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Timestamp: 2019-04-22 14:31:28+00:00

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The IEEE Standard 1459: What and Why?
Abstract-Multilevel voltage source converters are emerging as a new breed of power converter options for high-power applications. The multilevel voltage source converters typically synthesize the staircase voltage wave from several levels of dc capacitor voltages. One of the major limitations of the multilevel converters is the voltage unbalance between different levels. The techniques to balance the voltage between different levels normally involve voltage clamping or capacitor charge control. There are several ways of implementing voltage balance in multilevel converters. Without considering the traditional magnetic coupled converters, this paper presents three recently developed multilevel voltage source converters: (1) diode-clamp, (2) flying-capacitors, and (3) cascadedinverters with separate dc sources. The operating principle, features, constraints, and potential applications of these converters will be discussed.
voltages can be spanned by series devices without device voltage sharing problems. Unfortunately, the number of the achievable voltage levels is quite limited not only due to voltage unbalance problems but also due to voltage clamping requirement, circuit layout, and packaging constraints. To date, hardware implementation has only been reported up to six levels for a back-to-back intertie application , in which the voltage unbalance problem has been successfully overcome. The magnetic transformer coupled multi-pulse voltage source converter has been a well-known method and has been implemented in 18- and 48-pulse converters for battery energy storage and static condenser (STATCON) applications, respectively [ 15,161. Traditional magnetic coupled multipulse converters typically synthesize the staircase voltage wave by varying transformer turns ratio with complicated zig-zag connections. Problems of the magnetic transformer coupling method are bulky, heavy, and lossy. The capacitor voltage synthesis method is thus preferred to the magnetic coupling method. There are three reported capacitor voltage synthesis based multilevel converters: (1) diode-clamp, (2) flying-capacitors, and (3) cascaded-inverters with separated dc sources. This paper will describe operating principles of these capacitor voltage synthesis multilevel converters. Based on the features and constraints, the application areas of these multilevel converters will be addressed.
An m-level diode-clamp converter typically consists of m-1 capacitors on the dc bus and produces m levels of the phase voltage. Fig. 1 shows a single-phase full bridge fiven which the dc bus consists of level diode-clamp converter i four capacitors, C,, C , C , and C , . For a dc bus voltage V,,, the voltage across each capacitor is VJ4, and each device voltage stress will be limited to one capacitor voltage level. VdJ4, through clamping diodes.
Prepared by the Oak Ridge National Laboratory, Oak Ridge. Tennessee 37831-7258, managed by Martin Marietta Energy Systems. Inc. for the U. S. Department of Energy under contract DE-AC05-830R2 1400. The submitted manuscript has been authored by a contractor of the U. S. Government under contract No. DE-AC05-840R21400. Accordingly! the U. S . Government retains a nonexclusive. royalty-free license to publish or reproduce the published form of this contribution. or allow others to do so. for U. S. Government purposes.
and (Sa. This number represents a quadratic increase in m. then there will be (m-l)x(r. The: resulting line voltage is a 9-level staircase wave.through Sa. Da2 and Dal need to block 2 Vdc/4. D. (5’02. A diode-clamp 5-level converter circuit diagram To explain how the staircase voltage is synthesized. Using the 5-level converter shown in Fig. the number of diodes required for each phase will be (m-l)x(m-2). through Sa. (2) For voltage level Va0=3VdJ4. or 3 VdJ4. This implies that an m-level conwerter has an m-level output phase voltaige and a (2m-l)-level output line voltage. are turned on.. Features H i g h g e Rating Required for Blocking Diodes Although each active switclhing device is only required to block a voltage level of Vd~’(m-l).n-2)/2 devices oversized.S a ’ j ) . it can be seen that switch Sa. Sl... (4) For voltage level Va0=VdJ4. when all lower devices. TABLE I Diode-clamp 5-level converter voltage levels and their corresponding switch states Fig. B.. When the inverter design is to use the average duty for all devices. 