978-1-4799-5115-4/14/$31.00 ©2014 IEEE

A Single-stage Isolated Three-phase Bidirectional AC/DC Converter

Ling Gu, Ke Jin, Hong Zhu, and Chenghua Wang Jiangsu Key Laboratory of New Energy Generation and Power Conversion

Nanjing University of Aeronautics and Astronautics Nanjing, 210016, P.R.China

Email: guling@nuaa.edu.cn, jinke@nuaa.edu.cn

Abstract—In this paper, a single-stage isolated bidirectional AC/DC converter is proposed. The converter can achieve high power factor, sinusoidal input currents, adjustable output voltage and high frequency electrical isolation with single-stage structure. The converter is modulated by Space Vector Pulse Width Modulation (SVPWM) algorithm in dq rotary two-phase coordinates. Both PI control and deadbeat control method can be applied on this converter. To verify the theoretical analysis, the simulation was done by MATLAB/SIMULINK and a 3kW hardware based on the proposed converter was built in the lab with the digital controller TMS320F2812. The simulation and experimental results show the high power factor and the low harmonic distortion characteristics of the circuit.

Keywords—DC microgrid; bidirectional AC/DC converter; electrical isolation; SVPWM

I. INTRODUCTION Nowadays, the application of renewable energy, such as

photovoltaic power generation and wind power generation, has attracted more and more attention. The renewable energy is always connected to the grid to constitute the microgrid, which can effectively solve the power fluctuation of the system. Compared with traditional AC microgrid, DC microgrid has fewer converters and it presents better efficiency and reliability[1-4]. High voltage (380V) DC microgrid is preferable to the lower voltage (48V) DC microgrid as fewer coaxial cables and lower distribution loss [5-6]. Therefore, the research for 380V DC microgrid is popular with a great many experts around the world. Although there exist more and more DC powered electrical equipments, several equipments still need AC power supply. Therefore, AC-DC hybrid microgrid is an alternative choice which satisfies different power supply requirements.

A bidirectional AC/DC converter is needed to transfer the energy between the AC bus and DC bus. The converter plays the role of energy management of the whole system and is the only interface of AC and DC microgrid[7]. The basic requirement for the converter is converting three-phase 380V AC voltage to 380V DC voltage and electrical isolation. The traditional solution is three-phase PWM converter, which generally converts three-phase 380V AC voltage to 600V~800V DC voltage and doesn’t achieve electrical isolation [8-10]. Accordingly, it is necessary that a DC/DC

converter and a three-phase PWM converter constitute a two-stage structure or a power frequency transformer is added, which increases the volume and reduces the efficiency.

VIENNA rectifier II with a single-stage structure achieves electrical isolation and buck-boost type output by introducing a transformer [11]. However, its bidirectional switch consists of four diodes and one active switch, which brings large conduction loss in high power application. And it is a unidirectional converter and can’t transfer energy from dc side to ac side.

An isolated three-phase soft-switched buck rectifier and a three-phase buck rectifier with high-frequency isolation by single-stage are proposed by [12] and [13] to achieve high-frequency isolation and buck AC/DC conversion. The control strategy is simple with only three switches in primary side. However, the conduction loss of the converters is large and neither forward converter nor forward-flyback converter is suitable for high power application. Furthermore, it still can’t achieve bidirectional energy flow.

Quasi-single-stage isolated three-phase ZVZCS buck PWM rectifier is proposed by [14]. It consists of a three-phase buck bridge switching under zero current and a phase-shift-controlled full-bridge with ZVZCS, while no intermediate dc-link is involved. The converter achieves high-frequency electrical isolation. But its input current is discontinuous and has lower power factor.

This paper proposes a single-stage isolated three-phase bidirectional AC/DC converter with high power factor and adjustable output voltage, and then builds its control system based on the SVPWM algorithm. Both traditional PI control strategy and deadbeat control strategy are applied on the proposed converter. The operation principles are presented specifically. Finally, the converter was simulated by MATLAB/SIMULINK and a 3kW hardware controlled by TMS320F2812 was built in the lab to verify the theoretical analysis.

