Design and characterization of a HMPPT technique for PV applications

Design and characterization of a HMPPT technique for PV applications Gianluca Aurilio, Marco Balato, Daniele Gallo, Carmine Landi, Mario Luiso, Massimo Vitelli Department of Industrial and Information Engineering - Second University of Naples Via Roma 29, 81031 Aversa (CE)-Italy {gianluca.aurilio, marco.balato, daniele.gallo, carmine.landi, mario.luiso, massimo.vitelli}@unina2.it

Abstract— In the last years, with the focus on greener sources of power, photovoltaic has become an important source of power for a wide range of applications. Even with higher efficiency and lower cost, the goal remains to maximize the power from the PV system under various lighting and temperature conditions. In PV applications, under mismatching conditions, the Maximum Power Point Tracking technique both to regulate the voltages of the PV modules of the array and the DC input voltage of the inverter is used. In this paper a new hybrid method based on the periodic scan of the Power vs. Voltage characteristic at the input of the inverter is presented to improve the performance of MPPT for solar PV system. Its main advantages are the high MPPT efficiency and the high speed of tracking. Design, simulation and laboratory characterization of the presented technique are discussed. Keywords—PV application measurements; voltage measurement; current measurement; power measurement; Maximum Power Point Tracking (MPPT); Hybrid MPPT (HMPPT); mismatching; grid connected PV systems

I.

INTRODUCTION

The power delivered by a PV system of one or more photovoltaic cells is dependent on the irradiance, temperature, and the current drawn from the cells. At a certain weather condition, the output power of a PV panel depends on the terminal voltage of the PV system. To maximize the output power, a high-efficiency, low-cost DC/DC converter with an appropriate maximum power point tracking (MPPT) algorithm is commonly employed to control the terminal voltage of the PV system at optimal values in various solar radiation conditions. The output of a solar module is characterized by a performance curve of voltage versus current, called the I-V curve. The maximum power point of a solar module is the point along the I-V curve that corresponds to the maximum output power. Traditional solar inverters perform MPPT for an entire array as a whole. In such systems the same current, dictated by the inverter, flows through all panels in the string. Because different panels have different I-V curves and different MPPs (due to manufacturing tolerance, partial shading, etc.) this architecture means some panels will be performing below their MPP, resulting in the loss of energy and in reduction of the overall system efficiency [1]-[4]. Many MPP Tracking (MPPT) techniques have been presented in the literature [1]-[4]. In case of mismatch operating conditions, the Power-Voltage (P-V) characteristic may exhibit more peaks, due to the presence of bypass diodes,

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and MPPT algorithms can fail. Moreover the absolute maximum power of the mismatched PV field is lower than the sum of the available maximum powers that the mismatched modules are able to provide. In order to allow each PV module of the array to provide its own maximum power it is possible to use module-dedicated DC/AC converters (micro inverters [5], [6]) or module dedicated DC/DC converters and central inverters ([7]-[10]). In this paper the attention is focused on Distributed MPPT technique (DMPPT ) that adopting module-dedicated DC/DC converters and central inverters. In [7] advantages and drawbacks of such basic topologies are discussed, and the conclusion is that a practical compromise solution for seriesconnected DC/DC converters installed on all the PV modules of the string is generally represented just by the boost converter ([11]-[21]). The system presented in this paper is composed by a PV module equipped with a dedicated boost DC/DC converter will be called Self Controlled PV Module (SCPVM). The term Central MPPT (CMPPT) will be used with reference to a MPPT technique acting on the output voltage of a whole array of SCPVMs and carried out by the central controller equipping the inverter. Instead the term Distributed MPPT (DMPPT) will be used with reference to a MPPT technique, simultaneously acting on the output voltage of each PV module representing the source of each SCPVM of the array. The DMPPT technique is simultaneously carried out by all the controllers equipping the DC/DC converters of the array of SCPVMs. In [10] the reasons why the joint adoption of both CMPPT and DMPPT is necessary are discussed in detail. As shown in [10], in order to obtain full profit from DMPPT, it is necessary that the bulk inverter voltage belongs to an optimal range whose position and amplitude depend on the number of SCPVMs in the string, on the atmospheric operating conditions characterizing each PV module (irradiance and temperature values), on the voltage and current ratings of the physical devices the power stages of SCPVMs are made of, and on the adopted DC/DC converter topology ([22]-[35]). In [10] it has been clarified that the central controller cannot adopt the P&O technique (as well as any other standard MPPT technique) in order to avoid that the PV system operates in the proximity of a suboptimal point thus wasting the presence of the distributed MPPT DC/DC converters. Instead it is possible to adopt a technique based on the scan of the Power versus Voltage (P-V) characteristic of the array of SCPVMs. In this paper a Modified P&O (MP&O) technique has been used as

