Showing papers by "Seung-Ki Sul published in 2016"

A Comprehensive DC Short-Circuit Fault Ride Through Strategy of Hybrid Modular Multilevel Converters (MMCs) for Overhead Line Transmission

Abstract: The modular multilevel converter (MMC) is a promising candidate for voltage-sourced-converter-based high-voltage direct current (VSC-HVDC) transmission. The dc short-circuit fault management is a crucial issue especially for overhead line transmission, where the nonpermanent dc short-circuit faults occasionally occur. In this paper, a comprehensive dc short-circuit fault ride through (FRT) strategy is proposed for a hybrid MMC which combines half-bridge submodules and full-bridge submodules. By the proposed method, the hybrid MMC-based VSC-HVDC system is able to ride through a pole-to-pole short-circuit fault supporting the ac grid as a static synchronous compensator. Moreover, the transmission system can still transmit one-half of the rated active power in case of a pole-to-ground short-circuit fault without increasing power semiconductor stresses or requiring higher insulation levels of the electric equipment. The proposed method ensures successful regulation of arm currents and submodule capacitor voltages during the FRT process. Validity of the proposed strategy is verified by both computer simulation and experiment results.

Suppression of Injection Voltage Disturbance for High-Frequency Square-Wave Injection Sensorless Drive With Regulation of Induced High-Frequency Current Ripple

Abstract: In the square-wave voltage signal injection sensorless drive of the interior permanent-magnet synchronous machine, the injection voltage can be distorted by the inverter nonlinearity effects especially when the injection voltage is low. If that happens, a 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 on injection voltage, on induced current ripple, and on the position estimation performance in sequence and proposes a voltage injection method to minimize the impact of the inverter nonlinearity by the regulation of the HF current ripple. By simulations and experiments, the performance of the proposed method has been verified. The experimental results show 19.7% reduction of position estimation error under 15% of rated voltage signal injection without increasing the current ripple at a certain operating condition. Moreover, the performance of position estimation is kept uniform regardless of the loading condition.

A new topology of multilevel VSC converter for hybrid HVDC transmission system

Abstract: In this paper, the existing Modular Multilevel Converter (MMC) topologies for Line Commutated Converter (LCC)-Voltage Source Converter(VSC) connected as a hybrid High Voltage DC (HVDC) transmission system are reviewed and a new topology of multilevel converter for a hybrid HVDC is introduced. Among the existing MMC topologies for the hybrid HVDC, an MMC structure consisted of Half-Bridge SubModule(HBSM)s and Full-Bridge SubModule(FBSM)s has characteristics such as reduced system cost, low operation loss, but still keeping capability to cope with DC short circuit fault. However, it is very difficult for the conventional hybrid MMC structure, where each arm of MMC is consisted of mixed HBSM and FBSM, to balance the submodule capacitor voltages under sliding of DC bus voltage. For solving the defect of the conventional structure of MMC, an asymmetric MMC, where in a leg an arm is consisted of HBSM and other arm of FBSM, is devised. The proposed asymmetric MMC can regulate the DC bus voltage freely without uncontrollable submodule capacitor voltages. The problems of the conventional structure of MMC and the validity of asymmetric MMC are verified by both computer simulation and experiment results.

Design, Modeling, and Control of an IPMSM With an Asymmetric Rotor and Search Coils for Absolute Position Sensorless Drive

Abstract: In this paper, a design methodology and control algorithms of an interior permanent-magnet synchronous machines (IPMSM) for absolute position sensorless drive are proposed. In conventional research on absolute position sensorless control, initial motion of the rotor was inevitable, which was not preferred in many applications. In order to eliminate the initial motion, an IPMSM with an asymmetric rotor and search coils is devised. In the proposed IPMSM, the information of the absolute position is included in the voltage signals at the search coils. By measuring and analyzing the search coil voltages, the absolute position can be estimated without any initial motion of the rotor. Overall performances of the proposed motor are evaluated through rigorous finite element analyses and experimental tests.

