CONVERTION HVAC INTO HVDC OF POWER TRANSMISSION LINES WITH USING VOLTAGE SOURCE CONVERTER

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1 No. E-14-PTL-1646 CONVERTION HVAC INTO HVDC OF POWER TRANSMISSION LINES WITH USING VOLTAGE SOURCE CONVERTER Aziz Aghazadeh Electrical Power Distribution of Tehran Province Company TVEDC Tehran, Islamic Republic of Iran Abstract The conversion of existing AC lines into DC lines represents an interesting alternative for upgrading the power carrying capability for the existing rights of way and at the same time obtaining other added values. This way, these conversions can contribute to the creation of the HVDC Super Grid, which is one possible option to integrate massive renewable energy sources. With this objective, it is necessary to adapt the AC lines to the requirements of DC. The conversion can be carried out changing the head of the line towers or maintaining the towers structure and just making the minimum changes. This paper presents an analysis of the different configurations available for converting AC lines into DC lines and also includes a new proposal for configuration. Finally, a comparison of the presented configurations, in terms of power increase and losses is included. Keywords Conversion; Distribution lines; HVAC; HVDC; Voltage Source Converter I. INTRODUCTION Building a new transmission line in industrialized countries is a difficult process that takes a long time because the necessary permits can be delayed during an uncertain period. Last years, the environmental requirements have made this process even more difficult and expensive. Moreover, sometimes it is just impossible to build a new transmission line. This fact can cause difficulties, such as meeting peak demand. Also, problems can appear to integrate renewable energy sources in power systems [1,2]. Consequently, nowadays there is a major interest in achieving a better use of the existing infrastructures. With that objective, there are different alternatives for upgrading the transmission capacity of the existing lines. Those upgrading alternatives include increase of ampacity, which has been traditionally achieved by means of increasing the conductor section or the number of conductors per phase [3], rated voltage level increase [4,5] and conductor replacement with better performance conductors, such as High Temperature Low Sag (HTLS) conductors [5 7]. There are other techniques, for example, line current maximization for a safe operation, which include deterministic methods, probabilistic methods or real time monitoring [8,9]. Additionally, High Surge Impedance Loading (HSIL) can be applied to transmission lines by means of electric fields optimization and bundle expansion techniques [10]. Finally, it is also possible to use AC lines to transmit DC power [11 19]. The conversion of AC lines into DC ones can be considered as a solution to bottlenecks, by means of upgrading the transmission capacity of the line. Moreover, this option also provides added values that distinguish it from other methods [20,21]. Among them, stability and controllability can be mentioned as the most outstanding ones [22]. Thus, depending on the characteristics of the original line, this upgrading method can result in an attractive option, especially when there are other limiting factors. Besides, the properties of the converted DC lines fulfill the requirements of HVDC grids [23]. Therefore, the proposed conversions can constitute some of the preliminary steps for creating the HVDC Super- Grid which is intended to cover several countries. However, when an AC line is converted into DC, the AC line must be adapted to the requirements of DC and converter stations must be installed at both sides of the line. The cost of a converter station increases with the power square root, therefore, conversion of lines with lower power ratings will have lower costs. In the case of distribution lines, the voltage level is lower than in long transmission lines, lowering the cost of electronic devices. Thus, the cost of the distribution lines conversion will be lower than the cost of the transmission lines conversion [11 19]. For that reason, even though the conversion of existing AC lines into DC lines can be applied to any voltage level, this paper is focused on distribution lines. To this extent, considering the option of converting AC lines into DC lines, the paper starts with a review of the converter stations technologies. Following, the possible configurations for performing the conversion of AC lines into DC lines are identified for VSC transmission, related to the original AC line type. Among these possible options, a new proposal of configuration is included. Finally, the aforementioned configurations are compared in terms of power increase and losses. II. HVDC TECHNOLOGY High Voltage Direct Current (HVDC) links have been used for more than 50 years. The first commercial HVDC system, based on mercury valves, went into service in 1954 between the island of Gotland and Sweden. From that moment on, HVDC technology has suffered major evolutions, 1

