- Steady State Stability Definition: Steady state stability is the ability of a power system to remain in synchronism after small, gradual changes in operating conditions.
- Importance of Steady State Stability: It determines the maximum load a power system can handle without losing synchronism.
- Factors Affecting Stability: Important factors include mechanical power (Pm), electrical power (Pe), load angle (δ), inertia constant (H), and synchronous speed (ωs).
- Conditions for Stability: The system must maintain maximum power transfer without losing stability, keeping voltage (|Vt|) constant for each load.
- Improving Stability: Methods to improve stability include reducing reactance (X), increasing excitation voltage (|E|), and using parallel lines in high reactance transmission lines.
As an introduction, we need to know about power state stability. It is really the capability of the system to return to its steady state condition after subjected to certain disturbances. We can now consider a synchronous generator to understand the power system stability. The generator is in synchronism with the other system connected to it. The bus connected to it and the generator will have same phase sequence, voltage and the frequency. So, we can say that the power system stability here is the capability of the power system to come back to its steady condition without affecting synchronism when subjected to any disturbances. This system stability is classified into – Transient Stability, Dynamic Stability and Steady State Stability.
Transient Stability: Study of power system which are subjected to sudden major disturbances.
Dynamic Stability: Study of power system which are subjected to small continuous disturbances.
Steady State Stability
Steady state stability involves studying small, gradual changes in the system’s working state. It aims to find the maximum load the machine can handle before losing synchronism. This is done by slowly increasing the load.
The highest power that can be transferred to the system’s receiving end without losing synchronism is called the Steady State Stability limit.
The Swings equation is known by
Pm → Mechanical power
Pe → Electrical power
δ → Load angle
H → Inertia constant
ωs → Synchronous speed
Consider the above system (figure above) which is operating on steady state power transfer of 
Assume the power is increased by a small amount say Δ Pe. As a result, the rotor angle becomes 
from δ0.
p → frequency of oscillation.
The characteristic equation is used to determine the system stability due to small changes.
Conditions for System Stability
Without loss of stability, the Maximum power transfer is given by
If the system operates below the steady state stability limit, it may oscillate for a long time if damping is low, posing a hazard to system security. To maintain the steady state stability limit, the voltage (|Vt|) should be kept constant for each load by adjusting the excitation.
- A system can never be operated higher than its steady state stability limit but it can operate beyond the transient stability limit.
- By reducing the X (reactance) or by raising the |E| or by increasing the |V|, the improvement of steady state stability limit of the system is possible.
- Two systems to improve the stability limit are quick excitation voltage and higher excitation voltage.
- To reduce the X in the transmission line which is having high reactance, we can employ parallel line.
