1. Power System Quality and Reliability
VOLTAGE SAGS AND INTERRUPTIONS
Dr. Prathibha
1

2. > Voltage sags and interruptions are related power quality
problems.
> Both are usually the result of faults in the power system
and switching actions to isolate the faulted sections.
> They are characterized by rms voltage variations outside
the normal operating range of voltages.
Introduction
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3. > A voltage sag is a short-
duration typically 0.5 to 30
cycles) reduction in rms
voltage caused by faults on
the power system and the
starting of large loads, such
as motors.
> Typical end-use equipment
sensitive to voltage sags
are computers,
programmable logic
controllers, controller power
supplies, motor starter
contactors, control relays
and adjustable speed
drives.
Voltage sag
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4. > Voltage swell is
an increase in
RMS voltage at
the power
frequency for
duration of 0.5
cycles to 300
cycles.
Voltage swell
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5. > An interruption is defined
as a reduction in line-
voltage or current to less
than 10 percent of
nominal, not exceeding
60 seconds in length.
> Interruptions can be a
result of control
malfunction, faults, or
improper breaker
tripping. Figure shows
an interruption of
approximately 1.7
seconds in length.
Interruption
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6. > Momentary interruptions (typically no more
than 2 to 5 s) cause a complete loss of voltage
and are a common result of the actions taken
by utilities to clear transient faults on their
systems.
> Sustained interruptions of longer than 1 min
are generally due to permanent faults.
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7. Seven types of dip h, residual
voltage (pu)
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10. > Voltage sags and
interruptions are
generally caused
by faults (short
circuits) on the
utility system.
Sources of sags and interruptions
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11. The following is a general procedure for working with industrial
customers to assure compatibility between the supply system
characteristics and the facility operation:
> Determine the number and characteristics of voltage sags that
result from transmission system faults.
> Determine the number and characteristics of voltage sags that
result from distribution system faults.
> Determine the equipment sensitivity to voltage sags. This will
determine the actual performance of the production process
based on voltage sag performance calculated in steps 1 and 2.
> Evaluate the economics of different solutions that could
improve the performance, either on the supply system (fewer
voltage sags) or within the customer facility.
Estimating Voltage Sag Performance
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12. > Equipment within an end-user facility may have different sensitivity to
voltage sags.
> Equipment sensitivity to voltage sags is very dependent on the specific
load type, control settings, and applications.
> Generally, equipment sensitivity to voltage sags can be divided
into three categories:
(i) Equipment sensitive to only the magnitude of voltage sag.
(ii)Equipment sensitive to both the magnitude and duration of
a voltage sag.
(iii) Equipment sensitive to characteristics other than
magnitude and duration.
Equipment Sensitivity to Voltage Sags
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13. Equipment sensitive to only the magnitude of voltage sag.
> This group includes devices such as under voltage relays, process
controls, motor drive controls and many types of automated machines
Devices in this group are sensitive to the minimum (or maximum)
voltage magnitude experienced during a sag (or swell).
> The duration of the disturbance is usually of secondary importance for
these devices.
Equipment sensitive to both the magnitude and duration of a
voltage sag.
> This group includes virtually all equipment that uses electronic power
supplies.
> Such equipment mis-operates or fails when the power supply output
voltage drops below specified values.
> Thus, the important characteristic for this type of equipment is the
duration that the rms voltage is below a specified threshold at which
the equipment trips.
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14. Equipment sensitive to characteristics other than magnitude and
duration.
> Some devices are affected by other sag characteristics such as the
phase unbalance during the sag event, the point-in-the wave at which
the sag is initiated, or any transient oscillations occurring during the
disturbance.
> These characteristics are more suitable than magnitude and duration,
and their impacts are much more difficult to generalize.
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15. > The voltage sag performance for a given customer facility will depend
on whether the customer is supplied from the transmission system or
from the distribution system.
> For a customer supplied from the transmission system, the voltage sag
performance will depend on only the transmission system fault
performance.
> On the other hand, for a customer supplied from the distribution
system, the voltage sag performance will depend on the fault
performance on both the transmission and distribution systems.
