Introduction to Power Quality Problems
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The document provides an in-depth overview of power quality, defining it as the quality of voltage and current in electrical systems, with emphasis on various power quality problems like transients, short and long duration variations, and waveform distortion. It details their causes, effects on equipment, and classifications, as well as guidelines from CBEMA and ITIC curves for acceptable power conditions. The information aims to educate on how to ensure reliable operation of sensitive electronic equipment in diverse environments.
Introduction to Power Quality Problems
- 1. INTRODUCTION TO POWER QUALITY Dr. S. Rajalingam Senior Lecturer/EEE Sunyani Technical University rajalingamstu@gmail.com For explanation: https://youtu.be/VX69IzldHJw
- 2. What is Power Quality? • Power Quality expresses the quality of voltage or the quality of current (quality of sine waves). • Electric power quality involves voltage, current frequency, and its waveform. Dr.S.Rajalingam/EEE/STU
- 3. Power Quality-Definition • As per IEEE, power quality is defined as “The concept of powering and grounding sensitive equipment in a manner that is suitable to the operation of that equipment” – IEEE (Institute of Electrical and Electronic Engineers) www.ieee.org Dr.S.Rajalingam/EEE/STU
- 4. Power quality problem • “Any power problem manifested in voltage, current or frequency deviations that result in failure or mal-operation of customer equipment” is called power quality problem. • The most common source of power quality problems are Power electronic devices, Arching devices, Load switching and other natural/ environmental accidents(trees). Dr.S.Rajalingam/EEE/STU
- 5. Types of Power quality problem 1. Transients 2. Short duration variations 3. Long duration variations 4. Voltage unbalance 5. Waveform distortion 6. Voltage fluctuations 7. Power frequency variations Dr.S.Rajalingam/EEE/STU
- 6. Transients in Power Quality For explanation: https://youtu.be/VX69IzldHJw
- 7. Transient/Surge • Transients are defined as the power quality disturbances that involve destructive high magnitudes of current and voltage or even both which is undesirable and momentary in nature. • It may reach thousands of volts and amps even in low voltage systems. However, such phenomena only exist in a very short duration from less than 50 nanoseconds to as long as 50 milliseconds.
- 8. Power Quality Transient- Sources • Lightning Strikes • Switching activities – Capacitor bank switching – Opening and closing of disconnects on energized lines – Tap changing on transformers • Loose connections in the distribution system that results to arcing • Accidents, human error, animals and bad weather conditions
- 9. Power Quality Transients- Effects • Electronic Equipment – Equipment will malfunction and produces corrupted results – Efficiency of electronic devices will be reduced • Motors – Transients will make motors run at higher temperatures – Degrades the insulation of the motor winding resulting to equipment failure. – Increases the motor’s losses (hysteresis) and its operating temperature • Lights – Fluorescent bulb and ballast failure – Appearance of black rings at the fluorescent tube ends (indicator of transients) – Premature filament damage leading to failure of the incandescent light.
- 10. Transients - Classification • They are classified as – Impulsive transient – Oscillatory transient • An impulsive transient (IEEE 1159) is defined as a sudden change in the steady-state condition of voltage, current, or both that is unidirectional in polarity (primarily either positive or negative). • It is normally a single, very high impulse like lightning. Impulsive transients are generally described by their rise and decay times. • An oscillatory transient is a sudden change in the steady- state condition of voltage, current, or both, that includes both positive and negative polarity values.
- 11. Impulsive and Oscillatory Transients
- 12. Transients - Classification • The impulsive transient can be further classified into – Nanosecond impulsive transient – Microsecond impulsive transient – Millisecond impulsive transient • The Oscillatory transient can be further classified into – Low frequency oscillatory transient – Medium frequency oscillatory transient – High frequency oscillatory transient
- 13. • Long duration voltage variations • Short duration voltage variations For more explanation: https://youtu.be/fD5UD87epZ4
- 14. Long duration voltage variation • When the voltage deviations are exceeded for greater than 1 minute, then it is said to be Long duration voltage variation. • The Long duration voltage variation can be classified into three types – Overvoltage – Undervoltage – Sustained interruption Consider the Supply frequency = 50Hz i.e., 50 cycles per second 1 minute = 60 seconds Hence Number of cycles per minute = 50 × 60 = 3000 cycles per minute
- 15. Long duration voltage variation • Overvoltage: An increase in the RMS AC voltage greater than 110% at power frequency for duration more than 1 minute Causes: Switching off a large load or energizing a large capacitor bank. • Undervoltage: A decrease in RMS AC voltage to less than 90% at power frequency for duration more than 1 minute Causes: Switching on a large load or Switching off a large capacitor bank. • Sustained interruption: When voltage is 0 for duration more than 1 minute.
- 16. Long duration voltage variation-Effects • The overvoltage and undervoltage is not the result of system faults but they are caused by the load variations on the system. • Incorrect tap setting on transformers can also cause undervoltage and over voltage • Effects: Since they are greater than one minute, they stress computers, controllers and motors and shorten the life of power system equipment's and motors.
