Correct Answer  Option 4 : Load bus
Classification of Buses:
The power system is nothing but the interconnection of the various bus.
Each of these buses is associated with four electrical parameters namely voltage with magnitude and phase angle, active power, and reactive power.
Slack Bus:
 Slack Bus is also known as Swing or Reference Bus.
 One of the generator buses is considered as a slack bus.
 The slack bus does not exist in real rather it is assumed for the consideration of losses occurring during power transmission.
 Actually, there exist only two buses in the 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 are specified, and active power and reactive power are to be calculated.
 This active power of slack bus is the equivalent power loss occurring in different systems.
 Generally, the phase angle of the slack bus is taken for reference for the entire load flow solution. Therefore this bus is also called Reference Bus.
Load Bus:
 For load bus, real power P and reactive power Q are known but the magnitude and phase angle of bus voltage is unknown.
 It is desired to find the bus voltage using load flow analysis. At load bus, voltage is allowed to vary within some specified limit.
Generator Bus:
 Generator Bus is also called a voltagecontrolled bus.
 The generator is connected to this bus.
 Therefore the bus voltage corresponding to generation voltage and active power generation corresponding to generator rating is specified for this bus.
The quantities specified for different type of buses shown in the below table.
Type of bus

Specified quantities

Unknown quantities

Generator bus (PV bus)

P, V

Q, δ

Load bus (PQ bus)

P, Q

V, δ

Slack bus

V, δ

P, Q
