Medium Voltage Underground Cable Guide

Table of Contents

CHAPTER 1:   WHAT IS THE BACKGROUND OF MEDIUM VOLTAGE UNDERGROUND CABLES?

Medium voltage cables were expected to have a life duration of 40 years during the initial license period. However, the medium voltage cables did not meet life expectations initially.

During the mid-1980s, there were different improvements made to the medium voltage cables, such as :

All of these were the features incorporated into the manufacturing of medium voltage cables.  As a result, the medium voltage underground cables developed a very long service life.

The concept of medium voltage came as the level of energy increased, and there was a need for a more excellent range in classification, which now includes extra-low and extra-high voltage.

Medium voltage cables are so durable that they can work efficiently, even if there are flaws in their manufacture.

The materials and coatings used for the construction of these cables make them extremely durable. Thus, the medium voltage underground cables require very little maintenance.

Medium voltage cables have a life expectancy of 40 years. The 40-year life expectancy in is cases where these cables get exposure to continuous wetness or deteriorating conditions.

If the conditions are less risky, the medium voltage cables possess an even more exceptional life service, which can exceed 60 years or more.

CHAPTER 2:   WHAT ARE MEDIUM VOLTAGE UNDERGROUND CABLES?

According to the International Electrotechnical Commission (IEC), medium voltage cables are those cables that have a voltage rating of above 1kV or up to 100kV.

Medium voltage cables are used mainly for power distribution or in mobile substations.

In general, medium voltage underground cables are less prone to moisture-related degradation than similar systems in the distribution box.

This feature of medium voltage underground cables is due to their low electrical stress levels.  It is because of the use of rubber insulations, overall jackets, duct bank systems, and well-shielded terminations.

Medium voltage underground cables are useful in power circuits in the open air, conduit, or duct systems.

Medium voltage underground cables are available in a variety of constructions and types based on the application needs.

Medium voltage underground cables are useful in numerous ways. They range from chemical plants, refineries, steel mills, industrial plants to commercial buildings, utility substations, and generating stations.

Medium voltage underground cables are less applicable to those stations that utilize rubber insulation systems.

There is known to be a shortcoming regarding the mechanism of degradation of black and long black rubber insulations. Degradation occurs when insulations exposed to long-term immersion. This degradation remains a significant concern in the development of meaningful tests for this class of cables.

CHAPTER 3:   WHAT ARE THE VARIOUS STANDARDS FOR THE MANUFACTURE OF MEDIUM VOLTAGE UNDERGROUND CABLES?

Medium voltage underground cables get manufactured as per various British and International Standards. These include:

CHAPTER 4: WHAT ARE THE MAIN FEATURES OF MEDIUM VOLTAGE UNDERGROUND CABLES?

Medium voltage underground cables possess a variety of features, including:

In medium voltage underground cables, degradation from the water takes the form of water-treeing in which the gradient forces the water to create small channels in the polymer. These channels look like trees under magnification.

Medium voltage underground cables are available in various versions, which is per specific environmental conditions.

Contaminants and voids are the main problems in wetted extruded cable insulations. These problems are because they disturb the potential gradient within the insulation.

These contaminations also increase the potential across the remaining good insulation.

Different types of Ethylene Propylene Rubber (EPR) are in use nowadays. EPR sub-types have various vulnerabilities to water-enhanced degradation:

• Pink (red) EPR is useful in most EPR medium voltage cable designs. The EPR has treated clay fillers to prevent water absorption. This EPR makes the insulation less prone to water-enhanced degradation than black EPR.
• Brown EPR cables are designed to have small leakage currents through the insulation to prevent charge buildup. This design allows for water degradation.
• Water degradation reduces the dielectric strength of the insulation. As a result of this, instant failure during lighting does not take place.

CHAPTER 5   WHAT IS THE STRUCTURE OF MEDIUM VOLTAGE UNDERGROUND CABLES?

The structure of medium voltage cables includes the following parts:

CHAPTER 5.1  WHAT IS THE CONDUCTOR CORE?

The rating of conductors for medium voltage underground cables depends upon the cross-sectional area in mm2. This cross-sectional area means how much current can flow through the conductor.

Which means that the higher is the conductor, the higher is the amount of current it allows.

For medium voltage underground cables, the conductors in use range from 35mm2 up to 1000mm2. These cables may be compacted stranded or can either be solid construction.

Some of the conductors in medium voltage underground cables make use of three core designs. Sector shaped conductors are useful in reducing the overall diameter of the wire.

