estudo capacitor

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Electric Power Systems Research 79 (2009) 12001208

Contents lists available at ScienceDirect

Electric Power Systems Research

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e p s r

Electronic power transformer with supercapacitors storage energy system

Haibo Liu, Chengxiong Mao, Jiming Lu , Dan Wang

Hubei Electric Power Security and High Efficiency Key Lab, Department of Electrical Engineering,

Huazhong University of Science and Technology (HUST), Wuhan 430074, PR China

a r t i c l e i n f o

Article history:

Received 31 July 2008

Received in revised form 20 January 2009

Accepted 28 February 2009Available online 3 April 2009

Keywords:

Bidirectional dcdc converter

Electronic power transformer (EPT)

Medium frequency transformer (MFT)

Storage energy

Supercapacitors

Voltage interruptions

a b s t r a c t

An electronic power transformer (EPT) with supercapacitors storage energy system is proposed in this

paper. The proposed system consists of an EPT, a supercapacitor bankand a bidirectional dcdcconverter.

The supercapacitor bank is connected to the dc link in EPT through the dcdc converter. The advantage

of the proposed systemover the EPT is to provide added ability to ride voltage momentary interruptions,

as well as the voltage sags and voltage swells. Furthermore, the main circuit parameters design and

controller design of proposed system are presented in detail. The performance of proposed system is

analyzed using simulations based on MATLAB/SIMULINK, and the proposed system is also implemented

in laboratory based on DSP TMS320F2812. Simulation and experimental results have verified the ability

of the proposed system and the validity of the design.

Crown Copyright 2009 Published by Elsevier B.V. All rights reserved.

1. Introduction

Electronic power transformer (EPT) is a new intelligent power

transformer enhancing the functions of the conventional power

transformer and achieving the flexible ac transmission systems

(FACTS) by power electronic conversion and has attracted much

attention from both academy and industry [15]. Most of the stud-

ies reported recently concentrated on its main circuit topology

design, power quality improvement for customers, parallel operat-

ing control technology and mathematical model establishing [69]

and the prototypes applied in the distribution system have been

implemented [1,4]. These researches show that EPT is employed

to not only realize the basic functions of traditional power trans-

former such as voltage transformation, isolation, power delivery,

but also perform many additionalfeatures like reactive power com-

pensation,preventing fromthe primary voltagesags, voltageswells,

voltage flickers, harmonics and voltage unbalance infecting the

output voltage etc.. However, EPT cannot compensate the voltage

interruption because it has no energy storage in the dc link.

Recently, supercapacitors (also known as ultracapacitors) are

paid attention as a new energy storage element. The supercapaci-

tors have a lot of advantages such as no maintenance, long lifetime

and quick charge/discharge characteristics with large current and

can operate effectively in diverse (hot, cold, and moist) environ-

ments [10]. At present, supercapacitors haveseen much application

in electric vehicles [11,12]. Near-term applications mostly use these

Corresponding author. Tel.: +86 27 87542669; fax: +86 27 87542669.

E-mail address: [email protected](J. Lu).

capacitors in power quality applications. For example, supercapac-

itors areadded to the dc link of uninterruptible powersupply (UPS)

to enhance power supply reliability in case of grid failure [12].

Supercapacitors are added to the dc link of motor drives to improveride-through times during voltage sags [13]. Supercapacitors are

also added to a dynamic voltage restorer (DVR) or interfaced to

the dc link of a distribution static compensator (DStatCom) [14,15].

However, UPS and DVR must try to synchronize their output volt-

age to the power grid voltage when the grid voltage is resumed to

normal operation because UPS and DVR are connected in parallel

with the grid [16].

We use the supercapacitors as an energy storage element

of EPT. In this paper, a combined operation system of EPT and

supercapacitors storage energy system, which is composed of

a supercapacitor bank and a bidirectional dcdc converter, is

proposed. The advantage of the proposed system over the EPT is to

provide additional ability to ride voltage momentary interruptions,

as well as the voltage sags and voltage swells. The advantageof the proposed system over the DVR and the UPS is to have

no above-mentioned synchronization problem because EPT is

connected in series with the grid. The operation of the proposed

system was verified through computer simulations based on

MATLAB/SIMULINK. The feasibility of hardware development was

confirmed through experimental works.

