1A High Energy Saving Interface System Usinga Matrix Converter between a Power Grid andan Engine Generator for Bio Diesel FuelJun-ichi Itoh, Member, IEEE, Hiroki Takahashi, and Junnosuke Haruna, Student Member, IEEE

Nagaoka University of Technology, Japan
Abstract--This paper discusses an interface power converter
between a power grid and an engine generator. A matrix
converter, which is one of the AC/AC direct converters, canachieve high efficiency without large energy storages. Therefore,
the volume of the converter becomes smaller and the reliability of
the interface system is improved due to no use of electrolytic
capacitor.
This paper discusses the control strategy of a matrix converter
for the power grid interconnection. In addition, the performances
are demonstrated by simulation and experiments with a 1.5-kW
prototype. The proposed control method is confirmed in thesimulation and experimental results. Furthermore, the maximum
efficiency of 96.8 % is obtained in the experiment. Total loss ofthe power converter is approximately half of the conventional
system that is using a Back-to-Back converter.
Index Terms--matrix converter,interface power converter, generator

BDF,

BTB

converter,

I. INTRODUCTION
The studies and activities about a smart grid are highly
conducting recently because this concept could achieves highenergy saving.
Bio Diesel Fuel (BDF) is one of renewable energy
resources. BDF is used for a diesel engine with a generator forelectric power conversion. BDF has similarity to the windturbines and photovoltaic cell systems in term of the carbon
neutral. However, an engine generator can provide more stable
power in comparison to the generation system that uses thenatural energy resources such as a solar power, a wind turbine
and other. Moreover, the biomass fuel which is typified as a
BDF does not influence human health and environmental.
Therefore, an engine generator using BDF is expected to apply
in a smart grid in the future.
In the smart grid, an interface power converter is required
between an engine generator and a power grid. This interface
power converter adjusts the frequency and the voltage
amplitude on both sides. The interface converters for theapplication of smart grid are expected to have high efficiency
and high reliability.
Generally, a back-to-back (BTB) converter, which consistsof a PWM rectifier and a PWM inverter, is applied in thissystem as the interface converter. However, the BTB converter
uses large amount of electrolytic capacitors as the smoothing
capacitors. As the results, the system size is big and regular

maintenance is required. That is, the electrolytic capacitors
reduce the system reliability. Moreover, the efficiency of the
BTB converter is low due to the two stages power conversion
within the converter. Recently, matrix converters attract a lotof attentions among the researchers because this converter
achieves high efficiency and small size due to no smoothing
capacitors. Therefore, the system with matrix converter is
expected to achieve high reliability and long-life time. Many
papers regarding to the matrix converter are being discussed in
the field of adjustable speed drive applications [1-4].
On the other hand, only small portions of the papers
describe using the matrix converter as a generator interface
converter in a distributed power supply and a micro grid
system, which discuss on the subjects of high efficiency andmaintenance issue. In these papers, a matrix converter isapplied to the interconnection system to adjust the frequency
and voltage amplitude between the generator and the grid
system. In Ref. [5-8], the control of the generator power basedon the generator parameters are being discussed. The control
method has to be changed regularly because it depends on thetypes and parameters of the generator. On the other hands,
BDF generators are formed by many kinds of generators suchas an induction generator, a synchronous generator and apermanent magnetic generator. These generators show adifference in terms of cost and field requirements upon to thecircumstances. Due to the varieties of generators, design
becomes complicated and therefore a robust interface
converter is required.
This paper proposes to apply the matrix converter as aninterface converter between the power grid and a diesel engine
with BDF. The key feature of this proposed system is that thematrix converter can control the generator power, regardless ofthe generator parameters and types. In the proposed control,
the generator is assumed as a simple variable amplitude andfrequency power supply with the internal impedance. Then, acurrent regulator in the generator side controls the input filter
current. As a result, the proposed system can connect voltage
source at the both input and output side.
In this paper, first, the feature of the proposed interface
system is introduced. Second, the control strategy of the matrix
converter for the interface converter is described. Finally, thesimulation results and also the experimental results which are
obtained from a 1.5-kW prototype are demonstrated

2

accordingly.
II. SYSTEM CONFIGURATION

Fig. 1. Configuration diagram of an interconnection system using aninterface converter for a smart grid.

