Seung-Ki Sul

Seoul National University, Sŏul, Seoul, South Korea

Are you Seung-Ki Sul?

Claim your profile

Publications (322)357.96 Total impact

  • Byung-Geuk Cho, Seung-Ki Sul
    [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.
    2014 International Power Electronics Conference (IPEC-Hiroshima 2014 ECCE-ASIA); 05/2014
  • Yongsoon Park, Seung-Ki Sul, Ki-Nam Hong
    [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.
    2014 International Power Electronics Conference (IPEC-Hiroshima 2014 ECCE-ASIA); 05/2014
  • [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.
    2014 IEEE 5th International Symposium on Sensorless Control for Electrical Drives (SLED); 05/2014
  • [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.
    2014 International Power Electronics Conference (IPEC-Hiroshima 2014 ECCE-ASIA); 05/2014
  • [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.
    2014 International Power Electronics Conference (IPEC-Hiroshima 2014 ECCE-ASIA); 05/2014
  • [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.
    2014 IEEE Applied Power Electronics Conference and Exposition - APEC 2014; 03/2014
  • Source
    [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.
    Journal of power electronics 01/2014; 14(2). · 0.78 Impact Factor
  • Yongsoon Park, Seung-Ki Sul
    [Show abstract] [Hide abstract]
    ABSTRACT: The rotor angle can be estimated from the stator flux in permanent magnet synchronous motor (PMSM). Although integrations are essential to estimate the stator flux from the voltages and currents in the stationary reference frame, disturbances can arise in the process of integrations due to practical reasons. In this paper, a frequency-adaptive disturbance observer has been proposed to remove the disturbances in estimating the stator flux and to enhance the accuracy of the rotor angle estimation. The design and utilization of the proposed observer are detailed under the consideration of its application to the practical system driving PMSM. The performance of the proposed sensorless method has been mainly assessed through experiments at low speed operations, where the sensorless drive of PMSM is regarded as being extremely difficult without the signal injection.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
  • [Show abstract] [Hide abstract]
    ABSTRACT: It is important to improve the overall efficiency of a photovoltaic (PV) inverter when it is connected to the grid. Fundamentally, the conversion efficiency from dc to ac power of an inverter is important. However, in the presence of partial shading, maximum power point tracking (MPPT) on PV modules is more important than the conversion efficiency. In this paper, a new control method for a three-level inverter is proposed. With the proposed method, each dc-link voltage of the three-level inverter can be asymmetrically regulated. When PV modules are split into two and each split module is connected to the respective dc-link capacitors of the inverter, the asymmetric control can be helpful because separate MPPTs are possible. The effectiveness of the proposed method was examined through experiments with a T-type three-level inverter, where each dc-link capacitor was supplied by a PV simulator emulating two separate PV modules under different shading conditions.
    IEEE Transactions on Power Electronics 01/2013; 28(6):2760-2769. · 5.73 Impact Factor
  • S. Choe, Eunsoo Jung, Seung-Ki Sul
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper proposes a new sensorless method for a Synchronous Machine with an Inverter Integrated Rotor (SMIIR). The SMIIR is a newly developed machine based on the wound rotor synchronous machine (WRSM) but has no brushes and slip-rings. And conventional high-frequency signal injection sensorless methods are based on physical magnetic saliency. Therefore, conventional methods cannot be applied to the SMIIR which has no sufficient magnetic saliency. The proposed sensorless method suggests virtual resistance saliency created by the rotor-side inverter, whose axes can be placed arbitrarily. Based on this virtual saliency the rotor position information is included and excluded in the relationship between the high-frequency stator voltage and stator current. Because the SMIIR already injects high-frequency voltage for the power transfer, there is no need to inject additional voltage for the proposed high-frequency sensorless method. And initial +d axis detection algorithm using the rotor-side inverter without using magnetic saturation is also proposed. The feasibility of the proposed sensorless method was verified by experimental results with the prototype SMIIR.
    Applied Power Electronics Conference and Exposition (APEC), 2013 Twenty-Eighth Annual IEEE; 01/2013
  • Myoungho Kim, Seung-Ki Sul, Anno Yoo
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper conducts modeling and controller design for SEries-connected Output Universal Link(SEOUL) converter. The SEOUL converter was proposed to interface multiple distributed power sources to the grid with reduced number of reactive components. It employs high-frequency transformer instead of bulky line-frequency transformer as Solid State Transformers(SSTs). Besides, it does not use large DC-link capacitor at the input-side and reduce the size of the input filter. This is inherently possible due to its original operating characteristic. The H-bridges connected to the grid does not perform PWM operation to make the input phase current sinusoidal. Instead, their switches operate synchronized to the grid frequency and the input phase currents are synthesized sinusoidally by the high-frequency power transfer part of the SEOUL converter. This paper focuses on the power transfer of SEOUl converter and develops the analytic model of it. Based on the developed model, a DC-link voltage control strategy is proposed with the decoupling control method. The developed model and proposed control method was verified by experiments. The resultant input phase current is kept as sinusoidal at various operating conditions.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
  • Source
    Byung-Geuk Cho, Jung-Ik Ha, Seung-Ki Sul
    [Show abstract] [Hide abstract]
    ABSTRACT: To obtain phase currents information in AC drives, shunt sensing technology is known to show great performance in cost-effectiveness and therefore it is widely used in low cost applications. However, shunt sensing methods are unable to acquire phase currents in certain operation conditions. This paper deals with the derivation of the boundary conditions for phase current reconstruction in three-shunt sensing inverters and proposes a voltage injection method to expand the measurable areas. As the boundary conditions are deeply dependent on the switching patterns, they are typically analyzed on the voltage vector plane for space vector pulse width modulation (SVPWM) and discontinuous pulse width modulation (DPWM). In the proposed method, the voltage injection and its compensation are conducted within one sampling period. This guarantees fast current reconstruction and the injected voltage is decided so as to minimize the current ripple. In addition to the voltage injection method, a sampling point shifting method is also introduced to improve the boundary conditions. Simulation and experimental results are presented to verify the boundary condition derivation and the effectiveness of the proposed voltage injection method.
    Journal of power electronics 01/2013; 13(2). · 0.78 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The aim of this study was to develop a new method to operate an interior permanent magnet synchronous machine (IPMSM) on the maximum torque per ampere (MTPA) condition. The characteristics of the MTPA condition were analyzed and the MTPA condition was derived based on the input electric power. The proposed method injects a small current signal used for tracking the MTPA operating point along with the fundamental current for torque generation. This method does not require any machine parameters or premade lookup table. The frequency of the injected signal is several hundred hertz, and the performance of the MTPA tracking is almost free from load torque disturbance. The feasibility of the proposed method was verified under various operating conditions with computer simulation and testing with an 11 kW IPMSM drive system.
