Evaluation of High Surge Impedance Loading (HSIL) solutions for increased natural capacity of 500 kV Overhead Lines. Study Committee B2 Working Group WG B2.38 Members Angel Gallego Del Monte, Danna Liebhaber, Hee-Sung Ahn, Jarlath Doyle, José Manuel Frenandez Davila, Oswaldo Junior Regis, Paul Penserini, Tsinghua University, Vivendhra Naidoo. Abstract The third generation of 500 kV lines were developed after several studies of High Surge Impedance Loading - HSIL, for a new level of 1200MW, applying the concept of compaction of the distance between phases, or the concept of expanded bundles, or a mix of both concepts. This report compares six different 500kV Transmission Lines Projects, each with four subconductors per phase and Surge Impedance Loading (natural capacity) of 1200 MW, which are currently operating in more than 10.000 km of extension in Brazil. Transmission Line compaction and the use of expanded bundle techniques are analyzed focusing on electrical coupling and its influence on positive sequence and zero sequence impedance. The electric field on the conductors’ surface and the electric and magnetic fields at ground level are evaluated for each configuration. 2. Theoretical aspects The basic theory of optimization [1] of HSIL shows that, for the same voltage level, the equalization and maximization of the electric field on the sub-conductor’s surface, and/or an increase of the number of subconductors per phase, increases the Surge Impedance Loading – SIL of the lines. 1. Introduction Considering the impedance theory, it is also possible to show that the Natural Capacity of transmission increases as the line phases are compacted [2] (i.e. the distance between the phases is reduced), and as the bundle dimensions are expanded [3] (i.e. the distance between sub conductors of a phase is increased). The Surge Impedance Loading (SIL), also called Natural Capacity, of an alternating current (AC) transmission line can be defined as the active power that, being transported through the line, causes a consumption of reactive power equal to the value of the reactive power generated by the capacitance of the same line. Power flows below the SIL produce minimum voltage drop and stability problems on the line. 2.1 Diagram of a transmission line The diagram below represents a transmission line in a system study, where P is the transmitted power and Vs and Vr are the sending and receiving end voltages. The parameters of the line are: C (the shunt capacitance) that provides reactive power and Z (the series impedance). The series inductance X consumes reactive power and is responsible for the drop of the voltage and for stability problems on the long distance transmission. The line’s thermal capacity, which concerns premature aging of conductors and connectors as well as maintaining minimum electrical safety clearances, is generally higher than the natural capacity and is not covered here. The systems analysis view is especially important for long distance interconnections where increased Surge Impedance Loading (SIL) of the lines can reduce or even remove restrictions on power flow. In Brazil, the first generation of 500kV lines had a 3-conductor bundle per phase and a SIL of 900 MW. The second generation of 500 kV lines had 4-conductor bundles per phase and a SIL of 1000 MW. The need for long lines, mainly interconnections between the north, northeast and southeast regions, has motivated the development of lines with higher transmission capacity. Diagram : Schematic representation of a Transmission Line in electric systems studies Cigre Science & Engineering • N°2 June 2015 63 Figure 1 – Racket Tower; the drawings distances and heights are in meters Figure 2 – Cross-Rope Tower (Chainette) Figure 3 – VX-Asymmetrical Tower self impedance and the positive sequence impedance, Zl, increasing the capacity of the transmission line. For steady state and transient studies, the TL is represented by its positive and zero sequence constants. In the load-flow studies that analyze the voltage control in the system, only the positive sequence constants are used. Therefore, a line that has smaller positive sequence impedance per unit length (Z1) has higher capacity of transmission for the same length reference. The mutual impedance (Zm) depends on the distance between the phases. As the distance between the phases decreases, the higher will be the line’s mutual impedance. Therefore, the impedance Z1 will again be smaller cause Zm has a negative signal in relation (Z1 = Zs - Zm), increasing the capacity of power transmission. 2.2 Analisys of self and mutual impedance, and zero and positive sequence These two techniques, compaction of phase spacing and expansion of the bundles, were used in different projects to increase the Natural Power (SIL) of 500 kV interconnection lines in Brazil, as shown in the next topic. The positive sequence impedance Z1 of a line can be obtained from their self and mutual impedances (Zs and Zm) by the relation Z1= Zs-Zm. The self impedance (Zs) depends on the conductor, but mainly depends on the geometry of the bundle of each phase. The higher the bundle dimension, the smaller the While the reduction in phase-phase spacing and the increase in bundle diameter increases the Natural Power Cigre Science & Engineering • N°2 June 2015 64 Figure 4 – VX-Symmetrical Tower Figure 5 – Cat Face Tower Figure 6 – Monopole tower Distances and heights are in meters. In all cases, the phase bundles consist of 4 Rail, 954MCM sub-conductors, but with different bundle diameters and different distances between the phases, in order to obtain a Natural Power of 1200MW. on the other hand, the influence of those variations of the self and mutual impedances (Zs and Zm) in the impedance of zero sequence (Zo) shows a different effect as the relation is Zo = Zs + 2 x Zm. Differently from de effect on Z1, the application of compaction or expansion techniques of the bundles results in variations of the impedance Zo in both ways, increasing or decreasing , depending on the case. The following item (7) shows the Table II with the parameters of sequence and the analysis of their variations. 