Power Systems - Overhead Insulators & Underground Cables Complete Study Notes

The electrical power is transmission (or distribution) is carried out either by underground cables or by overhead lines. Underground cables are less used for power transmission because the power being transmitted has to cover long distances to meet load centers which obviously costs very high when compared to overhead lines. Another reason being power which is being transmitted is kept at high voltages for economic considerations, therefore a proper insulation is required to cables so that they could withstand higher pressures. Hence, power transmission over long distances is carried out by using overhead lines.

Also, an overhead line is subjected to uncertain weather conditions and other external interferences, which in turn need to use of proper mechanical factors of safety in order to ensure the continuity of operation in the line.

In general, the mechanical strength of the line is designed such that it must sustain against worst weather conditions.

Main Components of Overhead Lines:

The main components of an overhead line are:

(i) Conductors which carry electric power from the sending end to the receiving end.

(ii) Supports which may be poles or towers and keep the conductors at a suitable level (or height) above the ground.

(iii) Insulators which are attached to supports and insulate the conductors from the ground.

(iv) Cross arms which provide support to the insulators.

(v) Others such as lightning arrestors, anti-climbing wires, etc.

Types of Insulators:

Pin Type Insulators:

The figure shown below is pin type insulator. As the name suggests, the pin type insulator is secured to the cross-arm on the pole. There is a groove on the upper end of the insulator for housing the conductor. The conductor passes through this groove and is bound by the annealed wire of the same material as the conductor.

Suspension Type Insulators

The cost of pin type insulator increases rapidly as the working voltage is increased. Therefore, this type of insulator is not economical beyond 33 kV. For high voltages (>33 kV), suspension type insulators are used as shown in figure below. Suspension type insulator is consisting of number of porcelain discs connected in series by metal links in the form of a string. The conductor is suspended at the bottom end of this string while the other end of the string is secured to the cross-arm of the tower.

Strain Insulators

When there is a dead end of the line or there is sharp curve, the line is subjected to greater tension. Therefore, strain insulators are used. The assembly for strain type insulator is shown in figure below.

Shackle Insulators

Shackle type insulators used for low voltage distribution lines and can be used either in a horizontal position or in a vertical position. Figure shown below shows a shackle insulator fixed to the pole. The conductor in the groove is fixed with a soft binding wire.

Potential Distribution Over Suspension Insulator String

A string of suspension insulators consists of a number of porcelain discs connected in series through metallic links.

The porcelain portion of each disc is in between two metal links. Therefore, each disc forms a capacitor ‘C’ as shown in figure below, which is known as mutual capacitance or self-capacitance.

If there is only mutual capacitance, then charging current will be same through all the discs and therefore voltage across each unit would have been the same i.e., V/3.

But in practice, capacitance also exists between metal fitting of each disc and tower, known as shunt capacitance C₁. Due to shunt capacitance, charging current is not the same through all the discs of the string. Therefore, voltage across each disc will be different, i.e., disc nearest to the line conductor will have the maximum voltage. Thus, V₃ will be much more than V₂ or V₁ as shown in figure below.

where    n = number of discs in the string.

Methods of Improving String Efficiency:

The various methods for improving string efficiency are as follows:

1. By using longer cross-arms
2. By grading the insulators
3. By using a guard ring

Underground cables:

An underground cable essentially consists of one or more conductors covered with suitable insulation and surrounded by a protecting cover. The type of cable to be used depends on the working voltage and service requirements.

In general, a cable must fulfil below mentioned necessary requirements:

1. The conductor used in cables should be stranded type either made of copper or aluminum with high conductivity, the use of stranded conductor is done to reduce the skin effect.
2. The conductor size should be such that the cable carries the desired load current without overheating and causes voltage drop within permissible limits.
3. The cable must have proper thickness of insulation in order to give high degree of safety and reliability at the voltage for which it is used.
4. The cable must be provided with suitable mechanical protection so that it may withstand on rough use.

The materials used in the manufacture of cables should be such that there is complete chemical and physical stability throughout.

The various parts of the cable are as follows:

1. Cores or Conductors:

A cable may have one or more than one core (conductor) depending upon the type of application it is used for. The conductors are made of tinned copper or aluminium and are usually stranded in order to provide flexibility to the cable.

1. Insulation:

Each core or conductor is provided with a suitable thickness of insulation, the thickness of layer depends upon the voltage which cable need to withstand. The commonly used materials for insulation are impregnated paper, varnished cambric, or rubber mineral compound.

1. Metallic sheath:

In order to protect the cable from moisture, gases, or any other damaging liquids (such as acids or alkalis) in the soil and atmosphere, a metallic sheath of lead or aluminium is provided over the insulation.

1. Bedding:

Over the metallic sheath, a layer of bedding is applied which consists of a fibrous material like jute or hessian tape. The purpose of bedding is to protect the metallic sheath against corrosion and from mechanical injury due to armouring.

1. Armouring:

Over the bedding, armouring is provided which consists of one or two layers of galvanised steel wire or steel tape. Its purpose is to protect the cable from mechanical injury while laying it and during the course of handling.

