U14EET402: UNIT 3: 20 CONSTRUCTION OF UG CABLE

VIDEO: LAYING OF UNDERGROUND CABLE

Cables are mainly designed as per requirement. Power cables are mainly used for power transmission & distribution purpose. It is an assembly of one or more individually insulated electrical conductors, usually held together with an overall sheath. The assembly is used for transmission and distribution of electrical power. Electrical power cables may be installed as permanent wiring within buildings, buried in the ground and run overhead or exposed. Flexible power cables are used for portable devices, mobile tools and machinery.

These are designed and manufactured as per voltage, current to be carried, operating maximum temperature and purpose of applications desired by customer. For mining, we give extra mechanical strength to cable with double armouring. For wind power plant customers generally require flexible and UV protected cable with mechanical tough sheath so we design as per their requirement.

Rating of Power Cable

Short Circuit Rating

It happens frequently that the conductor size necessary for an installation is dictated by its ability to carry short-circuit current rather than sustained current. During a short –circuit, there is a sudden inrush of current for a few cycles followed by a steadier flow of current for a short period until the protection switchgear operators, normally between 0.1-0.3 seconds

Conductor Size & Material	Insulation Material	Operating Maximum Temperature	Short Circuit Rating
120 sq-mm Copper conductor	PVC Insulation	70° C	13.80 KA/SEC
120 sq-mm Aluminium conductor	PVC Insulation	70° C	9.12 KA/SEC
120 sq-mm Copper conductor	PVC Insulation	85° C	12.48 KA/SEC
120 sq-mm Aluminium conductor	PVC Insulation	85° C	8.28 KA/SEC

CURRENT CARRYING CAPACITY

Current carrying capacity is an important aspect is the selection of the optimum size of conductor. Voltage drop & short rating is also very important aspect to select the economical and optimum size of conductor.

Continous Current Rating of (Cables laid singly)	2 Core × 16 mm2	2 Core × 25 mm2
(i) In Ground ( Ground Temp 30°C)	103 A	131 A
(ii) In Duct ( Ground Temp 30°C)	86 A	111 A
(iii) In Air ( Ambient AirTemp 40°C)	94 A	125 A

VOLTAGE DROP

The allowable maximum voltage drops from source to load is another aspect of power cable conductor design. As per Ohm's law, V = IR. The first is the choice of material used for the wire. Copper is a better conductor than aluminium and will have less voltage drop than aluminium for a given length and wire size. Wire size is another important factor in determining voltage drop. Larger wire sizes (those with a greater diameter) will have less voltage drop than smaller wire sizes of the same length. In American wire gauge, every 6 gauge decrease gives a doubling of the wire diameter, and every 3 gauge decrease doubles the wire cross sectional area. In the Metric Gauge scale, the gauge is 10 times the diameter in millimetres, so a 50 gauge metric wire would be 5 mm in diameter.

Construction of Power Cable

There are various parts of a cable to be taken care of during construction. The power cable mainly consists of 1. CONDUCTOR 2. INSULATION 3. LAY for Multicore cables only 4. BEDDING 5. BRAIDING/ARMOURING (IF REQUIRED) 6. OUTER SHEATH

CONDUCTOR

Conductors are the only power carrying path in a power cable. Conductors are of different materials. Mainly in cable industry we use copper (ATC, ABC) and aluminium conductors for power cables. There are different types of conductor as Class 1: solid, Class 2 stranded, Class 5 flexible, Class 6 Extra flexible (Mostly used for cords & welding) etc. Conductor sizes are identified with conductor resistance.

INSULATION

The insulation provided on each conductor of a cable by mainly PVC (POLY VINYL CLORIDE ), XLPE (CROSSLINKED POLYETHYELENE), RUBBER (VARIUS TYPES OF RUBBER ). Insulating material is based on operating temperature.

