Direct Grid Connection of Matrix Converter withTransition Control for Flywheel UPS
Jun-ichi Itoh

Hisakatsu Igarashi

Dept. of Electrical, Electronics and Information Engineering
Nagaoka University of Technology
Nagaoka, Japan
itoh@vos.nagaokaut.ac.jp

Dept. of Electrical, Electronics and Information Engineering
Nagaoka University of Technology
Nagaoka, Japan
igarashi@stn.nagaokaut.ac.jp

Abstract— This paper discusses an uninterrupted power supply(UPS) with a flywheel and a matrix converter using a direct gridconnection. A flywheel type UPS has a problem where a standby
power is bigger than that of the battery or EDLC type UPS. To
solve this problem, an operation known as direct grid connection
mode is proposed, where a matrix converter is the interface
converter between the flywheel and grid. In the direct gridconnection mode, the matrix converter has no switching
operation. Thus, the converter loss is remarkably low in thestandby mode due to no switching loss. However, a rush output
current will occur in when the matrix converter is transited from
PWM to direct grid connection mode. In order to solve thisproblem, in this paper, a transition control which treats theoutput voltage to correspond to the input voltage is proposed.
Additionally, the validity of the proposed control is demonstrated
in the simulation and experimental results.

large electrolytic capacitor as an energy buffer and the size issmall. The matrix converter is very suitable interface converter
for the flywheel due to advantage of maintenance free for along time. Besides, a direct grid connection which connects
between power grid and matrix converter directly, can be usedduring the standby to reduce the switching loss. In the direct
grid connection mode, the matrix converter has no switching
operation. However, the voltage transfer ratio (vout/vin) of thematrix converter with PWM is limited up to 0.866 and as aresult transition control is required for direct grid connection.
Due to the above reason, the induction motor current changes
widely and over current will occur without the transition
control. In addition, changing the output current will affect thepower grid because the energy buffer is not located between
the input side (power grid) and output side (induction motor).

Keywords-component; Flywheel; Uninterrupted power supply;
Matrix converter; Direct grid connection;I.

INTRODUCTION

Recently, batteries and EDLCs are usually used as theenergy storage devices in the uninterrupted power supply(UPS). However, there are some problems such as short life
time and environmental impact in the waste dispose [1][2]. On
the other hand, studies on flywheel as energy storage havereceived significantly attentions recently. The flywheel has agood charging and discharging characteristics, and it has alonger life time because of no chemical objects. In addition, theflywheel can be used in hazard environments such as a desert
and a cold area because the output characteristics do notchange subjects to the ambient temperature [3] [4].
In general, a Back-to-Back (BTB) system, which iscomposed of a PWM rectifier and a PWM inverter, is used forflywheel system. However, the conventional BTB system has aproblem where the power loss increases because the BTB
system continues to perform switching during the idlingcondition, which is the standby mode. Some methods toimprove the efficiency in a light load are to apply MOSFETs[5][6]. However, the reduction of the power loss during thestandby mode is difficult because the BTB system needs todrive the AC motor at a constant speed.
On the other hand, a matrix converter has been studied
actively recently [7-10]. The matrix converter does not required

In this paper, the flywheel with an induction motor driven
by a matrix converter in the UPS is investigated. Additionally,
the transition control from PWM to the direct grid connection
mode is proposed. In the direct grid connection mode, thematrix converter has no switching operation, it is considered asa platform connected between the power grid and induction
motor. Thus the induction motor is driven by direct supply
power. Using this method, the converter loss is remarkably low
due to no switching loss. Besides, a transition control is
proposed, which the output voltage gradually corresponds tothe input voltage in transiting to the direct grid connection. The
rest of this paper is organized as follows. The system
configuration is explained in Section II. The operating
methodology of the proposed control is discussed in Section III.
The performance of the proposed control is verified by detailed
numerical simulation in Section IV. The experimental results
are presented in Section V. The loss analysis in the standby
mode is presented in Section VI. The summary in Section VII
concludes this paper.
II.

SYSTEM CONFIGURATION

Fig. 1 shows a UPS configuration with matrix converter.
The power grid side and the flywheel side are defined as an
input and an output of the matrix converter respectively.
The flywheel is driven by the induction motor that isconnected to the power grid though a matrix converter. Whilean interruption occurs, the flywheel operates as a generator
supplying the power to the load. In standby mode, theinduction motor is driven by direct power from the gird via thematrix converter. During this mode, the matrix converter
operates without switching, i.e. three legs are always turned on
to serve as the U, V and W port for the induction motor.
III.

