[Show abstract][Hide abstract] ABSTRACT: This paper presents a novel energy-saving method of a robotic system utilizing parallel mechanism by redundant actuation. The redundantly actuated system can distribute arbitrary actuating torques in a certain combination. By using this feature, the regenerative power dissipation and electric power loss are reduced. Moreover, the redundant actuation can reduce the peak torque of actuating joints. This feature reduces the friction loss because we can use a smaller gear reducer. A parallel manipulator with two degrees of freedom is simulated as a case study. The energy consumption of the robotic system is modeled by analyzing the type of energy consumption in actuation systems. An optimization process is proposed to maximize the energy-saving effect. The results show that 26.1% of electric energy can be saved by redundant actuation.
Preview · Article · Oct 2015 · International Journal of Precision Engineering and Manufacturing-Green Technology
[Show abstract][Hide abstract] ABSTRACT: Mechanical machinery has dominated the field of ship propulsion systems in the last century. Since fuel accounts for a large portion of the ship life-cycle cost, the current trend of moving toward fuel economy and environmental regulations in marine engineering has led to the efficient use of onboard energy. Incorporating power electronics into ship propulsion systems has been a very common method of improving fuel efficiency since approximately 1980. One high-profile example is the 1968-built Queen Elizabeth 2, whose steam turbines were replaced with a diesel-electric propulsion plant in 1986. Some commercial ships are now being built with electric drives, such as passenger ships, car ferries, shuttle tankers, cable-laying ships, icebreakers, and floating offshore platforms. Navies around the world have been actively exploring integrated power systems (IPSs) for use on future surface combatant ships since the late 20th ?century. Although the ship IPS has many distinct advantages, such as increased survivability and maneuverability, reliability inhibits further fuel economy improvement. The onboard battery energy storage system (BESS) was recently suggested to increase fuel economy and ensure reliability at the same time. Furthermore, an active front-end (AFE) converter can provide improved grid quality at transient states such as fault and future mission load in addition to steady state as with normal sea sailing.
[Show abstract][Hide abstract] ABSTRACT: This paper presents a carrier-based Pulse Width Modulation (PWM) method to reduce the common-mode voltage (CMV) of the three level four leg converter. The idea of the proposed PWM method is very intuitive and easy to be understood. Based on the analysis of the Space Vector PWM (SVPWM) and the Sinusoidal PWM (SPWM) switching patterns, 4th leg pole voltage of the three phase converter, so called 'f pole voltage' has been manipulated to reduce the common-mode voltage. To synthesize f pole voltage for the suppression of the CMV, positive and negative pole voltage references of f leg are calculated. Also an offset voltage to prevent the distortion of a, b, c phase voltages regarding to the neutral point is deduced. Not requiring extra hardware, the proposed PWM strategy can be easily implemented in the software of the DSP based converter control system without degradation of the control performance. The three level four leg converter with the proposed PWM algorithm results in a remarkable reduction in peak-to-peak value of the CMV. The feasibility of the proposed PWM method is verified by both simulation and experimental results. From the simulation and the experimental results, the peak-to-peak value of the CMV in the proposed PWM method is 33% comparing to that in the SVPWM method, and the transition number of the CMV during a switching period in the proposed PWM method is only 25% of that in the SVPWM method.
[Show abstract][Hide abstract] ABSTRACT: This paper proposes a power conversion topology involving a multi-winding transformer and converters. The fundamental idea is described with circuit diagrams, and the voltage output of the proposed topology is analyzed mathematically. The effectiveness of the topology is discussed with test results from a small-scale power conversion system. When conventional hardware consisting of two-level converters and a transformer is employed, multi-level voltage outputs can be applied to the transformer windings by the proposed method.
No preview · Article · Nov 2014 · Journal of power electronics
[Show abstract][Hide abstract] ABSTRACT: A switching loss of Modular Multilevel Converter(MMC) might increase drastically in HVDC system because the number of sub-module(SM) is proportional to the DC-link voltage. And, a special strategy for reducing switching frequency has been significant research issue in terms of overall operating efficiency of MMC for HVDC system. The voltage fluctuation of capacitor in SM, however, increases as the switching frequency decreases, and the capacitor with large capacitance which is the main portion of equipment cost for SM is required to mitigate the voltage fluctuation. In this paper, the switching frequency reduction strategy is proposed using the sorting method with a virtual capacitor voltage of individual SMs. In addition, this paper presents the 2nd order harmonic circulating current injection to suppress the voltage fluctuation. By numerical loss analysis, it is identified that the 2nd order harmonic current injection does not incur severe additional loss. Thanks to the harmonic current injection, the capacitance of SM capacitor could be reduced by 33% at the cost of only 0.05 % efficiency degradation in the given simulation condition. To evaluate the effectiveness of the proposed strategies, the computer simulation with 400 kV, 400MA, 221-level MMC has been performed and the results are discussed. Additionally, validity of the proposed strategies has been verified by 7-level down scaled prototype experimental setup.
