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الموضوع: stability

  1. #1
    Junior Engineer
    تاريخ التسجيل
    Jun 2006
    الدولة
    cairo
    المشاركات
    11

    stability

    بسم الله الرحمن الرحيم


    Power System Stability

    In radial Power system, large amount of power are transmitted from low cost power stations to major load centers over long transmission lines. Power systems with these features are commonly found in geographically broad countries with low population density. In these systems, stability problems are a major issue due to the fact that system disturbances on the interconnecting lines easily lead to loss of synchronism or sustained oscillations.

    Radial type systems are also rather susceptible of voltage stability problems, especially in a liberalized electricity market environment on which systems are being operated under increasingly stressed condition.

    Dynamic security assessment, i.e., the evaluation of the power system ability to withstand a set of severe disturbances , surviving the transient perturbation to recover acceptable steady-state conditions, is an essential task for remains dynamically secure , preventive and/or corrective remedial actions must be designed. Preventive actions are applied in the pre-contingency system, so that after any “credible” contingency, the system remains stable. Examples of these types of actions include restrictions on interface flows, generation active power re-dispatch and voltage rescheduling. Typically, reducing the available transfer capability (ATC) of the system is the most common preventive action take by operators, thus forcing expensive energy to be produced near load centers and hence increasing operation costs for the whole systems.

    What limits the loading capability?
    Three things that limit the loading capability of a line:
     Thermal
    • Thermal capability of an over head line is a function of the ambient temperature, wind conditions, condition of the conductor, and ground clearance.
    • The nominal rating of a line is generally decided on a conservative basis
    • Traditionally, increasing the loading capability could mean:
    o Upgrading the ratings of the transformer and other equipment as well.
    o Upgrading a line by changing the conductor to that of a higher current rating.
    o Converting a single circuit to a double circuit line.
     Dielectric
    • From an insulation point of view, many lines are designed very conservatively.
    • For a given nominal voltage rating, it is often possible to increase normal operation by 10% voltage.
    o Care is then needed to ensure dynamic and transient overvoltages are within limits.
     Stability
    • There are a number of stability issues that limit the transmission capability
    o Transient stability
    o Dynamic stability
    o Steady state stability
    o Frequency collapse
    o Voltage collapse
    o Subsynchronous resonance.


    Stability Limits

    Power system stability is a term applied to alternating current electric power systems, denoting a condition in which the various synchronous machines of the system remain in synchronism, or "in step" with each other. Conversely, instability denotes a condition involving loss of synchronism, or falling "out of step".

    Power systems are designed and operated so that they can survive large disturbances like storms, lightning strikes and equipment failures. This usually means that even though some power system equipment will be separated or isolated as a result of automatic protection and control actions, power supply to customers will not be disrupted or at least, that any such disruptions will be very localized.

    Operational limits for the transmission network can then be set using the following logic:
    • The maximum power each transmission line can transmit is limited by its current Carrying capacity, known as its thermal limit
    • The maximum loading of the transmission network is determined by any one of the lines hitting its thermal limit. So various combinations of generator and load injections at the nodes can produce this limiting condition. The SCADA continually checks for such thermal limit violations as the operating condition changes over time.
    • However, operating the system at such a limiting condition is not prudent because the loss of a transmission line or other equipment would probably overload some other lines.
    Thus the operating limit is not when the first line hits its thermal limit but when the loss of any one piece of equipment will make a line hit its thermal limit. This is known as the N-1 criterion for operation because any one of the N components of the power system can be lost without overloading any part of the system.

    • If a disturbance or short-circuit occurs, the first line of protection should isolate the faulted equipment. This is the rationale behind the N-1 criterion. It is possible; however, that the same disturbance may cause instability in which case more than just the faulted equipment will be lost. In such a case the maximum loading of the system will have to be lowered so that instability does not occur. This loading limit is thus set by the stability criterion rather than the thermal loading.
    • If better controls can increase this stability limit, then the system can be loaded at a higher level. This provides better utilization of the transmission network.

