Common-mode Overvoltage Mitigation in a Medium Voltage Pump Motor Transformerless Drive in a Mining Plant. Brenno Marcus Prado

Transcription

1 Page 1 of MC-0749 Common-mode Overvoltage Mitigation in a Medium Voltage Pump Motor Transformerless Drive in a Mining Plant Thiago Morais Parreiras Student Member, EEE Graduate Program in Electrical Eng Federal University of Minas Gerais Belo Horizonte, MG, Brazil thiagomparreiras@ieeeorg Brenno Marcus Prado Non-Member Drives Department GE Power Conversion Brazil Betim, MG, Brazil brennoprado@gmailcom Braz de J Cardoso Filho Mem ber, EEE Department of Electrical Eng Federal University of Minas Gerais Belo Horizonte, MG, Brazil brazcardoso@ieeeorg Abstract - Mining process requires several types of mining pumps to transfer a fluid from one point to another in the process n the majority of applications, those pumps are driven by variable speed motor drives in order to improve the energy efficiency of the overall system Large pumps, ranging from hundreds of kw to a few MW, usually demand medium voltage converters Transformerless converters reduce the footprint required for the installation, but produce additional concerns related to the common-mode voltages applied to the system and motor The absence of an isolation transformer along with the use of PWM rectifiers are the root causes of this issue This concern increase when long cables are needed to connect the converter to the motor This paper presents an application where 3L NPC transformerless medium voltage converters were applied to drive motor pumps in a mining plant The passive filter techniques used to address the common-mode voltage issue were analyzed through simulations and compared with the field results ndex Terms-mining pumps, common-mode voltage, transformerless motor drives, medium voltage converters NTRODUCTON Mining process requires several types of mining pumps to transfer a fluid from one point to another in the process n the majority of applications, those pumps are driven by variable speed motor drives in order to improve the energy efficiency of the overall system Large pumps, ranging from hundreds of kw to a few MW, usually demand Medium Voltage (MY) converters On these MY applications, a transformerless drive could be attractive due to the reduced footprint and better power quality at the grid side n mining applications, it is not rare to have long cables connecting the MY converter to the motor due to long distances between the process being driven and the electrical room, which leads to overvoltage and resonance problems [1] t has long been known that the application of variable speed drives (VSD) along with long cables impose peak voltage levels at the motor terminals that can be harmful to its insulation As in a voltage source converter (VSC) a series of voltage pulses with short rise times are applied to the cable, which has a surge impedance much lower than the motor leakage inductance, voltage peaks close to 2pu can be observed due to voltage reflections [2] Different from the ideal case where a motor is supplied by a sinusoidal waveform, pulse-width modulated (PWM) VSC apply not only differential-mode, but also common-mode voltages to the motor [3] These common-mode voltages can also be dangerous not only to winding insulations, due to over voltage from phase-to-ground, but also to the machine bearings due to a prohibited level of leakage currents The common-mode voltages applied to the MY motor will depend on the VSC topology, filtering and control mode [4] A transformerless drive will also be some kind more sensitive to this effect due to the presence of a PWM rectifier [5] and the absence of an isolation transformer [6] Therefore, additional filtering must be applied in order to overcome this problem and allow proper and safe operation of motor drive system This work presents a real case where a transformerless MY three-level (3L) neutral point clamped (NPC) VSC is used to feed water/slurry pumps in a mining plant through cables of length up to 390 meters After a brief explanation of the system and components modeling choice, simulation results are presented and compared to field results A Power System SYSTEM DESCRPTON The system is represented by the single line diagram of Fig l The main transformer supplies power to a 4l6kV bus and has its neutral grounded through a resistor The 416kV bus itself supply power to several linear and non-linear loads, including two MY water pumps drives Each drive is composed of a transformerless 3L NPC VSC, which supplies a 4kV induction motor of a water pump This water pump is part of the flotation process within the mining plant As it is a critical part of the whole process, the system relies on a spare inverter/motor pair n this application there is a 950cv (93663HP) motor for each pump Motors are connected through insulated cables coming from the electrical room to external area where the drives are located 390 meters away Thus, distance between drives and motors must be taken into consideration in any over voltage calculations /16/$ EEE

