21-08-2014, 12:10 PM
POWER QUALITY SEMINAR REPORT
[attachment=66824]
INTRODUCTION
Since last 25 years there has been an increase in the use of solid state electronic technology. This new, highly efficient, electronic technology provides product quality with increased productivity. Today, we are able to produce products at costs less than in the years passed, with the introduction of automation by using the solid state electronic technology .This new technology requires clear electric power.
The conventional speed control systems are being replaced by modern power electronic systems, bringing a verity of advantages to the users. Classic examples are DC $ AC drives, UPS, soft stators, etc. Since the thrusters converter technology is rapidly gaining in the modern industrial plants, the power supply systems are contaminated as the ideal sinusoidal current and voltage waveforms are getting distorted. This is in turn is affecting the performance of the equipment in the electrical network.
ABSTRACT
Power quality is essential for smooth functioning of industrial process. As industries expand, utilities become more interconnected and usage of electrically equipment increases, power quality is jeopardized. The quality of power in the power system is severely affected by the presence of harmonics. This harmonics adversely effects the power system performance. Some of the effects are over heating of metal parts, noise in motors, low efficiency in motors etc. The effects produced by the harmonics are reduced by adopting some corrective measures
WHAT IS POWER QUALITY?
Adequate to superior power quality is essential for the smooth functioning of critical industrial processes. As industries expand, utilities become more interconnected and usage of electronically controlled equipment increases, power quality is jeopardized. Most large industrial and commercial sites are served by overhead lines with feeders that are subject to unpredictable and sporadic events, e.g. lightning and contact with tree limbs. Most distribution circuits have resoling devices that clear temporary faults through a timed series of trip and close operations.
This minimizes the possibility of long-term outages but leads to a number of minor power disturbances. These typically occur several times a month. Many electric utilities have increased the voltage at which they distribute power. This allows a single circuit to serve more customers or deliver higher loads, and reduces energy losses in the system. But it often means the overhead distribution circuit is longer, with more exposure to disturbances. And disturbances travel farther because of lower system impedances associated with higher voltage circuits. Sophisticated new systems are providing vastly increased efficiency and control in critical processes. But with their high sensitivity even to brief variations in electric power quality, today's computer-driven devices fail when power is disturbed for even a few milliseconds.
HARMONICS-BASIC CONCEPTS
A pure sinusoidal voltage is conceptual quantity produced by an ideal AC generator build with finely distributed stator and field windings that operate in a uniform magnetic field. Since neither the winding distribution nor the magnetic field is uniform in a working AC machine, voltage waveform distortion is created, and the voltage time relation-ship deviates from the pure sine function. The distortion at the point of generation is very small (about 1%to 2%), but nonetheless it exists.
Because this is a deviation from a pure sine wave, the deviation is in the form of a periodic function and by definition, the voltage distortion contains harmonics. When a sinusoidal voltage is applied to a certain type of load, the current drawn by the load is proportional to the voltage and impedance and follows the envelope of the voltage wave form .These loads are referred to as linear loads (loads where the voltage and current follow one another without any distortion to their pure sine waves).examples of nonlinear loads are resistive heaters, incandescent lamps and constant speed induction and synchronous motors.
