Electrical Power Systems Quality, Second Edition phần 4 docx

charge voltage may be reduced only a few percent, the greatest benefit

of the scout scheme may be that it greatly reduces the rate of rise of

surge voltages entering the cable. These steep-fronted surges reflect off

the open point and frequently cause failures at the first or second pad￾mount transformer from the end. Because of lead lengths, arresters are

not always effective against such steep impulses. The scout scheme

practically eliminates these from the cable.

Many distribution feeders in densely populated areas will have scout

schemes by default. There are sufficient numbers of transformers that

there are already arresters on either side of the riser pole.

4.6 Managing Ferroresonance

Ferroresonance in a distribution system occurs mainly when a lightly

loaded, three-phase transformer becomes isolated on a cable with one

or two open phases. This can happen both accidentally and intention￾ally. Strategies for dealing with ferroresonance include

■ Preventing the open-phase condition

■ Damping the resonance with load

■ Limiting the overvoltages

■ Limiting cable lengths

■ Alternative cable-switching procedures

Most ferroresonance is a result of blown fuses in one or two of the

phases in response to faults, or some type of single-pole switching in

the primary circuit. A logical effective measure to guard against fer￾roresonance would be to use three-phase switching devices. For exam￾ple, a three-phase recloser or sectionalizer could be used at the riser

pole instead of fused cutouts. The main drawback is cost. Utilities could

not afford to do this at every riser pole, but this could be done in spe￾cial cases where there are particularly sensitive end users and frequent

fuse blowings.

Another strategy on troublesome cable drops is to simply replace the

fused cutouts with solid blades. This forces the upline recloser or

breaker to operate to clear faults on the cable. Of course, this subjects

many other utility customers to sustained interruptions when they

would have normally seen only a brief voltage sag. However, it is an

inexpensive way to handle the problem until a more permanent solu￾tion is implemented.

Manual, single-phase cable switching by pulling cutouts or cable

elbows is also a major source of ferroresonance. This is a particular

problem during new construction when there is a lot of activity and the

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transformers are not yet loaded. Some utilities have reported that line

crews carry a “light board” or some other type of resistive load bank in

their trucks for use in cable-switching activity when the transformers

have no other load attached. One must be particularly careful when

switching delta-connected transformers; such transformers should be

protected because voltages may get extremely high. The common

grounded wye-wye pad-mounted transformer may not be damaged

internally if the exposure time is brief, although it may make consid￾erable noise. When switching manually, the goal should be to open or

close all three phases as promptly as possible.

Ferroresonance can generally be damped out by a relatively small

amount of resistive load, although there are exceptions. For the typ￾ical case with one phase open, a resistive load of 1 to 4 percent of the

transformer capacity can greatly reduce the effects of ferroreso￾nance. The amount of load required is dependent on the length of

cable and the design of the transformers. Also, the two-phase open

case is sometimes more difficult to dampen with load. Figure 4.38

shows the effect of loading on ferroresonance overvoltages for a

transformer connected to approximately 1.0 mi (1.61 km) of cable

with one phase open. This was a particularly difficult case that dam￾aged end-user equipment. Note the different characteristics of the

phases. The transformer was of a five-legged core design, and the

middle phase presents a condition that is more difficult to control

158 Chapter Four

0 5 10 15 20 25 30

1

1.2

1.4

1.6

1.8

2

2.2

2.4

2.6

2.8

3

A

B

C

Ferroresonant Voltage (per Unit)

Resistive Load @ 480 V BUS (% Transformer Capacity)

Figure 4.38 Example illustrating the impact of loading on ferroresonance.

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with loading. Five percent resistive load reduces the overvoltage

from approximately 2.8 to 2 pu. The transformer would have to be

loaded approximately 20 to 25 percent of resistive equivalent load to

limit ferroresonance overvoltages to 125 percent, the commonly

accepted threshold. Since such a large load is required, a three-phase

recloser was used to switch the cable.

On many utility systems, arresters are not applied on every pad￾mounted distribution transformer due to costs. However, surge

arresters can be an effective tool for suppressing the effects of ferrores￾onance. This is particularly true for transformers with ungrounded pri￾mary connections where the voltages can easily reach 3 to 4 pu if

unchecked. Primary arresters will generally limit the voltages to 1.7 to

2.0 pu. There is some risk that arresters will fail if subjected to fer￾roresonance voltages for a long time. In fact, secondary arresters with

protective levels lower than the primary-side arresters are frequent

casualties of ferroresonance. Utility arresters are more robust, and

there often is relatively little energy involved. However, if line crews

encounter a transformer with arresters in ferroresonance, they should

always deenergize the unit and allow the arresters to cool. An over￾heated arrester could fail violently if suddenly reconnected to a source

with significant short-circuit capacity.

