Real-time Modelling, Diagnostics and Optimised MPPT for Residential PV Systems

The work documented in the thesis has been focused into two main sections. The first part is centred around Maximum Power Point Tracking (MPPT) techniques for photovoltaic arrays, optimised for fast-changing environmental conditions, and is described in Chapter 2. The second part is dedicated to diagnostic functions as an additional tool to maximise the energy yield of photovoltaic arrays (Chapter 4). Furthermore, mathematical models of PV panels and arrays have been developed and built (detailed in Chapter 3) for testing MPPT algorithms, and for diagnostic purposes.In Chapter 2 an overview of the today’s most popular MPPT algorithms is given, and, considering their difficulty in tracking under variable conditions, a simple technique is proposed to overcome this drawback. The method separates the MPPT perturbation effects from environmental changes and provides correct information to the tracker, which is therefore not affected by the environmental fluctuations. The method has been implemented based on the Perturb and Observe (P&O), and the experimental results demonstrate that it preserves the advantages of the existing tracker in being highly efficient during stable conditions, having a simple and generic nature, and has the benefit of also being efficient in fast-changing conditions. Furthermore, the algorithm has been successfully implemented on a commercial PV inverter, currently on the market. In Chapter 3, an overview of the existing mathematical models used to describe the electrical behaviour of PV panels is given, followed by the parameter determination for the five-parameter single-exponential model based on datasheet values, which has been used for the implementation of a PV simulator taking in account the shape, size ant intensity of partial shadow in respect to bypass diodes.In order to eliminate the iterative calculations for parameter determinations, a simplified three-parameter model is used throughout Chapter 4, dedicated to diagnostic functions of PV panels. Simple analytic expressions for the model important parameters, which could reflect deviations from the normal (e.g. from datasheet or reference measurement) I −V characteristic, is proposed.A considerable part of the thesis is dedicated to the diagnostic functions of crystalline photovoltaic panels, aimed to detect failures related to increased series resistance and partial shadowing, the two major factors responsible for yield-reduction of residential photovoltaic systems.Combining the model calculations with measurements, a method to detect changes in the panels’ series resistance based on the slope of the I − V curve in the vicinity of open-circuit conditions and scaled to Standard Test Conditions (STC) , is proposed. The results confirm the benefits of the proposed method in terms of robustness to irradiance changes and to partial shadows.In order to detect partial shadows on PV panels, a method based on equivalent thermal voltage (Vt) monitoring is proposed. Vt is calculated using the simplified three-parameter model, based on experimental curve. The main advantages of the method are the simple expression for Vt, high sensitivity to even a relatively small area of partial shadow and very good robustness against changes in series resistance.Finally, in order to quantify power losses due to different failures, e.g. partial shadows or increased series resistance, a model based approach has been proposed to estimate the panel rated power (in STC). Although it is known that the single-exponential model has low approximation precision at low irradiation conditions, using the previously determined parameters it was possible to achieve relatively good accuracy. The main advantage of the method is that it relies on already determined parameters (Rsm, Vt) based on measurements, therefore reducing the errors introduced by the limitation of the single-exponential model especially at low irradiation conditions

On the Impact of Partial Shading on PV Output Power

It is a well-documented fact that partial shading of a photovoltaic array reduces it output power capability. However, the relative amount of such degradation in energy production cannot be determined in a straight forward manner, as it is often not proportional to the shaded area. This paper clarifies the mechanism of partial PV shading on a number of PV cells connected in series and/or parallel with and without bypass diodes. The analysis is presented in simple terms and can be useful to someone who wishes to determine the impact of some shading geometry on a PV system. The analysis is illustrated by measurements on a commercial 70 W panel, and a 14.4 kW PV array

Influence of Resolution of the Input Data on Distributed Generation Integration Studies

One of the main issues concerning large penetration of the renewable energy based generators on the distribution network is related to the voltage variations due to intermittent character of the solar irradiance and wind. The actual power quality standards provide only general information regarding the evaluation procedure of the voltage fluctuations and no directions regarding the sampling frequency of the data used. As a consequence, most of the studies neglect effect of the solar irradiance and wind speed in fast changing conditions on the utility grid. This work proposes a methodology to evaluate the voltage fluctuations into the low voltage distribution network caused by variable generation and reveals the influence of data resolution on the final results. A short review regarding the assessment of the voltage variations is presented in advance and an appropriate model of the power system is build, including the generating units that are capable to operate with high resolution input data. Real parameters for the components of the simulated system are used in order to obtain realistic results.</p

