3 core cable vs. single core cable 2 - Eng-Tips
3 core cable vs. single core cable 2 - Eng-Tips
1) It allows stocking of a single conductor for all applications, rather than two.
2) When a fault occurs, the other two phases remain healthy. This is handy in underground residential dev't as 2/3 of the customers may stay energized, and you only need to repair one cable.
3) One heat-generating conductor usually allows for better heat dissipation, and a corresponding higher installed ampacity for a given wire size.
4)Can you imagine the weight and stiffness of >m of THREE-phase MCM cable? This is a very common length for utility-types to install in one 'pull'.
5) They were developed first, and utilities like sticking with 'what works' The more modern utility practice in the U.S. is to use single conductor cables on circuits that are over 5,000 volt phase to phase.
Cleveland Electric Illuminating Company uses both kinds going from substations to overhead distribution. There are instances of radial circuits to 3 phase transformers where they use 4 single conductor cables and make up the fourth to reach any of the primary terminals and all 4 conductors are energized.
The primary problem with single conductor cables is that you will get circulating current in the shields which increases losses if the shields are grounded at both ends. If grounded at one end only the other end of the shield can have a nasty tingle voltage and needs to be isolated.
Essentially all of their 4,400 volt substation exit cables are 3 conductor because jacketed type MC 5,000 volt cable only needs the shield provided by the overall sheath which eliminates circulating current in individual shields. These are only 500 KCM copper at the most and only 100 feet to 1/4 mile long. A lot of the older substation exit cables at 7,620Y13,200 volts are 3 conductor and the newer ones use triplexed single conductor cables. I have seen some substations that have both types of exit cables because the second transformer was installed later.
On CEI's older 11,500 volt system the oil filled cables are 3 conductor with would you believe a splice every 300 feet! What CEI uses for replacing a section is to use single conductor solid dielectric cables and a very funny looking heat shrink tube to adapt.
For 5,000 volts and less and 240 amps and less the favorite practice is still 3 conductor type MC cable. For everything else the preference is for single conductor or triplexed single conductor.
Mike Cole, Go over to Pirelli's website to view their pages on different types of medium voltage cables. 5,000 volt MC cable is just like the 600 volt kind but with 5,000 volt unshielded conductors inside with insulation packed between the conductors and the sheath to keep out air and moisture and to keep down corona. The sheath is of the welded corrugated type. There is usually an overall PVC jacket that makes pulling through conduits easier and to help prevent electrolysis betwwen the metal sheath and metal or concrete conduits.
A 3 conductor cable with individual shields around each conductor is going to have problems with circulating current in the shields. There is also a problems with keeping the individual shields in contact with each other and in contact with the overall shield.
Another advantage of single conductor cables is that if the run is in equal pieces that are a multiple of 3 you can bond the shields together once every 3 splices - at the 2 splices in between you can transpose the shields so as to reduce induced voltage and circulating current to almost zero. At each transposition the ground wire from the left A phase shield is connected to the right B phase shield, the left B connected to the right C, and the left C connected to the right A.
Both single and three core cables are installed in the Uk and the preferance varies between utilities.
The days of paper cables below 132kV are just about dead and the lower the voltage the colder the corpse.
Looking at cost first, three core paper cables were cheaper than three single core cables. Paper cables were provided with a lead sheath as a radial water barrier. Three core cables can also be steel wire armoured as the steel losses are small and do not have such a great effect on rating. Three core cables only need one cable entry into a termination box. Only one joint containing three cables at each joint bay (splice pit) means a smaller joint bay and lower cost. Pulling cables only requires one cable pull to install all three conductors. One duct (raceway) can contain all three conductors. Short circuit magnetic bursting forces are contained within the cable sheath. Shaped conductors may be used to reduce cable size. LPOF 3 core cables can utilise feed joints to reduce transient heating/cooling pressure effects.
Disadvantages of three core cables are that the individually the cables are larger and heavier than a single core cable which limits the length of the cable that can be installed on one drum before it exceeds vehicle height and weight. Manufacturers also have a limit on their drum twisting capabilities to lay up 3 core cables. The maximum conductor size of three core cables is thus lower than single core counterparts requiring more joint bays on a long route. Three core cable joints are more complex than single core joints.
Thus you would find that for small conductor installations 3 core cables were used and for large conductor installation single core cables were used.
Single core cables will be seriously derated by steel armour wire althogh aluminium armour is common. Larger conductor sizes can be handled, manufactured and carried with single core cables. Bigger conductors allow larger cable rating (ampacity). Single core cables can be installed in flat formation with spacing to allow better heat disipation and again a better rating. Induced voltages and currents in cable sheaths are important but there are ways around these on single core cables. Longitudinal water blocking of single core cables is easier to acheive.
Thus for low current short length requirements (e.g. industry) three core cables are "better" whereas for high power (e.g. transmission) circuits single core installations are more cost effective.
The relative cost of cables has reduced dramatically since the early days and civil installation costs have become more significant in Europe. Polymerics do not necessarily require a metallic sheath and where necessary low cost light weight radial water barriers are available.
This change in dynamic plus the development of polymeric cables has altered the cost balance but some arguements persist. The weight and size of cables per unit length still affect three core cable conductor sizes and typically 630sqmm is about the commercial limit.
There are other factors that may change the choice such as, a corporate stocking policy, spares requirements and special applications where, for instance, steel armour is required or where a triplex design is preferred.
Hope this helps.
Hi, sdlloyd, great to hear from you ...