> Transmission line faults and the subsequent opening of the protective
devices rarely cause an interruption for any customer because of the
interconnected nature of most modern-day transmission networks.
These faults do, however, cause voltage sags.
> Depending on the equipment sensitivity, the unit may trip off, resulting
in substantial momentary losses.
> The ability to estimate the expected voltage sags at an end-user
location is therefore very important.
Transmission System Sag Performance Evaluation
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16. > Customers that are supplied at distribution voltage levels are impacted
by faults on both the transmission system and the distribution system.
> The analysis at the distribution level must also include momentary
interruptions caused by the operation of protective devices to clear the
faults.
> These interruptions will most likely trip out sensitive equipment. The
overall voltage sag performance at an end-user facility is the total of
the expected voltage sag performance from the transmission and
distribution systems.
The critical information needed to compute voltage sag
performance can be summarized as follows:
> Number of feeders supplied from the substation.
> Average feeder length.
> Average feeder reactance.
> Short-circuit equivalent reactance at the substation.
Utility Distribution System Sag Performance Evaluation
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17. > Several things can be done by the
utility, end user, and equipment
manufacturer to reduce the number
and severity of voltage sags and to
reduce the sensitivity of equipment
to voltage sags.
> Figure illustrates voltage sag
solution alternatives and their
relative costs.
> As this chart indicates, it is generally
less costly to tackle the problem at
its lowest level, close to the load.
> The best answer is to incorporate
ride through capability into the
equipment specifications themselves.
> This essentially means keeping
problem equipment out of the plant,
or at least identifying ahead of time
power conditioning requirements.
Fundamental Principles of
Protection
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18. Any company’s equipment procurement specifications to
help alleviate problems associated with voltage sags:
> Equipment manufacturers should have voltage sag ride-
through capability curves available to their customers so
that an initial evaluation of the equipment can be
performed.
> The company procuring new equipment should establish
a procedure that rates the importance of the equipment.
> Equipment should at least be able to ride through
voltage sags with a minimum voltage of 70 percent.
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19. > Solutions to improve the reliability and performance of a process or facility
can be applied at many different levels.
> The different technologies available should be evaluated based on the
specific requirements of the process to determine the optimum solution for
improving the overall voltage sag performance.
The solutions can be discussed at the following different levels of
application:
1. Protection for small loads [e.g., less than 5 kilovoltamperes (kVA)].
> This usually involves protection for equipment controls or small, individual
machines. Many times, these are single-phase loads that need to be
protected.
2. Protection for individual equipment or groups of equipment up to
about 300 kVA.
> This usually represents applying power conditioning technologies within the
facility for protection of critical equipment that can be grouped together
conveniently. Since usually not all the loads in a facility need protection,
this can be a very economical method of dealing with the critical loads,
especially if the need for protection of these loads is addressed at the
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Solutions at the End-User Level

20. 3. Protection for large groups of loads or whole facilities at the low-
voltage level.
> Sometimes such a large portion of the facility is critical or needs
protection that it is reasonable to consider protecting large groups of
loads at a convenient location (usually the service entrance).
> New technologies are available for consideration when large groups of
loads need protection.
4. Protection at the medium-voltage level or on the supply system.
> If the whole facility needs protection or improved power quality,
solutions at the medium-voltage level can be considered.
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21. > Motor - generator set
> Ferroresonance transformer
> Electric tap changer
> Dynamic voltage restorer (DVR)
> Uninterrupted power supply
> Static transfer switch
> Series - connected voltage source converter
> Shunt connected back - up source
> Superconducting magnetic energy storage
(SMES) devices
Mitigation Methods
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22. > Motor-generator (M-G) sets come in a wide variety of sizes and configurations.
> This is amature technology that is still useful for isolating critical loads from
sags and interruptions on the power system.
> The concept is very simple, as illustrated in Fig. Motor Generator Sets are a
combination of an electrical generator and an engine mounted together to
form a single piece of equipment.
> Motor generator set is also referred to as a genset, or more commonly, a
generator.
> Motor generator sets are used throughout industry to provide electrical power
on demand.
> A motor powered by the line drives a generator that powers the load.
