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Transient Stability

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  • FAST TRANSIENT STABILITY SOLUTIONS

    H. W. Dommel N. Sato

    Bonneville Power AdministrationPortland, Oregon

    ABSTRACT

    Techniques are described for improving the speed of largetransient stability studies without sacrificing accuracy. A fast iterativemethod for solving the algebraic network equations, including theeffect of generator saliency, is explained. A new technique for solvingthe differential equations with the implicit trapezoidal rule ofintegration is introduced. These two techniques can be combined intoone simultaneous solution, thereby eliminating the problem ofinterface error between the differential and algebraic equationsolutions of the traditional approach.

    INTRODUCTION

    Transient stability studies now require considerable computertime, especially for solving large systems. This has motivated a searchfor faster solution techniques. Two possibilities for improving thespeed of transient stability studies are: (a) Reduction of the totalsystem to a smaller one, which could be solved faster, and(b) improvements in the numerical solution techniques.

    Reduction techniques produce equivalents which are normallyonly approximations. Therefore, in spite of recent progress in findingequivalents, the user must have a "feeling" for the problem whenequivalents are used. It should also be noted that solutions based onsparsity techniques become faster through reduction only to a certainpoint because of fill-in caused by the reduction processl. Judicioususe of equivalents can make stability solutions faster; however, thispaper is concerned with the second possibility, improved -numericalsolution techniques.

    Improvements in numerical solution techniques can speed up thesolution without placing any burden of judgment on the user. Thispaper describes experiments carried out at Bonneville Power Adminis-tration (BPA) over the last few years with the objective of obtainingfaster solutions for the steady-state equations as well as for thedifferential equations which arise in stability studies. Speeding up thesolution of the steady-state equations has been accomplished byapplying sparsity techniques and solution methods which requirefewer iterations. Speeding up the solution of the differentialequations has been achieved with the implicit trapezoidal rule ofintegration, which has already been used successfully for the solutionof switching transients at BPA2. This method is numerically stableand accurate enough. It should be noted that explicit techniques,including the Runge-Kutta methods, require higher accuracy primarilyto insure numerical stability. As a consequence, such techniques alsorequire longer solution times. It is expected that implementation ofthese ideas into BPA's production program will make the solutionabout five times faster.

    Brackets are used to indicate vector and matrix quantities.Parentheses usually indicate a functional relationship or the value fortime t. A super dot is used to indicate derivatives with respect to time.

    STRUCTURE OF TRANSIENTSTABILITY EQUATIONS

    The initial conditions for a stability study are determined by asteady-state power flow solution. Thereafter, two sets of equationsmust be solved simultaneously as a function of time, namely a system

    of steady-state equations (which describe the steady-state behavior ofthe network including steady-state models of loads and the algebraicequations of synchronous machines),

    gl(xl *.. wXn y,Yl* . .ym) = 0

    (1)gn(xl,-Xn,Yl,.. -Ym) = 0

    or [g([x],[y]) = 0

    and a system of differential equations (which describe the dynamicbehavior of the machines and their control circuits),

    ii = fi(xi') ...Xn'Yj I I IYMI t)......................... .............or[y] = [f([x], [y], t)1 (2)

    Ym =fm(Xl * xnyls....Ym- t)

    The structure of Eq. (1) will change at certain moments in time dueto fault initialization, fault clearing, line switching, etc. Such changes,which require re-solutions without advancing the time, producediscontinuities in the value of the vector [xl. No discontinuities canappear in [y].

    CLASSIFICATION OF SOLUTION METHODS

    On a digital computer, the values of [x ] and [yl are computed atdiscrete points in time, thereby producing a sequence of snapshotpictures at intervals At. The sampling rate 1/At determines the upperlimit of the frequencies which will be adequately represented in [x]and [y]. As a consequence of the discretization, interpolationassumptions must normally be made on [x] to define the statebetween discrete points. In most cases linear interpolation isadequate.

    The methods for solving Eqs. (1) and (2) simultaneously fallroughly into three categories: (a) Alternating solution of Eqs. (1)and (2), (b) elimination of [x], and (c) algebraization of Eq. (2) byimplicit integration. The distinction is not always clear. As anexample, the numerical solution of differential equations is in itselfbased on an algebraization process.

    (a) Alternating Solution Method

    This technique is probably used in most existing stabilityprograms. The algorithm is roughly as follows:

    1. Establish initial conditions by solving Eq. (1) at t = 0.

    2. Predict how [x] behaves over the next time interval by usingzero, first or second order extrapolation.

    3. Solve Eq. (2) for [y(t)] by any numerical technique, using thepolynomial obtained in step 2 as an interpolation formula.