Suppression of Circulating Current in parallel operation of three-level converters

Abstract: The Zero Sequence Circulating Current (ZSCC) flows inevitably in parallel converters sharing common DC and AC sources. The currents flowing commonly in all converters increase losses and decrease the overall capacity of parallel converters. This paper proposes a simple and effective ZSCC suppression method based on the Space Vector PWM (SVPWM) method with the ZSCC controller. The zero sequence voltage for the proposed SVPWM is calculated based not on the phase voltage references, but on the grid voltage. The limit of linear modulation index of the converters with the proposed method is analyzed compared to other methods, and it is proven that the limit of linear modulation index can be maximized with the proposed method. The effectiveness of the proposed method has been verified through the experimental set-up consisted of the four parallel three-level converters. It has been confirmed that the ZSCC is well suppressed and larger linear modulation index is achieved at the same time with the proposed method. Moreover, the proposed control method does not require any communication between converters to suppress the ZSCC unlike the conventional methods.

Analysis and compensation of inverter nonlinearity for three-level T-type inverters

Abstract: This paper analyzes the inverter nonlinearity effect resulting in issues such as narrow pulses and the even order harmonics in three-level T-type inverters. These issues make the compensation of the inverter nonlinearity be difficult. Based on the analysis of the output voltage distortion, carrier-based PWM methods to avoid these issues and to balance dc-link voltages simultaneously are proposed using the concept of the offset voltage. The proposed PWM methods can be easily implemented by adding appropriate offset voltages to output voltage references. Also, a compensation method to alleviate inverter nonlinearity effects is proposed based on the modeling of the inverter nonlinearity. The effectiveness of proposed methods is verified by experimental results. Through the proposed algorithms, not only even harmonics but also 5th and 7th harmonic components of current are conspicuously reduced. At the same time, the neutral voltage of the inverter can be balanced effectively by the proposed PWM methods.

Improved Design of IPMSM for Sensorless Drive With Absolute Rotor Position Estimation Capability

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 proposed for the same purpose had a demerit that for the absolute position estimation, the rotor should initially stroke for 100 degrees in the worst case. Using the proposed IPMSMs featuring asymmetric winding and asymmetric rotor geometry, the initial stroke of the rotor can be reduced to 30 degrees. Moreover, basic characteristics of the motor such as torque ripples and harmonics of electromotive force (EMF) are conspicuously improved. The proposed designs are evaluated by rigorous finite-element analyses and experimental tests. An analytical model of the proposed motors considering their asymmetric structures is also developed and provided in this paper.

Linear Overmodulation Strategy for Current Control in Photovoltaic Inverter

Abstract: The maximum power point should be continuously tracked in a photovoltaic (PV) inverter. However, the maximum power point could be found at very low dc-link voltages around sunrise or sunset. Then, the overmodulation (OVM) of the inverter has to be considered to keep the power generation even at low dc-link voltages. In this paper, a series of implementation schemes is proposed to keep the current regulation, even if the inverter is under the OVM. These schemes can be helpful to increase the operating time of the PV inverter, which correlates with the daily amount of power generation. After being examined in a small-scale prototype inverter, the proposed method has been tested with a 250-kW PV inverter installed at a proving ground. In a sunset, the operation time of the inverter has been extended by about 12 min, and the generated energy has been increased by 234 Wh, when compared to the normal operation without the proposed methods.

Shaping of PWM Converter Admittance for Stabilizing Local Electric Power Systems

Abstract: This paper suggests the concept of shaping the pulsewidth-modulated (PWM) converter admittance. The analytic admittance matrix equation, which is derived from the small signal model of a PWM converter, is formulized. Based on the damping coefficient and the natural frequencies of phase-locked loop and dc voltage loop, the variations in admittance frequency responses are discussed. In addition, the frequency response can be adjusted through pole placement using the gain of the current controller. The controller gain for a stable operation is designed by detailed understanding of the frequency response shape. Furthermore, to stabilize the local electric power systems (EPSs) that invoke instability through the interaction between constant power load PWM converters, the admittance shaping loop is utilized for the distributed generator PWM converters. Through computer simulation and hardware experiments, the proposed shaping guidance can effectively stabilize local EPS.

Control and Analysis of Engine Governor for Improved Stability of DC Microgrid Against Load Disturbance

Abstract: This paper proposes an engine governor (GVR) controller of genset suitable to dc microgrid. The conventional genset controller is vulnerable to load disturbance and results in a considerable speed fluctuation, or frequency variation, of the generator. Moreover, variable speed operation of genset, which increases fuel efficiency of the genset compared with the conventional constant speed operation, would weaken the dc grid stability against severe load disturbance. This paper proposes an engine GVR controller designed with consideration of dynamic stability. The controller incorporates a dc grid voltage regulation as well as an additional acceleration capability, providing instant reaction to sudden load disturbance. The design and analysis of the proposed controller is performed theoretically, and its effectiveness is verified through a series of experimental tests.

