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2014 International Symposium on Power Electronics, Electrical Drives, Automation and Motion Experimental Characterization of Conducted EMI in Three-phase Power Electronics System using Terminal Model Junsheng Wei, Dieter Gerling Marek Galek Institute of Electrical Drives and Actuators Universitaet der Bundeswehr Muenchen Neubiberg, Germany Corporate Technology Siemens AG Munich, Germany to the requirement on measurement setup to extract model parameters, the modification and build-up of hardware measurement setup are described. The experimental characterization based on this setup is then carried out with procedure described and the data obtained is processed to calculate model parameters. At the end, model obtained is used to predict the noise level at LISN when connection conditions between LISN and converter is changed. Results are compared with measurement. Abstract— This paper investigates the relevant issues for experimental characterization of conducted EMI resulting from three-phase power electronics system. Terminal model is used to describe the EMI behavior. The theoretical analysis and equivalent terminal model for three-phase power electronics system are conducted and proposed respectively. The measurement setup which fulfills the requirement of parameter extraction is then discussed. Measurement procedure is described and the obtained data is processed to calculate the desired parameters. Using the model and parameters from experimental extraction, conducted EMI level can be predicted and the results are compared with measurement to validate the correctness of the characterization. II. Fig. 1 and Fig. 2 show the schematic and prototype of used research object. The investigated object consists of three-phase diode-bridge followed by boost converter and is used as a representative for three-phase system. Conducted EMI is usually measured in standard-defined environment, as shown in Fig. 3 [9]. Keywords—conducted EMI; three-phase system; terminal model I. INTRODUCTION Conducted EMI has been important concern during the design phase of power converters since certain limits are set by international standards committee to ensure that the emission will not endanger other equipment in the vicinity. In order to comply with the standards, certain measures have to be adopted and EMI filter is one of the most widely used methods [1]. Filter design has been a difficult task since long time ago and the trial-and-error routine makes it even more costly as well as time-consuming. There is a trend to design the filter in a virtual way [2]. By combining the estimated filter characteristic with models describing the EMI characteristic of power converter, the effectiveness of noise suppression can be estimated with very good accuracy. Because of the high frequency essence of EMI, the models of power converters, either based on timedomain simulation [3, 4] or frequency-domain simulation [5, 6], require detailed knowledge on the converters, which are not always easily accessible. Thus, terminal modeling has been proposed as an alternative [7]. Most of the researches of terminal modeling focus on single-phase system while threephase system plays also important role in industry and is worthy to be investigated. Fig. 1 Schematic of three-phase prototype This paper is organized in such way that firstly the terminal model of power converter with active source and passive impedance matrix is presented based on analysis of the measurement setup, as already proposed in [8]. Then according 978-1-4799-4749-2/14/$31.00 ©2014 IEEE TERMINAL MODEL OF THREE-PHASE SYSTEM Fig. 2 Photo of three-phase Prototype 1165 The passive admittance matrix in Boost part is a 2×2 matrix and active source is described with a 2×1 matrix. For the acquisition of LISN and cable matrix, it can be done with impedance analyzer and no further measurement setup is needed. As introduced in [8], the matrix of D6 can be obtained with special modeling technique. The parameters in Boost part must however be extracted based on terminal responses under different tests. III. HARDWARE MEASUREMENT SETUP To realize the parameter extraction for Boost part, several tests should be carried out. In order to fulfill the test condition, the LISN should be modified and according to the measurement environment in Fig. 3, additional components are needed. Fig. 3 Measurement environment defined by standard A. Terminal Model Considering the conducted EMI measurement environment and assuming that the line impedance stabilization network (LISN) works as expected in the discussed frequency range, the block diagram in Fig. 4 represents the flow of high frequency current. The diode bridge (D6) is separately considered, apart from other parts in converter. Reason is, under assumption that D6 only affects as propagation