The methods for solving the LFC problem will be illustrated for the sample power system given in following figure.

  • Problems:
  • With bus 1 as the slack, use the following methods to obtain a load flow solution:

–        Newton-Raphson using Ybus with tolerances of 0.01 per unit for the changes in the real and reactive bus powers.

–        Gauss-Seidel using Ybus with acceleration factors of 1.4 and 1.4 and tolerances of 0.0001 and 0.0001 per unit for the real and imaginary components of voltage.

–        Gauss-Seidel using Zbus with tolerances of 0.001 and 0.001 per unit.

–        Gauss using Yloop with loop voltage tolerances of 0.01 and 0.01 per unit.

Type of Bus

  • A ‘Slack’ bus (or ‘swing’ bus) is defined as V? bus, that is used to balance the active |P| & reactive power |Q| in the system while performing Load flow studies in Electrical Power Systems
  • Slack Bus is used to provide system losses by emitting or absorbing active/reactive power to/from the system
  • Load buses are of 3 types and are classified as :

–        PQ buses – here, the real power |P| and reactive power |Q| are specified. It is also known as Load Bus.

–        PV buses – here, the real power |P| and the voltage magnitude |V| are specified. It is also known as Generator Bus.

–        Slack bus – to balance the active and reactive power in the system. It is also known as the Reference Bus or the Swing Bus.

  • Magnitude and its phase angle at the buses, and also the active and reactive line flows for the specified terminal or bus conditions. Associated with each bus of a power system, there are 4 set of variables

–        magnitude of voltage i.e. |V|

–        phase angle i.e. |?|

–        active or real power i.e. |P|

–        reactive power i.e. |Q|

  • Based on these values, a bus may be classified into the above mentioned three categories as