Chapter III VOLTAGE SAG AND TRANSIENTS

OBJECTIVES

You will be able to

Understand the Voltage Sag & Causes of Voltage Sag Understand the Transients & Types and causes of transientsUnderstand Principles of over voltage protection

3. 0 INTRODUCTIONA sag is a decrease to between 0.1 and 0.9 pu in rms voltage or current at the power frequency

for durations from 0.5 cycle to 1 min. The power quality community has used the term sag for many

years to describe a short-duration voltage decrease. Although the term has not

been formally defined, it has been increasingly accepted and used byutilities, manufacturers, and end

users. The IEC definition for this phenomenon is dip. The two terms are considered interchangeable,

with sag being the preferred synonym in the U.S. power quality community.

Terminology used to describe the magnitude of a voltage sag is often confusing. A “20 percent sag” can

refer to a sag which results in a voltage of 0.8 or 0.2 pu. The preferred terminology would be one that

leaves no doubt as to the resulting voltage level: “a sag to 0.8 pu” or “a sag

whose magnitude was 20 percent.” When not specified otherwise, a 20 percent sag will be considered

an event during which the rms voltage decreased by 20 percent to 0.8 pu. The nominal, or base,

voltage level should also be specified. Voltage sags are usually associated with system faults but can

also be caused by energization of heavy loads or starting of large motors.

Figure 3.6 shows a typical voltage sag that can be associated with a single- line-to-ground (SLG) fault

on another feeder from the same substation. An 80 percent sag exists for about 3 cycles until the

substation breaker is able to interrupt the fault current. Typical fault clearing times range from 3 to 30

cycles, depending on the fault current magnitude and the type of overcurrent protection. Figure 3.6

illustrates the effect of a large motor starting. An induction motor will draw 6 to 10 times its full load

current during start-up. If the current magnitude is large relative to the available fault current in the

system at that point, the resulting voltage sag can be significant.

In this case, the voltage sags immediately to 80 percent and then grad-ually returns to normal in about

3 s. Note the difference in time frame between this and sags due to utility system faults.

Until recent efforts, the duration of sag events has not been clearly defined. Typical sag duration is

defined in some publications as ranging from 2 ms (about one-tenth of a cycle) to a couple of

minutes.Undervoltages that last less than one-half cycle cannot be characterized effectively by a

change in the rms value of the fundamental frequency value. Therefore, these events are considered

transients. Undervoltages that last longer than 1 min can typically be controlled

by voltage regulation equipment and may be associated with causes other than system faults.

Therefore, these are classified as long-duration variations. Sag durations are subdivided here into three

categories—instantaneous, momentary, and temporary—which coincide with the three

categories of interruptions and swells. These durations are intended to correspond to typical utility

protective device operation times as well as duration divisions recommended by international technical

organizations

Figure 3.6

3.1 VOLTAGE SAGA voltage sag as defined by IEEE Standard 1159-1995, IEEE Recommended Practice for

Monitoring Electric Power Quality, is a decrease in RMS voltage at the power frequency for durations

from 0.5 cycles to 1 minute, reported as the remaining voltage. The measurement of a voltage sag is

stated as a percentage of the nominal voltage, it is a measurement of the remaining voltage and is

stated as a sag to a percentage value. Thus a volt- age sag to 60% is equivalent to 60% of nominal

voltage, or 288 volts for a nominal 480 Volt system

3.1.1 Single-Phase Sags

The most common voltage sags, over 70%, are singlephase events which are typically due to a

phase-to-ground fault occurring somewhere on the system. This phase-to-ground fault appears as a

single phase voltage sag on other feeders from the same substation. Typical causes are lightning

strikes, tree branches, animal contact etc. It is not uncommon to see singlephase voltage sags to 30%

of nominal voltage or even lower in industrial plants.

3.1.2 Phase-to-Phase SagsTwo-phase, phase-to-phase sags may be caused by tree branches, adverse weather, animals

or vehicle collision with utility poles. The two-phase voltage sag will typically appear on other feeders

from the same substation.

