Jul-Ki Seok

Yeungnam University, Gyeongsan, Gyeongsangbuk-do, South Korea

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Publications (76)109.66 Total impact

  • SeHwan Kim, Jul-Ki Seok
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    ABSTRACT: This paper presents a small film capacitor inverter-based induction motor control approach to enhance with reliability and power density of three-phase variable speed drive applications. A robust hybrid motor controller is developed to prevent performance degradation caused by the electrolytic capacitor-less inverter fed by front-end diode rectifiers. The structure of the controller combines a model-based controller (MBC) and a hexagon voltage manipulating controller (HVC). The MBC determines the command output voltage with the intersection of the torque and rotor flux linkage command. In the HVC mode, the command voltage vector is determined simply by the torque command and the hexagon-shaped inverter voltage boundary. Successful application of the control approach is corroborated by a graphical and analytical means that naturally lead to a single voltage selection rule. This paper also examines the operation sensitivity under motor parameter drifts to determine how to decouple its effect using a voltage disturbance state-filter design.
    IEEE Transactions on Power Electronics 05/2015; 30(5):2713-2720. DOI:10.1109/TPEL.2014.2344693 · 5.73 Impact Factor
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    ABSTRACT: Because wind generators (WGs) in a wind power plant (WPP) produce different active powers due to wake effects, the reactive power capability of each WG is different. This paper proposes a hierarchical voltage control scheme for a WPP that uses a WPP controller and WG controller. In the proposed scheme, the WPP controller determines a voltage error signal by using a PI controller and sends it to a doubly-fed induction generator (DFIG). Based on the reactive current-voltage (IQ-V) characteristic of a DFIG, the DFIG injects an appropriate reactive power corresponding to the voltage error signal. To enhance the voltage recovery capability, the gains of the IQ-V characteristic of a DFIG are modified depending on its reactive current capability so that a DFIG with greater reactive current capability may inject more reactive power. The proposed scheme enables the WPP to recover the voltage at the point of common coupling (PCC) to the nominal value within a short time after a disturbance by using the adaptive IQ-V characteristics of a DFIG. The performance of the proposed scheme was investigated for a 100 MW WPP consisting of 20 units of 5 MW DFIGs for small and larger disturbances. The results show the proposed scheme successfully recovers the PCC voltage within a short time after a disturbance.
    Journal of Electrical Engineering and Technology 03/2015; 10(2):504-510. DOI:10.5370/JEET.2015.10.2.504 · 0.52 Impact Factor
  • Amir Parastar, Jul-Ki Seok
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    ABSTRACT: With the increasing integration of renewable energy generation into high-power grids, transmission at the dc level is becoming increasingly more useful than ac transmission. In this regard, emerging applications, such as offshore wind farms, require a high voltage gain dc/dc conversion system to interface with high-power transmission networks. This paper presents a new high-voltage gain resonant switched-capacitor dc/dc converter for high-power offshore wind energy systems. The proposed dc/dc converter is characterized by the resonant switching transitions to achieve minimal switching losses and maximum system efficiency. Therefore, a higher switching frequency is conceivable to attain a higher power density. The double stage output voltage of the proposed converter operates at seven times as high as the input voltage with a small device count. The output capacitors are charged and discharged continuously by a 180° phase shift with respect to each other to eliminate the output voltage ripples with the low capacitance requirements. The proposed series-modular and cascade configurations show the intrinsic advantage of being readily applicable to multistage power switching converters. The developed topology has been implemented on a 5-kW prototype converter to test its feasibility.