In comparison between copper and aluminum, copper has a lower electrical resistance than aluminum. Because of this, aluminum is a better and efficient conductor of electric current.

Aluminum can carry the same amount of electric current as copper through a small cross-sectional area.

For example, a copper conductor of cross-section 300mm2 can carry 670 Amps in a buried installation. However, an aluminum conductor can carry only 525 Amps under the same conditions.

Copper conductors would require a broader cross-section of 500mm2 to achieve the same rating. However, aluminum is lighter than copper, and thus, it has the advantage of enabling longer lengths to be safely handled.

The price of aluminum is lower than copper, which makes aluminum more economical per amp than copper.

Therefore, aluminum conductors are useful for medium voltage distribution networks that require long distances and extensive cabling.

Copper cables are useful for short links in substations and industrial installations. These copper cables are useful as smaller cables or places where higher power transmitting properties are required.

CHAPTER 5.2  WHAT ARE STRANDED CONDUCTORS?

Stranded conductors in medium voltage underground cables are those which consist of several layers of spiral wound wires. These wires are tightly compacted together.

The construction of stranded conductors in medium voltage underground cables is according to IEC 60228 (BS EN 60228), as class 2.

However, strander conductors of Class 5 rating are also in use. Their construction is in conjunction with Ethylene Propylene Rubber (EPR) insulation.

The stranded conductors have spaces in the intercedes; for this purpose, water-swellable powders or tapes are useful in the construction of conductors. This powder allows longitudinal blocking of water.

The powder or tape material will block and prevent any moisture from passing through the conductor. As a result, the joint termination position will stay protected.

The aluminum conductors in medium voltage underground cables prevent water penetration since they are water blocked.

However, copper cables do not make use of these materials. As such, the copper cables cannot be useful in very wet areas, e.g., subsea cables.

Furthermore, aluminum conductors in medium voltage underground cables that are water blocked also have decreased diameter than the stranded conductors.

CHAPTER 5.3  WHAT ARE THE DIFFERENT MATERIALS OF EXTRUSIONS USED AROUND MEDIUM VOLTAGE UNDERGROUND CONDUCTORS?

Three layers of extruded materials are useful around cables of medium voltage underground cables. These installations of these layers depend upon the standards of requirements.

These layers include :

• Conductor screen.
• And insulation screen.

These layers generally take extrusion in a single mechanism, known as triple-pass, or triple extruded.

CHAPTER 5.3 (1) SEMI-CONDUCTOR SCREEN ON THE CONDUCTOR (KNOWN AS THE “CONDUCTOR SCREEN”):

The semiconductor screen consists of a cross-linked compound. It is a layer of black semi-conductive cross-linked compound, which is less than 1.0mm.

The layer of the semiconductor screen has a particular thickness. This thickness acts as the interface between the conductor and the insulation.

Since the external surface of the conductor is not as smooth, the provision of this layer provides a seamless service.

Due to the installation of this smooth surface known as the semiconductor screen, the electric current can pass swiftly through the conductor without a barrier.

This layer adds additional protection. Without this layer, there are concentrations of electric energy, which leads to the failure of the medium voltage underground cables.

CHAPTER 5.3 (2)   INSULATION AROUND THE CONDUCTORS :

Insulation is carried out around the conductors in medium voltage underground cables, which provide protection from electrical short circuits and from the outer screens, which are at ground potential.

For insulation to work efficiently, it must have enough thickness to withstand electric fields under different environmental conditions.

Two main types of insulation materials are useful in medium voltage underground cables :

• XLPE – Cross-linked Polyethylene – Nowadays, XLPE is the most common material for medium voltage underground cables.
• EPR – Ethylene Propane Rubber –EPR is more flexible than XLPE but not as efficient at reducing losses in circuits as XLPE. EPR insulation cables usually are useful in marine and offshore applications.

The insulation that has a cross-linking structure of XLPE is better than the thermoplastic materials such as PVC. XLPE allows the cables to work efficiently under high-temperature conditions.

Insulations made of Cross-linked polyethylene (XLPE) also allow more current to pass through the conductor. XLPE material can allow a conductor temperature up to 250° C during short circuits.

The insulation in three core medium voltage cables has its natural color. This insulation is not useful for core identification.

This process takes place through printing on the insulation screen. Color-coded tubes between the cores are also useful.

CHAPTER 5.3  (3) SEMI-CONDUCTIVE SCREEN ON INSULATION (KNOWN AS THE “INSULATION SCREEN”):

The function of the semiconductive screen is similar to that of the conductor screen. This layer allows for a smooth transition from the insulating medium to the grounded metallic screen.

