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Electric field computation in 765 kV substation using charge simulation method with reference to occupational exposure D. Harimurugan1, G. S. Punekar2*, N. Srikanth Bhatt3 1
Department of Electrical and Electronics Engineering, National Institute of Technology Karnataka, Surathkal, Mangalore-575 025, Karnataka, India 2 Department of Electrical and Electronics Engineering, National Institute of Technology Karnataka, Surathkal, Mangalore-575 025, Karnataka, India 3 Department of Electrical and Electronics Engineering, National Institute of Technology Karnataka, Surathkal, Mangalore-575 025, Karnataka, India *
[email protected]
Abstract: With the increase in transmission voltage level, and the guidelines of the International Commission on NonIonizing Radiation Protection (ICNIRP), the effects of non-ionizing radiation on biological elements at high voltage (HV) substations have gained significant importance. The electric field (e-field) distribution in an upcoming 765 kV substation in the Indian subcontinent is computed using the charge simulation method (CSM). CSM is used to model the 765 kV bays, transmission-lines, buses, and ground-wires in the substation. The three-dimensional (3D) e-field is calculated through the superimposition of e-fields obtained in two orthogonal planes using infinite line charges. This proposed method of using infinite line charges gives realistic results. The simplistic model using infinite line charges greatly reduces the complexity of the CSM-based model (due to reduced number of charges) apart from increasing the CSM-based model accuracies. This fact has been demonstrated by comparing these results with those of CSM-3D-model of a detailed bay-model (including major equipment and associated support structures). The complex-charge-based CSM helps in computing the root mean square (rms) value of the e-field at a point, directly, as per ICNIRP guidelines. This rms value of the e-field is compared with the occupational exposure reference value prescribed in the ICNIRP guidelines.
1. Introduction The installed capacity of the Indian national grid in post-independence India was 1362 MW (December 1947); this value has steadily increased over the years to reach the current installed capacity of ~330 GW as of 30th April 2017 [1]-[2]. Although power generation has grown more than 100 times, post-independence, the current rate of increase in demand is even higher due to the significant growth in economic activities. In an attempt to meet the demand and to improve the efficiency of transmission and distribution systems, the country is reviewing and reforming its power infrastructure. Currently, the maximum capacity of the transmission voltage level in India is 1200 kV [3], and power is successfully transmitted at voltage levels of 765 kV, 400 kV, 220 kV, etc. With the increase in transmission voltage level (at 50 Hz as power frequency), the effect of the non-ionizing electromagnetic field (EMF) on biological elements has become a major concern. Various studies have been conducted by international organisations to investigate the potential health risks associated with EMF exposure [4]-[5]. IEEE has defined a safety standard for human exposure to electric and magnetic fields in the frequency ranges of 0 kHz to 3 kHz [6] and 3 kHz to 300 GHz [7]. The International Commission on NonIonizing Radiation Protection (ICNIRP) was established to provide scientific advice and guidance on the effects of nonionizing radiation. The ICNIRP guidelines suggest that the unperturbed root mean square (rms) value of a time-varying electric field (e-field) should not be more than 10 kV/m for occupational exposure and 5 kV/m for public exposure for the frequency of 50 Hz [8].
