Introduction to Power Transformer Frequency Response Analysis Signatures

Introduction

Frequency Response Analysis (FRA) is a technique employed to identify internal damage in transformers by measuring their frequency response. This method assesses the mechanical integrity of power transformer components, including the core, windings, and clamping structures, by analyzing their electrical transfer functions across a wide frequency range.

FRA was initially introduced by Dick and Erven in 1978. It is a non-intrusive and non-destructive test that can be employed either independently to detect failures or in conjunction with other tests to pinpoint and assess any damage detected by them.

Transformer conditions can deteriorate due to three primary types of faults:

• Electrical
• Mechanical and
• Thermal-induced

Among these, mechanical faults, primarily affecting the winding and core, are the most significant. Consequently, FRA is a widely employed diagnostic tool for identifying deformations in the winding and core.

The FRA technique is favored for its simplicity and ease of use. It operates by comparing the current response to the transformer's behavior with its previous responses, making it a straightforward and efficient method for fault diagnosis.

Purpose of Frequency Response Measurements?

FRA is typically employed to identify geometrical changes and electrical short-circuits in the windings, as referenced in IEC 60076-18, Annex B. The test involves measuring the transformer's response across a wide frequency range. These responses yield diagnostic information in the form of a transfer function, which is related to the RLC network structure of the transformer under examination. On the contrary, the RLC network represents the physical geometry and construction of the test specimen. For instance, issues related to the winding wires, such as short circuits or conductor breaks, primarily affect the resistance and inductance of the transformer's RLC network. Similarly, faults that result in changes to the shape and position of the windings, such as axial and radial deformations, lead to variations in the network's capacitance or inductance. Consequently, comparing the current frequency response with a reference response measured when the transformer was in optimal condition can uncover alterations in the power transformer's characteristics and facilitate fault diagnosis. This reference response is commonly referred to as the transformer's 'signature' or 'fingerprint.

FRA can be employed to assess a variety of conditions, including the following examples:?

• Damage following a through fault or other high current event (including short-circuit testing),
• Damage following a tap-changer fault,
• Damage during transportation, and
• Damage following a seismic event

For more detailed information on the application of frequency response measurements, please refer to IEC 60076-18, Annex C.?

Importance of Baseline Measurements

To effectively detect damage using FRA, it is highly advantageous to have baseline frequency response measurement data from the transformer when it is in a known good condition. Therefore, it is recommended to conduct these measurements on all large transformers, during their manufacture in the factory and when they are commissioned at the site. In cases where a baseline measurement is unavailable for a specific transformer, reference results may be obtained from a similar transformer or another phase of the same transformer as described in IEC 60076-18, Annex B.

In addition to damage detection, frequency response measurements can also serve a broader purpose in power system modeling, including applications like transient overvoltage studies.?

FRA, a Powerful Diagnostic Tool for in-service Transformers

The majority of transformers currently in service worldwide were installed prior to 1980, and as a result, most of them are approaching or have already exceeded their design lifetimes. This situation presents a significant risk for utilities and other stakeholders in power networks because the consequences of in-service transformer failures can be catastrophic. One of the most critical issues with in-service transformers is the movement of their windings, primarily caused by electromagnetic forces generated during short-circuit faults. Another potential problem arises from the reduction in clamping pressure due to insulation aging, which can also lead to winding movement and the risk of explosion. Mechanical faults in transformers can be triggered by various factors, such as earthquakes, combustible gas explosions in the transformer oil, short-circuit currents, or mishandling during transportation. While a transformer with minor winding deformation might still function satisfactorily, its ability to withstand further mechanical or electrical faults will gradually diminish.

Hence, it is crucial to promptly identify even the slightest winding deformations and initiate the necessary corrective measures. Winding deformations can manifest in various forms, including spiral tightening, conductor tilting, radial/hoop buckling, shorted or open turns, loosened clamping structures, axial displacement, core movement, and the collapse of winding end supports. Distinguishing between these internal faults using conventional testing methods can be challenging. FRA is a powerful diagnostic technique widely used to identify internal faults within power transformer.

P. Dick and C. Erven sub-divide the FRA frequency range as follows:

(a) The low frequency range

(<20 kHz), within which inductive components dominate the transformer winding response>

(b) The medium frequency range (20–400 kHz), within which the combination of inductive and capacitive components results in multiple resonances

(c) The high frequency range (>400 kHz), within which capacitive components dominate the FRA signature. These ranges and the associated fault types are summarized in Table 1.

IEEE Guide C57.149-2012 for the Application and Interpretation of Frequency Response Analysis for Oil-Immersed Transformers

The IEEE guide encompasses a comprehensive set of provisions and specifications for instrumentation, along with detailed procedures for conducting tests. It also delves into techniques for effectively analyzing the acquired data and offers valuable recommendations for the long-term storage of both data and results. Importantly, this guide finds applicability in a versatile range of scenarios, encompassing both field and factory applications.

IEEE guide is more sensitive to the condition of core and magnetic circuits, winding geometry and also interconnection. By using IEEE guide, the data from 20 Hz to 2 MHz can be analyzed thoroughly as each region of frequency sub-bands ranges cover overall part of transformer.

The IEEE guide is segmented into four main sub-bands, with a primary focus on addressing core, winding, and contact failures in transformers. In accordance with IEEE standards, Region 1 primarily deals with core and winding turn defects. Region 2 is sensitive to issues related to axial winding and the overall bulk winding of the transformer. For the detection of radial winding deformations, Region 3 comes into play, while Region 4 is associated with potential failures stemming from contact resistance, winding turn problems, or open circuit winding issues.

Table 2 provides a list of different fault types along with their corresponding effects on the FRA signature. This table can serve as a valuable reference for interpreting power transformer FRA signatures.

Conclusions

In conclusion, this article has provided a comprehensive introduction to Power Transformer Frequency Response Analysis signatures. We have explored the significance of FRA as a powerful diagnostic tool for assessing the health of transformers, especially given the aging infrastructure prevalent in the power industry today. Understanding the various fault types and their impact on FRA signatures, as detailed in Table 2, equips us with valuable insights into interpreting these signatures effectively. As we move forward in an era where reliability and efficiency in power distribution are of utmost importance, the knowledge and application of FRA signatures will undoubtedly play a pivotal role in ensuring the reliability and lifetime of our power transformers. With the continuous advancements in technology and the growing need for resilient power networks, embracing FRA as a critical asset in transformer maintenance and diagnosis is not only a necessity but also a step towards a more robust and sustainable electrical infrastructure.

REFERENCES

[1] Reza Khalilisenoberi et al., “Frequency response analysis (FRA) of transformers as a tool for fault detection and location: A review”, Electric Power Systems Research , Volume 155 ,?February 2018.

[2] IEC 60076-18:2012, “Power transformers - Part 18: Measurement of frequency response”.

[3] IEEE C57.149-2012 - IEEE Guide for the Application and Interpretation of Frequency Response Analysis for Oil-Immersed Transformers

[4] Abu-Siada et al., “Understanding Power Transformer Frequency Response Analysis Signatures”, 2013, Electrical and Computer Engineering Department, Curtin University, Western Australia.

[5] E. P. Dick and C. C. Erven, "Transformer Diagnostic Testing by Frequency Response Analysis," Power Apparatus and Systems, IEEE Transactions on, vol. PAS-97, pp. 2144-2153, 1978.

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