A Hybrid Capacitor-Clamp Cascade Multilevel Converter Anees Abu Sneineh
Ming-yan Wang
Kai Tian
Harbin Institute of Technology School of Electrical Engineering and Automation Habin, China, 150001
[email protected]
[email protected]
Abstract — Use of multilevel converter has become popular in recent years. This paper will present a new topology of a Hybrid capacitor-clamp cascade multilevel converter that is derived from two popular topologies. The new concept of the converter is based on the connection of multiple three-level capacitorclamp converter modules with different DC bus voltages. With the novel topology consisting of higher voltage modules and lower voltage modules, realization of multilevel converters using a hybrid approach involving higher voltage devices and faster devices operating in synergism is possible. A detailed example of HCCMC is given. The proposed converter is also verified by computer simulation using MATLAB-Simulink. Simulation results are also presented in this paper. Keywords — Cascade; Capacitor-clamp; Multilevel Converter; Topology; Sub-Harmonic PWM; Hybrid Modulation
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–2E. Obviously by using proper control method applied to this topology, the stepped output waveform can be approximate to a sinusoidal waveform.
I. INTRODUCTION Multilevel power conversion has become increasingly popular in recent years due to its advantages [1-12]. The general concept of the researches in power conversions involves producing ac waveform from small voltage steps by utilizing isolated dc sources or a bank of series capacitors. The small voltage steps yield waveforms with low harmonic distortion as well as low dv/dt. The advantages of multilevel converters if they are compared with conventional two-level converters are the capability of increasing the output voltage magnitude and reducing the output voltage and current harmonic content, the switching frequency and the voltage supported by each power semiconductors. Three main types of multilevel converters, i.e. diodeclamp, capacitor-clamp, and cascade converter with separated DC sources, have been developed. In fact, the cascade multilevel converter with separated DC sources has been shown many advantages over the other two. Specially, the modularized circuit layout and packaging are possible. This makes the cascade multilevel converter feasible for manufacturing. II. CASCADE MULTILEVE CONVERTER A. Traditional Cascade Converter Fig.1 shows a schematic of a single-phase cascade converter in which two cell of traditional two-level power converters with separated DC sources are series-connected [1, 2]. The output waveform is synthesized by adding of each converter output voltage. Assuming each DC source has the same Dc voltage, E. Based on switch combinations, five output voltage levels can be synthesized, viz., +2E, +E, 0, –E, 1-4244-0136-4/06/$20.00 '2006 IEEE
Fig.1. Single phase Cascade multilevel converter
In general, the output voltage of a given multilevel converter can be calculated [3] from: n −1 Vo = (S − )E (1) 2 Where Vo is the output of the multilevel converter, n is the number of the output levels; S is the switching state that ranges from 0 to (n-1). E is the minimum voltage level the multilevel converter can produce. B. Hybrid Multilevel Converter (HMC) Hybrid multilevel converter is derived from the traditional cascade converter. In traditional cascade converter, the dc bus voltage of each module has the same value, and switching frequency and voltage blocking capability of all switches are the same. As shown in Fig.1, HMC has the same configuration as the traditional cascade converter, but the dc bus voltage as well as the devices, switching frequency and voltage stress across the devices are different [4, 5, 6]. A single-phase HMC consists of an IGCT converter with a 2.2kV bus and an IGBT converter with 1.1kV bus in series is introduced in ref. [4, 5]. A hybrid modulation strategy is also employed in the converter.
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C. Capacitor-Clamp Cascade Multilevel Converter Fig.2 shows a schematic of a single phase full-bridge capacitor-clamp multilevel converter. Assuming the DC bus voltage of the converter is 2E, it can be easily found that there are five-levels in the output waveform, according to (1), obtains: (2) Vo = (S − 2) E Where S=0, 1, 2, 3, 4. For S select, five different values of the output voltage can be achieved, viz., +2E, +E, 0, –E, –2E accordingly.