1 as an example.I... and three lower switches Sa..Fig. turn on three upper switches Sa. Sa. Using phase-leg a as the example.). and Sa. Assuming that each blocking diode voltage ratiing is the same as the active device voltage rating. Similarly.the clamping diodes need to have different voltage ratings for reverse voltage blocking. The complimentary switch pair is defined such that turning on one of the pair switches will exclude the other from being turned on. The line voltage consists of a positive phase-leg a voltage and a negative phase-leg b voltage.. Sa. of Fig. I ... the charging time for rectifier operation (or discharging time for inverter operation) for each capacitor is different.. the number of diodes required will make the system impractical to implement. through Sa. State condition 1 means the switch is on. turn on one upper switches Sa.. conducts only during Vao = Vdc. 3(a).. S a 219 (SJj. 0. and the inner switches may be undersized. conducts over the entire cycle except Vao= 0. (1) For voltage level Voo=Vd0 turn on all upper switches So. Notice that each switch is only switched once per cycle. Each phase voltage tracks one-half of the sinusoidal wave. Phase. When operating at unity power factor. Table I lists the voltage levels and their corresponding switch states. there are five switch combinations to synthesize five level voltages across a and 0. Capacitor ’Voltape Unba lance In mo’st applications. When m is sufficiently high. a powier converter needs to transfer real power from ac to dc (rectifier) or dc to ac (inverter operation). needs to block three capacitor voltages. 2 shows phase and line voltage waveforms of the example 5-level converter.. while switch So..and line voltage wavefomis of a 5-level diode-clamp voltage source converter.j4. and 0 means the switch is off. and one lower switch So. as shown in Fig. unequal Device Rating From Table 1. Using Da. and two lower switches San.. (3) For voltage level Vo0=VdJ2. There exist four complimentary switch pairs in each phase. Such a capacitor chiarging profile repeats every Switch State 2349 .. turn on all lower half switches Sa. the negative dc rail. is considered as the output phase voltage reference point.. the four complementary pairs are (Sa.through So. ( 5 ) For voltage level Vao=O. 2.). and Da3 needs to block 3 Vd.-Sa.. Fig. the outer switches may be oversized. If the design is to suit the worst case. Such an unequal conduction duty requires different current ratings for switching devices. 1 a!$an example. turn on two upper switches So3 and Sa.
harmonic content will be low enough to avoid the need for filters. Vaa. and the result is unbalanced capacitor voltages between different levels.=3VdJ4. 0 Efficiency is high because all devices are switched at the fundamental frequency.can be synthesized by the following switch combinations: (1) For voltage level Vuo=Vdc.3 (3) For voltage level Va0=VdJ2. the capacitor voltages can be balanced by equal charge and discharge in one-half cycle. s a 29 Su.. there are six combinations: (a>Sa/. Assuming that each capacitor has the same voltage rating as the switching device. (b) S O 2 9 Sa+>Sa. Three inner-loop balancing capacitors for phase leg Q. s a 2 . Su'+ (Va.2 ( V a . s a 3 ( V a ~ V 3d 4 . the dc bus needs (m-I) capacitors for an m-level converter.=Vdc-3 Vd4+V&-Vd4). su. Ca. Su.2 ( v a ~ V d c . (d) s a / .sa33 sa. The phase voltage of an m-level converter has m levels including the reference level. ( 2 ) For voltage level Va. The use of a controlled dc voltage will result in system complexity and cost penalties. This indicates that the converter can transfer pure reactive power without the voltage unbalance problem.?. v 5 (a> (b) Fig. With the high power nature of utility power systems. In summary. (b) voltage and current are 90" out of phase. S a 3 . It is difficult to do real power flow control for the individual converter. 3(b). Su3. Advantages: When the number of levels is high enough. 4 is to indicate the voltage level between the clamping points..and Ca3are independent from those for phase leg b. When operating at zero power factor. 4 as the example. S a 3 . = V d ) .. s a s a 4 ( V a ~ V 3d 4 . Each phase-leg has an identical structure.-C. 4 illustrates the fundamental building block of a single-phase full-bridge flying-capacitor based 5-level converter . su. 3 Waveforms showing capacitor charging profile (a) voltage and current in phase. the voltage of the 5-level phase-leg Q output with respect to the negative dc rail. and (f) s u 2 .. The control method is simple for a back-to-back intertie system. however. and the line voltage has (2m-1) levels. sa.. such as replacing capacitors by a controlled constant dc voltage source such as pulse-width modulation (PWM) voltage regulators or batteries..