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Dr. Mahalingam College of Engineering and Technology, Pollachi-3

(An Autonomous Institution affiliated to Anna University)

C C E T / ( 2 0 1 4 R e g u l a t i o n )

Course Code & Course Title: 11 EE911 Power System Quality

Name of Programme: BE EEE

Sem: VII Date& Session: 03/08/16 AN Duration: 1½ hours Max. Marks: 40

Part- A Objective Questions (10X1=10 Marks)

Q. No Question CO

No

Blooms

Level

1 ----------------- standard provides guidelines for harmonic current distortion levels on distribution and transmission system.

1 1

2

Typical duration of long duration variation is (A) < 60sec (B) > 1 min (C) 0.5-30 cycles (D) 3sec–1min

1 2

3

The major reason for voltage sag is (A) Harmonics (B) Notch (C) Energizing larger loads (D) Lightning

1 1

4 Crest factor for pure sinusoidal wave is (A) 1.414 (B) 1.8 (C) 2.414 (D) 2.8

1 2

5

Which one of the following equipment act as a source of inter harmonics? (A) Adjustable speed drives (B) Electric heater (C) Cycloconverter (D) Fluorescent lighting

1 2

6

NEC is intended to (A) Protect people from fire and elocution (B) Protect sensitive electronic equipment (C) Protect utility equipments (D) Protect PQ monitoring equipments

1 2

7

Which of the following is an example of non-linear load? (A) Electric heater (B) Induction motor (C) SMPS (D)Incandescent lamp

2 2

8 The presence of a DC voltage or current in an AC power system is termed as …………………..

2 2

9

The sum of sinusoids are analyzed using (A) Laplace transform (B) DFT (C) Fourier transform (D)Fourier series

2 2

10 ……………… is the 13th order harmonics (assume system frequency is 60Hz)

2 3

Part- B Short Answer Questions (5X2=10 Marks)

Q. No Question CO

No

Blooms

Level

11 Define power quality. 1 1

12 State the equations for 1) THD 2)TDD 1 2

13 Draw CBEMA curve. 1 2

14 Comment on harmonic phase sequence. 2 2

15 Compare transients and harmonics. 2 2

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Part- C Descriptive – either or questions X10=20 Marks)

Q. No Question CO

No

Blooms

Level

16. (a)

Explain the different types of wave form distortion.

1 2

OR 16.(b) Describe the various power quality terms in detail.

1 1

17. (a)

Discuss the sources and effects of power quality problems. 2 2

OR

17.(b)

Explain in detail the sources and effects of harmonic distortion problem.

2 2

Note:

Code for Blooms Levels:

Sl.

No. Blooms Level Code

1 Remember R

2 Understand U

3 Apply Ap

4 Analyze An

5 Evaluate E

6 Create C

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11 EE911 Power System Quality – Answer Key

Part- A Objective Questions (10X1=10

Marks)

1. IEEE 519-1992

2. (B) > 1 min

3. (C) Energizing larger loads

4. (A) 1.414

5. (C) Cycloconverter

6. (A) Protect people from fire and elocution

7. (C) SMPS

8. DC offset

9. (D)Fourier series

10. 780 Hz

Part- B Short Answer Questions (5X2=10

Marks)

11. Power Quality: Any power problem leads to voltage, current, or frequency deviations, that results in failure or misoperation of customer equipment.

12

Write the equations for 1) THD 2)TDD

13 Draw CBEMA curve.

14. Comment on harmonic phase sequence.

• Positive sequence : 1, 4, 7, 10, 13, … (A-B-C phase rotation (e.g., 0°, -120°, 120°))

• Negative sequence: 2, 5, 8, 11, 14, … (negative sequence set are also displaced 120°, but have opposite phase rotation (A-C-B, e.g., 0°, 120°, -120°)).

