ESL-IE-98-04-53
VOLTAGE SAG-RELATED UPSETS OF INDUSTRIAL PROCESS CONTROLS
IN PETROLEUM AND CHEMICAL INDUSTRIES
Arshad Mansoor
Tom Key
Engineering Manager
Technical Director
EPRI Power Electronics Applications Center
Knoxville, TN
ABSTRACT
In the modem industrial facility, many electrical
and electronic control devices are routinely
integrated into automated processes, increasing the
potential for electrical compatibility problems. Hard­
wired motor control circuits are now retrofitted with
PLC controls. The sensitivity of these process
controls can stop an essential service motor required
for a continuous process such as in a refinery or
chemical plant. Typically the controls are sensitive
to the common momentary voltage sag caused by
power system faults many miles away from the plant
location. The number of upsets is directly related to
the number of these events and the sensitivity of
process control equipment. This paper describes the
sag-related upset problem and explains how today's
motor control technology determines the immunity of
entire petroleum and chemical process and plants.
OVERVIEW
Chemicals and Petroleum industry account for
over half of the energy consumed by U.S.
Manufacturers [I]. A major portion of this energy
consumption is in the form of electricity supplied by
the local utility company. Maintaining process
continuity in Petroleum Chemical industry is not only
important for the industry profitability but it also
impacts the electric supply company since, these
industries represent a major portion of the customer
load in terms of total kWh sales. This situation is
further complicated when a refmery electrical
distribution system experiences a voltage sag due to
faults in the power line on the utility distribution or
transmission grid that affects the production in the
plant. With the integration of sophisticated process
controls in these industries the impact of these
momentary disturbances are becoming even more
critical. In most cases these incidents leads to a
finger-pointing scenario between the customer and
the utility which ultimately does not help any of the
parties involved.
Sam Woinsky
Manager of Chemicals and Petroleum Program
EPRI Chemicals, Petroleum and Natural Gas Center
Houston, TX
The best way to resolve the incompatibility gap
between the electric supply system variations and
process equipment limits, is for engineers on both
sides to share information and work to bridge the
gap. On the utility side this involves trying to
minimize the number of events by taking pro-active
action in upgrading system protection and
maintenance. In the plant it entails understanding the
characteristics of the momentary disturbances and
taking steps to de-sensitizing the weak links in a
process. In subsequent sections we will discuss the
characteristics of voltage sags and momentary
interruption due to faults on the power line and their
impact on process motors and drives. Also covered is
details regarding voltage sag tolerance of typical
hardwired motor control circuits and of
programmable logic controllers (PLCs).
The main objective of this paper is to pinpoint
problem areas in typical process control designs. It is
intended to provide insight to facility engineers on
how voltage sags may impact their industrial process
control equipment and what can be done to identify
costs and benefits of in-plant modifications. Voltage
sag tolerance envelop of various industrial control
equipment will be presented from actual test data, the
impact of interconnectivity of sensitive equipment in
maintaining process continuity will be reviewed in
order to determine the weakest link in a process. It is
expected that this paper will serve as a useful
reference to industrial customers helping them to
identify and diagnose voltage sag related process
upset problems.
PO Disturbance: Voltage Sag
A voltage sag is defined as a decrease in RMS
voltage magnitude lasting from 0.5 to 30 cycles.
Voltage sags are usually caused by a fault in the
utility transmission or distribution system. Power line
faults can be due to animals in lines, car striking
utility pole, or lightning strikes to power lines. While
proper maintenance and grounding and arrester
location can minimize the number of faults, these
phenomena can never be eliminated.
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ESL-IE-98-04-53
voltage sag, the duration of which will depend on e
recloser clearing time. The depth of the sag will
depend on how close the customer is to the fault
location, resistance at the fault and the available fau t
current. Faults on the transmission system on other
hand will cause a sag to all the customers
downstream of that location. Figure 2 shows a
voltage sag that caused the system voltage to fall to
approximately 78% of nominal voltage for three or
four cycles. Based on the above discussion it can b
seen that customers can be affected by sags due to
faults at numerous other locations in the system
besides the feeder that is supplying them.
Most of the faults in the utility system are
cleared by the operation of utility system protective
devices. Substation breakers are typically set up with
a re-closing relay to open momentarily during a fault
condition and allow the fault to clear. The effects of
a temporary fault and recloser operation on an end
user vary, depending on the relative location of that
user in the system.
