Dynamic voltage restorer employing multilevel cascaded H-bridge inverter

IET Power Electronics, 2016

ABSTRACT: This study presents design and analysis of a dynamic voltage restorer (DVR) which employs a cascaded multilevel inverter with capacitors as energy sources. The multilevel inverter enables the DVR to connect directly to the medium voltage networks, hence, eliminating the series injection transformer. Using zero energy compensation method, the DVR does not need active energy storage systems, such as batteries. Since the energy storage system only includes capacitors, the control system will face some additional challenges compared with other DVR systems. Controlling the voltage of capacitors around a reference voltage and keeping the balance between them, in standby and compensation period, is one of them. A control scheme is presented in this study that overcomes the challenges. Additionally, a fast three-phase estimation method is employed to minimize the delay of DVR and to mitigate the voltage sags as fast as possible. Performance of the control scheme and estimation method is assessed using several simulations in MATLAB / SIMULINK environments.

KEYWORDS:

  1. Multilevel inverter
  2. cascaded H-bridge inverter
  3. Dynamic Voltage Restorer

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

 Multilevel inverter

 Fig. 1 DVR strcuctures  a) Conventional DVR b) CHB-based DVR

 EXPECTED SIMULATION RESULTS:

Fig. 2 Three-phase voltage sag a) Network voltage b) Injected voltage by the DVR c) Load-side voltage

 Fig. 3 Unbalanced voltage sag (a 20% voltage sag on phase A) a) Source voltage b) Injected voltage by the DVR c) Load-side voltage

Fig. 4 Voltages of the DC link capacitors

Fig. 5 Three-phase 20% voltage sag with voltage harmonics a) Network voltage b) Injected voltage by the DVR c) Load-side voltage

 

CONCLUSION:

This paper presented design and performance assessment of a DVR based on the voltage sag data collected from MWPI. Using a multilevel converter, the proposed DVR was capable of direct connection to the medium voltage-level network without a series injection transformer. In addition, development of zero active power compensation technique helps to achieve voltage restoration goal just by the capacitors as energy storages. Due to internal losses of H-bridge cells and probable inaccuracies in measurements, voltage of DC link capacitors may become unequal, which prevents proper operation of the converter. A voltage control scheme, comprised of three separate controllers, was proposed in this paper for keeping voltage balance among the DC link capacitors within nominal range. A fast estimation method was also employed for calculation of phase and magnitude terms in an unbalanced three-phase system. This estimation method is able to recognise voltage sags in approximately half a cycle. Several simulations were performed in PSCAD/EMTDC environment to verify the performance of CHB-based DVR. Additionally, a laboratory-scale prototype of the proposed DVR was built and tested. Results of the experimental test also confirmed validity of the proposed control system.

 REFERENCES:

1 Chapman, D.: ‘The cost of poor power quality’ (European Copper Institute, Copper Development Association, 2001), March

2 Radmehr, M., Farhangi, S., Nasiri, A.: ‘Effects of power quality distortions on electrical drives and transformer life in paper industries’, IEEE Ind. Appl. Mag., 2007, 13, (5), pp. 38–48

3 Lamoree, J., Mueller, D., Vinett, P.: ‘Voltage sag analysis case studies’, IEEE Trans. Ind. Appl., 1994, 30, (4), pp. 1083–1089

4 Bollen, M.H.J.: ‘Understanding power quality problems: voltage sags and interruptions’ (New York, Saranarce University of Technology, 2000)

5 Ghosh, A., Ledwich, G.: ‘Power quality enhancement using custom power devices’ (Berlin, Kluwer Academic Publications, 2002)

Design and Performance of a Bidirectional Isolated DC–DC Converter for a Battery Energy Storage System

 

ABSTRACT:

This paper describes the design and performance of a 6-kW, full-bridge, bidirectional isolated dc–dc converter using a 20-kHz transformer for a 53.2-V, 2-kWh lithium-ion (Li-ion) battery energy storage system. The dc voltage at the high-voltage side is controlled from 305 to 355 V, as the battery voltage at the low voltage side (LVS) varies from 50 to 59 V. The maximal efficiency of the dc–dc converter is measured to be 96.0% during battery charging, and 96.9% during battery discharging. Moreover, this paper analyzes the effect of unavoidable dc-bias currents on the magnetic-flux saturation of the transformer. Finally, it provides the dc–dc converter loss breakdown with more focus on the LVS converter.

