Modern Starting Methods of Large Thrusters supplied by the Power Network of a Ship
Ioannis K. Pallis, Ilias P. Georgakopoulos, Emmanuel C. Tatakis University of Patras, National Technical University of Athens, Greece ipallis@upatras.gr
XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Thrusters • Thrusters are transversal propulsion systems built into the bow or stern of the ship. • Maneuvering the ship inside the ports. • Avoid collision accidents. • Electric motor Induction machine 0.5 to 2.5MW
XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
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 lower the inrush current is lower XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Thruster startup effects • 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.
XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Thruster Soft Starting • • • • •
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Several soft starting 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 XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Simulations • A real power grid network from a new built Ro-Ro ship was simulated using MATLAB/SIMULINK • Two 2.1 MVA gensets and two 2.4 MVA shaft generators (AC bus at 440V/60Hz). • Low bus voltage is the worst case scenario in comparison to 3.3kV or 6.6 kV AC bus voltage. Higher thruster start up currents.
XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Simulations
• From onboard measurements, it was known which loads were operating during the maneuvering and included to the simulations. • Thruster’s 1 MW induction machine equivalent circuit was estimated from the manufacturer’s datasheet and a precise model has been created.
XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Thruster Direct on line (DOL) operation
• In this topology, thruster motor is directly connected to the main voltage bus. ▫ motor start current is six times greater than the nominal one. ▫ major voltage drop is noted XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Thruster Direct on line (DOL) operation •
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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 over-magnetized, a voltage overshoot occurs. As a consequence, high inrush currents during this transient period may cause a significant voltage disturbance in the power system. XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Thruster Soft Starting In AES the thruster motor has to be interconnected to the ship power network. – An electronic power converter can be used in order to start smoothly the large induction motor. The electronic soft starter may be bypassed after the start up due to energy saving.
XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Control of Soft starters
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Controlled parameters: ▫ Voltage Timed Voltage Ramp Method ▫ Current Current limiting ▫ (Also : Torque, Acceleration)
XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Timed Voltage Ramp Method •
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Control parameters • Voltage vs Time Ramp slope • Voltage boost Effects – reduces the start up current – reduces start up torque – slows motor’s acceleration rate
XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Timed Voltage Ramp Method – Zero voltage boost •
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Several slope rates Greater parameter value greater slope rate For greater slope rates: – Shorter startup duration – Higher currents – Greater voltage drop
XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Timed Voltage Ramp Method – Voltage boost
Soft starter can offer initially enough voltage to the motor in order to produce acceleration torque startup time period can become shorter.
XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Current Limiting Method
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Control parameters – Ramp slope – Voltage boost – Current threshold
XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
(TVR) vs. (CLM)
XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
(TVR) vs. (CLM) Timed Voltage Ramp o Open-loop control o No current measurement o Shorter startup time
Current Limiting o Limited thruster startup current o Minimized voltage drop o Improved power quality
XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Soft Starting via AC/DC/AC converter
Used in many industrial drive systems. • Scalar or Field Oriented Control (FOC) can be applied. • Increases the cost of the thruster drive. •
XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
AC/DC/AC converter simulations The same thruster was driven by the AC/DC/AC converter. • Scalar (V/f) and Field Oriented Control (FOC) was implemented. • The torque applied to the shaft of the induction motor was considered linear and reaches 2500Nm at steady state after 8 sec. • The acceleration ramp of the machine was set to 200 rpm/sec. •
XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Scalar (V/f) control strategy
• Large current pulsations occur during the startup. There are lower than that existing on the direct online method. • Moreover, the voltage bus suffers from severe pulsation and additionally the voltage value tends to pulsate above 1 p.u. XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Field Oriented Control
• The FOC controls exact the acceleration needed for the startup and as a result the startup current is limited by a significant portion. XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Conclusions •
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One of the most severe power quality issues is voltage dip when large loads are connected in the ship weak power grid. Thruster start up can challenge the ship power systems and control. Electronic soft starters can reduce the voltage dips at thruster start up. Especially, when current limiting control methods are used, the voltage drop can be substantially reduced. When ac/dc/ac power converters are used, despite the higher cost, can meet even the more strict demand concerning the voltage dips. XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Acknowledgments
The methodology presented in this paper has been developed within the framework of the Thales-DEFKALION project (www.defkalion-thalis.org). This research has been co-financed 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
XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014
Thank you!
XXIth International Conference on Electrical Machines Berlin, Germany – September 2-5, 2014