ee801 power quality

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MAHAKAVI BHARATHIYAR COLLEGE OF ENGINEERING &

TECHNOLOGY, VASUDEVANALLUR 627 758

DEPARTMENT OF EEE VIII SEM EEE EE801-POWER QUALITY

POWER QUALITY (EE801)

SEM 8 EEE

Two Marks Questions and Answers

Prepared by:

C.MALLIKA DEVI

AP/ EEE, MBCET.



MAHAKAVI BHARATHIYAR COLLEGE OF ENGINEERING & TECHNOLOGY,

VASUDEVANALLUR 627 758


UNIT- I INTORDUCTION TO POWER QUALITY

1.What is Power Quality?

From the utility perspective, Power Quality has been defined as the parameters of the voltage that

affect the customer’s supersensitive equipment.From the power user perspective, Power Quality

may be defined as any electricalparameter or connection that affects the operation of the equipment. This

included allelectrical parameters, connections and grounds, whether the source from the utility,

localequipment or other users.

2.How can Power Quality problems be detected?

Determining the exact problems requires sophisticated electronic test equipment. Thefollowing

symptoms are indicators of Power Quality problems:

A piece of equipment misoperates at the same time of day.

Circuit breakers trip without being overloaded.

Equipment fails during a thunderstorm.

Automated systems stop for no apparent reason.

Electronic systems fail or fail to operate on a frequent basis.

Electronic systems work in one location but not in another location.

Lights dim or blink and electronic systems misoperate.

3.What are Harmonics?

Harmonics are distortions in the AC waveform. These distortions are caused by loadson the electrical

system that use the electrical power at a different frequency than thefundamental 50 or 60 Hz. An

example of such a load is the common PC, televisionreceiver or any other electronic load.

4.What are the various power quality issues?

Power frequency disturbances

Power system transients

Grounding and Bonding

Electromagnetic interference

Power system harmonics

Electrostatic discharge

Power factor






5.Classify power quality events in long duration events and shout duration events.

The short duration variations are further classified into the following three categories:

1.Sag

2.Swell

3.Interruption

6.Differentiate between sag and swell

.1.Voltage Sag is an event in which the RMS voltage decreases between 0.1 and 0.9 perunit at the

power frequency. It lasts for durations of 0.5 cycles to 1 min.

2.Swell is an event in which the RMS voltage increases between 1.1 and 1.8 per unit atthe power

frequency. It lasts for durations of 0.5 cycles to 1 min.

7. Mention the types of sag.

Based on the time duration and voltage magnitude, sag is further classified as:

1.Instantaneous Sag

2.Momentary Sag

3.Temporary Sag

8.What are the causes of sags?

1.Voltage sags are usually associated with system faults but can also be caused by theswitching of heavy

loads.

2.Voltage sags are caused by motor starting, for example, an induction motor will drawsix to ten times

its full load current. This lagging current causes a voltage drop acrossthe impedance of the system.

9.Define Waveform Distortion.

It is the deviation from an ideal sine wave of power frequency principallycharacterized by the spectral

content of the deviation. Harmonics is one of the causes of many types of waveform distortion.

10.What are the components of waveform distortion?

DC offset

Notches

Flickering

Harmonics

Noises

Interharmonics

11.Define Total harmonic distortion and mention the formula used to calculate






THD.

Total harmonic distortion is the term used to describe the net deviation of a non-linearwaveform from

ideal sine wave characteristics. THD is the ratio between the RMS valueof the harmonics and the RMS

value of the fundamental.Mention the formula used to calculate THD.

THD=100√(

Where Uh represents harmonic components, U1 represents the fundamentalcomponent and h

represents harmonic order.

12.Define Total Demand Distortion (TDD).

The total demand distortion is defined as the square root of the sum of the squares of the RMS value of

the currents from 2nd to the highest harmonic (say 25thmaximum inpower system) divided by the peak

demand load current and is expressed as a percent.

