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Transformers Measurements


We provide

  • commissioning works for medium and high voltage transformers
  • prior to initial energization measurements
  • complete maintenance and preventive program
  • services in case of damage, malfunctions or any other circumstance  

All the above are provided according to the latest applicable international standards and will be followed by relevant test reports.

High Voltage Transformer - ATE’s Testing VAN ready for measurements 
  • Insulation resistance measurements (IR – DAR – PI)

These are cornerstone tests for transformer condition monitoring. These tests indicate the insulating quality of a dielectric by applying a DC voltage and measuring the current flow to present the insulation value in ohms and trend this value over a time interval of 10 minutes. ATE uses a series of testers (up to 5kV) to perform the measurements



  • Transformers’ ac withstand test

In a medium voltage transformer it is essential to perform an ac withstand test to both low and high voltage side. Test voltage up to 6kV ac for low voltage side and to 50kV for high voltage side and test duration 60 secs.  ATE uses a series of testers (up to 60kV) to perform the measurements

  • Transformers’ capacitance and tan δ measurements – frequency sweep

The measurement and trending of tan δ values and the capacitances of the insulation of the transformer is an important tool in the condition monitoring of the equipment. The values and the trend will show degradation of the insulation and can be used in predictive assessment to determine the best time for costly maintenance. ATE uses the MEGGER DELTA 4000 test unit to perform measurements to the highest accuracy.



More over ATE performs frequency sweep, a MEGGER patent test method in order to trend tan δ values from 0.1 Hz to 480 Hz. Having formed the curve one can draw additional conclusions for the transformers’ insulation condition



  • HV Bushings capacitance and tan δ measurements

Measurement and trending of tan δ values and the capacitances of HV Bushings in order to verify bushings’ condition. ATE uses the MEGGER DELTA 4000 test unit to perform measurements to the highest accuracy.


  • Excitation current measurements

The exciting current is the current that maintains the magnetic flux excitation in the core of a transformer. An accurate measurement of exciting current will provide information about the condition of transformer's core. Unwanted circulating currents or unintentional grounds can affect the exciting current and indicate a problem. ATE uses the MEGGER DELTA 4000 test unit to perform excitation current measurements.


  • Insulating oil measurements

Insulating oil measurements are very critical for the monitoring of transformers’ condition. ATE uses DTS 60A unit and performs

  • dielectric strength – mean value – standard deviation
  • oil tan δ measurement

Oil chemical tests are performed by well known and established chemical laboratory


  • Transformer’s vector group verification – turns ratio measurements

Transformer Turns Ratio is the ratio of the number of turns in the high voltage winding to that of the low voltage winding. Upon delivery of new apparatus it is important to test the turns ratio and to ensure that the internal connections have been made correctly and that the transformer matches the specification. Transformer ratio can change due to several factors, including physical damage from faults, deteriorated insulation, contamination and shipping damage. The ratio should be routinely tested, as a transformer ratio changes of more than 0.5 percent from the rated voltage ratio, may hamper reliable operation. ATE uses 3ph turns ratio meter type ATTR-03B manufactured by VANGUARD INSTRUMENTS Inc.



  •  DC winding resistance tests

 Winding resistance measurements in transformers are of fundamental importance for the following purposes:

  • Calculations of the I2R component of conductor losses.
  • Calculation of winding temperature at the end of a temperature test cycle
  • As a base for assessing possible damage in the field

Transformers are subject to vibration. Problems or faults occur due to poor design, assembly, handling, poor environments, overloading or poor maintenance. Measuring the resistance of the windings assures that the connections are correct and the resistance measurements indicates that there are no severe mismatches or opens. Many transformers have taps built into them. These taps allow ratio to be increased or decreased by fractions of a percent. Any of the ratio changes involves a mechanical movement of a contact from one position to another. These tap changes will also be checked during a winding resistance test. ATE uses micro-ohmmeter WRM-40 manufactured by VANGUARD INSTRUMENTS Inc to perform the measurements



  • Transformers’ Impedance

This is an important test to perform as the short-circuit impedance tests on transformers is a vital tool in diagnosing winding deformation. All the flux in a transformer will not be able to link with both the primary and secondary windings. A small portion of the flux will link with either winding but not both. This portion of flux is called leakage flux. 

Due to this leakage flux in a transformer there will be a self - reactance in the concerned winding, which is alternatively known as the leakage reactance. The self-reactance associated with the resistance of the transformer is actually impedance. Due to this impedance there will be voltage drops in both primary and secondary transformer windings. ATE uses the MEGGER MLR 10 test unit to perform excitation current measurements.



  • Transformers’ overall wiring drawings check and functional tests 

It is very critical to perform all functional checks regarding correct wiring and appropriate operation to all alarm, trip and control circuits of a power transformer. Before first energization or during a maintenance program in a substation, ATE’s commissioning team perform the following tests and measurements


  • Thorough check of all wiring drawings regarding transformers control circuits



 Check and test 

  • Transformers’ alarms and trip indications  
  • Bucholz relay correct operation (alarm and trip)
  • Oil thermometer correct settings (alarm and trip) 
  • Oil thermometer correct operation
  • Winding thermometer correct settings and operation (signalling, alarm and trip)
  • Transformers’ cooling fans correct operation (first and second stage)
  • Transformers’ oil level correct operationOLTC oil level correct operation
  • Transformers’ AVR correct wiring, setting, program and operation