Location

This example model can be found in the software under the category "Renewable Energy" with the file name "Photovoltaic_Generation_System.ecf".

Description

This example shows the control operations of a PV generation system (PVGS) connected to a typical distribution feeder with active and reactive loads. The interface converters of the PV and the boost converter use switching functions (SwF) suitable for real-time simulation. The switching function presents a good compromise between the real-time performance of an average model and the accuracy of using detailed switches. This is used to study harmonic currents generated by the nature of switching devices. Several tests are performed to show the PV generation both in curtailment and MPPT mode. Also, a comparison is made between the model with the PVGS using SwF (Page 2) and the same model with the PVGS using average converters (Page 1). For more information regarding the PVGS example model with an average converter, refer to Examples | Distribution Connected Photovoltaic Generation System with Average Converter. 

The figure above shows the example of a distribution network with PV generation. The PVGS has a nominal power of 750 kW and is rated at 1050 V DC link. The system features a PV array, a coupling inductor, a step-up transformer, a boost converter, and an inverter. It is connected to a simplified three phase bus 22.9 kV, 60 Hz distribution system. The internal topology of the PVGS with SwF is explained in Photovoltaic Generation System (PVGS) - Switching Function. 

The PVGS is connected to the PCC via a 22.9 kV/480 V, 0.9 MVA star-delta distribution transformer. The secondary transformer is connected to the grid-side converter with a 250 µH choke filter. The switching frequency is 1620 Hz.  The PVGS features a PV array that is connected to an inverter via a boost DC-DC converter. The boost converter controls the PV array in two modes of operation: 1) maximum power point tracking (MPPT); and 2) curtailment. In MPPT mode, the boost converter duty cycle is calculated by a perturb-and-observe (P&O) MPPT algorithm to ensure maximum power extraction from the array. In the curtailment mode, the duty cycle of the boost converter is controlled to follow the active power reference. The inverter’s primary control objective is to maintain and regulate the DC link voltage and the reactive power at their respective commanded values at the point of common coupling (PCC). 

For more details about the PVGS with SwF, please refer to Photovoltaic Generation System (PVGS) - Switching Function.

Simulation and Results

The PV system can operate in MPPT (curtailment input equals to 0) or curtailment mode (curtailment input equals to 1). When curtailment mode is disabled, the converter injects its maximum available power, which is subject to the solar irradiance. On the other hand, when curtailment mode is enabled, the maximum power injection becomes limited by the Pref input. The control sequence displaying different scenarios is summarized in the table below. The same control sequence is used for both PVGS models (average model and SwF). The models were tested at a time-step of 50 µs.

The purpose of this simulation is to compare the results obtained with the PVGS example model using an averaged converter against the results obtained with the same example model using the PVGS with SwF. Simulation results presented below were obtained using the ScopeView template provided with the example. 

During the first MPPT window (operating points 1-3), the active power reference is ignored and the maximum available power at 800 W/m² is drawn from the PV array. As the curtailment signal goes high at 4 seconds (operating point 4), the active power reference is again ignored as the demanded power is greater than the available power. At 5 seconds (operating point 5), as the demanded power goes to 0.5 pu, the power drawn from the array is curtailed to meet the reference. At 6.5 seconds (operating point 6) the active power reference again goes beyond the available power, therefore, the system again operates in MPPT mode and starts to raise the active power. The active power jumps to 1 pu at 15 seconds (operating point 10) as the irradiation jumps to 1000 W/m², then as the temperature increases from 25°C to 35°C at 18 seconds (operating point 11), the active power reduces due to the temperature dependence of the model. 

It is noted that the duty cycle increases and decreases between 5 s to 10 s to curtail the output power to a reference value. In a typical power voltage curve of a PV array, the output power can be obtained at two different voltages. Therefore, for the second voltage value the duty cycle will first decrease and then increase, while maintaining same the output power of the system. It is also noted that the measure of reactive power always follows the reference of reactive power (operating points 1,2 and 5-9) except where the demanded power or the measured active power is high enough to violate the system ratings. That is when the reactive power is reduced to respect the system rating (operating points 3,4, 10 and 11). 

For the output power dynamics, it is noted that the active and reactive power of the PVGS with SwF (PVGS2.P(SwF) and PVGS2.Q(SwF)) have similar transient and steady state responses compared to the PVGS with the average model (PVGS1.P(Avg) and PVGS1.Q(Avg)). In the DC link side, the voltage of the DC link of the PVGS with SwF (PVGS2.Vdc(SwF)) contains harmonic components with a ripple factor of 2.7% caused by the dynamics of the boost converter switching function. However, the transient and steady state responses are the same compared to the PVGS with the average model (PVGS1.Vdc(Avg)). Similarly, the grid-side current, PCC current, and PCC voltage of the PVGS with SwF (CB2_2(SwF), PVGS2_Iabc(SwF), and PCC_2.V(SwF)) contain harmonic components caused by the switching dynamics of the DC-AC converter. However, the main frequency components (60Hz) of these signals are similar to the grid-side currents, PCC currents, and PCC voltages of the PVGS with the average model (CB2_1(Avg), PVGS1_Iabc(Avg), and PCC_1.V(Avg)). It is important to note that the choke filter used in this example model is used to allow the PVGS to inject current harmonics. This is done for illustrative purposes. However, the choke filter value van be modified to have better harmonic rejection compared to this example model.

The aforementioned results demonstrate that the PVGS model with SwF model provide similar results compared to the PVGS with average model. However, the PVGS with SwF provide more accurate results regarding harmonic components injection caused by the internal switching functions of the DC-AC and DC-DC converters. 

References

[1] J. Rocabert, A. Luna, F. Blaabjerg and P. Rodríguez, "Control of Power Converters in AC Microgrids," in IEEE Transactions on Power Electronics, vol. 27, no. 11, pp. 4734-4749, Nov. 2012.
doi: 10.1109/TPEL.2012.2199334

[2] Gray, J.L, “The Physics of the Solar Cell”, in Handbook of Photovoltaic Science and Engineering, A. Luque, Hegedus, S., Editor. 2011, John Wiley and Sons.