How do low-voltage static var generators (SVGs) maintain their reactive power output capacity during grid voltage sags?
Publish Time: 2025-08-14
Maintaining the reactive power output capacity of low-voltage static var generators (SVGs) during grid voltage sags is a critical issue for ensuring stable power system operation. Voltage sags are often caused by short-circuit faults, the start-up of large-capacity motors, or lightning strikes. They can cause a sharp drop in grid voltage over a short period of time, severely impacting the normal operation of sensitive loads. During these events, the system urgently needs a large amount of reactive power to support voltage recovery. As a dynamic reactive power compensation device, low-voltage static var generators (SVGs) must continuously provide reactive power during this critical period.The basic operating principle of a low-voltage static var generator (SVG) is to use a voltage source inverter composed of fully controlled power electronic devices (such as IGBTs) to convert the energy in DC-link capacitors into AC current and inject it into the grid. The reactive current output is continuously adjustable, with extremely fast response times, typically completing dynamic compensation within milliseconds. However, when grid voltage sags occur, the output capacity of SVGs is limited because their reactive power output is proportional to the grid voltage. If voltage V drops, the available reactive power will be significantly reduced given the current I.To maintain or even enhance reactive power output during voltage sags, low-voltage static var generators (SVGs) typically feature short-term overcurrent protection. Most industrial-grade SVG designs allow them to output 1.2 to 1.5 times their rated current for a short period of time. For example, if the SVG can increase its output current to 1.25 times the rated value when the grid voltage drops to 80%, its reactive power output can still approach the rated level. This overload capability relies on the thermal design of the IGBT module, the efficiency of the heat dissipation system, and the precise current management of the control system.In terms of control strategy, SVGs employ advanced algorithms to cope with low-voltage conditions. Dual closed-loop control based on a synchronously rotating coordinate system (dq coordinate system) is widely used. The outer voltage loop regulates the DC bus voltage or reactive power command, while the inner current loop rapidly tracks the d- and q-axis currents. During voltage sags, the control system can switch to "Low Voltage Ride-Through Mode" (LVRT Mode), prioritizing the output of q-axis reactive current while appropriately adjusting the d-axis current to maintain DC-side voltage stability.In addition, SVGs typically incorporate a grid voltage feedforward control mechanism. This technology detects grid voltage changes in real time and feeds this into the modulation process as a feedforward signal, proactively adjusting the amplitude and phase of the modulation wave. This accelerates the SVG's response to voltage fluctuations, reduces dynamic errors, and improves its reactive power support capability during voltage dips.Stability of the DC-side voltage is also crucial for maintaining SVG reactive power output. During voltage sags, the DC bus voltage may fluctuate due to reduced AC-side energy input. To address this, the SVG can absorb a small amount of active power from the grid by adjusting its active current component to compensate for switching losses and maintain a constant capacitor voltage. This coordinated active and reactive power control strategy ensures that the SVG does not cease operation due to DC undervoltage or overvoltage during a fault.Modern low-voltage static var generators (SVGs) are also equipped with comprehensive protection and monitoring systems. When a voltage sag is detected, the system initiates emergency response procedures within 1-2 milliseconds, blocking the fault signal, activating the overcurrent protection threshold, and entering dynamic reactive power priority mode. Simultaneously, real-time monitoring of parameters such as IGBT temperature, capacitor voltage, and cooling system status prevents thermal damage to the equipment under overload conditions, ensuring safe operation under extreme conditions.The low-voltage reactive power support capabilities of low-voltage static var generators (SVGs) have been widely verified in practical applications. For example, in a manufacturing company's power distribution system, when a short circuit in the external grid caused the bus voltage to drop to 70%, the configured 3Mvar SVG responded within 5 milliseconds, outputting capacitive reactive power at 1.4 times the rated current, quickly restoring the bus voltage to normal, effectively avoiding unplanned production line downtime.In summary, low-voltage static var generators (SVGs) can effectively maintain or even improve their reactive power output capacity during grid voltage sags by providing short-term overcurrent protection, employing advanced control strategies (such as direct-current (DQ) control, voltage feedforward, and LVRT), maintaining DC-side voltage stability, and implementing system-level protection and monitoring. This not only enhances the SVG's own operational reliability but also provides critical dynamic voltage support for the grid, making it a core technical means of improving power quality and system resilience.
