Sizhao Lu

Switching Performance Optimization of a High Power High Frequency Three-Level Active Neutral Point Clamped Phase Leg

This paper introduces a three-level neutral point clamped (NPC) converter as a high power renewable energy grid interface. The phase leg building block is carefully designed to achieve high switching frequency and high efficiency. Different operating modes and switching loops in the phase leg are identified. Switching characteristics of each loop is evaluated by a double pulse test. Various parameters are taken into consideration as influential factors to loss and stress. An in-depth analysis of some special switching transients is also given. The test shows significant switching performance variance for different switching loops. Performance for each switch is optimized separately considering loss and stress tradeoff. A detailed loss model is built based on the test to calculate system loss distribution and loss breakdown under different switching modes and different operating conditions. The ANPC phase leg can be used to avoid undesirable switching loop and to reduce system loss under certain operating modes. The high-frequency NPC phase leg is tested under full power and the high efficiency design objective is verified.

An improved virtual resistance damping method for grid-connected inverters with LCL filters

In renewable energy generations, LCL filters are commonly used to connect in series with the output ports of the converter to smooth the currents flowing into the grid due to their enhanced attenuation ability and smaller inductance compared to single L filters. However, the inherent resonance of an LCL filter makes the control of such a system challenging, and different passive and active damping strategies have been developed till now. This paper aims to the damping problems of LCL filters and proposes a new approach to realize active damping. Based on traditional virtual resistance damping strategy, the new algorithm employs a lead compensation block in the capacitor current feedback loop so as to alleviate the impact of control delay. It is easy to be transformed from conventional virtual resistance damping method since just an additional digital filter is needed. Theoretical analyses for the essence equivalent relationship between passive damping and virtual resistance damping, as well as the impact investigation of control delay are made in details. The results show that the traditional virtual resistance active damping method is equivalent to passive damping of connecting damping resistance in parallel with the capacitor in the control block diagram, while the proposed virtual resistance active damping method is equivalent to passive damping of connecting damping resistance in series with the capacitor in the control block diagram. Both simulation and experimental results have verified the feasibility.

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.

An Improved Phase-Shifted Carrier Modulation Scheme for a Hybrid Modular Multilevel Converter

This paper presents an improved phase-shifted carrier pulse width modulation (PSC-PWM) scheme for the hybrid modular multilevel converter (MMC) consisting of half-bridge submodules (HBSMs) and full-bridge submodules (FBSMs). When the traditional PSC-PWM schemes for the HBSMs-based MMC and the FBSMs-based MMC are directly applied to the hybrid MMC, some mismatch pulses will occur on the arm voltages because the HBSMs and the FBSMs have different characteristics. These mismatch pulses not only generate undesirable harmonics on the output voltages, but also induce the significantly uneven loss distributions between the HBSMs and the FBSMs. Therefore, an improved PSC-PWM scheme for the hybrid MMC is proposed in this paper to deal with these issues. The improved PSC-PWM scheme can eliminate the mismatch pluses in the output voltages, alleviate the uneven loss distributions between the HBSMs and the FBSMs, and reduce the total switching loss of the converter. The mathematical analysis of the improved PSC-PWM scheme for the hybrid MMC is conducted and its harmonic characters are investigated. The analysis results are verified and compared with the traditional PSC-PWM scheme by the simulated and experimental results.

Coupled Inductors in Interleaved Multiphase Three-Level DC–DC Converter for High-Power Applications

This paper investigates and evaluates coupled inductors (CIs) in the interleaved multiphase three-level dc-dc converter. If non-CIs are used in the multiphase three-level dc-dc converter, interleaving operation of the converter will increase inductor current ripple, although the overall output current ripple and common-mode (CM) voltage will become smaller. To reduce inductor current ripple, inverse-CIs are employed. The current ripple in the CI is analyzed in detail. The benefits of the three-level dc-dc converter with CIs under interleaving operation are evaluated. By adding CIs and working under interleaving operation, smaller inductor current ripple, smaller overall output current ripple, and smaller CM voltage can be achieved simultaneously compared with the noninterleaving case. The analysis results are verified by simulations and 10 kW scale-down experiments.

Analysis and design of coupled inductor for interleaved multiphase three-level DC-DC converters

The coupled inductor can reduce the large inductor current ripple in the interleaved multiphase three-level converter, while keeping the benefits of interleaving. The design of the coupled inductor is different from conventional two-level converters. This paper analyzes the flux in the coupled inductors of interleaved multiphase three-level DC-DC converters. Based on the flux analysis, a design procedure is introduced. A new structure of the coupled inductor is also proposed which can significantly reduce the volume and is easy to fabricate.

A new power circuit topology for energy router

A new power circuit topology for energy router is presented in this paper. The new topology employs modular multilevel converter with three-level sub-modules, so less submodules are needed to achieve the same high voltage level compared with two-level sub-modules based modular multilevel converter. The new topology provides high voltage AC port, high voltage DC port, low voltage DC port and low voltage AC port in a single device. Meanwhile, it realizes voltage transformation, galvanic isolation, and good power quality.

Switching performance optimization for high frequency high power 3-level neutral point clamped phase leg

A 3-level neutral point clamped (NPC) phase leg was designed and tested as modularized building block for renewable energy grid interface converter. To achieve high switching frequency and high efficiency, the phase leg was carefully designed and evaluated. Different operating modes and switching loops in the phase leg were identified. Double pulse tests were conducted for turn on and off transient of each loop. Various parameters like gate resistance and parasitic inductance were taken into consideration as influential factors to loss and stress. An in-depth analysis of their impact to the switching transient is also given. The distinctive switching characteristics of each switch provide guidance for its gate resistor selection and loss-stress tradeoff. The phase leg loss distribution and system loss for diode clamped and active clamped NPC under different power factors are calculated and compared based on the loss data from experiment. Based on the result, the overall switching performance for 3-level NPC phase leg can be optimized under various operating condition.

Space vector modulation for 3-level NPC converter with neutral voltage balancing and switching loss/noise reduction

This paper proposes an improved space vector modulation (SVM) method for the 3-level neutral point clamped (NPC) inverter that serves as a bidirectional power conversion system for renewable energy micro-grid. Conventional SVM for the NPC converter majorly deals with neutral point (NP) balance issue without considering switching loss. The proposed SVM method balances the NP voltage and minimize the switching loss/noise simultaneously. The coordinately selected redundant vectors consider both the NP charge and the pulse sequence so that the minimized NP ripple and switching events is guaranteed in one switching cycle. In addition, the switching events between switching cycles are also considered to reduce the total switching loss and noise. This method also guarantees evenly distributed phase leg loss and constant system efficiency under different power factors (PF). The control result for NP balance and loss reduction is verified by both simulation model and experiment on a 200kVA converter hardware.

Coupled inductors in interleaved multiphase three-level DC-DC converter for high power energy storage applications

This paper investigates and evaluates coupled inductors in interleaved multiphase three-level DC-DC converters. If non-coupled inductor is used and maintaining the inductance of inductor, multiphase three-level DC-DC converter interleaving operation will increase the inductor current ripple, although the overall output current ripple and common mode voltage will be smaller. To reduce the inductor current ripple, inverse-coupled inductor is employed. The current ripple in the coupled inductor is analyzed and verified by simulations. By adding coupled inductor and interleaving, we can achieve smaller inductor current ripple, smaller total output current ripple as well as smaller CM voltage simultaneously.