This paper deals with the analysis, design and control of a three-level 24-pulse Voltage Source Converter (VSC) based High Voltage Direct Current (HVDC) system. A three level VSC operating at fundamental frequency switching (FFS) is proposed with 24-pulse VSC structure to improve the power quality and reduce the converter switching losses for high power applications. The design of three-level VSC converter and system parameters such as ac inductor and dc capacitor is presented for the proposed VSC based HVDC system. It consists of two converter stations fed from two different ac systems. The active power is transferred between the stations either way. The reactive power is independently controlled in each converter station. The three-level VSC is operated at optimized dead angle (β). A coordinated control algorithm for both the rectifier and an inverter stations for bidirectional active power flow is developed based on FFS and local reactive power generation. This results in a substantial reduction in switching losses and avoiding the reactive power plant. Simulation is carried to verify the performance of the proposed control algorithm of the VSC based HVDC system for bidirectional active power flow and their independent reactive power control.
Voltage Source Converter (VSC), Three-level VSC, Fundamental Frequency Switching (FFS), HVDC System, Power Flow Control, Reactive Power Control, Power Quality, Total Harmonic Distortion (THD), Dead Angle (β).
Fig. 1 Three-level 24-pulse double bridge VSC based HVDC system
EXPECTED SIMULATION RESULTS:
Fig. 2a Performance of rectifier station during reactive power control of three level 24-pulse VSC HVDC system
Fig. 2b Performance of Inverter station during reactive power control at rectifier station of three-level 24 pulse VSC HVDC system
Fig. 2c Variation of (δ) and (α) values for rectifier and inverter Stations for reactive power variation of a three-level 24-pulse VSC HVDC system
Fig. 3a Rectifier station during active power reversal of three-level 24-pulse VSC HVDC system
Fig. 3b Inverter station during active power reversal of three-level 24-pulse VSC HVDC system
Fig. 3c Variation of (δ) and (α) values during active power reversal of three level 24-pulse VSC HVDC system.
A new three-level, 24-pulse voltage source converter based HVDC system operating at fundamental frequency switching has been designed and its model has been developed and it is successfully tested for the independent control of active and reactive powers and acceptable level harmonic requirements. The reactive power has been controlled independent of the active power at both conditions. The converter has been successfully operated in all four quadrants of active and reactive powers with the proposed control. The reversal of the active power flow has been implemented by reversing the direction of dc current without changing the polarity of dc voltage which is very difficult in conventional HVDC systems. The power quality of the HVDC system has also improved with three-level 24-pulse converter operation. The harmonic performance of this three-level, 24-pulse VSC has been observed to an equivalent to two-level 48-pulse voltage source converter.
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