13.Define for the following terms:(i)DC offset, (ii)Interharmonics, (iii)Voltage

Flicker

DC offset:

The presence of a dc voltage or current in an ac power system is termed DC offset.This can occur as the

result of a geomagnetic disturbance or asymmetry of electronicpower converters.

Interharmonics:

Voltages or currents having frequency components that are not integer multiples of thefrequency at

which the supply system is designed to operate (e.g., 50 or 60 Hz0 are calledinterharmonics. They

can appear as discrete frequencies or as a wideband spectrum.Interharmonics can be found in networks

of all voltage classes.

Voltage Flicker:

Voltage flicker is rapidly occurring voltage sags caused by sudden and large increasesin load

current. Voltage flicker is most commonly caused by rapidly varying loads thatrequire a large amount of

reactive power such as arc furnaces, electric welders, rock crushers, sawmills, wood chippers, metal

shredders, and amusement rides. It can causevisible flicker in lights and cause other processes to shut

down or malfunction.

14. List any four standards available in power quality

.IEEE Power Quality Standards:The Institute of Electrical and Electronics EngineerIEC Power Quality






Standards:The International Electro technical CommissionSEMI Power Quality Standards:The

Semiconductor Wquipment and Material InternationalUIE Power Quality Standards:The International

Union for Electricity Applications

UNIT- II VOLTAGE SAG & INTERRUPTIONS

15.What is Voltage Sag?

A sag or dip, as defined by IEEE Standard 1159-1995, IEEE Recommended Practicefor Monitoring

Electric Power Quality, is a decrease in RMS voltage or current at thepower frequency for durations

from 0.5 cycles to 1 minute, reported as the remainingvoltage. Typical values are between 0.1 pu and

0.9 pu.

16.What are the causes of sag?

Voltage sags are usually associated with system faults

It can also be caused by energization of heavy load

Starting of large motors

17.What are three levels of possible solutions to voltage sag and

momentaryinterruption problems?

Power System Design

Equipment Design

Power Conditioning Equipment

18.List some IEEE Standards Associated with Voltage Sags.

1.IEEE 1250-1995, “IEEE guide for service to equipment sensitive to momentaryvoltage

disturbances”

2.IEEE 493-1990, “Recommended practice for the design of reliable industrial and

commercial power systems

3.IEEE 1100-1999. “IEEE recommended practice for powering and groundingelectronic

equipment”

4.IEE 446-

1995, “IEEE recommended practice for emergency and standby power

systems for industrial and commercial applications range of sensibility loads”

19.What are the sources of sags and Interruption?

1.A sudden increase in load results in a Corresponding sudden drop in voltage






2.Any sudden increase in load, if large enough, will cause a voltage sag in

1.Motors

2.Faults.

3.Switching

20.Name the different motor starting methods.

1.Resistance and reactance starters

2.Autotransformer starters

3.Star-Delta starters

21.Name any four types of sag mitigation devices.

1.Dynamic Voltage Restorer (DVR)

2.Active Series Compensators (Transformer less series injection)

3.Solid State (static) Transfer Switches (SSTS)

22.Define active series compensation devices.

1.One of the important new options is a device that can boost the voltage by injecting avoltage in series

with the remaining voltage during a voltage sag condition. These arereferred to as active series

compensation devices.

2.They are available in size ranges from small single-phase devices to very largedevices that can be

applied on the medium-voltage systems.

23.What is the main function of DSTATCOM?

Voltage regulation and compensation of reactive power

Correction of power factor

Elimination of current harmonics

UNIT- III OVER VOLTAGES

24.Define transient over voltages.

A transient over voltage can be defined as the response of an electrical network to asudden change in






network conditions, either intended or accidental, (e.g. a switchingoperation or a fault) or network stimuli

(e.g. lightning strike).

25.Define voltage magnification phenomena?

The highest transient voltages occur at the low voltage capacitor bank when thecharacteristic frequency

of the switching transient is nearly equal to the resonantfrequency of the low voltage system and when

the switched capacitor is ten or moretimes the size of the low-voltage capacitor

26.Give the various aspects of equipment specific design and protection issues for

thecapacitor switching transients.

