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)

Leave a Reply

Your email address will not be published. Required fields are marked *