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1 Medium Voltage Current Source Converter Drives for Marine Propulsion System using aDual-winding Synchronous Machine Abstract Medium Voltage (MV) AC drives are being increasingly used in high-power marine applications for runningthrusters and main propulsion motors. In this paper, an MV drive solution employing active front-end current sourceconverters is proposed for a 7 MW synchronous motor based propulsion system. The proposed solution includes twoindependent drives, each to control one of the two sets of three-phase windings of the synchronous machine. Adedicated communication link between the drives allows continuous load sharing and robust system operation in amaster-slave drive configuration. Field oriented control with the use of an absolute encoder is implemented forproviding high starting torque and smooth speed control over a wide speed range including a 30% over-speed region.Major advantages of the proposed solution include simple structure, increased system power rating, redundantoperation, low-harmonic input/output waveforms, improved reliability, elimination of a bulky input isolationtransformer, and parallel drive control without the need of a complex coordinated inverter gating system. In addition,the system also offers input power factor compensation and dynamic braking to allow operation on a generatorbased supply system. Field test results obtained on a ship are provided to demonstrate the system performance. I. Introduction In high-power marine applications, dual-winding synchronous machines are widely used not only to achieve higherpower levels with compact motor size but also to offer redundancy operation for maximizing the overall shipperformance. Traditionally, high-power synchronous motors are driven by phase-controlled cycloconverters [1,2].The cycloconverter drives save the initial converter cost but provide poor harmonic profiles in both line- and motor-side waveforms. In order to improve the waveforms, multi-pulse transformer and multiple converter configurationsare required to filter out certain low-order harmonics [2]. In addition to the cycloconverter solution, multi-levelvoltage source converters (VSCs) with direct torque control (DTC) have also found presence in the literature andreal industrial marine applications [3-5]. As an alternative solution, this paper proposes a marine drive system usingcurrent source converters (CSCs). The proposed solution provides sinusoidal motor voltage and current waveforms.The 5 th and 7 th order harmonics in the motor currents are eliminated or very low in magnitude, making the systemsuitable for dual-winding motors with either zero-degree or thirty-degree phase angle shift. A patented integrated dc-link choke with common-mode inductance can also be employed to reduce the common-mode voltage stress on themotor insulation system. Having a low component count with simple mechanical packaging and similar rectifier andinverter packaging provides increased reliability and system availability.In the following sections, detailed description of the system and the controller is provided. Test results from a real 7MW ship propulsion system are given to illustrate the validity of the proposed solution. II. System overview 2The proposed system configuration is shown in Fig.1.The two parallel drives, one acting as the master driveand the other acting as the slave drive, communicate overa drive area network link (DANL). The master drivetakes the responsibility for torque and flux regulation,while the slave drive serves as a torque follower andreceives the commands from the master drive. A systemcontroller (typically PLC) communicates with each drivefor supervisory control and serves as the interface to thecustomer system. In case of single drive failure, thesystem allows the remaining healthy drive to continuerunning with reduced capacity. If the master drive isfaulted, the slave drive will then become the actingmaster and start performing motor control. In addition,the system is built with two exciter packages whichallows full redundant operation. III. Current source converters The current source converter employed in each drive iscomposed of a pulse-width-modulated (PWM) current source rectifier (CSR) and a PWM current source inverter(CSI) connected through a dc-link choke. The switching device used in both converters are symmetric gatecommutated thyristors (SGCTs) operating at a few hundred Hz (400~600Hz). Active PWM switching with low-order harmonic elimination at the line side allows to improve the harmonic performance without multi-pulsetransformers. Detailed converter configurations anddescriptions can be found in Fig.3 and [6,7]. The powercomponent design will be discussed in the full paper. IV. Dual-winding synchronous motor The dual-winding synchronous motor discussed in theproposed configuration can have either zero or thirtydegree phase shifted between the two sets of electricalwindings. The equivalent circuit in the dq synchronousframe is shown in Fig.2 [1]. Assuming the statorwinding resistance and leakage inductance are the samefor the two sets of windings, the equivalent circuit of the stator can be further simplified with consideration of phase angle shift. Detailed equations and derivations for Fig.1 System configuration of parallel CSC drives for dual-winding synchronous motor Fig.2 Equivalent circuits of the dual-winding synchronousmachine [1]. 