Energy Grids
Jason Runge and Hossam Gabbar University of Ontario Institute of Technology, Oshawa, Canada
Email: {jason.runge, hossam.gabbar}@uoit.ca
Abstract—This paper presents an inverter design for high
performance Micro Energy Grid (MEG). The proposed
system consists of a DC bus line bridged to an AC line with
a single phase two stage inverter. The configuration of the
MEG is first introduced, then subsequently the design for its
components. The system requirements and inputs are also
described herein. The inverter is rated to output 1.5kW of
power in as a small scale as possible. To produce the desired
output voltage wave form, Sine Pulse Width Modulation
(SPWM) is utilized. The results demonstrate that the system
can achieve an efficiency of approximately 98% with a THD
of approximately 4%.
Index Terms—inverter, micro energy grid, low THD, high
efficiency, transformerless, micro energy grid simulation
module, building energy conservation
I. INTRODUCTION
Climate change has been called the one of the most
serious threats to humanity of our time. Whether this is
true or not, it is definitely one of the most challenging
problems we face today. It has been shown that there is a
direct correlation between climate change and greenhouse
gas emissions [1]. According to the International Energy
Agency (IEA) the building sector accounts for 35% of
our worldwide energy consumption, followed by Industry
(31%), Transport (30%), and other sectors (4%) [1].
Energy consumption in buildings can be as high as 40%
in most IEA countries [1]. As such, the building sector
represents the largest consumer of energy and greenhouse
gas emissions. Therefore, improving energy efficiency
within this sector can be quite beneficial to help reduce
our consumption of energy and thus greenhouse gas
emissions. In order to help reduce the energy
consumption, engineers are trying to find new and
effective means to deploy energy sources to buildings.
While software for simulation is easily available through
programs such as HOMER and EnergyPlus, there is a
lack of available tools which can verify the results of
these programs in real world simulations. This has driven
the need for a device which users can demonstrate the
energy generation/consumption scenarios simulated by
the software in a physical model.
Manuscript received June 2, 2015, revised August 18, 2015.
II. BACKGROUND AND LITERATURE REVIEW
A. History
An inverter is essentially an electronic device which
converts direct current (DC) into alternating current (AC).
There are many different types and applications of
inverters; from single phase types feeding off grid
appliances, to polyphase (multiple phases) and motor
drives.
No one knows the exact origins of the word inverter
with absolute certainty. David Prince is the person who is
most likely credited with coining the term [2]. In 1925,
Prince published an article in the GE review titled “The
Inverter” [2]. In this article, Prince explained that he had
took the rectifier circuit and inverted it, taking in DC
current and outputting AC current. He did not physically
mean he inverted the rectifier devices, but rather he
inverted the function or operations of the rectifier [2]. By
1936, Princes’ term inverter had spread throughout the
world and was a term in common usage [2]. Originally,
rotary converters were manufactured until the 1950’s
which would transform the DC into AC (effectively
called “inverted rotaries”) [2]. However in the 1950’s, as
semiconductor technology began to emerge, they quickly
replaced the inverter rotaries. The IEEE defines inverters
as “a machine, device, or system that changes direct-
current power to alternating-current power” [2].
Today, there are many different classifications of
inverters and inverter designs. The broadest two
classifications for inverters are current source inverters
(CSI) and voltage source inverters (VSI) [3]. Further
classifications can then progress based off of the
technology device used, circuit design, nature of output
voltage, firing circuits, power stages, etc. [3].
B. Objective
The design of the inverter is an essential component to
the Micro Energy Grid (MEG). It is the only DC to AC
electric power conversion mechanism to be used within
the MEG. As such it plays a significant role the design
and operation of the device. The objective of this project
is to design a small scale high efficiency inverter which
can then be used within the design of the MEG (see Fig.
1).
C. Goal Requirments
After a consideration of the existing technologies in
the market place, with their respective parameters, the
International Journal of Electrical Energy, Vol. 3, No. 3, September 2015
©2015 International Journal of Electrical Energy 191
Inverter Design for High Performance Micro
doi: 10.18178/ijoee.3.3.191-196
specific design requirements of the inverter have been
derived into the following:
Must be able to handle up to 1.5kW loads
48VDC input
Output must comply with Canadian standard of
120V, 60Hz AC single phase power
Must have a total harmonic distortion + noise on
both voltage and current of <5%
Must have a DC-AC efficiency of greater than
90%
By meeting the above requirements it will be
concluded that the work presented herein has been
successful.
