UNIT I INTRODUCTION TO POWER QUALITY. Part A (2-Marks)

Transcription

1 UNIT I INTRODUCTION TO POWER QUALITY Part A (2-Marks) 1. What is Power Quality? Any power problem manifested in voltage, current or frequency deviations that result in failure or misoperation of customer equipment. Power Quality = voltage quality, because power system can control over the quality of voltage not particular load currents. 2. Define transients. Transients are undesirable but momentary in natures which produce the power supply variations. It is also called as surge. 3.What are the types of transients? Impulsive transients Oscillatory transients 4. Define Impulsive transients: It is a sudden non-power frequency change in steady state condition of voltage, current or both that is unidirectional in polarity value.(either in +ve or ve). 5. Define Oscillatory transients: It is a sudden non-power frequency change in steady state condition of voltage, current or both that includes both positive and negative polarity values. 6. Define over voltage and under voltage. An over voltage is an increase in the rms ac voltage greater than 110%at the power frequency for a duration longer than 1 minute. Under voltage: An under voltage is an increase in the rms ac voltage less than 90% at the power frequency for a duration longer than 1 minute.

2 7. Define Sustained interruption. When the power supply voltage has been zero for a time of time in excess of 1 minute, the long duration voltage variation is considered as a sustained interruption. 8. Define Interruption. (APRIL/MAY 2008) It occurs when the supply voltage or load current decreases to less than 0.1 pu for a period of time not exceeding 1minute. 9. Define Voltage sag (dip). (APRIL/MAY 2008) Sag is defined as 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. 10. Define Voltage swell. Swell is defined as a increase 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. 11. Define CBEMA curve. (APRIL/MAY 2008) A set of curves representing the withstand capabilities of computers in terms of the magnitude and duration of the voltage disturbance. Developed by the Computer Business Equipment Manufacturers Association (CBEMA), it has become a de facto standard for measuring the performance of all types of equipment and power systems. 12. Define Total demand distortion (TDD). The ratio of root mean square of the harmonic current to the rms value of the rated or maximum demand fundamental current, expressed as a percent. 13. Define Total harmonic distortion (THD)

3 The ratio of rms of the harmonic content to the rms value of the fundamental quantity, expressed as a percent of the fundamental. 14. Define Harmonic filter. In power systems a device for filtering one or more harmonics from the power system. Most are passive combinations of inductance, capacitance and resitance.newer technologies include active filter that can address reactive power needs. 15. Define Inter harmonic. A frequency component of a periodic quantity that is not an integer multiple of the frequency at which the supply system is designed to operate (50 or 60 HZ). 16. Define Notch. A switching disturbance of the normal power voltage wave forms, lasting less than a half cycle; which is initially of opposite polarity than the wave form and is subtracted from the normal waveform in terms of peak value of the disturbance voltage. It includes the complete loss of voltage for up to a half cycle. 17. Define Flicker. Impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time. 18. What are primary types of wave form distortion? Dc offset Harmonics inter harmonics notching noise 19. Define Voltage imbalance (unbalance). = max.deviation from the average of the 3phase voltages or currents/ the average of the 3phase voltages or currents 20. Define Wave form distortion.

4 It is defined as a steady state deviation from an ideal sine wave of power frequency principally characterized by the spectral content of the deviation. 21. Define Voltage fluctuation. Systematic variations of the voltage envelope or a series of random voltage changes, the magnitude of which does not normally exceed the voltage ranges of 0.9 to 1.1 pu specified by ANSIC. 22. Define Crest factor. A value reported by many power quality monitoring instruments representing the ratio of the crest value of the measured wave form to the rms value of the wave form. For example, crest factor for sine wave is Define Drop out. A loss of equipment operation (discrete data signals) due to noise, sag or interruption. 24. Define Drop out voltage The voltage at which a device will release to its deenergized position. 25. Define Harmonic. A component of order greater than one of the Fourier series of a periodic series of a periodic quantity. PART B(16 marks) 1.(a) Describe the EMC phenomena as per IEEE Standard (May / June 2012)(10) Explain What is EMC Explain EMC for Utility Explain EMC for Equipment manufacturers

5 (b) Describe the sources of power quality problem with suitable examples (May / June 2012)(6) Name the sources with examples Explain each sources 2.(a)Explain transients and wave form distortions with suitable wave forms (May / June 2012)(8) Define transient Draw the Wave form Define wave form distortion Draw the wave form (b)describe the power acceptability curves (8) Draw the curve Explain the importance of curve in terms of power quality 3. What are the major power quality issues? Explain in detail Name all the power quality problems Explain above all with (i) Wave forms (ii) Causes/Source (iii) Effects (iv) Mitigation methods 4. Discuss the following characteristics of power quality events (i)long duration voltage variation (ii)short duration voltage variation Name all LONG duration voltage variations Explain all the above with (i) Wave forms (ii) Causes/Source (iii) Effects (iv) Mitigation methods Name all SHORT duration voltage variations Explain all the above with (i) Wave forms (ii) Causes/Source (iii) Effects (iv) Mitigation methods

