Instruments for transformer LTC diagnosis


Despite the fact that some power transformers in the energy systems were initially furnished with LTC devices, energy companies did not tend to apply them in the on-line mode. In case of considerable voltage change, (e.g., due to seasonal load changes) the transformers were shutdown, the necessary switchings were made, and they were put into operation again. To a great extent it is due to relatively low (with respect to a transformer as a whole) reliability of switching mechanisms, lack of equipment for their condition diagnosis, complexity of the required parameters measurement.

With more stringent power quality requirements (to the network voltage in particular) the energy companies have to use LTC according to their purpose (for voltage control under load). This originated demand for the corresponding analyzers and, as a consequence, a large number of different instruments and methods for LTC control emerged. In order to get a better idea on the instruments of Russian and foreign manufacture available at the market, let us first consider each method of LTC state diagnosis separately.

A complete list of required tests is given in RD 34.45-51.300-97 "Scope and standards of electric equipment testing"; methodical guidelines on the transformer condition analysis and on extension of its service life are given in RD EO 0410-02; and methodology of adjusting the switches of windings leads under load (manufactured by Bulgaria and GDR) are given in SO 34.46.606.

It should be mentioned that repair of "old" transformers without appropriate diagnosis and details of LTC restoration technology may be not only useless, but even  harmful and would reduce the transformer reliability. For example, widely used LTC of RS-3 and RS-4 types fail mainly due to their design drawbacks. Failures of the elements of those devices have the following priorities: contactors; pre-selectors, selectors, and then failures of other elements. 

Contactor defects that may result in its leaving the 'latch' are most frequent; they are often caused by nuts self-unscrewing that may lead to contacts burn and misalignment of kinematic elements; current-limiting resistors often fail as well. Along with the above faults the LTC devices of RS-3 and RS-4 type often have other drawbacks:   leakage between transformer contactor tanks, switchover delay due to spring metal aging (fatigue of metal), damage of a protecting membrane, damage of the insulation shaft of the selector; misalignment of the contactor kinematics. Selector contacts misalignment, contacts burn due to weak contact pressure, insufficiently cleaned contacts surface are most frequent defects of a selector and pre-selector.

Table 1. Main methods of LTC diagnosis


Method merits

Method demerits


Visual inspection

Simple in use, qualification is needed Only an experienced person may notice that something is wrong.

Only LTC nodes accessible for inspection can be examined. An interval between inspections is determined by the number of switches or by time, whatever comes first. Not all the nodes are accessible for inspection. Selector contacts are not accessible, for example, for visual inspection as they are at the bottom of the LTV tank.

Gas analysis

This method is sufficiently sensible towards different contact faults that lead to their overheating.

Limited area of application: if LTC and a transformer have common oil, you can never be certain that the problem lies withing the LTC contacts. Gas analysis can say nothing about mechanical defects of a drive.


Vibro changes

Transformer operates in the conditions of vibrations that can be considered as a testing impact during vibration tests.  This allows detection of problems caused by slackening or unscrewing of different joints without transformer shutdown, which allows early diagnosis of defects and avoidance of emergencies.  Switchings (on the loaded or unloaded transformer) give additional information on LTC condition (it is available in the sound signal spectrum).

For correct diagnosis you need a spectrum of a properly operating LTC and a corresponding base of typical faults spectra.


Temperature measurements

Temperature of different parts of a transformer is often the fist sign of higher contacts resistance. The measurements are made on the transformer under load.

Condition of contacts that are not loaded cannot be assessed.

Thermal imagers: 
Testo 875-1, 
Testo 875-2, 
Testo 881-1, 
FLIR B660 24, 
Fluke Ti32, 
Fluke TiR32, 
Fluke Ti25 
and others.


Power consumed allows assessment of springs condition, detection of drive seizers and other defects that raise or decrease power consumption by an electric motor.

It says practically nothing about electric resistance of contacts.

Watt-hour meters

Measurement of electric resistance of contacts

It characterizes the condition of selector, pre-selectors and contactor contacts. Measurements shall be taken in all the positions of selector and pre-selector contacts. Measurements are performed without opening the LTC tank by measuring the phase-to-zero or phase-to-phase resistance of transformer windings, or directly by connecting to LTC contacts.

In the former case (without tank opening) special procedures are needed to single out the electric contacts resistance out of the full circuit resistance; in the latter case (with LTC tank opened) the direct measurements are not always possible due to lack of access to the contacts to be tested.

Simple milliohmmeters

Non-destructive control (without opening the LTC cover): recording the time diagram of contactor operation of a high-speed LTC allows check of the current-limiting LTC resistors integrity, and assessment of the time of contactor switchover from one position to the other one.

Non-destructive control does not always allow checking the adequacy of selector and pre-selector operation.


Circular diagram recording

A circular diagram allows checking the lack of fixed contacts displacement, size of clearances, etc. Comparison of time and statuses of switches allows detection of abnormalities in the directional contactor operation and, thus, prevention of contacts sticking. This defect occurs if contacts were not operating for a long time. Faults in the contact system of a selector may be caused by incorrect contacts alignment (insufficient or excessive pressing, misalignments, etc.), by formation of an oxide film due to infrequent switches, by untimely motoring of a device, and by violations in the kinematic scheme. Therefore, if during the transformer operation the switchings using LTS are not applied (or if the number of switchings is less than 300 per year), and if current load exceeds 0.7 of the rated one, then 10 cycles of switchings using LTC device shall be done every six (6) months in the controlled range to clean contacts from oxide and sludge.

LTC cover needs to be opened.

and others.

Recording the time diagram of contacts operation

Oscillograms obtained allow one to check:

  • lack of breaks in the electric circuit;
  • time of arch-extinction contacts operation in the 'bridge' position;
  • switching time between opening and closing of auxiliary and arch-extinction contacts of different shoulders during which the arch is extinguished;
  • lack of impermissible vibrations in the moving arch-extinction contacts of a contactor.

In the majority of cases the transformer LTC cover shall be opened.

and others.

Table 1 shows that the market of special-purpose equipment for transformer LTC diagnosis offers a large range of instruments, both of Russian and foreign manufacture, that differ in methods for standard LTC parameters control and in the degree of convenience. Some instruments duplicate the LTC control methods prescribed in their operations manuals (but on a more advanced base), others are based on innovations that do not require connection of external resistors or connection elements. There are also instruments that require additional settings prior to measurements, whereas in others the basic settings are in the instrument's memory, which considerably simplifies their use. Instruments that integrate several methods for LTC control deserve special attention. Procurement of such equipment allows complex approach to transformer LTC diagnosis problems.

When selecting an instrument pay attention to availability of procedures for making measurements on your particular LTC and availability of fasteners for mounting an angle displacement transducer when recoding the circular diagram. Otherwise you will have to solve those problems yourself as due to design peculiarities you cannot always connect the measurement probes to the points indicated on the LTC electric circuit.

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