Menu
Search
Power Plant
Power Plant

Basic Concepts

Power system stability is the ability of the system, for a given initial operating condition, to regain a normal state of equilibrium after being subjected to a disturbance.

Stability is a condition of equilibrium between opposing forces; instability results when a disturbance leads to a sustained imbalance between the opposing forces.

The power system is a highly nonlinear system that operates in a constantly changing environment; loads, generator outputs, topology, and key operating parameters change continually.

When subjected to a transient disturbance, the stability of the system depends on the nature of the disturbance as well as the initial operating condition. The disturbance may be small or large. Small disturbances in the form of load changes occur continually, and the system adjusts to the changing conditions. The system must be able to operate satisfactorily under these conditions and successfully meet the load demand. It must also be able to survive numerous disturbances of a severe nature, such as a short-circuit on a transmission line or loss of a large generator.

Following a transient disturbance, if the power system is stable, it will reach a new equilibrium state with practically the entire system intact; the actions of automatic controls and possibly human operators will eventually restore the system to normal state. On the other hand, if the system is unstable, it will result in a run-away or run-down situation; for example, a progressive increase in angular separation of generator rotors, or a progressive decrease in bus voltages.

An unstable system condition could lead to cascading outages and a shut-down of a major portion of the power system.

The response of the power system to a disturbance may involve much of the equipment. For instance, a fault on a critical element followed by its isolation by protective relays will cause variations in power flows, network bus voltages, and machine rotor speeds; the voltage variations will actuate both generator and transmission network voltage regulators; the generator speed variations will actuate prime mover governors; and the voltage and frequency variations will affect the system loads to varying degrees depending on their individual characteristics.

Further, devices used to protect individual equipment may respond to variations in system variables and thereby affect the power system performance. A typical modern power system is thus a very high-order multivariable process whose dynamic performance is influenced by a wide array of devices with different response rates and characteristics. Hence, instability in a power system may occur in many different ways depending on the system topology, operating mode, and the form of the disturbance.

Traditionally, the stability problem has been one of maintaining synchronous operation. Since power systems rely on synchronous machines for generation of electrical power, a necessary condition for satisfactory system operation is that all synchronous machines remain in synchronism or, colloquially, “in step.”

This aspect of stability is influenced by the dynamics of generator rotor angles and power-angle relationships. Instability may also be encountered without the loss of synchronism. For example, a system consisting of a generator feeding an induction motor can become unstable due to collapse of load voltage. In this instance, it is the stability and control of voltage that is the issue, rather than the maintenance of synchronism. This type of instability can also occur in the case of loads covering an extensive area in a
large system.

In the event of a significant load/generation mismatch, generator and prime mover controls become important, as well as system controls and special protections. If not properly coordinated, it is possible for the system frequency to become unstable, and generating units and/or loads may ultimately be tripped possibly leading to a system blackout. This is another case where units may remain in synchronism (until tripped by such protections as under-frequency), but the system becomes unstable.

Because of the high dimensionality and complexity of stability problems, it is essential to make simplifying assumptions and to analyze specific types of problems using the right degree of detail of system representation. The following subsection describes the classification of power system stability into different categories.

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

12 Comments


  1. Shyam Sundar BR
    Jun 12, 2015

    Could i get a good technical article on understanding of Inertia constant, GD square, axial and radial loads, short circuit ratio and time constant etc., of a synchronous generator (preferably hydro generators)

  2. […] enduse equipment cannot tolerate even a momentary power outage and, further, even relatively minor disturbances in the power system can cause computer systems to re-boot, causing operational […]

  3. […] infrequent, circuit breakers occasionally fail to trip, or fail to clear a fault. Depending on the power system network topology other circuit breakers must then be called upon to trip and isolate the sources contributing to the […]

  4. […] = 60; Process flow diagram 1. PurposeThe purpose of this article is to establish the design basis electrical system Front End Engineering Design (FEED) and data design or standards uses in the gathering stations […]

  5. […] the extra time is not significant in terms of overall tripping time and possible effects of power system stability. Operation of RelayDigital relay consists of:Analogue input subsystem,Digital input […]

  6. […] Flexible AC Transmission System (FACTS) have been evolving to a mature technology with high power rating. This technology has wide spread application, became a top rate, most reliable one, based on […]

  7. […] transient reactance’’ model. Loads were represented as constant impedances.Improvements in system stability came about by way of faster fault clearing and fast acting excitation systems. Steady-state […]

  8. […] and audit trails can also be replicated.TopSecure NetworkingNetwork Encryption and Controlled Topology. To protect your data assets, iFIX offers a high degree of network security with a proprietary set […]

  9. […] electromagnetic poles, torque being transmitted magnetically across the ‘‘air gap’’ power angle, lagging in generators and leading in motors.Synchronous machine sizes range from fractional watts, […]

  10. […] by back feeding the city’s power generators, and blacked out the city. The city had a backup generator and company officials denied Tesla further access to their feed if he did not repair the […]

  11. […] and  symbols on the nameplate.  These marks show the connections where  the input and output voltages (and currents) have the same  instantaneous polarity.Figure 1 – Polarity Illustrated | Test […]

  12. […] of the new generation.Occasional tripping of these units is feasible and can become an attractive stability control in the future. Most generator tripping controls are event-based (based on outage of generating […]

Leave a Comment

Tell us what you're thinking... we care about your opinion!


Get PDF