university of idaho department of … report i/final...interfacing an avista sr-12 hydrogen fuel...
TRANSCRIPT
UNIVERSITY OF IDAHO DEPARTMENT OF ELECTRICAL AND COMPUTER ENGINEERING
Interfacing an Avista SR-12 Hydrogen Fuel Cell to the Analog Model Power System (AMPS)
May 13, 2005
Project by: Daniel Hubbard Sponsors/Advisors: Dr. Brian Johnson [email protected] ECE Department University of Idaho Adam Lint Dr. Herb Hess [email protected] ECE Department University of Idaho Chris Cockrell [email protected]
Abstract Alternative energy sources are of great focus in the energy industry as the world realizes that its dependency on fossil fuels must diminish because of pollution, oil prices, or other factors. For this reason it is important that students at the University of Idaho have exposure to some of the alternative energy sources available in the 21st century. This report outlines a plan for interfacing an Avista SR-12 hydrogen fuel cell to the UI analog model power system. Fuel cell characteristics and system specifications are discussed along with simulation data for the entire system. However, the primary focus of the simulation data discussion is on the zero detection circuitry used for synchronizing the AC signal from the DC/AC converter with the utility frequency. The results of this simulation provide a method for implementing the design in hardware and show that the system will need to be designed to transfer a maximum of 200W to the AMPS. Key Terms: Fuel Cell, Alternative Energy, Zero-Crossing Detection, Synchronization, Voltage Regulation, Voltage Conversion, Model Power System.
i
TABLE OF CONTENTS:
1 PROJECT DESCRIPTION ................................................................................................... 1
1.1 OBJECTIVE AND SCOPE ........................................................................................................ 1 1.2 REQUIREMENTS .................................................................................................................... 1 1.3 SIGNIFICANCE OF THE PROJECT.......................................................................................... 2 1.4 BACKGROUND/HISTORY ...................................................................................................... 3 1.5 FUNCTIONAL SPECS.............................................................................................................. 3 1.6 CONTRAINTS ......................................................................................................................... 3 1.7 SOLUTION.............................................................................................................................. 4
2 STATUS ................................................................................................................................... 5
2.1 DESIGNED AND WORKING.................................................................................................... 5 2.2 DESIGNED AND NOT WORKING ........................................................................................... 5 2.3 DESIGNED AND NOT TESTED................................................................................................ 6 2.4 NOT DESIGNED...................................................................................................................... 6
3 METHOD OF SOLUTION.................................................................................................... 7
3.1 FUEL CELL ............................................................................................................................ 7 3.1.1 TYPES OF FUEL CELLS [3]................................................................................................. 7 3.1.2 OPERATION OF THE PEM FUEL CELL ............................................................................. 8 3.1.3 THE AVISTA SR-12 PEM FUEL CELL .............................................................................. 9 3.1.4 CHARACTERIZING THE SR-12 FUEL CELL ...................................................................... 9 3.1.5 ADDITIONAL CONSIDERATIONS ...................................................................................... 11 3.2 DC/DC CONVERTER........................................................................................................... 12 3.3 FLYBACK CONVERTER ....................................................................................................... 13 3.4 DC/AC CONVERTER........................................................................................................... 13 3.5 TRANSFORMER.................................................................................................................... 14 3.6 SYSTEM CONTROL.............................................................................................................. 15 3.6.1 DC TO AC CONVERTER .................................................................................................. 16 3.6.2 ZERO-CROSSING DETECTION ......................................................................................... 19 3.7 PROTECTION ....................................................................................................................... 24
4 VALIDATION....................................................................................................................... 25
4.1 FUEL CELL .......................................................................................................................... 25 4.2 DC/DC CONVERTER........................................................................................................... 26 4.3 DC/AC CONVERTER........................................................................................................... 26 4.4 ZERO-CROSSING DETECTION CIRCUITRY ........................................................................ 26
5 RESULTS .............................................................................................................................. 27
ii
5.1 FUEL CELL OPERATING PROCEDURE ............................................................................... 27 5.1.1 SAFETY AND CARE OF THE FUEL CELL.......................................................................... 27 5.1.2 STARTUP PROCEDURES ................................................................................................... 27 5.1.3 TROUBLESHOOTING ........................................................................................................ 28 5.2 VALIDATION RESULTS........................................................................................................ 29 5.2.1 DC/DC CONVERTER........................................................................................................ 29 5.2.2 DC/AC CONVERTER........................................................................................................ 30 5.2.3 ZERO DETECTION CIRCUITS........................................................................................... 32 5.3 COST ANALYSIS .................................................................................................................. 34
BIBLIOGRAPHY ....................................................................................................................... 35
