SWITCHING AND REDUCTION OF COMMON MODE VOLTAGE OF MULTILEVEL- H-CASCADED CONVERTER FOR MEDIUM VOLTAGES

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1 1 SWITCHING AND REDUCTION OF COMMON MODE VOLTAGE OF MULTILEVEL- H-CASCADED CONVERTER FOR MEDIUM VOLTAGES AUTHOR: MUHAMMAD JAMIL Faculty of Electrical Engineering and Information Technology, Chemnitz Uniersity of Technology, Germany Tel.: +49-(0) e. Mail: Abstract: This paper presents reduction of common mode oltage in a multileel H-cascaded conerter for medium oltages. It is well known that a conentional two-leel pulse width modulated (PWM) conerter generates high frequency common mode oltage with high d/dt. In the same way, commonly used multileel conerter modulation schemes generate common mode oltage. Multileel oltage source conerters are getting increased importance for applications in the medium and high oltage range. Due to the easy construction of H-cascaded conerter, it can play an important role in the industry. Common mode oltage may cause motor shaft oltages, bearing currents and electromagnetic interference (EMI). The common mode oltage depends on the switching method and earth mass. In order to reduce the common mode oltage, a suitable filter has been used and common mode oltage has been reduced to zero. Sinusoidal (sine-triangle) PWM scheme is being used for this purpose and simulation results are being presented in this paper by using software Simplorer. Key words: cascaded conerter, common mode oltage, motor shaft oltage, bearing current, electromagnetic interference (EMI) 1

2 2 I INTRODUCTION The importance of adanced power electronic systems is increasing due to the trend of decentralised power generation and the deregulation of energy markets. Seeral types of multileel conerters hae been inestigated for these new applications [1-6]. The basic requirements for this application field are high oltage and high power. Besides this, many other aspects, regarding to the industrial implementation of these conerters, hae to be taken into consideration. A particular adantage of this topology is that the modulation, control and modulation requirements of each bridge are modular [7]. Howeer, each single phase inerter has its own supply. Multileel inerter structures hae been deeloped to oercome the shortcomings in solid state switching deice ratings so that they can be applied to high oltage electrical systems. There hae been a number of mitigation techniques suggested for the bearing current and conducted EMI. Howeer, few of them hae addressed the common mode oltage successfully and directly none of them hae been found within the context of multileel PWM inerters. At sinusoidal supply oltages, bearing currents may flow in a closed loop comprising the shaft, both end-shields and the housing. These circulating currents are caused by magnetic asymmetries of the stator yoke, which result in a ring flux in the yoke inducing the so-called shaft oltage in the loop. Up to a certain alue of the shaft oltage the circulating current is zero; howeer, at higher shaft oltages the circulating currents destroy the bearing within a short period of time. In many cases the shaft oltage can be limited to uncritical alues by optimisation of the yoke geometry; otherwise the insulation of one bearing is a common measure of protection. Noel effects occur at supply by modern oltage source conerters (VSC), caused by capacitie coupled bearing oltages, which initiate the so-called EDM-currents (Electric Discharge Machining) through one motor bearing, and by shaft oltages of high frequency. This effects result from the common mode oltage of the inerter which represents a zero sequence component of the oltages and which is inherent to all control schemes of PWM inerters. In [8], a dual bridge inerter (DBI) is presented to generate zero common mode oltage. For this work many filters, described in [9-10], hae been researched to reduce the common mode oltage. With the help of a filter (fig. 13), the common mode oltage has been reduced to zero (fig. 14). All simulation results are being presented in this paper. II STRUCTURE AND OPERTATION OF H-CASCADED BRIDGES A cascaded multileel inerter (fig. 1b) consists of H-bridge (single-phase full bridge) inerter units. The general function of this multileel inerter is to synthesize a desired oltage from seeral separate dc sources. Fig. 1a shows a rectifier with separate supply sources. Each inerter leel can generate three different output phase oltages, +V dc, 0 and -V dc by connecting the dc sources to the ac output side by different combination of four switches S 1, S 2, S 3 und S 4. The phase oltage depends on the number of leels. With enough leels, using this fundamental switching technique results in an output oltage of the inerter that is almost sinusoidal. From fig. 1b it is clear to see that there are four possible switching states of a single phase conerter, which are stated in fig. 2a-d. To obtain the dc-bus oltage -V dc, switches S 1 and S 4 are turned on simultaneously. Turning on switches S 2 and S 3 yields V dc. By turning on S 1 and S 3 or S 2 and S 4, the output oltage is 0. The fundamental frequency for this circuit is 50Hz, while the switching frequency is 3kHz.. Electromotie Force (EMF) e can be 2

