Dr Andrey A. Reykherdt, Senior Condition Monitoring Engineer, Select Solutions

One major issue with the SFRA measurements is their repeatability on-site in the range of high frequencies when it comes to periodic condition monitoring of transformer windings. SFRA measurements can be affected by a number of factors such as layout and grounding resistance of the ground extensions of SFRA cables which may lead to misinterpretation of the test results.

Power transformers are a key element of power networks and therefore their periodic condition assessment becomes essential. One of the most important aspects of the transformer assessment is the detection of mechanical deformations of transformer windings which can result in many cases, from short circuits in the network as well as reduced clamping pressure due to insulation aging.

Several methods exist for detection of displacement or deformation of transformer windings. Among all the most popular are three phase and single phase leakage inductance or impedance measurements. However, because these measurements utilise fixed frequency of 50 Hz or 60 Hz, the detection sensitivity is much lower in comparison with the SFRA method, where the measurements are typically performed at frequencies from few Hz up to 2 MHz [1].

As a result diagnostic frequencies in SFRA can be divided into the following three frequency ranges:

  • 5 Hz up to about 5 kHz, this range shows the effect of the transformer core, including core magnetization and residual flux;
  • 5 kHz up to about 500 kHz, indicates the effect of winding relationships incl. radial geometrical movements of windings relative to each other and
  • Above 500 kHz, indicates the effect of axial movements of windings, internal leads and tapping circuits.

The SFRA method is very sensitive to any mechanical movements in transformer windings due to the changes occurring in the distributed winding inductance and capacitance, which in turn can influence the characteristic frequencies of the input and output signal measured in terms of the magnitude and phase of its admittance [2-5].

However a number of different factors may influence the SFRA measurements and among the common factors are the following: (i) the layout of ground extensions of SFRA test cables across high voltage bushings, (ii) type of the ground extension in the measurements (round type or flat ground extensions) and (iii) the resistance between ground extensions and the transformer tank [6].

Figure 1 shows a common connection of the test cables and their ground extensions during SFRA measurements on a transformer while testing one of the phases. Figure 2 demonstrates two ways commonly used for connections of the ground extensions across the bushings with the ground extension away from the bushing (not recommended) and along the bushing (preferred).

As can be seen from Figure 2, the ground extensions can be connected in a different way along the bushing from the test cables at the top of the bushing to a flange base. It is important that the length of the ground extension corresponds to the length of the bushing every time the FRA measurements performed as the repeatability of the SFRA measurements in the high frequency range strongly depends on the layout of these ground extensions and the selected grounding points around turrets and flanges.

Figure 1. A common method of connecting test cables and their ground extensions to a power transformer for SFRA measurements

Figure 2. Ground extensions of SFRA test cables quite often connected along transformer bushings in such ways (earth away from HV bushing is not recommended grounding practice)

Figure 3 demonstrates a possible effect of the layout of the ground extensions with two different earthing points. The first measurement was performed with the ground extension ‘away from the HV bushing’ (A phase to Neutral) and then the earth position was corrected to ‘along the bushing’. Similar SFRA response was received on other phases providing good correlation of SFRA signatures (only C-N measurement is shown in Figure 3 for comparison purpose).

Likewise when earthing the ground braids to the transformer tank (turret or flange) ground resistance also plays a very important role as many old transformers have painted or rusted surfaces which should be cleaned to make a lower ground resistance for the ground extensions. Figure 4 shows this effect of grounding resistance between the ground extensions and the transformer tank on SFRA signatures.

Figure 3. Effect of different earth point and layout of ground extensions on FRA measurements; C phase to Neutral (C-N) measurement shows good correlation with A-N when using similar layout of ground extensions

Figure 4. Poor ground resistance at transformer tank when earthing ground extensions resulted in higher deviation of LV c-n measurement which was successfully repeated with lower earthing resistance of ground extensions

As mentioned before, a different type of ground extension can also affect the FRA measurements. Figure 5 demonstrates the calculated internal impedance of the round and flat type ground extensions according to [6]. Flat (braid type) ground extension of the measurement cables has the lowest internal inductance and as a result provides smaller influence on SFRA test results. Figure 6 shows the FRA results from 330 kV winding performed using two different ground extensions providing high and lower test cable ground impedance.

Figure 5. Calculated internal impedance of two types of ground extensions (round & flat) versus frequency for the same size of their cross section (20 mm2)

Figure 6. SFRA measurements using two ground extensions with high (round type) and lower impedance (wide flat type) of ground extensions

Conclusion

Experience of Select Solutions with SFRA measurements demonstrates that the SFRA results can be adversely affected if many aspects of the measurements are neglected. Misleading diagnostic results can be obtained at the frequencies as low as around 400 kHz possibly leading to a wrong conclusion on the condition of the transformer.

To achieve high repeatability of SFRA measurements the following practices are recommended:

  • Keep the ground extensions straight along the bushings and make them as short as possible.
  • Perform the measurements using only flat type ground extensions with low impedance.
  • Make sure connections are made to bare metal not to painted nor rusted surfaces.
  • Before starting SFRA measurements, connect the source, reference and test clamps together clamped on the highest voltage HV bushing with the ground extensions along the bushing. The resulting graphs should be a straight line of zero magnitude across the whole range of measurement frequencies. On completion of all SFRA measurements, perform another calibration check in the same way it was done initially. This will determine the highest repeatable frequency by comparison of the initial and the final SFRA measurements.
  • Perform the SFRA measurements in a consistent way to minimise external interferences.

High Voltage Test Services of Select Solutions is committed to perform SFRA measurements to the best abilities to achieve a high level of SFRA repeatability and quality of the measurements that allows users to obtain better interpretation of the test results.

 

 

[1] “Mechanical-condition assessment of transformer windings using frequency response analysis (FRA)”, CIGRE Brochure 342, WG A2.26, 2008.

[2] K. Feser, J. Christian, T. Leibfried, A. Kachler, C. Neumann, U. Sundermann and M. Loppacher, “The transfer function method for detection of winding displacements on power transformers after transport, short circuit or 30 years of service”, 12/33-04, CIGRE Paris, France, 2000.

[3] S.A. Ryder, ALSTOM Transformer Research Centre, Saint-Quen, France, “Diagnosing a wider range of transformer faults using frequency response analysis”, XIII International Symposium on High Voltage Engineering, Netherlands, 2003.

[4] J.A.S.B. Jayasinghe, Z.D. Wang, P.N. Jarman and A.W. Darwin, “Investigations on sensitivity of FRA technique in diagnostic of transformer winding deformations”, IEEE International Symposium on Electrical Insulation, 19 – 22 Sep 2004, Indiana USA, pp. 496 – 499.

[5] Zhongdong Wang, Jie Li, Dahlin and M. Sofian, “Interpretation of transformer FRA responses – Part 1: Influence of winding structure”, IEEE Transactions on Power Delivery, vol. 24, No.2, pp. 703-710, Apr 2009.

[6] Andrey A. Reykherdt and Valery Davydov, “Effects of Test Cable Ground Extensions on Repeatability of Frequency Response Analysis Measurements on Power Transformers”, IEEE Electrical Insulation Magazine, Vol. 28, No.3, pp. 26-31, May – June 2012.