Hojoon Shin

Low-Common Mode Voltage H-Bridge Converter with Additional Switch Legs

H-bridge converter with additional switch legs (HA converter) and its offspring circuit are proposed in this paper with the intent to reduce the common mode noise. The proposed topology connects grounds of the input and output terminals, which gives zero common mode current in the ideal case. The operation of the proposed circuit is flexible and allows for the circuit to be capable of both ac–dc and dc–ac conversions. The proposed topology is especially advantageous when it is applied to the photovoltaic power conditioning system in dc distribution system or stand-alone power system because they include large stray capacitances and are prone to common mode EMI. In this paper, a 4-switch HA (HA4S) converter for both ac–dc rectification and dc–ac inversion is derived from the proposed HA converter as the implementation example. The experimental results based on the proposed and the conventional prototype circuits prove that the HA4S converter outperforms the conventional counterparts.

Phase Current Reconstructions from DC-Link Currents in Three-Phase Three-Level PWM Inverters

This paper describes phase current reconstruction methods from dc-link currents in three-phase three-level PWM inverters. Since the three-level inverters have three paths between dc-link and power switches, the reconstructions differ from that in two-level inverters which have two paths there. Considering these paths in three-level inverter, this paper proposes the methods for phase current reconstruction. While the reconstruction method is executed by the currents sampled on the dc-links, the system encounters current-unmeasurable areas where the dc currents cannot be sampled due to the lack of measurement time according to the PWM status. This paper analyzes the areas and proposes a minimum voltage injection method for minimizing them to guarantee accurate current reconstruction. The method provides the expanded operating areas especially in low-output voltage operation and the minimized distortions in output voltages. The feasibility of the proposed method has been assessed in experiments.

Low Voltage Ride Through(LVRT) control strategy of grid-connected variable speed Wind Turbine Generator System

In this paper, a Low Voltage Ride Through (LVRT) control strategy for variable speed full scale Wind Turbine Generator System (WTGS) is proposed. Grid-connected wind power system should satisfy LVRT requirement when grid fault occur, and the variable speed full scale WTGS has advantage of satisfying LVRT compared with the fixed speed WTGS. The proposed LVRT control strategy satisfies not only LVRT requirement but also stabilizing a DC link voltage regardless of reduced DC link capacitance. During the grid faults, especially three phase to ground fault, the grid side converter cannot control the DC link voltage because it is impossible to control grid side power. In this situation, the DC link voltage can be controlled by the generator side inverter using the proposed DC link voltage control scheme. The proposed control scheme can keep the power balance between grid and generator side power in any case of grid fault condition for LVRT. The compliance to Grid Codes has been evaluated by computer simulations. Though the experimental results, it is verified that the proposed method has satisfied the LVRT requirements and DC link voltage control specification simultaneously.

System configuration and control strategy using FPGA for Hybrid vehicle power Control Unit

In this paper, it is presented a configuration to miniaturize HPCU (Hybrid vehicle Power Control Unit) and a control strategy using FPGA logic. To miniaturize the HPCU, the film capacitor is used as a DC-link capacitor instead of the electrolytic capacitor. Additionally, for the minimization of control hardware, a FPGA based controller is used to control multi-module of power converter. To verify the feasibility of the control strategy and FPGA based control hardware, the laboratory based load test is performed. With FPGA based hardware, the sampling frequency of the digital current control loop can be increased by 30% compared to DSP software based control system.

Active DC-link circuit for single-phase diode rectifier system with small capacitance

This paper introduces the multifunctional circuit which is called as active DC-link circuit (ADLC) in single-phase diode rectifier system without electrolytic capacitor. This circuit is located to DC-link node in parallel and shows many performances such as energy buffer for decoupling ripple power and power factor correction (PFC) by controlling instantaneous system power. Therefore the diode rectifier system with ADLC does not need to equip additional PFC for satisfying grid regulations. Furthermore, since the ADLC can boost the DC-link voltage and supply the energy to the motor drive system while the DC-link voltage is lower than back electromotive force (EMF) voltage of motor, the drive system features constant torque and does not need excessive motor current for flux-weakening control in comparison with the conventional small capacitance motor drive system. This paper presents such a power control method and offers the design guideline of active DC-link circuit. The feasibility of the active DC-link circuit are verified by simulation results.