Suppression of circulating current in paralleled inverters with isolated DC-link

Abstract: In this paper, it is described how the circulating current flows between inverters which have isolated DC-link such as Cascaded H Bridge (CHB) topology, when they operating in parallel. In this case, Zero Sequence Circulating Current (ZSCC) flowing through shared DC-link cannot flow between inverters in parallel. However, circulating current is provoked by asynchronous switching instant of devices and difference of DC-link voltages of parallel inverters. The circulating current should be suppressed by sharing inductor which is inserted between inverters. Generally, all of the inverters should synthesize the same output voltage reference for load current control, to minimize the size of this sharing reactor. However, this conventional method cannot guarantee to diminish circulating current in the transient state and even in the steady state. In this paper, to reduce circulating current, after deriving circulating current model, circulating current control method is devised based on the model. This proposed algorithm is applied to Active Front End 5Level-CHB inverter system for medium voltage drive. Simulation and experimental results are provided to verify the effectiveness of the proposed control scheme.

DC capacitor voltage balancing control for delta-connected cascaded h-bridge STATCOM considering the unbalanced grid and load conditions

Abstract: In this paper, a comprehensive control scheme for a delta-connected cascaded h-bridge (CHB) converter based static synchronous compensator(STATCOM) is presented, especially focusing on improving dynamic performance by novel feedforward control method. The method can conspicuously improve the dynamics of circulating current regulation of delta connected CHB STATCOM especially under grid fault condition as well as load unbalance without excessive DC cell capacitor voltage fluctuation. The full scaled simulation results and the down scaled experimental results verify that stable operation is guaranteed for both emulated grid and load unbalance conditions.

New maximum power extraction algorithm for single stage solar water pump system working effectively for dynamically changing conditions

Abstract: This paper proposes a new maximum power point tracking (MPPT) scheme, based on perturb and observe concept, for a single stage PV water pump system working effectively under various environmental conditions. The selection of frequency step size is an important parameter for single stage solar water pump system. Larger step size may lead to a sudden drop in DC link voltage due to oscillations around the maximum power point. Smaller step size takes longer time to reach MPP or may get trapped at lower power point. The system may also fail under dynamic change of environmental conditions (particularly during rapid fall in solar irradiation). The proposed algorithm is able to identify the conditions under which DC link voltage may fall and operate the system at a point where it can work efficiently without halt of system. The detailed analytical, simulation and experimental results of this work under different environmental conditions are presented in this paper.

DC link voltage control of single phase back-to-back converter for medium voltage motor drive

Abstract: This paper describes DC link voltage control method of single phase back-to-back converter for medium voltage motor drive system. Cascaded H-Bridge (CHB) topology is widely used to achieve medium voltage operation and low harmonic distortion, simultaneously. Each power cell of CHB should have the isolated DC link voltage by using input transformer and rectifier. Instead of three phase Active Front-End (AFE) which is mostly used, using single phase AFE could make the structure of input transformer and power cell be simple. However, even in the same three phase windings of the input transformer, the DC link voltage of each single phase AFE might be different due to the power imbalance of each phase. In this paper, DC link voltage balancing control method that uses the negative sequence voltage of converter is described. The proposed method is verified with experimental results. The DC link voltage of each AFE has been controlled within 1% error of its reference value.

Pulse-width modulation strategy in double-delta sourced winding

Abstract: A topology, so called, Double-Delta Sourced Winding (DDSW) has been proposed to improve harmonic properties in high-power conversion systems. In this paper, a pulse-width modulation method is proposed to further optimize the harmonic properties in DDSW-based systems. Specifically, it is described through mathematical analyses how to modify voltage references for reducing harmonics in combination with DDSW. The effectiveness of the proposed method is assessed with experimental results. As a result, the proposed method revealed 40% further reduction of harmonic current.

Analysis on current flowing through deactivated modules in parallel connected AC/DC converters

Abstract: In this paper, the problem with the “unexpected current” flowing in parallel connected AC/DC converter system was discussed. The above mentioned “unexpected current” is the current that flows through the diodes of the turned-off converters where some converters are operating but others are not. Since this extra current degrades the efficiency of converters and distorts the grid current, it should be minimized. The phenomenon about the current which flows through the diodes of the turned-off converters was analyzed, and several ways to minimize this current have been reviewed and their effects were verified by simulation and experimental results.