3.1.3 Phase SagsSymmetrical three-phase sags account for less than 20% ofall sag events and are caused

either by switching or tripping of a three-phase circuit breaker, switch or recloser which will create a

three-phase voltage sag on other lines fed from the same substation. Three-phase sags will also be

caused by starting large motors, but this type of event typically causes voltage sags to approximately

80% of nominal voltage and are usually confined to an industrial plant or its immediate neighbours.

phase and multi-phase voltage sags can cause unplanned production stoppages but single-phase

(120V) control devices and electronic sensors can be very vulnerable to voltage sags. Modern

electronic equipment requires more precise voltage regulation than traditional devices such as induction

motors. When the manufacturing industry used mechanical devices and gearboxes to control the

speed of its processes, many of which were relatively slow and required manual operation or

intervention by operators, voltage variations were not such a serious issue. Automation has lead to high

speed processes, automatic electronic sensing and controls; precision machine tools have

sophisticated electronic controls, variable speed drives have replaced many gearboxes and any

unplanned manufacturing stoppage can be very expensive.

Electronic process controls, sensors, computer controls, PLCs and variable speed drives, even

conventional electrical relays are all to some degree susceptible to voltage sags. In many cases, one or

more of these devices may trip if there is a voltage sag to less than 90% of nominal voltage, even if the

duration is only for one or two cycles i.e. less than 100 milliseconds. The time to restart production after

such an unplanned stoppage can typically be measured in minutes, hours or even days. Costs per

event can be many tens of thousands of dollars.

3.1.4 Voltage Sags are caused

Voltage Sags are caused by abrupt increases in loads such as short circuits or faults, motor

starting, or electric heaters turning on, or they are caused by abrupt increases in source impedance,

typically caused by a loose connection.

3.2 Factors Contributing to the Causes of Voltage Sags

1. Rural location remote from power source

2. Long distance from a distribution transformer with interposed loads

3. Unreliable grid system

4. Power distributor’s tolerances not suitable for voltage sensitive equipment

5. Switching of heavy loads

6. Unbalanced load on a three phase system

7. Equipment not suitable for local supply

3.3 Symptoms of Sags and Swells

1. Production rates fluctuates

2. Equipment does not operate correctly

3. Dimming of lighting systems

4. Variable speed drives close down to prevent damage

5. Relays and contactors drop out

6. Unreliable data in equipment test

3.4 Methods of Dealing with Sags and Swells

1. Transformer with a tap changer

2. Constant voltage (ferro-resonant) transformer

3. Servo controlled voltage stabilizer

4. Switch mode power supply

5. Saturable reactor

6. Soft starters on larger electrical equipment

7. Connect larger loads to points of common coupling

8. Choose equipment with dip resilience

3.5 THE SOLUTION3.5.1 Equipment IdentificationIn order to provide an optimal and cost effective solution to voltage sag problems, it is

necessary to determine which equipment is susceptible to unplanned stoppages. In most industries,

there is still a significant amount of electrical equipment which is not sensitive to voltage variation or

which can be restarted at little or no cost. Usually it is not necessary to protect an entire industrial

facility, it is sufficient to protect the key sensitive equipment.

3.5.2 Identify the Voltage SagsThe next stage is to determine the frequency, depth and duration of the voltage sags. These

can vary widely even in apparently similar industrial facilities. Collection of this data is essential if the

optimal solution is to be identified. In North America, only a small proportion of manufacturing

businesses

have installed electrical metering which is capable of measuring and recording the voltage

variations which are responsible for the majority of their very costly Unplanned Production Stoppages.

3.6 Classification of Power System DisturbancesPower quality problems occur due to various types of electrical disturbances. Most of the EPQ

disturbances depend on amplitude or frequency or on both frequency and amplitude. Based on the

duration of existence of EPQ disturbances, events can divided into short, medium or long type. The

disturbances causing power quality degradation arising in a power system and their classification

mainly include:

3.7 Interruption/under voltage/over voltage: these are very common type disturbances. During power interruption, voltage level of a

particular bus goes downto zero. The interruption may occur for short or medium or long period. Under

voltage and over voltage are fall and rise of voltage levels of a particular bus with respect to standard

bus voltage. Sometimes under and over voltages of little percentage is allowable; but when they cross

the limit of desired voltage level, they are treated as disturbances. Such disturbances are increasing the

amount of reactive power drawn or deliver by a system, insulation problems and voltage stability.

