Examples of magnetic field calculations in indoor distribution substations

Transcription

1 Examples of magnetic field calculations in indoor distribution substations T. Keikko, S. Kuusiluoma, M. Suojanen, P. Menonen, L. Korpinen Department of Electrical Engineering, Tanzpere University of Technology, Firzland Abstract The magnetic fields from indoor distribution substations have been studied for several years now at Tampere University of Technology (TUT). The need to study the fields caused by distribution substations and the means of reducing them, has mainly arisen from practical problems. One reason for this is disturbances caused by power frequency magnetic fields in some sensitive appliances. Another reason for calculations is the EMC standards, which set immunity levels for magnetic fields (generally 3.8 pt and for displays 1.3 PT) [l, 21. The topic has been discussed for several years now, and a more accurate way of calculating the magnetic fields in a room above or beside a substation has been developed. The aim of this study is to compare the measured and calculated values of magnetic field in rooms above or next to distribution substations. 1 Introduction Magnetic fields from indoor distribution substations can cause disturbances in electrical devices, e.g., in computer monitors. Also, the possibility of healtt, effects of magnetic fields has been discussed, and that possibility has not yet been ruled out. The means of reducing magnetic fields from substations has been studied at Tampere University of Technology (TUT) for several years. In Finland, the number of the substations in national grid is 57 and in regional networks about 750. The number of the indoor distribution substations is PI

2 The aim of [his study is to compare the calculated and measured magnetic fields above or beside indoor distribution substation and to analyze the differences between them. 2 Measurements First the magnetic fields of indoor distribution substations were measured. The room near the transfosmer room was mapped, and the measurement points were marked. The distance between the measurement points was 1 meter. Each of the measurement points were measured at the height of 0 m (the floor level), I m and 2 m. The currents were also measured during the magnetic field measurements. Those currents were used in calculations to have similar circumstances in calculations and measurements. Figure 1 on the left presents an example of the transformer room and on the right the measurement points in a room above it. Magnetic fields could not be measured in all of the measurement points because there were immovable objects in the way, such as cupboards and shelves. / LV- switchgefr Figure l : An example of the transformer room and the measurement points. Measurements were performed with a 3-axial EFA-3 (accuracy +5%) which can record the measured values. In addition, each value of magnetic field was written down in the field book. Afterwards the recordings of the meter were uploaded to a computer, and the data was analyzed. 3 Calculation The substations have been calculated with finite element method (FEM) and analytical method. FEM was used to evaluate the dampening of the fields, when

3 Cornplrturiorlal Methods arid E.vperimetrml Measlrra 929 shielding is used around the conductors [4]. Analytically, the magnetic field produced by a single electric conductor can be determined acc~~rately with the following law of Biot and Savart [5]. where 7 is viewing point, p. is permeability of vacuum, 7' is point with source current density, 7 is current density and v' is volume with source current density. Each conductor is calculated separately, and finally the total field is summed as vectors. FEM calculation is based on Maxwell's equations. Static and dynamic fields are considered and displacement currents are omitted. This yields equation 2 in a homogeneous area. where A is magnetic vector potential, ois electrical conductivity, at is differential time, VU is gradient of electric scalar potential and & is source current density. In two-dimensional calculations the electric scalar potential is zero. Shielding Effectiveness (SE) has been calculated from the FEM results : B,, SE = 20 log,,, - Bs where B,, is magnetic flux density without the shield and Bs with the shield [6,7]. The SE can be used 10 analytically calculate the magnetic field Bs from the shielded conductors on the other side of the shield. 3.1 Realization of calculations In calculations the programs utilized were AutoCad R14, Excel 97, Matlab 5.2 and MagNet First, the secondary conductors of the substation were drawn to AutoCad R14. Second, Excel 97 transferred the co-ordinates of the conductor system to matrix. Third, the magnetic field was calculated analytically with Matlab 5.2. In case of any shielding, the shielding effectiveness was calculated with MagNet.