Different numerical techniques have been adopted by researchers to calculate electric and magnetic field strength due to high voltages (HVs) and currents in substations. Few of the earlier works involving e-field computation in substations are stated herein. The finite element method (FEM) was used to calculate the e-field in 1000 kV ultrahigh voltage (UHV) substation [9] and 765 kV substation [10]. The boundary element method (BEM) was employed to calculate the power frequency e-field in a 500 kV substation, considering the effect of switching equipment [11]. Further, a combination of BEM and the Galerkin method [12], and BEM and wavelet transform [13] also were proposed for computation of e-fields in substations. The charge simulation method (CSM) is a preferred numerical method for open boundary problems as it does not involve discretisation of the solution region [14]-[16]. CSM was used to calculate the e-field in a 400 kV substation, which consists of two buses and transmission lines. The ring charges and linear charges were used to model the various components, and the results were compared with the measured field values [17]. The more accurate case-specific CSM model of a substation was developed [18]. It features a finite line charge in modelling a 500 kV substation, which consists of a single main bus with incoming and outgoing feeders. The effect of changing the spacing between the conductors was discussed, and complex charges were used in simulation to calculate the e-field [18]. The incoming and outgoing lines may not always be a straight line; in such cases, finite slant line charges have been opted to model the lines more effectively [19]. In reference [20], to calculate the induced current in the human body due to transmission
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line or substation equipment, the human body was modelled with ring charges, and CSM was used to calculate the efields. A computer program was developed using CSM along with BEM to calculate the e-field in a 400 kV substation consisting of three busbars and a bay arrangement [21]. The substation as a system is an open boundary problem, and hence, CSM is more suitable as a numerical technique. This paper investigates the e-field stress using CSM-based models in an extra high voltage (EHV), 765 kV substation in India for an actual layout plan, which is under construction. Apart from reporting these results, as a case study, following two aspects are dealt in this paper. This substation is unique in its own way; it consists of four bays, two buses, and four transmission lines with overhead ground-wires. In addition, HV buses run transverse to the remaining components (bays, transmission lines, and overhead ground-wires). The CSM with infinite line charge is used to compute the e-fields in the substation. The results of the two-dimensional (2D) e-field obtained in two orthogonal planes are superimposed to obtain the estimates of three-dimensional (3D) e-field distribution in the substation area. This approach is unique and generic. The e-fields are also computed and reported for a single bay (with major bay components included; detailed model) by directly implementing the 3D CSM model. These results are compared with the results of CSM-based model with infinite line charges (excluding bay equipment) in view of ICNIRP guidelines. The merit of using the simplified model (as a compromise) is demonstrated quantitatively in the present work, probably for the first time. 2. Details of EHV substation under study The substation under study is a 765 kV generating substation of a 3 x 600 MW thermal power plant in India. The total area of the substation is 124944 m2 (30.87 acre). The substation has four bays, among which three are generating bays and one is a reactor bay. All bays are charged to 765 kV. The transmission lines run in parallel with the bay conductors of 456 m (length). The overhead ground-wires also run in parallel to the transmission lines, at a height of 6 m above the transmission lines. Two buses (Bus-1 and Bus-2) run at right angles to the bays and have a length of 274 m (breadth). The clearance between the phase conductor in bay, transmission line, and bus is 15 m. The bay-to-bay clearance is 24 m. Quad Bull AAC conductor is used for bus and transmission lines, which have a spacing of 0.45 m between each of the four conductors. Two overhead ground-wires are associated with each transmission line. These overhead ground-wires have a spacing of 30 m between them. Spacing between the overhead ground-wires associated with the two adjacent transmission lines is 24 m. The schematic given in Fig. 1 depicts the 3D view of the substation indicating bays, transmission line, overhead ground-wires, and buses. In Fig. 1, the X-axis denotes the breadth of the substation, Y-axis indicates the length of the substation, and Z-axis presents the clearance (height) from the ground plane. The 2D view of the XZ and YZ planes of the substation are given in Fig. 2 and Fig. 3, respectively. The clearance from the ground plane, type of conductor used, and diameter of the conductor for the lines, bays, and buses are given in Table 1.
