True dynamic resistance measurement: Pinpointing concealed problems inside a resistive type OLTC before physical inspection

15 Sep 2021
True dynamic resistance measurement: Pinpointing concealed problems inside a resistive type OLTC before physical inspection

- The maintenance team at the Electric Power Transmission Network in the City of Karbala performed routine electrical tests on the transformer including: winding resistance measurements (WRM), transformer turns ratio (TTR), short circuit impedance (SCI), excitation current, and line-frequency power factor (a.k.a.dissipation factor or tan delta).

- The maintenance team’s TRAX instrument afforded the opportunity to complete advanced testing also, including: magnetic balance, Frequency Response of Stray Losses (FRSL) and OLTC Dynamic Resistance Measurement (DRM) tests.

- Only the DRM test results clearly flagged a deficiency in Phase B of the OLTC, which was not observed in static measurements. Static measurements are those that are performed while the OLTC (and DETC) is stationary at each of a number of selected tap positions. Examples of tests that provide static tap changer measurements include excitation current, TTR, WRM, and sweep frequency response analysis (SFRA).

- Dynamic tap changer measurements, such as provided by a DRM test, are critical to assess a resistive-type OLTC’s bridging or ‘transition’ components, which only appear in a test circuit during the OLTC’s transition from one tap position to the next. A resistive-type OLTC does not use its bridging state as a viable tap service arrangement/position, so these components never appear in the test circuit of a static measurement.

- Figure 5 is a typical transition DRM response for a 2-resistor switch that has been in service for some time. Resistance values and transition times for both resistors are similar.

- Figure 6 shows something that is far from normal, wherein the apparent resistance goes all the way up to 300 Ω. After 20 ms, the resistance drops to a stable 12 Ω, indicating that the second transition resistor has made contact.

- The DRM test results lead to a much more specific conclusion (than that rendered by the ripple test results alone). The diverter switch operates in both directions; therefore, the fault sometimes comes at the second transition resistance and not at the first one.

- The DRM analysis identifies a problem/deficiency with one of the transition resistors of phase B.

Action Taken:
- Based on the DRM analysis, the owner decided to inspect the OLTC thoroughly. After cleaning the OLTC, a loose connector was found (Figure 7).
- The broken contact (Figure 8) was welded back in place (Figure 9) and the OLTC was reinstalled.
- A verification test was performed to verify the OLTC condition after repair. Figure 10 shows the ripple for each transition for all three phases. Now things look normal. Routine electrical and dielectric testing on the YNd1 25 MVA transformer did not provide clear indication of mechanical damage in the seventeen-position resistive type OLTC.

- True DRM allows for identification of faulty components affecting the resistive characteristic of the transition resistors in the OLTC.
-Megger suggests that a make-before-break test is included as part of the static winding resistance test.
- The DRM procedure does not add significant testing time, but it shows great value in identifying defects in transition resistors or their connections to the transition arcing contact.
- The normal WRM test did not indicate any problem, nor did any of the other tests performed, except for the DRM test. DRM testing is useful for power transformer OLTC assessment.

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