INSULATED MEDIUM VOLTAGE CABLES INSULATED MEDIUM VOLTAGE CABLES
UNDERGROUND CABLE INSTALLATION. SAFE DISTANCE
in urban or high density of population is not recommended the laying of high voltage overhead lines for reasons of safety for residents. If we add the environmental and aesthetic
involving supports and lines, it is advisable and mandatory installation of underground lines in those areas.
Under these conditions, the processing centers is located at various points throughout the villages and are fed by underground medium voltage lines. On a smaller scale, they also begin to settle
these lines in rural and industrial areas.
drivers used for this purpose must be clearly isolated, and installed underground (directly or within protective tubes) arranged in galleries built under ground level.
The initial problem lies in the adequate protection of these drivers, both with respect to electrical insulation as mechanical or chemical attacks which may come from underground.
The increase in underground lines and high voltage is due to development and new designs from power quality insulating materials that guarantee reliable and sustainable supplies.
Needless to say that underground lines are more expensive than air. The implementation of adequate ditches and galleries is more expensive than installation of supports and, above all, the cables are much more complicated (and expensive) than just the bare conductors used in the airlines. Election
drivers of medium voltage underground line.
For the choice of drivers in these lines to keep in mind the specifications of the particular rules of distribution companies. They made a list of types
preferred to install cables, both overhead and underground, either in medium voltage or low voltage. This will standardize and simplify the selection criteria.
-Prescriptions
. Drivers will be aluminum.
. Tensions have nominal values \u200b\u200b12/20625/45 kV.
. Sections nominal core cables have the values: 150-240 mm
-400. Cables rated section exceeding 400 mm shall be unipolar.
Having decided the course of the line, has to choose:
. The insulation material of cables.
. The nominal section of drivers.
underground cable
Choice of cable sizes.
For the cable section the fundamental fact of departure is the expected intensity of the current will circulate around them. We get that intensity
from:
-expected power to be transmitted.
-The rated voltage of the line.
-The estimated power factor. Except in very specific cases takes the value of 0.90.
Knowing the expected intensity value is chosen as the initial value of the section:
"He who gives us a current-carrying capacity (properly corrected, if necessary) of equal value or the
immediately above the predictable.
-El that meet the above condition, provides a voltage drop between the substation supply and processing center that feeds, no more than 5% of rated voltage.
Except in very specific cases, the voltage drops in MV lines are small, so that the criterion for selection is based practically on the value of the expected intensity.
If a ring network consists of an underground line, the section of their drivers will remain constant throughout its length. Like
lines air, can be justified the choice of a section than necessary in order to reduce energy losses
. The justification is twofold if, as a result of the calculation, we obtain a section that is missing from the standard by the distributor.
For the calculation of the losses we can use the scales of resistance / length provided by cable manufacturers.
Safety distances.
assembly and maintenance.
Drivers today are electric cables, are the result of the large studies that manufacturers charge for their R & D, so they can work on these cables under the harshest conditions at all times will be sued.
The high technology that have these cables, requires that management and instalaci6n of these, it has to be done in an appropriate way that can not be damaged their technical characteristics. If these cables are inadequately treated can cause damage, which, if not immediately detected and installed, could shorten its life considerably.
The greatest dangers may suffer cables occur in the transport and laying of them: The transport
distinguish three main periods
a) The transport from the factory to the warehouse.
b) the warehouse itself.
c). The transfer to the place of lying.
The cables are supplied mainly in wooden bobbins wing whose overall diameters ranging, from 250 cm in the largest cases to 60 cm in the case of the smallest, all depending on the length, weight and the outside diameter of the cables
transport of different forms of packaging to the different types of construction of cables, such as boxes, rolls or coils, and especially the latter, will be made so that, should always be feet and never supported one side, so that means of transportation used (train, trucks, etc.) must have the tools necessary to anchor these are not rolling.
To proceed with loading and unloading of reels in the mode of transportation selected should be hung on a rod appropriate to the weight of the order to place them in the right place or if we used ramps and docks, they will be constructed so they can slip the coils. The recommended maximum slope in the case of ramps shall not exceed 25%.
should never be thrown or coils or rolls from the vehicle to the ground, although both their size and weight are small, because the shock or impact might damage the cables.
In storage, never be stored in rolls or boxes outside, and whenever possible, also the coils, since the presence of the sun and humidity can get to cause damage. In the case of coils, the wood may suffer serious harm, involving major problems both for transport and for the subsequent laying of the same.
When the cables housed in the coils have to remain outdoors, must be fitted caps that cover completely, ie all drivers and the outer covering of the cord.
