What can be seen on the PKR-2M DRM graph?

The OLTC device in a transformer is one of its most vulnerable construction unit, which requires constant and careful monitoring. Strict requirements are imposed on it in terms of reliability, therefore, special attention is paid to its diagnostics. Timely check of the condition of the OLTC will reduce the cost of maintenance and repair of the transformer, and help to detect all malfunctions in advance, which will prevent a possible accident.

There are two methods for diagnosing the OLTCs: demountable – with the opening of the tank (partial or complete draining of the oil) and non-demountable, which is called DRM (Dynamic Resistance Measurement). The method consists in measuring the current through the winding when the OLTC is switching from one tap to another and the results can be shown in a view of oscillogram.

The use of non-demountable DRM diagnostic method reduces employee’s time and labor costs for the enterprise and the company as a whole. Of course, there is an important demountable diagnostic, but it is quite a long and technically challenging way to check the OLTC. Using the DRM method, it will only be necessary to open the contactor tank or pull out the OLTC if a malfunction is detected.

The SKB EP company takes into account modern needs, therefore it produces specialized instruments that have the non-demountable DRM diagnostics mode:

One mode in devices for various purposes identifying the different parameters:

  • The PKR-2M device is intended directly for checking the technical condition of all types of OLTCs devices – resistor and reactor ones. The OLTC diagnostic is not the priority function in milli-ohmmeters, but an additional tool for checking a transformer while measuring winding resistance.
  • All configurations for a specific OLTC are performed automatically in the PKR-2M, circular diagrams and oscillography are taken in three phases at the same time. Milli-ohmmeters perform phase-by-phase diagnostic.
  • An oscillogram DRM taken with PKR-2M as close as possible to the manufacture’s OLTC oscillogram taken in a demountable mode, which makes it easier for specialists to understand the results.
  • If necessary, it is possible to carry out additional, more extended tests using the demountable diagnostic method (oscillography, circular diagrams) with the same instrument PKR-2M. Using milli-ohmmeters you can only use DRM method of oscillography.

According to the tests results, it is necessary to read an oscillogram and figure out what is hidden on each segment (pic.3).

  • А-а: switching from the main contact to the arc-extinguishing contact;

  • а-а1: switching of the arc-extinguishing contact;

  • а1-b1: "bridge" position;

  • b1-b: switching of the arc-extinguishing contact;

  • b-В: switching to the main current-conducting contact.

Note: All listed segments can be defined using cursors in the software application for a PC for the PKR-2M.

In order to learn how to identify malfunctions, we need to monitor the dynamics and collect a database. We recommend to start such work from the moment when a new equipment is put into operation or after it is checked out with a complete demounting.

The ability to analyze DRM graphs makes it possible not only to identify if the OLTC devices is broken or not, but also to indicate the nature of the defect. That gives you an opportunity to exclude unnecessary disassembly of serviceable OLTCs.

We are waiting for your questions by mail skb@skbep.com or by phone +7 (812) 500-25-48.

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