In power-flow studies, define PQ, PV, and Slack buses and indicate which variables are known or unknown at each bus type.

In power-flow studies, define PQ, PV, and Slack buses and indicate which variables are known or unknown at each bus type.

• A. PQ bus: P and Q specified; V and δ unknown. PV bus: P and V specified; Q and δ unknown. Slack bus: V and δ specified; P and Q adjust.
• B. PQ bus: P and Q specified; V and δ unknown. PV bus: P and V specified; Q and δ unknown. Slack bus: V and δ specified; P and Q adjust to balance.
• C. PQ bus: V specified; P unknown; Slack bus: P and Q specified.
• D. PQ bus: P and Q specified; V known; Slack bus: V and δ unknown.

Answer: (B)

The essential idea is which quantities are held fixed at each bus and which are solved for in a power-flow calculation. At a PQ bus, both active and reactive power are specified, so the voltage magnitude and the voltage angle at that bus are the unknowns the solver determines. At a PV bus, the active power and the voltage magnitude are specified, so the reactive power and the voltage angle are unknowns to be found by the solution. At the Slack (reference) bus, the voltage magnitude and the voltage angle are fixed, and the solver adjusts the active and reactive power at that bus to balance the overall power in the network. This matches the given answer because it correctly states which quantities are known or unknown for each bus type. The other options mix up which variables are fixed, especially for the Slack bus and PV bus, making them inconsistent with the standard definitions used in power-flow studies.

The essential idea is which quantities are held fixed at each bus and which are solved for in a power-flow calculation. At a PQ bus, both active and reactive power are specified, so the voltage magnitude and the voltage angle at that bus are the unknowns the solver determines. At a PV bus, the active power and the voltage magnitude are specified, so the reactive power and the voltage angle are unknowns to be found by the solution. At the Slack (reference) bus, the voltage magnitude and the voltage angle are fixed, and the solver adjusts the active and reactive power at that bus to balance the overall power in the network. This matches the given answer because it correctly states which quantities are known or unknown for each bus type. The other options mix up which variables are fixed, especially for the Slack bus and PV bus, making them inconsistent with the standard definitions used in power-flow studies.