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| A Harmonic Compensation Modulation Strategy for Extending Steady Operation Range of Modular Multilevel Converter-Based Photovoltaic System |
| Pan Yao, Sun Xiaofeng, Cai Yao, Li Xin, Zhao Wei |
| Key Lab of Power Electronics for Energy Conservation and Motor Drive of Hebei Province Yanshan University Qinhuangdao 066004 China |
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Abstract Harmonic compensation modulation (HCM) strategies are commonly used solutions for photovoltaic (PV) power imbalance conditions of a modular multilevel converter-based photovoltaic system (MMC-PVS). The maximum equivalent fundamental-frequency modulation index is extended to 1.273, and the system’s stable operation range is expanded. According to the principle of HCM strategy, harmonics are injected into submodules (SMs) with high-power PVs to prevent over-modulation, and the rest of SMs corresponding to low-power PVs (referred as normal SMs) compensate for the same amount of opposite harmonics to avoid distortions on phase arm voltages and currents. However, existing HCM strategies may lead to over-modulation on normal SMs and distortions on phase arm voltages and currents when facing severe PV power imbalance. This paper proposes a novel HCM strategy to prevent the over-modulation on SMs and normal SMs simultaneously.
The principle of HCM strategy is introduced, and an HCM strategy for expanding the system’s stable operation range is proposed. Then, relationships between the injected harmonic amplitude and equivalent fundamental-frequency modulation index in over-modulation SM are analyzed. On this basis, operation constraints of HCM strategies in MMC-PVS are derived as the criteria of system stable operation. Combined with the concepts of stable operation space (SOS) and stable operation factor (SOF), stable operation ranges of different HCM strategies are evaluated qualitatively and quantitatively. The results show that the SOF of the proposed HCS strategy is improved by up to 71.57%, which is 4.61 times that of the traditional sinusoidal pulse width modulation (SPWM) strategy and significantly larger than those of most existing HCM strategies.
When PV power is slightly imbalanced, the proposed HCM strategy and existing HCM strategies achieve smooth transitions under various working conditions with negligible current distortion. Under severe PV power imbalance, the proposed HCM strategy maintains system stability, while other HCM strategies exhibit current distortions. The HCM strategies can normally operate when the number of SMs of each phase arm is 3 and 4, respectively.
Compared with the existing HCM strategies, the proposed HCM strategy extends the stable operation range of MMC-PVS, avoiding over-modulation on normal SMs and SMs with high power PVs simultaneously. The HCM strategies maintain the normal operation of MMC-PVS with odd or even numbers of SMs in each phase arm under PV power imbalance conditions. The proposed HCM strategy can also be applied in cascaded H-bridge converter-based PV systems with the capability of stable operation range extension.
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Received: 03 August 2023
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2024, Vol. 39 