In contrast some loads cause the current to vary disproportionately with the voltage during each half cycle. These loads are classified as nonlinear loads and the current and voltage have waveforms that are non sinusoidal containing distortions where by 50 Hz waveform has numerous additional waveforms superimposed
THE AFFECTS
The actual problems of any building/industry will vary depending on the type and number of installed harmonics producing loads. Most electrical network can withstand nonlinear loads of up to 15% of the total electrical system capacity without concern but when the nonlinear loads exceed 15% some non expected negative consequences can be expected. .for electrical networks , they have on linear loading of more than 25% particular problems can be apparent
SOLUTION
Users of variable frequency drives often have strict demands placed on them to mitigate harmonic distortion caused by the nonlinear loads. Many choices are available to them including line reactors, harmonic traps, 12 pulse rectifier, 18 pulse rectifiers, and low pass filters
HARMONIC FILTERS
In some cases, reactors alone will not be capable of reducing the harmonic current distortion to the desired levels. In these cases, a more sophisticated filter will be required. The common choices include shunt connected, tuned harmonic filters (harmonic traps) and series connected low pass filters (broad band suppressors). They consist of a capacitor and an inductor which are tuned to a single harmonic frequency. Since they offer very low impedance to that frequency, the specific (tuned) harmonic current is supplied to the drive by the filter rather than from the power source. If tuned harmonic filters (traps) are selected as the mitigation technique, then multiple tuned filters are needed to meet the distortion limits which are imposed
PULSE RECTIFIERS
Figure shows the actual measurement of input current harmonic distortion for 12 pulse rectifier supplied from a balanced 3 phase voltage source while operating at full load conditions. For test purpose transformer has delta primary and delta,wye secondary windings. To obtain the best results, the bridge rectifier is connected in series so equal dc windings. To obtain the best results, the bridge rectifier is connected in series so equal dc
PULSE RECTIFIER
To determine how an eighteen-pulse drive system operates under unbalanced line voltage conditions, we constructed a 30 HP eighteen-pulse drive from a conventional isolation transformer and standard six-pulse drive using the series bridge connection shown in figure 1. An auto transformer could have been used in place of the isolation transformer. The auto transformer costs less and requires less mounting space, but the isolation transformer was selected because it provides better performance and is readily available as a modified standard transformer. Care was taken in the physical construction of the transformer to balance the leakage reactance and output voltage of the three secondary windings. The system was tested with line voltage unbalance ranging from 0% to 3% and with loads ranging from 5% to 100%. The input total harmonic current distortion, THID, is shown in figure 3. THID varied from 7.4% at full load with balanced line voltages to 59% at 30% load with a 3% unbalance
LOW PASS HARMONIC SUPPRESSORS
Low pass harmonic filters, also referred to as broad band harmonic suppressors, offer a non-invasive approach to harmonic mitigation. Rather than being tuned for a specific harmonic, they filter all harmonic frequencies, including the third harmonic. They are connected in series with the non-linear load with a large series connected impedance, therefore they don’t create system resonance problems. No field tuning is required with the low pass filter.
Due to the presence of the large series impedance, it is extremely difficult for harmonics to enter the filter / drive from the power source. Rather they are supplied to the drive via the filter capacitor. For this reason, it is very easy to predict the distortion levels which will be achieved and to guarantee the results
MOTOR TEMPERATURE REDUCTION
Motors operated on a VFD tend to run warmer than when they are operated on pure 60hz, such as in an across-the-line stator application. The reason is that the output waveform of the VFD is not pure 60hz,, but rather it contains harmonics which are currents flowing at higher frequencies. The higher frequencies cause additional watts loss and heat to be dissipated by the iron of the motor, while the higher currents cause additional watts loss and heat to be dissipated by the copper windings of the motor. Typically the larger horsepower motors (lower inductance motors) will experience the greatest heating when operated on a VFD.
Reactors installed on the output of a VFD will reduce the motor operating temperature by actually reducing the harmonic content in the output waveform. A five percent impedance, harmonic compensated reactor will typically reduce the motor temperature by 20 degrees Celsius or more. If we consider that the typical motor insulation system has a "Ten Degree C Half Life" (Continual operation at 10 degrees C above rated temperature results in one half expected motor life), then we can see that motor life in VFD applications can easily be doubled. Harmonic compensated reactors are actually designed for the harmonic currents and frequencies whereas the motor is not.
MOTOR EFFICIENCY
Because harmonic currents and frequencies cause additional watts loss in both the copper windings and the iron of a motor, the actual mechanical ability of the motor is reduced. These watts are expended as heat instead of as mechanical power. When a harmonic compensated reactor is added to the VFD output, harmonics are reduced, causing motor watts loss to be reduced. The motor is able to deliver more power to the load at greater efficiency. Utility tests conducted on VFD's with and without output reactors have documented efficiency increases of as much as eight percent (at 75% load) when the harmonic compensated reactors were used. Even greater efficiency improvements are realized as the load is increased.
CONCLUSION
VFD users have many choices when it comes to harmonic filtering. Of course they may do nothing, or they may choose to employ one of the many techniques of filtering available. Each filtering technique offers specific benefits and has a different cost associated with it. Some may have the potential to interfere with the power system while others will not.
For best overall results when using reactors or harmonic filters, be sure to install them as close as possible to the non-linear loads which they are filtering. When you minimize harmonics directly at their source you will be cleaning up the internal facility mains wiring. This will also reduce the burden on upstream electrical equipment such as circuit breakers, fuses, disconnect switches, conductors and transformers. The proper application of harmonic filtering techniques can extend equipment life and will often improve equipment reliability and facility productivity.
