18-06-2013, 04:35 PM
POWER ELECTRONICS GENESIS, TRENDS AND IMPROVEMENT.
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ABSTRACT
The latest buzzword, in energy conscious industries, is the improvement of power quality. Good quality of electric power can not only result in handsome savings in power bills but also helps in avoiding unwarranted breakdowns. This paper aims at analysing the reasons for poor quality of power and solutions for improving it. The change in magnitude and power factor can be corrected without much of difficulty with only problem being that of maintenance of shape, which is distorted by harmonics. We look at the factors causing in the production of harmonics, what impact these harmonics have on other equipments in the load serenity. We look at the types of harmonics, their effects and problems caused by them. Active filters are the ultimate solution for reduction of harmonics which use the IGBT technology in additions to the pulse width modulation technology (PWM) which combines with control techniques to emerge successfully with improved power quality by fighting with the problems of harmonics to an extent that the THD (total harmonic distortion) is within acceptable limits i.e. <5%, these filters are the ultimate solution and far win over their counter parts i.e. passive filters.
INTRODUCTION:
Electric power quality, in general, means how pure is the quality of power being used, without any abnormalities, which cause disturbances in it.
he voltage, which is the primary driving force behind all electrical equipment in the system, is an alternating sinusoidal and is characterized by its magnitude, frequency and wave shape. Sags, Surges, spikes and power failures all these disturbances are created due to modification of amplitude and can be measured and removed by using voltage stabilizing, conditioning and UPS systems. To ensure good power quality apart from the requirement of magnitude and frequency to be within acceptable limits, it is important that the wave shape is maintained as a ‘sine wave’ throughout the electrical network.
HOW POWER QUALITY DETERIORATES:
Although power is generated in the pure form, electrical consumers or loads distort it progressively due to the manner in which it is consumed. This problem of power pollution was not heard of in the past but is taking a serious form due to extensive use of energy eficient power electronic (non-linear) loads.
In the past major loading of supply system was made of large slowly changing electrical loads like motor, transformer, which were linear in nature i.e. the current consumed was directly proportional to the supply voltage. The current waveforms being sinusoidal with only power quality problem being that of power factor.
In the last 10 to 15 years, the gradual and never ending prolification of computers, micro-processor system and power electronics in all kinds of industrial, commercial and domestic facilities have completely changed the nature and profile of supply loading. These nonlinear loads consume non-sinusoidal current when sinusoidal voltage is applied to them by conducting only during a fraction of the fundamental period. The current drawn is characterized by sharp rise and fall during only a fraction of the fundamental period .The current waveform contains a considerable amount of odd harmonics, the magnitude of which may be higher than the fundamental current component. In the past all these non-linear loads were confined to industries and hence the power quality by and large was satisfactory.
SOURCES OF HARMONICS
• Switched mode power supplies (SMPS)
• Electronic fluorescent lighting ballets
• Variable speed drive
• Uninterrupted power supply (UPS)
EFFECT OF HARMONICS ON CAPACITORS:
Electric Utilities i.e. Electricity boards in India are offering attractive incentives for power factor (PF) improvement to industrial consumers installing power capacitors in their electrical network to achieve power factor to unity [1.0] Power factor is displacement of current waveform in time with reference to voltage wave forms. Capacitor banks are even present in many industrial plants for reactive power compensation purposes.
The power factor correction i.e. improvement in power factor is basically done by capacitors. These power factor correction capacitors react adversely in the presence of harmonic distortion in the system. Although capacitors, being linear loads, do not generate harmonics, they tend to amplify existing harmonics.
The amplification of harmonics by capacitors is caused by the electrical phenomenon of ‘Resonance’. Which is similar to symphonic vibration in a stringed musical instrument. These ingredients cause electrical resonance viz., capacitors, inductors, non-linear loads. Elimination of any one of these avoids occurrence of resonance. As majority of modern electrical loads are both inductive and non-linear, their elimination is not possible. Elimination of capacitance is impossible, as it would reduce PF below acceptable limits.