2. System configuration

Fig. 1(a) shows circuit diagram of EPT. As can be seen from the

Fig. 1(a), this is a three-stage design that includes an input stage,

0378-7796/$ see front matter. Crown Copyright 2009 Published by Elsevier B.V. All rights reserved.

doi:10.1016/j.epsr.2009.02.012
http://www.sciencedirect.com/science/journal/03787796http://www.elsevier.com/locate/epsrmailto:[email protected]://dx.doi.org/10.1016/j.epsr.2009.02.012http://dx.doi.org/10.1016/j.epsr.2009.02.012mailto:[email protected]://www.elsevier.com/locate/epsrhttp://www.sciencedirect.com/science/journal/03787796


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H. Liu et al. / Electric Power Systems Research 79 (2009) 12001208 1201

Fig. 1. Circuit diagram of EPT and block diagram of the proposed system (a) circuit diagram of EPT, (b) block diagram of EPT with supercapacitor storage energy system and

(c) bidirectional dcdc converter with supercapacitors.

an isolation stage and an output stage. In the input stage, there isa three-phase high frequency converter, which converts the input

ac voltage to dc voltage. The isolation stage consists of H-bridge-1

and medium frequency transformer (MFT) and H-bridge-2. The dc

voltage from the input stage is fed to the H-bridge-1 and is modu-

lated to a medium frequency square wave. Then the square wave is

provided to theMFT andis rectifiedas dc voltage bythe H-bridge-2.

In the output stage, there are three single-phase inverters, which

convert the dc voltages from the isolation stage to three phase ac

sinusoidal voltages. In Fig. 1(a), L1 is the input inductor, CDC1 is the

dc capacitor of the input stage, Lf, Rf and Cf are the filter induc-

tor, the filter resistor and the filter capacitor of the output stage

respectively.

The system configuration of EPT with energy storage element is

shownin Fig.1(b).Main circuit ofthe proposed systemconsistsof an

EPT, a bidirectional dcdc converterand a supercapacitor bank. Thesimple equivalent circuit of thesupercapacitorbank is alsoshown in

Fig. 1(b), where R is the equivalent resistance, CSC is the equivalent

capacitor.

Here, the supercapacitor bank is connected to dc link through a

bidirectional dcdc converter. The dcdc converter is used to con-

trol the charging current to the supercapacitors and also used to

keep the dc link voltage constant for the discharge of the super-

capacitors. Furthermore, the high voltage supercapacitors are not

required because of the boost function of the dcdc converter, also

there is a possibility to limit a charging current and accordingly

to limit a starting current due to the inductor L comparing to the

supercapacitors being directly connected to dc link.

In normal situations, the electric power to the load is provided

from power grid through EPT, the supercapacitors are charged with


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1202 H. Liu et al. / Electric Power Systems Research 79 (2009) 12001208

constant current. After the voltage becomes full, the voltage across

the supercapacitors is kept constant by the dcdc converter. And

the dc link voltage is also kept constant by the isolation stage

of EPT. When power failure occurs, the power from the superca-

pacitors is provided to the load through the dcdc converter, and

the dc link voltage is kept constant by the dcdc converter. When

the power grid is resumed to normal operation, the proposed sys-

tem has no synchronization problem because EPT is connected

in series with the grid. Therefore, the proposed system can com-

pensate not only the voltage sags but also the short-term power

interruption.

3. Analysis and design of supercapacitors storage energy

system

3.1. Supercapacitors

Since several years, supercapacitors are paid attention as a new

energy storage element. Supercapacitors have some advantages

over batteries [10]:

Rapid charge/recharge capability within minutes.

High power density. Low degradation after 100,000 cycles. No maintenance, high reliability. Environment safety. Wider operating temperature range.