Referring to Figure I, a customer would actually
experience a brief interruption if a fault occurred on
the supplying distribution feeder. All other
customers on adjacent feeders will experience a
F au~ on transmission
system deared by
transmission line breake",
i
Transmission
Distribution
Feeder breakers
F au~ on fused brandl
cleared by fuse (may also
cause feeder breaker 10
operate)
T
End user affected
by voltage sags
Fau~ on main feeder deared by
feedar breaker or redoser
Figure 1.
Example Distribution System showing Fault Location and Customer Impact
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Example Voltage Sag Characteristics During a Fault
Voltage sags by no means are a new
phenomenon. However, due to increasing sensitivity
of sophisticated process controls, very-short-duration
sag events can shut down a complete system.
Equipment manufacturers typically specify limits of
proper equipment operation based on an expected
steady-state voltage range, such as +/- 5% or 10%.
However, the effect of voltage sags on equipment is
not usually considered as a design criterion. This
lack of consideration for the normal electrical
environment, coupled with the increased sensitivity
of equipment have caused a wide-spread
incompatibility problem that is affecting industrial
productivity.
While nobody expects an industrial process to
continue operating during an interruption, a
momentary voltage sag of 30% for as short as three
cycles perhaps should not, but often does cause a
complete plant shutdown. The control equipment
used in many processes, such as the PLC and the
remote block I/O, are extremely sensitive to voltage
sags and can shut down the process even if motor­
drives and other heavy equipment withstands the
disturbances. The effect of voltage sags on
continuous processes, such as chemical refmeries,
may have even more damaging consequences when
the process unexpectedly shuts down.
Vo1taee Sae Statistics
Voltage sags affecting industrial plants and
process equipment are by no means a unique
situation. The Electric Power Research Institute
(EPRI) Distribution Power Quality (DPQ)
Monitoring Project has collected data from
distribution systems around the country[2]. Figure 3
summarizes the result of the three-year monitoring
project with sample points taken from over 250 sites.
This data provides an estimate of the typical
magnitude and frequency of voltage sags and
momentary interruptions that may be expected at a
given site. As can be seen from Figure 3, an average
of 66 voltage sags between 10% and 90% is expected
at a given site. Also, the average of number of
momentary interruption at a given site is between 8
and 9
However, it should be noted that at any given
site, the actual sag rate may vary depending on the
configuration of the distribution and transmission
system, customer density, geographical location, and
utility preventive-maintenance policy.
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ESL-IE-98-04-53
Interruption and Sag Rate as a Function of Voltage Magnitude
·Customer Events· for All Sites, Average of Felder Sections
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Figure 3.
Sag and Interruption Rate Magnitude Histogram from DPQ Data
Impact of Voltage Sags on Motors and Drives
The primary process loads are motor-driven in a
typical petroleum or chemical plant. Low voltage
and medium voltage induction motors, medium
voltage synchronous motors, and motors controlled
by electronic ac or dc drives make up the motor­
driven process in a typical plant. Motors ranging
from a few horsepower to thousands of horsepower,
direct connected to the utility system or through a
variable speed drive provide a variety of process and
auxiliary functions.
of the motors and drives, but mainly the response
the controls.
0'
The impact of voltage sag and momentary
interruption on an induction motor-driven process
has been explained in great detail in a number of
references [4,5,6]. A complete analysis of the subj ct
is beyond the scope of this paper. However, the
consensus among all the references is that keeping
motor connected to a line is the best strategy.
Allowing the motor to ride-through the voltage sag
works well as long as the process can handle the
speed loss. It is nearly always a better alternative
than taking the motor off line and reclosing on the
motor upon restoration of voltage. In a case wher
the motor is taken off line, due to concern for
excessive transient torque from out of phase
reclosing, the safe approach for reconnection is to
wait for the voltage at the motor terminal to decay 0
at least 133 percent ofnarneplate voltage before
attempting reconnection [5].
In addition to the specific process equipment, the
in-plant utility system is extremely important in
maintaining process continuity. Motors are once
again the main category of electrical equipment used
as ill fans, FD fans, boiler feed water pumps and in
many other applications. Because of the
interrelationship between plant utility system and the
process requirement any electrical disturbance that
will disrupt the utility operation will likely cause
process upset. For example, if a voltage sag causes a
loss of a feed water pump the resultant loss of steam
might trigger the shutdown of a process equipment
which in tum could start a chain reaction of process
disruption[3].
