Dedicated AC Power Flow Solver

[jchen975]

Hi, I just realized this problem, but when I run a solved Matpower case, it takes the same number of iterations as it would with default values. In Matpower,

results = runpf('caseX') 
savecase('caseX', results)
runpf('caseX')

should result in iterations: 1 in the end. When running with default/solved values for case118, and case300, it takes 4 and 5 iterations respectively. I tried both with the case saved from Matpower and with manually updating the vm, va fields of all buses from the solutions dict, and got the same results.

Any idea why this is happening? I know Matpower uses NR for acpf by default, so I'm assuming it has something to do with Ipopt. I'd appreciate any clarifications on this.

[ccoffrin]

This is a good question and there are quite a few details to get the most out of a warm start in PowerModels. At a high level these are the key points,

1. you have to provide special values in the data model for providing an initial starting point. In the case of the ACPPowerModel these are pg_start, qg_start, vm_start, va_start, pf_start, pt_start, qf_start, qt_start (note that the ACPPowerModel adds explicit flow variables to the branches). <var name>_start is the pattern for all variables.
2. After setting these values you then need to set Ipopt's parameters carefully. This includes setting smaller values for mu_init, bound_frac and bound_push. For very large models you might also try hessian_approximation to reduce the time for derivative computations.

I hope that helps. I'll also note that PowerModels was originally designed for solving OPF without a good guess for a starting point. Due to that focus, doing very fast PF solves is not super easy in PowerModels. In the future I plan to add some dedicated AC-PF solvers that assume a good starting point and are much faster and simpler to use for that use case.

[ccoffrin]

@jchen975, can we close this issue?

[jchen975]

Thank you for your answer and sorry for taking so long to get back to you, I was on vacation and didn't have access to internet until now. If you don't mind, I'll take some time tomorrow to figure out how to do warm-start ACPF properly and close the issue then.

One last thing and please correct me if I'm wrong, currently run_ac_pf() is designed for cold-start ACPF (e.g. vary some PD, QD values of a Matpower case and run ACPF with the default vm, va values), and the summary information printed from Ipopt would be the same from Matpower, right? It's just the warm-start ACPF that I need to set up myself.

[ccoffrin]

No problem. run_ac_pf solves the feasibility problem that is defined here, which I would characterize as a fairly standard power flow formulation. It does not attempt to change pd or qd values. For a formulation that will consider pd/qd changes I would refer you to the PowerModelsMLD package.

The equations implemented by PowerModels are identical to Matpower when reading matpower case files (as of Matpower v6), so for an identical problem specification they should be the same. You can try the calc_branch_flow_ac and calc_power_balance functions to verify this using a solution that was generated by Matpower.

[jchen975]

Thanks again for your time! I took a look at the Ipopt documentation and adjusted those parameters, but I couldn't really find anything on <var>_start for ACPPowerModel. Any hints?

[ccoffrin]

So far there is no documentation on setting the starting point in PowerModels. I am still exploring how to best initialize Ipopt.

You can have a look in this file to see the names of the parameter you need to add in the network data to specify the starting value for each variable.

[jchen975]

Hi Dr. Coffrin, regarding your previous comment on your plan to add an ACPF solver that accommodates warm-start conditions, has there been any updates? If that's not a short-term priority, I was actually thinking about creating a new package that only solves the PF problem, by rewriting the Matpower runpf related functions in Julia, sometime early next year (I'm finishing my undergrad this coming April so I probably won't be able to finish it while I'm still in school). Unfortunately I eventually switched back to Matpower to continue with my project, but I'd like to get your opinions on this. Thanks!

[ccoffrin]

@jchen975 there are not updates per-se, it is still possible to warm-start Ipopt with the current version of PowerModels, although it is not well documented.

Please feel free you write your own power flow solver, I hope to implement something similar in PowerModels in the next 6-12 months, so I might draw some inspiration from your implementation.

[ccoffrin]

a native DC power flow solver is not available. The AdmittanceMatrix data structure as been added to help support this feature.

[timueh]

Hi @jchen975, is this still an open issue? I'd be interested in a dedicated power flow solver in Julia that uses PowerModels.jl. A first step might be to set up the system of nonlinear equations, and then use it with one of Julia's Newton-type solvers. Then, one could go about implementing decoupled power flow etc..

Any thoughts?