unit.1 power quality - welcome to srm university1).pdf · professor & hod/eee, ... much more...

04/06/2012 UNIT.1 POWER QUALITY 1

Prepared byPrepared by

Dr.S.S.DashDr.S.S.Dash

Professor & HOD/EEE,Professor & HOD/EEE,Department of Electrical and Electronics Department of Electrical and Electronics EnggEngg.,.,

SRM University,SRM University,

ChennaiChennai--203203

Unit.1 Introduction to Power QualityUnit.1 Introduction to Power Quality


Unit.1 Power Quality-syllabus

• Power Quality phenomenon

• Terms and definitions

• Various Power events in power quality

• causes for reduction in power quality.


POWER QUALITY

�Power quality refers to maintaining a sinusoidal waveform of bus voltages at rated voltage and frequency.

� The waveform of electric power at generation stage is purely sinusoidal and free from any distortion.


�Many devices that distort the waveform.

� These distortions may propagate all over the electrical network.


Modern Utility System


1. Deregulation of electricity market

2. Customer demand

3. Distributed generations

• Wind Energy• Solar Energy• Co-generation plants

Modern Utility System


PQ Problems


Other linear loads, such as electrical motors driving fans, water pumps, oil pumps, cranes, elevators, etc., not supplied through power conversion devices like variable frequency drives or any other form or rectification/inversion of current will incorporate magnetic core losses that depend on iron and copper physical characteristics.


�Classification of power quality areas may be made according to the source of the problem such as,

�Converters �Magnetic circuit non linearity�Arc furnace or by the wave shape of the signal such as harmonics, �Flicker or by the frequency spectrum (radio frequency interference).


� The wave shape phenomena associated with power quality may be characterized into synchronous and non- synchronous phenomena.

�Synchronous phenomena refer to those in synchronism with A.C waveform at power frequency.


CAUSES OF POWER QUALITY DETERIORATION

1. Natural causes:

• Faults or lighting strikes on transmission lines or distribution feeders

• Falling of trees or branches on distribution feeders during stormy conditions, equipment failure etc.


2. Due to load or transmission line / feeder operation:

• Transformer energisation

• Capacitor or feeder switching

• Power electronic loads (UPS, ASD, converters etc.)

• Arc furnaces and induction heating systems

• Switching on or off of large loads etc.


FOUR MAJOR REASONS FOR THE INCREASED CONCERN:

1. Newer-generation load equipment

2. Increasing harmonic levels on power systems

3. End users have an increased awareness of power quality issues.

4. Many things are now interconnected in a network. Integrated processes mean that the failure of any component has much more important consequences.


Increased concern about the quality of electric power is the continued push for increasing productivity for all utility customers.

�Utility customers - always want to increase productivity

�Manufacturers - want faster, more productive, more efficient machinery

�Utilities - encourage this effort because it helps their customers become more profitable


POWER QUALITY DEFINITION

Whole of power engineering, in one way or other is related to power quality.

There is no universal agreement for the definition of power quality.

�A Utility may define power quality as reliability and show statistics demonstrating that its system is 99.98 percent reliable.


POWER QUALITY DEFINITION

A manufacturer of load equipment may define power quality as those characteristics of the power supply that enable the equipment to work properly. These characteristics can be very different for different criteria.


� Power quality is ultimately a consumer-driven issue, we define power quality as,

�Any power problem manifested in voltage, current, or frequency deviations that result in failure or misoperation of customer equipment.


Power Quality = Voltage Quality

The power supply system can only control the quality of the voltage; it has no control over the currents that particular loads might draw.

Therefore, the standards in the power quality area are devoted to maintaining the supply voltage within certain limits.


Generators may provide a near-perfect sine wave voltage, the current passing through the impedance of the system can cause a variety of disturbances to the voltage. For example,

1. Current resulting from a short circuit causes the voltage to sag or disappear completely.


2. Currents from lightning strokes passing through the power system cause high-impulse voltages that frequently flash over insulation and lead to other phenomena, such as short circuits.

3. Distorted currents from harmonic-producing loads also distort the voltageas they pass through the system impedance. Thus a distorted voltage is presented to other end users.


