Everything you need to know about Medium Voltage Cable
Many of our valued customers have raised queries about the best type of cable for their application and are often confused about what medium voltage cables are, what are their applications and what are their benefits.
This article addresses all the common concerns of our customers and beyond related to medium voltage cables.
Table of Contents
What are Medium Voltage cables?
Medium Voltage (MV) cables are defined by the International Electrotechnical Commissions (IEC) as the cables having a voltage rating of more than 1kV to 100kV.
Initially there only existed high and low voltage cables; however, as the level of the voltage increased over time, the need for a higher classification range for the cables also increased.
This allowed for three main categories of cables: low voltage cables, medium voltage cables, and high voltage cables.
Components of Medium Voltage Cables
Though each cable has a slightly different configuration, the basic components for the medium voltage cables remain more or less the same;
The conductor in all the cables, let it be low voltage, medium voltage or high voltage cable, serves the same purpose.
That is, to transport current from one part of the cable to the other.
Therefore, the conductor is a key component in all types of cables.
Conductors in the cable are usually aluminum or copper and may have platings.
The conductors in medium voltage cables are either solid in small sizes or are stranded in larger sizes.
This distribution allows the cables to have better flexibility and durability.
Though each manufacturer has its own method of determining the size of the conductor in the wire, it are usually determined via ampacity tables or other related computer-based studies.
The size of the conductor is also dependent on the cable surroundings and the current-carrying requirements.
The conductor in the cables are well insulated and hardly anything can go wrong with them, except for rare cases of corrosion.
The conductor shield is used over stranded conductors in medium voltage cables.
This shield provides a radial, smooth electric field within the insulation.
In MV cables having polymeric insulation, the conductor shield is usually of the same material as the insulation but is impregnated with carbon particles so that the shield remains a semiconductor.
After the conductor, insulation is the most important part of the medium voltage cables.
Most of the medium voltage cables have insulations of Either polyethylene (PE), polymeric, Ethylene propylene rubber (EPR).
The majority of the modern medium voltage cables are cross-linked.
The insulation shield is similar to the conductor shield and is compulsory to be used in the medium voltage cables.
A properly fitted insulation shield provides a radial and smooth electric field within the insulation. One key characteristic of the insulation shield is that they are strippable.
This property of insulation shield allows for cables to be easily spliced or terminated.
However, it must be noted that while stripping off the insulation shield, the conductor shield should remain intact.
A damaged conductor shield can cause small pockets in the cable resulting in partial discharges.
The metallic shield is applied over the insulation shield. The metallic shield is applied because of it;
- Provides a path for the fault current to flow
- Reduces the touch potential in cases where there is a dig-in into the cables
- Provides a path for the charging current to flow
If the metallic shield used in the cables are of sufficient conductivity and size, it can provide a path for the return current.
This ensures that the medium voltage cables become a neutral system. It is due to this reason that the metallic shields are commonly referred to as neutral shields.
The metallic shield is commonly made with copper and in some cases, with aluminum as well.
There is no fixed design for the metallic shields. Common designs include flat strap, round wire, taped, or longitudinally corrugated.
The longitudinally corrugated shield can also act as a hermetic seal and thus, prevent any seepage of water into the insulations.
There are cases when multiple single-phase medium voltage cables are put together in the multi-phase cable. In such cases, the metallic shield on each phase is thin copper wires called drain wires.
The outermost layer of the medium voltage cables is either a plastic jacket, metal sheath, or both.
The main purpose of the outer layer is to provide the cable with mechanical protection.
If the sheath is solid metal, it can also aid in protecting the cables from water ingress.
However, solid outer covering reduces the flexibility of the cable significantly.
Though plastic jacketing gives excellent mechanical protection, they do not hermetically seal the cable.
Basic components of a medium voltage cable
Apart from the above components, some cables may have special components that are added by the manufactures at the request of the users. These may include;
- Water absorbing materials – different water-absorbing materials can be added within the medium voltage cables to effectively absorb or slow down the moisture ingress in the cables
- Strand blocking materials – these materials are added to keep the water from flowing through the conductor via the insulating layers
- Additional layers – more layers are added to either aid in the cable manufacturing or to give more strength to the cable.
Components of MV cable systems
The medium voltage cable systems comprise of different components. The major components include;
The connection of the cable conductors can be carried out by either welding, compression soldering, or using shear-bolt type connectors.
For the medium voltage cables jacketed with XLPE insulation, the use of heat for making the connections can be an issue.
Therefore, the use of mechanical connectors or compression has become an international standard for almost all types of cables. Especially for the medium voltage cables, the classic compression connector is the widely used connection method instead of the shear bolt connectors.
