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| Method for Characterizing the Safety Region of HVAC-Connected Offshore Wind Power Plants under Multi-Harmonic Constraints |
| Zhao Wenrui1, Qin Qianqian1, Wang Tong1, Huang Xiaoming2, Xu Qunwei2 |
1. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources North China Electric Power University Beijing 102206 China;
2. State Grid Zhejiang Electric Power Research Institute Hangzhou 310014 China |
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Abstract During the 14th Five-Year Plan, China plans to build five offshore wind power bases with a total capacity exceeding 12 GW. Large-capacity, long-distance offshore wind power marks the future of wind energy development. Among the three mainstream grid integration technologies—HVDC, HVAC, and fractional-frequency transmission—HVAC is widely adopted in nearshore projects due to its technological maturity. However, when large-scale offshore wind farms connect to the grid via HVAC (OWFs-HVAC), the coupling between the dynamic characteristics of long-distance AC cables and the complex frequency-domain behavior of nonlinear power electronic devices leads to multimodal and weakly damped system features. The capacitive effect of the cables further shifts natural resonance points toward low-frequency harmonics, amplifying these harmonics and creating significant challenges in offshore wind integration.
To address harmonic risks arising from HVAC integration under varying conditions, this study proposes a method to analyze harmonic levels and define safe operating regions. By characterizing the harmonic safety region for HVAC-connected offshore wind systems, a harmonic frequency-domain model is established to derive the nodal admittance matrix, incorporating harmonic sources from both the onshore grid and offshore wind farms. Using harmonic nodal voltage equations, harmonic levels under background harmonics are quantified. Additionally, a harmonic impedance sensitivity analysis is conducted through the inverse matrix differentiation method, with a comprehensive index introduced to evaluate harmonic risks and safety margins under voltage and current constraints. Simulations of a 1 100 MW offshore wind power grid-connected system are carried out to examine harmonic impedance sensitivity and the harmonic safety region under both normal operating conditions and PCC near-zone disconnection scenarios.
Simulations based on actual wind farm data reveal that under normal conditions, the 11th harmonic is the primary amplified frequency, with currents reaching 60 A in some scenarios—over six times the limit. The 5th and 7th harmonics are also significant. Both safety region and sensitivity analyses confirm a strong correlation between harmonic risks and the number of operating turbines. In PCC near-zone disconnection scenarios, the 5th harmonic dominates, with harmonic voltage and current showing higher sensitivity to AC cables. Reducing the cable length by 20%, while maintaining turbine numbers, significantly reduces the 5th harmonic current, validating the safety region characterization. Frequency-domain multimodal features from long-distance AC cables result in irregular harmonic safety region distributions. The turbine operation ratio impacts both the harmonic admittance matrix and the magnitude of harmonic sources, leading to significant coupling between harmonic levels and turbine operations. The dominant harmonic frequency shifts between the 5th, 7th, 11th, and 13th across different scenarios. This method supports planning and construction of offshore wind power projects by optimizing offshore distances and installed capacities.
The following conclusions can be drawn from the simulation analysis: (1) The proposed comprehensive harmonic evaluation index accurately assesses the harmonic safety of the system and can effectively identifies harmonic risk scenarios. (2) As the total parallel length of high-voltage AC cables increases and the grid strength decreases, the harmonic safety level of OWFs-HVAC systems declines. The extent of harmonic amplification is not directly related to the number of wind turbines connected or disconnected, but the dominant harmonic frequencies vary under different turbine connection and disconnection combinations. This variability complicates harmonic suppression efforts for offshore wind integration, challenging the effectiveness of traditional single-tuned passive filters. (3) For the OWFs-HVAC systems, the harmonic current and voltage safety margins at the point of interconnection are low, harmonic compensation devices of appropriate capacity should be configured based on the cable length and wind farm capacity.
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Received: 12 October 2024
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2025, Vol. 40 
