Liqiang Yuan

Hybrid Selective Harmonic Elimination PWM for Common-Mode Voltage Reduction in Three-Level Neutral-Point-Clamped Inverters for Variable Speed Induction Drives

This paper proposes a hybrid selective harmonic elimination pulsewidth modulation (SHEPWM) scheme for common-mode voltage reduction in three-level neutral-point-clamped inverter-based induction motor drives. The scheme uses the conventional SHEPWM (C-SHEPWM) to control the inverter at high frequency (≥ 0.9 motor rated frequency) and uses the modified SHEPWM (M-SHEPWM) to control the inverter at low frequency. It also uses a scheme to ensure the smooth transition between the two SHEPWM schemes. As a result, at high frequency, the C-SHEPWM provides the required high modulation index for the motor, while at low frequency, when a passive filter is less effective for common-mode voltage reduction, the M-SHEPWM is used to suppress the common-mode voltage. Experimental results show that the proposed hybrid SHEPWM scheme could meet the modulation index need of the motor and reduce the common-mode voltage in the drive, and the two SHEPWM schemes could transition smoothly.

A comparative study of Luenberger observer, sliding mode observer and extended Kalman filter for sensorless vector control of induction motor drives

Observer-based sensorless techniques are becoming more and more popular and universal. This paper presents a comparative study of three kinds of observers for direct field oriented controlled (DFOC) induction motor (IM) drive: Luenberger observer (LO), sliding mode observer (SMO) and extended Kalman filter (EKF). Each kind of observer employs the dynamic full-order IM model and mechanical equation is introduced to improve the performance of speed estimation. Theoretical principles of the three observers are illustrated. Not only computer simulations, but also a series of experimental results are presented to evaluate the performances of the three observers. A comprehensive analysis and comparisons are given from several aspects, such as steady state accuracy, dynamic performance, low speed operation, parameter sensitivity, noise sensitivity and complexity. The advantages and disadvantages of each observer are summarized and a comprehensive conclusion is given.

Selective Wireless Power Transfer to Multiple Loads Using Receivers of Different Resonant Frequencies

In multiple receivers of resonant wireless power transfer, selective power flow among the loads is an important issue. This paper proposes a new method to control power division. The two-coil structure with different resonant frequencies of the sending and receiving loops is modeled and analyzed. The efficiency is proved to peak at the resonant frequency of the receiving loop, regardless of the resonant frequency of the sending loop. Using this feature, selective power transfer can be achieved by setting the receiving loops at different resonant frequencies. The efficiency of a particular load is greatly influenced by the driving frequency. The multiple-load system with different resonant frequencies is modeled and the efficiency expression of each load is deduced. The mutual inductances of the receiving coils have a small impact on the efficiency distribution. The closer the resonant frequencies of the receiving loops, the less isolated the related loads. The calculations and the experiments confirm the analysis.

Closed-Form Oriented Modeling and Analysis of Wireless Power Transfer System With Constant-Voltage Source and Load

In many practical applications of wireless power transfer, the battery, which can be modeled as a voltage source, is charged wirelessly from the voltage-source inverter via the transmitter and the receiver. Therefore, it is crucial to analyze such a wireless power transfer system. In this paper, the closed-form oriented modeling and analysis of the wireless power transfer system with the constant-voltage source and load are conducted. Two cases are studied: both the transmitter and the receiver are under resonance, and only the receiver is under resonance. In the latter case, the transmitter is set to be inductive for the implementation of zero voltage switching. The battery current, the output power, and the transfer efficiency of both cases are analyzed and compared in detail. The voltage gain, the power factor, and the output power of the latter case are studied to offer physical insights and design guidelines. An experimental prototype is implemented to verify the analysis. The experiments agree with the calculations.

The Impact of Nonlinear Junction Capacitance on Switching Transient and Its Modeling for SiC MOSFET

The nonlinear junction capacitances of power devices are critical for the switching transient, which should be fully considered in the modeling and transient analysis, especially for high-frequency applications. The silicon carbide (SiC) MOSFET combined with SiC Schottky Barrier Diode (SBD) is recognized as the proposed choice for high-power and high-frequency converters. However, in the existing SiC MOSFET models only the nonlinearity of gate-drain capacitance is considered meticulously, but the drain-source capacitance, which affects the switching commutation process significantly, is generally regarded as constant. In addition, the nonlinearity of diode junction capacitance is neglected in some simplified analysis. Experiments show that without full consideration of nonlinear junction capacitances, some significant deviations between simulated and measured results will emerge in the switching waveforms. In this paper, the nonlinear characteristics of drain-source capacitance in SiC MOSFET are studied in detail, and the simplified modeling methods for engineering applications are presented. On this basis, the SiC MOSFET model is improved and the simulation results with improved model correspond with the measured results much better than before, which verify the analysis and modeling.

