The purpose of this article is to provide a list of the standard battery of tests performed on new and remanufactured transformers, while also providing an introductory explanation of the purpose and scope of the most common routine factory and diagnostic field tests. Factory testing is performed according to IEEE C57.12.00 and IEEE C57.12.90 standards for liquid-immersed distribution, power, and regulating transformers and IEEE C57.12.01 and IEEE C57.12.91 standards for dry-type distribution transformers and power transformers.
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(as defined in IEEE C57.12.80-):
Tests made for quality control by the manufacturer on every device or representative samples, or on parts or materials as required, to verify during production that the product meets the design specifications” (Section 3.393).
IEEE C57.12.00 & IEEE C57.12.90 (Liquid-Immersed)
IEEE C57.12.01 & IEEE C57.12.91 (Dry-Type)
Those tests made to determine the adequacy of the design of a particular type, style, or model of equipment or its component parts to meet its assigned ratings and to operate satisfactorily under normal service conditions or under special conditions if specified, and to demonstrate compliance with appropriate standards of the industry. Syn: type test (IEC)” (Section 3.99).
IEEE C57.12.00 & IEEE C57.12.90 (Liquid-Immersed)
IEEE C57.12.01 & IEEE C57.12.91 (Dry-Type)
Tests so identified in individual product standards that may be specified by the purchaser in addition to design and routine tests (Examples: impulse, insulation power factor, audible sound.)” (Section 3.311).
IEEE C57.12.00 & IEEE C57.12.90 (Liquid-Immersed)
IEEE C57.12.01 & IEEE C57.12.91 (Dry-Type)
Every two-winding transformer has a ratio. The ratio is the relationship between the number of turns on the primary and secondary windings of a transformer. To understand the basic function of a transformer, you could think of it as a ratio box. No matter what you put into it, it will always produce a result proportionate to the ratio. An example of a 1:1 ratio would be where the input and output voltages are the same (for every 1 turn on the primary winding, you would have 1 corresponding turn on the secondary). For a 2:1 ratio, the secondary (output) voltage is half of the primary (input) voltage—for every two turns on the primary winding, you have one corresponding turn on the secondary side, and so on. If you applied 10 volts to the primary of a transformer with a 2:1 ratio, the result would be 5 volts on the secondary; if you put 20 volts into the same transformer, you would get 10 volts out. This predetermined relationship between the primary and secondary windings for any given transformer is called the calculated ratio. The turns ratio test (TTR) is performed to confirm that the unit’s tested ratio lies close enough to the calculated value per IEEE standards
To find the calculated ratio, divide the rated primary phase voltage by the rated secondary phase voltage as depicted on the nameplate of the transformer. When determining the calculated ratio for a transformer, it is important to refer to the coil (phase) voltage—the phase voltage determines the number of turns at the transformer coils. For a delta-connected winding, the phase voltage is the same as the line-to-line voltage, but for a wye-connected winding, the line and phase voltages are different. For a wye winding, the coil or phase voltage is represented by the second smaller number (line-to-neutral) and is obtained by dividing the phase-to-phase voltage by the square root of 3 and is written as follows: Y/ . For example, a transformer that is Y/ - 480 Y/ 277 would have a calculated ratio of 27.51, whereas a transformer with a delta primary, such as D - 480 Y/ 277, would have a calculated ratio of 47.65.
When a transformer is built at the factory, the actual ratio at the coils will differ slightly from the calculated value due mainly to the fact that you cannot have partial turns. IEEE standards allow a 0.5% variance above and below the calculated value for tested ratios. This standard is used by Maddox, and it is the same standard employed by field testing companies and associations such as NETA. Maddox performs a standard TTR test on all used units when they are brought into inventory and again after the remanufacturing process is complete. Test values are provided for all tap positions on remanufactured units.
A winding resistance test helps evaluate the condition and quality of the current-carrying path of the windings in a transformer. For new factory-built padmount transformers, this test is only required for sizes above 2,500 kVA (IEEE C57.12.00), but Maddox utilizes the winding resistance test for all remanufactured medium-voltage units. Winding resistance provides essential diagnostic information, which can aid in evaluating whether a unit is suitable for repair or remanufacturing. Issues such as loose internal connections, faulty tap changers, open circuits, and broken conductor strands or crimp connections may be identified with this test. In the case of a delta connection, measurements are made phase-to-phase (H1-H2, H2-H3, H1-H3). With a wye connection, measurements may also be made phase-to-phase (H1-H2, H2-H3, H1-H3), as well as phase-to-neutral (H1-H0, H2-H0, H3-H0).
