id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
6196	Undetected high-index DAE or incorrectly handled overdetermined initialization in Buildings model (was: Bogus linear tearing in Buildings model)	Francesco Casella	Karim Adbdelhak	"Please check [https://libraries.openmodelica.org/branches/master/Buildings_7.0.0/files/Buildings_7.0.0_Buildings.Electrical.AC.ThreePhasesUnbalanced.Validation.IEEETests.Test4NodesFeeder.BalancedStepDown.YD.sim Buildings.Electrical.AC.ThreePhasesUnbalanced.Validation.IEEETests.Test4NodesFeeder.BalancedStepDown.YD]. The model fails during lambda0 initialization with
{{{
Failed to solve linear system of equations (no. 569) at time 0.000000, system is singular for U[2, 2].
Matrix U
        -0 
Output vector x
[ 1]                    0
The default linear solver fails, the fallback solver with total pivoting is started at time 0.000000. That might raise performance issues, for more information use -lv LOG_LS.
}}}
The debugger shows that system 569 is torn in this way:
{{{
linear (torn), unknowns: 3, iteration variables: 1 (transformer.conv2.terminal_p.i[1])
(torn) transformer.conv3.terminal_p.i[1] := transformer.conv2.terminal_p.i[1] - line2.i3[1]
(torn) transformer.conv1.terminal_p.i[1] := transformer.conv2.terminal_p.i[1] + line2.i2[1]
(residual) line2.i1[1] + transformer.conv1.terminal_p.i[1] - transformer.conv3.terminal_p.i[1] = 0
}}}
It is immediate to see that the term {{{transformer.conv2.terminal_p.i[1]}}} appears both in {{{transformer.conv3.terminal_p.i[1]}}} and in {{{transformer.conv1.terminal_p.i[1]}}}, so when those two are subtracted to compute the residual, its sensitivity w.r.t. its unknown is zero.

I don't know what criterion is used to perform linear tearing, but it's obviously not a good one. Linear systems should be solved with proper pivoting; solving some equations according only to structural information but disregarding the coefficient values can be very dangerous.

Until this issue is solved, I would recommend to disable linear tearing by default. This will sure enough solve a lot of problems in the testsuite. BTW, now that we have sparse solvers to deal with large linear systems, the usefulness of linear tearing is probably quite limited.
"	defect	assigned	blocker	1.19.0	Backend				Karim Adbdelhak Michael Wetter