Low-Voltage Ride Through Strategy for MMC With Y0/Y0 Arrangement Transformer Under Single-Line-to-Ground Fault

 ABSTRACT:

In the offshore wind farm high-voltage direct-current (HVDC) system, the power delivery capability of the onshore modular multilevel converter (MMC) decreases severely under grid fault, which makes the DC-bus voltage increase rapidly and threatens the safe operation of the system. This paper proposes a low-voltage ride through (LVRT) strategy for MMC with Y₀/Y₀ arrangement transformer under single-line-to-ground (SLG) fault. The influence of different transformer arrangements to the MMC under SLG fault is analyzed. On this basis, a power delivery capability enhanced method is proposed for MMC with Y₀/Y₀ arrangement transformer to take advantage of its control ability on zero sequence current. In addition, an optimized LVRT strategy based on resonant controller is proposed, which has simple control structure and can ride through the SLG fault without DC chopper. The offshore wind farm MMC-HVDC simulation system is established in PSCAD/EMTDC and simulation studies are conducted to validate the effectiveness of the proposed LVRT strategy.

KEYWORDS:

1.      Modular multilevel converter (MMC)

2.      Grid fault

3.      High-voltage direct-current (HVDC)

4.      Low-voltage ride through (LVRT)

SOFTWARE: MATLAB/SIMULINK

 

BLOCK DIAGRAM:

 

 

Figure 1. Block Diagram Of The Conventional Control Strategy Of Mmc Under Slg Fault.

EXPECTED SIMULATION RESULTS:

Figure 2. Simulation Results Of Mmc With Y₀/1 Arrangement Transformer Using The Conventional Strategy (P D 935mw).

Figure 3. Simulation Results Of Mmc With Y₀/Y₀ Arrangement Transformer Using The Conventional Strategy (P D 935mw).

 

Figure 4. Simulation Results Of Mmc With Y₀/Y₀ Arrangement Transformer Using The Proposed Strategy (P D 935mw).

Figure 5. Simulation Results Of Mmc With Y₀/1 Arrangement Transformer Using The Conventional Strategy (P D

750mw).

Figure 6. Simulation Results Of Mmc With Y₀/Y₀ Arrangement Transformer Using The Conventional Strategy (P D 750mw).

 

Figure 7. Simulation Results Of Mmc With Y₀/Y₀ Arrangement Transformer Using The Proposed Strategy (P D 750mw).

 CONCLUSION:

In this paper, the influence of different transformer arrangements to MMC under SLG fault has been analyzed, and an LVRT strategy for MMC with Y₀/Y₀ arrangement transformer has been proposed. Comparative simulation studies have been conducted under SLG fault. The conclusions can be summarized as follow. (1) Compared with the Y₀/1 arrangement transformer, the grid-side zero sequence current can be restrained by using Y₀/Y₀ arrangement transformer, and the power delivery capability can be enhanced. However, the zero sequence current is transferred to the MMC side. (2) The proposed LVRT strategy can restrain the zero sequence current and enhance the power delivery capability for MMC with Y₀/Y₀ arrangement transformer effectively. The MMC can ride through SLG fault without DC chopper by using the proposed LVRT strategy when the wind farm works in the full-power mode. (3) The proposed LVRT strategy can work well under different power factors, which means the MMC using the proposed strategy can not only ride through the grid fault,but also provide reactive power support to the grid within its capability when the wind farm doesn't work in the full-power mode.

REFERENCES:

[1] S. M. Muyeen, R. Takahashi, and J. Tamura, ``Operation and control of HVDC-connected offshore wind farm,'' IEEE Trans. Sustain. Energy, vol. 1, no. 1, pp. 30_37, Apr. 2010.

[2] R. Shah, J. C. Sánchez, R. Preece, and M. Barnes, ``Stability and control of mixed AC-DC systems with VSC-HVDC: A review,'' IET Gener. Transm. Distrib., vol. 12, no. 10, pp. 2207_2219, 2018.

[3] X. Zeng, T. Liu, S. Wang, Y. Dong, B. Li, and Z. Chen, ``Coordinated control of MMC-HVDC system with offshore wind farm for providing emulated inertia support,'' IET Renew. Power Gener., vol. 14, no. 5, pp. 673_683, Apr. 2020.

[4] J. Lyu, X. Cai, M. Amin, and M. Molinas, ``Sub-synchronous oscillation mechanism and its suppression in MMC-based HVDC connected wind farms,'' IET Gener. Transmiss. Distrib., vol. 12, no. 4, pp. 1021_1029, Feb. 2018.

[5] S. Xue, C. Gu, B. Liu, and B. Fan, ``Analysis and protection scheme of station internal AC grounding faults in a bipolar MMC-HVDC system,'' IEEE Access, vol. 8, pp. 26536_26548, 2020.