Experimental EMF Characterization of a Secondary Substation

Transcription

1 Experimental EMF Characterization of a Secondary Substation Giovanni Cangemi, Antonio Cataliotti, Member IEEE, Giovanni Cipriani, Valentina Cosentino, Vincenzo Di Dio, Salvatore Nuccio Department of Energy, Information Engineering and Mathematic Models (DEIM), Università di Palermo, Viale delle Scienze, Palermo, Italy giovanni.cangemi01@unipa.it, acataliotti@ieee.org, giovanni.cipriani@unipa.it, vincenzo.didio@unipa.it, salvatore.nuccio@unipa.it. Dario Di Cara, Giovanni Tinè, Member IEEE, National Research Council (CNR), Institute of Intelligent System for Automation (ISSIA) Via Dante Alighieri, 12, Palermo, Italy dicara@pa.issia.cnr.it, tine@pa.issia.cnr.it Mario Melodia CEP s.r.l. Zona Industriale Fegotto 91013, Calatafimi- Segesta (TP) Italy melodia.mario@cepsrl.it Abstract This paper presents the results of the experimental EMC characterization of a new medium/low voltage (MV/LV) secondary substation, which was developed in the framework of the research project "NEW CAB ELARC". It has two MV/LV transformers, with total rated power of 1000 kva. To perform the experimental characterization, firstly an analysis was performed of the Italian and European standards recommendations. Different approaches were highlighted and the most appropriate procedures were identified for the measurement of the magnetic fields emissions, at power systems frequency (50 Hz) in steady-state conditions. A dedicated test lab was set up for the secondary substation characterization. The measurement results showed that, thanks to the chosen layout, the emissions of the substation under test are lower than those of typical substations of the same power. Keywords Electromagnetic compatibility (EMC), secondary substations, induction magnetic field, electro-smog. I. INTRODUCTION In recent years, new technologies have been developed in industrialized countries for renewable energy sources (RES) exploitation for producing electricity. New issues have arisen concerning power plants connection to the grid and medium to low voltage (MV/LV) electricity transformation, for which the scientific community and companies must find innovative solutions. The research project "NEW CAB ELARC" is in this framework [1]; it has been developed by the companies CEP S.r.l., Isca Francesco S.r.l. and CIM S.r.l., in collaboration with the University of Palermo. The project is aimed at the development of new solutions of prefabricated substations with internal arc resistance and low electromagnetic emissions. The new substation are designed in compliance with the requirements of current Italian and European standardization and regulation, in the field of the electrosmog effects limitation and resistance against internal arcing [2 7]. As regards the first aspect, it has to be noted that the substations are often installed in places accessible to the public; therefore they must ensure the people safety in their normal use conditions. In this field, a significant aspect is related to the exposure to electric and magnetic fields. In more detail, it is important to evaluate the magnetic induction around the substation, while electric field magnitude is typically well below the values that deserve attention [8]. This paper describes some activities which have been carried out in the framework of the project "NEW CAB ELARC". They have been initially aimed at studying the magnetic induction field distribution in the neighbourhood of typical structures of secondary substations [9]. The study presented in this paper was carried out for a vibrated reinforced concrete prefabricated box substation, built by CEP S.r.l.. In order to evaluate the substation structure improvements for the electrosmog effects limitation, firstly a study was developed in order to identify the more appropriate measurement procedures between those suggested by the Italian and international regulation and standardization. Then the test laboratory is described which was set up within the premises of the CEP S.r.l.. In the paper some of the obtained results are reported, concerning the measurements of magnetic fields emissions of a secondary substation, at power systems frequency (50 Hz) and in steady-state conditions. II. ELECTROMAGNETIC POLLUTION. ITALIAN REGULATION AND STANDARDIZATION In Italy the electromagnetic pollution issues are regulated by the Framework Law n. 36 of 22 February 2001 [7], which concerns the protection from exposure to electric, magnetic and electromagnetic fields. It defines measures for both protection against acute health effects and caution against possible long-term effects produced by electromagnetic fields. It applies also to electrical substations.