The connectors in the MV cable systems are considered as one of the most important components in a joint.
There is a risk of thermal destruction if the insulations of the connector are faulty.
Therefore, it is just that good care and attention is taken to minimize the risk of accidents during their installations and subsequent uses.
Each type of connector must have the ability to carry the normal current of the cable alongside the maximum current without getting overheated.
Preliminary tests must be done on these cables by short term circuits tests and long-term usage tests.
There are several different joints that can be used for medium voltage cables.
The straight joint is the most common type of joint.
It is a connection between two similar cable ends. A straight joint consists of a connector, screen connector, insulating body, outer protection, and stress control device.
The transition joint is a type of connection between two different types of cables ends.
The transition joint is commonly used to link a paper and a polymeric insulated cable.
Medium voltage cables also use heat shrink joints for straight and transition joint applications.
The heat shrink joints are both time and cost-effective and are suitable for joints of all sorts.
The material used for heat shrink joints is typically EPDM or silicone.
Terminations and Plug-In Connectors
Terminations and plug-in connectors are required to properly connect the ends of cables to an overhead line or transformer or switchgear.
At medium voltage levels, all types of installation techniques are used, ranging from heat shrinking to slip-on cold shrink.
Modern medium voltage cables also use a ‘dry type’ termination technique.
This technique allows for easier installation, no requirement of oiling, and resistance to explosions and earthquakes.
Standard Termination of medium voltage cables
In cases where medium voltage cables are connected to SF6 insulated switchgear, plug-in connectors are used. For plug-in connections, there are two standardized plug-in systems available in the market: the inner cone system and the outer cone system.
The inner cone systems are widely used in medium voltage applications, whereas the outer core systems are used in high voltage applications.
Materials and Characteristics
Usually, the medium voltage cables are designed to have wet electricity stability along with all-condition reliability.
They are made of elastic insulations which allows the manufacturers to produce prefabricated and tested products.
Out of all the aforementioned materials, copper is the most cost-effective material, whereas, aluminum and its alloys have better performance capabilities.
Therefore, when in need of the cheapest reliable medium voltage cables, copper-based cables are the best option to take.
On the other hand, when the applications demand high-performance, aluminum-based, or aluminum alloy-based cables are a better option.
Three-phase medium voltage cable
For the jacketing of the cables, there are a variety of different options.
XLPE or cross-linked polyethylene is most commonly used. Other materials include Neoprene, chlorinated polyethylene, and chlorinated propylene (EPR).
Benefits of using medium voltage cables
The characteristics mentioned in the earlier section of this article enables medium voltage cables to have a wide array of different benefits.
One of the main benefits of using MV cables is primary metering. Primary metering can be done on the positive sides of the transformers. This saves around 5% of kWh charges.
It is because the medium voltage cables allow for metering right at the source rather than at the point of delivery.
Using medium voltage cables means that the voltage going through the cables has a minimal voltage drop.
The medium voltage feeder lines have a significantly lower load current therefore, fewer voltage drops.
The lowered voltages in the wires also eliminate the need for oversize conductors.
This makes the whole system efficient and requires less maintenance.
The use of medium voltage cables is also a cost-effective measure taken by engineers and contractors.
The cost of copper has shown a significant rise in recent years. A workable solution is to have medium voltage distribution cables with the correct rating cables.
The medium voltage in cables means that the current in the cables in low. Low current means fewer conductors needed to distribute the power. Lower conductors translate to less copper used.
The need for oversize cables lines is also reduced. With less copper used in the system, the installation costs, as well as the maintenance costs, are reduced for the cables and the overall system.
Other benefits of medium voltage cables include;
|Medium Voltage cables have high flexibility, therefore they can be easily used in tight spaces quite easily
|Extra protection to prevent charge built up anywhere along the cable
|These cables have good durability combined with excellent reliability
|MV cables are tough and can easily withstand extreme abuse
|Medium voltage cables are easy to install, replace and have a low overall maintenance cost
There are many different applications of medium voltage cables owing to its excellent characteristics and benefits.
Medium voltage cables can be used in;
|Power distribution to isolated areas.
|In different utility applications
|In power supply during wet or cold weather
|Performance demanding industrial-scale plants such as sewage treatment plants, paper mills, water treatment plants other such plants
|In the mining industry for energy supply to drills, haulers, and shovels
|In remote power generation locations such as substations
|Any location or environment that demands performance and reliable energy supply in the ‘medium voltage’ range.
|In energy supply from transformers, medium voltage generators, and small power plants.
With so many applications, one should be careful in purchasing the right cable for their applications. Our agents at SSGcables are always ready to assist you with your purchase.
