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Theory and application of distribution electronic power transformer
Electric Power Systems Research 77 (2007) 219–226
Theory and application of distribution electronic power transformer Dan Wang a,∗ , Chengxiong Mao a , Jiming Lu a , Shu Fan b , Fangzheng Peng c a
Department of Electrical Engineering, Huazhong University of Science & Technology, Wuhan 430074, China b Department of Electrical Engineering and Electronics, Osaka Sangyo University, Osaka 574-8530, Japan c Department of Electrical & Computer Engineering, Michigan State University, United States Received 13 June 2005; received in revised form 5 October 2005; accepted 28 February 2006 Available online 17 April 2006
Abstract Three-phase and 4-wire Distribution Electronic Power Transformer’s (DEPT’s) operation principle is analyzed in this paper. Based on the analysis, the control scheme is established. In this control scheme, the input stage is controlled as a three-phase balanced current source and makes the primary current sinusoidal and power factor easily adjusted. While the output stage is controlled as a three-phase balanced voltage source and keep the load voltage sinusoidal and nominal. In order to meet the requirement of the single phase or unbalanced loads, each phase is an independent voltage source. With the proposed control strategy, the characteristics of DEPT are studied by simulations. And further detailed simulations are carried out to validate the power quality control function of DEPT. The results show that DEPT has very good static and dynamic performances, and it can not only realize the functions of conventional power transformer, but also can prevent from voltage sags, swells, flickers and harmonics infecting the loads while avoid loads impacting the primary system. © 2006 Elsevier B.V. All rights reserved. Keywords: Distribution electronic power transformer; Distribution system; Input/output characteristic; Power quality
1. Introduction Transformers are widely used in electric power system to perform the primary functions, such as voltage transformation and isolation. Because of the bulky iron cores and heavy copper windings in the composition, transformers are one of the heaviest and most expensive parts in an electrical distribution system. The power throughput density of the transformer is inversely proportional to frequency, so increasing the frequency allows higher utilization of the steel magnetic core and reduction in transformer size [1]. In the recent decades, the power quality problem is becoming worse with increasing nonlinear loads in the distribution system. So many active power filters are introduced and the structure of the distribution system is becoming more complicated. Recently, a new type power transformer, which is based on power electronics and high frequency link, has been studied extensively [1–9]. ∗
Corresponding author at: 1037#, Luoyu Road, Hongshan district, Wuhan 430074, Hubei, China. Tel.: +86 27 87542669; fax: +86 27 87542669. E-mail addresses: [email protected] (D. Wang), [email protected] (C. Mao), [email protected] (J. Lu), [email protected] (S. Fan), [email protected] (F. Peng). 0378-7796/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsr.2006.02.012
Since Ref. [2] first introduced the concept of a high frequency link, many different configuration power electronics based transformers have been discussed. However, these discusses are emphasized on the configuration designs. In this paper, the power electronics based distribution transformer with the high frequency link, termed as Distribution Electronic Power Transformer (DEPT), is further explored. The focus of the paper is to study the load characteristics and application of the DEPT. This paper is organized as follows. A brief survey of the configuration and principle of DEPT is presented in Section 2. Section 3 introduces the control strategy of the DEPT. The characteristics of DEPT are analyzed in the following section. And the application as power quality controller is presented in Section 5. Finally, the relative simulations are carried out. 2. Principle and configuration of DEPT 2.1. Principle of DEPT Fig. 1 shows a basic diagram of the DEPT with a primary and secondary static converter and high frequency transformer. As can been seen, the power-frequency (50 or 60 Hz) input sin
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Fig. 1. Principle diagram of EPT.
wave voltage is first converted into a high frequency signal by the primary side converter, and then, magnetically coupled to the secondary. In the secondary side, the high frequency signal is unfolded into a power-frequency waveform. Here, the primary function of the high frequency transformer is to achieve isolation between the primary and secondary system. Because the transformer size is inversely proportional to the frequency, the high frequency transformer will be much smaller than the power-frequency transformer. There are two approaches to realize the DEPT. The one is without dc link [2–7], and the other is with dc link [8,9]. Compared with the first scheme, the second has many attractive characteristics. For instance, the voltage or current in either side of DEPT can be flexibly controlled through Pulse Width Modulation (PWM) technology. So, it is likely to become the mainstream of the future DEPT.
age. The isolation stage consists of a front-end H-bridge voltage source converter (VSC), a multi-windings high frequency transformer and three back-end H-bridge VSCs. The dc voltage from the input stage is fed to the front-end VSC, modulated into a high frequency square wave, coupled to the secondary of the high frequency transformer and rectified to form three dc link voltages. The output stage consists of three single-phase inverters with their output terminals YN-connection. They convert the three dc voltages into three phase balance ac sinusoidal voltages, including a neutral line. This type DEPT has the following features: (1) In terms of electrical performance, it and the conventional transformer are identical. Such as, they both can achieve the functions of voltage transformation and isolation. (2) It provides 3-phase and 4-wire electrical source for users. (3) It has good voltage regulation performance.
