In load flow analysis the load connected to a bus is represented as

In load flow analysis the load connected to a bus is represented as

Right Answer is:

Constant real and reactive drawn from the bus


Power-flow or load-flow studies are important for planning the future expansion of power systems as well as in determining the best operation of existing systems. Load flow studies determine if system voltages remain within specified limits under normal or emergency operating conditions and whether equipment such as transformers and conductors are overloaded. A load flow study is the steady-state analysis of the power system network.

In load flow study generators are modeled as a source of complex power, loads are modeled as the demand for complex power and transmission lines are modeled as π -model.

Bus types

Depending, upon which two variables you specify, the buses (nodes) can be categorized into three categories:

  1. Slack bus (swing or reference bus)
  2. PQ bus (also called as load bus)
  3. PV bus (generator bus)

PQ bus

Normally in a power system, the real and reactive power quantities are specified and so the load connected at the bus in load flow analysis is represented as constant real and reactive power drawn from the bus. PQ buses are also called load buses and may contain generators with specified real and reactive power outputs.

Slack bus

There is only one slack bus in the system under consideration. The slack bus always has a generator attached to it, with no exception. Normally this generator is the biggest in the system. Its two main tasks are to:

  • Serve as the reference for voltage angle
  • Balance generation, load, and losses, because the power losses are not known until the end of load flow calculation. The slack bus needs to supply losses.

PV bus

The PV buses can have voltage control capabilities and use a tap-adjustable transformer and/or VAR compensator instead of a generator. PV buses are also called voltage-controlled buses.

  • To solve non-linear algebraic equations it is important to have fast, efficient, and accurate numerical algorithms.
  • The output of the load flow analysis is the voltage and phase angle, real and reactive power (both sides in each line), line losses, and slack bus power.

Constant parameters of different buses in load flow studies:

Bus Name

Specified or constant values

Unknown values

Load bus

Real and reactive power

Voltage and load angle

Generator bus

Real power and voltage

Reactive power and load angle

Slack bus

Voltage, load angle is equal to 00

Real and reactive power

Voltage-controlled bus

Real power and voltage

Reactive power, load angle is equal to 00

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