Power Transformers Testing


Testing of transformer is finished to decide their electrical, warm and mechanical appropriateness for the framework where they will be applied or utilized. The vast majority of the tests performed on power transformers are characterized in public guidelines made by IEEE, NEMA and ANSI, whose object is to characterize a uniform arrangement of tests perceived by both the producer and the client.


Transformer Test Subtleties:


Field Testing. Field testing can be partitioned into three classes


Acknowledgment tests

Intermittent tests

Tests after disappointment

Acknowledgment tests ought to be performed 3 phase transformer manufacturers the item shows up at the objective. A couple of tests can be completed which are expressed underneath:

Turns proportion

Protection resistance(Winding and center)

Power factor

Opposition (winding)

Extremity and stage connection

Oil tests (DGA, dampness, dielectrics, and so forth.)

Visual assessment

Occasional tests are finished after the item is introduced in its long-lasting area. The primary reason for this test is to screen the state of the unit with the goal that any potential difficulty might be spotted right on time before a disappointment happens. A portion of these are recorded underneath:

Turns proportion

Protection obstruction

Power factor

Obstruction (winding)

Oil tests (DGA, dampness, dielectrics, and so on.)

Excitation current test

Visual examination

An unscheduled blackout and the capability of through and through disappointment can be forestalled by following an occasional test plan.

Disappointment tests directed on electric transformers are:


Turns proportion

Protection opposition

Power factor


Oil tests

Excitation current test

Flammable gas/gas-in-oil examination

Visual assessment (interior)

At the point when a transformer falls flat, the hour of disappointment tests will conclude whether the unit can be fixed at the site or whether it should be gotten back to the maker, or a specific place for fix. By contrasting the consequences of the tests and the laid out standards, a ‘past filled with’ the transformer can be gathered, and the explanations behind disappointment can be diagnosed.Here is a speedy outline of the previously mentioned tests:

Transformer Turns Proportion Test (Normal to all categories)The Transformer Turns Proportion test (TTR) is utilized to ensure that the Turns Proportion between the windings of the transformer is right. This proportion concludes what the result voltage of the transformer will accompany regard to the info voltage. The proportion is determined under no-heap conditions, with proportions determined at the tap positions for each twisting and for the twisting overall. A voltage is applied to one winding and the voltmeters associated with both low voltage and high voltage windings are perused all the while. The transformer proportion is the proportion of the HV voltmeter and the LV voltmeter readings. At the point when proportion tests are being made on three-stage transformers, the proportion is taken on each stage in turn, and the deliberate proportion ought to be contrasted and the proportion determined utilizing nameplate voltages. Any variety ought to be inside .5%.

Transformer Protection Opposition Test (Normal to all categories)The winding protection obstruction test (otherwise called the Meggar test) is a proportion of nature of protection inside the transformer. It can shift because of dampness content, neatness and the temperature of the protection parts. All estimations are rectified to 20’C for correlation purposes. It is suggested that tank and center are constantly grounded when this test is performed. Each winding ought to be shortcircuited at the bushing terminals. Protections are then estimated between each winding and any remaining windings and ground (for 2 winding transformer – H-LG, L-HG and HL-G and three winding transformer H-LTG, L-HTG, T-HLG, HL-TG, HT-LG, LT-HG and HLT-G ).

vPower Variable (Normal to everything categories)This test is made to screen the dryness of transformer protection. Power factor is characterized as the proportion of the power scattered separated by the information volt-ampere increased by 100. The estimation of force factor is made with a capacitance span and the associations are equivalent to for the protection obstruction tests.

Transformer Opposition (Normal to all categories)The obstruction of a transformer winding can be estimated after current has not gone through the transformer for a few hours, permitting it to arrive at similar temperature as its environmental factors. Winding opposition is determined by estimating the voltage and current all the while, with the current as near the evaluated current as could be expected. Working out the winding opposition can be useful as it allows you to compute and make up for I2R misfortunes, a significant part of burden misfortunes all in all. Winding opposition estimations can be made to decide whether any progressions have happened in the current conveying way. The winding obstruction estimations ought to be made with a Wheatstone span, Kelvin span or comparative scaffold fit for estimating fragmentary ohms precisely. For Wye associated values, estimations ought to be made between each sets of bushings, then, at that point, added and duplicated by three-parts to get the examination esteem.