Insulation Material	Maximum Operating Temperature
PVC TYPE A	75°C
PVC TYPE B	85°C
PVC TYPE C	85°C
XLPE	90°C
RUBBER – EPR IE-1	90°C
RUBBER – EPR IE-2, EPR IE-3, EPR IE-4, SILICON IE-5	150°C

Cores are identified by colour coding by using different colours on insulation or by number printing on cores

BEDDING (INNER SHEATH)

This portion of the cable is also known as inner sheath. Mostly it is used in Multi core cables. It works as binder for insulated conductors together in multi-core power cables and provides bedding to armour/braid. This portion of the cable is mainly made of PVC( PVC ST-1, PVC ST-2 ), RUBBER (CSP SE-3, CSP SE-4 & PCP SE-3, PCP SE-4, HOFR SE-3 HOFR SE-4, HD HOFR SE-3 ETC)

ARMOURING

There are mainly G.I. WIRE ARMOURING, G.I. STEEL STRIP armouring. It is done by placing G.I. WIREs, GI or STEEL STRIPs one by one on inner sheath. Armouring is a process which is done mainly for providing earthing shield to the current carrying conductors as well as it is also used for earthing purpose of the cable for safety. When there is any insulation failure in the conductor, the fault current gets enough paths to flow through the armour if it is properly earthed. Providing extra mechanical protection and strength to cable an important added advantage of armouring. In MINING CABLES it is done for conductance

BRAIDING

ANNEALED TINNED COPPER WIRE , NYLON BRAID , COTTON BRAID are mainly used for this purpose. Braiding is the process which gives high mechanical protection to cable and also used for earthing purpose. Significance of braiding is it is more flexible in comparison to armouring.

OUTER SHEATH

This is outermost cover of the cable normally made of PVC (POLYVINYL CLORIDE ), RUBBER (VARIUS TYPES OF RUBBER) and often the same material as the bedding. It is provided over the armour for overall mechanical, weather, chemical and electrical protection. Outer sheath is protection offered to cable not much electrically but more mechanically.

Material	Advantages	Disadvantages	Max Operating Temperature
PVC	Cheap, Durable, Widely available	Highest dielectric losses, Melts at high temperatures, Contains halogens	70°C for general purpose 85° C for heat resisting purpose
PE	Lowest dielectric losses, High initial dielectric strength	Highly sensitive to water treeing, Material breaks down at high temperatures
XLPE	Low dielectric losses, Improved material properties at high temperatures	Does not melt but thermal expansion occurs, Medium sensitivity to water treeing (although some XLPE polymers are water-tree resistant)	90° C
EPR	Increased flexibility, Reduced thermal expansion (relative to XLPE), Low sensitivity to water treeing	Medium-High dielectric losses, Requires inorganic filler / additive	90° C
Paper / Oil	Low-Medium dielectric losses, Not harmed by DC testing, Known history of reliability	High weight, High cost, Requires hydraulic pressure / pumps for insulating fluid, Difficult to repair, Degrades with moisture	70° C

Mainly above 6 sq mm cables are called power cables but it depends upon the use of cable. For PVC power cables we use IS:1554 and for XLPE power cables we use IS:7098 and for Rubber based power cables we use IS:9968 and other relevant specifications. Power cables are defined by voltage grade and nominal cross sectional area.

U14 EET 402: UNIT 3: 18- IMPROVEMENT OF STRING EFFICIENCY

The ratio of voltage across the whole string to the product of number of discs and the voltage across the disc nearest to the conductor is known as string efficiency i.e.,

where n = number of discs in the string.
String efficiency is an important consideration since it decides the potential distribution along the string. The greater the string efficiency, the more uniform is the voltage distribution. Thus 100% string efficiency is an ideal case for which the volatge across each disc will be exactly the same. Although it is impossible to achieve 100% string efficiency, yet efforts should be made to improve it
as close to this value as possible.

Methods of Improving String Efficiency

The maximum voltage appears across the insulator nearest to the line conductor and decreases progressively as the crossarm is approached. If the insulation of the highest stressed insulator (i.e. nearest to conductor) breaks down or flash over takes place, the breakdown of other units will take place in succession. This necessitates to equalise the potential across the various units of the string i.e. to improve the string efficiency.
The various methods for this purpose are :

