Power Transformers Testing


Testing of transformer is accomplished to figure out their electrical, thermal and mechanical suitability for the system exactly where they will be applied or employed. Most of the tests performed on energy transformers are defined in national standards designed by IEEE, NEMA and ANSI, whose objective is to define a uniform set of tests recognized by each the manufacturer and the user.

Transformer Test Information:

Field Testing. Field testing can be divided into 3 categories 

Acceptance tests
Periodic tests
Tests soon after failure

Acceptance tests ought to be performed instantly after the solution arrives at the location. A handful of tests can be carried out which are stated below:
Turns ratio
Insulation resistance(Winding and core)
Power issue
Resistance (winding)
Polarity and phase relation
Oil tests (DGA, moisture, dielectrics, etc.)
Visual inspection

Periodic tests are carried out right after the item is installed in its permanent location. The principal purpose of this test is to monitor the condition of the unit so that any prospective difficulty may possibly be spotted early prior to a failure happens. Some of these are listed below:
Turns ratio
Insulation resistance 
Power element
Resistance (winding)
Oil tests (DGA, moisture, dielectrics, and so on.)
Excitation existing test
Visual inspection

An unscheduled outage and the prospective of outright failure can be prevented by following a periodic test schedule.

Failure tests conducted on electric transformers are:

Turns ratio
Insulation  resistance 
Energy element
Oil tests
Excitation existing test
Combustible gas/ gas-in-oil evaluation
Visual inspection (internal)

When a transformer fails, the time of failure tests will make a decision whether or not the unit can be repaired at the internet site or no matter whether it needs to be returned to the manufacturer, or a specialized center for repair. By comparing the results of the tests with the established norms, a ‘history’ of the transformer can be compiled, and the motives for failure can be diagnosed.Here is a fast overview of the above described tests:
Transformer Turns Ratio Test (Typical to all categories)The Transformer Turns Ratio test (TTR) is used to make confident that the Turns Ratio between the windings of the transformer is appropriate. This ratio decides what the output voltage of the transformer will be with respect to the input voltage. The ratio is calculated beneath no-load conditions, with ratios calculated at the tap positions for every single winding and for the winding as a entire. A voltage is applied to one particular winding and the voltmeters connected to each low voltage and higher voltage windings are study simultaneously. The transformer ratio is the ratio of the HV voltmeter and the LV voltmeter readings. When ratio tests are getting produced on three-phase transformers, the ratio is taken on one phase at a time, and the measured ratio must be compared with the ratio calculated making use of nameplate voltages. Any variation ought to be within  .5%.
Transformer Insulation Resistance Test (Typical to all categories)The winding insulation resistance test (also recognized as the Meggar test) is a measure of good quality of insulation within the transformer. It can differ due to moisture content, cleanliness and the temperature of the insulation parts. All measurements are corrected to 20’C for comparison purposes. It is suggested that tank and core are always grounded when this test is performed. Every winding must be short-circuited at the bushing terminals. Resistances  are then measured in between each and every winding and all other windings and ground (for two winding transformer – H-LG, L-HG and HL-G and 3 winding transformer H-LTG, L-HTG, T-HLG, HL-TG, HT-LG, LT- HG and HLT-G ).
vPower Aspect (Frequent to all categories)This test is created to monitor the dryness of transformer insulation. Energy element is defined as the ratio of the power dissipated divided by the input volt-ampere multiplied by 100. The measurement of energy element is made with a capacitance bridge and the connections are the very same as for the insulation resistance tests.
Transformer Resistance (Common to all categories)The resistance of a transformer winding can be measured soon after current has not passed through the transformer for a number of hours, enabling it to attain the very same temperature as its surroundings. Winding resistance is calculated by measuring the voltage and existing simultaneously, with the present as close to the rated existing as feasible. Calculating the winding resistance can be helpful as it lets you calculate and compensate for I2R losses, a key component of load losses as a whole.  Winding resistance measurements can be produced to determine if any alterations have occurred in the present carrying path. The winding resistance measurements must be created with a Wheatstone bridge, Kelvin bridge or comparable bridge capable of measuring fractional ohms accurately.  For Wye connected values, measurements ought to be produced between every pair of bushings, then summed and multiplied by three-halves to get the comparison worth.
Transformer Oil Test (Common to all categories)A sample of insulating oil from a transformer in service can reveal much info about what is taking place inside the transformer. There are three simple enemies to insulating oil – oxidation, contamination and excessive temperature. The following tests can be completed:
Acid number
Dielectric breakdown
Power element
Moisture content material
Interfacial tension

After performing the tests the oil can classified as reusable reusable with minor reconditioning or disposable. 