6 5.(a)Write the various IEEE and IEC standards Explain what is IEEE and IEC standard Name and explain all the standard with respect to power quality including EMC (b)explain THD and TDD Define THD Write the formula for THD Discuss the importance of THD In power quality Define TDD Write the formula for TDD Discuss the importance of TDD In power quality 6.With wave form sketch explain the terms (i)voltage sag (ii)voltage interruption (iii)voltage swells (iv)sag with harmonics Explain all the above terms in the following order (i)definition (ii)wave form (iii)causes / sources (iv)effects (v)mitigation method UNIT 2 SHORT AND LONG INTERRUPTIONS PART A(2 MARKS) 1.What is long and short interruption Along interruption is a power quality problem during which the voltage at customer connection or that equipment terminals drops to zero and does not come back automatically 2.Define Motor starting sags Motors are drawing several times full load current while starting. when it flows through system impedance, this large current will cause a voltage sag which may dim lighs,cause contractors to drop out. V min (pu) = V (Pu) *KVA sc / (KVA LR +KVA sc ) Where V (Pu) = actual system voltage in per unit of nominal KVA LR = motor locked rotor KVA KVA sc = system short circuit KVA at the motor.

7 3.What are the datas are required for the simulation to estimate the sag severity during full voltage severity? Parameter values for the standard induction motor equivalent circuit R1, X1, R2, X2, R0, X0.where 1, 2 & 0 represents-positive, negative, zero respectively. Number of motor poles and rated rpm. Inertia constant values for the motor and the motor load. Torque vs. speed characteristic for the motor load. 4.What are the two basic options available to reduce the severity of faults on the power system? Prevent faults Modify fault clearing practices. 5.Define Reclosing. The common utility practice on over head lines of closing the breaker within a short time after clearing a fault, taking advantage of the fact that the most faults are transient or temporary. 6.What are the most common sequences in use of four shot reclosers? One fast operation, three delayed Two fast, two delayed.c 7.Define SAIFI: System Average Interruption Frequency Index. SAIFI= [(no. customers interrupted)/(no. interruption)]/total no. customers. 8.Define SAIDI: System Average Interruption Demand Index. SAIDI=Σ [(no. customers affected)/ (duration of outage)]/total no. customers. 9.Define CAIFI: Customer Average Interruption Frequency Index CAIFI=total no. customer interruptions/ no. customers affected

8 10.Define CAIDI: Customer Average Interruption Duration Index CAIDI= Σ customer interruption duration/total no. customer interruptions. 11.Define ASAI: Average System Availability Index ASAI=customer hours service availability/customer hours service demand 12.Define Adaptive relaying. It is the practice of changing the relaying characteristics of the over current protective device to suit the present system conditions. 13.Define Tree trimming. This is one of the more effective methods of reducing the number of faults on the overhead lines. It is a necessity some may complain about the environmental and aesthetic impact. 14.Define 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. 15.What are the types of fault locating techniques available? Thumping Cable radar and other pulse methods. tone Fault chasing with a fuse. 16.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.

9 17.What are the three levels of possible solutions to voltage sag and momentary interruption problems? Power System Design Equipment design Power conditioning equipment 18.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. 19.What are the origin of short interruptions? Lightning transients Small objects causing a temporary short circuit 20.What is monitoring of short interruption? Short interruption monitoring requires automatic monitoring devices Short interruptions are not noticeable one To record short interruption monitoring devices are installed on all feeders A number of survey have been performed to obtain statistical information about interruption 21. What is single phase tripping? Single phase tripping is used in the transmission system to maintain synchronicity between both sides of a line Single phase tripping is rarely used in low voltage and distribution side 22. Define post fault period The fault has extinguished, the short circuit has now become open circuit because the breaker in that phase is still open this period is called post fault period 23. What are the causes of long interruption? Component outage A protection relay intervenes incorrectly, thus causing component outage Operator action causes component outage 24. What are the devices used for voltage regulation Tap changing transformers Isolation devices with separate voltage regulators

10 Impedance compensating devices such as capacitors Ferro resonance transformers Magnetic synthesizer On Line UPS Motor Generator sets Static VAR compensators(svc) Dynamic voltage restorer(dvr) TCSC 25.Define Ferro resonant transformers and Magnetic synthesizers. It is also called constant voltage transformer (CVTs), can handle most voltage sag conditions.cvts are especially attractive for constant, low power loads. These are basically 1:1 transformers which are excited high on their saturation curves, there by providing an output voltage which is not affected by input voltage variations. It is an electromagnetic device which takes incoming power and regenerates a clean, 3 phase ac output wave form, regardless of input power quality. It is generally used for larger (several KVA) loads which make these unit cost effective. They are used for large computers and other electronic equipment that is voltage sensitive. Part B(16 Marks) 1(a).Compare short and long interruption and write the sources of such interruption(8)(may / june 2012) Compare above two terms in (i)definition (ii)wave forms (iii)sources (iv)effects (v)mitigation methods (b)discuss how induction motors influence on interruption of power quality(8)(may / june 2012) (i)definition (interruption) (ii)induction motor wave forms (iii)how the above form can affect the quality of power 2.Name and explain the voltage regulating devices(16) (May / june 2012) (i)explain the need for voltage regulating devices

