Transformer Testing

Last Updated : 02 May, 2024

Transformer testing is a process of examining a transformer to determine its health i.e., whether it is working properly or not. On an electrical transformer, we can perform various types of tests to measure its performance and efficiency and to take corrective actions.

As we know, in electrical systems, an electrical transformer is one of the most important and commonly used electrical machines that perform voltage and current transformation. Thus, the primary function of an electrical transformer is to increase or decrease the value of voltage or current in an electrical or electronic circuit. To perform this voltage step-up or step-down operation, the transformer utilizes the phenomenon of electromagnetic induction.

To ensure the desirable and proper operation of a transformer, we perform a variety of tests on it such as winding resistance measurement, insulation resistance measurement, dielectric strength measurement, and more. The group of transformer tests is referred to by using an umbrella term, transformer testing. This article will explain the concept of transformer testing and different tests that are to be performed on a transformer daily, quarterly basis, and yearly basis. Let’s get started with the basic definition of transformer testing.

Table of Content

Transformer Testing
Types
Type Test
Routine Tests
Special Tests
Pre Commissioning Test

Transformer Testing

What is Transformer Testing?

In electrical engineering, transformer testing is a process of evaluating the performance of a transformer and measuring its working efficiency. Transformer testing is one of the important practices that is used to ensure the reliability and safe operation of an electrical transformer. It provides valuable information about the transformer’s overall health that helps to take corrective actions accordingly.

We perform different types of tests on an electrical transformer to determine any issues that can impact the operation of the transformer and cause any kind of fault or abnormality in the electrical system. Transformer testing helps us to take corrective actions and maintain supply continuity. In electrical transformer testing, different types of tests are performed on a transformer depending on the type of transformer, the need for testing, and data for analyzing the transformer’s operation.

Type of Transformer Testing

It is important to note that transformer testing is an umbrella term which contains several different types of tests performed on a transformer. Here are some important transformer tests which are performed on a transformer to measure its performance, reliability, safety, and other health conditions

Test which are performed in the factory

Type Tests
Routine Tests
Special Tests

Test Which are performed in the sites

Pre-Commissioning Tests
Periodic/condition monitoring Tests
Emergency Test

Type Test of Transformer

It Refers to the tests which includes various tests that are conducted to check the quality, performance and the safety of the transformers. These tests are Important in the manufacturing process and during the commissioning of transformers.

Here are some common type tests:

Winding Resistance Test
Insulation Resistance Test
Temperature Rise Test
Partial Discharge Test
Impulse Voltage Withstand Test

Let us now discuss each type of transformer test in detail

Winding Resistance Test

In transformer testing, the winding test is an electrical test that performed on a transformer to measure the resistance of its primary and secondary windings. This test provides the information about the DC resistance of the transformer’s primary and secondary windings. This test helps to examine the following issues in transformer windings:

Loose connections between conductors
Damaged conductors in the windings
Transformer windings have specified electrical parameters to meet the industry standard

The winding resistance test of a transformer is basically a routine test which is performed during manufacturing and commissioning of the transformer. But it can also be performed during maintenance period to identify any abnormality.

Thus, the following are the two main purposes of the resistance winding test of transformer:

To identify or predict any kind of fault due to winding resistance
To calculate the temperature rise of the transformer windings, as the winding resistance is one of the key factor that influence their temperature.

Procedure of Winding Resistance Test

The step-by-step explanation to perform the winding resistance test on a transformer is explained below:

Isolate the transformer from the system and de-energize it.
Connect the required instrument to the transformer windings to measure their electrical resistances. It is important to note that the resistance testing is performed separately for primary and secondary winding.
The test equipment gives the voltage and current values for the winding. Then, we use the formula (R = V/I) to calculate the resistance of the winding.
These values are then compared with the standard values of the winding resistances mentioned in the design sheet of the transformer. If there is any significant difference between the measured values and specified values. Then, we take corrective actions.

While performing the winding resistance test, we take care of the following to ensure the accuracy in the test:

Winding resistance test should be performed on ambient temperature. However, it should also be performed at some different temperature values that occur during normal operation of the transformer.
Make proper connections between transformer windings and measuring instrument probes for accurate results.
Follow proper safety guidelines to avoid any electric shock or accident. It becomes more important when dealing with high-voltage transformers.

After performing the winding resistance test, if there is significant difference between the measured value and the expected value of the winding resistance. This indicates that there is some issues in the transformer winding such as loose connection, short-circuit between turns, or any other fault.

Therefore, the winding resistance test is one of the most important diagnostic tool to evaluate the performance, efficiency, and safety of a transformer.

Insulation Resistance Test

The insulation resistance test is another important test in transformer testing that is used to determine the strength and quality of insulation used in the transformer. This test is also performed during manufacturing and commissioning of the transformer. However, it may be performed during maintenance of the transformer if required. The primary purpose of the insulation resistance test is measure the electrical resistance of the insulating material used between various conducting components of the transformer. This tests helps to evaluate that how much electrical stress the insulation can handle without breakdown.

