Jinwu Gong

Analysis and realization of a fast repetitive controller in active power filter system

The repetitive control and resonant control are widely used in grid-connected voltage-source converters recently, such as uninterruptible power supplies (UPS), photovoltaic inverter (PV) and active power filter (APF). This paper introduces a new repetitive control strategy in APF system, whose repetitive period is only one half of the traditional and could eliminate all odd harmonics as a bank of resonant controllers. Compared to traditional repetitive control, proposed control strategy has faster tracking speed and smaller steady-state error; Compared to traditional PR control, it could be simply implemented in digital processor. Theory analysis with simulations and experiments demonstrate the validity and superiority of proposed fast repetitive control strategy.

Power recovery and cost reduction oriented optimization of regenerative cells embedded in cascaded multilevel converter

The novel hybrid cascaded multilevel converter, with partial regenerative cells supplied by bidirectional rectifiers, is cost effective to recover the regenerative energy. To apply it in industrial fields containing various motors with loads, a systematic scheme designing the power cells is indispensable. This paper reveals that the maximum power factor angle of the motor is determined by the deceleration velocity, and that the controllable domain of the converter is determined by the sum of the DC voltage of the regenerative cells. From these analyses derives a step-by-step designing scheme. Furthermore, the scheme is optimized to reduce the regenerative cells, so as to be more cost effective. The results of simulation and experiment demonstrate that the optimized scheme can enable the converter to cost effectively recover the regenerative energy.

Reliability-oriented design of LC filter in buck DC-DC converter

State-of-the-art LC filter design of buck DC-DC converter is based on the specifications of voltage and current ripples and constrains in power density and cost. Since lifetime is an important performance factor in reliability critical applications, this digest proposes a method to optimize the design of the LC filters from a reliability perspective, among other considerations. It investigates the design freedom between the values of inductor and capacitors, the physical formation of the LC network, and the corresponding electro-thermal stresses of the selected capacitors. The outcome enables an optimized LC filter design to fulfill the required lifetime. The theoretical analysis and simulation study are presented which are verified by the experimental results from a buck converter prototype.

High-reliability and harmonic-sharing dual parallel topology for active power filter

This paper chooses the proportional resonant controller for the controlling of Active Power Filter, and proposed a new control topology for dual parallel APF application. The APF_I near the load side employs the proportional resonant controller to compensate the 5th, 11th, 17th and 23rd harmonics; The APF_II near the system side also employs the proportional resonant controller to compensate the 7th, 13th, 19th and 25th harmonics. Both APF_I and APF_II choose the system side harmonic current for feedback control, and also have fast dynamic response due to the proportional resonant controllers. Compared with traditional dual parallel APF, this topology let two parallel APFs have different compensation characteristic, which could compensate selected harmonics and realize harmonic sharing. When one of them is out of working, the other APF could still compensate almost half of all harmonics. Proposed parallel system has advantage for system expansion, cost saving and power-rate improving. The circuit currents for different harmonics are also discussed. This paper analysis the system characteristic and design the system configuration and control parameters. The simulation on MATLAB/simulink also demonstrates the validity and superiority of proposed system configuration and control strategy.

Realization and improvement of repetitive control in rotating frame for active power filter system

The repetitive control, which is based on a delay of fundamental period, is widely used in grid-connected voltage-source converters, such as uninterruptible power supply (UPS), photovoltaic (PV) inverter and active power filter (APF). This paper presents four repetitive controllers in rotating frame for APF system, whose repetitive period is T, T/2, T/3, and T/6 respectively. Compared with traditional repetitive control, these repetitive controllers have fast dynamic response and also have similar structure, which could also be easily implemented in digital processor for APF system. Regeneration spectrum and sensitivity function based analysis methods provide a way for parameters design and performance comparison between each controller. Simulation and experiment results demonstrate the validity and superiority of each controller.