3.8 Voltage/Current unbalance: voltage and current unbalance may occur due to the unbalance in drop in the generating

system or transmission system and unbalanced loading. During unbalance, negative sequence

components appear. T hampers system performance may change loss and in some cases it may

hamper voltage

stability.

3.9 Discuss about the sources of sags and interruption.

Voltage sags and interruptions are generally caused by faults (short circuits) on the utility system.

Fig.3.6: Fault locations on the utility power system.

Figure.3.9.1 Voltage sag due to a short-circuit fault on a parallel utility feeder.

Figure.3.9.1: Utility short-circuit fault event with two fast trip operations of utility line recloser

Consider a customer that is supplied from the feeder supplied by circuit breaker 1 on the diagram

shown in Fig.3.9. If there is a fault on the same feeder, the customer will experience a voltage-sag

during the fault followed by an interruption when the breaker opens to clear the fault. If the fault is

temporary in nature, a reclosing operation on the breaker should be successful and the interruption

will only be temporary. It will usually require about 5 or 6 cycles for the breaker to operate, during

which time a voltage sag occurs. The breaker will remain open for typically a minimum of 12 cycles

up to 5 s depending on utility reclosing practices. Sensitive equipment will almost surely trip during

this interruption.

A much more common event would be a fault on one of the other feeders from the substation, i.e.,

a fault on a parallel feeder, or a fault somewhere on the transmission system (see the fault

locations shown in Fig.3.9.1). In either of these cases, the customer will experience a voltage-sag

during the period that the fault is actually on the system. As soon as breakers open to clear the

fault, normal voltage will be restored at the customer.

Note that to clear the fault shown on the transmission system, both breakers A and B must operate.

Transmission breakers will typically clear a fault in 5 or 6 cycles. In this case there are two lines

supplying the distribution substation and only one has a fault. Therefore, customers supplied from

the substation should expect to see only a sag and not an interruption. The distribution fault on

feeder 4 may be cleared either by the lateral fuse or the breaker, depending on the utility’s fuse

saving practice.

Any of these fault locations can cause equipment to misoperate in customer facilities. The relative

importance of faults on the transmission system and the distribution system will depend on the

specific characteristics of the systems (underground versus overhead distribution, lightning flash

densities, overhead exposure, etc.) and the sensitivity of the equipment to voltage sags.

Figure 3.9 shows the characteristic measured at a customer location on an unfaulted part of the

feeder. Figure 3.9 shows the momentary interruption (actually two separate interruptions) observed

downline from the fault. The interrupting device in this case was a line recloser that was able to

interrupt the fault very quickly in about 2.5 cycles.

3.10 Performance The Voltage Sag

It is important to understand the expected voltage sag performance of the supply system so that

facilities can be designed and equipment specifications developed to assure the optimum operation of

production facilities. The following is a general procedure for working with industrial customers to

assure compatibility between the supply system characteristics and the facility operation:

i. Determine the number and characteristics of voltage sags that result from transmission system

faults.

ii. Determine the number and characteristics of voltage sags that result from distribution system

faults (for facilities that are supplied from distribution systems).

iii. 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.

iv. Evaluate the economics of different solutions that could improve the performance, either on the

supply system (fewer voltage sags) or within the customer facility (better immunity).

3.11 Mitigation Of Voltage Sag.

There are many solutions to prevent damage due to voltage dips. Typically, these solutions can be

categorized into three classes:

Solutions in the manufacturing process itself;

Solutions between the process and the public electric grid;

Solutions in the grid.

3.12 Reduction of the Number of Faults

Short circuits cannot be entirely eliminated. The actions taken are: replacing overhead lines

with cables; the use of insulated conductors on overhead lines; regular tree cutting in the area of the

transmission line; fencing against animals; shielding overhead conductors with additional shield wires;

increased insulation levels; increased frequency of overhaul and periodic maintenance, cleaning

insulators, etc.

3.13 Reduction of the Fault Clearance Time

The duration of a voltage dip is largely determined by the speed at which short circuits are cleared.