Each bay has many equipment like lightning arrestors (LA), circuit breakers (CB), and current transformers (CT). The CSM-based 3D model using finite line charges for a single bay (with the bay equipment) is simulated and compared with the proposed CSM-based model results, obtained using the superimposed results of 2D models (without the bay equipment). In the detailed bay-model, each phase conductor is simulated along with one LA, three CB, and four CT as per the actual bay layout of the EHV substation under study (case study). The effort involved in forming a CSM-3D-model of a detailed bay-model (including major equipment and associated support structures) is compared with that of simplistic (proposed) model using infinite line charges to illustrate its advantages. Details are given in Section 4.4. 3. Computation of the e-field The investigation of the electric stress over the area of the substation is necessary to assess the exposure effects on biological elements. The e-field distribution in the present study is obtained using the semi analytical technique, namely CSM using infinite line charges. Use of infinite line charges in CSM for estimating the worst-case e-fields is justified, as the maximum stress occurs in the mid region (away from the ends) of the substation area. Use of infinite line charges also reduces the complexity in CSM (due to reduced number of charges) apart from increasing the CSM simulation accuracies (for the same number of simulating charges).
Fig. 1. Three-dimensional view of the 765 kV substation schematic showing four bays, two buses, four transmission lines, and overhead ground-wires
Fig. 2. Two-dimensional view of the substation in the XZ plane depicting bays, transmission-lines, and ground-wires (running parallel to Y-axis: into the plane of the paper) 2
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the error in the developed CSM-based model is estimated by choosing 20 test points on the surface of each conductor. The estimated worst-case potential (peak value) error at the surface of the conductors also is given in Table 2. As seen from the table, the worst-case potential (peak value) error is less than 1% (considering the maximum errors on all the conductors of each component). Thus, in CSM, having obtained a satisfactory set of charge arrangements and their charge magnitudes, the efield at any point in space can be calculated using [E] = [F][Q] Fig. 3. Two-dimensional view of the substation in the YZ plane depicting buses and ground-wires (running parallel to X-axis; into the plane of the paper)
where [E] is the vector of the e-field intensity, and [F] is the vector of the field coefficient matrix
Table 1 Type and specification of conductors used for the components in the 765 kV substation, and their ground clearances Component Conductor Clearance from ground Type Diameter plane (m) (m) Bay Aluminium 0.1143 14 Tubes Transmission Quad Bull 0.0400 39 line AAC Bus Quad Bull 0.0400 27 AAC Ground-wire G.S. shield 0.0101 45 wires 3.1. Advantages of CSM and its application In the present work, the various elements (bay, bus, transmission line, and overhead ground-wires) of the substation are simulated using the infinite line charges in CSM. The simulating charge magnitudes are obtained through numerical computation such that the integrated effect of the charges must satisfy the boundary conditions. This is given as [14, 22] [V] = [P] [Q]
(2)
(1)
where [P] is the potential coefficient matrix, [V] is the vector of potential values (conductor potentials), and [Q] is the vector of the unknown magnitude of infinite line charges. In the present study, magnitudes of simulating charges are computed with the appropriate potentials corresponding to 765 kV assigned to the conductors. In the modelling of the bay, each phase-conductor is represented by an infinite line charge. As Quad Bull AAC conductor is used for transmission lines and buses (see Table 1), each phase-conductor is represented by four infinite line charges (since it is a Quad conductor). Table 2 shows the number of simulating charges used in the developed CSM-based model for this scenario of substation involving bays, buses, transmission lines, and overhead ground-wires. The total number of simulating charges used in the CSM-based model is 92. The infinite ground plane is simulated using the image conductors and the corresponding image charges. Further,
3.2. E-field computation in the substation area In the present work, the e-field in this 765 kV substation with conductors (of the components listed in Table 1) at appropriate potentials is computed in two different planes (namely, XZ plane and YZ plane), separately. The superimposition of these two results will give the required 3D field. The 2D view of the substation in the XZ plane showing the conductors of bays, transmission lines, and overhead ground-wires is provided in Fig. 2. The 2D view of the substation in the YZ plane showing the conductors of buses and overhead ground-wires is illustrated in Fig. 3. In the XZ plane, the e-fields are computed with infinite line charges which are placed parallel to the Y-axis to simulate (i) bays, (ii) transmission lines, and (iii) overhead ground-wires in the CSM-based model. In the YZ plane, infinite line charges are kept parallel to the X-axis to simulate the bays and