Abstract：Active capacitors have demonstrated good performance in filtering applications. However, the voltage and current fluctuations of energy storage components are constrained by the voltage and current ratings of the switching devices, limiting the storage capacity. It typically restricts their application to medium and low-power scenarios. This paper proposes an active capacitor based on differential-frequency reactive power. By synthesizing low-frequency reactive power from two high-frequency voltage signals with different frequencies, the proposed active capacitor increases the operating frequency of the passive components and enhances their reactive power absorption capability. Additionally, a dual-resonance impedance network is designed to overcome the limitations imposed by the voltage ratings of the switching devices on the storage power. First, a high-frequency rotating voltage vector is generated using a three-phase bridge circuit, and the reactive power produced by this vector in the impedance network is analyzed. It is found that the high-frequency voltage not only generates high-frequency reactive power but also produces differential-frequency reactive power, with a frequency equal to the difference between the two voltage vectors. This differential-frequency reactive power is used to compensate for the second harmonic power of the inverter, enabling the conversion of low-frequency power into high-frequency processing. Secondly, an impedance network with dual resonance points is designed. Through resonance, the network eliminates high-frequency reactive power, retains differential-frequency reactive power, and amplifies the voltage and current amplitudes of the passive components, thereby improving the reactive power compensation capacity of the active capacitor. Moreover, the voltage stress on the switching devices is only related to the DC-side voltage, preventing the voltage rating limitations on storage power that are typically encountered with traditional active capacitors. Regarding the control strategy, a perturb and observe (P&O) algorithm-based control method is employed. This method generates differential-frequency reactive power by constructing high-frequency signals to compensate for fluctuating power. The active capacitor's DC side's real-time voltage and current measurements are sampled. Based on the DC bus voltage fluctuations, the amplitude and phase of one high-frequency vector are adjusted, while the other high-frequency vector is fixed, simplifying the control process. Simulation results show that when the DC-side voltage is 600 V and the second harmonic voltage ripple of the DC bus is 20 V, the proposed active capacitor effectively suppresses the ripple. After decoupling, the DC bus voltage ripple factor is reduced to less than 0.5%, and the voltage stress on the switching devices is only related to the DC voltage. As the target reactive power increases, the voltage and current fluctuations of the energy storage capacitor and inductor also increase. Additionally, a small-power experimental platform is set up. The active capacitor successfully suppressed the second harmonic voltage ripple of the DC bus to below 4 V. In summary, the active capacitor based on differential-frequency reactive power offers the following advantages. (1) The energy storage capacitor operates at higher frequencies with positive and negative voltages, improving capacitor utilization and enabling smaller capacitors of the same type. (2) Resonance amplifies the voltage fluctuations of the energy storage capacitor, enhancing decoupling ability. (3) Decoupling the capacitor voltage fluctuations does not impact the switching devices, and the current in the branches is positively correlated with the decoupling power, reducing the voltage and current stress on the switches, which lowers system cost.
刘鹏, 盛家英, 章勇高. 一种基于差频无功理论的有源电容及其控制策略[J]. 电工技术学报, 2025, 40(20): 6644-6657.
Liu Peng, Sheng Jiaying, Zhang Yonggao. An Active Capacitor and Its Control Strategy Based on Differential Frequency Reactive Power Theory. Transactions of China Electrotechnical Society, 2025, 40(20): 6644-6657.
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