If you can't explain it simply, then you don't know it well enough. — Albert Einstein

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Grading of underground cables

The electrostatic stress in a cable isn’t uniformly distributed. The potential gradient is inversely proportional to the distance from the center of the cable. Hence it will be maximum (gmax) at the surface of the conductor and goes on decreasing until it becomes minimum (gmin) at the surface of the sheath. That means electrostatic stress in the dielectric of a cable is maximum at the surface of the conductor and minimum at the surface of the sheath.
Obviously, for a safe cable, the dielectric strength of the insulation provided must be more than gmax i.e. maximum value of potential gradient. As the electrostatic stress in a cable isn’t uniformly distributed, the strength of the dielectric required isn’t uniform either. We need maximum dielectric strength only at the surface of the core. The remaining dielectric is unnecessarily strong and hence not utilized properly. This also causes the cable to be unnecessarily thick. A large size of electric equipment is always a disadvantage. Furthermore, the possibility of insulation breakdown is more if the stress distribution is non-uniform. These problems are rectified by Grading of the cables.

Grading of underground cables

Grading of a cable is nothing but the process of achieving uniform electrostatic stress in the dielectric of cable. This is achieved by making potential gradient equal throughout the dielectric layer. It can be done in two ways - (i) capacitance grading and (ii) intersheath grading.

Capacitance grading

Capacitance grading is done by employing various layers of different dielectrics having different permittivities between the core and the sheath. Hence the dielectric insulation provided is no longer homogeneous, but composite. The various layers are arranged so that the permittivity decreases from the surface of the conductor to the sheath of a cable i.e. the permittivity of dielectric is inversely proportional to the distance from the center (just like the potential gradient.)
capacitance grading of underground cables

Let an underground cable consist of three dielectric layers as shown in the above image. The inner conductor core is represented by the circle of radius r. The radii of the three dielectric layers are r1, r2 and R respectively. Similarly, let relative permittivities be ε1, ε2 and ε3 respectively. The relative permittivity values and their distances are ε1 > ε2 > ε3 and r1 < r2 < R. The uniform dielectric stress can be achieved by maintaining the product of permittivity and radius of each dielectric as same, i.e. ε1r1 = ε2r2 = ε3R.
Ideally, the dielectric stress will be uniform throughout the cable if we use infinite layers of dielectric. Practically, two or three layers are used. The chief disadvantage is that we require more number of dielectrics with their permittivities varying over a wide range. These are, of course, costly. An alternative is Inter sheath grading.
[Also read: Capacitance of underground cables]

Intersheath grading

In this method, instead of using various dielectrics and having a composite dielectric, we use a homogeneous dielectric material. However, in order to distribute the stress properly, we use extra metallic sheaths between the conductor and the main sheath. These intermediate sheaths are called ‘intersheaths’. These intersheaths are then held at adequate voltage levels. This method improves voltage distribution in the dielectric of the cable and consequently uniform potential gradient is obtained.
intersheath grading of underground cables

There are certain disadvantages of intersheath grading. The major limitations arise in fixing the intersheath potentials accurately and the losses encountered due to the increased charging currents of the various inter sheaths. For these reasons, this practice is rarely employed.

[Also read: Types of underground cables]
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Capacitance of underground cables

As we saw earlier in the construction of Underground cables, a cable is basically a set of one (or three) conductors surrounded by a metallic sheath. This arrangement can be considered as a set of two long, coaxial, cylinders, separated by insulation. The current carrying conductor forms the inner cylinder while the metallic sheath acts as the outer cylinder. The sheath is grounded, and hence voltage difference appears across the cylinders. The dielectric fills the space between the charged plates (cylinders), making it a capacitor. Hence, capacitance of the cable becomes a very important aspect, and must be calculated.
We can broadly classify cables as single-cored and three-cored. And the calculation of capacitance is different for both.

Capacitance of single core cable

A single core cable can be represented as shown below.
capacitance of single core cable
Let,
r = radius of the inner conductor and d = 2r
R = radius of the sheath and D = 2R
ε0 = permittivity of free space = 8.854 x 10-12
εr = relative permittivity of the medium
Consider a cylinder of radius x meters and axial length 1 meter. x be such that, r < x < R.
Now, electric intensity Ex at any point P on the considered cylinder is given as shown in the following equations.
Then, the potential difference between the conductor and sheath is V, as calculated in equations below.
After that, capacitance of the cable can be calculated as C= Q/V
calculation of capacitance of single core cable
When the capacitance of a cable is known, then its capacitive reactance is given by Xc = 1/(2πfC) Ω.
Then the charging current of the cable can be given as,
Ic= Vph / Xc        A

