Category Photovoltaic Sources

Power Stage Design

A switching DC/DC buck converter topology is chosen. The DC/DC converter is operated at a constant switching frequency and input DC voltage, while the regulation of the output voltage is obtained by varying the converter duty cycle (D).

The scheme of the DC/DC buck converter circuit is that illustrated in Fig. 7.13. In such a scheme, the resistive components rL and rC represent the parasitic resistance of the output filter inductor and capacitor, respectively.

The design of the converter output LC filter is carried out on the basis of the following conditions:

• continuous-current conduction operation of the converter;

• ripple on the output voltage not exceeding few percent.

The above-mentioned conditions can be obtained from Eqs. (7.23) and (7.19), respectively...

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Example of a PV Source Emulator Design

This section is dedicated to the design and practical setup of a PV emulator, devised by the authors, which is used to reproduce, in a laboratory frame, the electrical behavior of a real PV plant, previously modeled according to a single diode four parameter scheme. The PV generator’s I-V relationship is expressed by Eqs. (3.13) and (3.14).

The considered PV plant has been described in detail in Sect. 4.4.2.3, where the parameter identification for the corresponding model has been performed, as well. In particular, the adopted PV model uses a regression law to relate temperature and solar irradiance, as explained in Sect. 4.6

Here, the PV plant specific features are summarized in Table 8.1, for the sake of convenience.

Figure 8.16 shows the PV plant configuration.

image922

Fig. 8...

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Selection of the Best Topology

On the basis of the analysis developed in Chap. 7, the considered DC/DC converter topologies, i. e., the buck converter, the boost converter, and the buck-boost con­verter, are all, in principle, possible candidates to be used for the power amplifier stage of a PV emulator.

Anyway, the following practical considerations suggest the employment, when possible, of the buck topology.

• The DC voltage level for supplying the PV emulator can be easily obtained by the grid voltage through a simple bridge rectifier both in the case of single-phase grid and three-phase grid. In this last case, the obtained DC voltage of about

Table 8.1 Main features under standard test conditions (stc) of the PV plant for the emulator design

Parameter

Symbol

Value

Number of assemblies

Np

2

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Dynamic Response and Arbitrary Load Problem

Two different issues have to be handled when the PV emulator is required to correctly reproduce the dynamic behavior of a PV generator. The former is related to the need of following the response to rapidly changing environmental conditions or applied load. The latter is known as the arbitrary load problem and it is tied to the use of the PV emulator in a power conversion chain where it is connected to a power converter, intended as an electrical load.

With reference to the first issue, the response of the PV emulator should exhibit a time constant faster than the dynamic response of the model. This is achieved by suitably setting the emulator bandwidth and by setting a switching frequency much higher than the cut-off frequency of the emulator bandwidth.

It should be noted that these const...

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DC/DC Power Converter Design Constraints

8.5.1.1 Current and Voltage Output

The maximum output current and voltage deliverable by the PV emulator have to be defined on the basis of the operating conditions of the considered PV generator.

It should be observed that the maximum output current is the short circuit current at the highest solar irradiance, while the maximum output voltage is the maximum open circuit voltage.

Once these design constraints are fixed, the maximum allowable load variation must range from the infinity impedance (open circuit condition) to an impedance as much as low to permit approaching the above described short circuit condition.

It is worth noting that the mere short circuit condition, corresponding to a null load is not compatible with the converter operation as deduced by the transfer functions given ...

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DC/DC Converter-Based Emulation of a PV Source

The block diagram shown in Fig. 8.14 summarizes the concept of a PV source emulation, where the feedback structure of a DC/DC converter and the V = f(I) block are highlighted.

The feedback controlled DC/DC converter is that described in the previous section. It realizes an output equal to that delivered by the I-V relationship. Its output voltage is applied to the electrical load and the obtained current is used as an input for the V = f(I) block.

Figure 8.14 synthesises the content of the two parts of this book as well.

In particular, the block indicated as ‘‘PV source modelling’’ is the result of the I-V characteristic representation, obtained with one of the techniques described in part I.

On the other hand, the block indicated as ‘‘Feedback controlled DC/DC con­verter’’ is the ...

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Simulink® Implementation

With reference to the simplified circuit of the buck converter drawn in Fig. 8.12, the state variables can be expressed as in Eq. (8.42).

CO

image915

image916

R

Подпись: ic image918 Подпись: (8.42)

Fig. 8.13 Simulink® implementation of buck converter with pole placement control technique (top); Simulink® implementation of the transfer function ic versus. w allowing load variation (bottom)

x1 (vc Vref)

s

This formulation can be implemented in Simulink® as shown in Fig. 8.13, top scheme, where the gains are calculated by Eq. (8.37). It should be noted that the use of the block ‘‘transfer function’’ does not allow the parameters to be modified.

The transfer function ic versus w can be rearranged as in Fig. 8.13, bottom scheme, to simulate load variations.

image920

Fig. 8...

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Enhanced Pole Placement for Buck Converter

Figure 8.11 shows a buck converter similar to that analyzed in Sect. 7.4 with an additional current generator in parallel to the resistive load. It models the current supplied to an inverter connected to its output. The variable w is the mean voltage supplied by the generator Vs.

The buck converter can be considered as supplied by a generator:

w = DVS (8.23)

that represents the control variable.

The circuit is described by equations similar to Eqs. (7.38) and (7.39) slightly modified for the presence of the generator Io1.

< w = L + rLiL + Vo

t

Vo = Vc + rcic

(8.24)

.L = .c + Zo + Zo1

Vo = 4’L – C – ‘oO

(8.25)

The Eqs. (8.24) and (8.25) can be solved for the time derivative of the state variables, iL and vc.

{

diL _ Rrc + RrL + rcrL. R 1 , Rrc f

dt = L(R...

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