PQ Bus

A PQ bus (also called a load bus) is the most common bus type in power flow analysis, where both active power (P) and reactive power (Q) injections are specified as known inputs, while voltage magnitude and voltage angle are the unknowns to be solved.

Key Aspects of a PQ Bus:

• Prevalence: PQ buses represent approximately 80–85% of all buses in a practical power system, since the vast majority of network nodes are load delivery points without local voltage control.
• No Local Voltage Control: Because there is no voltage-regulating device at a PQ bus, voltage magnitude is free to vary depending on system conditions, making these buses where voltage problems such as low voltage or voltage collapse are most likely to manifest.
• PV-to-PQ Switching: During the iterative power flow solution, a PV (generator) bus can convert to a PQ bus if its reactive power output hits its upper or lower limit, at which point voltage can no longer be held constant.
• Data Requirements: Accurate specification of P and Q at each load bus is critical for realistic power flow results and is typically derived from load forecasting, SCADA measurements, or metering data.
• Applications: PQ bus modeling is used in power flow studies, voltage stability assessment, and planning studies to evaluate how load growth or new connections affect network voltages.