Xuancai Zhu

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.

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.

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.

Analysis of the Non-detection Zone with Passive Islanding Detection Methods for Current Control DG System

Islanding detection and anti-islanding protection are the essential functions of the gird-connected inverter in DG systems. This paper focuses on the Non-Detection Zone (NDZ) with passive islanding detection method for the DG system with the current control scheme. The parameters of the point of common coupling such as the voltage amplitude and frequency are studied through mathematical derivation. The NDZ regions of the passive islanding detection methods such as OVP/UVP, OFP/UFP are drawn base on the mathematical derivation. And the analysis is verified by the experiment results upon a fuel cell powered DG system with the current control scheme.

Evaluation of AFD islanding detection methods based on NDZs described in power mismatch space

Islanding detection and anti-islanding protection are the essential functions of the gird-connected inverter in DG systems. The active frequency drift (AFD) method is an effective islanding detection technology. In this paper, AFD methods are studied analytically. the boundaries of the non-detection zones which demonstrates the effectiveness of the methods is then obtained. For comparing with the passive islanding detection methods, the non-detection zones of the AFD methods are represented in a power mismatch space (ΔP versus ΔQ). Finally, the analysis of the NDZs of the system with AFD is verified with simulation results.

Energy management design for a 5kW fuel cell distributed power system

The proton exchange membranes fuel cell is known as a slow dynamic response application because of the existing of mechanical device such as air pressure, temperature and the humidity controllers. The transit load current may shorten the operation life of the fuel cell. In this paper, an energy management module with super capacitor as the auxiliary energy storage device is introduced to create a safe operation condition for the fuel cell during load transition. The energy management module design is base on the experiment results of the dynamic response of the fuel cell alone. The system structure and the model of the energy management are introduced. The controller design based on this model is given. The design of the bi-DC/DC converter and the energy management arithmetic are introduced for the practical realization. Finally, the experiment results upon a 5 kW fuel cell distributed power system with energy management module are given to verify the theoretical analysis.

Current-fed DC/DC converter with reverse block IGBT for fuel cell distributing power system

In this paper, the front-end-converter for fuel cell distributing power system is discussed. A current-fed phase shift full bridge (PSFB) ZCS DC/DC converter with reverse block IGBT (RB-IGBT), which is much suitable for fuel cell source, is investigated. Its ZCS operation condition and design consideration have also been discussed. Experimental results of a prototype for a fuel cell distributing power system are given to verify the expected effects.

Dynamic model of PWM plus phase-shift (PPS) control bidirectional DC-DC converters

In this paper, a modeling technique based on state-space averaging technique for PWM plus phase-shift (PPS) bidirectional DC-DC converter is presented. The simulation results and experiment results have proved the validity of the model.

Energy management analysis and design for a 5 kW PEMFC distributed power system

The proton exchange membranes fuel cell (PEMFC) is known as a slow dynamic response application. The transient load power may bring damage to the stack and shorten lifetime of the fuel cell. In this paper, an energy management design, which consists of a bi-DC/DC converter and super capacitor, is introduced to compensate the slow dynamic response of the PEMFC power source. The energy management principle is analyzed. The controller and the filter design based on these analyses are given. The capacity of the auxiliary energy buffer is calculated. The Bi-DC/DC converter, as the hardware realization is then introduced. Finally, the experiment results on a 5 kW fuel cell power system with energy management are given to verify the theoretical analysis.

A low cost solution to grid-connected distributed generation inverters

A low cost utility interactive inverter is valuable to the residential PV or fuel cell distributed generations. This paper presents a low cost solution to the single-phase grid-connected inverters, including the inverter topology, filter structure, and the current control methods. The reason for choosing the proposed converter topology is given. A novel current control method using the combinatorial feedback of the grid current and the capacitor voltage is introduced for the inverter, so that a low cost non-damped or slightly damped LCL-filter can be employed to attenuate the switch frequency current ripple and the system can be optimized easily for minimum steady-state error and current harmonic distortions, as well as the system stability. The characteristics of the inverter system with the proposed controller are investigated. Simulations and experimental results on a 5kW fuel cell inverter are provided to verify the proposed solution and its control strategy.