2.2. Configuration of DEPT
3. Proposed control strategy for DEPT
In a distribution system, in order to meet the requirement of the single-phase load or line-to-line load, the 3-phase and 4wire supply source is needed. So, DEPT must be designed as a 3-phase and 4-wire transformer. Fig. 2 shows a typical 3-phase and 4-wire DEPT topology. As can be seen, this is a three-part design that includes an input stage, an isolation stage and an output stage. The input stage is a 3-phase PWM rectifier, which is used to convert the primary power-frequency voltage into the dc volt-
As the basic conversion device in the distribution system, the DEPT should achieve the following demands. It can perform voltage transformation and isolation. For its secondary system, it can provide high quality electric power even much harmonic distortion exists in the primary system. For its primary system, it can’t inject the harmonics into the primary system. Because the control method directly influences the performance of the DEPT, we will design the control system to fulfill above demands.
Fig. 2. The typical circuit of 3-phase and 4-wire DEPT.
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Fig. 3. Mathematic model of the input stage. Fig. 4. Input stage control diagram.
3.1. Control scheme for the input stage The input stage of the DEPT is directly connected with the primary system. To prevent harmonics from being injected to the grid, the input current should be sinusoidal and in phase with the input voltage to achieve unity input power factor. To match the demand, the input stage is controlled as a controllable current source. The mathematic model of the input stage can be presented in stationary a–b–c reference frame or synchronous rotating d–q reference frame. But the control developed in rotating d–q reference frame, would get better performance [10]. The mathematic model of the input stage in synchronous rotating d–q reference frame is presented as (1) and shown in Fig. 3. di1d = ωL1 i1q − v1d + ed , dt di1q L1 = −ωL1 i1d − v1q + eq dt
L1
(1)
where L1 is the input inductance; and ω is the synchronous angular velocity of the grid voltage and ⎤ ⎡ ⎤ ⎡ i1a v1a v i1d 1d ⎥ ⎢ ⎥ ⎢ = T (ωt) ⎣ i1b ⎦ , = T (ωt) ⎣ v1b ⎦ , i1q v1q i1c v1c ⎡ ⎤ ea ed ⎢ ⎥ = T (ωt) ⎣ eb ⎦ eq ec and 2 T (ωt) = 3
sin ωt
sin(ωt − 120◦ )
sin(ωt + 120◦ )
cos ωt
cos(ωt − 120◦ )
cos(ωt + 120◦ )
Eq. (1) and Fig. 3 show that there is a cross coupling between the d axis and the q axis and that will influence the system dynamic performance. In order to solve this problem, the d–q voltage decouplers are designed and suitable feed-forward control components of the input source voltage are added in the control. To realize constant dc voltage and keep input current sinusoidal, the double control loops, a dc voltage outer loop
and an ac current inner loop, are adopted. The complete control diagram is shown in Fig. 4. As can be seen from Fig. 4, the reference for the active current i∗1d is derived from the dc voltage outer loop. The reference for the reactive current i∗1q is set independently or derived from the power factor loop. Here, i∗1q is set to zero to get unity power factor. The current error signals are input the current regulators and then form the modulation signals. If the d axis of the reference frame is aligned to the grid voltage, we obtain eq = 0. According to Fig. 4, the input stage is decoupled. Eq. (1) can be simplified as (2).
di1d KiI = KiP + (i∗1d − i1d ), L1 dt s
di1q KiI L1 (2) = KiP + i1q dt s where, KiP and KiI are control coefficients. 3.2. Control scheme for the isolation stage In the isolation stage, the dc voltage coming from input stage would be modulated to a high frequency square wave, coupled to the secondary and then reconverted into the low dc voltage. Here, the functions of the high frequency transformer are isolation and voltage levels transformation. The power electronic converter can change the voltage level directly, but, which would make semiconductor devices bear too high stress [11]. To simplify the control system design, an open loop PWM control is applied for the front-end H-bridge VSC. In the backend H-bridge VSCs, the diode rectifiers are adopted if only considering single-directional power flow. This control method provides an absorbing additional feature that the synchronization problem becomes easy to solve. So, the isolation stage can be seen as a proportional amplifier. The simplified model of the isolation stage is presented as (3): Vdc2 =
1 Vdc1 k
where k is the transformation ratio.
(3)
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Fig. 5. Output stage control strategy.
3.3. Control scheme for the output stage
Table 1 Principal parameters
The output stage of the DEPT is directly provided to the end users. Ideally, it should provide a clean and constant output voltage. As described above, there are three single-phase inverters in the output stage. Each inverter is an output phase independently, so the control of each inverter can be independent. To meet the requirement of the loads, each inverter is controlled as a single-phase sinusoidal voltage source. As we know, the inverter can be seen as a proportional amplifier when the switch frequency is much higher than the frequency of reference signal. So, the mathematic model of a phase of the output stage can be described as (4).
