This paper proposes an improved maximum power point tracking (MPPT) method for the photovoltaic (PV) system using a modified particle swarm optimization (PSO) algorithm. The main advantage of the method is the reduction of the steady- state oscillation (to practically zero) once the maximum power point (MPP) is located. Furthermore, the proposed method has the ability to track the MPP for the extreme environmental condition, e.g., large fluctuations of insolation and partial shading condition. The algorithm is simple and can be computed very rapidly; thus, its implementation using a low-cost microcontroller is possible. To evaluate the effectiveness of the proposed method, MATLAB simulations are carried out under very challenging conditions, namely step changes in irradiance, step changes in load, and partial shading of the PV array. Its performance is compared with the conventional Hill Climbing (HC) method. Finally, an experimental rig that comprises of a buck-boost converter fed by a custom-designed solar array simulator is set up to emulate the simulation. The soft- ware development is carried out in the Dspace 1104 environment using a TMS320F240 digital signal processor. The superiority of the proposed method over the HC in terms of tracking speed and steady-state oscillations is highlighted by simulation and experimental results.

An Improved Particle Swarm Optimization (PSO)–Based MPPT for PV With Reduced Steady-State Oscillation

This paper addresses the harmonic stability caused by the interactions among the wideband control of power converters and passive components in an ac power-electronics-based power system. The impedance-based analytical approach is employed and expanded to a meshed and balanced three-phase network which is dominated by multiple current- and voltage-controlled inverters with LCL- and LC-filters. A method of deriving the impedance ratios for the different inverters is proposed by means of the nodal admittance matrix. Thus, the contribution of each inverter to the harmonic stability of the power system can be readily predicted through Nyquist diagrams. Time-domain simulations and experimental tests on a three-inverter-based power system are presented. The results validate the effectiveness of the theoretical approach.

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Modeling and Analysis of Harmonic Stability in an AC Power-Electronics-Based Power System

Maximum power point (MPP) tracking (MPPT) techniques are widely applied in photovoltaic (PV) systems to make PV array generate peak power which depends on solar irradiation. Among all the MPPT strategies, the incremental-conductance (INC) algorithm is widely employed due to easy implementation and high tracking accuracy. In this paper, a novel variable step-size incremental-resistance MPPT algorithm is introduced, which not only has the merits of INC but also automatically adjusts the step size to track the PV array MPP. Compared with the variable step-size INC method, the proposed scheme can greatly improve the MPPT response speed and accuracy at steady state simultaneously. Moreover, it is more suitable for practical operating conditions due to a wider operating range. This paper provides the theoretical analysis and the design principle of the proposed MPPT strategy. Simulation and experimental results verify its feasibility.

A Novel Improved Variable Step-Size Incremental-Resistance MPPT Method for PV Systems

This paper investigates the capacitor-current-feedback active damping for the digitally controlled LCL-type grid-connected inverter. It turns out that proportional feedback of the capacitor current is equivalent to virtual impedance connected in parallel with the filter capacitor due to the computation and pulse width modulation (PWM) delays. The LCL-filter resonance frequency is changed by this virtual impedance. If the actual resonance frequency is higher than one-sixth of the sampling frequency (fs/6), where the virtual impedance contains a negative resistor component, a pair of open-loop unstable poles will be generated. As a result, the LCL-type grid-connected inverter becomes much easier to be unstable if the resonance frequency is moved closer to fs/6 due to the variation of grid impedance. To address this issue, this paper proposes a capacitor-current-feedback active damping with reduced computation delay, which is achieved by shifting the capacitor current sampling instant towards the PWM reference update instant. With this method, the virtual impedance exhibits more like a resistor in a wider frequency range, and the open-loop unstable poles are removed; thus, high robustness against the grid-impedance variation is acquired. Experimental results from a 6-kW prototype confirm the theoretical expectations.