• Triplens (h 3, 9, 15,…) are generally zero sequence (sinusoids of the zero sequence are in phase with each other (e.g., 0°, 0°, 0°))

• Phase sequence of harmonics in three phase balanced system +,-,0, +,-,0, +,-,0,…

15. Compare transients and harmonics.

Harmonics Transients

1. Steady state

phenomenon

2. Low frequency event

3. Due to nonlinear loads

4. Last for several

seconds

1. Transient phenomenon

2. Very high frequency

event

3. Due to switching actions

4. Dissipates with in few

cycles

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16) a) Explain the different types of wave form distortion. Waveform Distortion Waveform distortion is defined as a steady-state deviation from an ideal sine wave of power frequency characterized by the spectral content of the deviation. The different types of waveform distortion are,

1. DC offset 2. Harmonics 3. Interharmonics 4. Notching 5. Noise

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 electronic power converters. Incandescent light bulb life extenders, for example, may consist of diodes that reduce the rms voltage supplied to the light bulb by half-wave rectification. Direct current in ac networks can have a detrimental effect by biasing transformer cores so they saturate in normal operation. This causes additional heating and loss of transformer life. Direct current may also cause the electrolytic erosion of grounding electrodes and other connectors. Harmonics: It is defined as sinusoidal voltages or currents having frequencies that are integer multiples of the fundamental frequency (usually 50 or 60 Hz). Periodically distorted waveforms can be decomposed into a sum of the fundamental frequency and the harmonics. Harmonic distortion originates in the nonlinear characteristics of devices and loads on the power system. The harmonic indices used are total harmonic distortion (THD) and Total Demand Distortion (TDD. Figure 1.11 illustrates the waveform and harmonic spectrum for a typical adjustable-speed-drive (ASD) input current.

Figure 1.11 Current waveform and harmonic spectrum for an ASD Interharmonics: Voltages or currents having frequency components that are not integer multiples of fundamental frequency are called interharmonics. Interharmonics can be found in networks of all voltage classes. The main sources of interharmonic waveform distortion are static frequency converters, cycloconverters, induction furnaces, and arcing devices. Power line carrier signals can also be considered as interharmonics. Interharmonic distortion is generally the result of frequency conversion and is often not constant; it varies with load. Such interharmonic currents can excite quite severe resonances on the power system as the varying interharmonic frequency becomes coincident with natural frequencies of the system. They have been shown to affect power-line-carrier signaling and induce visual flicker in fluorescent and other arc lighting as well as in computer display devices.

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Notching: Notching is a periodic voltage disturbance caused by the normal operation of power electronic devices, when current is commutated from one phase to another. Since notching occurs continuously, it can be characterized through the harmonic spectrum of the affected voltage. Figure 1.12 shows an example of voltage notching from a three-phase converter that produces continuous dc current. The notches occur when the current commutates from one phase to another. During this period, there is a momentary short circuit between two phases, pulling the voltage as close to zero as permitted by system impedances.

Figure 1.11 Voltage notching due to 3-phase converter Noise. Noise is defined as unwanted electrical signals with broadband spectral content lower than 200 kHz superimposed upon the power system voltage or current in phase conductors, or found on neutral conductors or signal lines. Noise in power systems can be caused by power electronic devices, control circuits, arcing equipment, loads with solid-state rectifiers, and switching power supplies. Noise problems are often exacerbated by improper grounding that fails to conduct noise away from the power system. Basically, noise consists of any unwanted distortion of the power signal that cannot be classified as harmonic distortion or transients. Noise disturbs electronic devices such as microcomputer and programmable controllers. The problem can be mitigated by using filters, isolation transformers, and line conditioners.

