The power train technology development part of the electric ship research effort at the Center for Electromechanics includes the development of a comprehensive model of ship power system. The model will allow the study of various architectures and power system configurations and addresses issues associated with power management on board the all electric ship.

Fig.1: Notional DD power distribution components

In our initial effort, a power system model that reflects the notional DD power system architecture, shown in figure 1, was built in the Matlab/Simulink programming environment. Power electronics blocks and other components such as electric machines and transformers from Matlab/(Power system blockset) toolbox were used. The model’s components and their parameters are based, when available, on the published data related to the projected power system for DDX ships. The model consists of four synchronous generators, switchboards, two propulsion transformers, two propulsion rectifiers, two PWM drives, and two permanent-magnet propulsion motors. Realistic models for the projected prime movers, which consist of four gas turbines, have not yet been completed. The ship service section of the model has two load-center transformers, two rectifiers, a ship service transformer, an inverter, a DC-DC converter, several switches and breakers, and eight different loads. The top level model of the power system is shown on figure 2.

Fig 2: Top-level Matlab/Simulink power system model.

To illustrate the use of the model, an example simulating fault a scenario is presented. In this scenario, the ship is assigned a mission profile in which it is accelerated from rest to a speed of 30 knots, holds this speed for a short period of time, then decelerates to a cruising speed of 20 knots. During this period a ground fault at one of the propulsion motor terminals is initiated then removed 20 milliseconds later. The effects of the fault are observed by monitoring currents and voltages at relevant places in the model. The following figures show some results of the event just described.

Figure 3 displays the ship's mission profile showing acceleration from rest to top speed (30 knots), steady motion at top speed, deceleration to crusing speed (20 knots), and ground fault while at cruising speed. This also shows the ship’s speed profile, and motor current profiles for the three phases are presented in figure 4. The results show the currents increasing and decreasing during the acceleration and deceleration segments of the mission, while they remain steady during the two cruising periods, as expected. The currents response to the ground fault shows a gradual recovery after several oscillations. It is important to note that in this example ship speed rates and simulation times were adjusted in order to run the full mission in a reasonable amount of time. Obviously, it takes a much longer time to accelerate the ship from rest to full speed. The goal of this exercise is to demonstrate the capabilities of the model and point out its shortcomings when appropriate.

Fig. 3: Propulsion motor 3-phase current during motion and their response to the ground fault

As mentioned earlier, this model uses component models from the Power System Blockset toolbox. While these pre-programmed blocks are useful in terms of ease of modeling, they are often limited in scope and flexibility and do not always run as expected. Simulation errors are often attributed to blocks with no means to correct the problem since we do not have access to them. A fact that often requires modifications of the model in order to get around the difficulties. This is a typical drawback for all pre-packaged programs to which Simulink and the Power System Blockset toolbox are not immune. In order to improve the model performance and capabilities we are in the process of re-building the electric ship power system model without the use of the pre-programmed components from the Power System Blockset commercial toolbox.

In addition to Matlab/Simulink, other programming environments are being investigated, mainly, VTB and acsl. Our present effort is driven by the desire to build a power system model that can be used by the electric ship research community and the need to look at potential programming environments that allow better simulation capabilities and faster execution time. In fact, our initial assessment indicates that simulating realistic scenarios will require considerable computing resources that may not be readily available.

For more information, please contact:

Hamid Ouroua

See Also Related Topics on ONR Electric Ship:

Approaches to Shipboard Power Management by Kent Davey

Electromagnetic Materials for Machinery Development and Characteristics by Aleta Wilder