11 High Voltage Direct Current Transmission 11.0 Historical Background Power Transmission was initially carried out in the early 1880s using Direct Current (d.c.). With the availability of transformers (for stepping up the voltage for transmission over long distances and for stepping down the voltage for safe use), the development of robust induction motor (to serve the users of rotary power), the availability of the superior synchronous generator, and the facilities of converting a.c. to d.c. when required, a.c. gradually replaced d.c. However in 1928, arising out of the introduction of grid control to the mercury vapour rectifier around 1903, electronic devices began to show real prospects for high voltage direct current (HVDC) transmission, because of the ability of these devices for rectification and inversion. The most significant contribution to HVDC came when the Gotland Scheme in Sweden was commissioned in 1954 to be the World's first commercial HVDC transmission system. This was capable of transmitting 20 MW of power at a voltage of -100 kV and consisted of a single 96 km cable with sea return. With the fast development of converters (rectifiers and inverters) at higher voltages and larger currents, d.c. transmission has become a major factor in the planning of the power transmission. In the beginning all HVDC schemes used mercury arc valves, invariably single phase in construction, in contrast to the low voltage polyphase units used for industrial application. About 1960 control electrodes were added to silicon diodes, giving silicon-controlled-rectifiers (SCRs or Thyristors). In 1961 the cross channel link between England and France was put into operation. The a.c. systems were connected by two single conductor submarine cables (64km) at ± 100kV with two bridges each rated at 80 MW. The mid-point of the converters was grounded at one terminal only so as not to permit ground currents to flow. Sea return was not used because of its effect on the navigation of ships using compasses. The link is an asynchronous link between the two systems with the same nominal frequency (60Hz). The Sakuma Frequency Changer which was put into operation in 1965, interconnects the 50Hz and the 60Hz systems of Japan. It is the first d.c. link of zero length, and is confined to a single station. It is capable of transmitting 300 MW in either direction at a voltage of 250 kV. In 1968 the Vancouver Island scheme was operated at +250 kV to supply 300 MW and is the first d.c. link operating in parallel with an a.c. link. In 1970 a solid state addition (Thyristors) was made to the Gotland scheme with a rating of 30MW at 150kV. Also in 1970 the Kingsnorth scheme in England was operated on an experimental basis. In this scheme transmission of power by underground d.c. cable at ± 200 kV, 640 MW is used to reinforce the a.c. system without increasing the interrupting duty of a.c. circuit breakers. The first converter station using exclusively Thyristors was the Eel River scheme in Canada. Commissioned in 1972, it supplies 320 MW at 80 kV d.c. The link is of zero length and connects two a.c. systems of the same nominal frequency (60Hz).
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 GDWSI XQLW\ GDWSI 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.
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 GDWSI XQLW\ GDWSI 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.
High Voltage Direct Current Transmission 187 Also, the absence of continuous charging current in a d.c. cable permits higher active power transfer, especially over long lengths. (Charging current of the order of 6 A/km for 132 kV). Critical length at 132 kV ≈ 80 km for a.c cable. Beyond the critical length no power can be transmitted without series compensation in a.c. lines. Thus derating which is required in a.c. cables, thus does not limit the length of transmission in d.c. A comparison made between d.c. and a.c. for the transmission of about 1550 MVA is as follows. Six number a.c. 275 kV cables, in two groups of 3 cables in horizontal formation, require a total trench width of 5.2 m, whereas for two number d.c. ±500 kV cables with the same capacity require only a trench width of about 0.7 m. (e) No skin effect Under a.c. conditions, the current is not uniformly distributed over the cross section of the conductor. The current density is higher in the outer region (skin effect) and result in under utilisation of the conductor crosssection. Skin effect under conditions of smooth d.c. is completely absent and hence there is a uniform current in the conductor, and the conductor metal is better utilised. (f) Less corona and radio interference Since corona loss increases with frequency (in fact it is known to be proportional to f+25), for a given conductor diameter and applied voltage, there is much lower corona loss and hence more importantly less radio interference with d.c. Due to this bundle conductors become unnecessary and hence give a substantial saving in line costs. [Tests have also shown that bundle conductors would anyway not offer a significant advantage for d.c as the lower reactance effect so beneficial for a.c is not applicable for d.c.] (g) No Stability Problem The d.c. link is an asynchronous link and hence any a.c. supplied through converters or d.c. generation do not have to be synchronised with the link. Hence the length of d.c. link is not governed by stability. In a.c. links the phase angle between sending end and receiving end should not exceed 30o at full-load for o transient stability (maximum theoretical steady state limit is 90 ). Note: 5 5 0 θ = w √OFSHUNP [[ ) rad/km ≈ (2 x 180 x 50)/(3 x 10 ) ≈ 0.06 /km o The phase angle change at the natural load of a line is thus 0.6 per 10 km. The maximum permissible length without compensation ≈ 30/0.06 = 500 km With compensation, this length can be doubled to 1000 km. (h) Asynchronous interconnection possible With a.c. links, interconnections between power systems must be synchronous. Thus different frequency systems cannot be interconnected. Such systems can be easily interconnected through hvdc links. For different frequency interconnections both convertors can be confined to the same station. In addition, different power authorities may need to maintain different tolerances on their supplies, even though nominally of the same frequency. This option is not available with a.c. With d.c. there is no such problem.