Parameter
Value
Capability Grid frequency Input inductance DC-link capacitance Output filter inductance Output filter capacitance Operating frequency of high frequency transformer
1600 kVA 50 Hz 10.2 mH 5 mF 0.8 mH 470 F 1000 Hz
Cf
dvo = iL − io , dt
Lf
diL = −vo + vi dt
(4)
In the distribution system, the loads are regarded as a passive system. So the constant ac voltage control based on instantaneous value feedback can be implemented [12]. The control scheme of one phase of the output stage is shown in Fig. 5. According to Fig. 5, there are two loops in the control scheme. Two signals are introduced as feedback: output voltage RMS value (Vo ) and output voltage instantaneous value vo . The output voltage RMS value is introduced for the regulation of the outer loop to achieve constant output voltage RMS value. The output voltage instantaneous value is introduced for the inner loop to obtain output voltage sinusoidal. The output of the outer loop is taken as the reference of the inner loop after being multiplied by the ideal sinusoidal signal. Theoretically, the output voltage RMS value would keep constant even when the loads change. At the same time, the output voltage also can track the sinusoidal waveform. Here, to avoid the phase angle error enlarging, a P controller, but not a PI controller, is adopted for the inner loop. The control schemes are the same for the three phases. The only differences are phase angles of the ideal sinusoidal signals. For instance, the phase angle of phase a is set as 0◦ , so phase b is +120◦ and phase c is −120◦ .
4.1. Steady-state characteristics The steady-state characteristics of the DEPT include no load characteristic and full load characteristic. Figs. 6 and 7 illustrate several important waveforms recorded from the steadystate simulations at nominal voltage and under no load or rated loads with a lagging load power factor 0.8, respectively. From the figures, it is clear that the output voltages are constant and sinusoidal. Furthermore, the input voltage and current are in phase and the current is sinusoidal, regardless of output power factor. 4.2. Dynamic state characteristics Performance of the DEPT was also simulated with switching on full loads at 0.85 s. The output voltages and currents are shown in Fig. 8. It is evident that the output voltages have very little fluctuation and maintains very short time.
4. Characteristics of DEPT The input/output characteristics are important for the DEPT. Next we will investigate the input/output characteristics by the simulations, which are based on Matlab/Simulink. In the simulation, the primary voltage of the DEPT is 10 kV and the output voltage is 400 V. The principle parameters are shown in Table 1.
Fig. 6. Output phase voltage waveforms under no load.
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Fig. 7. Simulation results under full load: (a) input voltage and current of A phase, where, the current is enlarged eight times, (b) output phase voltages, (c) output phase currents.
Fig. 8. Simulation results under switching full load: (a) output phase voltages, (b) output phase currents.
Fig. 9. Simulation results under voltage sag and swell: (a) input phase voltages, (b) output phase voltages.
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Fig. 10. Simulation results under voltage flicker: (a) input phase voltages, (b) output phase voltages.
Fig. 11. Simulation results under voltage harmonics: (a) input phase voltages, (b) output phase voltages.
5. Power quality control In comparison to the conventional power transformer, the DEPT offers an attractive additional feature, which is to be used as power quality controller. Fast voltage regulation, reactive power compensation, harmonic suppression and waveform control etc., are incorporated into the DEPT. These additional features are guaranteed when the control system is designed in Section 3.
In order to verify the effectiveness of the proposed control scheme to ensure the DEPT realizing power quality control, some simulations are carried out under the conditions of voltage sag, voltage swell, voltage flicker, voltage harmonics and nonlinear loads. Fig. 9 shows the simulation results under the conditions of voltage sags and swells arising in the primary grid. At 0.82 s, a 20% voltage sag appears in the 10 kV distribution system and lasts 0.02 s. And then, at the moment of 0.86 s, a 20% voltage
Fig. 12. Simulation results under single-phase rectifier load: (a) output phase voltage, (b) output phase current.
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swell arises and also lasts 0.02 s in the primary system. The results show that the DEPT can prevent from voltage sag and swell of primary grid infecting the output voltages. Furthermore, the voltage sag and swell are compensated without any time delay. The simulation results for a 10 Hz flicker in the primary system are shown in Fig. 10. It can be seen that the output voltages of the DEPT are not infected. Fig. 11 shows the simulation results when the 20% 5th and the 7th order harmonics appear in the primary grid voltages. Under this condition, the output voltages can also keep sinusoidal and constant. The simulation results that the DEPT operates under singlephase rectifier load are shown in Fig. 12. Where, the load of the single-phase rectifier is 5 resister. According to Fig. 12, although the output current is severely distorted, the output voltage is still sinusoidal. The THD of the output voltage is less than 2.49%. 6. Conclusion DEPT can perform the functions of conventional transformer and power quality controller. Based on the principle analysis of the 3-phase and 4-wire DEPT, a simple and available control scheme is proposed in the paper. In this control scheme, the input stage is controlled as a 3-phase controllable current source and the output stage is controlled as three independent singlephase voltage sources. By simulations, the characteristics of the DEPT are analyzed. The results show the distribution EPT has good dynamic and steady state performances under proposed control strategy. Based on the proposed control, further detailed simulations confirm that the DEPT can realize power quality control: while realizing voltage levels change, magnetic isolation and energy transmission, the DEPT can prevent from dynamic power quality problems infecting output voltage and avoid load shock impacting the primary system. Acknowledgements The authors gratefully acknowledge the support by Program for New Century Excellent Talents in University and by the Excellent Young Teachers Program of M0E, PR China. Appendix A
