Wrong structural singular equation system

[Rüdiger Franke]

The current tunc release reports the following error for PowerSystems.Examples.Spot.Introduction.SimulationSteadyInitial:

Error: Initialization problem is structurally singular, error found sorting equations 
 78: $DER.lineB.i[3] = 0.0;
75: $DER.lineL.i[3] = 0.0;
14: $DER.lineR.i[3] + $DER.lineB.i[3] - $DER.lineL.i[3] = 0.0;
72: $DER.lineR.i[3] = 0.0;
 for variables 
 meter.i[1](63), voltageL.V(70), $DER.lineR.i[3](77), meter.i[3](61), transPh.y[2](72)

See
​https://test.openmodelica.org/libraries/PowerSystems/files/PowerSystems.Examples.Spot.Introduction.SimulationSteadyInitial.err

The solution to the reported equations appears obvious, even though it is for some reason overdetermined: all involved variables are zero. Why does OpenModelica report a structural singular equation system?

Note 1: the model was working until August 29, see:
​https://test.openmodelica.org/libraries/history/PowerSystems/PowerSystems-2016-08-29.html
The PowerSystems library changed around that time with the introduction of an enumeration for initialization options, replacing Booleans, see:
​https://github.com/modelica/PowerSystems/commit/7a8310bc7d36daee96538c4b0da4634d4a43343d

Note 2: the AC3ph models use the dq0 representation. The third coordinate is zero unless there is an unsymmetry between the three phases. Many models are symmetric by definition: sources assign 0.0 to variables of the third coordinate; component models pass the 0.0 through and never mix the third 0-coordinate with the d- or the q-coordinate.

Can't OpenModelica generally eliminate resulting equation systems with the rhs all zeros?

[Lennart Ochel]

This is related to an issue in the optimization module simplifyComplexFunction. As far as I can tell, this module should get removed anyway, because it is just a workaround for certain expressions that are handled wrongly in the Backend/SimCode phase. I assign this to Vitalij, because he developed that module, so probably he knows best how to fix this.

---

One can disable the module using following flag: --initOptModules-=simplifyComplexFunction.
Therewith, compilation works fine, but the initialization is not able to find a consistent solution:

Compilation output:

Notification: It was not possible to analyze the given system symbolically, because the relevant equations are part of an algebraic loop. This is not supported yet.
Warning: The initial conditions are over specified. Use +d=initialization for more information.
Warning: There are iteration variables with default zero start attribute. Use +d=initialization for more information.

Simulation output:

Simulation execution failed for model: PowerSystems.Examples.Spot.Introduction.SimulationSteadyInitial
LOG_INIT          | error   | The initialization problem is inconsistent due to the following equation: 0 != -10581.1 = voltageB.V * cos(voltageB.alpha0) - lineB.term_n.v[1]
stdout            | warning | Error in initialization. Storing results and exiting.
|                 | |       | Use -lv=LOG_INIT -w for more information.

[Lennart Ochel]

By the way, using the flag -d=initialization in addition will display the following message:

Notification: It was not possible to analyze the given system symbolically, because the relevant equations are part of an algebraic loop. This is not supported yet.
Warning: Assuming redundant initial conditions for the following 3 initial equations:
         lineR.omega[1] = 0.0
         lineL.omega[1] = 0.0
         lineB.term_n.v[1] = voltageB.V * cos(voltageB.alpha0)
[…]

[Vitalij Ruge]

Can't OpenModelica generally eliminate resulting equation systems with the rhs all zeros?

There no removeSimpleEquation for initilization. So Initilization don't find the equation
0 = 0.

[Martin Sjölund]

Ticket retargeted after milestone closed

[Martin Sjölund]

Milestone moved to 1.12.0 due to 1.11.0 already being released.

[Vitalij Ruge]

Unfortunately, I will not find time in the near future to fixed it.

[Francesco Casella]

Milestone moved to 1.13.0 due to 1.12.0 already being released.

[Francesco Casella]

Rescheduled to 1.14.0 after 1.13.0 releasee

[Francesco Casella]

Releasing 1.14.0 which is stable and has many improvements w.r.t. 1.13.2. This issue is rescheduled to 1.16.0

[Francesco Casella]

Retargeted to 1.17.0 after 1.16.0 release

[Francesco Casella]

Retargeted to 1.18.0 because of 1.17.0 timed release.

[Francesco Casella]

Ticket retargeted after milestone closed