2 2016-MC-0749 Page 2 of 9 MCC n TABLE MAN TRANSFORMER DATA 138kV 'l4e' :_kv Networ 1 A l---' Neutral 1 Neutr 1 grouṇding 6 r ; ::? resistor T (optional) MCC2 MCC 1 nverter 1 Power [kva] Voltage Ratio [V] Winding connections 7500 mpedance [%] Ground 4160 Dyl , Fig 1 System single line diagram nverter 2 The main transformer data is provided in Table Table provides the motor data and Table provides the cable data The converter is presented in details in the next section B Transformerless converter The transformerless converter is represented in the diagram of Fig 2, which is a VSC in a back-to-back configuration t has a capacity of driving motors up to 800kW 416kV and is composed mainly of: Linefilter (LF): composed by a grid inductance, a converter inductance and two resonant traps tuned in the 17th and 19th harmonics This type of filter represents a good solution for increased power quality at the grid side, especially when combined with the Selective Harmonic Elimination (SHE) PWM method [7] Active Front End (AFE): a 3L NPC VSC PWM rectifier using a current controlled five pulses SHE PWM that does not produce characteristic harmonics below the 17th order with a switching frequency of only 600Hz [8] Common-mode choke (CMC): a concept exported from the electromagnetic compatibility theory [9], which is also referred as a Zero Sequence Blocking Transformer (ZSBT) [4] and is used to provide a low impedance path for differentialmode currents while providing a high impedance path for common-mode currents A tertiary winding is included and wired to a damping resistor in order to damp possible resonances with the other impedances in the common-mode path Midpoint Grounding Resistor (MGR): in addition to the CMC, a resistor can be installed between the inverter midpoint and the ground in order to reduce the voltage floating of the inverter OV As the result, the common-mode voltage applied to the motor will be further reduced nverter: a 3L NPC VSC PWM inverter responsible for the DC AC conversion and to supply a variable frequency and voltage necessary for the application A carrier-based PWM method with zero sequence hannonics injection in the modulation signal is employed in this control [10] Output dv/dt filter: in order to reduce the dvldt and, therefore, the differential-mode voltage peak and rigging at the motor terminals, a low-pass LCR filter is included at the inverter output [11] The converter data is presented in Table V Rated Voltage [V] Rated Current [A] Rated Power [cv] Frequency [Hz] Nominal Speed [rpm] Cosq, [pu] Length [m] Capacitance [\F km] tem Rated Power Line Voltage Main Data Line Frequency DC Link Voltage nductance Ld dvldtjilter Capacitance Cd Resistance Rd TABLE MOTOR DATA 4000 Efficiency at full load [%] 125 p/n [pu] 950 Stator Resistance R [Q] 60 Stator nductance L [mh] Magnetizing 1192 nductance Lm [mh] 084 Rotor Resistance R2fQl TABLE CABLE DATA TABLE V CONVERTER DATA 800 kw 416 kv 60 Hz Referred to the stator tem 034 Common Mode Choke Leakage inductance Magnetezing nductance Transformer Ratio (P to S) 650l\H 650 mh V Resistor Rdm 0l375 Q Midpoint Grounding Resistor 600l\H Resistance 453 Q 0221\F Selflnductance s40nh 395Q /16/$ EEE

3 Page 3 of MC-0749 (+) (+) Une nput , Motor output Fuse '"OT lih AFE (-) NVERTER harmonic harmonic 19m LF CMC Fig 2 Converter diagram ---- dv/dt FLTER C ssues of concern and filtering techniques As long as there is no isolation transformer in this configuration, the main path for the common-mode currents is composed of the main transformer (source), converter, cables and motor stray capacitance to ground as indicated in Fig 3 This may increase phase-to-ground voltage peaks at the motor side (due to the voltage drop at its capacitance) and even to the grid side (due to the voltage drop at its grounding resistance) above the acceptable limits There are two main normative references for the withstand capabilities of winding insulation in motors: NEMA MG 1 [12] and EC [13] On the one hand, NEMA standard specifies that a motor designed to be supplied by a VSD must withstand the repetitive line voltage peak according to (1) This same standard states that phase-to-ground voltages according to (2) are typical to be applied in a motor supplied by a VSC On the other hand, EC states that motor windings are generally capable of withstand the line voltage peak and phase-to-ground voltage peak specified in (3) and (4), respecti vel y Where, ULL and ULF are the peak values of the line to line and line to frame maximum voltages Uns is the rated rms voltage of the insulation system, which is usually greater than the rated voltage in motors manufactured to be fed by a VSC Table V shows the maximum values informed by the 4kV motor manufacturer and compare them to the references in the aforementioned standards TABLE V 4KV MOTOR WNDNGS WTHSTAND VOLTAGE (3) (4) Peak voltage Manufacturer NEMAMGl lee t Phase to phase :: V :: V :: V Phase-to-ground :: V :: V :: V Fig 3 Common-mode current path Vt-peak 204 = Vrated Vph-peak J3 VPh-peak = Where, V-peak, V ph-peak and V ph-peak are the peak values of the line, phase and fundamental (sinusoidal) voltages, respectively Vrated is the line voltage rms value (1) (2) dv/dt ::02700 V lis ::02715 V lis :: V lis For an nsulaton voltage of 6kV rms For grid voltage, the restriction is even greater as long as standards allow typically less than 10% overvoltage in steadystate ANS C841, for example, states a maximum limit of 4400V (+58%) for a nominal voltage of 4160V at 60Hz [14] n order to avoid such prohibitive values, a combination of a CMC, a MGR at the converter and a capacitance in parallel with the main transformer grounding resistor can be applied /16/$ EEE