Capacitance of three core cable

Consider a three cored symmetric underground cable as shown in the following figure (i). Let Cs be the capacitance between any core and the sheath and Cc be the core to core capacitance (i.e. capacitance between any two conductors).
capacitance of three core cable

In the above figure (ii), the three Cc (core to core capacitance) are delta connected and the core to sheath capacitance Cs are star connected due to the sheath forming a single point N. The circuit in figure (ii) can be simplified as shown in figure (iii). Outer points A, B and C represent cable cores and the point N represents the sheath (shown at the middle for simplification of the circuit).
Therefore, the whole three core cable is equivalent to three star connected capacitors each of capacitance Cs + 3Cc as shown in fig. (iii).
The charging current can be given as,
Ic = 2πf(Cs+3Cc)Vph      A

Measurement of Cs and Cc

In order to calculate Cs and Cc we perform various experiments like:
  1. First, the three cores are connected together and capacitance between the shorted cores and the sheath is measured. Shorting the three cores eliminates all the three Cc capacitors, leaving the three Cs capacitors in parallel. Therefore, if C1 is the now measured capacitance, Cs can be calculated as, Cs = C1/3.
  2. In the second measurement, any two cores and the sheath are connected together and the capacitance between them and the remaining core is measured. If C2 is the measured capacitance, then C2 = 2Cc+Cs (imagine the above figure (iii) in which points A, B and N are short circuited). Now, as the value of Cs is known from the first measurement, Cc can be calculated.

Effects of capacitance in underground cables

We know that capacitance is inversely proportional to separation between plates. Hence, if the separation between the plates is large, capacitance will be less. This is the case in Overhead Lines where two conductors are separated by several meters. The converse, of course, is also true. If the separation is small, the capacitance is more. In Underground cables, obviously, the separation is relatively smaller. Hence capacitance of underground cables is much more than that of Overhead lines.
The most important factor that is affected by this is the Ferranti effect. It is more pronounced in cables than in lines. This induces several limitations.
Also, with increased capacitance, the charging current drawn is also increased. Underground cables have 20 to 75 times the line charging current compared to Overhead lines.
Due to these two conditions, the length of Underground cables is limited.
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Laying of underground cables

Underground cables are, of course, meant to be installed or laid under the ground. The reliability of underground cable network highly depends upon proper laying of cables, quality of cable joints and branch connections etc. There are three main methods of laying underground cables, which are - (i) direct laying, (ii) draw-in system and (iii) solid system. These three methods are explained below with their advantages and drawbacks.

Direct laying of underground cables

This method is the most popular as it is simple and cheap. The cables to be laid using this method must have the serving of bituminised paper and hessian tape so as to provide protection against corrosion and electrolysis. The direct laying procedure is as follows.

direct laying of underground cables

Laying procedure

  • A trench of about 1.5 meters deep and 45 cm wide is dug.
  • Then the trench is covered with a 10 cm thick layer of fine sand.
  • The cable is laid over the sand bed. The sand bed protects the cable from the moisture from the ground.
  • Then the laid cable is again covered with a layer of sand of about 10 cm thick.
  • When multiple cables are to be laid in the same trench, a horizontal or verticle spacing of about 30 cm is provided to reduce the effect of mutual heating. Spacing between the cables also ensures a fault occurring on one cable does not damage the adjacent cable.
  • The trench is then covered with bricks and soil to protect the cable from mechanical injury.

Advantages

  • Simpler and cheaper than the other two methods
  • Heat generated in cables is easily dissipated in the ground.

Disadvantages

  • To install new cables for fulfilling an increased load demand, completely new excavation has to be done which costs as much as the new installation.
  • Alterations in the cable network are not easy.
  • Maintenance cost is higher.
  • Identifying the location of a fault is difficult.
  • This method can not be used in congested areas such as metro cities where excavation is too expensive.
[Also read: Types of underground cables]

Draw-in system

In this method, cast iron or concrete pipes or ducts are laid underground with manholes at suitable positions along the cable route. The cables are then pulled into the pipes from the manholes. Usually, an additional pipe/duct is also provided along with the three cable ducts for carrying relay protection connections and pilot wires. Distance between the manholes should be such that pulling in the cables is easier. At corners or while changing the direction of route, radius of the corners must be longer. The cables that are to be laid in this way need not be armoured but must be provided with the serving of hessian and jute in order to protect them when being pulled.

draw in system of laying of underground cables

Advantages

  • Repairs, additions or alterations to the cable network can be easily made from manholes without re-excavation.
  • In this method, as the cables need not be armoured, the cable jointing procedure becomes simpler.
  • Maintenance cost is quite lower.
  • Fewer chances of fault occurrence due to the strong mechanical protection provided by the system.