SOURCES OF POWER QUALITY PROBLEMS

1. Load equipment and components

Converters, Pulse modulated loads, Machine drives, Arc furnaces, Computers, UPS, Television sets

Fluorescent and other gas discharge lighting

Certain components which employ magnetic circuits

2. Subsystems of the transmission and distribution system

Grounding systems, resonant systems


For steady-state phenomena, the following attributes can be used:

■ Amplitude■ Frequency■ Spectrum■ Modulation■ Source impedance■ Notch depth■ Notch area


For non-steady-state phenomena, other attributes may be required:

■ Rate of rise

■ Amplitude

■ Duration

■ Spectrum

■ Frequency

■ Rate of occurrence

■ Energy potential

■ Source impedance


Transients can be classified into two categories,

1.Impulsive

2.Oscillatory


Impulsive transient� It is a sudden, non–power frequency change in the steady-state condition of voltage, current, or both.

�It is unidirectional in polarity(primarily either positive or negative).

�Impulsive transients are normally characterized by their rise and decay times, which can also be revealed by their spectral content.


For example,

�1.2 *50-µs 2000-volt (V) impulsive transient nominally rises from zero to its peak value of 2000 V in 1.2µs and then decays to half its peak value in 50µs .

�The most common cause of impulsive transients is lightning.


Oscillatory Transient

�It is a sudden, non–power frequency change in the steady-state condition of voltage, current, or both.

�It includes both positive and negative polarity values.

�It consists of a voltage or current whose instantaneous value changes polarity rapidly.


�It is described by its spectral content (predominate frequency), duration, and magnitude.

�The spectral content subclasses defined in Table 2.2 are

�High

�Medium

�Low frequency


�HF: Primary Freq component > 500khz mesd in MicroSec duration -Local sys response to Imp Tr

�Med Freq: Primary Freq component 5-500khz mesd in MicroSecduration - Back-to-back capacitorenergization

�Low Freq: Primary Freq component <5khz mesd in MicroSec duration 0.3 to 50 ms - Cap Bank energization(T&D)


Long-Duration Voltage Variations

�Long-duration variations encompass root-mean-square (rms) deviations at power frequencies for longer than 1 min.

�It can be either overvoltages or undervoltages.


�Overvoltages and undervoltagesgenerally are not the result of system faults, but are caused by load variations on the system and system switching operations.

�Long-duration variations are typically displayed as plots of rmsvoltage versus time.


�OVERVOLTAGE

�Increase in the rms ac voltage greater than 110 percent at the power frequency for a duration longer than 1 min.

CAUSES

1.load switching (e.g., switchingoff a large load or energizing a capacitor bank)2. Incorrect tap settings on transformers

can also result in system overvoltages.

�EFFECT�The overvoltages result because either the system is too weak for the desired voltage regulation or voltage controls are inadequate.


�UNDERVOLTAGE

�Decrease in the rms ac voltage to less than 90 percent at the power frequency for a duration longer than 1 min.

�Due to switching events that are the opposite of the events that cause overvoltages.

�CAUSES

1.A load switching on or a capacitor bank switching off can cause an under voltage until voltage regulation equipment on the system can bring the voltage back to within tolerances.2.Overloaded circuits can result in undervoltages


SUSTAINED INTERRUPTION

�When the supply voltage has been zero for a period of time in excess of 1 min, the long-duration voltage variation is considered a sustained interruption.

�This term has been defined to be more specific regarding the absence of voltage for long periods.


SHORT DURATION VARIATIONS

This category encompasses the IEC category of voltage dips and short interruptions.

Each type of variation can be designated as,

1.Instantaneous, 2.Momentary, 3.Temporary, depending on its duration as defined in Table 2.2.

CAUSES1.Fault conditions2.The energization of large loads which require high starting currents3.Intermittent loose connections in power wiring. Depending on the fault location and the system conditions, the fault can cause either temporary voltage drops (sags), voltage rises (swells), or a complete loss of voltage (interruptions).


2.5.1 INTERRUPTION

An interruption occurs when the supply voltage or load current decreases to less than 0.1 pu for a period of time not exceeding 1 min.