path but not as noise source, its time varying essence prevents it to be included in the passive admittance matrix, which should be time invariant. The rest of converter can be described with active source and passive admittance matrix as in Fig. 5, according to Norton’s theorem. A. LISN The used LISN in the investigation is a commercial product, which is designed to fulfill the requirement for conducted EMI measurement in the frequency range from 9 kHz to 30 MHz as defined in [10]. The LISN has circuitry schematic for each phase as shown in Fig. 6. The series connected inductors are used to block high frequency current and parallel connected capacitors are for the purpose of bypassing noise. The LISN has generally two functions: 1. Avoid noise from grid to affect measurement results; 2. Provide constant impedance at different frequencies. Fig. 6 Schematic of one phase in LISN As discussed in [8], for the calculation of model parameters, the high and low impedance terminations at different ports should be applied under different test conditions. Therefore, at the stage of LISN, it should be modified so that these two terminations are realized and easily changed from one to the other. The two circuit connections chosen for achieving these two terminal conditions are shown in Fig. 7 Fig. 4 Block diagrams for high frequency current In the low impedance termination case, only the second capacitor leg is disconnected. This is mainly for the easier switch of impedance states. In the remaining two capacitor legs, the first on the grid side, which has high capacitance value, has actually small impact on overall impedance since the two series-connected inductors have a total inductance of 300 mH and effectively block the high frequency current. The LISN has a load of 50 Ω in each phase. The load is low compared to the other possible impedance in the propagation path of high frequency noise and thus it can be kept in low impedance termination case together with the series connected capacitor. Fig. 5 Equivalent model of Boost part B. Model Parameters As shown in Fig. 4 and Fig. 5, in order to describe the EMI behavior of the system, the passive matrix of LISN, cable, D6, Boost part and the active source matrix of Boost part must be known. The passive matrix of LISN is a 3×3 matrix while that of cable is 6×6 matrix and D6 is represented with a 5×5 matrix. 1166 (1) Fig. 9 Termination impedance with cable (green: high, blue: low) Low impedance termination A possibility to solve this problem is using high impedance between input cable and power converter, to ensure that the impedance seen from power converter is still high. Because the parasitic capacitance effects only at high frequency, the impedance of the additional component is not necessary high in low frequency and the use of inductor with low inductance value should be enough. (2) There will be power current flowing through the blocking inductors which could be critical since with DC-bias the permeability of magnetic material may be influenced. The power current is in the range of several Ampers depends on operation point. The impedance of blocking inductor is therefore tested under DC-bias in this range. It is however not possible to directly measure the impedance with impedance analyzer and thus the measurement setup using S-Parameter measurement is used as shown in Fig. 10. High impedance termination Fig. 7 Circuit connections in LISN for realization of test conditions In the high impedance termination case, besides the second capacitor leg, the third one is also disconnected. In order to facilitate the switch between impedance terminations, BNC connectors are used at the point of disconnection. The measured impedances at low as well as high impedance termination are shown in Fig. 8. Fig. 10 Measurement setup for impedance of blocking inductor under DC-bias S-Parameters are measured with network analyzer and based on characteristic impedance the impedance of device under test is calculated. Fig. 11 shows the impedance of the inductor under different DC-bias and even though the resulting impedance is lower with increasing DC-bias, the impedance value is still high enough for the application and therefore the test inductor is used as blocking inductor. Fig. 8 Measured impedances at different terminations (blue: low termination impedance, green: high termination impedance) B. Blocking Inductors The measurement setup in Fig. 3 shows the use of input cable and in reality, an input cable of about 2 meters is used to supply power from LISN to power converter. Because of the length of cable, the parasitic effects, including parasitic inductance and parasitic capacitance, cannot be neglected in the frequency concerned. Especially the parasitic capacitance induces problem for realizing high impedance termination, since it provides a low bypass path