Hong-Seok Song

Dual current control scheme for PWM converter under unbalanced input voltage conditions

Voltage unbalance in a three-phase system causes performance deterioration of PWM power converters by producing 120 Hz voltage ripples in the DC link and by increasing the reactive power. To eliminate the DC link voltage ripple and the DC component of the reactive power, both positive- and negative-sequence currents should be controlled simultaneously, according to the paper by Rioual et al (1996). The authors used two synchronous reference frames: a positive-sequence current regulated by a proportional integral (PI) controller in a positive synchronous reference frame (SRF); and a negative-sequence current regulated by a PI controller in a negative SRF. In the positive SRF, which rotates counterclockwise, the positive sequence appears as DC, while the negative sequence appears as 120 Hz. In contrast, in the negative SRF, which rotates clockwise, the negative sequence appears as DC, while the positive sequence appears as 120 Hz. By deleting 120 Hz components using a notch filter in each SRF, one can measure positive- and negative-sequence currents separately, and use them for constructing two feedback controllers. Since the negative-sequence current is also controlled in its own SRF by a DC command, this approach yields better performance without increasing the control gain. Note that, since the controller is implemented by a software routine in the digital signal professor chip, using two SRFs does not require additional hardware. The authors demonstrated the effectiveness of the proposed control scheme by using computer simulation and experiments.

An instantaneous phase angle detection algorithm under unbalanced line voltage condition

If a negative sequence is generated by a voltage sag and/or unbalance, it appears as an oscillating error in a synchronous reference frame (SRF). In power conditioning equipment, an exact value of positive sequence is needed for achieving the desired goal of the system, e.g., unity power factor and constant output voltage, whereas the exact value of negative sequence is needed for compensation. To measure the positive sequence separately from the negative sequence, one uses normally a low pass filter having a narrow bandwidth. However, such a filter causes a lot of phase delay or measurement delay, thus the response time of the system tends to be lengthened. The authors propose a method of estimating the positive and the negative sequence voltages separately without a significant delay by utilizing the weighted least-squares estimation (WLSE) method having the covariance resetting technique. They demonstrate through simulation and experiment the superior performance of the proposed scheme in measuring the positive and the negative sequence voltages at the time of abrupt transition. This method can be applied to UPSs, PWM AC/DC converters, active filters, series voltage compensators, etc.

Advanced control scheme for a single-phase PWM rectifier in traction applications

This work presents an advanced control scheme for a single-phase PWM rectifier. The controller consists of a PR (proportional-resonant) controller using the double sampling strategy, namely, a dual update scheme, a delayless feed-forward compensator, and a fast phase angle estimator. The PR controller is capable of regulating a sinusoidal line current without an additional prediction or an extremely high control gain under a medium switching frequency (0.5-5 kHz). The dual update scheme is the method that performs the sampling and control calculation twice in a switching period, which reduces the control time delay significantly and enables us to extend the maximum allowable control bandwidth. The proposed feed-forward compensator uses the estimated one-step predicted value of the source voltage and the source current to avoid adverse effects caused by the one-step delay, measurement noise, and harmonic component of the source voltage in the feedforward compensation process. A fast phase angle estimator is also presented, which is capable of estimating the phase angle and the frequency of the source voltage even under a highly distorted source voltage condition or a sudden amplitude, phase angle, or frequency changing condition. The feasibility of the proposed control scheme is confirmed by experiments.

Source voltage sensorless estimation scheme for PWM rectifiers under unbalanced conditions

A source voltage sensorless estimation scheme is proposed for a pulsewidth-modulation (PWM) rectifier in unbalanced circumstances. The negative sequence is accompanied by the unbalance among phases, and acts as an ac disturbance to a normal-mode estimator and controller. Hence, without considering voltage unbalance, a PWM rectifier yields a voltage ripple in the dc-link voltage and large reactive currents. With the proposed sensorless scheme, both positive and negative components are estimated separately by using a full- (or reduced-) order estimator. The feasibility of the proposed sensorless scheme is confirmed through computer simulation and experiment.

