Once oil filling is completed, various pre-commissioning checks/ tests are performed to ensure the healthiness of the Transformer/ Reactor prior to its energization. Various electrical tests are to be performed and their significance is given below in Table-8:
Table:8
|
Sr. No. |
Name of Test/ Check point |
Purpose of test/ check |
|
1. |
Core insulation tests |
Allows for investigating accidental grounds which results in circulating currents if there is more than one connection between the core and ground. |
|
2. |
Earth pit resistance measurement |
To check the resistance of earth pit provided for Transformer. In case, the resistance is more, proper treatment is to be given. |
|
3. |
Insulation Resistance (IR) measurement |
Test reveals the condition of insulation (i.e. degree of dryness of paper insulation), presence of any foreign contaminants in oil and also any gross defect inside the transformer (e.g. Failure to remove the temporary transportation bracket on the live portion of tap-changer part) |
|
4. |
Capacitance and Tan δ measurement of bushings |
Measurement of C1 & C2 Capacitance and Tand in UST mode. 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. |
|
5. |
Capacitance and Tan δ of windings |
Dissipation factor/Loss factor and capacitance measurement of winding is carried out to ascertain the general condition of the ground and inter-winding insulation |
|
6. |
Turns ratio (Voltage ratio) measurement |
To determine the turns ratio of transformers to identify any abnormality in tap changers/ shorted or open turns etc |
|
7. |
Vector Group & Polarity |
To determine the phase relationship and polarity of transformers |
|
8. |
Magnetic Balance test |
This test is conducted only in three phase transformers to check the imbalance in the magnetic circuit |
|
9. |
Floating Neutral point measurement |
This test is conducted to ascertain possibility of short circuit in a winding. |
|
10. |
Measurement of Short Circuit Impedance |
This test is used to detect winding movement that usually occurs due to heavy fault current or mechanical damage during transportation or installation since dispatch from the factory. |
|
11. |
Exciting/Magnetisin g current measurement |
To locate 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 flux in the core |
|
12. |
Operational checks on OLTCs |
To ensure smooth & trouble free operation of OLTC during operation. |
|
13. |
Tests/ Checks on Bushing Current Transformers (BCTs) |
To ascertain the healthiness of bushing current transformer at the time of erection |
|
14. |
Operational Checks on protection System |
Operational Checks on cooler bank (pumps & Fans), Breathers (Silicagel or Drycol), MOG, temperature gauges (WTI/OTI), gas actuated relays (Buchholz, PRD, SPR etc.) and simulation test of protection system |
|
15. |
Stability of Differential, REF of Transformer/ Reactor |
This test is performed to check the proper operation of Differential & REF protection of Transformer & Reactor by simulating actual conditions. Any problem in CT connection, wrong cabling, relay setting can be detected by this test. |
|
16. |
Frequency Response Analysis (FRA) measurement |
To assess the mechanical integrity of the transformer. Transformers while experiencing severity of short circuit current lo ses its mechanical property by way of deformation of the winding or core. During pre-commissioning this test is required to ascertain that Transformer active part has not suffered any severe impact/ jerk during transportation. |
|
17. |
Winding resistance measurement |
To check for any abnormalities due to loose connections, broken strands and high contact resistance in tap changers |
|
18. |
Dissolved Gas Analysis (DGA) of oil sample |
Oil sample for DGA to be drawn from transformer main tank before commissioning for having a base data and after 24 hrs. of charging subsequently to ensure no fault gas developed after first charging. DGA analysis helps the user to identify the reason for gas formation & materials involved and indicate urgency of corrective action to be taken |