Design and Implementation of the Digital Controller for a Fuel Cell DC-DC Power Converter system

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Design and Implementation of the Digital
Controller for a Fuel Cell DC-DC Power Converter
system
Department of Engineering
O.A. Ahmed, J.A.M. Bleijs
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• presentation Outline

• Introduction
• Dynamic Performance of PEMFC
• FC Converter Features
• Controller Design
• Simulation Results
• Control Algorithm Implementation
• Converter System Implementation
• Conclusions

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• Introduction

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DC Microgrid Power System
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• Introduction

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• DC Microgrid may comprise of
• A dispatchable power generator, such as the fuel
  cell (FC) or a back-up diesel generator.
• Non-dispatchable sources, such as solar PV
• and wind turbines.
• Energy storage, such as ultra-capacitor or
  battery.
• The overall efficiency of the distributed system
  depends on efficiency of power electronic
  converters.
• To obtain a cost-effective converter solution
• High efficiency.
• Low cost.
• Robust control system

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• objective

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Development of a Two-Loop Controller for a High
Efficiency FBCFC for a FC generator
Dynamic Response Analysis and Modeling of a
Ballard Nexa 1.2kW PEMFC
Digital Controller Design Based on a TMS320F2812
DSP
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• Dynamic Performance of PEMFC

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• Nexa data acquisition system is inadequate to
  show actual response.
• Fast-acting switches and high speed transducers
  are used.
• The gross stack current is 1.3 of the nominal
  current.

Experimental Dynamic Response Set-up for PEMFC
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• Dynamic Performance of PEMFC

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Accurate transient response using experimental
set-up
Data log file
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• Dynamic Performance of PEMFC

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Fuel cell model of test with resistive load
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• Dynamic Performance of PEMFC

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Simulated Transient Response of FC model
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• FC Converter Features

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• The clamp switch gives zero voltage switching
  (ZVS) for all switches at turn on and alleviates
  the usual voltage ringing across the bridge
  switches (S1S4)
• At turn off the voltage across all switching
  devices is clamped to the capacitor voltage.
• The output rectifier diodes operate with zero
  current switching (ZCS).

• Full bridge with voltage doubler technique
  results in reduced voltage across output
  capacitors and diode rectifier.
• Minimizes the voltage rating of the
  semiconductor devices.
• Reduces the turn ratio of high frequency
  transformer to half that of the conventional
  FBCFC.

• Untapped secondary winding transformer
  simplifies the transformer construction, improves
  window utilization, and results in lower leakage
  inductance.

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• Controller Design

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• Initially an analogue PI was designed for the
  FBCFC based on its small signal transfer
  functions .
• Digital average current (DAC) mode control and
  predictive current control (PCC) techniques can
  be used for the system.
• Since FC takes 0.24 second to respond after a
  sudden load application from no-load DAC is
  selected without need for a complex control
  algorithm.

Two-loop PI digital controller
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• Controller Design

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• The designed digital controller takes into
  account the sampling delay (Hsmp) and the
  computation delay (Hc).
• Two PI controllers with anti-windup protection
  were designed and their optimal parameters
  obtained using the Control Design Toolbox in
  Matlab.
• Based on the designed digital controller loop
  for the FBCFC, the transfer function for the
  current, voltage and overall open loop response
  of the system are obtained .

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• Controller Design

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• The plot indicate that inner loop Cid(z) gain
  has a desired crossover frequency at 5kHz with a
  phase margin of 590.
• Digital PI parameters for Cid(z) are Kpc
  0.1147, and Kic 506.66.

Bode Plot of Current Controller Open Loop
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• Controller Design

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• To accommodate the slow FC response, low
  bandwidth is used for voltage loop Cvi(z).
• The voltage loop has a crossover frequency of
  149Hz and a phase margin of 58.70.
• Digital PI parameters for Cvi(z) are Kpv
  1.08625,
• Kiv 100.748

Bode Plot of Voltage Controller Open Loop
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• Controller Design

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• Due to the active clamp the resonance is
  absence.
• The plots show that the bandwidth and phase
  margin of the controller system are designed as
  desired.

Bode Plot of Overall Open Loop System
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• simulation results

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• The simulated dynamic response of the converter
  at sudden load changes shows that the voltage
  recovers to its steady-state value within 15m
  sec.

Output voltage waveform during load changes

• Designed PI digital controller has been
  implemented in a DSP the on-chip ADC converter,
  PWM compare function and other functions were
  emulated in Simulink.

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• control algorithm implementation

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• IQ math library in CCS is used for the PI
  compensators, ADC results and ADC and PWM scaling
  factors.
• The number of GP timers in a DSP is limited to
  four. The PWM signals for all switches are
  generated using only one GP timer with dead-band
  zone.

Flow chart of the overlap PWM algorithm for FBCFC
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• control algorithm implementation

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Software block diagram of 32-bit PI digital
control for voltage and current loop
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• converter system implementation

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• The sampling frequencies are chosen 20 kHz for
  the current-loop and 2.5 kHz for the voltage-loop
  controllers.
• The FBCFC without clamp circuit shows high
  voltage overshoots across the MOSFETs resulting
  in increase switching and conduction losses and
  reducing the converters efficiency.

FBCFC with voltage doubler but without clamp
circuit secondary voltage (ch3), drain-source
voltage across S1 (ch4)
Proposed FBCFC primary voltage (ch3), FC current
(ch4), clamp switch voltage (ch2)
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• converter system implementation

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Simulated and Experimental Converter Efficiency
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• conclusions

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• FC experiments showed that the number of
  measurement points using Nexa log file is not
  sufficient to plot the accurate transient
  response characteristics of the FC stack. It is
  observed that the modelled FC matches very well
  with the real FC system results
• A two-loop digital controller with anti-windup
  protection has been developed for a 1.2kW active
  clamp FBCFC topology with a voltage doubler.
• The experimental results have validated the
  accurate modelling for the FC converter system.
• Simulation and practical measurements of the
  proposed converter and other configurations at
  different loads have shown that the proposed
  configuration has the highest efficiency.

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