ABSTRACT:
An investigation into solar photovoltaic (PV) system control topology selection, when partial shade is anticipated in the solar array, is presented. As available area is maximised in Building Integrated PV (BIPV) systems, shading is an inevitable on sequence.
The presence of partial shading in a PV array leads to multiple power peaks in the power-voltage curve, due to bypass diode sections being triggered, and an increase in module mismatch losses in the array.
A building energy design software, Integrated Environmental Solutions, is used to determine the shadowed area on PV modules throughout the year, incorporating the PV system location and geometrical models of obstacles around the array. Photovoltaic system topologies incorporate the following device options: central inverters, a power converter for each series string of PV modules, and a Module Integrated Converter (MIC).
A model has been developed to compare a DC-AC parallel MIC configuration with a series string DC to AC power inverter configuration, for a commonly occurring shading pattern, and a simulation is used to compare the power delivered by the topologies.
The model is broken up into the following stages: i) geometric modelling of array and surrounding obstacles, ii) solar PV power characteristic model as a function of temperature and irradiance, and iii) inverter efficiency model for each topology. A shade scenario is outlined to present data from each stage of the model. A discussion is presented to critically assess the strengths and weaknesses of the model.
PHOTOVOLTAIC SYSTEM TOPOLOGIES AND PROBLEM DESCRIPTION
Figure 1 PV System Components.
Figure 1 shows the typical components in a grid connected PV system. The power converters convert the DC power from the PV modules to AC power to be injected into the grid. The MPPT control algorithm is a searching algorithm aiming to obtain maximum power transfer from the PV, which is operating in a dynamic environment of changing irradiance and temperature. All modules are assumed to be mounted in fixed and stationary orientation.
MODELLING
Geometric Modelling of Array and Surrounding Obstacles:
Integrated Environmental Solutions (IES) is commercial software accredited for calculating Energy Ratings for non-domestic compliance in the Republic of Ireland of EPBD. The tool incorporates a weather database for your chosen location, and solar irradiance calculations to estimate energy falling on selected surfaces, enabling solar shading analysis. The tool is not typically used for the analysis of shade within PV systems and the present investigation adapts the application for this purpose.
Geometric Model in IES.
PV Model:
A model of a PV module is used to predict module power. Temperature variations are not considered in this analysis, and each module is assumed to be operated at 25°C. The model requirements are: i) to generate a power voltage curve with sufficient accuracy, ii) to be able to incorporate multiple bypass diodes and iii) to vary input environmental factors with ease. The Matlab / Simulink software has been chosen as the electrical components and environmental inputs can be modeled simultaneously.
Power Converter Efficiency Models:
The central inverter is a transformerless design, and the efficiency curve for the model is derived from experimental data from. The peak efficiency of the curve is 97.65%. The modular AC converter uses a high frequency transformer in its design, and an efficiency-power curve was derived from normalised experimental efficiency curves of a push pull AC converter.
The efficiency of these devices is typically lower due to the high voltage gain requirements of the hardware. The overall maximum efficiency of this design did not compare with micro-inverters currently on the market. The curve was shifted to a maximum peak power to match the commercial AC converter of 94.8%.
DISCUSSION AND CONCLUSION
If it is assumed that each PV configuration is being operated at the maximum available DC power, the string topology is the optimal selection for the shade scenario, as the topology can generate 36W in excess to the modular topology, although the margin is very small in this example.
The DC power available is almost identical in both topologies, assuming the global maximum is tracked. The modular topology is operating on a lower efficiency curve due to the high gain requirements of the hardware, and this has been to its overall disadvantage in this scenario.
However, if the local maxima is tracked, as is often the case in PV inverters, the string configuration can generate only 3 51 W of AC power, but the modular inverter maintains its power levels as the two power points are almost equal. There exists a trade-off between array capture losses, seen here in the difference between the global and local maximum in the string topology; and the transformation efficiency curves over the power range.
The string topology is highly dependent on having a MPPT algorithm that can recognise the existence of several operating points for partially shaded strings, namely to be able to distinguish the local maximum from the global maximum.
The modular topology is not as dependent on this function, as it is recouping losses by removing current and voltage mismatch losses down at the modular level. All unshaded modules are operating at their maximum; however the topology suffers due to its lower overall efficiency values, when compared to the string configuration.
The model is to be extended to include time, and the course of shadows over the PV array, to determine what length of time the shadow is impacting the system. This will enable a more accurate calculation of annual energy yield.
The results presented are simulation results, and the following validation procedures are in progress: i) the validation of shaded power-voltage curves, which incorporate bypass diodes, using a bespoke PV measurement test bedunder development in Bolton St, controlled by Lab VIEW software and DAQ hardware, and ii) the incorporation of measured experimental efficiency curvesinto the model, using off-the-shelf microinverter benchmarking data being monitored at the solar research facility in Kevin St.DIT.
This validated model can be used for further investigation of topology selection, incorporating variables such as the proportion of the shaded PV modules in relation to the total generator and the impact of the length of the string, within a variety of common shading scenarios.
The purpose of the investigations is to clarify some of the grey areas that occur when deciding on the most efficient control topology when designing a PV system which will incorporates shade to some degree; and to investigate the scope of application of the particular topologies.
Source: Dublin Institute of Technology
Author: Lynette O’Callaghan
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