3 3 calculated from equation (2), which for this work has been taken 3.81kV and output RMS-current (Root Mean Square) is selected 1.2kA. The oltages 1, 2, 3 and 12 in fig. 1b of three inerters are being described in equation (1): 1 = 2 = 3 = V dc and 12 = 2 1 = 2V dc (1) e = MV dc cos(ωt+φ) (2) In equation (2) is M = 1; and V dc,= 3.81kV. Abbreiations and symbols: M = Modulation index; and V dc,= dc-bus oltage, fig. = figure; equ. = equation; L L = Last inductor; L F = Filter inductor; C = Condensor; C F = Filter condensor III DESCRIPTION AND SIMULATION Typically, two kinds of bearing currents are caused by the bearing oltage which pass only one bearing of the motor and flow back to the conerter. The so-called du/dt-currents through the capactor of the bearing are less than maximal seeral hundred ma and therefore they cannot destroy the bearings. By contrast, the EDMcurrents, stochastically occurring break downs of the grease film at high peak alues of the bearing oltage, are of great practical importance. The endangering factors are the peak alues of the EDM-currents and its repetition rate. The repetition rate grows with the switching frequency. The common mode oltage causes common mode currents. It depends on the kind of grounding system whether these zero sequence components of the currents penetrate the actie parts of the motor. Statistics of bearing faults show that many bearing deficiencies at conerter supply are caused by poor grounding for frequencies in the khz range. The common mode currents penetrating the stator winding don t cause a linear oltage drop along the length of the winding conductors, because they flow partly through the capactor between winding and core. Consequently the currents in the two leads of each turn are not yet identical. The impulse shaped shaft oltage obiously acts as initiator for the circulating currents, the emf of which is the shaft oltage of basic frequency. This relationship was unknown up to now [11]. The power circuit of a three phase and three leel inerter is shown in fig. 1b. Assuming that of the two power switches in each leg of the inerter one and only one is always on, that is, by neglecting the time interals when both the switches are off (blanking time), three switching ariable phases a, b, and c can be assigned to the inerter. It is easy to show that the instantaneous line to line output oltages, as described in equ. (3), 12, 23 and 31 are gien by: = V dc a 1 b 2 c (3) An Bn Cn = V dc a b c (4) In a balanced three phase system, the instantaneous line to neutral output phase oltages, An, Bn and Cn has been expressed in equ. (4), where a, b and c are the phase legs in equation (3) and (4). 3