4 2016-MC-0749 Page 4 of , , T , M/2 Transformer Converter Cable Motor Lr/3 Lf/3 Llk/2 4k/2 4m/2 Ld/31 Lcs/3 LM/3 Rr/31 Rdm/2 Q MGR/2 3 X CcM G L 7 L Fig 4 Common-mode equivalent circuit The sum combination of these passive filtering techniques, which are intended to reduce peak voltages at both motor and grid side, is better understood through analysis ofthe commonmode equivalent circuit of Fig 4 This circuit was obtained according to the methodology presented in [15], (5) and (6) indicates the values for the common-mode sources as a function of the AFE and inverter phase-to-midpoint voltages (VaO, VbO, and V co - for the AFE - and Vuo, V vo, V wo - for the inverter) The nomenclature for the remaining parameters are indicated in Table V TABLE V COMMON-MODE CRCUT NOMENCLATURE Description Grounding resistor Grounding capacitor AFE Filter nductance AFE nductors Winding Resistance CMC Leakage inductance CMC Magnetizing inductance CMC Damping Resistor Damping Resistor Stray nductance Dvldt Filter nductance Motor Cable nductance Motor Cable Resistance Motor Cable Capacitance Motor Winding Leakage nductance Motor Common-mode Capacitance Nomenclature RG CG LF RF Llk M Rill" Ldlll Ld LeB RCB CCB LM CCM As one can observe from Fig 4, the CMC will substantially increase the impedance in the common-mode path The MGR (5) (6) will reduce the midpoint floating in reference to the ground at the motor side by providing a lower impedance circulation path back to the V CM2 source And the ground capacitor will provide an impedance path for the high-frequency commonmode currents lower than the grounding resistor, therefore reducing the voltage drop across it A System model SYSTEM MODEL AND SMULATONS To proper simulate the system behavior for high-frequency phenomena, the choice of the suitable models is a fundamental step At large frequencies, stray capacitances and inductances along with skin effect in the impedances may impact the overall result, but their full consideration in the simulation model can conduce to large simulation times A tradeoff between simulation accuracy and speed is always present The goal shall be to obtain simulation results that give the power electronics engineer the necessary data to designing a functional and safe system n the specific case under analysis, the interest is more in checking whether the voltage peaks applied to the equipment is below or not to the maximum allowed values The proximity to the actual shaping of the waveform, for example, is not a crucial factor in the design presented here Although the superposition theorem in linear systems allows the development of differential and common-mode models in separate [16], models that allows a simultaneous simulation of both modes are generally preferred due to simplicity of analysis Models considering the high-frequency differential and common-mode paths provided by several stray inductances and capacitances can be found in the literature for transformers [17] and converters [18] But these models are normally obtained for specific low power and low voltage components and could not be directly employed in the simulation Also it is not expected that the absence of such parameters have significant impact in the magnitude focused simulations presented by this work When dealing with the motor, while some complex models considering several non-intended high-frequency paths can be found [19], it is a common practice to only include a capacitance between the MV motor midpoint and the ground as used in [4] and [6] This model is presented in Fig 5 for /16/$ EEE

