EHV.ppt
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This document discusses trends in transmission line voltages and power transfer capabilities. It covers standard voltage levels used internationally, the highest voltage classes of 1000kV and 1150kV, factors that influence power transfer such as line length and voltage levels, and how higher voltages can reduce power losses during transmission.
EHV.ppt
- 2. • Voltages adopted for transmission of bulk power have to conform to standard specifications formulated in all countries and internationally. • They are necessary in view of import, export and domestic manufacture and use. • The following voltage levels are recognized in few countries for line-to-line voltages of 132 kV and higher
- 3. • There exist two further voltage classes which have found use in the world but have not been accepted as standard. They are: 1000 kV (1050 kV maximum) and 1150 kV (1200 Kv maximum). • The maximum operating voltages specified above should in no case be exceeded in any part of the system, since insulation levels of all equipment are based upon them • the primary responsibility of a design engineer to provide sufficient and proper type of reactive power at suitable places in the system. • For voltage rises, inductive compensation and for voltage drops, capacitive compensation must usually be provided
- 14. • In order to be able to estimate how much power a single-circuit at a given voltage can handle, • we need to know the value of positive-sequence line inductance and its reactance at power frequency. • in modern practice, line losses caused by I2R heating of the conductors is importance because of the need to conserve energy. • Therefore, the use of higher voltages than may be dictated by purely economic consideration • It might be found in order not only to lower the current I to be transmitted but also the conductor resistance R by using bundled conductors comprising of several sub-conductors in parallel.
- 16. When line resistance is neglected, the power that can be transmitted depends upon • magnitudes of voltages at the ends (Es, Er), • phase difference δ, • total positive sequence • reactance X per phase, when the shunt capacitive admittance is neglected
- 17. • P = power in MW, • Es, Er = voltages at the sending-end and receiving end, 3- phase, in kV line-line, • δ = phase difference between Es and Er • x = positive-sequence • reactance per phase, ohm/km, • L = line length, km.
- 18. • the power-handling capacity of a single circuit is P = E2 sin δ /Lx. • At unity power factor, at the load P, the current flowing is o the total power loss in the 3-phases will amount the percentage power loss is
- 20. • The power-handling capacity of line at a given voltage level decreases with line length,being inversely proportional to line length L. • if the conductor size is based on current rating, as line length increases, smaller sizes of conductor will be necessary. • This will increase the danger of high voltage effects caused by smaller diameter of conductor giving rise to corona on the conductors and intensifying radio interference levels and audible noise as well as corona loss
- 21. • the percentage power loss in transmission remains independent of line length • since it depends on the ratio of conductor resistance to the positive-sequence reactance per unit length, and the phase difference δ between Es and Er. • From the values of % p it is evident that it decreases as the system voltage is increased. • This is very strongly in favour of using higher voltages if energy is to be conserved. • With the enormous increase in world oil prices and the need for conserving natural resources
- 22. • In comparison to the % power loss at 400 kV, we observe that if the same power is transmitted • at 750 kV, the line loss is reduced to (2.5/4.76) = 0.525, • at 1000 kV it is0.78/4.76 = 0.165, and • at 1200 kV it is reduced further to 0.124.
- 23. • Decrease the cost of conductor • Increase the efficiency • Increase the capacity of the transmission line • Flexibility in the future development
- 24. • Corona loss • Skin effect • Heavy support • Stability • The capability of a conductor to carry current • Reactive loss • Ferranti effect
- 25. • Size of the conductor in DC transmission can be reduced as there is no skin effect • Cost is less as compared to the AC transmission • HVDC tower is less costly • No requirement of reactive power • No system stability problem • HVDC require less phase to phase and ground to ground clearance • Require less number of conductor for same power transfer • Improve line loading capacity
- 26. • HVDC is less reliable • IN HVDC very accurate and lossless power flows through DC link • The disadvantages of HVDC are in conversion, switching, control, maintenance • Lower availability than the AC system • HVDC is very complicated • HVDC does not have transformers for changing the voltage levels