DMPPT technique while the adopted CMPPT technique is based on the periodic scan of the P-V characteristic and will be identified with the acronym CMPPTS. II. CMPPT TECHNIQUE USED IN DMPPT APPLICATIONS The CMPPTS technique proposed in this paper is based on the periodic scan of the whole P-V characteristic of the string of SCPVMs in order to locate the operating value Vb_opt of vb in correspondence of which the power P provided by the string of SCPVMs is maximum ([36]-[40]). The scan is carried out by means of the CMPPT controller (see Fig. 1) which provides a staircase signal vb_ref which represents the reference voltage which must be followed by vb (Fig. 2). The amplitude of the step of vb_ref is equal to ∆vb_ref and the duration of each step is equal to Tb. The initial value Vb_min and the final value Vb_max of vb_ref must of course belong to the allowed inverter input voltage operating range vb_ref assumes n different values, where n is equal to: n=1 + (Vb_max - Vb_min)/∆vb_ref

(1)

The n steady-state values of the total power which is extracted in correspondence of the n values assumed by vb_ref are recorded, and, after the scan, the CMPPT controller sets the reference voltage for vb to Vb_opt. After these considerations, it is clear that, the while scan needs a total time Ttot equal to n·Tb. Two constraints requirements must be satisfied to choice ∆vb_ref. The first requirement can be explained on the basis of the following considerations. It is well-known that the voltage vb across the energy-storage bulk capacitor placed at the interface between the array of SCPVMs and the inverter (Fig. 1) exhibits an oscillation with a frequency equal to the second harmonic of the grid frequency fline and with a peak to peak amplitude ∆vb_fline equal, in accordance with [15], to: ∆v b_f

line

=

P Cb 2πflineVRMS

(2)

where VRMS is the root mean square value of the input inverter voltage and P is the DC power injected into the grid. The worst-case (highest) value of ∆vb_fline is obtained by assuming VRMS=N·VMPP and P=N·VMPP·IMPP. Of course such an oscillation can lead the CMPPT technique to the error due to the fact that, at the steady state, when vb_ref is fixed, the total power extracted from the SCPVMs is instead time varying.

Fig. 1. DMPPT grid-connected PV system adopting boost based SCPVMs with the output ports connected in series.

Fig. 2. Staircase signal vb_ref.

In fact there is an the unwanted additional local scan of the P-V characteristic in a voltage region around vb_ref caused by the above oscillation. In order to avoid error, it is necessary to adopt enough large values of ∆vb_ref: ∆v b_ref ≥ ∆v b_f line

(3)

The fulfilment of the eq. 3 assures us that the increase or the decrease of the power extracted from the SCPVMs, due to the variation (∆vb_ref) of the vb_ref, is respectively greater or lower than the variation of the power due to the oscillation ∆vb_fline of the input inverter voltage vb. The second requirement is explained as following. It must be ensured that the increase ∆vb_ref of vb_ref, taking place at the generic i-th sampling instant i·Tb, does not cause that the output voltage of one or more SCPVMs exceeds Vds max, that is the maximum voltage across the switches of the boost converter. When the operating conditions of the SCPVMs are uniform, then the variation ∆vb is equally shared among all the N output voltages of the string of SCPVMs. In such a case the increase ∆voutk of voutk (k=1,...,N), where voutk is the output voltage of SCPVMs, is equal to ∆vb_ref/N. Unfortunately, this happens only when the operating conditions of the SCPVMs are uniform, but, in mismatching operating conditions, the variation ∆vb=∆vb_ref is unequally shared among the N output voltages. In particular, in according with [10], under mismatching conditions the SCPVMs, which are characterised by very low irradiance levels, are forced to operate with very low output voltages which, in extreme mismatching conditions, can become nearly equal to 0. That is, the low irradiated SCPVMs may operate with their output ports in short circuit conditions while, at the same time, carrying the whole string current iout. In general, in order to grant the functionality of the rest of the PV system, a bypass diode should be connected at the output of each SCPVM. Indeed, since in the considered case the synchronous version of the boost topology has been chosen, the bypass function is automatically ensured by the presence of the two body diodes of the two MOSFETS or by the presence of the external diode which is generally added in antiparallel to each MOSFET in order to increase the overall efficiency of the power stage [15]. Therefore, as concerns the distribution of ∆vb among the N output voltages, the worst case takes place when the maximum possible number M of SCPVMs is working in short circuit conditions because, of course, in such a case ∆vb must be shared only among the remaining (N-M) SCPVMs which are instead working at the maximum allowed output voltage

(Vout lim). That is, in the worst case, it is voutk=Vout lim (k=1,...NM) while voutk=0 (k=N-M+1,...,N). It is: M=N-

Vb_min Vout lim

(4)

