What is MV vs HV cables?
Medium voltage cables and high voltage cables are power cables categorized by their rated working voltage and are mainly used in power transmission systems, each serving different voltage levels and application scenarios.
Medium Voltage Cables
Medium voltage cables usually refer to power cables with a rated voltage in the range of 3 kV to 35 kV. These cables are mainly used in urban distribution networks, power distribution in industrial areas, transmission lines between medium-sized substations and users, and power supply for industrial equipment with high voltage requirements. Medium voltage cables are designed with insulation requirements and mechanical strength in mind for their operation at medium voltage levels and may contain multi-layer insulation structures to ensure safe and reliable transmission of power. Under certain conditions, such as high current circuits or underwater laying, MV cables may be constructed with a single core to optimize heat dissipation or waterproofing.
Introduction to High Voltage Cables
High voltage cable is a kind of power cable, is used to transmit between 1kv-1000kv power cables, mostly used in power transmission and distribution, high-voltage cables in the laying of air chamber space of the building. High-voltage cable product implementation standards for GB12706.3-2002. high-voltage line usually refers to the transmission line that conveys more than 10KV (including 10KV) voltage, high-voltage usually does not contain 1000V.
High-voltage cable is a kind of power cable, high-voltage cable in the laying of air chamber space of the building. High-voltage cables are sheathed with low-smoke polyvinyl chloride (PVC) or fluorinated ethylene polymer (FEP) flame-retardant plastic sheath.
High-voltage cable structure composition
Conductor: At the center of a high-voltage cable is the conductor, which usually consists of multiple strands of copper or aluminum wire stranded to carry the current. The surface of the conductor may be coated with a thin conductive layer to improve electric field distribution.
Insulation: The outside of the conductor is covered with an insulating layer, which is one of the most critical parts of the cable to prevent current leakage and to isolate the conductor from other parts. Cross-linked polyethylene (XLPE) is commonly used as insulation in high-voltage cables because of its good electrical strength and mechanical properties.
Inner sheath (if any): There may be an inner sheath on top of the insulation to protect the insulation from mechanical damage and environmental influences.
Armor layer: The armor layer, usually made of steel tape or steel wire braid, is used to increase the mechanical strength of the cable to withstand external pressures and tensile forces, especially in buried applications, and to provide protection against rodent bites and mechanical damage.
Sheath: The outermost layer is the outer jacket, usually made of polyvinyl chloride (PVC) or other weatherproof materials, which is used to protect the cable from environmental factors such as water, UV rays, and chemicals.
High voltage cable characteristics and applications:
Low loss: high voltage cables can effectively reduce power loss in long-distance transmission, because high voltage transmission reduces the size of the current, thus reducing the resistance loss of the line.
Safe isolation: The insulation and armoring of high voltage cables are designed to ensure electrical isolation from the surrounding environment, reducing the risk of electric shock.
Environmental adaptability: Through the use of specific materials and structural design, high-voltage cables are able to adapt to a variety of complex environments, including underground, underwater, and extreme climates.
Installation: High-voltage cables can be installed in a variety of ways, such as directly buried, pipe-laying, overhead, etc., and can be flexibly selected according to specific environments and needs.
High-voltage cable types
The main types of medium voltage cables are YJV cable, VV cable, YJLV cable, VLV cable.
YJV cable full name cross-linked polyethylene insulation PVC sheathed power cable (copper core)
VV cable is fully known as PVC insulated PVC sheathed power cable (copper core)
YJLV cable is cross-linked polyethylene insulated PVC sheathed aluminum core power cable.
VLV cable is called PVC insulated PVC sheathed aluminum core power cable.
Due to the excellent conductivity of copper conductors, more and more projects use copper-core power cables as the backbone of the power supply system, while the application of aluminum-core power cables is less, especially in the more high-voltage power systems, the more copper-core cables are chosen.
High-voltage cable models and applications
NA-YJV, NB-YJV, cross-linked polyethylene insulated PVC sheathed class A(B) fire-resistant power cables can be laid in rooms, tunnels and ducts where fire resistance is required.
NA-YJV22, NB-YJV22, cross-linked polyethylene insulated steel tape armored PVC sheathed A (B) class fire-resistant power cables are suitable for fire-resistant requirements of buried laying, not suitable for pipeline laying.
NA-VV, NB-VV, PVC insulated PVC sheathed class A(B) fire-resistant power cables can be laid in the fire-resistant indoor requirements, tunnels and pipelines.