overhead ground-wires. The groundwires run perpendicular to the buses in the YZ plane. To obtain the effect of overhead ground-wires in the YZ plane, overhead ground-wires are assumed to be present along with the buses (above them). The equivalent effect of the overhead ground-wires, in the YZ plane, is achieved in the CSM-based model by assuming infinite line charges 5 m apart and parallel to the X-axis, as shown in Fig. 3. It is found that the existence of ground-wires (which are at a height of 45 m from the ground plane) in the YZ plane does not result in significant variation in the spatial distribution of the e-field at a height of 2 m. Hence, the effect of groundwire in the YZ plane is neglected in this work. The 3D efield distribution at a height of 2 m (approximation of human height) from the ground level over the area of the substation is calculated by superimposing 2D field results (namely, in YZ and XZ plane results). Table 2 Number of line charges used to simulate various conductors and estimated worst-case potential errors (peak value) in CSM-based model Component of the Number of Percentage error substation charges Bay 12 0.102 Transmission line 48 0.976 Bus 24 0.947 Ground-wire 08 0.000
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In the literature, e-field computation and analysis at varying heights of 1 m [18], 1.2 m [12], 1.75 m [13] ,1.8 m [21], and 2 m [23] above the ground plane can be found. The average human height being in the range of 1.7 m to 1.8 m, the e-field computed at 2 m height will yield cautious results for occupational exposure from the point of view of ICNIRP guidelines. Hence, the height of 2 m is preferred in the present study. All the reported results in this paper are computed at a height of 2 m above the ground plane. 4. Results and Discussion Results and discussion related to e-fields given in this section is divided into two segments. In the first segment, the results of proposed method of using infinite line charges in the CSM-based simplified modelling of the substation are reported. Simplified model implies modelling of the line conductors alone (excluding bay equipment and support structures). In this, the 3D efield results are obtained by using superposition of two orthogonal plane 2D e-field results. The e-field results obtained by using complex-infinite-line charges in the CSMbased modelling of the substation (including conductors of bay, bus, transmission line, and overhead ground-wire) are reported. In the second segment, the effectiveness of the proposed method (of using infinite line charges; simplified model) in estimating e-fields in view of ICNIRP guidelines is discussed by comparing the results with the detailed model. This comparison is carried out for a specific bay of the substation under study. The detailed model implies, modelling of the line conductors along with the major equipment (and their support structures) connected to the bay. The quantitative comparison of e-field deviations, CSM-based model errors, and computational time requirements of the detailed model and simplified model is also given.
Figure 4 shows the contour plot of rms values of the e-field calculated over the area of the substation using complex charges in CSM. The crossover points of bays and buses have a higher e-field stress. In addition to those points, the outer bays (bay-1 and bay-4) have higher e-field stress because of the absence of the cancellation effect normally present due to adjacent bays. The line graphic showing the variation of e-field over the breadth (X-axis) of the substation (at Y = 324 m; plane in which maximum e-field occurs) is shown in Fig. 5. The quantification of e-field distribution obtained through the complex charges in CSMbased model is given in Table 3. It is observed in the present case study that the computed rms value of the e-field (maximum over the substation area) is 12.86 kV/m and 13.06 kV/m for a height of 1 m and 2 m, respectively.
Fig. 4. The rms values of e-field intensity calculated over the area of the substation, obtained using CSM with complex charges
4.1. E-field distribution in the substation using simplified CSM-based model with infinite line charges With the specific code developed, the e-field distribution over the substation area is computed. This helps in assessing the threat posed to working personal due to non-ionizing radiation at 50 Hz. With respect to the ICNIRP guidelines, the reference value for occupational exposure to the time-varying e-field (unperturbed rms value) is 10 kV/m [8]. Hence, it is necessary to calculate the rms value of the e-field rather than the instantaneous e-field. Thus, the complex charge-based CSM is more useful in relating with the reference value for occupational exposure to the timevarying e-field. By using complex charges in the CSM-based model, the required e-field intensity is obtained in polar form for a given input voltage (phasor). With a single computation of e-field for any instant of input voltage, the magnitude (modulus) of the e-field gives the maximum value (amplitude) of the e-field in a cycle at a given point in space. Hence, the usage of complex charges in CSM aids significantly in reducing the computational time in calculating the maximum value of the e-field in both space (substation area) and time (over a cycle).
