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Description

The fuel cell generation system (FCGS) is shown in Figure 1. The FCGS features a fuel cell that is connected to an inverter via a boost DC-DC converter. The fuel cell modeled was implemented from a Matlab/Simulink model . The boost converter maintains and steps up the DC voltage from the fuel cell to the DC point of connection with the inverter. The inverter’s function 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). A RL choke filter is used to connect the inverter to the grid. The choke filter must be designed to limit the total harmonic distortion of the PVGS current injected into the grid at the point of common coupling (PCC) (although the average model used does not simulate switch events).

The FCGS must be connected to an external step up transformer to the grid. It is recommended to use a star-delta transformer with a base power of 1.2 times the nominal power of the FCGS. The model is not compatible with load flow. 

Figure 1 FCGS components schematic

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Mask and Parameters

System Parameters

Control Loops Parameters

Fuel Cell Parameters

Ports, Inputs, Outputs and Signals Available for Monitoring

Ports

Inputs

Outputs

None

Sensors

Modeling Details

Stack Voltage vs Current

The FCGS performs the control of its output currents in the dq reference frame. The structure of current regulator is shown in Figure 2. I_dq and V_dq are the grid currents and voltages, respectively, in the dq reference frame at the transformer primary while V_dqi are the inverter output voltages. The d axis current corresponds to the active power and the q axis current corresponds to the reactive power. 

Figure 2 Current regulator

The FCGS synchronizes to the grid using a phase locked loop (PLL) block. The d axis current references are generated in order to regulate the DC link voltage at the reference value (1050V in this example). The DC link voltage regulator is shown in Figure 3. 

Figure 3 DC link voltage regulator

The q axis current references are generated in order to follow the reactive power references provided to the inverter. The reactive power regulator is shown in Figure 4.

Figure 4 Reactive power regulator

The boost converter interfacing the fuel cell stack is operated with a duty cycle, d_boost, to control the current drawn from the fuel cell. The fuel cell current reference is generated by an active power regulator. Both the regulators are shown in Figure 5. The gain values for the current regulator should be slower than the response time of the fuel cell stack to ensure the control loop stability. The gains values of the active power regulator should be even slower than the fuel cell current regulator.

Figure 5 Active power regulator

References