NA-VV22, NB-VV22, PVC insulated steel tape armored PVC sheathed class A(B) fire-resistant power cables are suitable for fire-resistant requirements of buried laying, not suitable for laying in pipes.
WDNA-YJY23, WDNB-YJY23, cross-linked polyethylene insulated steel armored polyolefin sheath A (B) halogen-free and low-smoke fire-resistant power cables are suitable for halogen-free and low-smoke and fire-resistant requirements when buried, not suitable for pipeline laying.
ZA-YJV, ZA-YJLV, ZB-YJV, ZB-YJLV, ZC-YJV, ZC-YJLV, cross-linked polyethylene insulated PVC sheathed class A (B, C) flame retardant power cables can be laid in the requirements of flame retardant indoor, tunnels and pipes.
ZA-YJV22, ZA-YJLV22, ZB-YJV22, ZB-YJLV22, ZC-YJV22, ZC-YJLV22, cross-linked polyethylene insulated steel tape armored PVC sheathed class A (B, C) flame retardant power cables are suitable for underground laying when flame retardancy is required, and are not suitable for laying in pipes.
ZA-VV, ZA-VLV, ZB-VV, ZB-VLV, ZC-VV, ZC-VLV, PVC insulated PVC sheathed class A(B,C) flame retardant power cables can be laid in indoor, tunnels and pipelines where there is a requirement of flame retardancy.
ZA-VV22, ZA-VLV22, ZB-VV22, ZB-VLV22, ZC-VV22, ZC-VLV22, PVC insulated steel tape armored PVC sheathed A (B, C) class flame retardant power cables are suitable for buried laying when flame retardant requirements, not suitable for laying in pipes.
WDZA-YJY, WDZA-YJLY, WDZB-YJY, WDZB-YJLY, WDZC-YJY, WDZC-YJLY, cross-linked polyethylene insulated polyolefin sheathed class A(B,C) flame retardant power cables can be laid in the flame retardant and halogen free and low smoke requirements of indoor, tunnels and pipes.
WDZA-YJY23,WDZA-YJLY23,WDZB-YJY23,WDZB-YJLY23 , WDZC-YJY23,WDZC-YJLY23.
Cross-linked polyethylene insulated steel tape armored polyolefin sheathed A (B, C) class flame retardant power cables are suitable for flame retardant and halogen-free and low-smoke requirements of buried laying, not suitable for pipeline laying.
VV, VLV, copper (aluminum) core PVC insulated PVC sheathed power cables are laid indoors, tunnels and pipelines or outdoor bracket laying, do not bear the pressure and mechanical stress.
VY, VLY, copper (aluminum) core PVC insulated polyethylene sheathed power cables
VV22, VLV22, copper (aluminum) core PVC insulated steel belt armored PVC sheathed power cables are laid indoors, tunnels, cable trenches and direct burial in the soil, the cable can withstand pressure and other external forces
VV23, VLV23, copper (aluminum) core PVC insulated steel belt armored polyethylene sheathed power cables.
Characteristics of high-voltage cables
The product is suitable for AC rated voltage of 35KV and below for transmission and distribution of power can be fixed granular lines, the maximum long-term working temperature of the cable conductor 90 degrees, short-circuit (the longest time does not exceed 5S), the maximum temperature of the cable conductor does not exceed 250 degrees.
High-voltage cable advantages and disadvantages
Advantages: occupy less ground and space, by the weather and the external environment, can improve the system power factor, is conducive to personal safety, operation and maintenance work is simple and convenient, conducive to urban planning, is conducive to environmental protection.
Disadvantages: large construction investment costs, cable lines are not easy to change, branching technology is complex, cable joints require specialized technology, high cost, fault detection difficulties, repair time is long.
High voltage cable voltage division
1KV and below is known as low-voltage cable; 1KV ~ 10KV for medium-voltage cables; 10KV ~ 35KV for high-voltage cables; 35 ~ 220KV for ultra-high-voltage cables; ultra-high-voltage cables are with the continuous development of cable technology and the emergence of a power cable, ultra-high-voltage cables are generally used as a large-scale transmission system in the pivotal link, belonging to the technology content of a high-voltage cables, mainly used for Long-distance power transmission.
High-voltage cable use
Due to the excellent conductivity of the copper conductor, more and more projects use copper-core power cables as the backbone of the power supply system, while the application of aluminum-core power cables is less, especially in the more high-voltage power systems, the more the choice of copper-core cables. High-voltage cables are mainly used for AC rated voltage 35KV and below for transmission and distribution of power can be fixed granular line with the maximum long-term operating temperature of the cable conductor 90 degrees, short-circuit (the longest time does not exceed 5S), the maximum temperature of the cable conductor does not exceed 250 degrees.