A necessary feature of short-circuit protection is the graduation of the operating times of switches,

fuses, etc., in order to ensure that a short circuit is cleared at the most appropriate point in the supply

system. This means that the clearance time and, consequently, the duration of voltage dips and short

interruptions depend on the location where the short circuit has occurred. A reduction in fault-clearance

time does not mean a decrease in the number of faults but only a mitigation of their effects. It also does

not influence the number or the duration of supply interruptions, for the duration depends solely on the

speed of voltage recovery. Fast fault clearing does not influence the number of voltage dips, but can

significantly reduce their duration.

The basic method for reducing fault duration consists of the use of current-limiting fuses. These are

capable of clearing a fault in a very short time. Decreasing the short-circuit current and shortening its

duration significantly limit the duration of a voltage dip to rarely exceeding one cycle.

3.14 Modification of the Supply System ConfigurationThese operations allow for a reduction in the severity of the phenomenon, but at a high cost,

particularly in HV systems. The basic method of preventing voltage dips is to install elements of

redundancy, as follows:

Installing generators close to sensitive loads. They support the voltage during distant dips. The

voltage reduction equals the percentage share of the generator current in the short-circuit current.

Increasing the number of substations and busbars in order to limit the number of customers, who

potentially may be affected by the disturbance.

Installing current-limiting reactors at strategic points of the system in order to increase electrical

distance to the fault. It should, however, be remembered that this action may make a voltage dip

deeper for other customers.

Supplying sensitive customer’s busbars from several substations. The effects of a voltage dip on

one substation will be reduced by the influence of the others. The more independent these

substations are, the more effective the measure is. The best reduction effect can be achieved by

providing a power supply from two different supplying systems. The second supply increases the

number of dips but reduces their duration and depth.

3.15 Voltage StabilizersA more sophisticated way to eliminate the negative effects of dips is called custom power

technology. This technology is mainly based on power electronics and also, on some occasions,

electrical energy storage.

The most common method for mitigating the effects of the considered disturbances is the use of

additional equipment, namely voltage stabilizers. They can be installed on both the supplier’s or the

customer’s side but, as experience shows, the customer is the one who much more frequently does it,

since the improvement in supply conditions and increasing the equipment’s immunity are beyond the

customer’s control.

These systems can be generally termed as systems of improved power parameters.

3.16 Energy storage systems & Systems having no energy-storing capability.i. Energy storage systems. The stored energy is utilized to supply a critical load during the

disturbance. These systems can be used in the case of voltage dips with arbitrary residual

voltage, as well as during short supply interruptions. The immunity level of equipment depends

on the amount of energy stored and on the energy requirements of the protected process. In

many cases the reaction time of the compensation equipment should be considered critical.

Since the energy storage process is, as a rule, very costly, it is applied only to particularly

sensitive equipment. Examples of energy storage systems are: uninterruptible power supplies

(UPSs), superconducting magnetic energy storage (SMES), rotating machines with flywheels,

motor–generator systems, etc.

ii. Systems having no energy-storing capability. These can only be used to reduce the effects

of voltage dips (typically up to a maximum of 50 %) but not of supply interruptions. They differ in

depth of the voltage dip, which they are able to compensate. The duration of a dip is not a

critical parameter in these systems. Their cost, as a rule, is smaller than that of the energy-

storing systems.

Example of such solutions are:

o Constant voltage transformer (CVT);

o Static fast transfer switching (SFTS);

o Static generators of the fundamental harmonic currents and voltages.

3.17 Improvement in Equipment ImmunityOne of the most advantageous solutions, in both technical and economical terms, is the use of

equipment of a sufficient immunity level that is adequate for the intended operational environment. This

is an effective method which eliminates unwanted disconnections due to voltage dips (short

interruptions to a lesser extent). More and more frequently the immunity to a voltage dip of a specified

depth and duration becomes the basis of a manufacturer’s offer, determining its commercial success.

The level of compatibility of a sensitive load with the supply network is assessed prior to connection.

The possible procedure includes three stages:

i. Acquiring information on system operation. That is, the prospective number of voltage dips.

