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rapporti - Deliverable

Ottimizzazione e sperimentazione dell’algoritmo d’inseguimento del punto di massima potenza MPPT e studio di fattibilità di un DC/DC converter che implementa l’algoritmo e include funzioni diagnostiche dell’impianto fotovoltaico

rapporti - Deliverable

Ottimizzazione e sperimentazione dell’algoritmo d’inseguimento del punto di massima potenza MPPT e studio di fattibilità di un DC/DC converter che implementa l’algoritmo e include funzioni diagnostiche dell’impianto fotovoltaico

Il presente rapporto descrive lo studio di fattibilità di un inverter per fotovoltaico che implementa un nuovo algoritmo di inseguimento del punto di massima potenza MPPT ad alta efficienza sviluppato da RSE e al contempo alcune funzioni avanzate per la diagnosi ed il monitoraggio della stringa fotovoltaica in condizioni operative. Le funzioni diagnostiche riguardano la stima in tempo reale della temperatura di giunzione delle celle fotovoltaiche in condizioni operative, la valutazione in tempo reale della performance (potenza, efficienza) della stringa PV in condizioni, l’ identificazione automatica dei parametri modellistici delle celle solari mediante misurazione delle caratteristiche corrente – tensione di buio rilevate durante le ore notturne. Si riportano inoltre i risultati di alcuni test di efficienza dell’algoritmo MPPT eseguiti in laboratorio.

In 2014 RSE has started to develop an innovative algorithm which allows the Maximum Power Point Tracking MPPT for application in photovoltaic PV inverters. Such algorithm, completed in 2015, is particularly effective in the cases of not uniform illuminations of the photovoltaic cells (i.e. partial shadings caused by obstacles near flat plane PV installations installed on house’s roofs or module misalignments concerning concentrator photovoltaic systems). Under these conditions, the powervoltage characteristics of the photovoltaic string can exhibit several local maximum power points. Many conventional algorithms implemented in commercial inverters are incapable of determining the absolute maximum point with consequent loss of energy. Besides, if during the algorithm’s execution there are variations in solar irradiance which involves power variations, the conventional algorithms are often deceived and they could temporarily shift the working point to I-V curve’s areas that are far from the absolute maximum point. The new RSE’s MPPT algorithm, based on a mathematical modelling of the solar cell, is able to compare the relative peak power peaks without the need to move the working point to them. It identifies the voltage range where the absolute maximum power point is by a minimum number of iterations. It is worthwhile to point out that an efficient MPPT algorithm minimizes the search time and its skill to evaluate when it is convenient to change the MPP, is an important factor of efficiency. In the reference period, the new MPPT was optimized (especially under dynamic conditions) by studying new criteria which define trigger conditions. For the algorithm testing, a photovoltaic module simulator circuit consisting of real photovoltaic cells has been developed. It allows simulating the mismatch among cells by means appropriate potentiometers and multi-meters which allow adjusting and monitor the current that flow in each cell. A software (developed by RSE as well) allows simulating the MPPT algorithm with strings I-V curves affected by random mismatches and with variable solar irradiance. The results of experimental tests and simulations have shown that in the most critical conditions characterized by frequent irradiance variations and significant partial shading phenomena, the efficiency of the RSE MPPT algorithm is higher than 96% while in the same conditions the conventional MPPT Perturb & Observe has an efficiency lower than 65%. In less critical conditions, the efficiency of the RSE MPPT algorithm exceeds 98%. Under steady conditions, in the absence of partial shadows and with constant solar irradiance, the efficiency achieves the ideal value of 99.9% as the algorithm is able to recognize that it is not necessary to search power peaks elsewhere, avoiding unnecessary perturbations that would involve energy loss.

In the reference period, the feasibility study of an inverter implementing the new MPPT and some advanced functions for the diagnosis and monitoring of the photovoltaic string under operating conditions has been carried out. In particular, the following 4 advanced functions have been studied.

1. Function1: New RSE MPPT method (mentioned above).
2. Function 2: real-time estimation of the junction temperature of the photovoltaic cells under operating conditions.
3. Function 3: real-time performance evaluation (power, efficiency) of the PV string under operating conditions and performance’s translation to standard STC conditions and Standard Operating Conditions Condition.
4. Function 4: Automatically identifying of the modelling parameters required for the functions 1) 2) 3) by the measurements of the dark I-V curve during night hours.

The new MPPT algorithm requires:

• a set of electro-thermal parameters typical of photovoltaic cells;
• a set of operating parameters.

Regarding electro-thermal parameters, the methodologies for their identification are not reported in this report because they have been already defined in Standard Technical Guidelines (IEC EN) or in procedures already published by RSE. In the state of the art, however, such measures are carried out offline by specialized laboratories. On the other hands, the methods to identify the operating parameters are described in this report because they are based on an original model developed by RSE. The novelty proposed by function 4 is to equip the inverter with systems that automatically identify both electrothermal parameters and operating parameters by means of in-line measurements, which do not require to disconnect the PV system during the energy production. It’s worthwhile to note that the electro-thermal parameters during long periods may vary due to the degradation of the photovoltaic modules. The automatic identification of the electro-thermal parameters is therefore advantageous for both the new MPPT (because it continues to work with updated electro-thermal parameters), and for the monitoring and the diagnostics of photovoltaic modules which belong to the operating string.

Given the variability of meteorological conditions, the user of a photovoltaic plant is interested to know whether any deterioration in performance is due to adverse environmental conditions or to the degradation of its components. For this task, it is intended to equip the inverter with measuring and processing systems which, starting from the maximum power value (measured during the MPPT operation) under experimental environmental conditions, calculate the power value under Standard Test Conditions defined by the international technical standards. In order to do this, the measurement of some environmental variables (direct and global normal solar irradiance, ambient temperature, wind speed) and the solar cell’s junction temperature are required. The first ones are directly measurable by means of appropriate sensors connected to an integrated data acquisition system. Instead, the cell temperature, in the case of CPV systems, is not measurable directly since it is not possible to position a sensor over the cell. The international Standard IEC 62670-3 provides an indirect method for estimating the junction temperature based on the measurement of the short circuit current Isc and the open circuit voltage Voc of the string and the values of some electro-thermal parameters (that cab be automatically identified by function 4). The junction temperature measurement, at the state of the art, is performed offline by specialized laboratories since the methodology has never been implemented in an inverter. Since during the normal operations of the new MPPT method, the Isc and Voc measurements are periodically performed, the junction temperature value can be easily calculated by the inverter controller (function 2). After applying some translation formulas (defined by international technical standards) which use the junction temperature value and a set of environmental measurements, the inverter controller calculates the power value in Standard Conditions (function 3). It is worthwhile to say that the junction temperature monitoring is an important diagnostic information because it allows evaluating the possible deterioration of the module’s heat sinks.