1.Phase-to-ground and phase-to-phase insulation switching withstand to voltagestresses

2.Controlled closing for circuit breakers (pre-insertion resistors/reactors or synchronousswitching)

3.Capacitor bank and substation circuit breakers ANSI?IEEE C37 requirements

4.Current limiting reactor requirements

5.Surge arrester energy requirement

27.What are the various Causes of over voltages?

Over voltages, i.e. brief voltage peaks (transients, surges, spikes), can be attributed totheFollowing main

causes:

1.Atmospheric discharges, i.e. lightning (LEMP – Lightning Electro-Magnetic Pulse)

2.Switching operations in the public grid and low-voltage mains

3.Electrostatic Discharges (ESD)

4.Ferro resonance

28.What is the need of surge arrestors?

1.A surge arrester is a protective device for limiting surge voltages on equipment bydischarging or

bypassing surge current.

2.Surge arresters allow only minimal flow of the 50Hz/60Hz power current to ground.

29.What is metal-oxide surge-arrester?

A metal-oxide surge-arrester (MOSA) utilizing zinc-oxide block provides the bestperformance, as surge

voltage conduction starts and stops promptly at a precise voltagelevel, thereby improving system

protection.

30.What is the role of surge arrestor on shielded and unshielded transmission line?

1.On shielded transmission lines or under-built distribution circuits, the arresterprevents tower-to-phase

insulator back-flashovers during a lightning strike.

2.On unshielded sub transmission or distribution circuits, the arrester prevents phase-to-ground






flashover.

31.Define lightning phenomena.

1.Lightning is an electrical discharge in the air between clouds, between differentcharge centre within

the same cloud, or between cloud and earth (or earthed object).

2.Even though more discharges occur between or within clouds, there are enoughstrokes that terminate

on the earth to cause problems to power systems and sensitiveelectronic equipment

32.What is Ferro resonance?

Ferro resonance is a special case of series LC resonance where the inductanceinvolved is nonlinear and it

is usually related to equipment with iron cores. It occurswhen line capacitance resonates with the

magnetizing reactance of a core while it goes inthe out of saturation.

33.Give the cable life equation as a function of impulses.

The cable life is an exponential function of the number of impulses of a certainmagnitude that it receives,

according to Hopkinton. The damage to the cable is related byDc=P.VeWhere,Dc=constant,

representing cable damageP=Number of impulsesV=Magnitude of impulsesE=empirical constant

ranging from 10 to 15

34.What is the need of Computer analysis tools for transient studies?

Computer analysis simulation tool can simulate the time response of the transientphenomena in the

power system with a very high degree of accuracy.

35.Give any two analysis examples available in PSCAD/EMTDC?Transient Studies

1.Transient over voltage studies (TOV)

2.Line energizing (charging and discharging transients)

3.Capacitor bank back to back switching, selection of inrush and out-rush reactors

UNIT- IV HARMONICS

36.Define Harmonics.

Harmonics is a sinusoidal component of a periodic wave or quantity having afrequency that is an

integral multiple of the fundamental power frequency.The equation representing a harmonic frequency






is given by:

fh=f1*hWhere f1 is the fundamental frequency and h is the harmonic order

37.Define true power factor.

True power factor is calculated as the ratio between the total active power used in acircuit (including

harmonics) and the total apparent power (including harmonics)suppliedfrom the source.True power

factor = total active power (P)/Total apparent power (S)

38.Differentiate between linear loads and non-linear loads.Linear loads:

Any load that draws current at supply fundamental frequency only is a linear load.The current drawn

does not contain any harmonics (multiples of the supplyfrequency). Motors, resistors, inductors

and capacitors are all linear loads.

Non linear load:

Any load that draws harmonic currents from the supply is a nonlinear load.The current waveform of

such non-linear loads, is discontinuous and non sinusoidalbecause of the presence of harmonics.

