The diagnostic features of power transformer’s OLTC

On-load tap changer (OLTC) in a power transformer is a complex and insufficiently reliable power transformer component. Malfunction in this device may cause serious damage to the transformer as a whole. For reliable monitoring and diagnostics of such devices, the PKR-2(M) instrument is used. It can determine the main faults in the OLTC, control the operating sequence of its elements, record the actuation events, and record oscillogram for the switching process inside it. 

Diagnostics with the PKR-2M instrument

The presence of moving parts in transformers with the OLTC causes significantly higher labor costs for their repair and maintenance compared to a transformer without OLTC. One of the main checks of the OLTC during acceptance, after repair and during its revision is the inspection of the operating sequence of its elements. Comparing the actuation events compared with the angle of shaft rotation helps understand the relative position of the contacts and their deviation from the previously obtained diagram.

Oscillography of the switching process allows you to detect delayed actuation, non-simultaneous actuation by phases, repeated contact actuation.

Comparison of time points and circuit breaker positions makes it possible to detect incorrect operation of the reversing contactor and prevent "sticking" of contacts. This defect most often occurs if the contacts do not work for a long time.

To control the operating sequence of switching devices, the following checks are performed:

• for reactor type OLTC — a circular diagram of the actuation of contactors and selectors,
• for high-speed OLTC — also a circular diagram, and an oscillogram of contactors' operation is also required.
• to check the sequence of actions of contacts in an RNT-type OLTC, a circular diagram is recorded, and the contactor operation process is also oscillographed in high-speed devices of the RNOA, RNTA, RS, SCV, SAV, and SDV types.

  The obtained oscillograms are used to check:

• absence of breaks in the electrical circuit;
• duration of operation of arc extinguish contacts in the "bridge" position";
• duration of switching between the opening and closing events of auxiliary and arc extinguish contacts of different legs, during which the arc is extinguished;
• absence of unacceptable vibrations of the movable arc extinguish contacts of the contactor.

The main requirement for the monitoring instrument for OLTC is a reliable measurement of the switching sequence produced by the transformer voltage regulator.

All these characteristics of the OLTC can be measured by PKR-2(M), the monitoring instrument for OLTCs.

In addition, this instrument can be used to record contactor oscillograms and circular diagram simultaneously for all phases without installing additional external elements (jumpers or external resistors), unlike other competing instruments. Parameters of contactor oscillograms can be measured using cursors. However, while recording the circular diagram, the instrument can simultaneously record oscillograms with a sampling rate of 100 ms, which makes it possible to measure time intervals with high accuracy.

Regulatory documents for OLTC specify that current in the contactor circuit with closed contacts must be at least 2A. The measurement method used in the proposed instrument ensures this amperage for the main group of OLTCs not only through main contacts, but also through arc extinguish ones.

The PKR-2(M) instrument is a self-contained device in a protected case powered by AC 220 V mains. You do not need a laptop or personal computer (PC) to work with it. The device includes a rotation/angular movements transducer that can be installed on the drive shaft of the OLTC. A set of special attachments makes it possible to position the transducer on various drives of the OLTC. The transducer design is such that the measurement can be performed both when using an electric drive and a manual drive in case of a power outage.

PKR-2(M) offers a convenient user interface, a modern set of standards for communication with a PC (USB 1.1, LAN), and data transmission to a PC. It is used both in companies that operate transformers and in organizations that are engaged in major repairs of old and installation of new equipment. For example, the instrument is successfully operated at the Kostroma State District Power Station, a branch of OGK-3 JSC (Volgorechensk).

The opinion of employees of this company regarding this instrument:

"Our company uses the instrument with factory no. 1 to control the correct operation of the OLTC in the TRDNG-32000/220, TRDNS-32000, 40000, 63000/35, TRDN-25000/35 transformers and the ADOCTN-267000/500/220 autotransformer. I like the fact that it is possible to record the oscillograms of contactors and circular diagram simultaneously for all phases without installing additional external elements. You can view charts on the device display. The transducer design allows you to perform measurements both when using an electric drive and a manual drive of the OLTC in case of power outage." 

V.V. Svetushkov
Chief engineer, Kostroma State District Power Station, a branch of OGK-3 JSC, Volgorechensk

In the development of the PKR-2(M) instrument, we took into account the diverse wishes of users. Due to constant communication with users, SKB EP develops and produces modern, high-quality, and effective instruments for monitoring high-voltage switching equipment.