The following warning messages are shown (the first parts about the continuos states always shows up, even when there are no initial states, maybe it's related but I have tried discrete ODE solvers also and the problem persisted).
_Warning: The model 'example' does not have continuous states, hence Simulink is using the solver 'FixedStepDiscrete' instead of solver 'ode3'. You can disable this diagnostic by explicitly specifying a discrete solver in the solver tab of the Configuration Parameters dialog, or by setting the 'Automatic solver parameter selection' diagnostic to 'none' in the Diagnostics tab of the Configuration Parameters dialog > In C:\Program Files\MATLAB\R2012b\toolbox\physmod\powersys\powersys\private\power_init_pr.p>power_init_pr at 125 In power_init at 68 In C:\Program Files\MATLAB\R2012b\toolbox\physmod\powersys\powersys\private\power_initstates_pr.p>EvaluateTheModel at 636 In C:\Program Files\MATLAB\R2012b\toolbox\physmod\powersys\powersys\private\power_initstates_pr.p>power_initstates_pr at 35 In power_initstates at 24 In C:\Program Files\MATLAB\R2012b\toolbox\physmod\powersys\powersys\powergui.p>InitialStatesSettingButton_Callback at 132 In C:\Program Files\MATLAB\R2012b\toolbox\physmod\powersys\powersys\powergui.p>powergui at 87 _