Reliability-Oriented Optimization of the LC Filter in a Buck DC-DC Converter

Lifetime is an important performance factor in the reliable operation of power converters. However, the state-of-the-art LC filter design of a buck dc–dc converter is limited to the specifications of voltage and current ripples and constrains in power density and cost without reliability considerations. This paper proposes a method to optimize the design of the LC filters from a reliability perspective, besides other considerations. An enhanced model is derived to quantify the lifetime of the capacitor in the filter considering the electrothermal stress on it. Furthermore, the influence of different design aspects such as the value of capacitance, the value of inductance, and the type of the capacitor have been discussed, focusing on their impacts on the key design objectives, which are the cutoff frequency, lifetime, and volume. Based on the analysis, an optimized design is proposed among different parameter sets. A 1-kW converter prototype is applied to verify the theoretical analysis and simulation.

Asymmetrical phase-shifting carrier pulse-width modulation for harmonics suppression in cascaded multilevel converter under unbalanced DC-link voltages

In a hybrid cascaded multilevel converter subject to different DC-link voltage levels among cascaded cells, should conventional carrier phase-shifting pulse-width modulation be applied, it will fail to multiply the equivalent switching frequency of the overall output voltage. As a consequence, this makes it more difficult to design the output filter and limits the overall output bandwidth. Based on the mathematical analysis of the switching harmonics in the overall output voltage, three possible strategies of switching harmonics suppression are proposed and discussed. A general asymmetrical carrier phase-shifting PWM for an n-cell-structure is then derived as the most effective strategy to eliminate switching harmonics. Also, its maximum capacity of harmonic suppression is given. Moreover, the engineering implementation of asymmetrical carrier phase-shifting PWM is presented. Simulation and experiment results verify the validity of the proposed asymmetrical carrier phase-shifting PWM.

Adaptability of weighted average current control to the weak grid considering the effect of grid-voltage feedforward

The weighted average current control (WACC) strategy has been widely used in LCL-filter-based grid connected inverters, because it can reduce the order of grid connected inverter system to the first order. However, when the LCL-filter-based inverter is connected to a weak grid with grid-voltage feedforward, the phase and gain margins of WACC will be significantly reduced, which leads to limited gain of current control loop and finally affects the controllers performance. In this paper, a new WACC weighted factor calculation method is proposed to improve the adaptability of WACC to weak grid, by analyzing the effect of grid-voltage feedforward on the grid connected inverters control loop. The method is based on the optimization of the proportional feedforward, and the weighted factor calculation will not be affected by grid impedance. In this case, the stability of the controller is immune to the grid impedance and the current control loop gain is improved. Both theoretical analysis and experimental results are presented to validate proposed control method.

Modeling and damping strategy for active power oscillation of static var generator

The outer voltage control loop is critical important to Static Var Generator (SVG) because it directly relates to the stability of DC voltage. The low frequency active power oscillation of SVG indicates that there may be some interactions when the outer voltage loop is taken into consideration. However, there is no valid and concise model to analyze the mechanism and characteristics of the low-frequency oscillation. This paper firstly analyzes the active power balance between a single-phase SVGs AC and DC side, and establishes a general dc equivalent impedance model reflecting this power balance and DC voltage fluctuation. The Nyquist stability criterion is adopted to present this unstable low-frequency interaction characteristic. The analysis results show that the oscillation frequency is closely related to the outer voltage control loop and the impedance of grid. To suppress oscillation, a DC voltage droop control method is proposed to dampen out the power oscillation. Experiments are carried out on a SVG, and the results validate the correctness of the model, the analytical results and the suppressing scheme.

A segmented power distribution control system based on hybrid regenerative cascaded multilevel converter

In order to realize motors regenerated energy storage and reutilization, a segmented power distribution control system is proposed in this paper based on hybrid regenerative cascaded multilevel converter. The energy storage cells and ordinary cells are connected together to form a hybrid cascaded topology, which reduces the number of active front ends and secondary windings of phase-shifting transformer. According to motors different operation modes, a novel three-segmented control strategy based on active and reactive power control is proposed to realize the energy flow among the ordinary cells, storage cells and the motor, which can achieve the reutilization of regenerated energy and stabilization of DC voltages. A hybrid cascaded multilevel converter with two ordinary cells and two energy storage cells in each phase is taken as an example conducted on a 380V scale-down laboratory prototype. Experimental results demonstrate the effectiveness of the proposed topology and its control strategy.