Slack Bus / Swing Bus

The slack bus (also called swing bus, reference bus, or V-δ bus) serves a dual role in power flow calculations: it provides the voltage angle reference for the entire system and balances active power to account for transmission losses that cannot be known before the solution converges.

Key Aspects of the Slack Bus:

• Loss Compensation: Since total generation must equal total load plus losses, and losses depend on voltages and currents that are unknown until the power flow is solved, the slack bus absorbs this mismatch by allowing its active and reactive power to float as calculated outputs.
• Angular Reference: The slack bus voltage angle is fixed (typically at 0°), establishing the phase reference from which all other bus angles are measured.
• Selection Criteria: It is usually assigned to the largest generator or a strong interconnection point with sufficient capacity to absorb power swings without violating operating limits.
• Role in Iterative Solvers: In methods such as Newton-Raphson or Gauss-Seidel, the slack bus reduces the system of nonlinear equations by one unknown, enabling a deterministic solution.
• Distributed Slack: A known limitation is that the single slack bus absorbs all losses and mismatches, sometimes yielding unrealistic dispatch. In practice, engineers may redistribute the slack across multiple generators using a distributed slack formulation.
• Real-World Equivalent: In actual grid operation, the function of the slack bus is performed by generators on automatic generation control (AGC), which continuously adjust output to maintain frequency and balance supply with demand.