II. TOPOLOGY AND CONTROL STRATEGY Fig.1 shows the proposed single-stage isolated three-phase

bidirectional AC/DC converter as the interface converter of AC-DC hybrid microgrid. The converter can achieve high power factor, sinusoidal input currents, adjustable output

This work was supported by the National Natural Science Foundation ofChina under Grant 51377080, Fok Ying Tong Education Foundation underGrant 131056, Industry-Academia Cooperation Innovation Fund of JiangsuProvince-Prospective Joint Research Project(BY2012016), Natural ScienceFoundation for Distinguished Young Scholars of JiangsuProvince(BK20130036), Lite-On Power Electronics Technology ResearchFund, and Jiangsu Key Laboratory of New Energy Generation and PowerConversion Open Research Fund.

voltage and high frequency electrical isolation with single-stage structure. Fig.2 and Fig.3 shows the PI control system and deadbeat control system of the converter separately. With the dual-loop control system of current loop inside and voltage loop outside, the current controller is designed in dq rotary two-phase coordinates to maintain the adjustable dc voltage and ac current with unity power factor.

Fig. 1. A single-stage isolated three-phase bidirectional AC/DC converter

A. PI Control System The model of the converter under dq rotary two-

phase coordinates is as follows:

d d d

q q q

e i vLp R Le i vL Lp R

ωω

⎡ ⎤ ⎡ ⎤ ⎡ ⎤+ −⎡ ⎤= +⎢ ⎥ ⎢ ⎥ ⎢ ⎥⎢ ⎥+⎣ ⎦⎣ ⎦ ⎣ ⎦ ⎣ ⎦ (1)

From (1), it is seen that id and iq are coupled and can’t be controlled independently. So the feed-forward decoupling method is introduced which is shown by Fig.2 to achieve the separate control of id and iq. The voltage reference vd and vq can be calculated as follows:

_( )( )iid pi d ref d q d

Kv K i i Li e

sω= − + − + + (2)

_( )( )iiq pi q ref q d q

Kv K i i Li e

sω= − + − − + (3)

SVPW

M

Fig. 2. PI control system

B. Deadbeat Control System The deadbeat control method can also be applied to achieve

sinusoidal input current. Compared with PI control, it improves the accuracy and simplifies the control algorithm. The voltage reference vd and vq can be calculated as follows:

( ) [ ( 1) ( )] ( ) ( )dd d d qs

Lv k i k i k e k Li kT

ω= − + − + + (4)

( ) [ ( 1) ( )] ( ) ( )qq q q ds

Lv k i k i k e k Li kT

ω= − + − + − (5)

SVPW

M

Fig. 3. Deadbeat control system

C. Conducting Components As the different current direction under rectifier mode and

inverter mode, the conducting switches are also different, which is shown by Table.1.

TABLE I. THE CONDUCTING SWITCHES UNDER RECTIFIER MODE AND INVERTER MODE

Rectifier mode Inverter mode

Active switches

Qi2、Qi3、Qp1、Qp2

(i=a, b, c)

Qi1、Qi2、Qi3、Qi4、

Qs1、Qs2、Qs3、Qs4

Diodes Di1、Di2、Di3、Di4、

Ds1、Ds2、Ds3、Ds4 Dp1、Dp2、Di2、Di3

III. OPERATION PRINCIPLES All the current and voltage’s reference direction is shown

by Fig.1. Take ia>0, ib<0, ic<0 under rectifier mode and ia<0, ib>0, ic>0 under inverter mode based on purely sinusoidal mains current shapes (Fig.4) for example to analyze the operation principles.

ia ib ic

t0 / 3π 2 / 3π

ia>0,ib<0,ic<0

ia>0,ib>0,ic<0ia>0,ib<0,ic>0

ia<0,ib>0,ic<0

ia<0,ib>0,ic>0

ia<0,ib<0,ic>0

π 4 / 3π 5 / 3π 2π

Fig. 4. Three-phase ac current

A. Sector Partition As the voltage vectors formed by the switch state are also

influenced by the direction of current flowing in the transformer. So the sector partition should take both the basic vectors and current direction of ia, ib and ic into considerations. Therefore, set six intermediate variables as follows:

1ref sv v β= (6)

2ref sv v α= (7)

31 ( 3 )2ref s sv v vα β= − (8)

41 ( 3 )2ref s sv v vα β= − (9)

51 ( 3 )2ref s sv v vα β= − − (10)

61 ( 3 )2ref s sv v vα β= − − (11)

Where vα, vβ is the value of three-phase voltage in stationary two-phase coordinates.