This test is measured in ohms, and the value is typically low (tenths or hundredths of a decimal). Keep in mind that when this test is performed (especially in the field), it is typically done at the bushings of the transformer. As a result, measurements will include any components in the current-carrying path of the windings, such as tap changers, fuses, switches, and cable leads, which may affect test results. A questionable reading may not always indicate a problem with the coils themselves. For instance, if one phase of a transformer has a significantly longer section of internal bus work between the bushing and the winding lead it connects to, you may see a higher measured value across that particular phase. In this case, the test data would not indicate a problem, but a simple fact inherent to the mechanical design of the unit.
The same type of variance, however, could show up on a phase with a loose or frayed cable connection at the tap changer, where the variant reading would indicate a mechanical problem that would need to be addressed during the remanufacturing process. For this reason, the proper performance of a winding resistance test requires a proficient mechanical knowledge of the internal workings of the transformer, as well as an aptitude for evaluating the available test data. Results between phases that fall within 5% of each other are generally considered acceptable (IEEE Std 62-, p. 7, section 6.1.1).
While insulation resistance (or megger) testing is not recognized by IEEE for determining pass/fail criteria on newly manufactured transformers (discussed in the following paragraph), it is a useful supplementary test for units that have spent some time in service out in the field. As the title indicates, the purpose of this test is to determine the quality of the insulation on a piece of electrical equipment. Insulation resistance testing is used on a variety of electrical apparatus, such as conveyors, motors, fans, refrigerators, HVAC systems, and cables. In this test, we are measuring the resistive capability of the insulation material within the transformer between windings and core ground, commonly measured in megohms. The test is performed by applying a specified DC voltage through the conductor(s) of the transformer. Over time, the insulation may age or degrade from factors such as overheating, external physical stress, or moisture. This degradation can lead to a reduced capability of the insulation to withstand the required operating voltages of the transformer.
Transformers do not see the kind of mechanical and physical stressors common to motors and cables in a raceway. Due to the design of distribution class transformers, test results can yield inconclusive values and may puzzle a field technician who is used to meggering stand-alone cables in a raceway. Core grounding methods and components such as switches, fuses, and tap-changing devices may also affect the results of insulation resistance testing on transformers. For this reason, insulation resistance tests for transformers should be treated as supplementary to the battery of standard routine tests outlined under IEEE C57.12.00, and not the be-all and end-all of determining a good transformer from a bad one. Insulation resistance is performed on the high-side windings to low-side windings, high-side windings to ground, and low-side windings to ground.
Workers performing Megger on a dry-type transformer.
With an impedance test, we are measuring the losses in the transformer: the watts/power wasted or lost during electrical operation. The quality of construction along with the type of materials used in the building of the transformer’s coil assembly play a role in determining the results of this test. Unlike the insulation and winding resistance tests, which serve as supplementary evaluations for distribution class transformers, the impedance/load loss test yields concrete results that can be taken at face value. This test may be used to confirm the design values for a given unit where a certain number of losses is requested by a customer on a new factory-built unit.
IEEE lists standardized impedances for distribution class transformers above 500 kVA at 5.75% (+/-7.5%), but sometimes, customers will request something different. This can be largely accomplished with the design itself. The impedance (%IZ) of a transformer is affected by the resistive (%IR) and reactive (%IX) components of a transformer. It is during this test that the reactive and resistive components of impedance are identified, as well as the resulting X/R ratio of a unit. For remanufatured transformers, these values will be determined by how the unit was originally manufactured at the factory. Impedance/load loss testing is a standard routine factory test (IEEE C57.12.00), which is required for all new factory-built padmount distribution class transformers, and it is performed on all transformers that are repaired or remanufactured by Maddox. The tolerance from the specified value for two-winding transformers is ±7.5%; for zigzag units, autotransformers, and transformers with three or more windings, the tolerance is ±10% (IEEE C57.12.00-, p. 64, 9.2).
