Carl Ngai-Man Ho

Constant-Frequency Hysteresis Current Control of Grid-Connected VSI Without Bandwidth Control

The theory, design, and implementation of a constant-frequency hysteresis current control for grid-connected voltage source inverter (VSI) is presented. The proposed control technique retains the benefit of the hysteresis control having fast dynamic response and tackles the drawback of the standard hysteresis control having variable switching frequency. The concept is based on predicting the current reference, system dynamic behavior and past time to formulate the switching function for dictating the switching times of the switches in the inverter within a pre-defined switching period. Of particular importance, no hysteresis bandwidth (a challenge in the practical implementation) is needed in the entire control method. The operating principles of proposed technique and mathematical derivation of the switching functions will be given. The proposed method is successfully applied to a 300W, 110V, 60Hz grid-connected VSI prototype with the controller implemented by a simple analog circuit. The steady-state and large-signal dynamic response of the VSI are studied. Experimental results show that the inverter can reach the steady-state in two switching actions after the inverter is subject to large-signal input and output disturbances.

Practical Design and Implementation Procedure of an Interleaved Boost Converter Using SiC Diodes for PV Applications

The implementation of an interleaved boost converter (IBC) using SiC diodes for photovoltaic (PV) applications is presented in this paper. The converter consists of two switching cells sharing the PV panel output current. Their switching patterns are synchronized with 180° phase shift. Each switching cell has a SiC Schottky diode and a CoolMOS switching device. The SiC diodes provide zero reverse-recovery current ideally, which reduces the commutation losses of the switches. Such an advantage from the SiC diodes enables higher efficiency and higher power density of the converter system by reducing the requirement of the cooling system. This paper presents also an optimization study of the size and efficiency of the IBC. Based on 1) the steady-state characteristic of the topology; 2) the static and dynamic characteristics of the switching cells; 3) the loss model of the magnetic components; and 4) the cooling system design, the paper provides a set of design criteria, procedures, and experimental results for a 2.5 kW IBC prototype using SiC diodes.

Islanding detection in smart grids

All distributed generators (DG), especially those connected to low voltage distribution grids are required to detect islanding conditions. The methods for detecting islanding are classified in three main categories: passive, active and communication based. Passive methods are based on grid monitoring, are easy to implement but have a large non-detection zone in the case local generation meets the load demand. Active methods, which are commonly used today, may reduce the non-detection zone but in the case of large amount of DGs installed, power quality problems are foreseen. The communication based methods are seldom used today mainly because of high cost of communication.

Implementation and Performance Evaluation of a Fast Dynamic Control Scheme for Capacitor-Supported Interline DVR

The implementation of a fast dynamic control scheme for capacitor-supported interline dynamic voltage restorer (DVR) is presented in this paper. The power stage of the DVR consists of three inverters sharing the same dc link via a capacitor bank. Each inverter has an individual inner control loop for generating the gate signals for the switches. The inner loop is formed by a boundary controller with second-order switching surface, which can make the load voltage ideally revert to the steady state in two switching actions after supply voltage sags, and also gives output of low harmonic distortion. The load-voltage phase reference is common to all three inner loops and is generated by an outer control loop for regulating the dc-link capacitor voltage. Such structure can make the unsagged phase(s) and the dc-link capacitor to restore the sagged phase(s). Based on the steady-state and small-signal characteristics of the control loops, a set of design procedures will be provided. A 1.5-kVA, 220-V, 50-Hz prototype has been built and tested. The dynamic behaviors of the prototype under different sagged and swelled conditions and depths will be investigated. The quality of the load voltage under unbalanced and distorted phase voltages, and nonlinear inductive loads will be studied.

High-Frequency Modeling of the Long-Cable-Fed Induction Motor Drive System Using TLM Approach for Predicting Overvoltage Transients

Induction motor drive systems fed with cables are widely used in many industrial applications. Accurate prediction of motor terminal overvoltage, caused by impedance mismatch between the long cable and the motor, plays an important role for motor dielectric insulation and optimal design of dv/dt filters. In this paper, a novel modeling methodology for the investigation of long-cable-fed induction motor drive overvoltage is proposed. An improved high-frequency motor equivalent circuit model is developed to represent the motor high-frequency behavior for the time- and frequency-domain analyses. The motor equivalent circuit parameters for the differential mode (DM) and common mode (CM) are extracted based on the measurements. A high-frequency cable model based on improved high-order multiple-π sections is proposed. The cable model parameters are identified from the DM impedances in open circuit (OC) and short circuit (SC). To obtain a computationally efficient solution that could potentially be integrated with the motor drive controller, the system equations are discretized and solved using transmission-line modeling (TLM) approach. The proposed methodology is verified on an experimental 2.2-kW ABB motor drive benchmark system. The motor overvoltage transients predicted by the proposed model is in excellent agreement with the experimental results and represents a significant improvement compared with the conventional models.