2 particularly regarding electronic devices and control systems. As power electronics were developed, mercury valves gave way to thyristors and lately to IGBTs (Insulated Gate Bipolar Transistors), yielding to new technologies. Currently, HVDC can be considered a mature technology with numerous installations spread all over the world. In this section, the most relevant characteristics of the two main HVDC technologies are reviewed and compared. A. LCC technology Classic HVDC systems are Current Source Converters (CSC), in which DC current is constant (Fig. 1). LCC (Line Commutated Converter) systems are based on thyristors that offer a high power transmission capacity, being an economic alternative for the highest power transmissions [24]. As example, the biggest HVDC installation up to now using LCC, Jinping Sunan (China), presents the following parameters: 7200 MW, 800 kv and 2090 km [25]. The main characteristics of LCC technology are: The firing angle of thyristors can be selected during direct polarization, but the turn-off is only achieved when the device is inversely polarized, making their application range limited. Active power is controlled at will, but reactive power depends on the active power. Consequently, big capacitor banks are necessary and bulky filters are also required to attenuate the low order harmonics produced by the converter. It is possible to reverse the power flow in the DC link between two LCC converter stations. For performing this reversal, the DC voltage polarity must be inverted because thyristors are capable of withstanding voltage in both directions, but they are only able to conduct current in one direction [24]. Due to their characteristics, LCC converters cannot provide power to a system with no local generation [24], requiring a high enough short-circuit power source. _ Finally, commutations depend on the AC voltage provided to the converter, since LCC converters require a relatively strong synchronous voltage to perform commutations. As a result, one of the main disadvantages of LCC technology is the commutation failure risk. These failures are usually caused by AC faults and can produce lack of power transmission for several cycles. AC line connection point (voltage variations stabilization, voltage fluctuations mitigation, etc.). IGBTs exhibit a high commutation frequency, in the khz range. For that reason low order harmonics are reduced and the filters size is relatively small comparing with LCC. Other advantages, when compared with LCC are: the lack of commutation failure risk, black-start capability, no need for short-circuits ratio and smaller converter station size. VSC is also the most appropriate technology to implement multiterminal systems, because of their control capabilities [23,32]. The power flow can be reversed, with no need of reversing voltage polarity, as in LCC technology [23]. These properties make VSC transmissions suitable for several applications. For instance, it is possible to evacuate the energy from wind farms with no need of additional compensation. Even if the AC grid is weak in the connection point, there is no need of improving short-circuit power. Moreover, VSC-HVDC systems, on account of their flexibility, are the most adequate ones for future DC meshed grids development, which will improve the power system reliability. On the other hand, the numerous commutations cause higher total power losses than LCC converters. Also, problems related with semiconductor voltage withstand arise when VSCs are used in HVDC transmission systems. However, different converter structures using multilevel converters have been proposed to overcome such problems and to improve power grid quality [29]. Finally, VSC converters cannot extinguish DC fault currents by themselves. Trying to overcome these disadvantages, recently, Modular Multilevel Converters (MMCs) have been developed. This converters exhibit some promising features, such as the almost-sinusoidal voltage wave formed with lower commutation frequency. Consequently the power losses are smaller and the required filters are also smaller. Furthermore, the performance of the VSC converters against DC line faults is improved. Additionally, it must be pointed out that VSC is a continuously developing technology and thus, the drawbacks of VSC converters are being progressively reduced. Finally, in some cases, VSC can result in a cost effective solution since the drawbacks of LCC require relatively costly resources to overcome. Fig. 1. HVDC transmission based on LCC converters. B. VSC technology Nowadays VSC technology (Voltage Source Converters) is available for medium scale power transmission applications, in consequence, it fits entirely with distribution lines. VSC technology is based on IGBTs (Fig. 2), which can control both turn-on and turnoff. Accordingly, using VSC technology it is possible to control independently active and reactive power [26 31]. Furthermore, power quality can be improved in the III. CONVERSIONS OF AC LINES INTO DC LINES WITH TOWER HEAD MODIFICATIONS In conversions of AC lines into DC lines with tower head modifications, the existing tower head is changed to be adapted to the traditional HVDC configuration [11 16]. Fig. 2. HVDC transmission based on VSC converters. Nevertheless, the tower general structure is maintained and, consequently, the implied changes are not excessive. As 2

3 the required modifications in the towers imply new head geometry, new cross arms must replace the old ones and conductor configurations must be modified. Also, insulators must be replaced. Considering the required changes, the mechanical strength of the new design must be recalculated and, if necessary, the towers must be reinforced. When considering simple circuit AC lines, conversion into monopolar lines is the most feasible option [11 17]. Nevertheless, monopolar lines with electrode return can generate problems. They are attributed interferences and corrosion effects, especially when distribution lines cross gas pipes and telecommunication cables. Besides, there must be a good ground conductivity to obtain a good system operation and to avoid ground return problems. But this requirement is not always fulfilled. For example, in [17] problems arose because of a geological fault whose existence was unknown. To overcome this problem, metallic return conductors can be considered as an alternative to electrodes return path, although this option increases