Control scheme for cascaded H-bridge converter based distribution network static compensator

Control scheme for cascaded H-bridge converter based distribution network static compensator

 ABSTRACT:

In this study, a new control scheme is proposed for cascaded H-bridge (CHB)-based static compensator (STATCOM). The new indirect current control method does not require any current sensors, so the total system has enhanced reliability. Using the proposed control approach, dc-link voltages across dc-link capacitors are regulated to the reference value, even if the H-bridge cells dissipate different amounts of losses. However, regardless of the active power distribution, the reactive power is equally shared between the cells. The proposed scheme is investigated on distribution network STATCOM, but is not restricted to that and can be applied to STATCOMs at any voltage and power level. Multicarrier phase-shifted sinusoidal pulse-width modulation is used for modulation purpose, since it is naturally the best option for CHB converter. In this way, low-order harmonics are eliminated from the line side current and the current THD is below 5%, which satisfies IEEE-519 standard. To verify the feasibility of the control strategy, simulations as well as experimental implementations are carried out on a CHB converter having three cells in each phase.

 

SOFTWARE: MATLAB/SIMULINK

   

CIRCUIT DIAGRAM:

Fig. 1 Single phase view of the CHB converter

 

EXPECTED SIMULATION RESULTS:

Fig.2 Investigating the behaviour of STATCOM under unequal losses of H-bridge cells

a Grid voltage and current

b STATCOM input current spectrum

c Converter ac side voltage spectrum

 

Fig.3  Investigating the STATCOM behaviour when the grid voltage is distorted

a Grid voltage and STATCOM current under sag condition

b Dc-link voltage of first cell (under sag condition)

c Grid voltage and STATCOM current when the grid has low-order harmonics

 

Fig. 4 Dc-link voltages of a phase under off and on conditions of the controller

 

CONCLUSION:

In this paper, a new control strategy was introduced for CHB converter to operate as a D-STATCOM. Although the current sensor has been eliminated, the current has been sensitised to be sinusoidal and the reactive power follows its reference. The dc-link voltages are regulated to the reference value under unequal losses condition and the STATCOM does not require any dc power sources to work. The presented control strategy can be applied to CHB converter with any number of cells. Moreover, the control system is capable of being used in transmission system STATCOMs where STATCOMs are connected to the grid by a coupling transformer. The switching frequency is pretty low but at the same time, according to harmonic spectra of the input current waveform, it satisfies IEEE 519 from both THD and individual harmonics point of view. The stability analysis shows the guaranteed stable operation of the proposed method.

 

REFERENCES:

  • Kouro, S., Malinowski, M., Gopakumar, K., et al.: ‘Recent advances and industrial applications of multilevel converters’, IEEE Trans. Ind. Electron., 2010, 57, (8), pp. 2553–2580
  • Song, Q., Liu, W., Zhichang, Y., Wei, W., Chen, Y.: ‘DC voltage balancing technique using multi-pulse optimal PWM for cascade H-bridge inverters based STATCOM’. 2004 IEEE 35th Annual Power Electronics Specialists Conf., 2004 (PESC’04), 20–25 June 2004, vol. 6, pp. 4768–4772
  • Khomfoi, S., Tolbert, L.M.: ‘Multilevel power converters’ Power Electronics Handbook, (The University of Tennessee, Department of Electrical and Computer Engineering, Knoxville, Tennessee, USA, 2005)
  • Rodriguez, J.R., Dixon, J.W., Espinoza, J.R., Pontt, J., Lezana, P.: ‘PWM regenerative rectifiers: state of the art’, IEEE Trans. Ind. Electron., 2005, 52, (1), pp. 5–22
  • Malinowski, M., Gopakumar, K., Rodriguez, J., Pérez, M.A.: ‘A survey on cascaded multilevel inverters’, IEEE Trans. Ind. Electron., 2010, 57, (7), pp. 2197–2206

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