In the terms of energy density, supercapacitors are between an

ordinary capacitor and a battery. At first sight it is not profitable to

use supercapacitors for energy storage in EPT because of their high

price and low energy density.

However recent developments in nanotechnology help to

achieve a major breakthrough in the field of supercapacitors.

Nowadays supercapacitors represent an emerging technology of

electrochemical devices with very high capacitance values, which

allows reaching specific energy density of 4.5 Wh/kg and specific

power density of about 3500W/kg [17]. Using these new types of

supercapacitors that are suitable for high power and low energy

applications, it is possible to better exploit the energy reserve for

applications such as EPT.

Firstly, in power system, most of grid faults are very short (1 s)

[18]. Therefore, high power and low energy storage devices should

be employed, whereas supercapacitors have the characteristic of

high power density and low energy density. Secondly, its ability of

quick charge and discharge can provide compensation repeatedly

even if short-term voltage sag or momentary interruption occurs

again and again. Furthermore, the supercapacitors do not require

replacement for about 10 years or longer because the electrodes

show little deterioration. Finally, high price of the supercapacitors

defines high total price of the EPT, but the cost of the superca-

pacitors is less important comparing to the cost of the EPT itself.

Thus, it is reasonable to choose supercapacitor storage system in

EPT.

3.2. Bidirectional dcdc converter

Fig. 1(c) shows the bidirectional dcdc converter with superca-pacitors, which includes an inductor L, some supercapacitors CSC, a

dc link capacitor CDC, two insulated-gate bipolar transistors (IGBTs)

S1 and S2, and two diodes D1 and D2. Theconverter has two opera-

tion modes: Buck andBoost. Buck operation consists of transferring

energy from the power grid to the supercapacitors by triggering

IGBTS1. Boost operationresults fromtriggering IGBTS2, and energy

is transferred from the supercapacitor to power terminals.

3.3. Design of inductance L

The inductance L allows transient energy storage during the

operation of the dcdc converter. Its design is also related with

Fig. 2. Linear model of the dcdc converter control scheme and Bode plots of the closed-loop transfer function G(s) (a) linear model of the dcdc converter control scheme

and (b) bode plots of the closed-loop transfer function G(s).


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the current ripple amplitude, which is one of the variables that

has to be minimized, because it produces undesirable electromag-

netic interference, mechanical vibrations, and losses due to current

induction on the surrounding conducting material. On the other

hand, its weight, volume, and series resistance have to be as small

as possible.

The design of inductance L is a compromise between size,

current ripple, and control performance. Neglecting the internal

resistance of the inductance L and assuming a constant voltage VSC,the current ripple is given by

I=VDC

fPWMLD(1 D) (1)

where VDC is the dc link capacitor voltage; fPWM is the switching

frequencyof IGBTs; D is the dutycycleof the pulsewidth modulation

(PWM) applied (0 < D


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Fig. 4. Voltage interruption compensation (a) input voltage, (b) input current, (c) input stage dc voltage, (d) primary voltage of MFT, (e) secondary voltage of MFT, (f) output

stage dc voltage, (g) supercapacitors voltage, (h) supercapacitors current, (i) load voltage and (j) load current.

Accordingto Fig. 2(a),the control expressionof thedc link output

voltage can be derived

VPWM = kp2D

[kp1(VDC VDC) + ki1

(VDC VDC)dt iDC]

ki2D

[kp1(V

DC VDC) + ki1

(VDC VDC)dt iDCdt

(5)

where VDC

is the dc link voltage reference.

By Eqs. (4) and (5), the closed-loop dc link output-voltage

dynamic behavior takes the form

VDC = G(s)VDC Z(s)iDC (6)

Table 1

The parameters of the storage energy system.

