Xem mẫu
Long-Duration Voltage Variations
316 Chapter Seven
island. Therefore, some means of direct transfer trip is generally required to ensure that the generator disconnects from the system when certain utility breakers operate.
A more normal connection of DG is to use power and power factor control. This minimizes the risk of islanding. Although the DG no longer attempts to regulate the voltage, it is still useful for voltage reg-ulation purposes during constrained loading conditions by displacing some active and reactive power. Alternatively, customer-owned DG may be exploited simply by operating off-grid and supporting part or all of the customer’s load off-line. This avoids interconnection issues and provides some assistance to voltage regulation by reducing the load.
The controls of distributed sources must be carefully coordinated with existing line regulators and substation LTCs. Reverse power flow can sometimes fool voltage regulators into moving the tap changer in the wrong direction. Also, it is possible for the generator to cause regu-lators to change taps constantly, causing early failure of the tap-chang-ing mechanism. Fortunately, some regulator manufacturers have anticipated these problems and now provide sophisticated microcom-puter-based regulator controls that are able to compensate.
To exploit dispersed sources for voltage regulation, one is limited in options to the types of devices with steady, controllable outputs such as reciprocating engines, combustion turbines, fuel cells, and battery stor-age. Randomly varying sources such as wind turbines and photo-voltaics are unsatisfactory for this role and often must be placed on a relatively stiff part of the system or have special regulation to avoid voltage regulation difficulties. DG used for voltage regulation must also be large enough to accomplish the task.
Not all technologies are suitable for regulating voltage. They must be capable of producing a controlled amount of reactive power. Manufacturers of devices requiring inverters for interconnection some-times program the inverter controls to operate only at unity power factor while grid-connected. Simple induction generators consume reactive power like an induction motor, which can cause low voltage.
7.7 Flicker*
Although voltage flicker is not technically a long-term voltage varia-tion, it is included in this chapter because the root cause of problems is the same: The system is too weak to support the load. Also, some of the solutions are the same as for the slow-changing voltage regulation problems. The voltage variations resulting from flicker are often within the normal service voltage range, but the changes are sufficiently rapid to be irritating to certain end users.
*This section was contributed by Jeff W. Smith.
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.
Long-Duration Voltage Variations
Long-Duration Voltage Variations 317
Flicker is a relatively old subject that has gained considerable attention recently due to the increased awareness of issues concern-ing power quality. Power engineers first dealt with flicker in the 1880s when the decision of using ac over dc was of concern.2 Low-fre-quency ac voltage resulted in a “flickering” of the lights. To avoid this problem, a higher 60-Hz frequency was chosen as the standard in North America.
The term flicker is sometimes considered synonymous with voltage fluctuations, voltage flicker, light flicker, or lamp flicker. The phenom-enon being referred to can be defined as a fluctuation in system voltage that can result in observable changes (flickering) in light output. Because flicker is mostly a problem when the human eye observes it, it is considered to be a problem of perception.
In the early 1900s, many studies were done on humans to deter-mine observable and objectionable levels of flicker. Many curves, such as the one shown in Fig. 7.14, were developed by various companies to determine the severity of flicker. The flicker curve shown in Fig. 7.14 was developed by C. P. Xenis and W. Perine in 1937 and was based upon data obtained from 21 groups of observers. In order to account for the nature of flicker, the observers were exposed to vari-ous waveshape voltage variations, levels of illumination, and types of lighting.3
7.0
6.0
5.0
4.0
3.0
2.0
1.0
0.1 1.0 10.0 100.0 Frequency of Flicker in Seconds
Figure 7.14 General flicker curve.
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.
Long-Duration Voltage Variations
318 Chapter Seven
Flicker can be separated into two types: cyclic and noncyclic. Cyclic flicker is a result of periodic voltage fluctuations on the system, while noncyclic is a result of occasional voltage fluctuations.
An example of sinusoidal-cyclic flicker is shown in Fig. 7.15. This type of flicker is simply amplitude modulation where the main signal (60 Hz for North America) is the carrier signal and flicker is the modu-lating signal. Flicker signals are usually specified as a percentage of the normal operating voltage. By using a percentage, the flicker signal is independent of peak, peak-to-peak, rms, line-to-neutral, etc. Typically, percent voltage modulation is expressed by
Percent voltage modulation Vmax Vmin 100% 0
where Vmax maximum value of modulated signal Vmin minimum value of modulated signal
V0 average value of normal operating voltage
The usual method for expressing flicker is similar to that of percent voltage modulation. It is usually expressed as a percent of the total change in voltage with respect to the average voltage (V/V) over a cer-tain period of time.