1. By using longer cross-arms. The value of string efficiency depends upon the value of K i.e., ratio of shunt capacitance to mutual capacitance. The lesser the value of K, the greater is the string efficiency and more uniform is the voltage distribution. The value of K
   can be decreased by reducing the shunt capacitance. In order to reduce shunt capacitance, the distance of conductor from tower must be increased i.e., longer cross-arms should be used. However, limitations of cost and strength of tower do not allow the use of very long cross-arms. In practice, K = 0·1 is the limit that can be achieved by this method.
2. By grading the insulators. In this method, insulators of different dimensions are so chosen that each has a different capacitance. The insulators are capacitance graded i.e. they are assembled in the string in such a way that the top unit has the minimum capacitance, increasing progressively as the bottom unit (i.e., nearest to conductor) is reached. Since voltage is inversely proportional to capacitance, this method tends to equalise the potential distribution across the units in the string. This method has the disadvantage that a large number of different-sized insulators are required. However, good results can be obtained by using standard insulators for most of the string and larger units for that near to the line conductor.
3. By using a guard ring. The potential across each unit in a string can be equalised by using a guard ring which is a metal ring electrically connected to the conductor and surrounding the bottom insulator. The guard ring introduces capacitance between metal fittings and the line conductor. The guard ring is contoured in such a way that shunt capacitance currents i1, i2 etc. are equal to metal fitting line capacitance currents i′1, i′2 etc. The result is that same charging current I flows through each unit of string. Consequently, there will be uniform potential distribution across the units.

U14EET402-UNIT 3: 17- TYPES OF INSULATORS

There are mainly three types of insulator used as overhead 

video: insulator installation
Types of insulator
1. Pin Insulator 2. Suspension Insulator 3. Strain Insulator In addition to that there are other two types of electrical insulator available mainly for low voltage application, e.i. Stay Insulator and Shackle Insulator.

Pin Insulator

Pin Insulator is earliest developed overhead insulator,

It is popularly used in power network up to 33KV system. 

Pin type insulator can be one part, two parts or three parts type, depending upon application voltage.

In 11KV system we generally use one part type insulator where whole pin insulator is one piece of properly shaped porcelain or glass. As the leakage path of insulator is through its surface, it is desirable to increase the vertical length of the insulator surface area for lengthening leakage path. In order to obtain lengthy leakage path, one, tow or more rain sheds or petticoats are provided on the insulator body. In addition to that rain shed or petticoats on an insulator serve another purpose. These rain sheds or petticoats are so designed, that during raining the outer surface of the rain shed becomes wet but the inner surface remains dry and non-conductive. So there will be discontinuations of conducting path through the wet pin insulator surface. In higher voltage like 33KV and 66KV manufacturing of one part porcelain pin insulator becomes difficult. Because in higher voltage, the thickness of the insulator become more and a quite thick single piece porcelain insulator can not manufactured practically. In this case we use multiple part pin insulator, where a number of properly designed porcelain shells are fixed together by Portland cement to form one complete insulator unit. For 33KV tow parts and for 66KV three parts pin insulator are generally used.

Designing Consideration of Electrical Insulator

The live conductor attached to the top of the pin insulator is at a potential and bottom of the insulator fixed to supporting structure of earth potential. The insulator has to withstand the potential stresses between conductor and earth. The shortest distance between conductor and earth, surrounding the insulator body, along which electrical discharge may take place through air, is known as flash over distance. 1. When insulator is wet, its outer surface becomes almost conducting. Hence the flash over distance of insulator is decreased. The design of an electrical insulator should be such that the decrease of flash over distance is minimum when the insulator is wet. That is why the upper most petticoat of a pin insulator has umbrella type designed so that it can protect, the rest lower part of the insulator from rain. The upper surface of top most petticoat is inclined as less as possible to maintain maximum flash over voltage during raining. 2. To keep the inner side of the insulator dry, the rain sheds are made in order that these rain sheds should not disturb the voltage distribution they are so designed that their subsurface at right angle to the electromagnetic lines of force.

Post Insulator

Post insulator is more or less similar to Pin insulator but former is suitable for higher voltage application. Post insulator has higher numbers of petticoats and has greater height. This type of insulator can be mounted on supporting structure horizontally as well as vertically. The insulator is made of one piece of porcelain but has fixing clamp arrangement are in both top and bottom end. The main differences between pin insulator and post insulator are,

SL	Pin Insulator	Post Insulator
1	It is generally used up to 33KV system	It is suitable for lower voltage and also for higher voltage
2	It is single stag	It can be single stag as well as multiple stags
3	Conductor is fixed on the top of the insulator by binding	Conductor is fixed on the top of the insulator with help of connector clamp
4	Two insulators cannot be fixed together for higher voltage application	Two or more insulators can be fixed together one above other for higher voltage application
4	Metallic fixing arrangement provided only on bottom end of the insulator	Metallic fixing arrangement provided on both top and bottom ends of the insulator

Suspension Insulator

In higher voltage, beyond 33KV, it becomes uneconomical to use pin insulator because size, weight of the insulator become more. Handling and replacing bigger size single unit insulator are quite difficult task. For overcoming these difficulties, suspension insulator was developed. In suspension insulator numbers of insulators are connected in series to form a string and the line conductor is carried by the bottom most insulator. Each insulator of a suspension string is called disc insulator because of their disc like shape.

