Transformer OLTC diagnostic: what instruments to use, what to check?

SKB EP specializes in high-voltage equipment diagnostic and produces the instruments for high-voltage circuit breakers technical condition control, micro- and milli-ohmmeters for the resistance measurement in inductive and non-inductive objects and also the transformer OLTC control instruments.

Among the most developing defects of transformers the malfunction of OLTC devices takes the second place after the malfunction of high-voltage bushing and often becomes the reason of transformer incapacitation in general. The transformer OLTCs have a high degree of sensitivity and require the timely diagnostics to prevent the accidental condition.

Among the most widespread reasons of transformer OLTC device malfunctions are:

  • disturbances of contactors and switching devices work,
  • contact air burn of contactors,
  • contactor mechanisms jamming.

To check the technical condition of transformer OLTCs of all types (both resistor and reactor), SKB EP company manufactures two devices PKR-2 and PKR-2M.

PKR-2 enables the direct diagnostic method oscillography with the tank cover opening, partial or full oil drain and also take the circular diagrams.

In addition to possibility of direct method diagnostic PKR-2M has the non-demountable check mode and allows the diagnostic without the tank removal and opening (DRM method). DRM graph bases on the measuring results and shows the current changing in the moment of contact switching. PKR-2M builds two graphs based on the DRM mode checking results: resistance-time relation and the conductivity-time relation.

Furthermore, the instrument is equipped with a built-in battery, which makes it more portable and easy-to-use. The internal battery power source allows for instrument exploitation during the work day without charging.




OLTC check mode


Demountable and non-demountable (the DRM method)

Built-in battery



Data transfer to external Flash drive



These instruments have no analogs on the market and are indispensable helpers for the companies specializing on transformers repair, especially, on replacement, modernization and adjustment of OLTC devices.

The instruments allow for identification of the response delay, time difference by phases, chatter during switching and other malfunctions. In dependence of the OLTC type the diagnostic results allow to receive various information. For example, for the devices of RS, RNOA and similar types of transformers it’s possible to identify the work correctness of the main and arc extinguishing contactors contacts, their state (burned or not), the integrity of current-limiting resistors. For the instruments of RNTA-35/320 type and analogic equipment, in which the contactor and the selection detector are replaced with the fine control switching device, it makes possible to check the main and arc extinguishing contacts, current-limiting resistor and also the state of all the switching device fixed contacts, through which the unfixed contacts move.

Besides the specialized equipment for the transformer OLTC devices diagnostic, SKB EP has two milli-ohmmeters MIKO-9A and MIKO-8M(A), in which non-demountable control of transformer OLTC devices (DRM mode) is an additional feature, but this doesn’t decrease its potential and allows to enlarge the control of transformer technical condition by measuring the resistance and implementing an additional control.

The main differences of the DRM mode in PKR and MIKO instruments are:

  • Milli-ohmmeters allow to implement the single-phase diagnostic, whereas PKR-2M implements the oscillography either (and taking circular diagrams) simultaneously in three phases.
  • The results of presenting the data (oscillogram/diagram) are obtained at the same format, but visually the DRM mode oscillogram of PKR-2M is maximum close to the form of graphs, taken by demountable method during the direct transformer OLTCs diagnostic, that makes the understanding of specialists easier.
OLTC diagnostic is an important and necessary process, therefore it’s important to understand issues, all the more considering the existence of automated digital instruments, which are aimed to simplify the control process and differ by the functionality for particular client’s tasks.

If you deal with OLTCs control or you have to implement its diagnostic, but don’t know what instruments to use, and in case you are a specialized contracting organization, which carries out the assessment of state and the repair of transformers, you can call the specialists of SKB EP at +7 (812) 500-25-48 or mail to and select to instrument for your particular goals and objectives!

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