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Transformer testing and classification

Author: Tanssion Date: 2023-08-23 Hits: 0

Ⅰ. Voltage withstand test and insulation test of transformer
Ⅱ. How transformers are used to adjust voltage and supply power
Ⅲ. Classification of transformers
Ⅳ. Characteristics of transformers

Ⅰ. Voltage withstand test and insulation test of transformer

Transformers are important electrical equipment in power systems. To ensure their safe operation and reliability, withstand voltage tests and insulation tests are required.

Transformer testing and classification

1. Withstanding voltage test:

The withstand voltage test is to test the withstand voltage capability of the transformer insulation system by applying a high voltage to verify that it can operate normally at the rated voltage. This helps detect if there is an insulation breakdown, discharge or leakage problem. The transformer withstand voltage test refers to the inspection of the transformer winding, insulation and insulation capabilities between each other at a specific time and at a specific voltage. The purpose of this test is to find insulation defects, such as partial discharge, leakage, etc., to ensure that the transformer can operate safely and stably. High-voltage testing equipment is required for testing, and the voltage and time must be set according to standard requirements. The withstand voltage test is designed to evaluate the insulation strength of the equipment under the rated voltage to ensure that the equipment will not cause accidents due to insulation breakdown during normal operation.

Common pressure test methods:

Partial Discharge Testing: Evaluates the state of insulation performance by detecting partial discharge activity in an insulation system.

Power frequency withstand voltage test: Apply a power frequency AC voltage of rated voltage between the winding and the casing of the transformer for a period of time to detect whether there is current leakage or breakdown.

Impulse withstand voltage test: Apply a short-term high peak voltage pulse to test whether the insulation system can withstand transient overvoltage.

2. Insulation test

The transformer insulation test refers to checking the normal and abnormal conditions of the transformer insulation state by measuring the winding, ground insulation resistance and dielectric loss factor. The test is designed to check the reliability and stability of the winding and ground insulation, as well as its resistance to insulation and damaging external agents such as water, gas and dust. Different test methods can be used for testing, including insulation resistance test, dielectric loss test, etc.

Insulation testing is to evaluate the insulation performance of transformers by measuring insulation resistance. This test can detect if there is leakage or insulation aging problems in the insulation system. Insulation testing is to evaluate the insulation performance of materials or equipment by measuring their insulation resistance. Insulation resistance indicates how insulating a material is, i.e. its ability to not conduct electricity.

Common insulation test methods:

Polarization Index Test: Evaluate the stability of an insulation system by measuring the change in insulation resistance over time.

Insulation resistance measurement: Insulation resistance is measured using an insulation resistance tester, usually at rated voltage, to assess the integrity of the insulation system.

Absorption Ratio Test: Measures the absorptivity of insulating materials, which is an indicator of an insulating material's ability to absorb and release electrical charge.

3. The difference between withstand voltage test and insulation test

Although both the transformer withstand voltage test and the insulation test are to ensure the safe and reliable operation of the transformer, their test purposes and test methods are slightly different. The transformer withstand voltage test is mainly to detect whether the voltage between the winding and the insulation is normal, focusing on the discovery of insulation defects and partial discharges. The transformer insulation test is to detect the state and performance of the insulation, focusing on the reliability of the winding and ground insulation.

The main feature of the withstand voltage test is to use high voltage, usually 1.5 to 2 times the rated voltage, and the goal is to test the ability of the insulation system to withstand high voltage to avoid breakdown and discharge. The main feature of insulation testing is to measure insulation resistance, which is usually carried out at normal operating voltage. The goal is to evaluate the state of the insulation system to detect problems such as leakage and aging.

4. Precautions during testing

Hipot test: Make sure to use the withstand voltage test instruments and equipment that meet the standard requirements to ensure the accuracy and reliability of the test. According to the rated voltage of the equipment and relevant standards, set the appropriate test voltage. Typically, the test voltage is set to 1.5 to 2 times the rated voltage. Maintain a good ground at both the test equipment and the device under test to prevent voltage leakage and risk of electric shock. Hipot test time should not be too long to avoid possible overheating and damage. Set the appropriate test time according to the standard requirements. Carry out regular calibration and maintenance on the pressure test equipment used to ensure its performance and accuracy.

Insulation test: Conduct insulation test in a relatively dry environment without electromagnetic interference to avoid errors in test results. Insulation testing should be performed at normal operating voltage to more realistically assess the condition of the insulation system. When testing insulation, ensure that the equipment is completely de-energized to prevent the risk of electric shock. During the test, avoid current flowing through the device under test nearby to prevent external current from interfering with the test results. When performing insulation tests, make sure that both the test equipment and the device under test are free from moisture and dust.

