Solver was unable to reduce the step size without violating the minimum step size value of 4.9717E-17 for 1 consecutive times at time 0.0139941.

I'm trying to simulate a test bench of a transcritical CO2 compressor in which is present also an expander in order to retrieve some power. The model is basically a transcritical heat pump with an expander replacing the valve. I need to evaluate the power recovered from the expander for different values of the maximum pressure of the thermodynamic cycle. The model runs for un upper pressure of 90 bar but doesn't work when i change this value for example to 95 bar. I think that the parametrization of the different blocks is correct and i've also tried decreasing the minimum step value but it doesn't seem to change during simulation. I've also tried increasing the number of consecutive minimum steps but the results is the same.

I attach the simscape model below.

Thanks for your help

Answer 1

[sai charan sampara]

Hello Luca,

The warning “Solver was unable to reduce the step size without violating the minimum step size value” can be caused due to several reasons. Some of them are high stiffness of the model, presence of algebraic loops, solver finding Differential Algebraic Equation (DAE) exceptions.

In your case during simulation at the time t=0.007638 the “Positive-Displacement Compressor(2P)” is showing a warning that the assertion “Fluid at port A must be fully vapor” is failing. Fixing this may solve the issue as this warning may be causing solver exceptions that are coming up in subsequent time in other components. In the time t=0 to 0.01 solver is throwing several DAE exceptions firstly in the “System-Level Condenser Evaporator (2P-TL)” and thereby in the subsequent components in the circuit. Check the values and units of block parameters and examine upstream blocks to identify potential modelling improvements. From the stiffness analysis it shows that the “rotational velocity” of the turbine is causing the most impact on the stiffness. Adjusting the parameters of the inertia block attached to the turbine may be helpful.

You can use the “Solver Profile” to perform a “Stiffness Impact Analysis” to identify the potential components causing the issue. It can be done as follows:

https://www.mathworks.com/help/simscape/ug/simscape-stiffness-analysis.html

You can also refer to the following resources for more information:

• https://www.mathworks.com/help/simulink/ug/solver-exception-events.html
• https://www.mathworks.com/matlabcentral/answers/1598074