Fault Diagnosis of a ZVS DC–DC Converter Based on DC-Link Current Pulse Shapes

The dc-dc converter is a critical component in a hybrid electric vehicle since it supplies power to an electronic control unit, as well as chassis electric components such as power windows, wipers, etc. In this paper, a low-cost diagnostic method for MOSFET faults in a zero- voltage-switching dc-dc converter is proposed. The proposed method utilizes the dc-link current patterns as the signatures of faults of MOSFETs. A presignal processing circuit consists of a peak detector and an integrator circuit. The ratio of peak-to-integral values, which is similar to the crest factor, is useful for diagnosis.

Very fast phase angle estimation algorithm for a single-phase system having sudden phase angle jumps

This work proposes a very fast phase angle estimation algorithm for a single-phase system under a sudden phase angle jump condition. When a source voltage changes in a step manner, the normal phase angle detector, such as a phase locked loop (PLL) and a (low pass) filter, typically generates phase delay and results in a sluggish response, and causes some time-critical machine to malfunction. This work proposes a method of estimating the fundamental component, the phase angle and the frequency of a single-phase line voltage without a significant delay, even when the amplitude and the phase angle of the line voltage changes in a step manner, by using the weighted least-squares estimation (WLSE) method with the covariance resetting technique. An example application of a single-phase pulse width modulation (PWM) rectifier for a traction is presented in an appendix. This method can be applied to uninterruptable power supply (UPS), PWM rectifier, active filters, etc.

Idling Port Isolation Control of Three-Port Bidirectional Converter for EVs

The battery charger and the dc-dc converter can be combined in a single unit with the three-port converter topology. In the three-port converter, one port is idling, while the other two are operating actively. To block power flowing into an idling port, a virtual isolation scheme is proposed. It is also illustrated by a phase analysis. Effectiveness of the proposed isolation scheme is verified by simulation and experimental results.

Asymmetric Duty Control of a Dual-Half-Bridge DC/DC Converter for Single-Phase Distributed Generators

A dual-half-bridge (DHB) converter is integrated with a half-bridge 60-Hz inverter as a converter/inverter system for a small distributed generator. This topology provides an isolation between the power source and the load with a 100-kHz transformer, and therefore, the system volume is small. On the other hand, it reduces the number of switching devices greatly. However, the half-bridge inverter causes severe capacitor-voltage fluctuations. The unbalanced voltage problem can be solved by controlling the converter switching: In addition to the conventional phase-shift control method, an asymmetric charging method is used that charges the upper and lower secondary capacitors differently. This means that the voltage imbalance is corrected by adjusting the switching time of the secondary switches. Furthermore, a decoupling control algorithm can be derived from this approach. The usefulness of this method is then validated by simulation and experimental results.

Locating power supplies on a personal rapid transit system to minimize system losses

A personal rapid transit (PRT) system is a computer-controlled and lightweight railway vehicle using linear motors. The speeds of vehicles are controlled to be constant, and the distance between vehicles is also strictly controlled to be only 12 m at 40 km/h. Thus, the vehicle density of the PRT system is much higher than that of the mass railway train (MRT) system. Moreover, the required power of a PRT vehicle is much smaller than that of an MRT vehicle. Therefore, the methodology to allocate the position of power supplies for an MRT system is thought to be improper for a PRT system. This paper proposes a practical method that locates power supplies optimally for a PRT system to achieve a load power balance and a total power loss minimization. This method uses an iterative approach based on a new virtual isolation concept among power supplies. The effectiveness and feasibility of the proposed method is confirmed by simulation results and example studies.

Optimal position finding algorithm for the power sources in the PRT system

The PRT (personal rapid transit) system is a computer controlled and light weighted railway vehicle using linear motors. The speeds of vehicles are controlled to be constant, and the distances between vehicles, namely, headway is also strictly controlled to be only 12 m at 40 km/hr. Thus, the vehicle density of the PRT system is much higher than that of the mass railway train (MRT) system. Moreover, the required power of a PRT vehicle is much smaller than that of the MRT. Therefore, the methodology of the power supply location for the MRT system is thought to be improper to the PRT system. This work proposes a practical method that finds the optimal locations of the power supplies for the PRT system to achieve a balanced load sharing and the total power loss minimization. This method is an iterative algorithm based on a new virtual isolation concept between the power sections. The effectiveness and the feasibility of the method is confirmed by simulation results and case studies.