Parameter Value Parameter Value

L 0.3mH R 0.02

CDC 0.05F CSC 60 F

kp1 5 ki1 0.5

kp2 19.5 ki2 0.45

where G(s) is the voltage gain and Z(s) is the output impedance, as

shown in Eqs. (7a) and (7b)

G(s) =VDCVDC

iDC=0

=kp1kp2s

2 + (kp1ki2 + kp2ki1)s + ki1ki2

LCDCs4

+ RCDCs3

+ (kp1kp2 + CDC/CSC)s2

+ (kp1ki2 + kp2ki1)s + ki1ki2

(7a)

Z(s) =VDCiDC

V

DC=0

=Ls3 + (R + kp2)s

2 + (ki2 + 1/CSC)s

LCDCs4 + RCDCs

3 + (kp1kp2 + CDC/CSC)s2

+ (kp1ki2 + kp2ki1)s + ki1ki2

(7b)

Fig. 2(b) shows the typical Bode plots ofG(s). We set the param-

eters of the dcdc converterto thevalues listedin Table1 according

to the above-mentioned design principle and the proposed control

scheme.


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4. EPT main circuit design

Hereinafter, the main circuit parameters design of EPT is dis-

cussed and the corresponding control scheme is also proposed.

Section 4.1 introduces the design of input stage, and the designs

of isolation stage and output stage are presented in Section 4.2 and

Section 4.3 respectively.

4.1. Design of input stage

4.1.1. Design of input inductor

Design of the input inductance is related with not only the

dynamic response and steady-state response of the current loop,

but also the output power, the power factor and the dc voltage of

three-phase high frequency converter. The main functions of the

inductance are as follows: isolating the ac side voltage Va1, Vb1 and

Vc1 from the input voltage ea, eb and ec, eliminating the harmonic

current of the ac side, making the converter have the boost voltage

characteristic and achieving good damp characteristic.

The inputinductance shouldbe chosen large enough to keep the

input current sinusoidal. On the other hand, the input inductance

should be chosen small enough to achieve a fast current tracking

response. According to [19], its design should meet the inequality

Eq. (8):

(2VDC1 3Em)EmTs2VDC1imax

L1 2VDC13Im

(8)

where Em is the peak value of the grid voltage, Ts is the equivalent

switching frequency, imax is the maximum value of the input rip-ple current, Im is the amplitude of the input current, VDC1 is the dc

side voltage of the input stage.

4.1.2. Design of dc capacitor of input stage

The dc capacitor of input stage has two main functions. One is

to keep the dc voltage be a constant. The other is to eliminate the

dc side harmonic voltage. Generally, considering the tracking per-

formance of the voltage loop control, the capacitor is to be chosen

Fig. 5. Voltage sag and swell compensation (a) input voltage, (b) load voltage and

(c) Load current.

small enough. However, in view of the disturbance rejection char-

acteristic of the voltage loop control, the capacitor is to be chosen

large enough. According to [19], its designmay choosethe following

experience Eq. (9):

CDC1 tr

0.74ReL(9)

where tr is the rising time, ReL is the equivalent load resistance.

4.1.3. Input stage control scheme

Fig. 3(a) shows the input stage control diagram of EPT. The input

stage of EPT described in this paper has two main functions. One

is to keep the input currents ia1, ib1 and ic1 sinusoidal and in phase

with the input voltage to achieve unity input power factor and pre-

vent harmonics from being injected to the grid. The other is to keep

the dc side voltage of the input stage be a constant. In Fig. 3(a), is

the synchronous angular velocity.

As can be seen from Fig. 3(a), a dc voltage outer loop and an

ac current inner loop are adopted to realize constant dc voltage

control and keep the input current sinusoidal, and the referencefor

the reactive power i1q is set to zero in order to achieve unity input

power factor.

4.2. Design of isolation stage

The main functions of the isolation stage are as follows: voltage

transformation, isolation and power delivery. Tosimplifythe design

of the control system, open loop PWM control is applied for the H-

bridge-1 and H-bridge-2. In the H-bridge-2, the diode rectifiers are

adopted if only considering single-directional power flow. There-

fore, neglecting the losses of MFT, the isolation stage can be treated

as a proportional amplifier. The simplified model of the isolation

stage is presented as:

VDC =1

kVDC1 (10)

where k is the transformation ratio, VDC1 and VDC denote thedc side

voltages of the input stage and the output stage respectively.