CAUSES

1.Power system faults

2.Equipment failures

3.Control malfunctions


2.5.1 INTERRUPTION

�The interruptions are measured by their duration since the voltage magnitude is always less than 10 percent of nominal.

�The duration of an interruption due to a fault on the utility system is determined by the operating time of utility protective devices.

�Instantaneous reclosing generally will limit the interruption caused by a nonpermanent fault to less than 30 cycles.


�Delayed reclosing of the protective device may cause a momentary or temporary interruption.

�The duration of an interruption due to equipment malfunctions or loose connections can be irregular.

�Figure shows such a momentary interruption during which voltage on one phase sags to about 20 percent for about 3 cycles and then drops to zero for about 1.8 s until the recloser closes back in.


Sags (dips)

A 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.

Causes of Voltage sags

–Associated with system faults –Energization of heavy loads –Starting of large motors.


Figure shows typical voltage sag that can be associated with a single- line-to-ground (SLG) fault on another feeder from the same substation.


•Figure illustrates the effect of a large motor starting. An induction motor will draw 6 to 10 times its full load current during start-up.

•In this case, the voltage sags immediately to 80 percent and then gradually returns to normal in about 3 s.• Note the difference in time frame between this and sags due to utility system faults.


•Sag durations are subdivided here into three categories such as,

•Instantaneous (0.5-30 Cycles)

•Momentary (30 Cycles-3sec)

•Temporary (3sec – 1 min)

Swells

•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.

•Swells are characterized by their magnitude (rms value) and duration.


Causes of Voltage Swell

-Associated with system faults –Energization of large Capacitor bank –Switching off large load

–The severity of a voltage swell during a fault condition is a function of

•Fault location•System impedance•Grounding


Voltage Imbalance (voltage unbalance)

•Voltage imbalance (or unbalance) is defined as the ratio of the negative or zero sequence component to the positive sequence component.

•The negative or zero sequence voltages in a power system generally result from unbalanced loads causing negative or zero sequence currents to flow.

Source

•Single-phase loads on a three-phase circuit (<2%) • Result of blown fuses in one phase of a three-phase capacitor bank• Severe voltage unbalance (>5%) can result from single-phasing conditions


Waveform Distortion

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.

5 types of waveform distortion

–DC offset

–Harmonics

–Inter harmonics

–Notching

–Noise


DC offset

The presence of a dc voltage or current in an ac power system is termed dc offset.

HARMONICS

• Harmonics are sinusoidal voltages or currents having frequencies that are integer multiples of the frequency at which the supply system is designed to operate.

•IEEE Standard 519-1992 provides guidelines for harmonic current and voltage distortion levels on distribution and transmission circuits.


•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.

•Harmonic distortion levels are described by the complete harmonic spectrum with magnitudes and phase angles of each individual harmonic component.


•Total harmonic distortion (THD), as a measure of the effective value of harmonic distortion.

•THD - used to characterize both current and voltage waves. However THD refers distortion in voltage wave

•Figure illustrates the waveform and harmonic spectrum for a typical adjustable-speed-drive(ASD) input current.


•Total Harmonic distortion (THD)•IEEE 519 sets limits on total harmonic distortion (THD) for the utility side of the meter

•Utility is responsible for the voltage distortion at the point of common coupling (PCC) between the utility and the end user.

•Total harmonic distortion is a way to evaluate the voltage distortion effects of injecting harmonic currents into the utility’s system.


Total Harmonic distortion (THD) =

•(RMS of the harmonic content / RMS value of the fundamental) * 100

•Total harmonic distortion (THD) is a term used to describe the net deviation of a nonlinear waveform from ideal sine waveform characteristics.


Example: Find the total harmonic distortion of a voltage waveform with the following harmonic frequency make up:

Fundamental = V1 = 114 V

3rd harmonic = V3 = 4 V

5th harmonic = V5 = 2 V

7th harmonic = V7 = 1.5 V

9th harmonic = V9 = 1 V

THD = (4.82/114) × 100 =4.23%


Total Demand Distortion (TDD)

•IEEE 519 sets limits total demand distortion (TDD) for the end-user side of the meter.

•(RMS of the harmonic current / RMS value of MD of Load Current ) * 100

•Expressed as a percent of rated load current.