4 4 Equation (3) and (4) allow easy determination of the line to line and line to neutral output oltages for all states of the conerter. The line to line oltages can assume fie alues (±2V dc, ±V dc and 0), while the line to neutral oltages assume only three alues (±V dc and 0) as shown in fig. 4 and 5. For a gien input phase oltage s (also called line to neutral oltage) of 2,2kV for a single conerter (fig. 1a), the dc-bus oltage has been calculated as gien in equation (5) and shown in fig. 3b: Vdc = 3 * s = 3 *2.2kV = 3. 81kV (5) In equation (5) s is the peak alue of input phase oltage. In equations (3-5) it has been assumed that dc-bus oltage is, dc = V dc = constant. But it is not quite correct because the simulation results hae shown that dc is the sum of oltages [12], as described in equation (6): dc = V + dc i= 1 sin( ω dc, it + ϕ dc, ) dc, i i The reason for equation (6) is that as soon as the switches are being switched on, the dc-current takes a part of pulses and this part of pulses produces the ripples. Therefor the dc-bus oltage shows sine waes. The lowest frequency of a dc-bus oltage should be twice (100Hz) of a single phase (fig. 3b) and 6 times (300Hz) of a three phase conerter than that of basic frequency [12]. The amplitude of the input line to line oltage of one inerter is also equal to 3 * s. The maximum amplitude of a dc-bus oltage dc should be 2.34* s [13]. The output line to line oltages 12, 23 and 31 (fig. 1b) hae the maximum amplitude of 2 dc, whose simulation results are described in fig. 4. The maximum amplitude of output phase oltages (fig.5) An, Bn and Cn is 1/2 of the output line to line oltages. The common mode oltage ( cm ) is howeer 1/3 of the line to line oltages. The simulation results of output phase oltages and common mode oltage can be seen in fig. 5 and 7. By applying the Kirchhof s law of oltages (KLV) = 0 for the loops from fig. 1b, described in equ. (7) and (8), the line to line oltage 12 is equal to: (6) = 0 12 = 2 1 (7) and 12 + Bn An = 0 12 = An Bn (8) From fig. 1b it is to see that the sum of the output phase oltages ( An + Bn + Cn = 0) is equal to zero, which can be erified from fig. 6. Similarly, the sum of the output phase currents (i An + i Bn + i Cn = 0) is equal to zero. The fig. 12 and 8 show the output phase currents with and without filter. With the help of a filter and earth mass (fig. 9), the harmonics of common mode oltage has been reduced (fig.11) but the amplitude remains constant. The mathematical description for reduction of common mode oltage (fig. 14) according to fig. 13 has been gien in the following equations (9-13). The fig. 10 shows the oltages on the filter capactors. By using the law of oltages (Σ = 0) for fig. 13 the equations (9-11) hae been achieed for common mode oltage. cm = CF1 L1 eu (9) cm = CF2 L2 e (10) and cm = CF3 L3 ew (11) By adding the equations (9-11) : 1 cm = [ CF1 + CF2 + CF3 ( L1 + L2 + L3) ( eu + e + ew)] (12) and cm 0 3 = (13) 4

5 5 In equation (12) the electromotie forces e u, e and e w are free from common mode oltage and therefore the sum of e u, e and e w is zero. As the last is symmetric in fig. 13, it makes a symmetric system and therefore the sum of the oltages ( CF1 + CF2 + CF3 = 0, L1 + L2 + L3 = 0) is zero and the common mode oltage cm between the point n and 0 is zero. The equ. 13 and fig. 14 show that the common mode oltage is zero. V s RF 1.55kV Rectifier (RF) N S1 S3 RF dc dc dc C RF C RF C S2 S4 a 12 b 23 c L L 31 An Bn Cn e + V Common mode oltage ( cm ) Parameters: V s = 1.55kV, f 1 = 50Hz, f s = 3kHz, M = 1, e = 3.81kV, C = 2.46mF, L L =7.14mH Fig 1: Rectifier Three phase and three leel H-cascaded multileel conerter 5

6 6 ij = -V dc ij = V dc ij = 0 ij = 0 S1 S3 S1 S3 V dc ij S4 S2 ij S2 ij S4 ij (c) (d) Fig. 2: Four possible switching state (a-d) Voltages (V) Voltages ( V) Line to linevoltages DC-bus oltage Fig. 3: Input phase oltages line to line oltages and dc bus oltage of a single phase conerter (c) (c) Fig. 4: Output line to line oltages (c) 31 Fig. 5: Output phase oltages An Bn (c) Cn 6

7 7 L F1 L L1 e u Conerter n + V C F1,2,3 cm 0 Parameters: L L = 7.14mH, L F = 1.42mH, C F = 197µF, e = 3.81kV Fig. 9: Circuit of H-cascaded conerter with filter and earth mass Voltages (V) (c) Fig.6: Phase Voltages An Bn (c) Cn Fig. 10: Voltages on the condensors (C F ) Currents (A) Fig. 7: common mode oltage without filter Currents (A) Fig. 11: Common mode oltage with Filter and earth mass according to fig. 9 Fig. 8: Output phase currents (i An, i Bn, i Cn ) without filter according to fig. 1b Fig. 12: Output phase currents (i An, i Bn, i Cn ) according to fig. 9 In fig. 7 and 11 the common mode oltage has been explained according to the circuits from fig. 1 and 9. By comparing these common mode oltages it can be seen that although throug a filter and earth mass (fig. 9) the 7

