Power circuits

Copper and silver combined with refractory metals, such as tungsten, tungsten carbide, and molybdenum, are the principal materials for electrical contacts. A mixture of the powders is pressed and sintered, or a previously pressed and sintered refractory matrix is infiltrated with molten copper or silver in a separate heating operation. The composition is controlled by the porosity of the refractory matrix. Copper—tungsten contacts are used primarily in power-circuit breakers and transformer-tap charges. They are confined to an oil bath because of the rapid oxidation of copper in air. Copper—tungsten carbide compositions are used where greater mechanical wear resistance is necessary. 

ANSI/IEEE-C62.41-1991 Recommended practice for surge voltages in low voltage a.c. power circuits ... 

NEMA/lCS-9/1993 Industrial control and systems. Power circuit accessories (requirement for brakes) ... 

With these relays, there is no need to use HRC fuses or thermal OCRs. The power circuit is thus devoid of any heat generation in these components and provides energy conservation. 

Figure 12.44 A simple power circuit diagram for a breaker-operated HT motor starter...

Figure 12.45 Typical schematic and control wiring diagram lor power circuit of Figure 12,44...

A power circuit is basically an R-L circuit. In the event of a fault, the system voltage (V , sin ft))) may occur somewhere between V = 0 and V = on its voltage wave. This will cause a shift in the zero axis of the fault current, 7sc> and give rise to a d.c. component. The fault current will generally assume an asymmetrical waveform as illustrated in Figure 13.27. 

The power circuit single-line diagram is show n in Figure 13.50. The following are the controls and protections that may be genertilly required for such a plant. 

Metal enclosed low voltage power circuit breaker switchgear Metal clad and station type cubicle switchgear Metal enclosed interrupter switchgear Guide for calculating losses in isolated phase bus... 

Preferred ratings, related requirements and application recommendations for LV power circuit breakers and a.c. power circuit protectors... 

Table 14.3 For series I voltage systems. Dielectric test voltages for control and auxiliary circuits and also LT power circuits...

Figure 14.3 General arrangement of a power circuit to conduct a short-circuit test...

These are employed for the measurement of power circuit currents through an ammeter, kW, kWh or KVAr and power factor meter, or similar instruments requiring a current measurement. They must have a specified accuracy class as in lEC 60044-1 and the secondary current substantially proportional to the primary within a working range of about 5-120% of its primary rated current. They... 

Fach phase of these circuits is provided with an appropriate main CT, the secondary of which is connected to the primary of the summation CT. Summation is possible of many circuits through one summation CT alone per phase. The primary of summation CTs can be designed to accommodate up to ten power circuits easily. If more feeders are likely to be added it is possible to leave space for these on the same summation CT. 

Both requirements of measuring and protection cannot be met through one transformer generally. Thus two sets of transformers are required for a power circuit associated with a protection scheme, one for measurement and the other for protection. 

Figure 15.26 Power circuit and control scheme during a very severe external fault condition...

Making or breaking a power circuit causes a change in... 

Figure 17.24(b) Typical power circuit of a C-fl-type surge suppressor... 

Figure 18.1(a) A typical power circuit of a non-linear resistor-type surge arrester... 

To determine this, consider the simple power circuit diagram of Figure 18.18(a). where... 

ANSI C 37.16/1998 Low voltage power circuit breakers - preferred ratings and application recommendations ... 

In the above situation, even an overspeeding motor on a fault elsewhere would feed back the supply source and require such protection. The protective scheme isolates the faulty source from being fed by the healthy sources. Figure 21,18 illustrates a simple power circuit provided with a directional G/F relay. In the event of a fault in system B, source B alone would isolate. Source A would not feed the fault as relay b would trip the breaker B and eliminate /g. The relays are necessarily set at lower settings and at lower tripping times than the non-directional... 

Yet another reason for harmonic disorder in a power circuit is the occurrence of series or parallel resonance or a combination of both. Such a situation may occur at certiiin frequencies generated by the harmonic generating sources in the system. The capacitors installed in the system magnify the same. 

When a single uncharged capacitor unit is switched on a power circuit, as illustrated in Figure 23.20, then... 

We have considered the switching of capacitor banks that are installed on one power circuit and mounted in... 

Consider Figure 23.29(a), where, the p.f. of a power circuit is to be improved from cos 0, to cos 02- If kVAri is the reactive component of power at p.f. cos 0, which is to be improved to kVAr2, at p.f. co.s 0i, through the reactive power coinpensation, then the reactive component of power compensated or kVAr rating of the required capacitor banks... 

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