The behavior of synchronous motors during
voltage sag depends on the excitation control [6]. f
the voltage sag causes the field current to decay th
this could cause the motor to pullout of
synchronization with the system, resulting in tripp'
of the motor circuit breaker by the pull-out (power
e
factor relay). Because of that current regulated
e
excitation system are preferred alternative. This
of excitation system prevent the synchronous mot
field current to decrease during a voltage sag and
prevent unnecessary pull-out of the synchronous
motors during momentary voltage sags.
In order to analyze the sensitivity of these motor­
driven processes the main issues is the sensitivity of
the motor control scheme and the drop-out
characteristics of the motor contactors. However, a
brief overview of the impact of voltage sags and
momentary interruption on the motors and drives is
necessary to understand why in most cases,
disruption of process is not because of the response
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Proceedings from the Twentieth National Industrial Energy Technology Conference, Houston, TX, April 22-23, 1998
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DC LINK
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SUPPLY
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(UNCONTROLLED)
Figure 4:
INVFRTfR STAr.E
(CONTROLLED)
Schematic of a Typical AC Drive
AC
SUPPLY
DC WOTOR
ARWATURE
Figure 5:
Schematic of a Typical DC Drive
Typically an ac drive consists of a rectifier stage
that converts the incoming ac voltage to a dc voltage
across dc-bus capacitor and then the inverter stage
that converts this dc voltage to ac voltage of desired
frequency and magnitude using the pulse width
modulated scheme (see Figure 4). The dc-bus
capacitor is the primary energy storage element in an
ac drive. During a voltage sag the dc-bus voltage
decreases and when it reaches the undervoltage trip
point (typically set anywhere between 75 and 85
percent of the nominal dc bus voltage) of the drive
will trip on undervoltage [7]. Sine the dc bus charges
to the peak of the line to line voltage a single phase
line-ground sag in most cases will Dot cause the bus
voltage to fall below the trip point because enough
voltage remains between one line to line. However,
in a majority of cases drives do trip for such sags
because either the control circuit in the drive is
affected by the sag or interface to the drive from
relays or other control equipment does not have the
required ride-through for voltage sags.
The dc drive behaves much differently than the
typical ac drive due to its different topology and its
lack of inherent energy storage [7]. A typical dc drive
layout is shown in Fig. 8. The ac source is coupled to
the armature of the dc motor via a controlled
rectifier. Although it has many variations, this six­
pulse, six-SCR topology is the most common. During
normal operation, an SCR is fired every 1/6 ofa
cycle in a sequence determined by the phase rotation
of the ac supply, with each SCR conducting for 2/6
of a cycle. The gate timing of the SCRs is precisely
controlled relative to the supply voltage waveform to
yield an output with the desired average voltage.
Usually a feedback loop (voltage or speed) controls
this firing angle.
Modem dc drives incorporate timing circuitry
that normally synchronizes to the zero crossing of
one of the line voltages. A phase-locked loop (PLL)
stabilizes the timing circuitry. Phase rotation is
determined automatically, thus the firing sequence is
determined by what the drive's intemal sensing
circuitry. Most drives monitor the RMS value of the
incoming waveform (usually through a peak
detecting circuit) and will trip for undervoltage,
overvoltage, phase loss, and excessive current, either
line current and/or armature current.
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Proceedings from the Twentieth National Industrial Energy Technology Conference, Houston, TX, April 22-23, 1998
ESL-IE-98-04-53
Impact of Voltage Sag on Motor Control Circuits
(Hardwired, without PLC)
Motor Control Centers for low voltage and
medium voltage motors are often the weakest link
that disrupts process operation due to momentary
voltage disturbances. Low-voltage motors are
typically controlled by contactors, held closed by an
a-c magnet coil. A sudden drop in voltage reduces
the magnet force sufficiently to allow the starting
contactor to" drop out," the armature drops free and
the contacts open. This event can lead to several
different out comes and impacts on the overall
process, depending on the control design philosophy.
