Won-Sang Im

Diagnosis and Fault-Tolerant Control of Three-Phase AC–DC PWM Converter Systems

There is the possibility of an open-switch fault in a three-phase ac-dc pulsewidth-modulation (PWM) converter due to problems with the switching devices. In this case, the converter causes unbalanced input ac currents including increased harmonics and dc-link voltage ripples. In this paper, the one-open-switch fault case of six switching elements is analyzed by considering the switching patterns of space vector PWM and the directions of a faulty phase current to prove the cases of the fault. In addition, this paper proposes a detection method for open-switch faults and a fault-tolerant control method to minimize the imbalance of the input ac currents and the voltage ripple of the dc link in a three-phase ac-dc PWM converter. The usefulness of this paper is verified through experimental results.

Torque Maximization Control of 3-Phase BLDC Motors in the High Speed Region

This paper proposes a new torque control algorithm for BLDC motors to get the maximum torque in the high speed region. The delay of the phase currents is severe due to the stator reactance. The torque fluctuations of BLDC motors increase and the average torque is decreases due to a slow rise in the phase current when compared to the back EMF. In this paper, the phase current of BLDC motors under the high speed condition is analyzed and a torque maximization control is developed on the basis of using numerical analysis. Computer simulations and experimental results show the usefulness of the proposed control algorithm.

Fault detection and tolerant control of 3-phase NPC active rectifier

Compared to conventional 2-level AC/DC PWM converter, the 3-phase NPC active rectifier has a high number of power semi-conductors. Hence the possibility of the open-switch faults is much higher. Due to the many number of active elements, the detection and the tolerant control of the open-switch faults are more complicated than the conventional 2-level converter. Furthermore, the open-switch faults give rise to imbalance of the AC input currents and the fluctuation DC-link voltage. This paper proposes the open-switch fault detection method and the tolerant control method. In particular, open-switch fault case of one switch among inner six switches of NPC active rectifier is analyzed. The usefulness of this paper is verified through the computer simulation and experimentations, respectively.

Analysis and compensation of current measurement errors in a doubly fed induction generator

This paper proposes a compensator to reduce the power ripple of DFIG generated by offset and scaling errors of the rotor current sensors. The input signal of the proposed compensator adopts the synchronous d-axis current of the rotor. The amplitude of the stator current is low but that of the rotor current is high. Also stator current is variable according to the wind intensity but the rotor current is almost constant. Therefore the rotor current is more suitable for the input signal of the compensator. Two errors of the sensor contain one and two times ripple of the slip frequency, respectively. Each ripple component can be obtained by integrating the synchronous d-axis current of the rotor according to the slip angle. These two errors can be compensated by subtracting those from the measured rotor currents. The feasibility and effectiveness of the proposed algorithm can be verified through experimentation.

An Optimal Current Distribution Method of Dual-Rotor BLDC Machines

This paper proposes an optimal current distribution method of dual-rotor brushless DC machines (DR-BLDCMs) which have inner and outer surface-mounted permanent-magnet rotors. The DR-BLDCM has high power density and high torque density compare to the conventional single rotor BLDCM. To drive the DR-BLDCM, dual 3-phase PWM inverters are required to excite the currents of a dual stator of the DR-BLDCM and an optimal current distribution algorithm is also needed to enhance the system efficiency. In this paper, the copper loss and the switching loss of a DR-BLDCM drive system are analyzed according to the motor parameters and the switching frequency. Moreover, the optimal current distribution method is proposed to minimize the total electrical loss. The validity of the proposed method was verified through several experiments.

Diagnosis and fault-tolerant control of 3-phase AC-DC PWM converter system

There is a possibility of open-switch fault of 3-phase AC-DC PWM (Pulse Width Modulation) converter due to some troubles of the switching devices. In this case, the converter causes the unbalanced input AC currents including harmonics and the ripple of DC-link voltage. This paper proposes the phase detection method of open-switch fault and the fault-tolerant control algorithm to minimize the unbalance of the input AC current and the ripple voltage of the DC-link in 3-phase AC-DC PWM converter. In particular, open-switch fault case of one switch among six switches of PWM converter is analyzed. The usefulness of this paper is verified through the computer simulation and experimentation, respectively.

Cooperative Controls for Pulsed Power Load Accommodation in a Shipboard Power System

To isolate the negative impacts of pulsed power loads (PPL) in a shipboard power system (SPS), energy storage systems (ESS) are usually equipped for online ESS charging and offline ESS discharging for PPL deployment. In order to achieve fast and smooth ESS charging, the generation control and ESS charging control need to be well coordinated. In addition, control constraints, such as generation and charging current bounds and ramp rates, and changes of operating conditions during charging, should also be properly considered. To better solve this important but insufficiently investigated problem, two control algorithms are presented in this paper. The first PI-based control algorithm is designed based on analysis of the desired performance and rules for coordination. The second feedback linearization based control algorithm is designed based on a simplified SPS model and ramp rates difference of supply and demand. The algorithms are simple to implement and can effectively reduce the negative impacts during supercapacitor charging. The algorithms are tested with detailed single-generator and multiple-generator SPS models. Power hardware-in-the-loop and real-time digital simulation studies are performed to demonstrate the effectiveness of the proposed algorithms.

A Capacitance Estimation of Film Capacitors in an LCL-Filter of Grid-Connected PWM Converters

A capacitor deterioration of LCL-filter grid-connected PWM converters is progressed by the self-healing mechanism. It leads to the degradation of the filter performance and drop of power factor. Thus, it is required to diagnose fault-point of capacitors and determine the replacement time. Typically, the fault of capacitors is determined when the capacitance is reduced up to 80% from initial value. This paper proposes algorithm to the determine capacitor replacement time of an LCL filter. The algorithm takes the advantage of change of the response on the injected resonant frequency corresponding to 80% value from the initial capacitance. The results of the algorithm are demonstrated through simulations and experiments.

Novel phase advance method of BLDC motors for wide range speed operations

This paper proposes a novel phase advance method of brushless DC motors (BLDCMs) for wide range speed operations. In order to extend the speed of BLDCMs, it is necessary to control phase currents to lead the phase back EMF by a phase advance method. Many phase advance methods have been studied for the speed expansion of the BLDCM during the last 20 years. However, these conventional methods increase the torque ripple even if it widens the range of speed for the BLDCM. As compared with the conventional method, the proposed method takes definite advantages. it can obtain wider range speed and lower torque ripple. A key of this method is to broaden conduction area of phase currents by using an advance angle. The usefulness of the proposed phase advance method is verified through simulation results.

An overmodulation method for space vector PWM inverters with dc-link shunt resistor

In order to lower the cost and expand the working range of pulse width modulation voltage source inverter (PWM-VSI), the control method under the overmodulation mode with a dc-link shunt resistor is developed. The voltage utilization according to un-implementable region is analyzed in overmodulation region. By using the conventional phase reconstruction method, the minimized un-implementable region can be achieved for maximizing voltage utilization and the minimizing total harmonic distortion (THD) under the overmodulation region. The new overmodulation voltage reference which has the same voltage utilization with original voltage reference is produced to minimize THD by using Fourier analysis. The effectiveness of proposed algorithm is verified through the THD comparison between conventional and proposed method. The experimental results verify the validity of new overmodulation control and behavior of phase current reconstruction method.