•TDD deals with evaluating the current distortions caused by harmonic currents in the end-user facilities.


INTERHARMONICS

Voltages or currents having frequency components that are non-integer multiples of the fundamental frequency.

Sources of Interharmonic Waveform Distortion

•Static frequency converters

•Cycloconverters

•Induction furnaces

•Arcing devices


NOTCH

Notching is a periodic voltage disturbance caused by the normal operation of power electronic devices when current is commutated from one phase to another


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.

SOURCES

Power electronic devices, Control circuits, Arcing equipment, Loads with solid-state rectifiers, and Switching power supplies.


VOLTAGE FLUCTUATION(VOLTAGE FLICKER)

• Voltage fluctuations are systematic variations of the voltage envelope or a series of random voltage changes, the magnitude of which does not normally exceed the voltage ranges specified by ANSI C84.1 of 0.9 to 1.1 pu.•SOURCE

•Loads that can exhibit continuous, rapid variations in the load current magnitude can cause voltage variations that are often referred to as flicker.


Power Frequency Variations

Power frequency variations are defined as the deviation of the power system fundamental frequency from it specified nominal value.


CBEMA Curve

•A set of curves representing the withstanding capabilities of computers in terms of the magnitude and duration of the voltage disturbance.

•Developed by the Computer Business Equipment Manufacturers Association (CBEMA)


•The axes represent magnitude and duration of the event.

•Points below the envelope are presumed to cause the load to drop out due to lack of energy.

•Points above the envelope are presumed to cause other malfunctions such as insulation failure, over voltage trip, and over excitation.


ITI Curve

•A set of curves published by the Information Technology Industry Council (ITIC) representing the withstand capabilities of computers connected to 120-V power systems in terms of the magnitude and duration of the voltage disturbance.

• The ITI curve replaces the curves originally developed by the ITI’spredecessor organization CBEMA.


SOURCES OF PQ PROBLEMS•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.1.Utility side of the meter includes

Switching operationsPower system faultsLightning.

2.Other category is on the end-user side of the meter and includes

Non-linear loadsPoor groundingElectromagnetic interferenceStatic electricity


Utility side of the meter

•Sources of PQ problems on 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.

Figure 2.20 shows a utility breaker.


Figure shows a utility breaker & Utility PF Improvement Capacitor



•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 PQ 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:

•Non-linear inrush current from the start-up of large motors

•Static electricity

•Power factor improvement capacitors amplifying harmonics

•Poor wiring and grounding techniques


Nonlinear loads

•There are today many types of nonlinear loads. All these devices change a smooth sinusoidal wave into irregular distorted wave shapes. The distorted wave shapes produce harmonics. They include all types of electronic equipment that use

•SMPS, ASDs•Rectifiers, Inverters •Arc welders and Arc furnaces•Electronic and magnetic ballast in fluorescent lighting•Medical equipment like MRI (magnetic radiation imaging) and x-ray machines. •Other devices that convert ac to dc and generate harmonics include battery chargers, UPS, electron beam furnaces, and induction furnaces.


•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).

What is a switched-mode power supply? How does it produce harmonics?

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 requires 5 V dc to operate.

Go inside a switched mode power supply and you’ll find a switching circuit that takes stored energy from a capacitor in short pulses and delivers voltage at a frequency of 20 to 100 kHz to a transformer in the form of a square wave.

The high-frequency switching requires a small and light transformer. However, the pulsed square wave distorts the sine wave and produces harmonics.


•Adjustable-speed drives save energy by adjusting the speed of the motor to fit the load.

•However, ASDs 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 irritating to humans. Arc furnaces also generate harmonic currents.


Figure 2.24 illustrates the configuration of a one-electrode dc electric arc furnace.


•Most nonlinear loads not only generate harmonics but cause low power factor. They cause low power factor by shifting the phase angle between the voltage and current.

Power factor

Power factor is a way to measure the amount of reactive power required to supply an electrical system and an end-user’s facility.

Reactive power represents wasted electrical energy, because it does no useful work.

Inductive loads require reactive power and constitute a major portion of the power consumed in industrial plants. Motors, transformers, fluorescent lights, arc welders, and induction heating furnaces all use reactive power.