Design and Implementation of a Fast Dynamic Control Scheme for Capacitor-Supported Dynamic Voltage Restorers

This paper presents a fast dynamic control scheme for capacitor-supported single-phase dynamic voltage restorers (DVRs) for inductive loads. The scheme consists of two main control loops as inner and outer loops. The inner loop is used to dictate the gate signals for the switches in the DVR. It is based on the boundary control method with the second-order switching surface. The load voltage can ideally be reverted to the steady state in two switching actions during a supply voltage dip. The outer loop is used to generate the DVR output reference for the inner loop. It has three control modes for achieving two different functions, including the output regulation and output restoration. The first mode is for regulating the capacitor voltage on the dc side of the inverter, so that the output of the DVR is regulated at the nominal voltage. The second mode is for restoring the output with the near minimum energy injection by the DVR during a voltage dip. The third mode is in maximum voltage injection and will be activated when the capacitor voltage is reduced to a level that starts distorting the output voltage in the second mode. The mode boundaries will be derived in this paper. By studying the small-signal characteristics of the control loops, a set of design procedures will be derived. A 500 VA, 110 V, 60 Hz prototype has been built and tested with nonlinear inductive loads. The dynamic behaviors of the prototype under different voltage dip depths will be investigated.

A comparative performance study of an interleaved boost converter using commercialized Si and SiC diodes for PV applications

A performance comparison of an interleaved boost converter (IBC) using Si and SiC diodes for PV energy conversion systems is presented in this paper. Performance attributes under investigation include the device behavior, thermal requirement, system efficiency and power density. The interleaved boost converter is designed for sustaining the dc link voltage in the energy conversion system. Due to the absence of reverse recovery current in SiC Schottky diodes, low switching loss is generated in the diodes and the switches. This benefit causes higher system efficiency and lower cooling system design requirement. As a benefit, the volume and weight of the heatsink can be further reduced. Furthermore, behaviors of semiconductors and steady-state characteristics of IBC are discussed in the paper. The validity of the analyses is confirmed experimentally by using a 2.5 kW IBC prototype with wide power and input voltage operating range.

Practical implementation of an interleaved boost converter using SiC diodes for PV applications

The implementation of an interleaved boost converter using SiC diodes for PV applications is presented in this paper. The converter consists of two switching cells sharing the PV panel output current. Their switching patterns are synchronized with 180 degree phase shift. Each switching cell has a SiC Schottky diode and a CoolMOS switching device. The SiC diodes provide zero reverse recovery current ideally, which reduces the commutation losses of the switches. Such advantage from the SiC diodes can make the converter system achieve higher efficiency and higher power density by reducing the requirement of the cooling system. This paper presents also an optimization study of the size and efficiency of the interleaved boost converter. Based on 1) the steady-state characteristic of the topology, 2) the static and dynamic characteristics of the switching cells, 3) the loss model of the magnetic components and 4) the cooling system design, the paper provides a set of design criteria, procedures and experimental results for a 2.5 kW interleaved boost converter using SiC diodes prototype.

Fixed-frequency boundary control of buck converters with second-order switching surface

A fixed-frequency boundary control of buck converter will be presented in this paper. The methodology is based on integrating the concept of variable hysteresis into the previously proposed boundary control technique with second-order switching surface. The switching frequency is maintained constant by using a frequency-to-voltage converter and comparing its output with a reference voltage to control the width of the hysteresis in the boundary controller. Compared with the methods using phase-locked loops, the proposed method has a wider lock-in range. Moreover, it integrates the advantages of the boundary control that the converter can settle into the steady state in two switching actions after large-signal input or output disturbances. The basic operating principles, stability analysis and design procedures will be given. The proposed control method has been successfully applied to control a 140 W, 24 V/12 V buck converter. The large-signal dynamic responses under supply voltage and load disturbances will be given.

A Circuit-Level Analytical Study on Switching Behaviors of SiC Diode at Basic Cell for Power Converters

This paper presents a strategy for the analytic determination of the dynamic switching behaviors of the SiC Diode- Si MOSFET basic cell. The approach employs the semiconductor device electrical parameters from data sheets and static characteristic measurements and testing conditions to determine the behaviors of the basic cell. The presented equations can explain the phenomenon of basic cell such as SiC Diode negative current overshoot. A detailed analysis of the typical waveforms during switching is presented stage by stage. Furthermore, a 400V, 4A test bench has been built and tested under different testing conditions in order to determine the main effects. There is good agreement between theoretical analysis and experimental results.