There are a number of ways to get such data: contacting the electric power supplier, monitoring

the power supply over an extended period of time, analysis of faults, etc. Obtaining credible

information requires the measurements to be performed for a long time. An alternative is the

use of statistical methods of prediction.

ii. Acquiring information on equipment sensitivity. This information can be obtained from the

manufacturer, by conducting tests or assuming typical sensitivity characteristics. In practice, it

frequently happens that the user learns about the limited immunity of the equipment only after

installing it.

iii. Determination of the potential effect. If the foregoing information is available, thereis the

possibility to assess the potential threat of equipment failure (failure rate) and evaluate the

economic effect of its occurrence (Section 4.6.1). On that basis a method of proceeding can be

chosen: improvement of supply conditions, better (i.e. less sensitive) equipment and application

of a stabilizer or acceptance of the existing situation.

3.18 Power Quality Improvement Active Series Compensators In Power Quality ImprovementAdvances in power electronic technologies and new topologies for these devices have resulted in

new options for providing voltage sag ride-through support to critical loads. One of the important new

options is a device that can boost the voltage by injecting a voltage in series with the remaining voltage

during a voltage sag condition. These are referred to as active series compensation devices. They are

available in size ranges from small single-phase devices (1 to 5 kVA) to very large devices that can be

applied on the medium-voltage systems (2 MVA and larger).

Figure3.18 : Active Series Compensator

A one-line diagram illustrating the power electronics that are used to achieve the compensation is

shown in Fig. 3.18. When a disturbance to the input voltage is detected, a fast switch opens and the

power is supplied through the series-connected electronics. This circuit adds or subtracts a voltage

signal to the input voltage so that the output voltage remains within a specified tolerance during the

disturbance. The switch is very fast so that the disturbance seen by the load is less than a quarter cycle

in duration. This is fast enough to avoid problems with almost all sensitive loads. The circuit can provide

voltage boosting of about 50 percent, which is sufficient for almost all voltage sag conditions.

3.19 Static transfer switches and Fast transfer switches.There are a number of alternatives for protection of an entire facility that may be sensitive to voltage

sags. These include dynamic voltage restorers (DVRs) and UPS systems that use technology similar to

the systems described previously but applied at the medium-voltage level.

Another alternative that can be applied at either the low-voltage level or the medium-voltage level is

the automatic transfer switch. Automatic transfer switches can be of various technologies, ranging from

conventional breakers to static switches. Conventional transfer switches will switch from the primary

supply to a backup supply in seconds. Fast transfer switches that use vacuum breaker technology are

available that can transfer in about 2 electrical cycles. This can be fast enough to protect many

sensitive loads. Static switches use power electronic switches to accomplish the transfer within about a

quarter of an electrical cycle. The transfer switch configuration is shown in Fig. 3.18. An example

medium-voltage installation is shown in Fig. 3.18. The most important consideration in the effectiveness

of a transfer switch for protection of sensitive loads is that it requires two independent supplies to the

facility. For instance, if both supplies come from the same substation bus, then they will both be

exposed to the same voltage sags when there is a fault condition somewhere in the supply system. If a

significant percentage of the events affecting the facility are caused by faults on the transmission

system, the fast transfer switch might have little benefit for protection of the equipment in the facility.

Fig.3.19 Configuration of a static transfer switch used to switch between a primary supply and a backup

supply in the event of a disturbance. The controls would switch back to the primary supply after normal

power is restored.

3.20 ferroresonant transformer

Ferroresonant transformers, also called constant-voltage transformers (CVTs), can handle most

voltage sag conditions.

Fig.3.20. Ferroresonant Transformer

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 providing an output voltage

which is not significantly affected by input voltage variations

Figure 3.20 shows the voltage sag ride-through improvement of a process controller fed from a

120-VA ferroresonant transformer. With the CVT, the process controller can ride through a voltage

sag down to 30 percent of nominal, as opposed to 82 percent without one.

Fig.3.20 Voltage Sag improvement using ferroresonant transformer

From the above figure, it is clear that the ride-through capability is held constant at a certain level.

The reason for this is the small power requirement of the process controller, only 15 VA.

Ferroresonant transformers should be sized significantly larger than the load.

