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The 19th International Symposium on High Voltage Engineering, Pilsen, Czech Republic, August, 23 – 28, 2015 GUIDE TO EHV/HV CABLE SHEATH BONDING APPLICATION SOFTWARE 1 1 1 1* F. Garnacho , A. Khamlichi and A. Valero High Voltage Technological Centre (LCOE-F2I2), Eric Kandel, 1 - 28906 Madrid, Spain *[email protected] Abstract: This paper introduces a software tool for the sheath bonding design of high voltage power cable systems. The calculation tools for the design of the bonding system are required for supplying the maximum value of the temporary overvoltages obtained for the worst short circuit that can appear in the grid. Sheath overvoltages depend on different factors: on the place where a short circuit appears, on the earth resistance value at each grounding connection and on the designed sheath bonding configuration. The software package uses a general method of circuit analysis adapted to cable systems to determine temporary overvoltages in complex sheath bonding configurations. The developed software allows defining any arbitrary architecture of linked elemental cable sections in order to simulate the real sheath bonding configuration. Temporary overvoltages in cable joints, terminations and link boxes are determined in this paper. These values should be used for the correct selection of the sheath voltage limiters used for protection of the outer sheath, joints, terminations and link boxes insulation. An example of two cable systems is analysed to demonstrate that small modifications on the sheath bonding configuration can reduce significantly the temporary overvoltages and permits the viable selection of the surge arresters used for protection. 1 INTRODUCTION Experience has proven that guidelines are needed whereby engineering companies can select the best sheath bonding configuration and the correct surge arrester (SVL) to protect cable sheaths for each particular cable system. The IEEE Standards Association has recently approved a guide  for helping the development of sheath bonding configuration in cable system projects. In spite of the fundamentals of calculating sheath voltages and currents defined for many years, many failures in the cable systems occur due to overvoltages. The continuous increasing of short circuit capabilities of modern power systems has accentuated these problems, requiring the application of more efficient bonding design criteria and numerical tools for the design of sheath bonding configurations. The numerical tools are required for supplying the maximum value of the overvoltage obtained for the worst fault conditions. Figure 1: Overvoltages due to short circuit The maximum absolute and local temporary voltages shown in figure 2 must be determined. In particular, the maximum value of the local temporary overvoltage that can appear on the link boxes where the surge arresters are placed must be determined to avoid the failure of these SVLs. Jc Ulocal When a single-phase short circuit occurs in a HV network significant overvoltages appear on power cable sheaths, especially in the cable terminations that are not connected to the earth and in the sheaths of cross bonding configurations. In the first instants a transient damped overvoltage of several tens of kilohertz and up to several tens of kilovolts is superimposed to a temporary overvoltage of power frequency that disappears when the short circuit is removed when the current is extinguished (see figure 1). Both transient and temporary overvoltages provoke significant stress to be considered in insulation coordination of cable sheaths, cable accessories, link boxes and SVLs. Je m Umn R em Uabsolute n R en Figure 2: Absolute and local temporary voltages Although ATP software can be used to determine temporary overvoltages, in practice, the user interface is not intuitive enough for the most engineering companies. The developed software permits determining temporary overvoltages in a user-friendly manner. 