Thus, the following are the primary objectives of the insulation resistance test of a transformer:

To determine the weakness of the insulation that may cause faults in the transformer.
To ensure electrical safety of the transformer, as a weak insulation between conducting components can cause an electric shock.
To measure moisture content and degradation of the insulation.

Procedure of Insulation Resistance Test

We perform the insulation resistance test on an electrical transformer as per the following steps:

Isolate the transformer from the power supply and properly de-energize it.
Connect the measuring instrument to the transformer for measuring its insulation resistance.
Take the following three readings:
- Measure the insulation resistance between the high voltage winding and transformer tank. During this measurement ground the low voltage winding.
- Measure the insulation resistance between the low voltage winding and the transformer take. During this measurement ground the high voltage winding.
- Measure the insulation resistance between the low voltage winding and the high voltage winding. During this measurement ground the transformer tank.

After measuring all these insulation resistance values, we have to compare them with the expected values. If the measured and desirable values are equal or nearly equal, it indicates that the insulation is in good condition, otherwise it need some corrective actions.

Hence, the insulation resistance test plays an important role to judge the quality, reliability, and safety of a transformer and it prevents any kind of unexpected failure of the transformer.

Temperature Rise Test

The temperature rise test is an important test performed on electrical transformers to determine their thermal efficiency and performance when they are operating under different load conditions. Under temperature rise test, we measure the temperature of different components of a transformer such as windings, core, oil, tank, etc. under operating conditions.

The data about temperature of the transformer components help us to optimize the performance and efficiency of the transformer and load the transformer safely without breakdown. For an electrical transformer, it is important to limit the temperature rise within a specified limit so that it can operate efficiently.

Thus, the primary goals of temperature rise test of a transformer are listed below:

To determine the thermal performance of a transformer so that the temperature of different components does not exceed a specified limit.
It helps to evaluate that the transformers insulation is in good quality and the cooling system is working properly.

Procedure of Temperature Rise Test

The step-by-step procedure of temperature rise test of electrical transformer is explained below:

Bring the transformer in the condition of stable and normal temperature.
Connect an electrical load of rated capacity of the transformer.
Measure the temperature of various components of the transformer such as primary and secondary windings, magnetic core, oil, and other components.
Calculate the rise in the temperature of different components. For this simply, subtract the initial temperature from the measured value during the test.
Compare the calculated temperature rise values with the standard values specified by the manufacturer of the transformer.

While performing the temperature rise test on a transformer, take care that the external environmental factors do not affect the test’s accuracy. If the values of temperature rise are as per the design specifications, then the transformer can operate safely at the rated load without any failure due to excessive heating.

The temperature rise test of transformer is one of the important transformer tests that helps to determine the loading capacity of the transformer and ensure that the transformer is safer to operate. It also helps to improve the lifespan of the transformer.

Partial Discharge Test

In transformer testing, the partial discharge test is a type of test that is used to determine the major defects in the insulation material used between different conductive components of the transformer. Partial discharge is nothing but a local electrical discharge that occurs within the insulating material used in the transformer and it provides information about the quality of the insulating material. The partial discharge is considered an early indicator of degradation of the insulation material.

This test becomes more important in the case of high voltage transformers. It is performed during manufacturing, commissioning, and maintenance periods.

The main purposes of the partial discharge test are the following:

To detect any kind of defects in the insulation material used in the transformer.
To determine the overall condition of the insulation material used.

Procedure of Partial Discharge Test

The following steps are followed to perform the partial discharge test on a transformer:

Properly isolate the transformer from the power supply and bring it in the stable condition.
Apply a high voltage across the transformer insulation. This applied voltage is greater than the normal operating voltage of the given transformer.
Increase the applied voltage gradually to produce a partial discharge.
Detect the partial discharge occuring in the insulating material during the test. For this, use the specialized sensors.
Record the readings of the testing instruments and perform data analysis to obtain the test results.

It is always important to note that the partial discharge test is performed on a voltage higher than the normal voltage but lower than the breakdown voltage of the insulating material used in the transformer. Also, this test is performed at different voltages to determine the quality of insulating material used in the transformer under different electrical stresses. The results of this test provides complete information about the health of the insulating material used in the transformer.

During the partial discharge test, if low or no partial discharge occurs, then it shows that the insulation is in healthy condition. On the other hand, if a high or abnormal partial discharge occurs, then it shows that the insulation has become weak and may cause transformer failure. Hence, the partial discharge test helps us to make decisions about maintenance and repair of the transformer insulation.

Impulse Voltage Withstand Test

In transformer testing, the Impulse voltage withstand test is done on the transformer for Critical evaluation, It is used to check their ability to withstand Transient Voltage surges or impulses. These Impulses can occur due to multiple factor such as lightning strikes, Switching operations, or other transient disturbances in the electrical network. This test is basically done to check Equipment’s Insulation can withstand Such high Voltage Surges Without breakdown or failure.

The Following are the main purpose of Impulse Voltages Withstand Test:

To determine the ability of the insulation system of the transformer to withstand high voltage impulses such as lighting strikes or other impulse surge factors.
To determine integrity of the Transformer by subjecting them to Extreme Transient Voltage conditions.
To determine the Reliability and durability of the insulation material and components used in the transformer.
To determine the Quality Assurance in the Transformer to ensure that it meets Specific Performance Standards and Requirements.