    IEEE Transactions on Power Electronics 02/2015; 30(2):644-656. DOI:10.1109/TPEL.2014.2314110 · 5.73 Impact Factor
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    ABSTRACT: Recently, the interest in offshore wind farms has been significantly increased because of the stronger and more stable winds at sea, which will lead to a higher power production. DC/DC power conversion solutions are becoming more popular for fulfilling the growing challenges in the high-voltage (HV) dc-connected offshore wind power industry. This paper presents several multilevel modular dc/dc conversion systems based on the capacitor-clamped (CC) module concept for high-power offshore wind energy applications. Two types of the CC modules, namely, the double-switch (DS) module and the switchless (SL) module, are discussed. A soft-switching technique is adopted to achieve minimal switching losses and the maximum system efficiency. Theoretical analysis is carried out for the -level cascaded configurations based on the CC modules. The inherent interleaving property of the proposed configurations effectively reduces the output voltage ripple without adding extra components. A cascaded hybrid topology is developed by the combination of DS and SL modules. The proposed hybrid topology achieves higher efficiency and lower component count. The cascaded hybrid approach is evaluated in terms of the power device count, reliability, and efficiency against other HV dc/dc topologies to demonstrate its advantage for HVDC-connected offshore wind farms. The experimental results of two 5-kW prototype CC converters are presented to validate the theoretical analysis and principles as well as attest the feasibility of the proposed topologies.
    IEEE Transactions on Industrial Electronics 01/2015; 62(5):2879-2890. DOI:10.1109/TIE.2014.2363818 · 6.50 Impact Factor
  • SeHwan Kim, Jul-Ki Seok
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    ABSTRACT: This paper proposes a voltage-limited finite-settling-step direct torque and flux control (FSS-DTFC) method with a constant switching frequency for torque-controlled interior permanent-magnet synchronous motors (IPMSMs). The proposed control law dynamically scales the voltage vectors on the hexagonal voltage boundary to ensure the maximum torque capabilities under the given operating conditions while simultaneously regulating the stator flux linkage magnitude under flux-weakening operation. Instead of relying on classical overmodulation methods at voltage limits, this paper developed two independent voltage truncation rules to facilitate the possible voltage vector choices. The analytical solution led to the dynamic voltage modification at each time step with respect to the available inverter voltage. The voltage-limited FSS-DTFC approach has potential advantages in achieving a fast transient torque trajectory and direct manipulation of the stator flux linkage while exploiting the maximum voltage excitation.
    IEEE Transactions on Industry Applications 09/2014; 50(5):3374-3381. DOI:10.1109/TIA.2014.2303424 · 2.05 Impact Factor
  • SeHwan Kim, Jul-Ki Seok
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    ABSTRACT: A hybrid maximum voltage utilization controller is developed for interior permanent-magnet synchronous motors over a wide operating range. The structure of the controller combines the current vector control (CVC)-type maximum torque per ampere controller and the modulating voltage-scaled controller (MVSC). The hybrid structure provides a smooth transition from the CVC to the proposed MVSC mode by deactivating the current regulator in the flux weakening region. A seamless transition to a full six-step modulation can be realized by adjusting the scaling gain, which is a significant merit in terms of power utilization for wide flux weakening applications. This paper also examines the torque control accuracy under motor parameter drifts to determine how to decouple its effect using a voltage disturbance state-filter design.
    IEEE Transactions on Power Electronics 12/2013; 28(12):5639-5646. DOI:10.1109/TPEL.2013.2253802 · 5.73 Impact Factor
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    Amir Parastar, Jul-Ki Seok
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    ABSTRACT: Offshore wind farms are rapidly growing owing to their comparatively more stable wind conditions than onshore and land-based wind farms. The power capacity of offshore wind turbines has been increased to 5MW in order to capture a larger amount of wind energy, which results in an increase of each component's size. Furthermore, the weight of the marine turbine components installed in the nacelle directly influences the total mechanical design, as well as the operation and maintenance (O&M) costs. A reduction in the weight of the nacelle allows for cost-effective tower and foundation structures. On the other hand, longer transmission distances from an offshore wind turbine to the load leads to higher energy losses. In this regard, DC transmission is more useful than AC transmission in terms of efficiency because no reactive power is generated/consumed by DC transmission cables. This paper describes some of the challenges and difficulties faced in designing high-power-density power conversion systems (HPDPCSs) for offshore wind turbines. A new approach for high gain/high voltage systems is introduced using transformerless power conversion technologies. Finally, the proposed converter is evaluated in terms of step-up conversion ratio, device number, modulation, and costs.