39.What is voltage and current distortion?

1.Voltage distortion is any deviation from the nominal sine waveform of the linevoltage.

2.Current distortion is any deviation from the nominal sine waveform of the AC linecurrent.

40.What is total harmonic distortion?

THD is the ratio between the RMS value of the harmonics and the RMS value of thefundamental.The

amount of harmonic distortion can be measured by means of a factor known asthe total harmonic

distortion (THD), which is given by the relation,THD%=100*Where U1 represents the fundamental

components and h represents harmonic number

41.What is total demand distortion?

The total demand distortion is defined as the square root of the sum of the squares of the RMS value of

the currents from 2nd

to the hth harmonics divided by the peak demandload current and is expressed as a

percentage.TDD% of peak demand=Where, IRMS distorted is the RMS value of the distorted

waveform with the fundamentallift out of the summation.

42.Mention the harmonic effects on devices and loads.

1.Insulation stress (voltage effect)

2.Thermal stress (current effect)

3.Load ruptures (abnormal operation)

43.Mention the harmonic sources from industrial loads.

1.Three phase converter with Adjustable speed drives (DC drives and AC drives)






2.Arcing Devices (Arc furnaces, weiders, Discharge lamps etc)

3.Saturable devices (transformer, electromagnetic devices etc with steel core)

44.State the different types of inverters

1.Variable voltage inverter(VVI)

2.Current source inverter(CSI)

3.Pulse width modulated(PWM)

45.What is Variable Voltage Inverter?

The variable voltage inverter (VVI). Or square-wave six-step voltage source inverter(VSI), receives DC

power from an adjustable voltage source (either from thyristorconverter or DC-DC converter fed by

Diode Bridge) and adjusts the frequency andvoltage.

46.What are the objectives of IEEE standard?

Provide general harmonic distortion evaluation procedures for different classes of customers (industrial,

commercial, residential) and for the application of equipments onutility system.

47. What are the applications of active filters?

Passive tuned filters introduce new resonances which can cause additional harmonicproblemsActive

filters will provide compensation for harmonic components on the utilitysystem based on the existing

harmonic generation at any given moment in time

48.Give at least two IEC standards for EMC.

1.IEC 610000-2-2 (1993): Electromagnetic Compatibility (ECM). Part 2: Environment.Section 2:

Compatibility Levels for Low-Frequency Conducted Disturbances andSignaling in Public Low-Voltage

Power Supply Systems.

2.IEC 610000-3-2 (2000): Electromagnetic Compatibility (EMC). Part 3: LimitsSection 2: Limits for

Harmonic Current.

49.What is the need of filtering in harmonic studies?

Filtering is a method to reduce harmonics in an industrial plant when the harmonicdistortion has been

gradually increased or as a total solution in a new plant. There aretwo basic methods: passive and active

filters.

50.List the some dynamic correction of power quality events.

1.Resonance Prevention.

2.Power Factor Correction.

3.Dynamic VAR Compensation.






UNIT- V POWER QUALITY MONOTORING

51.What are the importance of power quality monitoring?

1.Power Quality Monitoring is necessary to detect and classify disturbance at aparticular location on the

power system.

2.PQ monitoring assists in preventive and predictive maintenance.

3.Problems can be detected before they cause widespread damage by sendingautomated alerts.

4.PQ Monitoring can be used to determine the need for mitigation equipment.

52.What are the monitoring objectives?

1.Continuous evaluation of the electric supply system for disturbances and powerquality

variations.2.Document performance of power conditioning equipment, such as static switches,UPS

systems, other ride through technologies, and backup generators.53.What are the requirements of monitoring for a voltage regulation and

unbalance?

1.3 phase voltages

2.RMS magnitudes

3.Continuous monitoring with periodic max/min/avg samples

4.Currents for response of equipment.

4.What are the requirements of monitoring for a harmonic distortion?