A PWM method reducing harmonics of two interleaved converters

Abstract: This paper proposes a PWM method reducing harmonics of two interleaved converters at twice the switching frequency region. If harmonics at twice the switching frequency region can be reduced by the PWM method, the size of filter interfacing the grid and converter would be reduced. Based on Fourier series of phase voltage in every switching period, offset voltage for PWM minimizing harmonics at twice the switching frequency region can be calculated in real time. With the proposed PWM, the size and volume of inductive filters of two interleaved parallel converters can be shrunken conspicuously. Effectiveness of the proposed method is verified by the computer simulation and experimental results. The maximum harmonic at twice the switching frequency region has been reduced by 58% at modulation index 0.8.

Model-based sensorless control of IPMSM enhancing robustness based on the estimation of speed error

Abstract: In this paper, new model-based sensorless control methods are proposed, including estimation methods for the position and speed errors and compatible position and speed estimators. In proposed methods, the speed error is estimated as well as the position error in the conventional model-based sensorless methods. By the proposed methods, unity transfer function from actual position to estimated position can be achieved eliminating the effects of load disturbances. It means that the position error would be ideally zero even in transient situations. Experimental results verify the effectiveness of the proposed methods under severe speed transient (20,000 r/min/s) and load torque transient (20 p.u./s). With the proposed methods, position error has been conspicuously reduced by 60% at speed transient and by 70% at load torque transient in high speed region.

A three phase current regulation strategy with inductor over saturation region

Abstract: This paper presents an improved current regulating strategy with a feed-forward term in stationary reference frame to compensate widely varying inductance of three phase AC inductor over magnetic saturation region. In conventional current regulators such as state decoupled PI controller and complex vector PI controller, inductor in three phase circuit is presumed as linear and constant, and always operating far lower than the magnetic saturation level region. However, because of three phase instantaneous current difference and nonlinear characteristics of magnetic material, inductance of each phase would not be the same among three phase and would be time varying. Moreover, because of saturation characteristics, inductors are oversized to avoid the saturation region. With the proposed control strategy by extracted self and mutual inductance look-up table, not only inductance variation can be compensated, but also low order harmonics, especially 3rd order harmonics, are considerably reduced as well when operating over saturation current level. Experiment results are presented to verify the validation of the proposed control strategy.

Suppression of Circulating Current in Parallel Operation of Three-Level AC/DC Converters

Abstract: Zero-sequence Circulating Current (ZSCC) flows inevitably in parallel converters that share common DC and AC sources. The ZSCC commonly flowing in all converters increases loss and decreases the overall capacity of parallel converters. This paper proposes a simple and effective ZSCC suppression method based on the Space Vector PWM (SVPWM) with the ZSCC controller. The zero-sequence voltage for the proposed SVPWM is calculated on the basis of the grid voltage and not on the phase voltage references. The limit of the linear modulation region of the converters with the proposed method is analyzed and compared with other methods, thereby proving that the limit of the region can be extended with the proposed method. The effectiveness of the proposed method has been verified through the experimental setup comprising four parallel three-level converters. The ZSCC is confirmed to be well suppressed, and the linear modulation region is extended simultaneously with the proposed method. Moreover, the proposed control method does not require any communication between the converters to suppress the ZSCC unlike other conventional methods.

Control strategy of single phase back-to-back converter for medium voltage drive under cell fault condition

Abstract: Cascaded H-Bridge (CHB) inverter is the most widely used topology for Medium Voltage (MV) drive system due to the high degree of modularity, easier implementation of medium output voltage, and ability to continuous operation under the cell fault condition. Because each power cell of CHB should have isolated DC source, multi-winding input transformer and three phase Active Front-End (AFE) are generally used for regenerative applications. The whole system can be reduced by replacing the three phase AFE with single phase AFE. However, input power imbalance among three phases under the cell fault condition inevitably occurs, if control strategy of normal operation is employed in system. This paper proposes a control scheme for the cell fault condition of single phase AFE CHB. Not only DC-link voltages of each cell but also grid current are regulated well without imbalance by applying the proposed control scheme to the system, even in the cell fault condition. Simulation and experimental results are provided to verify the effectiveness of the proposed scheme.