Ⅱ. How transformers are used to adjust voltage and supply power

1. The voltage regulation principle of the transformer

Transformers work on the principle of electromagnetic induction. When an alternating current is passed through the input coil (also known as the primary coil), an alternating magnetic field is generated in the core. This alternating magnetic field passes through the output coil (also known as the secondary coil), which induces an electromotive force in the secondary coil, which in turn generates the output current. Magnetic coupling between the two coils allows voltage and current to pass between the primary and secondary.

The principle of transformer voltage regulation is to change the number of turns of the winding, that is, to change the voltage ratio between the primary and secondary sides of the transformer. According to the working principle of the transformer, when the internal impedance voltage drop of the transformer is ignored, there is U1/U2=N1/N2= K. In the formula, U1 and U2 are the primary and secondary terminal voltages of the transformer respectively; N1 and N2 are the turns of the primary and secondary windings of the transformer respectively; K is the transformation ratio of the transformer.

The transformer tap is on the primary side, changing the number of turns of the primary winding of the transformer, the transformation ratio K also changes accordingly, and because U2=U1/K, the secondary voltage also changes, which plays a role in voltage regulation effect. The transformation ratio of a transformer refers to the ratio of the output voltage to the input voltage. The transformation ratio of the transformer can be adjusted by the winding ratio of the coil. If the secondary coil winding is larger than the primary, the output voltage will increase, otherwise it will decrease.

2. The voltage regulation method of the transformer

Off-field voltage regulation:

Generally, most small power transformers are non-excitation voltage regulating tap-changers, and the power must be cut off first when adjustment is required. Taking the 10kV transformer as an example, it has three gears, I gear: 10.5kV, 400V; II gear: 10kV, 400V: III gear: 9.5kV, 400V. It can be seen that when the input voltage is constant, the output voltage of gear I is the lowest, and the output voltage of gear III is the highest.

When the output voltage of the transformer is lower than the allowable value, adjust the position of the tap changer from gear I to gear I, or from gear II to gear II, and vice versa. That is, "low to low, high to high". "Low to low-key" means that the primary winding is reduced, the transformation ratio is reduced, the power supply voltage remains unchanged, and the secondary voltage becomes higher. "High to high pitch" means that the number of turns of the primary winding increases, the power supply voltage remains unchanged, the transformation ratio increases, and the secondary voltage decreases.

Open the cover of the transformer tap changer, and turn the adjusting handle to the desired position. When adjusting the non-excitation tap changer, generally the tap changer should be rotated forward and reverse for three cycles to eliminate the oxide film and oil on the contacts, and then the tap changer should be formally changed.

On-load voltage regulation:

There are two types of on-load voltage-regulating musical instruments, one is an on-load voltage-regulating musical instrument with tuning windings, and the other is an on-load transformer with a voltage regulator. On-load tap-changing transformers with tap-changing windings have tap-changing devices, which can be used to switch taps when there is a load.
The on-load tap changer can use reactance or resistance transition during the tap change process to limit the circulating current during the transition, so there are reactance and resistance types.

The characteristic of the reactance type is that if the reactor is designed for continuous operation, it can stay at the position of bridging the two taps during the tap change process. Under the condition that the required voltage regulation stages are the same, the transformer The number of taps of the coil is reduced by half, and at the same time, even if the power supply of the tap changer operating mechanism fails at any position during the transition process, the transformer can still continue to operate. However, the power factor of the circulating current is low during the transition, and the electrical life of the arc contact of the diverter switch is short; due to the use of a reactor, the volume of the transformer is increased and the manufacturing cost is high.

The characteristic of the resistive type is that the transition time is short, the power factor of the circulating current is 1, and the electrical life of the diverter switch arc contact can be increased from 10,000 to 20,000 times of the reactive type to 100,000 to 200,000 times. However, since the resistance works for a short time, once the operating mechanism is operated, it must be completed continuously. If it is interrupted due to unreliable mechanism and stays in the transition position, it will cause the resistance to burn out and cause an accident.

The on-load tap changer is composed of a diverter switch, a selector switch and an operating mechanism. The changeover switch is a part specially responsible for switching the load current, and its action is quickly completed according to a predetermined program through a fast mechanism. The selector switch is in the order of tapping, so that the adjacent taps to be switched immediately are connected in advance and bear the continuous load. Its action is carried out without electricity. In order to meet this requirement, the selector switch is divided into odd and even numbers, and the two operate step by step. In order to increase the range of voltage regulation, sometimes the selector switch can have one or several range switches, which are connected into positive and negative voltage regulation, coarse and fine voltage regulation, etc. to increase the number of voltage regulation stages.

The diverter switch and selector switch, collectively referred to as the switch body, are generally installed in the transformer oil tank. The diverter switch generates an arc when switching the load current, which will deteriorate the oil quality, so it must be installed in a separate insulating cylinder to separate it from the transformer oil tank.