Fig. 5. E-field intensity variation along the breadth (Xaxis) of the substation at a fixed length (Y=324 m) obtained using CSM with complex charges Table 3 Quantification of e-field distribution (considering the entire substation area) with infinite line charges used in CSM taken from complex-number field Quantity Computed value Maximum e-field (kV/m; rms) 13.06 Average e-field (kV/m; rms) 5.921 Standard deviation 2.156 4
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The above analysis of e-field is carried out without considering the effect of major equipment and their support structures beneath the bays. Also, the effect of jumpers and conductor sag are not included in the proposed simplified model. In order to substantiate the effectiveness of the proposed simplified model, a single bay is modelled with inclusion of major equipment (along with their support structures) beneath it. Bay conductors being the lowermost conductor with least clearance from ground (see Fig. 1), their contribution to the e-field is higher compared to the other components (transmission lines and buses). Hence, the detailed CSM-based model of a single bay (neglecting buses, transmission lines, and other bays) alone is attempted and the results are compared with proposed simplified model of a bay. Details of this comparison are given in the following section. 4.2 Modelling of a single bay to substantiate the effectiveness of the proposed simplified model A single bay (three conductors) with an arena under study of 120 m x 456 m is considered for the present analysis. The bay conductors are modelled as per the actual layout plan of the substation under study (case study). The type of conductor used, diameter of the conductor, and ground clearance of the bay conductors are given in Table 1. The clearance between the phase conductors is 15 m. The complex charges are used in the CSM-based models. Two CSM models, namely, (i) Simplified model (infinite length bay-conductors with no bay equipment) (ii) Detailed model (with the finite length bay-conductors of 305 m) are developed for a single bay. The results are compared to prove the advantages of the proposed simplified model in view of ICNIRP guidelines. The details of the model developed are given in the following subsections. 4.2.1 Simplified CSM modelling of the bay: In the simplified CSM modelling of the bay, the major equipment and their support structures are not considered. Infinite line charges (taken from complex number field) are used to model the bay conductors. Hence, the total number of infinite line charges used in the simplified CSM-based modelling of a bay is three (only). The unknown charge magnitudes are obtained using (1). The e-field is computed over the arena under study (120 m x 456 m) of the substation with this simplified model. The e-field results of this model are compared and discussed along with the detailed model in Section 4.2.3. 4.2.2 Detailed CSM modelling of the bay: In the detailed CSM model, the major equipment and their support structures beneath the bay conductors are also modelled, along with the bay conductors. As per the actual layout plan of the substation under study, three CBs, four CTs, and one LA are located beneath the bay conductors, in each phase. The bay conductors are modelled using the finite line charges. The vertical support structure of the equipment (which leads to the elevation of the ground point) is modelled using the ring charges. The equipment are made of ceramic (єr = 4.5) insulating material. Hence, the CSMbased model is a multi-dielectric one. In such cases, in addition to the potential boundary condition at the conductor
boundary, the system of simulating charges must also satisfy the dielectric boundary conditions [14, 22]. In the present work, the solid dielectric insulation structure of the equipment is assumed to be cylindrical and are simulated using the ring charges. The vertical height of the support structure and the diameter of the equipment used in the CSM-based model are given in Table 4. These dimensions are taken from the manufactures catalogue. The mechanical sag of the conductors and the effect of jumpers are neglected in the present analysis. The type and number of simulating charges used in the modelling of bay equipment are given in Table 5. The simulating charge magnitudes of the detailed CSM model implemented are obtained using (1). The contour plot showing the e-field distribution (over the region of a bay) obtained through detailed CSM-based model is given in Fig. 6. The line graphic showing