Neutral Conductor Research Articles

Design of flexible coaxial cores for ac superconducting cables

The phase (inner) and neutral (outer) tubular conductors are each formed from two layers of niobium-clad copper strips laid in helices of opposite sense. The dielectric is lapped high density polyethylene tape impregnated with helium. The lay angles of the conductor strips are chosen to meet the requirements of thermal contraction and magnetic pressure from large fault currents. The main advantage of using twin, instead of single conductor layers is that there is no axial magnetic flux within the core and hence the associated problems are avoided. A further advantage is that when the thickness of the two copper layers is optimized there is a useful reduction in power dissipation when the superconductor is normal. This improves both the cable's ability to carry fault currents and the stability of the conductor against reversion to the normal state when carrying the rated current. Since the optimum thickness of each layer (∼0.5 mm) is about half that in a single layer cable, the double layer would not be significantly more expensive.

Modifications of Commutation Circuits for a Three-Phase Inverter, Connected with the Neutral Conductor

The self-commutated three-phase bridge connection with a common turn-off device using the neutral conductor of the three-phase system is chosen as an example to illustrate modified commutation circuits. These can extend the possibilities of operation of the three-phase bridge. although they may have othersuitable applications too. A particular turn-off device is first described for the use of this bridge as inverter for three-phase drive systems, which enables this inverter to be used for multiple pulse current control. Subsequently various commutation circuits are described which are to avoid a forced charge reversal of a large highly energized commutation capacitor. Instead of the normally used resonant circuit by which the capacitor will be recharged with a high current amplitude and therefore with high losses. a small additional auxiliary capaCitor will be used in these new circuits to reverse the voltage in the commutation circuit with a lower current and with lower losses. Moreover these circuits will allow a replacement of the large expensive turn-off capacitor by a cheaper d.c. capacitor.

Ground-Fault Interrupters

In a distribution circuit for connecting a load to an a-c source, it is known that a ground-to-neutral short tends to reduce the sensitivity of ground fault detectors that use a differential current transformer to sense line-to-ground leakage. A resonated coil on a core links the neutral conductor forming part of a protective device for interrupting the load circuit in the event of reduced Q of the resonated coil caused by a neutralto-ground fault. The core must also link the line conductor, for otherwise a loop including the line, the load and the neutral would wrongly have a like effect. The same core and coil are used with a common amplifier for the neutral-to-ground fault detector and for the line-to-ground leakage detector, so that the apparatus can be economical and compact. Furthermore, so long as there is no neutral-to-ground fault, the amplifier delivers a signal of normal strength that monitors the integrity of both detectors. The detector also responds to ''''run-away'''' of the monitoring signal generator.

Device for determining the neutral conductor in Ek--10 commutator motors.

Device for determining the neutral conductor in Ek--10 commutator motors.

Current in the neutral conductor

Current in the neutral conductor

Convective Flexure of a Plasma Conductor

The basic magneto-fluid-dynamical and electromagnetic equations are applied to study the flexure of an electrically neutral plasma conductor subjected to low Mach number subsonic forced convection. The basic equations, in particular the equation of energy balance, are shown to provide a linear relation between the curvature of the conductor and the mass flux of convection. For a given mass flux the curvature is found to vary inversely with a parameter introduced as the ``flexural rigidity'' of the conductor. The flexural rigidity is proportional to the coefficient which relates to the thermal conduction and the enthalpy flux due to diffusion. The flexural rigidity also increases with radiation losses and is inversely proportional to the specific heat at constant pressure. The theory is compared favorably with the results of an experimental investigation of the deflection of an arc positive column drawn between local, collinear electrodes in argon near atmospheric pressure. The arc current was varied from 2 to 6 A for various incident convection velocities normal to the interelectrode axis up to approximately 200 cm/sec.

Phase unbalance in low-voltage distribution systems

An analytical study by statistical methods is made of the unbalance in l.v. distribution systems caused by unequal loads on the different phases of the supply network. It is shown that the current in the neutral conductor is statistically distributed according to the X2 distribution on two degrees of freedom and that the probability distribution of the voltage across the consumer's terminals is closely approximated by the normal distribution. The results confirm the conclusions that have been derived from field data and that have previously been used as the basis for estimating the unbalanced voltage regulation in l.v. distribution systems for new housing estates. Correction factors are given in graphical form, which can be used to correct the balanced regulation for the effects of phase unbalance at different risk levels.