Some common approaches observed in the petro­
chern industry are:
• Manual, push-button reset required after drop
out
(sometimes called" 3-wire control")
• Auto restart after a set time delay (sometimes
called" time-delay on")
• Auto restart if duration of low voltage is less
than setting (" time-delay off')
• Auto restart immediately when power returns
(" maintain on," or" 2-wire control")
Properly employed these techniques can improve
the protection and control of process. However in
many applications other electrical control devices'
individual response to low voltages interfere with the
intended outcome. For example the output of
electrical sensors, interposing relays, PLCs may not
provide the appropriate command during these
abnonnal voltage events. As discussed earlier,
perhaps the best case for motor ride-thru is to remain
connected to the power system during a sag event,
however most motor control systems are designed to
prevent that outcome.
which voltage sag happens has a significant impact
on the drop-out level [8].
3 WIRE CONTROL
M
START
--l.­ ()
....
--<1....L~_~.()
OL
--I
STOP
M
Figure 6.
Basic Manual Restart Required, Thre Wire Control Circuit
Most often large-size contactors have enough
magnetic energy stored in the coil to prevent
premature dropout. In most cases ride-through for
these coils extends up to 40-50 percent of nominal
voltage. However, the use of interposing relays to
switch the main contactor can significantly reduce
the drop-out characteristics of the motor starter. F r
example, as shown in Figure 7 the interposing rela
can be an .. ice-cube" type small relay with
significantly less magnetic energy stored in the coi .
Figure 8 shows the relative ride-through
characteristic of a main NEMA Size 1 contactor co;"
and a small .. ice cube" relay. Even though the rna
contactor coil can survive a voltage sag up to 40%
nominal voltage, because of the sensitivity of the
interposing relay the starter will drop out for a
voltage sag down to approximately 75% of nomin I
voltage.
In a three-wire control scheme as shown in
Figure 6, commonly used for low voltage motors,
undervoltage protection is provided by means of a
seal-in contact across the momentary contact push­
button (start-stop) station. In such an arrangement
the drop-out characteristic of the contact coil
determines how severe a voltage sag the motor can
withstand before dropping off-line.
Unfortunately, no standard governs the specific
voltage at which a motor contaetorlstarter will drop
out. Few manufacturer publish such data and in may
cases whatever data is provided can be subject to
misinterpretation. For example, if the voltage is
gradually lowered from the rated value the drop out
point for a contactor will be quiet different then the
drop-out point if the voltage drops suddenly as is the
case during a voltage sag. This situation is further
complicated by the fact that the point on wave at
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Figure 7.
Three-Wire Control Circuit With
Control Transformer and Interposing Relay
The ride-through characteristics even for similar
rated relays and contactors can be widely differen
depending on manufacture to manufacturer. Also,
the push for compactness and space optimization ,as
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Proceedings from the Twentieth National Industrial Energy Technology Conference, Houston, TX, April 22-23, 1998
ESL-IE-98-04-53
120 Vac 60 Hz
led to smaller and smaller devices which has even
less capability of energy storage.
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Figure 8.
Ride-Through Characteristics of
Control relay and Size I Contactor
Starters used for medium voltage motor typically
use a main holding coil, which is a dc coil that
derives its power from a rectifier power supply. The
drop-out characteristic of dc coils are more robust
than its ac counter part. However, the control circuit
often uses ac relays that can interrupt the power
supplied to the rectifier during a voltage sag. The
drop-out characteristics of medium voltage starters is
typically governed by the auxiliary ac control relay.
A common method of keeping critical process motors
running after a voltage sag is to apply a normally­
open, timed open (NOTO) or a "time delay off'
contact across the start push-button. This
arrangement, shown in Figure 9, allows the motor
starter to drop out during a voltage sag and then, if
voltage is restored within a preset time, the starter is
automatically reclosed. Randomly applied for
numerous motors in a plant this arrangement can
often cause more problems then it was suppose to
solve. Simultaneous reclosing of a large number of
motors can by itself cause a voltage sag if the supply
system is not stiff enough to handle the resulting
inrush current. The resulting secondary sag may be
more severe than the first one, and may cause other
motors to drop out.
Figure 9.