[Show abstract][Hide abstract] ABSTRACT: Nowadays, a Line Commutated Converter (LCC)-HVDC is a mature technology with decades industry experience. Even though it's with several advantages such as higher reliability, lower cost, and higher efficiency, it presents some drawbacks such as large converter station size and lack of black starting capability. So the LCC-HVDC is not suitable for some applications in which black starting capability is necessary or high power density is highly emphasized. A LCC-Voltage Source Converter(VSC) hybrid HVDC transmission system combines the advantages of both technologies. In this paper, a comprehensive control strategy of a LCC and Full Bridge Modular Multi-level Converter(FB-MMC) hybrid HVDC transmission system, including converter control schemes, power flow control strategy, black starting strategy, and DC short circuit fault ride through strategy is proposed. Also, DC short circuit fault characteristics depending on AC grid types of the FB-MMC are analyzed and appropriate ride through strategies for each case are established. Validity of the proposed control strategies are verified by computer simulation and tested at reduced scale prototype set-up.
[Show abstract][Hide abstract] ABSTRACT: To increase the popularity of HVDC power transmission system, the Multi-Terminal Direct Current (MTDC) power transmission system is expected to be installed. Since the Voltage Source Converter HVDC (VSC-HVDC) is available, the MTDC systems are much more likely to be installed soon. However, an efficient Direct Current blocking measure such as DC circuit breaker remains as the most critical hurdle to implement MTDC system. In this paper, a new DC current suppression circuit is proposed. This circuit is a kind of the semiconductor DC circuit breaker, and can be used for very quickly interrupting the DC current against the short circuit fault condition at the cost of minimum conduction loss at the normal operation. Because of the modular configuration, the proposed circuit has inherent redundancy and no need of the series-connection of the semiconductor switches and bulky surge voltage arrestor bank.
[Show abstract][Hide abstract] ABSTRACT: This paper proposes a control method of the whole ship Integrated Power System (IPS) with an Active Front End (AFE) converter and a Battery Energy Storage System (BESS). The propulsion inverter which drives the propulsion motor controls the ship speed by controlling the torque of the motor. The AFE converter controls not only the active and reactive power flow at the Point of Common Coupling (PCC) to suppress the transients of the both frequency and voltage of the PCC simultaneously, but also the State Of Charge (SOC) of the BESS. The components of the ship IPS are modeled partially by software or implemented partially as hardware for the real time Power Hardware-In-The Loop Simulation (PHILS) to verify the proposed algorithm. The parallel operating generators and their associate controllers, ship-service load, and pulse load are modeled in C code, after partitioned to distributed computational burden, and simulated in low cost multicore DSP. The hardware is down scaled because of the cost, time, space and safety issues while preserving the electrical and mechanical dynamics. The connection between HILS and hardware is achieved via a custom designed voltage amplifier. The dynamics and load sharing properties of the parallel operating generators, the proposed propulsion inverter controller, and the AFE converter controller have been verified successfully.
[Show abstract][Hide abstract] ABSTRACT: In the Multi-Terminal DC (MTDC) power transmission system, the power flows according to the DC voltages of the terminals and the line impedances. And, the power flowing through each DC line in the mesh MTDC system is not able to be controlled independently. In this paper, a Modular Multilevel Converter based on Full Bridge cells (MMC-FB) is proposed for auxiliary voltage control in the mesh type MTDC system. To control the MMC-FB in four-quadrant, the capacitor voltage balancing method is also proposed with three equivalent circuit models. All control algorithm associated with the proposed MTDC has been confirmed by computer simulation.
[Show abstract][Hide abstract] ABSTRACT: Photovoltaic (PV) inverters autonomously adjust their DC-link voltages to maximize power generation. Around sunrise or sunset, a PV inverter may operate at much lower DC-link voltage than the nominal level due to the low irradiance. The inverter would be under over-modulation if the DC-link voltage is relatively low to the grid voltage at the point of common coupling. In this paper, a series of implementation schemes are proposed to keep the current regulation under over-modulation. After the proposed method is detailed, its fundamental operations are verified by a small-scale prototype inverter and further evaluated by the 250 kW PV inverter installed at a proving ground.
[Show abstract][Hide abstract] ABSTRACT: In square wave voltage signal injection sensorless drive, the injection voltage can be distorted by the inverter nonlinearity effects especially when the injection voltage is low. If that happens, High Frequency (HF) current signal which contains the rotor position information could be also distorted, which directly leads to an error in the position estimation. This paper analyzes the effects of the inverter nonlinearity to injection voltage, to induced current ripple, and to the position estimation performance in sequence and proposes a voltage injection method to minimize the impact of the inverter nonlinearity by the regulation of HF current ripple. By simulations and experiments, performance of the proposed method has been verified. The experimental results show 34.9% reduction of noise input in the position estimation and 19.7% improvement of the position estimation performance under 15% of rated voltage signal injection.