2 BASIC THEORY For each single-point bonding (SP) configuration the following equation can be written: The application software uses the general theory of circuit analysis by the mesh current method. The developed application software permits linking different elemental bonding connections, named “major sections”, with overhead lines to simulate the real sheath bonding configuration. For each major section of a cable system a set of equations can be written with a specific boundary condition. For instance, each sectionalized cross bonding (SCB) configuration as the shown in figure 3, satisfies the following system of three equations: [Umn] = [Zc( abc)s(αβγ) ]⋅ [Jc( abc) ] +[Zss(αβγ) ]⋅ [Js(αβγ) ] (1) [ ][ ] U mn = Z ec ( abc) ⋅ J c ( abc) + Z ee ⋅ J e (3) where: [Zec(abc)] = coupling impedances between the GCC and each phase conductor. Zee = self-impedance of the GCC. Je = current through the GCC. which unknown variables are the current Je through the GCC, and the voltage Umn. In addition, the following boundary condition must be also satisfied: U mn = Rem · J 'm − Ren · J 'n −( Rem + Ren )· J e where: (4) Ja [Zc(abc)s(αβγ)] = coupling impedances between each phase conductor and each phase sheath, that take into account sheath transpositions. [Zss(αβγ)] = coupling impedances between cable sheaths, that take into account sheath transpositions. [Jc(abc)] = conductor currents Ja, Jb and Jc. [Js(αβγ)] = sheath currents Jα, Jβ and Jγ. which unknown variables are sheath currents Jα, Jβ and Jγ and voltage, Umn between the ends of the SCB. In addition, the following boundary condition must be also satisfied: Umn = Rem·J 'm − Ren·J 'n −(Rem + Ren )·(Jα + J β + Jγ ) (2) where: Rem = earth resistance of the ground continuity conductor (GCC) on the left side. Ren = earth resistance of the GCC on the right side. J’m = current through GCC of the previous section on the left side. J’n = current through GCC of the following section on the right side. Minor section Jα Ja Jb Jc Jα+Jβ+Jγ m Umn Ren Rem Figure 4: Major section defined by a SP After Je determination sheath voltages Usa, Usb and Usc can be determined by means of the following equations: [U s ( abc ) ] = [Z sc( abc ) ][J c ( abc ) ] + [Z J’n (5) e Different major sections can be linked in series to simulate the real sheath bonding configuration of the cable system to be designed. For each major section a set of equations can be entered in the global array that defines the circuit. Jc Major section 2: Single-Point Jα JF Jβ Jγ n JM=Jm1 Rem ]J [Zsc(abc)] = coupling impedances between the phase conductors and each phase sheath. [Zs(abc)e] = coupling impedances between the phase sheaths and the GCC. Ja Jα+Jβ+Jγ s ( abc ) e where: Jb J’m n Umn Major section 1: Cross-Bonding Jγ Jc J’n m Jβ Jb Je J’m Ren Re1 Figure 3: Major section defined by a SCB JN=Jn3 Je J1 J2 R e2 Re3 Figure 5: Simple circuit composed by a SCB + SP For a better explanation the analysis method using mesh equations is presented for a simple sheath bonding configuration composed by two basic major sections (see figure 5). The equivalent circuit of the figure 5 is shown in figure 6. ][· J ] U = −[Z U = −[Z ][· J ] [Z JM ss ( αβγ ) ] 1 c ( abc) s ( αβγ ) c ( abc) 2 Z tt 2 + ce ( abc ) c ( abc ) + JN JM - Single-point bonding (SP) - Solid bonding (SB) - Sectionalized cross bonding (SCB), with three minor sections. - Continuous cross bonding (CBC), with any number of minor sections. - Overhead line (OL), with any number of spans. JN Re1 J1 J2 Re2 Re3 Figure 6: Equivalent circuit of analysed example The mesh equations that define the figure 6 circuit are the following: [U ] = [M ] ⋅ [J ] In SP sections is possible to add a second GCC and both can be transpose at any point along the cable run, while in SCB and CBC sections the user could connect a parallel GCC along the major section. Meanwhile, OL sections can be with or without grounded wire. (6) where: Re1 + Re2 + Zαα R + R + Z [M ] = e1 e2 αβ Re1 + Re2 + Zαγ − Re2 Re1 + Re2 + Z βα Re1 + Re2 + Zγα Re1 + Re2 + Zββ Re1 + Re2 + Zγα Re1 + Re2 + Z βγ Re1 + Re2 + Zγγ − Re2 − Re2 − Re2 − Re2 Re2 + Re3 + Zee − Re2 Zaα Zbα Zcα Re1 ⋅ J M Z J a Z Z R ⋅J [U ] = aβ bβ cβ ⋅ Jb + e1 M Zaγ Zbγ Zcγ Re1 ⋅ J M J c − Re3 ⋅ J N Zae Zbe Zce Figure 7: Example of inserting a SCB section In any major section joint or termination, earth resistance value is easily entered. The resolution of the following linear equation system allows determining the unknown variables [J]: [J ] = [M ]−1 ⋅ [U ] (7) where: