Sub-Synchronous Oscillations Caused by Interaction of a PMSG-Based Wind Farm with a Four-Terminal MMC-HVDC Grid

doi:10.16183/j.cnki.jsjtu.2023.574

Abstract

Abstract:

Multiple occurrences of sub-synchronous oscillations have been observed following the integration of a wind farm into a four-terminal ring-structured modular multilevel converter (MMC)-high-voltage direct current (HVDC) grid in China. To address repeated oscillation, this paper develops a small-signal model of a direct-drive wind farm integrated into the four-terminal MMC-HVDC system, and reveals the underlying mechanism of the sub-synchronous oscillations observed in the actual project. It is found that a direct-drive wind farm is closely coupled only with the directly connected MMC, while other MMCs have little impact on sub-synchronous oscillation modes. Therefore, when studying the sub-synchronous oscillation issues involving the interaction between the wind farm and the flexible DC system, a simplified model can be established using a single-ended flexible DC system directly connected to the wind farm, ignoring the influence of other MMCs. It is also found that in the system where a direct-drive wind farm interacts with a directly connected MMC, both active power-coupled sub-synchronous oscillation modes and reactive power-coupled sub-synchronous oscillation modes coexist. As the output power of the wind farm gradually increases, the damping of the active power-coupled sub-synchronous mode increases, while the damping of the reactive power-coupled sub-synchronous mode decreases, reproducing the onsite phenomenon where an increase in wind farm active power aggravates sub-synchronous oscillations. The smaller the integral gain of the outer loop of the wind farm’s reactive power controller and the proportional gain of the phase-locked loop (PLL), and the larger the integral gain of the q-axis component of the AC voltage outer loop at the wind farm-side converter station, the more stable the reactive power-coupled oscillation mode becomes. By optimizing these parameters, the risk of system instability can be reduced. A simulation model of a direct-drive wind farm connected to a four-terminal flexible DC grid was built using the electromagnetic transient simulation software PSCAD (power systems computer aided design), and the correctness of the above theoretical analysis was verified.

Key words: modular multilevel converter (MMC)-based high-voltage direct current (HVDC) grid, direct-drive permanent magnet synchronous generator (PMSG), sub-synchronous oscillation, small-signal model

CLC Number:

TM712

BAI Feng, CHEN Wuhui, QIN Wei. Sub-Synchronous Oscillations Caused by Interaction of a PMSG-Based Wind Farm with a Four-Terminal MMC-HVDC Grid[J]. Journal of Shanghai Jiao Tong University, 2025, 59(11): 1707-1719.

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模态	特征值	f/Hz	ξ/%	相关性
λ₁₍₂₎	-11.55±j5623.34	895.436	0.2	直流架空线
λ₃₍₄₎	-7.67±j3385.38	539.07	0.2	直流架空线
λ₅₍₆₎	-5.98±j2290.21	364.68	0.2	直流架空线
λ₇₍₈₎	-2.30±j1380.48	219.82	0.1	直流架空线
λ₉₍₁₀₎	-3.77±j89.63	14.27	4.2	站间直流耦合
λ₁₁₍₁₂₎	-6.45±j85.19	13.46	7.5	站间直流耦合
λ₁₃₍₁₄₎	-1.57±j84.79	13.50	1.9	站间直流耦合
λ₁₅₍₁₆₎	-2.01±j25.01	3.98	8.0	站间直流耦合
λ₁₇₍₁₈₎	-10.86±j949.83	151.25	1.1	WFMMC_A内部谐波
λ₁₉₍₂₀₎	-5.78±j954.67	152.02	0.6	WFMMC_B内部谐波
λ₂₁₍₂₂₎	-8.42±j948.38	151.02	0.9	MMC_C内部谐波
λ₂₃₍₂₄₎	-2.51±j946.69	150.75	0.3	MMC_D内部谐波
λ₂₅₍₂₆₎	-10±j84.67	13.48	10.0	风电场A与WFMMC_A有功耦合
λ₂₇₍₁₈₎	-0.48±j47.68	7.50	1.0	风电场A与WFMMC_A无功耦合
λ₂₉₍₃₀₎	-9.58±j83.65	13.32	10.0	风电场B与WFMMC_B有功耦合
λ₃₁₍₃₂₎	-0.56±j46.78	7.45	1.2	风电场B与WFMMC_B无功耦合