Advantages of Suspension Insulator

1. Each suspension disc is designed for normal voltage rating 11KV(Higher voltage rating 15KV), so by using different numbers of discs, a suspension string can be made suitable for any voltage level.

2. If any one of the disc insulators in a suspension string is damaged, it can be replaced much easily.

3. Mechanical stresses on the suspension insulator is less since the line hanged on a flexible suspension string.

4. As the current carrying conductors are suspended from supporting structure by suspension string, the height of the conductor position is always less than the total height of the supporting structure. Therefore, the conductors may be safe from lightening.

U14EET402- UNIT 3: 17 INSULATOR MATERIAL

Electrical Insulator must be used in electrical system to prevent unwanted flow of current to the earth from its supporting points. The insulator plays a vital role in electrical system. Electrical Insulator is a very high resistive path through which practically no current can flow. In transmission and distribution system, the overhead conductors are generally supported by supporting towers or poles. The towers and poles both are properly grounded. So there must be insulator between tower or pole body and current carrying conductors to prevent the flow of current from conductor to earth through the grounded supporting towers or poles.

Insulating Material

The main cause of failure of overhead line insulator, is flash over, occurs in between line and earth during abnormal over voltage in the system. During this flash over, the huge heat produced by arcing, causes puncher in insulator body. Viewing this phenomenon the materials used for electrical insulator, has to posses some specific properties.

Properties of Insulating Material

The materials generally used for insulating purpose is called insulating material. For successful utilization, this material should have some specific properties as listed below- 1. It must be mechanically strong enough to carry tension and weight of conductors. 2. It must have very high dielectric strength to withstand the voltage stresses in High Voltage system. 3. It must possesses high Insulation Resistance to prevent leakage current to the earth. 4. The insulating material must be free from unwanted impurities. 5. It should not be porous. 6. There must not be any entrance on the surface of electrical insulator so that the moisture or gases can enter in it. 7. There physical as well as electrical properties must be less effected by changing temperature.

Porcelain Insulator

Porcelain in most commonly used material for over head insulator in present days. The porcelain is aluminium silicate. The aluminium silicate is mixed with plastic kaolin, feldspar and quartz to obtain final hard and glazed porcelain insulator material. The surface of the insulator should be glazed enough so that water should not be traced on it. Porcelain also should be free from porosity since porosity is the main cause of deterioration of its dielectric property. It must also be free from any impurity and air bubble inside the material which may affect the insulator properties.

Properties of Porcelain Insulator

Property	Value(Approximate)
Dielectric Straingth	60 KV / cm
Compressive Strength	70,000 Kg / cm2
Tensile Strength	500 Kg / cm2

Glass Insulator

Now days glass insulator has become popular in transmission and distribution system. Annealed tough glass is used for insulating purpose. Glass insulator has numbers of advantages over conventional porcelain insulator<

Advantages of Glass Insulator

1. It has very high dielectric strength compared to porcelain. 2. Its resistivity is also very high. 3. It has low coefficient of thermal expansion. 4. It has higher tensile strength compared to porcelain insulator. 5. As it is transparent in nature the is not heated up in sunlight as porcelain. 6. The impurities and air bubble can be easily detected inside the glass insulator body because of its transparency. 7. Glass has very long service life as because mechanical and electrical properties of glass do not be affected by ageing. 8. After all, glass is cheaper than porcelain.

Disadvantages of Glass Insulator

1. Moisture can easily condensed on glass surface and hence air dust will be deposited on the wed glass surface which will provide path to the leakage current of the system. 2. For higher voltage glass can not be cast in irregular shapes since due to irregular cooling internal cooling internal strains are caused.

Properties of Glass Insulator

Property	Value(Approximate)
Dielectric Straingth	140 KV / cm
Compressive Strength	10,000 Kg / cm2
Tensile Strength	35,000 Kg / cm2

Polymer Insulator

In a polymer insulator has two parts, one is glass fiber reinforced epoxy resin rod shaped core and other is silicone rubber or EPDM (Ethylene Propylene Diene Monomer) made weather sheds. Rod shaped core is covered by weather sheds. Weather sheds protect the insulator core from outside environment. As it is made of two parts, core and weather sheds, polymer insulator is also called composite insulator. The rod shaped core is fixed with Hop dip galvanized cast steel made end fittings in both sides.

