Taku Takaku

Improved wind power conversion system using magnetic energy recovery switch (MERS)

This paper presents experimental results on an innovative power conversion technology using magnetic energy recovery switch (MERS) of a wind turbine system with a synchronous generator to improve the output power and the efficiency. An output voltage of the synchronous generator decreases with the increase of current because of synchronous reactance. The MERS, which consists of four MOSFET or IGBT elements and one small DC capacitor just like as a full bridge configuration, is inserted in series between the generator and ac-dc converter. The capacitor absorbs the magnetic energy stored in the synchronous inductance by forced LC resonance. Since MERS compensates the reactance voltage of the synchronous generator by the capacitor voltage, the output voltage of the generator increases and the excitation current of the generator can be extremely reduced. Also, the switching loss of converter in the MERS system is very small because the MERS is not required PWM control and the switching frequency is the same as the generator, and the downsizing of converter is realized. The effect of MERS is verified in a small scale experimental set-up of wind power generation with a permanent magnet type synchronous generator and a dc-excitation type synchronous generator. The data indicate a great potential of the new power conversion technology to make the actual wind turbine system compact and to improve the efficiency.

Demonstration of the stress-minimized force-balanced coil concept for SMES

Strong electromagnetic force caused by high magnetic field and coil current is a serious problem in SMES systems. In facing this problem, we proposed the concept of Force-Balanced Coil (FBC) which is a helically wound toroidal coil. Based on the virial theorem, the FBC can minimize structure requirements for energy storage by selecting an optimal number of poloidal turns. We designed and fabricated a small experimental device which is composed of inner and outer helical coils mutually wound in opposite toroidal directions using NbTi superconductors. The distribution of the working stresses in this device can be changed by selecting the optimal current ratios between inner and outer coil currents. From the experimental results, we demonstrated the validity of the FBC concept.

A New AC Current Switch Called MERS with Low On-State Voltage IGBTs (1.54 V) for Renewable Energy and Power Saving Applications

Emergence of new power electronics configurations have historically been one of the important drivers for improvement of the IGBT technology. Development of new IGBTs is said to be a trade-off between saturation voltage, short-circuit capability and switching losses. With the common applications requiring high switching frequency and short-circuit capability, the saturation voltage performance has not been fully optimized. This paper describes a new configuration called the Magnetic Energy Recovery Switch (MERS). It is characterized by using simple control and low switching frequency, where saturation voltage is the main contributor to losses. The semiconductor requirements of this configuration have led to the development of a new low on-state voltage IGBT. Application in the area of wind power conversion shows potential for efficiency improvements. Additionally, due to the soft-switching nature of the MERS application, series connection of the new IGBTs in variable frequency induction heating application is shown to be easy without voltage sharing problems.

Application of parallel connected power-MOSFET elements to high current d.c. power supply

The low aspect ratio spherical torus (ST), which has single turn toroidal field coil, requires the extremely high d.c. current like as 20 MA to energize the coil. Considering the ratings of such extremely high current and low voltage, power-MOSFET element is employed as the switching device for the a.c./d.c. converter of power supply. One of the advantages of power-MOSFET element is low on-state resistance, which is to meet the high current and low voltage operation. Recently, the capacity of power-MOSFET element has been increased and its on-state resistance has been decreased, so that the possibility of construction of high current and low voltage a.c./d.c. converter with parallel connected power-MOSFET elements has been growing. With the aim of developing the high current d.c. power supply using power-MOSFET, the basic characteristics of parallel operation with power-MOSFET elements are experimentally investigated. And, the synchronous rectifier type and the bi-directional self commutated type a.c./d.c. converters using parallel connected power-MOSFET elements are proposed.

Voltage Rating Reduction of Magnet Power Supplies Using a Magnetic Energy Recovery Switch

A new concept of magnet power supplies that can reduce voltage ratings of the power supplies is proposed. Circuit diagram and operation principles of magnetic energy recovery switch (MERS) are described. MERS consists of a capacitor and four semi-conductor devices such as MOSFETs and IGBTs. It is connected in series to a power supply and a coil. MERS is a switch module and it has no power supply in itself. Because MERS generates a voltage required for the inductance of the coil, the power supply only has to supply a voltage required for the resistance of the coil. Therefore, using MERS can reduce voltage rating and capacity of the power supply. Two types of power supply using MERS and voltage rating reduction are discussed. Comparatively small power supplies for high-repetition pulsed magnets and alternating magnetic field coils can be designed. Some experiments were carried out and confirmed that MERS can reduce voltage ratings of power supplies

