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.

Power System Stability – Basic concepts and definition:

  • Ability of an electric power system, for a given operating condition, to regain a state of operating equilibrium after being subjected to a disturbance, with all system variables bounded so that the entire system remains intact.
• Following a disturbance, the dynamic system performance is concern of system stability
– System is stable if the oscillatory response of a power system during the transient period (following a disturbance) is damped and the system settles in a finite time to a new steady operating condition
• Stability is a condition where there is an equilibrium between opposing forces:
– Instability occurs when a disturbance causes a sustained imbalance between the opposing forces.
– Instability is either a run-down or a run a way situation.
   • Electric power systems are highly non-linear systems that operate in constantly changing environment since loads, generator output and key operating parameters change continually.
• When electric power system is subjected to disturbance, the system stability depends on the nature of the disturbance, as well as the initial operating conditions.
• The disturbances may be of large or small scale:
– Small disturbances in the form of load changes.
– Large disturbances of severe nature, such as a loss of large generator or a short circuit on a transmission line.   • If the power system is stable it will reach a new equilibrium state when subjected to a transient disturbance and the entire system will practically be intact after the disturbance:
– Faulted elements and any connected load are though disconnected in order to isolate the fault.
– Actions of automatic control and system operator actions will eventually restore the system to normal state.
• If the power system is unstable, a transient disturbance will result in a run-away or a run-down situation:
– A progressive increase in angular separation between the generator rotors.
– A progressive voltage decrease in the bus voltages.
• The consequences of a unstable condition could be cascading outages and a shut down of a large part of the power system.
   • Instability in a power system can be manifested in many different ways depending on the system configuration and operating mode:
– Mode of instability depends of which set of balancing forces experience a sustained imbalance.
• Traditionally, the stability problem has been one of maintaining synchronous operations of the generators (rotor angle stability).
• Instability may also be encountered without the loss of synchronism, for example:
– System consisting of synchronous generator feeding a induction motor through a transmission line can become unstable because of the collapse of the load voltage. 

  Power System Stability Problem :

  • Concerned with the behavior of the synchronous machines after disturbance
– System is stable when all machines remain in synchronism (operating in parallel at same speed) at a new steady state; as excessive oscillating transients fluctuates power flow on lines results in line outage
• Stability of power system depends on:
– Operating condition
– Type of disturbance
– Line loading
– Nature of impact
• Need to study the disturbance which can result in instability
 

Design and Operating Criteria for Stability :
 

  • The normal design contingencies include the following:
– Three-phase fault on any generator, transmission circuit, transformer or bus section, with normal fault clearing.
– Simultaneous phase-to-ground faults on different phase of each of two adjacent transmission circuit on a multiple-circuit tower, cleared in normal time.
– A permanent phase-to-ground fault on any transmission circuit, transformer or bus section with delayed clearing because of malfunction of circuit breakers, relay or signal channel.
– Loss of any element without a fault
– A permanent phase-to-ground fault on a circuit breaker, cleared in normal time.
• The criteria requires that the stability of the bulk power system is maintained during and after the most severe of the contingencies specified above
 
 

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