16) b) Describe the various power quality terms in detail. Explanation of minimum 20 PQ terminologies. (Each carries 0.5 marks) Active filter: Any of a number of sophisticated power electronic devices for eliminating harmonic distortion. CBEMA curve: 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 had become the de facto standard for measuring the performance of all types of equipment and power systems and is commonly referred to by this name. CBEMA has been replaced by the Information Technology Industry Council (ITI), and a new curve has been developed that is commonly referred to as the ITI curve. COMMON MODE VOLTAGE: The noise voltage that appears equally from current carrying conductor to ground. COUPLING: A circuit element, or elements, or a network that may be considered common to the input mesh and the output mesh and through which energy may be transferred from one to another. Crest Factor: A value reported by many power quality monitoring instruments representing the ratio of the crest value of the measured waveform to the root mean square of the fundamental. For example, the crest factor of a sinusoidal wave is 1.414. CRITICAL LOAD: Devices and equipment whose failure to operate satisfactorily jeopardizes the health or safety of personnel, and/or results in loss of function, financial loss, or damage to property deemed critical by the user. CURRENT DISTORTION: Distortion in the ac line current. Differential Mode Voltage: The voltage between any two of a specified set of active conductors. Distortion: Any deviation from the normal sine wave for an ac quantity. Distributed Generation (DG): Generation dispersed throughout the power systemas opposed to large, central station power plants. DG typically refers to units less than 10 megawatts (MW) in size that are interconnected with the distribution system rather than the transmissionsystem. Dropout: A loss of equipment operation (discrete data signals) due to noise, sag, or interruption. Dropout Voltage: The voltage at which a device will release to its deenergized position. Electromagnetic Compatibility: The ability of a device, equipment, or system to function satisfactorily in its electromagnetic environment without

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introducing intolerable electromagnetic disturbances to anything in that environment. Equipment Grounding Conductor: The conductor used to connect the non–current carrying parts of conduits, raceways, and equipment enclosures to the grounded conductor (neutral) and the grounding electrode at the service equipment (main panel) or secondary of a separately derived system (e.g., isolation transformer). Failure Mode: The effect by which failure is observed. Fast Tripping: Refers to the common utility protective relaying practice in which the circuit breaker or line recloser operates faster than a fuse can blow. Also called fuse saving. Effective for clearing transient faults without a sustained interruption, but is somewhat controversial because industrial loads are subjected to a momentary or temporary interruption. Fault: Generally refers to a short circuit on the power system. Fault, Transient: A short circuit on the power system usually induced by lightning, tree branches, or animals, which can be cleared by momentarily interrupting the current. 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. Flicker: An impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time. Frequency Deviation An increase or decrease in the power frequency. The duration of a frequency deviation can be from several cycles to several hours. Ground: A conducting connection, whether intentional or accidental, by which an electric circuit or electrical equipment is connected to the earth, or to some conducting body of relatively large extent that serves in place of the earth. Harmonic Content: The quantity obtained by subtracting the fundamental component from an alternating quantity. Harmonic Distortion: Periodic distortion of the sine wave. Harmonic Filter: On power systems, a device for filtering one or more harmonics from the power system. Most are passive combinations of inductance, capacitance, and resistance. Newer technologies include active filters that can also address reactive power needs. Harmonic Resonance: A condition in which the power system is resonating near one of the major harmonics being produced by nonlinear elements in the system, thus exacerbating the harmonic distortion.

Impulsive Transient: A sudden, non power frequency change in the steady state condition of voltage or current that is unidirectional in polarity (primarily either positive or negative). Interruption, Momentary (Electrical Power Systems): An interruption of a duration limited to the period required to restore service by automatic or supervisory- controlled switching operations or by manual switching at locations where an operator is immediately available. Nonlinear Load: Electrical load that draws current discontinuously or whose impedance varies throughout the cycle of the input ac voltage waveform. Normal Mode Voltage: A voltage that appears between or among active circuit conductors. Notch: A switching (or other) disturbance of the normal power voltage waveform, lasting less than a half-cycle, which is initially of opposite polarity than the waveform and is thus subtracted from the normal waveform in terms of the peak value of the disturbance voltage. This includes complete loss of voltage for up to a half-cycle. Sag: A decrease to between 0.1 and 0.9 pu in rms voltage or current at the power frequency for durations of 0.5 cycle to 1 min. Shield: As normally applied to instrumentation cables, refers to a conductive sheath (usually metallic) applied, over the insulation of a conductor or conductors, for the purpose of providing means to reduce coupling between the conductors so shielded and other conductors that may be susceptible to, or which may be generating, unwanted electrostatic or electromagnetic fields (noise). TOTAL DEMAND DISTORTION (TDD): The ratio of the root mean square of the harmonic current to the rms value of the rated or maximum demand fundamental current, expressed as a percent. TOTAL HARMONIC DISTORTION (THD): 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. Voltage Regulation: The degree of control or stability of the rms voltage at the load. Waveform Distortion: A steady-state deviation from an ideal sine wave of power frequency principally characterized by the spectral content of the deviation. 17) a) Discuss the sources and effects of power quality problems. Sources of Power Quality Problems Power quality experts find it a challenge to analyze any power quality problem and determine the source of the problem. They usually measure the