The capacitor-clamp multilevel converter [7] is based on the connection of several three-level capacitor-clamp converter modules, and the multilevel waveform is synthesized by adding of each converter output voltage. An example of the converter is shown in Fig.3. It consists of two capacitor-clamp multilevel converters that are seriesconnected together. Suppose the DC bus voltage of each capacitor-clamp converter is 2E, five output voltage levels of each cell can be synthesized, viz., +2E, +E, 0, –E, –2E. The synthesized output waveform of the whole converter is nine voltage levels in total (+E, +2E, +3E, +4E, 0, -E, –2E, –3E, –4E). According to (1) we get: 9 −1 Vo = (S − ) E = ( S − 4) E (3) 2 Where S=0, 1, 2, 3, 4, 5, 6, 7, 8. From (3), the output level of the proposed converter can be defined by the switching states S, such as, when S=7, from (3), Vo= +3E. III. HYBRID CAPACITOR-CLAMP CASCADE MULTILEVE CONVERTER
Fig.2. Capacitor-clamp multilevel converter
By employing hybrid modulation method and capacitorclamp cascade multilevel converter with different dc bus voltage, a hybrid capacitor-clamp cascade multilevel converter (HCCMC) is created. GTO and IGBT are employed in the higher voltage module and lower voltage module respectively. To clearly explain the proposed converter, an example of the novel converter is studied in the following sections, as shown in Fig.4. Synthesize stepped waveforms with 13 voltage levels, viz., –6kV, –5kV, –4kV, –3kV, –2kV, –1kV, 0, +1kV, +2kV, +3kV, +4kV, +5kV, +6kV at the phase leg output. Obviously it produces more voltage levels when compared to conventional configuration with a same number of dc sources, detailed comparison will presents in the conclusion. A. Hybrid Modulation Strategy It is well known that the voltage blocking capability of faster devices such as IGBT and the switching speed of high voltage devices like GTO is found to be limited. Hence a hybrid modulation strategy that incorporates stepped synthesis in conjunction with variable pulse width of consecutive steps such as Sub-harmonic PWM method will be employed in HCCMC. Under this modulation strategy, the GTO module is modulated to switch only at fundamental frequency of the converter output, while the IGBT module is used to switch at a higher frequency. The proposed static transfer characteristics for the GTO and IGBT modules are illustrated in Fig.5 and Fig.6. As may be observed from Fig.5, when the command signal (desired output) is greater than 2kV or smaller than –2kV, the GTO module contributes to the output with 2kV or –2kV, respectively, and when the command signal is greater than 4kV or smaller than –4kV, the GTO module contributes to the output with 4kV or –4kV, respectively. As to IGBT module, Sub-harmonic PWM (SHPWM) method is employed.
Fig.3. capacitor-Clamp Cascade Multilevel converter
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SHPWM is a conventional control method suit for multilevel converter [5-8]. The control principle of the SHPWM method is to use several triangular carrier signals with only one modulation wave per phase. For an n-level inverter, (n-1) triangular carrier of the same frequency fc, and the same peak-to-peak amplitude Ac, are disposed so that the bands they occupy are contiguous. The zero reference is placed in the middle of the carrier set. The modulation wave is a sinusoid waveform of frequency fm and amplitude Am. At every instant each carrier is compared with the modulation waveform.
Fig.4. Two cell of HCCMC converter
Fig.5. Static transfer characteristics of GTO module
Fig.7. Hybrid modulation theory for HCCMC (a) Desired output of the whole converter (b) Desired output voltage of GTO module (c) Desired output voltage of IGBT module (d) Desired output voltage of IGBT module compared with four triangular carriers
Fig.6. Static transfer characteristics of IGBT module
Fig.7 illustrates the hybrid modulation theory for HCCMC. Assuming a 6kV, 50Hz sinusoidal output is desired, as shown in Fig. 7(a). Fig. 7(b) shows the desired output waveform of GTO. After subtracting the desired output voltage of GTO module from the sinusoidal output command of whole converter, command signal of IGBT module is achieved, as shown is Fig. 7(c). The reference signal of IGBT module then compares to four triangular carriers as shown is Fig. 7(d), each comparison switches the device on if the 2033
reference signal is greater than the triangular carrier assigned to that device level; otherwise, the device switches off. A 13level output waveform is synthesized by adding of each module output voltage, as shown in Fig. 10. The modulation process and the state of the modules for various levels of command signals are summarized in Table I. Table I Hybrid Modulation Strategy Desired output GTO IGBT Module between Module -6kV and -5kV -4kV -2kV ↔ -1kV -5kV and -4kV -4kV -1kV ↔ 0kV -4kV and -3kV -2kV -2kV ↔ -1kV -3kV and -2kV -2kV -1kV ↔ 0kV -2kV and -1kV 0kV -2kV ↔ -1kV -1kV and 0kV 0kV -1kV ↔ 0kV 0kV and 1kV 0kV 0kV ↔ 1kV 1kV and 2kV 0kV 1kV ↔ 2kV 2kV and 3kV 2kV 0kV ↔ 1kV 3kV and 4kV 2kV 1kV ↔ 2kV 4kV and 5kV 4kV 0kV ↔ 1kV 5kV and 6kV 4kV 1kV ↔ 2kV a ↔ b: Switching between a and b.
around 37% and 78%. Hence, it is necessary for the IGBT inverter to cancel this excessive voltage. As may be seen from the fundamental voltage synthesized by the IGBT inverter, this inverter synthesizes negative voltage in this region of modulation depths. In terms of real power flow, which is represented by the current component that is in phase with the fundamental voltage, it appears that the IGCT inverter feeds the power into the IGBT inverter in this zone. The author of reference [4] adds a regenerative IGBT rectifier to solve the problem. As to the single-phase HCCMC shown in Fig.4, the ratio of dc bus voltages V1:V2= 2:1, same to the proposed converter in [4, 5]. It may be observed from Fig.8 that the GTO module synthesized voltage less than the command voltage. And it is not necessary for the IGBT module to cancel the excessive voltage. So in terms of real power flow, the GTO module does not feed the power into the IGBT module when M ranges from 0 to 1, as shown in Fig.8 and Fig.9.