Jα J [J ] = β Jγ Je J 1 = Jα + J β + J γ 3 SOFTWARE DESCRIPTION 3.1 DATABASES J 2 = Je 3.3 MINOR SECTIONS Each buried major section consists of at least one minor section which can also be divided in subsections in order to match the design to the real configuration (types of laying and cable). In the minor section menu the user can choose the power cable and GCC from the database, and the run length and the soil resistivity value are entered directly by the user. In the case of OL sections, the conductor, the bundled configuration, the grounded wire and the type of tower are eligible from the database, and the operating temperature, the line length and the mean span length are entered directly. Initially, the EHV/HV Cable Sheath Bonding Application Software requires the user to fill in a cables and overhead lines database. An Excel library should be completed with the information provided by cable and wire data sheet. Likewise, parallel ground continuity conductor (GCC) and overhead grounded wire specifications should be included in this library database. Figure 8: Example of editing a cable laying 3.2 MAJOR SECTIONS The software allows designing any power circuit with multiple bonded cable system and overhead lines by inserting in series all the desired major sections from the following: Any type of cable laying or OL laying to be used must be included in an Excel database, although they can be later modified directly on the layout viewer in order to fit the design to the real laying configuration. 3.4 VOLTAGE SOURCES 3.7 Two types of voltage sources can be considered: substation or power grid where cable is connected (see Figure 9). Source data can be entered either by their short circuit currents (three-phase and single-phase) or by their short circuit impedances (direct and homopolar). SIMULATION OUTPUTS When the case study is completely designed and the analysis scenario selected, the user can proceed with a full simulation which will be completed in a few seconds. The software allows evaluating the influence of the earth resistance values by reproducing the same case study with three different values in the same processing. The following results are obtained from the simulation: 3.7.1 Absolute sheath voltages COUPLING OF DOUBLE-CIRCUIT CABLE Since sheath standing voltages are modified by the presence of a closely second circuit, the software permits designing double-circuit geometries. Two independent circuits can be analysed with a common or different voltage source. Furthermore, any cable subsection can be coupled to another subsection or minor section of a second circuit system in order to simulated parallel double-circuit cable with any desired laying configuration. 3.7.2 Local sheath voltages The same outputs as for absolute voltages turn out for the maximum values of local overvoltage that can appear on the link boxes. 12 9 100.0% R t 150.0% R t 100.0% R t 200.0% R t 150.0% R t 200.0% R t 8 10 7 b) a) 8 6 5 U (k V ) 3.5 U (k V ) Figure 9: Types of voltage sources: a) substation source; b) power grid source The maximum absolute overvoltage values that can appear in the cable outer sheath of every joint or termination are shown directly over the circuit diagram, plotted on a line graph and exported numerically to Excel, in order to analyse the outer sheath insulator. 4 6 4 3 2 2 1 0 EI0 EI1_I EI1_D EI2 EI3 EI4 EI5 EI6 EI7 0 EI0 EI1_I EI1_D EI2 EI3 EI4 Empalme/Terminal EI5 EI6 EI7 Figure 11: Example of: a) absolute sheath voltage graph; b) local sheath voltage graph 3.7.3 Ground voltages Figure 10: Example of coupling a double circuit SIMULATION SCENARIOS 3.7.4 Induced sheath currents Two kinds of analysis can be performed: 3.6.1 Temporary analysis Sheath temporary overvoltages are caused by the following fault conditions: typically - Three-phase fault Single-phase fault to substation earth Single-phase fault to local earth Fault due to wire breakage in OL, from left or right side - Fault due to backflashover in OL - Internal cable fault In addition, induced current values are also shown over the circuit diagram by arrows indicating the flow direction. 9 100.0% R t Another possible scenario consists on inserting a three-phase load for steady-state simulation. 