4.3. Design of output stage

4.3.1. Design of filter inductor and filter capacitor

The main function of the filter inductor and the filter capacitor

is to reduce the output voltage harmonics, especially the low-order

harmonics.In theory,if the filter inductor and the filter capacitorare

selected large enough, the Total Harmonic Distortion factor (THD)

of the output voltage will be well below the allowed value. How-

ever, too large filter inductor will aggravate the change of the load

fundamental voltage, and too large filter capacitor will aggravate

the change of the inverter output current. Thus, the design of the

filter inductor and the filter capacitor is a compromise between the

load fundamental voltage and the inverter output current.As we know, for the single-phase inverter adopting PWM con-

trol scheme, the output voltage harmonics mainly concentrate on

the edge of the carrier frequency. Therefore, in order to reduce the

output voltage harmonics and avoid the resonance, the resonance

frequency should be far above the reference frequency, and should

be far below the carrier frequency. The resonance frequency can be

calculated by using Eq. (11).

f =1

2

LfCf(11)

4.3.2. Output stage control scheme

As described above, the output stage consists of three single-

phase inverters, and the control of each inverter is independent.


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Each inverter is controlled as a single-phase sinusoidal voltage

source to meet the requirement of the loads.

Because the switching frequencyis usually several orders higher

than the fundamental frequency of the single-phase inverter, the

switching dynamics of the inverter can be ignored. Thus, the out-

put stage inverter can be modeled as a simple proportional gain

block, as shown in Fig. 3 (b), where M is the proportional gain

of the output stage inverter, (For the simplicity of analysis, we

let M = 1.) kp3

and ki3

are the proportional and integrated gains

of the output voltage feedback loop, kp4 and ki4 are the pro-

portional and integrated gains of the inductor current feedback

loop.

The model shown in Fig. 3(b) has a multiloop controller, which

consists of an inner inductor current feedbackloop,an outer output

voltage feedback loop,and a reference voltage feedforward loop.An

instantaneous value of the feedback signals is adopted to enhance

the dynamic response in the control system, and the reference

voltage feedforward loop is added to provide a high tracking accu-

racy and velocity to the reference on the basis of the conventional

proportionalintegral (PI) control.

Fig. 6. Appearance of supercapacitor module, MFT and experimental results when EPT are in normal operation (a) appearance of supercapacitor module, (b) external view

of MFT, (c) load voltage, (d) input current, (e) input stage dc voltage, (f) primary voltage of MFT, (g) output stage dc voltage and (h) secondary voltage of MFT.


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The control schemes are the same for the three phases. The only

differences are thephaseanglesof thereference voltagesVa ,Vb

and

Vc , that is, the phase angle of phase A is set as 0, phase B is +120,

and phase C is 120.

5. Simulation results

Some computer simulations with MATLAB/SIMULINK software

were performed for the purpose of analyzing the operation of theproposed system andshowing the validityof the design. The super-

capacitor was modeled using a 60 F capacitor in series with a

0.02 resistor, and its nominal voltage is 200V. The controller wasmodeled using the built-in control block in the MATLAB/SIMULINK

software.The parameters of the storage energysystemwe designed

are listed in Table 1.

According to the above-mentionedEPT main circuit design prin-

ciple, the EPT main parameters that were used in the simulation

are as follows: the input inductor is 6 mH, the dc capacitor of input

stage is 4700 uF, the filter inductor is 2 mH, the filter capacitor is

330uF, the EPTs rated capacity is 500 kVA, the input voltage and

the output voltage are 10 kV and 400 V respectively, the input volt-

age frequency is 50Hz, the working frequency of MFT is 1 kHz, the

transformation ration of MFT is 50, the load capacity is 500 kVA(power factor is 0.8).