B. Spectral Analysis With the hybrid modulation strategy, the effective spectral response of the output depends on the IGBT switching, while the overall voltage generation is decided by the voltage ratings of GTO. For spectral analysis, reference command to the HCCMC can be represented as: Vref = M sin ωt (4) where M is the modulation depth which varies between 0≤M≤1 and ω is the angular frequency of the reference signal. Therefore, the GTO module output and the IGBT module command are given by equation (5) and (6) VGTO =
∞
∑
n =1, 3, 5...
4 1 2 [cos(n sin −1 ) + cos(n sin −1 )] sin nωt 3nπ 3M 3M
VIGBT = M sin ωt −
(5) Fig.8. GTO module fundamental voltage as a function of modulation depth
4 1 2 [cos(n sin −1 ) + cos(n sin −1 )] sin nωt 3M 3M n=1,3, 5... 3nπ ∞
∑
(6) At the fundamental frequency, from equation (5) and (6), the fundamental spectral analysis of GTO module and IGBT module are given by: 2 1 4 VGTO = )] sin ωt (7) ) + cos(sin −1 [cos(sin −1 3M 3M 3π 4 1 2 VIGBT = [M − (cos(sin −1 ) + cos(sin −1 ))] sin ωt (8) 3π 3M 3M The GTO module and IGBT module output under hybrid modulation are plotted against the modulation depth in Fig.8.and Fig.9. It is overlaid on a unity slope line that specifies the commanded fundamental voltage in Fig.8. In reference [4, 5], A single-phase HMC consists of an IGCT converter with a 2.2kV bus and an IGBT converter with 1.1kV bus in series is given. It may be observed from the HMC that the IGCT (or GTO) inverter synthesizes more voltage than necessary between the modulation depths 2034
Fig.9. IGBT module fundamental voltage as a function of modulation depth
The advantage of this topology is that it provides flexibility for expansion of the number of levels easily without introducing undue complexity in the power circuit [9]. Moreover, it requires the same number of switches as in diode-clamped cascade topology to achieve a given number of voltage levels [10]. Another advantage of this topology is that there is no unbalanced capacitor voltage problem because of its independent voltage source structure. However, a problem associated with this approach is the requirement of a complicated control strategy to regulate the floating capacitor voltages [11, 12].
(a)
IV. VALIDATION In order to validate the proposed converter, computer simulation using MATLAB-Simulink has been created. The 13-level converter shown in Fig.4 is considered in the simulation. Sub-harmonic method is applied to trigger the power switches for controlling the voltage levels generated on the ac side. The DC bus for the GTO is 4kV and for the IGBT is 2kV with 110Ω resistor and 100mH inductor were used as the load. The modulation wave is a sinusoid waveform of frequency fm = 50Hz. The modulation depths for this study were M=0.8, 0.9, 1.0, and the frequency of the triangle carrier waveform fc = 10 kHz. The synthesized output simulation waveform of the whole converter (two modules) is shown in Fig.10. Fig.7 shows the voltage output for the higher voltage module and lower voltage module individually. It can be seen that the higher voltage module always outputs low frequency stepped waveform and the output voltage of the whole converter is a full PWM waveform synthesized by adding output voltages of the two modules. Fig.11 shows Fourier analysis spectrums of the waveform shown in Fig.10. It can be seen from Fig.11 that the significant harmonics are near 10 kHz, and the components of these harmonics are low. As can be seen from Fig.10, 13voltage levels are achieved. The power quality is considerably high when compared to the conventional cascade converter.
(b)
(c)
Fig.11. Analytical voltage spectrum of HCCMC (a) M=1.0 (b) M=0.9 (c) M=0.8.