Power
grid
vin , iinrst

PROPOSED CONTROL

Fig. 2 shows a state transition diagram of the control mode.
In the automatic speed regulation mode (Mode I, the induction
motor is accelerated up to rating speed in starting up or after
recovery from short interruption. When the motor speed
reaches the rating, the mode state is changed to the transition
control mode (Mode II). In the transition control mode, theoutput voltage corresponds to the input voltage gradually. Afterthat the mode state is changed to the direct grid connection
mode (Mode III). In the direct grid connection mode, theconverter loss is reduced due to no switching operation. Whenthe short interruption occurs in the power grid, the mode stateis changed to UPS mode (Mode IV). In UPS mode, the load
power is supplied from the flywheel. After the shortinterruption recovery, the mode state is changed to theautomatic speed regulation mode.
Fig. 3 shows the relationship between the motor voltage
and the motor speed. The rated voltage of the induction motor
is to the grid voltage for direct grid connection. Since thevoltage utilization of the matrix converter is 0.866 or the input
voltage, the induction motor is driven using flux weakening inhigh speed in order to consistent the output voltage command.
Thus the output voltage is constant as flux weakening is
implemented in high speed. For this reason, when the mode
state is changed to the direct grid connection mode, the output
voltage will increase in a sudden without a proper transition
control. In addition, if the input voltage phase and the output
voltage phase are different, large rush current or overcurrent
might occur. Moreover, large transient variation of the output
current will affect the power grid because energy buffer are not
located between the input side (power grid) and output side
(induction motor). In order to solve these problems, thetransition control that treats the output voltage to correspond to
the input voltage is introduced between the automatic speed
regulation mode and the direct grid connection mode. Besides,
when the mode state is changed from the direct grid connection
mode to the UPS mode, the output voltage will be decreased
Nevertheless, the output current does not change widely due toan automatic current regulator (ACR) in motor controller.
Fig. 4 shows the control block diagram for the proposedsystem. The control method is based on a vector control of theinduction motor. The q axis current command Iq* into the ACR
and the switching signal of the matrix converter are changed
corresponding four control mode. Besides, ref is the output
phase command for the output phase control used in thetransition control. Other than the transition control, the output
phase control is not used, therefore, ref is set to out. Control
methods of each control modes are explained in follows.
A. Mode I – Automatic speed regulation mode
In the start-up of the induction motor, or after the UPS
mode, the mode state is changed to the automatic speed
regulation mode. In this mode, the induction motor iscontrolled to increase the motor speed up to rated speed by the

Sru
Ssu
Stu
Srv
Ssv
Stv
Srw
Ssw
Stw

Input filter

LOAD

vout , ioutuvw

Flywheel
IM

FW

Induction
motor

Matrix converter

Figure 1. Proposed system using flywheel energy storage with matrix
converter.
ModeI
Automatic speed
regulation mode

Power gridrecoveryStart
Transition control
ModeII
Transition control
mode

Finishtransition
control

ModeIVUPS modePower gridinterruption
ModeIII
Direct gridconnection mode

Figure 2. State transition diagram of the control mode.
Motor voltage
Rated voltage

1.0

Flux weakening 0.866
voltage

Mode III
Mode II

Mode Ior
Mode IV
Rotation speed

Figure 3. Relationship between motor voltage and rotation speed.

vector control as shown in Fig. 4. Iq* is set to the output of an
automatic speed regulator (ASR).
B. Mode II – Transition control mode
In transition control, the mode state is changed from theautomatic speed regulation to the direct grid connection. In
order to stabilize the output current, it is necessary that theoutput voltage corresponds to the input voltage gradually.
Thereby, first, the output voltage phase must be corresponding
to the input voltage phase using the output phase control.
Second, the amplitude of output voltage needs to becorresponding to the amplitude of input voltage using an AC
chopper switching mode.
Using the output phase control, the output phase out cancorrespond to the output phase command ref. First, Iq* is set tozero. Second, difference between out and ref is equaled to zerothrough a PI controller which is defined by a phase gain K anda phase integral time T. Finally, the final value is added with
an electrical angular velocity. A transfer function from ref toout is represented byK1  T s TG ( s ) K
s 2  K s  
T



qin





qout

Equation (1) is second order lag. Thus out corresponds to ref.
Beside, K and Tare designed byK  2w n 



T 

2

wn

din
vout
iout

Figure 5. Vector diagram when the output voltage phase corresponds tothe input voltage phase.

where  is damping coefficient, and wn is response speed of theoutput phase control.

