Depending upon which quantities have been specified, the buses are classified into three categories viz.: 1. Generation Bus 2. Load Bus and 3. Slack Bus.
Category # 1. Generation Bus or Voltage-Controlled Bus:
This is also called the P-V bus, and on this bus the voltage magnitude corresponding to generation voltage and true or active power P corresponding to its ratings are specified. Voltage magnitude is maintained constant at a specified value by injection of reactive power. The reactive power generation Q and phase angle δ of the voltage are to be computed.
Category # 2. Load Bus:
This is also called the P-Q bus and at this bus the total injected power is specified, i.e., the active and reactive power injected into the network at this bus. Magnitude and phase angle of the voltage are to be computed. It is required to specify only P and Q at such a bus as at a load bus voltage can be permitted to vary within the permissible values, i.e., 5%. Also phase angle of the voltage i.e., δ is not very important for the load.
Category # 3. Slack, Swing or Reference Bus:
One of the generation buses in a power system is chosen as slack or swing bus. At this bus, the magnitude and phase angle of the voltage are specified. The phase angle of the voltage is usually set equal to zero. The active and reactive powers at this bus are to be determined through the solution of equations.
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The swing bus is a fictitious concept in load flow studies and arises because the I2R losses of the system are not known precisely in advance for the load flow calculations. Therefore, the total injected power cannot be specified at every single bus. At the swing bus, it is customary to regard the active power as unknown.
The difference between the expected and solved output (MW) represents the error in the prior estimate of 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 the total system output plus losses. If slack bus is not specified, the generation bus usually with maximum active power P is taken as reference bus. There can be more than one slack bus in a given network.
Phase angle of the voltage at the slack bus is usually taken as reference or zero.
Power system can be analysed using either mesh current method or nodal voltage method but latter is usually employed. From the point of view of computer time and memory, the nodal admittance formulation, employing the nodal voltages as the independent variables is the most economical.