During the transfer of the cables from the warehouse to the point of lying, we must take the same precautions as when the factory moved from to the store
As regards the loading, transportation and unloading coils should be shot in the same sense as the manufacturer rolled to wires. If necessary
coil reversed itself, is done using a rod or stick
doing to facilitate the shift lever.
The preparations will be made for the laying of the cables is one of the most important tasks to perform, so that they do not suffer any damage. The first thing we will do is place the coil on a support shaft which must be located at a height that does not rotate freely to prevent the coil for proper laying of cables,
need to install a brake, but is in a very simple and elementary, allowing us to stop the coil in the event that we produce kinks or bends in the cable dangerous, and the inertia of rotation of the reel when the cable is tending to make endanger or cause injury to personnel who work there.
The location of the coil is such that the cable
not have to force yourself to make the alignment of lying.
The wiring cover is thermoplastic which must be made at temperatures above O ° C to prevent cracking. It is also important that the manufacturer indicates what the minimum radius of curvature with which the cables must be routed so that they not suffer, or change the specifications for which they were manufactured.
To install the cable, the first thing we do is drop the start winding wire, installing a head that will help us to pull it.
The cable can be run from one of these ways:
. Stretched by hand.
. Stretched from a moving vehicle.
. Lying with motor-driven rollers.
. Lying by winch or capstan.
In all cases, the routing is done using rolls prepared for the purpose, which serve to reduce friction on the ground for direct burial.
laying in hand, the rollers will prevent the cable drag along the ground or rubbing against the sidewalls of the trench in the direction changes. If there are no obstacles in the trench or near, the
can run wires directly from a vehicle, on which, using the appropriate jacks and the axis is placed the coil.
Laying the motorized roller is a newer system that is available along the trench, at distances between 20 and 30 feet, rollers driven by two electric motors. This traction system is evenly distributed along the cable.
The most common system is undoubtedly the mechanical shot by winch, whose security is total, if you previously, we have prepared the appropriate tools and appropriate action is taken. This system tended
can
made in the following ways:
a) Lying to stress applied to the cable end.
b) Construction with effort spread over the entire cable with the aid of cable ties suitable guarantor.
In the first case, the pull rope attached to the cable, as already indicated, by means of a sleeve or pull-cords, head traction, if it is longer cable section and weight to avoid damage caused by the sliding of the insulation on the conductors, by a head shot, attached directly to cable conductors, with the aid of an album where you will drill for those drivers who are held in position by means of hoses and screws.
In the second case, using a double length sear the trench, as the drive is from the opposite end of the coil and the beginning has to have covered the whole trench guarantor, more a side length equal to the full reel of cable. This is the second installment will be tying the cable as it unwinds the coil, through simple, rapid bonds that will be run by placing the cable every five meters.
The maximum tensile stresses are 5 Kg/mm2 desirable section in the case of copper and half in the case of aluminum conductors. Tended speed should not exceed 5 meters per minute. When laying will take the following precautions:
1). Constantly monitors a dynamometer tractive effort in order not to go for the maximum stresses allowed.
2). We have to put a pin protection calibrated rupture, so drive is interrupted at the time indicated efforts are exceeded.
3). Maintain the rollers at the points provided for the cable is not touching or rubbing the ground or the walls of the trench. If the cable route is winding duct, the total curves over 3000, the tensile stress can be equivalent to the weight of the cable, which may require limiting the length to build on each trade.
The drivers union is made by means of precast joints.
Once installed cables will perform the following checks:
. Insulation.
. Short.
. Adjournment.
. Succession of phases.
Maintenance.
Faults: Initially
drivers are in perfect condition, but the passage of time and actions caused by excavations next to them, landslides, fault isolation, overloads or shorts.
Oats is located where protections are triggered circuit. Many times these protections are triggered accidentally, so it is advisable to check the circuit before returning to reset this protection to ensure that no such damage.
We can distinguish two main types of faults:
a) Interruption of the continuity of the line.
a) Damage to the insulation.
In any type of cable is in good condition can be defined two parameters which are the conductor resistance and insulation resistance, so that in case of a fault these values \u200b\u200bvary and allow us to classify them as one of the two cases a) or b), or both at once.
If a driver at the time of measuring the strength of their drivers gives us an infinite value, while its insulation resistance gives an acceptable value, you are telling us what kind of damage is (broken driver), while if the value of the conductor resistance is normal, and the insulation resistance is low we also indicates that the type of problem is (damaged insulation or broken).