in high frequency range and reduces the effectiveness of high impedance. Fig. 9 shows the comparison between high and low impedance termination if cable is taken into account. Fig. 11 Measured impedance of blocking inductor under DC-bias 1167 considering the voltage level, input capacitance and if it is differential or not. Those possibilities are considered and tested to find out suitable probes for the purpose of conducted EMI characterization as shown in following. Comparing with Fig. 9 which shows the comparison between low and high impedance with cable, Fig. 12 shows the comparison between high termination impedance considering the cable and blocking inductor. Improvement of high impedance in high frequency range is obvious. The voltage probes are tested firstly. The tests have been made at the same source and with FFT the spectrums from two voltage probes are calculated. The oscilloscope with better performance as shown in last paragraph is used. Fig. 14 shows the spectrums. Fig. 12 Termination impedance with cable and blocking inductor (green: high, blue: low) C. Oscilloscope The oscilloscope is used to capture waveform in time domain, which is then post-processed and transformed into frequency domain by using Fast Fourier Transformation (FFT). From the theory of FFT, it is known that in order to achieve good resolution in the spectrum at high frequency, the recorded waveform must contain small steps and include accurate change in short time. These requirements on waveform can be interpreted as requirements on oscilloscope on two aspects: the storage capability and the resolution of Analogue-DigitalConverter (ADC). In the investigation, two oscilloscopes with different storage capabilities and ADC resolutions are used. Fig. 13 shows the spectrums calculated by FFT from recorded waveforms of the two oscilloscopes with the same source. Fig. 14 Calculated spectrums with different voltage probes From the spectrums, the resolution in high frequency range is obvious. The better performance is obtained using voltage probe with smaller capacitance and lower voltage rating. For current probes, tests have also been conducted and their spectrums are shown in Fig. 15. As the figure shows, neither the method of shunt resistor nor the hall-effect sensor has enough accuracy, considering that the current transformer is a well-calibrated device for high frequency application. Therefore, only the current transformer will be used for acquisition of current information. Fig. 13 Spectrums using two different oscilloscopes From the spectrums, it is obvious that the used oscilloscope has determinant influence on the accuracy of data acquisition, especially in the high frequency range. Fig. 15 Calculated spectrums with different current probes (red: Shunt resistor, blue: current transformer, green: Hall-effect sensor) Till now all the used hardware devices have been determined. D. Voltage Probe and Current Probe In order to transfer the electrical signal from measurement object to oscilloscope, voltage and current probes are used. There are different possibilities of measurement methods and different types of devices to be chosen. For instance, in order to measure the current, possible methods include shunt resistor, current transformer and hall-effect sensor. For voltage measurement, the choice of voltage probe is various IV. MEASUREMENT PROCEDURE AND DATA PROCESSING With the measurement setup and devices chosen from analysis as described in last chapter, the measurement procedure can be carried out. As explained in [8], to calculate the parameters in terminal model, the following tests must be conducted: 1168 • High impedance termination at all ports; • Low impedance termination at Port 1 and high impedance termination at all other ports; • Low impedance termination at Port 2 and high impedance termination at all other ports. With the hardware setup, the tests can be described as modification on measurement setups: • All BNC connectors at LISN are disconnected and all phase lines are fed through blocking inductors; • BNC connector at Phase 1 is connected and line of Phase 1 bypasses blocking inductor while other phases stay the same as first test; • BNC connector at Phase 2 is connected and line of Phase 2 bypasses blocking inductor while other phases stay the same as first test. Fig. 17 Phase alignment for waveforms (top: before, bottom: after) Since as discussed in [8], for each of these tests, all the terminal behaviors including terminal currents and voltages are needed for the derivation of model parameters while the oscilloscope as common devices has only four input ports, five measurements should be conducted for each test. The input ports of oscilloscope are arranged as following: one for voltage measurement, one for current measurement with high frequency transformer and two for current measurement