The measurement of earth-loop resistance

Regulation No. 507 in The Institution's Regulations for the Electrical Equipment of Buildings calls for a measurement of the earth-loop impedance and gives a method by which this can be done. Several instruments have been devised and are available to carry out this test, but all draw their testing current from the mains and are thus liable to serious errors. Records taken with a recording voltmeter show that in most cases there is already a voltage drop in the neutral conductor, which is varying continuously and rapidly. It is shown both theoretically and by practical tests that instruments of this character can have errors under these conditions amounting to 100% or more. Any instrument intended to carry out these tests must be such that it will give the correct value despite the presence of the continuously varying voltage drop in the neutral. An instrument is described which draws its testing current from its own hand-driven generator and is free from these errors. The Regulation also calls for a measurement of impedance, but it is suggested that in most cases the difference between impedance and resistance of an earth loop is so small that an instrument measuring resistance will be sufficient.

240/416-Volt 3-phase 4-wire power and lighting supply for modern industrial plants

Recently, in the technical magazines, articles have appeared advocating the use of a 277/480-volt 3-phase 4-wire grounded neutral distribution system for manufacturing plants. In these articles, it is pointed out that power and lighting service can be supplied from the same feeders by connecting motors, and other large loads requiring 3-phase electric supply, to the 3-phase conductors, and lamps to the phase and neutral conductors.

Corrosion Problems In Rural Power Distribution

Large scale construction of rural electric distribution lines in almost all parts of the United States has given rise to a variety of corrosion problems. Some are the result of special local conditions, while others are related to changes that came about when lines were designed for rural rather than urban construction. The corrosion problems are complicated by a wide range of atmospheric and soil conditions and by the difficulty of observing some kinds of damage before failure occurs. On multigrounded wye distribution lines all underground metal parts are generally connected to a common system neutral conductor in order to maintain the lowest ground resistance possible. Underground corrosion has been excessive in some locations because of galvanic action betwreen metal surfaces that assume varying potentials with respect to the soil. Atmospheric corrosion problems in pole line hardware are characterized by replacement costs that are very high compared with the value of the items replaced and by questions...

The phase/neutral system of supply for rural h.v. distribution

The suitability of the single-phase system of supply for distribution in remote ruraldistricts, with particular regard to utilization, has been cogently argued by Pickles and Wills. The present paper is an extension of the same argument. It deals particularly with supply, and advances the merits of the phase/neutral system of single-phase h.v. distribution as compared with the normal practice in this country of utilizing two live phases. It also considers the problems arising out of the application of the phase/neutral system in areas where the existing h.v. network is 3-phase, 3-wire, and where there is accordingly no neutral conductor available for the connection of phase/neutral spurs.

A report presented to the Council by the Committee on Multiple Earthing of the Neutral Conductor

A report presented to the Council by the Committee on Multiple Earthing of the Neutral Conductor

The use of protective multiple earthing and earth-leakage circuit-breakers in rural areas

This paper describes investigations made to ascertain to what extent consumers in rural areas are protected from risk of shock with methods of earthing at present in use, and what changes in practice might be desirable, with special reference to protective multiple earthing. The investigation shows that present safeguards are inadequate. Recommendations are made which it is believed will lead to material improvement, but further experience and experiment will be necessary before the matter can be put on an entirely satisfactory footing. It has been realized for some time that in rural areas fed by overhead lines where no water mains exist the protection afforded by ?ordinary earthing? is not entirely satisfactory. With this in mind an investigation was undertaken by the E.R.A. with a view to finding means for improving the situation. Alternative methods of protection are provided by earthleakage circuit breakers, or the use of the neutral to carry the fault current. This latter method, known throughout this paper as ?protective multiple earthing,? is subject to the disadvantage that the frameworks of apparatus connected to the neutral rise to the same voltage above earth as the neutral conductor. It was found from field tests, however, that the voltagerise of the neutral did not exceed a value dangerous to human beings under normal operation, and that even animals would be protected if the statutory voltage-drop were not exceeded. No interference with the satisfactory working of the telephone system was observed during any of the tests. Other possible sources of trouble due to the use of the protective multiple earthing system have been considered in some detail. An investigation has also been made to find the tripping voltage of earth-leakage circuit breakers which have been in use for a period of about 2 years. The tests were made by Mr. E. Fawssett in the electricity supply area of the Dumfriesshire County Council. The paper concludes with the major deductions reached from the practical tests, some conclusions respecting protective multiple earthing in general, and, finally, recommendations respecting existing and new installations and certain proposed conditions which should be complied with before protective multiple earthing is permitted. The Director of the E.R.A. will welcome comments from any who may have occasion to make use of this paper.