11 (ii)name all the voltage regulating devices (iii)explain each devices with individual sub headings 3.(a) Explain briefly about reliability evaluation and cost of interruption(8) (May / june 2012) Reliability Evaluation (i)explain the need of reliability studies (ii)define Failure (iii)define outage Cost of interruption (i)need for cost analysis (ii)classify and explain the various costs 4.Explain how the followings are influence to the power quality problem (i)induction motors (ii) Synchronous motors (iii) Adjustable speed drives (iv) Electronic equipments Explain all the four in terms of output wave forms Explain how the output wave forms of the above leads to power quality problem 5.Discuss about (i)monitoring of interruption(long / Short)(8) (ii)prediction of short interruption(8) Explain what is monitoring Need for interruption monitoring Methods used for monitoring Uses of interruption monitoring 6.Explain voltage regulation at (i)utility side (ii)end user side Explain what is mean by utility and end user side Define voltage regulation Need for voltage regulation How the voltage get regulated at utility and end user side

12 UNIT 3 VOLTAGE SAG AND TRANSIENTS PART A(2 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.What are the three levels of possible solutions to voltage sag and momentary interruption problems? Power System Design Equipment design Power conditioning equipment 3.List some industry standards associated with voltage sags. *SEMI F CBEMA curve 4.Give some economic impacts due to sag. Process outrages Damaged products Lost time for restarting 5.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 6.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 7.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.

13 Equipment sensitive to have characteristics other than magnitude and duration 8.Name the different motor starting methods. Resistance and reactance starters Autotransformer starters Star-Delta starters 9.What are the causes for voltage sags due to transformer energizing? Normal system operation, which includes manual energizing of a transformer. Reclosing actions. 10.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. 11.Name the three levels of possible solutions to voltage sag and momentary interruption problems. Equipment Design * Power conditioning equipment Power system design 12.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 13.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. 14.Define transient over voltages and its types A transient over voltage can be defined as the response of an electrical network to a sudden change in network conditions, either intended or accidental, (e.g. a switching operation or a fault) or network stimuli (e.g. lightning strike). types of transient over voltages 1) Impulsive 2) Oscillatory 15. Define impulsive transients. Give example for impulsive transient over

14 voltages. An impulsive transient is a sudden, non-power frequency change in the steady state condition of the voltage and/or current waveforms that is essentially in one direction, either positive or negative, with respect to those waveforms. The most common cause of this type of transient is lightning. 16.Give examples for oscillatory transient over voltages. Switching operations within the distribution network are a major cause of oscillatory transient over voltages. Such operations include (a) Switching of utility capacitor banks, (b) Switching of circuit breakers to clear network faults, and (c) Switching of distribution feeders to rearrange the network for maintenance or construction 17.What is the effect of capacitor switching transients on network? Transients of this magnitude and duration are usually not a problem on the utility system, but they can produce problems at a user facility. Severe over voltages can appear on user facility capacitors through a phenomenon known as voltage magnification 18. What are the causes of voltage magnification on network? The voltage magnification will not result in capacitor damage. The problem that usually occurs is the failure or mis-operation of sensitive loads in the facility where the low voltage capacitors are installed. 19.Define voltage magnification phenomena? The highest transient voltages occur at the low voltage capacitor bank when the characteristic frequency of the switching transient is nearly equal to the resonant frequency of the low voltage system and when the switched capacitor is ten or more times the size of the low-voltage capacitor 20.Mention the two important concerns for capacitor bank switching transients. Voltage transients at the capacitor bank substation and neighboring substations Power quality impact on sensitive customer loads due to variations in voltage when energizing capacitor banks

15 21.Give the various aspects of equipment specific design and protection issues for the capacitor switching transients. Phase-to-ground and phase-to-phase insulation switching withstand to voltage stresses Controlled closing for circuit breakers (pre-insertion resistors/reactors or synchronous switching) Capacitor bank and substation Circuit breakers ANSVIEEE C37 requirements./ Current limiting reactor requirements 22. What specify the IEEE standard for shunt power capacitors causing transient overvoltages? The IEEE Standard for Shunt Power Capacitors, ANSI/IEEE Std , specifies that capacitors "may reasonably be expected to withstand" transient overvoltages from 205% to 354% of rated peak voltage, depending on the number of times a year the overvoltage occurs. 23.What are the various Causes of overvoltages? Overvoltages, i.e. brief voltage peaks (transients, surges, spikes), can be attributed to the following 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.Ferroresonance 24.Give tile basic principles of overvoltage protection of load equipments. Limit the voltage across sensitive insulation. Divert the surge current away from the load. Block the surge current entering into the load. Bonding of equipment with ground 25.What is the need of surge arrestors? A surge arrester is a protective device for limiting surge voltages on equipment by discharging or bypassing surge current. Surge arresters allow only minimal flow of the 50Hz/60Hz power current to ground.