    Journal of power electronics 09/2013; 13(5). DOI:10.6113/JPE.2013.13.5.737 · 0.75 Impact Factor
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    ABSTRACT: The frequency of a power system should be kept within limits to produce high-quality electricity. For a power system with a high penetration of wind generators (WGs), difficulties might arise in maintaining the frequency, because modern variable speed WGs operate based on the maximum power point tracking control scheme. On the other hand, the wind speed that arrives at a downstream WG is decreased after having passed one WG due to the wake effect. The rotor speed of each WG may be different from others. This paper proposes an algorithm for assigning the droop of each WG in a wind power plant (WPP) based on the rotor speed for the virtual inertial control considering the wake effect. It assumes that each WG in the WPP has two auxiliary loops for the virtual inertial control, i.e. the frequency deviation loop and the rate of change of frequency (ROCOF) loop. To release more kinetic energy, the proposed algorithm assigns the droop of each WG, which is the gain of the frequency deviation loop, depending on the rotor speed of each WG, while the gains for the ROCOF loop of all WGs are set to be equal. The performance of the algorithm is investigated for a model system with five synchronous generators and a WPP, which consists of 15 doubly-fed induction generators, by varying the wind direction as well as the wind speed. The results clearly indicate that the algorithm successfully reduces the frequency nadir as a WG with high wind speed releases more kinetic energy for the virtual inertial control. The algorithm might help maximize the contribution of the WPP to the frequency support.
    Journal of Electrical Engineering and Technology 09/2013; 8(5). DOI:10.5370/JEET.2013.8.5.1021 · 0.52 Impact Factor
  • YoungSun Lee, SeYoung Park, Jul-Ki Seok
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    ABSTRACT: This paper evaluates the comprehensive optimization process of associated passive components of three-phase front-end interleaved boost converters (IBCs) to enhance the power density. It was found that the circulating zero-sequence current between rectifiers gives a negative impact on the low-order harmonics of the input current. The effect of circulating current was first analyzed in detail and the average input current model was developed in the presence of the circulating current. The model is subsequently incorporated into the development of the physics-based average current controller to compensate for harmonic distortions without sacrificing the system power density. The proposed method can be an attractive solution for enhancing the power density of low-power consumer electronic applications that need to meet the harmonic regulations.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
  • SeHwan Kim, Jul-Ki Seok
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    ABSTRACT: This paper proposes an enhanced direct torque and flux control method using the discrete-time version of the physical constraints to complete the finite-settling-step direct torque and flux control (FSS-DTFC) algorithm with a constant switching frequency. Successful application of the control approach has been corroborated by a graphical and analytical analysis that naturally leads to a single voltage selection policy for the discrete-time current limit (DTCL)-based FSS-DTFC. The algorithm has the advantage of providing adequate results over the number of potential secondary upsets found in the steady-state current limit (SSCL)-based DTFC. The proposed voltage selection rule enables the motor to track the fast changes in torque and stator flux linkage, which improves dynamic responses significantly over a wide constant-power operating region. The control strategy has been evaluated on a 900W IPMSM in both simulation and experiment.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
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    ABSTRACT: This paper presents a new solid-state step-up resonant Marx modulator (S3RM2) with a continuous output current for offshore wind energy applications. The developed topology is based on the Marx generator concept, where magnetic switches are replaced by solid-state switching devices. The proposed converter is characterized by resonant switching transitions to achieve minimal switching losses and maximum system efficiency. Therefore, a higher switching frequency is conceivable to attain a higher power density. A double module consists of the 4-active switches operating at the output voltage up to seven times as high as the input voltage. An appropriate output capacitor size is considered to eliminate output voltage ripples and work as charge storage. The series-modular and cascade configurations of the S3RM2 have the advantage of being readily applicable to multilevel power switching converters with an arbitrary number of levels. The developed topology has been implemented on a 5-kW prototype converter to verify its feasibility.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
  • SeHwan Kim, Jul-Ki Seok
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    ABSTRACT: A hybrid position sensorless motor control strategy is developed to meet the challenges of electrically assisted turbo-charger (EATC) systems integrated into an automotive engine plant. The proposed controller has a straightforward structure with properties that combines the conventional current vector controller (CVC) and the proposed hexagon voltage modulation controller (HVMC). The proposed HVMC accomplishes the maximum available voltage utilization, allowing a larger motor torque generation per ampere than that of current control alone in the 2nd flux weakening region. This feature leads to a higher PM flux linkage of the motor design, which lets us capture the reliable back-EMF information at the starting speed. In addition, setting a lower bound of the base speed can be achieved to avoid non-ideal current regulation problems at higher speeds. The proposed structure can offer further flexibilities in motor design and drive control, beyond a conventional turbo-charger.
