Diagnostic of transformer's OLTC with a modern instrument PKR-2

Power transformers are one of the most widespread and significant elements of power systems, so the reliability of networks, power plants, and power systems depends largely on the reliability of transformers. But at the moment, a significant number of transformers have already exceeded the minimum service life of 25 years stipulated by the standard, and the annual rate of renewal of the transformer fleet on average is no more than 3-5 %. In this regard, it is recommended to carry out planned preventive measures for timely detection of upcoming defects in transformers. This is especially true for on-load tap changers (OLTC), since up to 40% of the total catastrophic transformer failures are associated with this component.

SKB EP has developed a modern instrument PKR-2 (Fig. 1), which largely automates the process of checking the OLTC. PKR-2 is the second version of PKR-1 already proven in the market, which has received a lot of positive feedback and recommendations.

Fig.1 PKR-2 instrument for monitoring the transformer’s OLTC

Recording of the circular diagrams is a mandatory procedure during acceptance, after repair, and during inspection of the OLTC. Recording the actuation moments compared with recording the angle of shaft rotation helps understand the relative position of the contacts and their deviation from the correct position. In addition, by recording of the circular diagrams of the action sequence of the contacts of the selector and contactor during forward and reverse travel, you can judge the quality of the selector assembly by the backlash value. Then ten switching cycles are performed, and if there are no defects, it is considered that the switching device is repaired satisfactorily. Osсillography of the switching process allows you to detect delayed actuation, non-simultaneous actuation by phases, repeated contact actuation. Using PKR-2, it is possible to record the oscillogram and circular diagram simultaneously for three phases.

PKR-2 has the following unique features:

  1. Automatic adjustment to any type of OLTC without any additional external elements; 
  2. Recording of the circular diagrams for reactor type OLTC;  
  3. Recording of the circular diagrams for resistor type OLTC;
  4. Osсillography of contactor switching for resistor type OLTC;
  5. Estimating the value of current-limiting resistors, detecting bad contacts of contactors, selectors, and pre-selectors;
  6. These characteristics are measured simultaneously in 3 (three) phases for OTLC located "in neutral" of Y-connected windings;
  7. Characteristics are measured phase-by-phase for OTLC located "in line" of Y- or Δ-connected windings;
  8. The time for registering switching processes is adjustable from 0 to 20 minutes, which is enough to check all types of OLTC;
  9. To record the circular diagrams, the instrument is equipped with an angular movement transducer, which is installed on the extension of the drive shaft;
  10. The original locking of the transducer position eliminates the error of measuring the rotation angle due to the shaft eccentricity;
  11. The transducer is coupled with the shafts of various drives by a set of axes and bushings;
  12. Installation of the transducer, axes, and bushings is easy, simply put them on the shaft extension without using any tools. Adjustment springs ensure automatic locking of all parts on the shaft even if it has defects like bends, notches, and burrs; 
  13. Long contact probes allow you to measure the characteristics without draining the oil from the contactor tank;
  14. Without contact probes, the characteristics are measured when the oil is partially drained from the contactor tank, or after the OLTC is removed from the transformer;
  15. The resulting characteristics are displayed on a large color graphic display. The touch panel of the display simplifies and speeds up the user's interaction with the instrument when processing characteristics for determining the health of the OLTC.

The use of PKR-2 can significantly reduce the financial and resource costs for the organization, and improve the quality of diagnostics of transformers and avoid unscheduled repairs. As mentioned above, PKR-2 not only inherited the advantages of its predecessor PKR-1, but also acquired new additional features identified as a result of conversations with end users of equipment made by SKB EP.

When choosing SKB EP instruments, you rely on the time-proved quality and uniqueness of developments confirmed by feedback from large industrial facilities. In addition, SKB EP provides constant technical support to its clients and conducts field training seminars throughout Russia and neighboring countries. If you have any questions, please do not hesitate to contact us. We will definitely find a solution for you.

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