inverter design for high performance micro energy grids · the design of the inverter is an...

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.



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.



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.



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.



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.



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