Single-Phase Grid-Connected Motor Drive System With DC-Link Shunt Compensator and Small DC-Link Capacitor

The single-phase diode rectifier system with small dc-link capacitor shows wide diode conduction time and it improves the grid current harmonics. By shaping the output power, the system meets the grid current harmonics regulation without any power factor corrector or grid filter inductor. However, the system has torque ripple and suffers efficiency degradation due to the insufficient dc-link voltage. To solve this problem, this paper proposes the dc-link shunt compensator (DSC) for small dc-link capacitor systems. DSC is located on dc-node in parallel and operates as voltage source, improving the system performances. This circuit helps the grid current-shaping control during grid-connection time, and reduces the flux-weakening current by supplying the energy to the motor during grid-disconnection time. This paper presents a power control method and the design guideline of DSC. The feasibility of DSC is verified by simulation and experimental results.

DC-link shunt compensator for three-phase system with small DC-link capacitor

This paper proposes a control method of DC-link shunt compensator (DSC) located to DC-node in three-phase diode rectifier system. The circuit can substitute for bulky components such as DC reactor filter and DC-link capacitor from conventional three-phase system. Hence the system can be miniaturized and becomes lighter by reducing heavy passive components. DSC supplies harmonic current into diode rectifier system so that the shape of diode rectifier current can be adjusted, and the corresponding ripple power is transferred to floating capacitor. The action of DSC also intends to stabilize the system which has negative impedance characteristic caused by constant power load (CPL). In this paper, the control method is proposed and stability of the system with DSC is also analyzed. The feasibility of proposed method is verified by simulation results.

Active power control for minimum switching of three-phase electrolytic capacitor-less PWM converter

This paper presents active power control method for three-phase electrolytic capacitor-less PWM converter which is operated by minimum switching. In the proposed method, motor side inverter regulates DC-link voltage as the level optimized in the efficiency and then grid side converter always works in the boundary of the limit hexagon in the voltage vector space. By this operation, the switching number of grid side converter is minimized and the system efficiency is improved. Here, when this system operates for motoring mode, undesirable resonance occurs between small DC-link capacitor and filter inductor, and it makes the system unstable. This paper analyzes the resonance and proposes active power control method for DC-link voltage regulation and resonance suppression. This proposed method directly manipulates output voltage references and stably operates the system in both generating and motoring modes. The feasibility of proposed method is verified by simulation and experimental results.

LVRT Control Strategy of Grid-connected Wind Power System

This paper proposes a LVRT (Low Voltage Ride Through) control strategy which should be satisfied by grid-connected wind power system when grid faults occur. The LVRT regulation indicates rules or actions which have to be executed according to the voltage dip ratio and the fault duration. Especially the wind power system has to support the grid with specified reactive current to secure the grid stability when voltage reduction ratio is over 10%. The LVRT regulation in this paper is based on the German Grid Code and full-scale variable speed wind power conversion system is considered for LVRT control strategy. The proposed LVRT control strategy satisfies not only LVRT regulation but also makes power balance between wind turbine and power system through additional DC link voltage regulation algorithms. Because it is impossible to control grid side power when the 3-phase to ground fault occurs, the DC link voltage is controlled by a generator side inverter using the DC link voltage control strategy. Through the simulation and experiment result, the proposed LVRT control strategy is evaluated and its effectiveness is verified.

Grid Current Shaping Method with DC-Link Shunt Compensator for Three-Phase Diode Rectifier-Fed Motor Drive System

This paper proposes grid current shaping method using dc-link shunt compensator for three-phase diode rectifier system without electrolytic dc-link capacitor. In the proposed method, the diode rectifier system can satisfy the grid regulation IEC61000-3-12 without any power factor correction circuit or heavy grid filter inductor. The proposed grid current shaping method can be realized by applying DSC with reduced-rating components and without electrolytic capacitor, which is connected parallel to the dc link. Since DSC has no electrolytic capacitor, the system with DSC has high circuit reliability. Furthermore, DSC can enhance the system efficiency, especially in flux-weakening area, since the motor drive inverter recovers the reduced modulation index, which has been spent in the additional control for small passive components. This paper presents the control method for DSC and analyzes the effect of proposed method. The feasibility of proposed method is verified by simulation and experimental results.