Dynamical Model of Maritime Electric Power System
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A dynamical model of a maritime electric power system has been developed on assignment from ABB Marine. The model is implemented in Matlab Simulink. The model development and implementation have been done with regards to practical limitations for real time simulations, with special emphasis on simulation speed and choice of numerical solving method. Real-time simulations of maritime electric power systems is of great usefulness in system engineering, both during the design phase and the commissioning phase. Simulations provide a means of testing the designed system before installation, and hence minimizes the risk of unexpected problems to occur during or after installation. In addition, the simulation model can be used for tuning system and controller parameters before physical testing at sea trials. This reduces both the work load at service engineers, and the risk of damaging equipment. The main components of a maritime electric power system on the generation side are prime movers, synchronous generators and the main switchboard. Each prime mover is connected to one synchronous generator through a shaft, and converts fuel energy to mechanical energy by rotating the shaft. This makes the connected generator able to produce electrical energy. Normally, four to eight generators are paralleled to two or four main switchboards, to provide centralized system voltage sources. The electrical energy from the switchboard is fed to various loads. A small part of total system load is constant with respect to power, current or voltage consumption, and is used for ''hotel purposes'' like lighting and heating. The major part of the total electric power is used for various motor loads, like powering of propulsion drives, thrusters, pumps and fans. Electricity is an energy form that cannot easily be stored in large quantities, and hence the energy production in the maritime electric power system must at all times meet the demand from the loads. Synchronous generators must operate at the same average speed to keep stable co-operation, and produce constant frequency, voltage and current. The voltage level should be equal across the switchboard, to prevent power from flowing between parallel generators instead of being fed to the loads. Especially motor loads will be affected by, and themselves affect, both system voltage and frequency. To ensure that the power demand is always met by corresponding generation, and that system frequency and voltage are kept within specified limits, many control functions must be included in the maritime electrical power system. Each prime mover has a speed controller, called governor, to provide correct frequency, and each generator has an Automatic Voltage Controller (AVR) to provide the correct terminal voltage level. In addition, system level control and protection functions must be provided to minimize the consequences of faults within the system. The main dynamical effects to influence the system frequency and voltage in a maritime electric power system are disturbances and load variations. In this assignment, the focus is load variations. Examples of extreme load variations are sudden increase or decrease in power consumption, tripping of generators which have lost synchronism with the system, and tripping of loads. This will usually happen as protection against major system faults. The system limitations with respect to load application are mainly determined by the prime movers, since these have upper limitations with respect to torque development. For stable load application within specified constraints, procedures for start-up and guidelines for load application rates given by the prime mover producer must be followed.