3.1 Racket tower The Racket Tower is aself-supporting structure with square conventional bundles, 18 inches (0,457m) on each side. The distance between the phases is very reduced (bigger Zm) in a triangular arrangement called Compact Tower. This leads to an increased SIL. 3. Towers and arrangements analyzed 3.2 Cross-rope tower (chainette) Pictures of Towers and drawings of phase’s bundles showing their sub-conductors are presented below. The Cross-Rope Tower is supported by two guyed poles, with the electric phases between them. The bundles are Cigre Science & Engineering • N°2 June 2015 65 Figure 7 – Electric superficial gradients in 500kV guyed “V” supports, with horizontal phases having structure masts between them. In order to obtain a higher SIL, a regular square expanded bundle is used (lower Zs), 1,20m/side. This allows the use of standard hardware which the previous design did not. On the other hand, as it is shown later, the electric field is higher in some sub-conductors, since it is not the optimized electric distribution. conventional (square with 0,457 meters), with a nearly flat arrangement of phases and with very short distances between them, which gives a higher SIL due to the compaction. 3.3 VX-ASYMMETRICAL (V guyed tower with Asymmetrical Expanded Bundle) The VX-Asymmetrical Tower has a guyed “V” support for the center phase, with phases on the same level and structure masts between them, which prevent compaction. To obtain a higher SIL an asymmetrical expanded bundle (lower Zs) which optimizes the electric fields on each sub-conductor’s surface. 3.5 Cat face tower The Cat Face Tower is self-supported with metal parts between phases. This limits its compaction. In order to obtain higher SIL, a regular expanded bundle was used (lower Zs) with a square form, 1,20m/side. The phases are arranged in a triangle. This provides a more equalized distribution of electric fields in the sub-conductors. 3.4 VX-SYMMETRICAL (V guyed tower with Symmetrical Expanded Bundle) The VX-Symmetric Tower has also its conception in Electric field kV/cm Cross Rope Racket VX Symmetric Monopole VX asymmetric Cat face Maximum value 18.00 17.76 17.02 16.88 16.05 16.36 Minimum value 15.04 15.00 14.69 14.52 14.67 14.47 Max / Min (%) 19.7 18.4 15.9 16.3 9.4 13.1 Average - phase A 15.7 15.7 15.3 15.1 15.1 15.0 Average - phase B 17.9 17.3 17.0 16.5 16.0 16.0 Average - phase C 15.7 15.7 15.3 15.1 15.1 15.0 Average - General 16.4 16.2 15.9 15.6 15.4 15.3 Table I: Minimum, Maximum and Average values of electric field on conductor’s surface. Cigre Science & Engineering • N°2 June 2015 66 Figure 8 – Graphic with the audible noise profiles (AN) Figure 9 – Graphic with radio interference profiles (RI) 3.6 Monopole tower The Table I below presents, for each configuration, the values of maximum and minimum gradients, and the ratio of these values. It also presents the average of the gradients on sub-conductors of each phase, and the general average of all sub-conductors of each line type. Notice that even though the VX asymmetric tower has the lowest maximum gradient value, the Cat face tower presents a general average which is slightly lower. The Monopole Tower uses guyed supports with a single lattice pole and structural parts between phases. Nevertheless, the spacing between phases has been minimized for the triangular phase arrangement. To produce a natural capacity of 1200 MW, an expanded square bundle, 0.90m/side is used. In this case, the SIL gain was reached as a combination of a light compaction (higher Zm) and the use of a “semi-expanded” bundle (lower Zs). 5. Comparative analysis of audible noise (an) and radio interference (ri) 4 Comparative analyses of the superficial electric fields The following graphs, the electric field on each subconductor’s surface, are calculated for a 500 kV line. It is important to note that when these fields are higher, they result in more corona activity and, hence, in higher levels of audible noise (AN) and radio interference (RI) along the right of way (r.o.w.) and nearby. The Figure 8 shows the audible noise (AN) profile of all lines analyzed. It is noticeable that the values of the AN profile decrease proportionally to the gradients average of all the sub-conductors, starting with the highest value of the Cross-Rope, to the smallest value of the Cat Face. The AN phenomenon generated by the transmission lines decreases slowly with distance outside of the line corridor. Figure 7 is arranged in descending order for the highest superficial electric field (gradient) calculated with each design configuration. Figure 10 – Graphic with the electric field profiles Figure 11 – Graphic with the magnetic field profiles Cigre Science & Engineering • N°2 June 2015 67 500 kV TL Parameters