16 26.Differentiate between transient voltage surge suppressors (TVSS) and surge arrestors. Arresters and TVSS devices protect equipment from transient overvoltages by limiting the maximum voltage, and the terms are sometimes used interchangeably. However, TVSSs are generally associated with devices used at the load equipment. A TVSS will sometimes have more surge-limiting elements than an arrester. 27Mention the types of surge arrestors Metal-oxide varistor type Gapped silicon - carbide type 28.What is metal-oxide surge-arrester? Mention its advantages A metal-oxide surge-arrester (MOSA) utilizing zinc-oxide block provides the best performance, as surge voltage conduction starts and stops promptly at a precise voltage level, thereby improving system protection Advantages Improved Surge Duty Capability Improved Temporary Overvoltage Capability 29.What is the need of Transmission Line Arresters? Mention its benifits '. Transmission Line Surge Arresters conduct lightning surges around the protected insulator so that a lightning flashover is not created. They are designed to be installed functionally in parallel with the line insulator. The arrester conducts the lightning surges around the protected insulator so that a subsequent 50Hz / 60 Hz fault on the circuit is not created. Benefits of Transmission Line Surge Arresters Lowers initial cost of new or transmission line upgrades by making construction more compact and transmitting more energy in the same right of way. Reduces the height of transmission lines by eliminating shield wire Improves outage statistics by eliminating back flashover from the tower ground lead to the phase conductor 30.What is the role of surge arrestor on shielded and unshielded transmission line? On shielded transmission lines or under-built distribution circuits,

17 the arrester prevents tower to phase insulator back-flashovers during a lightning strike. On unshielded sub transmission or distribution circuits, the arrester prevents phase-to-ground flashover. 31.Define lightning phenomena. Lightning is an electrical discharge in the air between clouds, between different charge centre within the same cloud, or between cloud and earth (or earthed object). Even though more discharges occur between or within clouds, there are enough strokes that terminate on the earth to cause problems to power systems and sensitive electronic equipment 32. How Overvoltages are induced due to lightning? When lightning strikes occur in or near an electricity distribution system, lightning currents are generated and conducted through the power system into connected equipment. Large impulsive transient over voltages are produced as a result of this current 33. What are the various causes due to lightning over voltages? In transmission systems, the insulation is generally sufficient enough not to be endangered by induced voltages. However, distribution systems in which the insulation level is low, induced voltages are hazardous. When the induced voltage caused by lightning exceeds the strength of the insulation, a line flashover results, causing either temporary faults or disruption of services to customers 34.What is Ferro resonance? Ferro resonance is a special case of series LC resonance where the inductance involved is nonlinear and it is usually related to equipment with iron cores. It occurs when line capacitance resonates with the magnetizing reactance of a core while it goes in and out of saturation. 35. List the advantages of computer analysis tools for transient studies. The application of commercial time-domain simulation packages (PSCAD/EMTDC) has many advantages over the conventional mathematical analytic methods, in terms of: 1. There being no need to develop a complex nonlinear differential equation for the system; 2. The complex power system phenomena such as ferroresonance, harmonics,

18 etc being easily modeled with modules of lines, transformers, power converters, loads and protection equipment from the library. PART - B 1.(a)Explain the role of power electronics load in voltage sag(8) (May / june 2012) (i) Name all the power electronic loads which causes voltage sag (ii)explain the above with the followings (a)schematic diagram / Circuit diagram with explanation (b)output waveform of the above (c)how the waveform causes the voltage sag (b)explain the method of stochastic assessment ofvoltagesag(8)(may/june2012) (i)need for voltage sag assessment (ii)types of assessment (iii)stochastic assessment 2.What is transient/explain different types of transient with suitable example(16) (May/june2012) (i)define transients and its causes (ii)types of transients (iii)explain each type with diagram by sub title 3.(a)Discuss the sources of sags and interruption(8) (i)define sag and interruption (ii)name the sources (ii)explain each and every source (b)what is the need of estimating sag performance? Explain the different methods of estimating voltage sag performance(8) (i)procedure for estimation (ii)area of vulnerability (iii)equipment sesnsityvity 4.Discuss the following transients (i)capacitor switching transient (ii)lightning transient Explain how capacitor switching influence the transient Switching time Preinsertion resistors Capacitor location Types of lightning transient

