Previous Year Question

Correct answer: A — |V|, δ

Role of the slack bus in load flow

Each bus in an AC load-flow problem is described by four main quantities: active power P, reactive power Q, voltage magnitude |V| and voltage angle δ. Two are normally specified and the other two are determined. At the slack bus, also called the swing or reference bus, the specified quantities are |V| and δ.

The reference angle is commonly chosen as 0°. This establishes the angular reference for all other buses. Only differences in bus-voltage angles affect power flow, so one angle must be fixed to make the solution definite.

Network losses are not known accurately before the bus voltages and branch currents have been solved. The slack-bus power injection is allowed to adjust to the resulting mismatch between generation, demand and network losses. Thus P and Q at this bus are outputs of the conventional load-flow calculation.

For a system represented with one slack generator, its active-power contribution satisfies the system balance:

This is a mathematical role in the load-flow model. It does not mean that the physical generator has unlimited capability; its computed operating point must still satisfy equipment limits.