Output filter

Figure 1 shows an interconnection system using an interface
converter among a BDF generator, a load and a power grid.
the conventional interface converter between an engine
generator and the power grid. In order to connect a generator
to the power grid, the interface converter must provide agenerator power in spite of the differences of the frequency,
phase and voltage amplitude between the generator and thepower grid. In addition, the interface converter controls theactive power according to the load demands. From here, theseinterface converters are used to satisfy the above requirements.
An engine generator using the BDF is usually operating at aconstant speed for obtaining a constant output power.
Therefore, the operation condition of the generator can be set
to the highest efficiency condition.
Figure 2 (a) shows a conventional system using a BTB
converter. The PWM inverter needs to control theinterconnection current to sinusoidal waveform for the grid.
On the other hand, the PWM rectifier needs to control thegenerator current to sinusoidal waveform. Both input andoutput of the BTB converter do not interfere with each otherbecause the smoothing capacitors in the DC link could
separate the controls. Therefore, the interconnection current
and the generator can be controlled easily. As a result, a BTB
converter is usually used as an interface converter. On theother hand, the problems of this system are as following; thesmoothing capacitor in DC link is bulky; the smoothing
capacitor is the shortest life-time element in the system; thesystem efficiency becomes lower because of two times
conversion between the generator and the power grid.
Figure 2 (b) shows a conventional system with a matrix
converter using nine bidirectional switches as an interface
converter. The smoothing capacitor is not required because thematrix converter has no DC stage. In addition, the matrix
converter can achieve the minimum loss due to only one stage
conversion in term of AC to AC conversion. The power loss ofthe matrix converter is approximately half of that of the BTB
system [9].
However, these conventional interface systems in Figure 2
have another problem in term of the generator control. In order
to control the power from a generator, the generator
parameters first must be designed and known. The mostcommon generators seen in this application are such as theinduction generator, the synchronous generator with external
exciter, or the permanent magnetic generator. Therefore, it iscomplicated to change the control methods according to thegenerators, especially in a low cost system.
Figure 3 shows the proposed interconnection system with amatrix converter. A LC filter is added between the generatorside and the input line of the matrix converter. The generator
power is controlled by adjusting the input filter current insteadof the generator current. The generator current is decided by
the amplitude and phase angle of the input filter capacitor
current. The matrix converter can control the filter reactor(a) BTB converter
Matrix converter
Current
BDF
generator source side

Voltagesource side

Power
grid

G
Interconnection
ReactorInput filter(b) Matrix Converter
Fig. 2. Configuration diagram of the conventional power grid connected
system using a power converter.
Matrix converter
BDF
Generator

Voltagesource side

Current
source side

Power
grid

G

Input filter

Output filter

Fig. 3. Configuration diagram of the proposed power grid connected systemusing a matrix converter.

current with the control block diagram by not using thegenerator parameters, such as internal impedance and backelectromotive force of the generator. Moreover, the operating
speed and phase angle of the generator from its encoder are
unnecessary because the active power can be controlled by thedetection of the generator terminal voltage from the input filter
capacitors. As a result, the generator can be assumed as asimple variable frequency power supply with internal
impedance. From this, all types of generator, especially a very
low cost generator, can be used in this system.
However, the proposed interface system has technical3

