Slack bus

In power system analysis, a slack bus (also known as a swing bus or reference bus) is a key component in power flow studies. It serves as the reference node for voltage magnitude and angle, providing the necessary power balance for the entire system. Since power losses occur during transmission and distribution, and these losses are not known a priori, the slack bus compensates for these unknown losses.

Specifically, the slack bus performs the following functions:

• Voltage Magnitude and Angle Reference: It is assigned a fixed voltage magnitude (typically 1.0 per unit) and a fixed voltage angle (typically 0 degrees). All other bus voltages are solved relative to this reference.
• Power Balance: It absorbs or supplies the difference between the total power demand and the total power generation in the system. This ensures that the power balance equation (total generation = total load + total losses) is satisfied. The actual power injected or absorbed by the slack bus is calculated as part of the power flow solution.
• Loss Compensation: Since transmission losses are unknown before the power flow is solved, the slack bus implicitly accounts for these losses. It effectively injects or absorbs the power necessary to cover these losses.
• Ensuring Convergence: The slack bus plays a crucial role in the convergence of the iterative power flow solution algorithms (e.g., Newton-Raphson, Gauss-Seidel). By fixing the voltage magnitude and angle at one bus, the algorithm has a defined starting point and a stable reference to converge towards.

In essence, the slack bus acts as an infinite source or sink of power, ensuring that the power balance is maintained in the system and that the power flow solution converges. It is a mathematical construct used in power system simulations and does not necessarily correspond to a specific physical generator or load in the real-world power grid. The choice of which bus to designate as the slack bus can sometimes influence the convergence rate of the power flow solution, but generally any adequately sized generator bus can be selected.