Guoqiao Shen

A New Feedback Method for PR Current Control of LCL-Filter-Based Grid-Connected Inverter

For a grid-connected converter with an LCL filter, the harmonic compensators of a proportional-resonant (PR) controller are usually limited to several low-order current harmonics due to system instability when the compensated frequency is out of the bandwidth of the system control loop. In this paper, a new current feedback method for PR current control is proposed. The weighted average value of the currents flowing through the two inductors of the LCL filter is used as the feedback to the current PR regulator. Consequently, the control system with the LCL filter is degraded from a third-order function to a first-order one. A large proportional control-loop gain can be chosen to obtain a wide control-loop bandwidth, and the system can be optimized easily for minimum current harmonic distortions, as well as system stability. The inverter system with the proposed controller is investigated and compared with those using traditional control methods. Experimental results on a 5-kW fuel-cell inverter are provided, and the new current control strategy has been verified.

An improved control strategy for grid-connected voltage source inverters with a LCL filter

A novel current control strategy based on a new current feedback for grid-connected voltage source inverters with an LCL-filter is proposed in this paper. By splitting the capacitor of the LCL-filter into two parts, each with the proportional division of the capacitance, the current flowing between these two parts is measured and used as the feedback to a current regulator to stabilize and improve the system performances. Consequently, the V-1 transfer function of the grid-connected inverter system with the LCL-filter is degraded from a third-order function to a first-order one, therefore the closed-loop current feedback control system can be optimized easily for minimum steady-state error and current harmonic distortion, as well as the system stability. The characteristics of the inverter system with the proposed controller are investigated and compared with those using traditional control strategies. Experimental results are provided, and the new current control strategy has been verified on a 5 kW fuel cell inverter.

Weighted Average Current Control in a Three-Phase Grid Inverter With an LCL Filter

A novel control method, named Weighted Average Current control (WACC) is proposed for damping control of three-phase grid inverter with LCL-filter. With WACC, three-phase current control of the grid inverter can be decoupled under synchronous rotating frame (SRF). Besides the system transfer function is degraded from the 6th order system to the 1st order one. Consequently, the control loop gain and bandwidth can be increased. Therefore the current control of the grid system is able to be optimized easily for higher power quality and good dynamics. WACC is verified with the experiment on a 30kW three-phase grid inverter prototype.

Design of the Dynamic Power Compensation for PEMFC Distributed Power System

Transient load power may bring damage to the proton exchange membrane fuel cell (PEMFC) and shorten the lifetime of the stack. This paper introduces a dynamic power compensation unit consisting of a bidirectional dc/dc converter and a supercapacitor pack, used for compensating the slow dynamic response of the PEMFC and guaranteeing the operation safety of the fuel cell (FC) during load transitions. In this paper, the characteristics of PEMFC are first studied by an experiment, and then, the target of the dynamic power compensation is set based on the experimental results. Subsequently, an analysis on the dynamic power compensation and derivation of the design target is presented. The controller and the filter design based on these analyses are given. With regard to the hardware realization, the bidirectional dc/dc converter is then introduced. Finally, the experimental results on a 5-kW FC power system with dynamic power compensation are given to verify the theoretical analysis and the design. With the dynamic power compensation unit, the FC only needs to supply a slowly changing output power during the sharp load transition process. A more reliable operation condition can be achieved for the FC.

Power Management Unit With Its Control for a Three-Phase Fuel Cell Power System Without Large Electrolytic Capacitors

This paper investigates a three-phase four-wire fuel cell power system. The system is sensitive to the load characteristic in a stand-alone mode, because variation or unbalance of the three-phase load will affect the safety and lifespan of the fuel cell. A novel power management unit (PMU) with its control strategy is proposed. The PMU can compensate fuel cell slow dynamics. In addition, the PMU acts as dc bus capacitors to deal with the low-frequency current ripple. Therefore, the large dc bus electrolytic capacitors can be eliminated in the fuel cell power system. The control strategy of the PMU is derived and the whole system is verified by the experimental results.

