Technical Articles

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An Example of Choosing 138 kV Line Circuit Breaker

Let’s choose 138 kV line #3 circuit breaker To better understand a process of high voltage equipment selection, let’s discuss an example of choosing 138 kV Line #3 Circuit Breaker, shown in Figure 1, assuming that the following information about the system is available: Continuous current for all the lines: Line #1 – 1,000 A, […]

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Testing of Power Transformer – Measurement of Zero-Sequence Impedance

Earth-fault protection Purpose of the measurement The zero-sequence impedance is usually measured for all star-connected windings of the transformer. The measurement is carried out by supplying a current of rated frequency between the parallell connected phase terminals and the neutral terminal. The zero-sequence impedance per phase is three times the impedance measured in this way. […]

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10 Important Precautions When Working On Low Voltage Energized Equipment

Have eyes in the back of head… For most work, the electrical equipment must be de-energized because there is a high risk of injury to workers if they work on energized equipment. It may be possible to schedule such work outside of normal work hours to limit the inconvenience. Sometimes it is not practicable to completely disconnect low-voltage […]

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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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13 features and advances of using electronic soft starters for a motor starting

Starting characteristic Soft starters serve for a continuous adjustment of the starting characteristic of three-phase asynchronous motors to the requirements of the load by controlling the voltage across the motor and enable for an optimum integration of the drives in process control by means of various complementary functions. While when star-delta starters are used, the […]

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Cable spacing as a means of noise mitigation

Separation distances In situations where there are a large number of cables varying in voltage and current levels, the IEEE 518-1982 standard has developed a useful set of tables indicating separation distances for the various classes of cables. There are four classification levels of susceptibility for cables. Susceptibility, in this context, is understood to be […]

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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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Harmonic Distortion and High Frequency Applications

Distortion of the fundamental sine wave The presence of harmonic frequencies in addition to the fundamental 50Hz, bring about distortion of the fundamental sine wave. Considerable problems may arise dependant upon the level of this distortion. Sources of harmonic distortion can be: Variable Speed Drives UPS equipment Non linear load (switch mode power supplies) Special […]

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How the Weather Affects Conductors

Sag Notice how a conductor is strung between poles. It is not pulled tight; it sags. In hot weather it sags even more than in cold weather. Consider the analogy of the clothesline. A tightly strung clothesline as shown in Figure 1a puts a much greater strain on the poles which hold it at either […]

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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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