Cascaded Two-Level Inverter-Based Multilevel STATCOM for High-Power Applications

ABSTRACT

In this paper, a simple static var compensating system using a cascaded two-level inverter-located multilevel inverter is planned. The topology consists of two standard two-level inverters linked in cascade done open-end windings of a three-phase transformer. The dc link voltages of the inverters are controlled at different levels to get four-level operation. The simulation study is transported out in MATLAB/SIMULINK to conclude the work of the proposed scheme under balanced and unbalanced supply-voltage conditions.

CONTROL SCHEME

A lab prototype is grown to validate the simulation results. The control scheme is appreciate using the TMS320F28335 digital signal processor. Further, stregth behavior of the topology is checked. The dynamic model is developed and transfer functions are derived. The system act is consider for various operating conditions.

 KEYWORDS

  1. DC-link voltage balance
  2. Multilevel inverter
  3. Power quality (PQ)
  4. Static compensator (STATCOM)

 SOFTWARE:  MATLAB/SIMULINK

BLOCK DIAGRAM

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Fig. 1. Power system and the STATCOM model.

EXPECTED SIMULATION RESULTS

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Fig. 2. Frequency response ∆Vdc1(s) /∆δ1(s) at  iq0 =1.02 p.u., δ1=-0.902=178.90,R1=80 p.u., R2=60 p.u.

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Fig. 3. Root locus of the transfer function  ∆Vdc1(s) /∆δ1(s) at  iq0 = – 0.75 p.u., δ1=-0.5702=179.60,R1 =80 p.u., R2=60 p.u.

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Fig. 4. Reactive power control. (a) Source voltage and inverter current. (b) DC-link voltages of two inverters.

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Fig. 5. Operation during fault. (a) Grid voltages on the LV side of the transformer. (b) -axis negative-sequence current component idn. (c) -axis negative- sequence current component iqn.

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Fig. 6. Experimental result: Capacitive mode of operation. (a) Source voltage (50 V/div) and STATCOM current (5 A/div). (b) DC-link voltages of inverter-1 and inverter-2 (20 V/div). Time scale: 5 ms/div. (c) Harmonic spectrum of current.

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Fig. 7. Experimental result: Mode change from capacitive to inductive. (a) DC-link voltages of inverter-1 and inverter-2 (20 V/div). Time scale: 100 ms/div. (b) Source voltage (100 V/div) and STATCOM current (5 A/div) in steady state. Time scale: 100 ms/div.

CONCLUSION

DC-link voltage balance is one of the major spread in cascaded inverter-based STATCOMs. In this paper, a simple var compensating system is proposed for a cascaded two-level inverter- located multilevel inverter. The scheme provide regulation of dc-link voltages of inverters at asymmetrical levels and reactive power benefit. The work of the system is verify by simulation and test under balanced and unbalanced voltage conditions. Further, the cause for insecurity when there is a change in reference current is examined.

DYNAMIC

The dynamic model is grown and transfer functions are derived. System behavior is consider for various operating conditions. From the search, it is implicit that the system is a non minimum phase type, that is, post of the transfer function always distortion on the left half of the -plane. However, zeros shift to the right half of the -plane for certain operating conditions. For such a system, oscillatory insecurity for high controller gains exists.

REFERENCES

[1] N. G. Hingorani and L. Gyugyi, Understanding FACTS. Delhi, India: IEEE, 2001, Standard publishers distributors.

[2] B. Singh, R. Saha, A. Chandra, and K. Al-Haddad, “Static synchronous compensators (STATCOM): A review,” IET Power Electron., vol. 2, no. 4, pp. 297–324, 2009.

[3] H. Akagi, H. Fujita, S. Yonetani, and Y. Kondo, “A 6.6-kV transformerless STATCOM based on a five-level diode-clamped PWMconverter: System design and experimentation of a 200-V 10-kVA laboratory model,” IEEE Trans. Ind. Appl., vol. 44, no. 2, pp. 672–680, Mar./Apr. 2008.

[4] A. Shukla, A. Ghosh, and A. Joshi, “Hysteresis current control operation of flying capacitor multilevel inverter and its application in shunt compensation of distribution systems,” IEEE Trans. Power Del., vol. 22, no. 1, pp. 396–405, Jan. 2007.

[5] H. Akagi, S. Inoue, and T. Yoshii, “Control and performance of a transformerless cascaded PWM STATCOM with star configuration,” IEEE Trans. Ind. Appl., vol. 43, no. 4, pp. 1041–1049, Jul./Aug. 2007.

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