Optimal Capacity Proportion of GFL/GFM Wind Turbines Considering Power Transmission Capability Improvement
Mei Ziting1, Shen Xiaolin2, Liu Ziwen1, Zhang Yiqiong1, Meng Xiaoge2
1. School of Electrical and Power Engineering Hohai University Nanjing 211100 China;
2. State Grid Economic and Technological Research Institute Co. Ltd Beijing 102211 China
Insufficient small-signal stability margin can restrict the active-power transfer capability of wind farms connected to weak grids. The objective was to determine the minimum proportion of GFM wind turbines required to satisfy a specified power-transfer demand while maintaining system stability and operating constraints in a hybrid GFL/GFM direct-drive wind farm.
To address this objective, small-signal impedance models were derived for GFL and GFM direct-drive wind turbines and then combined to obtain the equivalent impedance of the hybrid grid-connected system. The hybrid model was verified by frequency-sweep simulations over 10 Hz to 1 kHz under different GFL/GFM capacity ratios. Based on the impedance model, a transmission-power feasible region was defined as the set of active-power operating points satisfying small-signal stability, point-of-common-coupling voltage, wind-turbine output, and power-balance constraints. The damping ratio of the dominant eigenvalue was used to quantify the stability margin, with 0.03 adopted as the lower limit, while the short-circuit ratio (SCR) was used to characterize grid strength. The effects of GFM capacity proportion and SCR on the feasible-region boundary were then evaluated. On this basis, an optimization model was formulated to minimize the GFM capacity proportion subject to the required transmission power and the above constraints, and particle swarm optimization was used to obtain the minimum feasible configuration.
The frequency-domain results show that increasing the GFM proportion changes the hybrid system from an unstable to a stable operating condition, while the subsequent improvement in stability margin gradually diminishes. Under the representative weak-grid condition used for impedance analysis, the pure-GFL system has a magnitude crossover at 501 Hz and a phase difference of 193°, indicating instability. When the GFM proportion increases from 20% to 40%, the crossover frequency decreases from 70 Hz to 10 Hz and the phase difference decreases from 181° to 167°, corresponding to a transition from instability to stability. When the GFM proportion further increases from 60% to 100%, the crossover frequency decreases from 8 Hz to 5 Hz and the phase difference decreases from 145° to 124°, indicating a slower increase in stability margin. Consistent with these frequency-domain characteristics, the feasible-region analysis shows that weaker grids require a larger GFM proportion to maintain the same transmission level. At SCR=1.5 and SCR=1.0, the GFM proportions required for the transmission capability to reach the rated wind-farm capacity are approximately 34% and 40%, respectively. For a pure-GFL wind farm, reducing SCR from 5.20 to 1.50 decreases the maximum transferable active power to 0.69 per unit (pu). At SCR=1.50, increasing the GFM proportion to 10%, 20%, and 30% raises the corresponding maximum transferable powers to 0.76 pu, 0.82 pu, and 0.86 pu.
To further examine the configuration results, time-domain case studies were conducted at different system scales. In the 6 MW case with SCR=1.0, the optimized 40% GFM configuration enables the active power to rise from 0.6 pu to 0.8 pu and settle at a new operating point by 2.1 s, whereas the lower-capacity comparison case exhibits divergent oscillation. Similarly, in the 40 MW case with SCR=1.8, seven GFM turbines allow the wind-farm output to increase from 0.7 pu to 1.0 pu and settle by 4.7 s, while the six-GFM comparison case does not converge. In the 80-turbine, 160 MW wind-farm model with SCR=1.5, the optimized GFM proportion is 25%. Following the fault, the active power recovers with an approximately 4%-5% short-duration overshoot and settles at about 1.75 s, whereas the 18.75% GFM comparison case shows sustained near-constant-amplitude oscillation after fault clearance.
Overall, the results establish a quantitative relationship among grid strength, GFM capacity proportion, stability margin, and transferable active power. The feasible-region formulation converts the stability effect of GFM integration into an explicit power-transfer boundary. Accordingly, the optimization procedure determines the minimum GFM proportion required to satisfy a prescribed transmission requirement under the specified stability and operating constraints.
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