What is rated power frequency withstand voltage?

What is rated power frequecy withstand voltage

1) Defination

Rated Frequency Withstand Voltage is a key concept in the field of electrical equipment. First, “industrial frequency” refers to the standard AC frequency of 50Hz or 60Hz used in the power systems of most countries. For example, the frequency of our power system is 50Hz.

Withstand Voltage refers to the maximum voltage that a device or insulation system can tolerate. This is similar to the maximum pressure a person can withstand; if this pressure limit is exceeded, the person may collapse. Likewise, when the withstand voltage limit of equipment insulation is exceeded, the insulation will break down, resulting in equipment failure.

Rated frequency withstand voltage is the maximum voltage that electrical equipment or its insulation system can safely handle during normal operation at the rated frequency (such as 50Hz or 60Hz). Under this voltage, the equipment can operate normally without experiencing insulation breakdown or other failures.

2) Relationship with Other Parameters

• Relationship with Rated Voltage: Rated voltage is the standard voltage for equipment design and operation. For example, the rated voltage of a motor is 380V, which is the ideal voltage for its normal operation. Rated frequency withstand voltage, on the other hand, is the maximum voltage that the insulation of the equipment can withstand, usually higher than the rated voltage. This is to accommodate situations such as voltage fluctuations in the power system. For example, lightning strikes or other faults in the power grid can cause transient overvoltages. The existence of rated frequency withstand voltage ensures that the equipment’s insulation will not be damaged in such cases.
• Relationship with Power Supply Frequency: The power supply frequency must match the rated frequency of the equipment design because the voltage withstand performance of insulating materials is also frequency-dependent. Even if the voltage does not exceed the rated frequency withstand voltage, operating at a non-rated frequency can affect the performance of the insulation materials, leading to equipment failure. For instance, some insulating materials experience increased dielectric loss at higher frequencies, causing heating and potentially reducing their voltage withstand capacity.

3) The Importance of Practical Application

In the Power System: Various electrical equipment, such as transformers, circuit breakers, and insulators, are rated with a specified industrial frequency withstand voltage. For example, if the rated frequency withstand voltage of a transformer is 72.5kV, it means that, under the 50Hz frequency (typical in our country’s power grid), its insulation can withstand up to 72.5kV without breakdown. This is essential for ensuring the safe operation of the transformer. If the insulation fails, it can cause short-circuit faults, destabilizing the entire power system and potentially damaging other connected electrical equipment.

In Power Equipment Production and Testing: The rated frequency withstand voltage is a parameter that must be tested during the production of electrical equipment. After the equipment is manufactured, it must undergo a withstand voltage test to verify that the insulation meets the design specifications. In this test, the equipment is subjected to an environment close to or equal to the rated frequency withstand voltage to observe whether the equipment operates normally and whether the insulation remains intact. This is a critical step in ensuring the quality and safety of the equipment.

4) What is the relationship between insulation level and rated frequency withstand voltage of a power transformer?

1. Insulation Level Determines Rated Frequency Withstand Voltage
   • The insulation level of a power transformer refers to its ability to withstand various overvoltage effects. The quality, thickness, and structure of the insulating materials collectively determine the transformer’s insulation level. For example, using high-performance insulation materials and ensuring sufficient insulation thickness can result in a higher insulation level for the transformer.
   • Rated frequency withstand voltage is a key reflection of the insulation level. A transformer with a high insulation level can withstand higher-frequency voltages without failure, and its rated frequency withstand voltage will correspondingly be higher. Conversely, if the insulation level is low—such as in cases of defective or aging insulation materials—the rated frequency withstand voltage will be lower.
2. Rated Frequency Withstand Voltage Reflects the Insulation Level’s Voltage Withstand Capability
   • Rated frequency withstand voltage, measured at the standard frequency (such as 50Hz or 60Hz), serves as a quantitative index of the transformer’s ability to withstand voltage. It directly indicates the maximum voltage the transformer’s insulation system can handle. For example, a transformer with a rated frequency withstand voltage of 300kV can endure higher voltage levels under standard frequency conditions compared to a transformer with a rated withstand voltage of 200kV, indicating it has a higher level of insulation.
3. The Synergistic Effect of Insulation Level and Rated Frequency Withstand Voltage for Ensuring Safe Transformer Operation
   • Transformers are subject to various voltage stresses during operation, including normal operating voltage, overvoltage conditions, and lightning-induced overvoltage. The insulation level should account for these factors in its design, while the rated frequency withstand voltage primarily focuses on insulation performance under normal frequency voltage fluctuations.
   • When grid voltage fluctuates, such as an increase in industrial frequency, the transformer’s insulation will ensure normal operation as long as the voltage remains below the rated industrial frequency withstand voltage. For instance, in the case of system faults that cause a voltage rise, as long as the elevated frequency voltage stays below the rated frequency withstand voltage of the transformer, the transformer’s insulation will prevent breakdowns, such as short-circuits between the windings or between the windings and the core, thereby ensuring the safe and reliable operation of the transformer.

How is the insulation level of a power transformer determined?

1）Determining Insulation Level Based on Voltage Level

1. Power transformers of different voltage levels require different insulation standards. For example, a 10kV transformer has relatively low insulation requirements due to its lower operating voltage, whereas high-voltage transformers, such as those rated for 500kV, require much higher insulation levels.
2. The rated voltage of the transformer is the basis for determining the insulation level. When designing a transformer, it is essential to consider its stable operation at the rated voltage while also accommodating a certain range of voltage fluctuations. For example, the grid voltage might increase by 10%, and the transformer’s insulation should be capable of withstanding this rise. For high-voltage transformers, the insulation design must also account for potential overvoltage situations, such as operational overvoltage and lightning-induced overvoltage.

