Electrical Power Systems Quality, Second Edition phần 7 pdf

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.

316 Chapter Seven

*This section was contributed by Jeff W. Smith.

Long-Duration Voltage Variations

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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

Long-Duration Voltage Variations 317

1.0

2.0

3.0

4.0

5.0

6.0

7.0

0.1 1.0 10.0 100.0

Frequency of Flicker in Seconds

Voltage Change (in Volts) on 120-V System

Threshold of Perception

Threshold of Objection

Figure 7.14 General flicker curve.

Long-Duration Voltage Variations

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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  100%

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.



Vmax Vmin

V0

318 Chapter Seven

–200

–150

–100

–50

0

50

100

150

200

0.000

0.058

0.117

0.175

0.233

0.292

0.350

0.408

0.467

0.525

0.583

0.642

0.700

0.758

0.817

0.875

0.933

Time (s)

Voltage (V)

Figure 7.15 Example flicker waveform.

Long-Duration Voltage Variations

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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

Long-Duration Voltage Variations 319

Long-Duration Voltage Variations

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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

320 Chapter Seven

1.0 0.9 0.8 0.7 0.6 0.5

Slip

0.4 0.3 0.2 0.1 0.0

Active Power

Reactive Power

Q

P

Figure 7.16 Active and reactive power during induction machine

start-up.

Long-Duration Voltage Variations

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