Electrical power transformers transforms electrical energy from one circuit to another using mutual induction between two windings. It transforms power from one circuit to another without changing its frequency. Transformers are tested to give an indication to which extent they comply with specified requirements set out by the Utility or Customer. It also determines their electrical, thermal and mechanical suitability for the system in which they will be applied. Certain tests are done at the factory (manufacturer) before the transformer is delivered to the customer and some tests are carried out at the consumer site on commissioning and periodically.



Ratio Test


The ratio test ensures that the transformer windings have the proper turns to produce the voltages required. The ratio is a measure of the voltage applied to the primary terminals to the voltage measured at the secondary terminals.


Winding Resistance Test


Measuring the resistance of the windings assures that the connections are correct and that there are no anomalies such as loose connections, high contact resistance in tap changers.


Short Circuit Impedance Test


This test is performed to ensure the stated nameplate impedance and the measured transformer impedance match. A high power voltage source is required to excite the HV winding of the transformer, whilst the LV is short circuited.


Load Tap Changer Test


The tap-changer contacts are tested for electrical continuity during tap-changing. By applying a constant current while tapping, the tester looks for open circuit transients during tap-changing. Bouncing or High Resistance Contacts cause arcing and possible tap changer or transformer failure if left undetected.


No Load Test


This test verifies the design and core performance by indication of the no load current drawn by the transformer. The magnetisation current is a factor of losses in the electric circuit, losses in the magnetic circuit, and losses in the dielectric circuit. A high power voltage source is required to excite the HV winding of the transformer at the impedance voltage whilst the LV is left open circuited


Tan Delta Test


The Tan Delta is a measure of the dielectric loss when the dielectric is subjected to an AC electric field. The AC field is a result of voltage applied between a conductor and the outer capacitive foil in the case of the bushing or C.T. and for transformers between the conductor and the tank or core or other windings. Results recorded on site should be compared to nameplate or factory recordings.


Sweep Frequency Response Analysis (SFRA)


SFRA tests are special tests, sometimes performed as part of the commissioning tests, during preventative maintenance and after fault trips. SFRA assesses the mechanical condition of transformers, and is one of several methods used to detect mechanical failures. The tests are to be done on the transformer prior to dispatch from the factory and similar tests done when the transformer is finally installed and commissioned on site. A comparison is then made of the reference thumb prints obtained prior to the transformer leaving the factory and the thumb print obtained during commissioning.



Insulation Resistance and Polarization Index Test (PI)


The purpose of this test is to check that the transformer is not put into service with compromised insulation. The insulation of a transformer can be compromised by the following causes:


  • Water contaminated paper or oil.
  • Heat damaged insulation due to severe overloading or a transformer fault.
  • Mechanical damage due to poor transport preparation or a fault.



  • Partial discharge
  • Oil dielectric
  • Functional/ Operational checks on Auxillary devices




    Specialists in High Voltage Testing & Diagnostics


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