Peculiarities of Checking the State of the RNTA 35/320 OLTC Closing Switch

Parameters of the closing switch change-over oscillogram are among the indicators reflecting the state of high-speed OLTC devices:

  • For direct oscillogram recording (destructive diagnosis) of the OLTC devices of RS-3/RS-4 type the cover of the closing switch hatchway must be removed, test cables of MIKO-8 or PKR-2 (PKR-2M) analyzers must be connected to static contacts of a closing switch using special probes (delivered upon order) by fixing them on the wall of the closing switch tank. 
  • When oscillogram is recorded in the DRM mode (non-destructive diagnosis) using the MIKO-8 analyzer, a current-limiting resistor shall be connected (delivered together with the analyzer upon order).  If the closing switch oscillogram is recorded in the DRM mode using PKR-2 or PKR-2M analyzers, the measurement shall be performed in the mode of testing the OLTC’s resistor devices.

When recording the oscillograms of the OLTC devices of RNTA (RNOA) type in a similar way (via the upper hatchway of the closing switch) the problem of connecting the test cables to the OLTC device arises that implies the following:

  • First, the node of the OLTC devices of RNTA/RNOA type whose change-over oscillogram needs to be taken is combined with the node of selecting a branch, and during changing-over it moves from one moving contact to the other one. Therefore, cables cannot be directly connected to this node.
  • Second, in the tank of the OLTC of this type the static contacts belonging to different phases are located on different levels (e.g., Phase A contacts are on the upper level, Phase B contacts are on the middle level, and Phase C contacts are on the lower level).  It is, in principle, possible to connect to the upper level contacts (in some OLTC modifications of RNTA type), whereas it is rather problematic to connect to the lower level contacts.

Thus, the DRM (Dynamic Resistance Measurement) method incorporated in the MIKO-8 and PKR-2M analyzers is, actually, the main method of recording the oscillogram of the closing switch change-over for the OLTC devices of this type. When this method is used, the test cables of devices are connected to the HV and neutral terminals. LV windings are short-circuited (more detailed description of diagrams for analyzers connection to transformer terminals at different diagrams of its windings is provided in the Manuals supplied with the analyzers).

If works on the transformer are performed after oil discharge, you can connect to the OLTC contacts via a side hatchway of the transformer (Fig. 1).

In this case for recording the oscillogram of the closing switch chane-over using the PKR-2M analyzer, the clamps of A, B, C test cables are to be connected to any contacts of the appropriate phase (Fig. 2); COMMON (Общ) cable shall be connected to the Neutral terminal or to a zero bus of the closing switch. Measurements are performed in the mode of checking the OLTC’s resistor devices over three phases simultaneously.

If the MIKO-8 is used (Fig. 3), the measurements are performed phase-by-phase. One branch of its test cable is connected to any contact of the tested phase, and the second branch is connected to the Neutral terminal or to a zero bus of the closing switch. Measurements are performed in the DRM mode with mandatory connection of a current-limiting resistor included in the device kit.

For recording the oscillograms of the closing switch change-over of the RNTA-type OLTC device removed from the transformer tank, the groups of contacts belonging to the same phases shall be connected by a 1.5 mmcopper wire (Fig. 4) thus simulating their connection to the transformer windings taps. Points of test cables connection remain the same as when they are connected to the closing switch via a side hatchway of the tranasformer.

Conclusions. Despite considerable differences between operation of OLTC and RS3/RS4 and RNOA/RNTA types, recording the oscillograms of closing switch change-over using PKR-2M and MIKO-8 analyzers does not cause any difficulties. Despite different physical phenomena used by analyzers for destructive and non-destructive control methods of the oscillograms recording and different principles of measurements incorporated in the analyzers, the differences in the results of measurements are not considerable, which proves their equality for oscillograms recording.

To facilitate the choice of devices the table below gives comparison of the usability properties of MIKO-8PKR-2 and PKR-2M devices.

Usability properties
Analyzers manufactured by SKB EP
MIKO-8
PKR-2
PKR-2M
Measurement of electric resistance of windings
1.gif 2.gif 2.gif
Ability to record the oscillograms of the closing switch change-over at direct connection to contacts of the closing switch
1.gif
1.gif
1.gif
Ability to record the oscillograms using DRM-method
1.gif
2.gif 1.gif
The number of phases for simultaneous oscillogram recording
1 3 3
Ability of recording the circular diagrams of the OLTC devices
2.gif 1.gif
1.gif
Power supply from the mains
1.gif
1.gif
1.gif
Power supply from an in-built battery
2.gif
2.gif
1.gif
Power supply from an external battery (e.g., from a car battery)
1.gif
2.gif
2.gif

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