power quality voltage fluctuation UNIT - I.pptx

Power quality

1. EE8006 -Power Quality UNIT– I INTRODUCTION TO POWER QUALITY

2. Power Quality Definition Power quality is a set of electrical boundaries that allows a piece of equipment to function in its intended(own way) manner without significant loss of performance or life expectancy. This definition embraces (Accept or support or include) two things that we demand from an electrical devices – performance and life expectancy.

3. Power Quality Definition The term Power quality is used to describe the extent of variation of the voltage, current and frequency on the power system. The variation of voltage and current can either be in terms of magnitude or waveform shape/distortion. Power Quality broadly refers to maintaining a near sinusoidal power distribution bus voltage at rated magnitude and frequency Power quality is any abnormal behavior on a power system arising in the form of voltage, current or frequency deviations that results in failure or mis - operation of customer equipment's

4. Power quality has been defined as the parameters of the voltage that affect the customers supersensitive equipment. Power quality may be defined as any electrical parameter or connection that affects the operation of the equipment. This included all electrical parameters, connections and grounds, whether the source from the utility , local equipment or other users

5. Power Quality Problem Reliability point of view , Energy supplied to customer should not be interrupted Power quality problem mainly occurred in power distribution side Power transmission systems may also have an impact on the quality of power Transmission System Problem For long distance power transmission voltage is stepped up by transformer High voltage transmission has its own problem due to corona and other losses.

6. Power Quality Disturbances Power Quality is the degree to which both the utilization and delivery of electric power affects the performance of electrical equipment. Any deviation to the magnitude or frequency of the ideal sinusoidal voltage waveform called power Quality disturbance. The various types of power quality disturbances can be classified into the following categories.

7. Short Duration Variations The variation of the RMS voltage from its nominal value for a time greater than 0.5 cycles of the power frequency but less than or equal to 60 seconds is called short duration variation. Short duration voltage variation caused by fault conditions. - The energization of large loads which require high starting currents - Intermittent loose connection in power wiring. Short duration variations are further classified using two categories.

8. The first category indicates - The magnitude of the voltage variation - Sag,Swell or Interruption The second category indicates – The duration of the voltage variation - Instantaneous, Momentary or Temporary . 1. Sag (a) Instantaneous (b) Momentary (c) Temporary

9. 2. Swell (a) Instantaneous (b) Momentary (c) Temporary 3. Interruption (a) Momentary (b) Temporary

10. Long Duration Variations The variation of the RMS value of the voltage from its nominal values for a time greater than 60 seconds is called long duration variation. These variations are further classified - by using magnitude of a voltage variation 1.Under voltage 2. Over voltage 3. Sustained interruption

11. Over voltages and under voltages generally are not the result of system faults, but are caused by load variations on the system and system switching operations. Such variations are typically displayed as plots of RMS voltage versus time.

12. Transients Transients are disturbances that occur for a very short duration. These may be of either polarity and can be additive or subtractive from the nominal waveform. Transients can be further classified into: 1. Impulsive Transients (a) Nanosecond Impulsive Transients (b) Microsecond Impulsive Transients (c) Millisecond Impulsive Transients

13. 2. Oscillatory Transients (a) Low Frequency Oscillatory Transients (b) Medium Frequency Oscillatory Transients (c) High Frequency Oscillatory Transients Waveform Distortion It is the deviation from an ideal sine wave of power frequency principally characterized by the spectral content of the deviation. Harmonics is one of the causes of many types of waveform distortion.

14. CHARACTERISTICS OF POWER QUALITY EVENTS Voltage sags are the most common power problem encountered. Sags are a short- term reduction in voltage. It cause interruptions to sensitive equipment such as adjustable-speed drives, relays, and robots. Sags are most often caused by fuse or breaker operation, motor starting, or capacitor switching. Voltage sags typically are non-repetitive, or repeat only a few times due to recloser operation .

15. Sags multiple phases or on a single phase and can be accompanied by voltage swells on other phases. Statistics shows that 40% to 60 % of the time a power quality event is a voltage sag event. Voltage Sag is an which the RMS voltage decrease between 0.1 and 0.9 per unit at the power frequency. It lasts for duration of 0.5 cycles to 1 min. Classification of sag is shown in fig below

16. Types of Sag Based on the time duration and voltage magnitude, sag is further classified as: 1. Instantaneous Sag 2.Momentary Sag 3. Temporary Sag Instantaneous Sag : Instantaneous Sag is said to occur when the RMS voltage decreases between 0.1 and 0.9 per unit for time duration of 0.00833 second to 0.5 second.