issues in term of the interconnection. The matrix converter
controls both input and output waveforms at the same time.
Therefore, the input side control affects the output side control
due to the lack of energy buffer, which is different from a BTB
converter. Therefore, if the generator terminal voltage andcurrent are distorting, the fluctuation will affect the power gridconditions. In addition, the proposed system uses LC filters at
both input and output sides, which the resonance between L
and C is possible to occur at both sides. The resonance in the
output side is suppressed by the damping resistor and theresonance in the input side is suppressed by voltage andcurrent feedback control which will be described in next
chapter.
On the other hand, the matrix converter must compensate
the load power even if a trouble such as interruption occurs at
the power grid. When the trouble occurs at the power grid,
circuit breakers shown in the Figure 1 are opened. Therefore,
the matrix converter must control the load power using thepower provided from the generator only. Thus, the control forindependent operation of the matrix converter is required incase a trouble occurs at the power grid.

e

q

pLgig

vc

pLi

iq

vmc
ig ic

ig

i

ig
id

d

Fig. 4. Relationship among the voltages and the currents on the generatorside with dq axis based on the terminal voltage and γδ axis based on the backelectromotive force of the generator.

III. CONTROL STRATEGY
There are two types of power source in a matrix converter
known as, the voltage and current source. Usually, the voltage
source side is connected to reactors, and the current side isconnected to capacitors. In general, the current source side islocated on the voltage power source i.e. a power grid. In an
adjustable speed drive system, the current source side islocated on the power grid side. In the proposed system,
however, the current source side of the matrix converter isconnected to the power grid, which is treated as a load side.
That is, this system is operated similar to the regeneration
mode of an adjustable speed drive system.
The generator side becomes the voltage source side alike a
voltage source inverter. The reactor current and the capacitor
voltage in the input filter are controlled by the proposed
method. Note that the input voltage of the matrix converter islower than 86.6% of the power grid voltage because of thelimitation in the voltage transfer ratio of a matrix converter. In
other words, the proposed system boosts up the generator
terminal voltage. Thus, the matrix converter of the proposedsystem can operate even if the generator voltage lowers due tothe generator frequency fluctuation. On the other hand, theconventional interface system using a matrix converter such as
Figure 2 (b) has the lower limitation of the generator terminal
voltage which the matrix converter can operate at, because of
the limitation of the voltage transfer ratio.
Figure 4 shows the relationship between the γδ and the dq
axis to consider the active power and reactive power on the
proposed control. Note that γδ axis is based on the backelectromotive force of the generator and dq axis is based on
the terminal voltage of the generator. In Figure 4, e is the backelectromotive force vector of the generator, ig is the generator
current vector, ic is the capacitor current vector, vc is theFig. 5. Control block diagram of the proposed interface system between an
engine generator and the power grid.

voltage vector of the input filter capacitor, i is the current
vector of the input filter reactor, vmc is the input voltage vectorof the matrix converter, pLi is the voltage drop vector at the
input filter reactor, and pLgig is the voltage drop vector at thegenerator inductance. Note that p is a differential operator.
Generally, the dq axis is based on the back electromotive forceof the generator. In this paper; however, the terminal voltageof the generator is defined as the basis of the dq axis. Theseaxes are obtained from the rotating frame and rotate at thesame speed.
Figure 5 shows the proposed control block diagram. The
active and reactive current controls are implemented on therotating frame. The direction of the q-axis is defined as thesame direction of the input filter capacitor voltage. The active
and reactive power on the dq axis is indicated by p dq = v c ⋅ i = v cd i d + v cq i q(1)
q dq = v c × i = v cd i q − v cq i dwhere pdq is the active power, qdq is the reactive power of the
input of the matrix converter. The direction of the voltage
vector of the input filter capacitor conforms to the q-axis. Then,(1) is calculated by4

