TRANSFORMER MAINTENANCE GUIDELINES

TRANSFORMER MAINTENANCE GUIDELINES

Following specific checking and maintenance guidelines as well as conducting routine inspections will help ensure the prolonged life and increased reliability of a transformer. The frequency of periodic checks will depend on the degree of atmospheric contamination and the type of load applied to the transformer.

Routine checks and resultant maintenance

Sl No	Inspection Frequency	Items to be inspected	Inspection Notes	Action required if inspection shows unsatisfactory conditions
1.1	Hourly	Ambient Temperature	-	-
1.2	Hourly	Oil & Winding Temperature	Check that temperature rise is reasonable	Shutdown the transformer and investigate if either is persistently higher than normal
1.3	Hourly	Load (Amperes) and Voltage	Check against rated figures	Shutdown the transformer and investigate if either is persistently higher than normal
2.1	Daily	Oil level in transformer	Check against transformer oil level	If low, top up with dry oil examine transformer for leaks
2.2	Daily	Oil level in bushing
2.3	Daily	Relief diaphragm		Relief diaphragm
3.1	Quarterly	Bushing	Examine for cracks and dirt deposits	Clean or replace
3.2	Quarterly	Oil in transformer	Check for dielectric strength & water content	Take suitable action
3.3	Quarterly	Cooler fan bearings, motors and operating mechanisms,	Lubricate bearings, check gear boxes, examine contacts	Replace burnt or worn contact or other parts
4.1	Yearly	Oil in transformer	Check for acidity and sludge	Filter or replace
4.2	Yearly	Oil filled bushing	Test oil	Filter or replace
4.3	Yearly	Gasket Joints	-	Tighten the bolts evenly to avoib uneven pressure
4.4	Yearly	Cable	boxes Check for sealing arrangements for filling holes.	Replace gasket, if leaking
4.5	Yearly	Surge Diverter and gaps	Examine for cracks and dirt deposits	Clean or replace
4.6	Yearly	Relays, alarms & control circuits	Examine relays and alarm contacts, their operation, fuses etc. Test relays	Clean the components and replace contacts & fuses, if required.
4.7	Yearly	Earth resistance		Take suitable action, if earth resistance is high

IR testing:

The transformer should be de-energized and electrically isolated with all terminals of each

winding shorted together. The windings not being tested should be grounded. The meg-ohmmeter

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 meg-ohm values

along with the description of the instrument, voltage level, humidity, and temperature should be

recorded for future reference.

The minimum megaohm 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 megaohms ([13,200V x 200] / 1000). If previously recorded readings taken under similar

conditions are more than 50% higher, the transformer should be 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. 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

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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.

Acceptance testing

Acceptance tests 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 drytype

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.

Winding resistance measurement:

Accurate measurement of the resistance between winding terminals can give 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 multi-strand 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, the transformer is de-energized and disconnected 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 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,

the transformer is disconnected 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 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.

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.

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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, a

measurement should be taken during the transformer's initial installation to establish a benchmark

value.

An impedance test is performed by electrically connecting the secondary terminals together with aconductor 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.

These values shall be recorded 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.