As machinery becomes more advanced, choosing the right medium voltage cable is critical for performance and safety.
Inadequate cables can lead to power loss, equipment damage, and dangerous failures.
Understanding the construction and components of MV cables, from conductors to armor, allows you to select cables optimized for your application, ensuring reliable and efficient operation.
Medium Voltage
The term “medium voltage” can have slightly different definitions depending on the context and governing standards. According to IEEE, medium voltage typically refers to equipment designations between 601 volts and 69,000 volts. However, some IEEE standards propose alternative ranges, such as above 600V up to 35,999V or even 1 kV to 35 kV.
In the context of distribution systems, the National Electrical Code (NEC) classifies medium voltage distribution as voltages between 50 and 1000 volts, while high voltage distribution spans from 1000 to 4160 volts.
The International Electrotechnical Commission (IEC) defines medium voltage cable as any voltage level between low voltage and high voltage, with a rated voltage range from 1 kV to 100 kV. The most common range for medium voltage cables is 3 kV to 69 kV. IEC standards also specify voltage ratings for medium voltage cables used in large, high-power equipment and machinery, ranging from 1.8/3 kV to 18/30 kV.
Construction and Components of Medium Voltage Cables
Conductor Materials
The conductor forms the core of a medium voltage cable and is responsible for carrying the electric current. The choice of conductor material depends on the specific application and requirements:
- Copper: Favored for its excellent conductivity and flexibility, copper is suitable for applications requiring high current carrying capacity. It is widely used in medium voltage cables due to its superior performance.
- Aluminum: As a lightweight and cost-effective alternative, aluminum offers good conductivity. It is often preferred for overhead wires and large distribution projects where weight and cost are important factors.
Conductor Screen
The conductor is wrapped with a semiconducting layer called the conductor shield or screen. By ensuring a more uniform and radial electric field inside the insulation layer, the conductor screen effectively confines the electric field to the cable core, enhancing the cable’s performance and longevity.
Insulation Materials
The insulation layer in a medium voltage cable is designed to withstand high electric fields inside the cable and prevent current leakage. Several insulation materials are commonly used:
- Cross-linked polyethylene (XLPE): XLPE is highly regarded for its excellent electrical, mechanical, and thermal properties, as well as its resistance to chemicals and moisture. XLPE-insulated cables are usually designed to be longitudinally waterproof and have a low dielectric constant.
- Tree-resistant cross-linked polyethylene (TR-XLPE): This special formulation of XLPE contains additives that slow the growth of water trees, thereby extending the cable’s service life and reducing electrical losses.
- Ethylene propylene rubber (EPR): EPR offers better flexibility and performance at high temperatures compared to XLPE. It is commonly used in applications where the cable may be subjected to higher operating temperatures.
- High modulus ethylene propylene rubber (HEPR): HEPR provides enhanced mechanical strength and resistance to deformation, making it suitable for applications with demanding physical requirements.
Insulation Screen
A second semiconducting layer, called the insulation screen, is applied around the insulation layer. This screen is in perfect contact with the insulation layer and serves to prevent electrical interference and minimize the cable’s interference with external devices. The insulation screen works in conjunction with the conductor screen to ensure optimal electric field distribution within the cable.
Metallic Shielding
Medium voltage cables typically include a metallic shielding layer in contact with the outer semiconducting layer. This shielding layer is usually made of copper fibers arranged in a spiral pattern, evenly covering the entire circumference of the cable. Alternatively, it can be composed of copper wire or copper tape wrapped around the cable core or individual cable cores. The metallic shielding provides additional protection against electromagnetic interference and helps to contain the electric field within the cable.
Water Blocking Layers
For cable designs intended for use in humid or underground environments, waterproof layers are incorporated to prevent the intrusion and spread of moisture.
Inner Sheath
In armored medium voltage cables, an inner sheath, also known as an armor pad, is applied between the metal shield and the armor layer. This inner sheath acts as a barrier and is usually made of the same material as the outer sheath. Its purpose is to provide a cushion for the armor and prevent direct contact and potential damage to the underlying metal shield during installation or due to external forces.
Armor
The armor layer in medium voltage cables is designed to significantly increase the cable’s protection against external aggression and mechanical damage. It provides an additional layer of defense, ensuring the cable’s durability and reliability in harsh environments.
Outer Sheath Materials
The outermost layer of a medium voltage cable is the outer sheath, which offers comprehensive protection for the cable from mechanical and chemical attacks. This layer is typically a uniform and continuous coating, often in a distinctive red color. The outer sheath serves as the first line of defense against external factors and helps maintain the cable’s overall integrity.