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Third-Party Inspection Transformer

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Third Party Inspection Transformer

Transformer is a static device that transfers AC electrical energy from one circuit to another through electromagnetic induction. It is either used to increase i.e. Step up or to decrease i.e. Step down the voltage level.

There are various types of transformer that can be described as below

Phase Concerning

  • Single phase 1F Transformer
  • Three phase 3F Transformer

Winding position concerning

  • Core Type
  • Shell Type

Transformation ratio concerning

  • Step up
  • Step down

Instrumental concerning

  • Current Transformer CT
  • Voltage Transformer PT
  • Auto Transformer

Services concerning

  • Power Transformer
  • Distribution Transformer
  • Instrument Transformer

Cooling base concerning

  • Air Natural (AN) or self-air-cooled dry type
  • Air forced (AF) or air blast type
  • Oil Natural Air Natural (ONAN)
  • Oil Natural Air Forced (ONAF)
  • Oil Forced Air Forced (OFAF)
  • Oil Natural Water Forced (ONWF)
  • Oil Forced Water Forced (OFWF)


  • To step up and step down the voltage level in Electric transmission, distribution.
  • Voltage Regulator for over voltage protection
  • Welding Machine: Transformer reduces the voltage level and increases the Current in welding process.
  • Rectification: The rectification process does AC to DC conversion. Rectification is important for HVDC transmission. The most common type of rectifier is your mobile charger.

The defects of transformer can be classified as

Internal defects are –

  • Dielectric interruption / insulation breakdown
  • Oil contamination and leakage
  • Rupture and twist of the winding
  • Tap changer open connection
  • Grounding mistake

External defects are –

  • Oil leaks
  • bushing of the breathers, over pressure valve, thermometers, oil level gauge, etc.
  • Defects on forced cooling fans, bucholz relay, exit of the current transformers of the bushings, etc.

Sudden Failures –

  • Most of the dielectric interruptions occur suddenly, especially due to lightning or to an abnormal tension, causing a direct failure. Excessive current by an external short circuit or by a mechanical hit also happen suddenly.
  • Disturbances by earthquakes and fires can accidentally damage the transformer.

Defects that develop slowly –

  • Deformation of the insulation materials and of the transformer windings
  • Insulation of the core.
  • Poor insulation due to a harsh operational condition such as excessive load.
  • Deterioration of the insulation materials.
  • Deterioration of the external insulation.
  • Defects in accessories.

Different types of National / International standard for transformer

IEC                  IS                     Subject

IEC 71                                     Insulation Co-Ordination

IEC 76 (1-5)    2026                Power Transformers

IEC 137           2099                Bushings for Alternating voltages above 1000 Volts.

IEC 214                                   On Load Tap Changers

IEC 296           335                  Specification for unused mineral oils for transformers and                                                                switchgear.

IEC 354           6600                Loading guide for oil immersed transformers.

IEC 437                                   Radio influence Voltage measurement.

IEC 518                                   Dimensional standardization of terminals for high voltage                                                                switchgear and control gear

IEC 529           2147                Degrees of protection provided by enclosures.

IEC 542                                   Application guide for On Load Tap Changers.

IEC 551                                   Measurement of transformer and reactor sound levels.

IEC 606                                   Application guide for power transformers.

IEC 616           557                  Terminal and tapping markings for power transformers.

Third Party Inspection for Transformers

All power transformers inspection and testing is done against the approved drawings, purchase order specifications, purchasers or company standards, and within the practices and rules of the country, state or province and any government decrees, laws, ordinance or regulation as may apply.

The applicable codes and specifications for a power transformer that is under the construction process is:

  • Design code
  • Purchase order specification
  • Purchaser’s standards
  • Approved drawings

The applicable codes and standards may be based on any international standards. This content is general and can be useful for all recognized standards.

Documents Required for Third Party Inspection of transformer

The list of documents normally is agreed upon in the Pre-Inspection meeting; the parties that participate in this meeting are the manufacturer, purchaser and third-party inspection agency representatives.

The list of documents required at the time of Third-Party Inspection

  • Power Transformer Purchase Order or Contract
  • Power Transformer Manufacture Quality Control Plan
  • Power Transformer Inspection and Test Plan
  • Power Transformer Data Sheet
  • Power Transformer Approved Drawings, (arrangements, schematics, wiring, single line)
  • Power Transformer Weld Procedures and Procedure Qualification Records (if applicable)
  • Power Transformer Material Test Reports
  • Power Transformer Calibration Certificates for Test Equipment
  • Power Transformer Special Procedures
  • Power Transformer Special Qualifications
  • Power Transformer Preparation and Painting/finishing Procedure
  • Power Transformer Packing List

Third Party Inspection for Transformer can be done at below stages

  • Material Inspection
  • Fabrication
  • Construction check
  • General Arrangement Check
  • Winding Resistance Test
  • Winding Ratios Test
  • Temperature Rise Test -Type Test
  • Load Loss Test (Copper Loss)-Type Test
  • No Load Loss Test (Iron Loss)-Type Test
  • Transformer Efficiency –Type Test
  • Induced Overvoltage Withstand Test (Overpotential Test)
  • Dielectric Test
  • Separate Source Voltage Withstand Test
  • Reporting
  • Release Note
  • Packing, Marking and Shipping

Material Inspection

This is inspection for inward / process material to verify its effectiveness that may be through material test certificates for compliance with specifications, and where appropriate, drawings.

