How can a static var generator (SVG) avoid the risk of resonance with the grid impedance through direct current control?
Publish Time: 2025-12-04
Static var generators (SVGs), as core equipment in modern power quality management, are widely used in industrial, commercial, and public power distribution systems due to their millisecond-level response, continuous adjustability, and bidirectional reactive power capability. Their core principle is to generate a controllable compensation current through a self-commutating bridge circuit, which is then connected to the grid in parallel via a reactor to dynamically offset reactive power and harmonics generated by the load. However, in actual operation, improper parameter matching between the SVG and the grid can induce parallel or series resonance, leading to voltage distortion, equipment overload, or even system collapse.1. Causes of Resonance: The Interaction Hazards Between SVG and Grid ImpedanceThe power grid is not an ideal power source; it possesses a certain equivalent impedance, especially under conditions of long-distance transmission, multiple transformers in parallel, or weak grids, where system impedance increases significantly. When the frequency of the compensation current output by the SVG approaches the inherent resonant frequency of the grid, the two may form a parallel resonant circuit, amplifying specific harmonic currents. If the parameters of the reactor inside the SVG are improperly matched with the grid capacitance, series resonance may occur. These resonance phenomena not only weaken the compensation effect but may also cause SVG overcurrent protection to trip and even damage power devices.2. Direct Current Control: Suppressing Resonant Excitation at the SourceUnlike traditional voltage control methods, modern low-voltage SVGs generally adopt a direct current control strategy based on instantaneous reactive power theory. This method uses a high-precision current sensor to collect the load current in real time, quickly separates the reactive and harmonic components through dq transformation or ip-iq algorithm, and then generates a precise reference compensation current command by a PI or PR controller. Since the control objective is to "track a specified current" rather than adjust the voltage amplitude, the SVG essentially acts as a controlled current source with extremely high output impedance, making it insensitive to changes in grid impedance, thus naturally possessing the ability to suppress resonance.3. Active Damping Technology: Actively Injecting Virtual ResistanceTo further improve system robustness, advanced SVG control systems often integrate active damping algorithms. This technology introduces a feedback signal proportional to the reactor current or capacitor voltage into the current control loop, equivalent to "virtually" adding a resistor to the resonant circuit, consuming resonant energy, and thus damping oscillations. For example, active damping methods using reactor current feedback can effectively suppress potential resonance peaks in the 2nd–15th order range without increasing hardware costs, and without affecting the fundamental reactive power compensation performance.4. Intelligent Resonance Scanning and Adaptive AdjustmentHigh-end SVG equipment also features online grid impedance identification. During startup or operation, the system can actively inject weak test signals, analyze the voltage and current response spectrum, estimate the grid equivalent impedance in real time, and automatically adjust the control bandwidth, filter parameters, or damping coefficient to achieve "adaptive resonance avoidance." Some products even have a built-in resonance early warning mechanism; when an abnormal increase in gain is detected in a certain frequency band, the compensation gain for that band is reduced in advance to prevent problems before they occur.5. System-Level Collaborative Design: End-to-End Protection from Hardware to ControlBesides control strategies, hardware design is equally crucial. Appropriately selecting reactor inductance values to keep their resonant frequency away from the main harmonic frequency bands; using LCL or LLCL filter structures instead of single reactors can further improve high-frequency attenuation capabilities; simultaneously, ensuring the SVG installation location is far from large-capacity capacitor compensation cabinets to avoid forming LC resonant circuits. These measures, in conjunction with advanced control algorithms, construct a multi-layered resonance protection system.Static var generators (SVGs), through direct current control combined with active damping, adaptive adjustment, and system-level optimization, not only efficiently provide dynamic reactive power support but also fundamentally avoid the risk of resonance with grid impedance. This allows SVGs to maintain high reliability and high power quality management capabilities even in complex and weak grid environments, truly becoming an indispensable "stabilizer" and "purifier" in modern smart distribution systems.
