There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

The solid-state transformer (SST), which has been regarded as one of the 10 most emerging technologies by Massachusetts Institute of Technology (MIT) Technology Review in 2010, has gained increasing importance in the future power distribution system. This paper presents a systematical technology review essential for the development and application of SST in the distribution system. The state-of-the-art technologies of four critical areas are reviewed, including high-voltage power devices, high-power and high-frequency transformers, ac/ac converter topologies, and applications of SST in the distribution system. In addition, future research directions are presented. It is concluded that the SST is an emerging technology for the future distribution system.

Review of Solid-State Transformer Technologies and Their Application in Power Distribution Systems

The solid-state transformer (SST) is one of the key elements in power electronic-based microgrid systems. The single-phase SST consists of a modular multilevel ac-dc rectifier, a modular dual active bridge (DAB) dc-dc converter with high-frequency transformers, and a dc-ac inverter stage. However, due to dc bus voltage and power unbalancing in each module, the modular SST often presents instability problems making its design difficult and causing unpredictable behavior. Moreover, the unbalanced dc-link voltages increase the stress of the semiconductor devices, and also cause high harmonic distortions of grid current, therefore, necessitating the use of a bigger ac filter. This paper presents a novel single-phase d-q vector-based common-duty-ratio control method for the multilevel rectifier, and a voltage feedforward and feedback based controller for the modular DAB converter. With the proposed control methods, the dc-link voltage and power in each module can be balanced. In addition, the low-distortion grid current, unity power factor, and bidirectional power flow can be achieved. Simulation and experimental results are presented to validate the proposed control methods.

Research on voltage and power balance control for cascaded modular solid-state transformer

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 conventional 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 regulation 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.

Analysis and design of electronic transformers for electric power distribution system

The method of the invention in one aspect involves electronic power control by varying the amplitude of an electrical power supply voltage, independent of frequency, whereby the output frequency will always be the same as the input frequency. An electrical circuit apparatus for accomplishing this function in a preferred embodiment is also disclosed herein. The preferred circuitry of this aspect of the invention uses four solid state switches, such as IGBT's, four diodes, an inductor, input and output filters and novel controlling circuitry. The controller apparatus and methods of the invention may be used to implement all otherwise conventional converter types, buck, boost, and inverting (and duals of these) versions to obtain different regulating characteristics, including galvanic isolation of the output from the input. The inventive methods and devices may be used in power factor correction, voltage and/or current harmonic filtering and neutralization, line and load conditioning, control of power transfer between two power grids, and programmable control of surges, sags, dropouts and most other voltage regulation problems.

Method and apparatus for electronic power control

A new concept of an intelligent power transformer is presented in this paper, where the size of a power transformer of commercial frequency is miniaturized by introducing a modulator. In this circuit, various functions, such as constant voltage, constant power and power factor correction are realized by a phase control system.

Intelligent transformer

Research on the reverse current phenomenon in synchronous rectifiers (SRs) is presented in this paper. For loss reduction, the SRs composed of metal-oxide-semiconductor field-effect transistors (MOSFETs) have recently been employed to replace the conventional rectifiers with diodes in low-voltage and high-current applications. Because the MOSFETs in the SRs are used as bidirectional switches, reverse current flow will probably occur. The reverse current phenomenon will cause undesired power loss. To clarify the effects of the reverse current on the forward converter with an SR, both the experiment and the analysis are performed. Furthermore, the concepts and methods for dealing with this phenomenon are clearly described in this paper.

Research on the Reverse Conduction of Synchronous Rectifiers

Synchronous rectifiers (SRs) composed of MOSFETs have recently been employed to replace the conventional rectifiers with diodes. SRs are widely used in switched-mode power supplies with low output voltage and high output current for efficiency improvement. Owing to the high-efficiency characteristic, it is adequate to use an SR buck converter in a voltage regulator for powering a central processing unit. Normally, such SR buck converter must operate at fairly high switching frequency for miniaturizing a whole circuit and achieving a fast response. However, at the conditions of low output voltage, high output current, and high switching frequency, the influence of parasitic elements to circuit operation will become extremely obvious. Therefore, the design considerations concerning the ripple voltage and the stability of such SR converters should be carefully investigated and clarified. By establishing the equivalent circuit and using a state-space averaged method, the ripple ratio of output voltage and the static and dynamic characteristics of the SR buck converter with nonnegligible parasitic elements are obtained. Thus, the design criteria concerning the output ripple voltage and the stability are clarified.

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Investigation on the Ripple Voltage and the Stability of SR Buck Converters With High Output Current and Low Output Voltage

In order to simplify circuit complexity, reduce cost and improve system efficiency, this paper proposes an interleaved single-stage LLC resonant converter to drive multi-channel light emitting diodes (LEDs) with high power-factor and current-sharing features. Two derivative buck-boost typed power-factor-correctors (PFCs) with interleaving operation are integrated with an LLC resonant converter to form the presented circuit. The proposed single-stage converter has the features of simple circuit structure, balanced switch currents, low dc-bus voltage, low input current harmonics, and high system reliability. With proper analysis and design, the power switches can turn on with zero-voltage-switching (ZVS), and the output rectifier diodes can turn off with zero-current-switching (ZCS), which results in low switching losses. Additionally, by connecting the primary windings of transformers in series, the current-sharing among all the LED channels can be achieved naturally. Operation principles and circuit analysis are addressed. Finally, a laboratory prototype with 220 V
 rms
 utility-line voltage is implemented correspondingly to verify the validity of the theoretical predictions and the feasibility of the proposed circuit.

An interleaved single-stage LLC resonant converter used for multi-channel LED driving

A study on the reverse current phenomenon in synchronous rectifiers is presented in this paper. For loss reduction, the synchronous rectifiers composed of MOSFETs have recently been employed to replace the conventional rectifiers with diodes in low voltage and high current applications. Because the MOSFETs in the synchronous rectifiers are used as bidirectional switches, reverse current flow will probably occur. The reverse current phenomenon will cause undesired loss. To clarify the effects of reverse current on the forward converter with synchronous rectifier, both the experiment and the analysis are performed in this research.

Masahito Jinno

Papers

Intelligent transformer

Research on the Reverse Conduction of Synchronous Rectifiers

Investigation on the Ripple Voltage and the Stability of SR Buck Converters With High Output Current and Low Output Voltage

An interleaved single-stage LLC resonant converter used for multi-channel LED driving

Study on the Reverse Conduction of Synchronous Rectifiers