Power Electronics Final Project Any data that are not explicitly provided below are intended for you to assume. System Description: A DC microgrid is connected to the main AC grid through a 3-phase PWM inverter as shown in Figure 1. A circuit breaker (CB) at the point of common coupling (PCC) enables switching between the grid-connected and islanded modes of operation. The DC bus voltage is 318V and the AC grid is at 208V. A photovoltaic system, consisting of 10 parallel strings with 5 series-connected modules per string, is interfaced with the DC bus through a DC-DC converter. This converter is controlled to track the maximum power point (MPP) of the photovoltaic system. Solaria 280 modules are used (P = 280.2 W, VOC = 44.6 V, temperature coefficient of VOC = -0.33 %/deg, VMP=35.9, ISC = 8.27, temperature coefficient of ISC = 0.05 %/deg, and IMP = 7.8A). A 48V, 100Ah Lithium-Ion battery is connected to the common DC bus through a bidirectional DC-DC converter. This converter is controlled to regulate the charging and discharging current of the battery. A DC load is connected to the DC bus through a DC-DC converter. The load is modeled as a 20 W resistor, and the converter is responsible for regulating the load voltage to any desired value between 200V and 400V. A balanced 3-phase Y-connected resistive AC load, of 40 ohms per phase, is connected at the terminals of the inverter.
Figure 1. DC Microgrid.
DC
DC
DC
DC
MPPT
P
P
Charge/Discharge Control
Solar
Battery
DC Loads
AC Loads
Grid
DC
AC
P
Inverter
PCC
DC
CB
Prof. A. Mohamed Electrical Engineering Department, CCNY
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Design Problems (Use a switching frequency of around 10 kHz for all converters/inverters): • Design and simulate all the required DC-DC converters for the solar photovoltaic system (with MPPT algorithm), for the battery (with its charge/discharge control), and for the DC load. • Design and simulate the required DC-AC inverter. Use any suitable pulse width modulation technique. Design the controller such that independent control of active and reactive power can be achieved. Use an L-filter (consider its internal resistance).
Simulation Scenarios: 1. Simulate the microgrid for 12s in the grid-connected mode, during which the inverter is regulating the DC bus voltage. The DC load is operated at a constant 250V. The solar radiation is varying. The inverter is operating at unity power factor. The battery is initially charging with 40A and the DC load voltage is 200V. After 4s, the battery starts to discharge 30A and the load voltage is increased to 400V. After 8s, the battery current is set to zero. Show the DC bus voltage, and the current at the DC bus side of the PV system, battery, and DC load. Show the inverter AC output voltage and current (inspecting the AC voltage and current waveforms, you should be able to verify unity power factor operation), inverter output active and reactive power, and inverter direct-axis and quadratic-axis currents. Show the harmonics spectrum of the inverter current, and the value of the THD. 2. Simulate the microgrid for 6s in the grid-connected mode, during which the inverter is regulating the DC bus voltage. The DC load is operated at a constant 250V. The solar radiation is varying. The battery is discharging 20A. The inverter is initially operating at unity power factor. After 2s, the inverter starts to draw reactive power of 500 VAR. After 4s, the inverter starts to inject reactive power of 500 VAR. Show the inverter AC output voltage and current, inverter output active and reactive power, and inverter direct-axis and quadratic-axis currents. Show the harmonics spectrum of the inverter current, and the value of the THD. 3. Simulate the microgrid for 8s. Initially, the DC load is operated at a constant 250V. The solar radiation is varying. The battery is charging with 20A. The inverter is initially operating at unity power factor. Assume that a blackout takes place at 4s, which triggers the CB to open transitioning the microgrid to an islanded mode. Microgrid inverters have black-start capability. Develop the needed logic that will enable a smooth transition to the islanded mode. Show the DC bus voltage, and the current at the DC bus side of the PV system, battery, and DC load. Show the inverter AC output voltage and current (inspecting the AC voltage and current waveforms, you should be able to verify unity power factor operation), inverter output active and reactive power, and inverter direct-axis and quadratic-axis currents. Show the harmonics spectrum of the inverter current, and the value of the THD. Hint: To expedite the running time of the model, consider fixing the sampling time (e.g., at 1e-5 s) and use a simple solver (e.g., Ode1/Euler).
Prof. A. Mohamed Electrical Engineering Department, CCNY
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Deliverables: • A brief report including the required results and plots. Analyze the scenarios commenting on the response of the various converters. • Three SIMLINK models, a working model (any MATLAB version) for each of the simulation scenarios.