- 17. Short duration voltage variation • “The variation of RMS voltage for a time greater than 0.5 cycle to one minute/60 seconds” is called short duration variation. Causes: fault conditions, intermittent loose connections in wiring. • Short Duration Voltage Variations are defined as the variations in the supply voltage for durations not exceeding one minute. • The Short duration voltage variation can be classified into three types – Sag (dip) – Swell – Interruption
- 18. Short duration voltage variation • Sag/Dip: A sag is a decrease to between 0.1 and 0.9 p.u in RMS voltage or current at the power frequency for durations from 0.5 cycle to 1 min. Causes: voltage drop due to fault current or starting of large motors. • Swell: An increase in RMS voltage in the range of 1.1 to 1.8 p.u. for duration from 0.5 cycles to 1 minute. • Interruption: a reduction in the supply voltage, or load current, to a level less than 0.1 p.u for a time not exceeding 1 minute.
- 19. Short duration voltage variation • They are further classified into Instantaneous, momentary and Temporary
- 20. Long/short duration voltage variation
- 21. Voltage unbalance, Waveform distortion, Voltage fluctuation, and power frequency variation For more explanation: https://youtu.be/2iI5Cy6SYx8
- 22. Voltage unbalance • It is defined as the variation in the amplitudes of three phase voltages relative to one another. (or) • It is defined as the deviation of each phase from the average voltage of all three phases.
- 23. Voltage unbalance • Causes: unequal distribution of loads in the distribution system, Large single phase loads (induction furnace, traction loads) • Effects: It can produce network problems such as mal-operation of protection relays and voltage regulation equipment, and also overheat of motor and transformer. It affects three phase loads (three phase induction machine) • The effect can be mitigated by voltage regulators.
- 24. Voltage unbalance • Voltage unbalance can be estimated as the maximum deviation from the average of the three-phase voltages divided by the average of the three-phase voltages, expressed in percent. Voltage unbalance = Max deviation from Average Voltage Average Voltage Example: • Assume the following phase-to-phase voltage readings of 226, 232, and 235. • Average Voltage = (226 + 232 + 235) / 3 = 231 • Maximum Deviation from Average Voltage = 231 - 226 = 5 V • Voltage Unbalance = 5 / 231 • Voltage Unbalance = 0.0216 or 2.16%
- 25. Waveform distortion • Waveform distortion is defined as a steady- state deviation from an ideal sine wave of power frequency. • There are 5 types of waveform distortion – DC offset – Harmonics – Inter-Harmonics – Notching – Noise
- 26. DC Offset • DC offset: It is the defined as the presence of DC voltage/current in the AC power system. Causes: Asymmetry of power converters Effects: heating and reduce the life span
- 27. Harmonics & Inter-harmonics • Harmonics: It is the sinusoidal voltages or currents having frequencies that are integer multiples of the fundamental frequency. Sources: Non linear devices like computers, VFD, UPS • Inter-Harmonics: It is the sinusoidal voltages or currents having frequencies that are not the integer multiples of the fundamental frequency. Sources: Static frequency converters, cyclo- converters, power line carrier signals.
- 28. Harmonics
- 29. Harmonic indices • The THD and TDD are the commonly used harmonic indices. • The amount of harmonic distortion is measured by a factor called Total harmonic distortion(THD%). • The THD is defined as the ratio between the RMS value of the harmonics to the RMS value of the fundamental.
- 30. THD • The formulas to calculate the voltage THD, Current THD are as follows
- 31. TDD • The Total Demand Distortion (TDD) is defined as the square root of the sum of the squares of the RMS value of the currents from 2nd harmonic current to the highest harmonic divided by the peak demand load current and is expressed as a percent. • The TDD index most often describe the current harmonic distortion level
- 32. Notching • Notching: It is a periodic voltage disturbance caused by normal operation of power electronic devices when current is commutated from one phase to another (two phases of supply are effectively short- circuited for a short time).
- 33. Noise • Noise: It 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. • Electrical noise adds “hash” or mess onto the fundamental sine wave Causes: – Power electronic devices – Arcing equipment Effects: – Affects Microcomputer – Affects Programmable controllers
- 34. Voltage fluctuation/flicker • Rapid changes in voltage within the allowable limits of the nominal voltage, e.g. 0.9 to 1.1 p.u. Causes: loads that exhibit continuous rapid variations in current magnitude. • If the impact of the voltage fluctuation on lamps perceived by the human eye then it is called flicker. • Flicker : undesirable result of fluctuation • These two terms are linked together in standards and hence the common term voltage flicker is used.
- 35. Power frequency variation • It is defined as the “Deviation of power system fundamental frequency from its specified nominal value” (50Hz to 60Hz). • Frequency variations that go outside of accepted limits for normal steady-state operation of the power system can be caused by faults on the bulk power transmission system, a large block of load being disconnected, or a large source of generation going off-line. • On modern interconnected power systems, significant frequency variations are rare.
