How do power capacitors provide instantaneous reactive power support to stabilize the system during grid voltage sags?
Release Time : 2025-10-10
In modern power systems, voltage stability is a key indicator for ensuring power supply quality and safe equipment operation. Grid voltage sags are a common power quality issue, often caused by short-circuit faults, large-capacity motor startup, or lightning strikes. Voltage sags can not only cause sensitive equipment to shut down and production lines to interrupt, but can also lead to serious accidents such as voltage collapse. As a key reactive power compensation device, power capacitors can quickly provide instantaneous reactive power support during voltage sags, effectively suppressing voltage drops and improving system dynamic stability.
1. Causes and Dangers of Voltage Sags
A voltage sag is a sudden, short-term drop in the effective value of the grid voltage to 10% to 90% of the rated value, lasting from 0.5 cycles to 1 minute. Primary causes include short circuits in transmission and distribution lines, large load surges, or the recovery process after a system fault. When a voltage sag occurs, inductive loads absorb more reactive power from the grid to maintain magnetic flux, further exacerbating the voltage drop and creating a vicious cycle. Without timely compensation, voltage instability and even cascading failures can occur. Therefore, rapidly injecting reactive power at the initial stage of a voltage sag is a key measure to curb its progression.
2. Principle of Reactive Power Compensation with Power Capacitors
Power capacitors are essentially capacitive reactive components. Their operating principle is based on the characteristic of current leading voltage by 90 degrees in AC circuits. When connected to the grid, capacitors "send" capacitive reactive power to the system, offsetting the reactive power "absorbed" by inductive loads, thereby reducing the reactive transmission burden on the grid and maintaining voltage levels. As can be seen, even when voltage drops, capacitors can still provide a certain amount of reactive power output, and their output capacity is proportional to the square of the voltage. Although a voltage drop will reduce the output of a single capacitor, its fast response time, simple structure, and low cost make it an ideal choice for addressing transient voltage fluctuations.
3. Implementation Mechanism of Instantaneous Reactive Power Support
During a voltage sag, power capacitors can provide nearly "instant" reactive power support. The response speed depends on the type of switching device. Traditional contactor-switched capacitor banks have a response time of tens to hundreds of milliseconds. Although there is some delay, they can still be operational within a few cycles after a voltage drop, effectively mitigating the voltage drop. However, the rapid switching technology used with thyristors achieves zero-crossing triggering, shortening the response time to one to two cycles, operating almost synchronously with the fault, significantly improving support effectiveness. Once the capacitors are switched on, the capacitive reactive power they generate is directly injected into the busbar near the fault point, boosting the local voltage, alleviating pressure on the main grid, and buying valuable time for system recovery.
4. Advantages of Application in Distribution Networks and Industrial Systems
In distribution networks, power capacitors are often installed centrally at substations or distributed at load centers. When a fault on a remote line causes a voltage drop, locally compensated capacitors can quickly provide a local reactive power source, preventing deep voltage drops along the line. On the industrial user side, large motor clusters are voltage-sensitive. Automatic power factor compensation cabinets can quickly activate backup capacitors when voltage anomalies are detected, maintaining stable busbar voltage within the factory and preventing production equipment from tripping. Furthermore, capacitors can be used in conjunction with on-load tap-changing transformers and dynamic reactive power compensation devices to form a multi-tiered voltage support system, further enhancing system resilience.
Power capacitors rapidly release capacitive reactive power, playing a key role as voltage support devices during grid voltage dips. Their simple principle, mature technology, and cost-effectiveness make them a fundamental means of enhancing the dynamic stability of power systems. In distribution networks and industrial power applications, the rational deployment and rapid switching of power capacitors can effectively mitigate voltage drops, prevent equipment malfunctions, and ensure power supply continuity.
1. Causes and Dangers of Voltage Sags
A voltage sag is a sudden, short-term drop in the effective value of the grid voltage to 10% to 90% of the rated value, lasting from 0.5 cycles to 1 minute. Primary causes include short circuits in transmission and distribution lines, large load surges, or the recovery process after a system fault. When a voltage sag occurs, inductive loads absorb more reactive power from the grid to maintain magnetic flux, further exacerbating the voltage drop and creating a vicious cycle. Without timely compensation, voltage instability and even cascading failures can occur. Therefore, rapidly injecting reactive power at the initial stage of a voltage sag is a key measure to curb its progression.
2. Principle of Reactive Power Compensation with Power Capacitors
Power capacitors are essentially capacitive reactive components. Their operating principle is based on the characteristic of current leading voltage by 90 degrees in AC circuits. When connected to the grid, capacitors "send" capacitive reactive power to the system, offsetting the reactive power "absorbed" by inductive loads, thereby reducing the reactive transmission burden on the grid and maintaining voltage levels. As can be seen, even when voltage drops, capacitors can still provide a certain amount of reactive power output, and their output capacity is proportional to the square of the voltage. Although a voltage drop will reduce the output of a single capacitor, its fast response time, simple structure, and low cost make it an ideal choice for addressing transient voltage fluctuations.
3. Implementation Mechanism of Instantaneous Reactive Power Support
During a voltage sag, power capacitors can provide nearly "instant" reactive power support. The response speed depends on the type of switching device. Traditional contactor-switched capacitor banks have a response time of tens to hundreds of milliseconds. Although there is some delay, they can still be operational within a few cycles after a voltage drop, effectively mitigating the voltage drop. However, the rapid switching technology used with thyristors achieves zero-crossing triggering, shortening the response time to one to two cycles, operating almost synchronously with the fault, significantly improving support effectiveness. Once the capacitors are switched on, the capacitive reactive power they generate is directly injected into the busbar near the fault point, boosting the local voltage, alleviating pressure on the main grid, and buying valuable time for system recovery.
4. Advantages of Application in Distribution Networks and Industrial Systems
In distribution networks, power capacitors are often installed centrally at substations or distributed at load centers. When a fault on a remote line causes a voltage drop, locally compensated capacitors can quickly provide a local reactive power source, preventing deep voltage drops along the line. On the industrial user side, large motor clusters are voltage-sensitive. Automatic power factor compensation cabinets can quickly activate backup capacitors when voltage anomalies are detected, maintaining stable busbar voltage within the factory and preventing production equipment from tripping. Furthermore, capacitors can be used in conjunction with on-load tap-changing transformers and dynamic reactive power compensation devices to form a multi-tiered voltage support system, further enhancing system resilience.
Power capacitors rapidly release capacitive reactive power, playing a key role as voltage support devices during grid voltage dips. Their simple principle, mature technology, and cost-effectiveness make them a fundamental means of enhancing the dynamic stability of power systems. In distribution networks and industrial power applications, the rational deployment and rapid switching of power capacitors can effectively mitigate voltage drops, prevent equipment malfunctions, and ensure power supply continuity.