AC-AC CONVERTER WITH LOAD POWER ESTIMATOR
Marian Gaiceanu
”Dunarea de Jos” University of Galati,
Department of Electrical Engineering,
Domneasca Street, 111, 6200 Galati- ROMANIA

Key words: PWM ac-ac converter, power balance, rotor field oriented control.

Abstract
In this paper a three-phase quasi-direct ac-ac converter system by means of pulse-width modulation (PWM) is reported. The control of the source converter is based on the minor current loop in a synchronous rotating-frame with a feed forward load current component added in the reference, completed with the dc voltage control loop in a cascaded manner. The synchronous rotor based reference frame was used for the field-oriented controllers involved in the load side. Two control boards, based on the dSMC (digital Smart Motion Controller) 30 MIPS 32-bit fixed-point digital signal processor (DSP), were involved in the ac-ac system driving. By using the power balance control, the DC link voltage variation at the load changes can be reduced. In this way a small DC link capacitor is required to handle the dc voltage error control. In order to obtain the feed-forward current component, the load power must to be known. In this paper the load power is estimated from the dc link, indirectly, through a dc load current estimator. In this way the author overcomes the use of the serial communication between control boards in order to deliver the load power information from the inverter side. The load current estimator is based on the DC link voltage and on the load current of the supply converter. By a proper control sinusoidal input current, a nearly unity power factor (0,998), bi-directional power flow, small (up to 5%) ripple in the dc-link voltage in any operated conditions high quality balanced three-phase output voltages were obtained. The performances of the ac-ac converter system were shown through the experimental results.

Introduction
The use of the force-commutated [1] PWM source converters is related to attaining of the bi-directional power flow and unity power factor, respectively. The first approaches in the ac/ac conversion with PWM modulators were done in [2]-[4]. The ac-ac converters are spited in indirect ac-dc-ac and quasi-direct ac-ac converters, respectively. The quasi ac-ac converter system has the advantage of the systems energy storage devices reduction. The load feed-forward current component was introduced to force the converter power to match the power [5] required by the inverter. By controlling the power flow in ac-ac converter system, the unidirectional DC-link voltage can be kept at the constant value. Reversing the flow direction of the dc-link current ensures the reversibility of the power. The ac-ac converter is used in drives that require regenerating power capability like elevators, crane, centrifuge, wind turbine and so on. The PWM ac-ac converter system is mainly used in industrial ac drive systems, where control of the ac voltage and frequency at the induction machine terminals are needed.

The ac-ac converter
The block control diagram of the ac-ac system is shown in the Fig.1. On the basis of a DC voltage reference V*dc, dc voltage feedback signal (Vdc), current feedback signals (Ia, Ib for supply converter, and Iu, Iv for the load converter), reference and the feedback speed, flux reference and the magnetizing current, and load power signal (obtained through the load power estimator), the software operates the ac-ac control (voltage, current, flux and speed) system and generates the firing gate signals to the corresponding PWM modulator.
The input boost inductor is designed from the THD (Total Harmonic Distortion) factor point of view. Therefore, the input current THD factor is constrained to be less than 5% at the full load. The boost inductor limits the peak switching currents to an acceptable value and smoothing the line current. The dc link capacitor design is carried out under the assumption that the ripple currents generated by the supply converter and load converter are equal in magnitude and are phase shifted by 180˚. The DC link capacitor provides a decoupling function between the grid side and load side, respectively. Thus, the dc capacitor provides a reduced value for the ripple current generated by both converters and maintains a controllable dc link voltage. The phase-locked loop (PLL) is necessary for the proper current controllers decoupling, and for d-q axes synchronization with the grid reference voltage, respectively. The PLL tracks the grid frequency. Because of the system stability limit [6], [7] the proportional feedback gain of the PI dc voltage controller must be small according to [8]. In order to obtain a sinusoidal ac current shape, in motoring mode operation, the dc link voltage is regulated at a higher level than the peak of the incoming line such that the IGBTs can control the current.

Control of the voltage source converter
The detailed design methodology for the ac-ac converter system regulators is presented in [9]. In ac drive a unity power factor is necessary. This implies a proper orientation of the reference frame and a zero reference value for d-axis line current, Id:

Low overshoot and fast response are the design criteria.

Voltage controller
By model linearizing through the small perturbation method around the equilibrium point the parameters of the dc-voltage controller were derived [9]. The main task of the voltage controller is to maintain the dc link voltage to a certain value. Another task is to control the power flow of the voltage converter.

Current controllers
The internal (current) loops with wide bandwidth are needed to regulate the reactive power in order to achieve unity power factor, and to regulate the active power such that low dc voltage variation is ensured. At the same time, the ac input current is regulated to be sinusoidal. The proportional integral (PI) controllers were used in [9]. By using a decoupling method the cross coupling between the d and q axes due to boost inductance was compensated. Thus, by decoupling the d and q axes of the synchronous reference frame model, the current control loop is obtained. The feed-forward current component was added to the reference. Their value is provided from the power balance equation, i.e. the power converter must meet the load power requirements. Therefore,

in which the proportional constant (considering that the supply voltage has the constant value) is given by:

Through the feed-forward component the power flow is controlled indirectly by the reference current. The output dc link power is estimated as:

The estimated dc link current is given by:

Mathematical model of the three-phase load converter side
By aligning the rotor flux vector to the d-axis and setting the rotor flux linkage to be constant when the motor is operating in its constant-torque region, the following equations are available:

in which ψqr (ψdr): q-axis (d-axis) rotor flux linkage. The d-q model of the induction machine in the rotor field oriented frame is :

In the stator voltage equations, the d and q axes components are coupled. They can be decoupled by introducing the corresponding coupling compensation terms and the back-EMF feedforward voltages, Ed and Eq, in the d-q axes voltage commands (V*ds, V*qs)

where eds= i*ds- ids, eqs= i*qs- iqs denote the d-q axes current errors, and i*ds, i*qs the d-q axes current commands, respectively. Kp and Ki denote the proportional gain and the integral gain, respectively.

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