Trouble balancing a simple power flow model in modelica - modelica

Question

I try to set up a simple model with electrical or thermal power flows between sources and sinks.
I seem to have the same problem as treated in this topic although I used only one pair of flow and potential variables in my connector:
connector PowerPortE
flow SI.Power P;
SI.Voltage v "Dummy potential-variable to balance flow-variable P";
end PowerPortE;
A simple example with a signal responsed power sink looks like this:
model PowerSinkE
SimplePowerSystem.PowerPortE Port;
Modelica.Blocks.Interfaces.RealInput P(unit = "W");
SI.Voltage v(start = 230);
equation
Port.P = P;
Port.v = v;
end PowerSinkE;
model Test
SimplePowerSystem.PowerSinkE Verbraucher ;
Modelica.Blocks.Sources.Sine sine1(freqHz = 50) ;
equation
connect(sine1.y,Verbraucher.P);
end Test;
Checking PowerSinkE goes well, but when trying to simulate, I get the following errors:
Internal error pre-optimization module removeSimpleEquations failed.
Internal error Found Equation without time dependent variables Verbraucher.Port.P = const.k
An independent subset of the model has imbalanced number of equations (1) and variables (2).
variables:
Verbraucher.v
Verbraucher.Port.v
equations:
1 : Verbraucher.Port.v = Verbraucher.v
An independent subset of the model has imbalanced number of equations (4) and variables (3).
variables:
sine1.y
Verbraucher.P
Verbraucher.Port.P
equations:
1 : Verbraucher.Port.P = Verbraucher.P
2 : sine1.y = sine1.offset + (if time < sine1.startTime then 0.0 else sine1.amplitude * sin(6.283185307179586 * sine1.freqHz * (time - sine1.startTime) + sine1.phase))
3 : Verbraucher.Port.P = 0.0
4 : Verbraucher.P = sine1.y
Initially I wanted to leave the variable v completely out of the model (though I had to leave it in the connector to be balanced) but this didn't work out either:
Model is structurally singular, error found sorting equations
1: 0.0 = sine1.offset + (if time < sine1.startTime then 0.0 else sine1.amplitude * sin(6.283185307179586 * sine1.freqHz * (time - sine1.startTime) + sine1.phase));
for variables
Verbraucher.Port.v(1)
The problem seems to be that I need the flow variable power but don't have a corresponding potential variable. I am running out of ideas how to fix this, so thanks for the help.

Answer 1

Why do you try to use a connector in this case? If you don't need the "pyhsical meaning" of flow and potential variables inside the connector, you just can use real inputs and outputs to handle signals.
package SimplePowerSystem
model PowerSinkE
import SI = Modelica.SIunits;
SI.Power P;
Modelica.Blocks.Interfaces.RealInput P_in(unit="W");
equation
P = P_in;
end PowerSinkE;
model Test
SimplePowerSystem.PowerSinkE Verbraucher;
Modelica.Blocks.Sources.Sine sine1(freqHz = 50);
equation
connect(sine1.y, Verbraucher.P_in);
end Test;
end SimplePowerSystem;

Answer 2

My initial thought is that port in the consumer is unconnected. This adds the equation consumer.port.P = 0.0. But what you really need is an equation for the voltage in the port.

Answer 3

You need to use voltage and current on your electrical connector and you need an electrical ground. I suggest you have a look at Modelica by Example for more about both electrical and thermal component modeling.