200
150
100
50
0
–50
–100
–150
–200
Time (s)
Figure 7.15 Example flicker waveform.
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.
Long-Duration Voltage Variations
Long-Duration Voltage Variations 319
The frequency content of flicker is extremely important in determin-ing whether or not flicker levels are observable (or objectionable). Describing the frequency content of the flicker signal in terms of mod-ulation would mean that the flicker frequency is essentially the fre-quency of the modulating signal. The typical frequency range of observable flicker is from 0.5 to 30.0 Hz, with observable magnitudes starting at less than 1.0 percent.
As shown in Fig. 7.14, the human eye is more sensitive to luminance fluctuations in the 5- to 10-Hz range. As the frequency of flicker increases or decreases away from this range, the human eye generally becomes more tolerable of fluctuations.
One issue that was not considered in the development of the tradi-tional flicker curve is that of multiple flicker signals. Generally, most flicker-producing loads contain multiple flicker signals (of varying magnitudes and frequencies), thus making it very difficult to accu-rately quantify flicker using flicker curves.
7.7.1 Sources of flicker
Typically, flicker occurs on systems that are weak relative to the amount of power required by the load, resulting in a low short-circuit ratio. This, in combination with considerable variations in current over a short period of time, results in flicker. As the load increases, the cur-rent in the line increases, thus increasing the voltage drop across the line. This phenomenon results in a sudden reduction in bus voltage. Depending upon the change in magnitude of voltage and frequency of occurrence, this could result in observable amounts of flicker. If a light-ing load were connected to the system in relatively close proximity to the fluctuating load, observers could see this as a dimming of the lights. A common situation, which could result in flicker, would be a large industrial plant located at the end of a weak distribution feeder.
Whether the resulting voltage fluctuations cause observable or objec-tionable flicker is dependent upon the following parameters:
Size (VA) of potential flicker-producing source System impedance (stiffness of utility) Frequency of resulting voltage fluctuations
A common load that can often cause flicker is an electric arc furnace (EAF). EAFs are nonlinear, time-varying loads that often cause large voltage fluctuations and harmonic distortion. Most of the large current fluctuations occur at the beginning of the melting cycle. During this period, pieces of scrap steel can actually bridge the gap between the elec-trodes, resulting in a highly reactive short circuit on the secondary side
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.
Long-Duration Voltage Variations
320 Chapter Seven
of the furnace transformer. This meltdown period can generally result in flicker in the 1.0- to 10.0-Hz range. Once the melting cycle is over and the refining period is reached, stable arcs can usually be held on the elec-trodes resulting in a steady, three-phase load with high power factor.4
Large induction machines undergoing start-up or widely varying load torque changes are also known to produce voltage fluctuations on systems. As a motor is started up, most of the power drawn by the motor is reactive (see Fig. 7.16). This results in a large voltage drop across distribution lines. The most severe case would be when a motor is started across the line. This type of start-up can result in current drawn by the motor up to multiples of the full load current.
An example illustrating the impact motor starting and torque changes can have on system voltage is shown in Fig. 7.17. In this case, a large industrial plant is located at the end of a weak distribution feeder. Within the plant are four relatively large induction machines that are frequently restarted and undergo relatively large load torque variations.5
Although starting large induction machines across the line is gener-ally not a recommended practice, it does occur. To reduce flicker, large motors are brought up to speed using various soft-start techniques such as reduced-voltage starters or variable-speed drives.
In certain circumstances, superimposed interharmonics in the sup-ply voltage can lead to oscillating luminous flux and cause flicker. Voltage interharmonics are components in the harmonic spectrum that are noninteger multiples of the fundamental frequency. This phenom-enon can be observed with incandescent lamps as well as with fluores-cent lamps. Sources of interharmonics include static frequency converters, cycloconverters, subsynchronous converter cascades, induction furnaces, and arc furnaces.6
Q
P
1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0
Slip
Figure 7.16 Active and reactive power during induction machine start-up.
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.
...
- tailieumienphi.vn
nguon tai.lieu . vn