Control design and experimental verification of a series compensated 50 kW permanent magnet wind power generator

The increase in converter rating when going to permanent magnet generators for wind power turbines motivates the search for low loss and compact power electronic solutions. Previous investigations show that by using a combination of a diode bridge rectifier and an active series compensation device, the losses can be reduced. This paper investigates the control issues of this concept further. By applying appropriate control modes, the size of the dc-dc chopper can be reduced with 50 percent. A control method when using an active series compensator called magnetic energy recover switch is demonstrated. Small scale experiments verify the control concepts without the use of rotary encoder. Simulations show that maximum power point tracking can be achieved with simple control. Potential for implementing active drive train damping is also indicated.

Power supply for pulsed magnets with magnetic energy recovery current switch

In this paper, we propose a power supply with magnetic energy recovery current switch for pulsed magnets, such as the synchrotron accelerator bending magnets, magnetizer. The switch which consists of four MOSFET elements and one capacitor, generates a fast pulsed current with low voltage, and it improves the power factor. The switch absorbs the magnetic energy stored in the inductance of the load into the capacitor. And in next time on, it regenerates the energy to the load. In addition, this switch operates in zero-voltage switching and zero-current switching, and the switching loss is very small. In order to turn on the load current at high speed in the circuit with an inductance, high voltage of several times higher than the voltage which maintains steady current. Therefore, by adopting this switch in the power source for pulsed power supply, high-speed pulsed current is efficiently generated by recovering the magnetic energy which has been stored in the inductance to the load in the next time on. As an application of DC circuit, a semiconductor Marx-generator which generates the high voltage pulse composed of a multistage magnetic energy recovery is described.

Application of SiC-based power element to high current AC/DC converter system

Abstract It is proposed that SiC-based power-MOSFET is applied to the high current AC/DC converter system. The possibility of using SiC-based power-MOSFET depends on operational loss reduction and circuit configuration simplification of the high current AC/DC converter system. Concerning the operational loss reduction, the lower on-state resistance of SiC-based power-MOSFET can decrease the conduction loss of switching unit, and it means that the temperature rise of switching unit in operation period is lowered. Therefore, the auxiliary cooling equipment for AC/DC converter will be minimized. The higher operational temperature of SiC-based power-MOSFET will allow also the cooling capacity minimization. As many parallel connected power-MOSFET elements are required for high current operation, the parallel connected elements should be cooled down to liquid nitrogen temperature for on-state resistance reduction.

Advanced application of power-MOSFET to large capacity converter

Advanced application of power-MOSFET to a large capacity novel converter system is studied. Parallel operation of many power-MOSFET elements is investigated in room and liquid nitrogen temperatures. The total on-state resistance shows lowered value in parallel connected and cooled down conditions, and the reduced resistance is effective for operational loss reduction of the converter circuit. The bi-directional switching unit with four power-MOSFET elements is proposed, which can be applied to the self commutated type AC/DC converter. Another type of AC/DC converter using power-MOSFET consists of a three phase synchronous rectifier circuit, which is suitable to output one directional high DC current.

Loss and Rating Considerations of a Wind Energy Conversion System with Reactive Compensation by Magnetic Energy Recovery Switch (MERS)

Summary Electrical conversion system for permanent magnet wind generators is challenging due to the large rating of power semiconductor devices needed and the associated costs and losses. This motivates the search for new configurations improving these relationships. Additionally, due to the large inductance present in the generator (synchronous reactance), voltage drops results with resistive load and the generator output power is limited. Traditionally, a conventional active rectifier is used, with a 3-level type illustrated in Fig. 1a. The main challenges with this solution are the costs and also increased losses resulting from high speed switching and filter operation. A solution utilizing an active series compensation device combined with a diode bridge is investigated in this paper as shown in Fig. 1b. The series compensator supplies only reactive power with a resulting good generator utilization. This also means that the VA capability of the active semiconductor part can be reduced compared to an active rectifier since the active rectifier also needs to supply the full active power. The use of an active series compensator called Magnetic Energy Recovery Switch (MERS) is studied. MERS is a simple configuration with low switching losses and simple control acting as a variable capacitor. Due to the special characteristics of this configuration, a new IGBT with low on-state voltage has been developed. The performance of the configuration has been confirmed with experiments on a 50 kW multi-pole permanent magnet generator. Numerical investigations on large scale system indicate potential for loss reductions and power semiconductor rating reduction compared to using an active rectifier solution.