How can plant personnel successfully maintain low-voltage dry type transformers?

 

Low-voltage dry type transformer (LVDTT) insights

• With life expectancies typically in the 30-40 year range, low-voltage dry type transformers (LVDTT) remain ubiquitous electrical gear with little to no interaction with humans until something goes wrong. Like other electrical equipment, periodic maintenance is recommended to provide a safer and longer-lasting service environment.
• Having an electrical equipment preventive maintenance program that includes LVDTTs will help prevent premature failures and provide the best opportunity for a 40-year life span. How can LVDTTs be properly maintained?

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Low-voltage dry type transformers (LVDTT) are a low-voltage class of distribution transformers operating at voltages below 1.2 kilovolts (kV) with most operating at or below 600 volts alternating current. The U.S. Department of Energy and the Natural Resources Canada define LVDTTs as distribution transformers that consume energy.

Therefore, both the U.S. and Canada have regulated minimum efficiency levels for these transformers and the testing and minimum efficiency levels are the same in both countries. Nevertheless, the reason for the regulation is that transformers consume energy and they give off this energy as heat. As a result, the heat byproduct of transformer energization is a factor that must be carefully observed. Over time, the accumulation of heat will degrade the transformer’s essential insulation system while also contributing to premature failure or end of life. There are three major contributors to heat accumulation — loading, connections and restricted airflow.

Loading the LVDTT

Transformers have maximum load ratings known as its capacity. This capacity is measured in apparent power in kilo-volt amps (kVA). The capacity is stated on a typical transformer nameplate (see Figure 1).

This transformer shows a capacity of 75 kilovolt amperes (kVA). It is important to understand that this kVA rating is based upon the temperature rise also stated on the nameplate — in this example 302°F rise. At 75 kVA, a 208-volt secondary will provide 208 full-load amps (FLA) of current and this transformer will exhibit a ≤302°F temperature — this is almost hot enough to bake a cake.

As we mentioned earlier, heat is a critical factor in the longevity of a transformer. Applying more load to this transformer for even short periods of time (less than four hours) will increase the heat of this transformer exceeding its nameplate rating. Therefore, it is wise to periodically check the loading of the transformer, especially as different renovations may occur during the building’s life.

It is important to consider the loads operating during a 24-hour period. For example, to get a higher load, measure the loads during peak usage, like summer for air conditioning or winter for heat. In fact, a 24-72 hour or more period of load study would reveal periods of peak load. To check for loading, the transformer must be energized; therefore, only a qualified electrician should do this testing and evaluation. The electrician should compare the overall results of the load study to the nameplate of the transformer to assess any overload periods and the duration of the overload.

Transformer connections

These are the electrical connections of the transformer. In a typical three-phase, delta-wye transformer, there will be at least seven connections and often 20 or more. Over time, these electrical connections degrade and must be maintained at a minimum to the manufacturer’s torque requirements. This connection degradation occurs over time due to changes in temperature and mechanical vibration.

All energized LVDTTs vibrate and this ongoing vibration, along with temperature changes, will cause the metallic connections to flex and become loose. Therefore, it is important to inspect and even test the connections periodically.

The ANSI/NETA MTS: Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems (Section 7.2.1.1) calls for a more thorough analysis of the LVDTT connections using at least one of these three methods:

• Thermography readings (energized test).

• Low-resistance (milliohm) meter (nonenergized test).

• Verify connection tightness using a calibrated torque wrench to the manufacturer’s specification (nonenergized service).

Thermography reading allows the measurement of the heat being generated at the connection points. This test is done while the transformer is energized and under load (normal operating conditions). If infrared (IR) windows are installed in the transformer covers, these readings can be done without removing the covers, providing a greater element of safety for the technician. This is a fast and easy way to provide preventive maintenance documentation of the transformer. If there are no IR windows installed, then the front cover needs to be removed to do thermography measurements — this will require qualified electrical technicians and the use of proper personal protective equipment (PPE).

The low resistance (milliohm) meter method requires the de-energization of the transformer. A skilled and qualified technician should perform this test to ensure accurate results in a safe environment. Typically, the desired reading should be in the very low milliohm or, even better, microohm ranges.

For example, a 75 kVA transformer can provide 208 FLA per phase. If the main connection terminals to the secondary of the transformer are measured at 1 milliohm, the connection point will generate 43.3 watts of power (W)=I²R; 208² x 0.001 = 43.3 W — just at the connection point.

Remember, heat accumulation can degrade and lead to the failure of the transformer. In Figure 2, this point of degradation and failure at a connection point is very clear. The heat accumulation at the connection point far exceeded the 428°F temperature rating of the insulation system, causing insulation burning and transformer failure. The burned connection points in Figure 2 are one of the three transformer primary winding taps. The position of the taps can be arranged during the installation to best match the incoming power supply voltage.