=3 VdJ4). s u 2 . 111. as shown in Fig. Y charged Fig. (C)s a / . Sa. Ca2. s a 1. All phase legs share the same dc link capacitors.4 (Va. turn on all upper switches SUI through Sa. there are three combinations: (a) s a / . Basic Principle Fig. Assuming that each capacitor has the same voltage rating. series connection of capacitors in Fig. s a Sa.half cycle. (Vao=Vdc-Vd4). the converter switching frequency must be kept to a minimum to avoid switching losses and electromagnetic interference (EMI) problems. Using Fig. Disadvantages: Excessive clamping diodes are required when the number of levels is high. 4 Circuit diagram of a flying capacitor based 5-level single-phase voltage source converter. advantages and disadvantages of a diodeclamp multilevel voltage source converter are as follows./ (Va..V d 4 ) .. and VdJ4+VdJ2).V d 2 + V d 4 ) . (b) Sa37 Sa+. the 2350 . Reactive power flow can be controlled. MULTILEVEL CONVERTER USISG FLYIKG- The voltage level defined in the flying-capacitor converter is similar to that of the diode-clamp type converter. 17 CAPACITORS A . The voltage unbalance problem in a multilevel converter can be solved by several approaches. C. The voltage synthesis in a flying-capacitor converter has more flexibility than a diode-clamp converter. s u 3 ( Va~Vdc-3 V ~ J 4 + V d 4 ) 9 (e> s a z . sa.=Vdc-V&). (c>Sa/.
one may employ two or more switch combinations for middle voltage levels (i. The inverter control will be very complicated. Turning off all switches yields v. Sa. by 90°. (c) so39 s ( 5 ) For voltage level Vao=O. 5(b).=O. Each level of the full-bridge converter consists of four switches. Iv. tuming on SI and S. and s is the number of dc sources.. when it involves real power conversions. = 0 B.dl SDC capacitor voltages can be balanced. and the switching frequency needs to be higher than the fundamental frequency. the average charge to each dc capacitor is equal to zero over one line cycle. and Vd&4) in one or several fundamental cycles. harmonic content will be low enough to avoid the need for filters. With the assumption that all capacitors have the same voltage rating.v. . Fig. Turing on SI and S. (b) Sa. S. Sa. Therefore. and the switching frequency and switching losses will be high for rleal power transmission. leading or lagging the phase voltage v . 5( b) shows the synthesi:zed phase voltage waveform of a 9-level cascaded inverter with four SDCs. yields vI=+Vdc. Each single-phase full bridge inverter can generate threle level outputs./. where m is the output phase voltage level. +Vdc. This is made possible by connecting the dc sources sequentially to the ac side via the four gate-tum-off devices. van= vi + v2 + v3 4.1) capacitors.and -Vdc.. For a three-phase system. an m-level diode-clamp inverter only requires (m. TABLE I1 A possible switch combination for the flying capacitor based 5-level When the number of levels is high enough. (Vue= V d D . SI. s u 29 so. Disadvantages: An e!xcessive number of storage capacitors is required when. 0.Sa.=-Vdc. the "level" in a cascaded-inverters based converter is defined by m=2s+l. each device needs to be switched only once per cycle. The phase output voltage is synthesized by the sum of four inverter outputs. the flying-capacitor multilevel converter also has unequal device duty problems. an m-level converter will require a total of (m-l)x(m-2)/2 auxiliary capacitors per phase leg in addition to (m-1)main dc bus capacitors. In summary.ively new converter structure..INVERTERS WITH SEPARATE DC SOURCES converter. (VaFVdc-3 Vdc/4). by proper selection of switch combinations. there are three 0 (a) Sa/>Sa'.*. Thus. Similarly.3. the: output voltages of the three cascaded inverters can be connected in either Y. Provided that the voltage rating of each capacitor used is the same as that of the main power switch. I OUtDUt I Switch State i v. 3 VdJ4. l ~ l Each ~ SDC l is associated ~ l with a single-phase full-bridge inverter. For example. Basic Principle A relal. With the phase current.Sa. In order to balance the capacitor charge and discharge. Advantages: 0 Large amount of storage capacitors provides extra ride through capabilities during power outage. The ac terminal voltages of different level inverters are connected in series.or A- 2351 . VdJ2.. Using the top level as the example. M~JLTILEVEL CONVERTER USING CASCADED. 