    IEEE Transactions on Power Electronics 01/2013; 28(1):488-497. · 5.73 Impact Factor
  • Byung-Geuk Cho, Seung-Ki Sul
    [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, a reactive power sharing method is proposed for the operation of multiple distributed generation units. When the line impedance is unbalanced, accurate reactive power sharing is challenging with conventional current sharing or droop control strategies. This paper describes the effect of the unequal line impedances on the reactive power sharing accuracy, especially for mainly inductive line impedance. The proposed method compensates the impedance effect by modifying the droop slope and is shown to be effective through two parallel 1MVA units. The accuracy of the reactive power sharing is improved by 26.7% and 42.5% for each DG unit in the grid-connected mode and 11.9% in the islanding mode.
    ECCE Asia Downunder (ECCE Asia), 2013 IEEE; 01/2013
  • Jae-Jung Jung, Hak-Jun Lee, Seung-Ki Sul
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper presents a control scheme for the Modular Multilevel Converter (MMC) to drive a variable-speed AC machine, especially focusing on improving dynamic performance. Theoretically, the energy balance in the cell capacitors is prone to be unstable at startup and low frequency operation. Therefore, the MMC topology essentially requires advanced strategies for energy balancing so as to suppress the voltage pulsation of each cell capacitor. The pulsation is getting larger as the output frequency of MMC is getting lower. This paper proposes a strategy for robust dynamic response even at zero output frequency of MMC employing leg offset voltage injection. With this proposed strategy, an AC machine has been driven from standstill to rated speed without excessive cell capacitor voltage ripples. The experimental results verify that stable operation is guaranteed for both a 15r/min operation with 40% step load torque and the entire frequency operation with varying load torque.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
  • Myoungho Kim, Seung-Ki Sul, Junggi Lee
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper presents a turn-fault detection method for inverter-fed Interior Permanent Magnet Synchronous Machines(IPMSMs) using high-frequency current injection when the motor is at standstill. When the motor is stopped, the fault current which flows in the faulty turns and the phase currents can be expressed with the injected phase current and the mutual inductance between each phase winding and the faulty turns. Because of the turn-fault, the injected high frequency currents cause different losses according to the direction of the injection. The proposed detection method exploits the difference of the losses of each case to diagnose the turn-fault. Experimental results are provided to verify the proposed detection method. The resultant loss pattern agrees well with that of the electrical model and shows reasonable sensitivity to the turn-fault. Two-turn fault among 27 turns of a phase winding of the motor under test has been detected easily with existing hardware of the drive system.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
  • So-Yeon Kim, Byung-Geuk Cho, Seung-Ki Sul
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper proposes a control strategy of an active front end (AFE) rectifier to cope with voltage deviation at the shipboard AC-power system. The voltage sags in naval ships may occur when the step load such as a large pulse power weapon is applied or when faults happen in the grid or generators themselves. In this paper, the capability of AFEs to support voltage is presented for the voltage sags. In the proposed control scheme, reactive power is utilized to achieve voltage compensation. For the verification of the effectiveness, simulation using MATLAB/Simulink and PLECS has been conducted.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
  • [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, a design and control method of Interior Permanent Magnet Synchronous Machines (IPMSMs), which is controlled in position sensorless mode based on high frequency signal injection, for absolute position detection and control capability has been addressed. Although conventional motor designs with symmetric structure work well in sensorless speed and torque control modes, their actual mechanical rotor position is unidentifiable since their inductance profile is repeated for every electrical revolution. In order to realize the mechanical rotor position detection capability, from a given template 6-pole/9-slot IPMSM with symmetric structure, an IPMSM design with asymmetric winding and asymmetric rotor geometry is proposed and constructed as a prototype. Based on the proposed design, the mechanical rotor position can be identified at the cost of slight degradation of basic motor characteristics such as torque ripples, harmonics in back EMF, and etc. The overall performances of the proposed IPMSM have been evaluated by rigorous FEM and experimental test.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
  • Jae-Jung Jung, Hak-Jun Lee, Seung-Ki Sul
    [Show abstract] [Hide abstract]
    ABSTRACT: This paper presents a control strategy of the entire frequency range operation for Modular Multilevel Converter (MMC), especially focusing on variable speed drive of an AC machine. The structure of MMC essentially requires energy balancing control so as to mitigate the voltage pulsation of each cell capacitor in converter arms. In the proposed control strategy, two operation modes are employed. One is a low frequency operation mode for start-up and low speed operation of the AC machine, and the other is a normal frequency operation mode from medium to rated speed of the AC machine. To reduce the pulsation, this paper proposes the energy balancing control strategies at each operation mode. Theoretically, the energy balancing control of the capacitors is prone to be unstable at low frequency operation. In order to prevent the instability, a special control strategy is introduced. The strategy exploits a common mode voltage and a circulating current with high frequency component in low frequency operation mode. With the proposed control scheme, the speed control range of the AC machine driven by MMC can be down to zero speed without instability of voltage of the cell capacitors. Experimental results for the energy balancing control are shown to demonstrate the effectiveness of the proposed control strategy.
    2012 IEEE International Conference on Power Electronics, Drives and Energy Systems (PEDES); 12/2012
  • [Show abstract] [Hide abstract]
    ABSTRACT: Photovoltaic (PV) panels are subject to partial shading in many practical situations due to nearby obstacles like trees, buildings and the adjacent PV panels. Multiple peaks can exist on the resulting power-voltage (P-V) characteristics of the PV panel under partial shading conditions. This may disturb the efficient maximum power point tracking (MPPT) operation as many existing MPPT methods can converge to local peaks. In this paper, effects of different shading conditions on the characteristics of the PV panel have been investigated. Based on the results and observations, a simple MPPT technique is proposed to circumvent the trappings into the local peaks. The proposed method is based on several critical observations under common shading conditions. The proposed method uses variable step approach which considerably reduces the time to track the global peak. Effectiveness of the proposed method has been verified by computer simulations and experimental results.
    2012 7th International Power Electronics and Motion Control Conference (IPEMC 2012); 06/2012