the magnitude of e-field variation along the length of the bay at a distance of 1 m from the support structure of R-phase (at X = 40.5 m; see Fig. 6) is given in Fig. 7. Table 4 The height of support structure and the diameter of the equipment connected to the bay in the 765 kV substation under study. Equipment Support Equipment structure diameter (m) height (m) Circuit Breaker 7.265 0.235 Current Transformer 8.236 0.215 Lightning Arrestor 8.010 0.160 Table 5 Number of simulating charges used in the detailed modelling of the bay conductors with major components and their support structures. Substation Type of Total number Components simulating of simulating charges charges Bay conductors Finite line charges 1836 Bay equipment Ring charges 462 Support structure Ring charges 231 (of bay equipment) Total number of charges 2529
Fig. 6. E-field intensity computed over the considered arena under study, obtained using detailed CSM model 5
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Fig. 7. E-field intensity computed at 1 m from the support structure (left of R-phase, at X=40.5 m; see Fig. 6) over the length of the bay 4.2.3 Comparison of e-field results of simplified model and detailed model: From the results of detailed CSM-based model of the bay, the maximum value of e-field obtained along the length of the bay (at X = 40.5 m) is 9.648 kV/m. The corresponding maximum value of efield obtained using the simplified model is 8.235 kV/m. In Fig. 7, the e-fields computed at X = 40.5 m using the detailed model are compared with that of the simplified model. In Table 6, the computed maximum e-field values along the length of bay at different discrete distance (at breadthwise distance of 1 m, 3 m, and 5 m to the left of Rphase of the bay conductor) are compared. The deviation in the computed e-field, as obtained from the two models, decreases as the point of computation moves away from the support structures. The difference in the e-field is reduced below 3.4 % as the distance from point of computation to the support structure is greater than 3 m in the present study. Thus, the effect of major equipment and their vertical structures on e-field distribution is limited and prominent only at close proximity of the equipment. The CSM programs are case specific. The programming aspects of the detailed CSM-based model (involving finite line charge) are intricate as it involves (i) multi-dielectrics (ii) a large number of finite-length line charges. Hence, it also significantly increases the computational-time apart from the increased programming complexity. Simulation error comparison: The potential errors at the conductor surface are assessed for both the CSMbased models (simplified and detailed) by considering a large number of test points on the boundary of the bay conductors. The percentage rms potential error in the simplified model and in the detailed model is 1.798 % and 0.043 %, respectively. The maximum percentage deviation (between two models) in the e-field estimation, as seen from Table 6, is 14.65 % at 1 m distance away from the support structure along the breadth (X-axis) of the substation. This better e-field estimate with the detailed modelling of the substation (if at all it is to be considered as an improvement!) is with the maximum potential error of 19.25 % in CSMbased model (see Table 7). Also, as one moves away from the bay (breadth wise), the deviation in e-field results in the
simplified model reduces, when compared with that of a detailed model (see Table 6). In case of the simplified model, the maximum potential error is 0.071 % (significantly low). Thus, the value of e-field calculated is more realistic in terms of overall error in the case of simplified model. Computational time comparison: Time taken to calculate the magnitude of simulating charges in the CSMbased models, and the time taken to compute e-field over the considered arena under study of the substation is given in Table 7. The total time taken to compute the e-fields with the simplified model of the bay is 6.20 seconds, whereas, it is 7 hour 23 minutes in the detailed modelling of a bay (for the same area). In this computational time comparison, the e-field computations are carried out over the considered area with grid spacing of 0.5 m, identically for both the models. The computer with Intel® core™ Duo 2.2 GHz processor is used in the present work. Hence, this comparison of the simplistic model of a bay (using only three infinite line charges) with the detailed bay model (including the bay equipment and their support