2) Considering the Performance of Insulation Materials

1. The type and quality of insulation materials play a critical role in determining the transformer’s insulation level. Common materials used in transformers include insulating paper and insulating oil. Insulating paper has excellent electrical insulation properties and mechanical strength, allowing it to withstand significant electric field strengths. Insulating oil, on the other hand, is used to fill the gaps between the transformer windings and the core, enhancing insulation properties and helping dissipate heat.
2. Different insulation materials have varying properties such as voltage withstand strength and dielectric constant. For instance, high-quality insulating paper may have a higher voltage withstand strength compared to ordinary paper, allowing transformers with high-quality insulation to handle higher voltages. Additionally, the thickness of the insulating material is also critical. Increasing the thickness improves the voltage withstand capability, but it also increases the transformer’s size and cost.

3）The Synergistic Effect of Insulation Level and Rated Frequency Withstand Voltage in Ensuring Safe Transformer Operation

1. .During operation, a transformer is subject to various types of voltage, including normal operating voltage, overvoltage during operation, and lightning-induced overvoltage. The insulation level must be designed with these factors in mind. While the insulation level is designed to account for these variables, the rated frequency withstand voltage primarily focuses on insulation performance under normal frequency voltage fluctuations.
2. When the grid voltage fluctuates, such as during an increase in industrial frequency, as long as the voltage does not exceed the rated industrial frequency withstand voltage, the transformer’s insulation will ensure the proper operation of the equipment. For example, in the event of system faults causing a voltage rise, as long as the elevated frequency voltage stays below the transformer’s rated frequency withstand voltage, the transformer insulation will prevent breakdowns, such as short circuits between the windings or between the windings and the core. This ensures the safe and reliable operation of the transformer.

How is the Insulation Level of a Power Transformer Determined?

1. Based on the Voltage Level
   1. Power transformers require different insulation levels based on their voltage rating. For example, a 10kV transformer has relatively lower insulation requirements due to its lower operating voltage, whereas high-voltage transformers, such as those rated for 500kV, require much higher insulation levels.
   2. The rated voltage of the transformer serves as the foundation for determining its insulation level. When designing a transformer, it is essential to ensure its stable operation at the rated voltage while considering potential voltage fluctuations. For instance, the grid voltage may increase by up to 10%, and the transformer’s insulation must be capable of withstanding this increase. For high-voltage transformers, the insulation design should also account for potential overvoltage scenarios, such as operational overvoltage or lightning-induced overvoltage.
2. Considering the Performance of Insulation Materials
   1. The type and quality of insulation materials play a critical role in determining the transformer’s insulation level. Common insulation materials used in transformers include insulating paper and insulating oil. Insulating paper provides good electrical insulation properties and mechanical strength, with the ability to withstand a certain electric field strength. Insulating oil is used to fill gaps between the transformer windings and core, improving insulation properties and helping with heat dissipation.
   2. Different insulation materials have varying parameters, such as voltage resistance and dielectric constant. For example, high-quality insulating paper may have a higher voltage withstand strength than standard paper, allowing transformers with superior insulating paper to handle higher voltages. The thickness of the insulating material is also critical. Increasing the thickness improves voltage resistance but also increases the transformer’s size and cost.
3. Considering Environmental Factors
   1. The operating environment also impacts the determination of the transformer’s insulation level. If the transformer is installed outdoors, factors such as humidity, temperature fluctuations, and pollution levels must be considered. For instance, in high-humidity environments, moisture can penetrate the transformer’s insulation system, reducing its effectiveness. In such cases, the insulation level must be improved by using better sealing measures to prevent moisture intrusion.
   2. Temperature is another important factor. The transformer heats up during operation, and increased temperature can affect the performance of the insulation material. In high-temperature environments, the voltage withstand capacity of the insulating material may be reduced. Therefore, the insulation level must be determined based on the transformer’s heat dissipation capacity and the expected operating temperature. For transformers with poor heat dissipation, it may be necessary to use more heat-resistant insulation materials or increase the insulation thickness to maintain an adequate insulation level even in higher temperatures.

What is the Rated Frequency Withstand Voltage of a Power Transformer?

The rated frequency withstand voltage of a power transformer varies based on its voltage level. Below is an approximate range of the rated frequency withstand voltages for transformers at various common voltage levels:

10kV Transformer

• For typical 10kV equipment used in building electrical systems, the rated short-time frequency withstand voltage is generally between 30kV and 42kV, or 35kV.
• Some 10kV transformers may have an impulse withstand voltage of approximately 35kV at the high-voltage winding lead end.

35kV Transformer

• The frequency withstand voltage for the neutral insulation level of a 35kV transformer is typically around 73kV.
• For fully insulated 35kV transformers, the frequency withstand voltage at the line end is usually around 85kV.

110kV Transformer

• For 110kV transformers, the line-end withstand voltage typically ranges from 185kV to 200kV.

220kV Transformer

• For a 220kV main transformer with three-phase insulation, the rated frequency withstand voltage varies as follows:
  • At the high-voltage lead end, it is generally around 360kV.
  • At the neutral lead end of the high-voltage side, it is about 200kV.
  • At the lead end of the medium-voltage side, the withstand voltage is around 200kV, while at the neutral lead end of the medium-voltage side, it is about 95kV.
  • For the low-voltage side, the withstand voltage at the lead end is typically around 95kV, and the withstand voltage at the neutral lead end is approximately 100kV.

500kV Transformer

• 500kV transformers have higher rated frequency withstand voltages, typically ranging from 510kV to 630kV at the line end.