•Power factor is also a way of measuring the phase difference between voltage and current.

•Just as a rotating alternating current and voltage can be represented by a sine wave, the phase difference between voltage and current can be represented by the cosine of the phase shift angle.

•Nonlinear loads often shift the phase angle between the load current and voltage, require reactive power to serve them, and cause low power factor.

•Linear motor loads require reactive power to turn the rotating magnetic field in the motor and cause low power factor.

•Nonlinear and linear loads that cause low power factorinclude induction motors of all types, power electronic power converters, arc welding machines, electric arc and induction furnaces, and fluorescent and other types of arc lighting.


Active power is the power to do useful work, such as turning a motor or running a pump, and is measured in kilowatts (kW).

Electrical equipment needs active power to convert electrical energy into mechanical energy.

Reactive power is the power required to provide amagnetic field to ferromagnetic equipment, like motors and transformers, and does no useful work.

Reactive power is measured in kilovolt-amperes–reactive (kVAR)s.


Apparent power or demand power is the total power needed to serve a load. It is measured in kilovoltamperes (kVA) and is the vector sum of reactive and active power:

•Reactive power takes up capacity on the utility’s and end-user’s electrical distribution systems.

•Reactive power also increases transmission and distribution losses.


Reactive power is frequently described as analogous to the foam in a beer mug.

It comes with the beer and takes up capacity in the mug but does not quench the beer drinker’s thirst.

As can be seen from the power triangle in Figure 2.26, power factor measures the reactive efficiency of a power system.

At maximum efficiency the reactive power is zero, and the power factor is unity.


•As a general rule, an electrical system using motors exhibits a low power factor.

•Low power factors result in overall low power system efficiency, including increased conductor and transformer losses and low voltage. Low power factor also reduces line and transformer capacity.

•Utilities must supply both the active and reactive power and compensate for these losses. For this reason, most utilities charge their customers a penalty for low power factor.

•Many utilities increase the demand charge for every percent the power factor drops below a set value, say 95 percent.


• However, more and more utilities are charging for kVAR-hours just like they charge for kW-hours. These charges provide utility customers an incentive to increase their power factor by the use of power factor improvement capacitors.

•Otherwise, the utility has to install power factor improvement capacitors on its own power system. But how do capacitors improve power factor?

•Improving power factor can be accomplished through the addition of shunt capacitors.


Power factor improvement capacitors

•Power factor improvement capacitors improve the power factor by providing the reactive power needed by the load.

•They also reduce the phase shift difference between voltage and current.

•Like a battery, they store electrical energy. Unlike a battery, they store energy on thin metal foil plates separated by a sheet of polymer material.

•They release the energy every half-cycle of voltage.



They cause the current to lead the voltage by 90°. This subtracts from the phase angle shift of induction loads that cause the current to lag the voltage by 90°.

This is how capacitors reduce the phase shift between current and voltage and provide the magnetization that motors and transformers need to operate.

Therefore, capacitors are an inexpensive way to provide reactive power at the load and increase power factor.


•They supply the reactive, magnetized power required by electric loads, especially industrial loads that use inductive motors.

•Motors with their inductive, magnetizing, reactive power cause current to “lag” behind voltage.

•Capacitors create “leading” current. Capacitors act in opposition to inductive loads, thereby minimizing the reactive power required.

•When carefully controlled, the capacitor lead can match the motor lag, eliminate the need for reactive power, and increase the power factor toward unity.


•Both fixed and dynamic shunt capacitors applied to inductive loads increase the power factor.

•Fixed capacitors are switched on manually and apply a constant capacitance; dynamic capacitors can be switched on automatically and adjust their capacitance according to the inductive load.

•Both types have advantages and disadvantages, but both types provide similar benefits. In raising the power factor, shunt capacitors release energy to the system, raise system voltage, reduce system losses and, ultimately, reduce power costs.

•However, capacitors have a downside. They can amplify harmonics through harmonic resonance.


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.

The formulas for inductive and capacitive reactance illustrate this relationship:

XL =2*Π*f*L

XC =1/(2* Π *f*C)


•Capacitors can cause two types of resonance: parallel and series resonance.