APPENDIX .................................................................................................................................. 36
APPENDIX A – COMPAC DC/DC CONVERTER DATA SHEET................................................... 37 APPENDIX B – LF356 OP-AMP DATA SHEET............................................................................. 42 APPENDIX C – H11L1 OPTOISOLATOR DATA SHEET ............................................................... 52 APPENDIX D – ZERO DETECTION: CIRCUIT ANALYSIS ............................................................ 62 APPENDIX E – ZERO DETECTION: SIMULATION ....................................................................... 65 APPENDIX F – INDIVIDUAL PROJECT CONTRIBUTIONS ............................................................ 72
FIGURES AND TABLES:
TABLE 1 - PRELIMINARY FUEL CELL VOLTAGE LEVELS ............................................................................... 10 TABLE 2 - FUEL CELL LOADING CHARACTERISTICS ..................................................................................... 11 TABLE 3 - TROUBLESHOOTING THE FUEL CELL ............................................................................................ 28 TABLE 4 - BUDGET........................................................................................................................................ 34
FIGURE 1 - FUNCTIONAL BLOCK DIAGRAM .................................................................................................... 4 FIGURE 2 - PEM CHEMICAL REACTION .......................................................................................................... 8 FIGURE 3 - FUEL CELL LOADING CURVE ...................................................................................................... 11 FIGURE 4 - SCHEMATIC OF A FLYBACK CONVERTER..................................................................................... 13 FIGURE 5: TRIANGLE WAVE GENERATOR ..................................................................................................... 16 FIGURE 6: SINE WAVE GENERATOR.............................................................................................................. 17 FIGURE 7: DELTA ANGLE CONTROLLER........................................................................................................ 17 FIGURE 8: CONVERTER VOLTAGE CONTROLLER........................................................................................... 18 FIGURE 9: GATE SWITCHING CONTROL......................................................................................................... 18 FIGURE 10 - ZERO DETECTION CIRCUITS [4]................................................................................................. 20 FIGURE 11 - ZERO DETECTION CIRCUIT INPUT STATE [4] ............................................................................. 22 FIGURE 12 - ZERO DETECTION CIRCUIT MIDDLE STATE [4] ......................................................................... 23 FIGURE 13 - DC/DC CONVERTER ................................................................................................................. 29 FIGURE 14 - DC TO DC CONVERTER OUTPUT .............................................................................................. 30 FIGURE 15 - SYSTEM SIMULATION MODEL ................................................................................................... 31 FIGURE 16 - PWM INPUT: 1-PHASE .............................................................................................................. 31 FIGURE 17 - THREE PHASE AC OUTPUT ....................................................................................................... 31 FIGURE 18 - OP-AMP INPUT PIN SIMULATION............................................................................................... 33 FIGURE 19 - ZERO DETECTION CIRCUITS OUTPUT SWITCHING ..................................................................... 33
iii
1 Project Description
1.1 Objective and Scope The project objective is to interface an Avista SR-12 hydrogen fuel cell to the University
of Idaho Analog Model Power System (AMPS). This project will allow the fuel cell to
provide supplemental power to AMPS. Overall power quality and capacity is not
important in this project, as our main goal is to simply create the interface. Protection
circuitry is important to shield the fuel cell and the interface design from power surges or
faults on the AMPS.
1.2 Requirements
The requirements for this project are as follows:
1. Operation of the Fuel Cell must be better documented.
a. ECE students must be able to initialize and shut down the fuel cell in a
safe and timely manner.
b. ECE students must be able to identify and efficiently correct problems that
arise as a result of fuel cell operation.
2. The power converters must be simulated to show knowledge of their operation.
a. The DC to DC power converter for the fuel cell must accept 18-36V DC
and regulate it to 12V DC at the output. A tolerance of ±1V is acceptable.
b. The DC to AC power converter for the fuel cell must invert the regulated
DC voltage to a sinusoidal three-phase 208 V line-to-line AC voltage
±2%.
1
c. All power converters, transformers, and other circuitry used in this design
must be capable of handling 200W peak power though, ideally, only
100W will ever pass through them.
3. The overall system simulation must provide power to the AMPS.
a. The fuel cell, when interfaced to the AMPS, must provide positive three
phase AC power flow into the model power system.
b. The interface design must include protective circuitry to ensure that surges
on the AMPS do not damage the fuel cell, either of the converters, the
transformer, or the zero detection circuitry.
4. The fuel cell system must be able to synchronize with the AMPS without drawing
excessive current.
a. “Excessive” will be classified as any current above the rated conditions for
the fuel cell: approximately 11A.
b. Maximum phase error will need to be determined by circuit analysis after
measuring the line impedance of both the fuel cell system and the AMPS.
1.3 Significance of the Project
The AMPS provides students with the opportunity to explore and understand a typical
power transmission system. The ECE department wishes to expand and improve this
system to include alternative energy sources. This project will further facilitate the
learning experience for students and enable them to experiment with new technologies in
the power industry. Ultimately, this project will benefit power companies and the people
of our nation as UI’s students can apply their knowledge in the workforce.
2
1.4 Background/History In the mid-1990’s, the University of Idaho acquired the Analog Model Power System
(AMPS) from Idaho Power [1]. The AMPS is located in the basement of the Buchanan
Engineering Laboratory on UI’s main campus in Moscow, Idaho. The purpose of AMPS
is to provide insight into the workings of a power transmission and distribution system.
The main source of power is from the local utility company, Avista. Currently, a
generator is also interfaced with the AMPS to provide additional power to the system.
In addition to the generator, the UI has obtained an Avista SR-12 Hydrogen Fuel Cell
from Genesis Fueltech [2]. This fuel cell represents one of many alternative energy
sources available in the 21st
century.