Capacitor-Current-Feedback Active Damping With Reduced Computation Delay for Improving Robustness of LCL-Type Grid-Connected Inverter

This paper addresses the resonance problem in a parallel-inverter-based grid-interactive microgrid. Unlike the single grid-connected inverter system where the resonance frequency is mainly fixed by the inverter output LCL filter parameters, the parallel-inverter-based grid-interactive microgrid system presents a more challenging picture where inverter interactions will excite complex resonances at various frequencies. As a result, line currents of inverters can be severely distorted even when the control schemes and filter circuits are properly designed based on the single-inverter model. This paper first develops a microgrid model using discrete time-domain closed-loop Norton's equivalent circuit. Multiple resonances can then be evaluated with the developed model. To improve the microgrid power quality, this paper also designs a virtual-harmonic-resistance-based active damping method. The proposed damping method can be seamlessly incorporated into the conventional deadbeat control scheme through the direct control reference modification. Therefore, the active damping method is able to address both the transient and steady-state resonances within the deadbeat current control bandwidth. Simulation and experimental results are provided to validate the correctness of the developed resonance modeling and active damping methods.

Investigation and Active Damping of Multiple Resonances in a Parallel-Inverter-Based Microgrid

Designing adequate control laws for grid-connected inverters with LCL filters is complicated. The power quality standards and the system resonances burden the task. In order to deal with resonances, system damping has to be implemented. Active damping is preferred to passive damping so as to improve the efficiency of the conversion. In addition, paralleled grid-connected inverters in photovoltaic (PV) plants are coupled due to grid impedance. Generally, this coupling is not taken into account when designing the control laws. In consequence, depending on the number of paralleled grid-connected inverters and the grid impedance, the inverters installed in PV plants do not behave as expected. In this paper, the inverters of a PV plant are modeled as a multivariable system. The analysis carried out enables to obtain an equivalent inverter that describes the totality of inverters of a PV plant. The study is validated through simulation and field experiments. The coupling effect is described and the control law design of paralleled grid-connected inverters with LCL filters in PV plants is clarified.

Modeling and Control of $N$ -Paralleled Grid-Connected Inverters With LCL Filter Coupled Due to Grid Impedance in PV Plants

In order to maximize the profitability of big photovoltaic (PV) plants, high-power PV inverters of more than 500 kW are becoming attractive. The master-slave (MS) inverter is one of the most interesting architectures. Usually, it is composed of N-paralleled three-phase inverters connected to the medium voltage grid through a transformer with N secondaries on the low-voltage side. This type of transformer is up to 40% more expensive than a single secondary one. This paper analyses the case in which the N inverters are connected to a single secondary transformer so that the N inverters share the ac and dc sides. Furthermore, three-phase three-limb inductors are used, which are around 25% cheaper than single-phase equivalents. Although the use of this type of transformer and inductors is highly attractive in economic terms, it does generate two types of couplings between the different inverters, compromising the system stability. This paper presents a new control technique that achieves a stable system, invariant with N, and independent of the type of inductors and transformer used. Stability of the system is guaranteed in any situation and it can be implemented with a low computational cost. The simulations and the practical results obtained with an Ingecon Sun 625HE-MS confirm the correct functioning of this control technique.

Modeling and Control of a Master–Slave PV Inverter With N-Paralleled Inverters and Three-Phase Three-Limb Inductors

Due to the variation of the maximum power point (MPP) of photovoltaic (PV) generators with solar radiation and temperature, boost DC-DC converters placed between PV modules and inverters in grid-connected PV systems have to be controlled in a variable operating-point condition. In addition, inductor current dynamics changes suddenly when moving from continuous to discontinuous conduction mode. The previous difficulties make the design of reliable and fast control laws for the input voltage of boost converters complicated. The aim of this paper is to propose a control algorithm based on cascaded-loop control. The input voltage is controlled by the outer loop. The inductor current is controlled by an inner loop strategy which is able to perform in mixed conduction mode, owing to the fuzzy switching technique. Simulation and experimental results for a 10-kW boost converter show that the proposed strategy achieves an accurate and robust performance at every operating point, even if the inductor value varies in a wide range; thus, fast MPP tracking techniques can be implemented. An additional advantage is that constant switching frequency is achieved.

J. L. Agorreta

Papers

Modeling and Control of $N$ -Paralleled Grid-Connected Inverters With LCL Filter Coupled Due to Grid Impedance in PV Plants

Modeling and Control of a Master–Slave PV Inverter With N-Paralleled Inverters and Three-Phase Three-Limb Inductors

Fuzzy Switching Technique Applied to PWM Boost Converter Operating in Mixed Conduction Mode for PV Systems