Rs

Fig. 5 shows the vector diagram on a d-q rotation frame
when the output voltage phase corresponds to the input voltage
phase. The input din axis is aligned with U phase of the input
voltage vector vin. In the output phase control, the output
current vector iout corresponds to the output dout axis because Iq
is controlled to zero. Additionally the output voltage vector voutleads from iout by load angle IM.

 ref   in   IM

  in  tan 1*

PL
Vd_n-1*
Vq_n-1*
Id

ASR

3

Iq* cal.
eq.(7)



Rs

Flux
wakening

Id *wr

w e l s  M  

Id

ls

Rr
s

M

vout

(a) In acceleration or deceleration

vout

ls
M

(b) In constant speed

Figure 6. Equivalent circuit of an induction motor verified by thefundamental component.

where in is the phase angle of input voltage, and we is theelectrical angular velocity. When the output voltage phasecorresponds to the input voltage phase, matrix converter will be
operated in an AC chopper switching mode.
Fig. 7 (a) shows a configuration of a V connection AC
chopper, and Fig. 7 (b) shows a control block diagram [11-13].
The output voltage is PWM waveform that the envelope curveis sinusoidal. The output frequency is same as the input
frequency. Moreover the voltage transfer ratio of the Vconnection AC chopper is zero to 1. Thus, switching operationof the matrix converter is fixed to correspond to the switching
operation of the V connection AC chopper, so that the
maximum transfer ratio of the matrix converter is improved up
to 1.

ACR

ACR

Vd*

vu*, vv*, vw*
dq
uvw

V q*

3

Mode wre

we

1 s

wse
slip
cal.

1 sT

MC
SW
cal.

AC chopper
SW cal.

Duty
p

Rs

lr

Fig. 8 shows the control block diagram of the switching

*’

M *
U Iq
X

0

out

dout



Fig. 6 shows a one single T-type equivalent circuit of theinduction motor. Rs and Rr are the stator resistance and therotor resistance, respectively, then, ls, lr and M the a stator
leakage inductance, the rotor leakage inductance, and themutual inductance, and s is the slip of the induction motor.
Fig. 6 (a) is the equivalent circuit in acceleration or
deceleration. Fig. 6 (b) is the equivalent circuit in constant
speed. In the transition control, the motor speed is constant at
rated speed. Fig. 6 (b) is applied in transition control. Thereby,
the output phase command ref which the output voltage phasecorresponds to the input voltage phase is expressed by

vin

IM





in

out

SW

M
U
9 X

Mode

K

ref

Output phase
control
Id
Iq

9

iu
uvw
dq

iv
iw

Figure 4. Control block diagram for the proposed system.

MatrixConverter

SensorIMFW

wrm

pulse of the matrix converter in the AC chopper mode. Duty
ratio is increased from an initial value Dint to 1. Dint isexpressed by2

Dint 

Vd _ int  Vq _ int





where Vin is the RMS value of the input phase voltage, Vd_int
and Vq_int are output voltage commands on d-q axis at the
moment of the operation mode changing to the AC chopper
mode. The duty ratio is increased up to 1, in other words,
Switches turn on constantly. After that, the mode state ischanged to the direct grid connection mode.
C. Mode III – Direct grid connection mode
In the direct grid connection mode, the induction motor isdriven by the power from the power grid via the non-switchingstate of the matrix converter. Switches Sru, Ssv, and Stw areturned on and the remaining switches are turned off. Moreover,
if a magnetic contactor shunting the input and output of thematrix converter are added, the conduction loss is reduced
drastically. It is noted that the response time for UPS mode will
be delay when the magnetic contactor is used.
D. Mode IV – UPS mode
When the short interruption occurs in the power grid, themode state is changed to UPS mode. In this mode, the motor
speed is decreased from rated speed. The power that use tocompensating the short interruption is regenerated from theflywheel to the load. Iq* is set to required value to supply theload power PL. Beside, an active power P in rotating reference
frame is expressed by


