Chapter 01 Intro to Power Quality GV Nguyễn Hữu Phúc PQ is a Business Problem Power Quality issues cause business problems such as • Lost productivity, idle people and equipment • Lost orders, good wi.
Chapter 01_ Intro to Power Quality GV: Nguyễn Hữu Phúc PQ is a Business Problem Power Quality issues cause business problems such as: • Lost productivity, idle people and equipment • Lost orders, good will, customers and profits • Lost transactions and orders not being processed • Revenue and accounting problems • Customer and/or management dissatisfaction • Overtime required to make up for lost work time According to Electric Light and Power Magazine, 30 to 40 Percent of All Business Downtime Is Related to Power Quality Problems Is Power Quality Such a Big Problem? Why PQ is such a Big Problem? • The sensitivity of today’s electronic equipment makes it susceptible to power disturbances • For some devices, a momentary disturbance can cause o scrambled data o interrupted communications o a frozen mouse o system crashes and equipment failure PQ Problems are Expensive • Berkeley Lab Study Estimates $80 Billion Annual Cost of Power Interruptions … Research News, Berkeley Lab, February 2, 2005 • $50 billon per year in the USA is lost as a results of power quality breakdowns … Bank of America Report • A manufacturing company lost more than $3 million one day last summer in Silicon Valley when the “lights went out.” … New York Times January 2000 • “A voltage sag in a paper mill can waste a whole day of production - $250,000 loss” … Business Week, June 17,, 1996 • Half of all computer problems and one-third of all data loss can be traced back to the power line … Contingency Planning Research, LAN Times Who is Affected? • High Cost Facilities o Semiconductor plants o Pharmaceuticals o Data centers • Medium Cost Facilities o Automotive manufacturing o Glass plants o Plastics & Chemicals o Textiles • • • • • • Lost production Scrap Costs to restart Labor costs Equipment damage and repair Other costs PQ Definitions Revisited IEEE Categories Std 1159-1995 Short Duration Variations Typical Duration Instantaneous Sag 0.5 – 30 cycles Momentary Sag 30 cycles – sec Temporary Sag sec – PQ Definitions Revisited IEEE Categories Std 1159-1995 Short Duration Variations Typical Duration Instantaneous Sag 0.5 – 30 cycles Momentary Sag 30 cycles – sec Temporary Sag sec – Instantaneous Swell 0.5 – 30 cycles Momentary Swell 30 cycles – sec Temporary Swell sec – PQ Definitions Revisited IEEE Categories Std 1159-1995 Short Duration Variations Typical Duration Instantaneous Sag 0.5 – 30 cycles Momentary Sag 30 cycles – sec Temporary Sag sec – Instantaneous Swell 0.5 – 30 cycles Momentary Swell 30 cycles – sec Temporary Swell sec – Momentary Interruptions 0.5 – 30 cycles Temporary Interruptions 30 cycles – sec PQ Definitions Revisited IEEE Categories Std 1159-1995 Long Duration Variations Typical Duration Sustained interruptions > Under voltages > Over voltages > Oscillatory transient •An oscillatory transient is a sudden, non–power frequency change in the steady-state condition of voltage, current, or both, that includes both positive and negative polarity values Low-frequency oscillatory transients caused by ferroresonance of an unloaded transformer Low-frequency oscillatory transients caused by capacitor bank energization onto 34.5 kV bus voltage Medium-frequency oscillatory transients caused by back-to-back capacitor bank switching Long-Duration Voltage Variations •Long-duration variations encompass root-mean-square (rms) deviations at power frequencies for longer than Long-duration variations can be either overvoltages or undervoltages Overvoltages and undervoltages generally are not the result of system faults, but are caused by load variations on the system and systemswitching operations •Overvoltage •An overvoltage is an increase in the rms ac voltage greater than 110 percent at the power frequency for a duration longer than •Overvoltages are usually the result of load switching (e.g., switching off a large load or energizing a capacitor bank) •Undervoltage An undervoltage is a decrease in the rms ac voltage to less than 90 percent at the power frequency for a duration longer than Undervoltages are the result of switching events that are the opposite of the events that cause overvoltages A load switching on or a capacitor bank switching off can cause an undervoltage until voltage regulation equipment on the system can bring the voltage back to within tolerances Overloaded circuits can result in undervoltages Sustained interruptions •When the supply voltage has been zero for a period of time in excess of1 min, the long-duration voltage variation is considered a sustained interruption •Voltage interruptions longer than are often permanent and require human intervention to repair the system for restoration Short-Duration Voltage Variations •This category encompasses the IEC category of voltage dips and short interruptions Each type of variation can be designated as instantaneous, momentary, or temporary, depending on its duration as defined in the following table •Short-duration voltage variations are caused by fault conditions, the energization of large loads which require high starting currents, or 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) 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 Sags [US] (dips[IEC]) •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 •Three-phase rms voltages for a momentary interruption due to a fault and subsequent recloser operation Voltage sag caused by an SLG fault (a) RMS waveform for voltage sag event (b) Voltage sag waveform Temporary voltage sag caused by motor starting 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 Voltage Imbalance Voltage imbalance (also called voltage unbalance) is sometimes defined as the maximum deviation from the average of the three-phase voltages or currents, divided by the average of the three-phase voltages or currents, expressed in percent Imbalance is more rigorously defined in the standards using symmetrical components Instantaneous voltage swell caused by an SLG fault The ratio of either the negative- or zero sequence component to the positive-sequence component can be used to specify the percent unbalance The most recent standards specify that the negative-sequence method be used Voltage