Xiang Hao

A Sliding-Mode Controller With Multiresonant Sliding Surface for Single-Phase Grid-Connected VSI With an LCL Filter

This paper presents a sliding-mode control (SMC) scheme via multiresonant sliding surface for single-phase grid-connected voltage source inverter with an LCL filter to eliminate the grid current tracking error as well as suppress its total harmonic distortion (THD). In general, the design of SMC leads to a sliding surface that is a linear combination of the system state variables and the generated references. The sliding surface would drift while the system parameters change or external disturbance exists, which affects the tracking error and THD of system output seriously. Moreover, for ac tracking system, integral SMC can reduce, but not fully alleviate the sinusoidal tracking error and has limited ability to suppress the grid current harmonics, especially for high-order harmonics. In order to fully eliminate the grid current tracking error and suppress its THD effectively, multiple resonant terms of the grid current error are added to the sliding function. It is used for the first time in the SMC. This method can be used for an arbitrary ac tracking system. Simulation and experimental results on a 5-kVA single-phase grid-connected inverter prototype show the effectiveness of the proposed control strategy. The tracking precision of the grid current is about 0.91% and the THD of the grid current is 0.76%.

A novel repetitive control scheme for three-phase grid-connected inverter with LCL filter

Grid voltage distortion gives rise to extra grid current harmonics in grid-connected inverter system. To damp these harmonics, this paper proposes a novel control strategy in which inverter output current is regulated by PI controller and grid current is controlled by repetitive controller (RC) for three-phase grid-connected voltage source inverter (VSI) with LCL filter under d-q rotating frame. This paper implements RC to regulate grid current based on the stability of the system under inverter output current control loop. This paper discusses the design of RC and analyzes the stability and effectiveness of the proposed control strategy in damping the grid current harmonics arising from grid voltage distortion. At last, proposed scheme is verified on a 100 kW three-phase grid-connected PV inverter prototype. Simulation and experimental results show that the proposed scheme has a good performance.

A novel grid voltage feed forward control strategy for three-phase grid-connected VSI with LCCL filter

In this paper, a novel grid voltage feed forward control strategy under d-q rotating frame is proposed based on three-phase grid-connected voltage source inverter (VSI) with LCCL filter to suppress the grid current harmonics caused by the grid voltage distortion. Although the control strategy of grid-connected inverter with LCCL filter has many advantages, this control strategy fails to control grid current directly making the current sensitive to grid voltage distortion. A feed forward component of grid voltage is derived to completely eliminate the grid voltage impact on grid current. To make this control strategy applicable, an equivalent scheme in which grid voltage feed forward components are separately inserted into current reference and modulation is proposed. The results of both simulation and experiment of 100kW prototype verify the effectiveness of the proposed scheme.

A discrete-time integral sliding-mode controller with nonlinearity compensation for three-phase grid-connected photovoltaic inverter

This paper presents a novel control strategy that combines a discrete-time integral sliding-mode controller (DISMC) with nonlinearity compensation under d-q rotating frame for three-phase grid-connected photovoltaic inverter to suppress the current harmonics as well as improve the dynamic response and robustness. Because of the nonlinearity introduced by dead time and turn on/off delay of three-phase grid-connected inverter, the output voltage of inverter is distorted seriously. This distortion increases the THD of the grid current. In this paper, the nonlinearity of inverter output voltage is analyzed and the model of inverter with the nonlinearity effect is established accordingly. In addition, a discrete-time integral sliding-mode controller is designed according to the new model to regulate the grid current. The novel control strategy achieves both exact nonlinearity compensation and strong robustness control of the whole system. Finally the control scheme is verified in a 100 kW three-phase grid-connected photovoltaic inverter prototype. The simulation and experimental results show that the proposed control strategy has fast dynamic response, strong robustness and good current harmonics rejection.

Voltage control and fluctuation suppression of the three-phase SST with DC bus in dual rotating reference frames

The three-phase solid-state transformers(SST) with DC bus have been widely studied in micro-grid systems. The double line frequency component of AC power on DC-link in SST is unavoidable because of the inherent single phase structure of the HV-side cascaded H-Bridge(CHB) converter. However, the AC power at double line frequency of three phases can be mutually eliminated by enabling DAB converters with the ability to process the power. This paper develops a closed multi-loops method of voltage control and fluctuation suppression for the three phase SST system with DC-bus in dual rotating reference frames. Theoretical analysis and modeling are provided. The performance of the proposed control strategy is verified through simulation results and the experiment is under way.

