Load Flow Study

The flow of electrical power in any interconnected electrical system is termed as Load Flow. The Load Flow Study is conducted to calculate the voltages at the various buses. The following variables given below are associated with each bus.

  • Voltage magnitude
  • Voltage phase angle
  • Active power and reactive volt amperes.

In the load flow studies in a power systems three types of buses are identified. In each bus two variables are known and the other two are to be calculated. The buses are classified as follows:-

Swing Bus

The swing bus is the first one to respond to a changing load condition. The voltage magnitude and phase angle are specified in this type of bus.

Generator Bus

In this type of bus, voltage magnitude and active power are given and the phase angle and voltage are to be determined.

Load Bus

In this bus active power and reactive volt amperes are specified. Buses with neither generator nor load may be considered as load buses. If any bus in a power system has both load and generator, then the load is generally treated as negative generation.

One of the generator buses is selected as the reference bus for the various reasons such as – The losses in the system remain unknown until the load flow solution is complete. Hence, one of the generator buses is made to take the additional real and reactive powers to supply the losses. This bus is, therefore, known as Swing bus. The voltage throughout the system should be close to 1, hence the voltage on the slack bus is assigned to 1 per unit.

The voltage of the slack bus is taken as reference and thus its angle is equal to zero. Generally the bus connected to the largest generating station is selected as the slack bus. This bus is numbered as bus 1. In load flow problem the single phase representation with positive sequence network is used since power system is usually balanced under normal condition of operation.

The load flow problem has a following solution.

  • Consider some values of all the unknown values. It is good to set all the voltage angle to zero value and all the voltage magnitude to 1 per unit value.
  • Solve the load flow equations using all the values of voltages, angle and magnitude.
  • Solve and update the changes of the values of the voltage and magnitude.
  • Stopping condition is checked, if fulfilled, then the process is stopped otherwise repeat the steps again.

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