    Energy Conversion Congress and Exposition (ECCE), 2013 IEEE; 01/2013
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    ABSTRACT: Recent power electronics and variable-frequency motor drive technologies have been applied to air conditioners to improve efficiency and power density. However, the mechanical vibrations and acoustic noise resulting from the compressor still remain as a serious problem. This paper presents the development and implementation of an online disturbance state-filter for the suppression of multiple unknown and time-varying vibrations of air conditioning systems. The proposed design has a form of the state-filter based on a Luenburger-style closed-loop speed observer. An active vibration decoupling strategy with an estimated disturbance is provided, which manipulates a motor torque command. Since the proposed estimation does not require any additional transducers or hardware for obtaining real-time information upon disturbances, it is suitable for retrofitting industrial air conditioners.
    Journal of power electronics 11/2012; 12(6). DOI:10.6113/JPE.2012.12.6.1003 · 0.75 Impact Factor
  • Sehwan Kim, Jul-Ki Seok
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    ABSTRACT: This paper proposes a voltage-limited finite-settling-step direct torque and flux control (FSS-DTFC) method with a constant switching frequency for torque-controlled interior permanent-magnet synchronous motors (IPMSMs). The proposed control law dynamically scales voltage vectors on the hexagonal voltage boundary to ensure maximum torque capabilities at given operating conditions, while simultaneously regulating the stator flux linkage magnitude under flux-weakening operation. Instead of evaluating control performance based on intuitive voltage selection rule, this paper focuses on developing a discrete time function of the rate of change of an air-gap torque for facilitating optimal voltage vector choices. Then, this analytical solution leads to optimal voltage modification at each time step with respect to the available inverter voltage. The voltage-limited FSS-DTFC approach has potential advantages of achieving fastest transient torque trajectories and direct manipulation of the stator flux linkage while exploiting maximum voltage excitation.
    Energy Conversion Congress and Exposition (ECCE), 2012 IEEE; 01/2012
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    ABSTRACT: This paper proposes the development and implementation of a hybrid maximum voltage utilization controller for interior permanent magnet synchronous motors (IPMSMs) over a wide operating region. It has a structure that combines the current vector control (CVC)-type Maximum Torque Per Ampere (MTPA) controller and the modulating voltage scaled controller (MVSC). Particularly, the current regulator is deactivated for achieving the true maximum voltage utilization in the MVSC region. The hybrid structure provides a smooth transition from the CVC to the proposed MVSC mode by deactivating the current regulator in the flux weakening region. A seamless transition to the full six-step modulation can be easily realized by adjusting a scaling gain, which can be considered a very significant merit in terms of power utilization for wide flux weakening applications. This paper also attempts to investigate the torque control accuracy under motor parameter drifts and provide how to decouple its influence using a voltage disturbance state-filter design.
    Energy Conversion Congress and Exposition (ECCE), 2012 IEEE; 01/2012
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    SeHwan Kim, Chan-Hee Choi, Jul-Ki Seok
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    ABSTRACT: This paper presents the development and implementation of an online voltage disturbance estimator to achieve the precise torque control of interior permanent magnet synchronous motors (IPMSMs) over a wide operating region. Different design methodologies for disturbance extraction are analyzed using a complex vector frequency-response function (FRF) and evaluated in terms of estimation accuracy. The proposed design has a form of the state-filter based on a Luenburger-style closed-loop stator current vector observer. We provide an accurate torque control strategy with the estimated disturbance, which is based on a torque command manipulation. In addition, the developed estimator is proven to work with any torque control schemes incorporating even with numerous lookup tables. The proposed disturbance estimation scheme has the advantage of being general and readily applicable to a wide range of other systems, such as PWM converters and symmetric machines.