1.3 phase voltages and currents

2.Waveform characteristics

3.128 samples per cycle minimum

4.Synchronized sampling of all voltages and currents

5.Configurable sampling characteristics

55.What are the Characteristics of power line monitors?

1.Portable, rugged, lightweight

2.Simple to use, with proper training

3.Designed for long-term unattended recording

4.Definition of line disturbance parameters varies between manufacturers

56.What is the use of oscilloscope?

Oscilloscopes with fast sampling rates and automatic triggering function can be veryuseful for trace of

transients.






57.What is Spectrum analyzer?

1.An instrument used for the analysis and measurement of signals throughout theelectromagnetic

spectrum.

2.Spectrum analyzers are available for sub audio, audio, and radio-frequencymeasurements, as well as

for microwave and optical signal measurements.

58.What is FFT (or) digital technique used for harmonic analysis?

The signal to be analyzed is converted to a digital signal by using an analog-to-digitalconverter, and the

digital signal is processed by using the FFT algorithm.The algorithm analyzes the time domain

waveform, computes the frequencycomponents present, and displays the results.

59.What is tracking generator?

The tracking generator enhances the applications of spectrum analyzers. Ins outputdelivers a swept

signal whose instantaneous frequency is always equal to the input tunedfrequency of the analyzer.

60.What is the purpose of SVC?

The SVC is mainly used for compensation of large fluctuating reactive industrial loadslike arc furnaces

etc

The purpose of the SVC is to maintain a constant voltage level at PCC by dynamicallyinjecting reactive

power and compensation for the fluctuation power demand of the load.

61.What are the components of flicker meter?

The flicker meter consists of a lamp model, a model representing the human eye-brainchain and a

statistical evaluation block where the flicker parameters PST and PLT arecalculated.

62.What is total error?

The total error in a measurement is a sum of errors that can (mainly) be divided intothree different

categories:1.Instrument errors (quantization, offset- and linearity errors etc.)

2.Transducer errors

3.Errors due to the measure signal (low signal level etc.)

63.What are the advantages of expert systems?

1.Expert systems help users cope with large volumes of EIA work;

2.Expert systems deliver EIA expertise to the non-expert;

3.Expert systems enhance user accountability for decisions reached; and

4.Expert systems provide a structured approach to EIA.






UNIVERSITY QUESTIONS

1. Define sag.

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.

2.What is Power Quality?

From the utility perspective, Power Quality has been defined as the parameters of the voltage that

affect the customer’s supersensitive equipment.From the power user perspective, Power Quality

may be defined as any electricalparameter or connection that affects the operation of the equipment. This

included allelectrical parameters, connections and grounds, whether the source from the utility,

localequipment or other users.

3. When sag leads to interruption?

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.

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.

4. How fast transfer switches are used in minimizing the severity of sags?

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.

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 most important consideration in

the effectiveness of a transfer switch for protection of sensitive loads is that it

requirestwo 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.

5. When on the device to position the power conditioners to avoid voltage swells.

Low-impedance power conditioners (LIPCs) are used primarily to interface with the

switch-mode power supplies found in electronic equipment. LIPCs differ from isolation

transformers in that these conditioners have much lower impedance and have a filter as

part of their design (Fig. 4.22). The filter is on the output side and protects against

highfrequency,

source-side, common-mode, and normal-mode disturbances (i.e., noise and

impulses). Note the new neutral-to-ground connection that can be made on the load side

because of the existence of an isolation transformer. However, low- to medium-frequency

transients (capacitor switching) can cause problems for LIPCs: The transient can be

magnified by the output filter capacitor.

6. How to model a surge arrestor in PSCAD?

This program features a very sophisticated graphical user interface that enables the user

to be very productive in this difficult analysis. Time-domain solution is required to model

nonlinear elements such as surge arresters and transformer magnetizing characteristics.

