Transformer insulation materials refer to the materials used both inside and outside the transformer. The main function of these materials is to isolate electrical components to prevent the uncontrolled conduction of current between windings of different voltage levels, between windings and the core, between windings and the casing, and between windings and other conductive parts. The use of insulating materials ensures that the transformer can operate safely within the designed electrical stress range, preventing electrical breakdown, short circuits and other electrical faults.
• Electrical isolation
Insulating materials prevent electrical short circuits or breakdowns by providing a high-resistance path, which prevents current from flowing from one electrical component to another or ground.
• High-pressure resistance performance
Insulating materials must be capable of withstanding the high voltage and instantaneous overvoltage during the operation of the transformer (such as voltage spikes caused by lightning strikes or switch operations) without electrical breakdown.
• Heat resistance performance
Transformers generate heat during operation, so insulating materials must have sufficient heat resistance to prevent degradation or failure at high temperatures.
• Mechanical protection
Insulating materials also need to provide mechanical support and protection to prevent windings or other electrical components from being damaged due to vibration, shock or external stress.
• Long-term stability
Insulating materials should have good anti-aging performance and be able to maintain their electrical and mechanical properties during long-term operation.
• Winding insulation materials: enameled wire, paper-clad wire, fiberglass tape, etc
• Main insulating materials: electrical paper, epoxy resin, insulating oil
• Spacers and supporting materials: Pressed wood boards, epoxy glass fiber boards
• Insulation between windings: Interlayer insulation paper, polyester film
• End insulation material: Insulating sleeve, end insulation
• Lead insulation materials: insulating sheath, insulating tape
• Additional insulating materials: mica tape, high-temperature resistant gasket
• Enclosure and structural insulation: Insulating partition boards, insulating support strips
• Insulating oil: Mainly used in oil-immersed transformers, it serves a dual purpose of cooling and insulation
• Transformer external insulation: Porcelain bushings, used at the high-voltage lead outlet, providing electrical insulation and mechanical support
Insulation level
The insulation level of a transformer refers to the ability of the internal insulation system of the transformer to withstand specific voltage stresses (such as power frequency voltage, lightning impulse voltage or switch operation voltage) without electrical breakdown. It is a key indicator for measuring the transformer’s resistance to electrical stress such as electrical faults and overvoltage events (such as lightning strikes and switch operations) during operation. The insulation level directly affects the reliability, safety and service life of transformers.
• The tolerance capacity of electrical stress
The main purpose of the insulation level is to ensure that the windings, core and other electrical components of the transformer do not experience breakdown, partial discharge or other forms of electrical faults when subjected to electrical stress.
• The properties of insulating materials
The insulation level is directly related to the quality of the insulating material. The dielectric strength, heat resistance, aging resistance, moisture resistance and other properties of the material determine the electrical stress that the transformer can withstand.
• Insulation design
The insulation level also depends on the design of the transformer, including the selection of materials, thickness, layout method, etc. Reasonable insulation design can effectively enhance the insulation level of transformers and ensure the stability of the insulation system under high voltage and high temperature conditions.
• The overall reliability of the insulation system
Insulation level not only refers to the tolerance capacity of a single material, but also includes the reliability of the entire insulation system in design, manufacture and operation. A high-quality insulation system can maintain its electrical performance during long-term operation and avoid performance degradation caused by aging or environmental changes.
The importance of insulation level
Insulation level is a key indicator for measuring whether a transformer can operate safely under different electrical stress conditions, such as power frequency voltage, lightning impulse voltage and operating impulse voltage. High insulation levels mean that transformers can withstand these stresses without breakdown or short circuit, thereby ensuring the overall safety of the power system. The reliability of the insulation system is directly related to whether the transformer will experience power outages, equipment damage or more serious power accidents due to electrical faults.
During operation, transformers will encounter various electrical stresses, such as overvoltage and instantaneous voltage spikes. High insulation levels enable transformers to maintain normal operation when facing these challenges, avoiding insulation failure or partial discharge. This not only enhances the reliability of the transformer, but also reduces the maintenance and replacement costs caused by shutdown due to faults.