17. Momentary Sag : Momentary Sag is said to occur when the RMS voltage decreases between 0.1 and 0.9 per unit for time duration of 0.5 second to 3 seconds. Temporary Sag : Temporary Sag is said to occur when the RMS voltage decreases tween 0.1 and 0.9 per unit for time duration of 3 to 60 seconds. The Sag characteristics are shown in Table

18. Causes of Sag Voltage sags are caused by the switching of heavy loads. Voltage sags are caused by motor starting. For example, an induction motor will draw six to ten times its full load current. - This lagging current causes a voltage drop across the impedance of the system. - If the current magnitude is large relative to the system available fault current, the resulting voltage sag can be significant.

19. Swell Swell is an event in which the RMS voltage increases between 1.1 and 1.8 per unit at the power frequency. It lasts for durations of 0.5 cycles to1 min. Following fig shows the Swell classification.

20. Types of Swell Based on the time duration and voltage magnitude, Swell is further classified as: 1. Instantaneous Swell 2. Momentary Swell 3. Temporary Swell Instantaneous Swell - Instantaneous Swell is said to occur when the RMS voltage decreases between 1.1 and 1.8 per unit for time duration of 0.008333 second to 0.5 second.

21. Momentary Swell - Momentary Swell is said to occur when the RMS volta decreases between 1.1 and 14 per unit for time duration of 0.5 second to 3 seconds. Temporary Swell -Temporary Swell is said to occur when the RMS voltage decreases between 1.1 and 1.2 per unit for time duration of 3 to 60 seconds.

22. Causes of Swell Swells are also associated with system fault conditions, but they are much less common than voltage sags. For example, faults on one line cause voltage rise on other phases. Swells can be caused by switching off a large load and by switching on a large capacitor bank.

23. Swell characteristics

24. Interruption Power interruptions are zero-voltage events that can be caused by weather. equipment malfunction, recloser operations, or transmission outages. Interruptions can occur on one or more phases and are typically short duration events, the vast majority of power interruptions are less than 60 seconds. An interruption is said to have occurred when the supply voltage decreases to less than 0.1 per unit

25. Interruption Classification

26. Types of Interruption Based on the time duration and voltage magnitude, Interruption is further classified as: 1. Momentary Interruption 2. Temporary Interruption 3. Sustained Interruption Momentary Interruption - Momentary Interruption is said to occur when the RMS voltage decreases less than 0.1 per unit for time duration of 0.008333 second to 3 second. - Causes: Utility recloser operation, faulty circuit breakers, bad wiring connections. - Effects: Lost data, destruction of files, damaged hard disk .

27. Temporary Interruption - Temporary Interruption is said to occur when the RMS voltage decreases less than. 0.1 per unit for time duration of 3 second to 60 seconds. Sustained Interruption - Sustained Interruption is said to occur when the RMS voltage decreases 0.0 per unit for time duration greater than 60 seconds. Voltage interruptions longer than 60 seconds are often permanent in nature and require manual intervention for restoration.

28. Interruption characteristics

29. Causes Interruption Voltage Interruptions are caused by the operation of protective devices such as breakers and fuses Voltage interruptions longer than 1 minute are often permanent in nature and require manual intervention and restoration. Sometimes system maintenance can require voltage interruption in certain sections of power systems.

30. Under Voltage Under voltage is an event in which the RMS voltage decreases between 0.8 and 0.9 per unit at the power frequency for a period of time greater than 1 min.

31. Under voltage classification

32. Under voltage characteristics

33. Causes of Under voltage Under voltages can be the result of load switching. Overloaded circuits can also lead to under voltage. Sometimes faulty connections or wiring and Loose or corroded connections can also cause under voltage.

34. Overvoltage Overvoltage is an event in which the RMS voltage increase between 1.1 and 1.2 per unit at the power frequency for a period of time greater than 1min.

35. Overvoltage Classification

36. Overvoltage Characteristics

37. Causes of Over Voltages Overvoltage can be the result of load switching for Example - Switching OFF a large load Switching ON large capacitor bank. Poor system voltage regulation capabilities or controls result in over voltages . Incorrect tap settings on transformers can also result in system over voltages .

38. Transients Transients are disturbances that occur for a very short duration. This may be of either polarity and can be additive or subtractive from the nominal waveform. The main reason for the occurrence of the transients is a sudden change in the voltage or current in power system. The primary characteristics that define a transient are the peak amplitude, the rise time, the fall time and the frequency of oscillation Sub cycle (lasting less than one cycle) transients are the most difficult to detect.

