the power flow problem scott norr for ee 4501 april, 2015
TRANSCRIPT
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The Power Flow Problem
Scott NorrFor
EE 4501April, 2015
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Electric Concepts
• Ohm’s Law: V = IR (V = IZ)• Kirchoff: KCL: ∑ i = 0 (at any node)
∑ V = 0 (on closed path)• Power: P = VI (S = VI*)
= V2/R = I2R• NOTE: i means electric current, j2 = -1
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Previously: DC Circuits (RI=V)
Resistance Matrix
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AC Circuits: Phasor AnalysisZI = V (Thanks to Euler, Steinmetz)
Impedance Matrix
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DC Power:All electrical systems naturally seek an equilibrium point of lowest entropy
Important to recognize that P ᴕ V2
(try to find the proportional
symbol in powerpoint
sometime…..)
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AC Power
• The complex power, S = VI* = P + jQ• P is the average power (“real” power) in watts,
attributable to resistive loads • Q is the reactive power (“imaginary” power) in
VAr, attributable to capacitive and inductive loads
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The Power Problem:
• On AC power systems, we don’t pre-determine the phase angles on the sources, they are determined by the system (additional unknowns to solve for!)
• Power is injected into nodes in the system via sources and is removed at nodes via loads (consumption points)
• Additionally, power is lost in the network
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Consider an Example: 3 Node System
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Unknowns:
• At each bus (node) there are 4 parameters: P, Q, V and Ө
• There are three types of buses:– Load Buses: P, Q are known, V, Ө are unknown– Generator Buses: P, V are known, Q, Ө are unknown– Slack Bus: (unique) V, Ө are known, P, Q unknown(this special generator node is allowed to accumulate errors in the iterative solution of the system of equations)
So, for N nodes, 2N unknown node parameters
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Balancing Power at Each Node:• ∑Si = o• SG-SL = Vi∑Ip*
• SG-SL = Vi∑Vp*Yp*
• Can separate the real (P) from the
imaginary (Q)to form two equations at each
Bus
• A system of 2N equations
• Sparse, largely diagonalized matrices
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Solve for Node Voltages and Angles:
• Vi new = (1/Yii)(Si/Viold - ∑Vp*Yp*)
• An iterative process, involving an initial starting estimate and convergence to a pre-determined tolerance.
• This is called the Gauss-Seidel Solution Method
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A better Method:
• For analytic, complex differentiable systems, can compute the low order terms of the Taylor series and solve using Netwon’s method.
• In two variables, an iterative approach:f1(x,y) = K = f(xo + Δxo, yo + Δyo)g1(x,y) = L = g(xo + Δxo, yo + Δyo)
Computing the Taylor Series, and truncating ityields an equation exploiting a Jacobian matrix
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Newton – Raphson Solution:
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Conclusions:• Powerflow Software is used by every electric
utility in the world. Many models contain 10,000 nodes or more.
• There are quite a few solution techniques that are more efficient than the G-S and N-R methods outlined here:
Fast-Decoupled N-R – decouples P,O from Q,V and solves the two, smaller systemsInterior Point Newton - calculates a Hessian Mtx!
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PowerWorld Simulator
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References:
• Stevenson, William D., Elements of Power System Analysis, McGraw-Hill, 1982
• Tylavsky, Daniel, Lecture Notes #19, EEE 574, Arizona State University, 1999
• PowerWorld Simulator, www.powerworld.com,2014