Shanxu Duan

Comparison of P&O and hill climbing MPPT methods for grid-connected PV converter

Maximum power point tracking (MPPT) techniques are employed in photovoltaic (PV) systems to make full utilization of the PV array output power which depends on solar irradiation and ambient temperature. Among all the MPPT strategies, perturbation and observation (P&O) and hill climbing methods are widely applied in the MPPT controllers due to their simplicity and easy implementation. In this paper, both P&O and hill climbing methods are adopted to implement a grid-connected PV system. Their performance is evaluated and compared through theoretical analysis and digital simulation. P&O MPPT method exhibits fast dynamic performance and well regulated PV output voltage, which is more suitable than hill climbing method for grid-connected PV system.

Analysis and Improvement of Maximum Power Point Tracking Algorithm Based on Incremental Conductance Method for Photovoltaic Array

Photovoltaic (PV) array in the photovoltaic power system (PVPS) has the nonlinear current-voltage characteristic which is affected by the panels temperature and irradiance conditions. To improve the energy converter efficiency of the PVPS, the maximum power point tracking algorithm should be adopted to maximize the yield of the PV array. The incremental conductance (INC) method is widely used in the PVPS due to its succinctness and high tracking efficiency. The conventional INC algorithm using a fixed iteration step-size is impossible to achieve rapid dynamic response and good steady tracking accuracy simultaneously, because if the step-size is increased for rapid dynamic response, the tracking accuracy is decreased and vice versa. An improved algorithm with variable step- size is proposed to solve the problem in this paper. The contrastive experimental results between the improved and the conventional INC algorithm are presented to demonstrate its effectiveness.

A novel solid state fault current limiter for DC power distribution network

DC power distribution network is specifically used in industrial manufactures and aircraft/ship power distribution since it can provide flexible interface to various power converters with high efficiency and less conversion stage. The protection for DC power distribution system is more difficult compared with those AC systems since theres no natural zero crossing point for the current. Low shortcut impedance will cause high current rising rate, which can bring heat, arc and electrical force damage to the DC power distribution grid in a quite short time that the DC breakers hardly act. This paper proposed a solid state fault current limiter (FCL) using a combined power switch. Solid state FCL can achieve fast response to the fault current in the DC power grid, without special requirement of the cooling system. Using a combined power switch instead of single fully controlled power electronics component can reduce the steady power loss while normal working states. When fault states occurs, the power electronics control technique can strictly limits the fault current beneath a certain desired range. In that way, the fault damage can be limited. Experiment results are taken to prove this design.

Design considerations and topology selection for dc-module-based building integrated photovoltaic system

The photovoltaic modules using in the building integrated photovoltaic system generally have different installation orientations and angles, moreover these modules are easy to be affected by partial shadows and mismatch of their electrical parameters. Consequently the conventional power configurations based on centralized, string and multi-string technology is difficulty to obtain high energy conversion efficiency, in severe cases, the hot-spot problem can arise and the photovoltaic modules can be irreversibly damaged. A novel power configuration based on dc module technology is proposed to improve the reliability and maximize the output capability of the photovoltaic power system, and it is of potential interest in residential generation system utilizing multiple sustainable energy sources. The dc-module-based system integrates a high step-up dc-dc converter with a photovoltaic module into one electrical device, then assigned to a common inverter, and a compact and cost effective solution is achieved. This paper investigates the design criterion of photovoltaic module integrated dc-dc converter according demands and specifications of the system. The performance and characteristic of the suitable converter topologies is evaluated. Based on the evaluation results, a current-fed half bridge topology is selected for this application, and an experimental prototype was built and tested in the laboratory to verify the results.

An active islanding detection method for grid-connected converters

Islanding, detection is a mandatory function for gird-connected converters. The popular slip mode frequency shift and auto phase shift active islanding detection methods are investigated and an improved slip mode frequency shift (IM-SMS) strategy is proposed in this paper. In the proposed method, additional phase shift is introduced to help stimulating the islanding detection function and the algorithm is simplified as well. When the utility grid is disconnected, the algorithm keeps the frequency of the converter output voltage deviating until the frequency protection relay is triggered. The working principle of the method is introduced and the guidance of parameters selection is also provided. The islanding detection performance is evaluated through theoretical analysis, digital simulation and experiment. The IM-SMS method exhibits features of simplicity, easy implementation and high reliability and is expected to be an effective active islanding method.

