IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 14, NO. 6, NOVEMBER 1999 1133 Analysis and Design of Electronic Transformers for Electric Power Distribution System Moonshik Kang, Student Member, IEEE, Prasad N. Enjeti, Senior Member, IEEE, and Ira J. Pitel, Fellow, IEEE Abstract— A transformer performs many functions such as voltage transformation, isolation, and noise decoupling, and it is an indispensable component in electric power distribution systems. However, at low frequencies (60/50 Hz), it is a bulky and expensive component. In this paper, the concept of electronic transformers is further extended and explored for its suitability in power distribution systems. It should be noted that from the input/output behavior, the electronic transformer and the con- ventional transformer are identical. Possible topologies employing static converters connected on the primary and secondary sides are explored to realize high-frequency operation of the magnetic core. To assist the commutation process, a four-step switching has been developed which does not require the use of snubbers. Reduced size, losses, higher efficiency, and better voltage regu- lation are some of the advantages of this approach. A 10-kVA design example along with experiment results are discussed. It is shown that a transformer designed with a conventional grain- oriented silicon–steel core can process three times the power at 1-kHz operating frequency as compared to 60 Hz. The proposed method is scalable in voltage/current with the currently available insulated gate bipolar transistor (IGBT) devices connected in series without special snubbers. Index Terms— Electronic transformers, four-step switching, high-frequency switching. I. INTRODUCTION T RANSFORMS are used widely in electric power distribu- tion/conversion systems to perform many functions, such as isolation, voltage transformation, noise decoupling, etc. Transformers are one of the heaviest and most expensive parts in an electrical distribution system. The size of transformer is a function of the saturation flux density of the core material and maximum allowable core and winding temperature rise. Saturation flux density is inversely proportional to frequency and increasing the frequency allows higher utilization of the steel magnetic core and reduction in transformer size. The subject of a high-frequency link has been studied extensively in power electronic systems [1]–[5]. Reference [1] first in- troduced the concept of a high-frequency ac/ac link, termed as electronic transformer, and a recent paper [2], discussed the implementation of the concept on a 200-V 3-kVA unit operating at 15 kHz. The overall efficiency was reported to be about 80% and is viewed as a major disadvantage. In this paper, the concept of a high-frequency ac link is further explored. The focus of this paper is to realize an elec- Manuscript received August 7, 1998; revised April 26, 1999. Recommended by Associate Editor, A. Kawamura. The authors are with the Electrical Engineering Department, Texas A&M University, College Station, TX 77843-3128 USA. Publisher Item Identifier S 0885-8993(99)08893-6. tronic transformer as a power delivery component in electrical distribution systems. The primary purpose is to reduce the size, weight, and volume and to improve efficiency dramatically. In order to achieve this, a standard grain-oriented silicon–steel core transformer is used and an operating frequency of 600 Hz to 1.2 kHz is chosen. The proposed system consists of static converters on both the primary and secondary side exciting the transformer synchronously. The low-frequency input sine wave voltage (60 Hz) is first inverted at 600 Hz to 1.2 kHz by the primary-side static power converter, magnetically coupled to the secondary and then unfolded into a low-frequency (60 Hz) waveform by the secondary-side static converter. This operation requires both primary- and secondary-side static converters to be operated synchronously. In terms of electrical performance, the proposed electronic transformer and the conventional transformer are identical. The proposed electronic transformer has the following advantages: • identical input/output characteristic as a conventional transformer; • smaller size and weight (nearly 1/3 size reduction with standard steel core transformer); • efficiency compatible with conventional transformer; • electronic current limiting feature; • does not require the use of input/output filters; • good voltage regulation; • snubberless operation due to four-step switching strategy; • no additional harmonics are generated due to switching. Fig. 1 shows the proposed topologies for single-phase elec- tronic transformers. In Fig. 1(a), switches SW1, SW1 ; SW2, SW2 ; SW3, SW3 ; and SW4, SW4 are operated synchro- nously and the transformer core is operated at high frequency. The topology shown in Fig. 1(b) employs a multilevel static converter to achieve step down operations. The split trans- former core on the primary side ensures equal voltage sharing among the switches employed in the high-voltage primary converter. II. ELECTRONIC TRANSFORMER OPERATING PRINCIPLES Fig. 1(a) shows a single-phase electronic transformer topol- ogy with a primary/secondary-side static converter. The static converters employ bidirectional switches as shown. Another topology shown in Fig. 1(b) employs a multilevel static con- verter on the primary side of the transformer for voltage step down application. The voltages across capacitors C1 and C2 effectively divide the source voltage into half, and each primary-side converter processes half of the source voltage. 0885–8993/99$10.00  1999 IEEE