Transformer testing and classification

Ⅲ. Classification of transformers

A special type of transformer used in switch mode power supplies. A switch-mode power supply is a highly efficient means of supplying power by periodically turning on and off electronic switches to transfer power from an input to an output, enabling voltage conversion and power conversion. The working principle of SMPS transformers is different from traditional transformers, and it is usually used for high-frequency switching operations to achieve efficient energy conversion. SMPS transformers work in a higher frequency range, usually between tens of kHz and hundreds of kHz, which is higher than the operating frequency of traditional power transformers. Due to high frequency operation, SMPS transformers can use smaller magnetic core materials, resulting in smaller size and weight. SMPS transformers combine high-frequency operation and switching technology to convert electrical energy from input to output with high efficiency, thereby reducing energy loss.

The key electrical equipment used to change the AC voltage and current levels, it plays an important role in the power system. Power transformers transfer electrical energy from one voltage level to another through the principle of electromagnetic induction to achieve energy transmission, distribution, and conversion.

Power transformers are mainly oil-immersed, and the product structure is divided into two types: core type and shell type. The production volume of core type accounts for 95%, and that of shell type only accounts for 5%. There is no overwhelming advantage between the core type and the shell type, but the core type process is relatively simple, so it is adopted by most enterprises; while the shell type structure and process are more complicated, only traditional factories use it. The shell type is especially suitable for high voltage and large capacity. It has advantages in insulation, machinery and heat dissipation, and is suitable for transportation of hydropower stations in mountainous areas.

The operation of power transformers is based on the principle of electromagnetic induction. When an alternating current passes through the main winding, an alternating magnetic field is generated in the iron core. This magnetic field will pass through the secondary winding, thereby inducing an electromotive force in the secondary winding, generating an output current. The transformation ratio of the transformer (the ratio of the number of windings in the primary winding to the number of windings in the secondary winding) determines the relationship between the input voltage and the output voltage.

3. Distribution Transformer

It is a dedicated power transformer mainly used to step down high-voltage electrical energy to low-voltage electrical energy suitable for domestic, commercial and industrial purposes.
The transformation ratio of a distribution transformer determines the relationship between input voltage and output voltage. Also, transformers can be single-phase (for single-phase loads) or three-phase (for three-phase loads). Distribution transformers usually use insulating materials and transformer oil to ensure electrical insulation and heat dissipation.

The capacity of foreign distribution transformers can reach 2500KV·A, and there are circular and oval core forms. The circular ones account for the vast majority, and the elliptical ones can reduce the material consumption due to the small M0 (the distance between the iron core columns), and the corresponding coils are elliptical. Low-voltage coils are wirewound and foil, and tanks are piped (few) and corrugated (most).

It is an electronic component specially used for audio signal transmission and coupling. It plays an important role in audio equipment, used to match the impedance between different circuits, isolate two circuits, realize signal transmission and reduce noise.

Audio transformers can be used to match impedance between different circuits to achieve maximum power transfer for signal transmission. This is especially important in audio equipment connection and tuning to avoid distortion and loss of signal. Audio transformers provide electrical isolation, separating the electrical connections between the input and output circuits. This can reduce noise and interference caused by ground loops in some applications. Through electrical isolation and impedance matching, audio transformers reduce the effect of external noise and interference on audio signals.

Refers to customized transformers for special application areas, whose design and characteristics are optimized for specific needs and requirements. The aerospace field requires special transformers that can function properly under high temperature, high voltage and harsh environmental conditions. These transformers need to be highly reliable and robust. In the field of nuclear energy, transformers need to withstand severe radiation and environmental tests. They need to have properties such as radiation resistance, high temperature resistance and high reliability. Transformers used in the oil well and mining industries need to be able to withstand harsh underground environments including high temperature, high humidity and corrosion. Transformers used in power electronics need to be able to handle special requirements such as high frequency, high voltage pulses and fast switching.

6. Dry type transformer

It is a type of transformer that does not use liquid insulation (such as transformer oil), and its winding and iron core are exposed to the air and cooled naturally by the air. There are four structures of dry change: epoxy resin casting, filling casting, wrapping and impregnation. At present, open and ventilated H-class dry-type transformers are widely used in Europe and the United States. It is a new type of H-class dry transformer developed after absorbing the characteristics of the wrapped structure and using Nomex paper on the basis of the impregnated type.

Since no transformer oil is required, dry-type transformers eliminate the need for maintenance and disposal of transformer oil, reducing the risk of environmental pollution. Since it does not involve the use of transformer oil, dry-type transformers have less impact on the environment, and are suitable for some application scenarios that require high environmental protection.

