Yukihiko Sato

A new control strategy for voltage type PWM rectifiers to realise zero steady-state control error in input current

In this paper, a new simple control strategy for AC input current of voltage type PWM rectifiers which can eliminate the steady state control error completely is proposed. This control method requires neither the instantaneous value of the supply voltage nor any accurate circuit parameters on the AC side of the rectifier. Thus, a robust operation against the variation of the circuit parameters can be achieved. In the proposed control system, a digital resonant element implemented by a digital signal processor is introduced as a feedback controller. The digital resonant element exhibits a function similar to an integrator for the fundamental frequency components. Thus, it can eliminate the steady state control error of the input current completely. The principle of the proposed control method is discussed and its effectiveness is shown theoretically. The detailed method of the implementation of the loss-less digital resonant element is explained. The effects of the harmonics in the supply voltage on the AC input current waveform are clarified. To confirm the effectiveness of the proposed control method, some experimental results from two laboratory test systems are shown.

A current-type PWM rectifier with active damping function

A new control method for current-type pulse-width modulation (PWM) rectifiers which can provide active damping function is presented. This damping function is effective only on the harmonic components of AC input current selectively. Thus steady-state waveform distortion and transient oscillation of the input current are reduced by the active damping effects. The active damping function can be realized by feedback control of an LC filter connected to the AC side of the rectifier, and it does not require any additional components in the main circuits, permitting a simple circuit configuration. The control system of the proposed PWM rectifier is analyzed by using a simple block diagram developed in the present paper. From the analytical results, the influence of the circuit parameters and control delay on the active damping effects and the stability of the operation are clarified to establish the design method. To confirm the effectiveness of the active damping function, some results of basic experiments are included. As an example of application of the active damping function, the proposed rectifier is applied to reduce the harmonic currents generated by conventional rectifiers operating in parallel with the proposed rectifier. Some experimental results in this application are also included.

Damping of transient oscillations on the output LC filter of PWM inverters by using a virtual resistor

This paper presents a method to damp transient oscillations on the output LC filter of PWM inverters by using a virtual resistor. A virtual resistor is an additional control algorithm that simulates the roles of a resistor in the output LC filters. Because no real resistor is used, the oscillations can be damped effectively without sacrificing the power efficiency. The implementation of the virtual resistor is determined by how the resistor is connected to the LC filter. Experimental results are included to verify the proposed method.

State feedback control of current type PWM AC-to-DC converters

A control method of current type PWM AC-to-DC converters which realize sinusoidal AC input current and unity power factor is discussed. In such a type of converter, an LC filter required on the AC side may cause a resonant problem especially in the transient condition. To overcome this problem, state feedback control is introduced, and a control strategy which is suitable for the DC output current control as well as the AC side current control is proposed. Circuit parameters and feedback coefficients in the AC side current control system are optimized on the basis of an analysis in which the system is treated as a sampled data system. This system is applied to current pattern control of a reactor.<<ETX>>

A new state feedback based transient control of PWM AC to DC voltage type converters

This paper presents a new state feedback based control strategy for a PWM AC to DC voltage type converter with phase and amplitude control. In this control strategy the state variables of the LC filter connected to the AC side of the converter are fed back to the PWM pattern generator, thereby eliminating a DC offset of the AC input currents as well as oscillations of the DC output current during transients. Computer simulation of the converter system with the proposed control strategy shows that the transient waveforms of AC input and DC output currents are improved greatly even if the damping effect of the AC side resistance can not be expected. The DC voltage regulation with good dynamic response is also achieved even if DC capacitance is substantially reduced. Experimental results from a low power laboratory model are also included to confirm the simulated results and to demonstrate the effectiveness of the proposed control strategy. >

Analysis and minimization of ripple components of input current and voltage of PWM inverters

Analysis and minimization of the ripple components of the input current and voltage of three-phase voltage-source pulsewidth modulated (PWM) inverters are presented in this paper. The analytical expressions for the rms values of the input current and voltage ripples as a function of the shape of the PWM reference signal are derived. It is found that the reference signal which produces a minimum ripple in the input voltage varies as a function of the load power factor. It is also found that the rms value of the inverter input current ripple does not depend on the shape of the reference signal. In respect of the inverter input current and voltage ripples, the addition of harmonics other than the third into the sinusoidal reference is neither useful nor necessary. Experimental results are included to verify the derived expressions.

A control strategy for general-purpose active filters based on voltage detection

A new control strategy to realize general-purpose active filters based on voltage detection is proposed as an economical solution to harmonic compensation in distribution lines. The proposed control method is based on real-time digital simulation of an LC filter accomplished by a digital signal processor. The adaptive gain control to realize the automatic on-line adjustment of the compensating characteristics is introduced. This achieves the optimum operation according to the condition of the AC lines. Thus the proposed active filter can be used as a general-purpose active filter. In this paper, the compensation characteristics of plural units of the proposed active filters connected to a distribution line are investigated. The theoretical investigation of the compensation characteristics is presented. Furthermore, to confirm the effectiveness of the proposed control strategy, some experimental results employing a test system are included.

Force-balanced coil for large scale SMES

In large scale SMES, huge electromagnetic force caused by high magnetic fields and coil currents is a serious problem. In order to solve this problem, we propose a concept of force-balanced coil (FBC) applied to the SMES. The FBC balances the centering force with the hoop force, both of which are exerted in the major radius direction. Moreover, for the optimization of large aspect ratio superconducting coils, we propose a stress-balanced coil (SEC) concept, improving the concept of FBC, which balances magnetic pressures at the coil nose part where the produced magnetic field reaches its maximum value. Comparing the toroidal magnetic field coil (TFC) and FBC with SEC, the last can reduce coil stresses and obtain large stored energy with shorter length conductors and/or lower magnetic fields.

Volt-ampere rating of air-core superconducting power transformers

The authors point out that air-core superconducting power transformers have the potential of serving not only as power transformers but also as shunt reactors when used in power transmission systems having large capacitances. Here, the rated primary volt-amperes, volt-amperes as a shunt reactor, and related quantities are analyzed and their relations to the magnetic coupling factor and magnetizing reactance of the transformer are clarified. >

Analysis of a 3-phase air-core superconducting power transformer

The air-core superconducting power transformer has been investigated as one type of superconducting transformer. According to previous investigations, the air-core superconducting power transformer has many advantages compared to an iron-core superconducting or conventional power transformers. However, since these investigations about air-core superconducting power transformers have mainly been done for single-phase transformers, investigations of 3-phase transformers are indispensable as the next step. In this paper, results of previous investigations about single-phase air-core superconducting transformers, such as theoretical characteristics, are expanded into 3-phase. Then, the results of load tests obtained by using an experimental 3-phase air-core superconducting transformer are shown to confirm the results of the theoretical investigations.