Introduction,Classification of Power System Stability & Stability Limit

Introduction:

The ability of a power system to maintain synchronism during normal operation and after being subjected to some disturbance is called power system stability.Stability studies which evaluate the impact of disturbances on the electromechanical dynamic behavior of the power system are of two types : transient and steady state. Transient stability studies are very commonly undertaken by electric utility planning departments responsible for ensuring the proper dynamic performance of the system. The system models used in such studies are extensive because present-day power systems are vast, heavily interconnected systems with hundreds of machines which can interact through the medium of their extra-high-voltage and ultra-high-voltage networks. These machines have associated excitation systems and turbine-governing control systems which in some but not all cases are modeled in order to reflect the properly correct dynamic performance of the system. If the resultant nonlinear differential and algebraic equations of the overall system are to be solved, then either direct methods or iterative step-by-step procedures must be used. In this chapter, we emphasize transient stability considerations and introduce basic iterative procedures used in transient stability studies.

Classification of power system stability:

It considers the in balancing of active demand and supply only and avoids the imbalance of reactive power.

Transient stability:

Since transient stability studies involve large disturbances, linearization of the system equations is not permitted. Transient stability is sometimes studied on a first-swing rather than a multi-swing basis. First-swing transient stability studies use a reasonably simple generator model consisting of the transient internal voltage ; behind transient reactance ; in such studies the excitation systems and turbine-governing control systems of the generating units are not represented. Usually, the time period under study is the first second following a system fault or other large disturbance. If the machines of the system are found to remain essentially in synchronism within the first second, the system is regarded as being transiently stable.

It can be defined as the ability of power system to remain in synchronism after subject to a sudden large disturbance such as short circuit fault, switching large load etc. is called transient stability. Analysis of transient stability is carried for a very short time such as the function of second. Operation of protective and controlling devices (protective device such as controlling governor, AVR etc. are excluded in the transient stability analysis because they slow to react in transient phase.

Steady state stability:

The ability of power system to remain in synchronism after subjected to gradually increasing small disturbances such as a small continuous change in load.Steady-state stability studies are usually less extensive in scope than transient stability studies and often involve a single machine operating into an infinite bus or just a few machines undergoing one or more small disturbances. Thus, steady-state stability studies examine the stability of the system under small incremental variations in parameters or operating conditions about a steady-state equilibrium point. The nonlinear differential and algebraic equations of the system are replaced by a set of linear equations which are then solved by methods of linear analysis to determine if the system is steady-state stable.

Dynamic stability:

It is the ability of power system to remain in synchronism after subjected to disturbance lasting for a long time. Dynamic stability studies are carried out including the operation of controlling and protective devices present in the system. The study is carried out for a longer period.

Stability Limit:

Stability limit is the maximum power that can be transferred in the network between sources and load without loss of synchronism.

Transient stability limit is the maximum power that can be transferred without the system being unstable when a sudden large disturbance occurs.

Steady state stability limit is the maximum power flow beyond which system becomes unstable due to gradually increasing disturbances.