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Factors that affecting the repeatability of frequency response

Introduction

The comparison of SFRA results measured under different conditions could be one of the possible reasons for getting the wrong SFRA results assessment. Attention must be given to some factors, which are influencing the response of the SFRA traces. In this paper, it will be described how to identify those different factors and avoid their influence on the SFRA plots. Without this knowledge, it is often very critical or even impossible to distinguish between measurement mistakes and real damage inside the tested transformer.

SFRA is a powerful diagnostic tool because SFRA trace represents a fingerprint of a transformer winding’s construction and physical position within the main tank of the transformer. Therefore, if the SFRA trace deviates from a baseline trace, then it is probable that some components of the transformer are physically changed or the test was not repeated in the same way as the first time. Some of the following factors are influencing the results:

  • Residual magnetism effect
  • Tap-changer position
  • Bushings condition effect
  • Measurement direction
  • Measurement direction
  • Test voltage value
  • Poor grounding and measurement lead connection
  • Effect of the insulating oil
  • Effects of the tertiary winding

1. Residual magnetism effect

The residual magnetism influences the SFRA results. It is highly recommended that the SFRA test is performed before the DC tests on the power transformer or immediately after the transformer core has been successfully demagnetized. The magnetized core causes the traces to deviate (shifted to the right side) in frequencies less than 10 kHz only for the open-circuit type of tests, as shown in Figure 3. Moreover, the trace response in this area is mostly affected by the core. Short-circuit SFRA tests are not influenced by residual magnetism.

Shifting of low-frequency core resonance
Figure 1: Shifting of low-frequency core resonance to the right due to residual magnetism
(red trace – core magnetized, blue trace – core demagnetized)

2. Tap changer position

To compare FRA trace to a baseline measurement, the transformer must be tested in the same tap-changer positions as the baseline measurement. Changing the tap changer position will cause continuous changes of the curve shape over a wide frequency range, as shown in Figures 1 and 2. It is suggested to perform measurements for each relevant phase on the lowest, highest, and middle positions of the tap changer.

If the maximum tap is chosen, then the whole tap changer is examined by the test. This applies to both DETC and OLTC. Typically, the test will be carried out at a full winding position and neutral.

U-N open circuit test for three different positions
Figure 2: U-N open circuit test for three different positions – DETC; position 1 blue, position 3 red, position 5 green, 235 kV
U-V open circuit test for three different positions
Figure 3: U-V open circuit test for three different positions – OLTC; position 1 blue, position 9 green, position 17 red, 15.75 kV

3. Bushing’s condition effect

According to the relevant standards, SFRA tests must be performed with the same transformer configuration to get repeatable results. Sometimes, different transformer bushings could be used during factory acceptance tests and on-site tests, or for some reason, the bushings could be replaced on the transformer during the service life. SFRA tests on different bushings may cause differences in the high frequencies, as shown in Figure 4.

Bushing replacement effect on SFRA
Figure 4: Bushing replacement effect on SFRA (blue trace – new bushing, red trace – old bushing, 110 kV)

4. Measurement direction

One of the important details that should be included in the SFRA test report is on which terminal the signal is being injected and from which terminal the response signal is being measured (e.g. H1-H0 injection or H0-H1 injection). If not specified otherwise, it is recommended to connect the generator signal on the phase terminal and measure response on the neutral terminal. Comparing measurements made on the same side on power transformer but in different directions, discrepancies in the higher frequencies, as shown in Figure 5 could be visible.

Figure 5: Measurement direction effect on frequency response (blue H1 – H0, red H0 – H1, 110 kV)

5. Test voltage value

As previously explained, the response of the traces in low-frequency areas can be influenced by the remanence magnetism. Furthermore, since in the low-frequency region, the frequency response is dominated by core inductance it also depends on the output test voltages, as shown in Figure 6.

There is no influence of measurement voltage in higher frequencies. When a measurement is repeated on the same test object, it must be ensured that the same measurement voltage is used. Otherwise, there will be deviations in the low-frequency range that might lead to the wrong conclusion.

Influence of different output voltages on SFRA traces
Figure 6: Influence of different output voltages on SFRA traces (LV side – open circuit test)

The transformer core permeability varies with the applied voltage, which makes the SFRA responses at low frequencies voltage dependent. Consistent voltage is, therefore, very important for SFRA testing. A test instrument with variable output voltage (like DV Power’s FRA500) is suggested so that the test can be performed at the same voltage as the reference curve. FRA500 instrument offers test voltages in a range from 0.1 V up to 12 V (or 0.2 Vp-p up to 24 Vp-p).