Remote and Centralized Monitoring of PV Power Plants

This paper presents the concept and operating principles of a low-cost and flexible monitoring system for PV plants. Compared to classical solutions which can require dedicated hardware and/or specialized data logging systems, the monitoring system we propose allows parallel monitoring of PV plants with different architectures and locations by taking advantage of the intrinsic monitoring capabilities of the inverters and their internet connectivity. The backbone of the system is a software system capable of collecting production measurements and current-voltage (I-V) characteristic curve measurements from the inverters within each PV plant. The monitoring software stores the PV measurements in a data warehouse optimized for managing and data mining large amounts of data, from where it can be later visualized, analyzed and exported. By combining PV production measurements data with I-V curve measurements the diagnostic and condition monitoring capabilities of the PV system can be greatly enhanced. The practical implementation and operation of the monitoring system is demonstrated with a study case system deployed at Aalborg University.</p

Intrinsic-Capacitance-based Differential Power Processing for Photovoltaic Modules

Partial shading reduces energy production and affects the lifetime of the overall PV system. To mitigate the mismatch effects caused by partial shading, several PV cell- or sub-panel-level techniques employing power electronics have been proposed in the literature, where discrete passive components, e.g., inductors and capacitors, are also used. In this paper, a differential power processing (DPP) technique, which utilizes only the intrinsic capacitance of solar cells, is introduced for small-scale PV applications. The developed DPP topology mitigates the mismatch effects by operating all PV cells at or near to their corresponding maximum power points (MPPs) even under mismatch conditions. The analysis of the topology and its comparison with the frequently used series- and series-parallel (SP)-connected techniques, are presented to validate its efficacy and operational capabilities, through simulation results. Moreover, a prototype is built to verify the topology. The experimental results confirm the elimination of multiple power peaks under mismatch along with maintaining the same voltages across the PV panels.</p

Three-Phase Unbalanced Load Flow Tool for Distribution Networks

This work develops a three-phase unbalanced load flow tool tailored for radial distribution networks based on Matlab®. The tool can be used to assess steady-state voltage variations, thermal limits of grid components and power losses in radial MV-LV networks with photovoltaic (PV) generators where most of the systems are single phase. New ancillary service such as static reactive power support by PV inverters can be also merged together with the load flow solution tool and thus, the impact of the various reactive power control strategies on the steady-state grid operation can be simply investigated. Performance of the load flow solution tool in the sense of resulting bus voltage magnitudes is compared and validated with IEEE 13-bus test feeder

A Simple Mismatch Mitigating Partial Power Processing Converter for Solar PV Modules

Partial shading affects the energy harvested from photovoltaic (PV) modules, leading to a mismatch in PV systems and causing energy losses. For this purpose, differential power processing (DPP) converters are the emerging power electronic-based topologies used to address the mismatch issues. Normally, PV modules are connected in series and DPP converters are used to extract the power from these PV modules by only processing the fraction of power called mismatched power. In this work, a switched-capacitor-inductor (SCL)-based DPP converter is presented, which mitigates the non-ideal conditions in solar PV systems. A proposed SCL-based DPP technique utilizes a simple control strategy to extract the maximum power from the partially shaded PV modules by only processing a fraction of the power. Furthermore, an operational principle and loss analysis for the proposed converter is presented. The proposed topology is examined and compared with the traditional bypass diode technique through simulations and experimental tests. The efficiency of the proposed DPP is validated by the experiment and simulation. The results demonstrate the performance in terms of higher energy yield without bypassing the low-producing PV module by using a simple control. The results indicate that achieved efficiency is higher than 98% under severe mismatch (higher than 50%)

Internal active power reserve management in Large scale PV Power Plants

Active Power Reserves (APRs) provided through curtailment (iAPRs) or through auxiliary storage systems (aAPR) with Large scale PV Power Plants (LPVPPs) becomes a reality in the near future with high penetration levels of Photovoltaic (PV) power into the grid. Therefore, this paper analyses the solutions for iAPR fulfilment in central inverter based - LPVPPs in terms of their layout, configuration and control architecture. During iAPR supply, the LPVPP has to operate under its Maximum Power Point (MPP), which means that the MPP algorithm has to undergo several changes. For this purpose, the paper proposes to supply iAPRs by means of a sweep function distributed over the PV inverters used in the central inverter station which has the potential to guarantee a maximum energy extraction along with a secured supply of the APR in any given meteorological condition

Test Platform for Photovoltaic Systems with Integrated Battery Energy Storage Applications

We present a hybrid simulation and a real-time test platform for developing control systems for photovoltaic (PV) inverters with integrated battery energy storage (BES). The platform consists of a dual-stage single-phase PV inverter system, DC coupled with a full-bridge grid connected inverter, which emulates the charge regulator and battery bank. The real-time control of the two power electronic converters is implemented in a Simulink/dSpace platform, together with the real-time simulation model of the battery pack. The input power can be provided by either a high performance PV emulator or by a physical PV array. The platform enables real-time testing of PV+textbf{{BES control systems and energy management systems (EMS), for a variety of battery technologies, which can be modelled in detail and emulated by the full-bridge grid connected inverter. Such flexibility is difficult to achieve with real BES systems, due to electrical safety and cost constrains of high power charge regulators and battery packs.</p