5 Page 5 of MC-0749 common-mode current and winding voltage analysis The main issue for this model is to obtain a correct value for the capacitance CCM, which is not usually provided by the manufacturer References [4] and [6] present a value of 65nF for such capacitance in similar voltage and motor rated power Therefore, this value is considered in the simulations presented in this work U L, R, Lm L, R,/s inductance in series with this resistor was performed The results are presented in Fig 8 (a) and Fig 8 (b) & Tme[s] V R, L, L, R,/s Time [$1 Fig 7 Voltages at motor terminals L, R, Lm L, R,/s W = CeM 9000 '500 & 0 > Lm nne s] Fig 5 Motor model Finally, for the cable a very accurate model was proposed in [20], which take into account parameters variation due to skin effect in a broad range of frequencies As the interest in terms of permissible voltages relies more on the magnitude of the values rather than in the waveform exact shape, this work will consider a simple model of ten Pi-cells connected in series The model is represented in Fig 6 and the parameters are the ones provided by the cable manufacturer (Table ) at fundamental frequency B Simulation results Fig 6 Cable model Simulations were performed in a MATLAB / Simulink platform considering the models and parameters from previous sections First the simulation was performed without the MGR and the results for the motor line and phase voltages are presented in Fig 7 (a) and Fig 7 (b), respectively These results, 8555V maximum peak for line voltage and 5555 V maximum peak for phase voltage, show the system operating already above the limits stated by the motor supplier (Table V) After that, it was verified that the damp resistor (Rdm) wired to the CMC secondary was not specified as a low inductance resistor So, a simulation considering 254flH of parasitic 0025 Time (s] Fig 8 Voltages at motor terminals with inductive damping resistor Now, one can observe the phase voltage peak is about 6000V, which is above the insulation limits and additional measures should be taken As expected, no significant difference was observed in the line voltage This result can be understood by looking back at Fig 4 and noting that these kind of high stray inductance in the damping resistor will cause this inductance to not be negligible in relation the resistor itself This condition (negligible value for Ldrn) is usually also considered in the transfer function used to dimensioning a CMC capable of reducing both peak and rms values of the common-mode current [21] Then, a simulation was performed considering the installation of a non-inductive MGR at the inverter side The new motor voltages result is presented in Fig 9 The current at the MGR is presented in Fig 10 (b) and has an rms value of 58A '500 f o > Timersl (b) Phase to Ground Voltage Tme(,] Fig 9 Voltages at motor terminals with MGR installed /16/$ EEE

6 2016-MC-0749 Page 6 of 9 ;ll: Time [ms] r : ('l MGR Cu, ", u " 'r 1 n'1 f r Tr Tm'!m ntr TTn:nrn n 250L-'---:- 0""OO"-, --="00'-' --,-00 "-15 ::-----:-' 00,,-'--,-0 "'0:':---0, "::----:-:000'='='---:01 ":---0,-'"-::- '---="005 1me!s] Fig 10 (a) Line voltage dv/dt and (b) MGR current These results show a maximum phase voltage peak of 4690 V, well below the insulation limit The cost of such solution is additional 152kW loss at the MGR Finally, Fig 10 (a) shows a dv/dt of 1758 V lls This value is far below from the motor insulation limit Then, with the solution defined at the motor side, simulations were performed for the voltages at the grid side Fig 11 shows the voltages with only the original grounding resistor at the transformer secondary The maximum grid phase voltage peak was reduced to values below 3594V (+58%), which is the limit established by [14] n all case scenarios, the line voltage remained below the +58% limit (6225 V) No substantial change was observed in voltages or currents at the motor side V FELD RESULTS A Line and phase voltages without MGR Below, in Fig 13 are presented voltages samples at the motor output, full speed, before the OV grounding resistor was installed in the drive These and the following measurements were carried out using the following main items: 1 One Fluke 125 series portable oscilloscope and its respective acquisition software; 2 One 10kV differential voltage probe; 3 One Rogowski coil for current measurement; 4 One bench top Tektronix oscilloscope; 5 One laptop computer One can see that in this case the line overvoltage is not reached, as per Table V previously presented Tme!s] Fig 1 L Voltages at the grid side (main transformer terminals) As the grid phase maximum peak voltage is about 3900 V, which is 148% above the rated value, a 10 lf capacitor was inserted in the simulation in parallel to the transformer grounding resistor in order to provide a low impedance path for high frequency currents The result is presented in Fig 12 Fig 13 Line Voltage 3504 '000 ' 2: \ & 0 / \ ' > '000 ",)(,< Time(s) '--_ _ _-L '--_ _ _-L '--_ _ Tme]s] Fig 12 Voltages at the grid side with grounding capacitor Wh ereas for line voltages m easurem ents there was no concern about reaching peak values stated by motor supplier, measurements for phase voltages made clear that maximum limits were reached Fig 14 shows phase-to-ground voltages without OV grounding resistor One can notice that the peak voltage reaches and surpasses 60kV in certain moments, which is not allowed by motor main insulation as per Table V /16/$ EEE