Flywheels on the same shaft provide greater inertia to increase ride-through
time.
> When the line suffers a disturbance, the inertia of the machines and the
flywheels maintains the power supply for several seconds.
> This arrangement may also be used to separate sensitive loads from other
classes of disturbances such as harmonic distortion and switching transients.
Motor Generator Set
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24. > Ferroresonant transformers, also called constant-voltage
transformers (CVTs), can handle most voltage sag conditions.
(See Fig.) CVTs are especially attractive for constant, low
power loads.
> Variable loads, especially with high inrush currents, present
more of a problem for CVTs because of the tuned circuit on the
output.
> Ferroresonant transformers are basically 1:1 transformers
which are excited high on their saturation curves, thereby
providing an output voltage which is not significantly affected
by input voltage variations.
> A typical ferroresonant transformer schematic circuit diagram
is shown in Fig.
Ferroresonance Transformer
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26. > The actual tap-changer which is used in the Indian Railways
locos, it will better to understand in general what tap-changers
are.
> The output voltage of a transformer varies according to the
turn’s ratio of the primary and the secondary windings of the
transformer.
> It can appreciated that at any point of the primary or the
secondary winding the voltage is different from any other point
on the same winding because these points are at different
ratios with respect to the other winding.
> Hence each and every tap brought out from the winding gives
a different voltage.
Electric Tap Changer
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27. Dynamic voltage restorer (DVR)
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28. connecting a series voltage source
between the critical load and disturbed
power supply source, these systems can
also alter the equivalent reactance of a
power system, function as a phase
shifter, provide balancing and active
elimination of voltage distortion at the
load terminals, etc.
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

29. > An Uninterruptible Power Supply (UPS), also known as an
Uninterruptible Power Source, Uninterruptible Power System,
Continuous Power Supply (CPS) or a battery backup is a device
which maintains a continuous supply of electric power to
connected equipment by supplying power from a separate
source when utility power is not available.
> There are two distinct types of UPS: off-line and line-
interactive (also called on-line).
> An off-line UPS remains idle until a power failure occurs, and
then switches from utility power to its own power source,
almost instantaneously.
> An on-line UPS continuously powers the protected load from its
reserves (usually lead-acid batteries or stored kinetic energy),
while simultaneously replenishing the reserves from the AC
power.
Uninterrupted Power Supply
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30. > In this design, the load is always
fed through the UPS.
> The incoming ac power is rectified
into dc power, which charges a
bank of batteries.
> This dc power is then inverted
back into ac power, to feed the
load.
> If the incoming ac power fails, the
inverter is fed from the batteries
and continues to supply the load.
> In addition to providing ride-
through for power outages, an on-
line UPS provides very high
isolation of the critical load from
all power line disturbances.
However, the on-line operation
increases the losses and may be
unnecessary for protection of
On-line UPS
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y
loads. 30

31. > A standby power supply (Fig.)
is sometimes termed off-line
UPS since the normal line
power is used to power the
equipment until a disturbance
is detected and a switch
transfers the load to the
battery backed inverter.
> The transfer time from the
normal source to the battery
backed inverter is important.
OFF LINE UPS
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32. > The Static Transfer Switch provides break-before-make switching
between two independent AC power sources for uninterrupted power to
sensitive electronic equipment.
> When used with redundant AC power sources, the switch permits
maintenance without shutting down critical equipment.
> The switch utilizes solid-state switching devices close to the critical load,
thus producing high levels of power availability and power system
tolerance.
> The switch is suited for data processing, distributed computing,
telecommunications equipment, and high-tech manufacturing
applications.
Features include:
> 0.25 cycle maximum transfers between AC power sources;
> Manual and automatic transfers;
> Selectable preferred input sources;
> AccuVar TVSS for both AC inputs;
Static Transfer Switch and fast transfer switches
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Clear LCD monitoring panel with on screen instructions;
>

33. 5/13/2020 33

34. > A device and a method for controlling the flow of electric power in a
transmission line carrying alternating current, in which a first voltage
source converter is connected to the transmission line at a first point
and a second voltage source converter is connected to the
transmission line at a second point.
