“ISSUES RELATED TO PROPULSION SYSTEMS FOR ALL ELECTRIC SHIP OPERATION” Pallis I.,Georgakopoulos I., Tatakis E., Dallas S., Prousalidis J.
2nd MARINELIVE International conference on “All Electric Ship” February 12-13, 2014, Athens, Greece
All-Electric-Ship (AES) • AES power system ▫ All generators and electric loads are interconnected into a single grid (AC Bus) ▫ A stand-alone system of several MWs.
• Benefits ▫ ▫ ▫ ▫
Designing Operational Economical Environmental
All-Electric-Ship (AES) • Electric loads ▫ ▫ ▫ ▫ ▫ ▫ ▫ ▫
prime movers fans pumps compressors elevators lightning systems electronic devices thrusters
Thrusters • maneuvering the ship inside the ports • avoid collision accidents • Electric motor ▫ 0.5 to 2.5MW ▫ induction machine. ▫ Torque ~ V2 High inrush currents during startup
Induction machine as thruster • Starting-up, ▫ high inrush current ▫ Significant voltage drop to the main bus affecting all electric loads.
• Startup current value ▫ Voltage rms ▫ (Mechanical load + rotor) inertia reduce voltage rms or disconnect motor load
Thruster startup • Thruster is usually connected with a pitch controlled propeller. • To counterbalance the high load torque at zero speed, the propeller of the thruster starts with its blade pitch at 0 degrees. ▫ the startup load is lowered
Thruster startup effects • High inrush currents voltage drop ▫ less motor breakaway torque ▫ all ship loads affected ▫ electric distribution system affected • In a ship’s electrical power grid, power demand is covered by diesel generator sets and shaft generators. ▫ high instantaneous reactive power demand from thrusters can stress the ship’s generators to its limits. ▫ Generator AVR has to be designed for fast and accurate response
Conventional ship • Isolating the thrusters from the electric network. ▫ rest electric loads are immune to thruster transients. ▫ Thruster motors are driven by separate generators, usually SGs
• To control the motor ▫ the generator increases gradually its terminal voltage by excitation voltage regulation, while rotor speed is almost constant.
• Unfortunately, in implementable
(all loads are interconnected with the generators)
Thruster operation in AES • Thruster operation modes ▫ the thruster motor is directly connected to the vessel’s electric network ▫ an electronic soft-starter drives the thruster motor.
• The system studied via simulation is implemented in a new built Ro-Ro (main bus voltage is set to 440V) • Low bus voltage worst case scenario in comparison with 3.3kV or 6.6 kV AC bus voltage greater thruster start up currents.
Overview of electric power system • A real power grid network from a ferry, was simulated using MATLAB/SIMULINK
• Two 2.1 MVA gensets and two 2.4 MVA shaft generators (AC bus at 440V/60Hz). • From on board measurements it was known which loads where operating during the manoeuvring and included to the simulations. • Thruster’s 1 MW induction machine equivalent circuit was estimated from the manufacture’s datasheet and a precise model has been created.
Overview of electric power system
Figure 1. Ship electric power system model.
Thruster Direct on line (DOL) operation â€˘ In this topology, thruster motor is directly connected to the main voltage bus. â–Ť motor start current is six times greater than the nominal one. â–Ť major voltage drop is noted
Thruster Direct on line (DOL) operation • As a voltage drop appears, the generators’ AVR start to increase magnetization in order to compensate for the increased load currents of the generators. • As the motor reaches the pitching moment, the stator currents quickly reduce and since the generators are now temporarily overmagnetized, a voltage overshoot occurs. • As a consequence, high inrush currents during this transient period may cause a significant voltage disturbance in the power system.
Thruster Direct on line (DOL) operation
Figure 2. Motor start current in DOL operation.
Figure 3. Voltage rms waveform during motor start up in DOL operation.
Thruster SoftStarting • Several softstarting methods, found in several types of ships, using : • autotransformers • capacitors • power electronic devices • generator AVR control. • AVR control of a dedicated to thruster start up generator has been pointed out as a low cost and space saving solution. not implementable in AES
Thruster Soft Starting â€˘ In AES the thruster motor has to interconnected to the ship power network.
â–Ť An electronic power converter may be used in order to start smoothly the large induction motor. The electronic softstarter may be bypassed after the start up due to energy saving. R S T
Figure 4. Solid-state softstarter circuit diagram
Soft starters • Motor controlled parameters: ▫ Voltage Timed Voltage Ramp Method ▫ Current Current limiting ▫ (Also : Torque, Acceleration)
Timed Voltage Ramp Method • Effects Vo - - -
▫ reduces the start current ▫ reduces start torque ▫ slows motor’s acceleration rate
• Control parameters Vrms =0, if t<0 Vrms= Vo+Kv.t, if t>0,  where t: motor starting-up time Vo: voltage boost Kv: voltage increment gradient
▫ Ramp slope ▫ Voltage boost
Timed Voltage Ramp Method
Figure 5. Thruster Drive â€“ Softstarter block parameters.
Timed Voltage Ramp Method – Zero voltage boost • Several slope rates Kv
Greater parameter value greater slope rate
• For greater slope rates: ▫ Shorter startup duration ▫ Higher currents ▫ Greater voltage drop Figure 6. Voltage rms waveform during motor start up in TVR operation.
Soft starter can offer initially enough voltage to the motor in order to produce acceleration torque startup time period can become shorter.
Timed Voltage Ramp Method â€˘ Several voltage boost values
Figure 7. Voltage rms waveform during motor start up in TVR operation.
Timed Voltage Ramp Method – Voltage boost
Figure 9. Thruster speed during start up for several voltage boost values. Figure 8. Voltage rms waveform during motor start up in boosted TVR operation (slope rate “20”).
Figure 10. Motor start current in boosted TVR operation.
Current Limiting Method Vrms waveform during start up
Vo - - -
Current rms value during start up
• Control parameters ▫ Ramp slope ▫ Voltage boost ▫ Current threshold
(TVR) vs. (CLM)
(TVR) vs. (CLM) Timed Voltage Ramp
•Limited thruster startup current
•No current measurement
•Calculated motor thermal stress
•Shorter startup time
•Minimized voltage drop
•Improved power quality
Thruster speed control • After the nominal speed is reached, there is a need of maneuvering operation during docking • Conventional approach is done mechanically. Pitch controlled thrusters • Modern speed and torque control of induction machine can be used • Scalar or Vector control in conjunction with power converters
Field Oriented control • Park Transformation • High dynamic response
• Desired speed in less than 4 seconds compared to several minutes required by the conventional mechanical methods.
This work has been done within the frame of "DEFKALION" project, which is funded by both European and Hellenic National resources ( "ESPA-Thalis" projects).
WORK PRESENTED IN THIS PAPER HAS BEEN DEVELOPED WITHIN THE
THALES-DEFKALION PROJECT. THIS RESEARCH HAS BEEN COFINANCED BY THE EUROPEAN UNION (EUROPEAN SOCIAL FUND – ESF) AND GREEK NATIONAL FUNDS THROUGH THE OPERATIONAL PROGRAM "EDUCATION AND LIFELONG LEARNING" OF THE NATIONAL STRATEGIC REFERENCE FRAMEWORK (NSRF) - RESEARCH FUNDING PROGRAM: THALES: REINFORCEMENT OF THE INTERDISCIPLINARY AND/OR INTER-INSTITUTIONAL RESEARCH AND INNOVATION.
FRAMEWORK OF THE
Thank you !