Power-frequency magnetic field calculation around an indoor transformer substation

Transcription

1 Boundary Elements XXVII 695 Power-frequency magnetic field calculation around an indoor transformer substation A. Geri & G. M. Veca Department of Electrical Engineering, University of Rome, Italy Abstract The authors propose a full three-dimensional (3D) calculation model based on a multipole technique able to predict the magnetic field pollution outside of indoor medium voltage/low voltage (MV/LV) transformer substations. The authors demonstrate that the proposed model may be advantageously used in all problems related to the characterization of the electromagnetic environment around a MV/LV transformer substation. They also show how the model may be used to support the optimised design of the cabin layout when this solution is sufficient to assure the reduction (below the exposure limits fixed by law) of the magnetic flux density in target volumes within civil buildings. In particular, the proposed algorithm has been applied to evaluate the magnetic flux density levels in the neighbourhood of a typical indoor MV/LV cabin placed at ground zero of a civil building (and therefore bordering on houses and/or offices). In addition, in order to mitigate the magnetic field outside the transformer substation, using the proposed calculation code an attempt was made to design the layout of the cabin by rearranging the position of the LV distribution switchboard (i.e., the power center, PC). Keywords: magnetic pollution, magnetic field calculation, magnetic mitigation, MV/LV transformer substation, distribution power systems. 1 Introduction The installation of a new indoor MV/LV transformer substation (see fig. 1), in urban areas may be a very hard task with respect to the problems related to human exposure to power frequency magnetic fields. In fact, these cabins are often placed in civil buildings therefore they border on houses or offices (i.e., places where people live for a large part of the day). In addition, during the last

2 696 Boundary Elements XXVII years, the attention of the public opinion, with respect to this possible risk for human health, is growing up (e.g., during this year the Italian law concerning the limits of exposure has been revisited, [1]). Consequently, today, it is fundamental to predict the electromagnetic pollution due to a new cabin placed within civil structures or residential buildings, in order to evaluate the exposure levels of population living or working near the substation. For these reasons, the authors describe in this paper an efficient and reliable tool for the prediction of the magnetic pollution around a cabin. Because of several sources contribute to generate the magnetic flux density in the vicinity of a cabin, its detailed calculation is therefore rather complicated. According to numerous measurements in different type of MV/LV transformer substations, the main sources of magnetic field may be reduced to: the in and out MV power cables, the connection bus-bars, the MV and the LV panels and the outlet LV power cables (i.e., the contribution of the transformer has been neglected). The magnetic field is computed using a powerful 3D computational model described in companion publications, [2,3]. In particular, in this paper, a typical indoor MV/LV transformer substation has been studied. From electromagnetic point of view, the environment around the cabin has been completely characterized by means of the proposed tool. The authors have also demonstrated that the overcome of exposure limits, fixed by law, may be very frequent in the neighbourhood of the substation. Some possible design solutions able to mitigate the magnetic pollution around the cabin has been evaluated and discussed: in fact, in many cases, simple rearrangement of the layout of the substation may be sufficient to reduce the magnetic field values outside the cabin. TR 70 A MV LV 38 A 700 A 900 A Figure 1: Schematic 3D view of a typical indoor MV/LV transformer substation (e.g., located at ground zero of a civil building). Excitation currents are also highlighted.

3 Boundary Elements XXVII MV/LV cabin configuration A typical indoor MV/LV transformer substation has been considered in this study, fig. 1. It has input and output MV supply power cables, and two LV outgoing distribution cables. These distribution cables start from the PC, which is a LV distribution switchboard installed downstream the MV/LV transformer. The PC and the LV distribution cables are usually critical sources in a MV/LV cabin, with respect to the problems related to magnetic pollution at power frequency. For this reason, the target volume has been deliberately located near both the PC and the LV outgoing distribution cables, fig. 2. (a) (a) (b) Figure 2: Geometrical configuration of a typical indoor MV/LV transformer substation: (a) schematic 3D view of main busbars, distribution busbars, connection cables and supply power cables and its layout (b). The target volume is also highlighted. All lengths are in m.

