Medium Voltage cable: Design & Construction

A medium voltage cable is a cable used to distribute electricity from power substations to transformer stations. It has a minimum power rating of 1kV and goes up to 36kV. Top Cable&#;s entire range of medium voltage cables is designed according to the most demanding international standards. You can consult it all here.

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LAYERS OF MATERIALS MAKING UP A MEDIUM VOLTAGE CABLE

The composition and design of medium voltage cables is defined by the following elements or layers:

1. CONDUCTOR

It carries the electric current. It is the central element of the cable. The conductors are class 2 and are made of electrolytic copper or high purity electrolytic aluminium.

2. INTERNAL SEMICONDUCTOR SCREEN

It completely covers the conductor. It is made to improve the distribution of the electric field on the surface of the conductor.

3. CABLE INSULATION

Insulation is the critical component of the MV cable, since it has to withstand the high electric field inside. The maximum voltage that a cable can withstand depends on the material and the insulation thickness, which increases with the cable rated voltage. The materials used in the insulation of Medium Voltage cables are as follows:

• Crosslinked polyethylene (XLPE)
• High modulus Ethylene-Propylene (HEPR)
• Ethylene-Propylene (EPR)

4. CABLE EXTERNAL SEMICONDUCTOR

It completely covers the insulation. This material is cross-linked and in perfect contact with the insulation. Peelable semiconductors, partially bonded to the insulation, are usually used to make the preparation of the connections as easy as possible.

5. CABLE METAL SCREEN

Medium voltage cables have a metal screen in contact with the external semiconductor. This screen is made up of copper fibres placed in a helix, uniformly covering the whole perimeter of the cable.

6. PROTECTION AGAINST WATER (ONLY -OL AND -2OL TYPE CABLES)

In cables with a longitudinal seal (type -OL), a tape of hygroscopic material is placed under the sheath, which prevents the longitudinal propagation of water. In cables with Double Sealing, a series of hygroscopic wires are also placed on the conductor (type -2OL).

7. INNER SHEATH (ONLY FOR ARMOURED CABLES)

All armoured cables have a separation cover (or armour pad) between the metal screen and the armour. This inner sheath is made of the same material as the outer sheath.

8. ARMOUR (ONLY FOR ARMOURED CABLES)

Armours improve the protection of the cable against external aggressions. Their use is recommended in all installations where there is a high risk of mechanical aggression. The armour consists of metal wires or strips arranged on the inner sheath. The metal used in single-core cables is aluminium.

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9. CABLE OUTER SHEATH

This is the outermost layer of the cable and protects the cable from the mechanical and chemical aggressions. It is formed by a uniform and continuous coating, usually red colour, which is totally watertight and highly resistant to knocks and abrasions, as well as to the action of the weather. The materials normally used are PVC and halogen-free polyolefins.

TOP CABLE, MANUFACTURER AND DISTRIBUTOR OF MEDIUM VOLTAGE CABLE

Top Cable&#;s Medium Voltage cables are designed according to the most demanding international standards. We manufacture X-VOLT® Medium Voltage Cables: RHZ1 and HEPR. 6/10kV, 8.7/15kV, 12/20kV, 18/30kV, in copper and aluminium.

Medium voltage is defined by NEMA in North America as a utilization voltage between volts and 13,800 volts. Although 60Hz is considered a standard frequency in medium voltage, many facilities still generate their own MV at frequencies as low as 25Hz. Medium voltage is typically used in applications requiring long power distribution lengths and/or controlling motor loads over 200HP. Medium voltage allows for less current draw, resulting in smaller cables and less voltage sag caused by high inrush currents of high HP loads. In general, there are five items to consider when starting a medium voltage project: Transformers/Gear, MCCs, Drives, Wire and Motors.

1) MV Transformers/Gear - Choosing the right MV gear is extremely important for future expansion and ensuring parts and service remain available. Eaton Gear/Transformers can accommodate any application from utility and residential to commercial and industrial. There are also special application to consider, such as operating environment & temperature, K rated and Drive Isolation type transformers.

2) MV Motor Control Centers - Motor Control Centers fed from distribution gear allow for high density layout and control of motor starters, transformer and drive feeders and their electronic protection relays. MV MCCs are comprised of integrated power wiring, vacuum contactor, CPT, fusing and low voltage controls. AB/Rockwell Centerline ® Series MV MCCs are available up to a two high and have been utilizing the same Center Line® bus design since .

3) MV Drives - Mechanics of a design dictate whether a variable speed range or a ramped/reduced voltage start is necessary to protect equipment from the stresses of starting and stopping. Medium voltage drives and soft starts, fed from input contactors, are specified to protect and drive equipment in these situations. The AB/Rockwell MV PowerFlex® is the only MV drive on the market to offer current source technology that reduces size and cost of installation and maintenance. The MV PowerFlex® drive offers a lower cost option and smaller feature set for low complexity fan and pump applications.

4) MV Wire/Cables - Medium voltage wire/cabling can come in a wide range of options, Aluminum or Copper conductors to shielded or non-shielded cable. They have a voltage from 2.4kv all the way to 35kv that are suited for anything over the volt listed above. The most notable medium voltage cable would be the copper tape shielded cable. This comes in 5kv &#; 35kv and can be used in wet or dry locations when installed in accordance with the NEC.

5) MV Motors &#; To specify the right medium voltage motor, requires an evaluation of the load characteristics, inertia, duty cycle of the application and correcting sizing the horsepower based on the application&#;s requirements. The evaluation should also include the area of installation, temperature, altitude and environmental factors (humidity, salt laden air, corrosive, dust, hazardous location, etc.). If the motor is going to be powered by a VFD (Other than an AB PowerFlex® ), common-mode voltage filtering should be considered. Special insulation systems are available to handle high common-mode voltages, but are a costly alternative to isolation transformers. These situations can be mitigated when using current sourced drives such as the AB PowerFlex® .  

Allen-Bradley Medium Voltage: It's Use, Control and Applications. ,,.

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