Certificates

The information presented here is only a summary of key points and is not comprehensive. The standards and requirements for maintenance and testing of medium voltage dry type transformers contained in NEC Article 450, Chapter 21.3 and IEEE Std. 57.12.91 should be followed.

1. Maintain a Temperature Log

Periodically read and record the transformer temperature controller at normal load and compare the reading with the log history.. All coils should read approximately the same temperature with a 3 phase load. A large difference or a persistent increase in operating temperature at constant load may indicate a problem with either the air cooling system or the transformer.

2. Perform Maintenance Only On De-Energized Units

Caution: The transformer must be de-energized (both primary and secondary circuits disconnected) and all terminals grounded before performing any inspection, cleaning, servicing, or testing on the unit. An Arc Flash suit should be worn when de-energizing the transformer, but once it’s de-energized, locked out, tagged out and grounded, an Arc Flash suit is not necessary.

2.1 Visual Inspection

Caution: This is a visual inspection only. It is not necessary to touch any part of the transformer when performing the inspection.

An air-cooled transformer requires periodic visual inspections. The frequency of inspection depends on how intensively the transformer is used. A transformer operating on load continuously 24/7 needs to be inspected more frequently than one operating on load only 8 hours 5 days a week. Since the transformer needs to de-energized for inspection and cleaning, maintenance should be scheduled during a normal shut-down period. Maintain a log of all observations and maintenance operations performed.

Look for adequate ventilation space around the transformer. Remove anything in the area that impedes airflow. Nothing should be stored in the transformer room. (NEC 450.21.3.5.1)

Open the enclosure and look for accumulation of dust or dirt on the surfaces of the transformer. These must be cleaned off while the unit is de-energized .

The presence of dust indicates that the transformer will require inspections at least every three to six months.

Look for any obvious discoloration of the windings which is a sign of overheating. Take a photo of the windings and report any discoloration to the plant engineer.

2.2 Cleaning the Unit

Caution: For safety reasons, physical contact with the transformer should be minimized during cleaning even if the unit has been de-energized , locked out and and grounded.

Clean any dust or dirt from the windings and cooling fans using either a vacuum or compressed air to avoid directly touching the components. Start by vacuuming off the parts followed by spraying them with dry compressed air with a pressure of no more than 20 to 25 psi. Do not use chemical cleaners to clean the surfaces of the unit.

Inspect all de-energized electrical connections, tighten all accessible hardware and replace any loose insulation or parts. This will prevent screws or nuts from working loose during regular operation.

2.3 Checking for Potential Problems or Defects

While cleaning the transformer, look for rust on the clamps and core steel, any carbonization or tracking on the windings and insulation.

Examine the windings and surfaces more closely for discoloration which could be a sign of overheating or a loose connection.

Check for cracked, discolored or loose insulators.

Check for loose, pitted, corroded or discolored electrical connections.

If possible, tighten or replace any hardware components; otherwise you should contact a professional to evaluate and repair the transformer.

Fans, motors and other auxiliary devices should be inspected and checked for proper operation.

Report all abnormal or suspicious conditions in writing to the plant engineer or facilities manager, who will determine if any additional testing or service is needed before re-energizing the unit.

3. Tests During Routine Maintenance

During routine maintenance, several tests of the transformer may be made addition to the inspection and cleaning. Performance of these tests are regulated by NEC 450.11.11. These tests are necessary only if discoloration, carbonization or other evidence of overheating or shorting is found. They will verify the correct operation of the unit and can detect issues that are not apparent from a visual inspection. Failure of any test requires further investigation and service before the unit is put back in service.

These tests are to be performed only while the transformer is de-energized (disconnected from both the primary and secondary circuits) and for safety and accuracy they should be performed only by a qualified engineer wearing protective clothing and using a Megger (or equivalent ) test kit.

3.1 Transformer Turns Ratio (TTR)

A TTR assesses the condition of the transformer windings and core. It tests whether the actual voltage ratio between the primary and secondary windings (for each phase and tap point) agrees with the nameplate or certified values. They should agree within ± 0.5% (NEC 450.11.2 & IEEC57.12.91.)

3.2 Insulation Resistance Test (IR)

The IR tests the integrity of the insulation between coil winding conductors. Each transformer rating has a minimum required resistance level measured in Mega- Ohms. Readings below this level indicate a degradation of dielectric strength. This is a warning sign of a potential transformer failure. (NEC 450.11.11.3)

3.3 Winding Resistance Test (WR)

The measured resistance of the coil windings for each the voltage taps being used should be within 1% of the factory tested value or the most recent test. (NEC 450/11.11.7)

The normal insulation and winding resistance levels of your ELSCO transformer can be obtained from the documentation supplied with the transformer, or by calling ELSCO at 513-275-5781 and supplying the transformer serial number.

3.3 Other Tests

Other diagnostic tests require special equipment and facilities, and should be performed experienced professional engineers.

ELSCO performs a series of certification tests on every new dry type transformer and documents the results in the certificate that is shipped with the unit. If you have questions about the test performance of your ELSCO dry type transformer or need advice regarding additional diagnostic testing, call ELSCO at 513-275-5781 and talk with one of our transformer engineers.

Useful Economic Life of ELSCO Dry Types

Ownership of an ELSCO dry type transformer is an investment in quality that pays off in four ways:

Lower installed cost due to ELSCO’s simple installation protocol that can save thousands of dollars at the outset,

Higher power efficiency that yields significant savings in annual utility bills,

Lower maintenance costs of a high-quality dry type,

Longer useful life – 35 years or more – due to a cooler design and high-performance insulation.

These benefit payoffs from an ELSCO dry type transformer result in a lower Total Cost of Ownership compared to alternatives.