REASONS FOR LIMITING HARMONICS
Harmonic pollution causes a number of problems. A first effect is the increase of RMS value and the peak value of distorted waveform. This is illustrated in the figure below (next page), which shows the increase of these values as more harmonic components are added to an initially undistorted waveform. The RMS-value and peak value of undistorted waveform are defined as 100%. For example let us consider fundemental, 3rd, 5th harmonics with the help of the fig 1. in the next page. The increase of RMS-value increases heating of the electrical equipment. Furthermore, circuit breakers may trip due to higher thermal or instantaneous levels. All fuses may blow and capacitors may get damaged. KWH meters may get fault readings. The winding and iron losses of motors increase and they experience perturbing torques on the shaft. Sensitive electronic equipment may be damaged. Equipment, which uses supply voltage, as reference may not be able to synchronize properly, and either apply wrong firing pulses to switching elements or switch off. Interference with electronic communication equipment may occur. Excessive amount of harmonics leads to premature ageing of the equipment.
ACTIVE FILTERS – THE ULTIMATE SOLUTION
Active filters are a relatively new technology and overcome the disadvantages of passive filters. They make use of IGBT semiconductors with various control loops to increase power factor and reduce harmonics, thereby reducing amperage, KVA, circulating currents, harmonic resonance, line voltage imbalance and closed delta-winding losses. The active filter measures the harmonic currents and generates actively a harmonic current spectrum in opposite phase to the original distorting current that was measured. The original harmonics are thereby cancelled. THE PRINCIPLE OF ACTIVE HARMONIC FILTERING IS AS SHOWN BELOW
CONCLUSION
Considering all the above facts, it may not be surprising if electrical utilities across the country start penalizing industrial consumers for high harmonic levels like they do for poor power factor. This is because high harmonics are similar to poor power factor in terms of reducing the efficiency of power distribution system. These days when industrial efficiency standards are being continuously improved, the concept of power quality management is going to play a very significant role in the near future. Thankfully there are qualified consulting engineering firms who should be able to help decide the right solution for a particular situation.
[attachment=55657]
ABSTRACT
The latest buzzword, in energy conscious industries, is the improvement of power quality. Good quality of electric power can not only result in handsome savings in power bills but also helps in avoiding unwarranted breakdowns. This paper aims at analysing the reasons for poor quality of power and solutions for improving it. The change in magnitude and power factor can be corrected without much of difficulty with only problem being that of maintenance of shape, which is distorted by harmonics. We look at the factors causing in the production of harmonics, what impact these harmonics have on other equipments in the load serenity. We look at the types of harmonics, their effects and problems caused by them. Active filters are the ultimate solution for reduction of harmonics which use the IGBT technology in additions to the pulse width modulation technology (PWM) which combines with control techniques to emerge successfully with improved power quality by fighting with the problems of harmonics to an extent that the THD (total harmonic distortion) is within acceptable limits i.e. <5%, these filters are the ultimate solution and far win over their counter parts i.e. passive filters.
INTRODUCTION:
Electric power quality, in general, means how pure is the quality of power being used, without any abnormalities, which cause disturbances in it.
he voltage, which is the primary driving force behind all electrical equipment in the system, is an alternating sinusoidal and is characterized by its magnitude, frequency and wave shape. Sags, Surges, spikes and power failures all these disturbances are created due to modification of amplitude and can be measured and removed by using voltage stabilizing, conditioning and UPS systems. To ensure good power quality apart from the requirement of magnitude and frequency to be within acceptable limits, it is important that the wave shape is maintained as a ‘sine wave’ throughout the electrical network.
HOW POWER QUALITY DETERIORATES:
Although power is generated in the pure form, electrical consumers or loads distort it progressively due to the manner in which it is consumed. This problem of power pollution was not heard of in the past but is taking a serious form due to extensive use of energy eficient power electronic (non-linear) loads.
In the past major loading of supply system was made of large slowly changing electrical loads like motor, transformer, which were linear in nature i.e. the current consumed was directly proportional to the supply voltage. The current waveforms being sinusoidal with only power quality problem being that of power factor.