7. What is Total harmonic distortion?

Total Harmonic Distortion: The ratio of the root mean square of the harmonic content

to the rms value of the fundamental quantity, expressed as a percent of the

fundamental

8. Give the IEEE and IEC standards for EMC.






IEC 61000-2-2 (1993): Electromagnetic Compatibility (EMC). Part 2:

Environment. Section 2: Compatibility Levels for Low-Frequency Conducted

Disturbances and Signaling in Public Low-Voltage Power Supply Systems.

IEC 61000-3-2 (2000): Electromagnetic Compatibility (EMC). Part 3: Limits.

Section 2: Limits for Harmonic Current Emissions (Equipment Input Current Up

to and Including 16 A per Phase).


Section 4: Limitation of Emission of Harmonic Currents in Low-Voltage Power

Supply Systems for Equipment with Rated Current Greater Than 16 A.


Section 6: Assessment of Emission Limits for Distorting Loads in MV and HV

Power Systems. Basic EMC publication

9. What are the necessary measurements to be made to know about the power

quality of a product?

1. Nature of the problems (data loss, nuisance trips, component failures, control system

malfunctions, etc.)

2. Characteristics of the sensitive equipment experiencing problems (equipment design

information or at least application guide information)

3. The times at which problems occur

4. Coincident problems or known operations (e.g., capacitor switching) that occur at the

same time

5. Possible sources of power quality variations within the facility (motor starting,

capacitor switching, power electronic equipment operation, arcing equipment, etc.)

6. Existing power conditioning equipment being used

7. Electrical system data (one-line diagrams, transformer sizes and impedances, load

information, capacitor information, cable data, etc.)

10. What is meant by online power quality monitoring?

On-line power quality data assessment analyzes data as they are captured. The analysis

results are available immediately for rapid dissemination. Complexity in the software

design requirement for on-line assessment is usually higher than that of off-line. Most

features available in off-line analysis software can also be made available in an online

system. One of the primary advantages of on-line data analysis is that it can provide






instant message delivery to notify users of specific events of interest. Users can then take

immediate actions upon receiving the notifications.

11. (A) Define distortion factor.

Distortion is defined as a steady-state deviation from an ideal sine wave of power

frequency principally characterized by the spectral content of the deviation.

11. (B) What are the power quality indices?

Power quality indices are the indices for assessing the quality of service with respect to

harmonic voltage distortion.

12. What are the sources of sag interruptions?

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

system.

fault on the same feeder,

a fault on one of the other feeders from the substation,(a fault on a parallel feeder)

a fault somewhere on the transmission system

4. Define sag severity.

If full-voltage starting is used, the sag voltage, in per unit of nominal system voltage, is

16. Define ferroresonance.

An irregular, often chaotic type of resonance that involves the nonlinear characteristic of

iron-core (ferrous) inductors. It is nearly always undesirable when it occurs in the power

delivery system, but it is exploited in technologies such as constant-voltage transformers

to improve the power quality.

17. What are the advantages of computer analyzing tools?

low cost

Very sophisticated graphical user interface that enables the user to be very

productive in this difficult analysis.

Ability of the program to solve some classes of nonlinear problems but is not

usually a significant constraint for most power system problems.

18. Define individual and total harmonic distortion.

Harmonic distortion is caused by nonlinear devices in the power system. A nonlinear

device is one in which the current is not proportional to the applied voltage. While the

applied voltage is perfectly sinusoidal, the resulting current is distorted. Increasing the

voltage by a few percent may cause the current to double and take on a different wave






shape.

2 Total Harmonic Distortion: The ratio of the root mean square of the harmonic content

to the rms value of the fundamental quantity, expressed as a percent of the fundamental.

19. Name the devices for controlling harmonic distortion.

In-line reactors or chokes

Zigzag transformers

Passive filters

Active filters

20. List the advantages of automatic power quality monitoring system.

The capability to provide systemwide analysis of the power quality information. The

future of these systems involves integration with other data-collection devices in the

substation and the facility. Standard interfaces like the Power Quality Data Interchange

Format (PQDIF) and COMTRADE are used to share the information, and standard

protocols like UCA are used for the communications. The intelligent applications

described will be applied at both the substation level and at the enterprise level, as

appropriate.

