Veröffentlichungen
- 9.J. Holtz and K. Werner, "Multi-inverter UPS system with redundant load sharing control" in 15th Annual Conference of IEEE Industrial Electronics Society, 1989, pp. 159-164 vol.1.
Abstract:
The concept of a redundant multi-inverter UPS (uninterruptible power supply) system includes extended monitoring of the status and the operating conditions of all power electronic equipment. Each block of the UPS system is monitored by two independent microcomputers that process the same data. The microcomputers are part of a redundant distributed monitoring system, being separately interlinked by two serial data buses through which they communicate. They establish a hierarchy among the participating blocks, defining one of the healthy inverter blocks as the master. The actual master runs the central synchronizing unit for the entire system, while the slave units perform the control of equal active and reactive load sharing. Operation and fault detection are experimentally exemplified in a dual inverter system having a rating of 10 kVA of redundant power.{\textless}{\textgreater} - 8.J. Holtz, S. Salama and K. H. Werner, "A nondissipative snubber circuit for high-power GTO inverters", IEEE Transactions on Industry Applications, vol. 25, no. 4, pp. 620--626, 1989.
Abstract:
A novel snubber circuit for high-power GTO inverters is composed of only passive components which, idealized, produce no losses. The stored energy in the turn-on and turn-off snubbers is completely recovered during a switching cycle. In the absence of a permanent circulating current in the recovery transformer, the minimum on-time duration of one half-bridge is very low. The nondissipative operation of the snubber circuit permits a liberal design of the snubber components by which the switching losses in the power semiconductors are greatly reduced.{\textless}{\textgreater} - 7.J. Holtz and H. Kelin, "The propagation of harmonic currents generated by inverter-fed locomotives in the distributed overhead supply system", IEEE Transactions on Power Electronics, vol. 4, no. 2, pp. 168--174, 1989.
Abstract:
Pulse-width-modulated converter-fed locomotives generate current harmonics that give rise to traveling waves in the overhead supply system. The waves are partially reflected at the feeding substances, causing parallel and series resonances at various discrete frequencies. An investigation based on the wave propagation approach demonstrates the influence of the track topography and the varying position of locomotives within the track. The distribution of harmonic currents in the overhead supply system is evaluated and discussed. These currents are shown to be much higher in certain locations of the railway track than the harmonic current injected by a locomotive. They may also appear at distant tract locations. The natural resonances in the overhead supply system determine the intensity of electromagnetic interference with the track-side communication lines.{\textless}{\textgreater} - 6.J. Holtz and U. Boelkens, "Direct frequency convertor with sinusoidal line currents for speed-variable AC motors", IEEE Transactions on Industrial Electronics, vol. 36, no. 4, pp. 475--479, 1989.
Abstract:
A novel concept for a static three-phase to three-phase power converter for an AC drive with a unity power factor and reduced harmonics on the line side is presented. The power circuit comprises two back-to-back connected six-pulse bridges having no energy storage elements in the DC link. This permits pulse-width modulation (PWM) control in both bridges while requiring active turn-off semiconductor switches in only one bridge. The line-side harmonics are suppressed by a three-phase second-order filter. The method of predictive optimization is used for the control of the power converter. The complex control structure of the system is based on an online prediction of space vector trajectories. The steady-state operation of the system is exemplified by simulation results.{\textless}{\textgreater} - 5.J. Holtz, W. Lotzkat and K. Werner, "A high-power multitransistor-inverter uninterruptable power supply system", IEEE Transactions on Power Electronics, vol. 3, no. 3, pp. 278--285, 1988.
Abstract:
The active and reactive load distribution between n paralleled single-phase uninterruptible power supply (UPS) inverters is equalized by virtue of n-1 load-sharing control loops. The approach permits the construction of UPS systems of any desired power rating at maximum utilization of the power components. The method of harmonic cancellation decreases the switching frequency of the power devices while maintaining good dynamic performance. The design details of a 45 kVA UPS inverter system with 150{%} steady-state overload capability are presented. The performance under various operating conditions is illustrated by oscillograms.{\textless}{\textgreater} - 4.J. Holtz and E. Bube, "Field-oriented asynchronous pulsewidth modulation for high performance AC machine drives operating at low switching frequency" in Conference Record of the 1988 IEEE Industry Applications Society Annual Meeting, 1988, pp. 412-417 vol.1.
Abstract:
A novel pulsewidth-modulation technique is reported that is based on the prediction of the trajectory patterns which the stator current vector describes in a field-oriented coordinate system. The method limits only the error of the torque-producing current component while exerting no constraint on the harmonics of the field-producing current. High dynamic performance and minimum switching frequency is achieved with the help of precalculated memory data, permitting implementation in low-cost hardware.{\textless}{\textgreater} - 3.J. Holtz and S. F. Salama, "Megawatt GTO-inverter with three-level PWM control and regenerative snubber circuits" in PESC '88 Record., 19th Annual IEEE Power Electronics Specialists Conference, 1988, pp. 1263-1270 vol.2.
Abstract:
Regenerative turn-on and turn-of snubbers for a three-level PWM inverter using gate turn-off thyristors (GTOs) are analyzed and discussed. Problems encountered in high-power GTO inverters, namely, trapped energy in the snubbers and switching losses, are discussed. The circuit configuration and operation are described and a comparison with a two-level inverter is made. Experimental results from the operation at reduced voltage of a 4 MVA phase-leg circuit are presented.{\textless}{\textgreater} - 2.J. Holtz, P. Lammert and W. Lotzkat, "High-Speed Drive System with Ultrasonic MOSFET PWM Inverter and Single-Chip Microprocessor Control", IEEE Transactions on Industry Applications, vol. IA-23, no. 6, pp. 1010--1015, 1987.
Abstract:
The design of high-current high-voltage MOSFET inverters requires measures to reduce the switching losses of the flyback diodes. Saturable reactances have been used in the case of a 4.5-kVA MOSFET inverter to limit the di/dt during commutations. The switching overvoltages of the reactances are absorbed by one common clamping circuit per bridge leg. The drive control system and the space vector modulator for the generation of the PWM switching sequences are implemented in a single-chip microcomputer. - 1.R. Venkataraman, B. Ramaswami and J. Holtz, "Electronic Analog Slip Calculator for Induction Motor Drives", IEEE Transactions on Industrial Electronics and Control Instrumentation, vol. IECI-27, no. 2, pp. 110--116, 1980.
Abstract:
For some schemes of variable speed control of squirrel-cage induction motor fed by a current-source inverter, accurate evaluation of motor slip frequency is essential for obtaining optimum torque output. For evaluating slip accurately, a digital speed transducer coupled to the shaft is generally required. Such a transducer and the associated digital circuits make the system complex and expensive. An analog speed transducer which is relatively cheap could be used only if the slip could be obtained accurately by some other means. This paper presents an inexpensive and accurate method of obtaining an analog signal proportional to the slip by using a simple calculator circuit that uses the dc link current, dc link voltage, and inverter frequency as its inputs. The slip calculator described here is capable of giving the correct output only under steady-state conditions. The design of the slip calculator is illustrated in the Appendix with the aid of a numerical example.
