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A Carrier-Based PWM Strategy With the Offset
Voltage Injection for Single-Phase Three-Level
Neutral-Point-Clamped Converters
Abstract:
Single-phase three-level neutral point clamped (NPC) converters are widely applied in
high-speed railway electrical traction drive systems. A significant problem related to the
single-phase three-level NPC converters is the fluctuation of the neutral-point voltage. In
this paper, a capacitor voltage balancing technique is proposed that injects an offset voltage
into the sinusoidal modulating signals of the conventional carrier-based pulsewidth modulation (CBPWM) method. Furthermore, when the injected offset voltage is maximized, it
cannot only balance the dc-link capacitors voltages, but also reduce switching losses.
Theoretical analysis has shown that both methods can control the neutral point voltage effectively, but the neutral point voltage controller in the CBPWM with maximum offset
voltage injection (CBPWM-MOVI) has a faster dynamic response. It was observed that the
high-order harmonics frequencies of the line current are centered around the twice
switching frequency in the CBPWM with the offset voltage injection (CBPWM-OVI) but are
centered around the switching frequency in the CBPWM-MOVI. And also, the CBPWM-
MOVI has switching commutations number at least 25% below that of the CBPWM-OVI in
one modulating signal period. The performances of the two strategies were verified by
simulation and experimental tests.
INTRODUCTION:
A
RELIABLE high-speed traction drive system for railway transportation is in demand in
many countries. This type of system is comprised of traction transformers, single-phase
rectifier, three-phase pulsewidth modulation (PWM) inverters, and induction motors
[1]-[3].
The single-phase rectifiers may use the three-level neutral point clamped (NPC) converter
[4], [5] or two level converter
Manuscript received January 12, 2012; revised April 2, 2012 and May 29, 2012; accepted
July 7, 2012. Date of current version October 12, 2012. This work was supported in part by
the Key Project of the National Key Technology Support Program of China under Grant
2009BAG12A05, and in part by the National Natural Science Foundation of China under
Grant U11344205. Recommended for publication by Associate Editor S. K. Mazumder.
W. Song and X. Feng are with the School of Electrical Engineering, Southwest Jiaotong
University, Chengdu 610031, China (e-mail: songwengsheng@ 163.com;
[email protected]).
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K. M. Smedley is with the Department of Electrical Engineering and Computer Science, and
Power Electronics Laboratory, University of California- Irvine, CA 92697 USA (e-mail:
[email protected]).
Color versions of one or more of the figures in this paper are available online at
http://ieeexplore.ieee.org.
Architecture Diagram:
CONCLUSION
This paper proposes CBPWM strategies in conjunction with an offset voltage injection for a
single-phase three-level NPC converter to achieve neutral point voltage control and PWM
drive signals generation. The restriction range of the offset voltage is discussed in details.
Based on this, this paper presents a CBPWM strategy with the maximum offset voltage
injection. The salient features of the proposed CBPWM-OVI and CBPWM-MOVI strategies
are as follows:
1.
both methods guarantee to achieve voltage balancing, while the CBPWM-MOVI has
a faster dynamic response of the neutral point voltage controller than the CBPWMOVI;
2.
the high-order harmonics of the line current distribute around at twice switching
frequency 2 f s in the CBPWM- OVI, and the same as the switching frequency in the
CBPWM-MOVI;
3.
the total number of switching commutations of CBPWM- MOVI is 25% below that
of the CBPWM-OVI, at least in a modulating signal period;
4.
both CBPWM-OVI and CBPWM-MOVI with voltage step compensation can
guarantee the maximum voltage level step to be half of the dc-link voltage compared with
the existing CBPWM strategy.
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Simulation and experimental results verify the validity and feasibility of these conclusions,
and the proposed CBWM- OVI and CBPWM-MOVI strategies are also desirable for singlephase three-level NPC UPS inverter or solar inverter applications.
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