High-voltage cable technical knowledge
First of all, to ensure that the electrical parameters of the stress tube must reach the values specified in the above standards to work reliably. Be careful to use silicone grease to fill the cable insulation semi-conductive layer fracture out of the air gap to exclude gas, to reduce the purpose of partial discharge. Cross-linked cables due to poor internal stress treatment will occur in the operation of a large contraction, and thus in the installation of accessories to pay attention to the stress pipe and insulation shielding lap not less than 20mm, in order to prevent contraction of the stress pipe and insulation shielding detachment. Heat shrinkage accessories because of the elasticity is small, the operation of the thermal expansion and contraction of the interface may produce air gap, so the sealing technology is very important to prevent moisture immersion.
High-voltage cable laying selection
The selection of high-voltage cable is mainly related to the means of laying, the surrounding environment.
Buried: refers to the high-voltage cables directly laid in the ground, the laying method of strong aesthetics, does not affect the traffic and cityscape. Due to the complexity of the underground environment, with corrosion, mechanical damage, pressure and many other unstable factors, suitable for the use of armored cables with external protective layer.
Overhead: refers to the laying of high-voltage cables through the poles, this cable laying method is mostly used in the edge of the city, no residential areas and the ground leveling zone. This type of laying method due to the cable is exposed for a long time, easy to be affected by external influences, selection should be chosen to have the outer layer of the cable or all-plastic cable.
Tunnel type: refers to the high-voltage cable laid in the cable tunnel bridge or support. This kind of laying method is used in the city, so for the safety has high requirements, to choose flame retardant cable or armored cable.
Pipeline: refers to the high-voltage cable laid in a prefabricated pipe, the choice of pipe material in order to prevent power loss, pipe to choose non-conductive or non-magnetic, the cable to choose a plastic protective sleeve cable or bare armored cable.
High-voltage cable failure performance
High-voltage cable damage performance mainly cable sheath damage, cable trench collapse and cable body breakdown, etc., the consequences may be: transmission, transformation, power supply system paralyzed, engineering machinery and equipment burned, and serious may be: inflatable equipment burst, resulting in the expansion of the accident or chain disaster, or even into a major safety accident resulting in life-threatening. Therefore, the necessary high-voltage cable line inspection and protective measures are very important.
Causes of High Voltage Cable Failures
According to the cause of failure is broadly divided into four categories: manufacturer manufacturing, construction quality, design unit design and external damage.
High-voltage cable manufacturer causes
According to different manufacturing causes, manufacturers can be categorized into cable body, cable joints and cable grounding system.
High-voltage cable body reasons
Generally speaking, problems that tend to occur during cable manufacturing include insulation eccentricity, uneven thickness of insulation shielding, insulation impurities, protrusion of internal and external shielding, uneven cross-linking, cable moisture, and poor sealing of the cable metal sheath. In some cases, failures may occur shortly after completion tests or operation, and most cable systems are defective, posing serious risks to the long-term safe operation of cables.
Causes of high-voltage cable joints
In the past, high-voltage cable joints used winding, mold casting, molding and other types of on-site production workload. Due to the limitations of on-site conditions and production processes, there are inevitably gaps and impurities between the insulation tape layers, and thus problems are prone to occur. The commonly used types in China are assembly type and prefabricated type.
Cable joints are divided into cable terminal joints and cable intermediate joints. Regardless of the form of the joint, cable joint failure generally occurs at the cable insulation shield break, as it is the part where the electrical stress is concentrated. Causes of cable joint failures include manufacturing defects in stress cones, insulation packing problems, and oil leakage from seals.
High-voltage cable grounding system
The cable grounding system includes components such as cable grounding boxes, cable grounding protection boxes (with protective layer protectors), cable cross interconnection boxes, and protective layer protectors. Generally prone to problems mainly due to poor sealing of the box, resulting in multi-point grounding, leading to excessive metal protective layer induced current. In addition, irrational selection of parameters or poor quality of the protective layer protector, and unstable zinc oxide crystals are also likely to cause damage to the protective layer protector.
High-voltage cable construction reasons
There are many cases of high-voltage cable system failures due to construction quality, and the main reasons are as follows.
First, poor site conditions, cables and joints in the factory manufacturing environment and process requirements, the construction site temperature, humidity and dust is difficult to control.
