id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
5049	Numerical issues close to steady-state in ODE mode	Francesco Casella	Willi Braun	"Please check the {{{PowerGrids.IPSLValidation.Examples.SmallCase.OpenLoopLoadStepPowerGrids}}} model. One can monitor the status of the simulation by looking at {{{gen_pwGenerator4W__GEN______SM.CePu}}} which is the electrical torque in per unit. The model starts in steady-state, then a step change is applied to the load at time = 1. What should ensue is a damped oscillation which eventually dies out to a new steady state. 

The simulation currently runs fine because it uses {{{--daeMode=new}}}. However, if the corresponding vendor annotation is removed and the standard ODE mode integration is used, a weird thing happens: at time = 21.31 the {{{CePu}}} variable suddenly drops to zero for a short time, then it does so again around time = 23.8, and eventually a new transient similar to the first is actually triggered (see dassl.png attachment). This makes absolutely no sense, as the system should stay at steady state indefinitely for time > 20.

If I change the solver from DASSL to IDA, the result is different but qualitatively similar, see ida.png.

I guess for some reason the causalized code is numerically unstable close to steady state, and thus tricks the ODE solver in some unpredictable way.

The root cause of this behaviour should be identified and the issue removed before the 1.13.0 release, because it may also affect other models belonging to different domains."	defect	closed	blocker	1.13.0	Run-time		fixed		marianne.saugier@… adrien.guironnet@… Andrea Bartolini