A Novel Modulation Technique and a New Balancing Control Strategy for a Single-Phase Five-Level ANPC Converter

ABSTRACT:  

This paper proposes a novel modulation technique and a new balancing control strategy for a single-phase five-level flying-capacitor (FC)-based active-neutral-point-clamped (ANPC) converter. The proposed modulator can control the FC voltage to follow the requested reference value and simultaneously generate the required ac output voltage regardless of the values of the dc capacitor voltages of the converter. By implementing this method, smaller values of the dc-link capacitor and FC can be used even in applications that could experience ripple or transient in the capacitor voltage.

In a single-phase five-level ANPC converter applications, where the capacitors can experience pulsation power and dc-link balancing issues, such as grid-connected photovoltaic system, the selection of the reference voltage value for the FC can play an important role to balance the average values of the dc-link  capacitor voltage. The proposed new control strategy uses a new reference voltage for the FC to be applied by the new modulator to have an average balanced dc-link voltages as well as an ac output voltage with good power quality. Simulation studies and experimental results demonstrate the effectiveness of the proposed modulation technique and control strategy even with relatively small dc capacitors to produce high-quality output voltage and current waveforms while maintaining an average balanced dc-link voltages.

KEYWORDS:
  1. Active-neutral-point-clamped (ANPC) converter
  2. Flying capacitor (FC)
  3. Multilevel converters
  4. Photovoltaic (PV) power system
  5. Pulse width modulation
  6. Voltage balancing

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

 Fig. 1. Single-phase 5L-ANPC converter.

 EXPECTED SIMULATION RESULTS:

Fig. 2. Results of applying conventional modulation technique with dc-link capacitors equal to 300 μF. (a) DC-link voltage, dc-link capacitor voltages, and FC voltage. (b) Inverter output voltage. (c) Inverter output and ac grid current. (d) Magnitude of current harmonics (p.u.).

Fig. 3. Results of applying conventional modulation technique with dc-link

capacitors equal to 3 mF. (a) DC-link voltage, DC-link capacitor voltages, and

FC voltage. (b) Inverter output voltage. (c) Inverter output and ac grid current.

(d) Magnitude of current harmonics (p.u.).

Fig. 4. Results of applying proposed modulation technique with dc-link

capacitors equal to 300 μF and FC reference voltage equal to Vdc/4. (a) DClink

voltage, dc-link capacitor voltages, and FC voltage. (b) Inverter output

voltage. (c) Inverter output and ac grid current. (d) Magnitude of current

harmonics (p.u.).

Fig. 5. Results of applying proposed modulation technique with dc-link

capacitors equal to 300 μF and FC reference voltage equal to half of the engaged

dc-link capacitor in each output half-cycle. (a) DC-link voltage, dc-link

capacitors voltages, and FC voltage. (b) Inverter output voltage. (c) Inverter

output and ac grid current. (d) Magnitude of current harmonics (p.u.).

Fig. 6. Results of applying the proposed modulation technique with the proposed

FC reference voltage with having step power in the PV and transmission

power to the grid at t = 37 ms: (a) DC-link voltage, dc-link capacitors voltage,

and FC voltage. (b) Inverter output and grid current.

 CONCLUSION:

 A novel modulation and control strategy for a five-level FC based ANPC converter has been presented. A theoretical framework of a novel extended modulation technique for unsymmetrical and symmetrical voltage conditions of a 5L-ANPC converter has been proposed. The application of the proposed modulation and control strategy, for a single phase grid-connected PV system using a five-level FC-based ANPC converter to produce ac output voltages with good power quality under both symmetrical and unsymmetrical conditions, has been investigated.

Issues related to the balancing of dc-link voltages and its associated problems are discussed, and a new control strategy has been introduced to solve the dc-link voltage divergence problem. The proposed strategy is applicable for other applications of the five-level FC-based ANPC converter.  The effectiveness of the proposed modulation technique and control strategy was demonstrated by the simulation and experimental results in the laboratory, demonstrating the ability of the system to operate properly using smaller size dc-link capacitors to produce ac output voltage and current waveforms with good power quality while maintaining dc-link average voltage balancing.

REFERENCES:

[1] J. I. Leon, L. G. Franquelo, S. Kouro, B. Wu, and S. Vazquez, “Simple modulator with voltage balancing control for the hybrid five-level flying-capacitor based ANPC converter,” in Proc. IEEE ISIE, 2011, pp. 1887–1892.

[2] P. Barbosa et al., “Active neutral-point-clamped multilevel converters,” in Proc. 36th IEEE PESC, 2005, pp. 2296–2301.

[3] F. Kieferndorf et al., “ANPC-5L technology applied to medium voltage variable speed drives applications,” in Proc. Int. SPEEDAM, 2010, pp. 1718–1725.

[4] S. A. Gonzalez, M. I. Valla, and C. F. Christiansen, “Five-level cascade asymmetric multilevel converter,” IET Power Electron., vol. 3, no. 1,   pp. 120–128, Jan. 2010.

[5] Y. Kashihara and J. Itoh, “Parameter design of a five-level inverter for PV systems,” in Proc. 8th IEEE ICPE ECCE, 2011, pp. 1886–1893.

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