 KEYWORDS:

  1. Bidirectional isolated dc–dc converters
  2. Dc-bias currents
  3. Energy storage systems
  4. Lithium-ion (Li-ion) battery

 SOFTWARE: MATLAB/SIMULINK

 CIRCUIT DIAGRAM:


Fig. 1. Li-ion battery bank of 53.2 V, 40 A·h connected to the 6-kW bidirectional isolated dc–dc converter, where LS is the background system impedance (<1%). LAC = 280 μH (1.3%), LF = 44μH (0.2%), RF = 0.2Ω (3%), and CF = 150 μF (33%) on a three-phase 200 V, 6-kW, and 50-Hz base.

EXPECTED SIMULATION RESULTS:

 

 Fig. 2. Experimental waveforms with dc-voltage control at the HVS. (a) Charging mode at PB = 5.9 kW (VD1 = 355 V). (b) Discharging mode at PB = 5.9 kW (VD1 = 305 V).

Fig. 3. Waveforms of vD1, vB , and iB . (a) Battery charging at PB = 5.9 kW.

(b) Battery discharging at PB = 5.9 kW.

Fig. 4. Drain–source and gate–source voltages of a leg in bridge 2 at PB =

5.9 kW, VD1 = 355 V, and VB = 59V

Fig. 5. Effects of the RC-snubber on a MOSFET in bridge 2 during battery charging at PB = 5.9 kW. (a) Drain–source voltage and RC-snubber current. (b) Time-expanded waveform of vDS and iRC .

CONCLUSION:

This paper has presented the experimental results from the combination of a 53.2-V, 40-A·h Li-ion battery bank with a single-phase full-bridge bidirectional isolated dc–dc converter. The results have verified the proper operation of the Li-ion battery energy storage system. Discussions focusing on magnetic flux saturation due to unavoidable dc-bias currents at the high voltage and LVSs have been carried out. The transformer with an air-gap length of 1 mm has been shown experimentally to be robust against magnetic-flux saturation, even in the worst cases. The bidirectional isolated dc–dc converter exhibits high efficiency in the low-voltage and high-current operation. From the estimation of loss distribution in the dc–dc converter, a large portion of the loss at the rated power is caused by the turn off switching loss at the LVS. One of the best methods of improving the efficiency of the dc–dc converter is to operate it at a lower switching frequency. However, this method is accompanied by acoustic noise generation and a bulky transformer.

REFERENCES:

[1] New Energy and Industrial Technology Development Organization (NEDO). (2008). Global warming counter measures: Japanese technologies for energy savings/GHG (greenhouse gases) emissions reduction (Revised ed.), [Online]. Available: http://www.nedo.go.jp

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[3] P. F. Ribeiro, B. K. Johnson, M. L. Crow, A. Arsoy, and Y. Liu, “Energy storage systems for advanced power applications,” Proc. IEEE, vol. 89, no. 12, pp. 1744–1756, Dec. 2001.

[4] R.W. A. A. De Doncker, D. M. Divan, and M. H. Kheraluwala, “A threephase soft-switched high-power-density dc/dc converter for high power applications,” IEEE Trans. Ind. Appl., vol. 27, no. 1, pp. 63–73, Feb. 1991.

[5] M. H. Kheraluwala, R. W. Gascoigne, D. M. Divan, and E. D. Baumann, “Performance characterization of a high-power dual active bridge dc-todc converter,” IEEE Trans. Ind. Appl., vol. 28, no. 6, pp. 1294–1301, Nov./Dec. 1992.