•Since most power factor improvement capacitors are in parallel with the inductance of the power system, as shown in the schematic of a parallel resonant circuit (Figure 2.28), parallel resonance•occurs most often.

•When capacitive and inductive reactance connect in parallel in the power system, the magnitude of the total reactance or impedance becomes,


Harmonic resonance occurs when XL XC and XTbecomes a pure resistance (R) and from Ohm’s law (I V/XT) the harmonic current I reaches a maximum.


How do you prevent resonance?

We prevent 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.


True power factor

•True power factor is the power factor caused by harmonics and the fundamental, while the standard or displacement power factor described previously is caused by the fundamental power at 60 Hz.

•It is not measured by standard VAR or power factor meters.


True power factor

•As can be seen from the diagram, the true kVA is larger than the displacement kVA because of the effect of the harmonic distortion.

•Even though there is no penalty associated with true power factor, it still has a detrimental effect on the power system.

•Low true power factor means increased losses and reduced system capacity. True power factor is increased not by the addition of capacitors but by the elimination of harmonics through the use of filters.

•The addition of capacitors can cause the true power factor to be worse by magnifying the harmonic distortion.


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.


Symptoms of Poor Wiring and Grounding includes

•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.


• Guidelines will help to identify and prevent problems caused by inadequate wiring and grounding.

• Guidelines can be divided into three categories:

(1)Wiring

(2) Grounding

(3) Lightning protection


• Intermixing loads can cause power quality problems in any facility.

• When nonsensitive and sensitive loads are connected to the same circuit, they often interact with one another.

• For example, when a large motor on an elevator or an air conditioner starts, it causes a large inrush current that can cause a voltage sag.

• The voltage sag inside a facility has the same effect that a voltage sag has outside of the facility. It causes lights to dim and computer equipment to malfunction.


• Solution is to not connect nonsensitive loads that will interact with sensitive loads.

• Wiring sensitive loads to separate circuits connected to the main electrical service panel separates sensitive loads from nonsensitiveloads.

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


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.

• Michael Faraday’s transformer law explains this phenomenon.

• 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.

• 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 PQ 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.

• Discharge of static electricity can occur when you then touch a grounded object, like another person or a metal object.

• Although static electricity PQ problems are infrequent, they are often overlooked.

• Static electricity can create voltages of 3000 V or more and damage sensitive electronic equipment.


• We can minimize static electricity problems by

– Increasing the humidity

– Changing the carpet

– Clothing

– Furniture to non static types

– 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. Often a utility customer calls the utility in an attempt to determine the cause of a power quality problem.

• However, most power quality problems manifest themselves as some effect on an end-user’s electrical equipment.

• These symptoms include motors overheating, adjustable speed drives tripping off, computers shutting down, flickering lights, and stopped production.



• Effects of PQ 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 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 - 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 - give erroneous readings in the presence of harmonics.

Ferromagnetic equipment include transformers and motors -overheat and lose life when harmonic currents increase the loading on them.

Telephones - experience noise induced by adjacent electrical equipment.

Adjustable-speed drives not only cause harmonics but are affected by them- frequent shutdown of an ASDs is usually an indication of excessive harmonics.



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.


Need for PQ Standards

• PQ industry recognizes that PQ standards are critical to the viability of the industry.

• Stakeholders in the PQ industry have developed several PQ standards in recent years.

• Increased interest in power quality has resulted in the need to develop corresponding standards.

• Increased use of sensitive electronic equipment, increased application of nonlinear devices to improve energy efficiency, the advent of deregulation, and the increasingly complex and interconnected power system all contribute to the need for power quality standards.



• Standards set voltage and current limits that sensitive electronic equipment can tolerate from

• electrical disturbances.

• Utilities need standards that set limits on the amount of voltage distortion their power systems can tolerate from harmonics produced by their customers with nonlinear loads.

• End users need standards that set limits not only for electrical disturbances produced by utilities but also for harmonics generated by other end users.



• Deregulation increases the need for standards so that the offending organization causing poor quality problems is held accountable for fixing the problems.

• Standards also allow utilities to provide different levels of power quality service.

