Power System

The simple answer:

Slack bus is the reference bus for the load flow calculation in power system. It is also known as swing bus. Generally it is the bus to which the largest generator of the system is connected and its governor is set to isochronous mode (see http://cr4.globalspec.com/thread/42805 for isochronous mode) for load control.

The detail answer:

For load flow study, four values of a bus are used: Real power (P), Reactive Power (Q), Voltage magnitude (V) and Voltage angle (∂). There are three categories of buses are identified for load flow study based on the known values of P, Q, V and ∂. They are (1) Load bus or PQ bus, (2) Voltage Controlled bus or Generator bus or PV bus and (3) Slack bus or Swing bus or Reference bus.

(1) Load bus or PQ bus: The bus where only the loads of the system are connected. The real and reactive power (P and Q) of the load bus is known, so it is also called PQ bus. There may be more than one Load bus in a power system.

(2) Voltage Controlled bus or Generator bus or PV bus: The buses where the generators (having the load control system set in droop mode) are connected called Voltage Controlled bus or Generator bus. The voltage of the generators bus remains constant with AVR control and the delivered real power is set constant with droop control mode of the generator governors. The real power (P) and voltage (V) of these buses are known, so they are also called PV bus. There may be more than one Voltage Controlled bus in a power system.

(3) Slack bus or Swing bus or Reference bus: Only one bus in a power system is considered Slack bus or Swing bus. Generally, the bus where the largest generator (called swing generator) is connected is the swing bus. The constant voltage (V) is controlled by AVR and hence the voltage of this bus is known. The generator of this bus undertakes the changes of the load in the system and hence the real power (P) is not known. However, the voltage angle (∂) of swing bus is considered 0 as a reference and so it is also called Reference bus. The voltage angle of all other buses is calculated with the reference angle ∂=0 of this bus.

See the link below for more detail:

http://www.rose-hulman.edu/class/ee/HTML/ECE471/PDFs/Lect08.pdf

- MS