Fig.3.20.1. Voltage sag versus ferroresonant transformer loading

Figure 3.22 shows the allowable voltage sag as a percentage of nominal voltage (that will result in

at least 90 percent voltage on the CVT output) versus ferroresonant transformer loading, as

specified by one manufacturer. At 25 percent of loading, the allowable voltage sag is 30 percent of

nominal, which means that the CVT will output over 90 percent normal voltage as long as the input

voltage is above 30 percent.

This is important since the plant voltage rarely falls below 30 percent of nominal during voltage sag

conditions. As the loading is increased, the corresponding ride-through capability is reduced, and

when the ferroresonant transformer is overloaded (e.g., 150 percent loading), the voltage will

collapse to zero.

3.21 Motor Starting methods:

Starting Method Reduction on L-L Voltage Reduction in starting current

Autotransformer starting Taps provide a motor voltage of 80, 65, or 50 percent of system voltage during start-up.

Vary with the square of the voltage applied

Resistance and reactance starting

Varies with the resistance/ reactance value. Reactors are typically provided with 50, 45, and 37.5 percent taps.

Varies with the resistance/ reactance value.

Part-winding starting Energies one part of winding 50%

Star-Delta starting 57% 33%

Estimation of the sag severity during full-voltage starting:

Voltage sag in pu, Vmin ( pu )=

V ( pu ). kVASCkVALR+kVASC

where,V(pu) – Actual system voltage, in per unit of normalkVALR - Motor locked rotor kVA kVASC – System short circuit kVA at motor

Computation for sag to 90 percent of nominal voltage, using typical system impedances and motor characteristics.

If the sag is above the minimum allowable steady state value of the affected equipment, then the full voltage starting is acceptable.

Otherwise, voltage sag – duration characteristics to be compared with the voltage tolerance envelope of the affected equipment.

Such complicated analysis may be left to computer analysis.

Fig.3.21. Typical motor versus transformer size for full-voltage starting sags of 90%. Computer simulation requires the following data:

o Parameter values for the standard induction motor equivalent circuit: R1, X1, R2, X2 and XM.

o Number of motor poles and rated rpm (or slip).o WK2 (inertia constant) values for the motor and the motor load.o Torque versus speed characteristic for the motor load.

3.22 Magnetic synthesizer.

Magnetic synthesizers can handle three phase and provide improved voltage sag support and

regulation for three-phase loads.

They use resonant circuits made of nonlinear inductors and capacitors to store energy, pulsating

saturation transformers to modify the voltage waveform, and filters to filter out harmonic distortion.

They are applicable over a size range from about 15 to 200 kVA and are typically applied for

process loads.

They supply power through a zigzag transformer. The zigzag name comes from the way the

transformer changes the phase angle between voltage and current. The zigzag transformer traps

triplen harmonic currents and prevents them from reaching the power source.

Applications of magnetic synthesizers include protection of large computer installations,

computerized medical imaging equipment, and industrial processes, like plastic extruders,

especially from voltage sags. They protect sensitive loads not only from voltage sags but also from

transients, overvoltage, undervoltage, and voltage surges. However, they can be bulky and noisy.

The block diagram in Figure 3.22.1 illustrates the main components of a magnetic synthesizer.

Figure.3.22.1. Block diagram of Magnetic Synthesizer

3.23 Power Quality Improvement Using Motor Generators Sets.

Motor-Generator sets are available in various sizes and configurations. This is one of the

established technologies for preventing sensitive loads from sags and interruptions.

Fig.3.23 Typical M-G set with flywheel. Fig.2.10 shows the arrangement of M-G set in which the motor is powered by a driver circuit from

line. The motor drive a generator that energize 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.

Disadvantages:o Losseso Noise and maintenanceo Frequency and voltage drops with the speed. This may not desirable for some loads.

Written-pole synchronous machine are also used. In this machine, the number of poles getting varied according the speed of the machine to maintain the frequency as well the voltage constant.

Solid state inverters are also preferred for some cases. But the loss and cost associated with this arrangement is high.

3.24 Motor Starting Sags.The motors are drawing more current during starting. This large current will, by flowing through

system impedances, causes a voltage sag.Effects:

Dim lights Contactor drop-outs Disturbance to sensitive equipments.