the cost and losses. Summarizing, it is possible to obtain a big power upgrading, recycling the existing AC line into DC, with tower modifications. However the implied modifications can be substantial. Next section analyses the conversions that achieve similar power increase with simpler modifications. alternative presents the complication of adapting the existing three or six AC conductors to the two DC poles. Following, the different available configurations for converting simple and double circuit AC lines, with no tower modifications, are presented and compared by means of transmitted power and redundancy. Also, a new proposal of configuration is included. Redundancy for a conductor loss is calculated as indicated in(1), assuming 15% of temporary overload capacity. This concept prevents the converted line from becoming a limiting outage case where a considerable increase in the transmission capacity is developed. where R C-L is the redundancy for a conductor loss, P C.out is the maximum power with one conductor out and P is the total power. A. Conversion of simple circuit AC lines Simple circuit AC lines have three conductors. Consequently, it is compulsory to adapt the existing towers designed for three phase lines, to the two-poles DC line. The different configurations that make possible this adaptation are analyzed in this subsection. (1) Fig. 3. Conversion of AC line substituting the tower head (a) double circuit AC line and (b) bipolar DC line. IV. CONVERSIONS OF AC LINES INTO DC LINES WITH NO TOWER HEAD MODIFICATIONS Conversions with no tower head modifications imply that the original tower structure is maintained and only the necessary modifications to adapt the existing line to the new requirements are made. Consequently, the investment is lower than in the previous case. This way, insulators may have to be replaced depending on the new DC voltage and fittings for conductors should be revised and changed if necessary. But it is not compulsory to change the conductors. However, the power increase can be greater if the conductors are changed. For instance, sub-conductors can be added or a new conductor with a bigger section can replace the old one. An attractive option is to use high temperature low sag conductors [16]. This paper is focused on converting AC lines, which are provided with three phase circuits, into DC lines using the existing towers. Although this option is cheaper, this 1) Simple circuit AC line converted into bipolar DC line A conductor per pole is used, whilst the third conductor is free(fig. 4a). The free conductor can work as a return conductor in an emergency case. Therefore, the redundancy in case of conductor failure is 100%. But if a circuit is lost, this configuration cannot continue transmitting power. This is a simple and economical configuration but its main drawback is that the transmitted power is reduced, due to the lower number of conductors. The power transmitted by the DC bipolar line is shown in (2). where P b is the power transmitted by the DC bipolar line, UPG is the pole to ground voltage and I is the rated current of pole 1 and pole 2. The simple circuit AC line can also be converted into a bipolar DC line with dedicated metallic return path. In this configuration, the three conductors are considered, in such a way that a conductor per pole is used, with positive and negative polarity and the third one is neutral (Fig. 4b). If necessary, an asymmetrical operation is feasible, with positive and negative asymmetrical voltages. Fig. 4. VSC-HVDC bipolar configuration (a) free middle conductor and (b) using middle conductor. (2) 3

4 Fig. 7. VSC-HVDC modulated bipolar configuration. Fig. 5. VSC-HVDC tripolar configuration The main advantage of the second configuration is the possibility of getting 57% of redundancy when a circuit is lost, as shown in (3) (assuming 15% of overload). This possibility exists because when a circuit is lost, the other circuit can keep operating. The power increase available is similar to VSC conventional configuration (Fig. 4a). 2) Simple circuit AC line converted into tripolar DC line The tripole is composed of the two poles of a conventional VSCHVDC system (which have opposite polarities) and a third pole that changes polarity, as shown in Fig. 5. Pole 3 is formed by a monopolar system based on thyristors, which has an extra valve that provides capacity to reverse current and voltage. Although tripoles were initially designed for LCC systems, they are also possible with VSC technology. The operation of the three poles together is similar to a bipolar operation, where pole 1 is positive, pole 2 is negative and pole 3 shares alternatively positive and negative current with poles 1 and 2, as shown in Fig. 6. These poles change periodically high current states over the conductors thermal limit, when they are consequently overheated, and low current states, when they are cooled. Pole 3 shares current with the low current pole and inverts the polarity periodically. If the length of those periods oscillates between four and five minutes, the conductors overheating will be similar to that of a day with wind or clouds changes. A characteristic of this configuration is that all conductors use the full thermal capacity. The power upgrading that can be achieved is about 1.37 times higher than the bipolar one (4) and the redundancy is 84% (5), being these figures similar to the ones of conventional LCC tripoles. (3) where P DC is the power transmitted by the DC line, 1.37I is the current of pole 1 at a particular instant, 0.37I is the minimum current of pole 2 at a particular instant and I is the rated current of pole 3. 