7 Page 7 of MC-0749 [6] 244 7kU: _!DlE Fig 16 Grounding resistor assembly - "'lsj( r " Fig 14 Phase Voltage t was also obtained a measurement of the voltage rise time characteristic of these operational conditions n Fig 15 it is possible to notice that the voltage rises from approximate 32kY to 76kY in 25/1s, ie, 176Y//1s This is far below from what is permitted by motor's main insulation Based on these measurements it is clear that main problem for this application was on phase-to-ground voltages Kl Lei'" K2 : =PEi Lei- L J Fig 17 Grounding resistor circuit The next step after installing this assembly was carrying out a new set of measurements in order to verify the effects of the new connection Fig S presents the line voltage for the new condition of operation at full speed rms ku", Dl!!l ' " A Fig 15 dv/dt characteristic B Line and phase voltages with MGR After taking previous measurements into consideration and simulating the circuit to check the influence of common mode voltages in the DC link, it was implemented the solution including a grounding resistor between midpoint of DC link and ground This resistor was assembled directly in the existing drives as presented in Fig 16 below The assembly is composed of an association of 6S0 low inductance resistors resulting in a 4530 total resistance, as seen in Fig 17 Fig 18 Line voltage with MGR One can see that no noticeable change is achieved The insulation levels are not reached, with almost any difference from previous line voltage from Fig 13 Maximum voltage levels are kept around SOkY Fig 19 indicates the phase-to-ground voltage obtained after the MGR installation /16/$ EEE

8 2016-MC-0749 Page 8 of 9 rms rms A Dlll ku 't l "'ll"' [[lljl l, Fig 19 Phase voltage with MGR Fig 21 Current at MGR Now it is clear that maximum voltages are kept around 40kV as desired with occasional peaks up to 48kV or 49kV This is a safe operational condition below the 54kV limit stated by the motor supplier n order to ensure all insulation levels were acceptable another important measurement was the dv/dt characteristic n Fig 20 this measurement can be evaluated HGH LOW + C Phase voltage at the grid side The phase-to-ground voltage at the grid side (VSC input) was also measured and indicated in Fig 22 A peak voltage of about 38kV was reached Although this value is above the limits specified by ANS C841, the mining plant engineering decided that this would be acceptable for operation So no capacitor was installed in parallel with the transformer grounding resistor in order to avoid reliability issues Ou + 1l:1!i1, rms ku-y LmlJD )' :;: S : : : : ; ; :, : : : ; ; j : : : : : - ' -: V Fig 20 dv/dt with MGR From the wave at the very right side of Fig 20 it is noticeable a voltage rise from OV to 40kV in approximately 25/ls This is a voltage rise of 1601/ls and the maximum dv/dt from motor datasheet is ensured for final operational condition Finally, Fig 21 presents the current the MGR itself Based on this current value, the power dissipated in each resistor in the assembly is about 240W Each resistor was specified to 300W of maximum power dissipation Fig 22 Phase-to-ground voltage at the grid V CONCLUSONS This paper presented a discussion of how overvoltages caused by common-mode currents were mitigated in an application of transformerless MV drives for a mining site Through theoretical and actual field results it was demonstrated the harmful effects for motor insulation when using long cables in such configuration as well as the solution applied to a real case /16/$ EEE