    We can see that reliability and transfer capability are limited by stability constraints. Maintaining system stability in a market environment will present new challenges, as power systems are operated with more uncertainty and less conservatism than in the past. If stability problems are accurately identified and properly mitigated, significant economic gains can be realized.
    it is important to distinguish between transient stability, voltage stability and steady-state stability. Transient stability, according to NERC, is "the ability of an electric system to maintain synchronism between its parts when subjected to a disturbance of specified severity and to regain a state of equilibrium following that disturbance." The disturbance" is usually a short circuit.
    The transient stability limit (TSL) is the value, in MW, below which the system would be able to regain equilibrium after the occurrence of any power system disturbance, regardless of size or location. To determine the TSL, the analysis should evaluate all disturbances for a succession of increased MW levels and reduced voltages until at least one disturbance has caused transient instability.
    Voltage stability can be considered a special case of steady-state stability. The sensitivities of system load to voltage will influence voltage stability and may cause a collapse of voltage in a load area caused by a small increase in power transfer. Voltage stability procedures are capable of finding the point of voltage collapse at individual buses by making certain assumptions about the nature of the load - but the process needs to be repeated to evaluate as many buses as possible or, at least, a minimum set of buses known a priori to be critical.
    Voltage stability describes the situation when the next increment of load causes a voltage collaps; this voltage reduction is generally a slow decay, occuring over time periods ranging from many seconds to minutes.
    In voltage stability Operator actions will be: watching voltage compared to experience-based criteria and redispatching or restricting power flow on highly stresses lines, adding series compensation to key lines, adding more voltage strengthening equipment, such as generators.
    Steady-state stability aims at computing the Steady-State Stability Limit (SSSL), which is the amount of MW (internal generation plus imports) such that, for any loading smaller than SSSL, the system is stable in the sense of small signal stability.
    Small signal stability refers to an occurrence of growing oscillations of system parameters during high power-transfer levels. These oscillations, typically in the 0.2- to 2-Hz range, can lead to loss of synchronism, or to line tripping and cascading outages caused by large power swings.
    Simply stated, below certain MW levels, the system is stable both during normal operation and in the presence of a disturbance, regardless how large. At higher load levels, certain large disturbances might cause instability - this is called transient instability. At even higher loadings, the system might still be operating, but a large disturbance is not required for a problem to occur. The smallest load change or voltage reduction may result in instability. This is called steady-state instability or voltage collapse, and is apparently what caused the Aug. 14, 2003, blackout.
    Transient Impact and Security Margin
    The TSL is an elusive target. It does exist, although for practical purposes, it cannot be computed exactly. However, analysis and practical experience suggests SSSL and TSL are interrelated. They change in the same direction: If SSSL is high, TSL is also high, and vice versa. For a given set of relay settings, TSL depends on the same factors that affect SSSL, including topology and voltage levels. It is not known whether a mathematical formula relating TSL and SSSL has been or can be found, but the TSL/SSSL ratio can be approximated empirically. In other words, it is possible to determine a safe MW system loading, referred to as security margin, such that for any state with a steady-state stability reserve smaller than this value, no contingency - no matter how severe - would cause transient instability. The security margin is expressed as a percentage of the SSSL.

    Improvement of steady state stability limit:
    The electric power generation-transmission-distribution grid constitutes a large system that exhibits a range of dynamic phenomena. Stability of this system needs to be maintained even when subjected to large low-probability disturbances so that the electricity can be supplied to consumers with high reliability. Various control methods and controllers have been developed over time that has been used for this purpose. New technologies, however, in the area of communications and power electronics, have raised the possibility of developing much faster and more wide-area stability control that can allow safe operation of the grid closer to its limits.

    Some of the technologies that improve the network stability are familiar to the industry, some are in design stages prior to field application, and some are still in the laboratories.

    The following discuss some of these technologies:
     Phase angle regulators
    The ability to transfer power between points over a line is limited by the constant physical, operating characteristics of the line but also by other variable characteristics. A phase angle regulator is an apparatus which is capable of changing the variable operating characteristics, but generally only within certain narrow range (for example, reducing the actual operating limits by a narrow range of (35-45)% below the physical operating limits).
    Phase angle regulators are installed in several locations and are used to control flows over both major paths and over minor paths. Such systems have also been used extensively in special situations where they provide a safe means of regulating the power flow on the underground, high voltage cable system where failure to control voltage could potentially be catastrophic. However, use of phase angle regulators, based on present-day technologies, has been somewhat limited because of their relatively large sizes, limited ability to control flows, and significant cost.

    Thyristor controlled phase angle regulator can theoretically achieve quick phase angle changes or the system but do not provide any advantages over the conventional tap changers for steady state power flows. They are not presently in commercial use and their relative cost can be higher than most of the FACTS devices of the same rating.