For location of different faults that we may have, we will have some equipment that is capable of providing a form as close as possible, the point where such damage has occurred.
is easier to find that the insulation has been pierced by a frank and be grounded or break free of a conductor, an intermittent contact. Therefore, sometimes causing the defect ex-desired, by passing a current such that char the insulation or cover the conductor at the point where it causes a local increase of the resistance.
classical location methods, introduction of the method above wing of pulse reflection tracking (radar), depend on the kind of damage.
1 º) The driver is not interrupted.
a) Short circuit or earth fault through of a small resistance, less than 500 n.
b) Referral to ground through a large resistance,
than 500 Ω.
2 °) The driver is stopped at point defects.
a) There earth fault on one or more drivers.
b) Drivers have good insulation values \u200b\u200b(no earth fault or short circuit).
first case (1 to)-If you have a driver in good condition and equal to the return, shall be measured to Murray Bridge. This system has been the most widely used for many years. In essence it is a wire bridge in which two of the resistors are formed: one for the return cable attached to the furthest stretch of damaged cable and the other in the next section, and two pieces of wire for calibration of the bridge.
When it is not possible to have a return line, using the method of measuring the direction of flow of Wurmbach.
second case (1 ° b)-is transformed in (1 º a) burning the failure to apply sufficient tension to form an arc that reduces their resistance to a value below 50 Ω.
third case (2 °)-was used Wurmbach the above method based on the principle that a current, which circulates inside the damaged cable, closed circuit through land use as the drilling path caused by the fault. At this point converge the two directions of current flowing through the cable. A galvanometer prepared to identify the direction of the current passes through a cable, provide a dial right or left contrary to the fault point and dial 0 (zero) at that point. The major drawback of this method is its hard, because it forces you to discover the cable in multiple locations in the field. For this reason, it has been replaced by more modern.
fourth case (2 ° b)-In this case, in which the conductor is cut, but has not deteriorated insulation, the method is based on the assumption The capacity of the stretch of broken cable is proportional to the length and the damaged cable. For the determination of these capabilities is used in bridge-building as a signal generator that uses a source of alternating current and a detector audio headphones. Regulates the strength and ability variables until it managed to extinguish the buzz on the phone. The magnitude of the capacity being on the bridge, when the sound ceases headphones must be equal to the capacity in the section of the conductor being measured. Today
modern methods are known by the generic name of trackers, which consists of crossing the path of the line, equipped special detectors to locate the point where the fault is located, being called these the acoustic method and the method of induction.
But the most effective troubleshooting techniques are based on the principle of reflection of a short electrical impulse is sent along the cable. Ce the cone will be the name of "RADAR" time for reflection of pulses or pulse echo reflectometer. Is to measure the time between the time of shipment and arrival of the reflected pulse. These two moments are represented on a screen as two peaks on a horizontal line. The distance between the two peaks represents the time between the output pulse and the arrival of the image reflected in the breakdown. To do this, it also presents a baseline graduated in microseconds, which can be zero to match with the output pulse. The elapsed time divided by two, gives us the time of arrival at the defect site.
The image of the screen provides more information on the nature of the fault, because if the peak of the reflected pulse is in the same direction as the original impetus is that the driver is stopped, as if in the opposite direction is that is shorted. However, it advises the person to find the point where the fault itself, that you read the instructions on the type of equipment to be used, and the method to be applied to your computer.
JUNCTION AND TERMINAL shrink.
Applying the experience of the polymers irradiated in the industry
power distribution cables, you get a wide range of heat shrinkable cable accessories for MV and LV.
Obtaining a shrinkable plastic material is made by electronic radiation prior to its crosslinking is achieved. Expanding
that once irradiated material at a certain temperature, with subsequent cooling and maintaining deformation, it has the effect of "elastic memory" such that the material regains its original form after a simple application of heat.
shrink feature has the following attributes:
-Because of its contraction, the same material can be adapted to different sizes of cables.
-Cross-linking gives the material a stability against external agents.
-Easy to install, since they will be perfectly matched with a simple heat input.
Installation Ease of installation makes them shrink materials used throughout the world, because they may be installed in any situation with nothing more than provide heat. Shrinkable materials, whether tubes or molded contract with the application of heat by means of a torch or hot air, achieving a perfect match to the cable material.
In the enforcement proceedings, you just have to place the material in its final position and caloric intake, to achieve a perfect contraction constantly moving the heat source so as not to cause overheating and focusing so as to preheat the area close to collapse .
joint system is based on a co-extruded tube with two layers, one prestressed elastomeric material (insulation) and other heat-shrinkable material (semiconductor).
shrink terminations.
shrink terminations for medium voltage cables are used in any type of cable, because its components can obviate the possible tolerances of the same, while providing ease of assembly.