List of symbols a, b, c denotes three quantities Cf output filter capacitance d, q denotes rotating reference frame quantities ed , eq grid voltage i1a , i1b , i1c instantaneous ac supply currents i1d , i1q instantaneous ac supply currents output inductance current iL io load current k transformation ratio of high frequency transformer
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integral constant of PI controller KiI KiP proportional constant of PI controller input inductance L1 t time v1a , v1b , v1c three phase rectifier instantaneous input voltages v1d , v1q three phase rectifier ac input voltages vi instantaneous output voltage of inverter vo instantaneous output voltage Vdc1 dc bus voltage of input stage Vdc2 dc bus voltage of output stage ω angular frequency of grid References [1] M. Kang, P.N. Enjeti, I.J. Pitel, Analysis and design of electronic transformers for electric power distribution system, IEEE Trans. Power Electr. 14 (1999) 1133–1141. [2] W. McMurray, Power converter circuits having a high frequency link, US Patent 3,517,300, June 23, 1970. [3] G. Venkataramanan, B.K. Johnson, A. Sundaram, An ac–ac power converter for custom power applications, IEEE Trans. Power Delivery 11 (1996) 1666–1671. [4] J.L. Brooks, Solid State Transformer Concept Development, Naval Material Command, Civil Engineering Laboratory, Naval Construction Battalion Ctr., Port Hueneme, CA, 1980. [5] EPRI Report, Proof of the principle of the solid-state transformer: the AC/AC switch mode regulator, EPRI TR-105067, Research Project 800113, Final Report, August 1995. [6] K. Harada, F. Anan, K. Yamasaki, M. Jinno, Y. Kawata, T. Nakashima, K. Murata, H. Sakamoto, Intelligent transformer, in: Proceedings of the 1996 IEEE PESC Conference, 1996, pp. 1337– 1341. [7] M.D. Manjrekar, R. Kieferndorf, G. Venkataramanan, Power electronic transformers for utility applications, in: Proceedings of the 2000 IEEEIAS Annual Meeting, 2000, pp. 2496–2502. [8] E.R. Ronan, S.D. Sudhoff, S.F. Glover, D.L. Galloway, A power electronicbased distribution transformer, IEEE Trans. Power Delivery 17 (2002) 537–543. [9] M. Marchesoni, R. Novaro, S. Savio, AC locomotive conversion systems without heavy transformers: is it a practicable solution, in: Proceedings of the 2002 IEEE International Symposium on Industrial Electronics, 2002, pp. 1172–1177. [10] M.E. Fraser, C.D. Manning, B.M. Wells, Transformerless four-wire PWM rectifier and its application in ac–dc–ac converters, IEEE Proc. Electr. Power Appl. 142 (6) (1995) 410–416. [11] J. Kassakian, M. Schlecht, G. Verghese, Principles of Power Electronics, Addison-Wesley, 1991. [12] G.B. Zhang, Z. Xu, G.Z. Wang, Steady-state model and its nonlinear control for VSC-HVDC system, in: Proceedings of the CSEE, vol. 22, no. 1, 2002, pp. 17–22. Dan Wang was born in Jiangxi, China, in 1977. He received his B.S. and M.S. degrees in Department of Electrical Engineer, from Huazhong University of Science & Technology (HUST), Hubei, China, in 1999 and 2002 respectively, and is pursuing the Ph.D. degree in HUST. His interest is the excitation control of synchronous generator and applications of high power electronic technology to power system. Chengxiong Mao was born in Hubei, China, in 1964. He received his B.S., M.S. and Ph.D. degrees in electrical engineering, from HUST, in 1984, 1987 and 1991, respectively. Presently, he is a Professor of HUST. His fields of interest are power system operation and control, the excitation control of synchronous generator and applications of high power electronic technology to power system. 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.
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degree from HUST. His research is focused on the excitation control based on microcomputer. Shu Fan received his B.S., M.S. and PhD. degrees in Department of Electrical Engineer, from HUST, in 1995, 2000 and 2004, respectively. Presently he works in Osaka Sangyo University, Japan. His research interests are power system operation and intelligent control system. Fangzheng Peng received the B.S. degree from Wuhan University, Wuhan, China, in 1983, and the M.S. and Ph.D. degrees from Nagaoka University of Technology, Nagaoka, Japan, in 1987 and 1990, respectively, all in electrical engineering. From 1990 to 1992, he was a Research Scientist with Toyo Electric Manufacturing Company, Ltd., engaged in research and development of active
power filters, flexible ac transmission systems (FACTS) applications, and motor drives. From 1992 to 1994, he was a Research Assistant Professor with Tokyo Institute of Technology, where he initiated a multilevel inverter program for FACTS applications and a speed-sensorless vector control project. From 1994 to 2000, he was with Oak Ridge National Laboratory (ORNL) and, from 1994 to 1997, he was a Research Assistant Professor at the University of Tennessee, Knoxville. He was a Staff Member and Lead (Principal) Scientist of the Power Electronics and Electric Machinery Research Center at ORNL from 1997 to 2000. In 2000, he joined Michigan State University, East Lansing, as an Associate Professor in the Department of Electrical and Computer Engineering. He is also currently a specially invited Adjunct Professor at Zhejiang University, Hangzhou, China.