A New Current Feedback PR Control Strategy for Grid-Connected VSI with an LCL Filter

Proportional Resonant (PR) controller with harmonic compensators has extremely higher gain at the desired frequencies to reduce the harmonic distortions in the alternative voltage or current waves. However, the harmonic compensators of the PR controller are limited to several low-order current harmonics, due to the system instability when the compensated frequency is out of the bandwidth of the system control loop, especially for the grid-connected converters with the use of an LCL-filter and conventional current feedback methods. In this paper, a new current feedback method and PR control strategy for grid-connected voltage source inverters (VSI) with an LCL-filter is proposed. The weighted average value of the currents flowing through the two inductors of the LCL-filter is used as the feedback to the current PR regulator. Consequently, the control system with the LCL-filter is degraded from a third-order function to a first-order one. In this way, a large proportional control loop gain can be chose to obtain a wide control loop bandwidth that is required by the harmonics compensations of the PR controller, the system can be optimized easily for minimum current harmonic distortions as well as the system stability. The characteristics of the inverter system with the proposed controller are investigated and compared with those using traditional control methods. Experimental results on a 5kW fuel cell inverter are provided, and the new current control strategy has been verified.

A Novel Seamless Transfer Control Strategy Based on Voltage Amplitude Regulation for Utility-interconnected Fuel cell Inverters with an LCL-filter

Fuel cell inverters can operate in both grid-tied and off-grid modes and a continuous AC power is required for sensitive and critical loads. This paper proposes a novel control strategy for the transition between grid-tied mode and off-grid mode of the utility-interconnected fuel cell inverters. By the means of output voltage amplitude regulation in the proposed new control strategy, the inverter is operated in voltage-controlled mode before the static transfer switch having been turned off, result in quickly grid current decrease and minimum voltage fluctuation on load. The algorithm is described and discussed in detail, along with the transition performances. Simulations and experiments on a 5kW inverter prototype are provided to validate the algorithm.

A High Step-Up DC to DC Converter Under Alternating Phase Shift Control for Fuel Cell Power System

This paper investigates a novel pulse width modulation (PWM) scheme for two-phase interleaved boost converter with voltage multiplier for fuel cell power system by combining alternating phase shift (APS) control and traditional interleaving PWM control. The APS control is used to reduce the voltage stress on switches in light load while the traditional interleaving control is used to keep better performance in heavy load. The boundary condition for swapping between APS and traditional interleaving PWM control is derived. Based on the aforementioned analysis, a full power range control combining APS and traditional interleaving control is proposed. Loss breakdown analysis is also given to explore the efficiency of the converter. Finally, it is verified by experimental results.

Novel seamless transfer strategies for fuel cell inverters from grid-tied mode to off-grid mode

This paper proposes two novel control methods for the transition of the fuel cell inverter from grid-tied mode to off-grid mode. By means of inverter output voltage amplitude regulation or phase regulation, the current through the static transfer switch is forced to decrease to zero. Since the inverter is changed from current control mode to voltage control mode before the static transfer switch is turned off, there is no voltage overshoot on the load on the instant when the static transfer switch is turned off, therefore smooth transfer is obtained. Simulations based on PSpice model are provided to verify the theory

Current control optimization for grid-tied inverters with grid impedance estimation

The control of a PWM inverter is affected by variable inductance present in the power line of the grid. This paper presents a new implementation of grid-tied inverter with weighted-average-current feedback control (WAC) and an embedded online technique to determine the grid line impedance. By adding an inter-harmonic voltage periodically in the modulation of the PWM and measuring the current response, it is possible to estimate the load impedance connected to the inverter, and the grid impedance can be obtained by subtracting the low-pass filter from the load impedance. Consequently, current control optimization can be carried out such as rearrangement of poles or zeroes and adjustment of the current loop gains to obtain the desired dynamic response even under changing line conditions. The presented technique can be implemented with the existing sensors and the digital processor of the inverter, provides a low cost and adaptive approach for current control of grid-tied inverters. Simulation on a 5kW grid-tied inverter demonstrates the control method and the impedance measurement.