QV Curve

A QV curve plots the relationship between reactive power injection or absorption at a specific bus and the resulting voltage magnitude at that bus. It is one of the standard tools for assessing voltage stability because it shows how much reactive support is available before the bus reaches an unstable operating condition.

Key Aspects of QV Curves:

• Axis Meaning: Voltage is plotted against reactive power at a selected bus, usually while the rest of the network operating point is held consistent. The curve shows how bus voltage responds as reactive support is added or withdrawn.
• Reactive Margin: The distance from the operating point to the minimum of the curve represents the reactive power margin. A small margin indicates that the bus is close to a voltage stability problem and has little reserve for further stress.
• Weak Bus Screening: Buses with shallow or unfavorable QV curves are usually the weakest voltage locations in the network. They are common candidates for capacitor banks, dynamic VAR devices, or generator voltage support.
• Relation to PV Analysis: A PV curve focuses on how voltage changes as active power transfer increases, while a QV curve focuses on reactive support at one bus. Engineers often use both to understand whether the limiting issue is transfer loading or local reactive deficiency.
• Planning Use: QV studies are used to place reactive compensation, evaluate post-contingency voltage margins, and confirm whether a bus can stay within limits after line outages or heavy loading.