Figure 2 and Figure 3 presents an example schematic for how a diesel electric propulsion system might be implemented in the VPF according to the standardization described in the introduction. The figures display the same system with different level of detail. In addition to the green boxes representing the model slaves there are also purple boxes representing the control system slaves and blue boxes representing the system topology and non-dynamic system behavior as Function units.

Exchange of information

The signal exchange follows the “standard” described in the introduction. Flow and effort signals between the dynamic slaves are represented by black arrows. For the electric side effort is voltage while flow is current. For rotational mechanical connections flow is ω or angular velocity and effort is torque. The direction of the arrows describes what variables are outputs and inputs to each slave. The two function units are thought of as two separate switch boards that may be connected. Note that for each of the switch boards there are one generator slave giving voltage while the rest of the generator slaves are giving current. Control signals are represented by purple arrows. Here it is assumed that each dynamic slave contains a local controller so that only a reference signal is sent from the higher level control slaves. For the diesel electric AC distribution an orange arrow with frequency information is also added. Generator models in this system are modeled with “two –axis theory” which eliminates the frequency information from the voltage and current signals. This is a desirable trait as the macro bus time steps limits the distribution of signals with high frequency. As other models in the system may need frequency information, frequency is sent as a signal. The red arrows are simulation control signals which control the model simulation. In Figure 3 the red arrows distribute information regarding model switching which is needed to enable change of generators that set the voltage. This is discussed further in section 2.2. The figures of the diesel electric system show the strength of having the Function units in addition to the slaves. There are no limits to the number of slaves to connect as the function unit is changeable. Non dynamic system behavior may also be included in the Function units like the breakers and additional signals may be distributed without problems. Simulation control like switching may also be included in the Function units.

Causality considerations

As there are no dynamics in the Function units, one of the feeding generators has to set the voltage, while the remaining generators feed current at the set voltage. This introduces the need for a model to switch between giving current at a set voltage and setting the voltage or the other way around. This functionality is needed if for some reason a generator setting voltage is taken offline, or if the breaker between the isolated switchboards with each voltage setting generator is closed leaving room for only one voltage setting generator. This means that a switched junction is also a part of the simulation control, sending signals to models which change their simulation behavior (red arrows in Figure 3).

Compatibility

In the current implementation the “two axis theory” is the foundation for the models. However there are other ways to model electric power systems. Further investigation is needed to determine if there will be any compatibility issues if other model approaches are to be connected to the suggested setup and if there are any work around for compatibility issues.

Guideline(s)
  1. Use two-axis theory if possible to eliminate the frequency information from the voltage and current signals
  2. Use power bonds to define the input/output interface (voltage and current)
  3. Implement causality switching in the model, to allow connecting several gen-sets to the same switchboard in a modular way
  • vpf/power_systems.txt
  • Last modified: 2017-02-12 20:24
  • by lars