19 Causes and effects 5.Explain the principle of over voltage protection(16) Schematic diagram Diagram description Explanation 6.With neat diagrams explain about the devices for over voltage protection(16) Name all the protective devices Explain each with neat diagram UNIT 4 WAVE FORM DISTORTIONS PART A(2 MARKS) 1. Differentiate between harmonics and transients Harmonics are integer multiples of the fundamental frequency in steady state condition. The wave form distortion that produces the harmonics is present continually, or atleast several seconds.hamonics are associated with continuing operation of a load. Transients disturbances contain high frequency components.transients are usually dissipated within a few cycles. Transients are associated with changes in the system such as switching a capacitor bank. 2. Define Triplen harmonics: Triplen harmonics are the odd multiples of the third harmonic (h=3, 9, 15, 21 ), which deserve special attention because of their natural tendency to be zero sequence. 3. What are the effects of harmonic in power system? Malfunction of sensitive equipment Random tripping of circuit breakers Flickering lights Very high neutral currents Overheated phase conductors, panels, and transformers

20 Premature failure of transformers and uninterruptible Power supplies (UPSs) Reduced power factor Reduced system capacity 4. How do you identify the harmonic presence? Measure the peak and root mean square (RMS) voltage at a sample of receptacles. The crest factor is the ratio of peak to RMS voltage. For a perfectly sinusoidal voltage, the crest factor will be 1.4. Low crest factories a clear indicator of the presence of harmonics. Inspect distribution panels: Remove panel covers and visually inspect components for signs of overheating, including discolored or receded insulation or discoloration of terminal screws Measure phase and neutral currents at the transformer secondary with clamp-on current probes. If no harmonics are being generated, the neutral current of a three-phase distribution system carries only the imbalance of the phase currents. In a well-balanced three-phase distribution system, phase currents will be very similar, and current in the neutral conductor should be much lower than phase current and far below its rated current capacity. In addition to these simple measurements, many power-monitoring devices are now commercially available from a variety of manufacturers to measure and record harmonic levels. These instruments provide detailed information on THD, as well as on the intensity of individual harmonic frequencies. 5. What are the sources of harmonic from commercial and industrial loads? Adjustable speed drives electronic power supplies dc motor drives Battery chargers electronic ballasts rectifier and inverter modern and micro processor based office equipment PWM modulated drives

21 arcing devices(arc furnaces, arc welders and discharge type lighting)& saturable devices(transformers and electro magnetic devices) 6. What is the effect of harmonic distortion on capacitor? The capacitor is subjected principally to two harmonics: 5th and 7 th. The voltage distortion consists of 4 % fifth and 3% seventh. This results in 20% fifth harmonic and 21% harmonic seventh current. 7. What is the effect of harmonic distortion on transformer? Harmonic current may result in the transformer rms current being higher than its capacity. The increased total rms current results in increased conductor losses.it increase the eddy current and core losses. Therefore harmonic current distortion produces additional heating in transformer. 8. What is the effect of harmonic distortion on motor? Harmonic voltage distortion at the motor terminals is translated in to harmonic fluxes with in the motor. The additional fluxes do little more than induce additional losses. The motor efficiency is decreased Heating is increased vibration High pitched noises. 9. When a harmonic distortion problem occurs, what are the basic options available for controlling the harmonics? Reduce the harmonic current produced by load. Add the filters siphon the harmonic currents off the system, block the currents from entering the system or supply the harmonic currents locally. Alter the frequency response of the system by filters, inductors and capacitors. 10. How to reduce the harmonic current in loads?

22 Adding a reactor in series will significantly reduce the harmonics and also provide the transient protection benefits. Use the phase shifting half of the six pulse power converters in a plant load by 30 degrees can approximate the benefits of 12 pulse loads by drastically reducing the 5 & 7 th harmonics. Delta connected transformers can block the flow zero sequence harmonics (triplen harmonics) from the line. Zig zag transformer connections, grounding transformer can shunt the triplens off the line. 11. How do you modify the system frequency response? Adding a shunt filter. Adding a reactor to detune the system. Changing the size of the capacitor. Moving a capacitor to a point on the system with different short circuit impedance or higher losses. Removing the capacitor and simply accepting the higher losses, lower voltage and power factor penalty. 12. What are the types of filters used to filter the harmonic distortion? Passive filter Active filter 13. Define Active filter: Any of a number of sophisticated power electronic devices for eliminating harmonic distortion. 14. Define Passive filter A combination of inductors, capacitors and resistors designed to eliminate one or more harmonics. The most common variety is simply an inductor in series with a shunt capacitor, which short circuits the major distorting harmonic component from the system. 15. What is the positive sequence harmonic order?

23 h=1, 7, What is the negative sequence harmonic order? h=5, 11, What is the zero sequence harmonic order? h=3, 9, 15 (Triplen) 18. What is the general procedure to design the harmonic filter? First apply one single tuned shunt filter and design it for the lowest generated frequency. Determine the voltage distortion level at the low voltage bus. Vary the filter elements according to the specified tolerances and check the filter s effectiveness. Check the frequency response characteristic to verify that the newly created parallel resonance is not close to a harmonic frequency. 19. Define quality factor. Q=n*Xl/R Where R=series resistance of filter elements n= tuning harmonic Xl=reactance of filter reactor at fundamental frequency. 20. What is the range of harmonics affect the telecommunication interference? Harmonics between 540 Hz (9 th harmonic) and 1200Hz are affecting the telecommunication interference And triplen harmonics (3, 9, 15...) has the greatest exposure in communication circuit in four wire system and directly in neutral circuit. 21. What are the datas are required for computer tools for harmonic analysis. Line and transformer impedances Transformer connections Capacitor values and location Harmonic spectra from nonlinear loads Power source voltages. 22. What are the computer tools are available for harmonic analysis? Transient Network Analyzer (TNA) Software tool: Network frequency response analysis program (NFRAP)