> Further, the first and second voltage source converters have their
direct current sides connected to a common capacitor unit.
> Also included is a by-pass switch connected to the transmission line
between the first point and the second point in parallel with the first
and second voltage source converters so that the first and second
voltage source converters will operate as a back-to-back station when
the bypass switch is open and as two parallel static var compensators
when the by-pass switch is closed.
Series connected voltage source converter
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35. > A shunt connected energy stabilizing system with isolation switching
for providing stored energy to loads or to a utility or industrial
electrical distribution system or source of electrical power.
> An energy backup and recovery system stores energy in a
superconducting magnet and releases the energy to a real
power/reactive power (VARs) generator which in turn delivers energy
to either the loads or to both the loads and the source of electrical
power.
> During periods of voltage sag or power outage, an isolation switch
provide a means for isolating the loads from the source of power so
that energy can be supplied to the loads only to provide "ride-thru".
> In effect, the isolation of the load sheds this load from the power
system, thereby boosting the rest of the electrical distribution to a
level so that other loads on the power system are not disturbed by the
voltage sags.
> By supplying energy to the loads only, small superconducting magnets
can be used thereby providing economic and size advantages.
Shunt connected back - up source
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36. > An SMES device can be used to alternate voltage sags and brief
interruptions.
> The energy storage in an SMES-based system is provided by the electric
energy stored in the current flowing in a superconducting magnet. Since
the coil is lossless, the energy can be released almost instantaneously.
> Through voltage regulator and inverter banks, this energy can be injected
into the protected electrical system in less than 1 cycle to compensate for
the missing voltage during a voltage sag event.
The SMES-based system has several advantages over battery-based
UPS systems:
> SMES-based systems have a much smaller footprint than batteries for
the same energy storage and power delivery capability.
> The stored energy can be delivered to the protected system more
quickly.
> The SMES system has virtually unlimited discharge and charge duty
cycles.
> The discharge and recharge cycles can be performed thousands of times
Superconducting magnetic energy storage (SMES) devices
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37. Typical power quality-voltage
regulator (PQ-VR) functional block
diagram
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38. The economic evaluation procedure to find the best option for
improving voltage sag performance consists of the following
steps:
1. Characterize the system power quality performance.
2. Estimate the costs associated with the power quality variations.
3. Characterize the solution alternatives in terms of costs and effectiveness.
4. Perform the comparative economic analysis.
Evaluating the Economics of Different Ride-Through
Alternatives
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39. The cost of a power quality disturbance can be captured
primarily through three major categories:
> Product-related losses, such as loss of product and
materials, lost production capacity, disposal charges, and
increased inventory requirements.
> Labor-related losses, such as idled employees, overtime,
cleanup, and repair.
> Ancillary costs such as damaged equipment, lost
opportunity cost, and penalties due to shipping delays.
Estimating the costs for the voltage sag
events
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40. > Each solution technology needs to be characterized in terms
of cost and effectiveness.
> In broad terms the solution cost should include initial
procurement and installation expenses, operating and
maintenance expenses, and any disposal and/or salvage
value considerations.
> A thorough evaluation would include less obvious costs such
as real estate or space-related expenses and tax
considerations.
> The cost of the extra space requirements can be
incorporated as a space rental charge and included with
other annual operating expenses.
> Tax considerations may have several components, and the
net benefit or cost can also be included with other annual
Characterizing the cost and
effectiveness for solution alternatives
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operating expenses.

41. Performing comparative economic
analysis
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42. > Motors have the undesirable effect of drawing several times their full
load current while starting.
> This large current will, by flowing through system impedances, cause a
voltage sag which may dim lights, cause contactors to drop out, and
disrupt sensitive equipment.
> The situation is made worse by an extremely poor starting
displacement factor usually in the range of 1 5 to 3 0 percent.
> The time required for the motor to accelerate to rated speed increases
with the magnitude of the sag, and an excessive sag may prevent the
motor from starting successfully.
Motor-Starting Sags
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43. > Energizing the motor in a single step (full-voltage starting) provides
low cost and allows the most rapid acceleration.