Theory and application of distribution electronic power transformer Dan Wang a,∗ , Chengxiong Mao a , Jiming Lu a , Shu Fan b , Fangzheng Peng c a
Department of Electrical Engineering, Huazhong University of Science & Technology, Wuhan 430074, China b Department of Electrical Engineering and Electronics, Osaka Sangyo University, Osaka 574-8530, Japan c Department of Electrical & Computer Engineering, Michigan State University, United States Received 13 June 2005; received in revised form 5 October 2005; accepted 28 February 2006 Available online 17 April 2006
Abstract Three-phase and 4-wire Distribution Electronic Power Transformer’s (DEPT’s) operation principle is analyzed in this paper. Based on the analysis, the control scheme is established. In this control scheme, the input stage is controlled as a three-phase balanced current source and makes the primary current sinusoidal and power factor easily adjusted. While the output stage is controlled as a three-phase balanced voltage source and keep the load voltage sinusoidal and nominal. In order to meet the requirement of the single phase or unbalanced loads, each phase is an independent voltage source. With the proposed control strategy, the characteristics of DEPT are studied by simulations. And further detailed simulations are carried out to validate the power quality control function of DEPT. The results show that DEPT has very good static and dynamic performances, and it can not only realize the functions of conventional power transformer, but also can prevent from voltage sags, swells, flickers and harmonics infecting the loads while avoid loads impacting the primary system. © 2006 Elsevier B.V. All rights reserved. Keywords: Distribution electronic power transformer; Distribution system; Input/output characteristic; Power quality
1. Introduction Transformers are widely used in electric power system to perform the primary functions, such as voltage transformation and isolation. Because of the bulky iron cores and heavy copper windings in the composition, transformers are one of the heaviest and most expensive parts in an electrical distribution system. The power throughput density of the transformer is inversely proportional to frequency, so increasing the frequency allows higher utilization of the steel magnetic core and reduction in transformer size [1]. In the recent decades, the power quality problem is becoming worse with increasing nonlinear loads in the distribution system. So many active power filters are introduced and the structure of the distribution system is becoming more complicated. Recently, a new type power transformer, which is based on power electronics and high frequency link, has been studied extensively [1–9]. ∗
Corresponding author at: 1037#, Luoyu Road, Hongshan district, Wuhan 430074, Hubei, China. Tel.: +86 27 87542669; fax: +86 27 87542669. E-mail addresses: [email protected] (D. Wang), [email protected] (C. Mao), [email protected] (J. Lu), [email protected] (S. Fan), [email protected] (F. Peng). 0378-7796/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsr.2006.02.012
Since Ref. [2] first introduced the concept of a high frequency link, many different configuration power electronics based transformers have been discussed. However, these discusses are emphasized on the configuration designs. In this paper, the power electronics based distribution transformer with the high frequency link, termed as Distribution Electronic Power Transformer (DEPT), is further explored. The focus of the paper is to study the load characteristics and application of the DEPT. This paper is organized as follows. A brief survey of the configuration and principle of DEPT is presented in Section 2. Section 3 introduces the control strategy of the DEPT. The characteristics of DEPT are analyzed in the following section. And the application as power quality controller is presented in Section 5. Finally, the relative simulations are carried out. 2. Principle and configuration of DEPT 2.1. Principle of DEPT Fig. 1 shows a basic diagram of the DEPT with a primary and secondary static converter and high frequency transformer. As can been seen, the power-frequency (50 or 60 Hz) input sin
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Fig. 1. Principle diagram of EPT.
wave voltage is first converted into a high frequency signal by the primary side converter, and then, magnetically coupled to the secondary. In the secondary side, the high frequency signal is unfolded into a power-frequency waveform. Here, the primary function of the high frequency transformer is to achieve isolation between the primary and secondary system. Because the transformer size is inversely proportional to the frequency, the high frequency transformer will be much smaller than the power-frequency transformer. There are two approaches to realize the DEPT. The one is without dc link [2–7], and the other is with dc link [8,9]. Compared with the first scheme, the second has many attractive characteristics. For instance, the voltage or current in either side of DEPT can be flexibly controlled through Pulse Width Modulation (PWM) technology. So, it is likely to become the mainstream of the future DEPT.
age. The isolation stage consists of a front-end H-bridge voltage source converter (VSC), a multi-windings high frequency transformer and three back-end H-bridge VSCs. The dc voltage from the input stage is fed to the front-end VSC, modulated into a high frequency square wave, coupled to the secondary of the high frequency transformer and rectified to form three dc link voltages. The output stage consists of three single-phase inverters with their output terminals YN-connection. They convert the three dc voltages into three phase balance ac sinusoidal voltages, including a neutral line. This type DEPT has the following features: (1) In terms of electrical performance, it and the conventional transformer are identical. Such as, they both can achieve the functions of voltage transformation and isolation. (2) It provides 3-phase and 4-wire electrical source for users. (3) It has good voltage regulation performance.
