Category Archive for: «Protection»

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Let’s analyse phase-to-earth faults in a single-incomer power system

Detect and isolate… Power-system protection should detect faults and isolate the faulty parts of the power system while keeping the fault-free parts in operation. Protection units are chosen according to the power-system configuration (parallel operation of generators or transformers, loop or radial power system, neutral earthing arrangement…). Resistance earthing on the transformer Earth fault protection units […]

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An Example of the Effectiveness of Directional Overcurrent Relays (ANSI 67, 67N)

When fault currents can flow in more than one direction… When fault currents can flow in more than one direction with respect to the load current it is often desirable to determine which direction the fault current is flowing and trip the appropriate devices accordingly. This is usually due to the need to de-energize only those parts […]

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Applications and Characteristics Of Overcurrent Relays (ANSI 50, 51)

50/51 and 50/51N relays Overcurrent relays are the most commonly-used protective relay type. Time-overcurrent relays are available with various timing characteristics to coordinate with other protective devices and to protect specific equipment. Instantaneous overcurrent relays have no inherent time delay and are used for fast short-circuit protection. Figure 1 below shows the timing characteristics of several typical […]

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14 terms of particular importance when discussing arc flash hazards

Electrical arcs Electrical arcs form when a medium that is normally an insulator, such as air, is subjected to an electric field strong enough to cause it to become ionized. This ionization causes the medium to become a conductor which can carry current. The phenomenon of electrical arcing is as old as the world itself. Lightning is […]

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Rating Definitions Applied to Medium Voltage Circuit Breaker

MV protection applications The medium voltage circuit breaker is the device of choice when sophisticated system protection at the medium voltage level is required. Most modern medium voltage circuit breakers use a vacuum as the interrupting means, although sulfur-hexafluoride (SF6) based units still exist and in use. Most modern medium voltage circuit breakers are rated on […]

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How to get rid off the harmonics in electrical system?

Nothing good from harmonics… The subject of harmonics is also a contributory factor in electrical noise I already wrote few days ago (Cable spacing as a means of noise mitigation). It also causes several other problems in power circuit components such as motors, transformers and capacitor banks. A load that is purely resistive has the same wave […]

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Microprocessor and conventional secondary systems compared

Reliable operation of the primary system Secondary systems are all those facilities needed to ensure reliable operation of the primary system, e.g. a high voltage substation. They cover the functions of controlling, interlocking, signalling and monitoring, measuring, counting, recording and protecting. With conventional secondary systems, the various functions are performed by separate devices (discrete components) […]

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Protection Against Overexcitation Of a Transformer

Increase of the magnetizing current Overexcitation of a transformer means that the magnetic flux in the core is increased above the normal design level. This will cause an increase of the magnetizing current and the transformer can be damaged if this situation isn’t taken care of. For step-up transformers connected to generators during start-up, overexcitation […]

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Differences between disconnectors, load switches, switch disconnectors and circuit breakers

Device Types Various types of device are available for carrying out the switching and protection tasks listed under title that are specially designed to fulfill the respective requirements. The various parts of IEC 60947 (Low voltage switchgear and controlgear) specify the design, performance and test features of the devices. The most important features of the […]

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How to mitigate effects of very fast transients (VFT) on HV equipment

VFT overvoltages The level reached by very fast transients (VFT) overvoltages originated by disconnector switching or line-to-ground faults inside a gas insulated switchgear (GIS) are below the basic insulation level (BIL) of substation and external equipment. However, aging of the insulation of external equipment due to frequent VFT must be considered. Transient enclosure voltages (TEV) […]

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