The need for electric power is increasing day by day throughout the world and in most countries, it is doubling every five to eight years. In some countries, excellent hydropower sites are available but at a far distance from the load centers. The thermal power plants are also required to be located near the coalfields, in order to avoid pollution – hazards and also from economic considerations, which are also at large distances from the load centers. Though the location of nuclear power stations is somewhat flexible the general practice is to site them far away from large cities and thickly populated areas. All these problems involve the transmission of large amounts of power over long distances, which can be done most economically only by using high voltages (or simply HV). In excess voltage of 230 kV fall in this category.
In 1952, Sweden successfully commissioned a 380 kV, 960 km long single circuit line transmitting 400 MW power. This event set the trend of adoption of still higher voltages.
In the world including our country nowadays many H.V. a.c. lines are in operation. In India, the first 400 kV transmission line from Obra to Sultanpur and 400 kV substation at Sultanpur (UP) was commissioned in December 1977. Many other 400 kV transmission lines and substations are under construction in our country.
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High Voltage D.C. Transmission
In the initial stages d.c. used to be a recruit for generation, transmission, and distribution of electric power. With the introduction of transformers and the 3 phase A.C. system the situation changed in favor of A.C., especially where the electric energy is generated from water the power which is usually available far from the load centers. For other many reasons, such as facility of the transformation of voltages from one value to another value, better performance of A.C. motors and superiority of A.C.generators in comparison to D.C. generators, the electric power was generated, transmitted, distributed, and utilized in the form of Alternating Current. The supporters of D.C. Although did not forget the advantages of D.C. transmission and extensive research has been carried out in this field and as a result, it has staged a sort of come – back to the field of electric power transmission at high a high D.C. voltage. Developments in the design of mercury arc rectifiers and silicon controlled rectifiers made H.V. D.C. transmission all the more feasible and attractive, through the generation and distribution still continue to be carried out by A.C.
H.V. d.c. transmission systems are classified as follows:
(i) Two – pole, one – wire
(ii) Two – pole, two-wire
(iii) Three – pole, three-wire
Standard voltages are 100, 200, 300, 400, 600 and 800 kV for both two poles and three – pole transmission.
The advantages and limitations of H.V. d.c. the transmission system is given below:
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Advantages:
1. Cheaper in Cost: It requires only two conductors for transmission and it is also possible to transmit the power through only one conductor by using earth as returning conductor while a.c. the system requires 3 conductors.
2. Lesser – Insulation Requirement: The potential stress on the insulation in case of d.c. the system is 1 / root 2 times of that an a.c. system for the same operation voltages. Hence for the same operating voltage, less insulation is required.
3. Simpler and Cheaper Tower Designs: Because of lesser load on the supporting structures, tower designs are simpler and cheaper.
4. Less Dielectric Power Loss and Higher Current Carrying Capacity: The cables have lesser dielectric power loss with d.c. in comparison with a.c. &, consequently, have a higher current carrying capacity.
5. Negligible Sheath Losses: In the case of d.c. only leakage current flows in the sheath of cable whereas in the case of a.c. charging, circulating and eddy currents flow through the sheath of the cable. Thus, in the case of d.c. transmission the sheath losses in the cables are negligible.
6. Higher Natural Dielectric Strength and Longer Life of Cable Insulation: The natural dielectric strength of cable insulation with d.c. is substantially greater than with a.c. the occurrence of dielectric exhaustion is also missing, therefore, the cable insulation has a longer life.
7. Absence of Charging Current and Limitations of Length of Cable: Because of large charging currents, the use of high voltage a.c. for underground transmission over a long distance is prohibited but because of the absence of charging current in the d.c system, there is no limit on the length of the cable.
8. No Stability Limit: There is no stability limit in d.c. system and therefore, transmission lines may be of any length whereas in a.c. systems the conceptual length of the transmission line which may be operated without loss of stability is only 1.43 times its natural impedance (mostly it is about 500 km).
9. Corona Loss and Radio Interference are less of a problem in d.c. systems as compared with that in a.c. systems.
10. Use as a Non – Synchronous Link Between Two A.C. Systems: Parallel operation of a.c. with d.c. which enlarges the stability limit of the system of two large a.c. system by d.c. transmission line. Here the d.c. the line is a non – synchronous link between two rigid (frequency constant) systems where an otherwise slight difference in frequency of the two large systems would produce serious problems of power transmission control in the smaller capacity link.
