# QV Curves

The QV Curves tool provides the ability to compute QV curves for any bus in the system. A QV analysis studies how variations in reactive power (Q) injection at a bus affects the voltage (V) at that same bus. Other system parameters can also be monitored as the reactive power injection changes.

To create a QV curve, a fictitious generator (synchronous condenser) is placed at a bus that is being studied. The voltage set-point of this generator is varied and its var output is allowed to be any valued needed to meet this voltage set-point. The vertical axis (y-axis) of a QV curve depicts the output of the fictitious generator in Mvar. The horizontal (x-axis) depicts the respective voltage under this condition. The base case operating point of the system is represented by the x-intercept of the curve. This is the point where the fictitious generator is at 0 Mvar output and represents the base case. There are situations in which the output of the fictitious generator is not 0 Mvar in the base case, but these will be explained with the appropriate option settings. **When considering a contingency scenario in the QV analysis, the term ***base case operating point*** will also be used. This is the starting case that represents the system prior to performing any analysis for the curve tracing.**

Tracing down the curve from higher to lower voltage set-points represents a decrease in the fictitious generator's Mvar output which is representative of an increase in Mvar load. The curve is then tracing what the voltage would be as the Mvar load increases. At some point the Mvar value of the generator will stop decreasing and the bottom of the curve will be reached. This point represents the maximum increase in load Mvar that can occur at this bus before voltage collapse is reached.

The following shows a typical QV curve. The plot is actually a VQ curve, but this is traditionally called the QV curve.

The following plot shows a situation in which there is not enough Mvar reserve. There is no base case operating point as the curve never crosses the x-axis. The difference between the bottom of the curve and the x-axis is the amount of Mvar injection needed to achieve a solvable case and to come out of collapse.

To use the QV Curves tool, select QV Curves on the **PV and QV Curves (PVQV)** ribbon group on the Add Ons ribbon tab. The QV Curves dialog will open from which buses to be monitored can be specified, options for the QV curves can be set, and the QV curve analysis can be run.