Disadvantages

  • The initial cost is very high.
  • Due to unfavourable conditions for dissipation of heat, current carrying capacity of the cables is reduced.
[Also read: Basics of electrical power transmission]

Solid system

In this method, the cable is laid into troughing of cast iron, stoneware, asphalt or treated wood. When the cable is laid into the position, the troughing is filled with a bituminous of asphaltic compound and then covered over. Cables to be laid in this manner could be just lead covered as the troughing provides a good mechanical protection.
This method is very rarely used nowadays as it is more expensive and requires skilled labour and favourable weather conditions.
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Types of underground cables

We have already seen what Underground cables are, and where they’re used. Moving forward, we must understand various types of Underground cables available to us.

Classification of underground cables

The classification of Underground cables can be done on the basis of several criteria. Various aspects are taken into account while classification and these include:
  1. Number of conductors in the cable
  2. Voltage rating of the cable
  3. Construction of cable
  4. Type and thickness of insulation used
  5. Installation and Laying of the cables

Classification based upon number of conductors in the cable

  1. Single core cable
  2. Three core cable
Typically, an Underground cable has either one, three or four cores. These cables are of course, constructed accordingly.
Underground cables are usually employed to deliver 3 phase power. A 3 cored cable is preferred up to 66 kV. Beyond that, insulation required for the cable is too much. For higher voltages, 3 cored constructions become too bulky, and hence, even with some limitations we employ single cored cables

[Also read: Electric power transmission system]

Classification based upon voltage rating of the cable

  1. Low tension cables: These have a maximum voltage handling capacity of 1000 V (1 kV)
  2. High tension cables: These have a maximum voltage handling capacity of 11 kV.
  3. Super tension cables: These have a maximum voltage handling capacity of 33 kV.
  4. Extra high tension cables: These have a maximum voltage handling capacity of 66 kV.
  5. Extra super voltage cables: These are used for applications with voltage requirement above 132 kV.

Classification based upon construction of the cable

1. Belted cable

In such cables, the conductors (usually three) are bunched together and then bounded with an insulating paper ‘belt’. In such cables, each conductor is insulated using paper impregnated with a suitable dielectric. The gaps between the conductors and the insulating paper belt are filled with a fibrous dielectric material such as Jute or Hessian. This provides flexibility as well as a circular shape. As we discussed earlier (in Construction of Cables), the jute layer is then covered by a metallic sheath and armouring for protection. One particular speciality of this cable is that its shape may not be perfectly circular. It is kept non-circular to use the available space more effectively.
underground cables belted type
There are some limitations of such construction. Since the electric field is tangential, the insulation provided is stressed. As a result, the dielectric strength falls over time. Hence, such construction isn’t preferred for voltage levels above 11 kV.

2. Screened cable

Further divided as H-type and S.L. - type cables.
  • H-Type Cables: It was first designed by M. Hochstadter. The three cores are individually insulated with paper and then covered by a metallic screen / cover. These metallic covers are perforated. As a result, such construction allows the three metallic screens to touch each other. These three metallic covers are then grouped together in a metallic tape usually made of copper. A lead sheath surrounds this construction. The metallic covers and the sheath are grounded.
    The obvious advantage is the electric stresses are radial, not tangential and hence of lesser magnitudes. Also, the metallic covers improve the heat dissipation.
  • S.L Type Cables: It is similar to the H type cables, with the difference that each of the three cores has its own lead sheath. With this provision, the need for the overall sheath used previously is eliminated. The advantage of such a construction is that the chances of a core-to-core breakdown are greatly minimized. Also, the flexibility of the cable is improved.
    The limitations are severe. Such construction is limited for voltages up to 66kV only. The individual sheaths are thinner, and if there are constructional defects, moisture may enter the cable and reduce its dielectric strength.
H type and SL type screened underground cables
  • H.S.L. Type Cables: This type of cable is combination of H type and S.L. type cable. In these cables each core is insulated with impregnated paper and provided with seperate lead sheaths.

3. Pressure cables

For voltages beyond 66 kV, the electrostatic stresses in the cables exceed the acceptable values and solid cables become unreliable. This occurs mainly because voids are created when voltages exceed 66 kV. Hence, instead of solid cables, we use Pressure cables. Typically, such cables are either oil filled or gas filled.
  • Oil Filled Cables: Oil is circulated under suitable pressure through ducts provided for such purpose. This oil supply and pressure are maintained through reservoirs kept at proper distances. The oil used is the same that is employed for impregnation of paper insulators.
  • Gas Filled Cables: Pressurized gas (usually dry nitrogen) is circulated around cables in an air-tight steel pipe. Such cables are cable of carrying higher values of load current and can operate at higher values of voltage. But the overall cost is more.
oil filled and gas filled underground pressure cables

