Journal of Electrical Engineering and Technology

The Korean Institute of Electrical Engineers (대한전기학회)
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Individual DC Voltage Balancing Method at Zero Current Mode for Cascaded H-bridge Based Static Synchronous Compensator

  • Received : 2017.03.23
  • Accepted : 2017.09.04
  • Published : 2018.01.01
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Abstract

Individual DC voltage balance problem is an inherent issue for cascaded H-bridge (CHB) based converter. When the CHB-based static synchronous compensator (STATCOM) is operating at zero current mode, the software-based individual DC voltage balancing control techniques may not work because of the infinitesimal output current. However, the different power losses of each cell would lead to the individual DC voltages unbalance. The uneven power losses on the local supplied cell-controllers (including the control circuit and drive circuit) would especially cause the divergence of individual DC voltages, due to their characteristic as constant power loads. To solve this problem, this paper proposes an adaptive voltage balancing module which is designed in the cell-controller board with small size and low cost circuits. It is controlled to make the power loss of the cell a constant resistance load, thus the DC voltages are balanced in zero current mode. Field test in a 10kV STATCOM confirms the performance of the proposed method.

Keywords

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Fig. 1. Structure of star-connected STATCOM and individual H-bridge cell

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Fig. 2. CHB based STATCOM control system composed of three loops, i.e., cluster voltage control loop (outer loop),current control loop (inner loop) and individual DC voltage balancing loop

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Fig. 3. Power loss model of the cell

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Fig. 4. Sketch of the software-based DC voltage balancingstrategy

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Fig. 5. Power loss model of the cell at zero current mode

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Fig. 6. Power losses under different DC voltages in zerocurrent mode

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Fig. 7. Equivalent resistors under different DC voltages inzero current

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Fig. 8. Equivalent circuit of the cascaded chain at zerocurrent mode

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Fig. 9. The schematic of the module in the cell-controllerboard

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Fig. 10. Impact of the adaptive load with different ratedpowers on the Hbridge cell and its local supplyconverter in the controller board

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Fig. 11. The layout of the module in the cell-controllerboard

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Fig. 12. 12-cell-cascaded 10 kV STATCOM

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Fig. 13. Basic operating conditions of the STATCOM atzero current mode

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Fig. 14. DC voltage divergency and the effection ofproposed module

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Fig. 15. DC voltgae divergency when the power lossdeviation exceeds the ability of the adaptive load

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Fig. 16. Other two hardware based individual DC voltagebalancing methods, i.e., (a) a single parallelresistor and (b) a parallel branch composed of aresistor and a switch in series

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Fig. 17. H-bridge cell with a parallel resistor on the DC bus

Table 1. parameters of the tested H-bridge cell

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Table 2. Power loss deviation at zero current mode

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Table 3. Paramiters of the fly-back converter

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Table 4. Parameters of the proposed module

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Table 5. Comparison of the three hardware based methods

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