2 The Law 36/01 [7] defines the following reference quantities: the exposure limit, the caution value and the quality objective. Their limit values are defined by two Prime Ministerial Decrees (DPCM), both issued on 8 July 2003 and published on the Official Gazette (G.U.) n. 200, on August 29, 2003 (for frequencies from 0 Hz to 100 khz) [3] and n. 199, on August 28, 2003 (for frequencies from 100 khz to 300 GHz) [10]. The exposure limit is the maximum value that must not be exceeded (even for short periods) and it is related to acute health effects. As regards the magnetic field, the limit value is 100 μt. The caution value is the limit that must not be exceeded in rooms used as prolonged stay (schools, homes, workplaces, etc.); it is intended for protection against possible long-term effects; the limit value is 10μT. The quality objective is a threshold for new plants and it is defined for the purposes of the progressive minimization of exposure; the limit value is 3μT. All the aforesaid limits are referred to 50 Hz induction magnetic fields [3]. As regards the measurement methodologies different testing procedure are suggested in the international standard [4]-[6], thus a deep analysis of the different prescriptions was developed and the conclusions are discussed in the section VI and VII. N.1 MV switchgear Rated voltage 24 kv; Nominal current 630 A; Flash current I cw = 16 ka (1s); N. 2 panels T-SFC (sectioning and earthing units) N. 1 panel S-SBR. III. SECONDARY SUBSTATION UNDER STUDY The tests presented in this paper were performed on a vibrated reinforced concrete prefabricated box substation, equipped with a double 2 shutter fiberglass door and two ventilation grids (model P54 CEP srl). The substation under test is composed by three rooms to allocate two MV/LV power transformers and the MV, LV switchgears. In Fig. 1 the layout and the equipment arrangement of the secondary substation under study is shown. In particular, two 500 kva power transformers were considered; they correspond to the maximum power configuration of the substation (1000 kva) and thus to the maximum magnetic field. The specifications of the substation are the following: N. 2 MV/LV power transformers: Rated power A n = 500 kva; Rated power ratio K n = 20000/400 ± 2x2.5% V/V; Nominal current I n,lv = 722A and I n,mv = 14.4A; Cooling ONAN; No-load losses P 0 = 910 W; No-load current I 0 % = 0.6%; Short circuit losses P cc = 6150 W; Short circuit voltage V cc % = 4.56%; Rated frequency f = 50 Hz; Vector group Dyn11. N.1 LV switchgear (model: ABB ArTu K) Rated voltage V n = 400 V; Nominal current I n = 1600 A; Short circuit current I cp = 31.7 ka. Fig. 1. Layout of the secondary substation under study. IV. TEST SYSTEM DESCRIPTION The experimental test were performed according to the procedures and the methodologies suggested by the in force standards and regulations [2 7]. The maximum external magnetic induction is obtained when the nominal currents are flowing into the two transformers. Thus, according to [6], a short circuit test can be performed. The connection scheme is shown in Fig. 2. The high voltage side of the power transformers is connected to an adjustable voltage source, while the low voltage bus-bars are short circuited. Since the secondary substation under test is a MV user substation, the maximum high voltage current is the sum of the transformer rated currents, thus no additional source have to be connected to the high voltage switchboards. To obtain the transformer rated currents, the voltage source has to be adjusted to give the transformer short circuit voltage (912 V). A 50 kva induction regulator is used to generate the adjustable supply voltage. It is supplied with the mains voltage of 400 V, giving a maximum output of 800 V. Even with the maximum output voltage, the transformer rated current could not be obtained. However the standards allow to perform the magnetic induction measurement at a different testing current (lower than the nominal one). The magnetic induction measured value has to

3 be then multiplied for the ratio between the rated current and the testing current (carry rate) [6], [9]. In this way an overestimation of the magnetic induction is obtained, which errs on the side of safety. A delta connected capacitor bank (34.25 F for each phase) is used to improve the power factor at the induction regulator output, thus reducing the current absorbed by the mains. Fig. 2. Short circuit test connection scheme. All the low and medium voltage equipment are earthed by connecting the secondary substation collector to the warehouse grounding system. In the same way, the induction regulator is also earthed. The substation under test is supported by four concrete cubes, thus guaranteeing enough distance from the floor for the entrance and exit of the cables. Both the supplying and the short circuit cables are laid on the floor. They have a direction orthogonal to the substation walls. Moreover, they enter the substation through the access hatches in the bottom. More in detail, the supplying cables are orthogonal to wall B, while the short circuit cables exit from the wall A, behind the medium voltage switch-boards. The exiting cables are laid inside a metal channel. the instrument bandwidth should be adequate to the frequency content of the magnetic induction to be measured (50 Hz - 1 khz in the case under study), in order to take into account both the fundamental and the first harmonics; measurements should be performed in the absence of disturbing elements, such as external source of magnetic induction, and far from ferromagnetic materials; the magnetic induction has to be evaluated at different heights, in order to consider the non-uniformity of the field generated by the