D. Overall Circuit
As stated previously, the purposes of this design is an
inverter to connect and the DC bus line to the AC bus line.
This can be utilized to supply excess or the total power
generated from the DC side.
Figure 1. The proposed MEG design.
Fig. 1 shows the application for this inverter system
integrated into the overall MEG. The MEG system
contains a DC bus line (48V, 1.5kW) seen on the right
hand side of Fig. 1. This line consists of programmable
DC sources (which can be used to simulate such things
like Solar, PV, batteries, etc.), programmable DC loads,
and a battery bank system. The main inputs to the DC
line are a rectified AC line input, the programmable
sources, battery and the grid. In addition to the DC line,
there is also an AC line (120VAC, 2kVA). All
connections can be connected and disconnected as per the
user’s desire. Similar to the DC line, this line contains
programmable AC sources and programmable AC loads.
Should excess power be generated in either of the lines,
the system will have the ability to transfer power over to
the other line. As such the inverter and rectifier are placed
in-between the bus lines in order transfer power between
them. Thus users will have the ability to simulate a wide
variety of consumption and generation patterns. This is
the platform the MEG creates for users to obtain then
experimental results demonstrating the effectiveness (or
ineffectiveness) of certain configurations of energy grids.
III. CIRCUITS AND SUBCIRCUITS
A. SPWM Firing Circuit
The firing circuits for the inverters vary depending on
the semiconductors used as well as the desired output.
For instance, square wave inverters can be triggered with
555 timer circuits, or by the use of more specialized
integrated chips (ICs). For the chosen inverter, using a
sine pulse width modulation (SPWM) circuit was chosen.
This can be seen in the Fig. 2 below.
Figure 2. SPWM firing circuit.
Fig. 2 shows the circuitry utilised for the triggering
circuit of the inverter. Two signals are generated and
compared to each other in order to produce a SPWM
output. This output will effectively trigger the switches
for the H-Bridge inverter. The first source is a sine wave
generator. This is connected to the top of the comparators
in Fig. 2. The second source is a triangular wave
generator which is connected to the bottom of the
comparators in Fig. 2. Both devices are operating with a
peak-to peak voltage of 8V. The sine wave generator is
operating at the desired output frequency of 60Hz, while
the triangular wave generator is operating at 10kHz.
These two signals are sent to the comparator and trigger
the inverter for every positive and negative cycle. For the
positive output cycle, the sine wave source generated is
directly compared against the triangular wave source.
This can be seen in the top comparator of Fig. 2. This
effectively triggers Q1 and Q4 for the H-bridge. For the
negative output cycle, the sine wave generator needs to
be first inverted and then sent to a comparator. This can
be seen in the bottom comparator of Fig. 2. This
effectively triggers Q2 and Q3 for the inverter. Working
together, both comparators output a full sine wave.
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©2015 International Journal of Electrical Energy 192
The governing equations for the SPWM are as follows
[3]:
The Amplitude Modulation Ratio:
c
a
vm
v
(1)
where:
cv is the sinusoidal reference wave amplitude
v is the triangular wave amplitude
The Frequency Modulation Ratio:
f
c
fm
f
(2)
where:
f is the triangular wave amplitude
cf is the triangular wave amplitude
B. Charge Controller
A boost converter is needed in order amplify the DC
bus line voltage to the required level for the inverter. For
this design it is slightly above the 120VAC in order to
account for voltage loses across the MOSFETS and filters
of the inverter.
Figure 3 Boost converter.
Fig. 3 shows the circuit design for the Boost converter
integrated in as the first power stage for the inverter. In
order of reduce the completity of parts, the same
MOSFET was used in the H-bridge inverter configuration
(explained further below).
The governing equations for the DC-DC converter are
as follows [4]:
Max Duty Cycle:
1in
out
VD
V
(3)
Maximum Output Current:
max min1 )
2(L L o
o
VI D
R
I I
(4)
Minimal Inductance:
2(1 )
2crit
RTL D D (5)
Output voltage ripple:
o o
DV V
RCF (6)
where:
inV = Input voltage,
outV = Output voltage, D = duty cycle,
R = output circuit resistance, C = capacitance, f =
frequency.