0 Provides switch combination redundancy for balancing different voltage levels. According to the device tum-on time requirement listed in Table 1 1 . and S.. advantages and disadvantages of a flyingcapacitor multilevel voltage source converter are as follows. To calmply with the delinition of the previously mentioned diode-clamp and flying-capacitor multilevel converters. and o ' I . Highlevell systems are more difficult to package and more expensive with the required bulky capacitors. the ac output voltage ait each level can be obtained in the same manner.V d 4 ) .?.e.. the selection of a switch combination becomes very complicated. Table I1 lists a possible combination of the voltage levels and their corresponding switch states.(4) For voltage combinations: level Va0=Vdc/4. 5(a) !jhows the basic structure of the cascaded-inverters with SDCs. the number of converter levels is high. This new converter can avoid extra clamping diodes or voltage balancing c:apacitors.. through Sa. the major problem in this inverter is the requirement of a large number of storage capacitors. s a ' 3 (Vao=Vdc/4).. a 9-level cascaded-inverters based converter will have four SDCs and four full bridges. Using such a switch combination. making a possible voltage source converter candidate for high voltage dc transmission. shown in a single-phase configuration. shown in Fig. Minimum harmonic distortion can be obtained b y controlling the cortducting angles at different inverter levels. i.. yields v. Features ~ O ~ O ~ O ~ O ~ Besides the difficulty of balancing voltage in real power conversion. However. the flying-capacitor multilevel converter may be used in real power conversions.e.. S. tum on all lower switches Sa.. ia . Both1 real and reactive power flow can be controlled. cascaded-inverters with separate dc sources (SDCs) is introduced here. A. Fig..
The polarity and the magnitude of the reactive current are controlled by the magnitude of the converter voltage. Fig. photovoltaic. V. is simply V5+Vc+jlc X. (ac to dc and dc to ac). the cascaded-inverter needs separate dc sources. the phase voltage and current are 90" apart. 1 (a) Circuit diagram (b) Waveform showing a 9-level converter phase voltage ' Fig. Fig. 8(b) indicates a lagging reactive current. 0 Modularized circuit layout and packaging is possible because each level has the same structure. where IC is the converter current vector. and X .-. S(a) indicates that the converter voltage is in phase with the source voltage with a leading reactive current. Features For real power conversions. and thus its applications are somewhat limited. 7 shows the circuit diagram of a multilevel converter directly connected to a power system for reactive power compensation. and biomass. which is a hnction of the dc bus voltage and the voltage modulation index. 7.5. when serving for reactive power compensation. The relationship of the source voltage vector. 2352 . . is the impedance of the inductor. 8 illustrates the phasor diagram of the source voltage.I I I Fig. Circuit diagram and the phase voltage waveform of a cascadedinverters based converter with separate dc sources.. V. A . I - I Load I Multilevel Converter I I . Connecting separated dc sources between two converters in a back-to-back fashion is not possible because a short circuit will be introduced when two back-to-back converters are not switching synchronously. Such a converter. and the capacitor charge and discharge can be balanced [2. Soft-switching can be used in this structure to avoid bulky and lossy resistor-capacitor-diode snubbers. APPLICATIONS I .. 5 .. Reactive Power Compensation When a multilevel converter draws pure reactive power.configuration . Fig. advantages and disadvantages of the cascaded-inverter based multilevel voltage source converter can be listed below. V. The multilevel structure allows the converter to be directly connected to a high voltage distribution or transmission system without the need of a step-down transformer.8]. 6 illustrates the connection diagram for a Y-configured 9-level converter using cascaded-inverters with four SDC capacitors. and the converter current. etc. I Advantages: 0 Requires the least number of components among all multilevel converters to achieve the same number of voltage levels. converter voltage. 6 . Fig. Fig.4. and the converter voltage vector. is called a static var generator (SVG). while Fig. Vs. and there are no extra clamping diodes or voltage balancing capacitors. L. I '-I '. B. A three-phase Y-configured cascaded-inverters based converter In summary. The structure of separate dc sources is well suited for various renewable energy sources such as fuel cell. Disadvantages: Needs separate dc sources for real power conversions. Circuit diagram showing a multilevel converter connected to a power system for reactive power compensation.
