Power System Stability

Some synchronous machines operate in synchronism as part of the power system. They must always remain in complete synchronism under steady state conditions in order for the power system to remain operational. When a disturbance happens in a system, the system creates a force that causes it to return to normal or stability. Stability is the capacity of the power system to resume normal or stable conditions following a disturbance. System disturbances can take many different forms, including switching, line-to-line faults, all three line faults, sudden changes in load, and sudden short circuits between a line and the ground.

The behaviour of the synchronous machines following a disturbance heavily influences the system’s stability. Depending on the size of the disturbances, there are primarily two types of power system stability.

• Steady state stability
• Transient stability

Steady-state stability

It describes the system’s capacity to recover from slow, minor disturbances brought on by gradual power changes so that all networks operate at the same speed and frequency. Two categories of steady-state stability are distinguished.

Dynamic stability

It indicates a system’s stability when it returns to its stable state following a brief disturbance (the disturbance lasts only 10 to 30 seconds). It also goes by the name “small signal stability.” The fluctuation in load or generation level is the main cause of it.

Static stability

Without the assistance (benefit) of automatic control devices like governors and voltage regulators, the system is stable.

Transient Stability

It is described as the power system’s capacity to resume normal operation following a significant disturbance. Large system disturbances are brought on by sudden load removal, line switching operations, system faults, unexpected line outages, etc.

When a new transmitting and generating system is planned, transient stability is tested. The synchronous machine’s behaviour in response to transient disturbances is described by the swing equation. The graph below depicts the transient and steady-state disturbances that the power system experiences. The machine’s synchronism is compromised by these disturbances, and the system becomes unstable.

So Stability studies are useful in determining the voltage levels, the transfer capacity of the systems, and the critical clearing time of circuit breakers.