1

Abstract--The paper’s aim is to analyze a power electronic

transformer (PET) able to deal with different sources at its input port, being able to distribute, according to the system’s strategies, where the power comes from. Thus, the multi-fed PET (MF-PET), is a power electronic transformer with two or more AC/DC rectifiers in its input port, and these may be connected to different feeders. High service quality to consumers is accomplished. Simulating results for the three-phase and single-phase topology are shown.

Index Terms—Flexible A.C. Transmission Systems, Power electronics, Power electronics transformer, Smart grid.

I. INTRODUCTION OWADAYS, there exists a great concern about the

electrical networks flexibility related with technological development and consumers’ demand. All over the world, the electrical infrastructure and supply sources are becoming less effective, which could have consequences.

Recently, novel approaches to the improvement of the grid’s operation are being researched, taking into account the consumers' demands such as: (i) service continuity, (ii) costs’ reduction, (iii) clearer communication between consumers and utilities. Likewise, the connection of small generating sources must be allowed, minimizing their environmental impact. Alongside other factors, consumer demand has given rise to the smart grid concept [1].

Despite smart grids have not a definitive definition, an intelligent network should satisfy the following minimal requirements: (i) efficient operation; (ii) power quality; (iii) resources’ optimization; (iv) open markets; (v) active relationship with the final consumer; and (vi) self-healing. Evolving from a conventional grid to a smart one needs the satisfaction of the above-mentioned requirements as well as some technological and economical developments. Within those developments, three are quite relevant: (i) power systems, (ii) digital communications, (iii) decentralization of processes.

Although there are transcendental advances related with networks planning, green power, power quality, measurements, digital transmission, more efforts are needed.

Johnny Posada C. is with Universidad Autonoma de Occidente, Cali, COLOMBIA (email: johnnyposada@gmail.com).

Juan M. Ramirez is with Centro de Investigacion y de Estudios Avanzados del I.P.N. – Unidad Guadalajara, MEXICO (e-mail: jramirez@gdl.cinvestav.mx).

Rosa E. Correa is with Universidad Nacional de Colombia – Sede Medellin, COLOMBIA (elvira.correa@gmail.com).

A gradual substitution of electromechanical elements is required, embedding novel power electronics-based devices in order to increase controllability [2]. With the appropriate rating, electronic power transformers (PET) may aid in a reliability increase [3-5]. One important issue about smart grids is its ability to use topology’s configuration so that auto-restoration, auto-monitoring, and power management can be allowed. These characteristics give to the network the capability of rapid responses under sudden load increases, balancing power supply through itself in order to offer a high service quality to the consumers, and a significant reduction in the non-supplied time. To attain this quality of service, power electronic-based devices, such as the Static Transfer Switch, Power Electronic Transformer (PET), Dynamic Voltage Restorer (DVR), can be quite useful. The development of new materials is a very promising pathway for reaching the above-mentioned objectives.

There is a variety of power electronics-based devices. For instance: (i) High Voltage DC (HVDC) systems and Flexible AC Transmission Systems (FACTS) that enable long distance transport and integration of renewable energy sources; (ii) different power electronic interfaces and power electronic supporting devices (STATCOM, Choppers) that provide efficient connection of renewable energy sources; (iii) Series capacitors, Unified Power Flow Controller (UPFC) and other FACTS devices that provide greater control over power flows on the grid; (iv) HVDC, FACTS, and active filters together with integrated communication and control that ensure greater flexibility, reliability and quality; (v) power electronic interfaces and integrated communication and control that support system operations by controlling renewable energy sources and loads [6].

This paper deals with a novel device, which works as a power transformer and have some other characteristics that make it quite attractive for the future grids, Fig. 1.

II. SOLID STATE TRANSFORMER In this paper, the MF-PET’s main idea is demonstrated on a

distribution system operating as a Power Electronics Building Block (PEBB), and capable to handle the electrical power in the grid. Fig. 2 shows that the system’s intelligence handles the power flow, and reconfigures the grid according to the power requirements.

