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 pulse width 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.
KEYWORDS:
1.
Carrier-based
pulse width modulation (CBPWM),
2.
Neutral-point
voltage balancing
3.
Single-phase
4.
The offset
voltage injection
5.
Three-level
converter
SOFTWARE: MATLAB/SIMULINK
CIRCUIT DIAGRAM:
Fig.
1. Single-phase three-level NPC converter.
Fig.
2. Simulation results of the main voltage us and the line current is .
(a)
CBPWM-OVI with k = 0.5. (b) CBPWM-MOVI.
Fig.
3. Simulation results of FFT analysis for frequency spectrum of the line
current
is . (a) CBPWM-OVI with k = 0.5. (b)
CBPWM-MOVI.
Fig.
4. Simulation results of the input port voltage uab . (a) CBPWM-OVI
with
k = 0.5. (b) CBPWM-MOVI.
Fig.
5. Simulation results of dc-link voltage u1 and u2 .
(a) CBPWM-OVI
with
k = 0.5. (b) CBPWM-MOVI.
Fig.
6. Simulation results of dc-link voltage error, the modulating signal
and
the offset voltage. (A) CBPWM-OVI with k = 0.5. (B)
CBPWM-MOVI.
(C)
CBPWM-MOVI (partial enlarged view).
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 2fs in the CBPWMOVI, and the same as the switching
frequency in the CBPWM-MOVI;
3)
the total number of switching commutations of CBPWMMOVI 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.
Simulation
and experimental results verify the validity and feasibility of these
conclusions, and the proposed CBWMOVI and CBPWM-MOVI strategies are also
desirable for single-phase three-level NPC UPS inverter or solar inverter applications.
REFERENCES:
[1]
R. Hill, “Electric railway traction—Part II. Traction drives with three phase induction
motors,” Power Eng. J., vol. 8, no. 3, pp. 143–152, Jun. 1994.
[2]
A. Steimel, “Electrical railway traction in Europe,” IEEE Ind. Appl.Mag.,
vol. 2, no. 6, pp. 6–17, Nov./Dec. 1996.
[3]
A. Cheok, S. Kawamoto, T.Matsumoto, and H. Obi, “High power AC/DC converter and
DC/AC inverter for high speed train applications,” in Proc. TENCON Conf.,
2000, pp. 423–428.
[4]
A. Nabae, I. Takahashi, and H. Akagi, “A new neutral-point-clamped PWM
inverter,” IEEE Trans. Indus. Appl., vol. IA-17, no. 5, pp. 518– 523,
Sep. 1981.
[5]
J. Lai and F. Peng, “Multilevel converters—A new breed of power converters,” IEEE
Trans. Ind. Appl., vol. 32, no. 3, pp. 509–517, May/Jun. 1996.