> It is the preferred method unless the resulting voltage sag or
mechanical stress is excessive.
> Autotransformer starters have two autotransformers connected in
open delta. Taps provide a motor voltage of 80, 65, or 50 percent of
system voltage during start-up.
> Line current and starting torque vary with the square of the voltage
applied to the motor, so the 50 percent tap will deliver only 25 percent
of the full-voltage starting current and torque.
> The lowest tap which will supply the required starting torque is
selected.
Motor-Starting Methods
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44. > Resistance and reactance starters initially insert an impedance in
series with the motor.
> After a time delay, this impedance is shorted out. Starting resistors
may be shorted out over several steps; starting reactors are shorted
out in a single step.
> Line current and starting torque vary directly with the voltage applied
to the motor, so for a given starting voltage, these starters draw more
current from the line than with autotransformer starters, but provide
higher starting torque.
> Reactors are typically provided with 50, 45, and 37.5 percent taps.
> Part-winding starters are attractive for use with dual-rated motors
(220/440 V or 230/460 V).
> The stator of a dual-rated motor consists of two windings connected in
parallel at the lower voltage rating, or in series at the higher voltage
rating.
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45. > When operated with a part winding starter at the lower voltage rating,
only one winding is energized initially, limiting starting current and
starting torque to 50 percent of the values seen when both windings
are energized simultaneously.
> Delta-wye starters connect the stator in wye for starting and then,
after a time delay, reconnect the windings in delta.
> The wye connection reduces the starting voltage to 57 percent of the
system line-line voltage; starting current and starting torque is
reduced to 33 percent of their values for full-voltage start.
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46. Estimating the sag severity during
full-voltage starting
> If full-voltage starting is used, the sag voltage, in per
unit of nominal system voltage, is
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47.  The formulation of numerical indices of voltage dips is a
compromise between the simplicity of calculations, their
mathematical correctness and representation of the physical
complexity of the phenomenon.
VOLTAGE DIP INDICES
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48. 5/13/2020 48

49. Utilities have two basic options to
continue to reduce the number and
severity of faults on their system:
1. Prevent faults.
2. Modify fault-clearing practices.
Utility System Fault-Clearing
Issues
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50. There are two fundamental types of faults on power
systems:
1.Transient (temporary) faults. These are faults due to such
things as overhead line flashovers that result in no
permanent damage to the system insulation. Power can be
restored as soon as the fault arc is extinguished. Automatic
switchgear can do this within a few seconds. Some transient
faults are self-clearing.
2.Permanent faults. These are faults due to physical
damage to some element of the insulation system that
requires intervention by a line crew to repair. The impact on
the end user is an outage that lasts from several minutes to
a few hours.
Overcurrent coordination
principles
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51. The two greatest concerns for damage are typically
1. Arcing damage to conductors and bushings.
2.Through-fault damage to substation transformers, where
the windings become displaced by excessive forces,
resulting in a major failure.
The typical hierarchy of overcurrent protection devices on a
feeder is
1. Feeder breaker in the substation. This is a circuit breaker
capable of interrupting typically 40 kA of current and
controlled by separate relays. When the available fault
current is less than 20 kA, it is common to find reclosers
used in this application.
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52. 2. Line reclosers mounted on poles at midfeeder. The
simplest are selfcontained with hydraulically operated
timing, interrupting, and reclosing mechanisms. Others have
separate electronic controls.
3. Fuses on many lateral taps off the main feeder.
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53. Reliability
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54. The actions that have the most effect on the number of
interruptions on the portion of the feeder that is downline
from the recloser are
1.Reduce the fault rate by tree trimming, line arresters,
animal guards, or other fault prevention techniques.
2. Provide more parallel paths into the service area.
3. Do not trip phases that are not involved in the fault.
There are at least two options for providing additional
parallel paths:
1. Build more conventional feeders from the substation.
2.Use more three-phase branches from the main feeder to
serve the load.
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55. Overhead line maintenance
> Tree trimming.
> Insulator washing.
> Shield wires.
> Improving pole grounds.
> Modified conductor spacing.
> Tree wire (insulated/covered conductor).
Utility fault prevention
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Thank you