Why some MV cables fail
As with most of the cables, the medium voltage cables tend to fail under certain conditions.
The common causes of medium voltage cables are;
The service life of the medium voltage cables tends to be reduced if it is operated beyond its optimal working conditions.
If used in extreme conditions, let it be high or low temperatures or voltages outside the recommended levels, cracking, embrittlement, and complete failure of the insulations can occur, resulting in exposure of the conductor.
This results in short-circuits or even fires and complete cable to fail.
If not installed by a professional, the medium voltage cables can be installed improperly and thus are likely to fail in their subsequent use. The integrity and reliability of the cable can also be adversely affected and cause the cable to fail abruptly.
Environmental factors such as external fires, extreme weather conditions, accidents, or rare cases such as trees falling over can all result in the mechanical failure of the medium voltage cable.
Mechanical failure of MV cable due to external fire
Moisture ingress is a leading cause of failure in medium voltage cables.
The moisture can corrode the copper conductors and even cause short circuits.
At SSGcables, we ensure that all our cables have proper insulations and protection to avoid any moisture ingress.
Electrical overloading in the medium voltage cables is also one of the common reasons for their failure.
The users often use medium voltage cables for high voltage applications resulting in the cable to come under huge stress and thus resulting in the cable to fail.
Electrical overloading can have dangerous consequences and must be avoided at all times.
Cable sheath degrading
There are several reasons why the sheath of the cable may degrade.
Factors such as excessive temperatures, exposure to UV light, abrasion, chemical, and extreme weather conditions can all degrade the cable sheath.
All these factors will eventually result in the medium voltage cable failure as the insulated cores are no longer protected by the cable sheath.
We at SSGcables are aware of this cause of failure and thus, make sure that each of our cables has premium sheath so that our customers can have a quality cable with little to no risk of cable failure.
Key differences between MV cables and low voltage cables
Apart from the obvious difference that the medium voltage cables are designed for medium voltages and low voltage cables for low, there are other significant differences between the two.
One key difference between MV cables and LV cables is that the medium voltage cables system has excellent designs of circuit breakers.
In low voltage cables, the circuit breakers are integrated with the system whereas, in medium voltage cables, the circuit breakers are housed separately.
The MV circuit breakers have a vacuum interpreter that can open or close the circuit whereas, separate relays guard against an overcurrent or short circuits.
Another key difference between the MV and LV cable systems is that medium voltage cable systems are much more compact than multiple low voltage cables.
However, the footprint of the medium voltage cable systems is significantly larger than LV systems.
Medium Voltage cable systems also have intelligent, microprocessor-based relays that ensure a safer and much more efficient operation as compared to the LV cable systems.
The relays in the medium voltage cable systems are accurate and rapid as compared with the conventional solid-state protections in the LV circuit breakers.
Future of medium voltage cables
Medium voltage cables can be considered a mature technology since it has been modified and perfected over time.
Despite the development of these cables, they still have room for improvement.
It is expected that in the future, these cables will be more power-efficient as well as will be much more well-insulated since some of the causes of failures are due to the insulation layer degradation.
In the future, medium voltage cables will see their major use in the smart grid application.
With the demand of electricity getting higher on a daily basis and the need for a much more efficient method of power delivery seems inevitable, the medium voltage cables will be in high demand.
The characteristics and the properties of medium voltage cables make them ideal for smart grid applications.
It is also expected that the overhead transmission of power via medium voltage cables will be completely replaced by the underground transmissions.
Currently, most of the underground cabling is done via open trenches however, in the future, it is expected that cabling of medium voltage cables will be carried out solely via direct plowing.
Direct plowing is both a cost and time effective measure of installing medium voltage cables without damaging the cables themselves.
Advancements in medium voltage cables will also lead to ‘hybrid power and communication medium voltage cables’.
This will enable a powerful information transmission system combined with power transmission and network monitoring.
The current stage of medium voltage cables does not have the capacity for the addition of fiber optics for the transmission of power.
With further advancements in the designing of these cables, it is expected that the hybrid power medium voltage cables will have blown fiber optics as their insulation sheath.
This will allow a cable that can efficiently transfer both the energy and data.
We at SSGcables are manufacturers and suppliers of high-quality medium voltage cables.
We have decades of experience in medium voltage cable research, development, and manufacturing.
Therefore, from conductor selection to the sheathing to complete cables, we execute a full set of strict control over the quality of our components at each stage of manufacturing.
At SSGcables, we have excellent sourcing and supply chain at each production stage, which ensures cost-savings, excellence, and value-adding solution so that we can deliver what we promise to our customers.
We would love to work closely with you to offer the best medium voltage cable or any other cable to fit your requirements and deliver you the best in class product.