with hall-effect sensor. The organization is shown in Fig. 16. The use of two current measurements with hall-effect sensors is mainly for time domain aligning of all the measurements and thus the accuracy of such measurement in high frequency is not critical. Fig. 18 Measured waveforms and zoomed-in waveform in two-diode conduction mode Once the frequency domain information about terminal behaviors in different tests is known, the following equations are used to calculate the model parameters, as explained in [8]. [YBoost ] = [Yall ] − [YLISN +Cable+ D 6 ] [ IsBoost ] = [ Isall ] (1) (2) ⎡ I p1 ⎤ ⎡ u p1 ⎤ ⎢ I ⎥ − [ IsBoost ] = ([YBoost ] + [YLISN +Cable+ D 6 ]) ⋅ ⎢u ⎥ (3) ⎣ p2 ⎦ ⎣ p2 ⎦ Fig. 16 Arrangement of oscillation input ports and overall measurement setup To validate the accuracy of model, it is used to predict the conducted EMI under different connection conditions and results are shown in Fig. 19 in comparison with measurement. If it is assumed that the usable range should have deviation within 6 dB, the accuracy of the model suggests the use availability up to 10 MHz. The reason for such accuracy limitation can be located at the realization of termination impedance. As shown in Fig. 12, at about 10 MHz, the Fig. 17 shows the measured waveforms before and after phase alignment. As can be seen, the waveforms are overlapped in time domain with DC deviation which will not influence the calculation of spectrum in frequency domain. The phase alignment is however critical for the correct calculation of model parameters. Fig. 18 shows the measured waveforms of current and voltage at Phase 1 in test 1, with zoomed waveforms in the time frame of two-diode conduction mode. 1169 parameters in terminal model are derived and the accuracy of model is validated by comparison between predicted and measured conducted EMI level under different connection conditions. Thus the model is most suitable for the design of EMI filter and can help to estimate the attenuation brought by filters. difference between high and low impedance termination is small. The better agreement between predicted and measured results in scenario 2 results from the fact that connection condition in scenario 2 is similar to those used in characterization procedure. REFERENCES [1] (1) Predicted noise level in scenario 1 (2) Predicted noise level in scenario 2 V. Serrao, A. Lidozzi, L. Solero, and A. Di Napoli: "Common and differential mode EMI filters for power electronics", Power Electronics, Electrical Drives, Automation and Motion, 2008. SPEEDAM 2008. International Symposium on, 2008. [2] I. F. Kovacevic, A. M. Musing, and J. W. Kolar: An Extension of PEEC Method for Magnetic Materials Modeling in Frequency Domain, Magnetics, IEEE Transactions on, 2011. [3] L. Ran, S. Gokani, J. Clare, et al.: Conducted electromagnetic emissions in induction motor drive systems. I. Time domain analysis and identification of dominant modes, Power Electronics, IEEE Transactions on, 1998. [4] J. Wei, D. Gerling, and S. P. Schmid: Prediction of Conducted EMI in Power Converters Using Numerical Methods, 15th International Power Electronics and Motion Control Conference, EPE-PEMC 2012 ECCE Europe, 2012, Novi Sad, Serbia. [5] J.-S. Lai, Huang, Xudong, Pepa, E., Chen, Shaotang, Nehl, T. W.: Inverter EMI modeling and simulation methodologies, Industrial Electronics, IEEE Transactions on, 2006. [6] J. Wei, D. Gerling, and M. Galek: Frequency Domain Prediction of Conducted EMI in Power Converters with front-end Three-phase Diodebridge ,15th European Conference on Power Electronics and Applications EPE 2013 - ECCE Europe, 3-5 September, 2013, Lille, France. [7] M. Foissac, J. L. Schanen, and C. Vollaire: "Black box" EMC model for power electronics converter, IEEE Energy Conversion Congress and Exposition ECCE, 20-24 September, 2009, San Jose, CA, USA . [8] J. Wei, D. Gerling, and M. Galek, "Terminal Characterization of Conducted EMI in Three-phase Power Converters," in Energy Conversion Congress and Exposition (ECCE), 2013 IEEE, Denver, USA,15-19.Sept.2013. [9] M.L.Heldwein, “EMC Filtering of Three-Phase PWM Converters”, Ph.D Dissertation, ETH Zurich, 2008. [10] "CISPR 16-1: Specification for radio disturbance and immunity measuring apparatus and methods", 2007. Fig. 19 Spectrums using two different oscilloscopes V. CONCLUSION This paper investigates the relevant issues of experimental characterization for terminal modeling of conducted EMI in three-phase power electronics converters. The investigation provides analysis and suggestions on hardware, measurement devices and the overall setup. Different possibilities of measurement devices are considered in order to guarantee best performance in data acquisition. Issues that should be paid attention to during data processing are also addressed. With measurement data from different test conditions, the 1170