An Improved Direct AC-AC Converter for Voltage Sag Mitigation

IEEE Transactions on Industrial Electronics, 2013 

ABSTRACT

Dynamic Voltage Restorer (DVR) is a definitive solution towards compensation of voltage sag with phase jump. Conventional DVR topologies however have dc-link and two stage power conversion. This increases its size, cost and associated losses. Therefore topologies without the dc-link, mitigating sag by utilizing direct ac-ac converters, are preferable over the conventional ones. As no storage device is employed, compensation by these topologies is limited only by the voltages at the point of common coupling that is feeding the converters. In this paper, a direct ac-ac converter based topology fed with line voltages is proposed. The arrangement provides increased range of compensation in terms of magnitude and phase angle correction. Detailed simulations have been carried out in MATLAB to compare the capability of the proposed topology with other similar topologies.

 

KEYWORDS:

  1. Dynamic voltage restorer (DVR)
  2. Voltage source inverter (VSI)
  3. Voltage sag compensation
  4. Voltage phase jump compensation.
  5. AC-AC converter

 

SOFTWARE: MATLAB/SIMULINK

 

BLOCK DIAGRAM:

Fig. 1. Interphase ac-ac converter topology

 

Fig. 2. Proposed converter topology.

  

EXPECTED SIMULATION RESULTS:

Fig. 3. Compensation of a sag type Ba. (a) Three phase voltage at the PCC with sag of 0.3 p.u. magnitude and 􀀀100 phase jump. (b) Three phase load voltage(c) Injected voltage. (d) The duty cycle of choppers in phase a sag supporter.

Fig. 4. Compensation of a sag type Ca. (a) Three phase voltage at the PCC with sag of 0.4 p.u. characteristic voltage magnitude and 􀀀200 phase jump. (b) Three phase load voltage at the PCC. (c) Injected voltages. (d) The duty cycle of voltages in phase b sag supporter. (e) The duty cycle of choppers in phase c sag supporter.

Fig. 5. Compensation  of  symmetrical  sag. (a) Three phase voltage at the PCC with sag of 0.5 p.u. magnitude and 􀀀600 phase jump. (b) Three phase load voltage at the PCC. (c) Injected voltages. (d) The duty cycle of voltages in all sag supporters.

 

CONCLUSION

In this paper, an ac-ac converter based voltage sag supporter fed with line voltage has been proposed to compensate voltage sag with phase jump. The operation and switching logic of this topology are explained in detail. The capability of the topology is tested for different types of voltage sags are compared with other topologies. This topology has the advantage of eliminating storage device and providing increased range of compensation. The efficacy of the proposed topology is validated through simulation and experimental studies. An intuitive method of classification of voltage sags [2], assorts sag into four basic types as shown in Fig. In the figure, the dashed lines represent the pre-sag voltage, and the solid lines represent the voltages during sag. The pre-sag voltages are given by V j , and during sag voltages by V0 j ,where j = a, b, and c. A single phase fault causes voltage sag in one phase (type B) at the terminals of a star connected load and in two phases (type C) at the terminals of a delta connected load. A phase-to-phase fault causes type C sag at the terminals of a star connected load and type D sag at the terminals of a delta connected load. A three phase symmetrical sag (type A) is caused by three phase fault. Further, voltage sag gets transformed into other sag types as it propagates in power system to lower voltage levels through transformers. Transformation of a voltage sag due to single phase fault i.e. type B sag, is illustrated in Fig. The type B sag when propagates through a star-delta transformer it transforms to a type C sag. When type C sag in-turn propagates through a star-delta transformer, it transforms to a type D sag. Each sag type is further classified into three subtypes based on the phase(s) that is/are affected. The subtypes are represented by a, b or c subscript, for easy reference. For instance, sag type Ba and Da have voltage sag in phase-a; while for sag type Ca, the line voltage bc is faulty and phase- a is healthy. Characterization of each type of sag is done in terms of the type and the complex characteristic voltage (V0 ch). The characteristic voltage defines three phase voltage sag. The phase voltages as a function of the characteristic voltage and the pre-fault voltage (which is usually 1 p.u.) is given in Table IV for the basic four types [2].

 

REFERENCES

  • S. Vedam and M. S. Sarma, Power Quality: VAR Compensation in Power Systems. CRC press, 2009.
  • H. J. Bollen, Understanding Power Quality Problems. New York: IEEE press, 2000.
  • Mohseni, S. M. Islam, and M. A. Masoum, “Impacts of symmetrical and asymmetrical voltage sags on dfig-based wind turbines considering phase-angle jump, voltage recovery, and sag parameters,” IEEE Trans. Power Electron., vol. 26, no. 5, pp. 1587–1598, May 2011.
  • Massoud, S. Ahmed, P. Enjeti, and B. Williams, “Evaluation of a multilevel cascaded-type dynamic voltage restorer employing discontinuous space vector modulation,” IEEE Trans. Ind. Electron., vol. 57, no. 7, pp. 2398–2410, Jul. 2010.
  • W. Li, D. Vilathgamuwa, F. Blaabjerg, and P. C. Loh, “A robust control scheme for medium-voltage-level dvr implementation,” IEEE Trans. Ind. Electron., vol. 54, no. 4, pp. 2249–2261, Aug. 2007.

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