Inverter topologies for DSTATCOM applications—a simulation study

ABSTRACT

This paper describes a few different topologies of inverters used for realizing a distribution static compensator (DSTATCOM). A brief introduction of the need for shunt compensation as well as the requirements of the shunt compensator has also been given. An algorithm for generating references has been described. Three major topologies of inverters – three-leg inverter with single dc capacitor, three-leg inverter with neutral clamped dc capacitors, four-leg inverter and three single-phase inverters with common dc capacitor – have been described and simulated. DSTATCOM topologies for high voltage distribution systems have also been described. The Voltage Source Converter (VSC) topologies have been compared on the basis of the performance of the inverter for certain chosen system conditions and the number of switch devices and dc capacitors used. All simulations have been performed using PSCAD/EMTDC.

KEYWORDS:

1.      DSTATCOM

2.      VSC

3.      Hysteresis current control

SOFTWARE: MATLAB/SIMULINK

CIRCUIT DIAGRAM:

Fig. 1. Single-phase circuit showing the DSTATCOM.

EXPECTED SIMULATION RESULTS:

Fig. 2. Simulation of three-leg VSC with balanced load: (a) source voltages; (b) source currents; (c) load currents; (d) source voltage and current.

Fig. 3. Simulation of three-leg VSC for unbalanced load: (a) phase a injected currents; (b) phase b injected currents; (c) phase c injected currents; (d)source currents; (e) load currents; (f) source voltage and current.

Fig. 4. Simulation of three-leg VSC for unbalanced load with modified reference currents: (a) phase a injected currents; (b) phase b injected currents; (c) phase c injected currents; (d) source currents; (e) load currents; (f) source voltage and current.

Fig. 5. Simulation of three-leg VSC with neutral clamped dc capacitors without dc in the load current: (a) source voltages; (b) source currents; (c) load currents; (d) source voltage and current; (e) dc link voltage; (f) dc capacitor voltages.

Fig. 6. Simulation of three-leg VSC with neutral clamped dc capacitors with dc in the load current: (a) source voltages; (b) source currents; (c) load currents; (d) source voltage and current; (e) dc link voltage; (f) dc capacitor voltages.

Fig. 7. Simulation of three-leg VSC with neutral clamped dc capacitors with dc in the load current using modified reference currents: (a) compensator current; (b) source current; (c) load current; (d) source voltage and current; (e) dc link voltage; (f) dc capacitor voltages.

Fig. 8. Simulation of four-leg VSC: (a) source voltages; (b) source currents; (c) load currents; (d) source voltage and current; (e) dc link voltage.

Fig. 9. Simulation of four-leg VSC for load currents with a dc component: (a) source voltages; (b) load currents; (c) source currents; (d) source voltage and current; (e) dc capacitor voltage; (f) phase c injected current (HV side); (g) phase c injected current (LV side); (h) transformer flux.

CONCLUSION

Based on the simulation results shown in the previous sections, a comparative study of the features of the DSTATCOM for low voltage distribution systems is given in Table 4. A comparison of DSTATCOM topologies for high voltage distribution systems can be made along similar lines with similar conclusions. The paper describes an algorithm [1] used to generate references for the currents to be injected by the DSTATCOM. It has been proved that the references would result in source currents that are balanced sinusoids. Limitations of the DSTATCOM topologies that are widely in use are highlighted and improvements have been suggested when a topology fails. From the comparison in Table 4 and the simulation results of Section 3.3, the four-leg VSC is the most suitable topology to realize a DSTATCOM. For high voltage distribution systems, it has been shown through simulation results that the DSTATCOM using the four-leg VSC is capable of compensating dc currents despite the VSC being connected to the distribution system through step-down transformers. It has been shown that the ampereturns on both windings of the transformer due to the currents injected by the DSTATCOM cancel each other and the transformer does not saturate.

REFERENCES

[1]   Ghosh, A. Joshi, A new approach to load balancing and power factor correction in power distribution system, IEEE Trans. Power Deliv. 15 (July (3)) (2000) 417–422.

[2]    S.-J. Huang, J.-C. Wu, A control algorithm for three-phase three wired active power filters under non-ideal mains voltages, IEEE Trans. Power Electron. 14 (July (4)) (1999) 753–760.

[3]    B. Singh, K. Al-Haddad, A. Chandra, A review of active power filters for power quality improvement, IEEE Trans. Ind. Electron. 46 (October (5)) (1999) 960–971.

[4]    M. Aredes, J. Hafner, K. Heumann, Three-phase four-wire shunt active filter control strategies, IEEE Trans. Power Electron. 12 (March (2)) (1997) 311–318.

[5]   M.K. Mishra, A. Ghosh, A. Joshi, A new STATCOM topology to compensate loads containing ac and dc components, in: IEEE Power Engineering Society Winter Meeting, Singapore, January 2000.