2.2. Configuration of DEPT
3. Proposed control strategy for DEPT
In a distribution system, in order to meet the requirement of the single-phase load or line-to-line load, the 3-phase and 4wire supply source is needed. So, DEPT must be designed as a 3-phase and 4-wire transformer. Fig. 2 shows a typical 3-phase and 4-wire DEPT topology. As can be seen, this is a three-part design that includes an input stage, an isolation stage and an output stage. The input stage is a 3-phase PWM rectifier, which is used to convert the primary power-frequency voltage into the dc volt-
As the basic conversion device in the distribution system, the DEPT should achieve the following demands. It can perform voltage transformation and isolation. For its secondary system, it can provide high quality electric power even much harmonic distortion exists in the primary system. For its primary system, it can’t inject the harmonics into the primary system. Because the control method directly influences the performance of the DEPT, we will design the control system to fulfill above demands.
Fig. 2. The typical circuit of 3-phase and 4-wire DEPT.
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Fig. 3. Mathematic model of the input stage. Fig. 4. Input stage control diagram.
3.1. Control scheme for the input stage The input stage of the DEPT is directly connected with the primary system. To prevent harmonics from being injected to the grid, the input current should be sinusoidal and in phase with the input voltage to achieve unity input power factor. To match the demand, the input stage is controlled as a controllable current source. The mathematic model of the input stage can be presented in stationary a–b–c reference frame or synchronous rotating d–q reference frame. But the control developed in rotating d–q reference frame, would get better performance [10]. The mathematic model of the input stage in synchronous rotating d–q reference frame is presented as (1) and shown in Fig. 3. di1d = ωL1 i1q − v1d + ed , dt di1q L1 = −ωL1 i1d − v1q + eq dt
L1
(1)
where L1 is the input inductance; and ω is the synchronous angular velocity of the grid voltage and ⎤ ⎡ ⎤ ⎡ i1a v1a v i1d 1d ⎥ ⎢ ⎥ ⎢ = T (ωt) ⎣ i1b ⎦ , = T (ωt) ⎣ v1b ⎦ , i1q v1q i1c v1c ⎡ ⎤ ea ed ⎢ ⎥ = T (ωt) ⎣ eb ⎦ eq ec and 2 T (ωt) = 3
sin ωt
sin(ωt − 120◦ )
sin(ωt + 120◦ )
cos ωt
cos(ωt − 120◦ )
cos(ωt + 120◦ )
Eq. (1) and Fig. 3 show that there is a cross coupling between the d axis and the q axis and that will influence the system dynamic performance. In order to solve this problem, the d–q voltage decouplers are designed and suitable feed-forward control components of the input source voltage are added in the control. To realize constant dc voltage and keep input current sinusoidal, the double control loops, a dc voltage outer loop
and an ac current inner loop, are adopted. The complete control diagram is shown in Fig. 4. As can be seen from Fig. 4, the reference for the active current i∗1d is derived from the dc voltage outer loop. The reference for the reactive current i∗1q is set independently or derived from the power factor loop. Here, i∗1q is set to zero to get unity power factor. The current error signals are input the current regulators and then form the modulation signals. If the d axis of the reference frame is aligned to the grid voltage, we obtain eq = 0. According to Fig. 4, the input stage is decoupled. Eq. (1) can be simplified as (2).
di1d KiI = KiP + (i∗1d − i1d ), L1 dt s
di1q KiI L1 (2) = KiP + i1q dt s where, KiP and KiI are control coefficients. 3.2. Control scheme for the isolation stage In the isolation stage, the dc voltage coming from input stage would be modulated to a high frequency square wave, coupled to the secondary and then reconverted into the low dc voltage. Here, the functions of the high frequency transformer are isolation and voltage levels transformation. The power electronic converter can change the voltage level directly, but, which would make semiconductor devices bear too high stress [11]. To simplify the control system design, an open loop PWM control is applied for the front-end H-bridge VSC. In the backend H-bridge VSCs, the diode rectifiers are adopted if only considering single-directional power flow. This control method provides an absorbing additional feature that the synchronization problem becomes easy to solve. So, the isolation stage can be seen as a proportional amplifier. The simplified model of the isolation stage is presented as (3): Vdc2 =
1 Vdc1 k
where k is the transformation ratio.
(3)
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Fig. 5. Output stage control strategy.
3.3. Control scheme for the output stage
Table 1 Principal parameters
The output stage of the DEPT is directly provided to the end users. Ideally, it should provide a clean and constant output voltage. As described above, there are three single-phase inverters in the output stage. Each inverter is an output phase independently, so the control of each inverter can be independent. To meet the requirement of the loads, each inverter is controlled as a single-phase sinusoidal voltage source. As we know, the inverter can be seen as a proportional amplifier when the switch frequency is much higher than the frequency of reference signal. So, the mathematic model of a phase of the output stage can be described as (4).
