Short T.A. Electric Power Distribution Handbook

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Voltage Sags and Momentary Interruptions 517

FIGURE 10.25

Ride-through duration for an interruption to several relays and contactors. (Data from [EPRI

PEAC Brief No. 44, 1998].)

FIGURE 10.26

Voltage magnitude for dropout of several relays and contactors for a ﬁve-cycle voltage sag.

(Data from [EPRI PEAC Brief No. 44, 1998].)

General purpose relays

Motor starters

Contactors

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Duration in 60-Hz cycles

90° point of wave

0° point of wave

General purpose relays

Motor starters

Contactors

0204060

Voltage, percent of nominal

90° point of wave

0° point of wave

0204060

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518 Electric Power Distribution Handbook

• Coil hold in — Use a coil hold-in device. Coil hold-in devices supply

current to keep a relay or contactor coil held in during a voltage sag

to about 25%.

• dc — Rectify the ac voltage and use a dc contactor. A dc contactor

generally has a longer ride through than an ac contactor. A capacitor

added in parallel with the contactor coil can extend the ride-through

time.

• Time-delay relay — Add a time-delay relay to the control circuit in

parallel with the contactor. If the contactor drops out because of a

voltage sag, the time-delay relay keeps the control circuit energized

(but the motor still drops out). If the sag ﬁnishes before the time-

delay setting has elapsed, the contactor pulls back in, and the motor

reconnects. This is not the best solution because the motor is discon-

nected and reconnected. The reconnection draws inrush current,

which can itself cause local disruptions, especially if multiple motors

are energized together. The high current may trip facility relays and

possibly damage motors. Additionally, the disconnection of many

motors within a facility removes the voltage support provided by

the motors feeding back into the utility system. Furthermore, when

the motors reconnect, the inrush creates a voltage sag.

• Power conditioner — Apply a constant voltage transformer or other

power conditioning device such as a dip-prooﬁng inverter to the

control circuit. This provides enough ride through for all but the

deepest voltage sags.

FIGURE 10.27

Ride-through capability of a NEMA size 1 contactor rated 0 to 50 hp at 480 V. (Data from [EPRI

PEAC Brief No. 10, 1993].)

0.00 0.05 0.10

0 2 4 6 8

Time, seconds

Voltage magnitude, percent

60-Hz cycles

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Voltage Sags and Momentary Interruptions 519

10.9.2.2 Adjustable-Speed Drives

Adjustable-speed drives (ASDs) are very common industrial tools used to

perform a variety of tasks. Figure 10.28 shows the most common drive

topology: the three-phase incoming supply is rectiﬁed to dc, and a pulse-

width modulation (PWM) inverter converts the dc to a variable frequency

three-phase ac voltage that drives an induction motor at variable speeds.

The capacitor on the dc bus provides some energy storage but not much.

Adjustable-speed drives are sensitive to voltage sags and almost always drop

out for momentary interruptions. A common cause for shutdown is that the

dc bus drops too low, and the drive shuts down on an undervoltage trip

(normally 70 to 85% of the nominal dc bus voltage). Even if a drive does not

actually shut down, the motor may stall and not be able to start without

removing the mechanical load. External factors can also cause shutdown: if

the drive is wired through a contactor and the contactor trips, the drive shuts

down (and the contactor may be more sensitive than the drive) (EPRI PEAC

commentary #3, 1998). Also, if programmable logic controllers provide stop,

start, or other signals, loss of the controller can shut the drive down. Fol-

lowing a voltage sag, the drive can draw large inrush, as much as three or

four times normal current. This can blow fuses or damage the input diodes

and trip the drive.

Drives have signiﬁcant variation. Of small drives tested by EPRI PEAC

(Brief No. 9, 1993) at full load, two could ride through ﬁve-cycle three-phase

sags down to zero volts; another two tripped at about 80% of rating. At half

load, three drives survived a ﬁve-cycle sag down to zero volts, and the other

tripped at 70% of rating. Some of the most sensitive drives are:

• Older drives — Many older drives power their electronics from the

ac system, which makes the controller more sensitive. Modern drives

power the controls from the dc bus.

• Higher horsepower drives — The front-end circuit normally uses thy-

ristors instead of diodes in a current-source topology. To prevent

FIGURE 10.28

Common adjustable-speed drive topology.

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520 Electric Power Distribution Handbook

commutation failure, the dc undervoltage relay is set more sensi-

tively, often 85 to 90%.

• dc drives — A thyristor-bridge feeds directly into the dc motor arma-

ture. dc drives have no built-in energy, and sags can disturb the

timing circuitry for ﬁring the thyristors. Regenerative converters

may be very sensitive, especially during regeneration (reverse power

ﬂow); a voltage sag can prevent a thyristor from shutting off, which

puts a short on the system that will blow a fuse.

