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Description

This network demonstrates the operation of a 2 MW, 1 Mvar photovoltaic power station.

The PV array can produce 2 MW at 1000 W/m² sun irradiance and a cell temperature of 25ºC. The figure below shows the P-V characteristics for the PV array.

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A boost converter (Switching Function Model) is connected to the PV array and it is controlled by a Maximum Power Point Tracker (MPPT) control system. The MPPT uses a technique called "Perturb and Observe" which allows extracting the maximum possible power of the PV by varying the voltage across its terminals.

The figure below shows how the Perturb and Observe technique works for extracting the maximum possible power of the PV.

There is a three-level NPC converter (switching function model fed by a PWM generator with pulse averaging) that converts the 1000 V DC (the output of the boost converter is connected to a common DC bus) to around 500 V AC. A DC voltage regulator is used to control the three-level NPC converter. This regulator maintains the DC link voltage to 1000 V whatever the amount of active power delivered by the PV array. In addition, the controller has a reactive power regulator allowing the converter to generate or absorb up to +/- 1 Mvar.

Finally, a 2.25-MVA 500V/25kV three-phase coupling transformer is used to connect the converter to the grid. The grid is modeled by 25 kV distribution feeders and a 120 kV equivalent transmission system.

The figure below illustrates each component of the example.

Location

This example model can be found in the examples under the category Renewable Energy with the file name 2MW_GridConnected_PVarray.ecf.

Simulation and Results

Before to start de simulation, select the options Activate iterative method and Solve control inputs before solving power in Simulation Settings in order to obtain accurate results.

Start the simulation and use the SCOPEVIEW template to see the resulting signals.

You can also program disturbances: 

• Irradiance variation

• DC link reference voltage step

• Reactive power set-point variation

• System fault

To select which disturbance to simulate, simply go to the Programming Sequences section of this example and set one or both Target Digital Output blocks. In SCOPEVIEW, select Trig option in Acquisition Parameters tab and use the template to study the impact of the selected disturbance in the network.

You can also simulate the model with the PV cells' temperature set to 45^oC instead of 25^oC by double-clicking the PV and Boost Converter Variable Settings block located in the Programming Sequences section of this example.

The results are shown in the figure below. The model was tested at a time-step of 25 us. In addition, active iterative method is enable and applied to all nonlinear elements.

When there is no disturbance presented in the network, the Network Measurements tab of the SCOPEVIEW template for this example shows the AC voltages and currents at the primary of the decoupling transformer (BUS_EAST) as well as the DC link voltage, which is maintained to 1000 V as requested by the DC voltage regulator (see figure below).

In addition, the next figure (Control Measurement tab of the SCOPEVIEW template) shows the measured and the referenec reactive power (0.5 MVar) for the Inverter Control block, the solar irradiance (set to 1000 W/m²) for the PV array and the boost input voltage (mean value) which should match the voltage at maximum power point since the solar irradiance is set at its maximum for the P-V characteristics curve of the array.

Finally, the next figure (Converters Power Balance tab of the SCOPEVIEW template) shows that there is no active power invention for both the boost and the inverter converters. The PV array supplies around 2 MW to the boost converter, which supplies around 1.975 MW to the inverter, which supplies around 1.925 MW to the grid (4% of losses between the PV and the grid).

References

L. G. Monteiro Oliveira et al., "Computational implementation of photovoltaic modules mathematical models for software application to estimate PV systems energy production," 2015 International Conference on Solar Energy and Building (ICSoEB), Sousse, 2015, pp. 1-6

W. De Soto, S. A. Klein and W. Beckman. « Improvement and validation of a model for photovoltaic array performance ». Solar energy, vol. 80, no. 1, pp. 78-88, 2007

M. H. Rashid, Power electronics handbook, 4^th ed. Butterworth-Heinemann, 2017

H. Islam, S. Mekhilef, N. Shah, T. Soon, M. Seyedmahmousian, B. Horan, and A. Stojcevski, “Performance Evaluation of Maximum Power Point Tracking Approaches and Photovoltaic Systems,” Energies, vol. 11, no. 2, p. 365, Apr. 2018