Janez Nastran

Synchronization of active power filter current reference to the network

This paper presents an improved method of current reference determination for a parallel active power filter. To insure sinusoidal line currents, thus reducing reactive power, an integrative method of reference calculation is applied. As a result, satisfactory response of line current to the load change is achieved. One of the major problems, the synchronization to the net voltage, is also considered. Classical solution is based on zero-cross detection of line voltage. Since this approach is very sensitive to the voltage distortions, a new method is suggested, relying on voltage fundamental harmonic calculation. These ideas were tested on a three-phase active power filter, controlled by 32-bit microcontroller.

Extending the low-speed operation range of PM Generator in automotive applications using novel AC-DC converter control

This paper outlines a case study on an integrated starter-generator-torque-booster (ISGtB) for a hybrid propulsion system, with a particular focus on the generator operation in the low-speed range. The propulsion system consists of an internal combustion (IC) engine and a brushless dc machine and is intended to drive a street scooter. The complex functionality of the ISGtB, some outstanding characteristics of modern IC engines, stringent demands on the generated supply voltage, and operational principle of the selected electrical machine are the reasons for our research on the generator operation. An ac-dc converter and its control are proposed. By utilizing MOSFET transistor reverse conduction characteristics, the idea of synchronous rectification, and the principle of boost switching conversion, several improvements in the generator characteristics are accomplished. The proposed solution, enabling each of the three-phase converter legs to operate as an autonomous boost converter with synchronously driven upper transistors instead of using body diodes, extends the generator operation range below the nominal rotational speed and offers simple yet efficient energy flow control.

Bi-directional power converter for wide speed range integrated starter-generator

The paper outlines a case study on an integrated starter-generator-torque-booster (ISGtB) for automotive applications, representing an electric part of a hybrid propulsion system of a street scooter. Requirements for a high power density of the electrical machine led to a permanent magnet brushless DC machine (BLDC) with an external rotor. The complex functionality of the ISGtB, some deciding characteristics of modern two-stroke internal combustion (IC) engines and the nature of the selected electrical machine with linear dependence between back EMF and rotational speed dictate the properties and capabilities of the switch-mode power converter (SPC) that must support all operating modes of the ISGtB and bi-directional energy flow. The research work involved the extensive study on different suitable SPC topologies, including also control strategies, in the sense of starting torque, motor as well as generator operation mode of ISGtB. The selection of the SPC topology was made on the basis of theoretical statements, computer simulations and measurement results on several ISGtB prototypes. It consists of a bi-directional buck/boost converter and of a common known six-step inverter, as an executing stage. The topology of the bi-directional buck-boost converter is derived mainly with regard to the conversion ratio between the supply battery voltage level and DC link voltage level. The instantaneous value of the transferred electric power is tuned to the torque/speed characteristic of the IC engine and, in the opposite energy flow direction, to the requested output power in the generator operation mode. The deciding parameter when selecting the executing stage of the SPC was the torque/current ratio of the BLDC machine for different inverter topologies and phase winding connections. A particular attention was paid also to an efficient operation of the ISGtB on the boundaries of the operation range, such as providing a low impedance current path at the starting maneuver and extending the low-speed generator mode. The phenomenon of reverse conduction of MOSFET transistors was effectively employed, thus improving the overall SPC efficiency and extending the ISGtB operation range. Measurement results clearly reveal the functionality of ISGtB over the whole rotational speed range of the prototype set-up. The employment of reverse conduction of MOSFET transistors significantly influences the torque characteristic in the starter mode of operation. The proposed inverter control principle in the generator operation mode of the ISGtB evidently improve the transition from generator to torque-boosting operation mode.

Power calibrator using switched mode voltage source

The paper is focused on a sophisticated switched mode power supply with a pure sinusoidal output voltage waveform with frequency (40-65 Hz) and voltage range (30-500 V). In order to cope with stringent voltage (10.2%) and distortion constraint (THD<0.7%) in a modern calibration system, traditional inverter design with an output LC filter is upgraded with a feedback-feedforward control principle. The paper presents the voltage source topology, analyzes its steady-state response and gives its comprehensive evaluation. Using a simple sensor that is capable to detect a small voltage asymmetry at the presence of a high AC component (500 VRMS), the parasitic DC component is limited below 5 mV. Experimental results demonstrate exceptional voltage accuracy (<0.2%) and low total harmonic distortion (THD).

Multilevel bipolar high voltage pulse source - interlock dead time reduction

In the paper, a distinctive method of generating high voltage pulses with high du/dt is investigated. The power stage of the proposed source consists of eight individual sources that can be arbitrarily connected in series to obtain the desired magnitude and a one-phase transistor bridge for the selection of the output pulse polarity. A special emphasis is put on the reduction of an interlock dead time when generating bipolar pulses.

Soft-magnetic ring core measuring system with a decreased number of primary and secondary winding turns

A computerized measuring system of magnetic properties of soft-magnetic ring cores according to the IEC 404-2 is presented. Special attention is paid to its robustness in order to assure the precise and fast measurement needed in the intensive magnetic component industry. To reduce the connection time of the specimen, the proposed test circuitry-in contrast to other well-known and applied solutions-uses a low number of primary and secondary turns. Such a solution increases demands that need to be fulfilled by a power supply generating the magnetizing current. This paper explains how a sinusoidal magnetic polarization of the core is maintained and introduces solutions to avoid a premagnetized magnetic core and power amplifier instability.

Symmetrization of line currents in three-phase four-wire loads

Single-phase loads and non-linear three-phase loads can produce unbalanced line currents, generating a current in the neutral line as a consequence. In some cases, these neutral currents, i.e. their RMS value, can be even higher than the nominal line currents, although the neutral lines are not designed to withstand these rates. Related problems can be partially resolved by a three-phase parallel active power filter, that reduces harmonic distortion, but the harmonic distortion is just one of the reasons for unbalance. The remaining unbalances can be successfully eliminated only when a proper method for filter current reference determination is applied. The paper introduces some characteristic load conditions and proposes a suitable filter configuration. Simulation results of symmetrization are shown, based on a proven filter current reference determination method, where synchronization with the line voltage is emphasized.

Synchronization to the network without voltage zero-cross detection

Many power electronics devices require to be synchronized to the network. Classical synchronization solutions are based on zero-cross detection of line voltage, but since this approach is very sensitive to the voltage distortion, a new method is suggested, attempting to perform on-line recalculation of line voltages fundamental harmonic. Besides the theoretical basis of the new synchronization method, experimental results are given. The method was tested with parallel active power filter, intended to compensate fundamental reactive power as well as reactive power due to line current harmonic distortion. Motorolas MC68332 microcontroller was applied, providing the filter current reference determination and concurrently performing the synchronization algorithm.

Sensorless control of induction machine with parameter adaptation

In this paper, a method for speed sensorless control of the induction machine is presented. The basic idea is to calculate the voltages induced by the main flux in the rotor field coordinates. This can be achieved by subtracting the stator voltage drops either in the stationary or in the field reference frame. The models main drawback is the poor stability in the braking region, where a stabilization algorithm has to be performed. Also, the influence of the stator and rotor resistances evaluation error on the speed estimation is analysed. Both parameters can be adapted using the direct component of the main flux induced voltage in the rotor coordinates.

Induction machine sensorless control with stator and rotor resistance adaptation

Two similar models for speed acquisition without a mechanical sensor, based on current and main flux induced voltage model, are presented for induction machine sensorless control schemes. The influence of stator and rotor resistance on speed error is analyzed and a method for their adaptation is proposed.