Different Buses in Load Flow Stuides ?

Bus is a node at which one or many loads and generators are connected. In a power system each node or bus is associated with four quantities, they are: 1. Magnitude of the voltage,2. Phase angle of the voltage, 3.Active or true power(P) and 4. Reactive power (Q). In a load flow problem two out of these four quantities are specified and remaining two quantities has to be determined through solving the load flow equations.Depending upon which quantities have been specified, the buses are classified into three categories.

Generator Bus or Voltage Controlled Bus:

This bus is also called as P-V bus. On this bus, voltage magnitude corresponds to the generation voltage and the true or active power (P) corresponding to its rating are specified. Voltage magnitude is maintained constant by at a specified value by injecting the reactive power. The Reactive Power generation and phase angle of the voltages are to be computed

Load Bus:

This is also called P-Q bus and at this bus total injected power is specified i.e, the active power and reactive power injected into the network at this bus. Magnitude and phase angle of the voltage are to be computed. In simple, at load bus only P and Q values are specified and bus voltage is computed as it is permitted to vary the bus voltages of the load bus with in permissible limits.

 Slack,Swing or Reference Bus:

One of the generation buses in a power system in a power system is chosen as Slack or Swing Bus. At this bus the magnitude and phase angle of the voltages are specified. The phase angle is usually set as zero.The active and reactive power at this bus are determined by solving the load flow equations. Swing bus is a fictitious concept in load flow studies arises because the I²R losses of the system are not known precisely in advance for the load flow calculations.At swing bus active power is considered as unknown variable, The difference between the expected and solved output (MW) represents the error in the prior estimate of the system I²R losses. The generators at the swing bus supply the difference of the specified active power to be injected into the system at other buses and total system output plus losses.