How is the Shielding Designed in High Voltage Power Cables?

High Voltage Power Cables are essential components in modern electrical systems, transmitting electricity efficiently and safely across long distances. One of the most critical aspects of these cables is their shielding design, which plays a vital role in ensuring both operational reliability and safety.

Understanding the Role of Shielding

Shielding in High Voltage Power Cables serves multiple purposes:

1. Electric Field Control: High voltage conductors generate strong electric fields. Shielding ensures these fields are evenly distributed, preventing localized stress that could degrade insulation or cause breakdowns.
2. Safety: By containing the electric field, shielding reduces the risk of accidental contact, which is especially critical for workers and nearby equipment.
3. Minimizing Electromagnetic Interference (EMI): High voltage currents can induce electromagnetic fields that interfere with nearby sensitive electronics. Proper shielding mitigates this issue.
4. Reducing Voltage Stress: Shielding helps maintain the uniform voltage gradient within the cable, prolonging the insulation life and enhancing reliability.

In essence, shielding is both a protective and performance-enhancing feature.

Components of Shielding in High Voltage Power Cables

High Voltage Power Cables are complex structures, and shielding is one of the integral layers. The main components associated with shielding include:

1. Conductor Screen:

   • This is a semi-conductive layer applied directly over the conductor.
   • Its primary function is to eliminate sharp edges on the conductor surface that could concentrate the electric field and stress the insulation.
   • Materials: Typically extruded semi-conductive polymer compounds.

2. Insulation:

   • While not a shield itself, the insulation layer works in tandem with the conductor and insulation screens to prevent electrical breakdown.

3. Insulation Screen:

   • This semi-conductive layer is applied over the insulation.
   • Its purpose is to ensure a smooth interface between insulation and the metallic shield, preventing voids or air gaps that can lead to partial discharge.

4. Metallic Shield (or Screen):

   • This layer consists of copper wires, tapes, or lead sheaths that completely surround the insulation.
   • It provides a path for fault currents, grounding, and electromagnetic shielding.
   • Metallic shields also protect the cable from external mechanical damage in some designs.

5. Outer Sheath:

   • The outer sheath is the final protective layer, shielding the metallic screen from environmental factors such as moisture, chemicals, and mechanical impacts.

Types of Shielding in High Voltage Power Cables

Shielding can vary depending on voltage level, cable application, and installation environment. The main types include:

1. Concentric Neutral (or Wire Shielding)

• Copper wires are helically wound around the insulation or conductor.
• Provides excellent grounding and is easy to terminate with standard connectors.
• Often used in medium and high voltage power cables in industrial and utility applications.

2. Tape Shielding

• Thin metallic tapes, usually copper or aluminum, are helically or longitudinally wrapped around the insulation.
• Offers uniform coverage and reduces impedance for fault currents.
• Common in high voltage and extra-high voltage cables due to its smooth interface with insulation.

3. Lead Sheath Shielding

• A layer of lead may be applied for both electrical shielding and moisture protection.
• Primarily used in underground or submarine high voltage cables where environmental protection is critical.
• Lead shields are highly durable but heavier and less flexible than copper-based shields.

4. Cross-Linked Polyethylene (XLPE) with Semi-Conductive Shields

• Modern high voltage cables often use XLPE insulation combined with semi-conductive shields.
• Semi-conductive layers reduce partial discharge risks while providing uniform voltage stress distribution.
• Lightweight and suitable for high-voltage underground applications.

Design Considerations for Shielding

Designing shielding for High Voltage Power Cables requires careful consideration of electrical, mechanical, and environmental factors.

1. Voltage Level

• The higher the voltage, the more critical the shielding becomes.
• High-voltage applications require precise control of electric fields to prevent insulation breakdown.

2. Current Carrying Capacity

• Shielding must accommodate fault currents safely.
• Designers calculate the required cross-sectional area of metallic shields to handle maximum prospective fault currents without overheating.

3. Environmental Conditions

• Underground cables may face moisture, soil chemicals, or mechanical pressure.
• Submarine cables require waterproof shielding and corrosion-resistant materials.

4. Electromagnetic Compatibility (EMC)

• High Voltage Power Cables can generate significant electromagnetic fields.
• Shielding design considers proximity to sensitive equipment to reduce interference.

5. Flexibility and Installation

• Cables may need to bend around corners or pass through conduits.
• Shielding must maintain electrical and mechanical integrity even under bending stresses.

6. Thermal Considerations

• Shields conduct heat away from the conductor.
• Proper thermal design ensures insulation longevity and prevents overheating during normal or fault conditions.

Shielding Performance Testing

Ensuring the shielding performs as intended is crucial for safe and reliable operation. Common tests include:

1. Partial Discharge Testing

   • Detects voids, defects, or gaps between insulation and shielding layers.

2. Voltage Withstand Test

   • High voltage is applied to confirm that the insulation and shielding can safely handle operational voltages.