Considering the nameplate of Figure 1, the primary voltage of 480 volts will pull from the incoming power supply 90.2 FLA per phase, much less than the secondary windings. Nevertheless, considering the same terminal resistance of 0.001 ohms, the resulting power will be 8 watts, much lower than the secondary, but still consuming power and generating heat.

Given that Figure 2 shows the primary taps, this connection must have been much higher than 0.001 ohms, perhaps in the several ohm range. For example, if the connection point resistance was 0.2 ohms, the resulting power dissipation would have been 1,627 watts — much hotter, which may have caused this burning and the insulation system failure.

Additionally, the low-resistance (milliohm) test can reveal any connections that have oxidized over time. The oxidation will increase the connection resistance (even if the connection torque is within range) resulting in higher temperatures at the connection points. Removing the oxidized connectors and removing/cleaning the oxidation before reinstalling the connectors will return the connection to its designed intent when reinstalled using the torque requirements of the manufacturer.

Because most transformers are wired using aluminum cables and connectors, when reinstalling the connectors, it is recommended to use an anti-oxidizing compound on the connection points and lugs. After cleaning and reinstalling the connectors, take a new resistance measurement of the connection point. The NASA-STD-4003A electrical bonding requirements standard recommends that electrical connection points should exhibit an electrical bond with a direct current resistance measurement of 0.0025 ohms (2.5 milliohms) or less.

Some transformer manufacturers provide a typical connection resistance and that should be followed as a guide. In practice, the lower the connection resistance point, the lower the wattage (and heat generation) of that connection point. At a minimum, the torque of the mechanical connections should be checked before reenergizing the transformers.

As a guide, both the NFPA 70B: Standard for Electrical Equipment Maintenance and NETA MTS mention that “as found” and “as left” tests should be recorded during the maintenance and cleaning.

Restricted airflow in LVDTTs

Restricted airflow is hazardous to LVDTTs and surrounding equipment. Two factors are most common to restricting airflow:

• Installation clutter.

• Dust and dirt.

Installation clutter is somewhat addressed in NFPA 70: National Electrical Code (NEC) in sections regarding working space or clearances. While transformers are not explicitly called out in the NEC working space definition, other electrical gear often located near transformers are included.

The common explanation of working clearances/spaces is that if the electrical gear needs to be accessible while energized for servicing, repair, testing or maintenance, then 36 inches of clearance is required from service entry doors or panels. Because we have already established that thermography and load testing require an energized transformer, it is prudent to install the transformer using the same working clearance requirements as other electrical gear.

In addition to working clearances, the NEC requires adequate space for heat ventilation of transformers. LVDTTs require space around the enclosure for airflow and preventing dangerous coupling of heat to combustible wall surfaces. Typically, transformer manufacturers require a 6-inch minimum spacing, with some as low as 3 inches from nearby noncombustible walls.

On the transformer nameplate seen in Figure 1, 6 inches is the required clearance from walls. Also, NEC Article 450.9 requires that transformers be marked as shown in Figure 3 with a label that states the top of the enclosure is not to be used as storage — remember these transformers can get as hot as a baking oven under full load.

It is, therefore, imperative that transformers be given the clearances for safe operation. Restricting the airflow of the transformer by not providing the necessary clearances, either due to clutter or inadequate clearances, will force the transformer to operate at or above design temperatures causing excessive heat accumulation and degradation of the transformer’s insulation system.

In addition, Figure 4 shows multiple violations of the NEC; although this is an older installation, it is risky at best and could lead to premature failure or worse.

The NFPA 70B Chapter 11 recommends that LVDTTs be cleaned once per year. The accumulation of dust and dirt within the transformer will inhibit airflow through the transformer. LVDTTs are air-cooled electrical products (Class AA — see Figure 1 nameplate) and the cooling works by what is known as the “chimney effect” where cooler ambient air enters the bottom of the transformer and through convection, rises through the core and coil structure exiting through the top vents or openings.

Transformer core and coil design and construction adds planned ventilation by providing cooling ducts as shown in Figure 5. Over time, through the movement of air and temperature changes, these cooling ducts can get clogged with dust and dirt. To clean these ducts, the transformer must be de-energized and allowed to cool before lightly blowing in air and vacuuming out the accumulated dust and dirt. Depending on the installation environment, this service should be annually for environments known to have significant amounts of airborne dust and dirt or every few years in environments where the airborne particulates are known to be moderate or low.

Preventive maintenance of LVDTTs

Remember to maintain dry-type transformers. Keep them clean, secure and clear. These simple rules will help give a transformer the best chance of providing 30-40 years of useful service life. Recommendations include: clean the transformer interior periodically, secure the connections at a minimum to the torque requirements of the transformer manufacturer and clear out the clutter around the transformer that may accumulate over time.

Adding the low-resistance connection measurements will help assure that connection points have not oxidized over time. It is best to refer to and follow the maintenance guidelines for LVDTTs in the ANSI/NETA MTS, NFPA 70B and the IEEE C57.94 standards.

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