How To Witness Test A Transformer—Part 1

Before a manufacturer sends a transformer to your site, it conducts various tests. To ensure performance requirements, you may want to witness the testing and inspect the unit yourself.

Does a new transformer represent a major expenditure for your company? Not only does such an investment cost thousands of dollars, but it's also vital to the ongoing operation of your business. Reliability is obviously a top priority here. So, how do you ensure equipment integrity and performance? Witness testing is one way to make sure your new transformer meets industry standards and will provide quality performance after installation.

About 75% of the transformers purchased by utilities undergo witness testing, compared to about 10% of those built for industrial/commercial applications. Witness testing is also more common for larger, complicated designs. Often, as customers forge a strong working relationship with a manufacturer, they're less likely to witness test every transformer. What does witness testing require? Just a visit to the plant to examine the new transformer and watch it perform during various tests. The success of a witness test depends on proper preparation by the manufacturer and you, the purchaser. Here are some useful tips.

For the manufacturer:

• Notify inspector, who will witness the test two weeks in advance of test date to confirm schedule. Then, verify this information three working days before the test.

• Complete routine tests before the inspector arrives to prevent delays in witness testing. However, you, as the inspector, may wish to witness the routine tests.

• Confirm calibration of instruments as scheduled. Use standard data sheets to record test values.

• Provide test certification and data sheets as standard documentation. (See ANSI/IEEE C57.12.90-1993, IEEE Standard Test Code for Liquid-Immersed Distribution, Power and Regulating Transformers and IEEE Guide for Short Circuit Testing of Distribution and Power Transformers, and C57.12.91-1995, IEEE Standard Test Code for Dry-Type Distribution and Power Transformers, for minimum information on test requirements.)

For the customer:

• Completely understand the tests you'll witness.

• Designate you or someone else as the inspector when you place the order for the transformer. Notify the manufacturer so you or your representative receives all applicable documentation.

• Notify the manufacturer of any discrepancies in documentation.

• Accommodate the manufacturer's schedule to avoid possible cost overruns.

• Review all test documentation before arriving at the plant so you're familiar with the design parameters.

What to do at the site:

• Review the purpose of the tests and any associated procedures with the manufacturer's test engineer prior to commencing the tests.

• Discuss which tests are of particular importance so the manufacturer can give them extra attention.

What to look for during testing:

• Understand how to read results and how they impact performance.

• Listen to the transformer's sound level when the technician applies voltage, particularly if you've requested a sound test.

• Allow sufficient time for tests. Some tests, such as an induced over voltage test, require 15 min of setup time and last as little as 18 sec. Others, such as taking temperature readings, can last overnight.

After the test:

• Conduct a physical inspection of the transformer, using the following checklist as a guide:

• Transformer grounding positions are correct.

• Paint color is as specified.

• Name plate, cautionary plates, etc. are correct and in an easily readable position.

• Transformer terminals allow for reliable connection and cable support in the field.

• Accessories are in the proper location.

• Gauges and monitors are positioned for readability and accessibility

Alternatives to witness testing. Although manufacturers usually don't charge for testing that doesn't incur unreasonable expenses, you must pay for your inspector's time and travel. If this is prohibitive, you should consider the following less expensive alternatives.

• Company audit.

  If you plan to or already work repeatedly with a single manufacturer, conduct a quality audit. Audits, which are standard among major corporations, include an in-depth inspection of the company's facilities and a review of procedures to determine if the manufacturer pays proper attention to all design and manufacturing processes. When you've identified, and the manufacturer has resolved, any issues of concern, you can forego future witness testing or do so only on an as-needed basis.

• Design and production partnership.

  Less formal than an audit, this partnership encourages you and the manufacturer to work together throughout the order-production-testing process. You receive detailed test reports before shipment. Careful examination and discussion between you and the test engineers, or manufacturer representative, can provide the same information as a witness test.

• Similar unit test.

  You'll receive results from tests conducted on similar units to provide an early view of your transformer's performance. These reports are available at any time during your transformer's construction life. Getting the transformer you want means effective coordination. Whatever level of product quality review you choose, remember partnering with your transformer manufacturer is key to getting the unit you want. If you change your specification requirements, let the manufacturer know of the changes immediately to minimize the cost later. Lastly, but most importantly, witness testing can provide you a better understanding of your transformer's operation.

Editor's Note: In Part 2 of this series next month, we'll feature a table describing routine and design tests. We'll also include information on specific test setups, procedures, test equipment, and pass/fail criteria. On a special note, we'll provide possible causes for failures and suggested corrective actions. This will be a definite clip-and-save reference for electrical engineers and designers.

Sidebar: Tests You Can Witness

All transformers undergo two kinds of industry-standard tests, routine and design, to ensure the transformer will perform as designed. Further optional tests explore the quality of the transformer's construction, assurance, and adherence to standards.

Manufacturers in the United States perform tests in accordance with applicable IEEE and NEMA standards (as approved by ANSI procedures). Overseas customers may require their transformers comply with IEC (International Electrotechnical Commission) specifications. Generally, the manufacturer repeats tests for witnessing for only those you specify. If you have no specific requests, the manufacturer will conduct routine ANSI tests. These include:

• Ratio and phase relation;

• Resistance;

• Excitation loss and current;

• Load loss and impedance;

• Applied voltage; and

• Induced voltage

Design tests, also called type tests, are required only on one of a series of similar or duplicate units. However, you may request the manufacturer perform any of the design tests (usually impulse tests) on any unit. These include:

• Impulse;

• Short circuit;

• Temperature (heat run);

• Insulation power factor;

• Insulation resistance;

• Sound; and

• Partial discharge