Sensible transformer maintenance. (part 2)

Following specific checking and maintenance guidelines as well as conducting routine inspections will help ensure the prolonged life and increased reliability of a dry-type transformer.

Because dry-type transformers are used so extensively in industrial, commercial, and institutional power distribution systems, their maintenance should be a top priority. As such, you should have a thorough knowledge of their maintenance requirements, which are similar to liquid-filled units in many ways but differ enough to warrant separate coverage. The following detailed discussion will help you attain the required knowledge.

Dry-type transformer classifications

Dry-type transformers are classified as ventilated, nonventilated, and sealed units, with each type detailed in the ANSI/IEEE C57.12.01-1989 standard, General Requirements for Dry-Type Distribution and Power Transformers. Because there are significant differences among these three groups and because some of these differences have an impact on maintenance procedures, it's important that you know the key aspects of each transformer type.

A ventilated dry-type transformer is constructed so that ambient air can circulate through vents in the surrounding enclosure and cool the transformer core and coil assembly.

A nonventilated transformer operates with air at atmospheric pressure in an enclosure that does not allow ambient air to circulate freely in and out.

A sealed transformer is self-cooled, with the enclosure sealed to prevent any entrance of ambient air. These transformers are filled with an inert gas and operate at a positive pressure.

While construction varies per transformer type, inspection and maintenance guidelines are somewhat similar.

Maintenance guidelines

As with liquid-filled transformers, a maintenance program for dry-type units should include routine inspections and periodic checks. Acceptance tests should be performed when new units are delivered as well as when the need is indicated by review of maintenance data and operating history. The frequency for these inspections and checks will depend on the transformer classification as well as the operating environment, load conditions, and requirements for safety and reliability.

A valuable reference source for maintenance procedures is the ANSI/IEEE C57.94-1982 Standard, Recommended Practice for Installation, Application, Operation, and Maintenance of Dry-Type General Purpose Distribution and Power Transformers, which covers many of the maintenance aspects that should be considered.

The frequency of periodic checks will depend on the degree of atmospheric contamination and the type of load applied to the transformer.

This is especially true for nonsealed transformers since ambient air and any contaminant dust or vapors it carries can contaminate the internal, electrically-stressed components. As routine inspections are made, the rate of accumulation of dust and moisture on the visible surfaces should serve as a guide for scheduling periodic maintenance. Thus, ventilated transformers will require more frequent periodic checks than nonventilated units. Sealed transformers will require less frequent periodic checks than either type, because of their construction.

Routine checks and resultant maintenance

Neither nonventilated nor ventilated dry-type transformers have indicating gauges, as are needed on liquid-filled transformers, to monitor temperature, pressure, and liquid level. Thus, routine checks are more subjective and consist mainly of visual and audible observations.

Sealed dry-type transformers do have pressure gauges and these should be routinely checked. Also, a complete checklist should be developed for each transformer and should include essential observations, with data recorded and preserved. A program recommending various type checks and their frequency is shown in the Table on page 86.

Dust accumulation. Visual inspections should cover louvers, screens, and any visible portions of internal coil cooling ducts for accumulated dust. Do not remove any panel or cover unless the transformer is deenergized. If dust accumulation is excessive, you should deenergize the transformer in accordance with established safety procedures, remove its side panels, and vacuum away as much of the dust as possible. Then, clean with lint free rags or soft bristled brushes. Do not use any solvents or detergents as these may react with the varnishes or insulating materials and lead to accelerated deterioration. They may also leave residues that will enhance future accumulation of dust and various contaminates.

If dust accumulation remains in inaccessible areas after vacuuming, you can blow dry air into the unit to clear ducts. You should use air or nitrogen that has a dew point of -50 [degrees] F or less and regulate the pressure at or below 25 psi.

Checks during deenergization. The following items should be done while the transformer is deenergized.

* When access panels are removed for cleaning, all insulation surfaces should be inspected for signs of discoloration, heat damage, or tree-like patterns etched into the surface that are characteristic of corona damage. The core laminations should be inspected for signs of arcing or over-heating.

* All accessible hardware should be checked for tightness.

* Isolation dampeners between the base of the transformer and the floor should be checked for deterioration.

* Cooling fans or auxiliary devices should be inspected and cleaned.

If the transformer is deenergized long enough so that it can cool to ambient temperature, make sure that the unit is kept dry. If the ambient air is very humid, you may have to heat the transformer with electrical strip heaters to avoid condensation of moisture on the winding insulation. This is very important because a large percentage of dry-type transformer failures occur after extended shutdowns, when the insulation is allowed to cool and moisture in the ambient air condenses on the insulation.

Checks with transformer energized. The following items should be done with the transformer energized.