[attachment=66824]
INTRODUCTION
Since last 25 years there has been an increase in the use of solid state electronic technology. This new, highly efficient, electronic technology provides product quality with increased productivity. Today, we are able to produce products at costs less than in the years passed, with the introduction of automation by using the solid state electronic technology .This new technology requires clear electric power.
The conventional speed control systems are being replaced by modern power electronic systems, bringing a verity of advantages to the users. Classic examples are DC $ AC drives, UPS, soft stators, etc. Since the thrusters converter technology is rapidly gaining in the modern industrial plants, the power supply systems are contaminated as the ideal sinusoidal current and voltage waveforms are getting distorted. This is in turn is affecting the performance of the equipment in the electrical network.
ABSTRACT
Power quality is essential for smooth functioning of industrial process. As industries expand, utilities become more interconnected and usage of electrically equipment increases, power quality is jeopardized. The quality of power in the power system is severely affected by the presence of harmonics. This harmonics adversely effects the power system performance. Some of the effects are over heating of metal parts, noise in motors, low efficiency in motors etc. The effects produced by the harmonics are reduced by adopting some corrective measures
WHAT IS POWER QUALITY?
Adequate to superior power quality is essential for the smooth functioning of critical industrial processes. As industries expand, utilities become more interconnected and usage of electronically controlled equipment increases, power quality is jeopardized. Most large industrial and commercial sites are served by overhead lines with feeders that are subject to unpredictable and sporadic events, e.g. lightning and contact with tree limbs. Most distribution circuits have resoling devices that clear temporary faults through a timed series of trip and close operations.
This minimizes the possibility of long-term outages but leads to a number of minor power disturbances. These typically occur several times a month. Many electric utilities have increased the voltage at which they distribute power. This allows a single circuit to serve more customers or deliver higher loads, and reduces energy losses in the system. But it often means the overhead distribution circuit is longer, with more exposure to disturbances. And disturbances travel farther because of lower system impedances associated with higher voltage circuits. Sophisticated new systems are providing vastly increased efficiency and control in critical processes. But with their high sensitivity even to brief variations in electric power quality, today's computer-driven devices fail when power is disturbed for even a few milliseconds.
HARMONICS-BASIC CONCEPTS
A pure sinusoidal voltage is conceptual quantity produced by an ideal AC generator build with finely distributed stator and field windings that operate in a uniform magnetic field. Since neither the winding distribution nor the magnetic field is uniform in a working AC machine, voltage waveform distortion is created, and the voltage time relation-ship deviates from the pure sine function. The distortion at the point of generation is very small (about 1%to 2%), but nonetheless it exists.
Because this is a deviation from a pure sine wave, the deviation is in the form of a periodic function and by definition, the voltage distortion contains harmonics. When a sinusoidal voltage is applied to a certain type of load, the current drawn by the load is proportional to the voltage and impedance and follows the envelope of the voltage wave form .These loads are referred to as linear loads (loads where the voltage and current follow one another without any distortion to their pure sine waves).examples of nonlinear loads are resistive heaters, incandescent lamps and constant speed induction and synchronous motors.
