Kui-Jun Lee

Robust Predictive Current Controller Based on a Disturbance Estimator in a Three-Phase Grid-Connected Inverter

This paper proposes a robust predictive current controller based on a disturbance estimator for a three-phase grid-connected inverter. The various components that deteriorate the performance of a conventional predictive current controller are regarded as disturbances, and a disturbance estimator is constructed using an inverter output voltage and current values. Furthermore, a grid angle is extracted using a phase-locked loop (PLL) with the estimated reactive component of the disturbance; therefore, sensorless grid voltage control is possible. In order to determine the relevant gains in the disturbance estimator, the frequency components corresponding to the unbalanced grid condition and low-order harmonics, as well as the PLL bandwidth, are considered. Moreover, the stability of the disturbance estimator due to parameter errors in the inductor filter is analyzed. The proposed method has an inherent rapid dynamic response due to the conventional predictive current controller, as well as low-cost implementation and robust control performance with regard to the disturbance and noise interference due to use of the combined estimation algorithm. The feasibility of the proposed predictive current control strategy is verified through the examination of experimental results.

Simple Fault Diagnosis Based on Operating Characteristic of Brushless Direct-Current Motor Drives

In this paper, a simple fault diagnosis scheme for brushless direct-current motor drives is proposed to maintain control performance under an open-circuit fault. The proposed scheme consists of a simple algorithm using the measured phase current information and detects open-circuit faults based on the operating characteristic of motors. It requires no additional sensors or electrical devices to detect open-circuit faults and can be embedded into the existing drive software as a subroutine without excessive computation effort. The feasibility of the proposed fault diagnosis algorithm is proven by simulation and experimental results.

Nonisolated ZVT two-inductor boost converter with a single resonant inductor for high step-up applications

The interleaved operation of a two-inductor boost converter system is a very attractive solution for converting the low input voltage (30–50V) of distributed power sources, such as photovoltaic and fuel cells, to the high output voltage (380V/760V) required in grid-connected power conversion applications. However, a soft switching method is required in the two main switches to increase the overall power conversion efficiency. This paper proposes a zero voltage transition (ZVT) two-inductor boost converter using a single resonant inductor to meet these needs. To satisfy the requirement of soft switching in the two main switches, a resonant cell is constructed at the output side with a bidirectional switch, two auxiliary diodes, and a single resonant inductor. This converter has advantages due to its simple circuitry, reduced size, and low cost due to a single resonant inductor. Compared to a case with a resonant cell at the input side, it does not perturb the input current of the distributed sources. Therefore, it is more suitable for stable maximum power point tracking (MPPT) operation. The validity of the proposed ZVT two-inductor boost converter is verified through experimental results.

An Adaptive Maximum Power Point Tracking Scheme Based on a Variable Scaling Factor for Photovoltaic Systems

An adaptive maximum power point tracking (MPPT) scheme employing a variable scaling factor is presented. An MPPT control loop was constructed analytically and the magnitude variation in the MPPT loop gain according to the operating point of the PV array was identified due to the nonlinear characteristics of the PV array output. To make the crossover frequency of the MPPT loop consistent, the variable scaling factor was determined using an approximate curve-fitted polynomial equation about linear expression of the error. Therefore, a desirable dynamic response and the stability of the MPPT scheme were maintained across the entire MPPT voltage range. The simulation and experimental results obtained from a 3-kW rated prototype demonstrated the effectiveness of the proposed MPPT scheme.

Design of an LCL filter employing a symmetric geometry and its control in grid-connected inverter applications

An inductor-capacitor-inductor (LCL) filter are widely adapted in grid-connected inverter applications. In this paper, the harmonic attenuations of the LCL filter are quantitatively analyzed, and then the design optimization of two inductance values, which are related on a cost and a size, is illustrated. Based on the design optimization, the LCL filter employing a symmetric geometry is proposed. Through the equivalent circuit analysis of the proposed LCL filter, the operating characteristics and validity are presented in detail. In addition, simple proportional-integral (PI) current controller suitable for the LCL Filter is designed to mitigate the resonance problem. From simulation results, it is seen that the proposed LCL filter and control method have a sufficient attenuation and stability for the high frequency distortions and load variations.

Simple Fault Detection and Tolerant Scheme in VSI-fed Switched Reluctance Motor

The Switched Reluctance Motor (SRM) has magnetic independence of the motor phases, so that it can continue to operate despite partial failures occurring in the motor-converter unit. Such inherent fault-tolerant characteristic is useful to the applications requiring high reliability, such as aircraft. However, on occurrence of fault, speed variation and torque ripple increase and it cause problems in applications. Therefore fault-tolerant SRM drive systems improving these problems are of extreme importance. This paper proposes the novel fault detection/isolation scheme using voltage of lower switches in each phase under the occurrence of inverter faults. It has benefit of simple realization and robustness in load/noise. The SRM operating principle with VSI-fed inverter and the proposed fault detection scheme is analyzed in detail. Also the validity of the proposed method is verified by simulation.

Modeling and control of a grid connected VSI using a delta connected LCL filter

In order to connect an LCL filter in a three phase system, two methods are possible. The first one is a wye connection and the second one is a delta connection. To study the stability problems, a mathematical model should be known, but the model of a delta connected LCL filter is not formulated. In this paper, a model with transfer functions is proposed for that purpose. Furthermore, this paper describes the design of a proportional plus resonant controller who regulates grid currents. The design is based on the frequency response analysis. Controller design is respectively carried out for a system with a delta connected LCL filter and a system with a wye connected LCL filter. To verify the proposed model and the designed controller, simulation and experimental results for a system with a delta connected LCL filter are compared with results for a system with a wye connected LCL filter.

Interleaved Bidirectional DC-DC Converter for Automotive Electric Systems

In this paper, interleaved bidirectional DC-DC converter with ZVT cell using a single resonant inductor in continuous conduction mode is proposed. The proposed interleaved bidirectional DC-DC converter has advantages such as a simple circuit, ZVS during wide load range and minimization of current injection effect by using dual current injection time. Also, it is possible to reduce diode drop voltage occurred during dead time of convectional synchronous buck converter. The validity of the proposed converter is verified through experimental results.

Low-Cost Fault Diagnosis Algorithm for Switch Open-Damage in BLDC Motor Drives

In this paper, a fault diagnosis algorithm for brushless DC (BLDC) motor drives is proposed to maintain control performance under switch open-damage. The proposed fault diagnosis algorithm consists of a simple algorithm using measured phase current information and it detects open-circuit faults based on the operating characteristic of BLDC motors. The proposed algorithm quickly recovers control performance due to its short detection time and its reconfiguration of the system topology. It can be embedded into existing BLDC drive software as a subroutine without additional sensors. The feasibility of the proposed fault diagnosis algorithm is proven by simulation and experimental results.

A three-port bidirectional modular circuit for Li-Ion battery strings charge/discharge equalization applications

In this paper, a three-port bidirectional modular circuit applied in charging and discharging equalization for lithium-ion battery strings is proposed. This circuit consists of four MOSFETs and one transformer which provide a simple structure to be easily modularized. Compared to conventional individual cell equalization schemes, it utilizes the transformer as the energy transfer element, allowing direct transfer of energy between arbitrary two cells of three-cell battery module, thus improving the equalization efficiency significantly by using much less number of equalizers for long battery strings. Simulation results are presented to validate the circuit operation and confirm its capability to equalize the three-cell battery module.