     Series Capacitors
    While the use of series compensation on transmission lines is not new, several technologies advances have made the economics more attractive and increased the range of beneficial applications. As a result, the use of this technology has been receiving increased interest in the search to increase the load carrying capacity of the existing transmission network. In general, as he reactance of the circuit decrease, the ability to transfer power between two points increase. Some of the improvements n the new capacitor technology that have made series capacitor more attractive are reduced losses, reduced weight and decreased space requirements. Improved performance and higher reliability have resulted fro improvements in efficiency, over-voltage protection and optical fiber technology based on microelectronics and digital technology.
    The application of series capacitors to increase transmission transfer capacity, however, is not without risk. One of the most common problems is known as subsynchronous resonance (SSR) which is attributed to dynamic energy exchanges between the series compensated system and the rotating shaft of turbine generators located close to such series capacitor installations. Without proper studies and appropriate safeguards, SSR can amplify the torsional response of the turbine generator shaft, and increase torsional stresses, which can result in shaft fatigue and failure. Improvements in providing such protections have been made since the catastrophic failure which occurred at the Mohave Generating Station, but extreme care in the design and continuing vigilance after installation are required in order to avoid such problems.
    Thyristor-Controlled Series Compensation (TCSC) is the “third generation” series capacitor application technology. Thyristor (silicon controlled rectifiers) are used to switch series capacitance into a transmission line. It is a powerful tool that can aid in relieving transmission loading that is limited by system stability or transient stability of generation. TCSCs have a high speed switching capability that provides a mechanism for controlling line power flow, readjusting power flow in response to various contingencies, and regulating steady-state power flow within its rating limits. The TCSC also has the ability to mitigate Subsynchronous resonance problems at thermal generator that are closely sited to transmission lines with series compensation.

     Static Compensator(STATCOM)
    The static compensator is another technology that has gone through some evolution. Its predecessor, the static var compensator (SVC), has been widely used by utilities since the mid-1970’s and is capable of providing voltage support during normal operation, or during times when the system is experiencing stress. It is a useful device that has found wide application in the power industry but has no ability to control active power flow.
    The static compensator is an improvement over the static var compensator because it can continue to deliver reactive power when the voltage is depressed. The SVC’s capability to deliver reactive power (and hence support voltage) deceases with the square of the voltage. The STATCOM has the capability to deliver reactive power (and hence support voltage) independent of the system voltage. By supporting the grid voltage, more power is allowed to flow through the transmission circuit, thereby increasing its utilization.
    Other technologies that could be used in conjunction with the STATCOM are battery energy storage (BES) and the superconducting magnetic energy storage (SMES) system. Both systems store direct current electrical energy; BES in batteries and SMES in an electromagnetic field. Neither have any moving parts and once in the battery or solenoid, energy can be stored indefinitely. When needed, it can be extracted for use almost instantaneously. A static condenser equipped with an either a BES or SMES system will provide voltage support while delivering energy for a short time. BES has been commercially available for some time while both the STATCOM and the SMES system are in the experimental stage. Prototypes have been built and are being tested in real utility settings. However, because of cost, their acceptance will be limited in the near future.
     Unified Power Flow Controller
    There are three basic parameters that affect the transfer capability of electrical transmission networks, voltage, line impedance, and phase angle. To increase transfer capability voltage can be increased, line impedance reduced, or phase angle increased. In the series capacitor application, line reactance (and therefore, line impedance) is reduced. In the application of a unified power flow controller (UPFC), we change both capacitance and line impedance, and indirectly, voltage.

  2. #2
    عضو فى رابطة مهندسى الكهرباء العرب الصورة الرمزية محمد الفيومي
    تاريخ التسجيل
    Jul 2006
    الدولة
    Amman
    المشاركات
    438

    رد: stability

    السلام عليكم ورحمة الله وبركاته:
    شكراً لك اخي الكريم

  3. #3
    V.I.P Member الصورة الرمزية dyiaeldeen
    تاريخ التسجيل
    Dec 2006
    الدولة
    مصر
    المشاركات
    1,795

    رد: stability

    شكراً لك اخي الكريم

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  1. TR STABILITY
    بواسطة Eng. Ahmed el mawee في المنتدى قسم وقاية نظم القوى الكهربائية Power System Protection
    مشاركات: 5
    آخر مشاركة: 04-07-10, 01:01 AM
  2. How can I do stability for this line
    بواسطة saud subaie في المنتدى قسم وقاية نظم القوى الكهربائية Power System Protection
    مشاركات: 1
    آخر مشاركة: 13-01-07, 01:06 AM
  3. how can we do the stability test
    بواسطة هانى بسيونى في المنتدى قسم وقاية نظم القوى الكهربائية Power System Protection
    مشاركات: 2
    آخر مشاركة: 24-09-06, 04:55 PM

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