9 Page 9 of MC-0749 Models for motor and cables were used to evaluate the highfrequency phenomena in the system Some proper simplifications were taken into consideration in order to achieve the design goals with reduced simulation times This analysis, therefore, led to the conception of a common-mode choke as a solution for motor safe operation Also during the simulations, it was observed that the use of an inductive damping resistor at the CMC caused the phase-toground peak voltages to surpass the winding insulation limit An alternative solution was proposed and analyzed: the midpoint grounding resistor This MGR applied to the inverter side was capable of providing further reduction in the phase peak voltages A phase-to-ground overvoltage effect was also observed in the system Although the magnitude of such effect is lower than in the motor side, more severe are also the standard requirements at the grid side due to its impact in other linear and non-linear loads supplied by the same bus For this case, a possible solution is the installation of a capacitance in parallel to the transformer grounding resistor in order to provide a low impedance path for high frequency currents Field results showed, in the end, that the simplifications assumed in simulations were acceptable for the goals established to obtain a functional and safe system The MGR was installed in the actual drive in order to implement the modifications suggested by models and achieved all the expected results Maximum insulation limits were preserved as well as safe motor and system operation [] A von Jouanne, P Enjeti, W Gray, "Application issues for PWM adjustable speed AC motor drives," EEE nd Appl Mag, vol 2, no 5, pp 10-18, Sept/Oct, 1996 [12] Motors and Generators, NEMA MG 1,2014 [13] Adjustable speed electrical power drive systems - Part 4: General Requirements - Rating specificationsfor ac power drive systems above 1000 V ac and not exceeding 35 kv, EC , 2002 [14] Electric Power Systems and Equipment - Voltage Ratings (60 Hertz), ANS C841, 20 [S] A D Brovont and S D Pekarek, "Equivalent circuits for common-mode analysis of naval power systems," in 2015 EEE Electric Ship Technologies Symposium (ESTS), 20 S, pp [16] P Pairodomonchai and S Sangwongwanich, "Exact common-mode and differential-mode equivalent circuits of inverters in motor drive systems taking into account input rectifiers ", in EEE 91 h nt Conj Power Electron and Drive Systems, 2011, pp [17] H de Paula, M V C Lisboa, JFR Guilherme, MLR Chaves, WP de Almeida, "Characterization of cable arrangements in terms of the generated high-frequency quantities in PWM motor drives," in Brazilian Power Electron Conj, 2009, pp [18] G Grandi, 1 Montanari, and U Reggiani, "Effects of power converter parasitic components on conducted EM," ieee Trans ind Appl, vol 41,no 5,p , Sept, 2005 [19] A F Moreira, T A Lipo, G Venkataramanan, and S Bernet, "Highfrequency modeling for cable and induction motor overvoltage studies in long cable drives," ieee Trans ind Appl, vol 38, no 5, pp , Sept/Oct,2002 [20] H de Paula, D A d Andrade, M 1 R Chaves, 1 L Domingos, and M A A de Freitas, "Methodology of cable modeling and simulation for high-frequency phenomena studies in PWM motor drives," EEE Trans Power Electron, vol 23, no 2, pp , Mar, 2008 [21] R de S Araujo, R de A Rodrigues, H de Paula, B J C Filho, 1 M R Baccarini, and A V Rocha, "Premature Wear and Recurring Bearing Failures in an nverter-driven nduction Motor - Part : The Proposed Solution," ieee Trans ind Appl, vol 51, no 1, Jan 2015 REFERENCES [1] J Rodriguez et a, "Resonances and overvoltages in a medium-voltage fan motor drive with long cables in an underground mine," ieee Trans nd Appl, vol 42, no 3, pp , May/Jun, 2006 [2] E Persson, "Transient Effects in Application of PWM nverters to nduction Motors," EEE Transnd Appl, vol 28, no 5, pp , Sept/Oct, 1992 [3] G Grandi, D Casadei, and U Reggiani, "Common- and differentialmode HF current components in AC motors supplied by voltage source inverters," EEE Trans Power Electron, vol 19, no 1, pp 16-24, Jan, 2004 [4] D Rendunsara, E Cengelci, P N Enjeti, V R Stefanovic, and 1 M Gray, "Analysis of common mode voltage - 'neutral shift' in medium voltage PWM adjustable speed drive (MV-ASD) systems," ieee Trans Power Electron, vol 15, no 6, pp , Nov, 2000 [5] D Rendunsara and P Enjeti, "A method to reduce common mode & differential mode dv/dt at the motor terminals in PWM rectifier / PWM inverter type adjustable speed drive systems," in Proc 1998 Applied Power Electronics Conj and Expo, vol 2, pp [6] B Horvath, "How isolation transformers in MV drives protect motor insulation," TM GE Automation Systems, Roanoke, VA, 2004 [7] A-SA Luiz and B 1 C Filho, "Analysis of passive filters for high power three-level rectifiers," in The 341 h Annu Conj EEE nd Electron Society, 2008, pp [8] T M Parreiras and B J C Filho, "Current control of three level neutral point clamped voltage source rectifiers using selective harmonic elimination," in The 40, h Annu Conj EEE nd Electron Society, 2014, pp [9] C R Paul, "Common-Mode Chokes," in ntroduction to Electromagnetic Compatibility, 2nd ed Hoboken, NJ, USA: Wiley, 2006, pp [10] A M Hava, R J Kerhman, T A Lipo, "Simple analytical and graphical methods for carrier-based PWM nverters," EEE Trans Power Electron, vol 14, no, pp 49-61, Jan, /16/$ EEE

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