Quasi-direct Currents Research Articles

Increasing Reactive Load of a Generator–Transformer Unit under the Influence of Quasi-Direct Currents

Increasing Reactive Load of a Generator–Transformer Unit under the Influence of Quasi-Direct Currents

INSTABILITIES OF SUPERSONIC COMBUSTION AT PLASMA-BASED FLAMEHOLDING

Two types of supersonic combustion instabilities identified in a M = 2 reacting flow at direct injection of gaseous hydrocarbon fuel and quasi-direct-current (Q-DC) plasma assistance have been considered: a global instability triggered by a mechanism of flow-combustion-plasma coupling and a fast instability of a thermoacoustic nature. The experiments were performed at SBR-50 supersonic combustion facility at variable conditions: pressureP0 = 1-4 bar, temperature T0 = 300-750 K, and fuel mass flow rate m‘ = 1-8.5 g/s. Diagnostics includedpressure measurements, filtered fast camera imaging, schlieren visualization, and spectroscopic observations. The global instability develops with a characteristic time of about 10 ms and is related to the interaction of the combustion-based separation zone with the reflected shock wave (SW) and electric discharge. It is shown that this instability could be effectively controlled by electrical discharge power. The thermoacoustic instability is developed with a characteristic time less than 1 ms. The analysis of pressure data reveals a resonant acoustic wave presence in the combustion zone between the fuel injection ports and the test section diffuser.

High frequency combustion instability control by discharge plasma in a model rocket engine combustor

To suppress the high frequency combustion instability of rocket engines, quasi-direct current (quasi-DC) discharge plasma is considered in combination with special arrangements in a rocket engine combustor. After establishing an internal reacting flow field model of a rocket combustor and a phenomenological plasma model, the influence of quasi-DC discharge plasma on high frequency combustion instability is numerically investigated. The results show that the characteristics of the high frequency combustion instability in the combustor are captured very well through the simulation. Based on the DES turbulence model, a positive feedback process is clearly observed for the self-excited combustion instability. The mean temperature and pressure of the combustor are not sensitive to the occurrence of quasi-DC discharge plasma. The pressure histories of the combustor and their characteristic oscillating frequencies are not affected by the plasma when only one actuator is in operation. However, the oscillating amplitudes of the pressure are reduced within a certain time when several actuators are in operation. The heat release becomes more diffusive and more eddies form around the injector faceplate with an increasing number of plasma filaments. Mainly through the reduction of local heat release, the plasma is capable of affecting the high frequency combustion instability in the combustor. To better control high frequency combustion instability, the optimal arrangement is a double-plasma filaments with symmetrical actuators.

The effect of quasi-direct currents on the generator-transformer unit

The effect of quasi-direct currents on the generator-transformer unit

An analytical model of the emission of magnetizing-current harmonics by a power transformer under the action of quasi-direct currents

This article validates the possibility of a piecewise linear approximation with one breakpoint of the magnetizing characteristic of a power transformer for studying the processes of unilateral saturation of a magnetic system during periods of a high geomagnetic activity, which is caused by space-weather disturbances. An integral quantitative characteristic of a piecewise linear approximation is proposed that takes the features of the main magnetization curve of electric steel and the structural features of the power-transformer magnetic system into account. An analytical model is developed that allows determination of the instantaneous values and the harmonic structure of the magnetizing current at different intensities of quasi-direct currents. Two specific levels of the geomagnetic activity are determined. At one of them, the even harmonics of the magnetizing current are maximized, while at the second level the fundamental harmonic of the magnetizing current reaches a maximum that exceeds the certified value of the open-circuit current of the power transformer by several times.

Preliminary Analysis on the Interplanetary Cause of Geomagnetically Induced Current and Its Effect on Power Systems