Adjustable-speed drives are less sensitive to single-phase sags than three-

phase sags because all three phases are rectiﬁed (Mansoor et al., 1997). A

three-phase sag sags the dc bus voltage down in similar proportions. With

a single-phase sag, the two “unsagged” phases can support the drive’s dc

bus. Whether the drive trips or not depends on how heavily it is loaded and

how the undervoltage detection circuitry works. Figure 10.29 shows an

example ride-through capability for a 60-kW drive.

Conﬁguration adjustments can sometimes improve ride through; reducing

the undervoltage trip setting signiﬁcantly improves ride through. Increasing

the overcurrent trip setting and setting appropriate restart parameters can

also help. Also, some models have ﬁrmware upgrades providing additional

ride through. Drives with a ﬂying restart feature (the drive can restart while

the motor is spinning) are better for critical loads. A drive with a synchronous

ﬂying restart following a sag to 50% voltage for ﬁve cycles allowed only a

5% decrease in motor speed and was fully restored in 1/2 sec (a nonsyn-

chronous ﬂying restart is not nearly as good) (EPRI PEAC Brief No. 30, 1995).

FIGURE 10.29

Ride-through capability of a 60-kW adjustable-speed drive under different voltage sag condi-

tions. (Data from [Abrahams et al., 1999].)

3-phase sags

line-to-line sags

line-to-ground sags

0.1 1 .00.02 0.20.05 0.5

100

Time, seconds

Voltage magnitude, percent

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Voltage Sags and Momentary Interruptions 521

10.9.2.3 Programmable-Logic Controllers

The performance of programmable-logic controllers (PLCs) varies widely. A

PLC is a hardened electronic controller used to control many types of indus-

trial processes and equipment. PLCs have multiple input and output chan-

nels (I/O racks) used to measure and control equipment. A dc power supply

powers the cpu and the I/O racks.

In most cases, the PLC power supply is the same as other computer power

supplies, a switched-mode power supply usually capable of riding through

an interruption of several cycles. The problem is that the power supply is

often not the weakest link in the system. A power supply monitoring circuit

that senses the input voltage may initiate a shutdown during a voltage sag.

PLCs are more sensitive as a result.

Another important concern is that sags or interruptions can not only cause

a shutdown of the PLC, a sag can produce faulty outputs on some PLCs.

Faulty outputs can cause more havoc with some processes than if the PLC

actually shut down. Figure 10.30 shows the sensitivity of several program-

mable-logic controllers.

10.9.3 Residential Equipment

The digital clock has been quoted as being “the world’s best-selling power

quality recorder.” The “blinking clocks” are a nuisance for customers and

generate many phone calls for utilities. That said, clocks have a wide range

of sensitivities, and many actually have very good ride-through capability.

Figure 10.31 shows ride-through capabilities for several digital clocks

tested by EPRI PEAC and Hydro Quebec. There is a wide range of voltage

sensitivity, but few of the digital clocks tested (25%) lose memory for a

complete interruption that is less than 0.5 sec. The main consideration for

distribution circuits is the dead time on the ﬁrst reclose attempt. This is

usually about 0.3 to 5 sec depending on the delay on the reclosing relay. An

immediate reclose attempt has a dead time of 0.3 to 0.5 sec, making it a good

option for reducing blinking clocks. Table 10.11 shows the ride through of

different residential devices from a study by Northeast Utilities that also

shows most devices have good ride through for events less than 0.5 sec.

Most residential devices have a fairly rectangular volt-time characteristic.

Figure 10.32 shows the characteristic of several residential devices. Only

longer-duration voltage sags affect most residential devices.

10.10 Solution Options

10.10.1 Utility Options for Momentary Interruptions

We can reduce momentary interruptions in several ways:

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522 Electric Power Distribution Handbook

• Immediate reclose

• Use of fuse blowing

• Single-phase reclosers

• Extra downstream devices (fuses or reclosers)

• Sequence coordination with downstream devices

• Reduce faults

Reducing faults is a universally good approach to improving power quality.

Other approaches target speciﬁc disturbances. For momentaries, the single

FIGURE 10.30

Sensitivity of six PLCs. (Adapted from [EPRI PEAC Brief No. 39, 1996].)

0.0 0.2 0.4 0.6 0.8 1.0

CPU

trips

Bad I/O

0.0 0.2 0.4 0.6 0.8 1.0

Time, seconds

Voltage, percent

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Voltage Sags and Momentary Interruptions 523

biggest improvement is to use fuse blowing instead of fuse saving (see

Chapter 8). Using an immediate reclose, while not reducing the number of

momentaries counted, reduces complaints from residential customers. Both

of these changes for improving momentaries are relatively easy to implement.

Other methods of reducing momentaries involve better application of

protection equipment. Extra protection devices that segment the circuit into

smaller sections help improve momentary interruptions (and long-duration

interruptions). Improving coordination between devices (including use of

sequence coordination to improve coordination between reclosers) helps

eliminate some unnecessary blinks. Single-phase reclosers instead of three-

phase reclosers or breakers helps reduce the number of phases interrupted

for single line-to-ground faults.

