Everything you need to know about High voltage underground cables
With the passage of time, with the rise of the demand for power and high voltage, the method of overhead power transmission is not only becoming obsolete but also dangerous and congested. An excellent solution to this is the high voltage underground cables. This article provides comprehensive information about the challenges and the installation techniques of high voltage underground cables as compared with the high voltage overhead cables. This article also covers the different types of high voltage cables available in the market today and how each differs from the rest.
Table of Contents
What are the high voltage underground cables?
High-voltage underground cables are cables that are used for the power transmission of high voltages. As the name suggests, they are designed and installed underground. These cables are suitable to use both underwater and underground and have thick insulations. High-voltage underground cables are used for a variety of different applications which include ignition systems, DC and AC power, and other high power instruments. High voltage underground cables are now being deployed in major industries and will one day soon replace the overhead cables.
High Voltage underground cables transmitting power in Sydney
Insulating high voltage underground cables
Overhead high voltage cables are insulated automatically by air whereas the underground high voltage cable conductors have to be wrapped in layers of thick insulating materials. Air is considered to be the simplest and cheapest type of insulation. The high voltage overhead cables are insulated by the air flowing over the conductors. However, when these cables are buried underground, robust installation procedures are required to control the very high voltages as well as high temperatures. The insulation on the high voltage underground cables is very much dependent on the method of installation.
High voltage underground cables due to their thick insulations and compact surrounding have the tendency to retain the heat produced inside the conductors hence this heat has to be distributed to the surrounding environment.
To compensate for the high heat built up, underground cables are usually bigger and thicker as compared to the high voltage overhead cables. This reduces the heat produced as well as the electrical resistance in the cables.
The high voltage underground cables that are buried directly in the ground needs to be well spaced from each other in order to have good heat dissipation. If they are grouped together the heat will not dissipate effectively causing damage to the cables. For instance, to match the overhead line thermal performance of a 400kV double circuit underground cable, as much as twelve separate cables in four different trenches would be needed. Along with the separation for the method, Water cooling can also be used. the high voltage underground cables that are installed indie boat rentals the cable cooling can be done with water cooling or proper air ventilation.
Things to keep in mind when installing high voltage underground cables
When designing a high voltage underground cable system there are a number of different factors that need to be considered relating to the physical environment of the cable. These considerations will directly help to optimize the electrical performance as well as the safe operation of the table. When installing a higher voltage underground cable, it must be ensured that;
| Heat dissipation is the most important factor to consider when installing high voltage underground cables. This consideration is important as adequate heat dissipation is necessary to prevent overheating and subsequent reduction in cable ratings |
| The maintenance crew must have proper access to the high voltage underground cables in order for them to inspect, repair and replace the cables when needed. |
| It is important that the cables are well protected from external physical damages. Adequate physical protection must be done in order to ensure that these cables do not pose a threat to people while in service. |
Types of high voltage underground cables
Currently, there is a limited number of high voltage underground cable manufacturers. Making high voltage underground cables requires expertise and the use of high industrial-grade materials in order for them to function properly. At SSGcables we make sure that our cables meet the high demands of the industry and perform as expected even under extreme conditions. The following are the different types of high-voltage underground cables;
XLPE cable
Since the advent of XLPE, the use of fluid-filled cables has been diminished. This is due to the immense advances in the field of cable technology in recent years. Cross-Linked polyethylene material is used to insulate the central conductor of these modern cables. The robust construction of the cable is achieved as the fluid is not present in these modern cable insulations. Furthermore, the XLPE high voltage underground cable does not require much maintenance as there is no ancillary fluid equipment present for monitoring or maintenance purposes. This cable can be installed in tunnels, ducts and can even be buried directly due to its simple structure and the absence of any fluid.
Fluid Filled Cables
The fluid-filled high voltage underground cables used to have paper insulation. This insulation was then wrapped around the copper conductor, which was present at the center. Afterward, it was infused with fluid under the pressure. In order to hold the fluid pressure, the metallic tapes were wrapped around the insulation, which was covered by a sheath of lead or aluminum. Water could enter the insulation if the cable was exposed to the groundwater. The reason behind it is the fact that the lead or aluminum will deteriorate when it would come in contact with the groundwater. To prevent this corrosion and the successive deterioration of the insulation, the insulation was further protected by an outer sheath. There were additional copper tapes around the lead sheath to provide it with further support. There was also a semi-conductive material present around the outer sheath, which would help deduct any defects that would occur on the outer sheath.