substation and to include all the possible effects on different parts of the human body. To this aim, the measurement are repeated at three different heights from the floor (0.5 m m m). The magnetic induction was measured with a PMM 8053 field meter whose specification are reported in TABLE I. The meter was connected via fiber optic to a PMM EHP-50C magnetic field analyzer, whose specification are reported in TABLE II. Registration number Frequency range Dynamic Measuring time Function Averaging mode Averaging time TABLE I. PMM 8053 FIELD METER Operating temperature -10 to +40 C Calibration certificate date 30/10/2014 Calibration expiration date 29/10/ WK Hz 40 GHz > 140 db 150 msec with 80 Hz filter 250 msec with 40 Hz filter 450 msec with 20 Hz filter 900 msec with 10 Hz filter Max., Min., Averaging Arithmetic, quadratic (RMS), manual, rolling average and spatial over Selectable from 30 sec, 1, 2, 3, 6, 10, 15, 30 min TABLE II. PMM EHP-50C ELECTRIC AND MAGNETIC FIELD ANALYZER Fig. 3. Secondary substation under test. V. MESUREMENT INSTRUMENTATION To perform the magnetic induction measurement according to the in force standard [2 7], the following issues should be considered: the measurement instrument should be connected to a isotropic tri-axial probe; moreover an overall measurement uncertainty lower than 10% should be guaranteed; Registration number Frequency range Level range Overload Dynamic Resolution Sensitivity Absolute error Linearity Electric field rejection SPAN 352WN Hz khz 1 nt 10 mt 20 mt at 50Hz > 140 db 1 nt 1 nt ± 0,5 db at 50 Hz and 0,1 mt ± 0,2 db (200 nt - 10 mt) > 20 db 100, 200, 500 Hz, 1, 2, 10, 100 khz FFT spectral analysis Start frequency Stop frequency 1.2% of the SPAN same of the SPAN Isotropicy ± 0.12 db Flatness (40 Hz 10 khz) ± 0.5 db Operating temperature -10 to +40 C Temperature error ± 0.01 db/ C Humidity error ± 0.05 db/ C Calibration certificate date 18/12/2014 Calibration expiration date 17/12/2016

4 The supplying current and voltage were monitored at the induction regulator output with a Multiver 3S Dossena power analyzer. The short circuit current was measured with a Fluke 435 power analyzer, which was coupled to the cables by means of four i430-flex current probes. During the tests, the temperature and the humidity were constantly monitored with a Vaisala HM34 HUMICAP Humidity & Temperature Meter. VI. MEASUREMENT METHODOLOGY AND RESULTS To evaluate the magnetic field emitted by the secondary substation, the power supply was set in order to have a LV current of I LV =1040 A (average value of the three phases), and a MV current I MV = 20.9 A. The climatic condition tests were: temperature 28 C and humidity 35%. The rms induction magnetic field measurements were carried out by means of the PMM 8053 and the EHP 50C analyzer linked by fiber optic. The instrument bandwidth frequency was set to 1 khz and the measurements were performed for different heights of the analyzer. During the tests the supply voltage and current of the substation were monitored and maintained constant in order to guarantee that the magnetic field measurements were performed in correspondence to the same reference currents. According to [6], a magnetic field inspection has to be performed all around the secondary substation. More in detail, a sequence of measurement points was identified in parallel direction to each side of the substation and with a step distance of 0.2 m. Moreover, the measurement points have a distance of 0.2 m from the substation. As regard the height, a similar step distance could have been used. On the other hand, the standard EN [4] recommends to measure the magnetic induction at three different heights (0.5 m, 1 m and 1.5 m from ground) which are related to the human body extension. Thus a first inspection was performed at an height of 1 m. Comparing all these measurements, the hot spot values, i. e. the point with the maximum measured magnetic field, can be identified for each side of the substation. Moreover, to determine the variation of the magnetic field as a function of the distance from the substation, starting at each hot spot the field value was measured along a line perpendicular to the cabin until the measured values was lower than 1/10 (-20dB) of the hot spot value [6]. This procedure was then repeated at the other heights, 0.5 m and 1.5 m, as suggested in [4]. The rms B values measured at the different heights, 0.5 m, 1 m and 1.5 m, are reported in Fig. 4, Fig. 5 and Fig. 6, respectively. The results show that the induction magnetic field measured in correspondence of the A wall, at the height of 0.5 m, does not decrease with the distance. This behaviour is justified by the presence of the LV cable disposed on the floor, as shown in Fig. 3. Taken into account that in a real installation the cables are under ground to a depth of 0.5 m, to evaluate the magnetic field for this wall it is possible to consider the measurements performed at 1 m and 1.5 m. As expected, the maximum induction magnetic field values was measured on the D side, in proximity of the LV switchgear and the LV cable harness. The short circuit LV current measured in the tests (about 1000 A) is lower than the sum of the transformer rated currents (1444 A). Therefore, the induction magnetic field measured in the tests has to be multiplied for the carry rate (1.44). For each hot spot the maximum induction magnetic field values are reported in Fig. 7 in dependence of the distance from the substation. Fig. 4. B values measured for each hot spot and wall of the secondary substation at different heights (h=0.5m), variables with the distance. Fig. 5. B values measured for each hot spot and wall of the secondary substation at different heights (h=1m), variables with the distance.