1.5 Functional Specs The specifications for the interface are:
• 18-36V DC input from the Fuel Cell • Output 208V +/- 2 % (L-L 3-phase) • Output Voltage at 60Hz +/- 0.05Hz • Max power flow of 200W for the interface • Max phase difference when synchronizing; to be determined
1.6 Contraints The system must not draw power from the AMPS. It must be easy to operate and simple
to maintain. All safety precautions must be properly labeled to ensure the user’s safety.
Other constraints for the system include but are not limited to cost, reliability and
performance.
3
1.7 Solution
In order to accomplish the project objective, a DC/DC was converter simulated to
regulate the DC output voltage of the fuel cell to a constant level (12V). After the
voltage has been regulated, it will be run through a custom-designed flyback converter to
step-up the voltage 120V DC. A custom-programmed DC/AC converter will then
convert the 120Vdc input to a three-phase AC signal at 60 hertz; the line to line output
voltage should be about 95 percent of the DC input due to switching losses in the
inverter. A parallel three-phase transformer will step up the voltage to 208V line to line
and connect to the AMPS to supply supplemental power. To correctly interface the
produced AC signal to the AMPS, the three phase signal must be synchronized with the
three phase signals already on AMPS. Zero detection circuitry connected to one of the
three phases on AMPS will provide the DC/AC converter with the required timings and
phase necessary for proper synchronization. A graphical representation of the design can
be seen below in a functional block diagram (Figure 1).
Figure 1 - Functional Block Diagram
4
2 Status
2.1 Designed and Working In this project, the most important module is the Avista SR-12 PEM fuel cell. Its
characteristics define the specifications of all the other modules required to successfully
interface our system to the AMPS. A couple of modifications have been made to render
the fuel cell functionally operational and to increase performance specifications, such as
the external batteries and the bubble humidifier. Currently, the hydrogen fuel cell will
operate steadily at a maximum of 150W.
The design simulations are also operational. For the PSCAD simulation, the system
outputs a 208V line-to-line three phase signal—meeting the specifications for connecting
to the AMPS. The PSpice simulation of the zero crossing detection circuit is also fully
functional, capable of successfully detecting the desired crossing within 22 microseconds.
The results of these simulations are discussed in section 5.2 of this report.
2.2 Designed and Not Working While the simulated design is functional, difficulties with finding a low voltage DC/AC
inverter have arisen. The supplier for this device, Tier Electronics, will be unable to
provide a device for a price within the proposed budget. However, Tier Electronics can
supply a 120V inverter at a reasonable price; and, after some consideration, another
design alternative has been considered. This design alternative would require that the DC
voltage from the output of the fuel cell be regulated and increased to 120V, requiring the
use of a boost or flyback converter instead of a buck converter. Because low voltage,
5
wide input boost converters would typically saturate before reaching the desired output, a
flyback converter is the most viable design option at this point. Further research will be
performed regarding the use of a flyback converter so that the overall design can be
adjusted to fit specifications and an appropriate power inverter can be purchased before
the start of the second phase of this project.
2.3 Designed and Not Tested The alternative design that incorporates the use of a flyback converter has not yet been
tested. Pending research and testing, proper functionality is expected and will allow us to
continue with the projected schedule. All other components, except for the fuel cell, have
not been physically tested since they are only available in simulation at this point.
2.4 Not Designed The control algorithm for the TI-2403 DSP has not yet been completely finalized. The
top down design has been started but does not define all of the present needs. Inquiries
and research into code algorithms and how the DSP will control the system components
have commenced but the algorithms are far from being designed. The greater portion of
next semester has been planned and set aside to meet these important criteria.
6
3 Method of Solution
3.1 Fuel Cell 3.1.1 Types of Fuel Cells [3] There are five different types of fuel cells currently being operated and manufactured.
Each is named for the type of electrolyte used in the individual cells. Alkaline fuel cells
(AFCs) are the oldest design. AFCs have been used in the U.S. space program since the
1960’s but are not widely used within the earth’s atmosphere because of their
susceptibility to contamination due to impurities in the air. Because they require pure
hydrogen and oxygen to operate, they must be operated in special containment
environments, which make them expensive and impractical for terrestrial use.
Solid oxide fuel cells (SOFCs) are capable of supplying as much power as large utility
grade generators but operate at much higher temperatures than other types of fuel cells—
around 1000°C. SOFCs require more maintenance than other fuel cells because of their
high temperature, but they are potentially more efficient because the steam generated by
the high temperature operation can be used to power turbines and generate more
electricity. Molten carbonate fuel cells (MCFCs) run at lower temperatures than SOFCs
but still have the capability of generating steam. Their lower temperature (around 600°C)
makes them cheaper than SOFCs, but they still surpass other types of fuel cells with
respect to cost.
Phosphoric acid fuel cells (PAFCs) have a much lower operating temperature than
SOFCs or MCFCs, but they have a longer warm-up time than proton exchange membrane
7
fuel cells (PEMs). PEMs are relatively cheap to manufacture and maintain, and they
operate at temperatures around 80°C. They also have a fast startup time and are small
and portable compared to the other types of fuel cells. This makes them suitable for a
wide variety of applications. A PEM fuel cell has been donated to the University of
Idaho ECE department and will be used to complete this project.
