I have been modelling and simulating a number of simple district heating networks in Dymola and am quite often faced with an error during initialisation.
The system we are simulating consists of
- A producer: two pressure boundaries - source and sink. Pressure at source is controlled via PI which ensures that the source pressure is such that the minimum differential pressure accross a consumer is greater than or equal to some set value.
- Consumers: They have a PI controlled valve and a heat exchanger with a fixed return temp. Valve controls mass flow and subsequently the heat flow to match the consumer load at any given time.
- Pipes: dynamic thermo-hydraulic model with heat loss and and Spatialdistribution to account for delay. Vectorised ports and mixing volumes used to reduce non linear equations.
The figure below is a heavily simplified version of the network (same error occuring here)
The consumer model looks as follows:
And the producer:
During initialisation, the following error occurs:
ERROR: Failed to solve non-linear system using Newton solver.
To get more information: Turn on Simulation/Setup/Debug/Nonlinear solver diagnostics/Details
Solution to systems of equations not found at time = 0
Nonlinear system of equations number = 3
Infinity-norm of residue = 118280
Iteration is not making good progress.
Accumulated number of residue calc.: 389
Accumulated number of symbolic Jacobian calc.: 5
Last values of solution vector:
L.PI.gainPID.y = 0
Last values of residual vector:
{ -118280 }
Trying to solve non-linear system using global homotopy-method.
... loading "data" from "C:/Users/Sim1/Desktop/Keith Dymola Files/GrazReininghaus_UseCase/PythonScriptsforTranslation/Reininghaus.txt"
ERROR: Failed to solve non-linear system using Newton solver.
To get more information: Turn on Simulation/Setup/Debug/Nonlinear solver diagnostics/Details
Solution to systems of equations not found at time = 0
Nonlinear system of equations number = 1
Infinity-norm of residue = 2.22814E+018
Iteration is not making good progress.
Accumulated number of residue calc.: 101
Accumulated number of symbolic Jacobian calc.: 9
Last values of solution vector:
M.port_a.m_flow = 0.000485868
N.valveLinear.dp = -55.8243
O.valveLinear.dp = -135.618
P.valveLinear.dp = 550.474
I.port_a.m_flow = 3.20321E-010
C.port_a.m_flow = 2.19343E-011
D.port_a.m_flow = 0.00208272
E.valveLinear.dp = 371.552
L.port_a.m_flow = -7.10982E-012
J.valveLinear.dp = 243.085
K.port_a.m_flow = 1.924E-005
Last values of residual vector:
{ 6.60393E+013, -1.14781E+018, -1.05438E+018, -2.58754E+016, -111988,
-1.56817E+010, 16024.9, 3.14411E+007, 3.99781E+008, 3.14412E+007,
-15012.9 }
Error: could not solve simplified initialization for homotopy method.
Error: could not solve simplified initialization for homotopy method.
FixInitials:Init
The components A,B,C e.t.c are the consumers in the network. I am using the Radau IIa 5th order solver with tol=1e-06. The PI controller in the consumer valves inittype is to integrate only with integrtor state and the PI in the producer is initialised with an output value. I have tried playing around with all sorts of nominal values for mass flows and pressure drops in the network, a well as initial values in the PI controllerrs but the same form ERROR is always returned. The model passes the error check but always fails at initialisation.
I would like to know if anybody has had experience in debugging such nonlinear systems, and if so, a few tips in how to initialise these models would be a great help with the debugging process.