During operation, transformers will encounter various electrical stresses, such as overvoltage and instantaneous voltage spikes. High insulation levels enable transformers to maintain normal operation when facing these challenges, avoiding insulation failure or partial discharge. This not only enhances the reliability of the transformer, but also reduces the maintenance and replacement costs caused by shutdown due to faults.
Transformers often need to operate under various complex environmental conditions, including high humidity, high pollution, extreme temperatures and other environments. The high insulation level enables the transformer to adapt to these environmental conditions without a decline in insulation performance or failure. This is particularly important for transformers operating in harsh environments, such as power equipment in coastal areas, industrial zones or high-altitude regions.
Sudden events such as lightning strikes and switch operations can exert extremely high voltage stress on transformers in a short period of time. The high insulation level of the transformer can ensure that it can still operate normally under these circumstances and will not cause insulation breakdown due to instantaneous voltage spikes. This is crucial for maintaining the stability of the power grid, especially in areas with extreme weather or frequent power grid operations.
The power industry has strict standards and regulatory requirements for the insulation level of transformers, such as IEC (International Electrotechnical Commission) or IEEE (Institute of Electrical and Electronics Engineers) standards. High insulation levels can ensure that transformers comply with these international standards, thereby guaranteeing their adaptability and acceptability in the global market. This not only helps ensure the compliance of the products, but also provides a guarantee for the application of transformers in different countries and regions.
Transformers with a high insulation level have fewer faults during operation, which means less maintenance requirements and downtime, thereby reducing operating costs. In addition, the high insulation level also reduces the maintenance and replacement costs caused by insulation aging or failure.
Transformers are key equipment in power systems, and their operational stability directly affects the overall stability of the power grid. A high insulation level can ensure that transformers operate safely and stably for a long time in the power grid, reduce the chain reaction caused by equipment failure, and thus guarantee the continuity and reliability of power supply.
The insulation level of a transformer is crucial for ensuring the safety, reliability, durability of the equipment and its adaptability to various operating environments. By designing and manufacturing transformers with high insulation levels, the overall performance of the power system can be effectively enhanced, the service life of equipment can be prolonged, and operation and maintenance costs can be reduced. These advantages have enabled high insulation levels to occupy a core position in the design, equipment manufacturing and operation and maintenance of power systems.
Core assessment of insulation level
The Applied Voltage Test of a transformer, also known as the Power frequency withstand test, is a test voltage applied to each winding of the transformer that is higher than its normal operating voltage. To verify the withstand capacity and reliability of its insulation system under extreme voltage conditions. This kind of test is usually carried out during the manufacturing process and on-site acceptance tests, and it is one of the important means to ensure the electrical safety of transformers in actual operation.
4.1.1 Applied Voltage Test – Purpose
• Verify the insulation strength
By applying a voltage higher than the normal operating voltage, check whether the insulation system between the transformer windings, between the windings and the core, and between the windings and the casing has sufficient strength to resist electrical breakdown
• Detect manufacturing defects
External withstand voltage testing can expose possible insulation defects in the transformer manufacturing process, such as partial discharge, deterioration or damage of insulation materials. These defects are more likely to be detected under high-voltage conditions
• Ensure operational safety
Through external withstand voltage testing, it is ensured that the transformer can operate safely under normal and abnormal voltage conditions during its working life, preventing equipment damage or power outages caused by electrical faults.
4.1.2 Applied Voltage Test – Method Overview
The Applied Voltage Test method involves applying a test voltage higher than the normal operating voltage to the transformer windings, such as twice the voltage, and maintaining it for one minute to verify the endurance and reliability of its insulation system under extreme voltage conditions.
The Lightning Impulse Test is a test method that simulates the withstand capacity of the insulation system of power equipment (such as transformers) under lightning strike conditions. This test is an important one to assess whether a transformer can avoid insulation breakdown when struck by lightning, ensuring the safety and reliability of the transformer.
4.2.1 Lightning impulse withstand test-BIL – Purpose
• Verify the insulation strength
By applying high-voltage pulses that simulate lightning strikes, it is tested whether the transformer insulation system can remain intact under extreme conditions to prevent electrical breakdown.