21. What are the specifications for the measuring equipment?

■ Number of channels (voltage and/or current)

■ Temperature specifications of the instrument

■ Ruggedness of the instrument

■ Input voltage range (e.g., 0 to 600 V)

■ Power requirements

■ Ability to measure three-phase voltages

■ Input isolation (isolation between input channels and from each input to ground)

■ Ability to measure currents

■ Housing of the instrument (portable, rack-mount, etc.)

■ Ease of use (user interface, graphics capability, etc.)

■ Documentation

■ Communication capability (modem, network interface)

■ Analysis software

22.Define Voltage swell and momentary interruption.

A swell is defined as an increase to between 1.1 and 1.8 pu in rms voltage or current at






the power frequency for durations from 0.5 cycle to 1 min. (1 mark)

Momentary interruption is a type of short-duration variation. The complete loss of

voltage (<0.1 pu) on one or more phase conductors for a time period between 30 cycles

and 3 s. (1 mark)

23. What are the causes of sag interruption?

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

system.

fault on the same feeder,

a fault on one of the other feeders from the substation,(a fault on a parallel feeder)

a fault somewhere on the transmission system

24. What are the mitigation techniques for voltage sags?

1. Equipment manufacturers should have voltage sag ride-through capability curves

(similar to the ones shown previously) available to their customers so that an initial

evaluation of the equipment can be performed. Customers should begin to demand that

these types of curves be made available so that they can properly evaluate equipment.

2. The company procuring new equipment should establish a procedure that rates the

importance of the equipment. If the equipment is critical in nature, the company must

make sure that adequate.

3.Equipment should at least be able to ride through voltage sags with a minimum voltage

of 70 percent (ITI curve). The relative probability of experiencing a voltage sag to 70

percent or less of nominal is much less than experiencing a sag to 90 percent or less of

nominal. A more ideal ride-through capability for short-duration voltage sags would be

50 percent, as specified by the semiconductor industry in Standard SEMI F-47.17

25. How does the lightning cause over voltages?

The most obvious conduction path occurs during a direct strike to a phase wire, either on

the primary or the secondary side of the transformer.This can generate very high

overvoltages. Transient overvoltages can be generated by lightning currents flowing

along ground conductor paths.

26. List the functions of line arresters.

To prevent overhead line faults, one must raise the insulation level of the line, prevent

lightning from striking the line, or prevent the voltage from exceeding the insulation

level.






The third idea is becoming more popular with improving surge arrester designs. To

accomplish this, surge arresters are placed every two or three poles along the feeder as

well

as on distribution transformers. Some utilities place them on all three phases, while other

utilities place them only on the phase most likely to be struck by lightning. To support

some

of the recent ideas about improving power quality, or providing custom power with super

reliable main feeders, it will be necessary to put arresters on every phase of every pole.

27. List the harmonic indices.

The two most commonly used indices for measuring the harmonic content of a waveform

are

the total harmonic distortion and the total demand distortion.

28. Name the devices for controlling harmonic distortion.

In-line reactors or chokes

Zigzag transformers

Passive filters

Active filters

29. What is meant by power quality monitoring?

Power quality monitoring is the process of gathering, analyzing, and interpreting raw

measurement data into useful information. The process of gathering data is usually

carried

out by continuous measurement of voltage and current over an extended period.

30.What are the requirements for the power quality measuring equipment?

Number of channels (voltage and/or current)

■ Temperature specifications of the instrument

■ Ruggedness of the instrument

■ Input voltage range (e.g., 0 to 600 V)

■ Power requirements

■ Ability to measure three-phase voltages

■ Input isolation (isolation between input channels and from each input to ground)

■ Ability to measure currents

■ Housing of the instrument (portable, rack-mount, etc.)






■ Ease of use (user interface, graphics capability, etc.)

■ Documentation

■ Communication capability (modem, network interface)

■ Analysis software

ee801 power quality

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