Fig. 4 shows the simulation results when the input voltage has a

voltage interruption for 0.2 s from 0.3 s to 0.5 s. In view of the bidi-

rectional power flow, when the voltage interruption happens, an

isolation switch Kshown in Fig. 1(b) preventing the energy of stor-

age energy system flowing the grid is required. Fig. 4(a), (i) and (j)

shows the input voltage, the load voltage and the load current. The

load voltage and current maintain a constant value by the support

of the supercapacitors storage energy system. Fig. 4(b)(e) shows

the input current, the input stage dc link voltage, the primary volt-

age and second voltage of MFT. Fig. 4(f)(h) shows output stage

dc link voltage, the supercapacitors voltage and the supercapac-

itors current. Due to adopting the dcdc converter, although the

supercapacitors are only charged to 200 V at the beginning, the dc

link voltage is kept constant at about 400 V for the discharge of the

supercapacitors.

Fig. 5 shows the simulation results when the input voltage has

a voltage sag for 0.1s from 0.25 s to 0.35 s and has a voltage swell

for 0.1 s from 0.45 s to 0.55 s. When the depth voltage sag or swell

happens, the input stage constant dc link voltage control would

not be obtained. So the input voltage should be switched off and

the isolation switch must be turned off at the same time. Also, the

simulation results are similar to that of voltage interruption. Thus,

for simplicity, only the simulation waveforms of the input voltage,

the load voltage and the load current are shown in Fig. 5(a)(c).

The load voltage maintains a constant value as expected. These

waveforms showthat the proposed EPT can compensate the voltage

interruption, as well as the voltage sags and voltage swells.

From Figs Fig. 44(i and j) and Fig. 55 (b and c), we also can see

that the load voltage and current are smooth and continuous when

theinputvoltageis resumed to normalcondition. Thereason is that

EPT is connected in series with the grid and the proposed system

has no synchronization problem when the power grid is resumed

to normal operation.

6. Experimental results

An experimental system was built and tested to confirmthe fea-

sibility of actual hardware implementation. EPT has a structure as

shown in Fig. 1(a) except that the diode rectifiers are adopted in

the H-bridge-2. The controller was implemented by means of a

TMS320F2812, fixed-point 150MHz digital signal processor (DSP)

from Texas Instruments. In view of the voltage interruptions being

more severe than the voltage sags, only experimental results based

on the voltage interruptions are given in the following section.

The following describes the supercapacitor module applied in

the storage energy system. One supercapacitor unit has a capacity

of 1200 F at 2.7 V, and twenty units are placed in series to make

one module of 60 F at 50V. The appearance of the module is shown

in Fig. 6 (a). A module has dimensions of 145 mm wide 275mm

deep 220mm high, with the weight being approximately 9 kg.

Thus, its energy density reaches 2.3 Wh/kg. In addition, its price

is about US $40.

EPT has thefollowing parameters: the inputvoltage equals 60V,

the output voltage equals 30 V, the working frequency of MFT as

shownin Fig.6 (b)is1kHzanditstransformationratiois3.Although

simulation and experiment systems are different, due to the work-

ingfrequencies of powerswitchdevices of thedcdc converterused

in simulation and experiment being different, one is 16 kHz, and

the other is 8 kHz. Also, the dc link voltages of the energy stor-

age system are different, one is 400 V, and the other is 50 V. Thus,

according to Eqs. (1) and (2), the inductance L and the dclinkcapac-

itance CDC of the energy storage system used in experiment can

be designed similar to that used in simulation. The working fre-

quencies of power switch devices, IGBT, used in EPT and the dcdc

converter are 1.2kHz and 8 kHz respectively. The load consists ofthree 20 resistors.

Fig. 7. Experimental results when voltage interruption occurs (a) load voltage and

(b) dc link voltage, supercapacitors voltage and supercapacitors current.


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Fig. 6 alsoshowsthe experimentalresults when EPTis in normal

operation with the proposed control scheme. Fig. 6(c) shows the

load voltage. The load voltage maintains a constant value with the

help of the inverters of the output stage. Fig. 6(d) and (e) shows

the input current and the dc voltage of the input stage. The input

current is kept sinusoidal and the dc voltage of the input stage is

kept constant at 160 V. Fig. 6(g) shows the dc voltage of the output

stage. Fig.6(f) and(h) shows theprimary voltage andthe secondary

voltage of MFT. All of these waveforms verify the validity of the

design.

