Transformer Site Testing and Pre-Commissioning Method Statement

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Transformer Site Testing and Pre-Commissioning Method Statement
Table of Contents

Purpose

This method statement provides brief steps for testing & pre-commissioning transformers.

Following is a standard method statement for testing & commissioning transformers:

Tools Required

Portable hand tools and toolbox. 

Safety Tools 

PPE such as 

  • Safety Shoes, 
  • Safety Helmet, 
  • Safety Glasses, 
  • Fluorescent Vest, & 
  • Safety Gloves 

To ensure maximum ability of safe work & dust mask.

Transformer Testing and Commissioning Procedure

  • Inspect the transformer & parts for shipment damage and moisture. 
  • Check the nameplate & prints for appropriate voltages & external phasing connections to the line or bus. 
  • Calibrate all thermal gauges, hot-spot heaters, bridge RTDs, and associated alarm contacts.
  • The contact settings must be similar to the following:
    • One stage runs continuously (forced cooling).
    • 2nd stage at 80°c
    • 3rd stage at 90°c
    • Hot-spot alarm at 100°c (triggers at 110°c if applicable).
    • Top-oil alarm temperatures rise to 80°c at 55°c and 75°c at 65°c.
    • OA – no fans (or) pumps.
    • FA – fans running 
    • FOA – fans & pumps running
  • Check and measure all wiring from point to point, including fans, pumps, alarms, heaters, tap chargers, & any other devices on the transformer & interconnecting cables. 
  • All banks over 150 MVA must be vacuum dried.
  • Do not use test voltages on the winding throughout the vacuum drying process and make sure the terminals are shorted and grounded during oil circulation due to the high amount of static charge that may create up on the winding.
  • After the tank is completely filled with oil, validate that an oil sample was sent to the chemical lab and that the results have been entered into the bank test reports.
  • Record the oil level & temperature at the completion of the filling.
  • Power operate to ensure proper rotation of pumps & fans, as well as correct operation of the under load (UL) tap charger, if provided. Also, check the heater, alarm, & all other devices to ensure proper operation.

Testing Preparation

  • Prior to the starting of any long-term works, areas & access will be inspected to confirm that they are ready for permanent works to start, and that the environment is clean & dry.
  • Testing will start only after the equipment has been completely installed in accordance with the approved method statement for transformer installation and has been released for testing commissioning.
  • All materials and relevant documentation for this testing works must be checked by the responsible employees prior to the start of testing activities.
  • Before any equipment is accepted for testing, a visual inspection will be conducted if defective equipment is found, it will be reported to the consultant for further action.
  • The testing electrical engineer must confirm the availability of the required three phase AC power supply & single phase for testing activities.
  • The testing electrical engineer must confirm the suitability of proper access to the site area where the equipment is to be tested.
  • When an appropriate pre-testing inspection is completed, the testing work will start.
  • The testing engineer shall instruct the supervisor on the testing works and ensure that all necessary
    • Schematic drawings, 
    • Data sheets, and 
    • Test formats are available at the testing area.
  • The supervisor will verify that all of the necessary tools & equipment will be utilized for testing are available in the testing area.
  • After the testing is completed, the supervisor inspects all of the testing equipment and returns to the storage cabin.  
  • While performing the testing work, the testing engineer will complete all of the required test formats. After completing the testing work, the testing engineer will get the signature of the related officials.

Method of Testing of Transformers

Visual Inspection

  • Correct installation, assembly.
  • Damage, paint, and crack checks.
  • Tightness checks.
  • Check the earthing connection.
  • Labels/markings rating plate correction checks.
  • Ferrules/terminal numbering checks.

Insulation Resistance Test (IR Test)

  • Insulation Resistance Test (HV-Earth)
    • Isolate the power source to the transformer. 
    • Connect the HV live side of the terminal to Earth using a 5 KV megger
    • Check the reading after 1 minute & record the consistent value.
  • Insulation Resistance Test (LV – Earth)
    • To conduct an insulation resistance test (LV – Earth), connect the terminal’s LV live section to the Earth using a 5 KV megger.
    • After one minute, check the value and note it as steady.
  • Insulation Resistance Test (HV – LV)
    • To conduct an insulation resistance test (HV-LV), connect the HV live terminal to the LV component terminal using a 5 KV megger.
    • Verify the value after1 minute & record the stable value.

Break Down Voltage Test

  • The break down value (BDV) for transformer oil is measured using a 2.5 mm electrode gap and a BDV tester for both the transformer and the HV cable box. 
  • The results are documented in the included test report.

Voltage Ratio Test 

  • Apply the 3 phase 440V to the primary side and measure the primary & secondary voltages with a multimeter, for each tap. 
  • Record the values in the form.

Magnetic Balance Test on Normal Tap 

  • Apply the supply voltage to any of the three phases of the transformer’s primary side & measure the voltage in the other two phases. 
  • The above test must be performed for all three phases in combination.
  • Record the values in the attached form.

Winding Resistance Measurement

  • Check for abnormalities in tap changers, such as loose connections, broken strands, and high contact resistance.
  • Check the contacts at bushings are clean.
  • Place THE sensing probes over the conduct in which the contact resistance is measured (test current 10A DC). 
  • Monitor the reading & recording in the test format.

NOTE: Wait 24 hours after oil filling for the power factor testing.

Vector Group Checks

  • This test determines the phase relationship & polarity of transformers. 
  • Ensure that neither the primary nor secondary windings are connected to any cables or busbars.
  • Connect the “R” phase of the primary and secondary windings (short R phase for primary & secondary termination). 
  • Apply 3 phase (415V AC) to the transformer’s primary termination.
  • Measure the voltage at the primary and secondary terminations of all phases.
  • Observe the reading & record it in the test format.
  • To obtain the vector group, either apply the formula (or) plot the values in the graph.

Pre-Commissioning Checks for Transformers

Oil Temperature Indicator 

The mentioned check is carried out by stimulating a value & determining the continuation of performance. All transformers’ alarm & trip set values are checked as described above.

Winding Temperature Indicator

The aforesaid check is carried out by stimulating a value & determining the continuation of performance. All transformers’ alarm & trip set values are checked as described above.

Electrical Pressure Relief Valve

The above inspection is performed by lifting manually the plunger of valve & checking for continuity of performance.

Cooling Gear

The values and settings for the operation of oil pumps & fans motors should be checked. 

Alarm trip contact settings for differential pressure, oil, and water flow indicators should be checked where provided. 

Marshalling Box

The wiring connecting different accessories to the marshalling kiosk must be checked.

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