3) Simple circuit AC line converted into modulated bipolar DC line This configuration uses one conductor per positive and negative pole and the third conductor changes, alternatively, positive and negative polarity. The operation of this new proposed configuration is similar to the tripole s, but instead of building a third pole based on thyristors, the two poles of the VSC-HVDC system are commutated. Positive and negative poles are placed in the outermost conductors and the polarity of the central conductor is commutated with electronic switches (IGBTs), as shown in Fig. 7. It is possible to switch the central conductor at the beginning and at the end of the line, but it is not necessary that the electronic device at the end of the line presents turn off capacity. For that reason, a thyristor can be used (Fig. 7). Fig. 8a shows the bipolar with one sending conductor, which withstands a higher current than the rated current and is, therefore, overheated. The other two conductors share the return current. Consequently, the current in these two conductors is lower than the rated value and they are cooled. This situation is maintained for some determined time, until IGBTs change their state. Fig. 8b shows two sending conductors and one return conductor. Fig. 9 shows the operation of the modulated bipolar, by means of the three poles current. It can be seen that poles 1 and 3 sequentially the high and low current state, whilst pole 2 changes polarity. (5) Fig. 6. Current diagram for a tripolar system. Fig. 8. Modulated bipolar system operation (a) one sending and two return conductors and (b) two sending and one return conductors. 4

5 Fig. 9. Current diagram for a modulated bipolar system. Therefore, the current level of one conductor can be over the rating of the line conductor for a certain period of time. Providing that conductor s rms current is 1,0 p.u. of continuous rating and assuming the same duration for all periods, the maximum current of poles 1 and 3, Imax, and the minimum current of poles 1 and 3, Imin, are determined by (6), (7). (6) B. Conversion of double circuit AC lines Double circuit lines have an even number of conductors, accordingly, it is easier to adapt these AC lines to the requirements of DC lines. A double circuit AC line can be converted into different bipolar DC lines, as shown in Fig ) Double circuit AC line converted into three bipoles DC line The double circuit AC line is converted into three bipoles, where each bipole is composed of two conductors, one with positive polarity and the other with negative polarity. Consequently, there are three positive poles and three negative poles, as shown in Fig. 10a. The three converters are connected on the AC side, to the same AC system. On the DC side, each pole is connected to a conductor of the original AC line. This way, one circuit of the original AC line, comprises the positive pole whilst the other circuit comprises the negative pole. This option has the added cost of the three bipoles (Fig. 11) but presents the same redundancy in case that a circuit or a conductor is lost. The transmitted power and redundancy are shown in (10) and(11). Eq. (8) shows the power that can be managed by the modulated bipole. where 1.26I is the current of pole 1 at a particular instant, and 0.63I is the current of pole 2 and pole 3 at a particular instant. Consequently, the power that can be transmitted is 1.26 times higher than the bipolar one. Besides, when a conductor is lost, 91% of the power can be restored, assuming 15% of overload (9). But in the case that a converter is lost, there is no redundancy. (7) (8) (9) (10) (11) 2) Double circuit AC line converted into a bipolar DC line The double circuit AC line is converted into one bipole. Accordingly, in this option, there is only one converter with two poles, where each DC pole is divided into three conductors, as shown in Figs. 10b and 12. Each pole is connected to one circuit of the original AC line. When there is a failure in a conductor, it is possible to disconnect one conductor of the opposite polarity and continue transmitting power. The transmitted power and redundancy for one conductor loss is similar to the previous configuration, but in case that a complete circuit is lost, there is no redundancy. 3) Double circuit AC line converted into two modulated bipolar DC lines Each AC circuit is converted into a modulated bipole similar to the aforementioned (Fig. 10c), where positive and negative poles are located at the extremes of the line, whereas the central conductors change alternatively positive and negative polarities. The power increase is smaller than the previous two options because the central conductors are not working at full capacity. However, the cost of this configuration is lower than the three bipoles and has 57% of redundancy, in case that one circuit is lost. Fig. 10. Conversion of a double circuit AC line into a DC line formed by (a) three bipoles, (b) one bipole and (c) two modulated bipoles. V. POWER UPGRADING OF CONVERTED DC LINES In this section, a comparative analysis of the transmission 5