Fig. 7(a) and (b) shows the experimental results when the input

voltage has a voltage interruption for about 5 s. For the analysis

of contrast, only the dc link of the output inverter of phase A is

connected to supercapacitors module through a dcdc converter.

Fig.7(a) shows that theloadvoltage of phase A maintains a constant

value because of using storage energy system, whereas the load

voltages of phase B and phase C cannot maintain a constant value.

Fig. 7(b) shows the dc link voltage, the supercapacitors voltage and

the supercapacitors current. All of these experimental results are

consistent with the simulation results. These waveforms also show

that the proposed system provides added ability to ride voltage

momentary interruptions.

Considering the limit of experimental condition,the paperillus-

trates a fairly small EPT with a supercapacitor. In fact, neglectingthe converters losses, the supercapacitor module adopted in this

paper can supply to the used 45 W load for about 1666 s and

the 75 kW load for about one second. For a 500kVA load used

in the simulation, the adopted modules are arrayed by four in

parallel and by four in series to constitute a block of 60 F at

200V, then the block can supply the 500kVA load for about

2.4s.

According to the energy density and the price of the adopted

module, the weight of the block is approximately 72 kg and the

price of the block is about US $320 respectively. Comparing the size

and the costof EPT itself, the sizeand the costof the block shouldbe

accepted. And there also is patented technology that allows create-

ing a supercapacitor with energy density 1.6MJ/kg [12]. In addition,

it must be noted that supercapacitors undergo intensive develop-ment andbecome more andmoresize andcostavailable.Therefore,

the utilization of thesupercapacitors storagesystemin EPThas good

prospects.

7. Conclusion

This paper describes the analysis results of a combined oper-

ation of the EPT with supercapacitors storage energy system.

The proposed system can compensate the voltage interruption

because of the use of supercapacitors storage energy system.

Compare to the DVR and the UPS, the proposed system has

no synchronization problem when compensating voltage inter-

ruption because EPT is connected in series with the grid. Both

simulation and experimental results have been reported to showthe performance of the proposed system and the validity of the

design.

Acknowledgements

This work was supported by Key Project of Ministry of Educa-

tion of China (107128) and National Natural Science Foundation of

China (50807020) and National Basic Research Program of China

(2009CB219702) and Program for New Century Excellent Talents in

University (NCET-04-0710).

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Haibo Liu was born in 1976. He is currently Working towards the Ph.D. degree in

collegeof electricand electronics engineering(CEEE)of HuazhongUniv. ofSci. & Tech

(HUST). His main interests are the applications of high power electronic technology

to power system.

Chengxiong Mao (M 1993) was born in Hubei, China, in 1964. He received his B.S.,

M.S. and Ph.D.degrees in electrical engineering, from Huazhong Univ. of Sci. & Tech.

(HUST), in 1984, 1987 and 1991 respectively. He was a visiting scholar in University

of Calgary, Canada, from Jan. 1989 to Jan. 1990 and in Queens University of Belfast

from Dec. 1994 to Dec. 1995 respectively. He was doing researches in Technische

Universitaet Berlin from April 1996 to April 1997 under the support of Humboldt

Foundation. Presently, he is a professor of HUST. His fields of interest are powersystem operation and control, the excitation control of synchronous generator and

the applications of high power electronic technology to power sytem

Jiming Lu was born in Jiangsu, China, in 1956. He received his B.S. degree from

Shanghai Jiaotong University, Shanghai, China, and received his M.S. degree from

HUST. His research is focused on the excitation control based on microcomputer.

Dan Wang was born in Jiangxi, China, in 1977. He received his B.S., M.S. and Ph. D.

degrees in Department of Electrical Engineer, from Huazhong University of Science

and Technology(HUST), Hubei,China, in 1999, 2002 and 2006 respectively.Presently,

heis doinghispostdoctoralresearchesin HUST. Hisinterestis theapplications ofhigh

power electronic technology to power system andexcitation control of synchronous

generator.

estudo capacitor

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