Fault Current Flows Research Articles

Fault current limiters using superconductors

Fault current limiters on power systems are to reduce damage by heating and electromechanical forces, to alleviate duty on switchgear used to clear the fault, and to mitigate disturbance to unfaulted parts of the system. A basic scheme involves a super-resistor which is a superconductor being driven to high resistance when fault current flows either when current is high during a cycle of a.c. or, if the temperature of the superconductive material rises, for the full cycle. Current may be commuted from superconductor to an impedance in parallel, thus reducing the energy dispersed at low temperature and saving refrigeration. In a super-shorted transformer the ambient temperature primary carries the power system current; the superconductive secondary goes to a resistive condition when excessive currents flow in the primary. A super-transformer has the advantage of not needing current leads from high temperature to low temperature; it behaves as a parallel super-resistor and inductor. The supertransductor with a superconductive d.c. bias winding is large and has small effect on the rate of fall of current at current zero; it does little to alleviate duty on switchgear but does reduce heating and electromechanical forces. It is fully active after a fault has been cleared. Other schemes depend on rapid recooling of the superconductor to achieve this.

Amplitude Comparator Based Algorithm for Directional Comparison Protection of Transmission Lines

When there is a source present at both terminals of the transmission line, the relays protecting the line are subjected to fault current flowing in both directions. Directional relays operate only when the fault current flows in the specified tripping direction. Using the quasi steady state components of the locally measured deviations of the voltage and phase shifted current from their prefault values, the direction to a fault is determined. This directional information is exchanged with the relay at the opposite end of the protected zone. If the combination of the locally detected direction and the information from the remote end indicates a fault inside the zone of protection, a trip signal is issued. The deviation signals of the voltage and phase shifted current are determined by subtracting the previous values from the corresponding values in the present cycle. The voltage and current deviation signals contain exponentially decaying dc and high frequency transient components in addition to the steady state component [1]. The exponentially decaying dc component in the current signal is eliminated by phase shifting the current signal by a replica impedance. The current signal is lowpass filtered in the analog part of the processor before phase shifting it. A 1st order digital lowpass filter is used to filter the voltage deviation signal. An amplitude comparator method is used to determine the direction to a fault. An amplitude directional discriminant value, ADIS, as given by the equation below is evaluated from the filtered deviation signals.

Effects of Sustained Ground Fault Current on Concrete Poles

The effects of sustained flow of ground fault current through concrete poles of power distribution overhead lines are discussed. Typical cases and actual damages due to sustained currents are described. It is shown that serious damages to concrete poles and surrounding soil, including excessive earth surface potential gradients, may occur when concrete pole ground resistance is higher than a critical value.

Effects of Sustained Ground Fault Current on Concrete Poles

The effects of sustained flow of ground fault current through concrete poles of power distribution overhead lines are discussed. Typical cases and actual damages due to sustained currents are described. It is shown that serious damages to concrete poles and surrounding soil, including excessive earth surface potential gradients, may occur when concrete pole ground resistance is higher than a critical value.

A mechanism for the fuse pre-arcing period

WHEN FAULT CURRENT FLOWS in a fuse element, the temperature distribution along the fuse wire, at the instant of melting, depends upon the fault magnitude and on the dimensions of the fuse cartridge. If the current is low, and near to minimum fusing current, and the cartridge dimensions are normal, or, if the current is high and the cartridge is very short, the distribution will assume a parabolic form, and melting will occur at the center of the wire. If the current is high and the cartridge dimensions are normal, or, if the current is low and the cartridge is very long, the distribution will have a rectangular form, and most of the wire will melt at the same time. The melting depends on current density and wire parameter.

