Novel Concept of Inverter-less Electric Drive Novel Concept of Inverter-less Electric Drive for Ship Propulsion for Ship System* Propulsion System *

Transcription

1 Novel Concept of Inverter-less Electric Drive Novel Concept of Inverter-less Electric Drive for Ship Propulsion for Ship System* Propulsion System * Oleksiy Bondarenko**, Tetsugo Fukuda**, Masanori Oleksiy Bondarenko Ito*** and Feifei ** Zhang*** Tetsugo Fukuda ** Masanori Ito *** Feifei Zhang *** Electric propulsion systems are found to be a more effective way of propulsion, especially in several applications such as supply vessels, shuttle-tankers, ice-breakers, etc., where a varying velocity profile is preferred. At the same time, the conventional design includes inverters to control propeller rotational speed and/or controllable pitch propeller, thus making the total system very expensive, complex and with additional space requirements. In this study, the authors propose a novel concept of electric propulsion direct drive consisting of a diesel engine and a synchronous generator which directly feeds the induction motor connected to a fixed pitch propeller. By changing the voltage applied to field windings of the generator and rotational speed of the diesel engine an inverter like control of propulsion motor rotational speed can be achieved. This paper presents general concept of a Direct Drive Electric Propulsion (DDEP) system and the simulation results of a complete diesel-electric propulsion system model. 1. Introduction Nowadays ever-more demanding regulations adopted in maritime industry encourage development of various technologies which are of prime importance in order to tackle environmental pollution and limited resources of fossil fuels. In this respect an electric propulsion system, in a field of special applications, is an effective way of propulsion and has been used widely in a supply vessels, ice breakers, research vessels, etc. Recently, however, electrical propulsion has found application in wider type of ships owing to better energy management and resulted fuel savings, improved maneuverability and machinery space design flexibility facilitating adaptation of the energy efficient hull design. The history of electric propulsion accounts for more than 100 years and in the end of 19 th century the electric motor directly connected to a generator was used as a speed reduction and reversion device to transmit power from high speed steam turbine to low speed propeller. The control was arranged by excitation adjustment. After certain period of Diesel engine design development, the interest to electric propulsion had been lost. Later in the middle of 20 th century electric propulsion again found application upon icebreakers, research ships *Received April 21, 2015 **Dept. of Envir. and Power Syst, National Maritime Research Institute, Japan ***Div. of Mar. Tech., Tokyo University of Marine Sci. and Tech. Journal of the JIME Vol.00, No.00(2005) and other specialized type of ships. Since then the configuration and performance of electrical propulsion has significantly evolved [1] and historical evolution is briefly summarized in Table 1. Typical configuration of present electrical propulsion system includes Diesel engines driving synchronous generators for power production, induction motors driving fixed pitch or controllable pitch propellers and various auxiliary power consumers connected to a common constant frequency/voltage bus. Propulsion motor speed and power can be controlled by means of various types of variable frequency drives [2] such as Voltage Source Inverter (VSI) with Pulse Width Modulated (PWM) voltage output, Load Commutated Inverters (LCI) or Cycloconverters. However, these are very much depended on the recent development of various electric devices and power electronics, moreover limited production and requirements for maritime application increases equipment cost and number of machinery components which contrast with benefits offered by the electric propulsion. Therefore, looking through the past and present perspective of electric propulsion system, a novel concept of Direct Drive Electric Propulsion (DDEP) is proposed, combining the merits of present systems and the simplicity of predecessor. The concept of DDEP consists of a Diesel engine driving a synchronous generator (SG) which directly feeds an induction motor