Publication Stats

6k Citations
357.96 Total Impact Points

Institutions

  • 1992–2012
    • Seoul National University
      • • School of Electrical Engineering and Computer Sciences
      • • Department of Electrical and Computer Engineering
      • • School of Computer Science and Engineering
      • • Korea Electrical Engineering & Sciences Research Institute
      Sŏul, Seoul, South Korea
  • 2005–2009
    • Kongju National University
      • Division of Electrical, Electronic and Control Engineering
      Kongju, South Chungcheong, South Korea
  • 2008
    • Georgia Institute of Technology
      Atlanta, Georgia, United States
  • 2007–2008
    • University of Padova
      Padua, Veneto, Italy
    • Samsung
      Sŏul, Seoul, South Korea
  • 2000–2007
    • Hyundai Motor Company
      Sŏul, Seoul, South Korea
  • 2006
    • RWTH Aachen University
      • Institute for Power Electronics and Electrical Drives
      Aachen, North Rhine-Westphalia, Germany
    • Virginia Polytechnic Institute and State University
      • Department of Electrical and Computer Engineering
      Blacksburg, VA, United States
  • 2003
    • Oregon Center for Advanced Technology Education
      Torrance, California, United States
  • 1999
    • University of Wisconsin, Madison
      • Department of Electrical and Computer Engineering
      Madison, MS, United States
  • 1995–1999
    • Soonchunhyang University
      • Department of Electrical and Electronic Engineering
      Asan, South Chungcheong, South Korea
  • 1998
    • R&D Systems
      Minneapolis, Minnesota, United States
  • 1997
    • Kangwon National University
      • Department of Electronics Engineering
      Kang-neung, Gangwon, South Korea
  • 1996
    • Seoul Semiconductor Co.
      Sŏul, Seoul, South Korea