Parameter
Value
Capability Grid frequency Input inductance DC-link capacitance Output filter inductance Output filter capacitance Operating frequency of high frequency transformer
1600 kVA 50 Hz 10.2 mH 5 mF 0.8 mH 470 F 1000 Hz
Cf
dvo = iL − io , dt
Lf
diL = −vo + vi dt
(4)
In the distribution system, the loads are regarded as a passive system. So the constant ac voltage control based on instantaneous value feedback can be implemented [12]. The control scheme of one phase of the output stage is shown in Fig. 5. According to Fig. 5, there are two loops in the control scheme. Two signals are introduced as feedback: output voltage RMS value (Vo ) and output voltage instantaneous value vo . The output voltage RMS value is introduced for the regulation of the outer loop to achieve constant output voltage RMS value. The output voltage instantaneous value is introduced for the inner loop to obtain output voltage sinusoidal. The output of the outer loop is taken as the reference of the inner loop after being multiplied by the ideal sinusoidal signal. Theoretically, the output voltage RMS value would keep constant even when the loads change. At the same time, the output voltage also can track the sinusoidal waveform. Here, to avoid the phase angle error enlarging, a P controller, but not a PI controller, is adopted for the inner loop. The control schemes are the same for the three phases. The only differences are phase angles of the ideal sinusoidal signals. For instance, the phase angle of phase a is set as 0◦ , so phase b is +120◦ and phase c is −120◦ .
4.1. Steady-state characteristics The steady-state characteristics of the DEPT include no load characteristic and full load characteristic. Figs. 6 and 7 illustrate several important waveforms recorded from the steadystate simulations at nominal voltage and under no load or rated loads with a lagging load power factor 0.8, respectively. From the figures, it is clear that the output voltages are constant and sinusoidal. Furthermore, the input voltage and current are in phase and the current is sinusoidal, regardless of output power factor. 4.2. Dynamic state characteristics Performance of the DEPT was also simulated with switching on full loads at 0.85 s. The output voltages and currents are shown in Fig. 8. It is evident that the output voltages have very little fluctuation and maintains very short time.
4. Characteristics of DEPT The input/output characteristics are important for the DEPT. Next we will investigate the input/output characteristics by the simulations, which are based on Matlab/Simulink. In the simulation, the primary voltage of the DEPT is 10 kV and the output voltage is 400 V. The principle parameters are shown in Table 1.
Fig. 6. Output phase voltage waveforms under no load.
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Fig. 7. Simulation results under full load: (a) input voltage and current of A phase, where, the current is enlarged eight times, (b) output phase voltages, (c) output phase currents.
Fig. 8. Simulation results under switching full load: (a) output phase voltages, (b) output phase currents.
Fig. 9. Simulation results under voltage sag and swell: (a) input phase voltages, (b) output phase voltages.
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Fig. 10. Simulation results under voltage flicker: (a) input phase voltages, (b) output phase voltages.
Fig. 11. Simulation results under voltage harmonics: (a) input phase voltages, (b) output phase voltages.
5. Power quality control In comparison to the conventional power transformer, the DEPT offers an attractive additional feature, which is to be used as power quality controller. Fast voltage regulation, reactive power compensation, harmonic suppression and waveform control etc., are incorporated into the DEPT. These additional features are guaranteed when the control system is designed in Section 3.
In order to verify the effectiveness of the proposed control scheme to ensure the DEPT realizing power quality control, some simulations are carried out under the conditions of voltage sag, voltage swell, voltage flicker, voltage harmonics and nonlinear loads. Fig. 9 shows the simulation results under the conditions of voltage sags and swells arising in the primary grid. At 0.82 s, a 20% voltage sag appears in the 10 kV distribution system and lasts 0.02 s. And then, at the moment of 0.86 s, a 20% voltage
Fig. 12. Simulation results under single-phase rectifier load: (a) output phase voltage, (b) output phase current.
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swell arises and also lasts 0.02 s in the primary system. The results show that the DEPT can prevent from voltage sag and swell of primary grid infecting the output voltages. Furthermore, the voltage sag and swell are compensated without any time delay. The simulation results for a 10 Hz flicker in the primary system are shown in Fig. 10. It can be seen that the output voltages of the DEPT are not infected. Fig. 11 shows the simulation results when the 20% 5th and the 7th order harmonics appear in the primary grid voltages. Under this condition, the output voltages can also keep sinusoidal and constant. The simulation results that the DEPT operates under singlephase rectifier load are shown in Fig. 12. Where, the load of the single-phase rectifier is 5 resister. According to Fig. 12, although the output current is severely distorted, the output voltage is still sinusoidal. The THD of the output voltage is less than 2.49%. 6. Conclusion DEPT can perform the functions of conventional transformer and power quality controller. Based on the principle analysis of the 3-phase and 4-wire DEPT, a simple and available control scheme is proposed in the paper. In this control scheme, the input stage is controlled as a 3-phase controllable current source and the output stage is controlled as three independent singlephase voltage sources. By simulations, the characteristics of the DEPT are analyzed. The results show the distribution EPT has good dynamic and steady state performances under proposed control strategy. Based on the proposed control, further detailed simulations confirm that the DEPT can realize power quality control: while realizing voltage levels change, magnetic isolation and energy transmission, the DEPT can prevent from dynamic power quality problems infecting output voltage and avoid load shock impacting the primary system. Acknowledgements The authors gratefully acknowledge the support by Program for New Century Excellent Talents in University and by the Excellent Young Teachers Program of M0E, PR China. Appendix A