 p dq = v cq i q(2)
 q dq = −v cq i d
Therefore, it is shown that id is the reactive current and iq is theactive current on the dq axis. That is, the d-axis current
command id* controls the reactive power; the q-axis current
command iq* controls the active power on the dq axis. This
current control is implemented with an automatic current
regulator (ACR). The output and input phase angles arecalculated by the arctangent function respectively. The dq axis
is obtained by the detected voltage of the input filter capacitor.
However, the γδ axis cannot be obtained without the
information from the generator encoder. Therefore, theproposed system adopts the dq axis to control the generator
power. A low-pass filter (LPF) is necessary after thegenerator
voltage detection because the filter capacitor voltage vcr, vct
may contain distortions due to the LC resonant. Note that theFig. 6 The vector diagram on the generator side when the generator current
resonant suppression control is applied with an ACR. The
vector and the voltage vector of the back electromotive force of the generator
voltage control of the capacitor is not necessary because theare in the same phase.
capacitor voltage is clumped by the generator voltage.
The control of the matrix converter is applied with a virtual
Input filter
Destination
vcd
AC/DC/AC PWM carrier comparison method [10]. The
PI controller
reactorfiltercontrol of the power grid current and the control of the input
id
*
*
vd
id
1
1
1 + sTi
Kp
voltage are separated by using a virtual rectifier command andsL
1 + sTisTia virtual inverter command in this method. As a result, thecontrol becomes simple. The power grid side can obtain aunity power factor.
1
Figure 6 shows the vector diagram on the generator side
1 + sTdamp
when the generator current vector and the back electromotive
Feed forward term
vcd
Kd
force vector of the generator are in the same phase. Note that
damping control
the current of the input filter capacitor is assumed to be zerovcq
Kd
for simplicity because the capacitor current is small
1
sufficiently. Therefore, the generator current vector conforms
1 + sTdamp
to the input current vector of the matrix converter. The active
vcq
power Pγδ on the γδ axis is obtained by
*
*
iq
iq
1
1 + sTi vq
1
iq
Kp
(3)
pγδ = e ⋅ i g = v cq cos φ ⋅= p dqsL
1 + sTisTi
cos φwhere φ is the phase angle between the dq axis and the γδ axis
Control
Circuitbecause of the internal impedance of the generator. The active
Fig. 7. Transfer function block diagram in ACR to control the input current
power on the γδ axis is equal to the active power on the dq axis.
of the matrix converter.
*
Therefore, the active current command id is calculated by**
id using (5) must use the generator parameters. Equation (5)
p dq
*
(4)
iq =
cannot be used in the control of the matrix converter becausev cq
the key feature of the proposed system is that the matrix
When the generator current vector and the voltage vector of converter controls power from the generator without thethe back electromotive force are in the same phase, the cupper generator parameters. However, it is useful for the generator
loss of the generator becomes the least because the generator efficiency to control i *. Therefore, the maximum power pointdprovides no reactive current on the γδ axis. The minimum tracking (MPPT) method which is applied to the generation
cupper loss is achieved when the reactive current of the system with the natural energy resources must be considered.
generator igγ becomes zero. The matrix converter controls igγ toFigure 7 shows a control block diagram which is used forzero by adjusting the reactive current command id* on the dq the control of the input current. The control strategy in Figureaxis. id* is calculated by
7 does not use the generator parameters. The destination filter*2
is used to prevent overshoot in the input current. The gain
X L i q v cq*
(5)
id = −
values of the PI controller are decided from natural angler2eδfrequency ωn and the damping factor ζ.
where XL is the reactance of the generator. Thus, derivation of

5
TABLE 1. GENERATOR PARAMETERS.
Rated terminal voltage (line to line)
176 Vrms

In addition, a feed forward term is applied into the damping
control to suppress the LC resonant at the PI output. The feed
forward term means the voltage of the input filter capacitor. It
is effective for stable operation on the generator side tosuppress the voltage ripple of the input filter capacitor. In
Figure 5, the fundamental frequency component of the input
filter capacitor is converted to a constant value alike a DC
component by the rotating frame. On the other hand, theresonant component of the input filter capacitor is converted toripple component. In the damping control for the feed forward
term in Figure 7, first, low pass filters, which have the time
constant Tdamp, separate the DC and ripple components of thevoltage of the input filter capacitor. Then only the ripple
component is multiplied by a damping gain Kd. As a result, thecompensated value for the ripple due to LC resonant isobtained.