Mass Impregnate Non-Draining cable (MIND)
MIND high voltage underground cables are used inside the substation on lower voltage systems such as LNR i.e. Liquid Neutral Resistor. The MIND cables are paper-insulated, and the paper in this insulation was made impregnable with a non-draining insulating fluid. The use of these types of cable as AC cables, and for the voltages above 33kV is not preferable due to the high stress that is involved. These cables are now replaced by the XLPE. The use of the MIND cables is now limited as the DC cables at higher voltages.
Gas-insulated lines
There are another alternative that can be used instead of the fluid-filled cables or XLPE, i.e. the gas-insulated lines (GIL). It is an emerging technology, and its installation can be seen only at a few places in the National Grid’s network. In these types of cables, the conductors are made up of aluminium or copper, which are then supported by the insulators that are contained within the sealed tubes. Nitrogen/Sulphur Hexafluoride (SF6) gas is used to provide the main insulation. But the backdrop of using this gas is that SF6 is a greenhouse gas, and unless it is cost-effective, it’s used is not advisable, as it is not suitable for our environment, which is already suffering from global warming. One of the biggest advantages of using the GIL is that a higher cable rating can be achieved. Furthermore, the terminations can be cost-effective too, as compared to the conventional sealing end compounds. These cables can be installed above ground in the tunnels and trenches.
High Voltage Cable Installation methods
There are various methods for the installation of high voltage cables. But the method that is used depends on a number of factors. Some of those factors include the land on which the cable will be installed, and the environmental conditions surrounding that area.
A list of options that are available is given below.
Direct buried high voltage cables
The direct burial method is commonly used in urban and rural areas for high voltage cables. Every single cable circuit requires a trench of about 1.5m wide and 1.2m deep. In order to maintain the cable rating, i.e. the capacity to carry current, the thermal conductivity around the cable should be maintained, which can be done by using a backfill of cement bound sand.
It usually depends on the number of circuits and the size of the conductor that is required to be installed to find out the measurement of the cable swathe. Normally the cable swathe can go up to 65m in width. In order to join the individual sections of the cables, joint bays are needed at an interval of about 500-1000m, where, for the arrangement of the joints, a widening of the easement corridor is also required.
The trenches are excavated for the purpose of the direct burial of the cables. Timber is then used to support the sides of the trenches. The winches or power rollers are used for the direct burial of the cables on a bed of cement bound sand. In order to ensure that no air pocket exists, the cement bound sand, or any other selected sand must be poured and arranged around the cables properly. Otherwise, the cable system rating would suffer and cause problems in the long run. To achieve the right level of compaction around the cables, regular tests are constantly carried out.
A typical high voltage underground cable installation
Some of the problems that would occur during the installation of the cables through the direct burial method include excessive noise, visual intrusion and dust generation. These problems would affect the environment, which in turn would affect the people living nearby. Moreover, these installations using the direct burial method also affects the traffic system. Safety issues can also take place due to the presence of construction equipment around the open excavations.
Although the direct burial method has its fair share of issues, it is also the cheapest method for the installation of the cables, only if there exists no restriction on the use of land. Hence the direct burial method for the high voltage underground cables is being commonly used in many parts of the world.
Ducted Methods
The method of the installation using ducts can be considered as another, a much more expensive method for the direct burial. The benefit of using the ducted methods is that the installation of the ducts can occur in shorter sections along the cable route. This would lead to a shorter section of the exposed trenches, and hence a decrease in the risk and disruption caused to the public. Another benefit of using the ducted method is that it saves the installation time. The companies start the manufacturing process of the cables once the contract is signed by all the parties involved. The manufacturing process would take time. In the direct burial method, this would cause long delays, because the burial process couldn’t start without the cables. In the case of ducted methods, the ducts can be installed while the cables are being manufactured. Hence, saving time and not causing delays. At SSGcables we make sure that our valued customers receive their high voltage underground cables on time to avoid delays.