2 753 Novel Concept of Inverter-less Journal Electric of the Drive JIME for Ship Propulsion System (IM) geared with the fixed pitch propeller (FPP). Assuming that the generator-motor system operates on the bus isolated from other consumers, by changing the voltage applied to the field windings of generator and the rotational speed of Diesel engine, an inverter like control (Variable Voltage Variable Frequency) of propulsion motor power and speed can be achieved. The following chapters give outline of DDEP concept supported by simulation results. Table 1. Historical evolution of electric propulsion system Propeller is driven by a synchronous motor (SM) and a propulsion power is controlled by the pitch of propeller Rotational speed of synchronous motor and thus a propeller is controlled by a Diesel engine speed governor Propeller is driven by an induction motor (IM) and propulsion power is controlled by the pitch of propeller IM speed and thus propeller speed is controlled by the number of poles switched IM speed and thus propeller speed is controlled by the Kramer method Converter-Inverter control of SM and thus propeller speed Thyristor frequency converter controls SM speed and thus propeller speed 2. Concept of DDEP From the study [3] on basic concept of DDEP the following case study of DDEP system has been evolved and it is based on the existing ship project of 749 gross ton chemical tanker with two propellers driven by the induction motors and three Diesel engine driven synchronous generators supply all necessary electricity for the ship. In Fig.1 the general topologies of propulsion plant systems are shown: Fig.1(a) shows the most common inverter system; Fig.1(b) shows the case of controllable pitch propellers where the ship speed is controlled by the pitch of propeller; Fig.1(c) shows the proposed new inverter-less concept of DDEP system. In all of the three cases, 3 sets of 660kW Diesel engine, 3 sets of 600kW SG direct coupled to the engines and 2 sets of 500kW IM are arranged. There are 170kW general electric consumers and 2 sets of 200kW cargo pumps apart from the main propulsion motors. In case of configurations in Fig.1(a) and Fig.1(b), a common bus with fixed frequency and voltage is used by keeping engine revolution constant. In case of configuration in Fig.1(c), the bus is divided into sections, providing isolation of generator-motor subsystems which are controlled independently, unless conditions for synchronous operation of generators are met. General electric consumers which need constant frequency and voltage are supplied through small inverter or separate harbor generator which is not shown in Fig.1(c). Journal of the JIME Vol.00, No.00(2005) -2-48

3 Novel Concept of Inverter-less Journal Electric of the Drive JIME for Ship Propulsion System 754 a) Inverter control of propulsion system changed while frequency is kept constant. A control algorithm of DDEP system is depicted in Fig.2. Point A corresponds to the idle speed of Diesel engine and thus the minimum control frequency. At this point gradual increase in voltage applied to field windings of generator cause increase of output terminal voltage and thus the motor voltage and torque, therefore soft start of propulsion motor is provided. As soon as voltage reaches a breakpoint B, the voltage and frequency start to change simultaneously, providing V/f constant control of propulsion motor torque. What is more, the relative position of control characteristic determined by the reference points A, B and C can be adjusted in order to match the Diesel engine performance to the generator-motor-propeller system and thus ensuring safe and reliable operation of propulsion plant. b) Pitch control of propulsion system Fig. 2 Inverter-less control algorithm c) Inverter-less control of propulsion system Fig.1 Electric propulsion system topologies As a result of power bus separation, an inverter-like control can be applied on propulsion motor: terminal voltage is control by means of voltage applied to generator field windings and frequency is controlled through rotational speed of Diesel engine. One should, however not forget that the engine minimum speed is limited by the idle speed which in general is not less than 1/2 of nominal. In this case, in order to start motor, the voltage is In order to confirm the practicability and performance of the concept DDEP system and its control strategy, it is necessary to analyze dynamic behavior of complete propulsion system. This can be achieved by implementing modelling and simulation methods in the designing of propulsion systems as explained in the subsequent chapter. 3. Simulation model of DDEP system Modeling and simulation are essential tools in the cost effective and rapid development of optimized or new concept. The model considered in this study, analyzes the dynamics of engine-generator-motor unit in the framework of ship propulsion plant. The model consists of a ship hull with single propeller geared with the IM and the SG is driven by a medium speed Diesel engine. The generator-motor system Journal of the JIME Vol.00, No.00(2005) -3-49