Rated back e.m.f. (line to line)

151 Vrms

Rated current

14 Arms (=1 p.u.)
3.7 kW

Rated speed

1800 rpm

Rated frequency

90 Hz

pole

6

Synchronous inductance

6.56 mH (50 %)TABLE 2. SIMULATION CONDITIONS.
Grid voltage (line to line)
200 Vrms
Grid frequency

50 Hz

Generator back e.m.f. (line to line)

150 Vrms

Generator frequency

90 Hz

MC rated input power

5.5 kVA

MC rated input voltage (line to line)

173 Vrms

Carrier frequency

IV. SIMULATION AND EXPERIMENTAL RESULTS
At first, the proposed system is evaluated in simulations inprobable situations for the interconnection between an engine
generator and the power grid. Table 1 shows the generator
parameters. Note that the generator is assumed as a voltage
source and a large reactor (50%) to evaluate the validity of the
proposed system in principle. Table 2 shows the simulation
conditions.
Figure 8 shows the operation waveforms of the proposedsystem with only active current command (iq*=1 p.u.) at steadystate. The input current of the matrix converter is controlled to
rated current 14 A (RMS) by the ACR. The unity power factor
is obtained on the power grid and the generator side. The unity
power factor on the power grid side is obtained easily by theopen loop control from the virtual current source rectifier. On
the other hand, the unity power factor on the generator side is
obtained by the ACR with no reactive current command (id*=0p.u.). In addition, both the grid current and input current obtainsinusoidal waveforms. Thus, the feature of the proposed
method, that is the simultaneous control on the input andoutput side without a DC link is confirmed.
Figure 9 shows the relationship between a zero point of theγ-axis current igγ and the input current commands of the matrix
converter. Note that back electromotive force of the generator
is set to 100 Vrms (line to line), the generator frequency is set to
60 Hz in Figure 9. The dq axis current id and iq are through a
LPF for observation. The matrix converter controls the input
power factor with the reactive current command id*. Note thatthe active current command iq* is a constant value and provides
constant active power to the power grid. When the reactive
current igγ on the γδ axis is zero, the reactive current id on dq
axis is -0.143 p.u.. Note that the theoretical value of id using(5) is -0.155 p.u. when igγ is zero. The generator current ispreferred to be a small value because the amplitude of thegenerator current affects copper loss of the generator. In otherwords, it is required to control igγ to zero by adjusting id*.
Figure 10 shows the matrix converter waveforms which isoperates under a starting sequence. The starting sequence isRated power

Grid side LC filter
Grid side damping resistor
Generator side LC filter

10 kHz2 mH (2.35 %)13.2 µF
47 Ω (ζ=0.131)
2 mH (15.2 %)6.6 µF

ACR damping factor ζ

0.7

ACR natural angular frequency ωn4000 rad/s

Fig. 8. Operation waveforms of the matrix converter in the proposed system
at steady state in simulation.

-id_ref_pu-id_pu_lpf

igq_puigd_puFig. 9 Relationship between a zero point of the γ-axis current and the input
current command of the matrix converter.