5 According to the first approach, the standard CEI EN [4] introduces the three-points average exposure level. This value is obtained as the arithmetic mean of the field measurements at three different heights (0.5 m, 1 m and 1.5 m from ground). For the case under study, the measurements results of the previous section were processed in order to determine the average three-points exposure level in the hot spots at different distances from the walls. The results obtained are shown in Fig. 8. It can be observed that field values below 3 μt were obtained at a distance above 1.7 m. Rounding to the upper half meter (in accordance with Decree 29 May 2008), the DPA was assumed to be 2 m. Fig. 6. B values measured for each hot spot and wall of the secondary substation at different heights (h=1.5m), variables with the distance. Induction Magnetic Field [mt] 40,0 35,0 30,0 25,0 20,0 15,0 10,0 5,0 0,0 0,0 0,5 1,0 1,5 2,0 2,5 3,0 Distance from the hot spot location [m] Side A Side B Side C Side D Quality target Fig. 7. Determination of the field variation as a function of the distance from the hot spot locations (perpendicular to the reference surface). After carrying out the measurements and switching off the equipment, the background noise level were measured and recorded. The measurements results showed that the background field level was below 1/10 of the lowest value measured at any of the hot spots found during the tests [6]. VII. DPA EVALUATION The aforementioned DPCM 2000/03 [3] introduces a buffer zone for electrical substations, which includes all points where the magnetic induction value may be greater than or equal to 3 T under normal operating conditions. The buffer zone can be evaluated following a simplified procedure, which makes use of the DPA parameter ( Distanza di Prima Approssimazione, first proximity distance). It is the distance from the substation at which the aforementioned requirement is fulfilled. Thus the substation can be characterized by determining the distance from walls and roof where the magnetic field value falls below the threshold of 3 T. The DPA parameter was evaluated following two different approach, according to the indications of the two different standard EN [4] and CEI [5]. Induction Magnetic Field [mt] 30,00 25,00 20,00 15,00 10,00 5,00 0,00 0,00 0,50 1,00 1,50 2,00 2,50 3,00 Distance from the hotspot location [m] quality target A Side B Side C Side D Side Fig. 8. Field variation as a function of the distance from the hot spot locations (perpendicular to the reference surface) and comparison with the quality target. According to the second approach, the standard [5] suggests, in the case of not uniform fields, to consider the position with the highest measured value, among those found at different heights. The considered heights are related to the human body extension (suggested heights between 1.10 m and 1.90 m). Following this approach, the DPA was evaluated by means of single-point measurements. The heights with the highest field (among those already chosen) and the hot spot on the roof were considered, by determining distance from walls and height from the roof, respectively, over which the field fell below 3 T (the measured value was carried at the rated current value). The obtained results showed that the maximum distance from walls and roof over which the field falls below the quality target of 3 T is equal to 1.82 m (see Fig. 7). Rounding to the upper half meter, the DPA was still equal to 2 m (worst case evaluation). VIII. CONCLUSION In this paper the experimental EMF characterization of a secondary MV/LV substation has been presented, which was developed in the framework of the research project "NEW CAB ELARC", with the aim of reducing the magnetic fields emissions. To carry out the study, the recommendations of the different national and international standards and regulations were analysed. Different approaches were highlighted and the most appropriate procedures were identified for the measurement of the magnetic fields emissions of the secondary substations, at power systems frequency (50 Hz) and in stationary conditions.

