Hello Steemians!!   

Hope you all are doing well and enjoying the bounties .Today i am  gonna share "Introduction to power system stability" with you guys. 

    Classification of Stability :

  • Power system performance is influenced by a wide array of devices with different characteristics and dynamic responses
• Due to the high complexity of the system, it is essential to make simplifying assumptions and analyze the problems using the right degree of detail.
• It is generally not very effective to study power system stability as a single problem.
• It is more effective to divide the stability problem into appropriate categories. 

 • Classification of stability into various categories greatly facilitates:
– The analysis of the stability problems
– Identification of essential factors which contribute to instability.
– Devising methods of improving stable operation.
• Classification of power system stability problems is based the following considerations:
– Physical nature of the disturbance resulting instability
– Size of the disturbance considered
– Devices, processes and the time span involved
– The most appropriate method of calculation and prediction of stability 

   Rotor Angle Stability :

  • Ability of interconnected synchronous machines to remain in synchronism under normal conditions and after being subjected to a disturbance
– Depends on the ability to maintain/restore equilibrium between electromagnetic torque and mechanical torque of each synchronous machine in the system
• If the generators become unstable when perturbed, it is as a result of a uncontrolled situation due to torque imbalance
– A fundamental factor is the manner in which power outputs of synchronous machines vary as their rotor angles swing
– Instability that may result occurs in the form of increasing angular swings of some generators leading to loss of synchronism with other generators
   • The relationship between interchange power and the angular positions of the rotor of synchronous
machines is highly non-linear.
• In order to illustrate this relationship, consider the system below, where machine 1 is operated as synchronous generator and machine 2 is operated as a synchronous motor.
                     

  • The Power transferred from the generator to the motor is a function of the angular separation (δ) between the machines.
• The angular separation is due to three components:
– The generator internal angle G
– Angular difference between the terminals of the generator and the motor L
– The motor internal angle M
• The figure below shows a model of the system, where the machines are modeled as an internal voltage behind an effective reactance.
                      

  • Phasor diagram showing the relationship between the generator and motor voltages

• There is a maximum steady state power that can be transmitted between the two machines. 

  • Under steady-state conditions, there is equilibrium between electromagnetic and mechanical torques
• If the system is perturbed, this equilibrium is upset, causing acceleration or deceleration of the rotor
– synchronism is maintained through development of restoring forces
• Change in electrical torque can be resolved into two components:
                   
   • System Stability depends on the existence of both components of torque for each of the synchronous machines.
– A lack of synchronizing torque (Ts) results in aperiodic instability.
– A lack of damping torque (TD) results in oscillatory instability.
• For convenience in analysis and for gaining inside into the nature of stability problems, it is usual to characterize the rotor angle stability phenomena in terms of two categories:
– Small signal stability, and
– Transient stability.
 

Hope you like my article and if you do please