A Frequency-Adaptive Wide Bandwidth Harmonic Voltage Suppression Strategy for Grid-Forming Converters
Zhao Tong1, Wu Chao1, Wang Yong1, Jiang Shunping2, Huang Qiuyan2
1. Department of Electrical Engineering Shanghai Jiaotong University Shanghai 200240 China; 2. Sieyuan Qingneng Electric and Electronics Co. Ltd Shanghai 201108 China
Abstract：Recently, more and more distributed generation sources and nonlinear loads are being connected to the grid, which generates current harmonics at the point of common coupling (PCC), and seriously worsens the power quality. However, the conventional grid-forming converter (GFM) mainly focuses on maintaining the amplitude and frequency of fundamental voltage and lacks the ability to suppress harmonic voltages, which no longer fulfills the demands of the power system containing distributed generation sources and non-linear loads. To compensate for the insufficient elimination capability of conventional GFM for harmonic voltage, this paper proposes a wide bandwidth harmonic voltage suppression strategy, which maintains sufficient gain in the frequency range of common voltage harmonics (200~1 000 Hz) and is decoupled from the fundamental voltage control. Besides, the proposed strategy simplifies the controller structure by eliminating the requirement for the harmonic voltage frequency extraction which is essential in adaptive resonant controllers and repetitive controllers. Firstly, the impact of the amplitude of the grid impedance Zg and the converter impedance ZVSC on the harmonic voltage at PCC is analyzed, which indicates that the elimination of harmonic voltage can be achieved by adjusting ZVSC. After that, based on further analysis of ZVSC, the design of the proposed wide bandwidth harmonic voltage elimination controller is carried out. Finally, a small-signal analysis is developed, which demonstrates that the proposed strategy effectively improves the harmonic voltage suppression capability while decoupling from the fundamental voltage control. After that, a 2 kV·A experimental platform has been built to verify the effectiveness of the proposed wide bandwidth harmonic voltage suppression strategy. The injected harmonic content of the voltage at PCC remains constant during the following three experimental validations (The THD of injected harmonic voltages is 30% for 4th, 5th, and 7th and 10% for 8th, 10th, 11th, and 13th). In the first experimental case, the grid fundamental voltage is balanced, where the proposed strategy decreases the THD of harmonic voltages from 14.58% to 2.29% at SCR=2 and from 21.25% to 3.68% at SCR=7.8. In the second experimental case, to analyze the impact of the proposed strategy on the ability to maintain the amplitude and frequency of grid fundamental voltage, the frequency of grid varies from 50 Hz to 49.8 Hz, whose experimental results reveal that both the supporting effect of GFM for fundamental voltage and the elimination of harmonic voltage are barely affected (ωg=50 Hz: THD of VAB is 2.32%; ωg=49.8 Hz:THD of VAB is 2.1%). In the last experimental case, the grid fundamental voltage is unbalanced (The amplitude of the voltage in phase A is reduced to 0.7 (pu), and the amplitudes of the voltage in the other two phases remain unchanged). and GFM is connected to the grid with and without the proposed strategy. The results indicate that the harmonic voltages in the frequency range of 200~1 000 Hz are effectively eliminated. However, the harmonic voltage at 100 Hz introduced by the unbalanced grid voltage only can be attenuated slightly. Based on the above experimental results, the correctness and effectiveness of the above theoretical analysis are verified. From what has been discussed above, the conclusion can be drawn as: (1) The proposed strategy owns a wider harmonic voltage suppression range than the conventional repetitive controller. It can realize the suppression of harmonic voltages with varying frequencies without the complex harmonic frequency extraction, which is necessary for conventional adaptive harmonic voltage suppression. Thus, the difficulty of the implementation of the proposed strategy is reduced. It also shows that this strategy has the capability of adapting to harmonic frequencies. (2) The wide bandwidth controller proposed in this paper eliminates the gain overshoot between resonant frequencies caused by phase jumps at the resonant frequency in conventional resonant and repetitive controllers, which is advantageous for the suppression of harmonic voltages that are not integer multiples of the fundamental frequency.
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2025, Vol. 40 