1. Serving:

In order to protect armouring from atmospheric conditions, a layer of fibrous material (like jute) similar to bedding is provided over the armouring, this is known as serving.

Types of Cables:

Cables for underground service may be classified in two ways i.e., according to

(i) the type of insulating material used

(ii) the voltage

Now, according to voltage cables can be divided into the following groups:

(i) Low-tension (L.T.) cables — up to 1000 V

(ii) High-tension (H.T.) cables — up to 11 kV

(iii) Super-tension (S.T.) cables — from 22 kV to 33 kV

(iv) Extra high-tension (E.H.T.) cables — from 33 kV to 66 kV

(v) Extra Online Classroom Program voltage cables — beyond 132 kV

Classification of Cables: 

Cables for underground service may be classified in two ways according to

(i) the type of insulating material used in their manufacture

(ii) the voltage for which they are manufactured.

However, the latter method of classification is generally preferred, according to which cables can be divided into the following groups:

• Low-tension (L.T.) cables — upto 1000 V
• High-tension (H.T.) cables — upto 11,000 V 
• Super-tension (S.T.) cables — from 22 kV to 33 kV
• Extra high-tension (E.H.T.) cables — from 33 kV to 66 kV
• Extra Online Classroom Program voltage cables — beyond 132 kV

A cable may have one or more than one core depending upon the type of service for which it is intended. It may be (i) single-core (ii) two-core (iii) three-core (iv) four-core etc. For a 3-phase service, either 3-single-core cables or three-core cable can be used depending upon the operating voltage and load demand.

Cable for 3-phase

In practice, underground cables are generally required to deliver 3-phase power. For the purpose, either three-core cable or three single core cables may be used. For voltage upto 66 kV, 3-core cable (i.e., multi-core construction) is preferred due to economic reasons.

However, for voltages beyond 66 kV, 3-core-cables become too large and unwieldy and, therefore, single-core cables are used. The following types of cables are generally used for 3-phase service:

• Belted cables — upto 11 kV
• Screened cables — from 22 kV to 66 kV 
• Pressure cables — beyond 66 kV

Dielectric Stress in Cable

The electric intensity at a point x meters from the center of the cable is

It is clear from the above equation that potential gradient varies inversely as the distance x. Therefore, the potential gradient will be maximum when x is minimum i.e. when x = d/2 or at the surface of the conductor. On the other hand, the potential gradient will be minimum at x = D/2 or at sheath surface.

Maximum potential gradient is 

Most Economical Size of Conductor:

For given values of V and D, the most economical conductor diameter will be one for which gmax has a minimum value. The value of gmax will be minimum when dln D/d is maximum i.e,

and the value of gmax under this condition is

g_max = 2V/d volt/meter

Grading of Cables

• The process of achieving uniform electrostatic stress in the dielectric of cables is known as grading of cables. It has already been shown that electrostatic stress in a single core cable has a maximum value (gmax) at the conductor surface and goes on decreasing as we move towards the sheath.
• The maximum voltage that can be safely applied to a cable depends upon gmax i.e., electrostatic stress at the conductor surface. For safe working of a cable having homogeneous dielectric, the strength of dielectric must be more than gmax.
• If a dielectric of high strength is used for a cable, it is used only near the conductor where stress is maximum. But as we move away from the conductor, the electrostatic stress decreases, so the dielectric will be unnecessarily over strong.

The following are the two main methods of grading of cables: (i) Capacitance grading (ii) Intersheath grading

Capacitance Grading

The process of achieving uniformity in the dielectric stress by using layers of different dielectrics is known as capacitance grading. 

In capacitance grading, the homogeneous dielectric is replaced by a composite dielectric. The composite dielectric consists of various layers of different dielectrics in such a manner that relative permittivity > r of any layer is inversely proportional to its distance from the center.

The capacitance grading can be explained as there are three dielectrics of outer diameter d1, d2, and D and of relative permittivity >1, >2 and >3 respectively. If the permittivity is such that >1 > 2 > 3 and the three dielectrics are worked at the same maximum stress, then

Total p.d. between core and the earthed sheath is

V = V₁ + V₂ + V₃ 

Intersheath Grading

In this method of cable grading, a homogeneous dielectric is used, but it is divided into various layers by placing metallic inters heaths between the core and lead sheath. The inter sheaths are held at suitable potentials which are in between the core potential and earth potential. This arrangement improves voltage distribution in the dielectric of the cable and consequently more uniform potential gradient is obtained.

Since the dielectric is homogeneous, the maximum stress in each layer is the same i.e.,
g_1max = g_2max = g_3max = g_max 

As the cable behaves like three capacitors in series, therefore, all the potentials are in phase i.e. Voltage between conductor and earthed lead sheath 

V = V₁ + V₂ + V₃ 

Inter sheath grading has three principal disadvantages.

• There are complications in fixing the sheath potentials.
• The inter sheaths are likely to be damaged during transportation and installation which might result in local concentrations of potential gradient.
• There are considerable losses in the inter sheaths due to charging currents. For these reasons, inter sheath grading is rarely used.

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