By utilizing a DC boost converter, this allows the
voltage for the inverter to reach the necessary output
voltage levels, and removes the need to add in a
transformer on the output AC line. By removing the
bulky transformer and replacing it with a small buck
converter the overall space required is minimalized and
allows the inverter to have a smaller footprint.
C. Full Bridge Inverter and MOSFET Choice
For this design, a full h-bridge inverter was chosen and
can be seen in Fig. 4 below. This is the second stage of
the inverter. The two wires on the far left hand side of Fig.
4 are the connections to the positive and negative DC
rails, while the two wires on the bottom are the
connections to the SPWM sub-circuits. Finally the two
wires on the right hand side are the output wires to
connect to the load.
In order to simulate the full bridge micro-inverter, non-
ideal MOSFET’s were used. It was known from the
requirements that the output of the inverter is to operate at
120Vrms, 1.5kW. Therefore the current was calculated. A
search was done among a wide range of MOSFET
manufactureres, to find the best possible choice for the
design. It was found that n-channel IXYS GigaMOS
MOSFET was the best choice due to it ability to handle
voltage, current, switching times, its minimal on-state
resistance and power losses. The current handling
capability of this MOSFET is rated at 320A continuous
current [5], which far exceeds the rated current need for
the project. However, this MOSFET had significantly
small resistance (avoiding losses) and can also safely
handle the applied voltage levels. It is possible to operate
the gate switch for this at 4V and operate it in the
required range, without excess thermal losses. The
current cost of one MOSFET is at $25.74/unit, [5] the net
cost just for MOSFETs is estimated to be $154.44. The
full cost of the entire inverter circuit is expected to be
around $250. By using the minimal amount of parts for
the conservation it is expected that the quality of the
device will not be compromised, and the resulting
compactness will aid in the reduced of overall footprint
size.
Fig. 4 shows the traditional H-bridge inverter
configuration used to create the AC output voltage [6].
The output of which is a PWM which is then sent to the
filter for smoothing and turning into the full AC signal.
International Journal of Electrical Energy, Vol. 3, No. 3, September 2015
©2015 International Journal of Electrical Energy 193
Figure 4. Inverter H-bridge.
D. Output Filter
The output of the inverter would otherwise be another
PWM signal. In order to turn the PWM back into sine
wave, a filter is needed.
Figure 5. Inverter output filter.
Fig. 5 shows the design of the filter for the inverter
system. The components were chosen and designed, such
that a 60Hz signal can pass through it and effectively and
remove the unwanted harmonics. In addition to
functioning values, these specific values were chosen due
to their availability from many different manufacturers.
I.E. they are readily and easily available in the market.
Figure 6. Bode plot filter.
Fig. 6 shows the frequency analysis of the output filter.
It can be seen that for the current (top graph) has its band
pass at 60Hz and the voltage (bottom graph) is the same
passing frequency. This filter helps smooth out and turn
the SPWM into a sinusoidal signal and help reduce
harmonics for the outputting alternating current.
E. Overvoltage Protection Circuit
IEEE Standard 519 – Must not have voltage +/-5% and
THD <5% [3]. As such an overvoltage/under voltage
protection circuit is needed. The circuit designed can be
seen in Fig. 7 below. The circuit turns on if the voltage
difference greater than 5% for 1ms has been achieved.
While this is a short time period, the time values can be
changed and modified easily as to be described below.
The inverter voltage reference is taking at the output of
the mosfets and compared to the reference sine wave. The reference sine wave is subtracted from the inverter sine
wave, thus producing and error signal. The error signal is then sent to a comparator. This device compares the
results to a reference set constant value (the 5% of
120Vrms). The output of the comparator is then sent to the C-Block module in PSIM. The results are delayed
from each other by a value of 0.00055 seconds. The time delays work as a signal storage section, taking previous
values of the cycle in order to see how long the pulse
(over threshold voltage) has been on for. The C-Block is monitoring the values of the three inputs each cycle, and
if all three signal inputs to the block are high, then the C-Block outputs a digital high in order to shut down the
inverter.