24 McGraw Edison Harmonic Analysis Program (MEHAP) V-HARM program Super harm program CYMHARMO program HARMFLO program 23.What are the important concepts to bear in mind to understand power system harmonics? There are two important concepts to bear in mind with regard to power system harmonics. The first is the nature of harmonic current producing loads (nonlinear loads) and the second is the way in which harmonic currents flow and how the resulting harmonic voltages develop. 24.What is the reason for existence of harmonic distortion? Harmonics distortion exists due to the nonlinear characteristics of the devices and loads on the power system. These devices act as current sources that inject harmonic currents into the power system. 25.Differentiate between linear loads and non-linear loads. Linear load: 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 supply frequency). 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 sinusoidal because of the presence of harmonics. 26.What is voltage and current distortion? Voltage distortion is any deviation from the nominal sine waveform of the AC line voltage. Current distortion is any deviation from the nominal sine waveform of the AC line current. 27.Mention the commonly used indices used for measuring harmonic component of waveform. The two most commonly used indices for measuring the ham10nic content of the waveform are the total harmonic distortion (THD) and total

25 demand distortion (TDD). 1. If a generator produces a non-ideal sinusoidal waveform, the voltage waveform will contain a certain amount of harmonics 2. In motors, decreased efficiency, excessive heating, and vibration are symptoms of harmonic voltage distortion. 28.Mention at least two causes of harmonics made on distribution systems. In the distribution system, transformers are capable of producing harmonics due to magnetic core saturation. This is more prevalent at a lighter loading of the transformer Large load currents in the neutral wires of a 3 phase system. Theoretically the neutral current can be up to the sum of all 3 phases therefore causing overheating of the neutral wires. Since only the phase wires are protected by circuit breakers of fuses, this can result in a potential fire hazard. 29.Mention the problems created by harmonics. A large load current flows in the neutral WIres of a 3 phase system. Theoretically the neutral current can be up to the sum of all 3 phases therefore causing overheating of the neutral wires. Poor power factor conditions that result in monthly utility penalty fees for major users (factories, manufacturing, and industrial) with a power factor less than What is the effect on transformer due to Harmonics? The primary effect of power system harmonics on transformers is the additional heat generated by the losses caused by the harmonic contents generated by the load current PART B(16 Marks) 1(a)Explain the standard and limitations of harmonics(8) (i)name IEEE and IEC standard about harmonic (ii)explain current and voltage limitation of the harmonics (b)explain sources and effects of harmonics distortion for different loads(8) (i)name the sources of harmonics Commercial loads Industrial loads (ii)explain the above with S Schematic diagram

26 Wave forms (iii)explain how the above wave form leads power quality problem 2(a).Define and explain different harmonic indices(8) (i)define THD and write the formula (II)Define TDD and write the formula (b)explain the harmonic phase rotation and phase angle relationship(8) (i)draw the vector diagram shows the harmonic contents (i)explain the effect of harmonics in case of phase angle variation 3.What is the principle of controlling harmonics? Explain about the devices for controlling harmonic distortion (i)define principle (ii) principle diagram (iii)name the devices used to control the harmonics (iv)explain each device with neat diagram 4.Explain the following terms (i)inter harmonics(4) (ii)notching(4) (iii)voltage vs Current harmonics(4) (iv)harmonics vs Transients(4) Define all with individual sub title Draw the waveform in the respective sub title Describe the concept in the respective sub title 5.Explain the sources and effects of harmonics from (i)commercial loads (ii)industrial loads Name all the commercial loads which produce harmonics Explain each with neat diagram and wave form Name all the industrial loads which produce harmonics Explain each with neat diagram and wave form 6(a)Explain the power system response characteristics under the presence of harmonics(8) Explain the following with neat diagram and wave form

27 System impedance Capacitor impedance Effect of resistance and resistive load Parallel resonance Series resonance (b)discuss effects of harmonic distortion(8) Explain the effect of harmonics in Impact of capacitor Impact on Transformer RMS current Eddy current losses Core losses Impact on motors Impact on telecommunication Impact on metering UNIT 5 POWER QUALITY SOLUTIONS PART A(2 MARKS) 1. What is the importance of power quality monitoring? Power Quality Monitoring is necessary to- detect and classify disturbance at a particular location on the power system. PQ monitoring assists in preventive and predictive maintenance. Problems can be detected before they cause widespread damage by sending automated alerts. PQ Monitoring can be used to determine the need for mitigation equipment. 2. What are the monitoring objectives? Continuous evaluation of the electric supply system for disturbances and power quality variations. Document performance of power conditioning equipment, such as static switches, UPS systems, other ride through technologies, and backup generators. 3. What are the purposes of power quality monitoring