[Also read: Types of conductors used in overhead power lines]

Classification based upon insulation of the cable

Various insulating materials used in cable construction are Rubber, Paper, PVC, XLPE (Cross linked Polyethene) etc. Such classification is based upon operating temperature limitations. Following are some insulating materials used and their maximum operating temperatures.
Insulation materialMax. operating temperature
PVC TYPE A75°C
PVC TYPE B85°C
PVC TYPE C85°C
XLPE90°C
RUBBER90°C
RUBBER – EPR IE-2, EPR IE-3, EPR IE-4, SILICON IE-5150°C

Classification based upon installation and laying of the cable

  • Direct Buried: As the name suggests, the conductors are buried underground in a trench without additional accessories. Sometimes cooling pipes are added if required. Once the cables are installed, there’s no visible sign above the ground.
  • Trough: Concrete troughs are dug and cables are installed in them. They’re visible on the surface. Maintenance is easier.
  • Tunnels: Sometimes, tunnels are dug up for this purpose. Such construction is mainly employed if a river needs to be crossed or if the intended power distribution is to a major city. Maintenance and future expansion is easier, but initial cost is higher.
  • Gas Insulated Lines: This is a relatively new technology. For cables operating at higher voltages and currents, and handling high power, such gas insulated line construction is safer. It is being employed nowadays for advanced projects.
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Underground Power Cables

In addition to the overhead lines we come across every day, electrical power can also be both transmitted and distributed using Underground cables. These underground cables, of course, come with their own set of advantage and limitations. Aside from better general appearance and lesser interference with other amenities, the advantages include smaller voltage drops and lesser probability of fault occurrence. On the other hand, they have higher production and installation costs, and hence are used wherever overhead lines aren’t viable due to practical limitations or risks involved. Hence we employ them in specific places such as urban areas with high population densities and across water bodies (as submarine cables).
A typical underground cable will consist of a conductor/s covered by a number of insulating and protective layers necessary for its satisfactory operation. Underground cables construction is explained below:

Construction of underground power cables


construction of underground power cable
Construction of underground power cable
  • Conductor: Usually, 1 or 3 conductors (depending upon the application) are used. These conductors are stranded to reduce skin effect, proximity effect and to keep it flexible. Conductors are made from electrolytic grade pure copper or aluminium.
  • Conductor screen: It is a semi-conducting tape or an extruded layer of a semi-conductive compound. Conductor screening is generally used in MV and HV cables to maintain uniform electric field and minimise electrostatic stresses.
  • Insulation: It is provided to withstand the electrostatic stress. Various types (and thickness) of insulators like VIR (Vulcanized India Rubber), Impregnated paper, PVC (polyvinyl chloride) and XLPE (Cross linked polyethylene) are utilised depending upon the applied voltage.
  • Insulation screen: A layer of semi-conductive material generally used in MV and HV lines. It serves the similar purpose as that of conductor screen.
  • Metallic Sheath: It provides protection to the cable from moisture and other chemicals (acids or alkalies) present in the environment or soil. It’s usually made up of Aluminium or Lead. It also provides a path for fault and leakage currents as the sheath is earthed at one cable end.
  • Bedding: It is a low grade insulator like Jute or Hessian which protects the metallic sheath from corrosion and from mechanical injury due to armouring.
  • Armouring: It provides mechanical protection from various stresses the cable may get exposed to during its installation and operational life. It’s usually a steel tape wound around the Bedding layer.
  • Serving: Another layer of low grade insulator like Jute or Hessian or a thermoplastic compound like PVC is again provided to protect the steel from atmospheric contaminants and agents.
[Also read: Corona Discharge in overhead power lines]

Types of underground cables

Underground cables are usually classified according to their Voltage ratings. They’re grouped as follows:
  1. Low tension cables which have a maximum voltage handling capacity of 1000V
  2. High tension cables which have a maximum voltage handling capacity of 11kV
  3. Super tension cables which have a maximum voltage handling capacity of 33kV
  4. Extra high tension cables which have a maximum voltage handling capacity of 66kV
  5. Extra super voltage cables which are used for applications with voltage requirement above 132kV.

Classification of 3 phase underground cables

1. Belted cables: As the name suggests, it has an additional layer of oil-impregnated paper which is wound around the insulated conductors. Such an arrangement is useful for low and medium voltage levels up to 11 kV.
2. Screened cables: Used only in particular applications with specialised construction, these Underground cables can be further divided as H-type and S.L-type cables.
3. Pressure cables: These are used when the voltage requirement exceeds 66kV and solid cables can't be used. Either pressurized gas or pressurized oil is used in these cables.
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