Creepage Distance for Insulators in Substation

Creepage Distance for Insulators:

The creepage distance for insulators is the shortest distance along the insulator surface between the metal parts at each end of the insulator. Creepage distance can also be refer as leakage distance for insulators.
Insulators in substation are provided to avoid any leakage current from live electrical conductors to flow to the earth through supports. The atmospheric dust sticks to the insulator surface forming a conducting layer. The leakage current flows from the live conductor to the earth through such surface layers. The leakage properties (creepage properties) of an insulator s in substation are characterized by the length of the leakage path. While designing the insulator sheds, the leakage distance for insulators requirement should be satisfied. The requirement of the leakage distance or creepage distance for insulators depends on the

• Rated phase to ground voltage
• Degree of atmospheric pollution

If the surface of the insulator is clean, smooth and well gazed the dust particles tend to fall down. When an a.c voltage is applied to the apparatus, the dust particles have a tendency to align to the direction of electrical field lines on the surface of the insulator. If the surface of the insulator is rough and moist this alignment will be faster. The continuous application of voltage causes slow alignment of these particles. To prevent a continuous conducting track, the insulator should have sufficient ceepage distance or leakage distance.

During power frequency withstand test of an unclean insulator, the flash-over occurs along the dirty surface of the insulator. In case of internal gas filled or oil filled apparatus, the internal surface should also be free from moisture and dust. Otherwise internal flash over can occur along the surface by tracking. Some of the typical creepage distance for insulators in substation provided based on the level of pollution are tabulated below

S No.	Degree of Pollution	Recommended Creepage Distance for Insulators
1	Clean areas	16 mm/kV
2	Moderately polluted areas	20 mm/kV
3	Industrial areas	22 mm/kV
4	Heavily polluted areas	25 mm/kV

Substation Interview Questions Answers

What are different insulators employed in substations?

Answer: Some of the insulators employed in substations are:

• Pin insulators
• Post insulators
• Strain insulators
• Suspension insulators
• Hallow apparatus insulators
• Dead-end insulators
• Solid core insulators

 What is creepage distance?

Answer: Creepage distance is the shortest distance between two conducting points along the surface of the insulating material. 

What is Basic Impulse Level (BIL)?

Answer: Basic Impulse Level (BIL) refer to the peak value of lightning impuse voltage withstand capability of the equipment

What are the disadvantages of corona?

Answer: Some of the disadvantages of corona are:

• Corona causes power loss
• Corona causes unacceptable noise
• Corona cause radio interference

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What is the significance of Corona rings in substations?

Answer: It is observed that corona discharge occurs mostly at sharp corners, edges near the conductor fixing points. Corona rings also called grading rings are hallow metallic rings having large diameter and will have a smooth surface. These corona rings are placed suitably to metal clamps, joints to encircle the conductors surface. This have the advantage of shifting of high stress point shifted from conductor surface to corona ring surface. As the diameter and surface area of the ring is higher than that of the conductor. This helps in:

• Field distribution is made uniform
• Surface stress is reduced to much lower value
• Corona discharge is eliminated
• Dust deposition is also minimized

What is the neutral point earthing and its advantages?

Answer: In substation all the neutrals of rotating machines, transformers, busbars and other equipment are connected to ground. Some of the advantages are:

• Earth fault protection is based on the method of neutral grounding
• The system voltage during earth faults depends on the neutral grounding. During line to ground faults, grounding of neutral helps to reduce the over-voltages which occur on the one healthy phases