• Several national and international organizations have developed power quality standards.


Power Quality Standards Organizations

Organizations responsible for developing power quality standards in the US include the following:

1.Institute of Electrical and Electronics Engineers (IEEE)

2. American National Standards Institute (ANSI)

3. National Institute of Standards and Technology (NIST)

4.National Fire Protection Association (NFPA)

5. National Electrical Manufacturers Association (NEMA),


Power Quality Standards Organizations

6. Electric Power Research Institute (EPRI)

7. Underwriters Laboratories (UL)

Outside the US, the primary organizations responsible for developing international PQ standards include the following:

• International Electrotechnical Commission (IEC)

• Euronorms

• ESKOM for South African standards


IEEE

• The IEEE was founded in 1963 from two organizations: the American Institute of Electrical Engineers (AIEE) and the Institute of Radio Engineers (IRE).

• In 1991, IEEE formed the Standards Coordinating Committee (SSC-22) to coordinate and oversee the myriad of IEEE PQ standards under development or revision.

• IEEE PQ standards deal primarily with the PQ limits of disturbances at the PCC (the point where the utility connects to its customer or end user).


IEEE

• IEEE power quality standards have a great impact in the electrical utility industry but lack official status, ANSI has the official responsibility to adopt standards for the US.

ANSI

• 5 engineering societies and 3 government agencies founded ANSI in 1918.

• It is a private, nonprofit organization with member organizations from the private and public sectors.

• It does not develop standards, but facilitates standards development by qualified groups, like the IEEE.


ANSI

Consequently, many officially authorized IEEE standards have the dual designation of ANSI/IEEE.

It is the sole United States representative to the two major international standards organizations,

1.International Organization for Standardization (ISO)

2.International Electrotechnical Commission (IEC).


IEC

• The genesis of the IEC occurred in 1890 at the Electrical Exposition and Conference held in St. Louis during a meeting of several famous electrical pioneers.

• It has since evolved into an organization with membership from 43 countries. The IEC Council heads the IEC and oversees 200 technical committees, subcommittees, and working groups.

• IEC PQ standards working groups are concerned mainly about standards that will enhance international trade.


IEC

They refer to power quality standards as so-called electromagnetic compatibility (EMC) standards.

IEC’s reference to PQ standards as EMC standards illustrates that IEC’s primary concern is the compatibility of end-user equipment with the utility’s electrical supply system.


What is Electromagnetics (EM)?

• It deals with analysis and application of electric and magnetic fields.

• Principle of EM applied in• Electrical Machines, electromechanical energy conservation, Satellite communication, remote sensing, fiber optics, electromagnetic interference and comaptability.

• EM Devices• Electric motors & Generators, Transformers, Electromagnets, Antennas, Radars, Microwave ovens, super conductors, Electro cardiogram


Other domestic standards organizations

In US, other organizations, like EPRI, UL, NEMA, NFPA, NIST, and some public utility commissions, have also developed PQ standards.

EPRI- Developed reliability indices for utility distribution systems and sponsored the System Compatibility Research Project to enhance the specifications of appliances and equipment to be more compatible with their electrical environment.

Underwriters Laboratories is concerned about the safety of various electrical appliances & developed a standard for the safety of transient voltage surge suppressors, UL 1449.


Other domestic standards organizations

NEMA has set PQ standards for motors, generators, and (UPSs).

NFPA - Concerned about electrical standards for fire safety. Consequently, developed PQ standards to protect computer equipment (NFPA-75) and building lighting (NFPA-780-95)from electrical fires.

NIST - Developed an information poster on power quality (NIST-SP768).


Other International standards organizations

IEC is the primary developer of international power quality standards, other organizations have developed their own standards.

ESKOM, the South African utility - Developed PQ standards based on the best of those in the US and the rest of the world, plus new requirements that other organizations have not developed yet.

• These standards have allowed ESKOM to provide enhanced power quality service at a premium cost.



The European Standards Community Standards Organization (CENELEC) - Developed PQ standards called Euronorms.

The International Union of Producers and Distributors of Electrical Energy (UNIPEDE)published, in 1995,“Measurement Guide for Voltage Characteristics.”