* Pressure readings should be checked and recorded for transformers with sealed [TABULAR DATA OMITTED] tank construction. The ambient temperature, time of day, and loading conditions should be recorded along with the pressure.

* Audible sound should be monitored, concentrating on the sound's characteristics as well as its level. Any noticeable change in the sound level or characteristics should be recorded. Significant changes could be indicative of loose clamping hardware, defective vibration isolators, over excitation, or possibly damage to the primary winding insulation.

* Proper ventilation should be verified. Although few dry-type transformers are equipped with temperature gauges, the effectiveness of ventilation can be verified by measuring the air temperature at the inlet (which should be near the floor) to an enclosed room and then measuring either the ambient temperature of the air in the enclosed space or the temperature of the air at the exhaust (which should be in the upper part of the room). The average temperature of the room should not increase more than 40 [degrees] F over the incoming air and the exhaust should not increase more than 60 [degrees] F. Additional details on ventilation requirements will be found in ANSI/IEEE C57.94.

Periodic tests

You should conduct periodic testing as often as needed. The frequency is usually dependent on the transformer's operating environment. If routine inspections indicate that cleaning is required, periodic tests should be made at the shutdown for the cleaning operation, after the transformer is thoroughly cleaned. The nominal period between scheduled tests is one year but this may be longer or shorter, depending on the observed accumulation of contamination on the cooling vents.

Sealed units should be opened only when the need is indicated by loss of pressure, operating abnormalities, or at intervals as recommended in the manufacturer's instructions. With these units, periodic tests should be confined to external inspections of the bushings and the enclosures. Also, readings at external terminals should be taken of insulation resistance (IR), power factor (PF), and turns ratio.

Section 6 (Testing) of ANSI/IEEE C57.94 states that induced or applied voltage tests also may be done periodically. These are overvoltage tests; as such, they are not necessarily nondestructive, even when skilled operators use the proper equipment. Damage may occur to the insulation, which would otherwise be serviceable at normal operating voltage levels. Therefore, these tests should not be considered as appropriate maintenance tests, which are by definition nondestructive.

IR testing. The IR of each winding should be measured using a megohmmeter in accordance with Sections 10.9 through 10.9.4 of the ANSI/IEEE C57.12.91-1979 Standard, Test Code for Dry-Type Distribution and Power Transformers. The transformer should be deenergized and electrically isolated with all terminals of each winding shorted together. The windings not being tested should be grounded. The megohmmeter should be applied between each winding and ground (high voltage to ground and low voltage to ground) and between each set of windings (high voltage to low voltage). The megohm values along with the description of the instrument, voltage level, humidity, and temperature should be recorded for future reference.

The minimum megohm value for a winding should be 200 times the rated voltage of the winding divided by 1000. For example, a winding rated at 13.2kV would have a minimum acceptable value of 2640 megohms ([13,200V x 200] / 1000). If previously recorded readings taken under similar conditions are more than 50% higher, you should have the transformer thoroughly inspected, with acceptance tests performed before reenergizing.

Turns ratio testing. The transformer turn ratio is the number of turns in the high voltage winding divided by the number of turns in the low voltage winding. This ratio is also equal to the rated phase voltage of the high voltage winding being measured divided by the rated phase voltage of the low voltage winding being measured.

Transformer turns ratio measurements are best made with specialized instruments that include detailed connection and operating instructions. ANSI/IEEE Standard C57.12.91 describes the performance and evaluation of these tests. The measured turns ratio should be within 0.5% of the calculated turns ratio. Ratios outside this limit may be the result of winding damage, which has shorted or opened some winding turns.

Insulation PF testing. Insulation PF is the ratio of the power dissipated in the resistive component of the insulation system, when tested under an applied AC voltage, divided by the total AC power dissipated. A perfect insulation would have no resistive current and the PF would be zero. As insulation PF increases, the concern for the integrity of the insulation does also. The PF of insulation systems of different vintages and manufacturers of transformers varies over a wide range (from under 1% to as high as 20%). As such, it's important that you establish a historic record for each transformer and use good judgment in analyzing the data for significant variations. ANSI/IEEE Standard C57.12.91 describes the performance and evaluation of insulation PF testing.

Acceptance testing

Acceptance tests (defined in Part 1, June 1994 issue, which concentrated on liquid-filled transformers) are those tests made at the time of installation of the unit or following a service interruption to demonstrate the serviceability of the transformer. This testing also applies to dry-type units. The acceptance tests should include IR testing, insulation PF measurement, and turns ratio testing, all as described under periodic tests. In addition, winding resistance measurements should be made and excitation current testing done. If you have a particular cause for concern, say a significant fault in the secondary circuit or a severe overload, you should make an impedance measurement and possibly an applied voltage test.