One of the most characteristic properties of these cable terminations is its moisture sealant, thank to use a special adhesive that is not carbon road under any pollution.
other terms are plug-in terminals, which have greater application in the cabins of the processing centers.
This plug-in system is compact and does not increase the total length of the terminal. The characteristics presented by these terminals are:
-Can be installed indoors and outdoors as well as upright, angled or inverted. -For installation
no special tools are needed or curb or fillers.
-not required to maintain minimum distances between stages.
-The connection can be put in tension immediately after installation.
-terminals can be applied to cables of any kind, such as polyethylene, crosslinked polyethylene, ethylene-.propileno and impregnated paper cables. He used
copper and aluminum.
-A capacitive voltage divider built to check if the line is in tension.
-The maximum operating voltage is 24 kV, with currents of 250-400 A.
There are three types of termination plug, which are: straight, angled and T.
cold shrink terminals
terminal system is based cold shrink in one piece that attaches to isolate and control the electric field performed on a prestressed core, which allows use in any situation easily, quickly and safe, without any equipment and tools.
The system is designed cable insulation impregnated paper dry up voltage 45 kV, installed either outdoors or indoors.
The isolation of the terminals is made of a silicone rubber, resistant to surface currents and corona, which provides better performance in humid atmospheres and high pollution. The silicone has a property that makes it unique and is to reject the water because its high surface tension causes the water on the surface of the insulator forming droplets instead of blades. This, coupled with that is a highly flexible material, makes it fit any curvature of the cable, providing a sealing effect of high reliability.
-The procedure achievement of this end! is, once completed, the cable, place the piece without any effort and manually remove the tape that makes up the inner core, which retracts into cold blend into cable, without gaps intermediate and ensuring a tight seal.
TERMINAL INSTALLATION PROCESS OF COLD INSIDE.
1 º) Remove the cable cover a length. A is a characteristic parameter of each product and B is the length of the terminal.
2 º) is folded back over the wires of the screen, it should be carefully fold one by one to be held constant calculated in paragraph lonitud above. Then fastened with a wire.
3 º) semiconducting screen is removed, leave 35mm bare in front of the cable jacket.
4 º) Remove the primary insulation on the length
5 º) Place and punching (aluminum) or press (copper) connection terminal: The rounded edges and clean all rebadas on insulation primary and the terminal.
6 º) Apply silicone grease on the primary isolation on the edge of the semiconductor layer.
7 º) terminal is introduced, is placed in position and the tape is withdrawn until the start of contraction. The insulation should be placed at the beginning the threads of the screen.
PROCESS OF INSTALLING A COLD JOINT.
1 º) cables are positioned so that splicing and intersect at right angles.
2 º) Clearing the deck of the wires in a length of about 600mm.
3 º) are tightened NUVI ends inside the joint to facilitate the introduction of the cable.
4 º) Pass the joint body in one of the cables.
Court of the housing, semiconductor and isolation:
5 º) Remove the cable cover a length of 240mm.
6 º) Cut the strip in contraespira flush deck.
7 º) A 50mm cut the deck, cut and remove the outer semiconducting layer.
8 º) Cut and remove the wire insulation over a length of A mm from the ends of the cable. Union
drivers:
9 º) is positioned and the nipple is made of understanding.
10 º) is checked after the union bound.
Lubrication
11 º) should be liberally lubricate the region comprising the joint. Positioned
joint body:
12 º) Remove the protective plastic inside.
13 º) Place the joint body, checking dimensions are equal. Withdrawn
internal support:
14 º) Remove the rubber band securing the support straps.
Application of the tape gun (or EPR):
15 º) were given three wraps of tape gun from 65mm to 50mm cutting deck to better accommodate the surrounding semiconductor joint body on the external semiconducting layer and cable insulation. Sealing
Internal
16 º) Place the sealing tape on the end of the joint, about 25 to 30mm.
Holding the cable screen:
17 º) Extends the Copper-Tin mesh on the screen cable wires.
18 °) is applied constant pressure spring on the whole (wire mesh + Cu-Sn)
external tightness:
19 º) Place the sealing tape in two layers.
Lubrication
20 °) is lubricated from the joint center to the ends of the sealing tape.
joint split:
21 º) are pulling the straps off the outer cover of the envelope. Then pulled longitudinally by sliding the outer sleeve over itself to seal coat the curb.
22 °) was taken in the same way on the other side of the joint. Mounting and anchoring
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