0. ECE 476 POWER SYSTEM ANALYSIS
Lecture 13 Newton-Raphson Power Flow
Professor Tom Overbye
Department of Electrical and Computer Engineering

1. Announcements Homework 6 is due on Thursday (Oct 18)
Be reading Chapter 6
Homework 7 is 6.12, 6.19, 6.22, 6.45 and Due date is October 25

2. Design Project
Design Project 2 from the book is assigned today (page 345 to 348). It will be due Nov 29. For a transmission tower configuration assume a symmetrical tower configuration with individual conductor spacings as provided on the handout sheet.

3. Newton-Raphson Algorithm
The second major power flow solution method is the Newton-Raphson algorithm
Key idea behind Newton-Raphson is to use sequential linearization

4. Newton-Raphson Method (scalar)

5. Newton-Raphson Method, cont’d

6. Newton-Raphson Example

7. Newton-Raphson Example, cont’d

8. Newton-Raphson Comments
When close to the solution the error decreases quite quickly -- method has quadratic convergence
f(x(v)) is known as the mismatch, which we would like to drive to zero
Stopping criteria is when f(x(v))  < 
Results are dependent upon the initial guess. What if we had guessed x(0) = 0, or x (0) = -1?
A solution’s region of attraction (ROA) is the set of initial guesses that converge to the particular solution. The ROA is often hard to determine

9. Multi-Variable Newton-Raphson

10. Multi-Variable Case, cont’d

11. Multi-Variable Case, cont’d

12. Jacobian Matrix

13. Multi-Variable N-R Procedure

14. Multi-Variable Example

15. Multi-variable Example, cont’d

16. Multi-variable Example, cont’d

17. NR Application to Power Flow

18. Real Power Balance Equations

19. Newton-Raphson Power Flow

20. Power Flow Variables

21. N-R Power Flow Solution

22. Power Flow Jacobian Matrix

23. Power Flow Jacobian Matrix, cont’d

24. Two Bus Newton-Raphson Example
For the two bus power system shown below, use the
Newton-Raphson power flow to determine the
voltage magnitude and angle at bus two. Assume
that bus one is the slack and SBase = 100 MVA.

25. Two Bus Example, cont’d

26. Two Bus Example, cont’d

27. Two Bus Example, First Iteration

28. Two Bus Example, Next Iterations

29. Two Bus Solved Values
Once the voltage angle and magnitude at bus 2 are
known we can calculate all the other system values,
such as the line flows and the generator reactive
power output

30. Two Bus Case Low Voltage Solution

31. Low Voltage Solution, cont'd

32. Two Bus Region of Convergence
Slide shows the region of convergence for different initial
guesses of bus 2 angle (x-axis) and magnitude (y-axis)
Red region
converges
to the high
voltage
solution,
while the
yellow region
to the low
solution

33. PV Buses
Since the voltage magnitude at PV buses is fixed there is no need to explicitly include these voltages in x or write the reactive power balance equations
the reactive power output of the generator varies to maintain the fixed terminal voltage (within limits)
optionally these variations/equations can be included by just writing the explicit voltage constraint for the generator bus |Vi | – Vi setpoint = 0

34. Three Bus PV Case Example

35. Modeling Voltage Dependent Load

36. Voltage Dependent Load Example

37. Voltage Dependent Load, cont'd

38. Voltage Dependent Load, cont'd
With constant impedance load the MW/Mvar load at
bus 2 varies with the square of the bus 2 voltage
magnitude. This if the voltage level is less than 1.0,
the load is lower than 200/100 MW/Mvar

39. Solving Large Power Systems
The most difficult computational task is inverting the Jacobian matrix
inverting a full matrix is an order n3 operation, meaning the amount of computation increases with the cube of the size size
this amount of computation can be decreased substantially by recognizing that since the Ybus is a sparse matrix, the Jacobian is also a sparse matrix
using sparse matrix methods results in a computational order of about n1.5.
this is a substantial savings when solving systems with tens of thousands of buses

40. Newton-Raphson Power Flow
Advantages
fast convergence as long as initial guess is close to solution
large region of convergence
Disadvantages
each iteration takes much longer than a Gauss-Seidel iteration
more complicated to code, particularly when implementing sparse matrix algorithms
Newton-Raphson algorithm is very common in power flow analysis