R1 (Ω/km) Positive Sequence Zero Sequence Ratio VX asymmetric VX Symmetric Cat face Racket Cross Rope Monopole 0.0171 0.0174 0.0175 0.0173 0.0173 0.0176 X1 (Ω/km) 0.269 0.261 0.268 0.267 0.268 0.265 C1 (nF/km) 16.57 17.07 16.56 16.29 16.36 16.63 SIL (MW) 1205 1240 1205 1199 1196 1214 R0 (Ω/km) 0.349 0.349 0.351 0.373 0.346 0.369 X0 (Ω/km) 1.336 1.342 1.329 1.478 1.496 1.361 C0 (nF/km) 9.840 9.764 9.708 7.620 7.251 9.347 X0/X1 4.98 5.14 4.96 5.55 5.59 5.13 C1/C0 1.68 1.75 1.71 2.14 2.26 1.78 Table II: Positive and Zero Sequence parameters of 500kV TLs The Figure 9 shows the Radio Interference profile (RI) for the lines analyzed. The phenomenon of RI generated by the transmission lines decreases more rapidly with distance than Audible Noise. r.o.w. The other towers have a higher value on the axis, with variations in the intermediate area, and significant reduction to the border of r.o.w. These calculations show that the triangular phase arrangements generate higher RI values under the line but lower levels at the edge of right-of-way. 35 meters from the axis, the Racket, Monopole and Cat Face designs have a RI value smaller than the others. The VX Asymmetric design has smaller RI values than the Racket in the middle area but levels that are 2 dB/1microV/m higher on at the edge of right-of-way. 7. Comparative analysis of the electric parameters As noted before, the techniques of TL compaction or TL bundles expansion, leading to an increase of their SIL, have an impact on the values of their electrical parameters of the positive and zero sequences (see Table II). It is observed that the values of reactance (X1) and capacitance (C1) of the positive sequence of each configuration are very close, since all designs have been adjusted to a same value of SIL (≈1200 MW). 6. Comparative analysis of the e&m fields near the ground On the other hand, it is observed that there are major differences in the zero sequence. Among the reactance (Xo) there are variations between the minimum (blue) and maximum (yellow) value on the order of 13%. In capacitance (Co), the variation reaches 36%. In the relations Xo/X1 and C1/Co the variations are similar. This indicates that the performance under unbalanced conditions and switching transients will have different responses depending on the maneuver considered and the adopted tower. All the field calculations, whose profiles are shown here, were made at 1.5 meter above ground level. The minimum phase height was approximately 10 meters to ground, to allow comparison of line designs. The recommendations of the ICNIRP limit values or other international bodies were not considered since these values must be met at time of executive project and according to the local regulations. In the electric field profile, Figure 10, it is verified that the more compact towers, i.e., the Racket and the CrossRope, present smaller field values in the middle and side areas of the r.o.w. The Cat Face and the Monopole tower have the smallest field values in the line axis, getting higher at 12m from the axis, and having slightly higher values than the compact towers at 35m from axis. The towers of plane configuration and with longer distances between phases have higher value of electric field under the phases, with reduced slope until the border, similar to other alternatives. 8. Conclusion Six TL design concepts of 500 kV TL have been analyzed, with Natural Power (SIL) of 1200 MW, and similar positive sequence electrical parameters for system studies and steady state conditions. The zero sequence impedance differs by up to 36% depending on the phases and sub-conductors arrangements. In the magnetic field profiles (Figure 11) it is verified that the compact towers have lower values throughout the TL electrical studies, such as superficial conductor’s Cigre Science & Engineering • N°2 June 2015 68 9. References gradient, environmental interference levels of audible noise and radio interference, and electromagnetic fields, showed the advantages and disadvantages of each design. [1] Alexandrov, Georgij N., et alii - The Increase of Effectiveness of Transmission Lines and Their Corridor Utilization - Cigre Paper 38-104, Paris 1996. [2] Fernandes, José H. et alii – Towers for Compact TL of the Second Circuit of the 500 kV North-Northeast Interconnection at Eletronorte – Electric Studies - VIII SNPTEE – 1986 (Paper in Portuguese). [3] Regis Jr., Oswaldo; Dart, F. C. et alii – Studies and Application of Expanded Bundle in 500 kV TL - XIV SNPTEE - 1997 (Paper in Portuguese) [4] Machado, Vanderlei G. et alii - 500 kV TL of Third North / South Interconnection – Tower Solution with Monomast Guyed Support and Expanded Bundle - XIX SNPTEE – 2007 (Paper in Portuguese) In specific applications, where terrain allow the use of guyed towers or requires self-supporting towers; or in areas sensitive to audible noise or electric fields at ground level, the best alternative may be practical. Finally, many references on switching transients studies shows that there is a strong dependence of surge levels of switching procedures (TL energizing and reclosing, circuit breakers opening under faults, load rejection, transient recovery voltage) with the sequence parameters of the TL nearby. This fact suggests further investigations of these phenomena in order to have an additional comparison of the behavior of the concepts of lines presented in this article. 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