Bus classification in load flow

In load-flow analysis each bus $i$ has four quantities: real power $P_i$, reactive power $Q_i$, voltage magnitude $|V_i|$ and voltage angle $\delta_i$. Two are specified and the other two are found by the load flow. On this basis buses are of three types.

1. Slack (swing / reference) bus

• Usually the largest generating station; only one in the system.
• Specified: $|V|$ and $\delta$ (normally $\delta = 0^\circ$, the reference angle).
• Found: $P$ and $Q$.
• Its generation supplies the difference between total load plus the (initially unknown) losses and the scheduled generation of other units.

2. PV (generator / voltage-controlled) bus

• Buses with generators, synchronous condensers or SVCs that can hold voltage.
• Specified: $P$ (set by turbine) and $|V|$ (set by AVR); limits $Q_{min}$, $Q_{max}$.
• Found: $Q$ and $\delta$.
• If computed Q goes outside its limits, Q is fixed at the limit and the bus becomes a PQ bus.

3. PQ (load) bus

• Buses with only loads (or fixed generation); about 80–90% of all buses.
• Specified: $P$ and $Q$ (net injection $P_G - P_D$, $Q_G - Q_D$).
• Found: $|V|$ and $\delta$.

Summary

In N-R analysis this gives $(N-1)$ P-equations (all except slack) and $(N-1-m)$ Q-equations (PQ buses only).