TRANSMISSION AND DISTRIBUTION

To Transmit electrical energy in bulk quantity from power station to the load centre at higher potential is known as transmission. To distribute electrical energy to the consumer is known as distribution.

(i) Primary transmission and secondary transmission,

(ii) Primary distribution and secondary distribution.

Methods of transmission and distribution of electrical energy are, i) overhead & ii) underground.

Standard voltage for (i) Generation, (ii) Transmission & (iii) Distribution: (i) 6.6kV, 11kV, 33kV,(ii) 33kV, 66kV, 132kV, 220kV & 400kV, (iii)  (a) Primary distribution 6.6kV, 11kV, 33kV, (b) Secondary distribution AC – 415/240V & DC – 450/225V.

The Basis of selection of voltage for transmission line: The basis of selecting the most economical transmission voltage is to use 650 volt per km. of transmission line.

Advantage of high voltage transmission: The advantages of high voltage transmission are as follows:

(a0 With the increase in voltage the size of the conductor is reduced which further reduces the cost of the supporting materials.

(b) With the increase in voltage the line current is reduced resulting reduction in line drop & line losses which causes higher efficiency.

© Due to low voltage drop better voltage regulation is achieved.

Limitation of high voltage transmission: In actual practice it is not possible to increase the transmission voltage beyond a certain limit because of the following facts:-

(a) The insulation required between the conductor & earthed tower increases which increases the cost of the tower.

(b) More clearance is required between conductors and conductor and earthed members. Accordingly the size of the tower is increased.

The DC is not used for transmission of power as it is very difficult to get sufficiently high voltage in DC generation and it is not easy to step up and step down the DC voltage.

Advantage of AC transmission over DC:

(a) The electrical power can be generated at high voltage easily.

(b) The voltage can be stepped up at generating end by means of step up transformer to the required value for transmission and then stepped down at distributing end by means of step down transformer for distribution.

© The maintenance of AC substation is easier and cheaper.

Merits and demerits of DC transmission over AC transmission:

Merits: (i) DC system requires only two conductors and hence much copper is saved.

(ii) It has no inductance, capacitance, phase displacement and surge problem.

(iii) As there is no skin effect in DC, full cross section of conductor is utilized.

(iv) Less insulation is required in DC for the same working voltage.

(v) Voltage regulation is better in DC transmission.

Demerits: (i) It is very difficult for high voltage DC generation due to commutating problem

(ii) It is not possible to step up the DC voltage.

Three phase three wire system is generally used for its better efficiency of operation and easy convenience. Underground system of transmission is not used beyond 66 kV due to insulation difficulties. Overhead line is preferred for power transmission because it is economical. The dielectric strength of air under normal condition is about 30kV/cm. Under ground cables are recommended up to the limit of 100 km.

Merits & demerits of underground system over O.H. system:

Merits: (i) It is more safer, (ii) Maintenance cost is low, (iii) Chances of power failure or faults and accident area very few, (iv) Voltage drop is low, (v) It is free from interruption of supply due to thunder storms, severe weather condition or such other causes.

Demerits: (i) It is more expensive, (ii) It is not flexible like O.H. line, (iii) For E.H.V. transmission above 66kV insulation difficulties will arise, (iv) Fault location and repairing are difficult and expensive, (v) It draws higher charging current due to high capacitance, (vi) Tapping and jointing are difficult.

Skin effect: When an alternating current passes through a conductor it does not flow uniformly all over the conductor. The concentration of current is higher on the surface of the conductor than at the centre, thus increasing the effective resistance. This effect is called the skin effect which is prominent at higher frequency. The skin effect increases with the increase of cross section and permeability of conductor and this effect is reduced in stranded conductors.

Proximity effect: The proximity effect is the distortions of the current distribution in the cross section of a conductor caused by the mutual induction between the currents in the go and return conductors arranged closely parallel to one another. The concentration of currents in the conductors is higher nearest each other thus increasing their effective resistance. It is directly proportional to the magnitude of the current and inversely proportional to the distance between conductors.

The effective AC resistance is more than DC resistance of transmission line due to skin effect & proximity effect.

