Transformer receiving inspection

Author: Lawrence Kirchner


The purpose of inspecting a transformer upon arrival is to determine and report damage (if any) to the manufacturer as a result of mishandling, vandalism, or an accident during transport. Inspection shall be performed before the transformer is removed from the train car or truck bed. The transformer can be off-loaded at the discretion of a trained Inspector, service manager, or representative from the manufacturer. Below you will find a number of tips. You should always consult with your manufacturer for any specific instructions regarding inspection and testing of your new transformer.


Necessary equipment

  • Insulation Resistance Tester (Megger).

  • Cosa dew point meter

  • Cell phone

  • Laptop computer with the correct software and cables to attach to the electronic impact recorder, if applicable

  • Oil sample kit including DGA syringe

  • Digital camera (with correct date / time stamp enabled)

  • Tape measure

  • Regulator and source of tested dry nitrogen

  • Extension Ladder and hand tools.



There are many potential safety hazards during transformer inspection. Below are a few things to consider.

  • Harness/lanyard must be used while working aloft

  • Ladder must be tied off to prevent lateral movement

  • Be aware of transformer tank pressure/vacuum

  • Block control cabinet doors when open to prevent accidental closure while inside

  • The bed of the transport may be slippery due to rain or snow, so be careful

  • Do not jump on or from the transport bed to the ground

  • Check cell phone service availability before starting work. It is not a good idea to be working alone in an area where there is no or poor cell phone service. If you are working alone, let someone know where/when you are working. Check in periodically.

Environmental concerns


If handling oil, use buckets, rags, and diapers to prevent any oil from contacting the ground, collect any and all trash around work site before leaving.


Transformer concerns

  •  An indication of ZERO on the transformer pressure/vacuum gauge is an indication the tank is not sealed

  • Never open the transformer during inclement weather

  • If leaning over an open manhole: make sure nothing falls into the unit. Empty pockets, tie eye-glasses and pencil to person.

Visual inspection

  • Is the transformer positioned symmetrically on the bed of transport? Has the unit shifted on the bed? Is there any evidence such as slide or skid marks on the bed surface? Quite often the shipping company will attach blocks to the transport bed against the transformer base to prevent movement. Are the blocks still in the intended position?

  • Inspect paint for signs (scrapes, dings, etc.) of impact from tree branches or other foreign obstacles.

  • Inspect all mounted accessories for damage (i.e. junction boxes, conduit runs, temperature gauges, valves, control cabinets and other types of enclosures)

  • Compare tension on all tie-down chains, rods, or straps. Are all comparatively tight? Any broken or missing?

  • Report any/all signs that unit has been vandalized such as graffiti, control cabinets opened or damaged, cover plates removed, gauges broken, copper buss or tubing removed, etc.

  • The trailer or rail car should also be inspected for evidence that it may have recently struck something. Look for areas of paint missing, fresh scrapes, fresh bends in supports/webbing, etc.

  • The top of the transformer is most vulnerable to shipping damage. Verify that bushing or bushing flanges have not been contacted by a foreign object.

  • Note the position of the DETC and LTC if so equipped. The manufacturer will often place a wire or wire tie through the locking tab on the DETC and/or control cabinet doors as an indication of “as shipped” condition. Have such indicators been removed?

  • Take photos of each of the 4 sides of the transformer and of the cover if possible. Take detailed photos of anything considered to be unusual or suspect to shipping damage.

  • If unit is shipped oil filled - Check entire unit including the cover for leaks. Check the oil level. If the unit is a nitrogen blanketed type, it is common to overfill the main tank the amount equal to the radiator capacity.

Take inventory upon arrival.

  • Using the shipping documentation, verify that all parts are with unit.

  • Insure that none of the parts are damaged due to improper packing. For example: Is the control cabinet wet inside?

  • Coolers, radiators, and pumps are generally shipped with cover plates and are positively charged with dry nitrogen though a Schrader valve. Do all items still have positive pressure?

  • Have items been damaged inside crates due to shifting?

  • Check oil level in bushings.

  • Check for leaks.

Impact recorders / indicators


Transformers will be equipped with devices to sense impacts or forces experienced during transportation. Such devices can be mechanical or digital impact recorders, impact indicators, or GPS tracking. These sensors are used as verification of careful shipping.


Mechanical recorders (Impact-o-graph) function similarly to a seisometer. A roll of paper is continuously pulled under a set of styluses to generate a chart. Three styluses are used to represent shock in the X, Y, and Z directions. Longitudinal (along length of car) in excess of 3 Gs is considered to be excessive and would be an indication that the car was Humped. Internal damage could result from improper car coupling. Vertical forces in excess of 1 G also indicate rough handling.