An improved constant on/off time control scheme for photovoltaic DC/DC MIC

In the medium and large PV power generation systems, a popular commercialized module integrated converter (MIC), comprised of H-bridge Buck-Boost topology with a pass through switch, would increase the MPPT efficiency when irradiation mismatch occurs. This paper proposes a novel control structure that, in Boost-mode, it works with constant on time control, in Buck-mode, and it works with constant off time control. The scheme can achieve ultra-fast transient response to track the fast-changing input voltage reference from MPPT block. In constant on time control, an improved ramp compensation which is from input capacitor current would increase the efficiency compared with the popular inductor current ramp compensation. A digital and analog mixed technology would be taken in this paper to realize a low cost and high performance structure. The simulation and experiment have been taken to verify the good performance of this control scheme.

A novel current dual-loop control strategy for three-phase grid-connected VSI with LCL filter

In grid-connected PV inverter system, grid voltage distortion gives rise to extra grid current harmonics. This paper first analyzes the shortage of inverter output current control scheme in suppressing the grid current harmonics caused by grid voltage distortion for grid-connected three-phase voltage source inverter (VSI) with LCL filter under d-q rotating frame. To damp these harmonics, we propose a novel dual-loop control scheme in which inverter output current inner loop is controlled by PI controller and grid current outer loop is regulated by the combination of PI and PR controller. PR controller yields a good performance in damping special order harmonics under this control scheme. Output admittance of the system is derived to analyze the impact of grid voltage distortion on grid current. Stability and effectiveness of this proposed strategy are verified with theoretical analysis as well as simulation and experimental results under 100kW PV inverter prototype.

An exact discrete-time model considering dead-time nonlinearity for an H-bridge grid-connected inverter

With the development of the renewable energy, grid-connected H-bridge inverter plays a more important role than ever. However, the dead-time and the zero crossing clamping (ZCC) effect caused by dead-time can cause THD problem. As a consequence, more and more researches have focused on this issue. Most of the researches use the harmonics to replace the effect of dead-time, which ignores ZCC effect. Some other researches use the average current to distinguish the ZCC effect, which is not accurate enough. This paper analyzes the whole changing process of inductor current ripples during power cycle and a complete discrete-time model of the H-bridge grid-connected inverter with dead-time is established. On the basis of the model, the analysis of dynamic behavior is carried out, the result of which shows that the length of dead-time can cause nonlinear phenomenon and reduces the stability region of the controller parameters. Besides, this model is also useful to the nonlinearity compensation of the system.

A multi-resonant sliding-mode controller for single-phase grid-connected inverter with LCL-filter

Sliding mode control (SMC) is recognized as robust controller with a high stability in a wide range of operating conditions, however it suffers from chattering problem. In addition, the sliding surface that is a linear combination of the system state variables and the generated references would drift while the system parameters change or external disturbance exists, which affects the tracking error and THD of system output seriously. In this paper, a fixed switching frequency multi-resonant SMC (MRSMC) is proposed for single-phase grid-connected inverter. The chattering problem of SMC is eliminated by adopting Gaos reaching law and the parameters of SMC are optimized according to the ripple of the system output based on pulse width modulation (PWM). In order to fully eliminate the grid current tracking error and suppress its THD effectively, multiple resonant terms of the grid current error are added to the sliding function. Simulation and experimental results on a 5-kVA single-phase grid-connected inverter prototype show the effectiveness of the proposed control strategy.

Design of auxiliary power supply for high voltage power electronics devices

High voltage and wide input range auxiliary power supply (APS) is widely used in high voltage power electronics devices. Due to a wide range of high input voltage, low power and significant parasitic effects, high voltage converter systems are more challenging to design. This paper proposes a solution to the design of APS which is based on flyback topology with a specially designed transformer. Because of the high input voltage, design of the transformer is a challenge. On the one hand, a proper transformer ratio is designed to decrease the requirement of the MOSFET. On the other hand, as the consideration of creepage effect in the windings of the transformer, a method of winding high input voltage and small volume transformer is proposed. In addition, the leakage inductance in the transformer is large, which will cause voltage spikes on the MOSFET and oscillations between the leakage inductance and the intrinsic capacitance of the MOSFET. The resistor in the RCD snubber should be small enough to absorb most of the energy stored in the leakage inductance. Simulation and experiment results are given to verify the correctness of the design.