P





3
Vd I d  Vq I q 
2



where 3/2 is the coefficient of a relative transformation to therotating reference frame. Equation (6) is transformed about Iq,
so that Iq* required to supply PL is expressed by
*

Iq 



1 2

*
PL  Vd _ n1 I d _ n1 * Vq _ n1  3






where Vd_n-1* is a last value of a d axis voltage command, Vq_n*
1 is a last value of a q axis voltage command, and Id_n-1 is a lastvalue of a d axis current. When the grid voltage returns tonormal, the mode state is changed to the automatic speed
regulation mode.
IV.

u
vuv
v

S2
S3

2

2Vin

S1

r
vrs
s

VERIFICATION BY SIMULATION

Table 1 shows parameters of the tested induction motor andthe flywheel. Simulation condition is as follows; grid line
voltage is 188 V and 50 Hz, as the grid voltage corresponds to
the rated values of a tested induction motor. The switchingfrequency of the matrix converter is 10 kHz. Each PI controllerof the vector control is designed, as the response of the ACR

t
vrs

S4

2

w

S2 , S3(b) Control block diagram(a) System configuration

V connection AC chopper.

1
Dint

2
2

AC chopper duty0Carrier

S1 , S4

Carrier

vuv

Figure 7.

2

Duty

1

4

Sru , Stw
Ssu , Ssw
Stu , Srv , Stv , Srw
Ssv

Figure 8. The control block diagram of the matrix converter in the AC
chopper mode.
TABLE I.

INDUCTION MOTOR AND FLYWHEEL PARAMETER

Motor rating
Motor and Flywheel parameter
Rated power
3.7kW
Stator resistance
0.334W
Rated speed 1500r/sec
Rotor resistance
0.266W
Rated voltage
188V
Stator leakage inductance 0.998mH
Rated current
18A
Rotor leakage inductance 0.580mH
28.8mH
Rated frequency 51Hz
Mutual inductance2
11.6A
Pairs of poles
Excitation current
motor moment of inertia 0.0163kgm2
Flywheel moment of inertia 0.608kgm2

and the ASR are 4000 rad/sec and 400 rad/sec. Parameters of
the output phase control are designed, as K and T are 14 and140 msec. The response of the output phase control is 10
rad/sec. Besides, following results are standardized based on
the rating of the induction motor.
Fig. 9 shows the simulation results including the transition
control. The transition starts from the automatic speed
regulation mode to direct grid connection mode. The mode
state is changed from automatic speed regulation mode to
direct grid connection mode at 0.1 sec, after that, the phaseangle of the output voltage phase is gradually corresponding to
the phase angle of input voltage phase, which is completed at
0.46 sec. At 0.46 sec, the voltage phase difference decreases tozero. Then the matrix converter is operated in the AC chopper
mode. In the AC chopper mode, the amplitude of output
voltage is increased gradually until it matches with theamplitude of input voltage amplitude. At 0.52 sec, the mode
state is changed to the direct grid connection. As a result, thetransition process is completed within 0.42 sec. Moreover it isconfirmed that surge current does not occur during thetransition period.
Fig. 10 shows the simulation result of the transition fromthe direct grid connection mode to the UPS mode. On theassumption that the short interruption occurs during 0.1 to 0.2
sec, the output q axis current command is set by - 0.3 p.u. As aresult, the output d-q axis currents are conformed to agree with

their commands. It is confirmed that the surge current does notoccur during transition period.
V.

EXPERIMENTAL RESULTS

Rotation speed wrm[p.u.]
1.00

In this section, the proposed control is confirmed byexperimental setup. Note that RL load (R is 10.3W, L is 10mH)
is used instead of the induction motor because the flywheel
system is constructing now. The parameters of each controller
are same as the simulation.

Fig. 13 shows the experimental results of the transition
process from the direct grid connection mode to the UPS mode.
In UPS mode, Iq* is set to 5A. As a result, the output q axis
currents are confirmed well agreed to the commands. Also, thesurge current is not occurred during transition period.
VI.