The review includes document review on:

  • Maker’s name and location
  • Certificate number, grade
  • Batch number
  • Chemical composition
  • Mechanical Properties
  • Cast number (for metal parts)
  • Heat treatment condition
  • Surface finish (if required)
  • Structural observations, deformities, etc.
  • Ancillary items may be supported by test certification, or by certificates of conformity

After satisfactory material inspection, inspection visit report is provided, that contains the following items:

  • Confirmation of satisfactory document review
  • Record of the endorsement of certification reviewed/witnessed
  • Record of all non-conformities
  • Record of any tests witnessed and the result


This inspection is based on the inspection and test plan. The third party inspector checks following in accordance of approved drawings and applicable standards:

  • Fabrication, protective finishes and cleanliness of all tank areas and of formers and frames
  • Correctness of windings, terminations, insulation, spacing and general construction
  • Freedom from damage (with particular attention to insulation), contamination and foreign objects
  • Manually check security of wedges

Construction check – This inspection involves following check points

  • Paint using specified colour swatch, measure thickness, if specified Name plate
  • Outline dimensions, interface dimensions and clearances and other specified dimensions

General Arrangement Check – This inspection is about all instruments, terminal boxes, compartments, control switches etc. are correctly rated and are correctly located, with specified labeling. The inspection is done based of approved drawings.

Types of testing for transformer –

Winding Resistance Test – In this inspection a resistance of all windings are measured, with winding temperature at recorded ambient, using a milliohm-meter and acceptance criteria is power transformer datasheet.

Winding Ratios Test – In this inspection Primary to secondary winding ratios, for each phase, for all tapings are measured using a ratio meter and acceptance criteria is power transformer datasheet.

Temperature Rise Test -Type Test – This inspection is subjected to full load current at rated voltage (various loading methods may apply), with “normal” tap selected, with monitoring of oil temperature (lower and upper); cooling air temperature measured by at least 3 thermometers in liquid providing a time constant of 2H, placed at 1 to 2 meters from the cooling surface for “Air Natural” cooling (for “Air Forced” cooling, the thermometers should be within the air input flow, at 1 to 2 meters).

Load Loss Test (Copper Loss)-Type Test – This inspection is carried out with shorting links are applied (either to the primaries or to the secondary’s, dependent on test supply voltages). Voltage supply is applied to the opposing windings and increased until rated current is achieved. Input and output powers are measured and loss calculated.

No Load Loss Test (Iron Loss)-Type Test

Rated voltage is applied to primaries or secondaries, with the opposing windings open circuit. Input and output powers are monitored and loss calculated.

Power Transformer Efficiency –Type Test

Efficiency is calculated from the losses.

Third party inspector reviews the calculation in manufacture quality control records.

Third Party Inspection for Induced Overvoltage Withstand Test (Overpotential Test)

There are three methods; dependent on winding category, the test would be acceptable if no collapse occurs in test voltage.

 Dielectric Test

Test requirements vary with winding category and acceptable level is in according to applicable standard.

 Separate Source Voltage Withstand Test

The test is applied between each winding and the terminals of all other windings, core, frame and tank, all connected together to earth.

Testing instruments used for transformer inspection – Multimeter – 3 Phase test panel for SC/OC Test – 1 phase test panel for SC/OC Test – HV Tester (0 – 5 KV) – Meggar – Tong Tester – Multimeter – Watt Meter – Ammeter – Voltmeter – Auto Transformer – Oscilloscope – CT/PT – Winding Resistance Meter – Ratio Meter – Temperature Measurement test set – Torque Wrench – Pressure Gauge – Oil Level Indicator – Die Electric Tester


The third-party inspector provides an Inspection Visit Report (IVR) after each visit, as well as a final report summarizing the activities carried out during the power transformer production in accordance with the contract requirements and circulated within the time limits specified in the contract.

Third Party Inspection for Release Note

When required by the contract or purchase order, a release note is issued by the third party inspection agency and given to the manufacturer when the power transformers have been accepted. 

Packing, Marking and Shipping

The following points are checked:

  • marking on transformer as per IS:2026 (Part IV) – Transformer markings and polarity – Conventional Labeling H = high voltage side & X = low voltage side, Transformer nameplate data – Voltage ratings high side and low side values (no-load values), Rated frequency of transformer, Rated Apparent power of transformer, Impedance measured at indicated temperature. Base S and V values are the ratings of the transformer, Maximum allowable temperature rise, Insulating medium and cooling ( see previous slide), (Basic Impulse Level) Measures the maximum voltage stresses that the transformer can handle, Transformer connections,
  • Verify that the packing materials conform to specifications and accepted practice for the mode of transport.
  • Verify protection against humidity, abrasion, distortion and other damage
  • Verify shipping marks to marking instructions and record.
  • Verify handling marks.

Third Party Inspection Agency in India

The Third-Party Inspection Agency in India who are involve in the Third-Party Inspection services of Transformer – can apply above technical information for Third-party Inspection services and prepared the Inspection Checklist of Transformer inspection referring the customer QAP/ Different Standards/ Code of Practices. Which Cloud help the Third-Party inspection services provider to make sound decisions while doing the Third-Party Inspection services in India

Benefits of Third-Party Inspection

It gives product conformity at its relevant stage; it conforms determined process flow. It identifies the defects of the product at its relevant stage. It reduces the cost of further inspection. IT enhances customer satisfaction and relevance.

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