3.1.2 Operation of the PEM Fuel Cell Proton exchange membrane fuel cells undergo a chemical reaction similar to that of a
battery. As shown in Figure 2, the hydrogen molecules are stripped of their electrons by
a thin membrane that has been specially treated to only conduct positively charged ions.
The electrons flow through a load and to the cathode where two oxygen atoms will
combine with the hydrogen ions to create water and heat.
Figure 2 - PEM Chemical Reaction
8
3.1.3 The Avista SR-12 PEM Fuel Cell In 2003, the University of Idaho ECE department acquired a 500W Avista SR-12 PEM
fuel cell from Genesis Fueltech. This unit contains 12 fuel cell cartridges, and was once
capable of providing 500W peak power with a variable DC voltage range of 23V to 43V
[2]. Since its manufacture in 1999, its capabilities have degraded and its characteristics
have changed. For this reason, several load tests were performed on the fuel cell to re-
characterize it. These tests have been discussed in the remainder of this section.
3.1.4 Characterizing the SR-12 Fuel Cell The values shown in Table 1 and Table 2 reveal that the actual voltage range of the fuel
cell differs from that discussed in “Avista SR-12 PEM Hydrogen Fuel Cell” [2]. Fletcher
claims that the voltage range is 23–43V DC, but by performing three separate tests, it can
be shown that the actual voltage range varies between 18V DC and 36V DC if not
severely overloaded. This smaller voltage range greatly reduces the cost of a suitable DC
to DC converter. Table 1 shows the results of tests one and two. Note that for the nine
and ten minute marks for the loaded test, the values were not available because the fuel
cell failed because of weak batteries. This will be discussed in the “Troubleshooting”
section of this report.
9
Table 1 - Preliminary Fuel Cell Voltage Levels
No Load >100 W Load time voltage time voltage
1 min. 29 1 min. 232 min. 31 2 min. 23.13 min. 30 3 min. 24.124 min. 28 4 min. 24.185 min. 29 5 min. 24.156 min. 30 6 min. 24.127 min. 31 7 min. 24.148 min. 29 8 min. 24.069 min. 29 9 min. N/A10 min. 31 10 min. N/A
The results of the third test, which include all ranges of the fuel cell voltage—including
overloaded conditions, are included shown in Table 2. This table shows load data
obtained by connecting a resistive load to the fuel cell while operating under nominal
conditions. Figure 3 shows a linear operation which is characteristic of most proton
exchange membrane fuel cells. This graph more concretely verifies the voltage range
presented in section 3.1.3, though an overload condition causes it to fall below the 18V
limit. When significant current is seen on the two 12V batteries used to power the
internal circuitry, the fuel cell should be considered overloaded. To reduce battery drain,
and to stay within specifications, the fuel cell should not be run in overloaded conditions.
10
Table 2 - Fuel Cell Loading Characteristics
V[V] I (Load)[A] P (load) [W] I (Batteries) [A] R [Ω
35 0.3 10.5 0.0002 116.67 34.8 0.4 13.92 0.0004 87.00
35 0.4 14 0.0003 87.50 34.5 0.5 17.25 0.0003 69.00 33.6 0.8 26.88 0.0003 42.00 31.7 1.8 57.06 0.0003 17.61 29.2 2.5 73 0.0003 11.68 30.5 2.4 73.2 0.0004 12.71
30 2.6 78 0.0003 11.54 28.9 2.8 80.92 0.0003 10.32 28.3 3.2 90.56 0.0003 8.84 27.8 3.3 91.74 0.0004 8.42 26.8 4.1 109.88 0.0001 6.54 26.6 4.4 117.04 0.0001 6.05
25 5 125 ~0.0517 5.00 25.7 4.9 125.93 ~.0093 5.24 23.4 6.8 159.12 1.11 3.44
23 7.1 163.3 ~1.23 3.24
0
5
10
15
20
25
30
35
40
0 1 2 3 4 5 6 7 8 9 10 11 12
Current (Amps)
Volta
ge (V
olts
)
Figure 3 - Fuel Cell Loading Curve
3.1.5 Additional Considerations The load tests reveal that the fuel cell is capable of supplying just over 100W of power
before overload conditions occur. This loss efficiency can be attributed to the aging fuel
11
cell cartridges. These cartridges depend on moisture content to operate well, and they
have lost some of their ability to self-humidify. To correct this, a bubble humidifier filled
with de-ionized water will be connected in line with the hydrogen to increase the
humidity in the system. Theoretically, this will noticeably increase the capabilities of the
fuel cell. This increase in power has been taken into account in the design of the
interface system. All components will be designed to withstand 200W peak power from
the fuel cell.
3.2 DC/DC Converter To successfully regulate the output voltage from the fuel cell, the DC/DC converter is
required to meet the following specifications:
• Wide Input Voltage Range: 20-36V • Constant Output Voltage: 12V • Power Rating: 200W • Over voltage/Current Protection
For the initial interface design, the ComPAC DC/DC power supply from Vicor
Electronics was chosen. The ComPAC, custom-built by Vicor, completely satisfies the
above specifications. This DC/DC converter has a wide input range (18-36V DC),
constant output (12V), protection circuitry, and a power rating of 200W. The low cost of
the ComPAC converter and relatively quick delivery time (2-6 weeks) also make this
desirable. A data sheet for the ComPAC DC/DC power supply is included in Appendix
A.