39. Transients Classification Impulsive Transients Oscillatory Transients

40. Impulsive transients An impulsive transient is a non-power frequency event. In which the steady state condition of the voltage, current or both change suddenly. The change is unidirectional in polarity either positive or negative as shown in Figure below. The impulsive transients are characterized by the rise and fall time, for example, a 1.2/50 µs ,2000 V impulsive transient rises to its peak value of 2000 V in 12 µs and then decays to half its peak value in 50 µs

41. Type of impulsive Transients Based on the rise time and duration, impulsive transients are further classified as 1.Nanosecond Impulsive Transient 2.Microsecond Impulsive Transient 3. Millisecond Impulsive Transient

42. Nanosecond Impulsive Transient -A transient with a rise time of 5 nano -second and a duration less than 50 nanoseconds is considered as a Nanosecond Impulsive transients. Microsecond Impulsive Transient - A transient with a rise time of 1 microsecond and a duration that lasts between 50 nano -seconds and 1 millisecond is considered a Microsecond. Millisecond Impulsive Transient - A transient with a rise time and duration greater than 1 millisecond is considered as Millisecond impulsive transients.

43. Characteristics of impulsive Transients

44. Causes of impulsive Transients The most common cause of impulsive transients is lightning. Due to the high frequencies involved, impulsive transients are damped quickly by resistive circuit and are not conducted far from their source. Impulsive transients can excite power system resonance circuits and produce oscillatory transients.

45. Oscillatory Transients - An oscillatory transient is a non-power frequency event in which the steady state condition of the voltage, current or both changes polarity rapidly. The change in polarity is bidirectional shown in figure below

46. Oscillatory Transients Classification

47. Types of Oscillatory Transients Based on the frequency content, time duration and voltage magnitude, oscillatory transients are further classified as: 1. Low Frequency Oscillatory Transient 2. Medium Frequency Oscillatory Transient 3. High Frequency Oscillatory Transient

48. Low Frequency Oscillatory Transient A transient with a primary frequency component less than 5 kHz and duration from 0.3 to 50 ms is considered a low frequency transient. This category of transients is encountered on utility sub-transmission and distribution systems. The most frequent cause is capacitor bank energization , which results in an oscillatory voltage transient with primary frequency between 300 and 900 Hz .

49. Medium Frequency Oscillatory Transient A transient with a primary frequency component between 5 and 500 kHz and duration of 20 µs is termed a medium frequency transient. They can be the result of system response to an impulsive transient or back to back capacitor energization .

50. High Frequency Oscillatory Transient A transient with a primary frequency component greater than 500 kHz and duration of 5 µs is termed as a high frequency transient. These transients are often (Frequently) the result of a local system response to an impulsive transient.

51. Oscillatory Transient Characteristics

52. Voltage Imbalance (Unbalance) Voltage unbalance is a steady-state quantity 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. Unbalance can also be quantified using symmetrical components. The ratio of the negative sequence (or zero sequence) component to the positive sequence component is used to specify the percent unbalance. The negative sequence (or zero sequence) voltages in a power system generally result from unbalanced loads causing negative sequence (or zero sequence) currents to flow.

53. Unbalance can be characterized by unbalance magnitude in percent versus time or can be summarized using statistics of the unbalance magnitude over some period. The primary source of voltage imbalances of less than 2% is single-phase loads on three phase circuit. Voltage imbalance can also be the result of capacitor bank anomalies, such as a blown fuse on one phase of a three-phase bank. Severe voltage imbalance (greater than 5%) can result from single-phasing conditions.

54. Voltage unbalance is most important for three phase motor loads. ANSI Std recommends that the maximum voltage unbalance under no load conditions should be 3%. Unbalance greater than this can result in significant motor heating &failure if there is no unbalance protection circuits to protect the motor . The voltage unbalanced factor can be expressed by:  

55. Power Frequency Variations Power frequency variations are a deviation from the nominal supply frequency (50 Hz or 60 Hz). The specified frequency variation should be within the limits of ± 2.5% Hz at all times for grid network. Frequency variation in 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 out of service. Frequency variations and its consequence are affects - electronic equipment, data corruption, hard drive crash, keyboard lockup, erratic operations and program failure and even component damage.

56. Waveform Distortion Generally Waveform distortion in a system is defined as "Steady state deviation from an ideal sinusoidal" waveform. The distortion of waveforms can be classed into six separate categories: Harmonic DC offset Inter-harmonic Notching Noises Flicker

57. The harmonic Components of a waveform are sinusoidal in shape, with a frequency equal to an integer multiple of the fundamental frequency The 5th harmonic has a frequency of 250 Hz (5 x 50 Hz) The 3rd harmonic has frequency of 150 Hz (3 x 50 Hz)

58. Harmonics Distortion Harmonics are sinusoidal currents and voltages with frequencies that are integral multiples of the fundamental power line frequency which is 50 Hz/60HZ. Harmonics distort the supplied 50 Hz voltage and current waveforms from their normal sinusoidal shapes. Order increases – Harmonics increases , Magnitude decreases Lower order harmonics most effect on the power system (5 th ,7 th )

59. Harmonic distortion originates from the non-linear characteristics of devices and loads on the power system. Harmonic distortion levels are described by the complete harmonic spectrum with magnitude and phase angle of each harmonic component despite its deficiency, The total harmonic distortion (THD) is frequently used as a measure of the degree of harmonic distortion of the system. These voltage harmonics can cause - communication errors and hardware damage due to unexpected overheating of components and also cause overheating, voltage fluctuation, and visual flicker in computer monitors (and fluorescent lighting and other arc lighting).