6

like following; contactors on the generator side in Figure 1 areclosed. Then, the matrix converter starts to operate with zerocurrent commands. After that, the current commands are fed
into the ACR. From 0 to 10 ms, the generator is not connectedto the matrix converter. The generator is already operating at
this period. However, the matrix converter does not operate.
At 10 ms, contactors on the generator side in Figure 1 areclosed to connect the generator and the matrix converter. After
this, the resonant current due to the internal impedance of thegenerator and input filter capacitor occurs. Note that the matrix
converter blocks off all the gates of IGBTs from 10 to 30 ms.
At 30 ms, the matrix converter starts to control the input
current to zero by the ACR. The LC resonant is suppressed by
the damping control in the ACR. Then, at 50 ms, a ramp
current command is inputted to suppress the surge voltage on
the input filter capacitor. As a result, the generator connection
is enabled by the above sequences. The generator terminal
voltage angle θin calculation for ACR is important for stable
control in the transient state. However, there is no surge
current except LC resonant current in the transient period.
Therefore, if the LPF for θin calculation eliminates thefrequency band over the rated generator frequency, the ACR
can start to operate stably right after the generator connection.
Figure 11 shows the generator frequency fluctuation withconstant input current commands. The generator terminal
voltage becomes lower as the generator frequency fluctuates
because the back electromotive force of a generator is
proportional to the operating speed. During the fluctuation
periods, the voltage and current on the grid side have no
distortion. The distortion due to the generator frequency
fluctuation does not occur because the fundamental frequency
current of the input filter reactor is controlled as a DC
component based on the conversion from the rotating frame.
Furthermore, the operation also confirms no fluctuation on thegrid power factor. Note that the grid current is reduced
because of the input power change due to the constant input
current commands and voltage fluctuation. Moreover, theconstant power control is available if the input current
commands are generated with the power commands.
Figure 12 shows the operation waveforms where the power
grid voltage is fluctuating. Note that the electromotive force ofthe generator is set to 100 Vrms (line to line), the frequency of
the generator is set to 60 Hz. The input current is controlled toa constant value because of the ACR against the fluctuation of
the grid voltage. On the other hand, the grid current increases
against the fluctuation of the grid voltage. Therefore, thematrix converter can control the generator power constantly
when the grid voltage fluctuates. As mentioned previously,
however, when a trouble occurs at the power grid such as
interruption, the circuit breakers in Figure 1 are opened.
Therefore, the control method of the matrix converter isneeded to study in the situation where an independent
operation condition in the proposed system is required.
A 1.5-kW prototype circuit was constructed and tested withthe proposed system to evaluate the efficiency. The
experimental conditions are almost the same as Table 2. Noteigu

-id_ref_pu-iq_ref_pu-id_pu-iq_puiu_obFig. 10. Voltage and current waveforms on the generator side in the starting
sequence of the proposed interface system.
Generator speed 100 rpm/div

1800

200 ms/div

vcu

Generator terminal voltage (phase) 50 V/div

Vr

Grid voltage (phase) 100 V/div00

Grid current 5 A/div

ir_ob

0
0.20

0.40

0.60

Fig. 11. Operation waveforms of the proposed system including no distortionin the period of the generator frequency fluctuation.

Vr

ir_ob

iu_ob

Fig. 12. Operation waveforms of the proposed system in period of thefluctuation of the power grid voltage.

7

V. CONCLUSIONSThis paper discusses an interface converter between an
engine generator and a power grid. The matrix converter isused as the interface converter in order to obtain highefficiency, reliability and size reduction in comparison to a
BTB converter. The proposed interface system is characterized
by the generator side control. The simulation results andexperimental results confirmed the validity of the proposedsystem. In the simulations, the control of the input current of
the matrix converter is confirmed for an interconnection
system. The maximum efficiency 96.8% is demonstrated in thepaper. Total power converter loss becomes approximately halfof the conventional converter loss.
In future work, the current waveforms will be improved andother type of experimental results including transient response
and revaluation of the system efficiency will be considered. In
addition, the control of the matrix converter in case of theinterruption will be investigated.
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Generator terminal voltage (phase) 250 V/div
0
Matrix converter input current

10 A/div00
Grid voltage (phase) 250 V/div
0
Grid current 5 A/div

10 ms

Fig. 13. Operation waveforms of the proposed system at steady state with anexperiment.

Efficiency

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Fig. 14. Efficiency characteristics with experiment.
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