[Show abstract][Hide abstract] ABSTRACT: This paper presents a control scheme for the regulation of cell capacitor energy balancing of a Modular Multilevel Converter (MMC) for HVDC transmission systems, considering the unbalanced AC grid conditions. It is essential that the MMC balancing control should be valid not only for the balanced normal operations but also for the asymmetrical grid fault conditions. This paper proposes the control scheme that has the ability of seamless mode change between balanced and unbalanced grid condition. Applying the proposed method, the capacitor energy balancing operation is successfully realized with improved dynamic responses. Finally, the simulation results verify the validity of the proposed method.
[Show abstract][Hide abstract] ABSTRACT: This paper describe a design method of an LCL filter for the three-phase two-level voltage-source PWM converters. The grid current harmonics regulation is the primary condition for the operation of the grid-connected converters to satisfy. If the grid voltage is assumed to be ideally sinusoidal, PWM switching is the only factor to produce harmonics on the grid current, especially around the bands of the switching frequency. However, every single harmonic cannot be evaluated for the filter design. In this paper, it is found that there exists the most significant harmonic frequency at which the harmonic current is dominant. Theoretical description of the PWM waveform and the transfer function of the filter's impedance are needed for the derivation of the most significant frequency. Once the most significant frequency is recognized, the total inductance to satisfy the grid current harmonic regulations is determined by selecting proper resonant frequency. Along with the information of the designed total inductance and preset resonant frequency, additional conditions such as the filter's total energy, converter current ripple and converter capacity are utilized to specify the values of each component of the filter. The effectiveness of the proposed method is verified with simulation and experimental results.
[Show abstract][Hide abstract] ABSTRACT: In this paper, two designs of Interior Permanent Magnet Synchronous Machine (IPMSM) for sensorless drive with absolute rotor position estimation capability are proposed. A conventional IPMSM design prepared for the same purpose had a demerit that for absolute position estimation, saying that the rotor should initially stroke for 100° in the worst case. In order to reduce the initial stroke of the rotor, two new IPMSM designs featuring asymmetric winding and asymmetric rotor geometry are proposed and constructed as prototype motors. With the prototype motors, initial stroke of the rotor has been reduced to 30°. Moreover, basic characteristics of the motor such as torque ripples and harmonics of EMF have been also conspicuously improved. The overall performances of the proposed designs have been evaluated by rigorous finite element analysis and experimental test.
[Show abstract][Hide abstract] ABSTRACT: This paper proposes a novel electric propulsion system for naval ships, which consists of Active Front End (AFE) converters directly connected to battery Energy Storage Modules (ESMs). Employing the proposed AFE converters with ESMs in the power systems of naval ships can enhance the reliability and quality of the electric power. Furthermore, the fuel-efficiency of the generator can be improved by a higher loading factor of the generator and its prime movers. The proposed AFE configuration does not require an additional dedicated DC/AC converter for the ESMs. Instead of that, the AFE converter itself can control the DC link voltage and the discharging and/or charging of the ESMs. A control scheme to achieve these control objectives is also presented in this paper. The overall power system, including the generators and electrical loads of a naval ship, is implemented by a small scaled Power Hardware-In-the-Loop (PHIL) simulator. Through this experimental setup, the proposed system configuration and the power control strategies are verified. It is shown that the fuel-efficiency and transient dynamics can be improved in the normal and contingency operation modes.
Preview · Article · Mar 2014 · Journal of power electronics
[Show abstract][Hide abstract] ABSTRACT: Cell capacitor energy control of a Modular Multilevel Converter (MMC) is conventionally done by controlling leg current and modulation strategy. In most of literatures, leg current transient is analyzed under an assumption that the DC bus is a stiff DC voltage source. In a real MMC-based HVDC transmission system, however, there's no such virtual stiff DC voltage source and the conventional regulation method can lead to poor dynamics of cell capacitor energy control and even make system unstable. In this paper, the MMC model is revised for circulating current transient analysis. Based on the revised model, a new comprehensive cell capacitor energy control strategy is proposed by updating leg capacitor energy reference on-line and injecting positive and negative sequence circulating currents. Validity of the proposed method is verified by a 7-level scaled version prototype experimental setup.
[Show abstract][Hide abstract] ABSTRACT: Lack of DC short circuit fault blocking and ride through capability is one of main issues in applications of Modular Multilevel Converter (MMC) to actual HVDC transmission system. Recently, several topologies have been proposed to provide DC short circuit fault blocking capability and/or DC fault ride through capability. In this paper, the operation principles, functionalities, and characteristics of several topologies are compared. And, it's revealed that the conventional leg capacitor energy balancing strategy by regulating DC component of leg current fails under DC short circuit fault. A corresponding new leg capacitor energy balancing method by common mode voltage injection is proposed. Validity of the proposed method is verified by computer simulation.