Transmission line possesses inductance and capacitance: When a conductor carries current it is surrounded by a magnetic field and in case of AC this magnetic field is not constant but changing and links with the same conductor and also other conductors. Due to these magnetic flux linkages the transmission line possesses inductance.

When two conductors are separated by an insulating medium it constitute a capacitor. In case of transmission line the two conductors form the two plate of a capacitor separated by air medium from one another thus possessing a capacitance. The existence of the capacitance in the transmission line is due to the potential difference between the conductors. The capacitance can be neglected up to 75km length of transmission.

The factors governing the inductance & capacitance of a transmission line:

(i) Distance between the conductor (inductance increases but capacitance decreases with the increase in distance).

(ii) Radius of conductor (inductance decreases but capacitance increase with the increase in radius).

(iii) Length of the line (both inductance & capacitance increase with the increase in length).

When capacitance of a transmission line exceeds, the line draws more leading current which compensates the lagging component of load current thus reducing the resultant current. The reduction in resultant current reduces the line losses, increases transmission efficiency as well as load capacity, improves voltage regulation and also power factor.

Corona effect: When a transmission line carries power at high voltage an electrostatic field is set up between the conductors and then the atmospheric air, the dielectric medium surrounding the conductors is subjected to electrostatic stresses.

If the voltage between the conductors exceeds the dielectric strength of air, the layers adjacent to the conductor break down giving rise to the ionization of an air column around the conductor which causes a leakage of energy i.e. power loss and gives rise to a hissing noise accompanied by the appearance of a luminous envelope of violet colour around the conductor and thus forming of ozone gas. The whole phenomenon, the hissing noise, the violet glow and production of ozone gas is known as corona effect.

Disadvantage of Corona effect: (i) It results in power loss in the line, (ii) It creates interference in neighboring radio communication circuits, (iii) The ozone gas is produced which chemically reacts with the conductor and causes corrosion.

Advantages of Corona effect: (i) It reduces the effect of high voltage travelling waves due to lightning discharge on terminal equipments such as transformers, (ii) As the air surrounding the conductor becomes conductive there is a virtual increase in conductor diameter due to which maximum electrostatic stress is reduced, thus reducing the probability of flash over and improving the system performance.

Factors, affecting corona: (i) Atmosphere, (ii) Conductor, (iii) Spacing between conductors & (iv) Line voltage.

The Maximum chances of occurrence of corona are during humid weather. T he Corona decreases with the increase in diameter of conductor.

Action for to minimize the effect of Corona: The corona loss can be minimized by raising the disruptive critical voltage which can be done by either increasing the diameter of the conductor voltage which can be done by either increasing the diameter of the conductor or the spacing between conductors. Practically the spacing can not be increased too much. So only increasing the diameter of the conductor is recommended for reducing corona effect.

Disruptive critical voltage: The minimum voltage or potential gradient, at which the disruption of the dielectric takes place i.e. ionization of the immediately adjacent air due to corona just commences without visual corona, is called the disruptive critical voltage.

Visual critical voltage: It is known that the corona discharge takes place at the disruptive critical voltage. The minimum voltage higher than this disruptive critical voltage, at which the corona just becomes visible, is called the visual critical voltage.

The glow of Corona is found brightest at the rough or dirty surface of the conductors.

The Corona effect depends on the following factors of the transmission line:-

(i) Voltage of the transmission line,

(ii) Ratio of the distance between two conductors to the radius of each conductor i.e.  D/r

(iii) Contour of transmission line and

(iv) Physical state of the surface of the conductor.

Ferranti effect: When the long transmission line having high capacitance is unloaded or lightly loaded it is sometimes observed that the receiving end voltage becomes more than the sending and voltage. This phenomenon of rise of voltage is called as Ferranti effect.

Regulation of transmission line: The regulation of transmission line is defined as the rise in voltage at the receiving end when the full load is thrown off, the sending end voltage being unaltered. The percentage regulation is this rise of voltage expressed as a percentage of the full load receiving end voltage.

The voltage regulation depends upon the resistance and reactance of the transmission line. Large values of resistance and reactance give greater percentage regulation. In other words it depends upon the current and power factor of the load.