Transformer Oil Test (Normal to all categories)A test of protecting oil from a transformer in help can uncover a lot of data about the thing is occurring inside the transformer. There are three fundamental adversaries to protecting oil – oxidation, tainting and exorbitant temperature. The accompanying tests should be possible:

Corrosive number

Dielectric breakdown

Power factor

Dampness content

Interfacial pressure


In the wake of playing out the tests the oil can named reusable; reusable with minor reconditioning; or expendable.

Transformer Extremity (Acknowledgment test)The extremity of a transformer is either added substance or subtractive. To figure out the extremity of a transformer, a voltage is applied between the essential bushings. Assuming the resultant voltage between the auxiliary bushings is more noteworthy than the applied voltage that implies that the transformer has added substance extremity. In the event that it is lower, the transformer has subtractive extremity. Extremity isn’t significant for a solitary associated dissemination transformer, however it is a crucial concern in the event that transformers are to be resembled or bank associated. Three stage transformers are additionally checked for extremity by similar means.

Transformer Stage Connection (Acknowledgment test)A stage connection test is done for polyphase (for example, three-stage) transformers to ensure that they have been associated so that their stage relationship is right. A stage connection test works out the precise removal and relative stage succession of the transformer, and can be completed related to proportion and extremity tests. The voltages of the period of essential and auxiliary can be recorded and examinations made to get the stage connection.

Visual Review (Intermittent and Disappointment tests)This might uncover either present or potential issues that may not be gotten by demonstrative testing. For instance, falling apart gaskets, low oil level or chipped bushing skirts. A standard rundown of designated spots ought to be laid out for every unit and afterward a record of every review kept up with.

Gas/Gas-in-Oil Test (Disappointment test):A investigation of gases either disintegrated in the oil or from the gas over the oil can likewise show unusual circumstances inside the transformers, like early blames. Three contemplations are vital:

The complete level of burnable gas

The level of each gas part

The pace of progress in flammable gas content

In the event that the level of burnable gases is above 5%, prompt activity is required


Excitation Current Test (Occasional and Disappointment tests)The excitation current is the base measure of current expected to keep up with the center in a condition of attractive excitation. It is estimated at the evaluated voltage, and generally given as a level of the appraised current.

The test is performed with a solitary stage supply with, ideally, a voltage evaluated at roughly 10% of the stage voltage of the twisting to which the stock is to be associated, in spite of the fact that lower voltages can be utilized.


There are 2 strategies that can be utilized: the first is to interface a solitary stage supply to any suitable twisting with an ammeter in the circuit to screen the thrilling current. Three such single-stage tests are essential for a three-stage transformer. The connection between the single stage readings is significant; it ought to be as per the following:

The readings taken on stage An and C ought to be inside 5% of one another.

The perusing on stage B ought to be somewhere in the range of 65 and 90% of the readings on stage An and C.

Readings that fall beyond the connections given above might be signs of a winding shortcoming. In the other strategy, a similar voltage level and ammeter necessities apply with the exception of the accompanying associations ought to be made:

Short one twisting on stage C and apply voltage and read the thrilling current on stage A.

Short one twisting on stage An and apply voltage and read the thrilling current on stage C.

Short one twisting on stage B and apply voltage and read the intriguing current on stage An or stage C.

Other Transformer Tests:

Different tests which can be performed are:


Center Misfortune Test Under no-heap conditions, a transformer will keep on depleting wellsprings of electrical energy. The main wellspring of this channel is center misfortune, which happens in the attractive center through a mix of hysteresis and swirl current misfortune, among others. Center misfortune is determined by applying the evaluated voltage and recurrence to a transformer under no-heap conditions. The resultant current is then estimated, from which the deficiency of energy can be extrapolated.

Load Misfortune Test Burden misfortune is a mix of I2R misfortunes, stray misfortunes and whirlpool misfortunes, all of which add to the deficiency of electrical energy that is viewed as flow moved starting with one twisting then onto the next. Load misfortune changes with the size of the heap: in other words, higher burdens see higher paces of misfortune. The heap misfortune is accordingly commonly determined for the evaluated load, while the transformer is under full-load conditions. It very well may be estimated by applying a voltage to one twisting while the other winding is shortcircuited. The voltage is changed

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