28 system? Preventive maintenance Load analysis Equipment diagnostics Long time surveys 4. What is proactive monitoring? The traditional approach to power quality monitoring is reactive. We need to know when a problem is going to occur before it happens. Permanent power quality monitoring systems are designed to help proactively identify conditions and events that may cause problems should be addressed. This is called proactive monitoring. 5.What are the steps involved in power quality monitoring? Planning for the monitoring Preparing for the monitoring Inspecting the site Monitoring the power Analyzing, monitoring and inspecting data Applying corrective solutions 6.What are the requirements of monitoring for a voltage regulation and unbalance? 3 phase voltages RMS magnitudes Continuous monitoring with periodic max/min/avg samples 7.What are the requirements of monitoring for a harmonic distortion? Currents for response of equipment 3 phase voltages and currents Waveform characteristics 128 samples per cycle minimum Synchronized sampling of all voltages and currents Configurable sampling characteristics

29 8. What are the Characteristics of power quality monitoring equipment? Harmonic Analysis Harmonic analyses are usually conducted by obtaining and interpreting measurements of waveforms. Equipment normally required to perform a harmonic study consists of a harmonic analyzer, an oscilloscope, and an RMS responding voltmeter and ammeter. Spectrum analysis is usually performed up to the 50th harmonic (3 khz). 9. What are the Characteristics of power line monitors? Portable, rugged, lightweight Simple to use, with proper training Designed for long-term unattended recording Definition of line disturbance parameters varies between manufacturers 10. What is the Types of power quality measurement equipment? Hand-held single-phase power quality monitors Portable three-phase power quality monitors Harmonic analyzers Distortion analyzers Multimeters 11. Mention the factors that should be considered for selecting the instrument. Number of channels (voltage and/or current) Temperature specifications of the instrument Input voltage range (e.g., a to 1000 V) Ability to measure three-phase voltages 12. What is the use of oscilloscope? Oscilloscopes with fast sampling rates and automatic triggering function can be very useful for trace of transients. 13. What is the use of spectrum analyzer? A spectrum analyzer can be used for trace of high frequency harmonics. 14. What is the use of simple single phase hand-held power quality monitor? Power quality problems like measuring the occurrence of harmonics or checking the voltage level or the power frequency can easily be made by using a simple single phase hand-held power quality monitor.

30 15. Mention the Instruments used for the analysis of non-sinusoidal voltage and currents? Oscilloscope Spectrum analyzer Harmonic analyzer 16. Mention the basic categories of instruments for harmonic analysis? Simple meters General-purpose spectrum analyzers Special-purpose power system harmonic analyzers Digital Harmonics Measuring Equipment Distortion Analyzers Data Logger 17. What is Spectrum analyzer? An instrument used for the analysis and measurement of signals throughout the electromagnetic spectrum. Spectrum analyzers are available for sub audio, audio, and radio-frequency measurements, as well as for microwave and optical signal measurements. 18. What is the operation of spectrum analyzer? A spectrum analyzer separates the signal into two components: amplitude (displayed vertically) and frequency (displayed horizontally). In some low frequency analyzers, phase information can also be displayed. Low-frequency analyzers are sometimes called as "Harmonic analyzers" Vertical scale displays the amplitude and horizontal scale displays the frequency. 19. What is Swept heterodyne technique? Any signal at the input, at a frequency such that the difference between its frequency and the local oscillator is within the bandwidth of an intermediate- frequency filter, will be detected and will vertically deflect the spot on the display by an amount proportional to the amplitude of the input signal being analyzed. 20. 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 digital converter, and the digital signal is processed by using the FFT algorithm. The algorithm analyzes the time domain waveform, computes the frequency components present, and displays the results. 21. What are the advantages of FFT? FFT technique is much faster. Measurement is virtually real time.

31 22. What are the disadvantages of FFT? Restricted to lower frequencies. Complex due to need of A/D converter. 23. What is the use of digital storage? Digital storage gives the effect of a constant display, even though a very slow sweep may have been used to acquire the displayed data. 24. What is tracking generator? The tracking generator enhances the applications of spectrum analyzers. Its output delivers a swept signal whose instantaneous frequency is always equal to the input tuned frequency of the analyzer. 25. What is harmonic analyzer? Spectrum analyzers covering up to typically 100 khz can also be called harmonic analyzers. PART B(16 Marks) 1(a)Explain the operation of power quality conditioning equipment(8)(may / june 2012) Need for power conditioning equipment End user circle diagram Explanation (b)discuss the evaluation of power quality monitoring(8)(may / june 2012) Need for evaluation Deregulation effect with diagram Explanation 2.Explain with necessary block diagram how active filters used to reduce the harmonics(16)(may / June 2012) 3.Explain the following Block diagram Block description Explain how the blocks are performed for harmonic reduction (i)harmonic analyzer (ii)flicker meters Schematic diagram for harmonic analyzer Diagram description and explanation

