Veröffentlichungen
- 134.G. Schmitt, R. Kennel and J. Holtz, "Voltage gradient limitation of IGBTS by optimised gate-current profiles" in 2008 IEEE Power Electronics Specialists Conference, 2008, pp. 3592--3596.
ISBN: 2377-6617
Abstract:
Using MOS-controlled semiconductors provide the opportunity to directly affect the voltage and currents gradients during the switching transients at the gate. An active gate driver is presented that imposes optimised gate current profiles in order to limit the dv/dt and di/dt. When limiting the dv/dt to 1 kV/mus the switching losses are be reduced by 35{%} in comparison to the common limitation method by gate resistor. The switch-off losses are improved about 10{%} by employing an optimised gate signal. - 133.J. Jiang and J. Holtz, "An efficient braking method for controlled AC drives with a diode rectifier front end" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 1446-1453 vol.3.
ISBN: 0197-2618
Abstract:
Standard PWM inverter fed induction motor drives employ a diode rectifier bridge to supply AC power from the utility to the DC link. Although a diode rectifier is the most cost-effective solution, it does not permit reversing the power flow. This prohibits operating the machine in the regenerative braking mode for active deceleration. An innovative control method substitutes conventional hardware, such as an active front-end rectifier or a chopper controlled braking resistor in the DC link circuit, by additional software that is implemented in the standard microprocessor control. The control algorithm maximizes the power losses in the machine and in the inverter. It enables regenerative braking operation of the induction motor at high torque. The algorithm conserves the high dynamic performance of a vector controlled drive system. - 132.T. Rösmann and S. Soter, "Regenerative Operation of DC-Series Machines in Pitchsystems for Multimegawatt Windturbines" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 1--7.
ISBN: 0197-2618
Abstract:
Modern windmills and multimegawatt turbines are nearly exclusively equipped with electrical pitchsystems.The pitchsystem itself has to fulfill the following functions: In the case of strong wind conditions during normal operation, the system is responsible to limit the generator torque. On the other hand, the feathering position of the blades is needed to brake down the turbine in stop or emergency situations. In modern windturbines this aerodynamical brake is necessary, because the mechanical brake is designed as a holding brake and is not able to prevent overspeed conditions. The last function described is safety relevant. Because of this, pitch systems are equipped with an backup energy storage. So the blade can be put into feathering position in any case. DC-machines are preferred in pitch systems because of their capability to be supplied directly by the DC-storage unit without any control or powerelectronics. The motor must be designed especially for this battery driven operation because current and speed during emergency battery drive are depending on the mechatronical behaviour of the machine. Actually two different DC machine types are used in pitch systems: DC-series and DC-compound machines. DC- compound machines were introduced because of their natural 4Q operation area. But an actual development allows the regenerative operation also for DC-series machines and makes them very cost attractive for multimegawatt pitchsystems. - 131.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. - 130.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. - 129.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. - 128.S. Soter, "Wind Converters and Farms -- Technologies and Control", Electrical Energy Systems -- University Enterprise Training Partnership, vol. 2008, no. 4, 2008.
- 127.J. Holtz, M. Stamm, J. Thur and A. Linder, "High-power pulsewidth controlled current source GTO inverter for high switching frequency" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 1330-1335 vol.2.
ISBN: 0197-2618
Abstract:
High-power GTO inverters require regenerative snubbers to ensure operation at predefined values of di/dt and du/dt. The use of regenerative snubbers for voltage source inverters is a well established technology. However, the proliferation of high-power PWM current source inverters is impeded by the nonavailability of an appropriate regenerative snubber scheme. The paper proposes a regenerative snubber for current source GTO inverters and reports experimental results of a 1.2 MVA prototype inverter. Energy recovery from the snubbers enables operation at extreme high switching frequency of 1 kHz and reduces the total filter kVA from 70{%} to 20{%} of the inverter power. - 126.J. Holtz and N. Oikonomou, "Synchronous optimal pulsewidth modulation and stator flux trajectory control for medium voltage drives" in 2008 IEEE Industry Applications Society Annual Meeting, 2008, pp. 1748-1791 Vol. 3.
ISBN: 0197-2618
Abstract:
High-power PWM inverters for medium voltage drives operate at low switching frequency to maintain the switching losses of the power semiconductor devices at an acceptable level. For these applications, synchronous optimal pulsewidth modulation methods permit a good trade-off between switching losses and harmonic distortion of the machine currents. The optimization of the pulse patterns is done off-line on the assumption of steady-state conditions. Dynamic modulation errors, and high overcurrents as a consequence, are encountered when the operating conditions change during operation. To overcome this problem, the stator flux linkage vector, including its harmonics, is subjected to closed loop control. A stator flux trajectory controller is employed for this purpose. Experimental results obtained from an industrial 1-MVA, 4.16 kV three-level inverter ac drive are presented. - 125.R. Wegener, S. Gruber, K. Nötzold and S. Soter, "Optimization of a Low-Cost Position Sensor for a Permanent Magnet Linear Drive" in PCIM Asia 2008, 2008.