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Limitations:
1. Costly Terminal Equipment: Converters and invertors required as terminal equipment in d.c. transmission system are more costlier in comparison with the transformers in a.c. systems.
2. Non – Availability of Reliable d.c. Circuit Breakers: Though it is true that circuit breakers for EHV d.c. are as yet to be developed lack of this facility is no hindrance to the operation of EHV d.c. system since selective fault clearance in a multi-terminal d.c. the system is possible by a combination of quick-acting isolators and grid – controlled action.
3. Distortion in Waveform, Presence of Ripples and Harmonics: Filtering and smoothing equipment are provided at the converter stations in order to remove ripples from the d.c. output. It can be also possible to offer filters on the a.c. side to absorb the harmonic currents.
4. Non – Transmission of Reactive Power Over the d.c. Link: It is obvious that no reactive power can be transmitted by HV d.c. transmission system and, therefore, devices such as synchronous capacitors are installed at the receiving end to produce reactive power as per the demand of the load and converting apparatus.
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High Voltage A.C. Transmission
As already mentioned that the technical and economic problems associated with the transmission of large amounts of power over long distances can be solved only by using high voltage (Voltage exceeding 230 kV). The requirement of EHV transmission is enlarging day by day for interconnections of two or more power systems to attain sharing of installed reserves; construction of large power plants, and for development of integrated systems and grids.
HV a.c. the transmission has many technical and economical advantages for transmitting bulk power over long distances, which are given below:
1. Reducind of Electrical Losses, Inlargeing in Transmission Efficiency, Changing of Voltage Regulation and Reduction in Conductor Material Requirement: For transmission of given amount of power over a given distance through the conductors of a given material and at a given power factor as the transmission voltage increases.
A.) Line losses are decreased thus line losses are inversely proportional to the transmission voltage,
B.) Transmission efficiency is in large because of reduction in line losses,
C.) Voltage regulation is enhanced because of decreasing of percentage line drop, and
D.) Less conductor material is needed to be inversely proportional of the square of transmission voltage.
2. Flexibility for Future System Growth: There is flexibility for future growth.
3. Increase in Transmission Capacity of the Line: Power transferred is expressed as
P = Vs . Vr/X sinα
Where Vs and Vr are the two terminal voltages, α is the load angle and X is the line reactance.
Thus the power transmission capacity of a transmission line increases with the increases in transmission voltage. No suspicion in the cost of transmission line and terminal devices also increases with the increase in the transmission voltage but normally these costs are proportional to the transmission voltage in place of the square of the transmission voltage. Moreover, there is also saving in cost due to a reduction in energy losses occurring in transmission lines. As a consequence, the total cost of transmission decreases with the increase in transmission voltages, as depicted in Figure below.
4. Reduction in Rights of Way: In some countries ‘rights of way’ are paid for at a rate proportional to the total width of the transmission lines. Even in many countries where right of way is not directly paid, there are normally strong pressures from the public towards fewer and fewer transmission lines. With the passage of time rights of way become either more costlier or difficult to obtain and, therefore, it is becoming necessary to have fewer transmission lines operating at EHV.
There are also some drawbacks and limitations in the transmission of power by 3 – phase EHV a.c. a system, which are given below:
1. Corona and Radio Interference: As we know that corona is not only a source of power loss but is also source of the interference with radio and television. The problem is more critical in case of the HV transmission. For limiting the corona loss, audible noise and radio – interference it is necessary to limit the r.m.s. electric stress at the surface of the conductor to 1.8 kV/mm, preferably to 1.5 kV/mm. Large size diameter conductors (hollow conductors or ACSR conductors) have been used to bring down the corona losses and radio – interference.
2. Heavy Supporting Structures and Erection Difficulties: Transmission line towers with fabricated steel members are usually used in HV transmission. The transmission lines are made more wind – resistant as they are to bear out the wind pressure during storms and cyclones.
3. Insulation Requirement: The level of insulation required depends upon the magnitude of likely voltage surges due to internal causes (switching operations) or due to external causes (lightning etc). The Switching surges are, however, more dangerous as they may cause overvoltages of 2 – 3 times the normal operating voltage. With the developments in the design of relay –breaker systems, however, it is possible to control and minimize switching overvoltages.