Typical "Time-Delay Off' in a
Medium Voltage Starter Circuit
One way to resolve this problem is to disable the
"time delay off' and allow the motor to ride-through
the voltage sag without disconnecting it from the
line. This can be done by introducing a delay in the
dc-coil circuit as shown in Figure 10 [9]. By
allowing a time delay in the DC coil circuit, power to
the rectifier is maintained even if the auxiliary coil
drops out. This type of control method typically
provides a 10 cycle ride-through and can be
retrofitted into existing starters.
120 Vec 60 Hz
Figure 10.
Through
Modified MY Starter Circuit with Ride­
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Proceedings from the Twentieth National Industrial Energy Technology Conference, Houston, TX, April 22-23, 1998
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Impact of Voltage Sag on Motor Control Circuits
(PLC Controlled)
With the increasing use ofprograrnmable Logic
Controllers (PLC) for motor control and other
process control there is an added element in
sensitivity to voltage sags besides the classical relay
and contractor drop-out problems that has been
discussed in the previous section. In order to
understand the voltage sag related issues with PLCs a
brief description of a typical PLC system along with
an understanding of how voltage sags might impact
the operation of the PLC is needed.
Voltage sags can affect the CPU, the PLC I/O
cards and also the PLC logic levels. Anyone of
these potential malfunction could disrupt the proce
continuity. One of the weak link in a PLC system i
the power supply. The power supply is typical of y
other electronic equipment such as a personal
computer (PC) taking 120V input and providing th
required low voltage DC output to the CPU and 0
PLC components. The ride-through characteristics f
the power supply depends mainly on the size of the
ripple control and energy storage capacitor used in
the typical switch-mode power supply.
The PLC is basically a hardened industrial
computer that has multi-channel input and output
modules to control process devices such as
conveyors, pumps, and control valves. Figure 11
shows the basic elements of a typical PLC. The PLC
controls these devices based upon a four-cycle step:
It 1) reads input data (Input Module) 2) Solves its
control program (CPU), 3) Diagnoses itself (CPU), 4)
modifies its output according to its program (Output
Module). The cycle time, the amount of time it takes
the PLC to complete all four steps can be as short as
20 milliseconds.
The I/O system form the interface by which fi d
devices are connected to the controller. Input devi e
such as push-buttons, limit switches sensors are
hardwired to the input terminal as shown in Figure
12. The most common type if input devices are
discrete type that senses the presence or absence of
voltage on the terminal like a logic 1 or O. Howeve ,
the voltage threshold for the logic states are defme
is not standardized. For example if a voltage sag
causes the voltage at the PLC input to drop to 60% of
nominal value for 6 cycles will that be considered
logic 0 or I ? If it is consider as a logic zero and it
falls within the PLC scan time then the PLC
controller will take action based on this roomen
disturbance and may cause the stoppage of a critic
process motor. Figure 13 and 14 shows the respo
of the discrete input modules and the CPU for two
the six PLCs tested to determine their response to
voltage sag[ 10]. As can be seen there is no stan
regarding how much ride-through the CPU has or
what the threshold level for the discrete input mod e
to sense the presence or absence of a signal is.
I
Figure 11.
Typical Programmable Logic Controller
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L2
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Figure 13. Voltage Sag Ride-Through Curves for Model D-1 PLC
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Proceedings from the Twentieth National Industrial Energy Technology Conference, Houston, TX, April 22-23, 1998
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100
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DURATION OF SAG (IN CYCLES)
Figure 14. Voltage Sag Ride-Through Curves for Model G-l PLC
Sometimes PLC I/O racks can be located near
the field devices to minimize the amount of wiring
required. This rack as shown in Figure 13 is referr d
to as remote I/O rack. In such a scenario, not only
the main power supply that provides power to the
CPU but also the power supply for the remote I/O
rack becomes critical in maintaining the integrity
the remote I/O module. In many plants, the main
CPU power supply may have back-up power in th
form of an Uninterruptible Power Supply (UPS),
however the remote I/O rack power supply can ca
process interruption.
Figure 15 shows the typical field connection for
a discrete output module. The output module can be
thought of as a switch that turns on a motor contactor
or a solenoid or a pilot light based upon the controller
decision. Typically relays or Traics are used as
switches inside the output module. If the power
supply of the PLC that provides power to the active
and passive elements inside the output module drops­
out, then the integrity of these switches are lost and
the output devices such as motor contactor may lose
power.