_. The .clamp 6-level converter for reactive power compensation. 10. IC.” 2353 . 2r. Vc. the left-hand side converter serves as the rectifier for utility interface. This B. This figure indicates that the line voltage ha:i 2m. 1 1.W Converter Cmnt 2 Ndiv Fig. . . Such a dc capacitor link is categorized a s the “back-to-back intertie.. J source & Converter Fig. I . As expected. VSg6. converter voltage. 8. a 6-level diode clamp converter and an 1 1-level cascade-inverters based converter with 5 separate dc sources have been constructed using the Insulated Gate Bipolar Transistor (IGBT) as the switching device and a digital signal processor. Each switch remains switching once per fundamental cycle. and the converter line voltage. is larger than the source voltage. - . The oscillogram indicates that the input source line voltage. is 90” leading the source voltage. I S v ‘1 I “1 - (a) Capacitor voltages at different levels 7 ’. Back-to-Back Intertie When interconnecting hvo diode-clamp multilevel converters together with a “dc capacitor link. .. The source phase voltage and the line current. I.- . Experimental results of an 1 I-level cascaded-inverterbased converter for reactive power compensation.4“ (a) Leading current (b) Lagging current slight phase difference is due to the lossy component in the inductor and the device voltage drops.. . . V V Y. Experimental results of a diode-. In hardware implementation. Phasor diagrams showing the relationship between the source and the converter voltages for reactive power compensation. The operating principle has been verified with computer simulations for all three types of multilevel converters. and the right-hand side converter serves as the inverter to supply the ac load. Fig. different Ievel capacitor voltages are well balanced without the need to vary the switch In actual combination for middle-voltage levels. as a fully digital controller. Fig. V and the converter current.. 9 shows the simulation results of a 5-level flying capacitor based converter using the switch combination stated in Table 2 for reactive power compensation. % I *r 4*1-2 _3+? (c) Source and converter voltages Fig 9 Simulated results of the flying capacitor based 5-level converter for reactive power compensation Fig. .. are slightly out of phase. are not quite 90” apart. implementation. v 17-Hay-95 13:46:35 I 200 I 260 I t 2 1 1 5 . . because the unbalance capacitor voltages on both sides tend to compensate each other. a pure 90” leading or lagging current may not be possible due to the lossy inductor and the device voltage drop..ub. 12. TMS320C3 1. Voltages 100 Vldi All three multilevel converters can be used in reactive power compensation without having the voltage unbalance problem. The result is a wellbalanced voltage across each capacitor while maintaining the staircase voltage wave. 11 shows the experimental line voltage and current waveforms of a 3-phase 1 1-level cascaded-inverters based converter for reactive power compensation. Fig. V .” as shown in Fig. .1 or 2 1 levels. 10 shows the experimental voltage and current waveforms of a 3-phase 6-level diode-clamp based converter using the switch combination listed in Table 1 for reactive power compensation._I IE V.
: : . Oscillogram of a 6-level back-to-back intertie system input line voltages and current operating at a lagging power factor condition. ..... the current is either 90" leading or lagging. This implies that phase a current is 24" lagging phase a voltage Vs-a.. .. . The purpose of the back-to-back intertie is to connect two asynchronous systems... SmSfdV Y input voltages lOOV/div . : .. the converter terminal line voltage.. 13 illustrates the phasor diagram for real power transmission from the source end to the load end.. . Fig. .. If the source voltage is constant.... ... Phasor diagram of the source voltage.. .. VC-ab: THD=7.... .. To show the superiority of the harmonic performance of the multilevel converter. . .Rectifier Operation DC Link Inverter Operation ...A. 6. 13. . The oscillogram indicates that the current... ... ..I 6 . ... and current showing real power conversions. Fig. . This unit also employs the IGBT as the switching device and TMS32031 as the controller. . cH2'"nv: .. . .. A 6-level "back-to-back" intertie hardware unit has been constructed and tested as a phase-shifter and a power flow controller. . and 120 blocking diodes. VS-b.... the voltages and current obtained in Fig. The power flow between two systems can be controlled bidirectionally. This diagram indicates that the source current can be leading or lagging the source voltage. Fig... .. The line current is 20" lagging the line voltage or 10" leading the phase voltage.19% THD=O. . ... VC-ab. . meaning that only reactive power is generated. ... Fig. . General structure of a back-to-back intertie system using two diode-clamp multilevel converters. Utility source voltage. input voltage 1001div input current 2Aldiv Fig 15 Experimental results of a 6-level back-to-back intertie s)stem input line voltages and current operating at a leadtng power factor condition 2354 . ... . . 60 built-in diodes. Y7-b: THD=1. . then the current or power flow will be controlled by the converter voltage. . c (a) Leading power factor (b) Unity power factor Source current Isa: (c) Lagging power factor Fig. and the source current.1992 [181. Fig. . .. 