The time required for the motor to accelerate to rated speed increases with the magnitude of the sag, and excessive sag may prevent the motor from starting successfully. Motor starting sags can persist for many seconds, as illustrated in Fig. 3.24

Figure.3.24 Typical motor starting voltage sag________

TWO MARKS

1. What is voltage sag?A sag or dip is a decrease in RMS voltage or current at the power frequency for

durations from 0.5 cycles to 1 minute, reported as the remaining voltage. Typical values are between 0.1 pu and 0.9 pu.

2. When sag leads to interruption.Voltage sag is a reduction in voltage for a short time. The voltage reduction magnitude

is between 10 % to 90% of the normal root mean square (RMS) voltage at 50 Hz. An interruption is a complete loss of voltage or a drop to less than 10 % of nominal voltage in one or more phases.

3. What are the causes of sag? Voltage sags are usually associated with voltage sag. Equipment sensitive to both the magnitude and duration of voltage sag. Equipment sensitive to have characteristics other than magnitude and duration.

4. What are the three levels of possible solutions to voltage sag and momentary interruption problems? Power System Design Equipment design Power conditioning equipment.

5. List some industry standards associated with voltage sags.*SEMI F47-0200 8CBEMA curve

6. What are the sources of sags and interruption? A sudden increase in load results in a corresponding sudden drop in voltage.

Any sudden increase in load, if large enough will cause a voltage sag in motors, faults, switching.

Recloser operation.

7. Give some economic impacts due to sag. Process outrages Damaged products Lost time for restarting.

8. What is the importance of estimating sag performance?It is important to understand the expected voltage sag performance of the supply

system so that facilities can be designed and equipment specifications developed to assure the optimum operation of production facilities.

9. What are the various factors affecting the sag magnitude due to faults at a certain point in the system.

Distance to the fault Fault impedance Type of fault Pre-sag voltage level System configuration System impedance Transformer connections.

10. Name the different motor starting methods.

Resistance and reactance startersAutotransformer startersStar-Delta starters

11. What are the causes for voltage sags due to transformer energizing? Normal system operation, which includes manual energizing of a transformer. Reclosing actions.

12. How voltage sag can be mitigated.Voltage sag can be mitigated by voltage and power injections into the distribution

system using power electronics based devices which are also known as custom power devices.

13. Name the three levels of possible solutions to voltage sag and momentary interruption problems.

Equipment Design * Power conditioning equipment Power system design

14. Name any four types of sag mitigation devices. Dynamic Voltage Restorer(DVR) Active Series Compensators Distribution Static Compensator(DSTATCOM) Solid State Transfer Switches(SSTS)

15. Define Dynamic Voltage Restorer (DVR).A DVR is a solid state power electronics switching device consisting of either GTO or

IGBT , a capacitor bank as an energy storage device and injection transformers. It is connected in series between a distributed system and a load.

16. What is the important role of a DVR?The basic idea of a DVR is to inject a controlled voltage generated generated by a

forced commuted converter in series to the bus voltage by means of an injecting transformer.

17. Define active series compensation devices.A device that can boost the voltage by injecting a voltage in series with the remaining

voltage during a voltage sag condition.

18. What is the need of DSTATCOM?It allows effective control of active and reactive power exchanges between the

DSTATCOM and the ac system.

19. What is the main function of DSTATCOM? Voltage regulation and compensation of reactive power Correction of power factor Elimination of current harmonics.

20. What is the role of SSTS? Can be used very effectively to protect sensitive loads against voltage sags, swells and

other electrical disturbance.

It ensures continuous high quality power supply to sensitive loads by transferring , within a time of milliseconds , the load from a faulted bus to a healthy one.

PART – B

1. Discuss the sources of sags and interruption.2. Discuss in detail about the sag performance evaluation indices.3. Explain the sag performance evaluation methods.4. Explain the various causes and effects of voltage sags.5. What are the different voltage sag mitigation techniques? Explain in detail.6. Explain the principle of DVR operation used for sag mitigation.7. Discuss in detail about the active series compensator.8. Explain the solid state transfer switch with the transfer operation.9. Explain the system adapted to estimate the severity of the sag occurred due to various sources.10. Mention the standards associated with the voltage sag.