Secondly, during the cable construction process, the insulation surface will inevitably leave small skid marks, semi-conductive particles and sand grains on the sand cloth may also be embedded in the insulation. In addition, as the insulation is exposed to the air, moisture from the joint construction process will also be drawn in, leaving hidden dangers for long-term safe operation.
Third, problems that may arise during installation are not strictly considered in accordance with the process construction or process regulations.
Fourth, DC voltage withstand test is used for completion and acceptance, which leads to the formation of anti-electric field in the joint, resulting in insulation damage.
Fifth, the sealing treatment is not good. Intermediate joints must be added to the metal copper shell PE or PVC insulation anti-corrosion layer of the sealing structure to ensure the sealing of the lead seal in the field construction, effectively ensuring the sealing and waterproof performance of the joint.
High voltage cable design reasons
Cable crush damage due to thermal expansion of the cable. When cross-linked cables are highly loaded, the core temperature rises and the cable heats up and expands. The top of the cable at the tunnel turnaround is located on the bracket elevation. The creep force of cables running under long term high loads is very high, resulting in the bracket elevation destroying the cable outer sheath and metal jacket, extruding into the cable insulation layer and causing the cable to break.
High-voltage cable protection measures
For high-voltage cable protection measures, in general, in the construction of the time must ensure the quality of construction, especially the production quality of the cable head, must comply with the provisions and requirements; in addition, in the construction of the cable line to strengthen the protection of the following commonly used methods:
First, isolation protection: cable lines buried deeper and near the pile foundation or pit cable protection methods, can be formed through steel sheet piles, tree root piles, deep mixing piles, etc. isolation body, to limit the underground cable lines around the soil only displacement, to prevent extrusion or vibration of the cable line; for the cable line depth is not too deep can be digging isolation tank method, isolation tank location can be selected between the construction point and the cable line, but also Can be excavated in the cable part, can also be dug out of the cable line suspension. Note that the isolation tank must be dug deep to the pipeline step, in order to play the role of isolation pressure and vibration force.
Second, suspension protection: in the cable protection area within the scope of the pit excavation, the need for cable trench hollowing to protect the safety of the cable trench, which will make the cable line exposed to the pit, and the cable line can not be set in the middle of the support, encountered in this case, you can use the suspension method of the cable line for fixed protection. Which must pay attention to: 1, the exposed cable must be buckled tube protection; 2, in the cable box culvert hollowing is, should be every 1 ~ 1.5 meters for suspension, and in the cable box culvert ground placed on the steel plate, to ensure that the cable box culvert ground force uniformity; 3, the deformation of the sling and the position of the sling fixing point should be unaffected by the deformation of the soil.
Third, support protection: the need for cable pipeline suspension, but also along the line to set up a number of support points to support the line, including temporary support such as easy to set up and remove the steel sheet piles, piers, etc., but also as a permanent building for the permanent erection of support points.
Fourth, cable trench reinforcement protection: taking into account the large-scale heavy machinery in the cable corridor protection within the scope of construction, generally used for cable trench reinforcement protection, in the original box culvert around plus a layer of “protective shell”, is the original cable trench load all unloaded to the newly constructed cable trench, to achieve the unloading of the cable box culvert protection, unloading the cable box culvert. Remove the load around the cable box culvert.
In addition, after the completion of the cable line installation, to strengthen the cable operation monitoring, to avoid overloading the cable operation, at the same time for the cable in accordance with a certain period of time for the cable test, if abnormal problems are found, then we must immediately take appropriate measures to achieve timely processing. For the cable trench to keep dry, to prevent the cable moisture, resulting in insulation degradation, resulting in short-circuit conditions.
Important reminder: to regularly sweep the dust accumulated on the high-voltage cable to prevent these dust because of the natural cause of the cable fire, to strengthen the cable circuit switch and protection of the regular calibration and maintenance to ensure its reliability.
The world’s leading high voltage cable manufacturer TST CABLES
High-voltage cable manufacturer TST CABLES gives high-voltage cables unparalleled power transmission efficiency, high-voltage cables using the most advanced cross-linked polyethylene insulation technology, with the pure conductivity of the copper core, matched with impenetrable armored shield, to ensure that every degree of power safely and efficiently to reach the faraway places. Even thousands of miles away from each other, the transmission of electric energy remains pure and unimpaired, and the power surges. If you have needs or questions about high voltage cables, please feel free to contact us by email, and you can also get free samples.