    4. Solve Eq. (1) for [x(t)] by any of the well-known power flowsolution techniques.

    Paper T72 137-3, recommended and approved by the Power System Engineering Committee of the IEEE Power Engineering Society for presentation at the IEEE WinterMeeting, New York, N.Y., January 30-February 4, 1972. Manuscript submitted September 20, 1971; made available for printing November 19, 1971.

    1643

  • 5. Use the computed value [x(t)] to correct the prediction over theinterval from t-At to t.

    6. Solve Eq. (2) for [y(t)] by any numerical technique, using thepolynomial obtained in step 5 as an interpolation formula.

    7. Check differences of two successive solutions for [y(t)]. If thedifferences are negligible proceed to step 8. Otherwise go tostep 4.

    8. Advance by one time step and loop back to step 2.

    Several variations are possible. In the version described above, thesteady-state equations (1) must be solved once, most of the time,since the differences in step 7 are normally sufficiently small on thefirst pass. The prediction and correction process of the alternatingsolution method should not be confused with predictor-correctormethods for solving differential equations; if such methods are used,steps 3 and 6 would also involve another kind of prediction-correctionprocess in itself.

    It is essential to include the check of step 7 to avoid so-called"interface" errors. If this is not done, a careful assessment should bemade about the acceptability of the accumulated errors. With theinterface loop closed, the quality of the prediction (step 2) willdetermine how often the loop must be executed and, in effect, howlong the study will take, but it will not degrade the accuracy of thesolution because of corrections initiated in step 7. This is not true foropen-loop solutions, where the quality of the prediction will have adecisive influence on the accuracy.

    (b) Elimination of [xl

    This approach is used for small disturbance stability studies3. Forsmall variations [Ax], [Ay] around an operating point, Eq. (1) canbe linearized, F ag ]1 ag] Ayl = 0

    TaXi +J ~ayJ Lr-a-- r-ag

    with the Jacobian matrices LaxJ,La yj being evaluated at the givensolution point. Eq. (2) can also be linearized around the operatingpoint,

    [Ar] = [A][Ay] + [B][Ax]Both linear equations combined become

    IaxjIaFyJII[~I[B]

    and after elimination of [xI by triangularization,

    [Aireduced [A]Jor

    [Ay] = [A]reduced - [Ay] (3)The eigenvalues of [Aireduced give the damping (real part) andnatural frequencies (imaginary part) of all possible modes ofoscillations without having to make any assumption about the specific(small) disturbance. The system will be stable as long as the real partsof all eigenvalues are negative (first method of Lyapunov).

    (c) Algebraization by Implicit Integration

    The step-by-step solution of Eq. (2) is best expressed in integralform, t

    [y(t)] = [y(t-At)] + f [f([y],[x],?] dTt-At

    Implicit integration techniques use interpolation functions for theexpressions under the integral. Interpolation means that the functionsmust pass through the yet unknown points at time t, which musttherefore be expressed as variables. In general, the solution mayrequire iterations. If the differential equations are linear, however,then a direct solution becomes possible.

    Let the differential equations (2) be linear of the form[y] = [A][y] + [B][x] (4a)

    Most differential equations are linear in stability studies. There aresome nonlinearities, however, but their inclusion poses no seriousproblem, as showni later for the excitation system. Eq. (4a) can berewritten as a step-by-step integration,

    t ~~~~t[y(t)] = [y(t-At)] + [A] f [yldr + [B] f [x]dr

    t-At t-At(4b)

    The simplest implicit integration scheme is the trapezoidal rule ofintegration.* It is based on the assumption that [x] and [y] varylinearly over the interval from t-At to t. Then Eq. (4b) becomes

    [y(t)] =[y(t-At)] + A [A] [[y(t-At)I + [y(t)I]

    +At [B] [[x(t-t)I + [x(t)j

    which is simply a system of linear algebraic equations

    [[U]-2t[A] [y(t)]- 2 [B][x(t)] = [c(t-At)] (5a)The vector on the right-hand side is known from the values alreadycomputed at the preceding time step:

    [ c(t-/t) I =At [B][x(t-At)] + U] + 2 [A [ y(t-At) ] (5b)

    [U] denotes the unit or identity matrix. Eq. (5a) can now be solveddirectly with Eq. (1) as one complete system of steady-stateequations

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