The regulation increases with the decrease in power factor in case of inductive loads. But in case of capacitive load the regulation decreases with the decreases in power factor till it becomes zero and then with further decrease in power factor the regulation becomes negative

The efficiency of transmission line is defined as the ratio of power received to the power sent out. The effect of load power factor on transmission efficiency is the efficiency of transmission line decreases with the fall in load power factor.

Necessity of keeping the receiving end voltage constant within specified limit: The receiving end voltage must be kept constant within specified limit in order to deliver good service to the consumers for satisfactory operation of lamps, motors and other loads, because abnormal decrease in voltage reduces the illuminating power of the lamp loads and also starting torque of the motor loads considerably and again on the other hand abnormal rise in voltage causes rapid deterioration of the filament in case of lamp and saturation in the magnetic circuit of the motor resulting large magnetizing current, undue heating and low power factor.

Methods of voltage control in a power system:

(i) By excitation control

(ii) By using tap changing transformer

(iii) By Auto transformer tap changing

(iv) By Booster transformers

(v) By Induction regulators

(vi) By synchronous condenser

Method (i) is applied at the generating station only whereas methods, (ii) to (v) can be used for transmission and primary distribution system. Method (vi) is reserved for voltage control of a transmission line.

Difference between surge and impulse: Though a surge and an impulse are waves of short duration, a surge appears in the electrical power system due to lightning strokes etc. whereas an impulse is obtained from impulse generator for testing.

Voltage surge and causes: A sudden rise in voltage for a very short duration on the power system is known as a voltage surge or transient voltage. The causes of voltage surge are as follows:-

(i) Direct lightning stroke.

(ii) Statically induced charges on he conductors due to the proximity of charged clouds.

(iii) Electro magnetically induced currents due to a lightning discharge in the immediate vicinity of the line.

(iv) Electro static charges imparted to the line due to the friction of dust on the conductors and variation of altitudes of the line along the route.

(v) Oscillations set up by sudden changes of circuit conditions e.g. switching on of unloaded line, sudden opening of a loaded line, insulation failure, arcing earth & resonance.

Harmful effect of voltage surge:

(i) Possible failure of insulation of electrical apparatus e.g. transformers, motor etc.

(ii) Possible flashover at very highly stressed points of equipments.

(iii) Possible damage or ageing to the insulating materials.

Ground wires area arranged above the power conductors on a transmission line and grounded at every tower or support:

(i) To protect the line conductor from direct stroke by carrying away the current from the direct stroke of lightning discharge.

(ii) To reduce the induced voltage in the conductor produced by electrically charged clouds drifting over the line.

(iii) To damp out the travelling wave on line conductors by its action as a short circuited secondary.

(iv) to save line insulator string from flashing over since induced over-voltages are greatly reduced.

Classification of distribution system according to the voltage:

(i) High voltage distribution and (ii) Low voltage distribution.

Classification of distribution system according to the type of current:

(i) AC distribution and  (ii) DC distribution.

Classification of distribution system according to the type of construction:

(i) Overhead distribution &  (ii) underground distribution.

Classification of distribution system according to the number of wires:

(i) Tow wire DC system, (ii) Three wire DC system, (iii) Single phase, Two wire AC system, (iv) Three phase Three wire AC system, (v) Three phase, Four wire AC system.

Classification of distribution system according to the scheme of connections:

(i) Radial system, (ii) Ring main system &  (iii) Interconnected system.

Primary distribution system & secondary distribution system: When the distributing lines carry power from the receiving substation to the big industrial and other consumers and to distribution transformers at 11 kV without any intermediate tapping it is known as primary distribution. When the lines carry power from the distribution transformer to the feeding point from which service tapping are taken at low and medium voltage, it is known as secondary distribution.

Difference between AC distribution and DC distribution:  AC distribution differs from DC distribution in number of wires. Three phase four wires in AC and three wires in DC are used for supplying power in low voltage & medium voltage at a time. During calculations of voltage drop and lines loss for AC distributors few more points are considered. The power factor of the load is considered in AC distributor which is absent in DC distributor. Total current in AC distributor is determined by the vector sum of currents of different sections and not by algebraic sum as in DC distributor. The voltage drop in AC distributor consists of inductive drop in addition to the DC resistive drop.