In the last 10 to 15 years, the gradual and never ending prolification of computers, micro-processor system and power electronics in all kinds of industrial, commercial and domestic facilities have completely changed the nature and profile of supply loading. These nonlinear loads consume non-sinusoidal current when sinusoidal voltage is applied to them by conducting only during a fraction of the fundamental period. The current drawn is characterized by sharp rise and fall during only a fraction of the fundamental period .The current waveform contains a considerable amount of odd harmonics, the magnitude of which may be higher than the fundamental current component. In the past all these non-linear loads were confined to industries and hence the power quality by and large was satisfactory.
SOURCES OF HARMONICS
• Switched mode power supplies (SMPS)
• Electronic fluorescent lighting ballets
• Variable speed drive
• Uninterrupted power supply (UPS)
EFFECT OF HARMONICS ON CAPACITORS:
Electric Utilities i.e. Electricity boards in India are offering attractive incentives for power factor (PF) improvement to industrial consumers installing power capacitors in their electrical network to achieve power factor to unity [1.0] Power factor is displacement of current waveform in time with reference to voltage wave forms. Capacitor banks are even present in many industrial plants for reactive power compensation purposes.
The power factor correction i.e. improvement in power factor is basically done by capacitors. These power factor correction capacitors react adversely in the presence of harmonic distortion in the system. Although capacitors, being linear loads, do not generate harmonics, they tend to amplify existing harmonics.
The amplification of harmonics by capacitors is caused by the electrical phenomenon of ‘Resonance’. Which is similar to symphonic vibration in a stringed musical instrument. These ingredients cause electrical resonance viz., capacitors, inductors, non-linear loads. Elimination of any one of these avoids occurrence of resonance. As majority of modern electrical loads are both inductive and non-linear, their elimination is not possible. Elimination of capacitance is impossible, as it would reduce PF below acceptable limits.
REASONS FOR LIMITING HARMONICS
Harmonic pollution causes a number of problems. A first effect is the increase of RMS value and the peak value of distorted waveform. This is illustrated in the figure below (next page), which shows the increase of these values as more harmonic components are added to an initially undistorted waveform. The RMS-value and peak value of undistorted waveform are defined as 100%. For example let us consider fundemental, 3rd, 5th harmonics with the help of the fig 1. in the next page. The increase of RMS-value increases heating of the electrical equipment. Furthermore, circuit breakers may trip due to higher thermal or instantaneous levels. All fuses may blow and capacitors may get damaged. KWH meters may get fault readings. The winding and iron losses of motors increase and they experience perturbing torques on the shaft. Sensitive electronic equipment may be damaged. Equipment, which uses supply voltage, as reference may not be able to synchronize properly, and either apply wrong firing pulses to switching elements or switch off. Interference with electronic communication equipment may occur. Excessive amount of harmonics leads to premature ageing of the equipment.
ACTIVE FILTERS – THE ULTIMATE SOLUTION
Active filters are a relatively new technology and overcome the disadvantages of passive filters. They make use of IGBT semiconductors with various control loops to increase power factor and reduce harmonics, thereby reducing amperage, KVA, circulating currents, harmonic resonance, line voltage imbalance and closed delta-winding losses. The active filter measures the harmonic currents and generates actively a harmonic current spectrum in opposite phase to the original distorting current that was measured. The original harmonics are thereby cancelled. THE PRINCIPLE OF ACTIVE HARMONIC FILTERING IS AS SHOWN BELOW
CONCLUSION
Considering all the above facts, it may not be surprising if electrical utilities across the country start penalizing industrial consumers for high harmonic levels like they do for poor power factor. This is because high harmonics are similar to poor power factor in terms of reducing the efficiency of power distribution system. These days when industrial efficiency standards are being continuously improved, the concept of power quality management is going to play a very significant role in the near future. Thankfully there are qualified consulting engineering firms who should be able to help decide the right solution for a particular situation.