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.
- Carrier-based pulse width modulation (CBPWM),
- Neutral-point voltage balancing
- The offset voltage injection
- Three-level converter
Fig. 1. Single-phase three-level NPC converter.
EXPECTED SIMULATION RESULTS:
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).
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.
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