Digital recorders electronically records impact data internally in three dimensions (Lat, Long, Vert). The acceptable limits are pre-programmed by the transformer manufacturer. These devices are not intended to be exposed to the elements and are therefore, generally mounted inside a weatherproof compartment. The receiving agent (you) cannot see the actual recorded data that the unit experienced. There are lights on the display of the unit that will indicate if the actual forces exceeded the pre-programmed limits. In the event that one or more of the indicating lights are lit, the manufacturer must be notified. The manufacturer has the ability to download the data for review and determination of acceptance.


Small indicators are normally installed on multiple locations around the transformer that range in force limits. Two different styles are often used, spring and ball or element type. These are simple pass or fail indicators that visually represent an exceeded force limit. At the time of inspection, the condition and G rating of all indicators should be recorded. If any are tripped, the factory is to be notified. When a spring & ball indicator fails, the inner pieces are visibly dislodged and jumbled about. Tripped element indicators will have a color alteration to visibly notify an inspector.


Global positioning features provide real time and place of event(s). GPS also serves to verify location of unit for receiving purposes. The device records impact data in 3 dimensions (Lat, Long, Vert.) and reports data via cell phone every 2 hours to a web based storage location. The manufacturer, and with permission, the buyer can access data via the internet at any time during shipment. The system can be set up to automatically notify the interested parties when and if the unit experiences an impact. There are no lights or indicators on the unit that would tell the receiver if the unit had experienced an impact exceeding limits set by the manufacturer. The web based data must be reviewed by the manufacturer and/or receiver.




It is recommended to consult with the manufacturer when conducting tests. Typical receiving tests would involve the following:


Core ground Megger


Often, larger units have the core grounded through an externally accessible bushing. It is most undesirable to have an inadvertent ground in addition to the intentional ground.

  1. Remove intentional ground from core bushing.

  2. Using Insulation Resistance tester, apply 1000VDC between core and ground.

  3. If both Core and Frame ground leads are brought out, use the following connections for testing:
    •  Core vs Ground          (guard Frame) 
    •  Frame vs Ground       (guard Core) 
    •  Core vs frame             (guard Ground)

  4. Contact manufacturer if measured value is less than 200 mega ohms.

  5. Reconnect intentional ground(s) when finished.

Dew point


Often, larger units are shipped with oil removed. These units are instead filled with dry air or dry nitrogen. The manufacturer will have noted in the shipping documents, the temperature/pressure/moisture ppm of the shipping gas.

  1. Pull a sample of the shipping gas and measure dew point with COSA or equivalent instrument. Moisture content in gas is directly proportional to the moisture content of the winding insulation.

  2. Attempt to perform the test when the ambient and gas temperature are close as possible to one another. i.e. early morning hours.

  3. Compare measured dew point of shipping gas to that noted in shipping documents. If no data exists or manufacturer cannot be contacted, values of -40 degrees F, or lower are generally acceptable.

Because of temperature differences between manufacturing location and delivery point, it is common for the unit pressure to be different (higher or lower) than documented in shipping papers.


The unit could potentially be under vacuum. If so, connect source of dry nitrogen through a regulator and increase tank pressure to approximately one psi. Wait a minimum of two hours before attempting dew point test.


A gauge reading of zero could be an indication of a tank leak. Make sure that the valve between the tank and the gauge is open for test. Contact manufacturer if measured value is greater than 0.5% RH.


Oil samples


If oil filled,

  1. Pull syringe sample from bottom of unit for DGA and water content analysis.

  2. Check instruction book and nameplate for certified PCB content. If you cannot find where the manufacturer performed a test, pull a sample to be tested.

  3. Pull another sample to run a complete battery of physical property tests including acidity, color, particulate count, etc.

Unit shipped empty


An oil sample from the new unit shall be collected from the bottom drain valve of the new unit during the receiving inspection. The sample shall be sent to a certified laboratory for the following minimum testing: moisture content, particulate count, and D1816 dielectric breakdown. Failure to meet values given in IEEE C57.106 may result in the need for additional oil circulation at additional cost. In these cases, please consult manufacturer for guidance.


Internal inspection


Caution: Transformer may be filled with nitrogen and not dry air. If this is the case, unit will have to be vacuumed for minimum of two hours and then backfilled with dry air.

  • Internal inspector must wear paper suit including paper covers over shoes.

  • Do not open unit if there is any potential for rain or snow.

  • No loose items can be taken inside unit. For example: Eye glasses must be tied to person with a strap. Flashlight must be connected to person in some fashion.

  • Be careful not to step on or otherwise damage lead supports, tap changer, leads, insulation, blocking, top core laminations, etc.

  • Check mounting of C&C to tank.

  • Check core laminations for evidence of shifting.

  • Check DETC for damage.

  • Check lead insulation / supports.

  • Check for loose components.

  • Check for dislodged / loose coil blocking.

  • Check for debris (paper, metal flakes, paint chips, etc).

  • Check for broken / cracked welds.