• Discover potential defects
Detect possible defects in the insulation system, such as bubbles, cracks or aging problems. These defects may not be visible during normal operation, but they may cause insulation failure under lightning strikes.
• Ensure the safety of the equipment
Ensure that the transformer can operate safely in actual lightning strike events to avoid equipment damage or power system failures caused by insulation failure.
4.2.2 Lightning impulse withstand test-BIL – Method Overview
Using an impulse voltage generator, a pulse voltage simulating lightning strikes is applied to the windings of the transformer. Tests are usually conducted several times (for example, 3 to 6 positive polarity impacts), and tests are carried out at different terminals. The response waveform of the transformer is monitored through equipment such as oscilloscopes to detect any abnormal phenomena (such as partial discharge and insulation breakdown). Record the voltage value, waveform and response of each impact.
• Lightning impulse level
It is represented by the symbol LI and the voltage unit is kV
• Applied Voltage level
It is represented by the symbol AC and the voltage unit is kV
For example:
An 80MVA 132/33KV power transformer
High voltage: LI/AC 650/275 kV
High voltage neutral point: LI/AC 325/140KV
Low voltage: LI/AC 170/70 kV
| IEC | IEEE | CSA |
| IEC 60076-3-2013 Power transformers – Part 3 Insulation levels, dielectric tests and external clearances in air | IEEE C57.12.00-2021 | CSA C2.1-06 (R2022) |
Other assessments of insulation level
• Definition
The maximum voltage that a transformer can withstand under overvoltage impact conditions caused by switch operations, etc. Compared with lightning impulse, the waveform of operational impulse is milder, but the duration is longer.
• Function
Ensure that the transformer can operate stably without insulation faults under overvoltage conditions caused by power system operations (such as the opening and closing of circuit breakers).
• Definition
Partial discharge refers to the phenomenon of partial dielectric breakdown that occurs inside or on the surface of an insulation system under high voltage conditions, usually not completely crossing the electrode spacing.
• Function
By measuring the level of partial discharge, potential defects in insulation systems, such as bubbles, cracks or material aging, can be detected to prevent these tiny discharges from developing into serious insulation faults.
• Definition
Measure the resistance value between the winding and ground or between different windings. The higher the insulation resistance, the better the insulation system.
• Function
The test of insulation resistance is used for daily maintenance and inspection, helping to assess the health status and moisture content of the insulation system and prevent insulation deterioration.
• Definition
The dielectric loss factor (tan δ) represents the electrical loss of insulating materials, reflecting the energy loss of the materials under the action of an electric field.
• Function
It is used to evaluate the electrical properties and aging degree of insulating materials. A higher tan δ value may indicate aging or defects in the insulation system.
• Definition
The maximum temperature that insulating materials can withstand over A long period of time is usually indicated by different letter grades (such as A, B, F, H), corresponding to different maximum allowable temperatures.
• Function
It is used for the selection and design of insulating materials to ensure that the materials do not lose their insulating performance at the expected operating temperature.
• Definition
Measure the temperature rise of the windings, core and insulation system of the transformer when it is operating at the rated load
• Function
Ensure that the transformer does not experience accelerated aging or failure of insulating materials due to overheating under normal operating conditions.
• Definition
Creepage distance is the shortest distance between two conductive parts along an insulating surface, and electrical clearance is the shortest distance through which air passes between two conductive parts.
• Function
Ensuring sufficient creepage distance and electrical clearance can prevent surface discharge and air breakdown, and guarantee the safety of the transformer in damp or polluted environments.
• Definition
It includes indicators such as breakdown voltage, acid value, and moisture content, reflecting the insulation performance and stability of insulating oil.
• Function
The quality of insulating oil has a direct impact on the overall insulation level of the transformer. Regular monitoring of the performance indicators of insulating oil can prevent electrical faults.
These parameters cover all aspects of the transformer insulation system, from material properties to overall design. Through comprehensive testing and evaluation, it ensures that the transformer has sufficient insulation levels under various working conditions, thereby guaranteeing its safe and reliable operation. Each parameter reflects a specific aspect of the insulation system. By integrating these indicators, the insulation level of the transformer can be comprehensively evaluated, ensuring its stability and safety in the power system.