6. Poor grounding and measurement leads connections

The IEC 60076-18 standard and CIGRÉ WG A2.26 guide describe the recommended procedure for a proper and reproducible measurement setup in detail. Ensuring the highest available frequency response in a noisy environment, the standards recommend for SFRA measurement the usage of shielded coaxial cables, which have to be grounded by wide flat aluminum braids. The center conductor of the cable is connected to the transformer terminal while the outer conductor is connected to the instrument chassis, which should be grounded. This practice prevents the center conductor from picking up noise. Separate measurement leads shall be used for each of the generator, reference, and measurement connections. Coaxial leads used for the measurement should be the same length and have a characteristic impedance of 50 Ohm.

To eliminate any influence of the grounding system on the SFRA results, the grounding braids should be always connected using the shortest path following the body of the bushing. This way of connecting is especially important for the repeatability of the traces in the high-frequency ranges.

6.1. Use the shortest ground braid connection

A large number of measurement errors occur due to inadequate shield grounding of the coaxial cables. To ensure the repeatability of the traces, especially at the high-frequency ranges, the shortest braid connection between the test clamp and bushing flange should be used. This is the only way to repeat the same connection for every measurement on a given transformer.

Good and bad grounding practice
Figure 7: Braid connection on the left bushing represent a good grounding practice, and connection on the right bushing represent poor grounding practice

6.2. Poor grounding of the transformer tank

transformer grounding
Figure 8: A very noisy curve is an indication for poor transformer grounding

6.3. Ground braid should not have electrical contact with the terminal contacts

Proper braid connection
Figure 9: Proper braid connection – left figure, bad braid connection – right figure (electrical contact between braid and terminal)
Red trace
Figure 10: Red trace – electrical contact between braid and terminal, black trace – proper connection

7. Effect of the insulating oil

Figure 11 shows the responses of the SFRA open-circuit tests performed on the HV side of the power transformer with and without insulating oil. The higher permittivity of the oil increases the capacitance, which means that the whole curve will be shifted towards lower frequencies since all the stray capacitances are increased by about the same factor. Changes in the frequency responses are expected in case the same transformer was tested in different conditions (e.g. was the transformer originally tested without the main tank insulating fluid and then later tested with the insulating fluid, was the transformer tested with the main tank filled with different types of insulating fluid, etc.). Such information should always be noted in the final test report.

HV open-circuit test with oil-filled and unfilled transformer tank
Figure 11: HV open-circuit test with oil-filled (blue trace) and unfilled (red trace) transformer tank

8. Effects of the tertiary winding

The phases of star-connected windings are only connected at the neutral (star-point), while the phases of a delta-connected winding are directly linked at the line terminals. This direct coupling has a profound influence on the frequency response of the phases of a transformer with a delta winding particularly in the frequency region dominated by the interaction between windings. Figure 12 shows the measurement results of the windings of the power transformer with the tertiary delta connection grounded and ungrounded.

If the tertiary delta connection is made outside the tank and earthed, for better phase comparison (in case of no other reference measurement available) the earth connection should be removed leaving the delta connection intact. Otherwise, the capacitive couplings among windings will be different for each phase, resulting in a very significant difference between the responses of three phases in the middle frequency ranges.

Open circuit test on LV side
Figure 12: Open circuit test on LV side (black trace – tertiary grounded, red trace – tertiary ungrounded)

9. Rules to achieve repeatability

  • Substation bus-bar connections (e.g., was the busbar disconnected or connected to the bushing terminals when the measurement was performed).
  • A high deviation in temperature can cause a minor shift of the resonances within the whole frequency range. The temperature differences cause changes in winding resistance and hence the amplitude of the frequency response.
  • All connections from the transformer except the tank ground should be removed.
  • The contacts of the bushings should be cleaned, and the connection clamps must be tightened firmly to assure reliable electrical contact.
  • Three shielded coaxial cables should be the same length.
  • Aluminum ground braids should not have electrical contact with the terminal contacts.
  • Aluminum ground braids must have low inductance, with a large surface and made of many small wires to reduce the skin effect at higher frequencies.
  • Aluminum ground braids connection to the base of the bushing (transformer tank is the reference potential) should be as short as possible.
  • Contact between braid and tank must be solid to get reliable results.
  • Detailed information about the test set-up should be stored together with the test data. Detailed photos of the connection setup are recommended as well.
  • The oil temperature does not affect the results considerably, but it is recommended to be documented.
  • To get the most suitable measuring conditions, the characteristic impedance of the measuring cables must be adjusted to fit the input impedance of the measuring instrument (usually a 50 Ohm impedance is widely accepted).
  • The experience has shown that in many cases, the condition of the tertiary winding (open/closed), during measurements in the primary or secondary windings could change the responses.

It is important to recognize measurement on-site mistakes and to repeat the questionable test after finishing the required corrections

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May 13, 2021

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