FIGURE 10.31

Capability of digital clocks to ride through an interruption (n = 8). (Data from [Courtois, 2001;

Courtois and Deslauriers, 1997; EPRI PEAC Brief No. 17, 1994].)

TABLE 10.11

Percentage of Devices That Were Able to Successfully

Ride Through a Momentary Interruption of the

Given Duration

Device 0.5 sec 2 sec 16.7 sec

Digital clock 70 60 0

Microwave oven 60 0 0

VCR 50 37.5 0

Computer 0 0 0

Source: Bowes, K. B., “Effects of Power Line Disturbances

on Electronic Products,” Power Quality Assurance Magazine,

vol. Premier V, pp. 296–310, 1990.

0 5 10

100

Ride-through duration for an interruption, seconds

Percent of tested samples

exceeding the x-axis value

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524 Electric Power Distribution Handbook

10.10.2 Utility Options for Voltage Sags

Utility-side options for reducing voltage sags are limited and are rarely done

(at least solely for the purposes of voltage sags). Some of the strategies and

equipment that help are:

• Use of fuse saving

• Current limiting fuses

• Smaller lateral fuses

• Faster breakers or reclosers

• Raise the nominal voltage

• Reduce faults

Faster relaying or faster interrupters — any changes in protection schemes

that clear faults faster — help reduce the sag’s duration. The next few sections

address some of the options for reducing the impact of power-system faults

on the voltage.

FIGURE 10.32

Ride-through capability of various residential devices. (Data from [Courtois, 2001; Courtois and

Deslauriers, 1997].)

0.0 0.5 1.0 1.5

020406080

Duration, seconds

Amplitude, percent

Computer

Fax machine

Microwave oven

Oven digital clock

Telephone

VCR

0.0 0.5 1.0 1.5

020406080

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Voltage Sags and Momentary Interruptions 525

10.10.2.1 Raising the Nominal Voltage

Raising the nominal voltage helps the ride through of many types of equip-

ment. Computers, adjustable-speed drives, and other equipment with capac-

itors beneﬁt if the voltage is regulated near the upper end of the ANSI range

A, or at least avoid the lower end. On the utility system, we use LTCs,

regulators, and switched capacitors. This approach is the opposite of

demand-side management programs which are designed to deliver low

voltages in order to reduce peak demand or customer energy usage.

10.10.2.2 Line Reactors

Series line reactors provide electrical separation between feeders off the

substation bus. Figure 10.33 shows the effect of line reactors on the station

bus voltage for different conﬁgurations. Reactors have the added beneﬁt of

limiting the fault current. The reactors provide good protection against some

FIGURE 10.33

Substation bus voltage for a three-phase fault at the given distance for different line reactor

conﬁgurations (for a 12.5-kV circuit with the same parameters as Figure 7.11).

No reactor

1Ω

2Ω

0 2 4 6

0.0

0.2

0.4

0.6

0.8

0 2 4 6 8

Distance from the substation, miles

Per unit line-to-ground voltage

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526 Electric Power Distribution Handbook

voltage sags but do not help with interruptions or sags caused by transmis-

sion-level faults. On the faulted feeder, line reactors make the voltage sag

worse, but much of the feeder may have very low voltage anyway (down-

stream of the fault), or the breaker may open and give all of the customers

an interruption.

Utilities have used line reactors in this application, mainly at urban sta-

tions to reduce high fault currents. Line reactors do not have the best rep-

utation; they are expensive, take up substation space, and increase voltage

drop. The reactor must be designed to withstand the fault currents that it

will regularly see.

10.10.3.2 Neutral Reactors

Phase reactors effectively isolate feeders and limit the voltage sag, so how

about the reactor sometimes used between the substation transformer neu-

tral and ground? The answer depends on the transformer connection serving

the customer and the connection of the load. Neutral reactors generally make

voltage sags worse for loads connected line-to-ground on the distribution

circuit because:

• Duration — A neutral reactor lowers the fault current, so tap fuses

and time overcurrent relays take slightly longer to operate, increas-

ing the sag duration.

• Magnitude — A neutral reactor increases the zero-sequence imped-

ance. The weaker ground source cannot hold up the station line-to-

ground voltage as well for a downstream line-to-ground fault.

For line-to-line connected loads, the neutral reactor signiﬁcantly improves

voltages to end-use equipment, because of

• Neutral shift — The reactor adds impedance which shifts the neutral

point. This raises the voltage on the unfaulted phases, but supports

the line-to-line voltages. (A very large reactor in a high-impedance

grounded system would have almost no drop in the line-to-line

voltages during a single line-to-ground fault.)

With a single line-to-ground fault, the line-to-line voltage in per unit is

(assuming that the circuit resistances are zero for simpliﬁcation)

The neutral reactor of X ohms adds 3jX ohms to the zero-sequence imped-

ance and raises the X

ratio. Table 10.12 and Figure 10.34 show that a

XXXX

01 1

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