of 28 specified identically 200-MVA power transformers.

Survey of excitation signals for FFT based signal analyzers

Abstract: The properties of ten different excitation signals are studied to analyze their suitability as excitation signals for fast Fourier transform (FFT)-based signal and network analyzers. Their influence on the measurement time, accuracy, and sensitivity to nonlinear distortions is described. The flexibility to create a customized amplitude spectrum is investigated. With this information it becomes possible to select the best excitation signal for many applications. >

A general method for determining resonances in transformer windings

Abstract: This paper presents a method for calculating terminal and internal impedance versus frequency for a lumped parameter model of a transformer The transformer's total frequency response can be accurately determined from this impedance data directly, ie, the terminal resonance and anti-resonance and internal amplification factor characteristic can be calculated for a single or three-phase transformer model Since most equipment associated with power system operation can be accurately modeled with lumped parameter networks, this method also provides an accurate, straightforward method for determining the resonance characteristic of those systems An additional significance of this method is that the accuracy of the calculation is limited only by the user's ability to represent the equipment or system Heretofore, the accuracy of this calculation was limited by the simplifying assumptions required of the network model by each solution method This paper presents the definitions and mathematical theory underlying the method Two examples are presented in which comparisons are made between measured and calculated values for a helical air core coil and a 200 MVA single-phase autotransformer The agreement is excellent

Application of numerical evaluation techniques for interpreting frequency response measurements in power transformers

Abstract: Frequency response analysis (FRA) is an emerging, powerful non-intrusive condition monitoring and diagnostic tool for verifying the mechanical integrity of power transformers. FRA results are graphical in nature and require trained experts to interpret test results. The work reported discusses numerical-criteria-based evaluation techniques. Persons not familiar with interpreting the FRA results can apply the evaluation criteria. The various criteria help in deriving proper conclusions. By evaluating correlation coefficient (CC), standard deviation and absolute sum of logarithmic error (ASLE) techniques, it is possible to discriminate between defective and non-defective windings. Experimental studies were conducted on two test transformers for axial and radial displacements, and additionally two sets of identical substation transformers. The techniques mentioned above are useful for interpreting frequency responses even in situations when a reference fingerprint was not available. However, it was concluded that if original fingerprints are available, the method gives very reliable indication for diagnosing the faulty winding. In addition, the severity of displacement/deformation can also be concluded from the amount of variation of the parameters from the suggested critical values.

Apparatus for Online Power Transformer Winding Monitoring Using Bushing Tap Injection

Abstract: Online transformer condition monitoring techniques based on transfer function methods, such as transmission-line diagnostics and swept frequency-response analysis, require the injection of a known test signal into the transformer On larger power transformers, a practical method is to use the available bushing tap connection In this paper, we will discuss a custom high-power signal generator that injects high-frequency signals on the bushing tap of the transformer under investigation, as well as a circuit to replace the bushing tap short and allow online operation of the system Finally, the system is demonstrated on a 650 kV transformer