As the core equipment connecting photovoltaic power generation systems and the power grid, the harmonic mitigation capability of photovoltaic grid cabinets directly affects the power quality and equipment safety of the grid. Harmonics are mainly generated by inverters, nonlinear loads, and other components. If not effectively mitigated, they can lead to problems such as equipment overheating, decreased power factor, and impaired grid stability. Harmonic mitigation for photovoltaic grid cabinets requires a comprehensive approach encompassing four dimensions: filtering technology, equipment selection, system design, and operation and maintenance management.
Filtering technology is the core method for harmonic mitigation. Commonly used filtering devices in photovoltaic grid cabinets include passive filters and active filters. Passive filters use capacitors and reactors connected in series to form an LC filter circuit, absorbing specific frequency harmonics (such as the 5th and 7th harmonics). They have advantages such as low cost and simple structure, but their mitigation range is limited, and they may experience resonance due to changes in grid parameters. Active power filters (APFs), on the other hand, monitor the grid current in real time and generate a compensation current with the same amplitude but opposite phase to the harmonics, achieving dynamic mitigation. Its advantages lie in its wide coverage (covering harmonics from 2nd to 51st order), fast response speed (millisecond level), and avoidance of resonance with the grid, but it is more expensive. In a photovoltaic grid cabinet, centralized control (installed at the low-voltage inlet) or local control (for specific circuits) can be selected based on harmonic characteristics; using both in combination can improve the control effect.
Equipment selection directly affects the source control of harmonic mitigation. As the main source of harmonics, the inverter's topology and control strategy play a decisive role in the harmonic content. Inverters using high switching frequencies and soft-switching technology can reduce harmonics generated by switching actions; simultaneously, selecting inverter models with low harmonic output characteristics can reduce subsequent mitigation pressure. Furthermore, the capacitor banks in the grid-connected cabinet must avoid resonance with the grid inductance. This can be achieved by configuring series reactors (e.g., 7% reactance rate) or selecting anti-harmonic capacitors to suppress harmonic amplification effects and extend equipment life.
System design must consider both harmonic mitigation and power factor optimization. The circuit layout of photovoltaic grid cabinets should minimize electromagnetic interference, for example, by using shielded cables and rationally planning wiring spacing to prevent harmonic propagation through coupling paths. Simultaneously, optimizing the power factor (PF) reduces harmonic generation—the closer the power factor is to 1, the lower the harmonic content. Inverters with high power factor characteristics can be selected, or static var generators (SVG) can be configured to compensate for reactive power in real time, stabilize voltage fluctuations, and suppress harmonics. SVG uses fully controlled power electronic devices (such as IGBTs), with response speeds reaching millisecond levels and a 20% harmonic mitigation function, suitable for scenarios where harmonics and reactive power issues coexist.
Operation and maintenance management is crucial to ensuring the long-term effectiveness of harmonic mitigation. Regularly monitoring the power quality parameters of the grid-connected cabinet (such as voltage distortion rate and total harmonic distortion rate THDi) can promptly detect harmonic exceedances. By installing power quality monitoring devices (such as APView500), harmonic events, voltage sags/boosts, and other data can be recorded in real time, providing a basis for adjusting mitigation strategies. In addition, regularly cleaning the equipment's heat dissipation devices and checking the health status of capacitors (such as capacitance value and loss tangent) can prevent a decline in harmonic mitigation capabilities due to equipment aging. For systems already experiencing harmonic problems, targeted improvements can be made by adjusting filter parameters, adding mitigation equipment, or optimizing operating strategies (such as limiting inverter output power).
Harmonic mitigation in photovoltaic grid cabinets requires a fully closed-loop system of "prevention-monitoring-mitigation-optimization." Suppressing harmonic propagation through filtering technology, reducing harmonic generation at the source through optimized selection, improving mitigation synergy through system design, and ensuring long-term stability through operation and maintenance management can significantly improve the power quality of photovoltaic power generation systems and facilitate the efficient grid connection of clean energy. With the advancement of power electronics technology, future harmonic mitigation in photovoltaic grid cabinets will develop towards intelligence and integration. For example, dynamic adjustment of mitigation strategies through AI algorithms or integration of monitoring, mitigation, and protection functions can further reduce operation and maintenance costs and improve system reliability.
