Understanding Rated Insulation Levels in High-Voltage Systems
High-voltage equipment needs strong insulation to work safely, reliably, and efficiently. Rated insulation levels show how much stress the insulation can handle, such as lightning strikes, sudden electrical surges, or power overloads. To ensure everything works consistently everywhere, organizations like the IEC and Indian Standards (IS: 13118) have created clear global rules for these insulation levels.
Actually, an insulation system may have to endure many high-voltage situations. The stresses imposed on the insulation due to various natural and operational activities are not permanent. Most of these stresses are transient in nature.
What Are Rated Insulation Levels?
Rated insulation levels refer to the specific voltage levels that a piece of electrical equipment can safely withstand without insulation failure under various conditions. These parameters are categorized into:
- Power-Frequency Withstand Voltage – The insulation’s ability to withstand sinusoidal voltages for a short period (e.g., 1 minute or 60 seconds) is an important characteristic. This time limit is typically specified as 1 minute. In power systems, when part of the load is disconnected or for other reasons, a higher voltage can momentarily occur. These voltage surges are brief and usually cleared within a few seconds by the protection system. For this reason, insulation systems must be designed to handle such short-term overvoltage situations for a specific duration. However, it is neither necessary nor cost-effective to design insulation for withstanding these overvoltages for an extended period.
- Lightning Impulse Withstand Voltage – The insulation’s ability to handle steep voltage surges caused by lightning (such as a 1.2/50 µs waveform) is crucial. Lightning impulses are extremely brief and sharp. Therefore, the insulation system must be capable of enduring these sudden, high-voltage spikes. However, it is not cost-effective to design an insulation system that can handle such high voltages for even a few milliseconds. These high-voltage surges typically last only for microseconds and dissipate very quickly once they occur in the system.
- Switching Impulse Withstand Voltage – This voltage rating is relevant for higher voltage systems. This rating defines the withstand capability of insulators against voltage surges due to switching operations. As per the Central Electricity Authority (CEA) Guidelines, the Switching Impulse Test is mandatory for equipment of 300kV and above. As per IEC 62271-1, the Switching Impulse Test is compulsory for equipment of 245kV and above.
Key Components of Rated Insulation Levels
- Rated Voltage (Ur): The highest voltage that equipment can safely handle continuously is called the rated maximum voltage. For systems up to 245kV, this is typically set about 10% higher than the nominal voltage to account for normal voltage fluctuations. For systems at 400kV and above, the margin is reduced to 5%, striking a balance between reliability and cost-effectiveness.
The Normal Voltage Rating column lists the nominal system voltages.Normal Voltage Rating (kV) Highest Voltage Rating (kV) Additional % 33 36 10% 132 145 10% 220 245 10% 400 420 5%
The Highest Voltage Rating column gives the maximum voltage for the system as per standard.
The Additional % column specifies the percentage increase over the nominal voltage. - Power-Frequency Withstand Voltage: This voltage rating evaluates the strength of insulation system under a power-frequency i.e. 50Hz voltage for 1 minute, under both dry and wet conditions. For example:
- A 36kV system has a power-frequency withstand voltage of 70kV (r.m.s).
- A 145kV system has a power-frequency withstand voltage of 275kV (r.m.s).
- A 245kV system has a power-frequency withstand voltage of 460kV (r.m.s).
- A 420kV system has a power-frequency withstand voltage of 630kV (r.m.s).
- Lightning Impulse Withstand Voltage: This is not possible to predict the exact amplitude and shape of a natural lightning surge. A lightning impulse withstand voltage simulating lightning surges, is expressed as the peak voltage of a 1.2/50 µs impulse waveform.
- For 36kV systems, the value is typically 170kV (peak).
- For 145kV systems, the value is typically 650kV (peak).
- For 245kV systems, the value is typically 1050kV (peak).
- For 420kV systems, the value is typically 1425kV (peak).
- Creepage Distance: The minimum distance along the surface of an insulator between two conductive parts, designed to handle pollution and environmental stresses. The Central Electricity Authority (CEA) guidelines classify creepage distance categories based on pollution levels and environmental conditions. These classifications ensure the insulation’s ability to handle surface leakage currents caused by contamination in various environments.
Pollution Level Minimum Creepage Distance (mm/kV) Light Pollution 16 Medium Pollution 20 Heavy Pollution 25 Very Heavy Pollution 31 - Altitude Considerations: Rated insulation levels are valid for installations up to 1000m above sea level. For higher altitudes, air density decreases, and derating factors are applied to maintain insulation reliability.