LOSS ANALYSIS

Fig. 14 shows the converter loss analysis in standby mode,as the matrix converter operates in the vector control mode or
the direct grid connection mode. The Switching power device
is FGW30N60VD (600V 30A, Fuji electric). In the vector
control mode, the exciting current in the induction motor isdecreased due to the flux weakening. Hence, the conduction
loss in the vector control is smaller than the direct gridconnection mode. Besides, the matrix converter has noswitching operation in direct grid connection mode, so that theswitching loss does not occur. As a result, the converter loss inthe direct grid connection mode is reduced by 16.5%,
compared to the vector control mode.
VII. CONCLUSION
This paper discusses the transition control method for thematrix converter with flywheel for an UPS. Using the direct
grid connection mode, the power loss of the matrix converter isreduced remarkably due to no switching operation. By loss
analysis, the converter loss in the direct grid connection mode

Mode II

Mode III

Input voltage vrs[p.u.] Output voltage (LPF) vuv_LPF [p.u.]

20-2
Output voltage vuv [p.u.]
2

Fig. 11 shows the experimental results of the output phase
control. The input voltage vrs and the output voltage vuv are
passed through the low pass filter (LPF), so that the phasedifference between the input voltage and the output voltage canbe identified. As a result, the output voltage phase corresponds
to the input voltage phase within 0.6sec. Moreover there is norush current in the process.
Fig. 12 shows the experimental results of the transition
control. At first, the current commands is set as, Id* is 0A, and
Iq* is 5A. Additionally the input voltage phase alreadycorresponds to the output voltage phase due to the output phase
control. During AC chopper operation mode, it is confirmedthat the output voltage waveform is similar to the output
voltage of V connection AC chopper. Moreover, the AC
chopper duty is increased gradually, so that the output current
is increased. After that the mode state is changed to the direct
grid connection mode. Therefore the output voltage waveform
corresponds to the input voltage waveform. As a result it isconfirmed that the surge current does not occur during thetransition period to direct grid connection mode.

Mode I

0.5

0-2

Output current iu[p.u.]

1
0
-1
0

0.1

0.2

0.3

0.4

0.5

0.6

Figure 9. Simulation result of transition from automatic speed regulation
mode to direct grid connection mode.
1.05

Rotation speed wrm[p.u.]Mode III
Mode IV

Mode I

1.00
0.95
Output voltage vuv [p.u.]
20-2

Output current iu[p.u.]

1
0
-1
1.0

Output d axis current Id[p.u.]

0.5
0.0
Output q axis current Iq[p.u.]
1.0
0.5
0.0
-0.5
0
0.05
0.10
0.15

0.20

0.25

0.30

Figure 10. Simulation result of transition from direct grid connection mode toautomatic speed rebulation mode.

is reduced by 16.5%, compared to the vector control mode.
However, the voltage transfer ratio of the matrix converter islimited up to 0.866, so that the rush output current will occur intransition from PWM to direct grid connection mode. To solve
this problem, the transition control which treats the output
voltage to correspond to the input voltage in term of phaseangle is proposed. From the simulation results, it was
confirmed that the mode state is changed from the automatic
speed regulation mode to the direct grid connection mode
within 0.42 sec, moreover the output current is shown stable
controllable. Besides, the experimental results also confirmedthat the surge current does not occur in transition.

In future work, the verification of the proposed control byexperiment with the induction motor will be shown.
ACKNOWLEDGMENT

Start output phase control
Input voltage (LPF) vrs[500V/deg]
0Output voltage (LPF) vuv[500V/deg]
0Phase difference between vrs and vuv [prad/deg]00100[ms/div]

Figure 11. Experimental result of the output phase control.
Output phase
control

AC chopper operation

REFERENCES[1]

A part of this study was supported by Industrial
Technology Grant Program in 2009 from New Energy andIndustrial Technology Development Organization (NEDO) of

Output current iu[10A/deg]

Japan.

Direct gridconnection

Input voltage vrs[500V/deg]
0
Input current ir[10A/deg]
0Output voltage vuv[500V/deg]
0Output current iu[10A/deg]
0

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20[ms/div]

Figure 12. Experimental result of the transition from the automatic speed
regulation mode to the direct grid connection mode.
Direct grid connection
Input voltage vrs[500V/deg]
Input current ir[10A/deg]

PWM mode

FWD switching loss

80

IGBT switching loss

70
60

0

FWD
conduction loss

50
40

0
0

90

30

Output voltage vuv [500V/deg]

IGBT
conduction loss

20
10

0

q axis current Iq[10A/deg]

0
10[ms/div]

Figure 13. Experimental result of the transition from the direct grid connection
mode to the UPS mode.

Vector control

Direct grid connection

Figure 14. Converter loss analysis in the vector control mode and the direct
grid connection mnode.