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effect of the problem and draw on their experience to identify the type of disturbance from the measurement. Even experienced power quality experts often find it is difficult to determine the source of the power quality problem. They know they need to understand the basic reasons why different devices and phenomena cause power quality problems. One common characteristic of sources of power quality problems is the interruption of the current or voltage sine wave. The major sources of power quality problems can be divided into two categories, depending on the location of the source in relationship to the power meter. One category is on the utility side of the meter and includes switching operations, power system faults, and lightning. The other category is on the end-user side of the meter and includes non-linear loads, poor grounding, electromagnetic interference, and static electricity. Utility side of the meter Sources of power quality problems on the utility side of the meter involve some type of activity on the utility’s electrical power system They can be either man-made or natural events. They all involve some type of interruption of the current or voltage. The most common manmade causes are switching operations. Utilities switch equipment on and off by the use of breakers, disconnect switches, or reclosers. Usually some type of fault on the power system causes a breaker to trip. Utilities trip breakers to perform routine maintenance. They also trip breakers to insert capacitors to improve the power factor. Lightning striking a power line or substation equipment, a tree touching a power line, a car hitting a power pole, or even an animal touching an energized line may cause the fault. The tripping of the breaker and the initiating fault can cause the voltage to sag or swell, depending on when in the periodic wave the tripping occurs. Utilities set breakers and reclosers to reclose on the fault to determine if the fault has cleared. If the fault has not cleared, the breaker or recloser trips again and stays open. Another type of utility activity that can cause oscillatory transients is the switching of power factor improvement capacitors. Utilities use power factor improvement capacitors to improve the power factor by adding capacitive reactance to the power system. This causes the current and voltage to be in phase and thus reduces losses in the power system. When utilities insert capacitors in the power system, they momentarily cause an increase in the voltage and cause transients. Capacitors, if tuned to harmonics on the power system, can also amplify the harmonics. This is especially true if the utility and end user both switch their capacitors on at the same time.

End-user side of the meter Sources of power quality problems on the end-user side of the meter usually involve a disruption of the sinusoidal voltage and current delivered to the end user by the utility. These disruptions can damage or cause misoperation of sensitive electronic equipment in not only the end-user’s facilities but also in another end-user’s facilities that is electrically connected. The following is a list of power quality problems caused by end users: nonlinear inrush current from the start-up of large motors, static electricity, power factor improvement capacitors amplifying harmonics, and poor wiring and grounding techniques. Nonlinear loads: There are today many types of nonlinear loads. They include all types of electronic equipment that use switched-mode power supplies, adjustable-speed drives, rectifiers converting ac to dc, inverters converting dc to ac, arc welders and arc furnaces, electronic and magnetic ballast in fluorescent lighting, and medical equipment like MRI (magnetic radiation imaging) and x-ray machines. Other devices that convert ac to dc and generate harmonics include battery chargers, UPSs, electron beam furnaces, and induction furnaces, to name just a few. All these devices change a smooth sinusoidal wave into irregular distorted wave shapes. The distorted wave shapes produce harmonics. Most electronic devices use switched-mode power supplies that produce harmonics. Manufacturers of electronic equipment have found that they can eliminate a filter and eliminate the power supply transformer (shown in Figure 2.22) by the use of a switched-mode power supply (shown in Figure 2.23). The switched-mode process converts ac to dc using a rectifier bridge, converts dc back to ac at a high frequency using a switcher, steps the ac voltage down to 5 V using a small transformer, and finally converts the ac to dc using another rectifier. Electronic equipment in the office includes computers, copiers, printers, and fax machines. Adjustable-speed drives save energy by adjusting the speed of the motor to fit the load. Residential heat pumps, commercial heating and ventilating systems, and factories that use motors in their processes benefit from the use of adjustable-speed drives. However, adjustable speed drives cause harmonics by varying the fundamental frequency in order to vary the speed of the drive. Arc furnaces use extreme heat (3000°F) to melt metal. The furnace uses an electrical arc striking from a high-voltage electrode to the grounded metal to create this extreme heat. The arc is extinguished every half-cycle. The short circuit to ground causes the voltage to dip each time the arc strikes. This causes the lights to flicker at a frequency typically less than 60 Hz that is

