Thomas J. Hotz

A DC Arc Model for Series Faults in Low Voltage Microgrids

This paper presents a dc arc model to simplify the study of a critical issue in dc microgrids: series faults. The model is derived from a hyperbolic approximation of observed arc voltage and current patterns, which permit analyzing the arc in terms of its resistance, power, energy, and quenching condition. Recent faults staged by the authors on a dc microgrid yielded enough data to develop an arc model for three fault types: constant-gap speed, fixed-gap distance, and accelerated gap. The results in this paper compare experimental and simulation results for the three fault types. It is concluded that because the instantaneous voltage, current, power, and energy waveforms produced by the model agree well with experimental results, the model is suitable for transient simulations.

Laboratory testing of the pulse power system for the cannon caliber electromagnetic gun system (CCEMG)

The team of (prime contractor) United Defense LP (UDLP) and The University of Texas at Austin Center for Electromechanics (UT-CEM) has completed a significant portion of the testing phase of a trailer mounted compulsator driven 35 mm (round bore equivalent) rapid fire railgun system. The objective of the program is to develop a compact, lightweight pulse power test bed capable of launching 3, 5 round salvos of 185-g integrated launch packages to 1.85 km/s at a firing rate of 5 Hz. Per contractual requirements, the pulse power system is also size compatible with the Amphibious Assault Vehicle (AAV). The pulse power system is developed around a second generation air-core, 4-pole rotating armature, self-excited, compulsator design. The 40 MJ at 12,000 rpm composite rotor stores all 15 shots inertially and is capable of 2.5 GW performance into the 2.21 m long series augmented railgun. This paper describes the CCEMG pulse power supply configuration and highlights important features of the commissioning test plan. The paper then presents test results from mechanical runs, stand alone compulsator (CPA) rectifier tests, short circuit tests, and single shot live fire tests. Finally, CPA performance is compared with predictions for the single shot tests presented.

Continued testing of the cannon caliber electromagnetic gun system (CCEMG)

The cannon caliber electromagnetic gun system is based upon a compulsator driven 30 mm rapid fire railgun system. The objective of the program was to develop a compact, lightweight test bed capable of launching three, five round salvoes of 185 g integrated launch packages to 1.85 km/s at a firing rate of 5 Hz. Per contractual requirements, the pulse power system is also size compatible with the amphibious assault vehicle. The pulse power system was developed around a fourth generation air-core, 4-pole rotating armature, self-excited, compulsator design. Although the contract for this effort has expired, the system continues to be used in part to demonstrate compulsator driven railgun technology. This system has performed seven single shots using identical control settings for each shot, which is the first such experience using a compulsator driven railgun system. This paper describes the experimental set-up for the demonstrations and compares the generator, converter, gun switch, and launcher performances for each shot.

Testing of the cannon caliber rapid fire railgun

A rapid fire launcher has been designed, built, and tested in single-shot mode for the Cannon Caliber Electromagnetic Gun (CCEMG) System. The 2.25-m long railgun has a rectangular cross-section (30 mm round bore equivalent) and has a series, two-turn augmented rail configuration. The gun is designed for rapid fire operation; three, five round salvos of 185 g integrated launch packages (ILPs) accelerated to 1,850 m/s with a minimum time between salvos of 2.5 s. Launch packages will be autoloaded at a repetition rate of 5 Hz via a hydraulic mechanism capable of up to 3,000 lb insertion forces. The railgun support structure and flexible buswork permit the railgun to recoil approximately 2 cm to mitigate the electromagnetic repulsion loads. Multiple 830 kA pulses provided from the CCEMG compulsator power supply require the gun to be liquid cooled for thermal management. Diagnostics for the single-shot tests include B-dots, flux rulers, voltage, and current measuring sensors. Other launcher diagnostics include rail conductor temperatures, coolant temperatures, and railgun preload mechanism (flatjacks) dynamic pressures. This paper presents the test results and general gun performance observations for single-shot, compulsator powered experiments.