12
3.3 Flyback Converter As an alternative to a low voltage buck converter, a flyback converter could be used if the
120V input DC/AC converter is purchased. Since flyback converters use a transformer
with a parallel tertiary winding on the primary side to operate, they are capable of
outputting a much higher voltage range than any other type of step-up converter. Figure
4 shows the schematic diagram of a flyback converter. Since the flyback converter is a
relatively new design option, its operation and specifications have not yet been studied.
Research regarding the flyback converter will continue as phase two of this project
begins.
Figure 4 - Schematic of a Flyback Converter
3.4 DC/AC Converter The next stage of the interface design requires converting the regulated voltage from the
fuel cell into a 208V three phase AC signal at 60 hertz. A DC/AC converter inverts the
DC signal using a switching scheme controlled by a digital signal processor (DSP). For
this project, a DSP provides the calculation speeds necessary to allow for synchronization
13
of the AC signals with AMPS. The DSP used by the DC/AC converter also needs to be
re-programmable to allow the converter controls to be adjusted in case of changes on the
system. As with the DC/DC converter, the DC/AC converter needs to have a power
rating of at least 200W.
The Tier Electronics Universal Power Converter (UPC) is a suitable DC/AC converter. It
will be custom-constructed for this project by Tier Electronics and is controlled by a
Texas Instrument 2403 DSP. Tier’s UPC meets the power requirements (1kW) and will
provide extra I/O pins on the DSP for zero detection signals.
3.5 Transformer Both single phase and three phase transformers were considered for this design. Single
phase transformers are normally used for large industrial applications due to the
economics associated with maintenance fees and replacement. However, this design
utilizes a three phase transformer because of its higher efficiency, smaller size, and lower
cost.
When considering three phase transformer options for this design, there were four
configuration choices: delta to delta, delta to wye, wye to delta, and wye to wye. The
simulated design incorporates a delta to wye configuration. There are two primary
reasons for this decision: a wye configuration on the high side allows the transformer to
be easily grounded. This adds a significant level of protection to the system. Because of
this grounding, a fault on any of the phases could be detected by breakers or relays in the
14
ground line. Without a ground, such as with a delta configuration, the fault would go
undetected on the secondary side of the transformer and would continue to draw high
current across the transformer. This high current could damage the transformer before
being detected by other circuit protection. Secondly, a delta connection must be used on
the low voltage side in order to filter the sine-triangle pulse width modulation produced
by the switching devices used in the inverter. This filtering removes a third harmonic
that circulates in the delta and is dissipated in the transformer’s energy losses. The wye
to wye configuration would not output our desired signal because the currents could not
circulate through the transformer legs. The design specifications are only met with a delta
to wye configuration, thus the final choice for the transformer type.
The physical transformer will be designed based on the output of the power inverter.
This transformer will step up the voltage to 208V line-to-line and account for the fact that
an inverter actually outputs a peak line to line voltage of about 95 percent of the DC
voltage input.
3.6 System Control System control is important for stable operation of this interface design. The DC to AC
converter and the synchronization circuitry both play an important role in controlling the
system. The DC to AC converter will be controlled by the TI-2403 DSP. This processor
is built in to the power inverter and will be programmed based on inputs from the
synchronization circuitry to ensure stable system operation.
15
3.6.1 DC to AC Converter Power inverters require a significant amount of control to operate safely and efficiently.
The simulation of the DC to AC converter contains five separate sub-systems to control
the inverter. These sub-systems, if present in hardware, will all be controlled by the TI-
2403 DSP.
Two of the systems control and regulate a synchronous sine-triangle pulse width
modulation scheme. Figure creates two separate triangle waves by tracking the power
frequency and outputting one triangle wave for turning on the switching devices and
another for turning them off. The frequency of these triangle waves is a product of a
user-defined integer and the power frequency. Figure creates two sinusoidal reference
signals that are later compared with the triangle signals.
Figure 5: Triangle Wave Generator
16
Figure 6: Sine Wave Generator
The third sub-system, shown in Figure 7, is used to control power flow by varying the
angle between the converter voltage and the system voltage. The control of power flow
is represented by the following equation where Vconv is the
effective converter voltage, Vsys is the system voltage and X is the line reactance.
PV conv V sys⋅
Xsin δ( )⋅
Figure 7: Delta Angle Controller
17
The forth sub-system, shown in figure 8, controls the converter Voltage, |Vconv| by
controlling the variable mi in the equation . This is an extra level of
control that can also be used to vary the power flow.
V conv mi k⋅ Vdc⋅
Figure 8: Converter Voltage Controller
Figure 9 controls the switching devices present in the DC to AC converter. It compares
the sine wave and the triangle wave and generates the PWM output necessary for turning
the switches on and off.