structure using finite line and ring charges totalling to 2529) proves that: one has to have a compromise between the ‘detailed bay model’ and the ‘CSM modelling complexity’. This has to be done keeping in view the overall improvement in the estimated e-fields and their accuracies. Also, with the entire modelling of the substation, with inclusion of all four bays, bay equipment, buses, and transmission lines (case study; see Fig. 1), the number of simulating charges required will be excessively large. Thus, the use of simplified model with infinite line charges and neglecting the effect of bay equipment in the present study seems to be reasonable. This also reduces the computational burden. Table 6 Comparison of maximum e-field obtained using the CSM-based simplified model and detailed model along the length of the bay, at different breadths (see Fig. 7) Breadthwise RMS e-field Percentage distance (left of (Maximum value) deviation in R-phase of the (kV/m) Maximum bay conductor) Detailed Simplified e-field (m) model model 1 9.648 8.235 14.65 3 8.347 8.067 3.355 5 7.597 7.531 0.869 Table 7 Estimated potential errors and computational time for the CSM-based simplified and detailed models of a bay. Computational Time (s) Percentage Error for CSM-based model in Maximum RMS obtaining computing Bay error error charge E-field Model magnitudes over the area Simplified model
0.071
0.043
1.137
5.0580
Detailed model
19.25
1.798
502.4 (8 min 22 s)
28517 (7 h 15 min) 6
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4.3 Magnetic fields in the substation under study The 765 kV substation under study (case study) is fed from 600 MW source and the corresponding full load current is 498 A. The worst-case magnetic field has been estimated by assuming the full load current flowing through the conductors. The maximum value of the magnetic flux density and magnetic field intensity assessed is found to be 39.97 µT and 31.81 A/m, respectively. These estimated maximum values are significantly lower compared to the ICNIRP occupational exposure reference value (at f = 50 Hz) of 500 µT (magnetic flux density) and 400 A/m (magnetic field intensity), respectively. These low magnitudes (in this case study scenario) are attributed to the following facts: (i) the current magnitudes are relatively lower (ii) ground clearances are higher, in the EHV substations. Hence, the hazards due to the magnetic field exposure being limited, it is not of great significance in the present case study. 5. Conclusion The e-field distribution for an actual 765 kV substation in India (as a case study), containing complex arrangement of buses, transmission lines, and bays is computed through CSM. The contour plot of e-field at a height of 2 m above the ground plane over the complete area of the substation of 124944 m2 (30.87 acre) is reported. The average e-field and maximum e-field estimated (rms values) over the substation area obtained using CSM with complex charges is 5.921 kV/m and 13.06 kV/m, respectively. The e-field due to the orthogonally oriented substation conductors (akin to the case study reported) has been effectively computed by two 2D analyses using the infinite line charges in the CSM-based model, probably for the first time. This is achieved by adopting the principle of superposition. With the simplified CSM-based model using infinite line charges for the bay conductors (and without inclusion of bay equipment), the maximum value of the rms e-field (at 1 m away from the bay conductor and 2 m above the ground plane) computed is 8.235 kV/m ±0.071%. On the contrary, in the 3D CSM-based detailed model, the computed maximum value of the rms e-field is 9.648 kV/m ±19.25%. Therefore, simplified model can be realistic in estimating efields. Hence, in view of the total number of simulating charges, CSM errors, computational time, and the CSMbased modelling complexity, the proposed method of simplified model provides a better compromise in assessing the occupational e-field exposure (ICNIRP guidelines) in the substations. 6. References
[1] ‘Growth of electricity sector in India from 1947-2016’, http://www.cea.nic.in/reports/others/planning/pdm/growth_2 016.pdf, accessed 12 May 2017 [2] ‘All India installed capacity (MW) of power stations’, http://www.cea.nic.in/reports/monthly/installedcapacity/201 7/installed_capacity-04.pdf, accessed 12 May 2017 [3] Bharti, S., Dubey, S. P.: ‘No-load performance study of 1200 kV Indian UHVAC transmission system’, IET High Volt., 2016, 1, (3), pp. 130–137
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