"TYPICAL
-GROUND CASU! SHIEU)
AT ONE END ONLY
(CHASSIS IIOUNTlNG BOLT)
AC OUTPUT MODULE
CONNECTION DIAGRAM
Figure 15.
TTL OlJTPUT MODULE
CONNECTION DIAGRAM
Typical connection ofPLC Discrete Output Module
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Local VO
Pusltlutton
Figure 16.
Typical Arrangement of Remote I/O Racks
Safety Versus Process Cogtinuity Considerations
A hardwired master control relay (MCR), as
shown in Figure 17, is included to provide a
convenient means for emergency controller
shutdown. Since the master control relay allows the
placement of several emergency-stop switches in
different locations, its installation is important from a
safety standpoint. However, the same safety feature
sometimes work as an undervoltage relay and stops
the entire process even if it was not intended to do so.
The main function of the safety circuit is to ensure
that the operators can quickly remove power from the
MCR and inhibit all machine motion by removing
power to the machine I/O devices when the relay is
de-energized. If a voltage sag on other hand causes
the MCR relay to de-energize, then this would cause
the same response as if somebody has hit the E-stop
button.
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Proceedings from the Twentieth National Industrial Energy Technology Conference, Houston, TX, April 22-23, 1998
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L1
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Figure 17.
'"
/
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Typical arrangement of MCR Relay in PLC wiring
would never be true. Now, if the normally closed
(NC) wiring is used, the input point receives powe
continuously unless the stop button is depressed.
However, the same effect can take place during
voltage sag if the PLC input module responds to
voltage sag as a logic 0 condition and thinks that e
push-button has been depressed.
Another safety consideration concerns the wiring
of the stop buttons. A stop button is generally
considered a safety function as well as an operating
function and due to safety consideration is wired as a
nonnally closed contact (NC). Using a normally
open (NO) contact to examine for an off condition
will produce the same logic but is not considered
safe. For example, Figure 18 shows the nonnally
open push-button stop configuration. If, by some
chain of events, the circuit between the button and
the input point were to be broken, the stop button
could be depressed forever, but the PLC logic could
never react to the stop command since the input
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Proceedings from the Twentieth National Industrial Energy Technology Conference, Houston, TX, April 22-23, 1998
ESL-IE-98-04-53
L,
L2
Input
Ladder logic Program
Output
REFERENCES
Slop
Stlp
1.
Start
1.
:=r
Start
M
1) Energy Infonnation Administration,
Manufacturing Energy Consumption Survey, EIA
846
M
Figure 18. Typical arrangement Nonnally Open
Push-Button Wiring
CONCLUSIONS
Many components and devices used in industrial
processes are sensitive to momentary voltage sags. In
industries such as petroleum and chemical, where
critical processes may be large and complex sag
related upsets can be costly, the sensitivity of the
control devices needs to be carefully evaluated.
Often the entire process is vulnerable at a threshold
equal to the weakest device in that process. Controls
are usually the fIrst to trip or to call for a trip in some
larger motor or drive device depending on the control
design philosophy. By understanding the nature of
sag events in a typical electrical environment and by
applying some ride-through enhancement devices
and design techniques signifIcant improvement in
plant performance can be achieved. More work is
needed to develop and standardize sag tolerant
factory process control and protection systems.
2) D. D. Sabin, T.E. Grebe, A. Sundaram,
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and Perspectives (PQA '95), New York, New York,
May 1995
3) K. W. Carrick, "Minimizing the Effects of
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Processes," IEEE Transactions on Industry
Applications, Vol. 32, No.6, NovemberlDecember
1996,pg.1424-1430
4) G. W. Botrel and 1. Y. Yu, "Motor Behavior
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no.5, Sept./Oct. 1980
5) R. H. Daugherty, "Bus Transfer of AC Induction
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6) 1. R. Linders, "Effects of Power Supply Variations
on A.c. Motor Characteristics," IEEE Transactions
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July/August, 1972
7) A. Mansoor, E. R. Collins, R.L. Morgan, "Effect
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on Harmonics and Quality of Power, ICHQP, Las
Vegas, NY, Octoberl6-18, 1996
8) A. E. Turner, E. R. Collins, "The Performance of
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10) EPRl PQTN Brief No. 39, "Ride-Through
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Proceedings from the Twentieth National Industrial Energy Technology Conference, Houston, TX, April 22-23, 1998