With an excitation test, we are testing the flow of magnetic flux in the transformer core. If the words magnetic flux sound a bit too technical, think about a simple magnet with two ends or poles (one south and the other north). If you sprinkled iron shavings on a table near the magnet, you would see the iron shavings line up in long oval loops springing from one end of the magnet to the other (these invisible phenomena are referred to as magnetic lines of flux). These magnetic fields are all around us, and it is this same principle which is behind the invention of the directional compass. A transformer’s ability to produce these lines of flux is what we are after here in the excitation test. To perform this test, voltage is applied to the low side of the transformer windings with the high-side windings open, which allows the amount of magnetic flux required for operation to flow through the core.
Another way to think of excitation is to think of it as the amount of work required to start the transformer. The quality of the core and assembly and its construction influence how much excitation is needed during energization. Imagine trying to roll a car with a dead battery down the street to a nearby parking lot. You would have to do some amount of work to get the car going, which would require a bit more heaving and grunting in the beginning; this extra heaving and grunting would merely be spent in getting the object out of its stationary state. In the same way, a poorly built (or damaged) core assembly will require more “heaving” and “grunting” when the transformer is energized. The additional work required at startup is what we refer to as inrush current. The excitation current and the associated no-load loss are the power that keeps the core energized during normal operation. The quality, orientation, and construction of the laminated core steel in a transformer will determine the exciting current. For new distribution class transformers, the DOE has set minimum efficiencies in distribution class transformers up to kVA.
Excitation/no-load loss testing is another standard factory routine test for new factory-built transformers (IEEE C57.12.00), and it is an essential part of the repair and remanufacturing process at Maddox. The presence of a higher-than-normal exciting current often can lead to the discovery of internal problems, such as shorted turns, a damaged core assembly, or a faulty tap changer. Some of these issues can be fixed in the repair process. This test is also used as a baseline for determining the viability of a unit for repair or remanufacturing, and it is performed on all 3-phase distribution class transformers.
Learn more about transformer cores in this article.
Loss testing being performed.
The phase relation test confirms the angular displacement and phase sequence between windings in a 3-phase transformer. In layman's terms, it confirms the coils are connected correctly inside the transformer tank. For example, if you apply a voltage across H1 and H2 at the primary winding, you would expect to measure a corresponding voltage across X1 and X2 at the secondary winding. Let’s say for the sake of this example that when you applied a voltage across H1 and H2, you instead found the corresponding voltage across X2 and X3 on the low winding. In this case, the primary and secondary windings would be out of sequence, and the internal winding connections would need to be fixed accordingly.
With two-winding transformers, the coils may be connected in delta or wye. For a 3-phase transformer connected delta on the primary and wye on the secondary, a 30-degree phase shift will typically be present, which can be either leading or lagging. For a transformer connected delta on the primary and delta on the secondary, there is typically a zero-degree (or no) phase shift (the same scenario exists where a wye connection is on both the primary and secondary side). The phase relation test confirms that the internal connection of the coils matches the vector grouping diagram on the nameplate of a given transformer. This information is vital to the proper operation of an electrical system—especially where multiple units are tied together. Verifying proper phasing is a basic part of the repair and remanufacturing process. This test is another routine test required for all new factory-built transformers (IEEE C57.12.00), and it is performed on all units brought into the Maddox facility for repair and remanufacturing.
Every liquid-filled transformer is tested to verify the transformer tank will hold and maintain pressure when put into service by adding 5 PSI of pressure to the tank and leaving it for 24 hours. A visual inspection then is made to verify that no fluid leaks exist around any gaskets or seals, and that no pressure has left the tank by checking the pressure gauge when possible. In the case where a radiator was repaired or replaced, additional care is taken to verify a successful repair that will hold up to usual service conditions in the field.
Liquid-filled transformer awaiting shipment.