Figure 9: Gate Switching Control
18
3.6.2 Zero-Crossing Detection For synchronization between the three phase signals produced by the DC/AC converter
and signals on the AMPS, accurate zero crossing detection is essential. If AC signals are
not synchronized when combined, current spikes and other undesirable transients will
appear on the system. In most common zero detection methods, a circuit is designed to
detect a zero crossing directly at its occurrence. This method is simple to implement but
is not relatively accurate when the precise location is necessary. The problem with single
output zero detection circuitry is that phase variations on the reference voltage will
translate into relatively inaccurate timings.
To maximize accuracy, this project utilizes a phase-locked loop algorithm that will create
two zero detection points, one right before the zero crossing and one directly after [4].
The DSP will count the time between the two signals, and then interpolate to find the
actual zero crossing. To interpolate the value, half of the count taken by the DSP will be
added to the pre-zero signal to estimate the actual zero crossing. Several readings will be
taken and then averaged to make the estimate more accurate. For this design, readings
will only be taken on the rising edge of the reference voltage.
The zero detection circuits designed for this project are shown in Figure 10. The top
circuit will give the pre-zero signal and the bottom the post-zero signal. The only
difference between the two circuits is the orientation of the biasing diodes on the input
stage.
19
Figure 10 - Zero Detection Circuits [4]
3.6.2.1 Design Constraints There are several constraints that control how a zero detection circuit must be designed
for an application. The most important constraint is the speed of the circuit, or how
much time it takes for a signal to be outputted, given a zero crossing. If the circuit is too
slow, timing specifications may be violated or the DSP used to control the DC/AC
converter will not have enough time to properly control the switches for synchronization.
Another factor is that the input of the circuit needs to function under large variations of
magnitude. In this project, the input needs to handle a 120VAC signal coming from the
AMPS. The final factor is that the circuit needs to be consistent and accurate. Therefore,
noise immunity needs to be incorporated into the design of the circuit. These constraints
will be discussed in the following sections.
20
3.6.2.2 Response Time To meet the timing requirements for this design, fast switching/responding devices were
chosen when creating the zero-crossing detection circuitry. The 1N914 diodes used in
this design have a max switching speed of 4ns. The 2N3904 switching transistor has rise
and fall times of 35ns and 50ns. The devices adding the most propagation delay to this
circuit are the LF356 op-amp and the H11L1 optoisolator. The LF356 op-amp has a
response time 12 V/µs (Appendix B). To reduce the response of the LF356 op-amp, the
output is clamped by a zener diode. The H11L1 has switching speed near 4µs (Appendix
C). The H11L1 optoisolator performs two functions: first, it provides isolation between
the zero detection circuit and the AMPS in case there is a surge or fault on the line; and,
second, it converts the output signal to logic levels that can be interpreted by the DSP.
3.6.2.3 Input Stage Both of the zero detection circuits utilize a resistance in series with diodes to
condition the input voltage to levels that can be used by the LF356 op-amp. The input
stage for the zero detection circuits can be seen in Figure 11. The only difference
between the input stages of the two zero detection circuits is the orientation of the biasing
diodes.
21
Figure 11 - Zero Detection Circuit Input State [4]
SPICE simulations show that the input range created from the conditioning for the pre-
zero circuit is about -2 to 1V and -1 to 2V for the post-zero circuit. If the resistance is
chosen properly, the voltage input to the op-amp will not change greatly despite relatively
large variations in current.
3.6.2.4 Active Feedback/Middle Stage To provide noise immunity to the zero detection circuit, dynamic positive feedback is
employed with the op-amp output and the reference voltage. By providing noise
immunity, the op-amp won’t output multiple transitions if the input voltage rises too
slowly or if there is significant noise present. If multiple transitions were to occur
because of noise, severe degradation would been seen with respect to the accuracy of the
circuit. The middle stage of our zero detection circuit can be seen in Figure 12.
22
Figure 12 - Zero Detection Circuit Middle State [4]
From an analysis of this stage, Vth, or the voltage on the positive terminal of the
op-amp has a relationship when there is a transition of:
Vth(t) = Vx + ∆Vz · e-t/RC (1)
A derivation of this equation can be seen in Appendix D.
This equation shows that immediately after a transition, the Vth jumps to (Vx+ ∆Vz), and
then decays back to Vx. When a transition occurs, Vth jumps out of the way to prevent
multiple transitions if there is noise on the signal or signal changes too slowly. After Vx
jumps by ∆Vz, it decays back to its original value without causing an extra, unwanted
transition. Equation (1) holds true for both circuits.
According to Equation (1), the time delay of the exponential decay depends on the
capacitor and resistor connected to the positive terminal of the op-amp. R and C must be
chosen to allow for the highest system frequency. If R is too small, the bias voltage, Vx,
will not hold true because of the large variation in current.
23
3.6.2.5 Optoisolator/Output State To allow the zero detection circuits to interface with DSP I/O pins, the output signal will
be sent through a switching transistor and a Schmitt-triggered optoisolator. The
optoisolator conditions the voltage to hi/low voltage levels that can be used by the DSP
I/O pins. A big advantage of using the optoisolator to condition the voltage is that it will
also provide optical isolation for our zero-detection circuit from the DSP.