Using the detected events of geomagnetically induced current (GIC) in the Ling’ao nuclear power plant from 2004 to 2005, and focusing on the interplanetary cause of GIC and its effect on power systems, we have analyzed the corresponding solar driving sources and interplanetary solar wind structures, and performed spectral analysis on the most intense GIC event by means of wavelet transform. The results of this study show that: (1) Most GIC events were driven mainly by the halo coronal mass ejections, the interplanetary cause of GIC events includes the shock sheath, magnetic cloud, and multiplex interplanetary solar wind structure. (2) Based on the strongest GIC event on 2001 November 9, we find that the fluctuation of GIC in the earlier stage was related to the magnetic cloud boundary layer, and the variation of GIC intensity in the later stage was caused by magnetic cloud itself. (3) Compared to the frequency of the power system (50 Hz), the GIC can be equivalent to a quasi direct current. The energy of the GIC is embodied in the two time intervals in the wavelet power spectrum: the first interval is shown as an impulsive type and with a weaker intensity, and the second one is stronger. Regarding to the cumulative time of the transformer temperature rise caused by GIC, the second interval has a longer duration than the first one. Hence, during the second interval, it is more harmful to the power systems and devices. (4) With a correlation analysis, the correlations of the SYM-H index and dBx/dt with the GIC are significantly stronger than those of other geomagnetic indices with the GIC.

A system for recording geomagnetically induced currents in neutrals of power autotransformers

A digital system for recording quasi-direct currents that arise in neutrals of autotransformers under the action of magentospheric disturbances (frequency range is 0–2 Hz) intended to perform synchronous measurements on several power substations is described. Methods of measuring, primary handling, and transmitting the measuring information from data acquisition stations to a remote central server by using a cellular channel are considered. Schematic diagrams of the units and examples of measurement results, compared with variations of the geomagnetic field, are given.

A novel all‐fiber high velocity interferometer for any reflector

AbstractA novel all‐fiber high velocity interferometer system for any reflector (AFHVISAR) is proposed and demonstrated. The system mainly consists of two 3 × 3 SM fiber couplers. A quasi‐direct current (QDC) signal directly from the moving surface is obtained and used to offset the QDC component in the interfering signals for more precise phase demodulation. The use of the diffuse surface solves the fringe loss problem induced by the nonparallel movements of the surface. The experiment with a one‐stage light gas gun shows that the AFHVISAR can successfully achieve the surface velocity profile of LY12 aluminum specimen. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 2032–2035, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22606

On Currents Induced in Power Transmission Systems During Geomagnetic Variations

Theoretical estimation of the electric field occurring at the earth's surface during a time variation in the geomagnetic field is discussed. Numerical computations, which have been made assuming an idealized behaviour for the magnetic field or using measured magnetic values, indicate the existence of horizontal electric fields in the order of 1 V/km in connection with geomagnetic disturbances. Calculation of quasi-direct currents produced in an arbitrary conductor network (e. g. a power transmission system) by a geomagnetically-induced electric field is discussed. Numerical results concerning the Finnish 400 kV grid are given. It is seen that currents in the order of 100 A can be expected to flow through transformers during geomagnetic disturbances. The largest current measured in the Finnish recordings so far has been 165 A. These recordings carried out since 1977 are also described in this paper.

On Currents Induced in Power Transmission Systems During Geomagnetic Variations

Theoretical estimation of the electric field occurring at the earth's surface during a time variation in the geomagnetic field is discussed. Numerical computations, which have been made assuming an idealized behaviour for the magnetic field or using measured magnetic values, indicate the existence of horizontal electric fields in the order of 1 V/km in connection with geomagnetic disturbances. Calculation of quasi-direct currents produced in an arbitrary conductor network (e. g. a power transmission system) by a geomagnetically-induced electric field is discussed. Numerical results concerning the Finnish 400 kV grid are given. It is seen that currents in the order of 100 A can be expected to flow through transformers during geomagnetic disturbances. The largest current measured in the Finnish recordings so far has been 165 A. These recordings carried out since 1977 are also described in this paper.

Critical Currents of Superconducting Nb 25 at.% Zr in High Magnetic Fields

Measurements of critical current as a function of applied magnetic field were made on wire samples of Nb 25% at. Zr alloy at 4.22°K and 2.24°K. Curves are plotted for the cases for which the applied magnetic field is parallel to or perpendicular to the current direction. At 4.22°K, Hc(I → 0) for H perpendicular to I is found to be 70 kG, while for H parallel to I, it is found to be 82 kG. Reduction of the temperature to 2.24°K results in an increase of 13 kG in both cases. For H perpendicular to I, current densities of 4.4×104 A/cm2 are observed at 60 kG and 4.22°K, and at 73 kG and 2.24°K. The effective demagnetization factor of the current carrying elements is 0.6 π. All measurements were made in a dc magnetic field with quasidirect currents by observation of the first resistive voltage to appear across the sample.