∆vb v outk +∆Voutk ≈ Vout lim +∆v outk ≈ Vout lim + ≤ Vds max N-M

16.0115 V ≤ ∆vb_ref ≤ 47.55 V (5)

(k=1, 2, ...., N-M)

it must be:

(

)

Vb_min Vout lim

(6)

As for the choice of Tb, the effect of the possible presence of nearly vertical portions in the P-V characteristic of the string of SCPVMs must be considered. In general, in correspondence of a given arbitrary value vbl assumed by vb during the scan, the worst-case (longest) times needed by the SCPVMs and inverter to reach the new steady-state condition (at vb=vbl+∆vb) in (7) is shown. T3 can be numerically evaluated as the settling time of the step response of the closed-loop transfer function W(s) between vb and vb_ref, where Tcv(s) is the loop gain of the inverter outer voltage feedback loop [15]. Then the value of Tb must be as in (7). T1 =T2 =

()

W s =

Vout lim ≤ 53.512 V

(9)

By applying (3) and (6), the allowed range of ∆vb_ref can be calculated:

Since it is desired that:

∆v b_ref =∆v b ≤ Vds max −Vout lim

79.9776 W (at Smax= 1000 W/m2 and Tambient = 25 °C); ∆vpan_ref = 0.15 V, Ta=1.5 ms. By applying (8) the allowed value of Vout lim can be evaluated:

VMPP ⋅ Ta ∆vpan_ref vˆ b ( s ) Tcv ( s ) = vˆ b_ref ( s ) 1+Tcv ( s )

(7)

The switching frequency fs of the boost converters and of the full-bridge inverter has been chosen equal to 30 kHz. Moreover L= 100 µH, Cin= 220 µF, Lf=330 µH, Cf= 47 µF, Cb= 1 mF, Cout= 330 µF. The PV voltage compensation network (Fig. 1) has been designed by choosing a crossover frequency equal to 5 kHz and a phase margin equal to 52°. The crossover frequency of the inverter outer voltage feedback loop is equal to about 10 Hz and the phase margin is equal to about 90°. As for the grid current compensation loop, the crossover frequency of the inverter inner current feedback loop is equal to about 1400 Hz and the phase margin equal to about 52°. The guidelines allowing the design of such three feedback lops can be found in [15]. By using (7) we get T1 = T2 ≈ 0.3 s, T3 ≈ 0.15 s. Hence, by taking into account (7), it must be: Tb > 0.75 s

It is worth noting that the values of Vout lim, ∆vb_ref and Tb respectively fulfill inequalities (9), (10) and (11); therefore a correct behavior of the CMPPTS technique is expected.

Tb ≥ T

If properly designed as discussed above, the CMPPT technique is able to avoid errors. Of course, the period Tr between two consecutive scans must be greater than T. As general rule, the lower Vout lim the lower the maximum power which can be extracted from the DMPPT PV systems. In particular, it can be shown that the value adopted by Vout lim must fulfill the following inequality [11]: Fig. 3. P-V characteristics.

(8)

III. SIMULATION RESULTS The system of Fig. 1 has been simulated by using PSIM (simulation software for power conversion and control) in a number of different cases. The PV array is composed by a string of N=11 SW225 PV modules. The values of the parameters appearing in the previous sections are: Vds max =60 V; Ppan1 = 192 W, Voc=32.7 V, ∆Ppan1 =8.8864 W, ∆Ptot = -

(11)

The allowed inverter input voltage range is [Vinverter_min, Vinverter_max]=[360, 660] V. The reported simulation refers to the following set of parameters: Vout lim = 53 V, ∆vb_ref = 25 V, Tb = 0.9 s, S=[1000 1000 1000 1000 1000 1000 1000 250 250 250 250] W/m2, Vb_min=360 V, Vb_max=560 V.

VMPP T=T1 +T2 +T3 = ⋅ Ta ∆vpan_ref

 ∆P  Vds max 1 − pan   Ppan   Vout lim ≤  Vds max ⋅ ∆Ptot  1 +  N ⋅ Voc ⋅ Ppan  

(10)

Fig. 4. Power P extracted during the scan.