List of symbols a, b, c denotes three quantities Cf output filter capacitance d, q denotes rotating reference frame quantities ed , eq grid voltage i1a , i1b , i1c instantaneous ac supply currents i1d , i1q instantaneous ac supply currents output inductance current iL io load current k transformation ratio of high frequency transformer
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integral constant of PI controller KiI KiP proportional constant of PI controller input inductance L1 t time v1a , v1b , v1c three phase rectifier instantaneous input voltages v1d , v1q three phase rectifier ac input voltages vi instantaneous output voltage of inverter vo instantaneous output voltage Vdc1 dc bus voltage of input stage Vdc2 dc bus voltage of output stage ω angular frequency of grid References [1] M. Kang, P.N. Enjeti, I.J. Pitel, Analysis and design of electronic transformers for electric power distribution system, IEEE Trans. Power Electr. 14 (1999) 1133–1141. [2] W. McMurray, Power converter circuits having a high frequency link, US Patent 3,517,300, June 23, 1970. [3] G. Venkataramanan, B.K. Johnson, A. Sundaram, An ac–ac power converter for custom power applications, IEEE Trans. Power Delivery 11 (1996) 1666–1671. [4] J.L. Brooks, Solid State Transformer Concept Development, Naval Material Command, Civil Engineering Laboratory, Naval Construction Battalion Ctr., Port Hueneme, CA, 1980. [5] EPRI Report, Proof of the principle of the solid-state transformer: the AC/AC switch mode regulator, EPRI TR-105067, Research Project 800113, Final Report, August 1995. [6] K. Harada, F. Anan, K. Yamasaki, M. Jinno, Y. Kawata, T. Nakashima, K. Murata, H. Sakamoto, Intelligent transformer, in: Proceedings of the 1996 IEEE PESC Conference, 1996, pp. 1337– 1341. [7] M.D. Manjrekar, R. Kieferndorf, G. Venkataramanan, Power electronic transformers for utility applications, in: Proceedings of the 2000 IEEEIAS Annual Meeting, 2000, pp. 2496–2502. [8] E.R. Ronan, S.D. Sudhoff, S.F. Glover, D.L. Galloway, A power electronicbased distribution transformer, IEEE Trans. Power Delivery 17 (2002) 537–543. [9] M. Marchesoni, R. Novaro, S. Savio, AC locomotive conversion systems without heavy transformers: is it a practicable solution, in: Proceedings of the 2002 IEEE International Symposium on Industrial Electronics, 2002, pp. 1172–1177. [10] M.E. Fraser, C.D. Manning, B.M. Wells, Transformerless four-wire PWM rectifier and its application in ac–dc–ac converters, IEEE Proc. Electr. Power Appl. 142 (6) (1995) 410–416. [11] J. Kassakian, M. Schlecht, G. Verghese, Principles of Power Electronics, Addison-Wesley, 1991. [12] G.B. Zhang, Z. Xu, G.Z. Wang, Steady-state model and its nonlinear control for VSC-HVDC system, in: Proceedings of the CSEE, vol. 22, no. 1, 2002, pp. 17–22. Dan Wang was born in Jiangxi, China, in 1977. He received his B.S. and M.S. degrees in Department of Electrical Engineer, from Huazhong University of Science & Technology (HUST), Hubei, China, in 1999 and 2002 respectively, and is pursuing the Ph.D. degree in HUST. His interest is the excitation control of synchronous generator and applications of high power electronic technology to power system. Chengxiong Mao was born in Hubei, China, in 1964. He received his B.S., M.S. and Ph.D. degrees in electrical engineering, from HUST, in 1984, 1987 and 1991, respectively. Presently, he is a Professor of HUST. His fields of interest are power system operation and control, the excitation control of synchronous generator and applications of high power electronic technology to power system. 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.
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degree from HUST. His research is focused on the excitation control based on microcomputer. Shu Fan received his B.S., M.S. and PhD. degrees in Department of Electrical Engineer, from HUST, in 1995, 2000 and 2004, respectively. Presently he works in Osaka Sangyo University, Japan. His research interests are power system operation and intelligent control system. Fangzheng Peng received the B.S. degree from Wuhan University, Wuhan, China, in 1983, and the M.S. and Ph.D. degrees from Nagaoka University of Technology, Nagaoka, Japan, in 1987 and 1990, respectively, all in electrical engineering. From 1990 to 1992, he was a Research Scientist with Toyo Electric Manufacturing Company, Ltd., engaged in research and development of active
power filters, flexible ac transmission systems (FACTS) applications, and motor drives. From 1992 to 1994, he was a Research Assistant Professor with Tokyo Institute of Technology, where he initiated a multilevel inverter program for FACTS applications and a speed-sensorless vector control project. From 1994 to 2000, he was with Oak Ridge National Laboratory (ORNL) and, from 1994 to 1997, he was a Research Assistant Professor at the University of Tennessee, Knoxville. He was a Staff Member and Lead (Principal) Scientist of the Power Electronics and Electric Machinery Research Center at ORNL from 1997 to 2000. In 2000, he joined Michigan State University, East Lansing, as an Associate Professor in the Department of Electrical and Computer Engineering. He is also currently a specially invited Adjunct Professor at Zhejiang University, Hangzhou, China.