7. Amorphous alloy transformer

It is a transformer made of amorphous alloy material. Amorphous alloy is a special metal material. Its atomic structure has no regular lattice arrangement, but presents an amorphous structure, which makes it have some special electromagnetic properties and is suitable for power transformers and other fields. Although the amorphous alloy transformer has poor short-circuit resistance and high noise, it can save energy. Amorphous alloy materials have higher saturation magnetic induction, which means that under the same output power, transformers made of amorphous alloy materials can be smaller and lighter. Amorphous alloy materials have good high-frequency characteristics, which makes amorphous alloy transformers suitable for some applications that require high-frequency operation, such as high-frequency transformers, frequency converters, etc.

Is a special type of transformer whose main function is to provide electrical isolation between the input and output. This means that there is no direct electrical connection between the input winding and the output winding, thereby achieving electrical isolation of the input and output circuits while also ensuring relatively high insulation performance.
An isolation transformer provides increased safety as it prevents current from flowing between different circuits, reducing the risk of electric shock and electrical accidents.

The isolation transformer also enables isolation of the input and output, further reducing possible interference and noise transmission. The isolation transformer can also provide certain common-mode rejection to reduce the impact of common-mode noise on the system.

It is a special type of transformer, which is different from ordinary isolation transformers in its working principle. Autotransformers share a part of the coil between the input and output, that is, there is a part of overlap between the input winding and the output winding. An autotransformer is different from a normal transformer in that it shares a part of the coil between the input winding and the output winding. This part of the coil is not only a part of the input winding, but also a part of the output winding. An autotransformer is often used where a large transformation ratio is required because it can achieve a large transformation ratio on one part of the winding and a smaller transformation ratio on another part of the winding. Because an autotransformer shares a portion of its windings, it is typically smaller and lighter than an equivalent isolation transformer.

A special type of transformer used to transmit pulsed signals in electronic circuits. Pulse transformers are commonly used in high frequency and radio frequency circuits to transmit and match pulse signals. Pulse transformers usually work in the high frequency and radio frequency range, so they need to have good high frequency characteristics, such as low loss and high frequency response. Pulse transformers can be used to match impedance between different circuits to ensure maximum power transfer and signal matching. Pulse transformers are used in communication systems, radar systems, etc. to transmit pulse signals, such as radar pulses, communication signals, radio frequency signals, etc.

It is a device specially designed for measuring and monitoring electric current. It is usually installed in power systems to convert high currents (usually proportional to the rated current) into relatively small measured currents. Current sense transformers usually have a specific transformation ratio, which represents the proportional relationship between high current and low current. Their accuracy is very important for accurate current measurement.

Current sense transformers need good insulation to prevent high voltages from intruding into the measurement system. In addition, they also help to increase the safety of electrical systems. Current detection transformers are mainly used for current measurement, such as monitoring load current and fault current in power systems. Current sensing transformers are also used for current protection in power systems, by monitoring changes in current to trigger protection devices to prevent current overload or short circuits. In addition, they are also used to control and regulate electric current.

Transformer testing and classification

Ⅳ. Characteristics of transformers

1. Power transfer: A transformer is capable of transferring power without changing the electrical power, i.e. the ratio between input power and output power remains constant.

2. Energy transfer: Transformers play the role of energy transfer in the power system, they can transmit electric energy from power plants to consumers, and at the same time realize voltage adaptation.

3. Voltage conversion: The main function of the transformer is to convert the voltage, which can reduce high voltage to low voltage or increase low voltage to high voltage to meet different power transmission and application requirements.

4. Current conversion: The transformer can also change the magnitude of the current while converting the voltage. According to the principle of energy conservation, the current ratio between input power and output power is opposite to the voltage ratio.

5. Electrical isolation: The isolation transformer can provide electrical isolation to prevent the flow of current between the input and output, thereby ensuring the safety of the system.

6. Loss: The transformer will generate certain losses during the voltage conversion process, mainly including copper loss and iron loss. These losses will lead to waste of energy, so it is necessary to consider how to reduce the loss in the design of the transformer.


Frequently Asked Questions

1、What is transformer ratio?
The transformation ratio is defined as the ratio of the number of turns in the secondary coil to the number of turns in the primary coil of the transformer. It is defined as the ratio of output voltage to the input voltage of the transformer.
2、Can a transformer work on DC?
Transformer does not work on DC. Direct current doesn't have a variable magnetic field induced if fed through the primary winding of a transformer. Thus, only a constant magnetic field will act on secondary and shall not induce any voltage across the terminals of the same.
3、What is the classification of transformer?
According to the supply used, the transformers are mainly classified as Single phase and three phase transformers. A normal transformer is a single phase transformer. It has a primary and a secondary winding and it is operated to either decrease or increase the secondary voltage.
4、How do transformers work?
A transformer is a machine that works using the principle of electromagnetic induction. Basically, this device allows you to increase or decrease the voltage and intensity of an electric current (alternating), but keeping the power constant.

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