Current sharing of IGBT modules in parallel with thermal imbalance

For large current, switching devices such as MOSFET and IGBT, often have to be connected in parallel. Due to this reason, derating and preselection of the switching devices become necessary to develop high-power converters. The current imbalance can be produced by stray inductances, device characteristic difference or asymmetric circuit. Moreover, thermal imbalance is anther important reason for current balancing. The static and transient characteristics of an IGBT vary sensitively with its junction temperature. This paper focuses on the current sharing of IGBTs in parallel with thermal imbalance. In this paper, an active gate control method which can achieve current balancing of the IGBTs in parallel with thermal imbalance, is explained and verified by experiments. This method can be applied to an actual 160kW/380V power electronics converter prototype for improving the utilization of the switching devices and enhancing system reliability.

Series-Connected HV-IGBTs Using Active Voltage Balancing Control With Status Feedback Circuit

Transient voltage unbalance is the major problem that limits the application of series-connected IGBTs in high-voltage and high-power converters. Asynchronous gate delay causes series-connected IGBTs not to turn-on and turn-off at the same time resulting in severely unbalanced voltage sharing. An active voltage balancing control technique is proposed in this paper to solve the asynchronous gate delay problem. By sampling the feedback signal caused by unbalanced voltage sharing, the microcontroller generates a time delay for the gate driver to compensate the asynchronous gate delay. The most vital part of active voltage balancing control, the status feedback circuit, is also discussed in detail in this paper. The function of the status feedback circuit and the effect of active voltage balancing control are verified in a two series-connected HV-IGBTs platform in rated operation (5 kV bus voltage and 600 A load current).

A DC-link voltage control scheme for single-phase grid-connected PV inverters

A fast DC-link voltage controller is desirable to reduce the DC-link capacitor for PV inverters. This paper proposes a method for single-phase grid-connected PV inverters to remove the ripple component from the DC-link voltage signal and a scheme to regulate the DC-link voltage based on energy-balance analysis. The proposed scheme provides excellent dynamic performance as well as a function to prevent DC-link overvoltage by limiting the input power, which is verified by both simulation and experiments.

Sensorless 3-level inverter-fed induction motor drive based on indirect torque control

A sensorless induction motor drive fed by 3-level inverter is presented, which combines the principle of indirect torque control, space vector modulation (SVM), speed adaptive flux observer and fuzzy logic controller (FLC). By using SVM, the switching frequency is kept constant and it is easy to solve the problems such as neutral point balance, voltage jump, etc, and more accurate results are obtained. A novel speed adaptive flux observer is proposed in this paper, which can observe flux and rotor speed accurately over a wide speed range. FLC is employed in the outer speed loop to provide excellent dynamic performance and steady state accuracy, especially for variable load changes. Large starting current is one of the problems in application for indirect torque control. By using pre-excitation technique and a ramp function, the stator flux is firstly established smoothly before motor starts, thus starting current is decreased while maintaining enough starting torque. Both simulation and experimental results are presented to validate the effectiveness of the techniques proposed in this paper.

Energy Feed-Forward and Direct Feed-Forward Control for Solid-State Transformer

There is large voltage deviation on the dc bus of the three-stage solid-state transformer (SST) when the load suddenly changes. The feed-forward control can effectively reduce the voltage deviation and transition time. However, conventional power feed-forward scheme of SST cannot develop the feed-forward control to the full without extra current sensor. In this letter, an energy feed-forward scheme, which takes the energy changes of inductors into consideration, is proposed for the dual active bridge (DAB) controller. A direct feed-forward scheme, which directly passes the power of DAB converter to the rectifier stage, is proposed for the rectifier controller. They can further improve the dynamic performances of the two dc bus voltages, respectively. The experimental results in a 2-kW SST prototype are provided to verify the proposed feed-forward schemes and show the superior performances.