Three wire distribution in DC power: It can be obtained by the following methods:-

(i) Two generators connected in series.

(ii) Storage battery connected across the lines with neutral connected at the mid point. l

(iii) Three wire generator or static balancer.

(iv) Rotary balancer sets.

Advantage of 3-wire distribution over 2-wire distribution: Tow types of voltages in low voltage & medium voltage can be obtained from 3-wire supply.

Why three phase transmission is always done by 3-wires whereas 3-phase distribution is always done by 4-wires?

Three phase transmission is always done by 3-wires not by 4-wires because of the better efficiency of operation and easy convenience of 3-wire system whereas 3-phase distribution is always done by 4-wires not by 3-wires i.e. one extra neutral wire is provided in addition to three phase wires to obtain two types of voltages in low voltage & medium voltage required for the consumer’s different types of lamps, household appliances and power loads.

Elements of distribution line: A distribution line has three elements e.g. (i) Feeders, (ii) Distributors and (iii) Service mains.

Feeder: The feeder may be defined as the line carrying power from the secondary substation to the distribution substation without being tapped at any intermediate points. It is designed mainly from the point of view of its current carrying capacity.

Distributor: A distributor is a line from which tapings are taken along its length to provide supply to various consumers. It is supplied from a distribution substation and designed from the point of view of the voltage drop in this line.

Features of good distribution:

(a) The voltage drop at he point of commencement of supply should remain within +6% of the declared voltage.

(b) There should be no power failure.

© The distribution line should not be overloaded.

(d) The insulation resistance of the distribution system should be kept as high as possible to avoid any leakage and danger to human life.

(e) The line loss should be kept as minimum as possible.

(f) The distribution system should be made as economical as possible.

(g) The whole system should require less maintenance.

Service line: Service line means the overhead line or underground cable connecting the supplier’s distribution line to the consumers’ wiring.

Reason to maintain high power factor in the AC transmission system: High power factor should be maintained to minimize the line loss and the volume of the conductor and also to increase the efficiency of the transmission line.

Capacitors used in transmission & distribution lines: Capacitors are used in two ways:

(i) Series capacitors are used for long AC transmission lines of rated voltage 220 kV and above for compensating series inductance of the line thereby increasing the power transfer ability.

(ii) Shunt capacitors are used in several locations near individual loads, group of loads, at receiving substation etc. for compensation of reactive power thereby improving the power factor. I

In case of long transmission lines shunt capacitors are connected at an interval of about 300 km. along the line in intermediate substation. Shunt capacitors are switched in during low voltage due to high lagging current and switched off during high voltage at light loads.

Ø  To design a feeder Voltage regulation is a main criteria,

Ø  To design a transmission line, Power loss is a main criteria,

Ø  The approximate value of surge impedance of a long Overhead transmission line is 400 to 600Ω,

Ø  The approximate value of surge impedance of a long underground cable is 40 to 60Ω,

Ø  While increasing the height of transmission tower its capacitance decreases but the line inductance remains constant.

Ø  The approx. value of inductance per km of 11 kv transmission line is 1mH and that of inductance is 0.01F respectively.

Ø  The voltage gradient is highest at the surface of the conductor.

Ø  The carrier current protection scheme is normally used for H.V. overhead transmission lines only.

Ø  The capacity of a transmission line if the voltages at the two ends if the line are 220 kV and its reactance is 48.4 ohms, will be 1000MW

Ø  As per I.E. rule,  minimum breaking strength of overhead conductor is 350 kg and maximum allowable voltage drop is 6%

Ø  Dielectric strength of porcelain is 60kV/cm of its thickness.

Reason for Failure of insulation in transmission line:

(i) Over voltage cause by switching, direct and indirect lightning stroke.

(ii) Bridging of conductors by birds,

(iii) Decrease of dielectric strength,

(iv)  External mechanical damages,

Causes of fault in Overhead transmission system:

(i) Lightning and tree falling on line,

(ii) Failure of apparatus,

(iii) Improper switching operation,

(iv) Sabotage

(v) Loose contacts of jump ring conductors due to storm and wind.