32 Schematic diagram for Flicker meter Diagram description and explanation 4. With a help of block diagram explain applications of expert system for power quality(nov /DEC 2009) o Draw the block diagram o Draw the data flow diagram o Explanation 5.Explain the harmonic analyzer and disturbance analyzer(apr / MAY 2008) o Draw the schematic of harmonic analyzer o Describe the diagram o Explain the working principle o Draw the schematic of disturbance analyzer o Describe the diagram o Explain the working principle 6.Discuss about planning and conducting power quality survey Need for power quality survey Method Explantion(Conducting)

33 B.E / B.Tech.DEGREE EXAMINATION, MAY/JUNE 2012 Seventh Semester Electrical and Electronics Engineering POWER QULITY ENGINEERING (Regulation 2008) Answer ALL questions PART A (10x2 = 20) 1. Why power quality issues are becoming important in today context? Power quality issue are becoming important because it deals whether the voltage or current is in pure sinusoidal form or not, and also power quality discover the proper operation of the equipment, if the power quality problem originate it leads to the mal operation of the SENSITIVE equipment 2. List the effect of power quality problem Data corruption Equipment damage Data transmission errors Intermittent equipment operation Reduced equipment life Irreproducible problems 3. How do the adjustable speed drives affect the power quality problem? Adjustable speed drives produce the wave form with lot of distortion.all the drives are produce the waveform with many harmonics so the fundamental wave form get affected 4. Name the methods of prediction of short interruption? o Failure rate per / km of the feeder o Length of the main feeder o Success rate of recloser o Position of recloser 5. What are the causes of sag interruption? Faults Increased load demand and Transitional events such as large motor starting

34 Sudden change in load current Fault on feeder Fault on parallel feeder Motor start Undersized distribution system 6.List the necessary critical information to compute the voltage sag performance 1. Determine the number and characteristics of voltage sags that result from transmission system faults. 3.Determine the number and characteristics of voltage sags that result from distribution system faults (for facilities that are supplied from distribution systems) 7. If the fundamental frequency is fo HZ, then find the 3 rd and 5 th harmonic frequency 3fo, 5fo 8. Define inter harmonics A frequency that is an Non integer multiple of the fundamental frequency is called a Inter harmonic frequency 9. Give ideal characteristics of power quality monitoring system Sensitivity Reliability Fast response 10. What is meant by mitigation Mitigation means how to reduce the concern power quality problem by using various devices and techniques. PART B-(5 X16 = 80 marks) 11.(a)(i) Describe the EMC phenomena as per IEEE Standard (May / June 2012)(10) Explain What is EMC Explain EMC for Utility Explain EMC for Equipment manufacturers (ii) Describe the sources of power quality problem with suitable examples (May / June 2012)(6) Name the sources with examples Explain each sources (or)

35 b.(i)explain transients and wave form distortions with suitable wave forms (May / June 2012)(8) Define transient Draw the Wave form Define wave form distortion Draw the wave form (ii)describe the power acceptability curves (8) Draw the curve Explain the importance of curve in terms of power quality 12(a)(i).Compare short and long interruption and write the sources of such interruption(8)(may / june 2012) Compare above two terms in (i)definition (ii)wave forms (iii)sources (iv)effects (v)mitigation methods (ii)discuss how induction motors influence on interruption of power quality(8)(may / june 2012) (i)definition (interruption) (ii)induction motor wave forms (iii)how the above form can affect the quality of power (Or) b.(i)name and explain the voltage regulating devices(8) (May / june 2012) (i)explain the need for voltage regulating devices (ii)name all the voltage regulating devices (iii)explain each devices with individual sub headings (ii) Explain briefly about reliability evaluation and cost of interruption(8) (May / june 2012) Reliability Evaluation (i)explain the need of reliability studies (ii)define Failure (iii)define outage Cost of interruption (i)need for cost analysis

36 (ii)classify and explain the various costs 13.(a)(i) Explain the role of power electronics load in voltage sag(8) (May / june 2012) (i) Name all the power electronic loads which causes voltage sag (ii)explain the above with the followings (a)schematic diagram / Circuit diagram with explanation (b)output waveform of the above (c)how the waveform causes the voltage sag (ii)explain the method of stochastic assessment ofvoltagesag(8)(may/june2012) (i)need for voltage sag assessment (ii)types of assessment (iii)stochastic assessment3 (Or) (b).what is transient/explain different types of transient with suitable example(16) (May/june2012) (i)define transients and its causes (ii)types of transients (iii)explain each type with diagram by sub title 14.(a)(i)Explain the standard and limitations of harmonics(8) (i)name IEEE and IEC standard about harmonic (ii)explain current and voltage limitation of the harmonics (ii)explain sources and effects of harmonics distortion for different loads(8) (i)name the sources of harmonics Commercial loads Industrial loads (ii)explain the above with S Schematic diagram Wave forms (iii)explain how the above wave form leads power quality problem (Or) b(i).define and explain different harmonic indices(8)