Advantages of Polymer Insulator

1. It is very light weight compared to porcelain and glass insulator.
2. As the composite insulator is flexible the chance of breakage becomes minimum. 3. Because of lighter in weight and smaller in size, this insulator has lower installation cost. 4. It has higher tensile strength compared to porcelain insulator. 5. Its performance is better particularly in polluted areas. 6. Due to lighter weight polymer insulator imposes less load to the supporting structure. 7. Less cleaning is required due to hydrophobic nature of the insulator.

Disadvantages of Polymer Insulator

1. Moisture may enter in the core if there is any unwanted gap between core and weather sheds. This may cause electrical failure of the insulator. 2. Over crimping in end fittings may result to cracks in the core which leads to mechanical failure of polymer insulator.

In addition to these, some other disadvantages might be experienced. Let us give a practical example where many difficulties are faced in maintaining a distribution network in Victoria Australia due to polymeric insulator.

There are many Cockatoos, Galahs & Parrots in that area of Australia, which love to chew on polymeric strain insulators. Here, the 22KV network has many of polymeric strain insulators installed and now after a few years of installing polymeric strain insulators, the authority is now replacing many of them back with Glass disc insulators.

Another disadvantage is that they have had post type polymeric insulators melt and bend in bushfire areas. They have a concrete pole and a steel cross arm that survives a bushfire, however the polymers in some cases fail. This would not be the case with glass or porcelain insulators.

They have also had polymeric insulators fail in areas close to the ocean coastline where there are high salt levels in the air.

1. Subject to bird attack by Parrots, Cockatoos & Galahs.
2. Not resilient to bushfire temperatures.
3. Not recommended for location near surf beaches due to salt spray.
The information is contributed by Robert Lancaster of Australian Electricity Supply Industry

Types of Insulator

There are mainly three types of insulator likewise
1. Pin Insulator 2. Suspension Insulator 3. Stray Insulator
In addition to that there are other two types of electrical insulator available mainly for low voltage application, e.i. stay insulator and shackle insulator.

U14EET402-UNIT 2- 11. FERRANTI EFFECT

Ferranti Effect in Power System

at light load or no load operation of transmission system, the receiving end voltage often increases beyond the sending end voltage, leading to a phenomena known as Ferranti effect in power system.

Why Ferranti Effect occurs in a Transmission Line?

A long transmission line can be considered to composed a considerably high amount of capacitance and inductor distributed across the entire length of the line. Ferranti Effect occurs when current drawn by the distributed capacitance of the line itself is greater than the current associated with the load at the receiving end of the line( during light or no load). This capacitor charging current leads to voltage drop across the line inductor of the transmission system which is in phase with the sending end voltages. This voltage drop keeps on increasing additively as we move towards the load end of the line and subsequently the receiving end voltage tends to get larger than applied voltage leading to the phenomena called Ferranti effect in power system. It is illustrated with the help of a phasor diagram below.

Thus both the capacitance and inductor effect of transmission line are equally responsible for this particular phenomena to occur, and hence Ferranti effect is negligible in case of a short transmission lines as the inductor of such a line is practically considered to be nearing zero. In general for a 300 Km line operating at a frequency of 50 Hz, the no load receiving end voltage has been found to be 5% higher than the sending end voltage.

Now for analysis of Ferranti effect let us consider the phasor diagrame shown above.

Here V_r is considered to be the reference phasor, represented by OA.

This is represented by the phasor OC.

Now in case of a long transmission line, it has been practically observed that the line electrical resistance is negligibly small compared to the line reactance, hence we can assume the length of the phasor I_c R = 0, we can consider the rise in the voltage is only due to OA - OC = reactive drop in the line.

Now if we consider c₀ and L₀ are the values of capacitance and inductor per km of the transmission line, where l is the length of the line.

Since, in case of a long transmission line, the capacitance is distributed throughout its length, the average current flowing is,

Thus the rise in voltage due to line inductor is given by,
From the above equation it is absolutely evident, that the rise in voltage at the receiving end is directly proportional to the square of the line length, and hence in case of a long transmission line it keeps increasing with length and even goes beyond the applied sending end voltage at times, leading to the phenomena called Ferranti effect in power system