unbalance trend for a residential feeder Waveform Distortion •Waveform distortion is defined as a steady-state deviation from an ideal sine wave of power frequency principally characterized by the •spectral content of the deviation There are five primary types of waveform distortion: ■ DC offset ■ Harmonics ■ Interharmonics ■ Notching ■ 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 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 (termed 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 total harmonic distortion (THD), as a measure of the effective value of harmonic distortion •the total demand distortion (TDD) This term is the same as the total harmonic distortion except that the distortion is expressed as a percent of some rated load current rather than as a percent of the fundamental current magnitude at the instant of measurement Current waveform and harmonic spectrum for an ASD input current Interharmonics •Voltages or currents having frequency components that are not integer multiples of the frequency at which the supply system is designed to operate (e.g., 50 or 60 Hz) are called interharmonics •They can appear as discrete frequencies or as a wideband spectrum 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 Noise Example of voltage notching caused by a threephase converter 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 Voltage Fluctuation 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 Example of voltage fluctuations caused by arc furnace operation Flicker Loads that can exhibit continuous, rapid variations in the load current magnitude can cause voltage variations that are often referred to as flicker Flicker (Pst) at 161-kV substation bus measured according to IEC Standard 61000-4-15 (Courtesy of Dranetz-BMI/Electrotek Concepts.) Power Frequency Variations Power frequency variations are defined as the deviation of the power system fundamental frequency from it specified nominal value (e.g., 50 or 60 Hz) Power frequency trend and statistical distribution at 13-kV substation bus (Courtesy of Dranetz-BMI/Electrotek Concepts.) Power Quality Terms •Ambiguous Terms •Blackout - Glitch •Blink - Outage •Brownout- Interruption •Bump- Power surge •Clean ground- Raw power •Clean power- Spike •Dirty ground- Surge •Dirty power- Wink CBEMA and ITI Curves http://www.powerstandards.com/cbema.htm One of the most frequently employed displays of data to represent the power quality is the socalled CBEMA (Computer and Business Equipment Manufacturers' Association) curve This curve was originally developed by CBEMA to describe the tolerance of mainframe computer equipment to the magnitude and duration of voltage variations on the power system While many modern computers have greater tolerance than this, the curve has become a standard design target for sensitive equipment to be applied on the power system and a common format for reporting power quality variation data 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, overvoltage trip, and overexcitation A portion of the CBEMA curve commonly used as a design target for equipment and a format for reporting power quality variation data ITI- Information Technology Industry Council The CBEMA organization has been replaced by ITI and a modified curve has been developed that specifically applies to common 120-V computer equipment CBEMA power quality graph The CBEMA (Computer and Business Equipment Manufacturers' Association, typically pronounced sa-BEEM-ah) power quality graph plots the depth of voltage sags on the vertical axis against the duration of voltage sags on the horizontal axis The green line on the graph shows the sag immunity suggested by CBEMA The sags above, shown as blue dots, were collected at worldwide semiconductor plants For more information, see SEMATECH TT#99063760B-TR Clearly, many of these sags were deeper and longer than the CBEMA recommendation This observation led the semiconductor industry to establish its own voltage immunity standard, SEMI F47 CBEMA has developed into ITIC; check the Tutorials & Info page for more information Power Standards Testing Labs specializes in testing to this SEMI standard, and offers sag generators to people who want to their own testing ITI curve for susceptibility of 120-V computer equipment •power quality teaching toy A fun and useful simulator for engineers who want to get an intuitive feel for power quality issues FREE! (but n power quality videos •Introduction to Voltage Dips/Sags: (a one minute video - 20MB) To save the video, right-click and select "Save as" brief power quality tutorials for engineers •Sags, dips, and swells: Introduction to the most common disturbance on AC mains •Transient overvoltages: Introduction to capacitor-switching and high-frequency transients •Harmonics: Introduction to voltage and current harmonics issues •Flicker: Introduction to voltage flicker •Voltage regulation: Introduction to voltage regulation issues •Other disturbances: Frequency variations, noise bursts, and other less common problems sag immunity tutorials •Tutorial #1: Common sources of voltage sags / dips •Tutorial #2: Five ways equipment fails due to voltage sags •Tutorial #3: Semiconductor industry voltage sag standard: SEMI F47 •Tutorial #4: Quick, inexpensive fixes that increase immunity to voltage sags electric power standards •IEEE power quality standards •IEC power quality standards •Utility power quality standards •Industry-specific power quality standards •U.S military power quality standards •Other power quality standards - ANSI, CBEMA, CIGRE, country-specific, etc Other Topics •How 4-terminal resistors work and why we use them Need copies of one of these standards? Please contact us, and we will be glad to help.Fun Stuff References ... Chapter 01 _Intro to Power Quality •Both electric utilities and end users of electric power are becoming increasingly concerned about the quality of electric power •The term power quality has... Is Related to Power Quality Problems Is Power Quality Such a Big Problem? Why PQ is such a Big Problem? • The sensitivity of today’s electronic equipment makes it susceptible to power disturbances... aspects of power quality What Is Power Quality? Any power problem manifested in voltage, current, or frequency deviations that results in failure or misoperation of customer equipment Power Quality=