Power System frequency

The typic medium-sized squirrel-cage motor is designed to operate at 2 to 3 percent shp (97 to 98 percent of synchronous speed). The synchronous speed is determined by the power-system frequency and the stator-winding configuration. If the stator is wound to produce one north and one south magnetic pole, it is a two-pole motor there is always an even number of poles (2, 4, 6, 8, etc.). The synchronous speed is... 

Cnrrents and voltages in a power system usually have a sinusoidal waveform of the fundamental power system frequency plus other normal harmon-... 

Preservation of acceptable levels of power quality is an essential requirement for the operation of power systems, APS included. Frequency stability and voltage control are the most important aspects of this issue. The European Standard EN-50160 (European Standard EN-50160, 1994) requires that for a non-interconnected (i.e. autonomous) power system, frequency and voltage should be within a range of ... 

At the threshold limit slight perception of pain or tingling near to the points of contact will occur. A continuous alternating current at a power system frequency, e.g. 50 or 60 Hz, of approximately 1 mA will cause this slight reaction. Increasing the current causes a greater intensity of reaction. At approximately 12 mA the muscles become very difficult to control, i.e. almost unable to Tet go of the contact. Between approximately 20 mA and 50 mA the current tends to cause difficulty in breathing, but not to an irreversible extent. A continuous current above 50 mA and up to 100 mA will tend to cause ventricular fibrillation and may lead to heart failure and death. 

The following resistances and reactances can be defined when referred to the stator at the nominal power system frequency -... 

Figure D.2 Over-loading a single-shaft gas-turbine generator. Power system frequency versus time.

A modification of this basic drive system uses solid-state rectifiers and thyristors to convert the wound-rotor, variable-frequency slip power first to direct current and then to hne-frequency power (60 Hz in the United States). This in turn is fed back to the power system as useful energy. 

These are required to sum the currents in a number of circuits at a time through the measuring CTs provided in each such circuit. The circuits may represent different feeders connected on the same bus of a power system (Figure 15.21(a)), or of two or more different power systems (Figure 15.21 (b)). A preeondition for summation of currents on different power systems is that all circuits must be operating on the same frequency and must relate to the same phase. The p.f. may be different. 

G) For a generator connected to a power system, abnormal frequency operation may be the result of a severe system disturbance. An isolated unit, however, can operate at a low or high frequency, due to an incorrect speed control adjustment or a malfunctioning of the speed control device. 

Consider the speed-load (drooping) characteristics of the two machines as shown in Figure 16.20. For ease of illustration, the slopes have been exaggerated. Normally they are within 4% of the rated speed, as discussed earlier. When both machines are loaded equally, the total load may be defined by the load line AA, at the bus frequency,/ When the power input to PMf is increased, so that the drooping curve AO shifts to curve BO, it shifts the load line AA also to BB, so that the total load shared by the two machines will still remain the same. The load shared by G is now more than before at so that , > /, and by Gt less than before at P, so that P < Pi The generators now operate at a higher system frequency, /i,. If the... 

In the first case, that is with dipoles integral with the main chain, in the absence of an electric field the dipoles will be randomly disposed but will be fixed by the disposition of the main chain atoms. On application of an electric field complete dipole orientation is not possible because of spatial requirements imposed by the chain structure. Furthermore in the polymeric system the different molecules are coiled in different ways and the time for orientation will be dependent on the particular disposition. Thus whereas simple polar molecules have a sharply defined power loss maxima the power loss-frequency curve of polar polymers is broad, due to the dispersion of orientation times. 

Two studies resolved the Unresolved Safety Issue A-44, "Station Blackout." The first siudy, The Reliability of Emergency AC Power Systems in Nuclear Power Plants," when combined uh die lelevant loss-oToffsite-power frequency, provides estimates of station-blackout frequencies lor 18 nuclear power plants and 10 generic designs. The study also identified the design and operational features most important to the reliability of AC power systems. The second study, "Station Blackout Accident Analysis" (NUREG/CR-3226), focused on the relative importance to risk of laiion blackout events and the plant design and operational features that would reduce this risk. 

The reliability of a modern electric power system depends on continuous real-time control of power and energy production, transmission line flows, system frequency, and voltage. This complex task will get more involved in the new environment with increased market participation on both the supply and the demand sides. 

A large number of X-band ENDOR spectrometers have been discussed in the literature. Instruments operating in the less common K-19), Q-20,21 and V-22 frequency bands have also been described. Most of the ENDOR spectrometers are so-called low-power systems which produce rf fields = 0.1 mTrot (rot rotating frame). These field strengths are often sufficient to achieve nuclear saturation in transition metal complexes, i.e. to meet the condition (1.2). 

The reliability of the power supply should be determined, taking into account the frequency of power outages and extent of interruption. Consideration should be given to connecting electrically driven fire water pumps to the emergency power system, where one exists. 

The systems can produce AC or DC power directly by simple electrical additions, and provide shaft power simultaneously. Frequency can be changed by frequency conversion. 

It is important to balance the electrical system. In the winter 2002-2003 the power system was timely out of balance. This resulted in severe deviation in transmission voltages and frequencies that were about 10% under the nominal design level, which in turn made the system unreliable. 

Transient — Subcycle disturbance in the AC waveform evidenced by a sharp, brief discontinuity of the waveform. This may be of either polarity and may be additive or subtractive from the nominal waveform. Transients occur when there is a sudden change in the voltage or the current in a power system. Transients are short-duration events, the characteristics of which are predominantly determined by the resistance, inductance, and capacitance of the power system network at the point of interest. The primary characteristics that define a transient are the peak amplitude, the rise time, the fall time, and the frequency of oscillation. Figure 1.12 shows a transient voltage waveform at the output of a power transformer as the result of switching-in of a motor containing power factor correction capacitors. 

Power frequency disturbances are low-frequency phenomena that result in voltage sags or swells. These may be source or load generated due to faults or switching operations in a power system. The end results are the same as far as the susceptibility of electrical equipment is concerned. Power system transients are fast, short-duration... 

The ITIC graph apples to 120-V circuits obtained from 120-V, 120/240-V, and 120/208-V distribution systems. Other nominal voltages and frequencies are not specifically considered, but their applicability may be determined in each case. The curve is useful in determining if problems could be expected under particular power system voltage conditions (see Chapter 9). 

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