Surface Trough
The excavation of the trench takes place on the surface trough the installation method. After the trench is excavated, at the bottom of the trench, a concrete base is laid. This is done to support the troughs. Afterward, the cables are placed inside the troughs. Later on, they are capped using the concrete covers. The benefits of using the surface trough are that it not only provides mechanical protection for the cables but also improves its thermal conductivity. The surface troughs are only used in secure places such as substations due to the high risk of vandals and thieves. Its impact on the environment is also minimum. A disadvantage of the surface trough is that it would cause dust and access restrictions, along with noise pollution during its construction, and later on, maintenance phases.
Deep bore Tunnels
If the direct burial method is causing a lot of disruption, then the deep-bore tunnels method is used, especially in the urban areas. The size of the tunnel and the type of ground in which the tunnel is to be excavated are important to be considered while deciding the method used for the excavation. In order to find out the best design suitable for the tunnels, detailed ground condition surveys are needed. Gaskets are used to seal the tunnels, while they are also linked with the bolted segments.
A significant amount of land is required around the tunnel construction i.e. 3,000m2. The tunnel that is constructed is usually around 25-30m deep. To provide drainage, a slope of about 1:1,000 is built. For the purpose of maintenance and installation of cables, two head house building, of dimensions 16m x 16m x 7m in height, are required by the tunnel at each end. In addition to this, the tunnels also require emergency access and exit points along the route to ensure the escape from the tunnel. This is compulsory for the tunnels with a considerable length. Electrically driven fans are used to provide forced-air cooling. The water cooling system can be used for additional cooling.
Some of the advantages of using the deep tunnels are that the water and sewage system are not affected, as a river or railway crossing can be built. There is also minimum disruption during the construction and maintenance phases of this method. But this method is also expensive due to its various steps such as planning consent and land acquisition. The planning and construction of a route to avoid major surface and underground structures would also require a considerable amount of money.
Cut and Cover Tunnels
The construction of the tunnel in this method takes place by using the pre-formed concrete sections. These pre-formed concrete sections are laid in a pre-excavated deep open trench. The conditions of the ground are important in order to find out the depth at which the pre-formed concrete sections are laid. Just like the deep-bore tunnels, they also require a head house at both ends of the tunnels. This is for the purpose of the ventilation fans and the entry points into the tunnels. On the basis of the length of the tunnel, the emergency exit and entry points are also built. The size of the tunnel must be sufficient in order to provide a considerable amount of space for the installation of the cables, and their subsequent maintenance. After all the tunneling has been carried out, the high voltage underground cables are then laid out.
Some of the disadvantages of the cut and cover tunnel include the environmental impacts that it has in the form of noise pollution, vibration, visual intrusion, dust generation, etc. The land that is present above the tunnel can be developed while following the restrictions and rules that may apply while doing so.
Benefits of High voltage underground cables
When considering the advantage of high voltage underground cables, one must compare how the high voltage underground cables will be beneficial against the high voltage overhead cables. The benefits are as follows;
| High voltage underground cable has a much longer life expectancy as opposed to the overhead cables. The reason for this is that the underground cables are not exposed to the different environmental conditions and these are well protected underground. |
| The high voltage underground cables have significantly lower maintenance costs as compared to other cables. The reason for this is because these wires are not exposed to different weather elements and thus require low maintenance. |
| Being not exposed to high winds, storms, and other such issues, the underground cables can keep on delivering power even if the weather on land is extreme. |
| The high voltage underground cables are also safer as compared with the overhead cables because they do not break down and causes electrical-related |
Why high voltage underground cables are important?
There are several reasons why high voltage underground cables are important. High voltage underground cables conserve land which is considered to be a valuable resource, especially in the urban regions. High voltage underground cables and systems allow for the construction of buildings along with other important structures without the overhead lines hindering the construction. SSGCables high voltage underground cables do not hamper the work of fire departments as well as the other hazards that are commonly faced by the search and rescue teams. These cables are also fully capable of delivering power in adverse weather conditions which makes sure that let the houses and the hospitals have the electricity running during emergencies. The high voltage underground cables are also considered the cables of the future because of the benefits and importance that they have.
Conclusion
We at SSGcables are manufacturers and suppliers of high-quality high voltage underground cables. We have decades of experience in high voltage underground cable research, development, and manufacturing. Therefore, from conductor selection to sheathing to complete cabling, we execute a full set of strict controls 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 high voltage underground cable or any other cable to fit your requirements and deliver you the best in class products.
SSG owns a manufacturing area over 40,000.00 SQM. Over 20 XLPE cable production lines imported from Germany, Finland, France etc.