3field oriented control (FOC) of the dual-winding synchronous machine will be provided in the full paper. V. Control of dual-winding synchronous motor using CSC The simplified block diagram of the drive control in the master drive is shown in Fig.3. In marine applications, thepropulsion drives are typically supplied by a generator system. Selective harmonic elimination (SHE) scheme withfixed modulation index is selected to eliminate low-order harmonics with limited device switching frequency. Themain purpose of the line-side control is to receive the dc-link current reference from the motor-side controller andensure desired dc-link current regulation through rectifier delay angle ( α rec ) control.Motor-side control is developed based on the air-gap flux oriented control. The motor voltage v ci and current i s aremeasured to obtain the electrical angular frequency ω e , the air-gap flux magnitude λ f and angle θ motor . In the zero orlow speed region, rotor position from absolute encoder is obtained for FOC control to offer high breaking torquecapability. The speed regulator issues the torque reference T e* and torque-producing current reference i sq* . Thetorque reference together with the motor speed feedback ω e defines the flux reference λ f * that is linearly interpolatedbetween no-load and rated values. The flux regulator provides references for both rotor field current i f * and statormagnetizing current i sd * . The stator magnetizing current helps improve the dynamic performance of the flux controlloop during transients and can be controlled to zero in steady state. The references for dc-link current i dc* andinverter delay angle α inv are then derived from converter currents, which are the sum of the motor and capacitorcurrents. The modulation schemes of the inverter devices incorporate space vector modulation (SVM) and SHEschemes in different speed ranges to provide desired overall performance in the full range. Fig.3 Simplified block diagram of the control system in the master drive. 4Other additional features, such as leading power factor compensation and dynamic braking are also integrated intothe control system to adapt the drives for generator based supply system. The power factor compensation (PFC)feature (shown in blue blocks in Fig.3) intentionally modifies inverter modulation index and motor statormagnetizing current to increase the dc-link current level and thereby reduce line-side leading VARs [8]. Dynamicbraking system [7] as indicated in red blocks in Fig.3 will take over the dc-link current control during regeneratingoperations such as rapid speed reduction and crash stop, so that the kinetic energy in the motor is dissipated in theresistor bank and not fed back to the generator system.In this parallel drive system, the speed and flux regulators are only active in the master drive and the outputs of theregulators are passed over to the slave drive to determine the motor current references in the slave drive. Each driveperforms its own independent dc-link current control, line PF control and dynamic braking control. More detaileddescription of the control blocks and features will be discussed in the full paper. VI. Experimental results The proposed system and control is verified on aship propulsion system with a dual-windingsynchronous motor rated at 6kV/7MW/36Hz.There is no phase shift between the two sets of three-phase windings. The following figuresdemonstrate the drive performance in steady stateand transients. The line- and motor-side currentand voltage waveforms of the master drive at fullpower 7MW and over speed 46Hz are shown inFig. 4. It can be observed that the motor sidewaveforms (Ch1&2) are smooth and sinusoidalwith low THD. At the line side, the voltagemeasurement (Ch4) is at the rectifier input ( v cr )instead of the generator output ( v g ). The line current waveform (Ch3) is with low harmonics and meets IEEE 519standard. Leading VAR associated with line capacitor is fully compensated by the converter.A crash stop operation of the ship is shown in Fig. 5. In this test, the propeller is srcinally operated at 29.3Hz, andis then commanded to go to reverse -40Hz at time around 27s. This simulates the scenario when an emergencystopping of the ship is required. The Speed Reference and Encoder Feedback shown in the second axis indicate theclose control of the propeller speed to its reference. Torque Reference varies from about 0.3pu to -0.8pu in order tofollow the speed reference. The rectifier controls Idc Feedback to follow Idc Reference (coming from motor-sidecurrent requirements) and thereby ensures precise control of the motor speed and torque. More test results of theparallel drive operation will be shown in the full paper. Fig.4 line- and motor-side waveforms at full power (7MW) over-speed region (46Hz). Ch1: line to line motor voltage ( v ci _ab );Ch2:motor phase current ( i s _a ); Ch3: input line current ( i g _a ); Ch4: lineto line rectifier input voltage ( v cr _ab ). Time: 10ms/div.