Procedure of Impulse Voltage Withstand Test

Ensure all the Connection are Secured and the test setup is free from any defects or damages.
Setup up impulse Voltage generator According to the Test requirements, Then configure the test parameters including peak voltages level ,waveform shape, polarity and repetition rate based on the test requirements.
Connect the Suitable measuring instruments such as oscilloscopes, voltages dividers and other measuring devices to monitor and measure the response of the equipment under test.
Apply the impulse voltage Waveform using the impulse voltage generator, gradually increase the voltage level and monitor the equipments response to the Applied response.
Analyze the recorded data from the measuring instruments to asses the transformers performance.

Routine Tests of Transformer

These Test are done to check their quality, reliability, and proper working with standards before they are put into service. These tests are conducted on each individual transformer unit during the manufacturing process or commissioning. Here are the common routine tests performed on transformers:

Winding Resistance Test
Insulation Resistance Test
Temperature Rise Test
Short Circuit Withstand Test
Transformer Oil Test

Winding Resistance Test, Insulation Resistance Test and Temperature Rise Test is same as Types Test

Short Circuit Withstand Test

In transformer testing, there is a test which is used to determine the ability of the transformer to withstand the thermal and mechanical stresses which are caused due to short-circuit faults in the electrical system. The primary purpose of the short-circuit test is to evaluate the mechanical strength and safety of the transformer under abnormal conditions. This test is generally performed during manufacturing and testing of the transformer.

The following are the main purposes of the short-circuit withstand test:

To determine mechanical strength of different components of the transformer under short-circuit fault condition.
To determine the heat dissipation ability of transformer during short-circuit fault.
To determine the weakness or deficiencies in the transformer’s design or Construction.
To determine the safe operation of transformer under adverse circumstances.

Procedure of Short Circuit Withstand Test

Ensure the the all connections in the Primary and Secondary winding Sides are Properly Configured and Connected.
Connect he High Current Source such as Short-Circuit generator or Specialized Short Circuit testing Device to the Primary winding.
Apply the Short Circuit current to the Transformer’s Primary Side for the require duration and monitor the Current waveform and magnitude Through out the test.
Analyze the Observed data to Evaluate the Transformer’s Performance during the Test.

Transformer Oil Test

In transformer testing, the Transformer oil test is done to check the health and performance of the transformer. In transformer, oil serves as multiple purpose such as insulation, cooling and arc suppression. Overtime the quality of the oil degrade due to various factors such as moisture, oxidation, and contamination, which leads to result in reducing the efficiency and reliability of the transformer.

The Following are the main purpose of Impulse Voltages Withstand Test

To determine the dielectric strength of the oil, which is important for evaluating the insulation purposes of the transformer.
To determine the Moisture content of the transformer as the Moisture is a common contaminant in transformer oil which can lead to degradation of the insulation of the transformer.
To determine the acidity levels in transformer oil and assess its neutralization capacity.
To determine the Insulation Degradation which can help to analyze the gases dissolve in oil which provides the insights into internal or potential issue in the Transformer.

Procedure of Transformer Oil Test

Collect the sample of transformer oil using proper sampling techniques.
Check all the testing equipment is clean and calibrated according to the testing Requirements.
Set up the test cell with electrodes and place the oil sample between them.
Apply increasing voltage to the oil sample until breakdown occurs, recording the voltage at breakdown.
Determine the acidity level of the oil by measuring the amount of potassium hydroxide (KOH) required to neutralize acidic components.
Analyze the test result to evaluate dielectric integrity and the oil condition of the transformer.

Special Tests of Transformer

Special tests of transformers are done under specific circumstances or when particular issues regarding the transformer’s performance, condition, or application. These tests are not be part of standard manufacturing or commissioning procedures but are carried out as needed to particular issues or to check the transformer’s continued safe operation.

In the special test it includes such as Partial Discharge Test which detects insulation defects or weaknesses in the transformed, Impulse Voltage Withstand Test to check the transformer’s ability to withstand high-voltage impulses, Frequency Response Analysis to detect mechanical defects or winding deformations in the transformer core and windings by checking their frequency response to a frequency signal.

Pre Commissioning Test of Transformer

Pre Commissioning test are done before connecting the transformer to the electrical system. These Tests are done to check the transformer is installed correctly and its in good working conditions that meets with the required standards and specifications. These test includes Visual Inspection to visually inspect the transformer, Polarity Check to verify the correct polarity of the transformer windings and connections, Insulation Resistance Test to measure the insulation resistance between various parts of the transformer, Winding Resistance Test to measure the resistance of transformer windings to ensure they match design specifications and estimate copper losses, Turns Ratio Test to verify the turns ratio of the transformer and ensure the correct voltage transformation.

Conclusion

In this Article, we have gone through the basics of the Transformer testing and learned about why it is important. we have also gone in brief through major testing methods for the transformer such as Winding Resistance Test, Insulation Resistance Test, Temperature Rise Test and other test. We have seen there full working with Procedure, At last with this we will conclude our Article.