Single shot switch performance on the cannon caliber electromagnetic gun program

The Cannon Caliber Electromagnetic Gun (CCEMG) program at The University of Texas at Austin Center for Electromechanics incorporates two solid state switch modules. The 95 MW rectifier/inverter bridge (RIB) generates the field inside of the compulsator (CPA) and the 825 kA gun switch module (GSM) is the main closing switch between the CPA and the launcher. Since commissioning the CCEMG system, nearly 400 discharges have been performed using these switches. These tests produced a significant amount of information which will be very useful for the design of future solid-state switching systems on other electromagnetic launch programs. This paper presents the basic design, testing and lessons learned from the commissioning of the CCEMG system switches.

Induction Motor Performance Testing With an Inverter Power Supply: Part 1

The development of high-power density electrical machines continues to accelerate, driven by military, transportation, and industrial needs to achieve more power in a smaller package. Higher speed electrical machines are a recognized path toward achieving higher power densities. Existing industry testing standards describe well-defined procedures for characterizing both synchronous and induction machines. However, these procedures are applicable primarily to fixed-frequency (usually 60 or 50 Hz) power supplies. As machine speeds increase well beyond the 3600-rpm limitation of 60-Hz machines, a need for performance testing at higher frequencies is emerging. An inverter power supply was used to conduct a complete series of tests on two induction motors (0.5 and 1.0 MW) with speeds up to ~5000 rpm. The use of a nonsinusoidal power supply with limited power output capability required the development of measurement techniques and testing strategies quite different than those typically used for 60/50 Hz testing. Instrumentation and techniques for measuring voltage, current, and power on harmonic rich waveforms with accuracies approaching 1% are described. Locked-rotor and breakdown torque tests typically require large kVA input to the motor, much higher than the rated load requirement. An inverter sized for the rated load requirements of the motor was adapted to perform locked-rotor and breakdown torque tests. Inverter drive protection features, such as anti-hunting and current limit that were built into the inverter had to be factored into the test planning and implementation. Test results are presented in two companion papers. This paper (Part 1) correlates test results with the results of an algorithmic induction motor analysis program. Part 2 presents the test results compared with a Matlab simulation program and also provides a comprehensive discussion of the instrumentation that was essential to achieve testing accuracy. Correlating test results with calculated values confirmed that the testing techniques developed during this testing program are useful for evaluating high-speed, high-power density electrical machinery

Design and testing of the power electronics for the cannon caliber electromagnetic gun system

The University of Texas Center for Electromechanics (UT-CEM) is in the final fabrication and testing phase of a power electronics system required to operate a skid mounted compulsator-driven railgun. Design goals for the self-excited air core compulsator include a 95 MW rectifier/inverter bridge for field coil self-excitation. Initial field coil seed energy is supplied by a 50 kJ capacitive discharge from the field initiation module. The field coil is passively protected from voltage transients by an array of metal oxide varistors. Other system power modules include the gun closing switch and explosive opening switch. This paper presents a brief system overview and detailed design of the rectifier/inverter bridge module with performance data from shots up to 6.

Development of a series fault model for DC microgrids

Series faults are self-sustained arcs produced in series with the normal path of supply power. The electromagnetic transients produced by these series faults are of interest during the design and analysis of existing and forthcoming dc microgrids. This paper presents a preliminary series fault simulation model to predict the electromagnetic transients introduced by series faults. Experimental results are compared with simulation results. The simulation results show the random nature of the arc, quenching attempts, arc energy consumption, and the influence of inductive loads.

Sensor development for compulsator driven railgun systems

As the technologies of rotating power supplies and thyristor switches advance, new methods of measuring various signals must be developed. The University of Texas at Austin Center for Electromechanics (UT-CEM) has developed new methods to measure compulsator position and speed and has made advancements in health monitoring of the thyristor switches. Technological advancements in machine design have enabled dramatic increases in machine speeds, which in turn increase electromagnetic interference. It is essential to a successful operation to have a method (reliable within this harsh environment) of measuring speed and sensing rotor position to generate gate signals for thyristor switch modules. As the modules switch larger amounts of current and voltage, it is correspondingly important to monitor the switching process, so that any damage to the system caused by a fault condition can be minimized. This paper describes sensors developed at UT-CEM to monitor speed, sense rotor position, and detect fault conditions.