The French standards organization, Union Internationale d’Electrothermie (UIE), is preparing a PQ guide on voltage dips, short-duration interruptions, harmonics, and imbalances.



• International standards tend to require more specific measurements of power quality than United States standards.

• International standards’ purpose is to ensure electromagnetic compatibility between utilities and their customers to help commerce and business, while United States standards’ purpose is usually to solve a power quality problem.

• US standards deal mostly with voltage quality, while international standards deal with compatibility limits between the electric utility power supply and the end-user equipment.

• Thus, international standards require more specificity than United States standards.


• Purpose of Power Quality Standards

• Purpose of power quality standards is to protect utility and end user equipment from failing or misoperating when the voltage, current, or frequency deviates from normal.

• Power quality standards provide this protection by setting measurable limits as to how far the voltage, current, or frequency can deviate from normal.

• By setting these limits, PQ standards help utilities and their customers gain agreement as to what are acceptable and unacceptable levels of service.


• Purpose of Power Quality Standards

To help the power quality industry compare the results of power quality measurements from different instruments, the IEEE developed IEEE Standard 1159-1995 copyright © 1995, Recommended Practice for Monitoring Electric Power Quality.


Voltage sag (dip) standards

Voltage sags are typically the most important power quality variation affecting industrial and commercial customers.

Standards for voltage sags or dips use reliability indices to set voltage sag limits

IEEE uses the term sag or momentary interruption

IEC uses the term dip or short-time interruption to refer to the same phenomenon


Voltage sag (dip) standards

• Most basic index for voltage sag performance is the system average rms (variation) frequency index voltage (SARFIx).

• SARFIx quantifies three voltage sag parameters into one index. The three parameters are the number of voltage sags, the period of measurement, and the number of end users affected by the voltage sag.

• SARFIx is defined as


•SARFIx can be used to assess the frequency of occurrence of sags, swells, and short-duration interruptions.

•Inclusion of the index threshold value x provides a means forassessing sags and swells of varying magnitudes.

•For example, SARFI70 represents the average number of sags below 70 percent experienced by the average customer served from the assessed system.


Transients or surges

ANSI/IEEE C62.41-1991, IEEE Guide for Surge Voltages in Low Voltage AC Power Circuits, deals with transients in a building.

Harmonic standards

The harmonic standard for the United States, IEEE 519-1992, Recommended Practices and Requirements for Harmonic Control in Electric Power Systems,recognizes that the primary source of harmonic currents is nonlinear loads located on the end-user (utility customer) side of the meter.

The utility can also transmit harmonic voltage distortion to other end users. IEEE 519-1992 sets current limits at the point of common coupling (PCC).


Harmonic standards

•IEEE 519-1992 defines harmonic limits on the utility side of the meter as the total harmonic distortion (THD) and on the end-user side of the meter as total distortion demand (TDD).

•This standard sets the voltage distortion limits or THD that the utility can supply to the end user at the point of common coupling.


Grounding and wiring standards

• The primary standards for wiring and grounding are IEEE Standard446, Emergency and Standby Power Systems for Industrial and

Commercial Applications (The Orange Book)

• IEEE Standard 141-1993, Electric Power Distribution for Industrial Plants (The Red Book)

• IEEE Standard 142-1991, Grounding of Industrial and Commercial Power Systems (The Green Book)

• IEEE Standard 1100, Powering and Grounding Sensitive Electronic Equipment, FIPS Pub. 94

• National Electrical Code® (NEC)®, ANSI/NFPA 70. While the NEC is concerned with providing adequate grounding that protects thepublic from electrical shock, these other standards are concerned with setting grounding standards that protect sensitive equipment from damage or misoperation caused by extraneous ground current.

• National Electrical Code® and (NEC)® are registered trademarks of the National Fire Protection Association


References

1. Electric Power System Quality – Roger C.Dugan,Mark F. McGranaghan, Mark McGranaghan, SuryaSantoso, H. Wayne Beaty, H. Beaty, Tata McGraw-Hill Education.

1. Power Quality Primer - Barry kennedy

unit.1 power quality - welcome to srm university1).pdf · professor & hod/eee, ... much more...

Documents