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irritating to humans. Arc furnaces also generate harmonic currents. Most nonlinear loads not only generate harmonics but cause low power factor. Harmonic resonance: Electrical harmonic resonance occurs when the inductive reactance of a power system equals the capacitive reactance of a power system. This is a good thing at the fundamental frequency of 60 Hz and results in the current and voltage being in phase and unity power factor. However, it is not so good when it occurs at a harmonic frequency. If resonance occurs at a harmonic frequency, the harmonic current reaches a maximum value and causes overheating of transformers, capacitors, and motors; tripping of relays; and incorrect meter readings. How does resonance occur at a harmonic frequency? The amount of inductive and capacitive reactance are dependent on the frequency of the current and voltage. Thus, resonance can occur at various harmonic frequencies. Resonance can be prevented resonance by sizing and locating capacitors to avoid the harmonic resonance frequency or by using filters. A filter is simply an inductor (reactor) in series with a capacitor, as shown in Figure 2.29. Filters detune the capacitor away from the resonant frequency. Filters usually cost twice as much as capacitors. Filters also remove the effect of distortion power factor and increase the true power factor. Poor wiring and grounding An EPRI survey found poor wiring and grounding in the end-user’s facilities cause 80 percent of all power quality problems. The National Electrical Code (NEC) determines the design of the wiring and grounding. However, the NEC, as described in Section 90-1(b), is intended to protect people from fire and electrocution, not to protect sensitive electronic equipment from damage. As a consequence there is a great need to establish guidelines for wiring and grounding that not only protects the public but prevents power quality problems. When poor wiring and grounding cause equipment to fail, utility customers often attribute the failure to the utility. They may even buy expensive power conditioning equipment that only treats the symptom of the power quality problem and does not solve the underlining cause of the problem. They should, instead, identify the effects of poor wiring and grounding, determine the cause of the power quality problem, and find a simple way to correct the problem.

Symptoms of poor wiring and grounding include computers that lose data or stop operating; telephone systems that lose calls or are noisy; industrial processes that suddenly stop; breaker boxes that get very hot; neutral leads that catch fire; and even power conditioning equipment, like transient voltage surge suppressors (TVSSs), that catch fire. Poor grounding can cause voltage potential differences, excessive ground loops, and interference with sensitive electronic equipment. Proper grounding not only protects people from shock but provides a reference point and a path for large currents caused by faults, like switching surges and lightning strokes. Poor grounding can result in lightning destroying equipment in a home, office, or factory. Lightning surges will take the path of least resistance. Wiring and grounding should be designed to divert lightning current away from sensitive equipment to ground through lightning protection devices, such as lightning arresters and surge protectors as shown in Figure 2.31, Electromagnetic interference (EMI) Another source of power quality problems is electromagnetic interference (EMI). Some devices, like a large motor during start-up, emit a magnetic field that intersects with an adjacent sensitive device, like a computer or telephone. Faraday’s transformer law says that when an alternating magnetic field cuts across an adjacent conductor, it will induce an alternating current and voltage in that conductor. The induced current and voltage can damage sensitive electronic equipment or cause it to malfunction. Sensitive equipment in hospitals often experiences EMI problems. For example, in one open-heart-surgery training center, electromagnetic fields from an adjacent electrical equipment room were causing heart monitors to read incorrectly. Moving cables emitting the electromagnetic fields a safe distance from the cables feeding the heart monitors solved this problem. Static electricity. Another cause of power quality problems is static electricity. Static electricity occurs when the rubbing of one object against another causes a voltage buildup. For example, you can build up an electric charge on your body when you rub your shoes on a carpet. A discharge of static electricity can occur when you then touch a grounded object, like another person or a metal object. Although static electricity power quality problems are infrequent, they are often overlooked. Static electricity can create voltages of 3000 V or more and damage sensitive electronic equipment. You can minimize static

