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)
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. In this case, the task of the MPPT algorithm is to properly impose the value of the duty cycle, at any one instant.
In our case, a boost converter has been devised to test the PV emulator behavior with an electronic load, which corresponds to most of the actual practical configuration of PV plants.
Once the PV emulator behavior is assessed, the whole system composed of the PV emulator and the boost converter is useful to test MPPT algorithm performance. In addition, if an inverter is included in the conversion chain, as illustrated in the example of Fig. 8.15, the inverter control algorithm can be tested, as well.
It is important to observe that, compared to a purely resistive load, an electronic load requires a high frequency current, in addition to the DC component, due to the switching operation. Hence, the high-frequency output impedance of the PV emulator should match the one of the real PV plant to achieve a realistic profile of delivered current and voltage.
The design of the boost converter is carried out imposing the conditions of continuous-current conduction operation and ripple on the output voltage lower than few percent.
The following values for the inductance and capacitance are fixed to: Lb = 2 mH and Cb = 23.5 pF.
In the case of a grid connection, by an active rectifier, the DC link voltage is mantained at a constant value, then it is possible to control the input voltage of the boost converter, and consequently the output voltage of the PV emulator, so to track the MPP.
The power switch is a SKM50GB123D power IGBT, switched at 10 kHz, as well as the sampling frequency of the control system.
A decoupling capacitance CPV = 650 iF is placed between the PV emulator and the boost converter in order to prevent oscillations toward the emulated PV source.
The boost circuit is allocated in the same case containing the power stage of the PV emulator. As shown in Fig. 8.36, the boost signal and power connectors are positioned in the right part of the front panel of the whole equipment.