You have certainly given us a wealth of information of how various issues are considered and addressed in making this decision, and how historically decisions were made considering these issues. You also indicated that with modern technology, a lot of changes seem to be in the air ...
I would really like to have you comment on the single phase fault replacing 3 core or single core cable economics which seem to be a prevalent consideration in the other comments from our American friends.
Do you have an opinion on the claim that 3 core cables have lower losses and lower voltage drops than single core cables ? There seems to be a perception [could be misconception] that this is why in Europe current limiting fuses are much more prevalent than in North America --- due to the lower impedances causing higher fault clearing capability for equipment such as Ring-Main-Units [thus the use of CL fuses in RMUs]
Do we have other European [or IEC countries] engineers caring to add their comment ? Come on, guys ... no time to be shy ...
What is an MV Cable and how is it used in PV power plants?
MV Cable definition: A medium voltage (MV) cable is a commonly used and highly adaptable form of electrical cabling. It is used within commercial, industrial, and electrical utility industries thanks to its durability and flexibility.
The International Electrotechnical Commission (IEC) guidelines classify an MV cable with a broad voltage rating, between 1kV and 100kV. Actual voltage ratings of an MV cable will vary depending on the manufacturer and are more often seen between 3kV and 69kV. There is often overlap between the voltage ratings of medium voltage cables and high voltage cables, with an IEC Standard High Voltage Cable being rated between 30kV and 150kV.
There are a lot of variables involved when choosing an MV cable, including construction, standards, voltage rating, and the materials used. Finding the right MV cable for your project will require research to find a cable that will balance performance, durability demands, and installation requirements.
What is an MV cable used for?
MV cables are usually designed with added protection in mind, so they can be used for a wide range of applications. We commonly see MV cables used for outdoor installations where weather conditions play a large part in the longevity of cabling.
Typical applications for MV cabling include:
Mobile substation equipment
Power distribution for industrial settings
Power distribution for tools and equipment used in mining settings
Maintenance and repairs, allowing for the downtime to be scheduled when needing to perform maintenance tasks or repairs
Wind farms
Solar PV plants
Public transportation infrastructures for supplying power to trains and metro lines
Weather-proof networks, allowing power to be transported underground so that severe storms do not disrupt the supply of power
Transformers
Medium voltage generator sets
Power distribution to isolated areas
MV cables come in various configurations and standards to suit different applications. Make sure to check that the MV cable you’re going to install is suited to the environmental challenges and performance requirements you need for your installation.
What are MV cables made of?
MV cables vary depending on the manufacturer and intended usage. There are many configurations available, with options for construction, standards, and materials used. There are also optional features, such as inner seething, additional armor, and water blocking. MV cables can be single-core, three-core, or single-core triplex.
Typically, an MV cable will have the following layers:
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Conductor — This is made of either copper or aluminum to allow the electrical current to pass through the cable. Class 1 solid, Class 2 stranded (circular, circular compacted, sectorial)
Conductor screen — A semiconductive layer which helps maintain a uniformly divergent electric field and contains the electric field within the core
Insulation — Typically made of XLPE (Cross-linked Polyethylene) or EPR (Ethylene Propylene Rubber)
Insulation screen — Another semiconductive layer that protects against electrical interference, and also prevents the cable from causing electrical interference
Metallic screen — Copper wire or tape is wrapped around the core, or individual cores. This provides additional insulation and prevents electromagnetic interference
Outer sheath — This layer wraps around the entire cable to provide protection against environmental hazards. Typical materials used for an outer sheath on an MV cable include LDPE, MDPE (Low/Medium Density Polyethylene), PVC (Polyvinyl Chloride), and LSZH (Low Smoke Zero Halogen)
What is the difference between LV, MV, and HV cables?
For a while, there were only low voltage (LV) and high voltage (HV) cables. HV cabling was used for power distribution, while LV cables were used for nearly everything else. As the amount of electricity we use increased dramatically, so did the amount of power we had to send through cables. This meant new cable classifications needed to be created to better reflect which situations each cable type could be used in. We now have LV, MV, HV, extra high voltage (EHV) and ultra-high voltage (UHV) cables.
LV cables are still used for domestic, office, and other low-power applications, while industries requiring more power have a range of options from MV cables to UHV cables.
Each type of cable will carry a different voltage. Typically the voltage rating of each cable type is as follows:
LV cables have a voltage rating between 300V and V
MV cables have a voltage rating between V and 30kV
HV cables have a voltage rating between 36kV and 150kV
EHV cables have a voltage rating between 89kV and 400kV
UHV cables have a voltage rating above 400kV
Most cable types are then split into smaller categories and voltage ratings can vary depending on usage and manufacturer. For example, MV cables often come with a voltage rating of 36kV to 66kV. Make sure to check the exact voltage rating when selecting a cable for your installation.
MV cables for solar PV installations
MV cables and solar PV installations go hand in hand. An MV cable is the perfect choice when it comes to interconnecting your power stations at the site and sending the power down to the local substation.
It’s important to consider the routing and grouping of MV cables within a solar PV plant. The goal is to reduce the length of cable required, limit energy loss, and allow for easy maintenance to reduce costly downtime.
This is something we have considered at RatedPower. We want to make it as easy as possible for engineers to design highly effective PV plants and reduce LCOE. RatedPower software allows you to fully customize MV routing between areas and the step-up substation, while our algorithms take the defined path and transfer the settings between the layout design, the bill of quantities, and the technical documentation.
Contact us to discuss your requirements of Three Cores Medium Voltage Cable. Our experienced sales team can help you identify the options that best suit your needs.
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