The P-V characteristic concerning this simulation is shown in Fig. 3; the n (n=9) steady-state operating points associated to the scan are marked by means of circle symbols. The corresponding waveform of the power P extracted during the scan is reported in Fig. 4. The values of P reported in Fig. 4 are in very good agreement with the theoretical predictions of Fig. 3. In the considered case, the operating voltage where the recorded value of P is maximum is vb= vb _ref=Vb_min, which corresponds to the first point of the scan (Figs. 3 and 4); therefore, at the end of the scan, the CMPPTS controller correctly sets vb_ref = Vb_min. IV. HARDWARE IMPLEMENTATION Every SCPVM [12-14] and the central inverter need a dedicated control electronic device (CED) which has to implement the control techniques briefly described before. In particular, the CED of a SCPVM has to implement the DMPPT technique, while the CED of the central inverter has to implement the CMPPT technique. The CED of SCPVM, in order to implement the DMPPT, has to measure input voltage and current and output voltage of the DC/DC module; the CED of the inverter has to measure input current and voltage and output voltage of the inverter. In order to prove the effectiveness of the proposed technique, a laboratory test system has been implemented. For sake of simplicity, it is composed by only two SCPVMs and a central inverter. In order to make the experimentation independent from solar irradiance, the PV module has been emulated with a numerically controlled power source. It is the Kepco BOP 3612M, a 400 W four-quadrants power supply. This power supply digitally communicates over an IEEE 488 digital bus, through which it can be programmed to reproduce the static and dynamic characteristics of a PV panel. As DC/DC converter, a step up regulator has been used. As CEDs the microcontrollers STM32F407 from ST Microeletronics have been used. The input port of the inverter has been emulated by means of a Chroma 63802 AC/DC Electronic Load.

current IMPP =4.67 A, Nominal Operating Cell Temperature NOCT=46 °C. Fig. 7 and Fig. 8 show the simulated and the emulated P-V characteristics of the PV modules exposed, respectively, to 1 sun and to 0.6 sun. A good agreement among simulated and emulated curve can be appreciated. The simulated P-V characteristics of both series connected SCPVMs as well as PV modules are shown in Fig.9. In the Fig.10 are, then, reported the time domain waveforms of vb voltage and the extracted input and output powers; the input power is the power extracted from SCPVMs in case of unit efficiency, while the output power is the real power extracted from SCPVMs, thus considering the actual efficiency of the power stage. The values of the input power reported in Fig. 10 are in very good agreement with the theoretical predictions of Fig.9; which involves a good implementation of the proposed technique. As concerns the output power, it results higher than the maximum power of the series connected PV modules. Moreover the maximum power is available for a wide range of the input inverter voltage vb. obviously such a power could be also higher using higher efficiency power stages.

Fig. 5. Block scheme of the measurement station

V. EXPERIMENTAL RESULTS In order to experimentally validate the realized embedded measurement system which implement the HMPPT technique, a measurement station has been set up. Its block scheme is represented in Fig. 5. It is composed of two emulated PV modules, two Boost converters, a load for the SCPVM array, a Personal Computer (PC), a data acquisition (DAQ) board, voltage and current transducers. The SCPVMs, the load and the PC are connected together through IEEE 488 bus; the DAQ board communicates with PC through USB bus. The voltage and current transducers are, respectively, LEM CV3 1000 and CT 10. The measurement software has been developed in LabView environment. A photo of the measurement setup is shown in Fig. 6. In the experimental test, the emulated PV modules, are exposed to two different irradiance levels: 1 sun, 0.6 sun, where 1 sun is equal to 1000 W/m2. The characteristics of the emulated PV modules in STC (that is AM=1.5, irradiance S=1000 W/m2, module temperature Tmodule =25 °C) are: open circuit voltage Voc =15 V, short circuit current Isc =5 A, MPP voltage VMPP =12.07 V, MPP

Fig. 6. A photo of the measurement station

Fig. 7. P-V characteristic of Emulated PV module n.1

which must be fulfilled by the two parameters of the CMPPTS technique (the amplitude ∆vb_ref of the variation of the reference voltage for vb and the sampling time Tb) in order to optimize the performance of the PV system have been identified. Their dependence on the main parameters on which the DMPPT MP&O technique is based has also been deeply discussed. The results of numerical simulations carried out in PSIM environment and the experimental results fully confirm the validity of the theoretical predictions. REFERENCES Fig. 8. P-V characteristic of Emulated PV module n.2

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Fig. 9. P-V Characteristic of a string of 2 SCPVMS [7]

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Fig. 10. Time domain waveforms of the inverter input voltage and power.

[11] [12]

VI. CONCLUSIONS Provided that the joint adoption of both a DMPPT technique and a suitable CMPPT technique is mandatory in PV applications operating under mismatching conditions, in this paper the proper optimization of the CMPPT technique has been shown. This technique is based on the periodic scan of the P-V characteristic of the string of SCPVMs in order to locate the optimal operating value, from the energetic efficiency point of view, of the bulk inverter voltage vb. In this paper it has been shown how the typical parameters of this technique must be chosen in order to avoid errors due to the peculiar shape assumed, under mismatching operating conditions, by the P-V characteristic of a string of SCPVMs. Such a curve is characterised by the possible presence of multiple peaks and/or flat regions and/or abrupt nearly vertical portions which may easily lead to the error of the CMPPTS technique, and, consequently, to a more or less consistent waste of the potentially available energy. The conditions

[13]

[14]

[15] [16]

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