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Figure 534 Inductive coupling of power system residual current to telephone circuit
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induction This was a severe problem in the days of open wire telephone circuits Now, with the prevalent use of shielded, twisted-pair conductors for telephone circuits, this mode of coupling is less significant The direct inductive coupling is equal in both conductors, resulting in zero net voltage in the loop formed by the conductors Inductive coupling can still be a problem if high currents are induced in the shield surrounding the telephone conductors Current flowing in the shield causes an IR drop (Fig 535), which results in a potential difference in the ground references at the ends of the telephone cable Shield currents can also be caused by direct conduction As illustrated in Fig 536, the shield is in parallel with the power system ground path If local ground conditions are such that a relatively large amount of current flows in the shield, high shield IR drop will again cause a potential difference in the ground references at the ends of the telephone cable
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Electric utility companies usually measure energy consumption in two quantities: the total cumulative energy consumed and the maximum power used for a given period Thus, there are two charges in any given billing period especially for larger industrial customers: energy charges and demand charges Residential customers are typically charged for the energy consumption only The energy charge represents the costs of producing and supplying the total energy consumed over a billing period and is measured in kilowatt-hours The second part of the bill, the demand charge, represents utility costs to maintain adequate elec-
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Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website
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Fundamentals of Harmonics 218 Five
d TWISTED PAIR VLOOP ISHIELD SHIELD VC = COMMUNICATION SIGNAL
Figure 535 IR drop in cable shield resulting in potential differences in ground references
at ends of cable
POWER SYSTEM NEUTRAL RESIDUAL CURRENT
COMMUNICATIONS CABLE
Figure 536 Conductive coupling through a common ground path
trical capacity at all times to meet each customer s peak demand for energy use The demand charge reflects the utility s fixed cost in providing peak power requirements The demand charge is usually determined by the highest 15- or 30-min peak demand of use in a billing period and is measured in kilowatts Both energy and demand charges are measured using the so-called watthour and demand meters A demand meter is usually integrated to a watthour meter with a timing device to register the peak power use and returns the demand pointer to zero at the end of each timing interval (typically 15 or 30 min) Harmonic currents from nonlinear loads can impact the accuracy of watthour and demand meters adversely Traditional watthour meters are based on the induction motor principle The rotor element or the rotating disk inside the meter revolves at a speed proportional to the power flow This disk in turn drives a series of gears that move dials on a register Conventional magnetic disk watthour meters tend to have a negative error at harmonic frequencies That is, they register low for power at harmonic frequencies if they are properly calibrated for fundamental frequency This error increases with increasing frequency In general, nonlinear loads tend to inject harmonic power back onto the supply system and linear loads absorb harmonic power due to the distortion in the voltage This is depicted in Fig 537 by showing the directions on the currents
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Fundamentals of Harmonics Fundamentals of Harmonics 219
etc
I7 etc
(a) load (voltage is distorted)
Figure 537 Nominal direction of harmonic currents in (a) nonlinear load and (b) linear
Thus for the nonlinear load in Fig 537, the meter would read Pmeasured P1 a3P 3 a5P 5 a7P7 (532)
where a3, a5, and a7 are multiplying factors ( 10) that represent the inaccuracy of the meter at harmonic frequency The measured power is a little greater than that actually used in the load because the meter does not subtract off quite all the harmonic powers However, these powers simply go to feed the line and transformer losses, and some would argue that they should not be subtracted at all That is, the customer injecting the harmonic currents should pay something additional for the increased losses in the power delivery system In the case of the linear load, the measured power is Pmeasured P1 a3P 3 a5P 5 a7P7 (533)
The linear load absorbs the additional energy, but the meter does not register as much energy as is actually consumed The question is, Does the customer really want the extra energy If the load consists of motors, the answer is no, because the extra energy results in losses induced in the motors from harmonic distortion If the load is resistive, the energy is likely to be efficiently consumed Fortunately, in most practical cases where the voltage distortion is within electricity supply recommended limits, the error is very small (much less than 1 percent) The latest electronic meters in use today are based on time-division and digital sampling These electronic meters are much more accurate than the conventional watthour meter based on induction motor principle Although these electronic watthour meters are able to measure harmonic components, they could be set to measure only the fundamental power The user should be careful to ascertain that the meters are measuring the desired quantity The greatest potential errors occur when metering demand The metering error is the result of ignoring the portion of the apparent power that is due solely to the harmonic distortion Some metering schemes accurately measure the active (P) and reactive power (Q), but
Downloaded from Digital Engineering Library @ McGraw-Hill (wwwdigitalengineeringlibrarycom) Copyright 2004 The McGraw-Hill Companies All rights reserved Any use is subject to the Terms of Use as given at the website
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