Harmonic-current-restrained relays for differential protection

DIFFERENTIAL relaying is the commonly accepted means of protecting large power transformers, a-c generators, and station bus systems against internal faults. <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> The methods and circuits employed in various instances, while differing in detail, are all fundamentally the same in basic principle. Briefly speaking, this principle consists in continuously comparing in each phase the current entering the protected equipment with that leaving. As illustrated in the generalized single-line diagram figure 1, the comparison is made by means of current transformers of suitable ratios placed in all the power circuits connecting with the protected equipment. If current transformation takes place within the equipment as in the case of a power transformer, the ratios of the current transformers placed in the various circuits are chosen to have relative values corresponding to the transformation ratios so that the current transformer secondary currents may be compared on a 1:1 basis. The secondary windings of all the current transformers in each phase are connected in parallel with each other to a special kind of current relay usually called a “differential” relay. The current transformers are all connected in the same polarity with respect to the direction of the protected zone so that currents entering and leaving the protected zone will be represented in the secondary circuit by currents of opposite polarities. Normally, with sound equipment these positive and negative components will be equal, except for negligible exciting currents, and their algebraic sum which by the connection is applied to the relay coil will be essentially zero. When a fault occurs within the protected zone, however, the balance is upset and a difference current proportional to the fault current flows in the coil of the relay, causing it to operate and trip circuit breakers in all the connecting circuits, removing the faulted equipment from service.

Harmonic-Current-Restrained Relays for Differential Protection

DIFFERENTIAL relaying is the commonly accepted means of protecting large power transformers, a-c generators, and station bus systems against internal faults.1 The methods and circuits employed in various instances, while differing in detail, are all fundamentally the same in basic principle. Briefly speaking, this principle consists in continuously comparing in each phase the current entering the protected equipment with that leaving. As illustrated in the generalized single-line diagram figure 1, the comparison is made by means of current transformers of suitable ratios placed in all the power circuits connecting with the protected equipment. If current transformation takes place within the equipment as in the case of a power transformer, the ratios of the current transformers placed in the various circuits are chosen to have relative values corresponding to the transformation ratios so that the current transformer secondary currents may be compared on a 1:1 basis. The secondary windings of all the current transformers in each phase are connected in parallel with each other to a special kind of current relay usually called a “differential” relay. The current transformers are all connected in the same polarity with respect to the direction of the protected zone so that currents entering and leaving the protected zone will be represented in the secondary circuit by currents of opposite polarities. Normally, with sound equipment these positive and negative components will be equal, except for negligible exciting currents, and their algebraic sum which by the connection is applied to the relay coil will be essentially zero. When a fault occurs within the protected zone, however, the balance is upset and a difference current proportional to the fault current flows in the coil of the relay, causing it to operate and trip circuit breakers in all the connecting circuits, removing the faulted equipment from service.

The Use of Communication Facilities in Transmission Line Relaying

This paper outlines the fundamentals of a method of obtaining overall protection of transmission lines by a comparison of the relative direction of fault current flow at the line ends by means of a communication system. The results obtained are equivalent to differential protection. The carrier-current pilot or d-c pilot wire schemes which have been introduced during the past few years are applications of this method. The optional use of a communication system to link the trip circuits at the ends of a transmission line provided with any conventional form of relay protection is described. This insures the simultaneous tripping of both line ends for all fault positions. The various types of communication systems adaptable for relay purposes are described and discussed in regard to coordination with the telephonic communication, telemetering or supervisory control systems employed as adjuncts of the power system.

Relay systems utilizing communication facilities

Overall protection of transmission lines may be secured by the use of a communication system to (1) compare the relative direction of fault current flow at line ends, or (2) link the tripping circuit at the line ends. The various types of communication systems adaptable for relay purposes are considerd in this article.

The use of communication facilities in transmission line relaying

This paper outlines the fundamentals of a method of obtaining overall protection of transmission lines by a comparison of the relative direction of fault current flow at the line ends by means of a communication system. The results obtained are equivalent to differential protection. The carrier-current pilot or d-c pilot wire schemes which have been introduced during the past few years are applications of this method. The optional use of a communication system to link the trip circuits at the ends of a transmission line provided with any conventional form of relay protection is described. This insures the simultaneous tripping of both line ends for all fault positions. The various types of communication systems adaptable for relay purposes are described and discussed in regard to coordination with the telephonic communication, telemetering or supervisory control systems employed as adjuncts of the power system.