If vref1>1, A=1; otherwise, A=0; If vref2>1, B=1; otherwise, B=0; If vref3>1, C=1; otherwise, C=0; If vref4>1, D=1; otherwise, D=0; If vref5>1, E=1; otherwise, E=0; If vref6>1, F=1; otherwise, F=0. The sector number is defined by

2 4 6 8 10N A B C D E F= + + + + + and the sector partition of the whole power frequency cycle is shown by Fig.5.The full lines divide the cycle into 6 sectors according to the basic voltage vectors as the traditional partition [15]. The dotted lines further divide it into 12 sectors according to the three-phase ac current direction.

23 dcnV

23 dcnVα

2 23 dcnVα

2-3 dcnV

2-3 dcnVα

2 2-3 dcnVα

Fig. 5. Sector partition

B. Vector Synthesis The switching states are described by 1{ }( )sign T

a b cj s s s=

[11]. When Qi1 is on, Si=0+; When Qi4 is on, Si=0-; When Qi2

or Qi3 is on, Si=1(i=a, b, c); otherwise, Si=0.The sign{T1}

represents the current direction of the transformer. Thus, when

ia>0, ib<0, ic<0 under rectifier mode, the conduction state in

Fig.6(a) is described as (011)j −= . And when ia<0, ib>0, ic>0

under rectifier mode, the conduction state in Fig.6(b) is

described as (0 11)j + += .To achieve the voltage balance of

transformer and minimize the switching loss, the switching

sequence of sector 13 under rectifier mode is as follows: 0

/2

0

(100) (110) (111) (011) |

(011) (111) (110) (100) |s

s

T

T

+ + −

− + +

→ → →

→ → → (12)

The switching sequence of sector 13 under inverter mode is as follows:

0/2

0

(10 0 ) (110 ) (111) (0 11) |

(0 11) (111) (110 ) (10 0 ) |s

s

T

T

− − − − − + +

+ + − − − − −

→ → →

→ → → (13)

(a) Rectifier mode

(b) Inverter mode

Fig. 6. Definition of switching states

C. Driving Signals Through analysis, the driving signals of Qp1, Qp2(rectifier

mode) and Qs1~4 (inverter mode) are related to the driving signals of Qi1~4(i=a, b, c), which is shown as Table.2.

TABLE II. DRIVING SIGNALS OF QP1, QP2 AND QS1~4

Sector Qp1 Qp2 Qs1,4 Qs2,3

13 Qc2,3 Qa2,3 Qc1,4 Qa1,4

7 Qc2,3 Qa2,3 Qc1,4 Qa1,4

3 Qc2,3 Qb2,3 Qc1,4 Qb1,4

1 Qc2,3 Qb2,3 Qc1,4 Qb1,4

9 Qa2,3 Qb2,3 Qa1,4 Qb1,4

19 Qa2,3 Qb2,3 Qa1,4 Qb1,4

18 Qa2,3 Qc2,3 Qa1,4 Qc1,4

24 Qa2,3 Qc2,3 Qa1,4 Qc1,4

28 Qb2,3 Qc2,3 Qb1,4 Qc1,4

30 Qb2,3 Qc2,3 Qb1,4 Qc1,4

22 Qb2,3 Qa2,3 Qb1,4 Qa1,4

12 Qb2,3 Qa2,3 Qb1,4 Qa1,4

IV. SIMULATION AND EXPERIMENTAL RESULTS The proposed converter based on the PI control system and

deadbeat control system was simulated by MATLAB/SIMULINK. Fig7 shows three-phase ac voltage and current of deadbeat control system. Fig.8 and Fig.9 shows the output voltage under rectifier mode and harmonic analysis of ac current. It is seen that the converter achieves high power factor.

Fig. 7. Three-phase voltage and current with deadbeat control system

0 0.1 0.2 0.3 0.4 0.5Time(s)

The

out

put v

olta

ge (V

)

0

200

400

600

0.6 0.7 0.8

Fig. 8. The output voltage under rectifier mode

A 3kW hardware with the digital controller TMS320F2812 was built in the lab to verify the theoretical analysis. Fig.10 and Fig.11 show the waveforms of ia, ib, ic under rectifier mode and the waveforms of va, vb, vc, ia under inverter mode. The experimental results verified the theoretical analysis.