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electricity problems by increasing the humidity, changing the carpet, clothing, and furniture to nonstatic types, and by grounding the person working on a piece of equipment to the equipment with a wrist strap. Effects of Power Quality Problems The effects of power quality problems are many and varied. These symptoms include motors overheating, adjustablespeed drives tripping off, computers shutting down, flickering lights, and stopped production. The effects of power quality problems can be best be understood by looking at the various types of loads that are affected by power quality problems, including computers, consumer products, lighting, meters, ferromagnetic equipment, telephones, manufacturing processes, and capacitors. Computers and computer-controlled equipment are most subject to power quality problems. They freeze up and lose data. Most power quality problems on computers are caused by voltage variations. Consumer products include digital clocks, microwave ovens, television sets, video cassette recorders, and stereo equipment. Most consumer products are affected by voltage sags and outages causing the electronic timer to shut down. This problem manifests itself by the blinking clock. Lighting includes incandescent, high-intensity discharge, and fluorescent lights. Incandescent lights often dim during a voltage sag. All lighting will flicker when arc furnaces and arc welders cause the voltage to fluctuate. Meters will give erroneous readings in the presence of harmonics. Ferromagnetic equipment include transformers and motors. They overheat and lose life when harmonic currents increase the loading on them. Telephones will experience noise induced by adjacent electrical equipment. Adjustable-speed drives not only cause harmonics but are affected by them. The frequent shutdown of an adjustable-speed drive is usually an indication of excessive harmonics.

Many manufacturing processes experience frequent shutdowns due to voltage sags. Capacitors can amplify as well as draw harmonic currents to themselves. This often causes the capacitors to fail or be tripped off-line.

17) b) Explain in detail the sources and effects of harmonic distortion problem.

SOURCES AND EFFECTS OF HARMONICS

1. Harmonics Sources from Commercial Loads

1. Single phase power supplies

2. Fluorescent Lighting

3. Adjustable-speed drives for HVAC and elevators

2. Harmonics Sources from Industrial Loads

• Very high third-harmonic content in the current

• Since third-harmonic current components are additive in the neutral of a three-phase system, the increasing application of switch-mode power supplies causes concern for overloading of neutral conductors, especially in older buildings where an undersized neutral may have been installed.

• Transformer overheating due to a combination of harmonic content of the current, stray flux, and high neutral currents.

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Fluorescent lights are discharge lamps; thus they require a ballast to provide a high initial voltage to initiate the discharge for the electric current to flow between two electrodes in the fluorescent tube.

Once the discharge is established, the voltage decreases as the arc current increases. It is essentially a short circuit between the two electrodes, and the ballast has to quickly reduce the current to a level to maintain the specified lumen output. Thus, a ballast is also a current-limiting device in lighting applications. Two types of ballasts,

1.Magnetic Ballast 2.Electronic Ballast

A standard magnetic ballast is simply made up of an iron-core transformer with a capacitor encased in an insulating material.

A single magnetic ballast can drive one or two fluorescent lamps, and it operates at the line fundamental frequency, i.e., 50 or 60 Hz.

The iron-core magnetic ballast contributes additional heat losses, which makes it inefficient compared to an electronic ballast.

