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Published byDamon Boyd Modified over 8 years ago
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A Power System Example Starrett Mini-Lecture #3
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Power System Equations Start with Newton again.... T = I We want to describe the motion of the rotating masses of the generators in the system
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The swing equation 2H d 2 = P acc o dt 2 P = T = d 2 /dt 2, acceleration is the second derivative of angular displacement w.r.t. time = d /dt, speed is the first derivative
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l Accelerating Power, P acc l P acc = P mech - P elec l Steady State => No acceleration l P acc = 0 => P mech = P elec
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Classical Generator Model l Generator connected to Infinite bus through 2 lossless transmission lines l E’ and x d ’ are constants is governed by the swing equation
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Simplifying the system... l Combine x d ’ & X L1 & X L2 l jX T = jx d ’ + jX L1 || jX L2 l The simplified system...
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Recall the power-angle curve P elec = E’ |V R | sin( ) X T
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Use power-angle curve l Determine steady state (SEP)
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Fault study l Pre-fault => system as given l Fault => Short circuit at infinite bus P elec = [E’(0)/ jX T ]sin( ) = 0 l Post-Fault => Open one transmission line yX T2 = x d ’ + X L2 > X T
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Power angle curves
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Graphical illustration of the fault study
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Equal Area Criterion 2H d 2 = P acc o dt 2 rearrange & multiply both sides by 2d /dt 2 d d 2 = o P acc d dt dt 2 H dt => d {d } 2 = o P acc d dt {dt } H dt
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Integrating, {d } 2 = o P acc d {dt} H dt For the system to be stable, must go through a maximum => d /dt must go through zero. Thus... m o P acc d = 0 = { d 2 l H { dt } o
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The equal area criterion... l For the total area to be zero, the positive part must equal the negative part. (A1 = A2) P acc d = A1 <= “Positive” Area P acc d = A2 <= “Negative” Area cl oo mm
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For the system to be stable for a given clearing angle , there must be sufficient area under the curve for A2 to “cover” A1.
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In-class Exercise... Draw a P- curve l For a clearing angle of 80 degrees yis the system stable? ywhat is the maximum angle? l For a clearing angle of 120 degrees yis the system stable? ywhat is the maximum angle?
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Clearing at 80 degrees
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Clearing at 120 degrees
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What would plots of vs. t look like for these 2 cases?
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