[Couenne] a non-convex problem

[Pietro Belotti]

Mahbube,

>  1. Can the limitaions on NEOS server be the reason of the infeasible 
> solution?

In general, no. The problem might be claimed infeasible by Couenne due to 
some numerical instability.

>  2. Does reducing the size of the problem by ommitting some buses and 
> lines can help to get a feasible solution?

It will reduce the time taken to reformulate and solve it, and yes, it 
may also reduce the possible numerical instabilities.

Hope this helps.
Pietro

--
Pietro Belotti
Dept. of Mathematical Sciences
Clemson University
email: pbelott at clemson.edu
phone: 864-656-6765
web:   myweb.clemson.edu/~pbelott

On Fri, 24 Jun 2011, mahbube zeraatzade wrote:

> Hi, I have been working on AC Optimal Power Flow for a while. The AC OPF is naturally a non-convex problem. The AMPL file that I have written works perfectly with all of the
> Mixed Integer Non-Linear solvers on the NEOS site. IEEE 5-Bus and 14-Bus are used to check the results.
> The referees of the Journal that I had sent my paper, believed that these test systems are small and the methodology should be checked on a real system. Therefore I am testing
> the real power system of a country with 1388 buses and 1212 lines.
> I have entered a Snap Shot of the network as the starting point and have run the program with all the possible assumptions but can't get any feasible solution.
> When I use Couenne as the solver, I get the following results:
> ****************************************************************************** 
> Job 2379178 has finished.
> Job 2379178 sent to e032.chtc.wisc.edu
> password: tYaAJeBv
> ---------- Begin Solver Output -----------
> Executing /opt/neos/Drivers/couenne-ampl/couenne-driver.py
> File exists
> You are using the solver couenne.
> Executing AMPL.
> processing data.
> processing commands.
> 
> Presolve eliminates 4268 constraints and 9191 variables.
> Adjusted problem:
> Error: Error at _cmdno 3 executing "solve" command
> 3913 variables:
> Error: (file amplin, line 2727, offset 199919):
> 3329 nonlinear variables
> Error: can't open /tmp/neos-29505/at29512.sol
> 584 linear variables
> 4547 constraints; 21611 nonzeros
> 4543 nonlinear constraints
> 4 linear constraints
> 1 linear objective; 584 nonzeros.
> 
> couenne:
> ANALYSIS TEST: Problem size before reformulation: 3913 variables (0 integer), 4547 constraints.
> Problem size after  reformulation: 26548 variables (0 integer), 3043 constraints.
> warning, tightened NLP is infeasible
> problem infeasible
> Error (512) in /opt/ampl/ampl -R amplin
> *******************************************************************************************
>  1. Can the limitaions on NEOS server be the reason of the infeasible solution?
>  2. Does reducing the size of the problem by ommitting some buses and lines can help to get a feasible solution?
> I thank you in advance for your help.
> Regards,
> Mahbube
>  
> 
>