V. CONCLUSION This paper proposes a single-stage isolated three-phase

bidirectional AC/DC converter. The converter achieves unity power factor and high frequency electric isolation. Its control strategy and operation principle are presented. A 3kW hardware with the digital controller TMS320F2812 was built in the lab to verify the theoretical analysis.

Fig. 9. The harmonic analysis of three-phase current

Fig. 10. Three-phase current under rectifier mode

Fig. 11. Three-phase voltage and current under inverter mode

REFERENCES [1] D. Salomonsson, L. Soder, A. Sannino, “An adaptive control system for

a DC microgrid for data centers,” IEEE Trans. Ind. Appl., vol.44, no.6, pp.1910-1917, Sept. 2008.

[2] A. Sanino, G. Postiglione, M. H. J. Bollen. “Feasibility of a DC network for commercial facilities,” IEEE Trans. Ind. Appl., vol.39, no.5, pp.1499-1507, Sept. 2003.

[3] A. Kwasinski, “Quantitative evaluation of DC microgrids availability: Effects of system architecture and converter topology design choices,” IEEE Trans. Power Electron., vol. 26, no. 3, pp. 835-851, Mar. 2011.

[4] C. Cho, J. Jeon, J. Kim, S. Kwon, K. Park, and S. Kim, “Active synchronizing control of a microgrid,” IEEE Trans. Power Electron., vol.20, no. 12, pp. 3707-3719, Dec. 2011.

[5] A. Pratt, P. Kumar, and T. V. Aldridge, “Evaluation of 400V DC distribution in telco and data centers to improve energy efficiency,” in Proc. 29th Int. Telecommun. Energy Conf., 2007, pp. 32–39.

[6] A. Fukui, T. Takeda, K. Hirose, and M. Yamasaki, “HVDC power distribution systems for telecom sites and data centers,” IEEE Int. Power Electron. Conf., 2010, pp. 874–880.

[7] D. Zhi, L. Xu, and B. W. Williams, “Improved direct power control of grid-connected DC/AC converters,” IEEE Trans. Power Electron., vol. 24, no. 5, pp. 1280–1292, May 2009.

[8] Zhonggang Yin, Jing Liu, and Yanru Zhong, “Study and control of three-phase PWM rectifier based on dual single-input single-output model,” IEEE Trans. Ind. Informatics, vol.9, no.2, pp. 1064-1073, May 2013.

[9] B. Yin, R. Oruganti, S. Kumar, and A. Bhat, “A simple single-input simgle-output (SISO) model for three-phase PWM rectifiers,” IEEE Trans.Power Electron., vol. 24, no. 3, pp. 620–631, Mar. 2009.

[10] Yi Yang, Poh Chiang Loh, Peng Wang, and Fook Hoong Choo, “One-cycle-controlled three-phase PWM rectifiers with improved regulation under unbalanced and distorted input-voltage conditions,” IEEE Trans.Power Electron., vol. 25, no. 11, pp. 2786–2796, Nov. 2010.

[11] J. W. Kolar, Uwe Drofenik, and Franz C. Zach, “VINNEA Rectifier II- a novel single-stage high-frequency isolated three-phase PWM rectifier

system,” IEEE Trans. Ind. Electron., vol. 46, no. 4, pp. 23-33, Aug. 1999.

[12] V. Vlatkovic, D. Borojevic, and F. C. Lee, “A zero-voltage switched, three-phase isolated PWM buck rectifier.” IEEE Trans. Power Electron., vol.10, no.2, pp.148-157, Mar. 1995.

[13] D. S. Greff, R. da Silva, S. A. Mussa, and et. al., “A three-phase buck rectifier with high-frequency isolation by single-stage,” in Proc. IEEE PESC 2008, pp.1129-1133.

[14] K.Wang, F. C. Lee, and D. Boroyevich, “A new quasi-single-stage isolated three-phase ZVZCS buck PWM rectifier,” in IEEE PESC, 1996, pp.449-455.

[15] M. Zhang, B. Li, L. Hang, L. M. Tolbert, and Z. Lu, “Performance study for high power density three-phase Vienna PFC rectifier by using SVPWM control method,” in Proc. APEC 2012, pp. 1187–1191.