15 were analyzed with the Fourier series analysis..:. ... ...95 y o These harmonic analysis results indicate that the THDs obtained from the multilevel converter are well within the limits of IEEE Std 5 19... .. The two intertie systems contain a total of 60 switching devices. The converter voltage is phaseshifted from the source voltage with a power angle. .& . 15 shows experimental results at leading power factor condition.. . . ..... 12. converter voltage. or (3) a power flow controller.. . .. For 6=0.. ... (2) a phase shifter. . Isa. . . is 54" lagging the line voltage Vs-b. operating at a lagging power factor condition... . The resulting total harmonic distortions (THDs) are as follows. .. .. .....'?i . ..Dc .. .39% Converter terminal voltage. .. These two experimental results indicate that both r e a l and r e a c t i v e power flows can be controlled in a multilevel voltage source converter. Isa. . . 14 shows experimental results of the utility input source line voltage. It can be treated as (1) a frequency changer. . .. 14. .
All three multilevel converters can be applied1 to SVGs without voltage unbalance problems because the SVG does not draw real power. TABLE 111 Comparison of power component requirements per phase leg among three multilevel converters. Two hardware models have been built and tested to veri@ the concept. can be formulated byp=(m-l)x6. The application on which the multilevel voltage source converter may have the most impact is the adjustable speed drive. VI. The structure that is most suitable for the back-toback intertie is the diode-clamp type. "Five level GTO inverters for large induction motor drives. Eloth simulation and experimental results prove that these multilevel converters are very promising for power system applications.bffg. The input frequency is 60 Hz. are the control design and the size of the capacitor. All three converters introduced in this paper can be used as the static var generator. the multilevel converter can be used for a utility compatible adjustable speed drive (ASD) with the input from the utility constant frequency ac source and the output to the variable frequency ac load. converters mentioned above. . (m. and high efficiency.1 L * & (m-l)x(m-2) ain diodes lamping (diodes E L apacitors B. p . when using the same structure for ASDs and for back-to-back interties. 5C. K. Utility Compatible Adjustable Speed Drives An ideal utility compatible system requires unity power factor." ConJ Rec. 2355 . The application that has been mentioned most frequently in the literature is SVG. Table I11 compares the power component requirements per phase leg among the three multilevel voltage source This paper has presented three transformerless multilevel voltage source converters that synthesize the converter voltage by equally divided capacitor voltages. The other two types may also be suitable for the back-to-back intertie. The second target could be the back-to-back intertie system for a unified power flow controller.1993. m. Using multilevel converters not only solves harmonics and EM1 problems. All these converters have been completely analyzed and simulated. I€€€ IAS Ann. the multilewel converters have shed a light in the power electronics arena and are emerging as a new breed of power converters for high-voltage high-power applicationis. The dc bus capacitor voltages are well balanced in the steady state. P. REFERENCES [I] R. Because the ASD needs to operate at different frequencies.1)/2 (m. The SVG is an excellent target for commercialization of these multilevel converters in high-voltage high-power systems. the relationship between the number of levels. and that the cascaded-inverters type uses a full bridge in each level ai compared to the half-bridge version used in the other two types. The multilevel converter using cascaded-inverters requires the least number of components and is most promising for utility interface applications because of its capabilities of modularization and soft-switch ing. Fig. DISCUSSION AND CONCLUSION A. but they require more switchings per cycle and more sophisticated control to balance the voltage. Steinke. no EMI.c 60Hz Voltages & Current at the Primary Side Fig. negligible harmonics. and the output frequency is 50 Hz. By extending the application of the back-to-back intertie. the dc link capacitor needs to be well-sized to avoid a large voltage swing under dynamic conditions. VI.l )x(m-2)/2 I-- 41 'I. With a balanced voltage stress in devices and utility compatible features. 595-601. Conclusion SoHz Voltages 8 Current at the Secondary Side I . -Type 1 diode-clamp( flying- I cascaded. Simulated voltage and current waveforms for a multilevel converter based variable frequency operation. Steimer.8 <I". Tlhis comparison assumes that all devices have the same voltage rating. The industry recently reported numerous ASD bearing failures arid winding insulation breakdowns due to high frequency switching PWM inverters.C.? I -c i3.. but not necessarily the same current rating. Discussion The multilevel converters can immediately replace the existing systems that use traditional multipulse converters without the need for transformers. Menzies. mid J. but also avoids possible high frequency switching dvldt induced motor failures. The major differences. 16. 16 shows the simulated input and output voltages and currents of a diode-clamp 5-level converter system for ASD applications. and the number of pulses. For a 3-phase system. pp.