8 8 harmonics hae been reduced but the amplitude remains the constant. In order to reduce the amplitude of common mode oltage the following circuit (fig. 13), with a filter connected with neutral ponit n and zero point 0, has been used and mathematical description has been gien in equations 9-13). L F1 L L1 e u Conerter n CF 0 cm Fig. 13: Circuit of H-cascaded conerter with filter Fig. 14: Common mode oltage according to fig. 13 Adantages of H-cascaded conerter Multileel H-cascaded conerter can oercome many problems i.e. dielectric stress and oltage harmonics. Due to the following adantages, the multileel H-conerter can play an important role in the industry. 1. As H-conerter based on two leel inerter, the topology is ery simple. 2. No additional diodes and capacitors are needed, as for other multileel conerters, i.e. neutral point clamped and flying capacitors inerters. 3. Voltage can be equally diided on all elements. Die structure can be extended up to 2s+1 leels. Die harmonics can be reduced by increasing the no. of leels. IV Discussion and Conclusion The H-conerter circuit under consideration proide high power dries at medium to high oltage leels, using IGBT technology. The H-bridge circuit has been exploited as a commercial product [6]. Switching methods, filters and earth masses influence the common mode oltage. With a proper switching methode, a suitable filter and choosing the zero ponit in the circuit the harmonics and common mode oltage can be reduced. The common mode oltage depends on switching method and earth mass. The research has been made by using many kinds of filters and earth masses and also modifying the circuit. With the help of a filter and finding the zero point in the circuit the common mode oltage has been reduced to zero. 8

9 9 References: [1] Nabae, A.; Takahashi, I.; Akagi, H.: A new neutral-point clamped PWM inerter IEEE Trans. Ind. Application, ol. IA-17, pp , Sep/Oct [2] Yuan, Xiaoming; Barbi, Io: A new diode clamping multileel conerter IEEE Trans. Ind. Application, ol. 15, pp , March [3] T.A. Meynard; H. Foch: Multileel choopers for high oltage applications, EPE. Dries Journal, ol. 2, no. 1, p. 41, March [4] C. Hochgraf; R. Lasseter; D. Dian; T.A. Lipo: Comparision of multileel inerter for static ar compensation, in Conf. Rec. IEEE-IAS Annu. Meeting pp , Oct [5] Peng, F.Z.; Lai, J.S.: Multileel cascade oltage source inerter with separate DC sources, U.S. Patent , July [6] P. Hammond: A new approach to enhance power quality for medium oltage ac dries, IEEE Trans. Ind. Appl., ol. 33, pp , Jan/Feb [7] M. Marchesoni; M. Mazzuchelli; and S. Tenconi: A non conentional power conerter for plasma stabilisation, in Proc. IEEE PESC 88, 1988, pp [8] A. on Juanne; H. Zhang: A dual- bridge inerter approach to eliminating common mode oltages and bearing and leakage currents, IEEE Trans. Power Elec., ol. 14, pp , Jan [9] M. D. Manjrekar; T. A. Lipo: Department of Electrical and Computer Engineering, Uniersity of Wisconsin-Madison, 1415 Engineering drie, Madison, WI USA., An Auxiliary Zero State Synthesizer to Reduce Common Mode Voltage in Three Phase Inerter, [10] N. Hanigoszki, J. Poulsen and F. Blaabjerg, Members IEEE; A noel output filter topology to reduce motor oeroltage Trans. Ind. Application IEEE, ol. 40 no. 3, May/June [11] A. Mertins, B. Ponick; Untersuchung und Berechnung on elektrischen Maschinen und Antriebssystemen Institute of drie system & power electronics, Hannoer uniersity- Germany, 11 May [12] F. Jenni, D. Wüest; Steuererfahren für selbstgeführte Stromrichter Hochschulerlag AG an der ETH Zürich, B.G. Tubner Stuttgart, Germany [13] K. Heumann: Grundlagen der Leistungselektronik, B.G. Teubner Stuttgart, Germany