Veröffentlichungen
- 130.J. Holtz and N. Oikonomou, "Optimal Control of a Dual Three-Level Inverter System for Medium-Voltage Drives" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 1--8.
ISBN: 0197-2618
Abstract:
Actual developments in medium-voltage drives aim at increasing the power capability of the PWM inverter. Parallel connection of power semiconductor devices permits increasing the output current, while series connected devices increase the output voltage. In either case are additional means required for balancing the current or voltage stress of the devices. The three-level neutral-point clamped (NPC) inverter topology offers an inherent means to ensure dynamic voltage balancing of a maximum of two series connected devices. Higher operating voltages can be achieved by series connection of entire inverter topologies. Two circuit topologies are considered that behave as five-level units. Synchronous optimal pulsewidth modulation is applied for their control. This permits reducing the switching frequency to the 100-Hz region without sacrificing on harmonic distortion. - 129.J. Holtz and N. Oikonomou, "Neutral point potential balancing algorithm at low modulation index for three-level inverter medium voltage drives" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 1246-1252 Vol. 2.
ISBN: 0197-2618
Abstract:
Three-level inverters produce low harmonic distortion of the ac currents even when operated at moderate switching frequency. This makes them the preferred candidates for high-power, medium voltage applications. To improve the utilization of the semiconductor devices, synchronous optimal pulsewidth modulation is employed. This permits reducing the switching frequency to very low values. Carrier modulation is maintained in the lower range of the modulation index. Operation at very low switching frequency increases the steady-state ripple of the neutral point potential. An intrinsic natural balancing mechanism of the neutral point clamped inverter topology eliminates long-term neutral point potential offsets. Transient conditions, however, may create successive increments of the offset to high values, which requires fast compensation. The novel method of selecting the appropriate redundant inverter subbridge meets this requirement without incurring additional penalties. The effectiveness of the approach is documented by experiments obtained from a medium voltage motor drive fed by a 1-MVA three-level inverter. - 128.J. Holtz, "Initial Rotor Polarity Detection and Sensorless Control of PM Synchronous Machines" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 2040--2047.
ISBN: 0197-2618
Abstract:
Sensorless control of a PM synchronous machine at low and zero speed is based on the injection of an oscillating high-frequency carrier signal. A particular demodulation technique serves to eliminate the estimation error introduced by the nonlinear characteristics of the inverter. The initial rotor position and the magnet polarity is detected before the drive is started. The initialization is performed by injecting an ac carrier and two short current pulses in a sequence - 127.J. Holtz and J. Quan, "Drift and parameter compensated flux estimator for persistent zero stator frequency operation of sensorless controlled induction motors" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 1687-1694 vol.3.
ISBN: 0197-2618
Abstract:
The performance of sensorless controlled induction motors is poor at very low speed. The reasons are the limited accuracy of stator voltage acquisition and the presence of offset and drift components in the acquired signals. To overcome these problems, a pure integrator is employed for stator flux estimation. The time-variable DC offset voltage is estimated from the flux drift in a parallel stator model and used to eliminate the offset by feedforward control. Residual high-frequency disturbances are compensated by feedback flux amplitude control. A linearization of the PWM inverter transfer function and an improved stator resistance estimation scheme further enhance the system performance. Experiments demonstrate high dynamic performance of sensorless control at extreme low speed and zero stator frequency. - 126.J. O. Krah and J. Holtz, "High-performance current regulation for low inductance servo motors" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 490-499 vol.1.
ISBN: 0197-2618
Abstract:
The paper reports on a standard microcontroller implementation of a pulsewidth modulator and near-deadbeat current regulator for high switching frequency. The application is in high-performance positioning systems. The control strategy relies on a simplified machine model without incurring performance degradations. Changes between different modulation strategies are programmed depending on the modulation index. The values of switching time durations are obtained exclusively by decision making, thus minimizing computational load. Features like overmodulation, dynamic overmodulation, anti-windup, and reduction of switching frequency at thermal overload are included. - 125.S. Soter, "Wind Converters and Farms -- Technologies and Control", Electrical Energy Systems -- University Enterprise Training Partnership, vol. 2008, no. 4, 2008.
- 124.J. Holtz, "Acquisition of Position Error and Magnet Polarity for Sensorless Control of PM Synchronous Machines", IEEE Transactions on Industry Applications, vol. 44, no. 4, pp. 1172--1180, 2008.
Abstract:
Sensorless control of a permanent-magnet synchronous machine at low and zero speed is based on the injection of an oscillating high-frequency carrier signal. A particular demodulation technique serves to eliminate the estimation error introduced by pulsewidth modulation delay and the nonlinear characteristics of the inverter. Before the drive is started, the initial rotor position and the magnet polarity are detected. The initialization is performed by injecting an AC carrier and two short current pulses in a sequence. - 123.J. Holtz and R. Rosner, "Gate drive power recovery and regenerative snubber scheme for series-connected GTOs in high voltage inverters" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 1535-1540 vol.3.
ISBN: 0197-2618
Abstract:
Pulsewidth modulated (PWM) inverters for power levels above 10 MW are presently the target of ongoing research. The implementation implies the series-connection of semiconductor devices, like GTOs or IGCTs, even if multilevel topologies are considered. Areas of application are flexible AC transmission systems (FACTS), transformerless reactive series compensators, HVDC transmission, static VAr compensators, and high-power AC drives. Existing problems are dynamic voltage balancing between the series-connected devices, snubber energy recovery and the transfer of gate drive power across potential barriers of more than 10 kV. The paper proposes an integrated solution for these problems. Experimental results from an inverter half-bridge composed of four IGCTs are presented. The respective device ratings are 4.5 kV and 4 kA. The DC-link voltage is 10 kV. - 122.J. Holtz and N. Oikonomou, "Estimation of the Fundamental Current in Low-Switching-Frequency High Dynamic Medium-Voltage Drives", IEEE Transactions on Industry Applications, vol. 44, no. 5, pp. 1597--1605, 2008.
Abstract:
The switching frequency of medium-voltage ac drives is limited to low values to restrain the dynamic losses of the power devices. This favors the use of synchronous optimal pulsewidth-modulation schemes that minimize the harmonic current. It is a drawback, though, that optimal algorithms do not have a means to extract the fundamental component of the load current. High-performance torque control is therefore difficult to obtain. This paper proposes a method to identify the instantaneous fundamental component of the stator currents. A novel observer is developed for this purpose. The approach enables fast torque control at very low switching frequency. Experimental results from a 30-kW induction motor drive are presented. - 121.J. Holtz and N. Oikonomou, "Fast Dynamic Control of Medium Voltage Drives Operating at Very Low Switching Frequency---An Overview", IEEE Transactions on Industrial Electronics, vol. 55, no. 3, pp. 1005--1013, 2008.
Abstract:
Medium voltage AC machines fed by high-power inverters operate at a low switching frequency to restrain the switching losses of power semiconductor devices. Particular care is thus required in the design of the drive control system. The signal delay caused by low switching frequency operation increases undesired cross-coupling effects in vector-controlled schemes. These are not sufficiently compensated by established methods like feedforward control. Improvements are achieved by a more accurate modeling of the machine and the inverter. An adequate controller is introduced, having a transfer function with complex coefficients. The high harmonic distortion due to the low switching frequency is a tradeoff. Using synchronous optimal pulsewidth as an alternative permits reducing the switching frequency without increasing the harmonics. The detrimental effects of conventional control methods are eliminated by forcing the harmonic components on an optimal spatial trajectory. Deadbeat behavior and complete decoupling are thus achieved. The performance of the aforementioned schemes is compared based on mathematical analyses and experimental results.