60. 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 . Interharmonics Voltages or current having the 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 inter harmonics

61. 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 . Voltage Flicker Voltage flicker is rapidly occurring voltage sags caused by sudden and large increases in load current.

62. Voltage flicker is most commonly caused by rapidly varying loads that require a large amount of reactive power such as are furnaces, electric welders, rock crushers, sawmills, wood chippers, metal shredders, and amusement rides. It can cause visible flicker in lights and cause other processes to shut down or malfunction. Flicker can be defined as small amplitude changes in voltage levels occurring at frequencies less then 25 Hertz .

63. Noises Noise is defined as unwanted electric signals with broadband spectral contents lower than 200 KHz superimposed upon the power system voltage or current in phase conductors, or found on neutral conductors or signal lines.

64. 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

65. Total harmonic distortion Total harmonic distortion is the term used to describe the net deviation of a non- linear waveform from ideal sine wave characteristics. THD is the ratio between the RMS value of the harmonics and the RMS value of the fundamental. The amount of harmonic distortion can be measured by means of a factor known as the total harmonic distortion (THD %)), which is given by the relation,

66. Where U represents the fundamental components U represents harmonic components andh represents harmonic order. Similarly Total harmonic voltage distortion is given by the relation.

67. Need For Standards and Guidelines Power quality standards gives the details about - Operational Characteristics - Tolerance and limits for sensitive devices Utility side and consumer side proper design ,installation, maintenance requires for highly sensitive loads High sensitive loads equipment's requires high degree of power quality.

68. The most universally accepted standards for power quality are IEC and IEEE Standards. IEEE adopts some of ANSI Standards

69. IEC Power Quality Standards IEC – International Electro Technical Commission Web – http://www.iecch/ .

70. IEC 61000 series Electro magnetic compatibility (EMC) defines for the following Part 1 : 61000 – 1 –X , Deals fundamental definitions Part 2 : 61000 - 2 – X , Characteristics of the environment Part 3: 61000 – 3 –X , Limits , permissible emissions that can be generated by the equipment connected Part 4: 61000 – 4 –X , Details about Tests and Measurements Part 5: 61000 – 5 –X , Provide guideline about Installation and mitigation Part 6: 61000 – 6 –X , Provide details about Generic immunity and emissions

71. CBEMA Curve and ITI Curve ITIC Curve – Information Technology Industry Council Curve , is published by Technical Committee 3 (TC 3) of the information technology industry council. ITIC formerly known as the Computer Business Equipment Manufactures Association ( CBEMA). CBEMA – was developed by collaboration with Electric power research institute’s (EPRI) and Power Electronics application center. This curve define the magnitude and duration of input AC Voltage that can be tolerated by computers, their peripherals and other information technology equipment (ITE) like copiers, fax machines, point of sales terminals

72. A set of curves representing the withstand capabilities of computer interms of the magnitude and duration of the voltage disturbance. In 1970- CBEMA – Developed curve , it had become standard for measuring the performance of all the types of equipment and power systems and is commonly referred as “CBEMA – Curve”. CBEMA Curve – Displays Power quality data's CBEMA Curve – Describe the tolerance of mainframe computer equipment to the magnitude and duration of voltage variations on the power system. CBEMA Curve - Drawn between Magnitude and duration of the voltage.

73. ITIC Curve is designed for computer equipment designed to operate at nominal voltage of 120V RMS systems. The above curve applicable for 120,120/208 and 120/240 Nominal voltage Horizontal axis - Represents duration for which an event lasts Vertical axis – Voltage magnitude of the event as a percent of the nominal voltage for the duration of event. Upper curve – defines disturbances that fall within the envelop Lower Curve – Not harmful to electrical equipment , disturbances that fall outside the envelope may disrupt or damage the equipment. -Envelope Presumed(Guess)

74. Points below the envelope are presumed to cause the load to drop out due to lack of energy (Fig 1.17) Points above the envelope are presumed to cause other malfunctions such as insulation failure , Over voltage trip and Over excitation (Fig 1.17) Seven types of events are described in this composite envelope