Short Answer:

In a power system, a bus is a point where different components like generators, loads, and transmission lines are connected. There are mainly three types of buses used in power system studies: Slack bus, PV bus, and PQ bus. Each type of bus is defined based on the known and unknown electrical quantities at that point.

The Slack bus maintains system voltage and balance, PV bus controls voltage and active power, and PQ bus represents most of the loads where power demand is fixed. These bus types are essential for load flow analysis and stable power system operation.

Detailed Explanation:

Types of buses in a power system

In power system analysis, especially during load flow studies, buses (or nodes) are key points in the network where lines, generators, transformers, and loads are connected. At each bus, different electrical quantities such as voltage magnitude, voltage angle, active power (P), and reactive power (Q) are either known or unknown.

To solve the power flow problem, we classify buses into three main types based on which values are specified and which need to be calculated. These are: Slack Bus, PV Bus, and PQ Bus.

Slack bus (also called Swing bus or Reference bus)

In every power system, there must be one slack bus.
At this bus, both voltage magnitude and voltage angle are known and fixed.
It acts as the reference for all other bus angles and balances the system’s active and reactive power.
The active (P) and reactive (Q) power at this bus are unknown and are calculated during load flow analysis.
The slack bus compensates for system losses and mismatch in power.

Example: Usually, the bus connected to the largest generator or central station is chosen as the slack bus.

PV bus (also called Generator bus or Voltage-controlled bus)

At this bus, active power (P) and voltage magnitude (V) are specified.
The voltage angle (δ) and reactive power (Q) are unknown and are calculated.
It represents buses where generators are connected and are used to control the voltage at that point.
The generator adjusts its excitation to maintain the voltage as specified.

Note: If the generator hits its reactive limit, the PV bus may be changed to a PQ bus.

PQ bus (also called Load bus)

At this bus, active power (P) and reactive power (Q) are specified.
The voltage magnitude (V) and voltage angle (δ) are unknown and are solved through the power flow calculation.
Most of the buses in a system are PQ buses as they represent the loads.

Example: Houses, factories, or commercial buildings receiving power are modeled as PQ buses.

Importance of bus classification

This classification helps simplify and solve large power system equations.
In load flow studies, knowing which values are known at each bus helps determine the unknowns using iterative methods.
It also helps in efficient system planning, operation, and control of voltage and power balance.
Correct bus type assignment ensures accurate load flow results and stable system operation.

Conclusion:

The three different types of buses in a power system are Slack bus, PV bus, and PQ bus. Each has a specific role in load flow analysis based on which electrical quantities are known or unknown. Understanding these bus types is essential for analyzing power systems accurately, ensuring proper power balance, and maintaining voltage stability.