200.0% R t b) 7 6 a) 5 4 3 2 1 0 EI0 3.6.2 Steady-state analysis 150.0% R t 8 U (k V ) 3.6 The software also provides the user maximum voltage values on the points connected directly to ground. EI1_D EI4 EI7 Figure 12: Example of: a) ground voltage graph; b) induced sheath current presentation 4 CASE STUDY - Fault due to wire breakage in OL from left side (WB-L), located in each span. - Fault due to wire breakage in OL from right side (WB-R), located in each span. - Fault due to backflashover in OL (BFO), located in each span. - Internal cable fault (IN), in two cases: located between Sub.1 and L1 at 450 m distance, and between Sub.3 and T1 at 50 m distance. In order to demonstrate the technical capabilities of the EHV/HV Cable Sheath Bonding Application Software, the local sheath voltages of a doublecircuit are analysed below for all possible fault conditions. As you can see in figure 13, there are in the example two circuits connected on their right side to the same voltage source reproducing a common substation (Sub.2). The first circuit (Sub.1-Sub.2a) is composed by two linked SCB followed by a long SP with the SVL at the right end. The second circuit (Sub.3-Sub.2b) is composed by one short SP followed by an OL of 30 km long. From the other side of the OL continues a SCB plus two SP with their not-grounded points coinciding at the middle joint (L10). Table 1: Local sheath voltages in the case study without improvements (kV) ULOCAL Furthermore, there are three coupled subsections reproducing a double-circuit laying configuration: minor section L5-L6 is coupled with a subsection of L8-L9 and the minor section L6-Sub.2a is divided in two equal length subsections, each coupled to the facing SP minor sections of the second circuit, L9-L10 and L10-Sub.2b respectively. Circuit Sub.1 – Sub.2a Circuit Sub.3 – Sub.2b Fault L1 L2 L4 L5 S/2a T1 L7 L8 3F-S 3,3 3,1 1,6 2,5 4,4 0,2 1,5 1,5 L10L L10R 0,7 0,7 1F-S 4,8 4,5 4,5 5,7 10,7 0,1 4,3 4,0 0,8 0,8 WB-L 1,1 1,1 1,2 1,5 0,1 0,2 1,1 1,2 0,1 0,1 WB-R 2,3 2,4 2,6 3,1 0,7 0,0 5,5 4,0 0,8 0,8 BFO 5,3 5,1 5,6 6,8 7,0 1,6 14,3 12,8 8,6 8,5 IN 6,0 5,4 4,8 5,8 10,8 0,2 2,5 2,0 0,7 0,7 Max. 6,0 5,4 5,6 6,8 10,8 1,6 14,3 12,8 8,6 8,5 Noting the last row of table 1, with the present sheath bonding design the outer sheath integrity is compromised and the selection of the SVLs is complicated in the link boxes of Sub.2a, L7, L8, L10 (left side) and L10 (right side). The proposals for improvement are described below in order to reduce these maximum local sheath voltages obtained in table 1 and select the SVLs correctly: The high voltage considered is 220 kV, having 40 kA of short circuit currents (three-phase and single-phase) in each substation. The same type of power cable and trefoil laying formation has been considered along the whole system in order to annul their influence on the local sheath voltages. - Improvement num. 1: reduce the earth resistance value in every transmission tower. In this example 20 Ω is changed by 10 Ω. - Improvement num. 2: add a parallel GCC along the SCB sections. In this case, this improvement is applied in the major sections Sub.1-L3, L3-L6 and T86-L9. - Improvement num. 3: add a second GCC in the SP sections. Is the case of the major sections L6-Sub.2a, Sub.3-T1, L9-L10 and L10-Sub.2b. - Improvement num. 4: break up facing SP sections and avoid having SVLs in substation terminations by switching the grounding connection points of the SP major sections. In this case study it is proposed moving the notgrounded point from Sub.2a to L6 and from L10 (left side) to L9 (right side). The OL is composed by 85 spans with 20 Ω of earth resistance value in each tower. The earth resistance value at substation considered is 1,5 Ω while at each joint is 5 Ω. The length of each minor section or subsection is shown in figure 13. The following fault conditions were analysed separately obtaining the maximum local sheath voltages shown in table 1: - Three-phase fault (3F-S), located in each substation. - Single-phase fault to each substation earth (1F-S). Sub.1: 220 kV Sub.1 L1 900m L2 300m L3 600m 1.5 Ω L4 200m L5 800m L6 400m 5Ω Sub.2a 450m 450m 5Ω Sub.2: 220 kV T1 AP AP T86 1.5 Ω Sub.3: 220 kV Sub.3 L7 30.00 km. 85 Spans 100m 1.5 Ω 700m 10 Ω 10 Ω 20 Ω Figure 