3.7 Protection
Protection is vital in a system such as this. When converting and controlling high power
signals with low power devices, like in this project, there needs to be protection to
prevent any part of the system from becoming damaged in the case of a fault or other
undesirable situation such as a voltage or current surge. For this design, protection will
be present in each stage, ensuring that each subsystem is adequately protected.
The Avista SR-12 fuel cell already contains internal protection circuitry. If there is
improper hydrogen flow into the fuel cell, it will automatically shut off. Internal circuitry
also monitors the load on fuel cell. If too much power is being produced by the fuel cell,
it will automatically disconnect the load from the system.
The DC/DC converter selected for this project also has its own internal protection
circuitry. There is internal monitoring that will detect overvoltage situations and
shutdown the converter. The DC/DC converter also has reverse polarity protection,
which will prevent a surge from coming from the AMPS and passing through the
interface design to the fuel cell.
24
The DC/AC converter protection will be controlled by our DSP. The DSP will take
continuous voltage measurements from the AMPS; and, if there is a sudden change in
voltage, indicating a fault on the line, it will open all the switches on the DC/AC
converter—disconnecting it from the system. The DSP will also be taking zero-crossing
measurements continuously from signals outputted from the zero detection circuitry. If
there is a sudden change in phase, the DSP will also disconnect the DC/AC converter
from the system. Besides controlling the switches on the DC/AC converter, the DSP will
also control switches between the DC/DC converter and DC/AC converter, isolating the
two components.
The zero detection circuit is designed to withstand wide variations of voltage and current
on its input. The optoisolator, used to generate the signals from the zero detection
circuits for the DSP, isolates the two subsystems from each other, protecting the DSP in
case of an accidental surge through the zero detection circuits.
4 Validation
4.1 Fuel Cell To validate the functionality and capabilities of the fuel cell, a thorough set of load tests
were performed. The load tests ranged from open circuit to a maximum of just over
150W. These load tests showed the output voltage range to be approximately 18-36V
DC and the max output power to be just over 150W. When a bubble humidifier is
installed during the next phase of the project, these tests will be performed again and any
design changes will be made accordingly.
25
4.2 DC/DC Converter A simulation of the DC/DC converter using PSCAD validated its theoretical operation.
The simulation data shows that by varying the duty cycle, the desired output of 12V can
be obtained regardless of the input from the fuel cell. This variation of duty cycle will be
automatically controlled using the proposed DC/DC converter discussed in this report.
4.3 DC/AC Converter The DC/AC converter was simulated using PSCAD and the PWM and three phase
outputs are shown in section 5.2.2 of this report. A second simulation, which includes
the DC/DC converter and a delta to wye transformer, was also performed to ensure
functionality of the system as a whole. The transformer successfully steps up the voltage
to 208V RMS and filters the PWM waveform from the inverter.
4.4 Zero-crossing Detection Circuitry Using a schematic provided by Dan Gordon, Ken Hollinger and Jon Leman [4], a zero-
crossing detection circuit was simulated using PSpice. This simulation utilizes pre-zero
and post-zero detection circuitry to successfully detect when zero crossings occur in the
voltage waveform. This circuit simulation will function properly if given a pure
sinusoidal input as well as a sinusoidal input with a high frequency ripple.
26
5 Results
5.1 Fuel Cell Operating Procedure 5.1.1 Safety and Care of the Fuel Cell As with all combustible gases, care should be exercised when working with hydrogen.
Only operate the fuel cell with the doors open, and never leave the fuel cell unattended
for long amounts of time. The fuel cell contains delicate circuitry and sensitive
components that could be damaged if operated improperly. To avoid damage to these
internal components, always follow the operating instructions, and never allow current to
flow into the fuel cell from an outside source.
5.1.2 Startup Procedures The startup procedures for the fuel cell outlined in “Avista SR-12 PEM Hydrogen Fuel
Cell” [2] have been summarized here to provide a more thorough understanding of the
operation of the fuel cell. In order to start the fuel cell in a safe and timely manner,
perform the following steps:
1. Turn the main flow regulator on the hydrogen tank to a minimum. This
requires counter-clockwise rotation of the black knob.
2. Turn on the main valve on the hydrogen tank and adjust the regulator to 6 or 7
psi. A flow rate in excess of 7 psi will cause the fuel cell to abort the
initialization process.
3. Hold down the “POWER” button on the fuel cell until all lights turn green.
Release the button to start the initialization.
27
4. Wait for the fuel cell to initialize and warm up. Once flow has started, this
should take less than ten minutes. When the status indicator reads “OFF-
LINE” the startup procedure has completed.
5. To switch in a load, press and briefly hold the “LOAD” button. Power will be
sent to the load, and the display will read “ON-LINE.”
5.1.3 Troubleshooting This section includes the troubleshooting procedure from “Avista SR-12 PEM Hydrogen
Fuel Cell” [2] as well as additional troubleshooting procedures that have been
encountered since the start of this current project.
Table 3 - Troubleshooting the Fuel Cell
Problem Solution
Unable to initialize fuel cell Low battery power: replace or charge the batteries
Unable to power a load for longer than approximately 3 minutes: Fuel cell shuts off and
no message is displayed.
Batteries are not supplying enough current to handle loading due to internal circuitry:
Charge the batteries.