4 755 Novel Concept of Inverter-less Journal Electric of the Drive JIME for Ship Propulsion System equipped with a governor which controls terminal voltage adjusting voltage applied to the generator field windings as well as manages speed set point of engine speed governor in accordance with control characteristics mentioned in Fig.2. The main parameters of model units are listed in Table 2. Table 2. Main particulars of simulated equipment Ship Hull Length, Lpp [m] Breadth, B [m] 11.5 Draft, D [m] 4.74 Gross ton 749 Design speed, Vs [kt] 11 Required Power, [kw] 750 Propeller Diameter, [m] 2.6 Speed, [rpm] 200 Number of blades 4 Pitch ratio 0.85 Expanded area ratio Motor Power, [kw] 750 Voltage, [V] 440 Frequency, ωb [Hz] 60 Poles, p 6 Speed, [rpm] 1200 Gear ratio, _ 6 Generator Power, [kva] 1000 Voltage, [V] 450 Frequency, ωb [Hz] 60 Poles, p 8 Speed, [rpm] 900 Gear ratio, _ 1 Diesel Engine Type Niigata 6L19HX Cylinders number 6 Speed, [rpm] 1000 Power, [kw] 750 Bore, [mm] 190 Stroke, [mm] 260 Here it should be noted that even though case study is based on the existing ship the simulated propulsion plant configuration differs from actual installation illustrated in Fig.1a. This fact accounts for the necessity of matching existing hull to the available data for simulation. Thus ship design speed and propeller parameters were selected to match hull towing power with available motor power as well as generator model is driven by two diesel engine models providing 1080 kw at 900 rpm. The models of SG and IM consist of a set of differential equations in the standard dq-axis form [4, 5]. The stator and rotor windings voltage equations for motor are as follows: - Stator - Rotor For generator the set of equations is the same as for the motor, only difference is that and voltage equation of field windings is added as follows: 3 where denote voltage, current, resistance and flux respectively; superscript denotes vector composed of dq-axis variables as follows, 0 1 is the 1 0 incidence matrix. The variables are presented in per unit base with the rated values of the SG as reference. The voltage equations set is completed with the equations of air-gap torque of motor and generator: 4 The motor is connected directly to generator, this results in same voltage and current between supplying and receiving elements _ _ _ _. Engine and generator are linked via the shaft rotational dynamics which can be described by the following differential equation: _ _ _ 5 As well motor and propeller shaft dynamics, taking into account reduction gear, is described by similar equation: _ _ _ 6 where are the inertia moments of engine-generator and motor-propeller systems reduced to a common shaft and is the gear Journal of the JIME Vol.00, No.00(2005) -4-50

5 Novel Concept of Inverter-less Journal Electric of the Drive JIME for Ship Propulsion System 756 ratio. Besides, with respect to ship hull only the longitudinal motion is considered, which results in differential equation for the ship speed as follow: 7 where is the hull mass, including added mass; is the calm water resistance of the hull. As an approximation of the medium speed diesel engine dynamics the quasi-steady cycle-mean value model is used [6]. This class of model combines simplicity of mathematical formulation with sufficient insight into engine operation including dynamics of turbocharged air supply. Thus engine torque is represented as a function of shaft speed, and fuel rack position. In turn fuel rack position is determined by the speed governor. Last but not least, in order to represent various modes of ship operation such as ahead/astern maneuvering, crash astern, etc., the four-quadrant [7] thrust and torque performances are adopted. Finally, control system includes PID voltage regulator of SG and PID regulator of Diesel engine speed. In addition, the PID voltage regulator is supplemented with the set-point low pass filter in order to avoid high-frequency oscillations in controller output and sharp change in generator load since dynamic response of electrical sub-system is much faster than that of Diesel engine and ship hull. The adopted form of the PID governor and low-pass filter are shown below: 1 8 where are the governor time constants; is the command from the ship s bridge and is the voltage regulator set-point. 4. Simulation of ship operation The developed complex model of DDEP system consists of set of differential and algebraic equations which is implemented in SimInTech software [8] similar to Matlab-Simulink. Regular ship operation, apart from sea voyage includes two important modes: 1) maneuvering in harbor - start/stop of propeller and ahead/astern changeover within narrow range of propeller speeds; 2) Crash Astern maneuver is a test of propulsion system consisting in ability of propeller reversal and ship stopping from full Ahead. The following sections discuss simulation results of mentioned above modes of operation. 4.1 Harbor Maneuvering During harbor maneuvering the voltage applied to field windings and thus terminal voltage is gradually increased whereas engine speed and thus generator frequency is kept constant until the pre-set break point B on the control characteristic in Fig.2. As a result of terminal voltage rise the stator windings current and thus air-gap torque also raise allowing soft control of motor and propeller speeds as shown in Fig.3. Propeller speed is determined by intersection point of motor torque curve with propeller torque curve at bollard pull condition (Vs 0). Here is should be noted that the stability of induction motor operation (especially on the left side of maximum torque point) is ensured by the speed dependence of propeller torque. Furthermore it can be confirmed by a stability factor which is positive at every intersection point. Fig.3 Soft control of motor start at Vs 0 Typical maneuver of a ship in harbor was simulated and results are depicted in Fig.4. The maneuver includes two steps of Ahead command followed by two steps Astern command and Ahead command then repeated again. Also in simulation the effect of low-pass filter time constant was investigated. The values of was set to 3 and 11 sec and denoted as fast and slow correspondingly. As can be seen from Fig.4 soft control of motor voltage by means of generator excitation provides soft control of propeller speed avoiding a sharp surge in motor current which is typical Journal of the JIME Vol.00, No.00(2005) -5-51

