http://ieeexplore.ieee.org/iel5/5610941/5624686/05624745.pdf

Power electronics

The photovoltaic (PV) grid-connected power system in the residential applications is becoming a fast growing segment in the PV market due to the shortage of the fossil fuel energy and the great environmental pollution. A new research trend in the residential generation system is to employ the PV parallel-connected configuration rather than the series-connected configuration to satisfy the safety requirements and to make full use of the PV generated power. How to achieve high-step-up, low-cost, and high-efficiency dc/dc conversion is the major consideration due to the low PV output voltage with the parallel-connected structure. The limitations of the conventional boost converters in these applications are analyzed. Then, most of the topologies with high-step-up, low-cost, and high-efficiency performance are covered and classified into several categories. The advantages and disadvantages of these converters are discussed. Furthermore, a general conceptual circuit for high-step-up, low-cost, and high-efficiency dc/dc conversion is proposed to derive the next-generation topologies for the PV grid-connected power system. Finally, the major challenges of high-step-up, low-cost, and high-efficiency dc/dc converters are summarized. This paper would like to make a clear picture on the general law and framework for the next-generation nonisolated high-step-up dc/dc converters.

Review of Nonisolated High-Step-Up DC/DC Converters in Photovoltaic Grid-Connected Applications

Traditional electric power systems are designed in large part to utilize large baseload power plants, with limited ability to rapidly ramp output or reduce output below a certain level. The increase in demand variability created by intermittent sources such as photovoltaic (PV) presents new challenges to increase system flexibility. This paper aims to investigate and emphasize the importance of the grid-connected PV system regarding the intermittent nature of renewable generation, and the characterization of PV generation with regard to grid code compliance. The investigation was conducted to critically review the literature on expected potential problems associated with high penetration levels and islanding prevention methods of grid tied PV. According to the survey, PV grid connection inverters have fairly good performance. They have high conversion efficiency and power factor exceeding 90% for wide operating range, while maintaining current harmonics THD less than 5%. Numerous large-scale projects are currently being commissioned, with more planned for the near future. Prices of both PV and balance of system components (BOS) are decreasing which will lead to further increase in use. The technical requirements from the utility power system side need to be satisfied to ensure the safety of the PV installer and the reliability of the utility grid. Identifying the technical requirements for grid interconnection and solving the interconnect problems such as islanding detection, harmonic distortion requirements and electromagnetic interference are therefore very important issues for widespread application of PV systems. The control circuit also provides sufficient control and protection functions like maximum power tracking, inverter current control and power factor control. Reliability, life span and maintenance needs should be certified through the long-term operation of PV system. Further reduction of cost, size and weight is required for more utilization of PV systems. Using PV inverters with a variable power factor at high penetration levels may increase the number of balanced conditions and subsequently increase the probability of islanding. It is strongly recommended that PV inverters should be operated at unity power factor.

Grid-connected photovoltaic power systems: Technical and potential problems—A review

The development of energy management tools for next-generation PhotoVoltaic (PV) installations, including storage units, provides flexibility to distribution system operators. In this paper, the aggregation and implementation of these determinist energy management methods for business customers in a microgrid power system are presented. This paper proposes a determinist energy management system for a microgrid, including advanced PV generators with embedded storage units and a gas microturbine. The system is organized according to different functions and is implemented in two parts: a central energy management of the microgrid and a local power management at the customer side. The power planning is designed according to the prediction for PV power production and the load forecasting. The central and local management systems exchange data and order through a communication network. According to received grid power references, additional functions are also designed to manage locally the power flows between the various sources. Application to the case of a hybrid supercapacitor battery-based PV active generator is presented.

/pdf/energy-management-and-operational-planning-of-a-microgrid-4bx5hk5h57.pdf

Energy Management and Operational Planning of a Microgrid With a PV-Based Active Generator for Smart Grid Applications

Multimegawatt wind-turbine systems, often organized in a wind park, are the backbone of the power generation based on renewable-energy systems. This paper reviews the most-adopted wind-turbine systems, the adopted generators, the topologies of the converters, the generator control and grid connection issues, as well as their arrangement in wind parks.