Message “Low Voltage” is displayed.
The fuel cell has been overloaded. Reduce the load to less than 100 Watts and press
“LOAD” to reinitialize.
Message “Shutdown=7a” is displayed.
This message indicated that the fuel cell is going to shutdown. This message will occur if “POWER” is pressed or if hydrogen flow is
lost. If hydrogen flow is lost, readjust the regulator to 6 or 7 psi.
Some of the cell status lights are red.
This most likely indicates an overload condition. Reduce the load and press
“Cartridge Reset” to restore the offline cells. If this step is not performed, the fuel cell will
automatically shut down.
28
5.2 Validation Results 5.2.1 DC/DC Converter As seen in previous sections of this report, the output voltage of the fuel cell varies from
18 to 36 volts. This requires a wide range voltage converter to regulate it to 12 volts. A
simulation of the DC/DC converter shows its general operation. Its duty cycle will be
automatically adjusted to obtain the required output voltage. This simulation data was
used to understand the operation of the DC/DC converter, so that the most efficient and
cost effective converter can be purchased during the next phase of this project. Figures
13 and 14 show the simulation results, along with the components necessary to construct
the buck converter.
Figure 13 - DC/DC Converter
29
Figure 14 - DC to DC Converter Output
5.2.2 DC/AC Converter The DC/AC converter is required for this project because the fuel cell can only provide
DC current. The AMPS operates on three phase AC current. The DC/AC converter, as
shown in Figure 15, will invert the DC voltage by using an appropriate pulse width
modulation switching scheme to control a series of power MOSFETs. The use of power
MOSFETs ensures that the 200W specification will be met since these devices are
capable of several hundred Amps. The DC/AC converter was simulated using PSCAD
and the PWM and three phase outputs are shown in Figures 16 and 17. The results and
components modeled in the simulation will be used to determine the parameters for the
purchase of a DC/AC converter in phase two of this project. Note that the DC/DC
converter has also been connected to the inverter and a delta to wye transformer is
modeled on the output to step up the voltage to 208V RMS and filter the PWM waveform
from the inverter.
30
Buck Converter DC to AC inverter
Transformer
Figure 15 - System Simulation Model
Figure 16 - PWM input: 1-Phase
Figure 17 - Three Phase AC Output
31
5.2.3 Zero Detection Circuits In order to complete the interface process, the waveforms from the fuel cell must be
synchronized with the waveforms on the AMPS. This is a crucial part of the interface
design because, without synchronization, massive current spikes will be seen on the
system. In addition, the system will take a long time to stabilize if not properly
synchronized. Current spikes and instability would push the converters beyond their
limits, and would overload the fuel cell. This would result in system shutdown or, worse,
system damage. By utilizing the high-speed digital signal processor on the DC/AC
converter, current and voltage levels can be sampled frequently, and zero-crossing
detection circuitry can locate the synchronization points to provide near-exact matching
of the two system signals. This will ensure a smooth transition when switching the fuel
cell system onto the model power grid. The zero-crossing detection circuitry was
simulated using PSpice, and the results seen in Figures 18 and 19 will be used to
determine component values required to build and optimize the hardware during phase
two of this project. Figure 18 shows the voltage level trends on the input pins of the op-
amp for each of the two zero detection circuits. Figure 19 shows the output of the zero
detection circuit with a rising clock edge. A complete set of simulation results is outlined
in Appendix E.
32
Figure 18 - Op-Amp Input Pin Simulation
Figure 19 - Zero Detection Circuits Output Switching
33
5.3 Cost Analysis Table 4 shows the updated budget for our project. No parts have been purchased yet but
there have been some updates in prices for the parts. The cost of the DC/DC converter
has dropped significantly through research of a new supplier.
Table 4 - Budget
Item Price DC/DC Converter (if needed) $216 DC/3-Phase AC Converter $1,800 Transformer $125 Flyback Converter (if needed) $500 Hydrogen $45 Design Poster/Report Binding $15 Project Display Costs $35 Protection Circuitry $100 Filtering $50 Software Licenses $250 Miscellaneous $300
Total $3,436
34
Bibliography [1] AMPS User Guide. University of Idaho. Moscow, ID. [2] Fletcher, Nathan. (2002, Mar). Avista SR-12 PEM Hydrogen Fuel Cell. University of
Idaho. Moscow, ID. [3] Nice, Karim. “How Fuel Cells Work.” How Stuff Works. 2000. Accessed: 6 April,
2005 <http://science.howstuffworks.com/fuel-cell.htm>. [4] Gordon, Dan., Hollinger, Ken., and Leman, Jon. “TNA Fault Controller.” Senior
Design Report, University of Idaho, 2001.
35
APPENDIX
36
Appendix A – ComPAC DC/DC Converter Data Sheet
37
38
39
40
41
Appendix B – LF356 Op-Amp Data Sheet
42
43
44
45
46
47
48
49
50
51
Appendix C – H11L1 Optoisolator Data Sheet
52
53
54
55
56
57
58
59
60
61
Appendix D – Zero Detection: Circuit Analysis
62
63
64
Appendix E – Zero Detection: Simulation
65
66
67
68
69
70
71
Appendix F – Individual Project Contributions
72