4 698 Boundary Elements XXVII 3 Computational model The magnetic flux density in each test point T of a target volume is computed by the classic multipole technique based on a discrete approximation of the Biot- Savart law. In particular, each long or short energized conductor (in which flow the currents i e ) is automatically divided into a suitable number of straight segments (e.g., the generic segment RS in fig. 3). Thereafter, in correspondence with the point T, the magnetic flux density components (i.e., B x, B y and B z ), due to each elementary segment, is computed as described in figure 3. Then, the magnetic flux density in the point T, generated by all energized conductors, can be simply obtained by superposing the field generated by each segment. a x x 2 x 1 b x x x 1 c x x x RS a x a y a z a y y 2 y 1 b y y y 1 c y y y TR b x b y b z a z z 2 z 1 b z z z 1 c z z z TS c x c y c z a x l 1 RS RP l 1 b x m 1 b y n 1 b z a y m 1 RS TP TR 2 RP 2 a z n 1 RS PS RS RP 0 i ek RP B 4 TP TR PS TS G a y c z a z c y 2 a z c x a x c z 2 2 a x c y a y c x l 2 B x a y c z l 2 B a z c y G m 2 B y a z c x m 2 B G a x c z n 2 B z a x c y n 2 B a y c x G Figure 3: Computational algorithm of the magnetic flux density components due to a generic small straight segment of an energized conductor located within a typical indoor MV/LV transformer substation.

5 Boundary Elements XXVII 699 Thus, starting from the layout of an indoor MV/LV transformer substation (i.e., for an assigned geometrical and electrical configuration of the main field sources consisting of main busbars, distribution busbars, connection cables and supply power cables, fig. 2) and knowing the current in each conductor inside the cabin, the prediction of the magnetic field levels in a target volume within a civil building may be easily computed by applying the proposed numerical model. This mathematical model has been chosen because it is compact and fast. The algorithm may be easily implemented in a calculation code by special high level programming languages (e.g., those embedded in general purpose commercial softwares such as Matlab or Matcad); the use of low level programming languages, such as FORTRAN or C++, is not necessary in this case. In addition, this code can be also simply coupled with genetic algorithms (GAs) with the aim to point out optimised design procedures of MV/LV transformer substation [4] with respect to the problems related the mitigation of the magnetic field inside target volumes. 4 Numerical results The proposed 3D model has been applied to study a typical indoor MV/LV transformer substation. The in and out current values have been chosen referring to usual nominal data of public distribution cabins in Italy and they are specified in fig. 1. The 3D geometric configuration of main busbars, distribution busbars, connection cables and supply power cables is shown in fig. 2. In this figure, the target volume has been also highlighted; it is located within the civil building and in the proximity of the PC, that is where the magnetic pollution is probably highest. The layout of this cabin is shown in fig. 4. The results of all simulations are plotted in fig. 5 where, referring to representative planes of the target volume (i.e., yz plane at x = 2 m, fig. 5a, xz plane at y = -1 m, fig. 5b; xy plane at z = 1 m, fig.5c), contour plots as well as surface plot of the magnetic flux density are drawn. It is possible to note that the magnetic flux density values overcome the exposure limits (in Italy, the values fixed by law are 3 µt, for new installations, and 10 µt, for existing installations) in relevant region of the target volume. In order to mitigate these magnetic field levels, an attempt was made to shift away the PC from the lateral wall, and then from the target volume. The modified layout of the cabin is shown in fig. 6. It is easy to observe that shifting away the PC from the wall of 500 mm, the maximum value of the magnetic flux density is reduced of about 53% (from 72.1 µt to 34 µt). A quite similar trend has been obtained for the mean value of the magnetic flux density. The results of the simulations pointed out for this arrangement are plotted in fig. 7 where, referring to the same representative planes previously defined, it is possible to note how the magnetic field map is changed with respect to the original configuration of the cabin. Even if the reduction of the magnetic field is relevant (for the peak value it is of about 53%), it is not sufficient to respect the limits fixed by Italian law. Then, some additional actions must be adopted. In particular, the authors are investigating the benefits of an active shielding by simple energized loops appropriately placed and feed [4].