13: Case study without improvements L8 700m L9 300m 400m L10 450m 5Ω Sub.2b 450m 5Ω Sub.1: 220 kV Sub.1 L1 900m L2 300m L3 600m 1.5 Ω L4 200m L5 L6 800m 400m 5Ω Sub.2a 450m 450m 5Ω Sub.2: 220 kV T1 AP AP T86 1.5 Ω Sub.3: 220 kV Sub.3 L7 30.00 km. 85 Spans 100m 1.5 Ω 700m 10 Ω L8 L9 700m 300m L10 400m 10 Ω 450m Sub.2b 450m 5Ω 5Ω 10 Ω Figure 14: Case study with improvements These proposals for improvement, represented in figure 14, are justified by countless real studies carried out by the authors of this article. The maximum local sheath voltages obtained for the case study with each proposed improvement are shown in table 2. In the last row the percentage difference between with and without improvements cases is calculated. Table 2: Local sheath voltages in the case study with improvements (kV) ULOCAL Circuit Sub.1 – Sub.2a Selected SVL residual voltage (Ures) for each link box must assure an appropriate protection margin taking into account the insulation level for transient overvoltages, and considering the effect of distance between the SVLs and the insulation to be protected. This topic is explained in detail in reference , therefore, in the improved case study of this paper, in particular, a correct selection of the SVLs would be as shown in the table 3: Table 3: Selection of SVLs in the final case study Circuit Sub.3 – Sub.2b SVL Circuit Sub.1 – Sub.2a Without Improv. 6,0 5,4 5,6 6,8 10,8 1,6 Improv. L1 L2 L4 L5 S/2a T1 ULOCAL 2,0 2,2 num.1 6,0 5,4 5,7 7,0 10,8 1,6 13,2 12,0 8,5 8,5 Ur ≥ 2,0 2,2 num.2 2,0 2,2 0,7 1,7 10,7 1,5 2,5 4,0 8,6 8,5 Ures ≤ 6,0 6,6 2,1 num.3 4,8 5,3 5,0 6,1 9,3 0,9 5,8 6,9 3,4 3,8 L1 L2 L4 L5 L6 T1 L7 L8 L9R L10R 6,0 5,4 5,8 7,1 10,8 0,1 13,4 12,0 8,5 8,5 2,0 2,2 0,7 1,7 9,3 0,9 2,5 3,4 3,8 num.4 With all Improv. % 14,3 12,8 L7 L8 4,0 8,6 8,5 L10L L10R -67% -59% -88% -75% -14% -44% -83% -69% -60% -55% Interpreting the results from table 2, it could be said that the improvements number 2 and 3 significantly reduce the local sheath overvoltages. In this case, the improvement number 1 can be not considered. Meanwhile, it is known that by removing facing SP sections (improvement num. 4) transient opposite polarities disappear, avoiding double overvoltage which would have endangered joint insulators. Additionally, removing SVLs from gas-insulated substation (GIS) terminations is proposed because the coordination between the cable SVL rated voltage and the GIS enclosure bypass SVL rated voltage is usually not easy. However, despite all the improvements the local sheath voltage obtained in the long SP section L6Sub.2a remains dangerous. The only possible solution would be to divide the major section in two separated SP sections just like the configuration in sections L9-L10 and L10-Sub.2b. Finally, selection criteria for surge voltage limiters should be considered for outer sheath protection. In this sense, SVL should withstand temporary overvoltages in order to be in the safety side. The SVL rated voltage (Ur) is chosen greater or equal to the local temporary sheath overvoltage. With all Improv. 6 L1 L2 L4 Circuit Sub.3 – Sub.2b L5 L6 T1 L7 L8 L9R L10R 0,7 1,7 9,3 0,9 2,5 4,0 3,4 3,8 0,7 1,7 9,3 0,9 2,5 4,0 3,4 3,8 5,1 27,9 2,7 7,5 12,0 10,2 11,4 CONCLUSION Significant temporary overvoltages can appear in cable sheaths when short circuits occur in high voltage power cable systems. The developed EHV/HV Cable Sheath Bonding Application Software allows designing any arbitrary architecture of linked elemental cable sections in order to simulate the real sheath bonding configuration and to determine the temporary overvoltages. The simulation outputs permits the viable selection of the surge arresters used for protection. This software is strongly recommended to cable project designers, cable manufacturers and transmission system operators. ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of Red Eléctrica de España (REE) and Unión Fenosa Distribución (UFD). REFERENCES  IEEE Std 575™-2014 “IEEE Guide for Bonding Shields and Sheaths of Single-Conductor Power Cables Rated 5 kV through 500 kV”.  F. Garnacho, A. Khamlichi, P. Simon and A. Gonzalez: “Guide to Sheath Bonding Design, in Distribution and Transmission Lines with HV Underground Cables”, CIGRE, B1105, 2012.