    Energy Conversion Congress and Exposition (ECCE), 2011 IEEE; 10/2011
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    Chan-Hee Choi, Jul-Ki Seok, R.D. Lorenz
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    ABSTRACT: This paper proposes a wide-speed deadbeat-direct torque and flux control (DB-DTFC) method associated with inverter voltage and current constraints of interior permanent-magnet synchronous motors (IPMSMs). The proposed approach has potential advantages controlling torque and flux linkage at voltage and current limits since integrators are not involved for torque control or flux weakening. An automatic transition between non-limited operation and six-step modulation can be achieved without modifying the control law. To support this, we provide a graphical and analytic analysis that naturally leads to a unique stator voltage vector selection on the hexagon. The proposed controller can maximize the available inverter voltage and generate higher output torque than conventional current vector controllers at high speeds. The method developed in this paper also maintains beneficial DTC features, such as fast dynamics and direct manipulation of the stator flux linkage for flux weakening.
    Energy Conversion Congress and Exposition (ECCE), 2011 IEEE; 10/2011
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    Rajendra L. Shrestha, Jul-Ki Seok
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    ABSTRACT: This paper presents an online voltage disturbance estimator to achieve precise torque control of IPMSMs over a high speed operating region. The proposed design has a type of state-filter based on a Luenburger-style closed loop stator current vector observer. Utilizing the frequency response plot (FRF) approach, the estimation accuracy and the parameter sensitivities are analyzed. Accurate torque control and improved efficiency are provided with the decoupling of the effect of the parameter variations. The feasibility of the presented idea is verified by laboratory experiments.
    Journal of power electronics 09/2011; 11(5). DOI:10.6113/JPE.2011.11.5.713 · 0.75 Impact Factor
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    Sung-Kuk Kim, Jul-Ki Seok
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    ABSTRACT: This paper presents a nonparametric approach to failure detection of broken rotor bars in inverter-fed induction motors (IMs). We lay the mathematical foundation for a diagnostic model of a rotor bar fault that captures the rotor bar high-frequency (HF) characteristics. The model shows that the HF equivalent motor resistance can be used as a direct indicator of broken rotor bars. It should be emphasized that the proposed detection methodology is applicable to any shape of rotor slot design by incorporating the idea of synchronous reference frame based injection and by taking the HF resistance as the fault detector. The proposed detection technique is also insensitive to other motor parameters and is effective under arbitrary load conditions. The full time-domain-based signature process provides efficient detection and enhances fault isolation. The identification scheme was implemented and tested on an inverter-fed 1.5-kW IM.
    IEEE Transactions on Industry Applications 09/2011; DOI:10.1109/TIA.2011.2153171 · 2.05 Impact Factor
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    ABSTRACT: In this paper, the feasibility of applying a position sensorless control technique to hybrid electric vehicles (HEVs) is practically evaluated. The proposed position estimator has a straightforward structure with properties that combines the model and the saliency tracking-based rotor position estimation for interior permanent magnet synchronous motors (IPMSMs). The proposed method can be used in the event of sensor loss or sensor recovery to sustain continuity of operations. The developed system takes into account the estimated position transition between two distinct sensorless methods. The transition is enhanced by introducing a synchronized transition algorithm based on a single tracking observer. Extensive experimental results are presented to verify the principles and show a reliable estimation performance over the entire speed range including standstill under 150% load conditions.
    Journal of power electronics 07/2011; 11(4). DOI:10.6113/JPE.2011.11.4.464 · 0.75 Impact Factor

Publication Stats

663 Citations
109.66 Total Impact Points

Institutions

  • 2001–2014
    • Yeungnam University
      • Department of Electrical Engineering
      Gyeongsan, Gyeongsangbuk-do, South Korea
  • 2007
    • BravoVax Co., Ltd.
      Wu-han-shih, Hubei, China
  • 1996–1997
    • Seoul National University
      • Department of Electrical and Computer Engineering
      Sŏul, Seoul, South Korea