4 930 Computational Methods and E.\perimental Measures The determination of the 3D-co-ordinates for the conductors was the most timeconsuming phase of the calculation procedure. A complete 3D AutoCad model of an example substation is presented in figure 2. Figure 2: An example display of AutoCad when the conductors are drawn In this study the analytical equations were scripted for Matlab 5.2 at Tampere University of Technology (TUT). The currents of the conductors and phases were needed to be able to complete the calculations. The high voltage conductors were disregarded because the currents are much smaller in them. 4 Results Table 1 presents measured maximum, calculated maximum, the difference between measured and calculated maximum AB,,,,, at the height of 1 m from the floor of the room above or next to indoor distribution substation for measurements S1... S20. Subscript A in substation name is original structure and B... D is reduction method.

5 Table 1. The highest measured value, the highest calculated value and the difference between measured and calculated maximum values AB,,,, 1) on the floor of the space above; 2) measured by an electric utility; 3) inside the substation; 4) in the space beside

6 The measured maximum values were PT, and the calculated values were PT. The differences between measured and calculated maximum were PT. If the difference is compared to measured values it means that the difference is %. 5 Discussion It was noticed that the thickness of the floor and walls was difficult to evaluate, which made the distances from the transformer room difficult to define. That could have affected the accuracy of the calculation results. Calculated and measured values differ most in cases where the magnetic values were very small. In the calculations, both analytical equations and 2D-FEM (Finite Element Method) were used. By using analytical methods, the bus bars can be modeled in three dimensions and the accuracy in simple bus bar constructions is quite good. When calculating the effect of shielding, 2D-FEM was used because material effects cannot be considered with the analytical method. Magnetic field is directly proportional to load current. Likewise the phase angle of the current affects the magnetic field. When calculating magnetic field or shielding effectiveness, momentary phase angles for the currents are chosen to represent the general situation. When the load changes, the phase angles usually change as well. That may cause the difference between calculated and measured values. In addition, change in the load current, sensitivity of magnetic field meter, measurement method and all the conductors and equipment which produce magnetic field, may have an effect on the difference between measured and calculated values. The load current changes in all conductors of the substation. That is difficult to take into account when calculating magnetic fields. Sometimes, in some substations, the change of the load current is very fast, which leads to a high difference between the load in momentary measurements. That also means a high difference in calculation errors between different calculation points. The wiring and the electric appliances that were not taken into account in calculation also affect the difference between calculated and measured values in all indoor distribution substations. Furthermore, the heat effect of the constructions should be considered when planning the shielding for high current systems. This kind of load may appear especially in industry. 6 Conclusion The method used in this study: the combination of analytical calculation method and the 2D-FEM was suitable, despite some inaccuracies in the measurement of the intermediate agent and specifically its thickness. With the 2D-FEM the different properties of the materials, e.g., the shielding material, could be taken into account.

7 More substations should be measured and calculated in order to make better comparisons between the measured and calculated values, and to sum up the accuracy of the calculation more comprehensively. From this study, it can be concluded that the calculation can be further developed to achieve better results by, e.g., evaluating the thickness of the walls better. Overall, the results of this study can be considered quite accurate, and the measurements show that the calculation method used is quite reliable. References [l] EN Electromagnetic compatibility - Generic immunity standard. Part 1: Residential, commercial and light industry [2] EN Electromagnetic compatibility - Generic immunity standard. Part 2: Industrial environment [3] Electricity Statistics for Finland Helsinki: Finnish Electricity Association, 1998,90 p. [4] Salinas E. Reduction of Power-Frequency Magnetic Fields from Electrical Secondary Substations. Chalmers University of Technology, Goteborg, Sweden p. [5] J.R. Reitz and F.J. Milford: "Fou~ldutions of Electromagnetic Theory", Addison-Wesley Publishing Company, Inc., [6] Hasselgren L. and Hamnerius Y. Calculation of Low Frequency Magnetic Shielding of a Substation Using a Two Dimensional Finite Element Method. Chalmers University of Technology, Goteborg, Sweden p. [7] W.W. Cooley, Low-Frequency Shielding Effectiveness of Nonuniform Enclosures, IEEE transaction.^ on E1rctrot)zagnetic Compatibility, 10(1), 1968,