In contrast some loads cause the current to vary disproportionately with the voltage during each half cycle. These loads are classified as nonlinear loads and the current and voltage have waveforms that are non sinusoidal containing distortions where by 50 Hz waveform has numerous additional waveforms superimposed
THE AFFECTS
The actual problems of any building/industry will vary depending on the type and number of installed harmonics producing loads. Most electrical network can withstand nonlinear loads of up to 15% of the total electrical system capacity without concern but when the nonlinear loads exceed 15% some non expected negative consequences can be expected. .for electrical networks , they have on linear loading of more than 25% particular problems can be apparent
SOLUTION
Users of variable frequency drives often have strict demands placed on them to mitigate harmonic distortion caused by the nonlinear loads. Many choices are available to them including line reactors, harmonic traps, 12 pulse rectifier, 18 pulse rectifiers, and low pass filters
HARMONIC FILTERS
In some cases, reactors alone will not be capable of reducing the harmonic current distortion to the desired levels. In these cases, a more sophisticated filter will be required. The common choices include shunt connected, tuned harmonic filters (harmonic traps) and series connected low pass filters (broad band suppressors). They consist of a capacitor and an inductor which are tuned to a single harmonic frequency. Since they offer very low impedance to that frequency, the specific (tuned) harmonic current is supplied to the drive by the filter rather than from the power source. If tuned harmonic filters (traps) are selected as the mitigation technique, then multiple tuned filters are needed to meet the distortion limits which are imposed
PULSE RECTIFIERS
Figure shows the actual measurement of input current harmonic distortion for 12 pulse rectifier supplied from a balanced 3 phase voltage source while operating at full load conditions. For test purpose transformer has delta primary and delta,wye secondary windings. To obtain the best results, the bridge rectifier is connected in series so equal dc windings. To obtain the best results, the bridge rectifier is connected in series so equal dc
PULSE RECTIFIER
To determine how an eighteen-pulse drive system operates under unbalanced line voltage conditions, we constructed a 30 HP eighteen-pulse drive from a conventional isolation transformer and standard six-pulse drive using the series bridge connection shown in figure 1. An auto transformer could have been used in place of the isolation transformer. The auto transformer costs less and requires less mounting space, but the isolation transformer was selected because it provides better performance and is readily available as a modified standard transformer. Care was taken in the physical construction of the transformer to balance the leakage reactance and output voltage of the three secondary windings. The system was tested with line voltage unbalance ranging from 0% to 3% and with loads ranging from 5% to 100%. The input total harmonic current distortion, THID, is shown in figure 3. THID varied from 7.4% at full load with balanced line voltages to 59% at 30% load with a 3% unbalance
LOW PASS HARMONIC SUPPRESSORS
Low pass harmonic filters, also referred to as broad band harmonic suppressors, offer a non-invasive approach to harmonic mitigation. Rather than being tuned for a specific harmonic, they filter all harmonic frequencies, including the third harmonic. They are connected in series with the non-linear load with a large series connected impedance, therefore they don’t create system resonance problems. No field tuning is required with the low pass filter.
Due to the presence of the large series impedance, it is extremely difficult for harmonics to enter the filter / drive from the power source. Rather they are supplied to the drive via the filter capacitor. For this reason, it is very easy to predict the distortion levels which will be achieved and to guarantee the results
MOTOR TEMPERATURE REDUCTION
Motors operated on a VFD tend to run warmer than when they are operated on pure 60hz, such as in an across-the-line stator application. The reason is that the output waveform of the VFD is not pure 60hz,, but rather it contains harmonics which are currents flowing at higher frequencies. The higher frequencies cause additional watts loss and heat to be dissipated by the iron of the motor, while the higher currents cause additional watts loss and heat to be dissipated by the copper windings of the motor. Typically the larger horsepower motors (lower inductance motors) will experience the greatest heating when operated on a VFD.
Reactors installed on the output of a VFD will reduce the motor operating temperature by actually reducing the harmonic content in the output waveform. A five percent impedance, harmonic compensated reactor will typically reduce the motor temperature by 20 degrees Celsius or more. If we consider that the typical motor insulation system has a "Ten Degree C Half Life" (Continual operation at 10 degrees C above rated temperature results in one half expected motor life), then we can see that motor life in VFD applications can easily be doubled. Harmonic compensated reactors are actually designed for the harmonic currents and frequencies whereas the motor is not.
MOTOR EFFICIENCY
Because harmonic currents and frequencies cause additional watts loss in both the copper windings and the iron of a motor, the actual mechanical ability of the motor is reduced. These watts are expended as heat instead of as mechanical power. When a harmonic compensated reactor is added to the VFD output, harmonics are reduced, causing motor watts loss to be reduced. The motor is able to deliver more power to the load at greater efficiency. Utility tests conducted on VFD's with and without output reactors have documented efficiency increases of as much as eight percent (at 75% load) when the harmonic compensated reactors were used. Even greater efficiency improvements are realized as the load is increased.
CONCLUSION
VFD users have many choices when it comes to harmonic filtering. Of course they may do nothing, or they may choose to employ one of the many techniques of filtering available. Each filtering technique offers specific benefits and has a different cost associated with it. Some may have the potential to interfere with the power system while others will not.
For best overall results when using reactors or harmonic filters, be sure to install them as close as possible to the non-linear loads which they are filtering. When you minimize harmonics directly at their source you will be cleaning up the internal facility mains wiring. This will also reduce the burden on upstream electrical equipment such as circuit breakers, fuses, disconnect switches, conductors and transformers. The proper application of harmonic filtering techniques can extend equipment life and will often improve equipment reliability and facility productivity.