This paper demonstrates that the PET owns properties that may help to provide higher grid's operation flexibility. Thus, it

Modeling and simulation of a solid state transformer for distribution systems

Johnny Posada C., Juan M. Ramirez, Member, IEEE, and Rosa E. Correa

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978-1-4673-2729-9/12/$31.00 ©2012 IEEE

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Fig. 4 Single-phase model

4

and are the feeder’s voltages and currents; are the reference currents in the hysteresis controller; are the reference currents in a regenerative mode of operation; ℎ allows select the proper rectifier’s controller; and allow select the power handle for each rectifier, their value lies on the interval [0, 1]; ℎ is the DC reference voltage.

B. DC/DC converter The DC/DC converter is a switching-mode source, which proportionates galvanic isolation. In this paper, this stage is simulated by the steady state gain a. Eqns. (3)-(4) show voltage and current in the inverter side, = ℎ

(3) ( ) = ℎ ( ) (4)

C. The DC/AC voltage inverter The MF-PET’s output stage is the DC/AC voltage inverter.

Its purpose is the constancy of the output voltage, no matter the load and the input voltage change. In this stage two cascade control loops are implemented. The inner loop is a current controller in the d-q reference frame; the outer loop is a voltage controller in the d-q reference frame, Fig. 3. The inner loop must be fast enough in order to track quickly the reference current, and stabilize the output voltage in few milliseconds.

Eqns. (5)-(6) describe the DC/AC converter model in the d-

q reference frame. , , and , are the output filter parameters; is the output voltage and current angular frequency; ( ) is the input current into the inverter’s filter; ( ) is the converter’s output voltage; ( ) is the load current, which is handled as a disturbance within the control algorithm; ( ) is the modulation index and it is the input control; is the desired output voltage, Fig. 3.

D. MF-PET model With the aim to obtain a simplified model able to represent

the MF-PET, a model based on current and voltage sources is built, Fig. 4. This model is used to simulate the MF-PET embedded into a distribution system. Several assumptions are made. Respect to the input rectifier, the hysteresis controller is modeled as a controllable current source in series with the RL input circuit, so that the input current is forced to track a specific reference, depending on the active and reactive power conditions. Eqn. (7) is used to calculate the DC voltage ℎ . The DC/DC converter is represented as a voltage controlled source, where , , , , are the peak value of the voltage and current in the two input rectifiers, respectively. The MF-PET’s DC/AC converter is modeled in a d-q reference frame. Two control loops are implemented, one for the output current, and the other one for the output voltage.

( ) = − ( ) + 2 ( )− ( ) − ( )

(5) ( ) = ( ) − ( )− ( )

(6) ℎ ( ) = ( ) + ( ) − ( ) ℎ ( ) = 2 ℎ ( ) + 2 ℎ ( ) − ( ) (7)

III. SIMULATIONS In this research the usefulness of the MF-PET is enhanced

through its use within a distribution electrical network, therefore the study of the MF-PET’s behavior embedded into a distribution system is shown.

A. Embedding a MF-PET into a grid A relevant application of the MF-PET is related to its ability

of power flow handling. This characteristic can be exhibited in a grid with two or more feeders, where the MF-PETs can carry out power transfer among feeders. Fig. 5 depicts the single-phase grid studied in this paper (7.6 kV). The high-voltage supply side at bus 1 and bus 2 is 19 kV, and the MF-PET-0 steps down this voltage to 7.6 kV. Its output is connected to bus 3 and transmitted through a 7.6kV line to the MF-PET-1 - MF-PET-3. An additional load at bus 6 is fed by the MF-PET-0. Notice the power handled by each MF-PET in Fig. 5. MF-PET-1 - MF-PET-3 are designed for 220 Vrms as output voltage. The transmission lines are simulated based on a π model. Lines connecting MF-PET-0 to feeders VA and VB are 5 km long, while lines connecting bus 3 to MF-PETs are 2 km long.

The simulation consists on the following events:

1. Based on a steady state, the MF-PET-1 to MF-PET-3 delivers 100% to their respective load by the rectifier connected at bus 3, in others words; the power in demand by each load is delivered by the MF-PET-0. The independent feeders (VA1, VA2 and VA3), not delivered power to the loads. Each rectifier in the MF-PET-0 delivers 50% to their respective load connected at bus 3. Notice that bus 6 has not an independent feeder.