- 36. CBEMA and ITEC Curves For more explanation: https://youtu.be/hSU0y7njh3U
- 37. CBEMA Curves • CBEMA Curve is one of the most frequently employed power acceptability curves. • This curve was originally developed by Computer & Business Equipment Manufacturers Association (CBEMA) in the year 1970 to describe the tolerance of mainframe computer equipment to the magnitude and duration of voltage variations on the power system. • The association designed the curve, to point out ways in which system reliability could be provided for electronic equipment. • The CBEMA curve was derived from experimental and historical data taken from mainframe computers.
- 38. CBEMA Curves • 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 CBEMA curve was adapted from IEEE Standard 446 which is typically used in the analysis of power quality monitoring results. • The horizontal axis represent the duration of the event. • The vertical axis represent the percentage of voltage applied to the power circuit.
- 39. CBEMA Curves
- 40. CBEMA Curves
- 41. CBEMA Curves • Voltage values above the envelope are supposed to cause malfunctions such as insulation failure, overexcitation and overvoltage trip. • Voltages below the envelope are assumed to cause the load to drop out due to lack of energy. • The concept is that if the supply voltage stays within the acceptable power area then the sensitive equipment will operate well. • In short, computers, programmable logic controllers (PLCs), power distribution units (PDUs), instrumentation, telecom and other solid-state systems will operate reliably when applied properly.
- 42. CBEMA and ITIC Curves • However, in 1994, the Information Technology Industry Council was formed by a working group of the CBEMA. They developed the curve called ITIC curve. ITIC Curve is a modified version of the CBEMA power acceptability curve • The intent was to derive a curve that can better reflect the performance of typical single-phase, 120 V, 60 Hz computers and their peripherals, and other information technology items like fax machines, copiers and point-of-sales terminals. • Both curves are used to define the withstand capability of various loads and devices for protection from power quality variations.
- 43. ITIC Curves • It is used as a reference to define the withstand capability of various loads and devices for protection from power quality problems. • This is because the curve is generally applicable to other equipment containing solid- state devices aside from being specifically applicable to computer-type equipment.
- 44. ITIC Curves
- 45. ITIC Curves • Compared to the old CBEMA curve, the ITIC curve has an expanded acceptable power area or operating region for the portions of ΔV − t plane. • The ITIC Curve illustrate an AC input voltage envelope, which typically can be tolerated by most Information Technology Equipment (ITE). • The ITIC Curve describes both steady-state and transitory conditions. • The ITIC Curve is applicable to 120 V nominal voltages obtained from 120 V, 208Y/120 V and 120/240 V 60 Hz systems.
- 46. ITIC Curves • Steady-State Tolerances: The steady-state range is ±10% from the nominal voltage. Any voltages in this range may be present for an indefinite period and are a function of the normal loadings and losses in the distribution system. • Voltage Swell: This region describes a voltage swell having an RMS amplitude of up to 120% of the RMS nominal voltage, with a duration of up to 0.5 seconds. This transient may occur when large loads are removed from the system or when voltage is supplied from sources other than the electric utility.
- 47. Low-Frequency Decaying Ring wave • This region describes a decaying ring wave transient which typically results from the connection of power factor correction capacitors to an AC distribution system. The frequency of this transient may range from 200 Hz to 5 kHz, depending upon the resonant frequency of the AC distribution system. • The magnitude of the transient is expressed as a percentage of the peak 60 Hz nominal voltage (not the RMS value). • The amplitude of the transient varies from 140% for 200 Hz ring waves to 200% for 5 KHz ringwaves, with a linear increase in amplitude and increasing frequency.
- 48. Low-Frequency Decaying Ring wave
- 49. High-Frequency Impulse and Ring wave • This region describes the transients that typically occur as a result of lightning strikes. • Wave shapes applicable to this transient and general test conditions are described in ANSI/IEEE C62.41-1991. • This region of the curve deals with both amplitude and duration (energy), rather than RMS amplitude. The intent is to provide an 80 Joule minimum transient immunity.
- 50. Voltage Sags • Two different RMS voltage sags are described in the curve. • Generally, these transients result from application of heavy loads, as well as fault conditions, at various points in the AC distribution system. • Sags to 80% of nominal are assumed to have a typical duration of up to 10 seconds, and sags to 70% of nominal are assumed to have a duration of up to 0.5 seconds.
- 51. No Damage Region • Events in this region include sags and dropouts which are more severe than others and continuously applied voltages which are less than the lower limit of the steady-state tolerance range. • The normal functional state of the ITE is not typically expected during these conditions, but no damage to the ITE should result.
- 52. Prohibited Region • This region includes any surge or swells, which exceeds the upper limit of the envelope. If ITE is subjected to such conditions, damage may result.
- 53. CBEMA & ITIC Curves