6 700 Boundary Elements XXVII z LV MV Lateral view y x LV y MV Plant view Figure 4: The original configuration of the typical indoor MV/LV transformer substation previously introduced. All lengths are in mm.

7 Boundary Elements XXVII 701 (a) (b) (c) Figure 5: Magnetic flux density (T) generated, in the target volume, by the original configuration of the MV/LV cabin: (a) contour plot on a yz plane at x = 2 m; (b) contour plot on a xz plane at y = -1 m; (c) surface plot on a xy plane at z = 1 m. All lengths are in m.

8 702 Boundary Elements XXVII LV MV z y y x LV MV Lateral view Plant view Figure 6: The modified configuration of the typical MV/LV cabin under analysis. The PC has been shifted away from the wall: in particular, the distance from the target volume has been increased of 500 mm. All lengths are in mm.

9 Boundary Elements XXVII 703 (a) (b) (c) Figure 7: Magnetic flux density (T) generated, in the target volume, by the modified configuration of the MV/LV cabin: (a) contour plot on a yz plane at x = 2 m; (b) contour plot on a xz plane at y = -1 m; (c) surface plot on a xy plane at z = 1 m. All lengths are in m.

10 704 Boundary Elements XXVII 5 Conclusions and remarks From the analysis of the results it is possible to conclude that the proposed model can be advantageously used for a complete characterization of the electromagnetic environment around indoor MV/LV transformer substations. Thus, this model may be applied during the design of new installations (when the magnetic flux density values are very critical in some regions outside the substations, especially if they are located within civil buildings) and the rearrangement of existing cabins (when they must be accommodated in order to respect more restrictive limits on the maximum magnetic flux density admissible for human exposure). The authors are working in order to generalize the proposed model accounting for more realistic cases. In particular, they are working to introduce the simulation of load diagrams (daily, monthly and yearly) as well as the simulation of time varying excitation currents (e.g., instantaneously unbalanced three phase systems). These improvements are necessary in order to predict the magnetic field levels (peak values and medium values) during fixed time ranges (days, months or years), with the aim to support epidemiological researches in the correlation between cause and effect. In addition, they are also working to couple the proposed model with GAs in order to develop automatic optimization procedures for the design of active shielding systems, which have the aim to mitigate, in assigned target volumes, the magnetic flux density produced by indoor MV/LV substations. Acknowledgments This study has been financially supported by Provincia di Roma, Dipartimento n. 2 Ambiente, Servizio n. 3 Tutela dell aria. References [1] DCPM 8 luglio 2003 (in G.U. n. 199 del 28 agosto 2003) Fissazione dei limiti di esposizione, dei valori di attenzione e degli obiettivi di qualità per la protezione della popolazione dalle esposizioni a campi elettrici, magnetici ed elettromagnetici generati a frequenze comprese tra 100 khz e 300 GHz. Italian law. [2] Geri, A., Locatelli, A., & Veca, G.M., Magnetic fields generated by power lines, IEEE Transactions on Magnetics, 31(3), pp , [3] A. Geri and G. M. Veca, Prediction of the magnetic pollution due to power lines, Proc. of III International Congress Energy, Environment and Technological Innovation, Caracas (Venezuela), pp , [4] F. Garzia, A. Geri, Active Shielding Design in a Full 3D Space of Indoor MV/LV Substations Using Genetic Algorithm Optimization, Proc. of 2003 IEEE Symposium on Electromagnetic Compatibility, Boston (USA), 1, pp , 2003.