International Journal of Electrical Energy, Vol. 3, No. 3, September 2015
©2015 International Journal of Electrical Energy 194
Figure 7 Overvoltage protection circuit.
Fig. 8 shows the outputs of the over voltage protection
sub-circuit. The top most figure shows a signal
representing the inverter voltage signal. The second from
the top shows a signal representing the reference grid
signal. The third from the bottom shows the difference
between the two signals. Finally the bottom most circuit
shows the output of the controller sub circuit. It can be
seen from the bottom most figure, that the output turns
high after approximately 1ms, the counter being started at
the time position of the vertical line on all four figures.
While a 1ms is a small time step, the step rise can be
easily increased by adjusting the delayed or adding
additional delays.
Figure 8. Results of overvoltage protection circuit.
Figure 9. Transient operation of inverter.
International Journal of Electrical Energy, Vol. 3, No. 3, September 2015
©2015 International Journal of Electrical Energy 195
Figure 10. Steady state operation inverter.
IV.
RESULTS AND CONCLUSIONS
In this section the results of operating the inverter
circuit are presented. The operation of the circuit under
the nominal conditions is shown in Fig. 9 and Fig. 10.
Fig. 9 shows the transiet operation of the inverter
during initial setup. The voltage is built up in the systems
and the current (red line) begins to flow.
Fig. 10 shows the results of the steady state operation
of the inverter design. The blue line represents the
voltage (120VAC) and the red line represents the current
measurment. It can be seen that the inverter is producing
the desired output power and voltages within IEEE
standards. The THD is approximately 4% for the inverter
design and the inverter effieincy is approximalty 98%.
The results show that the the inverter can meet the
design requirements to build the MEG and aid in the
conservation of energy in buildings.
REFERENCES
[1]
International Energy Agency. (Apr. 2014). FAQ: Energy
efficiency. International Energy Agency. [Online]. Available:
www.iea.org/aboutus/faqs/energyefficiency [2]
E. Owen, “History [origin of the inverter],”
IEEE Industry
Applications Magazine, vol. 2, no. 1, pp. 64-66, 1996.
[3]
R. Muhammad, Power Electronics Handbook, San Diego: Elseviser, 2007.
[4]
N. Mohan, T. Undeland, and W. Robbins, Power Electronics,
Converters, Applications and Design, New Jersey: Joh Wiley & Sons, 2003.
[5]
IXYS, GigaMOS TrenchT2 HiperFET, Milpitas: IXYS
Corporation, 2009. [6]
Y. Xue, et al., “Topologis of single phase inverters for samll
distributed power generators: An overview,” IEEE Transactions
on Power Electronics, vol. 19, no. 5, pp. 1305-1315, 2004.
Jason Runge was born in Toronto, Canada in
1987. He received a bachelor degree (with
honors) from University of Ontario Institute of Technology in 2014. He is currently
working towards his MASc degree in
Electrical Engineering.
Dr. Hossam A. Gabbar is a Professor in the
Faculty of Energy Systems and Nuclear
Science, and cross appointed in the Faculty of Engineering and Applied Science, University
of Ontario Institute of Technology (UOIT).
He obtained his Ph.D. degree (Energy Process Safety) from Okayama University (Japan),
while his undergrad degree (B.Sc., with First
Class of Honours) and Master degree courses are in the area of automatic control from
Alexandria University, Egypt. He is specialized in smart energy grids
with focus on safety, protection, and control engineering. Since 2004, he was an Associate Professor in the Division of Industrial Innovation
Sciences at Okayama University, Japan. And from 2001, he joined
Tokyo Institute of Technology and Japan Chemical Innovative Institute
(JCII), where he participated in national projects related to advanced
distributed control and safety systems for green energy and production
systems. He is founding general chair of the annual international conference on smart energy grid engineering, which is held at UOIT.
HE is the founding Editor-in-chief of International Journal of Process
Systems Engineering (IJPSE). He is regularly invited to give talks in scientific events and conferences, tutorials, and industrial development
programs in the area of energy safety and control. Dr. Gabbar is the
author of more than 210 publications, including books, book chapters, patent, and papers in the area of smart energy grids, safety and control
engineering.
International Journal of Electrical Energy, Vol. 3, No. 3, September 2015
©2015 International Journal of Electrical Energy 196