An electronic ballast employs a switch-mode–type power supply to convert the incoming fundamental frequency voltage to a much higher frequency voltage typically in the range of 25 to 40 kHz. This high frequency has two advantages.

1. A small inductor is sufficient to limit the arc current.

2. The high frequency eliminates or greatly reduces the 100- or 120-Hz flicker associated with an iron-core magnetic ballast.

A single electronic ballast typically can drive up to four fluorescent lamps.

Figure 5.12 shows a measured fluorescent lamp current and harmonic

spectrum. The current THD is a moderate 15 percent.

As a comparison, electronic ballasts, which employ switch-mode power supplies, can produce double or triple the standard magnetic ballast harmonic output.

Figure 5.13 shows a fluorescent lamp with an electronic ballast that has a current THD of 144.

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ASDs for HVAC and Elevators

Common applications of ASDs in commercial loads can be found in elevator motors and in pumps and fans in HVAC systems.

ASD consists of an electronic power converter that converts ac voltage and frequency into variable voltage and frequency.

The variable voltage and frequency allows the ASD to control motor speed to

match the application requirement such as slowing a pump or fan. ASDs also

find many applications in industrial loads. Harmonic Sources from

Industrial Loads

Modern industrial facilities are characterized by the widespread application of nonlinear loads.

These loads can make up a significant portion of the total facility loads and inject harmonic currents into the power system, causing harmonic distortion in the voltage. Industrial facilities often utilize capacitor banks to improve the power factor to avoid penalty charges.

The application of power factor correction capacitors can potentially magnify harmonic currents from the nonlinear loads, giving rise to resonance conditions within the facility.

Resonance conditions cause motor and transformer overheating, and misoperation of sensitive electronic equipment.

Nonlinear industrial loads can generally be grouped into three categories:

• Three-phase power converters

• Arcing devices

• Saturable devices

• 3 Phase Power Converters

• Three-phase electronic power converters differ from single-phase converters mainly because they do not generate third-harmonic currents.

3 Phase Power Converters

Voltage source inverter drives (such as PWM-type drives) can have much higher distortion levels as shown in Fig. 5.15.

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DC Drives

Rectification is the only step required for dc drives. Therefore, they have the advantage of relatively simple control systems.

Compared with ac drive systems, the dc drive offers a wider speed range and higher starting torque.

Purchase and maintenance costs for dc motors are high, while the cost of power electronic devices has been dropping year after year.

A 12-pulse rectifier in this application can be expected to eliminate about 90 percent of the fifth and seventh harmonics, depending on system imbalances. The disadvantages of the 12-pulse drive are that there is more cost in electronics and another transformer is generally required.

EFFECTS OF PQ PROBLEMS

The effects of power quality problems can be best be understood by looking at the various types of loads that are affected by power quality problems, including

Computers, consumer products, lighting, meters, ferromagnetic equipment, telephones, manufacturing processes, and capacitors.

Computers and computer-controlled equipment: freeze up and lose data.

Most PQ problems on computers are caused by voltage variations.

Consumer products: digital clocks, microwave ovens, television sets, video cassette recorders, and stereo equipment.

Most consumer products are affected by voltage sags and outages causing the electronic timer to shut down. This problem manifests itself by the

blinking clock.

Lighting: incandescent, high-intensity discharge, and fluorescent lights.

Incandescent lights often dim during a voltage sag.

All lighting will flicker when arc furnaces and arc welders cause the voltage to fluctuate.

Meters: Give erroneous readings in the presence of harmonics.

Ferromagnetic equipment: transformers and motors.

They overheat and lose life when harmonic currents increase the loading on them.

Telephones: Experience noise induced by adjacent electrical equipment.

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ASDs: Not only cause harmonics but are affected

by them. The frequent shutdown of an ASDs is usually an indication of excessive harmonics.

Many manufacturing processes: Experience frequent shutdowns due to voltage sags.

Capacitors: Amplify as well as draw harmonic currents to themselves.

This often causes the capacitors to fail or be tripped off-line.