ENGINEERING ARTICLES

INTELLIGENT : capable of sensing system overloads and rerouting power to prevent or minimize a potential outage; of working autonomously when conditions require resolution faster than humans can respond…and cooperatively in aligning the goals of utilities, consumers and regulators EFFICIENT: capable of meeting increased consumer demand without adding infrastructure ACCOMMODATING: accepting energy from virtually any fuel source including solar and wind as easily and transparently as coal and natural gas; capable of integrating any and all better ideas and technologies energy storage technologies, for example – as they are market-proven and ready to come online MOTIVATING: enabling real-time communication between the consumer and utility so consumers can tailor their energy consumption based on individual preferences, like price and/or environmental concerns OPPORTUNISTIC: creating new opportunities and markets by means of its ability to capitalize on plug-a...

A protective relay is a device that detects the fault and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system. The relays detect the abnormal conditions in the electrical circuits by constantly measuring the electrical quantities which are different under normal and fault conditions. The electrical quantities which may change under fault conditions are voltage, current, frequency and phase angle. Through the changes in one or more of these quantities, the faults signal their presence, type and location to the protective relays. Having detected the fault, the relay operates to close the trip circuit of the breaker. This results in the opening of the breaker and disconnection of the faulty circuit. A typical relay circuit is shown in Figure. This diagram shows one phase of 3-phase system for simplicity. The relay circuit connections can be divided into three parts viz. First part is the primary winding of a cur...

The use of semi-conductors in place of mechanical switches is what makes a circuit “electronic,” because they enable electrical signals to be switched at extremely high speeds, which is not possible with mechanical circuits. There are many different semi-conductor. DIODE: Like a one-way valve for electrical current, this device enables only electrical current to pass through it in one direction–extremely useful by itself, but also the basis for all solid state electronics. LIGHT EMITTING DIODE (LED): This type of diode emits a small amount of light when electrical current passes through it. LIGHT DEPENDENT RESISTOR (LDR): This type of semi-conductor has a changing resistance, depending on the amount of light present. BIPOLAR JUNCTION TRANSISTOR (BJT): This is a current-driven electronic switch used for its fast switching properties. METAL-OXIDE SEMICONDUCTOR FIELD-EFFECT TRANSISTOR (MOSET): This is a voltage-driven electronic s...

Although both transistors and grid-controlled tubes (e.g. triode, tetrode and pentode) can render the job of amplification, they differ in the following respects: The electron tube is a voltage driven device while transistor is a current operated device. The input and output impedances of the electron tubes are generally quite large. On the other hand, input and output impedances of transistors are relatively small. Voltages for transistor amplifiers are much smaller than those of tube amplifiers. Resistances of the components of a transistor amplifier are generally smaller than the resistances of the corresponding components of the tube amplifier. The capacitances of the components of a transistor amplifier are usually larger than the corresponding components of the tube amplifier.

It is assumed here that the transmission has multiple grounding points at wye connected transformer neutrals, located throughout the system. When this condition is satisfied, any arcing fault between a phase conductor and the ground will be supplied by zero-sequence currents originating in the neutral connection of the high-voltage transformer banks. We often refer to these neutral connections as the "sources" of ground current, since very little current would flow to the ground fault if there were no grounded neutrals to provide a complete circuit for the fault current. When there are multiple ground sources, the current flowing to the ground may be very large. Any current flowing to the ground contains zero-sequence components and, under grounded conditions, a zero-sequence voltage will be measured at any nearby relay installation. Negative-sequence currents and voltages will also be observed, and these are sometimes used by the protective system. However, most ground ...

An important aspect of transmission line protection is related to the fast detection and clearing of ground faults on transmission systems that have grounded neutrals. In the protection of transmission lines, ground faults are given special treatment. Ground faults are detected using different relays than those used for phase faults, although it is possible that phase relays may detect and properly clear ground faults. Ground relays, however, take advantage of unique features of the power system that make it possible to detect grounded conditions very quickly. IMPORTANCE OF GROUND FAULT PROTECTION: Most high-voltage and extra-high voltage transmission lines are grounded neutral transmission systems, either solidly grounded or grounded through a resistance or a reactance. It has been estimated that, on these high-voltage systems, over 90% of all transmission line faults are ground faults. It has been observed by one protection engineer that, on 500 kV transmission lines, one-line...

There are three basic types of relays that are used for ground relaying; overcurrent relays, distance relays, and pilot relays. Ground relays are almost always completely independent of phase relays, even though any fault current, including ground fault current, will flow through one or more of the phase relays. The ground relays, however, can be provided with much greater sensitivity to the zero-sequence currents by using higher tap settings. This means that the ground relays will pick up much faster than phase relays for a fault involving the ground. 1) OVERCURRENT RELAYS: Directional or non-directional overcurrent relays are widely used at most voltage levels because of their low cost and reliable service record. Many relay engineers prefer an overcurrent relay with an inverse or very inverse time-current characteristic. This means that the pickup will be very fast for close-in faults and delayed for faults at the end of the transmission line. This delay makes coordinatio...