6 757 Novel Concept of Inverter-less Journal Electric of the Drive JIME for Ship Propulsion System during direct start of IM. Furthermore, one should not forget that engine runs at idle speed with low power where allowable engine operation has narrow bounds. The results of harbor maneuvering simulation on engine performance are depicted in Fig.5 and as can be seen time constant of low-pass filter significantly influence diesel engine operation. even when no torque is applied by propulsion motor. The relationship between propeller speed and the torque required to reverse the direction of propeller rotation is indicated in Fig.6 by Robinson s curves, where the ship speed is used as a parameter. Fig.6 Robinson s curves Fig.4 Harbor maneuvering By applying reverse phase voltage to IM, the direction of revolving magnetic field opposes the direction of rotor rotation producing brake torque. This operation mode of IM is known as a plug braking. The IM operating in plugging mode absorbs power of moving hull and as soon as air-gap torque exceeds torque of propeller induced by inflow water, the direction of propeller rotation changes and opposite thrust is produced decelerating ship hull speed. The simulation results of this maneuver are depicted in Fig.7 where Robinson s curves are superposed with IM torque characteristics at different V/f combinations. Fig.5 Engine operation during harbor maneuvering 4.2 Crash Astern Maneuver Under this mode of operation the propulsion system must be able to brake the ship from full speed ahead in a reasonable time. Owing to the large inertia of the ship hull, the water flow at the velocity of ship continues to rotate propeller Fig.7 Crash astern maneuver Reverse-current braking (plugging) is very effective and simple method of braking but high current is generated unless is controlled by proper V/f ratio as shown in Fig.8. Journal of the JIME Vol.00, No.00(2005) -6-52

7 Novel Concept of Inverter-less Journal Electric of the Drive JIME for Ship Propulsion System 758 Last but not least concern is the safe operation of Diesel engine during crash astern maneuver and as shown in Fig.9 by selecting proper V/f combination the safe operation can be ensured. control, two and more generators run in parallel to avoid the bus voltage drop 4. The proposed inverter-less control algorithm should be checked on a real hardware at the same time the development of combined field voltage/engine speed control system is necessary 5. Performance assessment of motor-generator system running in plugging brake mode is necessary, since absorbed hull power is transformed to heat in a motor rotor. Acknowledgement Fig.8 Motor current during crash astern The authors wish to express their gratitude to Taiyo electric company for fruitful discussion on a matter of presented theme and technical support in a part of providing data for simulation model. References Fig.9 Engine operation during crash astern 5. Conclusions The mathematical model of the complete electric propulsion system and the conceptual inverter less control of propulsion motor had been developed which is useful for evaluation of the whole system behavior. The performed model based research outlined following key points of the proposed DDEP system: 1. The DDEP system combines simplicity of conventional Diesel engine direct drive with benefits of electric propulsion including flexible machinery space design and energy efficient stern part design 2. In propulsion plant design the expensive inverter-convertor system together with CPP system can be omitted thus releasing free space and reducing total cost of ship 3. Main bus separation and soft control of motor start allow to use only one generator per motor, whereas in system with inverter [1] Inoue K., Features and trend of electric propulsion system variety, (in Japanese), Journal of the MESJ, Vol.19, No2, (1984) pp [2] Paes Richard, "An Overview of Medium Voltage AC Adjustable Speed Drives and IEEE Std Standard for Performance of Adjustable Speed AC Drives Rated 375 kw and Larger". Technical Seminar, IEEE Southern Alberta (June 2011) Chapter: [3] Zhang F., Ito M., et.al., Development of Inverter-less FPP Electric Propulsion System Basic System, (in Japanese), Journal of the JIME, Vol 47, No1, (2012). [4] Krause P.C., Wasynczuk O. and Sudhoff S.D., Analysis of Electric Machinery and Drive Systems, Power Engineering Series, Willey-Interscience Press, (2002). [5] Vukosavic S.N., Electrical Machines, Power Electronics and Power Systems series, Springer Press, (2013). [6] Bondarenko O., Fukuda T., et al., Development of Diesel Engine Simulator for Use with Self-Propulsion Model Journal of the JIME, Vol. 48, No. 5, (2013). [7] Roddy R.F., Hess D.E. and Faller W., Neural Network prediction of the 4-quadrant wageningen propeller series, Hydrodynamics department report, David Taylor Model Basin, (2006). [8] SimInTech software on web: (in Russian). Journal of the JIME Vol.00, No.00(2005) -7-53