Category Photovoltaic Sources


This Chapter presents an analysis of the fundamentals of feedback control for DC/ DC converters and an example of emulator design realized by a buck converter.

The choice of the buck scheme is justified on the basis of the possibility to implement an appropriate control strategy for the emulation purpose.

Both the PV model and the power converter, used for the emulator set up, are first simulated in Matlab-PLECS® environment, then the practical implementation of the control algorithm on a DSP board and the overall PV emulator equipment are described.

Experimental results show that the developed PV emulator is able to reproduce correctly the electrical behavior of a real PV source under any environmental situation, including partial shading and rapidly changing conditions.

The pole placeme...

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. Experimental Results

Figure 8.37 shows a picture of the experimental rack containing the PV emulator. At the top of the rack the DC power supply can be observed.

The operation of the PV emulator has been tested by laboratory measurements. The used measurement system is composed by:

• a digital oscilloscope with a bandwidth of 1 GHz and a maximum sampling frequency of 2.5 GHz, 20 GS/s, i. e., the TEKTRONIX TDS7254B;

• a 100 MHz high voltage differential probe, i. e., the TEKTRONIX P5205;

• a current measurement system, including an amplifier, i. e. the Tektronix TCPA 300;

• a current probe, i. e., the Tektronix A6303.

First, the static I-V characteristics of the PV plant (described in Table 8.1 and Fig. 8.16) are determined under different uniform solar irradiance values...

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DC/DC Boost Converter for the MPPT

The optimal exploitation of a PV source is obtained by maximizing the delivered power. This means that the optimal resistance load must be set to VMP//MP. Since the MPP is variable with environmental parameters, the load value should be continuously changed to follow this optimal operating condition.

A possible way to realize a variable load is to use a further power converter connected to the output of the PV source.

If a DC/DC boost converter is used for this purpose, on the basis of (7.46) and (7.47), its input resistance is given by:

Ri = — = — (1 – D)2= R(1 – D)2 (8.45)

1 to

It can be noted that the boost input impedance can be regarded as a variable load resistance controlled by the duty cycle D for the PV source...

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Control Board

The PV emulator control is experimentally implemented using the DSP-2 board developed at the Institute of Robotica of Faculty of Electrical Engineering and Computer Science in Maribor, Slovenia.

The DSP-2 board is a high performance, floating-point digital signal processor – based inverter controller. This board, in combination with DSP-2 library, can be successfully used for several industrial applications.

In general, the DSP-2 board allows control algorithms, set up in Matlab/Simulink® environment, to be implemented and verified in suitable development systems, at a significantly low cost.

Подпись: Fig. 8.33 Picture of the DSP-2 board

The board is based on the Texas Instruments TMS320C32 DSP and the FPGA XCS40-4PQ240C, member of Xilinx Spartan family...

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DC/DC Buck Converter

The DC/DC buck converter is supplied by the TDK-Lambda GEN600-5.5 DC power supply because, with this solution, the maximum deliverable current can be elec­tronically limited and the PV emulator is galvanically isolated from the power grid.

As an alternative, an isolation transformer and a bridge rectifier can be used to obtain the supply voltage for the PV emulator.

The DC/DC converter employs the SKM50GB123D power IGBT whose rated current is equal to 40 A at 80° C. This high current value allows to manage powers up to 10 kW.

Fig. 8.32 REO load 302 three-phase resistive load bank

image946The used driver circuit is the hybrid dual MOSFET driver SKHI22AR.

A snubber circuit is realized by using a 0.22 pF polypropylene capacitor, i. e., the MKPC4BS, suitable for high-frequency applications.

As previ...

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Experimental Implementation of the PV Source Emulator

The principle block diagram of the PV emulator is represented in Fig. 8.31.

It encompasses the DC/DC buck converter and a control board, whose inputs are the converter output voltage and current and whose output is the control signal of the buck converter.

The control board implements both the PV model and the control algorithm. On the basis of the output current, the PV model calculates the reference voltage which is compared with the actual output voltage of the DC/DC converter.

The error signal is processed by the controller that outputs the command for the switching device.

In the following subsections, the experimental set-up of the whole PV emulator equipment is described in detail.

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PLECS-Based Simulation of PV Source Emulator

In order to evaluate the performance of the PV emulator, developed according to the previously described design constraints, a simulation analysis is carried out. It exploits the association of Matlab/Simulink® environment and PLECS® (Piece­wise Linear Electrical Circuit Simulation for Simulink) toolbox.

As specified in Chap. 5, PLECS® allows an actual plant, for example a power electronic circuit, to be implemented, as subsystem, while the related control is developed using standard Simulink® blocks. Its main advantage is the very short simulation time.

As for the considered application, the DC/DC buck converter has been built using PLECS® libraries, while the control algorithm has been implemented in Simulink® and directly interfaced to the circuit-based simulation model.

Figure 8...

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Pole Placement Voltage Controller

As previously highlighted, a good dynamic behavior is required in order to take into account the loading effect of the power converter supplied by the PV emulator.

The possibility to impose the closed-loop poles, by using the pole placement technique, described in Sect. 8.4, permits to obtain a fast dynamical response.

The output impedance of the DC/DC converter gives information on the range of frequencies in which the PV emulator behaves like an ideal voltage source.

As previously said, a low impedance value is desirable in order to correctly reproduce the current demand coming from the loading converter.

The closed-loop poles are chosen as a pair of complex and conjugate and a real one. The corresponding kt coefficients are chosen equal to k12 = 2я(500 ± j500) and k3 = 2я500...

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