Analysis of Non Detection Zone for multiple distributed PCS based on Equivalent Single PCS using reactive power approach

Abstract: In this paper, the methodology to analyze Non-Detection Zone (NDZ) of islanding detection algorithm for multiple Power Conditioning Systems (PCSs) connected to grid is proposed. The PCSs with renewable energy source or battery energy storage system are equipped with anti-islanding functionality and the method using frequency positive-feedback is one of the most popular methods to detect islanding condition. Because it is figured out that the positive feedback of grid frequency is only valid on reactive current, not the phase of current or active current, the reactive power according to the frequency drift algorithm is only considered for NDZ analysis in the proposed analysis. Additionally, the reactive power component by frequency drift anti-islanding can be linearized in normal operation range of grid frequency. By incorporated with this concept, Equivalent Single PCS (ESPCS), which equivalently supplies whole reactive current of multiple PCSs, is proposed and this could emulate the operation of multiple parallel PCSs without loss of the detection capability. The NDZ is analyzed by ESPCS and is compared to the NDZ of parallel PCSs. Through the computer simulation and hardware experiments, it is confirmed that NDZ based on ESPCS and operation dynamics is well matched to those of multiple PCSs.

Seamless transfer strategy considering power balance in parallel operation

Abstract: This paper presents a power reference modifier for seamless transfer considering the power balance in the parallel operation. When the PCS (Power Conditioning System) is disconnected from the grid, the PCS should operate in the stand-alone mode to supply energy to the critical loads. If multiple PCSs work as one PCS, slave PCSs maintain the current control mode even if master PCS change its control mode from the current control to the voltage control. If the power references of slave PCSs are not changed properly based on the load condition, the power balance condition may not be met. In that case, the voltage applied to the critical loads might be beyond or below the rated voltage. To avoid this phenomenon, the power values absorbed into the loads should be monitored consistently and the power references of the slave PCSs should be modified properly based on them. But the master PCS which transfers the power references to the slave PCSs and works as voltage source in stand-alone mode can obtain it indirectly by calculating its own power values without monitoring the power consumed by critical loads. Using those, simple controller for modifying the power references of the slave PCSs can be configured. The effectiveness of the proposed power reference modifier has been verified through the experimental results. By applying the proposed method, the transition from the grid-connected mode to the stand-alone mode works satisfying Computer Business Equipment Manufacturers Association (CBEMA) curve.

Design of the Single-loop Voltage Controller for Arbitrary Waveform Generator

Abstract: This study presents a design method for a single-loop voltage controller that is suitable for an arbitrary waveform generator (AWG). The voltage control algorithm of AWG should ensure high dynamic performance and should attain sufficient robustness to disturbances such as inverter nonlinearity, sensor noise, and load current. By analyzing the power circuit of AWG, control limitation and control target are presented to improve the dynamic performance of AWG. The proposed voltage control algorithm is composed of a single-loop output voltage control, an inverter current feedback term to improve transient response, and a load current feedforward term to prevent voltage distortion. The guideline for setting control gain is presented based on output filter parameters and digital time delay. The performance of the proposed algorithm is proven by experimental results through comparison with the conventional algorithm.

Motor, motor system, and detection method of mechanical angle of motor

Abstract: A motor includes a stator and a rotor that is arranged opposite to the stator through a predetermined air gap. The stator is such that a plurality of coils are wound around respective slots for each of a plurality of phases. In the stator, the number of turns of one of the plurality of coils is different from those of the others for each of the phases. In the rotor, among a plurality of magnetic poles formed of a plurality of permanent magnets arranged in a circumferential direction of a core, a magnetoresistance of at least one magnetic pole is different from those of the others.

Three-level back-to-back converters with ESS directly connected to the lower DC-link

Abstract: With the conventional circuit configuration of DC microgrids, two power conversion stages are required to transfer the energy from the Energy Storage System to loads or sources: DC/DC and DC/AC conversions. This paper proposes a new circuit configuration which puts the battery directly to the lower DC-link of 3-level back-to-back converters. The proposed circuit reduces the two-step power conversion stage to one-step. The working principle of the proposed circuit is analyzed, and the controllers for the circuit are designed. The validity of the control algorithm is verified by computer simulation.