8.1 Conclusions
The main objective of thesis is to develop a load/grid connected fuel cell power system which can be used for backup power supply. Digital control scheme of DC-DC converter and DC-AC inverter has been developed and the extensive simulation results are validated through experimental setup.
The general conclusions are drawn from the thesis
Fuel Cell Modelling: Mathematical modelling of solid oxide fuel cell (SOFC) is developed which considers different types of losses. Electrical characteristics (output voltage, efficiency, and power density and voltage losses) of SOFC are simulated through MATLAB.
FPGA Control of DC-DC Converter: A DC-DC boost converter is developed to
step up the output DC voltage of SOFC. Conventional PI controller and Sliding Mode Control (SMC) scheme has been designed for DC-DC converter. Conventionally the control circuit of DC-DC converter is implemented using analog components but due to the non-re-configurability nature of the analog control, it is not reliable in nature. So the control circuit of power converter is implemented using FPGA. FPGA is an ideal choice of digital computing device
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because it’s modular in nature and can be re-configured in real time. FPGA implementation of PI controller is designed and experimentally validated.
PWM-VSI Control Schemes: The fuel cell power system is implemented with
PWM-current controlled voltage source inverter. Different control schemes (TCCC, TPCC and Fixed-HCC, Fuzzy-HCC, Adaptive-HCC and Adaptive- Fuzzy-HCC) of PWM-VSI are summarized.
Fuel Cell as Back-up Power Unit: As fuel cell provides backup power, there can
be numerous real life scenarios such as non-availability of required number of fuel cell units, malfunction of fuel cell etc. This thesis considers the above mentioned scenarios. When the required numbers of fuel cells are not available, DC-DC converter is used to step up the output voltage of fuel cell. When there is a malfunction in fuel cell or shortage of hydrogen then a battery is used to provide backup power. The fuel cell power system is simulated through MATLAB/SIMULINK with different conditions.
In a first case, the DC-AC inverter is controlled using fixed-HCC and adaptive HCC. When there is any kind of malfunction of fuel cell then battery is connected to provide backup power supply. In this case, adaptive hysteresis controller is used for DC-AC inverter. When there is sufficient number of fuel cells is available then DC-AC inverter is controlled using fuzzy-HCC, Adaptive-fuzzy HCC, TCCC and TPCC.
The harmonics of line currents are analysed and it’s found that Adaptive-fuzzy- HCC technique reduces the switching losses and improves the fuel cell power system performance in comparisons with the TCCC, TPCC, fixed-HCC and the Adaptive-HCC. The measured total harmonic distortion of the source currents is in the compliance with IEEE 519 and IEC 61000-3 harmonic standards.
Fuel Cell Based Shunt APF: The fuel cell power system based on shunt active
power filter (APF) is designed for compensation of current harmonic and reactive power compensation in the AC power distribution system. APF based fuel cell reduces the real power flow from grid side, which eventually reduces the power rating of inverter.
Experimental Validation: The prototype model of single phase fuel cell power
system with hysteresis current control (HCC) technique is developed. FPGA implementation of HCC is done using NI-cRIO-9014. Three phase model of fuel
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cell power system is developed using adaptive-HCC using Xilinx/System Generator. The adaptive-HCC PWM-VSI control algorithm is designed and implemented in the FPGA-kit using Xilinx ISE. First, the controller is modeled using Xilinx Blockset. From the Simulink, VHDL code is generated for this particular controller using System Generator. The VHDL code is verified and synthesized in the Xilinx ISE 10.1. After synthesising the code, the programming bit file is generated and configured on the target device. The FPGA provides the switching pulses to drive the PWM-VSI inverter through gate driver circuits.
8.2 Scope for Future Work
Some suggested new directions of research in the area of fuel cell based power system are summarized in this section.
To improve the power quality of fuel cell power system by using dynamic voltage restorer (DVR).
To meet the higher power requirements, a modular and multilevel inverter can be used.
New control scheme for power electronic interface of FCPS system using soft computing technique can be developed by utility interactive operation.
A complete FPGA based controller for fuel cell power system can be developed that would add flexibility for controller design. This would significantly reduce hardware requirement and it can further be extended to ASIC development.
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