A Novel Decoupled Current-Sharing Scheme Based on Circulating-Impedance in Parallel multi-Inverter System

Parallel multi-inverter systems can be designed to have the advantages of expandable output power, improved reliability, and easy N+X redundancy operation. However, a current-sharing control scheme has to be employed to enable the inverters to share the load current equally. The concept of circulating impedance is proposed in the paper, and introduced into current-sharing control scheme in parallel multi-inverter systems. The mathematical model of circulating impedance is set up. By introducing circulating-impedance control regulator, the characteristic of circulating-impedance is controlled to be similar to that of the pure resistance or inductance. Then the relationship between the circulating-current phasor and the amplitude, the phase of inverter modules output voltage reference is decoupled. Experimental 220 Vac/3 KVA inverter modules are built and parallel operated. The results of experiment verify that the current-sharing scheme based on circulating impedance is available and efficient.

The Restrain of Harmonic Circulating Currents between Parallel Inverters

In inverters, distortions of output sine voltages caused by factors like dead-times of power switches can be well compensated by instantaneous voltages and/or currents feedback regulators such as PID or repetitive controllers. However, in our experiments, we found that the dead-time differences between inverters would result in significant harmonic circulating currents, and instantaneous regulators had limited effects on restraining them, esp. in large parallel power supply systems where the filter reactors and output impedances in each inverters must be small. In this paper, for analyses of harmonic circulating currents, an inverter model that considered the bias in SPWM voltage as a disturbance was established. The research on restraining of harmonic circulating currents by instantaneous feedback regulators indicated that there had been relationships in inverters between output voltage distortions and harmonic circulating currents. The experiments and simulations based on the duel loop voltage and current feedback controls demonstrated that instantaneous feedback regulators that achieve good waveforms could also lead to good restraining on the harmonic circulating currents

The Dynamic Model of Three-phase Inverters with Magnetic Couples

In three-phase inverters, three-pole reactors and transformers are frequently used for consideration of cost and size. However, the magnetic couples between three phases make dynamic analysis of three-phase inverters very difficult. This paper will establish the dynamic model of three-phase inverters with three-pole reactors and three-pole transformers based on instantaneous symmetrical components transformation. This simple and practical model will decouple magnet fluxes between three phases. Analyses of three-phase inverters with magnetic couple based on this model indicates that instantaneous zero sequence voltages output from inverter bridges can greatly influence upon three-phase inverters, esp. in parallel inverter systems. The simulations and experiments validated the above conclusion.

Genetic algorithm optimized fuzzy repetitive controller for low cost UPS inverter application

This paper presents a novel control strategy using a genetic algorithm optimized fuzzy logic controller with repetitive learning action for uninterruptible power supply (UPS) inverter applications. The repetitive learning action can reduce steady-state errors and distortions caused by unknown periodic disturbances. Considering its poor dynamic response with sudden load change, the fuzzy logic controller (FLC) is introduced. The parameters for the FLC are optimized by genetic algorithm (GA) in the proposed scheme. The presented digital control scheme measure only the output voltage, decreasing the amount of sensors and the overall system cost. The control laws and design procedure of the control scheme are discussed. Simulation and experimental results for a single-phase UPS inverter controlled by a TMS320F240 DSP are presented to verify the performance of the proposed control approach under different load conditions. It proves to be a cost-effective solution for common UPS products

Analysis and design of the DSP-based fully digital-controlled UPS

This paper proposes a new control scheme based on two parallel controllers to improve both the steady and transient state responses of close-loop regulated pulse-width-modulated (PWM) inverter for UPS. The controller consists of the repetitive and instantaneous controllers. The repetitive controller is designed to eliminate periodic disturbance and the instantaneous controller is synthesized for transient state response improvement. Design procedure is given for synthesizing the repetitive and instantaneous controllers. The proposed control scheme has been realized using a single-chip digital signal processor (DSP) TMS320F240 from Texas instruments. A 7.5 kVA UPS has been constructed to verify the proposed control scheme. Simulation and experimental results show that the DSP-based fully digital-controlled UPS can achieve both low total harmonics distortion and good dynamic response.