Electrical Transformers Pre‐Commissioning checks



Pre-commissioning tests are performed to ensure to the healthiness of the Transformer. Pre-commissioning tests start after oil filling process is completed. Various electrical tests are performed, they are listed below:

Operational Checks on protection System:

Checks whether the protection devices are working good. It checks on the cooler bank (pumps & Fans), Breathers (silica gel or drycol), MOG, temperature gauges (WTI/OTI), gas actuated relays (Buchholz, PRD, SPR etc.) and simulation test of the protection system.

Insulation Resistance(IR) measurement:

This is the test for checking the insulation (i.e. degree of dryness of paper insulation), in the presence of foreign contaminants in oil and also any gross defect inside the transformer.

Capacitance measurement of bushings:

Measurement of C1 & C2 Capacitance, Changes in the normal capacitance of an insulator indicate abnormal conditions such as the presence of moisture layer, short ‐ circuits or open circuits in the capacitance network.

Turns ratio (Voltage ratio) measurement:

Turns ratio between the windings is to identify any abnormality in tap changers/ shorted or open turns etc.

Vector Group & Polarity:

To determine the phase relationship and polarity of transformers.

Winding resistance measurement:

To check for any abnormalities due to loose connections, broken strands and high contact resistance in tap changers.

Vibration measurement:

The measurement of vibration is an important control procedure that must be performed periodically, which can be the result of the movement of the core-spiral assembly and the shielding structure caused by the variable magnetic forces in time that results in the vibration of the tank and auxiliary equipment. These vibrations have detrimental effects, such as excessive tension in the core-coil assembly.

Magnetic Balance test:

This test is conducted only in three-phase transformers to check the imbalance in the magnetic circuit.

Frequency Response Analysis (FRA) measurement:

To evaluate the mechanical integrity of the transformer. Transformers while experiencing the severity of the short-circuit current lose their mechanical properties by means of deformation of the winding or core. During the precommission, this test is required to determine that the active part of the Transformer has not suffered any severe impact / shake during transport.

Floating Neutral point measurement:

This test is conducted to ascertain the possibility of the short circuit in a winding.

Measurement of Short Circuit Impedance:

This test is used to detect the winding movement that usually occurs due to a heavy fault current or mechanical damage during transportation or installation from shipment from the factory.

Magnetising current measurement:

To locate the defect in magnetic core structure, shifting of windings, failures, in turn, to turn insulation or problems in tap changers. These conditions change the effective reluctance of the magnetic circuit thus affecting the current required to establish the flux in the core.

Operational check on OLTCs:

To ensure smooth & trouble free operation of OLTC during operation.

Stability of Differential, REF of Transformer/ Reactor:

This test is performed to verify the proper functioning of the differential protection and REF of the transformer and the reactor simulating the actual conditions. Any problem in the CT connection, incorrect wiring, retransmission adjustment can be detected by this test.

Tests/ Checks on Bushing Current Transformers (BCTs):

To ascertain the healthiness of bushing current transformer at the time of erection

Difference between shunt and series Reactors