Power System is nothing but the interconnection of various bus. Each of these buses are associated with four electrical parameters namely voltage with magnitude and phase angle, active power and reactive power. These fours parameters are not completely know but in practical situation only two are known and the remaining two parameters are calculated using Load Flow Analysis. Bus classification is based on the known parameters. On this ground, buses are classified into three types: Load Bus, Generator Bus and Slack Bus.

**Load Bus:** For load bus real power P and reactive power Q are known but magnitude and phase angle of bus voltage is unknown. It is desired to find bus voltage using load flow analysis. At load bus, voltage is allowed to vary within some specified limit say 5%. This is the reason bus voltage is not important and hence not mentioned.

**Generator Bus: **Generator Bus is also called voltage controlled bus. Generator is connected to this bus and therefore bus voltage corresponding to generation voltage and active power generation corresponding to generator rating is specified for this bus.

**Slack Bus:** Slack Bus is also known as Swing or Reference Bus. Slack bus does not exists in real life rather it is assumed for the consideration of losses occurring during power transmission. Actually there exists only two buses in power system, Load Bus and Generator Bus for which active power is specified. Since active power delivered by Generator Bus and consumed by Load Bus differ, this means that a power loss equal to the difference between Generator Bus P and Load Bus P is occurring. This loss can only be calculated after the solution of Load Flow. Therefore to supply power loss, an extra generator bus is considered for which bus magnitude and voltage is specified and active power and reactive power are to be calculated. Mind that this active power of slack bus is the equivalent power loss occurring in different system.

Generally phase angle of slack bus is taken for reference for the entire load flow solution. Therefore this bus is also called Reference Bus.

Table below gives a quick comparison of the three buses.

Bus Type |
Known Parameter |
Unknown Parameter |

Load Bus | P, Q | V, phase angle∠ |

Generator Bus | P, V (magnitude) | Q, Voltage phase angle∠ |

Slack Bus | Voltage magnitude & phase angle∠ | P, Q |

very clear and good article easy to understand. Thank you