6 A dedicated test set up was developed and real tests were performed on a vibrated reinforced concrete prefabricated box substation, developed by CEP S.r.l. It was equipped with two MV/LV power transformers with total rated power of 1000 kva. The measurement results showed that, thanks to the chosen layout, the emissions of the substation under test are lower than those of typical substations, with traditional building solutions and equipped with power transformers of equal power. As an example, the substation DPA is smaller (2 m) if compared to that of the traditional solutions. The measured DPA, in fact, is typical of substations with rated power (630 kva) lower than that of the substation under test (1000 kva) [2]. ACKNOWLEDGMENT This publication was supported by the POR titled NEW CAB ELARC (CUP: G93F ) PO FESR Sicilia Sicilian research program. This work was carried out with the contribution of Laboratorio tarature e prove per il settore elettrico ed elettronico - UNINETLAB - University of Palermo. REFERENCES [1] Cangemi, G., Cataliotti, A., Cipriani, G., Cosentino, V., Di Dio, V., Lipari, A., Di Cara, D. and Tinè, G. Critical issues and future prospects of the secondary substation in smart grid context. AEIT Annual Conference - From Research to Industry: The Need for a More Effective Technology Transfer, AEIT (2014). [2] Decreto 29 maggio 2008 Ministero dell Ambiente e della tutela del territorio e del mare Approvazione delle procedure di misura e valutazione dell induzione magnetica. (Decree of 29 May Ministry for the Environment, Land and Sea - Approval of procedures for measuring and assessing magnetic induction). In Italian. [3] Decreto del Presidente del Consiglio dei Ministri n luglio 2003 Fissazione dei limiti di esposizione, dei valori di attenzione e degli obiettivi di qualità per la protezione della popolazione dalle esposizioni ai campi elettrici e magnetici alla frequenza di rete (50 Hz) generati dagli elettrodotti. Pubblicata nella Gazzatte Ufficiale n. 200 del Decree of Chairman of the Board of Ministers 8 July 2003: Establishment of exposure limits, attention values and quality objectives for the protection of the population from exposure to electric and magnetic fields at mains frequency (50 Hz) generated by power line published in the Official Gazette of the Italian Republic of 29 August 2003, general series n In Italian. [4] Electric and magnetic field levels generated by a.c. power systems - Measurement procedures with regard to public exposure, Standard EN 62110, [5] Guida per la misura e per la valutazione dei campi elettrici e magnetici nell intervallo di frequenza 0 Hz 10 khz, con riferimento all esposizione umana, (Guide for the measurement and evaluation of electric and magnetic fields in the frequency range 0 Hz - 10 khz, for human exposure) Standard CEI 211-6, [6] High-voltage switchgear and controlgear Part 208: Methods to quantify the steady state, power-frequency electromagnetic fields generated by HV switchgear assemblies and HV/LV prefabricated substations, Standard CLC/TR , [7] Legge 22 febbraio 2001, n.36 Legge quadro sulla protezione dalle esposizioni a campi elettrici, magnetici ed elettromagnetici Pubblicata nella Gazzetta Ufficiale del 7 marzo 2001, n.55. (Italian law 22nd February 2001, no. 36: Framework Law on protection from exposure to electric, magnetic and Electromagnetic published in the Official Gazette of the Italian Republic no. 55 of 7 March In Italian. [8] Proios, A.N., Anagnostatos, S.D., Polikrati, A.D., Tsarabaris, P.T. and Koufakis, E.I. Magnetic field measurements near a compact kiosk type substation. MELECON th IEEE Mediterranean Electrotechnical Conference (Apr. 2010), [9] Cangemi, G., Cataliotti, A., Cipriani, G., Di Dio, V., Lipari, A., Miceli, R. and Tinè, G. Characterization of the Magnetic Induction Field produced by Secondary Substations. 20th IMEKO TC4 International Symposium and 18th International Workshop on ADC Modelling and Testing Research on Electric and Electronic Measurement for the Economic Upturn (2014), [10] Decreto del Presidente del Consiglio dei Ministri n luglio 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. - GU n. 199 del Decree of Chairman of the Board of Ministers 8 July 2003: Establishment of exposure limits, attention values and quality objectives for the protection of the population from exposure to electric, magnetic and electromagnetic fields generated at frequencies between 100 khz and 300 GHz published in the Official Gazette of the Italian Republic of 28 August 2003, general series n In Italian..