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# Note for High Voltage DC Transmission - HVDC By JNTU Heroes

• High Voltage DC Transmission - HVDC
• Note
• Jawaharlal Nehru Technological University Anantapur (JNTU) College of Engineering (CEP), Pulivendula, Pulivendula, Andhra Pradesh, India - JNTUACEP
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High Voltage Direct Current Transmission 185 The largest thyristors used in converter valves have blocking voltages of the order of kilovolts and currents of the order 100s of amperes. In order to obtain higher voltages the thyristor valve uses a single series string of these thyristors. With higher current ratings required, the valve utilizes thyristors directly connected in parallel on a common heat sink. The largest operational converter stations have ratings of the order of gigawatts and operate at voltages of 100s of kilovolts, and maybe up to 1000 km in length. The thyristors are mostly air cooled but may be oil cooled, water cooled or Freon cooled. With air cooled and oil cooled thyristors the same medium is used as insulant. With the Freon cooled thyristors, SF6 may be used for insulation, leading to the design of a compact thyristor valve. Unlike an a.c. transmission line which requires a transformer at each end, a d.c. transmission line requires a convertor at each end. At the sending end rectification is carried out, where as at the receiving end inversion is carried out. 11.1 Comparison of a.c and d.c transmission 11.1.1 Advantages of d.c. (a) More power can be transmitted per conductor per circuit. The capabilities of power transmission of an a.c. link and a d.c. link are different. For the same insulation, the direct voltage Vd is equal to the peak value (√2 x rms value) of the alternating voltage Vd. Vd = √2 Va For the same conductor size, the same current can transmitted with both d.c. and a.c. if skin effect is not considered. Id = Ia Thus the corresponding power transmission using 2 conductors with d.c. and a.c. are as follows. d c power per conductor Pd = Vd Id a c power per conductor Pa = Va Ia cos 3 The greater power transmission with d.c. over a.c. is given by the ratio of powers. Pd = _√2__ cos 3 Pa = H GDWSI XQLW\ GDWSI  J In practice, a.c. transmission is carried out using either single circuit or double circuit 3 phase transmission using 3 or 6 conductors. In such a case the above ratio for power must be multiplied by 2/3 or by 4/3. In general, we are interested in transmitting a given quantity of power at a given insulation level, at a given efficiency of transmission. Thus for the same power transmitted P, same losses PL and same peak voltage V, we can determine the reduction of conductor cross-section Ad over Aa.

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186 High Voltage Engineering - J R Lucas, 2001 Let Rd and Ra be the corresponding values of conductor resistance for d.c. and a.c. respectively, neglecting skin resistance. For d.c current = P Vm 2 2 power loss PL = (P/Vm) Rd = (P/Vm) .(!O\$d) For a.c current = power loss PL = = √2 P . Vm cos 3 P = (Vm/√2) cos 3 [√2P/(Vm cos3 @ Ra 2 2 (P/Vm) .(!O\$a cos 3 2 2 Equating power loss for d.c. and a.c. 2 (P/Vm) .(!O\$d) = 2 (P/Vm) .(!O\$a cos 3 2 2 This gives the result for the ratio of areas as H GDWSI XQLW\ GDWSI  J The result has been calculated at unity power factor and at 0.8 lag to illustrate the effect of power factor on the ratio. It is seen that only one-half the amount of copper is required for the same power transmission at unity power factor, and less than one-third is required at the power factor of 0.8 lag. Ad Aa = cos 3 2 2 = (b) Use of Ground Return Possible In the case of hvdc transmission, ground return (especially submarine crossing) may be used, as in the case of a monopolar d.c. link. Also the single circuit bipolar d.c. link is more reliable, than the corresponding a.c. link, as in the event of a fault on one conductor, the other conductor can continue to operate at reduced power with ground return. For the same length of transmission, the impedance of the ground path is much less for d.c. than for the corresponding a.c. because d.c. spreads over a much larger width and depth. In fact, in the case of d.c. the ground path resistance is almost entirely dependant on the earth electrode resistance at the two ends of the line, rather than on the line length. However it must be borne in mind that ground return has the following disadvantages. The ground currents cause electrolytic corrosion of buried metals, interfere with the operation of signalling and ships' compasses, and can cause dangerous step and touch potentials. (c) Smaller Tower Size The d.c. insulation level for the same power transmission is likely to be lower than the corresponding a.c. level. Also the d.c. line will only need two conductors whereas three conductors (if not six to obtain the same reliability) are required for a.c. Thus both electrical and mechanical considerations dictate a smaller tower. (d) Higher Capacity available for cables In contrast to the overhead line, in the cable breakdown occurs by puncture and not by external flashover. Mainly due to the absence of ionic motion, the working stress of the d.c. cable insulation may be 3 to 4 times higher than under a.c.