Significance of slack bus

The slack (swing) bus is the bus where the voltage magnitude and angle are specified (usually $1.0\angle 0^\circ$ pu or a set value) and the real and reactive power are unknown.

Its significance:

• Supplies the unknown losses. Line losses are not known until the load flow is solved, so one generator must supply the difference between total generation and total load plus losses.
• Phase angle reference. Its angle is taken as $0^\circ$ and all other bus angles are measured from it.
• Makes the equations solvable. It leaves two known and two unknown quantities at every bus.
• It is normally the largest generating station in the system. Its $P$ and $Q$ are found at the end from $P_1 - jQ_1 = V_1^{*}\sum_k Y_{1k}V_k$.