Transformer Polarity (Acceptance test)The polarity of a transformer is either additive or subtractive. In order to uncover out the polarity of a transformer, a voltage is applied in between the primary bushings.  If the resultant voltage amongst the secondary bushings is higher than the applied voltage that means that the transformer has additive polarity.  If it is lower, the transformer has subtractive polarity. Polarity is not critical for a single connected distribution transformer, but it is a important concern if transformers are to be paralleled or bank connected. Three phase transformers are also checked for polarity by the same indicates.
Transformer Phase Relation (Acceptance test)A phase relation test is carried out for polyphase (for instance, three-phase) transformers to make confident that they have been connected in such a way that their phase connection is correct.  A phase relation test calculates the angular displacement and relative phase sequence of the transformer, and can be carried out in conjunction with ratio and polarity tests.  The voltages of the phase of principal and secondary can be recorded and comparisons created to get the phase relation.
Visual Inspection (Periodic and Failure tests)This may possibly reveal either present or prospective issues that may possibly not be picked up by diagnostic testing. For instance, deteriorating gaskets, low oil level or chipped bushing skirts. A normal list of check points must be established for each unit and then a record of every single inspection maintained.
Gas/ Gas-in-Oil Test (Failure test):A study of gases either dissolved in the oil or from the gas above the oil can also show abnormal conditions within the transformers, such as incipient faults. 3 considerations are very critical:
The total percentage of combustible gas
The percentage of every gas component
The price of adjust in combustible gas content material
If the percentage of combustible gases is above five%, then quick action is needed

Excitation Current Test (Periodic and Failure tests)The excitation present is the minimum quantity of existing needed to sustain the core in a state of magnetic excitation. It is measured at the rated voltage, and generally given as a percentage of the rated present.

The test is performed with a single phase provide with, preferably, a voltage rated at roughly ten% of the phase voltage of the winding to which the supply is to be connected, even though lower voltages can be used.
There are 2 techniques that can be used: the initial is to connect a single-phase provide to any obtainable winding with an ammeter in the circuit to monitor the exciting present.  Three such single-phase tests are essential for a 3-phase transformer. The connection between the single phase readings is critical it must be as follows:

The readings taken on phase A and C must be inside five% of every other.
The reading on phase B ought to be between 65 and 90% of the readings on phase A and C.

Readings that fall outdoors of the relationships provided above may be indicators of a winding fault. In the other strategy, the identical voltage level and ammeter needs apply except the following connections must be made:
Short 1 winding on phase C and apply voltage and study the exciting present on phase A. 
Short one winding on phase A and apply voltage and read the fascinating existing on phase C. 
Short one winding on phase B and apply voltage and study the fascinating existing on phase A or phase C. 
Other Transformer Tests:

Other tests which can be performed are:

Core Loss Test Under no-load circumstances, a transformer will continue to drain sources of electrical energy. The chief source of this drain is core loss, which happens in the magnetic core via a mixture of hysteresis and eddy existing loss, amongst other folks.  Core-loss is calculated by applying the rated voltage and frequency to a transformer under no-load situations.  The resultant current is then measured, from which the loss of energy can be extrapolated.
Load Loss Test Load loss is a combination of I2R losses, stray losses and eddy losses, all of which contribute to the loss of electrical power that is noticed as current transferred from a single winding to one more. Load loss alterations with the magnitude of the load: that is to say, greater loads see greater rates of loss.  The load loss is as a result generally calculated for the rated load, even though the transformer is beneath full-load conditions. It can be measured by applying a voltage to 1 winding although the other winding is short-circuited. The voltage is adjusted till the current flowing by means of the circuit is the exact same as the rated current. The power loss measured at this time is the load loss.
Impedance Test Impedance is a measure of the resistance that leads to the loss of electrical power in a transformer at full load, causing the ratio of the input and output voltages to differ from the Turns Ratio. It can be measured at the very same time as load loss. Impedance is discovered by measuring the voltage necessary to pass the rated current by means of one particular winding of the transformer, even though the other winding is brief-circuited. This voltage is referred to as the impedance voltage.
Applied Potential Test The applied potential test is utilized to see how well the transformer’s insulation bargains with voltages higher than the rated voltage, for given periods of time. The applied possible test checks the insulation between person windings and between windings and ground by applying voltages to each of these areas.
Induced Possible Test The induced potential test is utilised to test the good quality of the transformer’s insulation, as with the applied prospective test above. It tests the insulation of the person windings of the transformer by applying voltages amongst turns, amongst layers and between lines. 
High quality manage impulse test Quality control impulse tests are made on transformers in order to simulate lightning to see how properly they withstand such higher bursts of voltage.  The electric impulses applied here can include reduced  full-wave tests, chopped-wave tests and front-of-wave tests, to simulate a range of extreme voltage scenarios.
Pressure Leak Test A transformer can be checked for stress leaks by pressurizing the tank and then leaving it alone for numerous hours. If the stress drops throughout the intervening time, or if there are signs of liquid leakage, than a leak is present.  Otherwise, the transformer is leakage cost-free.

Even though finding out about and overseeing the regular testing procedures of your transformer can be a laborious process, it certainly aids far better your understanding of the transformer’s operation, minimizes hazard to life and house, reduces downtime, minimizes the chance of sudden failure and as a result allows optimum use of the transformer.