Winding resistance measurement. Accurate measurement of the resistance between winding terminals can give you an indication of winding damage, which can cause changes to some or all of the winding conductors. Such damage might result from a transient winding fault that cleared; localized overheating that opened some of the strands of a multistrand winding conductor; or short circuiting of some of the winding conductors.

Sometimes, conductor strands will burn open like a fuse, decreasing the conductor cross section and resulting in an increase in resistance. Occasionally, there may be turn-to-turn shorts causing a current bypass in part of the winding; this usually results in a decrease of resistance.

To conduct this test, you should de-energize the transformer and disconnect it from all external circuit connections. A sensitive bridge or micro-ohmmeter capable of measuring in the micro-ohm range (for the secondary winding) and up to 20 ohms (for the primary winding) must be used. These values may be compared with original test data corrected for temperature variations between the factory values and the field measurement or they may be compared with prior maintenance measurements. On any single test, the measured values for each phase on a 3-phase transformer should be within 5% of the other phases.

Excitation on current measurement. The excitation current is the amperage drawn by each primary coil, with a voltage applied to the input terminals of the primary and the secondary or output terminals open-circuited. For this test, you should disconnect the transformer from all external circuit connections. With most transformers, the reduced voltage applied to the primary winding coils may be from a single-phase 120V supply. The voltage should be applied to each phase in succession, with the applied voltage and current measured and recorded.

If there is a defect in the winding, or in the magnetic circuit that is circulating a fault current, there will be a noticeable increase in the excitation current. There is normally a difference between the excitation current in the primary coil on the center leg compared to the that in the primary coils on the other legs; thus, it's preferable to have established benchmark readings for comparison.

Variation in current versus prior readings should not exceed 5%. On any single test, the current and voltage readings of the primary windings for each of the phases should be within 15% of each other.

Applied voltage testing. The applied voltage test is more commonly referred to as the "hi-pot test." This test is performed by connecting all terminals of each individual winding together and applying a voltage between windings as well as from each winding to ground, in separate tests. Untested windings are grounded during each application of voltage.

Although ANSI/IEEE C57.94 lists the applied voltage test as an optional pre-service or periodic test, this test should be used with caution as it can cause insulation failure. It should be regarded as a proof test to be conducted when there has been an event or pattern in the transformer's operating history that makes its insulation integrity suspect.

ANSI/IEEE C57.94 states that either AC or DC voltage tests are acceptable for applied potential testing but that the DC applied voltage should not exceed the rms value of the standard test level. AC voltage rms values are limited by C57.94 to 75% of the original test levels (these levels range from 2 to 4 times the operating voltage) for initial installation tests and 65% of the original test levels for routine maintenance tests. The original or factory test levels are specified in ANSI/IEEE C57.12.01 and the tests are described in ANSI/IEEE C57.12.91. You should review these standards carefully before conducting any applied potential tests. If the original factory test reports are available, you should consult them to determine the original factory test levels.

DC applied voltage tests are often conducted in the field because DC test sets are smaller and more readily available than AC applied voltage sets. With DC tests, the leakage current can be measured and is often taken as a quantitative measure. However, DC leakage current can vary considerably from test to test because of creepage across the complex surfaces between windings and between windings and ground.

The use of AC voltage is preferable since the transformer insulation structures were designed, constructed, and tested with the application of AC voltage intended.

Impedance testing. An impedance test may be useful in evaluating the condition of transformer windings, specifically for detecting mechanical damage following rough shipment or a service fault on the output side that caused high fault currents to flow through the transformer windings. Mechanical distortion of the windings will cause a change in their impedance. To maximize the effectiveness of this test, you should take a measurement during the transformer's initial installation to establish a benchmark value.

An impedance test is performed by electrically connecting the secondary terminals together with a conductor capable of carrying at least 10% of the line current and applying a reduced voltage to the primary windings. This is easily accomplished by applying a single-phase voltage to each phase in succession. The applied voltage is measured at the primary terminals and the current measured in each line.

You should record these values and then calculate the ratio of voltage to current for each phase. This ratio should be within 2% for each phase and should not vary more than 2% between tests. A variation of more than 2% indicates the possibility of mechanical distortion of the winding conductors, which should be investigated as soon as possible.

Forensic investigators continually encounter instances where transformers have failed following an extended period of proper maintenance. Some of these failures were unpredictable but many could have been prevented if available data had been carefully reviewed and results properly interpreted.

The final section of this article, scheduled for a future issue, will address the interpretation of maintenance records and how to use recorded data to identify trends that may indicate developing problems with any specific transformer.