The semi-conductive screen is a layer that is made up of semi-conductive compounds. This material has a thickness of 1mm. It is either fully bonded to the insulation layer. It is “cold stripped” by hand.

It is necessary to remove the insulation screen when you are joining or terminating the cables. For this purpose, rotatory tools with blades are useful.

Without removing or effecting the insulation, these blades remove the accumulated black semiconductive layer.

Cold Strippable screens do not need the use of such tools. The peeling of the cold stripped screen gives off the insulation of a clean layer of insulation.

However, you need to be careful when working with the cold strippable screen when using knives or other tools. This care is because the transition between screen and insulation is difficult.

CHAPTER 5.3  (4)  METALLIC SCREEN:

The metallic screen in medium voltage underground cables acts as a second electrode of the capacitor. The metallic screen is responsible for nullifying the electric field outside the wire.

For a metallic screen to perform its function, the filter needs to be attached to earth at one point or another.

Under normal operating conditions, this screen allows the drainage of the charging current and the circulating currents.

Another function of the metallic screen is that it drains the short circuit currents during faulty conditions or harsh environments.

The metallic screen in the medium voltage underground cables performs another essential function. This screen acts as a barrier and prevents the penetration of humidity into the insulation.

Humidity has a severe impact on the functioning of the metallic screen. When the moisture and strong electric fields join together, deterioration of the insulation takes place.

Thus, the insulation system needs protection for humidity, and this protection is through the metallic screen. The metallic screen keeps the insulation protected.

CHAPTER 5.3   (5)  DIFFERENT TYPES OF METALLIC SCREENS:

The different types of metallic screens in medium voltage underground cables are as follows :

CHAPTER 5.4   ANTI-CORROSION PROTECTIVE JACKET (OR SHEATH):

Sheaths are the outer coating in medium voltage underground cables, which protect the cables from environmental conditions and faults.

The sheath performs several functions, such as :

• It provides insulation to the metallic screen from the ground.
• The sheath also protects the metal elements from humidity.
• The cables get protection from the mechanical stresses that it faces during installation and service.
• It can also be designed to withstand termite attacks, resistance to hydrocarbons, etc.

CHAPTER 5.4   (1)   MATERIALS USED FRO CABLE SHEATHS:

Two primary materials are useful for cable sheaths:

• Poly-Vinyl Chloride (PVC), Polyethylene (PE) :
• The PE material used for sheaths can be of medium density (MDPE) or Linear low density (LLDPE) or high density (HDPE).
• PVC is softer than PE. Thus, it is used mainly in medium voltage underground cables.
• PVC has several advantages. It is fire resistance.
• However, it emits toxic gases after catching fire, which acts as a drawback when using PVC for cables.
• HFFR (HalogenFree Fire Retardant) materials :

• HFFR (HalogenFree Fire Retardant) materials are useful when the installation of cables is in confined places.
• These materials are fire-resistant and emit no harmful gases.
• Because of this, these cables are safer to use than PVC or PE.
• However, the mechanical properties of the HFFR materials are not as better as PVC or PE.
• These cables are also costly.
• However, you can make use of these cables in places where fire protection is the priority since these cables have excellent functionality.

CHAPTER 5 HOW ARE THE MEDIUM VOLTAGE UNDERGROUND CABLES CONSTRUCTED?

Medium voltage underground cable circuits consist of :

• Three single-core cables: Three single-core wires make use of aluminum rather than steel because steel can generate harmful power surges.
• One three core cable: Galvanised steel wires wound in helical around the screens in three core cables.

The main elements used for the construction of medium voltage underground cables are Conductor, insulation, metallic screen, sheath, etc.

However, some additional materials are useful in the construction of these cables :

• Bedding, which includes filters and tubes for constructing the circular design in three core cables.
• Separator tapes provide a barrier between layers to prevent any sticking during extrusion.
• Conductive tapes that bind three cores together in three core cables.
• Water swellable tapes provide a moisture-resistant barrier under the sheath.
• The Armoring, to carry away short circuit current.

CHAPTER 6   WHAT KIND OF MEDIUM VOLTAGE UNDERGROUND CABLES ARE SUITABLE FOR DIFFERENT ENVIRONMENTS?

Different environmental conditions require the use of different cables. The difference between these cables depends on the type of useful materials.

Lead sheathed cables are useful in those environments where hydrocarbons are present. The lead sheath cables provide a chemical resistant barrier, thereby protecting the cables.

Wire armored versions of medium voltage underground cables are useful in places where there are risks of mechanical damage.