The applied potential test is currently not part of the standard battery of tests for remanufactured transformers. It is a routine test per IEEE C57.12.00, and applied potential is performed on all new factory-built Maddox transformers. The purpose of this test is to ensure the integrity of the insulation system in a given unit by putting the insulation under short-term stress via an overvoltage. For this test, a voltage is applied and gradually increased between the windings being tested, with the starting voltage being no more than one-quarter of the full value. The duration of the test is one minute at the specified test voltage, as outlined in IEEE C57.12.90. This test is often omitted in the field for in-service transformers due to the difficulty of achieving the required test voltage levels. Applied potential testing is designed to fail an insulation system that is already compromised; it is generally agreed that it will not result in damage or failure when performed on a unit with good insulation.
The induced potential test is another overvoltage test, like the applied potential test. It is also a routine IEEE test performed on all newly manufactured transformers. Like applied potential testing, this particular test is currently not included among the list of tests performed on remanufactured transformers. To perform this test, a voltage “greater-than-rated volts per turn to the transformer” (IEEE C57.12.90-, p. 62) is applied and gradually increased for a designated period of time, depending on the frequency at which the test is performed (the frequency supplied must be raised to prevent over-excitation of the core, as the applied test voltage is significantly higher than the rated voltage). The formula for establishing the minimum test frequency is set forth in section 10.7.2 of IEEE C57.12.90.
Impulse testing is another test that is only performed on newly manufactured units at the factory. The purpose of this test is to analyze a transformer's ability to withstand large voltage surges, as would be common in a typical electrical system. During normal service conditions, transformers are often exposed to sudden high-voltage spikes resulting from lightning or the operation of switches. Along with induced and applied potential tests, we are again testing the dielectric strength of the insulation system in the transformer. In the case of lightning, the voltage wave can take a variety of forms. For this reason, the impulse test is designed to imitate both the form of the wave and the succession in which the various wave shapes may occur. For class I power transformers, these are one reduced full wave, one full wave, two chopped waves, and two full waves. For pad-mounted distribution transformers, these are one reduced wave and one full wave.
Power factor testing is most commonly associated with larger class I and II power transformers, and it is a standard routine test for power units per IEEE C57.12.00. It is important to note that this test is not recognized by IEEE as an accepted method for determining pass/fail criteria on distribution class transformers. The test code laid out in IEEE C57.12.90 also notes the difficulty and potential problems associated with attempting to establish absolute values to apply across the board for this test on distribution class transformers (See Notes 1, 2 & 3 of Table 4, IEEE C57.12.90-, p. 67). Although test results can be difficult to interpret at times, power factor testing provides a diagnostic benefit when comparing test data for a single unit over a period of time. For example, if a more recent set of test results contrasts significantly with data from an earlier test for the same unit, this could alert a technician to the possibility of an issue that may need attention. The use of an initial stand-alone test to establish pass/fail criteria in distribution class transformers, however, is not advisable. Due to the physical construction and the presence of other ancillary components within the path of the test voltage, such as tap changers and switches, results can vary widely for distribution class transformers.
While power factor testing is not performed under the standard battery of tests required by IEEE C57.12.00 for smaller transformers (generally below 10 MVA), it is required by NETA on all distribution class units. It’s important to note that the test values suggested by NETA for insulation power factor testing exist in lieu of the absence of an agreed-upon standard (See Table 100.3, NETA Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems). The values established by NETA do not remove the extant difficulties expressed under IEEE C57.12.90, but rather act as a general guide for technicians performing this test in the field. When field test values fall outside what is recommended by NETA, the next course of action should be to perform the remaining battery of standard tests for field commissioning and evaluate the results. A field technician may need to sign off on his end for any values that do not fall within NETA’s recommendations, and it may be necessary for Maddox to add clarification or verify that the unit’s test results are within acceptable limits according to factory standards. Insulation power factor testing is a standard test for all new and remanufactured class I power transformers supplied by Maddox.
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.
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.
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
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Test Which are performed in the sites
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:
Let us now discuss each type of transformer test in detail
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:
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:
The step-by-step explanation to perform the winding resistance test on a transformer is explained below:
While performing the winding resistance test, we take care of the following to ensure the accuracy in the test:
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.
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:
We perform the insulation resistance test on an electrical transformer as per the following steps:
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.
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:
The step-by-step procedure of temperature rise test of electrical transformer is explained below:
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.
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:
The following steps are followed to perform the partial discharge test on a transformer:
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.
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:
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 and Temperature Rise Test is same as Types 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:
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
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 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.
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.
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