6 upgrading when converting AC into DC lines is presented, for the configurations defined in the previous section. A. General aspects It is not easy to compare the transmission capacity of AC and DC lines. For AC lines, the power carrying capability depends on thermal limits, reactive power necessities and stability. In the case of DC lines, this capacity depends mainly on the thermal limit and it is proportional to the DC voltage and DC current. Fig. 11. Conversion of a double circuit AC line into a DC line with three bipoles. must also be considered: current, frequency, proximity effect, transformer effect, etc. There are experimental data that show the AC resistance of several conventional conductors, as well as calculation methods that consider all the related factors. Finally, for a complete comparison, it is not enough to consider the power losses produced in the conductors. AC substations and DC converter station losses must also be taken into account. AC substation losses are estimated approximately around 0.3%. Converter station losses for twolevel bridge VSC converters are about 3% and for three-level bridges vary between 1.6% and 1.8 %. In this paper, the considered voltages for AC original lines are 30, 45 and 66 kv. In these voltage levels there are several saturated lines which are not excessively branched, and could be appropriate for the proposed conversions. In this section, a graphical and numerical analysis of the power increase that can be obtained, by converting an existing AC line into a DC line, is presented. The maximum transmitted power of the original AC line can be calculated by substituting the line parameters in (13) and (14). Eq. (13) shows the maximum transmitted power limited by the maximum current. (13) And (11) shows the maximum power per circuit, according to the length and voltage drop. Fig. 12. Conversion of a double circuit AC line into one bipolar DC line. If the conductors are not changed in the conversion, the current, which depends on the maximum permissible current density, will remain constant. Nevertheless, current can be even slightly bigger because Also, current can increase considerably if the original AC line is limited of the lower resistance of conductors when DC is used. by certain constrictions or if the conductors are changed. Additionally, the upgrading level that can be obtained depends also on the adopted DC voltage. The higher the DC voltage, the higher the upgrading will be. But, increasing the DC voltage implies a revision of insulators, electric clearances and voltage gradient on conductor s surface. On the other hand, losses must also be taken into account to make an appropriate analysis of the power enhancement [42]. In distribution lines, variable power losses are two or three times bigger than fixed power losses. The most relevant ones for distribution lines are those produced due to Joule effect. Therefore, as this paper is focused on distribution lines, only Joule effect will be considered, which is shown in (12). (12) where P Joule is the Joule effect loss, I is the rated current and R is the conductor resistance per length unit To evaluate correctly their effect, it is necessary to analyze the conductors AC resistance and the relation with DC resistance. DC resistance depends on the resistivity, the conductor s cross-section and the temperature, as resistivity varies with temperature. For AC resistance, other parameters (14) where P AC is the transmitted power by the original AC line, U is the line voltage, Imax is the maximum withstand current for steady state, DU is the voltage drop expressed in % of the composed voltage, R is the phase resistance per length unit, X is the reactance per length unit and L is the line length. On the other hand, the power transmitted by DC lines in each configuration is defined in Section 4. Therefore, the power increase of the conversions and the Joule losses are shown in (15) and (16). (15) (16) The inclusion of too many variables minimizes clarity. Therefore, for each case, three configurations and two or three DC voltage levels have been considered. DC voltage levels have been selected to be similar to the rated AC voltage levels. Only one conductor has been considered, 147-AL1/34-ST1A, which is not changed in the conversion. For simple circuit AC lines the considered configurations are bipolar, tripolar and modulated bipolar. On the other hand, double circuit AC lines are converted into one bipole, three bipoles and two modulated bipoles. As example, the transmitted power of an original simple circuit AC line of 30 kv and the corresponding DC converted line are shown graphically in Fig. 13, for different 6

7 configurations and DC voltages. The x-axis represents the line length, in km. VI. CONCLUSIONS The incorporation of DC lines into the power system, including the conversion of strategic AC lines that are bottlenecks for the system, increase the reliability and stability. In this way, the dominant AC infrastructures are used to transmit in DC taking advantage of the added values of HVDC technology. One possible application of the proposed conversion is contributing to the development of the HVDC Super Grid. The increasing power demand, combined with structural changes in energy markets, requires the use of versatile power electronics. Therefore, VSC technology is considered the most adequate for converter stations when performing the conversions. These conversions do not involve excessive changes in the towers, so the reconstruction time is not too long. Additionally, if only the necessary changes to adapt the AC lines to DC are made, the costs are lower and the required time is smaller. In this paper, the possible configurations for converting AC into DC lines are analyzed, including a new proposed configuration, more adequate for converting simple circuit lines, which is denominated modulated bipolar configuration. This new topology requires simple modifications of the traditional converter stations and presents a significant power increase. Additionally, the power increase that can be achieved by each configuration has been analyzed and compared. Thus, it can be concluded that the conversion of existing distribution and transmission AC lines into DC lines provide a notable power enhancement. References [1] Xydis G. Comparison study between a renewable energy supply system and a supergrid for achieving 100% from renewable energy sources in Islands. Int J Electr Power Energy Syst 2013;46: [2] Xu M, Zhuan X. 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