Protection for a transmission line: (i) Time graded and current graded over current protection, (ii) Distance protection.

Time graded and current graded over current protection of a transmission line: In the transmission lines when the time setting and current setting of over current relays at different substations graded with diminishing progressively from the power source to the remote end of the line then they are said to be time graded over current protection and current graded over current protection respectively.

Problems of E.H.V. transmission:

(i) The radio interference due to corona effect becomes more prominent.

(ii) The cost of line support and insulation increases,

(iii) Reactive losses are more,

(iv) Stability limit is reduced,

(v) Ferranti effect is predominant.

Uses of different poles:

Wooden pole	For LT lines and HT lines up to 11 kV
Concrete pole	For LT lines and HT lines up to 11 kV
Steel tubular pole	For LT lines and HT lines up to 33 kV
Rail pole	For 33 kV HT lines
Steel tower	For extra high voltage lines.

Various types of conductors used in overhead line:

(i) Copper conductor, (ii) All-Aluminum conductor(AAC), (iii) Aluminum conductor steel reinforced (ACSR), (iv) Cadmium copper conductor, (v) Phosphor Bronze & (vi) Galvanized steel conductor.

ACSR Conductor: It means aluminum conductor steel reinforced. It has a core of one or two layers of galvanized steel stands surrounded by one, two or three layers of aluminum strands.

Advantage of ACSR conductor:

(i) It has much lighter weight as compared with copper conductors and greater tensile strength.

(ii) The sag can be reduced and accordingly shorter towers or longer spans may be used which in turn reduce the cost of transmission tower along with its accessories.

(iii) The failure in operation is very much less.

(iv) Maintenance cost is less.

(v) It has higher corona limit.

Disadvantage of ACSR conductor:

(i) Due to its lower electrical conductivity the relative cross sectional area will be more than that of copper,

(ii) It necessitates stronger supporting structures,

(iii) These conductors are susceptible to corrosion.

A.A.C.: It means All aluminum Conductor. These are stranded conductors made of aluminum wires. All the aluminum stranded conductors are mainly used on low voltage distribution system employing relatively shorter spans.

Stranded conductors are preferred due to following reasons:-

i)                    It becomes more flexible

ii)                  It can carry more current due to the elimination to a great extent of the skin effect.

iii)                The chance of its breaking under fatigue due to vibration is reduced so much.

Factors to decide the size of the low voltage overhead conductor: The size of the conductor depends upon the following factors e.g. conductivity, weight, mechanical strength, cost & voltage drop. But the important deciding factors in the low voltage lines are breaking strength and allowable voltage drop. As per I.E. rules minimum breaking strength is 350 kg and maximum allowable voltage drop is 6%.

Factor of safety: When the conductors or the metal supports are loaded beyond its capacity, then they ultimately break down either by tension, compression or shear. To keep the working stress safe its value should be kept below the ultimate breaking stress. The degree of reduction is expressed as a factor equal to the ratio of ultimate breaking stress to the safe working stress which is named as factor of safety.

Factor of safety of different conductors & supports:

i)                    For metal support                                                 - 1.5

ii)                  For mechanically processed concrete support     - 2.0

iii)                For hand molded concrete support                      - 2.5

iv)                For wood support                                                  - 3.0

v)                  Latticed structure                                                 - 1.5

vi)                Stay wires                                                              - 2.5

vii)              Conductors                                                            - 2.0

Dielectric strength of porcelain is 60 kV/cm of its thickness.

Dielectric strength of disc insulator is 11kV each.

             : DC supply is 100Kv, 200Kv, 300Kv, 400Kv, 600Kv 800Kv.

Difference between Overhead line & underground line.

sn	Overhead line	Underground line
1	Cheap to install	Costly to install
2	Easy disconnection	Disconnection is not possible
3	When load increases, extra connection is to be given & connections can be changed.	Not possible
4	Line voltage can be increased	For increasing line voltage, cable to be replaced
5	In emergency excess load can be given	Possibility of cable break
6	Fault can be easily detected	Hard to detect the fault