Overview of Multi-MW Wind Turbines and Wind Parks

The proper operation of grid-connected power electronics converters needs using a synchronization technique to estimate the phase of the grid voltage. The performance of this synchronization technique is related to the quality of the consumed or delivered electric power. The synchronous-reference-frame phase-locked loop (SRF-PLL) has been widely used due to its ease of operation and robust behavior. However, the estimated phase can have a considerable amount of unwanted ripple if the grid voltage disturbances are not properly rejected. The aim of this paper is to propose an adaptive SRF-PLL which strongly rejects these disturbances even if the fundamental frequency of the grid voltage varies. This is accomplished by using several adaptive infinite-impulse-response notch filters, implemented by means of an inherently stable Schur-lattice structure. This structure is perfectly suited to be programmed in fixed-point DSPs (i.e., it has high mapping precision, low roundoff accumulation, and suppression of quantization limit cycle oscillations). The proposed adaptive SRF-PLL has been tested by means of the TI TMS320F2812 DSP. The obtained experimental results show that the proposed synchronization method highly rejects the undesired harmonics even if the fundamental harmonic frequency of a highly polluted grid voltage abruptly varies.

/pdf/an-adaptive-synchronous-reference-frame-phase-locked-loop-4jl1lj2cho.pdf

An Adaptive Synchronous-Reference-Frame Phase-Locked Loop for Power Quality Improvement in a Polluted Utility Grid

An accurate small-signal model of three-phase photovoltaic (PV) inverters with a high-order grid filter is derived in this paper. The proposed model takes into account the influence of both the inverter operating point and the PV panel characteristics on the inverter dynamic response. A sensitivity study of the control loops to variations of the DC voltage, PV panel transconductance, supplied power, and grid inductance is performed using the proposed small-signal model. Analytical and experimental results carried out on a 100-kW PV inverter are presented.

/pdf/sensitivity-study-of-the-dynamics-of-three-phase-36np3hqr07.pdf

Sensitivity Study of the Dynamics of Three-Phase Photovoltaic Inverters With an LCL Grid Filter

In order to improve the efficiency and reduce the cost of a photovoltaic system, the use of transformerless photovoltaic inverters is an alternative of increasing interest. However, this topology needs to be studied in detail, as it presents some problems related to the galvanic connection between the grid and the photovoltaic generator (e.g. efficiency degradation and safety problems). In this paper, a review of grid-connected single-phase photovoltaic inverters based on transformerless topologies has been carried out. On the one hand, some alternatives based on classical topologies have been presented. On the other hand, alternatives based on multi-level inverter topologies have been studied, showing up that no leakage current is generated in comparison to classical topologies.

/pdf/transformerless-topologies-for-grid-connected-single-phase-3gu51aprh1.pdf

Transformerless topologies for grid-connected single-phase photovoltaic inverters

The proportional + resonant (PR) controller has been proposed in the past as a suitable method to control the current generated by the grid-connected photovoltaic voltage source inverters. Due to the fact that information regarding the frequency of the grid is needed to use this control technique, the synchronous reference frame phase-locked loop (SRF-PLL) is commonly used. To assure that the total harmonic distortion of the injected current (THDi) meets the appropriate standards, even if the grid voltage is polluted and its frequency varies, an adaptive control strategy is presented in this paper. This control strategy can improve the behavior of both, the conventional SRF-PLL and the conventional PR controller, when they are used in a polluted grid with a time varying frequency. The experimental results obtained by means of a digitally controlled 10-kVA inverter, show up that the THDi of the injected current is improved when the proposed adaptive control strategy replaces the conventional one.

/pdf/an-adaptive-control-system-for-three-phase-photovoltaic-1avdb1gsyv.pdf

An Adaptive Control System for Three-Phase Photovoltaic Inverters Working in a Polluted and Variable Frequency Electric Grid

This paper presents a robust control technique applied to modular uninterruptible power-supply (UPS) inverters operating in parallel. When compared to conventional proportional-integral (PI) control, the proposed technique improves the response of the output voltage to load steps and to high distorted output currents, reducing the distortion of the output voltage. Furthermore, an excellent distribution of currents between modules is achieved, resulting in fine power equalization between the inverters on stream. The crossover frequency of the different loop gains involved is moderate, so that robustness to variations of the operation point and to modeling uncertainties is achieved. A comparative study with a two-loop conventional PI control scheme is presented. Experimental results on a 1-kVA modular online UPS system confirm the viability of the proposed scheme.

Fran Gonzalez-Espin

Papers

An Adaptive Synchronous-Reference-Frame Phase-Locked Loop for Power Quality Improvement in a Polluted Utility Grid

Sensitivity Study of the Dynamics of Three-Phase Photovoltaic Inverters With an LCL Grid Filter

Transformerless topologies for grid-connected single-phase photovoltaic inverters

An Adaptive Control System for Three-Phase Photovoltaic Inverters Working in a Polluted and Variable Frequency Electric Grid

Robust Model-Following Control of Parallel UPS Single-Phase Inverters