Ferroresonance occurs when the cable capacitance reaches a critical

value sufficient to resonate with the transformer inductance (see Fig.

4.11). Therefore, one strategy to minimize the risk of frequent ferroreso￾nance problems is to limit the length of cable runs. This is difficult to do

for transformers with delta primary connections because with the high

magnetizing reactance of modern transformers, ferroresonance can

occur for cable runs of less than 100 ft. The grounded wye-wye connec￾tion will generally tolerate a few hundred feet of cable without exceeding

125 percent voltage during single-phasing situations. The allowable

length of cable is also dependent on the voltage level with the general

trend being that the higher the system voltage, the shorter the cable.

However, modern trends in transformer designs with lower losses and

exciting currents are making it more difficult to completely avoid fer￾roresonance at all primary distribution voltage levels.

The location of switching when energizing or deenergizing a trans￾former can play a critical role in reducing the likelihood of ferroreso￾nance. Consider the two cable-transformer switching sequences in Fig.

4.39. Figure 4.39a depicts switching at the transformer terminals after

the underground cable is energized, i.e., switch L is closed first, fol￾lowed by switch R. Ferroresonance is less likely to occur since the

equivalent capacitance seen from an open phase after each phase of

switch R closes is the transformer’s internal capacitance and does not

involve the cable capacitance. Figure 4.39b depicts energization of the

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transformer remotely from another point in the cable system. The

equivalent capacitance seen from switch L is the cable capacitance, and

the likelihood of ferroresonance is much greater. Thus, one of the com￾mon rules to prevent ferroresonance during cable switching is to switch

the transformer by pulling the elbows at the primary terminals. There

is little internal capacitance, and the losses of the transformers are

usually sufficient to prevent resonance with this small capacitance.

This is still a good general rule, although the reader should be aware

that some modern transformers violate this rule. Low-loss transform￾ers, particularly those built with an amorphous metal core, are prone

to ferroresonance with their internal capacitances.

4.7 Switching Transient Problems

with Loads

This section describes some transient problems related to loads and

load switching.

160 Chapter Four

(a)

underground cable

Switch L Switch R

(b)

underground cable

Switch L Switch R

Figure 4.39 Switching at the transformer terminals (a) reduces the risk of iso￾lating the transformer on sufficient capacitance to cause ferroresonance as

opposed to (b) switching at some other location upline.

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4.7.1 Nuisance tripping of ASDs

Most adjustable-speed drives typically use a voltage source inverter

(VSI) design with a capacitor in the dc link. The controls are sensitive

to dc overvoltages and may trip the drive at a level as low as 117 per￾cent. Since transient voltages due to utility capacitor switching typi￾cally exceed 130 percent, the probability of nuisance tripping of the

drive is high. One set of typical waveforms for this phenomenon is

shown in Fig. 4.40.

The most effective way to eliminate nuisance tripping of small drives

is to isolate them from the power system with ac line chokes. The addi￾tional series inductance of the choke will reduce the transient voltage

magnitude that appears at the input to the adjustable-speed drive.

Determining the precise inductor size required for a particular appli￾cation (based on utility capacitor size, transformer size, etc.) requires a

fairly detailed transient simulation. A series choke size of 3 percent

based on the drive kVA rating is usually sufficient.

4.7.2 Transients from load switching

Deenergizing inductive circuits with air-gap switches, such as relays

and contactors, can generate bursts of high-frequency impulses. Figure

4.41 shows an example. ANSI/IEEE C62.41-1991, Recommended

Practice for Surge Voltages in Low-Voltage AC Power Circuits, cites a

representative 15-ms burst composed of impulses having 5-ns rise

Transient Overvoltages 161

480-V Bus Voltage (phase-to-phase)

33.3 50.0 66.7 83.3 100.0 116.7

–1500

–1000

–500

0

500

1000

1500 Voltage (V)

Time (ms)

Figure 4.40 Effect of capacitor switching on adjustable-speed-drive ac current and dc

voltage.

Transient Overvoltages

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