2. At t = 0.3 s a power reference step is simulated in MF-PET-0, so that the power stemming from the feeder A increases to 75% of the power delivered by

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IV. CON

ed configurar transformer flexibility, incr flow modifnchrophasor an

wer flow’s fullerefore, some, which recei

order to attawork, the anelopment of itsa wide integratiower handling a greater robucharacteristicsrt grid concept

and currents.

ive power in thd voltage and c

voltage and curren

NCLUSIONS ation exhibits

functionality creasing servicfications. Thend transfer swl control, both feeders canive energy byain a dynami

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ustness of the ss of PET ant could be a rea

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PET egy. iers ork, ning ther

[1] F. L“Smon S

[2] A. AtechDist

[3] E. RElecPow

[4] M. SGhaTran2010

[5] A. MUnivInter2009

[6] M. SrealiSust

JohnnCorporaciM. Sc. in He is currprimary ar

Juan from UniEngineerinfrom UAEngineerinprofessor.systems.

Rosa Universidand Ph. D1992 she automatic

Li, W. Qiao, H. Smart Transmission Smart Grid, Vol. 1A. Edris, L. Gyu

hnology developmtribution Conferen

R. Roman, S. D. Suctronic-Based Diswer Delivery, Vol. Sabahi, A. Y. Go

arehpetian, “Flexnsactions on Powe0. Maitra, A. Sundaversal Transformernational Confere9, Paper 1032. Sooriyabandara, Jisation”, in Protainable Energy Te

ny Posada C. oión Universitaria AElectronics from

rently working towrea of interest is pM. Ramirez (M

iversidad de Guang from UNAM-M

ANL-México in ng of CINVESTA. His areas of inte

E. Correa obtdad Pontificia BoliD. from CINVEST

is with Universidac control and powe

V. REFERENC

Sun, H. Wan, J. WGrid: Vision and , No. 2, Sep 2010.ugyi, "Power ele

ments," in Proc. 20nce and Expositionudhoff, S. F. Glovestribution Transfo17, April 2002, Pa

oharrizi, S. H. Hoxible Power Eler Electronics, Vo

aram, M. Gandhier Design and Apence on Electrical

J. Ekanayake, “Smoc. 2010 IEEE echnologies (ICSE

VI. BIOGRAPH

obtained his BS Autónoma de OccUniversidad del V

wards his Ph. D. inower electronics. ’1986) obtained hanajuato, México México in 1987; 1992. He joinedAV in 1999, wh

erest are in operati

tained her BS iivariana in 1984,

TAV-Mexico in 19ad Nacional de Coer systems.

CES Wang, Y. Xia, Z.Framework”, IEE., Pages: 168 – 177ectronics-based T&003 IEEE PES Tran, vol. 3, pp. 1131-er and D. L. Galloormer", IEEE Trages: 537 - 543. osseini, M.B.B. Slectronic Transfool. 25, Issue 8, pp

, S. Bird, S. Dopplications”, in Prl Distribution, Pra

mart Grid - TechnInternational C

ET).

HIES in Industrial El

cidente, Valle, ColValle, Valle, Colon CINVESTAV-G

his BS in Electricin 1984; M. Sc

Ph. D. in Electricd the departmenthere he is currention and control of

in Electrical EngMedellin, Colomb

999 and 2004, respolombia. Her areas

6

Xu, P. Zhang, EE Transactions 7. &D controllers ansmission and - 1137. oway, "A Power ransactions on

Sharifian, G. B. former”, IEEE p. 2159 – 2169,

oss, “Intelligent roc. 2009 20th

ague, 8-11 June

nologies for its Conference on

ectronics from lombia in 2001; ombia, in 2007.

Guadalajara. His

cal Engineering c. in Electrical cal Engineering t of Electrical tly a full time f electric power

gineering from bia; her M. Sc. pectively. Since s of interest are

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