V. CONCLUSION
Fig.10. Output waveform Vo of the whole converter
This paper presented a Hybrid Capacitor-Clamp Cascade multilevel converter (HCCMC). The new concept of the proposed converter is based on the connection of several Capacitor-Clamp converter modules with different DC bus voltage. The output waveform is synthesized by adding of each capacitor-clamp converter output voltage. A two-cell HCCMC with dc bus 2:1 is studied. Unlike converter proposed in [4], it may be observed from Fig.8 that the GTO module of the novel converter synthesized voltage less than the command voltage; and it is not necessary for the IGBT module to cancel the excessive voltage. So in terms of real power flow, the GTO module does not feed the power into the IGBT module when M ranges from 0 to 1. 2035
The new converter was verified by Simulation using a combination of a capacitor-clamp converter with 4kV bus and 2kV respectively. Sub-harmonic PWM method is applied to the topology to trigger the power switches for controlling the voltage levels generated on the output. Simulation results are also presented. As can be seen from the simulation results, the output voltage waveforms exhibit low voltage dv/dt and low THD due to the extra voltage levels. Finally, a brief comparison of the proposed configuration for 13-level voltage generation with the topologies reported in literature is presented in Table II. It may be observed that the proposed approach offers the same number of levels at the output with a least number of primary devices and dc voltage sources. Table II Comparison of 13-Level converter Topologies Topology
Diode
Switches
24
capacitors
DC Bus
Levels in
(separated)
the output
1
13
12
Anees Abu Sneineh was born in 1976 in Palestine. He received the B.E.
degree in Industrial Automation Eng. from Palestine Polytechnic University (PPU), Hebron, Palestine, in 2000, and the M.S. degree in Mechatronics Eng. from Harbin Institute of technology, China, in 2004. He is currently working
clamp Capacitor
[10] Kai Ding, Yun-ping Zou, et al. “A Novel Hybrid DiodeClamp Cascade Multilevel Converter for High Power application,” IEEE Industry Applications Conference, 39th IAS Annual Meeting, Vol. 2, Oct. 2004, Page 820827. [11] J.L. Duarte, P.J.M. Jullicher, L.J.J. Offringa, and W.D.H. Groningen, “Stability Analysis of Multilevel Converters with Imbricated Cells,” EPE’97 Conference Proceedings, page 4.168-4.174, 1997. [12] Roberto Rojas and Tokuo Ohnishi, “PWM Control Method with Reduction of Total Capacitance Required in a Three-level Inverter,” COBEP’97 Conference Proceedings, page 103-108, 1997.
toward the Ph.D. degree at Harbin Institute of technology, China. 24
76
1
13
His research interests are modeling, design, and control of power
clamp
conversion systems of power electronics.
cascade
24
6
6
13
HCCMC
16
4
2
13
Ming-yan Wang was born in 1957 in Heilongjiang province, China. He
received B.E. and M.S. and Ph.D. degree in Electrical engineering and
REFERENCES
Automation from Harbin Institute of technology, China, in 1982, 1988 and
[1] Peter W. Hammond, “Medium Voltage PWM Drive and Method,” U. S. Patent 5 625 545, Apr.1997. [2] M.D. Manjrekar and T.A. Lipo, “A generalized structure of multilevel power converter,” in Proc. IEEE PEDES’98, 1998, pp. 62–67. [3] Keith Corzine, Yakov Familiant, “A New Cascaded Multilevel H-Bridge Drive,” IEEE Trans on Power Electronics, 2002, Vol. 17, No.1, page 125-131. [4] Madhav D. Manjrekar, Thomas A. Lipo, “Hybrid Multilevel Power Conversion System: A Competitive Solution for High-Power Applications,” IEEE Trans. on Industry application, Vol.36, No.3, MAY/JUNE 2000, page 834- 841. [5] Richard lund, Madhav D.Manjrekar, et al. “Control Strategies for a Hybrid Seven-level Inverter,” EPE’99 Conference proceedings, 1999. [6] Madhav D. Manjrekar, Thomas A. Lipo, “A hybrid multilevel inverter topology for drive applications,” IEEE APEC’98, 1998, page 523-529. [7] Anees Abu Sneineh, Ming-yan Wang, Kai Tian, “A New Topology of Capacitor-Clamp Cascade Multilevel Converters,” IEEE-IPEMC06, August, 2006. [8] Sung-Jun Park, Feel-Soon Kang, et al. “A New SinglePhase 5-Level PWM Inverter Employing a Deadbeat Control Scheme,” IEEE Trans. on Power Electronics, Vol.18, No.3, MAY 2003, Page 831-843. [9] L. Zhang, S. J. Watkins, W. Shepherd, “Analysis and Control of A Multi-level Flying Capacitor Inverter,” IEEE CIEP, Oct. 2002 Page 66 – 71.
electrical engineering in Harbin Institute of technology. In 1999 he becomes
2002, respectively. In 1999 he becomes the director of department of professor in Electrical engineering and Automation. His research interests include medium and high voltage rectifiers and inverters, multilevel converters. Kai Tian was born in Hebei province, China, in 1980. He received the B.E.
degree in Electrical engineering and Automation from Harbin Institute of technology, China, in 2004. He is currently working toward the master degree in power electronics and electrical drives at Harbin Institute of Technology, China. His research interests include medium and high voltage rectifiers and inverters, multilevel converters and modern digital devices.
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