S. Paper 14-202. Choi.’’ IEEE 95 WM 272-5 P WRD. “A novel resonant snubber based soft-switching inverter.. 1990. pp. “Integration of small taps into existing HVDC links. 518-523. Carpita and S. 285-294. ”A multilevel voltage-source converter system with balanced dc voltages. R. Hochgraf.” CIGRE 1994. Lai et. 1992. F. H. Z. 921-928. Z. Gole. C. IEEWIASAnn. IEEWIASAnn. Jan. 1993. Lasseter.” IEEE Paper No. M.S. “A new neutral-pointclamped PWM inverter. et al. Lefebvre. EPE 1991. C.. Sep. 1. Bhattacharya. pp. and J. pp. 19. S . Lipo. 1057-1069. Z. 58-66. Lai and F. J. S. F. “A multilevel voltage-source inverter with separate dc sources. Z. “A novel multilevel structure for voltage source inverter. W. PCIMMass Transit System Compatibility. D.No. VanCOevering. Schauder et al. Mtg. and H. 1983. 1994.” IEEE Trans. S. A. “Modeling and analysis of a static var compensator using multilevel voltage source inverter.G.” in Journal ofJapan IEE 1994./Oct. 1. Divan. L. to be presented in Con$ Rec. 1981. pp. TX. J. Jun. “Comparison of multilevel inverters for static var compensation. S. Menzies. pp. Woodford and R. Peng. 63-72./Dec. Lai. Peng and J. 1994. M. Teconi. 90-94.. Lai.“ in Conj Rec.” in Conj Rec.” I995 WM273-3 PWRD. 94SM479-6PWRD. and T. J. IEEE 5 19. Nabae. F. Industry Applications. 1994.. pp. L4-17. 26. L. Recommended practices and requirements for harmonic control in electric power systems. Lai. Peng and J. PCIWPower Quality. Peng. N. A. al. Mtg. . No 6. Martins. “A static var generator using a staircase waveform multilevel voltage-source converter. “10-MW GTO converter for battery peaking service. Peng and T. Industry Applications. C.” in Proc.” in Proc.F. S ./Feb.A. Vol. pp.. M. “Controlling a back-to-back link to operate as a phase shift transformer.” in press.. M. D. Applied Power Electronics Conference. J. McKeever.” IEEE Trans. Z. Nov. pp.” IEEE Trans. Bahman. “Power converter options for power system compatible mass transit systems. “Generalized structure of a multilevel PWM inverter. 797-803. and G. W. Stefanovic. Industry Applications. Reeve. to be presented in IEEE/IAS Annual Meeting 1995.’’ in Proc. S. ‘Walker. 1995. 1995. P. Takahashi. Fukao. “A multilevel inverter for static var generator applications. 5. 1994. “Development of a flOOMVAR static condenser for voltage control of transmission systems. no. vol.” in Conj Rec. Vol.. Cho. J. “Working group on dynamic performance and [Ill  [I31 1141 [I51 [I61 [I71 [ 18J modeling of dc systems and power electronics for transmission systems. R. IEEE Power Electronics Specialists Conference (PESC). Pilotto. Dallas. N. 901-908. Akagi. Cho. A. 2356 . pp.’’ in press. Bhagwat and V.

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